Thorax 1995;50:894-901

Airway smooth muscle relaxation

Alan J Knox, Anne E Tattersfield

Bronchodilators are important agents in thetreatment of asthma and chronic obstructiveairways disease. Beta agonists, theophyllines,and antimuscarinic agents are the main drugs incurrent use, but others are under investigation.Although other actions may be important, thesedrugs produce bronchodilation either by a dir-ect effect on airway smooth muscle or by in-hibiting neural pathways. As our understandingof airway smooth muscle relaxant mechanismsincreases, new therapeutic possibilities are

being realised. Several recent findings con-

tradict established dogma - for example, theassumption that ; agonists and theophyllinesact solely via cyclic adenosine monophosphate(cAMP)-linked intracellular pathways has beenquestioned and studies with atrial natriureticpeptide and nitrates suggest that cyclic gua-

nosine monophosphate (cGMP) may be an

important relaxant second messenger. The roleof potassium channel activators is being ex-

plored with the development of drugs whichrelax airway smooth muscle by a direct actionon cell membrane ion channels. In this reviewwe discuss what is known of the likely mech-anisms of action of the airway smooth muscle

Respiratory Medicine relaxants in current use and their effects onUnit, City Hospital, calcium homeostasis, and suggest how de-Nottingham NG5 IPB, velopments in our understanding of relaxantA J Knox mechanisms may aid the development of treat-A E Tattersfield ment for airways obstruction in the future.Reprint requests to: Agents such as the muscarinic, histamine or

Dr A J Knox. leukotriene antagonists which cause broncho-Received 7 November 1994 dilatation by blocking the effects of neuro-Returned to authors transmitters or mediators at the receptor level8 February 1995

Revised version received are not discussed, nor are the effects other than22 March 1995 those on airway smooth muscle of broncho-Accepted for publication1 May 1995 dilators in current use.

