Muscle

CHAPTER 9

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Cellular Physiologyof Skeletal, Cardi dc,and Smooth MuscleMichael Apkon

The primary function of each of the three fundamental types of muscle-. l ,e le t " . ca"J . rc . " d - roo th mus, l - r . to Bpne la lc lo r .e . ' r no , ,ene- tresponse Lo a physlological stimulus. All muscles transduce a chemical orelectrical command into a mechanical response. However, the unique phys-iologicaL lole ol each of the three basic muscle types dictates rnherentdillerences in the rate and duration of contraction, metabollsm, fatigabrLiry,and tl.re ability to regulare contractiLe srrengrh. For cxample. boLh skeletaLand cardiac muscle must be capable ol rapid force cler.eLopment and shorlening. However, skeletal muscle must be able to maintain contractile forcefor relatively long periods, whereas cardiac muscle need onLy contractbriefly wiLh each heartbeat, although it must sustain Lhis periodic acnvtyfor an entire lif.etime. Smooth muscle, like skeletal muscle, must be able toregulate contraction over a lvide rangr of folce Horvever', rn some tissues(e.g snhincrcrs) smoorh muscle rnust be able to sustain contraction with-o r r l : r i o r e l o ' . c n l n n o n p r i n d . l n . n r , o f r h r * n l f e r c n p r r h p r r. . . . . f lgqerfor muscle contraction is the same for all three types of muscle: a rise rnthe free cylosoLic Ca'z- concentration ([Ca'z-],).

In muscle of all types, certain comnonents of the mnsrle cell are highlyspecialized to accomplish the muscle's unique lunctiolr. Given the dispanryin their physiological roles, it is not surprising that eacl.r muscle type hasevolved unique anatomic structures and functionaL mechanisms. Thus, thevJr ou- \ ,pe> o l nu . . " ce l , L l ,e .pe . a l t - ,d phsmJ 1 . . rb r "ne ( ) lo , ,eeLons, endoplasmic reticulum, and metabolic pathways for energy genera-tion and utilizatLon.

EXCITATION OF MUSCLE CEttS

The plasma membrane or sarcoLemma-of each of the t1-pes of musclecelis has regions that are specialized Lo facihtate communication betweer.iceLls. ln skeletrl muscle, this specialization Lakes Lhe form of the postslnap-lic portion of the neuromuscular junctior-r Neuromuscular junctions (i.e.,chemical sy-Lapses) are also present in cardiac and smootlr muscle. lncardiac muscle, neuromuscular transmissiol-t cloes not iniLiaLe contraction; ilonly modulates il. On the other hand, neuromuscular transmission mayrnitiale contraction in smooth muscle ceLls, or it may modulate conlraclionthat has been initiared by another mechanism. In both smooth and cardiacmuscle cells, grp.lunLtions (i e . electrical synapses) couple the sarcolemmao[ nerghbonng ceL]s: these also play an rmportant role in intercellularcommunlcanon.

230

Branched structure of cardiac muscle

i Desmosome --:-*-;

lntercalaied disk

l\,4 icrofib ril Intercalated disk

Z-lineFICURE 9-1. Electrical coupling of cardiac myocyles

Skeletal Muscle Contracts in Response toNeuromuscular Synaptic TransmissionThe mature skeLetal muscle cell has a sinsle neuromuscu-d r - u n c L i o n \ l ' e r e d r e L ) . c L o l i n e r A ( h ' r e i e p t o r . r r e . o n -

centrated (p.209). A single muscLe cell responds to onlya single neuron. However, a single neuronal axon maybifurcate to innerrrate severa] individual muscle cells. Thegroup of muscle ceLls inneruated by a single neuron isreferred to as a motor unit.

The neuromuscular juncrion is the focus of Chapter B.Bnefiy, the ACh released by the preslnaptic nene termi-nal binds to inotropic (nicotinic) ACh receptors at theneuromuscular junction. These receptors are nonselectivecation channels that open when ACh binds to a specificsite on the channel, and a depola zation known as theend-plate potential is produced. I[ this end-plate potential exceeds the threshold for activatins Na+ channels. and. r ioF pore l , . j rp -u l l> t renera l ton o l in ac on po tenr ia linitiates the sequence of processes leading to contraction.The ACh ls rapidLy inaclivated by acetylcholinesterase, anenz)rme lhat is manufactured by the muscle ceLL, andmuscLe contraction stops a few milliseconds after neuronalacilvity ceases.

Cardiac Muscle Contracts in Response to thePropagation of Electrical Signals from OneCardiac Cell to Another Across Gap lunctionsCardiac muscle cells also have chemical sl'napses. but the5yn ld tLe l c a rd para . lnpathe t i , b ra rches t l rFe auLono lc ner \oL< <vsren l :ec Chapter l5 ) u ie rhese q)map-

Cellular Physiology of Skeletal, Cardiac, and Smooth Muscle / 9 2r1

ses to modulate, rather than to inltiate, cardiac musclefunction. ln contfast to skeletal muscLe, cardiac musclecontraction is triggered by electrical signals from neighboring cardiac muscle celLs. These electrical impulses orig-inale in the pacemaker region of the heart, the sinoatrialnode (p. 489), which spontaneously and periodically gen-erates aclion potentiaLs. To facilitate direct electrical com-munication between cardiac muscle cells, the sarcolemmaof ,a rJ t " , . musc le I< <pe. ia l i zed ro conLd in gap junc l ton .(p. 164), electrical s).Tiapses that couple neighboring cells(Fig. 9-1). When an action potential is initiated in onecell, current flows through the gap junctions and depolar-izes neighboring cells. If depolarization causes the mem-brane potential (V-) to be more positrve than threshold,self-propagating action potentials occur in the neighboringcells as well. Thus, the generation of an action potential isjust as critical 1or initiating contraction in cardiac muscleas it is in skeLetal rnuscle.

Smooth Muscles May Contract in Response toEither Neuromuscular Synaptic Transmissionor Electrical CouplingLike skeletal muscle, smooth rnuscle receives synaptic in-put from the neryous system. However, the q.naptic in-put to smooth muscle differs from that to skeletal musclein two ways- First, the neurons are part of the autonomicnpnouc sys lem ra ther tha- Lhe 'omat ic nervou. sys tem(see Chapter 15). Second, the neuron makes multiplecontacts with a smooth muscle cell. At each contactpornt, the axon diameter expands to form a varicositythat contains the presynaptic machinery. The varicosity isin close proximity io the postslrlaptic membrane of thesmooth muscle cell, but there is relatively little specializa-tion of the posts).naptic membrane. Rather than the neu-rotransmitter receptors being closely clustered at the neu-romuscular junction, as in skeLetal muscle, in smoothmuscle the receptors are spread more wideLy across theposts).naptrc membrane.

The mechanisms of intercellular communication amongsmoolh muscLe cells are more diverse than are those of.Le le ta l o r cardrac musc e In 'one o-ga- ' . 5moo h mu<-cle is innervated in a manner similar to skeletaL muscle inrh" r each smoorh mu"c le ce l l recer re< 5) 'nap l ' c inpu lHowever, a difference is that a smooth muscle cell mayreceive input from more than one neuron. Moreover,there is little electrical coupling among these smoothmuscle ceLls (i.e., few gap junctions). As a result, eachsmoolh muscle cell may contract independently of itsneighbor. Because this type of smooth rnuscle behaves asmultiple, independent celLs or groups of ceLls, it is calledmultiunit smooth muscle (Fig. 9-2A). Note that the"multi" in "multiunit" refers to lhe muscle fibers' actingindependently of one another as multiple unirs. Muhiunilsmooth muscles are capable of finer control. Indeed, mul-tiunit smooth muscle is found in the iris and ciliary bodyof the eye, the piloerector muscles of the skin, and someblood vessels.

ln contrast to multiunit smooth muscle, the smootllmuscle cells of most organs have extensive intercellularcommunication in the manner of cardiac muscle cells. ln

I,4ULIIUNIT

9 / Cellular Physiology of Skeletal, Cafdiac, and Smooth Muscle

A I,4ULIIUNIT

Elechical isolationof cells allows finermotor control,

UNITARY

Autonomic neurons/.

Smooth niuscle cetl Varicosilies(synaptic contacts)

FICURE 9-2. Smooth-muscle organization. A, Each smoolh muscle celL receives its own s'naptic inpuL B, Only a ferv of the snooth muscle cellsreceive direct s\taDlrc rnDut.

tl.ris tlpe of smooth muscle, gap junclions permit electdcal communication between neighboring cells. This com-munication alLows coordinated contractior-r of many celLs.Because these cells contract as a single unil, rhis tlpe ofsmoolh muscle is called unitary smooth muscle (Fig. 92B). Unitary smooth muscle is the predominant smoothr.ru5cle lpe wtthin Lhe wall, oI r s.e-a] orga . ,u, h a,the gastrointestinaL tract, the uterus, and many blood ves-sels. For this reason, unltary smooth muscle is often re-ferred to as visceral smooth muscle. Among unitarysmooth muscles, variation in the strength of intercellularcoupling from organ to organ leads to VariaLion in thespatial exlent ol a single unit. For example, in Lhe blad-der . e r ten- re coup l ing amorg , .e l l s denne, a ge lu r rLrional units, which allows the muscular wall o[ the blad-d c r t o . o n t r a , i n s ; n . h r o n 1 . O n r h e o r h e r " n d . r h ,-moorh mus. lp ce ls o l b lood ve>se l i coup le Lo orn-smaller, independently lunctioning units that are moreakin to multiunit smooth muscLe. ln fact, electrical coupling of smooth muscle unjts exhibits a tissue-specifrcconlinuum f.rom multiunit to unitary couphng.

Action Potentials of Smooth Muscles May BeBrief or Prolonged

Whereas both skeletal muscle and cardiac muscle procluceactron potentials that iniliale contraction, smooth musclecells produce a wide range o[ V., variations that caneither initiate or modulate contraction. Acrion poientialsthai are similar to rhose seen in skeletal muscle are obseNed in unitary smooth muscle and in some multlunitmuscle. Like cardiac muscLe celis, some smooth musclecells exhibit prolonged aclion potentials that are charac-terized by a prominent plateau. Still other smoorh musclecells cannot generate action potenrials at all. In thesecelLs, V^ changes in a graded fashion (p. 173) rather rhanin the all-or-none manner of action potentials. The stimulithat produce a graded response of V," include manycirculating and Local humoral factors, as welL as mechani-cal stimuli such as stretching the cell. These graded V..hange, . ra r be e i iher h \pe tpo ia r iz ing o ceoo lar rz rng .they sum lemporaily as well as spatially. lf the summationof graded depolarizations brings V," above threshold-in

a smooth muscle cell capable of producing an actionpolenlial an action potentlal will then ensue.

Action potentials are usualLy seen in unitary (visceral)smooth muscle. These action potentials typically have aslower upsLroke and longer duration (up to -100 ms)than do skeletal muscle acrion porentials (-2 ms). Theaction potential in a smoolh n]uscle celL can be a simpLespike, a spike foLlowed by a plateau, or a series of spikeson top of slow waves of V- (Fig. 9 3A). In any case, theupstroke or depolarizing phase of the action potentialreflects opening of voltage gated Ca2* channels. The in-ward Caz* current further depolarizes rhe cell andthereby causes still more voLtage-gated Ca2* channels toope . Thu. . 5one 5moorh musc le .e l l s ,an unocrgo thesame tlpe ol regenerative depoLadzation that is seen inskeletal muscle. However, the rate o[ rrse of the actionpotential in smooth muscle is lower because Ca2* channels open more slowly than do Na* channels in skeleraland cardiac muscle (p. 189). Repolarlzation of the smoothmuscle ceLl is also relatively slorv. Two explanations maybe o ' [e od [o - Lh is . lower repo la r - - t lon . t i - : r . \o l l Jgc-gated Ca']- channels, which are responsible lor the depo,larization phase of the action potential, inacrivare slowly.Seconcl, the repolarization phase of the action potenrialre l lec r ' Lhe de l r \ed dc t r \d t lon o l ro l rage-gaLeo K ' .han-nels and, in some cases, Ca2* activated K* channels.

Some smooth muscle cells have fast, voltage-gated Natchannels. Howerer, even when these channels arepresent, they do not appear to be necessary for generatingar.r aclion potenlial. Their main role may be to allow morerapid actnation of voLtage-gated Ca,+ channels and thuscontdbute to a faster rate of depolarization.

In some unilarl' smooth muscle, repolarization is sodelayed that the action porential contour displays a prom-inent plateau. These plateau porentials may be severalhundred mllliseconds in duration, as in cardiac muscle.Plateau action potenrials occur in smooth muscle of Lhegenilourinary tr-act, including the ureters, bladder, anduterus. The long V- plateau allows the entry of Ca2* tocontinue for a longer period and thus alLows lCa,*j, toremaln high for a longer period, thereby prolonging rhecontractlon-

A TYPES OF SI\,IOOTH.MUSCLE ACTION POTENTIALS

Cellular Physiology of Skeletal, Cardiaq and Smooth Muscle / 9

Slow waves

Time (sec)

+ 1 0

0

_10

-50

-90

0 100Time (rnsec)

B GENERATION OF SLOW WAVES

0 200 400Time (msec)

1 0

---------------- r -----€- rI

Action potential spile J

al

Voltage-gated Ca2+clnnnels open

Ca2t influx, [e in internalCa" concentration

Voltage-gated Ca" charmels close;internal Ca'- concentration decreases

Open Ca'--dependentK" channels

Slow hlTerpolarization

FICURE 9 3. Action pot€ntials and slow waves in smooth muscle

9 / Cellular Physiology of Skeletal, Cardiac, and Smooth l\4uscle

Some Smooth Muscle Cells Can InitiateSpontaneous Electrical ActivityAlthough smooth muscle cells undergo changes in V- inresponse to neuraL, hormonal, or mechanicaL stimulation,many smooth muscLe cells are capable of initiating spon-laneous electrical activity. In some ceLLs, this spontaneousactivity results from pacemaker currents. These currentsresult from time- and voltage-dependent properties of ioncurrents that produce either a spontaneous increase ininward or depo la i z rng . cur renr r .e .g ro l tage gared Ca-cufients) or a spontaneous decrease in ouiward, or hyper-po la - rz ing . cur ren ls re .g . . vo Lage gared K ' cur renLs) Thepacemaker curents cause the cell to depolarize unril V-reache ' th re .ho ld . I r igger -g Jn acr ron po tenL ia l .

ln other smooth muscle cells, this spontaneous electri-cal activity results in regular, repeddve oscillations in V-.These 4, osclllations occur at a frequency of severaL oscil-lations per minute and are referred to as slow waves(Fig. 9 3B). One hyporhesis for the origin of slow-wavepotentials suggests that voltage-gated Ca2+ channels-ac-r i v e / l r h c r e s t i n o V - d e n n l a t o e r h e c p I p r n r r o h r ^

activate more voltage-gated Ca2* channels. This activationresults in progressive depolarization and Ca'?* influx. Theincrease in [Ca'?+], activates Ca'z* dependenr Kn channels,which leads to progrcssive hyperpolarization and rermination of the depolarization phase of the wave. These peiodic depolarizations and [Ca'?t], increases cause periodic,tonic contmctions of the smooth muscle. When the am-plitude of the slow V", waves is suff,cienr to depolarizethe cell to threshold, the ensuing action potentials lead tof l r lLer ea ' tn f lux and oha5 ic cont racr ron . .

Other hlpotheses to ixplain spontaneous eLectrical andmechanical activity in smooth muscle cells are based onoscillaiorJ changes in other intracellular ions or mole-cules. For example, increased [Ca,*], during an actionpotenlial mighr stimulale Na-Ca exchange and lead to acyclic increase in lNa*1, and rhus an increase in the rateof Na* extrusron by the electrogenic Na-K pump Alterna-tively, the inositoL 1,4,5-triphosphate (lP) recepror chan-nel (p. 100) might spontaneousLy open and reLease Car*.The effect on [Ca2*], would be self-reinforcing because ofCa' -acrivaled Ca) release ria Lhe lP, recepLor Ar nigl-[Ca,*j,, this channel is inhibired and rhe Car+ releaseevent is terminated, followed by re-uptake of Ca2* intothe sarcoplasmic reticulum (SR). The fCa']*], increasesmay rhemselves lead to periodic electical activily by stimulating Ca'z*-activated inward and outward currents.

Some Smooth Muscles Contract WithoutAction Potentials

Whereas action-potential generation is essential for initiat-rng contraction of skeletal and cardiac muscle, manysmoolh muscle cells contract despite being unable to generate an aclion potential. As discussed previously, V- os-cillations can lead to tonic contractions in rhe absence ofaction potentials. Action potendals usually do not occurin multiunit smooth muscle. For example, ln the smoothmuscle that regulates the ids, excitatory neurotransmitterssuch as norepinephrine or ACh cause a local depolariza,tion, rhe junctional potential, which is similar to the

end-plate potential in skeletal muscle. Junctional poten-t ia l ' ' p read e lec t ro to - rca l l i , .e . in a g -aded fas" onrthroughout ihe muscle fiber, thereby altering V- and af-lec t ing rhe en t ry o f Ca2 th roug l vo l rage-garec : lo r , r(L+1pe) Ca':* channels. Changes in V--by an unknownTechan i )m ma1 a l ,o moduLare rhe a , r . r t y o f Lhe enz1-me phospholipase C, which cleaves phosphoinositidesto release the intracellular second messengers diacylglyc-erol (DAG) and IP, (p. 100). Both these second messen-gers are modulators of contractile force. In the absence ofaction potentials, some unitary smooth muscle, includingsome vascular smooth muscle, also contracts as a result ofgraded V- changes.

Some smooth muscle cells conrract without any changein V-. For example, a neurotransmifter can bind to areceptor, activate a G protein, and lead to the generationof lPr, which in turn leads to the release of Ca,* from theSR. The eventual depletion of Ca2* stores in the SR mayin tum stimulate Ca2* influx across the cell membranevia so-called store-operated Ca2t channels.

MUSCLE CONTRACTION

Striated Muscle Cells Are Densely Packedwith Myofibrils That Contain Ordered Arraysof Thick and Thin FilamentsAs summarized in Figure 9 44, each individual skeletalmuscle cell (or myacjte or rfiber) contains a dense parallelarray of smaller, cylindrical elements called myofibrilsthat have the diamerer of a Z disk (p. 45). Each of thesemyoEbrils compises repeating units, or sarcomeres, thatconsist o[ smaller interdigitating filaments called myofila-ments. These myofilaments come in two q.pes (see Fig.9-4B), thick fiLaments composed primarily of myosin andthin filaments composed primarily of actin (p. 27). Thesarcomere extends from one Z disk to another. Sarco-meres stacked end to end make uo a mvofibril. ThereoeaLing sarcomeres are rro5l l- ighli organrzec wiLhinskeletal and cardiac muscle and lmpan a striped appear-ance. Thus, both skeletal and cardiac muscle are referredto as striated muscle.

ln smooth muscle, striations are not vlsible. Althoughactin and myosin are present in smooth muscle cells, therelationship between actin and myosin (lhin and thickfilaments) is less highLy organized. The actin filaments areoriented mainLy parallel or obLique to rhe long axis of thecell. Multiple acdn filaments appear to join ar electron-dense regions called dense bodies, which are found immediateLy beneath the celL membrane, as well as withinthe inteior ol the myocyte. The rhick frlaments are lnter-spersed among the thin frlaments in smooth muscle, andare far less abundant than in skeletal or cardiac muscle.

Thin filaments are 5 ro B nm in diameter and, instriared muscle, 1 p,m in length. ln stiated muscle, thethin filaments are tethered together at one end, wherethey project from a dense disk known as the Z disk. TheZ disk is oriented perpendicular to the axis of the musclefiber; thin hlamenti pioject from both its faces. Nor onlydo Z disks lelher the thin fi.laments of a sinele mvofibriLtogether bu t connecr ronr oeLween the Z d i< l -s aso teLhe-

@

A FROM IV]USCLE TO I\ i ]YOFILAMENTS

Cellular Physiology of Skeletal, Cafdiac, and Smooth MLrscle / 9

B tvloDEL OF A SARCOI\iIERE A oano

Z ine

each myoflbril to its neighbors. These interconneclionsalign the sarcomeres and give skeletal and, to a lesserextent, cardiac muscle i|s slriated appearance.

The thick filaments are l0 nm in drameter and, instriated muscle, 1.6 pm in Length. They lie between andpartially interdigitate with the thin filaments. Thrs partialinrerdigitarlon results in ahernaung 1lghL and dark bandsalong the axis of the uryofibril (see Fig. 9 4C). The [ghtbands, which represent regions of the thin hlament thardo -o r l re a longs ioe t c ( h lamen ' . a re l . ow- a . Ibands because they are isorropic to polarized hght. The Zdisk is visibLe as a dark perpendicular Line at the center ofrhe I band. The dark bands, which represent the myosinfilaments. are known as A bands because they are anisorropic to polarized light. During contraction, the A bandsare unchanged in length whereas the I bands shorten.

Wirhin the A bands, the pivoting heads of the thickmyosin filaments, the moiecular motors, establish cross-bridges to the thin actin filaments. As drscussed Later, theadenosine triphosphate (ATP)-dependent cycle of n.iakingand breaking cross bridges causes the actin filament to be

cr-Actinin

C ELECTRON I\, l ICBOGBAPH OF SABCOIVEBf

drar,r,n over rhe myosin filament and rhereby resulb inmuscle contraction.

The Thin and Thick Fi laments AreSupramolecular Assembl ies ofProtein Subunits

THIN TILAMENTS. Tl.un filaments (Fig. 9 5A) consist ofactin, tropomyosrn, and troponin. The backbone of thefllament is a doubLe-stranded a-heLical pol1'rner of actinmolecules. Each helical turn of a single strand of filamen-tous or F-actin consists of 13 indivldual actin monomersand is approximately 70 nm lor.rg. F-actin is associatedr,vith trvo important regulatory, actin-binding proteins:

e r d t r n n n n i n

lndir'1dual tropomyosin molecules consist of two ider.r-tical o helices that coil around each other and sit near theLwo grooves that are tormed by the two hellcal actinstrands. Head-to-tail contacr between neighboring tropo-myosin molecuLes resuLts in two nearly continuous helical

I band H band

thin i i lamenls(myofilaments)

FICURE 9-4. Structure of the sarcomere.

One sarcomeTe

One sarcomere

Troponin^complex

TnT TnC TnI

236 9 / Cellular Physiology of Skeletal, Cardiac, and Smooth Muscle

A THIN FILAMENT

i<__ 70 nm

ca2* (oouno ito troponin \complex) \

myosin-ll molecules. Each myosin-ll molecule is a hex-amer (actually a double trimer) composed of two inter-twined heavy chriins, rwo regulatory light chains, and twoalhali (or esseixtial) lighr chcins. The two healry chainshave three regions: a rod, a hinge, and a head region. Therod portions are a helices thar wrap around each orher.At the hinge regions, rhe molecule flares open ro form twoglobular hec&, which are rhe cross bridges between thethick and thin hlaments of lhe sarcomere The heads ofthe healT chains also called 51 fragments-each possess a site for binding acrin as well as a site for bindingand hydrolyzing ATP. The head portion of each myosinlorms a complex with two light chains, one regulatoryand one alkali. The alkali light chain plays an essentialroLe in stabiLizing the myosin head region. The regulatorylight chain, as irs name implies, regulares rhe ATpaseacti\.ity o[ myosin. The activity of the myosin regulatorylight chain is in rum reguiared via phosphorylation byCa']* dependent and Ca'z*-independent kinases.

Figure 9 5C summa zes the interaction between athin hlament and a single pair of head groups from themyosin of a thick filament.

In All Three Muscle Types, An Increase in[Ca2+], Triggers Contraction By Removing theInhibition of Cross-Bridge CyclingUnderlying muscle contracrion is a cycle in which myo-sin Il heads bind to actin, rhese cross-bridges becomedistorted, and llnally rhe myosin heads detach from actin.Energy for ftis cycling comes from the hydrolysis of ATP.However, if unregulated, the cyciing would continue untilihe myocyte was depleted of ATP. It is not surpdsing,rhen, that skeletal, cardiac, and smooth muscLe each havemechanisms for regulating cross-bridge cycling. In ailLhree cell lypes, an increase in [Ca,+]i initiates and allowscross-bridge cycling to continue. During this excitatoryincrease, [Ca2*1, may rise lrom irs resling level of lessthan 10-7 M to greater than l0-5 M. The subsequentdecrease in [Ca2*], is the signaL ro cease cross-bridgecycling and relax.

Regardless of the muscle r1.pe, Ca,+ moduiates conlrac-tion through regulatory proteins rather than interactingdirectly with the contractile proteins. ln the absence ofCa2t, these regulatory proteins act in concert to inhibitactin myosin interactions, rhus inhibiting the contractileprocess. When Ca'?* binds to one or more of rhese pro-reins, a conformational change takes place in the regula-tory complex that releases the inhibition of contraction.

SKEIETAL MUSCLE. The heierotrimedc troponin mole-cule contains the key Ca']t-sensiiive regulator troponin C(Fig. 9-64). Each troponin C moLecule in skeleral muscle has two high-afftnity Car*-binding sires rhat parrici-pate in binding of troponin C to rhe thin fiLament. Caz*binding to these high affinity sires does nor change duringmuscle activation. Each troponin C molecule in skeletaLmuscle also has two additional, low-affinity Car*-bindingsites. Binding of Ca2* to these lo*affinity sites induces aconformational change in the troponin complex that hasrwo elfects. The f,rst effect is that the C terminus of the

Actin Tropomyosin

B MYOSIN MOLECULE

Heads of myosinheavy chain (Sr)

Regulatorylight chain

AIkalil ight chain

ot heavy chainsTail region of heavy chains

C INTERACTION OF THIN AND IHICK FILAIV]ENTS

fi lament)(thin

Myosin (thickfi lament)lvlyosin head bound

to actin filament atmyosin binding site

FICURE 9 5. Sructure ol ihin and rhick filalr€trls.

filanents that shadow the acrin double heLix. The lengrhof a single tropomyosin molecule corresponds to aboutseven actin monomers (i.e., a half tum of the actin helix).A . we sha l l see la rer . Lhe ro le o f L ropomlo- s to .n le r -fere with the binding of myosin to actin.