Ca2-

~ 5'AMP -5.GMVPPKA8__~~~~~~~~~KANT-P' c-A-,MP PDE

~~~AC~ C3

GC

Receptor Receptore.g. [32 e.g. ANP

Figure 1 Main pathways of airzvay smooth muscle relaxation: (1) via receptors linkedto cAMP; (2) via receptors linked to cGMP; (3) PDE (phosphodiesterase) inhibition;(4) activation of K+ channels; (5) inhibition of Ca2" channels. AC= adenylate cyclase;PKA =protein kinase A; GC=guanylate cyclase; PKG =protein kinase G.

Role of calcium in relaxation of airwaysmooth muscleThe drugs in current use cause relaxation ofairway smooth muscle by stimulating a cyclicnucleotide second messenger system, pre-dominantly cAMP and cGMP, by inhibitingthe breakdown of cyclic nucleotides or by adirect action on cell membrane ion channelsor transporters (table 1, fig 1).

Since the relation between an increase inintracellular calcium and contraction is wellestablished, it is generally assumed that re-laxation is preceded by a fall in intracellularcalcium, although relatively few studies haveinvestigated this. A fall in intracellular calcium(measured by aequorin luminescence)' ac-companied isoprenaline-induced relaxation incanine trachealis contracted with 5-hydroxy-tryptamine or cholinergic agonists and, sincethe fall in intracellular calcium correlatedstrongly with the reduction in tension, it wasthought to be responsible. A fall in cellularcalcium was also seen in cAMP-mediated re-laxation in canine airway smooth muscle2 andin cGMP-mediated relaxation in canine andbovine airway smooth muscle.34 There was nofall in intracellular calcium in response to eagonist in bovine tracheal smooth muscle inone study,5 and in some studies increases inintracellular calcium in response to ,B agonistshave been seen as a result of calcium influxthrough membrane channels under restingconditions.36 The inconsistencies seen in thesestudies may be due to the fact that the ex-periments which failed to show a reduction incalcium were carried out at 22°C when manyintracellular processes, includingNa +/K+ ATP-ase, would be inhibited. On balance it seemslikely that a fall in intracellular calcium is as-sociated with relaxation in the same way thatan increase in intracellular calcium is associatedwith contraction.

Table 1 Agents known to cause airway smooth musclerelaxation in vitro

Mechanism Agent

1. Agents acting through a agonists, vasoactive intestinalstimulating cAMP peptide, peptide histidine

isoleucine, pituitary adenylatecyclase activating peptidePGE2, PGI2

2. Agents acting through Nitric oxide, nitrosothiolsstimulating cGMP Nitrates, sodium nitroprusside

ANP, BNP, CNP3. Inhibitors of cAMP, Phosphodiesterase inhibitors

GMP breakdown4. Ion channels Calcium antagonists

Potassium channel activatorsSodium channel/transportmodulators

5. Miscellaneous Anaesthetic agentsLithiumProtein kinase C inhibitorsCalmodulin antagonists

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Airvay smooth muscle relaxation

Na+(3)K+(2)

H+(2)

Figure 2 The main calcium removal mechanisms in airway smooth muscle: (1) Ca2"Mg2" ATPase; (2) Na+ICa2+ exchanger which is linked to (3) Na+IK+ ATPase; and(4) Ca2 uptake by intracellular stores.

At least three calcium removal mechanismsare thought to lower intracellular calcium inairways smooth muscle: (1) calcium magnes-

ium ATPase, (2) sodium-calcium exchange,7and (3) uptake of calcium into intracellularorganelles such as sarcoplasmic reticulum andmitochondria (fig 2).

CALCIUM MAGNESIUM ATPASE (CALCIUMEFFLUX PUMP)The electroneutral calcium efflux pump foundin the plasma membrane ofmost cells includingsmooth muscle8 is a magnesium- and ATP-dependent enzyme which extrudes one calciumion for two hydrogen ions. It is stimulated bya calcium-calmodulin complex which may actas a feedback mechanism to limit the rise inintracellular calcium following stimulation.8-0Its activity also depends on the activity ofAMP-dependent protein kinases in several types ofsmooth muscle,"1 although the only study todate on airway smooth muscle failed to showthis.'2

SODIUM-CALCIUM EXCHANGE

The sodium-calcium exchanger transports one

calcium ion for three sodium ions and is drivenby the sodium gradient across the membranewhich, in turn, is maintained by Na+/K+ ATP-

ase."3 Although there is good evidence for

Receptor Contractile receptore.g. 02 ,- e.g. histamine

K (2) Na+ (3)

Figure 3 Schematic representation of the sites ofphosphorylation for protein kinase A.The combined effect ofphosphorylation at these sites is to lead to airway smooth musclerelaxation. MLCK= myosin light chain kinase; SR= sarcoplasmic reticulum; PLC=phospholipase C; DAG=diacylglycerol; Ins P3= inositol trisphosphate; PIP2= inositolbisphosphate; G =guanosine nucleotide binding protein; AC= adenylate cyclase.

sodium-calcium exchange in cardiac muscle,14its presence in smooth muscle is less wellestablished.'5 Such a mechanism probablyexists in the airways since reducing externalsodium causes contraction of canine,'6bovine,'7' 18 and human'9 airway smooth musclein vitro, and ion flux studies suggest that Na+/Ca2+ exchange sites are abundant in bovinetracheal smooth muscle.20