Troponin is a heterotrimer consisting of troponin T(which binds to a single molecule of rropomyosin), tropo-nin C (which binds Ca']*), and rroponin I (which binds toactin and inhibits contraction). Troponin C is closely re-lated lo another Ca2t-bincling protein, calmodulin (CaM;p. 102). Thus, each troponin heterotdmer interacts with asingle tropomyosir] molecule, whlch in turn interacts withseven actin monomers. The tropomn compiex also interacts directly with the actin filaments. The coordinatedInre I . l ( l l o - s t rq rg t ropon in . l ropo-nyor ln . a rd a tL 'n a l -lows actin-myosin interactions to be reguiated by changesin lCa']*1,.

THICK FILAMENTS. Like ihin acrin filaments, rhick hla-ments are poLymers of proteins (see Fig. 9 5B). Thickfilaments are bipolar assemblies composed of multlple

Cellular Physjology of Skeletal, Cardiac, and Smooth Muscle / 9

INITIATION OF CROSS-BRIDGE CYCLINGIN SKELETAL AND CARDIAC I\4USCLE

TropomyosinTroponincomPlex

B INITIATION OF CROSS-BRIDGE CYCLING IN SN/]OOTH MUSCLE

^ ^2+

A li .'r, *\ \-v\)

Calmodulin

^ ^2+

\ r lt

The movement of the tropomyosindeeper into the actinunmasks the mvosin bindins sites.

/)_

WAct ve calmodulin/MLCK complex

FIGURE 9-6. The role ol Car in triggerLng muscle contraction MLCK. m),osirl light chain kinase

inhrbitory troponin I moves awa)' lrom the actin/tropomy-Lr - l r F /T .n t . th , r . \ r -e | r ' t t ing .he rooomlos in mo ocule to rnove. According to one hypothesis, the othereffect, transmitted through troponin T, is to push tropo-myosjn away from the myosin bindilrg site on the actinand into the aclin groo\.e. With the steric hindrance re

* n o r e d t h e m 1 o : t n h , . d r . . r b l e r o i n t e r d . r r r i h l r t * r n d@ engug" n !ro\s bLidge (yLLlng

---s\{ \ Inactive

\!vosinAIr(

Regulatorylight chain

Activatedmyosin

(ARDlAc MUSCLE. The regulatory mechanism withincardiac muscle is similar to that of skeletal muscle, al-houg l - r -opontn \ rom . , d rc mus. le ha , 1 - - , , ' ; , ' *1 .

dc t v ' lou -a I t i t a / b rnd ' -g s i . .

sMooTH MUscLE. An entirely different mechanism con-t o l c . o . . b r J g c u " r o , e ' i n , n o o t h m J . ! l e H e r ! . a nincrease in lCaz-1, init lates a slow chain of eients thaL

9 / Cellular Physiology of Skeletal, Cardiac, and Smooth Muscle

ultimately increases the ATPase activity of the myosin (seeFig. 9-68). The first step is the binding of four Ca'?* ionsto calmodulin, which, as we noted earlier, is closely re-lated to troponin C. Next, the Ca2*-CaM complex activates an e.zyme known as myosin light chain kinase(MLCK), which in turn phosphorylates the regulatorylight chain that is associated with the myosin-ll molecule.Phosphorylation of the light chain alters the conformationof the myosin head, which greatly increases its ATPaseacrivity and aLlows it to interact with actin and act as amoLecular motor. Thus, in smooth muscLe, CaM ratherthan troponin C is the Ca']+-binding protein responsible[or tra-.ducing LLe Lontraction rriggering increases in[Ca,*],. Note rhat in smooth muscle, contraction cannolbegin until MLCK increases the ATPase activity of myo-sin, which is a time-consuming process. In skeLetal andcardiac muscle, on the oth€r hand, the ATPase activity ofrhe myosin head is constitutively high, and cross-bridgecycling can begin as soon as the tropomyosin is movedout of the way.

The mechanism just outlined acrivares rhe thich frIa-ments in smooth muscle. Other mechanisms act on therhin filaments of smooth muscle to remove the tonicinhibition ro actin-myosin interactions that are caused bysteric hlndrance. Two proteins-caldesmon and cal-oonin-tonically lnhibit the interaction between actinand myosin. Both are Ca2*-CaM-binding proteins, andboth bind to actin and tropomyosin. Calponin, which is{ound in a fixed stoichiometry with tropomyosin and ac-trn (one calPomn-one tropomyosln-seven acun mono-mers), tonically inhibits the ATPase activity of myosin. Aswe saw earlier, the increase in [Ca'?*], that triggers smoothmuscle contraction activates Ca2+-CaM. Besides activatingMLCK, this Ca'?* CaM complex has two effects on cal-ponin. First, Cd+ -CaM binds to calponin. Second, Ca2*-CaM activates Ca'z+-CaM dependent protein kinase,which phosphorylates calponin. Both effects reduce cal-ponin's inhibition of myosin's ATPase activity. Like cal-oonin. caldesmon also tonically inhibits the ATPase activ-ity of myosin in smooth muscle.

During the Cross-Bridge Cycle, ContractileProteins Convert the Energy of ATPHydrolysis Into Mechanical EnergyThe cross-bridge cycle that we introduced in the previousre lL 'o r oc lu -s rn f i ve sLeps (F 'g o T ln i r ia l l y . rhe myosin head is attached to an actin filament after the "powerstroke" from the previous cycle and after the actomyosincomplex has released adenosine diphosphate (ADP). lnthe absence of ATP, the system could remain in this rigidstate for an indefinitely long period, as is indeed the casein dgor mortis. In this dgid state, rhe myosin head is at a45-degree angle with respect to the actin and myoslnfilaments.

S lep l : ATP b ind ing ATP b ind ing to the head o f themyosin hear'ry chain (MHC) reduces the affinity of myo-sin for actin, causing the myosin head to release fromthe actin filament. lf ali cross-bridges in a muscle wereln rh i> r ta re . lhe mus( le ruou ld be fu i 1 re laxed.

Step 2: ATP hydrolysis. The breakdown of ATP to ADP

and inorganic phosphate (P,) occurs on the myosinhead; the products of hydrolysis are retained on them)os in . Ac a res- l t o l h ld ro)ys .s . the r ryosrn headpivots around the hinge into a "cocked" position (per-pendicular or at a 9O-degree angle to the thick andthin 6laments). This roiarion causes rhe tip of the myosin to move about ll nm along the actin filament soi l_a l r t no la l ines up wth a new ac t i r mono- re- t \ omonomers further along the actin hlament (see the boxr r led Vea.ur rng rhe Force o ' a 5 . -g le Cro>, -Br idge

r r - - - h r i d o p - i n r m r r < , l p\ ) \ , ( , v , r ( d a a r L i .

were in this state, the muscLe wouLd be fully relaxed.Step 3; Cross-bridge formation. The cocked myosin

head now binds to its new position on the actin frla-nen l . lh is b rndrng re f lec t< the rn . reased a f fn i t ; o l t \e

myosin-ADP-P, complex for actinStep 4: Release of P, from the myosin. Dlssociation of P,

f r n m r h e m t n . r n h e , r l r r i o o e r - r h e n n r v e r c r r n L e r

conformationaL change in which the myosin headbends approximateLy 45 degrees about the hinge andpulls rhe acrin ament about ll nm toward the tail ofrhe myosin nole.ule. lhrs Lonformarional changecauses the aclin filament to be drawn aLong the myosinfiLament, thereby generating force and motion.

Step 5: ADP release. Dissociation of ADP from myosincompletes the cycle, and the actomyosin complex is leftin a rigid state. The myosin head remains in the sameposition and at a 45-degree angle with respect to thethick and thin filaments. The ADP-free myosin com-plex remains bound to actin until another ATP bindsen/ ' l ,n, r i , rp< rnnthpr r . r lp

The ADP-free myosin complex ("attached state" in Fig.9-7) would quickly bind ATP at the concentrations ofATP normally found within cells. lf unrestrained, thisr ros , -b r idSe c ; .1 rng "ou ld cont rn re rn l 1 dep leL i rg Lhecytoplasm of ATP. At that time, the muscle would remainin the stiff "attached state" because release of the cross-bndge, Irom actin -equires binding of AIP o m)osrn.

Ac dis.ussed earlier. muscle cells do not egulare croc<-bridge cycling by modifleng lATPl,. lnstead, skeletal mus-cLe and cardiac muscle control this cycle at the third stepby preventing cross-bridge formation until the tropomyo-sin moves out of the way in response to an increase inlCa'?*],. Smooth muscle controls the cycle at the secondstep by prcventing ATP hydrolysis until the ATPase activ-ity of the myosin head increases in response to an in-crease in lCa'z*1,.

Although this general schema of cross-bridge cyclingoccurs in smooth muscle as well as skeletaL and cardiacmuscle, the frequency of cross-bddge cycling in smoothmuscle is less than one tenth the frequency encounteredin skeletal muscle. This variation reflecm differences inthe propeflies of the various myosin isoforms that areexpressed in various ceLl t1pes. Even though cross-bridgecycling occurs less frequently in smooth muscle, forcegeneration may be as great or greater, perhaps becausethe cross-bridges remain lntact for a longer period withearh Lycle lL .s l-kelv rhat thrs longer oenod duringwhlch the cross bridges are intact reflects a lower rate ofADP release from the smooth muscle isoform of myosin.

Cellulaf Physiology of Skeletal, Cardiac, and Smoorh Muscle / 9

Pl

gl- ,I ADP is released. ]

STATE

FICURE 9 /. The cross-bridge cycle in skeletal and cardiac muscle Each cycle advances the myosin head by rwo acrin monomers, or approximaiely1l nm.

POWER-STROKE

Because ATP Stores Are Small. the CellMust Regenerate the ATP Needed ForMuscle Contraction

Each cross-bridge cycle consumes one moLecule of ATP.ln skeletal muscle. the entire cellular store of ATP issufficient to aLlow only a few seconds of continuous max-imal contraction. Therefore, the muscie cell must resln-thesize ATP from ADP at a rate comparable to the rate ofATP consumption. Skeletal muscle has specialized energyslores that permit rapld regeneration of ATP. The mosrreaq i lv ave . lab le poo l o f rh is ene-g) r> LLe h igh-energyphosphare bond of phosphocreatine. The enz)rme creatinephosphotransJerae transfers the high-energy phosphate ofphosphocreatine to ADP, thereby rephosphorylating ADPto ATP. The phosphocreatine conient of skeletal muscle isadequate to repLenish the ATP pool several times, but it iss t i l l rnadequate fo - . r . ra -g rhe energ) need ' o .on-tractins muscle for more than 10 seconds.

ATTACHED STATE

COCKED STATE

Actin (thin filament)

Myosin (thick filament)

In comparison wlth the energy stored as phosphocrea,tine, glycogen is a far more abundant energy sourcewithin skeletal muscle. Glycogen that has been previouslystored by muscle can be enzl.rnatically degraded to pyru-vic acid. Degradation of gLycogen to pyruvate is rapid andliberates energy that the cel1 lnvests ln phosphoryLatingADP to rreld ATP. Pyruva.e can be furthe deg'adedalong with other foodstuffs by axidat:e metabolism, whichover the long term is the primary mechanism for theregeneration of ATP (p. 1220). The rate of ATP genera-tion by oxidative metabolism is limited by the rate ofoxygen delivery to the rnuscle. However, glycolytlc forma-tion of pyruvate occurs independently o[ oxygen, as doesthe conversion of pyruvate to lactate. Ihis anaerobic metqbolsm of muscle glycogen ensures that energy stores aresufficient to sustain muscle activity for nearLy a minuteeven when oxygen is unavailable. in Chapter-59 we willdiscuss the aerobic and anaerobic metabolism of exercis-ing muscle in more depth.

RELEASED STATE

P is released. Myosin heads changeconformatron, resulting in the power.troke. The f i laments 5l ide pasL each olher.

ATP is hydrolyzed, causingmyosin heads to return totheir resting conformation.


A SKELETAL MUSCLE Myofibril

EXCITATION-CONTRACTION COUPLING

ln our discussion of the mechanism o[ muscle contrac-tron, we saw that regardless of whether the muscle isskeletal, cardiac, or smoolh, it is an increase in [Ca']*],thar rriggels muscle contraction. The iime during which

[Ca'?r'], remains elevated delermines the duration of mus-cle contraction. The process by which "excitation" triSgersthe increase in lCa'z-l' is known as excitation contracLioncoupling. Diflerent kinds of myocytes have specializedmechar.iisms that regulate lhe entry of Ca2+ rnlo the cyloplasm, as rvelL as remove Ca2t lrom the cytoplasm oncethe stimulus 1br muscle contraction subsides. Caz- canenter the cytoplasm from the extracellular space throughvoltage-gated or ligand-gated ion channels, or alterna-tively, Ca'z* can be released into the cltoplasm from theSR. Thus, both extracellular and intracelLular soulces con-Ldbure to the ir.rcrease in lCa']*],. Hor,veYer, the relativeimportance of these [wo sources va es among the differenl muscLe lypes.

Invaginations of the Sarcolemma FacilitateCommunication Between the Surface of theCel l and l ts Inter ior

The plasma membranes of muscle cells display invagina-tions ihat extend the surface membrane into lhe musclece1L. In skeLetal and cardiac muscle, these invaginationsrake the form of radlalLy projecting tubes ca]led trans-verse tubules or T tubules (Fig. 9-BA). T tubules arehighly organized and penetrate the muscle at two pointsin each sarcomete: al the junctions of the A and I bands.A cross section through the A-l junction would show acomplex, branching array of T tubules penetrating to thecenter of the muscie cell and surrounding the individualmyohbrils. Along its Length lhe iubule associates wilh twocistemae, which are specialized regions of the SR. Thesarcoplasmic reticulum is the muscle equivalent of theendoplasmic reticulum, and it serves as a storage organe, le lo r u r t ra .e l lu l r r ca The .ombina t ron or th .T-tubule membrane and lts two nelghboring cistemae iscalled a triad: Lhis struclure plavs a cruciaL role in the

Plasmamembrane(sarcolemma)

Sarcomere

lnvaginationsof plasmamembrane(form lrans-verse tubules)

FTCURE 9 B. PLrsma-m€mbra e invaginarions A, The rransvcrse rubules (T Lubules) are exiensions ol the plasma membrane, penetrating the muscle

cell ar t\Vo points in each sarcomere: lhe iunctions ol the A and I bands. B, Smooth muscle cells have rudimentary in\'aginations of dre plasma

membrane. callect caveoli, contacting wrth rhe sarcoplasmic rclicuium

reticulumcisterna

Transversetuoute

Sarcoplasmicreticulumcrsterna

Cellular Physiology of Skeletal, Cafdiac, and Smooth Muscle / 9 241

channel (ryanodinereceptor) [tetramer]

J ca2'Mechanicalconnectton

SR terminal cisterna

rca2*

FICURE 9- 9. Excitation contraction coupling in skeletal muscle _A retrad ot four L lype Ca:* channels on the T tubules faces a single Ca2*-releasechamel ofthe SR, so that each L type Ca':* channel inreracts \!ith lhe Ioor of one olihe four subumts of rhe Ca, -release channel. N;!e thar half ofrhe car*-release channels lack associarions with L-rype caz' channels DHp, dihydroplridine; sR, sarcoplasmic r€rjculum

coupiing of excitation ro contraction in skeleLal and car-diac muscle. Smooth muscle, in contrast, has more rudi-mentary and shaLlow invaginations called caveoli (see Fig.o_ B8) .

In Skeletal Muscle, Depolarization of theT-Tubule Membrane Leads to Ca2* Releasefrom the Sarcoplasmic Reticulum at the TriadAction potentials originaring ftom depolarizations at themotor end plate propagate along the skeletal musclemembrane and down the T tubules. Depolarization oI thetriad region of the T tubules activates L-type Ca2* chan-ne ls rp . lo0 ' . wh i .h a re , l - : te red 'n q oup: o l lou-.a l led re r radc rFg o-or Ihe ,e ro l .age-gared t l - ,nne l ,play a pivotal role in coupling electrical excitation tocontraction because they function as the voltage sensorin rC coup l ing . E lecr ron r c ro .cop) revea ls , .he . [e -board pattern of projections arising from the T tubulemembrane and extending toward the cisternae of the SR;these projections probably represent the cytoplasmic faceoI these L-t1pe Ca']* channels. Each oI rhe four voltage-gared Ca cha nel" in a reLrad rs - lacr a reteropenia-

meric protein (see Fig. 7 12B). Each of the lour Car*channels is also called a DHP receptor, because it isinhibited by a class of antih)?ertensive drugs knowl asdihydropy-ridines. Depolarization of rhe T rubule mem-brane evokes conformationaL changes in each of the fourL-t1pe Ca'?t channels and has two effects. First, lhe con-formational changes allow Ca2* to enter through the fourchannel pores. Second, and much more important, theconformational changes in the four L-type Ca2t channelsincluce a conformational change in each of the lour subunirs of another channel-the Ca2*-release channel-that is located in the SR membrane

The Ca2*-release channel (Table 6-2, #18) has ahomotetramedc structure quite dillerenr from thar of theL-tl?e Ca'z+ channel that constitutes the voltage sensor.The Ca2*-release channel in the SR is aLso known as theryanodine receptor because ir is inhibited by a class ofdrugs that include the planr alkaloids ryanodine and cat'-Jeine. Ca2* -release channels cluster in the portion of theSR membrane thar faces the T tubules. Each of the foursubumts o[ these channeLs has a Large extension-alsoknown as a "foot." These feet projecr as a regular arrayinto rhe cytosol. The foot of each of the four Caz*-release

Membrane depolarization opensthe L-type Ca'- channel.

Mechanical coalpling betweenthe L-type Ca'- channel and theCa'--release channel causes theCa2+-release channel to open.

Ca'- entering the cell via L-typeCa'- charurels also can activatethe Ca2"-release channels.However, this pathway is notessential in skeletal muscle.

ca2* eits thisR via theCa2+-release channel andactivates troponin C, leadingto muscle contraction,

L-type ca2* \r ca2*^ h . n n a l / n H o T - f i ' h r r l a

receptor) [in arrays of 4]


channeL subunits is complementary to the cltoplasmicproleclion of one of the four L-type Ca2* channels in aie r rao on the T t - :bu le ' ' ee F8. o q r fhe c lo<e pnrsca lproximity of these two proteins, as well as rhe ability of

6oth DHP and ryanodine to block muscle contraction,suggests that inLeractlon belween these two different Ca']+channels underlies EC coupling. The precise mechanismof interaction between these Proteins is not yet fully un-derstood, although we know that it is not electrical inas-

much as ion conductance of the Ca'z--release channel is

not strongly voitage dependent. A large cytoplasmic pro-jecrion on the ar subunit of the L-type Ca'z* channelippeats to be necessary for interaction between the lwoCd* channels on opposing T-tubule and SR membranesThus, it is possible that direct mechanical coupling exists

between this projection and the Ca2*-release channelAs the L tlpe Ca'z* channel on the T-tubule membrane

mechanically opens the Ca2+-release channel in the SR,

Caz+ sequeslered in the SR rapidly ieaves via the Ca']+release channel. The resultant rapid increase in [Caz*]tactivales troponin C, thus inilialing cross-bridge cyclingas described earlier. The entire Process, extending fromdepoladzation of the T-tubule membmne to the initiationof cross-bridge cycling, is lermed excitation-contractioncouplinq.

A , r n o i g h w e n a ' e . t r e ' , e d a c l i \ a t i o n o l t r e C a r " - r e -

lease channel in the SR by mechanical coupling between itand the L-t1pe Ca2* channeL in the T-tubule membrane,local elevations in lCa']*]r can also aclivate the Caz*-re-lease channel in skeletaL muscle. When the L-t)?e Ca']+channel opens during depolarization, it al1ows an influx

of Ca2* that locally increases [Ca'z*]'. This mechanism ofactivating the Ca2*-release channel in the SR is known as

Ca2*-induced Ca2* release (CICR). Although L-tpe

Ca2* channels allow lCa"]r lo rise during action polen-

tials in skeletal muscle, CICR is nol necessary for conlrac-tion. Indeed, skeLetal muscle conlraction persists evenwhen Ca2* is absent from the extracellular fluid. How-ever, as we will see later, CICR plays a critical role in EC

coupling in cardiac muscle.

In Cardiac Muscle, Ca2* Entry Through L-TypeCa2* Channels ls Amplified By Ca2*-lnducedCa2* Release from the SarcoplasmicReticulumWhereas EC coupLing in skeletaL muscle does not requireCa2* influx through L-t)?e Ca'?n channels, cardiac con-

traction has an absolute requvement for Ca2* influxthrough these channels during the action potential. Be-cause the T-tubule Lumen is an exLension of the extracel-lular space, ir facilitates the diffusion of Ca2* from bulke"trace11uLa, fluid to the site of the L-type Ca'?* channeLson the T-tubule membrane. Thus, the Ca2* can simuhaneously reach superficial and deep regions of the muscLe.The increase in lCa'z*], resulting from Ca2* influx alone isnot, however, sufftcient to initiate contraclion. Rather, thelncrease in lCa']+lr that is produced by the L tlpe Ca'z*channels is greatly amplified by CICR from the SR via theCa2*-release channels. lndeed, because the Ca2*-releasechannels remain open for a longer period than do L{ype

Ca'?* channels, the contribution of CiCR to the rise in

[Ca'z*], ls greater than the flux contributed by the Ltype

Ca2* channels of the T tubules.It appears that each L-type Ca2* channel controls only

one SR Ca2*-release channel. The close physical proximityof L{}?e Ca2* channels of the T-tubule rnembrane andthe Caz*-release channel in the SR at the lriad junctions

aLlows for this tight local control. Although Ca'* diffusesin the cytosol away from its SR release site, Ca2* releaseat one site does not appear to be able to induce Ca2*release from a neighboring SR Ca2*-release channel. Thus,Ca2- release events are not proPagaled along the myocyte.ln fact, the SR Cazt-release channel does not appear to-espord to ge-era1 ,zed increa-es in ( ) lop lasmic lLa2 IGeneralized cardiac muscle contractions occur as a resultof the spatial and temporal summation of individual CICRevenls-

In Smooth Muscle, Both Extracellular andlntracellular Ca2* Activate Contraction

In smooth muscle cells, three major pathways-whichare not mutuaily exclusive-can lead to the increase in

lLa ' I Lna l l r .88ers co ' l tac l ion : I I ' Ca er t rv lh roughvoltage-gated channels in response to celL depolarization,

!21 Ca ' re lea ,e l rom the SR. and (J l Caz en t ry t l ^ roughvoltage-independent channels.

Ca'?* ENTRY THROUGiI VOLTAGE-GATED CIIANNELS. Asnoted earlier, smooth muscle cells respond to stimulationwith graded depolarizations or action potentiais. In eithercase, depolarization may produce an influx of Ca'?*through voltage-gated L-t1pe Ca2* channels (Fig. 9 l0)

ca,, RELEASE FROM THE SR. This ca2* release may occurby either of two mechanisms: CICR or IP3 mediated Ca']*release. As we have already seen, CICR plays a key role inEC coupling in cardiac muscle, where the L-type Ca'z*channels are highLy ordered and in close proximity to theCa2* release channels in the SR. Thus, Ca2* influxthrough L tlpe Ca2* channels can triSSer CICR lnsmoolh muscle, the relationship between the plasmamembrane and the SR is not as regularly organized as itis in striated muscle- Nevertheless, eleciron-dense cou-plings have been observed bndging the B- lo 10-nm gapbetween the cel1 membranes and elements of the SR insmooth muscle. Although CICR occurs in smooth musclecells under some conditions, it requires [Ca'?*], levels thatare higher than those that typically occur under physio-logical conditions, and its role remains unclear.

A more important mechanism for Caz* release from theSR of smooth muscle is the lP. pathway. The existence ofthis pathway is supported by the obsewation that someextracellular agonists can elicit smooth muscle conlractionwith minimal depoladzalion and negliglble Ca2+ influxFurthermore, even for agonists such as serotonin or norepinephrine, which activate a Ca']*-influx pathway, iheobser.red increase in [Ca'?*j, is out of proportion to thatexpected from Ca2* influx alone. Thus, another pathwaymust exisL for increasing lCa2*1,. Some agonisls causesmooth muscle contraction by triggering the productionof lP, (p. 100), which binds to a specific receptor on the

MALIGNANT HYPERTHERMIA

lvlalignant hyperthermia (MH) is a genetic disorder affect-ing between I in 10,000 and 1 in 50,000 individuals.Affected individuals are at risk for a potentially life-threat-ening syndrome when exposed to any of the variousinhalation anesthetjc agents, particularly halothane. Ad-ministration of succinylcholine can also trigger or exag-gerate MH. This drug is a short-acting inotropic (nico-tinic) acetylcholine-receptor antagonist that acts by firstopening the acetylcholine receptor channel and thenblocking it, thereby resulting in a burst of muscle activit,followed by paralysis. Onset of the syndrome in the set-ting of the operating room is typified by the develop-ment of tachypnea (rapid breathing), low plasma [Or],high plasma [COr], tachycardia (rapid heart rate), andhypefthermia (tising body temperature), as well as rigidit,sweating, and dramatic swings in blood pressure. Thepatient's temperature may rise as rapidly as

.i'C every 5

minutes. The onset of MH is usually during anesthesia,but it can occur up to several hours later. lf untreated,the patient will develop respiratory and lactic acidosis,muscular rigidity, and a breakdown of muscle tissue thatleads to the release of K* and thus profound hyperkale-mia. These episodes refiect a progressively severe hyper-metabolic state in the muscle tissues. Fortunatelv, ourevolving undersl.anding ot the physiology ol MH has ledto the development of a therapeuiic regimen that hasgreatly improved Lhe once-dismal prognosis.

The major features of the syndrome-hyperthermia,muscular rigidity, and an increased metabolic rate-ledearly investigators to suggest that MH was a disease ofabnormal regulation of muscle contraction. According tothis hypothesis, uncontrolled muscle contraction-some-how triggered by the administration of halothane andsuccinylcholine-causes excessive adenosine triphosphate(ATP) hydrolysis to provide energy for contraction. Theincreased rate of ATP hydrolysis leads to an increasedmetabolic rate as muscle tries to replenish and sustain itsATP stores. Hyperthermia develops because of the heatliberated by the hydrolysis of ATP.