INTRACELLULAR CALCIUM REMOVALMECHANISMSThe sarcoplasmic reticulum and mitochondriaare the most important intracellular organellesfor the removal of calcium. The sarcoplasmicreticulum accumulates calcium via a cAMP-dependent mechanism.8122' The mitochondriatake up calcium, store it as a non-ionic calcium-phosphate complex, and release it back intothe cytosol via a calcium efflux pathway.22 Themitochondrial calcium store may protect thecell against calcium overload in pathologicalconditions,7 at least in cardiac muscle.

Cyclic AMP-mediated relaxantmechanismsThe cAMP pathway is the most importantrelaxant signal transduction pathway in airwaysmooth muscle. The 12 adrenoceptor, the mainreceptor studied with respect to this pathway,is coupled through an intermediary stimulatoryG protein (Gj) to adenylate cyclase which cata-lyses the intracellular conversion of ATP tocAMP. Adenylate cyclase can also be inhibitedthrough muscarinic M2 receptors via an in-hibitory Gprotein (Gi). cAMP activates a groupof cyclic AMP-dependent protein kinases (pro-tein kinase A) which, in turn, phosphorylate anumber ofdifferent substrates which then causerelaxation. The precise way in which cAMPcauses relaxation is still not clear. Several ac-tions of cAMP-dependent protein kinaseswhich can cause relaxation have been identified(fig 3). These include inhibition of inositolphosphate hydrolysis,2324 increased calcium up-take by internal stores,25 inactivation of myosinlight chain kinase,26 and activation of cell mem-brane ion channels and transporters such asK+ channels27 and Na+/K+ ATPase.2829 Thenet effect of these processes would be to reduceintracellular calcium concentration and in-crease contractile protein phosphorylation,thereby producing relaxation. Although proteinkinase A has been shown to produce effects atthese sites in vitro, the relative importanceof each mechanism in mediating the relaxanteffects of ,B adrenoceptor agonists in vivo is stillunclear. Recent work in porcine and ferretairway smooth muscle suggests that 1 agonistscan produce relaxation independently from el-evation of cAMP via stimulation of the GTPbinding protein of adenylate cyclase (Gs)coupled to maxi-K+ channels.30 The effect ofG,and protein kinase A on the maxi-K+ channel isadditive.30Agents which act through stimulation of

cAMP production include 02 adrenoceptoragonists, vasoactive intestinal peptide andprostaglandins.

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12 ADRENOCEPTOR AGONISTSBeta2 agonists increase intracellular cAMPlevels3' and are potent relaxants of airwaysmooth muscle in several species includingman.32 The negative log molar EC50 value forisoprenaline-induced relaxation of bronchialsmooth muscle spontaneous tone is in the rangeof 9 to 7,32 with lower values (7 to 3 indicatingreduced potency) being seen when tissue ispreconstricted with contractile agents such ashistamine or acetylcholine. Most 12 selectiveagonists such as salbutamol are partial agonistswhen compared with isoprenaline, which is a

full but non-selective agonist in vitro. Theeffects of 12 agonists in vitro can be mimickedby direct activators of adenylate cyclase suchas forskolin.The new 02 adrenoceptor agonists, for-

moterol and salmeterol, produce more potentand prolonged relaxation of guinea pig tracheaand human bronchial smooth muscle in vitrothan drugs such as salbutamol.3334 The negativelog molar EC50 values for relaxation of humanbronchial smooth muscle is 9-6 for formoteroland 7-6 for salmeterol.35 Both drugs are morelipophilic than salbutamol and it has been pos-tulated that non-specific binding to the cellmembrane or binding to an exoreceptor mayprolong their action.35 Salmeterol is a partialagonist compared with both isoprenaline andsalbutamol."6

VASOACTIVE INTESTINAL PEPTIDE (VIP)VIP immunoreactivity has been demonstratedin nerve fibres in the airway37 and VIP receptorshave been identified on airway smoothmuscle.38 VIP relaxes airway smooth musclefrom several species including human airwaysand is a potent bronchodilator.39 Its effect onhuman airways is enhanced by epithelial re-moval and peptidase inhibition.40 Part of itsaction may be due to Na+/K+ ATPase ac-tivation since relaxation in rabbit airwaysmooth muscle is inhibited by the Na+/K+ATPase inhibitor ouabain.4'

PROSTAGLANDINS

Prostaglandin E2 (PGE2) relaxes canine airwaysmooth muscle.28 In human airway smoothmuscle at low concentrations it increases cAMP

N itratesANP

Guanytate.cyclase Soluble

ParticulatatGTP cGMP

Ca2+EvePROTEIN KINASE G

K+ve

Figure 4 Sites ofphosphorylation for protein kinase G which have been shown inairvay smooth muscle. SR = sarcoplasmic reticulum. Other possible sites demonstrated innon-airway smooth muscle include inhibition of inositol phospholipid turnover55 anddecreased sensitivity of contractile proteins to calcium."