Further support for this hypothesis came from the ob-servation that more tension developed in muscle fibersobtained by biopsy from susceptible individuals than infibers from normal individuals when the fibers were ex-posed to halothane. In muscle fibers from both humansand a strain of swine susceptible to MH, Ca,+-inducedCa2* release from the sarcoplasmic retjculum (SR) is en-hanced when compared with fibers from unaffected sub-jects. Furthermore, caffeine, which causes the Ca2n-re-lease channels to open, induced greater contractions infibers from susceptible subiects. Taken together, theseobservations suggested the possibil i ty that MH resultsfrom an abnormality in the Ca,+-release channel in theSR membrane.

In both humans and animals, inheritance of MH fol-

lows a mendelian autosomal-dominant pattern. Cloningof the gene (RyRl) encoding the Ca2*-release channel(ryanodine receptor) allowed genetic linkage analysis todemonstrate that human MH is closely l inked in somefamilies to the RyRl gene on chromosome 19. In swine,MH results from a single amino-acid substitution in RyRl(Cys for Arg at position 614). An analogous substitutionis present in some human kindreds as well. This substitu-tion increases the probability that the Ca2*-release chan-nel wil l be open. In other families, MH has been associ-ated with other genetic abnormalit ies in the RyR I gene.In sti l l others, MH does not appear to be geneticallylinked to the RyRl gene. lt is possible that defects inother steps along the excitation-contraction cascade canresult in abnormal regulation of muscle contraction andthe MH phenotype. For example, when under anesthesia,patients with some forms of muscular dystrophy mayhave metabolic crises that resemble MH.

MH also occurs in domestic livestock. The incidence ofMH i5 particularly high in swine, where episodes are trig-gered by a variety of physical and environmental stresses(porcine stress syndrome). MH in animals has significanteconomic importance in view of the potential loss fromfatal episodes and the devaluation of meat as a result ofmuscle destruction during non-fatal episodes.

ln humans, a condition similar to MH may occur inpatients treated with neuroleptic agenis such as the phe-nothiazines or haloperidol. lt is called lhe neuroleptic mo-lignont syndrome and appears to result from abnormallyhigh neuronal input to the muscle cells.

Therapy for MH now involves administration of thedrug dantrolene, cessation of anesthesia, and aggressiveefforts aimed at cooling the body. Dantrolene is aneffective therapeutic agent because it blocks excitation-contraction coupling between T tubules and the SR, thusinterrupting the otherwise uncontrolled progression ofmuscular contractions. The drug can be given acutely inan effort to abort an ongoing attack or, in a personknown to be at risk, it can be given before the initiationof anesthesia in order to prevent onset of ihe syndrome.Therapy also includes intravenous hydration and the judi-cious use of diuretics to keep the urine flowing. The latterlessens damage to the kidneys from the release of break-down products, such as myoglobin from the damagedmuscles. Sodium bicarbonate is given to counter the lac-tic acidosis, and patients may be mechanically hyperven-tilated to blow off the excess COr.

Despite the intensive protocol just outl ined, MH is sti l lassoclated with high mortality. The relatives of a patientwith a documented history of one episode of MH shouldbe carefully screened to see whether the, too, carry theinherited trait; many of the affected relatives may demon-strate baseline elevations in muscle enzyme leveis in theirblood (e.9., an increase in creatine kinase levels).

membrane of the smooth muscle SR (p. 100). This lP.-e ! epLo < r l 5 , l f a . r ga d -gd ted Cd ! r anne . . f hu< . areceptor in the plasma membrane can indirectly induceLa rel<lt , l rom t\c 5R anq hence conLra.Lron.

Ca2* ENTRY THROUGH VOLTAGE-rNDfPff\rDfwr CHAN-NELS. ln smooth muscle it is possibLe that extraceLlular

Cellular Physiology of Skeletal, Cardiac, and Smooth Muscle / 9

Ligands may rrigger the influx of Caz+ through eitherligand-gated channels (p. 170) or channels that are activaled via G-protein-coupled receptors. Nevertheless, it isnoi clear to what extent, if any. these tlpes of Ca']+chanre l - con t r .burc to [ , .a I rc -eases in . . r .oorh ru , -cle. However, another class of channeLs-store-operatedCa ' channe ls -apoears to p la r an mpo- td . lL roe Neu


ca2*t

ajSarcoplasmic reticulum

ro . rnsmi re 5 lLaL e- ( l o rhe d i " , harB. -anc Lh - rnedepletlon of SR Ca'z- stores (i.e., second pathu'a,v)somehow lead Lo the activatjon o[ store-operated Ca:*. h a n r r e L . n r h e D l . - m J r c n l - a n e l L e , I ' - l e - i n i

through these channels ailor,vs lCa']*1, to remarn elevaLedeven after SR depleLion and also appears to replenish SR

@ Cat' srores.Tog, th , . . Lhe >e.ur rd pd l l r r r , j ) 61 'n 1s3 ' g [ t a :

' ( . r ' re leJ ie fo r . l -e 'R l a rd rhc rh . rd p i - t \ \ . i ) , !2rn [ . ' , tL u r .Bh - to " -ope a ed cL . rn , l , h . ' . b , , n . . ,11 . .pharmacomechanical coupling because these excitatoryneurotransmitte$ and hormones can induce smooth muscle contraction that is independent of action poLenLialgeneration. Regardless of the source o[ the Ca']*, the re-suhant increase in [Ca']t], leads to contraction via a Ca2-CaM-dependent increase in MLCK actlvlty and myosinphosphory-latlon.

Smooth Muscle Contract ion May Also OccurIndependent ly of Increases in [Ca,*] ,\ \ h e r e a , - n " r ) e \ , r d r o ) - r . r r - l i r e l ) o n r r . , ( < e -

[Ca2'], to evoke conlraction, some stimuli appear to causcconlraction withoul a neasurable increase in [Ca']*],. Oneme.hdn.c rT t bv i lL ' ,h e r . . t " r to1 'L i ru l i - r g \ r .ndLr . ,r a 2 - . n d e p e n d e r r t . o , t r " . . i o n , r . b , r ' o d u : r - g t t r e r . t r, , ' n { . n n r a r t l p n r r c o r r l , r o n n r n , n i n - r l i ' . r l r l h , r n p

Extracellular space

Agonist

/Receprol

, t .a ,

/ r...-.'-<Y-

/ e1.,i-/ comprex

Phospho-lipase C

amount o[ lorce developed at any gl\'en [Cat-], may vary.This force/[Ca" l, ratio ].nay be increased or decreased andis generally higher dr-u.ing phar maconechanicaL activationthan during depolanzation-activated contractions. Becausephosphorylation of the my,osin lighL charn (N4LC) is amajor determinant of contracti le lorce in smooth muscle.! r i n J ! J ( n ( l p ' l . o r r J , r r o n r r . J 1 e . u l t e ' l e r [ - o r ' r r 'increase in the rate o1 MLC phosphorylatlon by MLCK orfrom a decrease in t].re rate of MLC dephosphorylation byMLC phospl-ratase. One second messengef system that candecrease the activity of pl.rosphatases is protein kinase C(PKC) (p. 102). Some excltaLory stir.nuli are therelore ca-pable oI initlatlng smooth muscle contraclion by induclngIPr-mediated release oI Ca2t from intraceLLuLar stores, asrvell as by producing PKC-mediated decreases in MLC

;hospLt t " 'e rc t t \ .Ly lLe5e pa t | , . t . r , . . T re fu t tLe t . r ;mp lc -of pharmacomechanical couphng.

TERMINATING CONTRACTION

In Skeletal , Cardiac, and Smooth Muscle,Terminating Contraction Requires Re-Uptakeof Ca2* Into the Sarcoplasmic ReticulumAfter the contracrion-activating stimulus has subsiclecl,Ca2' must be ren1oved from the c1'toplasm for contrac-

Caveolus

)-

I

F | c U R E 9 _ l 0 ' E X c i t a t i o n ' . o n t r a c t i o n ( E C ) c o r r p I r l g r r r s r r r o o t h m r r s c 1 e D A G ' d i a c y l g l y c e r o L l I P ] . i n o s i l o l ] ' , + ' 5 1 | i p h o s p h a t e ,inosiiol ,l,5 biphosphate; SR, sarcoplasmic fericulum

Voltage-gatedCa'- channel

When the SR Ca2+ stores becomedepleted, the SR signals-by an unknownmechanism a store-operated Car+channel to open, allowing Ca2* to enter.

Ca2* release fro]ithe sarcoplasmicreticulun can occur either via^ 2 + , . ^ 2 + .

importantly, via lP3 activation ofSR Ca'- channels.

Nd-Cd dnd e\Lhdnge'dnd Ca/ pump in thepiasma membrane both extrude Ca:+ ftom the cell.

Ca2* pump sequestersCa2+ within thesarcoplasmic reticulum.

Ca2+ is bound in thesarcoplasmic reticulum bycaheticulin and calsequestrin.

\ / l l _ . , : : . . .t ' t

. = - t | 1 , :I tLl"/ (_t ',

Calsequestrin/

Ca: removal liom Lhc cytqrhsm.FICURE 9 1 1. Mechanisms ol

tion lo cease and for relaxation Lo occur. Ca2t ma1' beextrudecl across the cell membrane or sequestered witl-rini n r , p l - r - : , ^ r ) r r F r F - \ f F r o o - l l )

l h e . c l l n a 1 r { t r r d C a r - r B c r t h e r , n \ ' - C " e xchanger (NCX, p. 68) or a Cart pump at the plasmamembrane (PMCA, p. 64). Extrusion across rhe cell membrane, however., rvould eventually deplete the cell of Ca:'and is tl-rerefore a minor mechanism for Ca2- removalfrom the cytoplasm. lnstead, Ca2* re uptake into the SRis the most important mechanism by r,".hichr . , e l l r c - r n - ( e 2 o r p 5 l r g l c \ e . . r J r q - . r p l a l (by the SR is mediated by a SERCA-type Ca2- pump( p . 6 4 ) .

It rs possible that the rate of Ca']= re rqrtake into LheSR may be regulated by modulating ihe activity of the SRr , ' . r - r a l . - e y r n n p I n . r , l r J . m u . c l e . S R ( : ,

lump ac l i r , ) r - r r b r ed bv the regu dL^n p fo te phos-pholamban. When phospholamban is phosphorylaLed bycyclic adenosine monophosphate (cAMP) dependenL pro-tein kinase (PKA), its ability to inhibit the SR Caz' punpLs lost. Thus, activatols of PKA, such as the neurotrans-r ' ' r ' l . ep ephr re r ra r cn arce Lhe taLe o [ ca d i . - ' . l y -ocyte relaxation (p. 525).

SR Ca']* pump acti\ ity is also inhjbjLed by high lCa':-l\ \ r l l L l r ( \ R l u n - . n . l h . s b r i o n o f ) R L r / - D U n l

activity is clelayecl by Ca']' bindrng proteins u'iLhin Lhe SRl u r r r r . l h e s . r r b d i n g p - o t e . n . 1 ' L r l l e r t h e t . r ' -

crease in tl.re SR during Ca'] re uptake and thus mark-edLy increase the Ca']t capacity of the SR. The principalCr o ' d nB pro r r - [ ( (12 n ] -c e . ca l :eques t r in . -also present ln carcliac and some smooth muscle. Calre-

Cellular Physiology of Skeletal, Cardiac, and Smooth Muscle / 9 245

ticulin is a ubiquitous Ca:- bincllng protein that is foundi n r - - i . r r l r r r h i . h , , n . . l . r ' r o n ) w - t ' - t h c 5 R o [- - r o o t h r u * e . l h e - c p r o r c , r - h r . e , t r " n e r d o r . , a l r . -i . 1 r o b r n d t a 2 ' ' r i t h u p t o 5 0 h i n d r n g - \ p e r p - o r e - rmolecule.

Ca'z' binding proteins are not located diflusely withinr L e s R R r h e r r r l . p n i p . r f . i . h o f l . l , , c , l : p d r o t h p

region ol the SR rmmediateLy beneath the triad junction.Calsequestrin appears to bind directly at the triad, wherer f , n< .nnn, ' r . rh h" a re lea , . .h rnne l an-

wirh two orher Lrlad proteins, juncrin and rriadin. Here,calsecluestrin is poised not only to aid n.ruscle relaxationby bultering Ca']' n'ithin the SR lumen but also to unload' r \ ( . , 1 r h p . , r r i ' n J r h p , 1 7 - - c l , a , e . h l - r e l a n d

thus facilitate EC coupling. lt has been hypothesized thatb( .o . f t t Pronotes ( r ' re e r )P -o n (a l )eqJe) t r ' - .making Ca:- available for exit from the SR.

In Smooth Muscle, Terminat ingContraction Also RequiresDephosphorylation of theMyosin Light ChainBecause Car- Lriggers smooth niuscle contraction by in-ducing phosphoq'lalion ol the myosin regulatory 11ght.nd i r ' ' re e ) e -Lnr t [ r a - I ro - lo \ \ -e>Lrng \d lL rema)' not allor,v muscle relaxatlon. Rather, relaxation ofsmooth muscle requlres MLC dephosphorylaLion, which rsaccomplished by myosin Iight chain phosphatase. Thisphosphatase is a heterotrimer consisting ol subunits withmolecular masses of 130, 20, and 37 kDa. The 130-kDasubunit confers specilicity by binding to myosin, whereas. l ^ e j 7 - 1 , D , p . o r e r r - r L e c r r ; 1 1 r r , . ' r b u r r r - e r p o - . b l c o -the dephosphorylating acriliti'.