via adenylate cyclase3' and causes relaxation,whereas at higher concentrations it causes con-traction.42"4 This can probably be explainedby its actions on different receptors.45 Thecontractile effects of prostaglandins arethought to be mediated by thromboxane re-ceptors at which PGE2 is a weak agonist. Thereare three prostaglandin E receptors - EPI, EP2,and EP3 - which are coupled to different cellsignalling systems. The relaxant effect ofPGE2is thought to be due to stimulation of the EP2receptor which is coupled to cAMP. PGI2 andberaprost (a PGI2 analogue) also cause re-laxation of canine airway smooth muscle whichcan be inhibited by inhibitors of adenylatecyclase, suggesting that cAMP pathways areinvolved.46

OTHERSPeptide histidine isoleucine (PHI) and itshuman equivalent, peptide histidine me-thionine (PHM), share structural similaritieswith VIP and have a similar immuno-cytochemical distribution in the lung. PHI isa potent relaxant of human bronchi invitro,39 whilst pituitary adenylate cyclase activat-ing peptide (PACAP) has been shown torelax rabbit and guinea pig airway smoothmuscle by increasing cAMP.4047

Cyclic GMP relaxant mechanisms(figs 1, 4)It has become clear that cGMP produced byguanylate cyclase is also a relaxant second mes-senger in airway smooth muscle.48 Guanylatecyclase exists in two forms - a soluble and aparticulate form - both of which have beenpurified and characterised.4950 Soluble guan-ylate cyclase is a haem protein which existsas a heterodimer composed of two subunits; itis activated by nitric oxide and the nitro groupof vaso/bronchodilators. Particulate guanylatecyclase is one of the receptors mediating theeffects of atrial natriuretic peptide.4950 Guan-ylate cyclase catalyses the conversion of GTPto cGMP which then activates a group of dis-tinct cGMP-dependent protein kinases. Con-siderable sequence homology exists betweencGMP-dependent protein kinases and otherkinases, particularly cAMP-dependent proteinkinases.50 In contrast to cAMP-dependent pro-tein kinases which are the major intracellulareffector system for cAMP, many cGMP effectsare not due to cGMP-dependent protein kin-ases, at least in non-muscle cells. cGMP-de-pendent protein kinases appear to have varioussites of action in airway smooth muscle as withcAMP-dependent protein kinases (fig 4)51-55activation of maxi-K+ channels and uptake ofcalcium by internal stores appear to be themost important for relaxation of airway smoothmuscle.5254The agents which act through stimulation of

cGMP production include nitrates and atrialnatriuretic peptide.

NITRATESGlyceryl nitrate, sodium nitroprusside and thecell permeable cGMP analogue, 8-bromo-

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Ainvay smooth muscle relaxation

Stimulus 2...Nos. R ....AX..ATICe.g. acetylcholine NO

ADP 'bradykinin :::L-citruine dshear stress L Airway smooth muscle cEglutamate _\

Generator cell

e.g. endotheliumepitheliumneurones

Figure 5 Mechanism whereby nitric oxide (NO) might bring about relaxation of airway smooth muscle. cNOS=constitutive nitric oxide synthase; GC=guanylate cyclase.

ell

cGMP, have been shown to relax canine andbovine airway smooth muscle56"5 and, in thecase of sodium nitroprusside, human airwaysmooth muscle.59 S-nitrosothiols, which consistof nitric oxide linked to a thiol group and arefound endogenously in airway secretions, relaxguinea pig and human airway smooth muscle.60Dissolved nitric oxide gas relaxed bovine trach-ealis in one study.6' Endogenous nitric oxideand S-nitrosothiols are produced by cells suchas airway epithelium and non-adrenergic non-cholinergic nerves and may be involved in theregulation ofbronchomotor tone (fig 5).62 Thiseffect might, however, be offset by pro-inflammatory actions of nitric oxide.62

ATRIAL NATRIURETIC PEPTIDEAtrial natriuretic peptide (ANP) relaxes guineapig63 and bovine airway smooth muscle andthis is associated with increased cGMP64 inbovine airway smooth muscle. ANP is degradedrapidly by epithelial peptidases and the re-sponse to ANP is enhanced by removing epi-thelium or adding a peptidase inhibitor.65 Theeffects of ANP appear to be less marked inhuman airway smooth muscle.6S65 Two relatedpeptides, brain natriuretic peptide (BNP) andC-type natriuretic peptide (CNP), have beenshown to increase cGMP and relax guineapig airway smooth muscle, but have not beenstudied in man.6970

Agents preventing the breakdown ofcAMP and cGMPA family of phosphodiesterase enzymes hy-drolyse the 3'-phosphoester bond on cAMP

Table 2 Phosphodiesterase isoenzymes in ainvay smoothmuscle

Isoenzymes Species

I Ca'+/calmodulin- Canine, bovine, humanstimulated

II cGMP-stimulated Canine, bovine, humanIII cGMP-inhibited Canine, guinea pig, humanIV cAMP-specific Canine, guinea pig, bovine,

humanV cGMP-specific Canine, humanVI cGMP-specific None to date

photoreceptorVII cAMP-specific, None to date

rolipram insensitive

and cGMP converting them to their inactive 5'-nucleotide metabolites 5'-AMP and 5'-GMP.Theophylline and other xanthine derivativescause airway smooth muscle relaxation in vitroand bronchodilatation in vivo, and althoughthey may have other mechanisms of action,phosphodiesterase (PDE) inhibition appears tobe important for smooth muscle relaxation.7'Theophylline has been shown to relax smoothmuscle from several species including manwhen contracted with a wide range of spas-mogens in vitro,72 although it is some 1000times less potent than isoprenaline in relaxinghuman bronchi contracted by moderate dosesof carbachol.73 Theophylline is a rather non-selective inhibitor of tissue phosphodiesterasesand this is one factor underlying its narrowtherapeutic index.