REGULATING MUSCLE CONTRACTION

Muscle Contract ions Produce Force and/orShortening and, in the Extreme, Can BeStudied Under Ei ther lsometr ic or lsotonicCondit ionsThe total lorce generated by a muscle is the sum of theb c e . e e r . r . t e J h ; . r " n 1 r n d c p . n J , n r l ; , 1 . 1 g a . i -

myosin cross bridges. The number of simultaneously cy-( . g , ro* -h rJg( ' d "perds -ub- r . r r t , . r l11 on the t ralengLh of tl.re rnllscle hber and on the pattern or [re-quency of muscle cell stimulation. When muscle is stimu-la re . l to .on t ra . . . i . exer '> d b rce .e lq r r 'g ro pu t l l . reathchment points at either end toward each other.This force is referred to as the tension developed by themuscLe.

Two mechanical and artificial - arrangements can beused to study muscle contraction. In one, the attachmentpoints are immobile, thereb,v fixing the uiuscle lenglh.Here, stinulation causes an increase in tension, but noshortening. Because these contractlons occur at constantier-rgth, they are relerred io as isometric contractions' T ' g o - 2 q l t h , - r . o n J r r i n g e r r n l . o " o l t h e t , r oattachmenL noints is n.Lobile. and a force or load-

Calreticulin

Sarcoplasmicreticulum

@

46 9 / Cellular Physiology of Skeletal, Cardiac, and smooth Muscle

ISOMETRIC c LENGTH-TENSTONDIAGRAI\,I( ISOMETBIC)

.100

75

Tension 50

Total

PassiveActrve

t1 0 0 1 1 5Lengrn

130 145

D "ACTIVE'LENGTH.TENSIONDIAGRAM (ISOMETBIC)

B ISOTONIC

F | C U R E 9 1 2 . 1 s o n e | r i c a n d i s o t o n 1 . c L r n r r a c t i o n ' l ' ] - 4 c r r m . n 1 a L P r e p ' l r o n 1 ! ) |

\elocit-r of mlrsc[_

qrcefer t€nsLoll tnar can r snortef nu:cLe.

,-- Muscle flber. ..-

Tension Tensionlow high

The maximal velocityis ihe same ai all lhrceinitial muscle lengths.

E LOAD-VELOCTTY D AGBAr\,1 (ISOTONTC)

Velocity ol sotonic coniracl on at 3dlf ferent in t a rest rg lenglhs

Cellular Physiology of SLeletal, Cdrdiac, and Smoolh Muscle / 9

MEASURING THE FORCE OF A SINGLE CNOSS-BRIDGE CYCLE

The force of a single cross-bridge cycle has been mea-sured directly. Finer, Simmons, and Spudich used optlcaltweezers to manipulate a single actin filament and placeit in proximity to a myosin molecule immobilized on abead (Fig. 9-13). With the use of video-enhanced mi-croscopy these investigators were able to detect move-ments of the actin fi lament as small as 1 nm. The oDticaltweezers could also exert an adjustable force opposingmovement of the actin filament. When the tweezers ap-plied only a small opposing force and the experimentwas conducted in the presence of adenosine triphosphate(ATP), they observed that the actin moved over the myo-sin bead in step-like displacements of 11 nm. This obser-vation, made under "microscopically isotonic" conditions,

suggests that the quantal displacement of a single cross-bridge cycle is approximately l1 nm. When the tweezersapplied a force sufficiently large to immobilize the actinfilament, the investigators observed stepiike impulses offorce that averaged around 5 pN. This observation, madeunder "microscopically isometric" conditions, suggeststhat the quantal force developed during a single cross-bridge cycle is about 5 pN. Interestingly, these isometricforce impulses lasted longer when the ATP concentrationwas lower. This last findino is consistent with the notionthat ATP binding to myosin must occur to allow detach-ment of the cross-bridges (step 7 in the cycle in Fig.9-7').

tends to pull this mobile point away from the 6xed one.Here, stimulation causes shortening, provided that thetension developed by the muscle is greater than the op-po> ing load. Because these shor ten ings o( (u r <1r (o -5 tdn lload, they are relerred to as isotonic contractions (seeFig. 9-128). Bolh isomeldc and isotonic contractions canbe examined at different initial muscle lengths. Moreover,they can be measured during individual muscle twitchesthar are evoked by single muscle action potentials, as wellas during other pattems of stimulatron.

Muscle Length Influences TensionDevelopment By Determining the Degree ofOverlap Between Actin and Myosin Filaments

The isometric force of contractions depends on the initiallength of the muscle flber. Unstimulated muscle may beelongated somewhat by applying tension and stretching it.The tension measured before muscle contraction is re-ferred to as passive tension (see Fig.9-12C). Becausemuscle gets stiffer as it is distended, it takes increasingamounrc of "passive" tension to progresslvely elongate themuscle cell. lf at any fixed lengLh (i.e., isometric condi-lions) the muscle is stimulated to contract, an additionalactive tension develops because of cross-bridge cycling.The total measured tension is thus the sum of the passiveand active tension. This incrementaL or dctiy€ tension-the diflerence between total tension and passive tension-is quite small when the muscle is less than ap-pror imaLe l l 709o o f r ts normal res t ing length tsee Frg . o12D). As muscle length increases toward its norrnallength, active tension increases. Active terlsion is maximalat a Length-fsually called l"-that is near the normalmuscle length. Active tension decreases with furtherlengthening, thus, active tension is again small when themuscle is stretched beyond 150o/o of its normal restinglength. Although the relationship between muscle lengthand tension has been best characterized lor skeletal mus-cle, the tension of cardiac and smooth muscle also ap-pears to depend on length in a similar manner.

This length-tension relationship is a direcr result of theanatomy of the thick and ihin filaments within individualsarcomeres (see Fig. 9-12D). As muscle length increases,the ends of the actin filaments arising ftom neighboring Zdisks are pul1ed away from each other. When length isincreased beyond 150o/o of its resting sarcomere length,the ends oI the actin filaments are nulled bevond theend. o l rhe myosr r F lamen l . Urder th is cond, t ion . nointeraction occurc between actin and myosin filamentsand hence no development of active tension. As musclelength shortens from this point, actin and myosin fila-ments begin to overlap and tension can develop; theamount of tension developed corresponds to the degeeof overlap between the actin and the myosin hlaments. Asthe muscle shortens further, opposing actin filamentsslide over one another and the ends of the myosin fila-ments and-with extreme degrees of shortening even-tually butt up against the opposing Z disks. Under theseconditions, the spatial relationship between actin and my-osin is distorted and active tension falls. The maximaldegree of overlap between actin and myosin filaments, and hence maximal active tension, corresponds toa sarcomere length that is near its normal restingLengrn.

At Higher Loads, the Velocity of Shorteningls Lower Because More Cross-Bridges AreSimultaneously Active

Under isotonic conditions, the velocity of shortening de-creases as the applied load opposing contraction of themuscie frber increases. This point is obvious; anyone canlift a single French fry much faster than a sack of pota-roes. As shown for any of the three blue curves in Figure9-l2E-each of which represents a different initiallength of muscle-the relationship between velocity andload is hlperbolic. Thus, the smaller the load applied tothe muscle, the greater its velocity of shortening. Con-ve$ely, the grcater the Load, the lower the veiocity o[shorteninq.


A EXPERII\,{ENTAL PREPARAIION

Actin filamenl with polystyrenebeads altached at each end

B ISOTONIC3020

Disp acement 1o{nm) o

c tsot\,1ETRtc1 0

0.0 0 .5 1 .0 1 .5Time (sec)

The load (or tension) velocity relationship is perhapsb, . t under> tood by .o - , der ing the s i ruaL ion d t m/x imunload for a resting muscle length (i.e., isonlelnc condirions).This situation is represented by rhe upper blue cume inFigure 9- 128. At any time, aLl the available cross-bridgesare engaged in resisting the opposing force. None are leftover to make the muscle shorten. If the number of en-gaged cross-bridges were decreased, the muscle wouLdlengthen. At a sLightly smaller load bur at rhe same iso-on ic mr .c le length . feue- , 'oss b r idge. reeo to oe en .

gaged to resist the opposing load. Thus, extra cross-bridges are available to ratchet the thick myosin filamentsover the thin actin nlaments, but at a very Low veLocity.At a still lower load, even more cross-bddges are availablefor ratcheting the myosin over the actin, and the veLocityincreases lurther. At very low Loads, it is reasonable toexpect that as lhe myosin filament slides along the actinfilament, only a tiny fuaction of the actin monomers need

' ) -force oI

I a stnole

l c ross -onogeJ cycre

0.2 0.4Time (sec)

to interact with myosin heads to overcome the Load. Under these conditions of vanishingly small loads, the speedwiLh which rhe rhick and thin filamenrs sLide over eachother is limited only by the time that it takes for rheATP-consuming cross-bridge cycle ro occur. With increas-i n g , e l o , i t 1 . r h e o - o b a b i l i t y o l a , r r n . n l o s i n L e r d r . i o n <decreases. Thus, fewer cross-bridges are simultaneouslyactive al higher shortening velocitres, and less tensiondevelops.

Note that rhe upper blue cui've in Figure 9 128 appLiesto a particular lnitlal length of the muscle, that is, theresting length. We already saw in Figure 9 12C that thetotal isometric lension (i.e., the maximal load that themuscle can sustain at zero veLociry) increases with initialru .c le lengt \ . lh is o - rnc .p .e i . con f i rmed r r f igure o-l 2 t : t h e l o n g e r r h e ' t r a e g L h . r ' r e l a r g e r - " r a r m a lload under zero veLocity conditions (i.e., the three differ-ent intercepts with the abscissa). In conrrast to this maxi

I " : . * - "

I acrn movestn one cross-

- bridge cycle

8

(pN) l

0

0 .0

FICURE 9-i3. Microscopic measurements of cross bridge force and dlsplacemenr. A, An aciin filanenr is anached at each end to a polysq.rene bead.The 'opncal iweezers, a 6nely focused beam of laser light, can '\rap rhe bead at lrs focal point and physically move rt. By adjuning the laserintensity, the €xpenm€nter can alter the strength of th€ tlap (i e , rhe lorce wirh which the bead is heid). In this experim€nr, rwo optical rweezers wereused to suspend the actin filament above a coverglass Atlached !o lhis coverslip is a silica beed, and myosrn molecules are bound ro rhe bead. B, hran "isoionic experim€nt, lhe lbrce between rhe actin filan1el1t and the fixed myosin/silica bead is kepr connan! by using a sLable laser imensity. Theexperim€nler e lunction of time, the displacement of the polystyrene bead (bhc) ar?y from the cer,rer of rl,e Lrap. Thus. in one crossbridge cyc1e, lhe myosin acljn interaction pulls the polyslyrene bead approximately 11 nm aNay frolr the cenrer of lhe !rap. C, In an "isomerricexperimen!, the expeimenler measures, as a function o[ !ime. lhe extra force that needs to be apphed (i.e., inc]ease in laser inrensiry) ro keep lhepolystyrene bead (blle) at a iixed position near lhe center of ihe trap. Thus. in one cross bddge c).cl€, ihe myosjn aclin interaction exerrs a lorce ofapproximately 5 pN. (Daia from Finer JT, Mehk AD, Spudrch JAr Characterization of single actin-myosin inreraclions Biophys J 68:29 1s 296s, 1995 )

A SINGLE I\,4USCLE TWITCHES B

Cellular Physiology of Skeletal, Cardiac. and Smooth Muscle / 9 249

TEMPORAL SUNII\,1ATION C UNFUSED IETANUS D FUSED TETANUS(10 Hz) (25 Hz) (50 Hz)(5 Hz)

Stimulus I Ilrequency Y Y l+ t t l+lt{' l{+t

FICURE 9 14. Frequency summarion of skeletal muscLe rwirches

mal /oad, which depends very much on length, maximalvelociiy is independent of length, as shown by the common intercept of the family of curves with the ordinate.The explanation lor this effect-as we have alreadynoLed-is thal maximal velocity (at no load) depends ontl-re maximal rate of cross bridge turnover, not on theiniLial overLap of the thin and thick filaments.

T h c r c l o , i r t . r p n < n n , r n e r p v e r l < r h i h r c ' , < i i n o r a , -

tionship between muscle power and applied load. Muscledoes measurable mechanicaL work only when it displacesa load. This mechanical work (W) is the product of load(F) and dispLacement (A{). Power (P) is Lhe rate at whichwork is performed, or work per unit time (At):

Equation 9- IP = w / A t : F X A x / A t

Because velocity (v) is Ar/At, it follows that

Equation 9- 2P : F X v

For a given load (F), we can calculaLe the power byreading the velocity (v) from the uppermost of the threeblue load-velocity relationships 1n Figure 9- 12E. Power ismaximal at intermediate loads (where both F and v aremoderate) and falls to zero at maximum load (wherer, : 9) and at zero Load (where F : 0)

In a Single Skeletal Muscle Fiber, the ForceDeveloped May Be Increased By SummingMultiple Twitches ln Time

At sufficier.rtly low srimulalion fiequencies, the tensiondeveLoped faLls to the resling leve1 between individualtwitches (Fig. 9-14A). Single skeLetal-muscle twitches lastl r p t * e r n ) 5 : r n d ) O t l m . e c d p n e n d i n o n n r h " , " ^ " ^ f

r ru ,c le . A l thoL,gL each Lw ' t .h i . c . rc . red hy a s -g le mr . -cle action potential, the duration of contracrion is Longwhen compared with the duration of the exciting actionpotential, which lasts only several milliseconds. Becausethe muscle twitch far exceeds the duration of the actionpoLential, it is possible to inillate a second dctioil pofentidlbefore a firsL contraction has fully subsided. When this

situation occurs, the second action potential stimulales atwitch that is superimposed on the residual tension of thefirst twitch, thereby achieving greater isometdc tensiont l -an rhc 5-s r (comnare I o o 144 and B) Th is e l ec r r :knowt] as summation.

lf multiple action potentials occur closely enough intime, lhe multiple twitches car.i summate and thus greatlyincrease lhe tension developed- Summation is more effec-tive at increasing tension when the action potentials aregrouped more closely in time, as in Figure 9 14C. lnother words, tension is higher when action potentials arcevoked at hlgher frequency. Because this type of tensionenhancement depends on lhe frequency of muscle stimu-lation, it is relerred to as frequency summation.

Wl -en the r l rmu lar ron f rcquenc l ' s in , reased .u l r -cientLy, the individual twitches occur so closely togetherin ITc tha t t ley luse (see F ig o . * r ' a -d cau 'e rhemuscle tensior-r to remain at a sLeady pLateau. The state inwhich the individual twitches are no longer disringuish,able lrom each other is referred to as tetanus. Tetanusarises wher-r the time between successive action potentialsis insulficient to retum enough Ca" back to the SR roJower lCa''1, below a Level that iniriates relaxation. lnlact, a sustained increase in [Ca'?*], persists untii the te,tanic stimuius ceases. At stimulation frequencies abovethe fusion frequency that causes tetanus, muscle frberrph<r .n ih . re i<p< \ ,en ' l i r r lp

In a Whole Skeletal Muscle, the ForceDeveloped May Be Increased By Summingthe Contractions of Multiple Fibers

In addition to determining the ftequency with which i[stimulates a single muscLe fiber, the central neryous sys-tem (CNS) can control muscle force by determining tl.renumber of indil'rduaL muscle flbers that it stimuLates at agiven time. As each additional motor-neuron ce1I bodywithin the spinal cord is excited, those muscLe hbers thatare part of the motor unif of that motor neuron are addedlo the LonLra( l ing poo l o l l i bc s fF iS o- l , l Thrs e f lec r i<known as multiple-fiber sumrnation. Generally, smallermotor neurons serye rrrotor units consisting of fewer indi-vidual muscle fibers. Because a given excltatory stimuluswill generate a larger excitatory postslnaptic potential (p.


212) in motor neurons with smaller cell bodies, Lhe smallmotor units are recruited even with minimal neuronalstimulation. As neuronal slimulatron intensi6es. largermotor neurons innewattng larger motor units are alsore . . . rpd lhe ) ourp . . vp rpc t t r - ren t O l l t r . r :na I rn -then larger and larger motor units is referred ro as thesize principle. The group of all motor neurons inneNar-ing a single muscle is called a motor-neuron pool.

Multiple-fiber summation, someLimes referred ro asspatial sumrnation, is an important mechanism that al-lows the force developed by a whole muscle to be rela-tively constant in time. lt is true thar rhe CNS coulddirect the lorce to be relatively constant over time merelyby driving a fixed number of moror units within themuscle to tetanus, where the force fluctuations are verysmall (see Fig. 9-14D). However, adding tetanic mororunils would increase toial muscle force by rather largeindividual incremen|s. lnstead, the CNS can activate indi-vidual motor units aslnchronously so that some units aredeveloping tension while others are relaxing. Thus,whole muscle lorce can be relativeLy constant wirh time,even when individual libers are not stimulared to tetanus.Smooth, nontetanic contraction is essential for fine motorcontrol.

In Cardiac Muscle, Increasing the Entry ofCa2* Enhances the Contractile ForceWhereas frequency summation and multiple-frber summa-tion are important mechanisms for regulating rhe strengthof skeletal-muscle contracLrons, these mechanisms would

not be consistent with the physiological demands of cardiac muscle. Because cardiac muscle must contract onlyonce wiLh each heartbeat and must fully relax betweeneach contractron, frequency summation is precluded. Fur,I e - r ' ' ro |e . lh ( r ren : i rc e lec l l r , ,1 \ ou ' l l nq he l$ee c - f -diac myocytes, as welL as Lhe requirement rhar cardiacmuscle contracl hourogeneously, eliminates the potentiallo r r r r r ip le [ ]1 , . r -unr rd r inn fh " r . io re the . r re rgLh o lcardiac muscle contracLion must be regulated by modulat-ing the contracrile force generated during each indlvldualmuscle twiLch. This tlpe of reguLation is an impo antpart of the adaptive response to exercise and is mediatedby norepinephrine, a neur otlansmilter released by thesympathetrc nen ous system.

Because an increase jn lCaz*], activates contraction byremoving thc inhibitory influence o[ rhe regulatory pro-Leins. it is reasonable to consider that conLractile functionmay be regulated eirher by modulatrng the magnitude o[the rise in [Ca']*], or by altering rhe Car' sensirivity ofthe reguiatory proteins. ln fact, both these urechanismsare important in controlling the force of cardiac n.rusclecontraction.

In cardiac muscle, a signilicanL proportion of rhe acti-vator Ca2t enters the cell via vohage gated Car* channelsthat open during the cardiac action potentlaL. Mosr of thisCa']* infiux occurs via L-type Caz' channels. How doesnorepinephrine increase the contractile force of the hearr?This hormone acts through rhe B+ipe aclrencrglc receptorLo increase the generation of cAMP, activate PKA (p. 97),and rn turn phosphorylate the L rype Ca2* channels.Lhereby increasing the passive influx of Carn. An in-creased lCaz*], leads to an increase in contractile force.The cAMP pathway aLso appearc to lncrease Lhe Caz*sensitivity oI the contractile apparatus by phosphorylaringone or more of the regulatory proteins Thus, cAMP causesan increase in the lorce generared for any given lCar']1.

Reciprocal control over Ca2t entry rs provided by cyclicguanosrne monophosphare (cGMP) dependenL phosphorylation o[ the L-r)?e Cazt channels. AcetylchoLine, acLingthrough muscarinic ACh receptors, raises inrraceLlular.CGMP concentrations. In turn, the CGMP dependentphospl.rorylaLion of L-t1pe Ca:- channels. at sites distinctfrom those phosphorylated by the cAMP dependent kinase,causes a decrease in Ca" influx during Lhe cardiac actionpotential and thus a decrease in the lorce of contraction.

Ca']* entry may also be regulated indirectly by modu-lating other ion channels so that they eirher change rheirCa']* permeabiliry or alter rhe duraLion of the action po-te - ta Norep in -ph- ine . lo r r r .ampe Tr ) in , rpd>e t teCa']t permeability of voltage-gaLed Na* channels. Receptortransduction mechanisms that inhibit voltage-gared K'curents may prolong the cardiac action potential andthereby increase net Caz+ influx through L t),?e Cartchannels without modulatlng rhe Ca,' channels themselves-

In Smooth Muscle, Contractile Force lsEnhanced By Increasing the Entry of Ca2+, AsWell As By Increasing the Ca2* Sensitivity ofthe Contractile Apparatus

Unlike skeletal muscle, in which force development results from the summatron of inclividual muscle twitches,

innervates one set ofmuscle fibers.

A pool consists ofmany motor neutons,each of whichinnervates a motorunit with the muscle.

FICURE 9 15. The moLor uni t and the moior ,neuron pool

indivldual smooth muscie cells can maintain a sustainedconL-a . l ron tha t ,an be graded in .1qngt \ q re . r wrderange. Contractile lbrce in smooth muscie largely dependson the relative balance between the phosphorylation anddephosphorylation of MLCs. The rate of MLC phospho-

rylation is regulated by the Ca']+-CaM complex, whlcl.r inturn depends on lertels oJ tntracellular Ca1'. Smootl.r n.rus-cle cells can regulate lCa'?*1, over a wider range rhan' , .e le ta l and .a rd i r . mr rs r le .an lo r se tera l ,easor - . F fs t ,<olTr€ :moolh ru,cle ,.ells do not gererate dLiion polentials. Rarher, their membrane potential varies sLowly inresponse to neurotransmltters or hormones. This gradedresponse of V- allows flner regulation of Ca'?+ influxthrough voltage-gated channels. Second, release of Caz*from intracellular stores may be modulated through neurotmnsmitter induced generation of intracellular secondmessengers such as lP3- This modulation allows finer con-troL of Ca']* release than occurs in the SR Ca2* releasechannel by L-t1pe Ca']* channels in skeietaL and cardiacmuscle.

A second level of control over contnctile force occursby reguLatlng the Cr12+ sensitivity of proteins that regulateconfraction. For example, inhibiting fiyosin lryht ch'tinphaspl:Lr'tast: alters the balance berween phosphoryLationand dephosphoryLation, in effecl alLowing a greater con-iraction at a lower [Ca']+],. Some neurotransmitters act byinhibiting the phosphatase, whlch appears to occurthrough activation of G-protein-coupled receptors. An-o ther n 'echan i .m 'o r goveming Lhe Ca .e ' " r rn ty o fproteins that regulate contraction is alteration of the Ca,tsensirivity of the myosi[ light chain hinase. For example,MLCK ltself is phosphorylaied at specilic sites by severaLproteln kinases, including PKA, protein kinase C, andCa'z*-CaM dependent kinases. Phosphorylation by any ofthese krnases decreases the sensitivity of MLCK to activa-

Smooth Muscle Maintains High Force at LowEnergy Consumption

Smooth muscle is oiten called upon to maintain hlghforce for long periods. lf smooth muscle consumed ATPat rates similar to striaied muscLe, metabolic demandswould be considerable and the muscle would be prone tofatigue. Unlike striated muscle, however, smoorh muscleis able to maintain high force at a low rate ol ATPhydrolysis. This low-energy consumptiontligh-tensionstate ls referred to as the latch state. The latch state in5r loorh mun le i - rn ique becau.e h igh ren ,or can bemaintained despite a decrease in the degree of muscleactivation by excitatory stimuli. As a result, force is main-'a ined a t " lower le rc o f M l t K phosphoq la ion .

The mechanism underLying the latch state ls not en-Lirely known, although it appears to be due in large part[o cL ,nges rn the k . re r ic , o I acr in -n 'yos in c o>s-br idgeformation and detachment. These changes may be a directresult of a decrease in the rate at which dephosphorylaredcro) ) -bndge. de tach . Ten, .on i - d r re r r l y re laLeq to r i -enumber o[ attached cross-bridges. Furthermore, the pro-portion o[ myosin heads cross-bridgecl to actin is relatedto lhe ratio of attachment fates to detachment rates

Cellular Physiology of skeletal, Cardiac, and Smooth Muscle / 9 257

Therefore, it is reasonable to expect that a decrease in thedetachment rate would allow a greater number of cross-bridges to be maintained and would resuLt in a lower rateol cross bridge cycling and ATP hydroLysis. Thus, smoorhmuscle appears to be able to slow down cross-bridgecyciing just belore detachment, a feat that can be accom-pLished in skeletal muscle (see Fig. 9-7) only at low ATPlevels (as in igor mortis).

DIVERSITY AMONG MUSCLES

Each muscle Lype (i.e., skeLetal, cardiac, and smooth) isdistinguishable on the basis of its unique histology, EC-coupling mechanisms, and regulation o[ contractiLe func,tion. However, even within each of the three categories,muscle in different locations must serve markedly differ-ent purposes, with different demands for srrengrh, speed,and fatigabiliry. This diversity is possible because of diflerences in the expression of specific isoforms for varlouscon[ractile and regulatory proteins (Table 9- L).

Skeletal Muscle ls Composed of Slow-Twitchand Fast-Twitch Fibers

Some skeletal muscles musl be resistant to fatigue and beable to maintain lension for relatively long periods, al,though they need not contract rapidly. Examples aremuscles that maintain body posture, such as the soleusnu>c le o l tLe

-o*er par t o l rhe leg .

'n cont ras t . some

muscles need to contract rapidly, yet infrequently. Exam-ples are the exlraocular muscles, which must contraclrapidly to redlrect the eye as an object oI vlsual intereslmoves about.

lndividual muscLe fibers are classified as siow twitch(type I) or.lcst twltch (type II), depending on rheir rate off n n p d p , e l n . - " " r T h p c p 6 h e r r l - ^ - ' - i i h

guished by their histologic appearance and their ability toresist fatigue.

Slow-twitch fibers (Tab1e 9-2) are generally thinnerand have a more dense capillary network surroundingthem. Slow twitch fibers also appear red because of alarge amount of the oxygen-binding protein myoglobin(p. 656) within the cytoplasm. This rich capil1ary networktogether with myoglobin lacilitates oxygen transpo tothe slow-twitch fibers, which mostly rely on oxidativemetabolism for energy. The metaboLic machinery of theslow-twitch fiber aLso favors oxidaiiye metabolism becauseit has low glycogen content and gLycolytic-enz1'rne activitybu . a - r .h mi to .hondr ia l and or rda t r re -enzyme con len t .

Fast-twitch fibers differ among themselves with re-spect to fatigability. Some fast-twitch fibers are fatigueresistant; they rely on oxidative metabolism (type IIa)and are quite similar to slow-twitch Iibers wirh respect tomyoglobin content (indeed, they are red) and metabolicmachinery. One important difference is that fast-twitchov ida t .ve f ibers contarn ab . rdant g lvcogen and have agreater number of mitochondria than slow-twitch fibersdo. lhe<e learures en 'u re adequr re ATP genera t on tocompensate for the increased rate of ATP hydrolysis infast-twitch fibers.

9 / Ce lulaf Physiology of Skeletal, Cardjac, and Smooth N4uscle

TABLE 9-I

SKELETAL SLOW 0)

SKELETAL FASTOXIDATIVE(ll.)

SKELETAL FASTFATICABLE0tb) CARDIAC SMOOTH

Myosin heavy chain

Myosin l ight chain

sR Ca ATPase

Phospholamban

Calsequestrin

Ca2* release mechanisms

Ca2t sensor

MHC. I

MLC.i aS, -1 bS

SERCA2a

Present

"Fast" and "car-diac"

RyRl (Ca,* releasechannel or "ry-anodine" recep-toD

Troponin Cr

MHC- b, ltx

MLC-It -3f

SERCAl

Absent

"Fast"

RyRl

MHC-c and -B

MLC-'l v, I a

SERCA2a

Present

"Cardiac"

RyR2

MHC-sM 1 , 2(multiple iso-forms)

MtC-17a, -17b

SERCA2a,2b( b > > > a )

Present

? "Cardiac"? "Fast"

lP3R (3 isoforms)RyR3

Calmodulin(multiple iso-forms)

MHC- l la

MLC- I t -3 f

5ERCA1

Absent

"Fast"

RyR'l

Troponin C, Troponin C2 Troponin Cl

lPrR, inositol 1,4,5"triphosphate receptor; SR, sarcoplasmic reticulum.

Orhcr las t n r iLch f iber . a re nor .apab le o l sJ lhc ie r loxidative metabolism to sustain contraction. Because theseflbers must rely on the energy that is stored within glyco-gen (and phosphocreatine), rhey are more easily fatigable.Fatigable fast-twitch fibers (rype IIb) have fewer mito-chondria and lower concentrations of myogLobin and oxi-dative enryTnes. Because of their 1ow myoglobin content,tlpe llb muscle fibers are white. They are, however, richerr g lyco l l t rc e rz lT le dc l l \ l \ rhan orher hber r lpes are

In rea l i t v . s lon- a rJ fasr rwrLch hber . represenr rheextremes of a continuum of muscle fiber characteristics.Moreover. eacl whole m-scle ': compoceo ol 5bei oleach twLtch t1pe, although in any given muscle one of thefiber qpes predominates. The differences between 6bert1.pes derive in large part from differences in isoformexpression of the various contractile and regulatory pro-teins (see Table 9- 1).

Differences in the rate of contraction, for example, maybe directly correlaled with the maximal rate of myosinATPase activity. As many as 10 different myosin heary

chain isoforms have been identified. lndividuaL isoformexpression varies among muscle q.pes and is developmen-tally regulated. At least four isoforms of the MHC proteinare expressed in skeLetal muscle (MHC-I, MHClla, MHC-ilb, MHC-llx/d). For the most part, a muscle fiber typeexpresses a single MHC isoform, the ATPase aciivity ofwhich appears to correspond to the rate of contraction inthat fiber t'?e. Whereas most frbers express one of theseisoforms, some fibers express a combination of rwo differ-ent isoforms. These hybrid cells have rales of contractionrhat are intemediate between the two pure fiber t1-pes.

Differences in the rates and strength of contraction mayalso resuh from differences in myosin light chain isoformexpression or from isoform differences among other components of the EC coupling process. Three skeLetaL mus-cle isoforms have been identifred. MLC-las and MLC-]bsare expressed in slow twitch flbers, whereas MLC,1f andMLC-3f are expressed in fast-twitch fibers.

Isolorm differences aLso exist for the SR Ca2+ pump(i.e., the SERCA), calsequestrin, the Ca2*-release channel,

TABLE 9_2

SLOW TWIT(H ]A5T TWITCH FAsT TWITCHSynonym

Fatigue

Color

Metabolism

Mitochondria

Clycogen

Type I

Resistant

Red (myoglobin)

Oxidative

High

Type lla

Resistant

Red (myoglobin)

Oxidative

Higher

Abundant

Type llb

Fatigable

White (low myoglobin)

Clycolytic

Fewer

High

dnd t roporn ( . Fur thermore 'ome pro tern . . - r - r "L asphospholamban, are expressed in one fiber type (sLowtwitch) and not the other.

One particuLarly inleresting feature of muscle differenti-. ' on rc rh / t lhe - t " . .e de ,ermt ra t ion s no l - ta tLL .Through exer r i ,e t ra in i rS or c . langes in pa t te rns o f neuronal stimulation, alterations in contractile and regulatoryp-o te .n .so lo rm expres( on m. jy ocLur l -o r c \a -npe. iL i spossible for a greater proportion of fast-twitch fibers todevelop in a specific muscle with reperltive rraining. lt iseven possible to induce cardiac-specif,c isolorms in skele-tal muscLe, grven appropriate stimulation pattems.

The Properties of Cardiac Cells Vary withLocation in the Heart

Just as skeletal muscle consists of mukiple Fber types, sotoo does heart muscle. The electrophystological and me-chanicaL properties of cardlac muscle vary with rheir loca-tion (i.e., atria versus conductrng system versus ventricle).Moreover, even among cells within one anatomic location,functional differences may exist between muscle cells nearthe surface of the heart (epicdrdlql cells) and rhose liningthe intedor of the same chambers (endoccrdial cells). As inskeletaL muscle, many of these differences reflecr differ-ences in isoform expression o[ lhe various contraclile andreguLatory proteins. Although some of the protein isoforms expressed in cardiac tissue are identical to those

Cellular Physjology of Skeletal, Cardiac, and Smooth Muscle / 9

expressed in skeletaL muscle, many of the proteins havecardrac- :pe t i1 . i ,o lo rm- l see Tab lp o - l l lhe VHC inheart, for example, exists in two isoforms, a and B,which may be expressed alone or in combination.

Smooth Muscle Cells May Differ MarkedlyAmong Tissues and May Adapt TheirProperties with Time Even in a Single TissueWhen one considers that smooth muscle has a broadrange of functions, including regulating the diameter ofblood vessels, propelling food through the gastrointesrinaltract, regulating the diameter of airways, and delivering anewborn infant from the uterus, ir is not surpising thatsmooth muc. lc r. , odrticu drly diver>e r;pe o, rr usc-le .naddition to being dlstinguished as unitary or multiunitmuscle (p. 231), smooth muscle in different organs diverges with respect to ner.,'e and hormonal conlrol, elecL- ica dc v l ty . a rd charac te- r , t . , o f Lon l racr ion .

Even among smooth muscle cells within the same sortof tlssue, important functional differences may exist. Forexample, vascular smooth muscle cells within rhe walls o[two arlerioles lhal peduse different organs may vary intheir contractile response to various srimuli. Differencesmay even exist between vascular smooth muscle cells attwo different points along one arterial pathway.

The phenotype of smooth muscle within a given organmay change with shifting demands. The uterus, for exam-

TABLE 9_3

SKELETAI. CARDIAC sMooTtlMechanism of excitation Neuromuscular transmission

Electrical activity of mus- Action potential spikescle cell

Ca2* sensor

Excitation-contractioncouplrng

Terminates contraction

Twitch duration

Regulation of force

Metabolism

Troponin

L-type Ca,* channel (DHPreceptor) in T-tubulemembrane coupling toCa2* release channel (ry-anodine receptor) in 5R

Breakdown of ACh by ace-tylcholinesterase

20-200 msec

Frequency and multif ibersummation

Oxidative, glycolytic

Pacemaker potentialsElectrotonic depolarization

via gap junctions

Action potential plateaus

Troponin

Ca'z- entry via L-type Ca'*channel (DHP receptor)triggers Ca2*-inducedCa2- release from SR

Action potential repolariza-UOn

200-400 msec

Regulation of calcium entry

Oxidative

Synaptic transmissionHormone-activated receptorsF l 6 . + r i . : l . ^ , ' n l i h ^

Pacemaker potentials

Action potential spikes, plateausCraded membrane potential

changesSlow waves

Calmodulin

Ca,* entry via voltage-gated Ca2*cnannets

Ca2*- and lP3-mediated Ca2* re-lease from 5R

Ca2* entry through store-operated Ca2* channels

Myosin l ight chain phosphatase

200 msec-sustained

Balance between MLCK phos-phorylation and dephosphory-lation

Latch state

Oxidative

ACh, acetylchol ine; DHP, dihydropyridine; lP3, inositol 1 ,4,5-tdphosphate receptor; MLCK, myosin l ight chain kinase; SR, sarcoplasmicreticulum.

9 / Cellular Physiology of skeletal, Cardiac, and Smooth Muscle

p le . i ' compo<ed o f - - roo th mu<c le- t re m)ometnu-n-that undergoes remarkable transformatlon during gesta-tion as it prepares tor parturition (see Chapter 55). lnaddition to hypertrophy, greater coupling develops be-tween smooth muscle cells through the increased forma-tion of gap junctions. The cells aLso undergo changes intheir expression of contractile protein isoforms. Changesin the expression of ion channels and hormone receptorsfac i r ra te rhyhmtr e lec t r .ca l ac t -v rLy . Th is ac l y i t y l s coo -dinated across the myometrium by propagation of actionpotentiaLs and increases in lCa']*1, through the gap junctions. These rhythmic, coordinated contractions developspontaneously, but they are strongly influenced by rhehormone oxylocin, levels of which increase just beforeand during labor and just after pa udlion.

These differences in smooth muscle function amonsvanou> l :s5ue< or eve- l over rhp l iLe tLme o l a ' ins le ce lp robab ly -e fec t d i f le rence, in p rore - ro r - rpos t r i ;n . Indeed, in comparison to striated muscle, smooth musclecells express a wider variety of isoforms of contractile andregu la to rv p roLe in ' r ' .ee Tab le o l r . th ts \a r ie ly .s aresult of both multlple genes and altemarive splicing (p.139). This richness in diversity is likely to have imponantconsequences for smooth muscle cell function, althoughthe precise relationship between the structure and function of these protein isoforms is not yer cLear.

Smooth Muscle Cells Express a Wide Varietyof Neurotransmitter and Hormone ReceptorsPerhaps one of the most impressive sources of diversityamong smooth muscle relates to differences in responseto neurotransmitters, environmental factors, and circulat-ing hormones. Smooth muscle cells differ widely withrcspect to the t),?es of cell-surface recepiors that mediatethe effects of these various mediators. ln general, smoothmuscle celLs each express a vadety of such receptors, andreceptor slimulalion may lead to eirher contractlon orreLaxation. Many substances act through different receptorsubqpes in ditferent cells, and these receptor subtlT)esmay acl through different mechanisms. For example,whereas some neurotransmitter,4:rormone receDtors mavbe . rgand-gared ion channe ls . o thers /c t th rough heLero t r i

meric G proteins that either act directly on targets or acrthrough intraceliular second messengers such as cAMP,cGMP, or IP, and DAG.

The list of neurotransmitterc, hormones, and environ-mentaL factors regulating rhe function of vascular smoothmuscle cells alone is vast (see Chapter 22). A few of thesevasoacLive subshnces include epinephrine, norepinephrine, serotonin, angiotensin, vasopressin, neuropeptide Y,nitric oxide, endothelin, and orygen. Identical slimuli,however, may result in remarkabLy differenr physiologicresponses by smooth muscle in different locations. Forexample, systemic afledal smooth muscle cel1s relax whenthe oxySen concentration around them decreases, whereaspulmonary artedal smooth muscle contracts when localorygen decreases (p. 699).

See Table 9-3 for a comparison of muscLe qpes.

R E F E R E N C E S

Books and Reviews

Farah CS, Reinach FCr The troponin conplex and regularion of musclecontracljon. FASEB J 9:755 767, 1995.

Franzini Armstrong C, Prorasi F: Ryanodine receplors of srriared muscles: A complex channel capable of muhiple interactions. Physiol Rev77:699 729, 1997.

Holda J, Klishin A, Sedova M, et ai: Capaciraiive calcium emry. NewsPhIsioL Sci l3:157-163, 1998.

Horo$.iu A, Menice CB. Lapone R, Morgan KG: Mechanisms of smoorhmuscle contraction. Physiol Rev 761967-1003, 1996.

Parekh AB, Penner Ri Store deplerion and calclum influx. Physiol Rev77:901-930. I997.

Striggos' F, Ehrlich BE: Ligand-gared calcium channels inside and our.Curr Opin Cell Biol 8:490-495. 1996.

Journal Articles

Cannel MB. Cheng H, Leder€r WJr The conrrol of calcium reiease inheart muscle. Science 26811045 1049, 1995.

Finer JT, Simmons RM. Spudich JA: Single myosin molecule mechanics:Picone$'lon forc€s and nanomerer steps. Narure 368:1I3-119, 1994.

Gordon AM, Huxley AF. Julian FJ: The vadarion in isometric rensionwiLh sarcomefe lengrh in verrebrate muscle. J Physiol 184:170-192,1966.

Mickelson JR, Louis CF: Malignant h)?erthermia: Excitation-conrractioncoupling. Ca'?. release channel. and cell Car* regulation defects PhysioL R€v 76i537 592, 1996.

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