Recent studies have identified at least sevenclasses of PDE isoenzymes, each of which hasseveral isoforms (table 2). These isoenzymesvary in their tissue distribution and their abilityto break down cAMP and cGMP.7475 Airwaysmooth muscle contains several PDE iso-enzymes in most species and in human airwaysmooth muscle types III, IV and, to a lesserextent, V are important for relaxation.76-79Drugs which inhibit specific isoenzymes maybe relatively tissue selective in their effects andthus have advantages over theophylline. Studiesof some of the more specific type IV inhibitorsare currently under investigation in man.

Drugs acting on cell membrane iontransportCALCIUM CHANNEL ANTAGONISTSVoltage-dependent calcium channel ant-agonists such as verapamil, gallopamil, ni-cardipine, and nifedipine inhibit the contractileresponse to depolarising agents and to variousspasmogens (histamine, cholinergic agonists,leukotriene D4) in isolated airway smoothmuscle from several species including man 8084They also reduce the response of sensitisedguinea pig and human airway smooth muscleto antigen, 586 the order of potency for thiseffect being nifedipine >verapamil >diltiazem.The effects of the drugs in vitro have generallybeen modest even with high drug con-centrations; this may be because the initiationof airway smooth muscle contraction is due to

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Ca2 voltage Ca receptor, dependernt I operated Na (3)

K (2)

;< > 4 ~~~~~~~Na+(3)>-ve2-ve'+

Ks channlel activators

:.:..:..~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.

:;.~~~~~~~~~~

..........s....:

s; ;1........;

voltage dependent

receptor operated

ATP regLillatedCa activated

Figure 6 Mechanisms whereby K+ channel activation is thought to produce airi

smooth muscle relaxation. SR=sarcoplasmic reticulum.

release of calcium from internalstoretthan calcium influx. A receptor operai

cium channel has been shown to be respfor calcium entry during the maintenanc

of the contractile response in cultured

airway smooth cells.87 Whether inhib:

this channel might have a role as airway

muscle relaxants awaits further study.

antagonists in current use have prov

appointing in studies of asthma in vivo

POTASSIUM CHANNEL ACTIVATORS

The electrical stability of airway smooth

is controlled by K+ channels.88h89 The tw

have generated the most interest with

to airway smooth muscle relaxationmaxi-K+ channel and the ATP regulatenel (fig 6). Activation of K+ channelsmembrane hyperpolarisation in airway

muscle.

CALCIUM ACTIVATED CHANNELS (MAXI-K

The large conductance K+ channels (nfound in airway smooth muscle fromspecies including ma99 are activated

cium and inhibited by charybdotoxin.

K+ channels may have a role in bothand cGMP-mediated airway smooth

relaxation. The channels are activa

cAMP-dependent protein kinase in ratchea27 and directly by cAMP in porci

ferret trachea30 and, when inhibited by

dotoxin, cAMP-mediated relaxation of

pig and human airway smooth muscle

cGMP-mediated relaxation in bovine

smooth muscle58 are inhibited. Friantagonism could be responsible foinhibitory effects, however. Charybcan cause calcium influx through i

dependent calcium channels, and theory effect of charybdotoxin on isoprcinduced relaxation in guinea pig trainhibited by nifedipine.96

METABOLICALLY CONTROLLED CHANNEL

ATP-sensitive K+ channels open in respa reduction in ATP and K+ channel ac

(cromakalim, nicorandil, pinacidil) and areblocked by the sulphonylureas (glibenclamide,tolbutamide, chlorpropamide). K+ channel ac-tivators have been shown to relax spontaneoustone and contraction induced by histamine,prostaglandin D2, and leukotriene C4 in guineapig airways, and histamine and cholinergic tonein human airways.97100 K+ channel openers arevery effective at reducing spontaneous tone inhuman airway smooth muscle but less effectiveat reversing receptor-mediated contraction.'The mechanisms by which K+ channel ac-

tivators cause airway smooth muscle relaxationis uncertain but are likely to include (a) mem-brane hyperpolarisation with reduction of cal-cium influx via voltage-dependent calciumchannels; (b) activation of Na+/K+ ATPase

way causing a decrease in intracellular Na+ and,as a consequence, Ca2+ efflux via Na+/Ca2+exchange; and (c) possibly the regulation ofintracellular calcium release (fig 6).102

s rather The drugs currently available lack specificityted cal- for the airways and, although some broncho-onsible dilatation has been seen in patients with:e phase asthma, systemic side effects such as hypo-human tension have been a problem.itors ofsmooth SODIUM,alcium Smooth muscle contains several exchangersred dis- which regulate intracellular sodium including

the Na+/Ca2+ exchanger, Na+/K+ ATPase, andthe Na+/H+ antiport. Intracellular sodium isclosely linked to intracellular calcium throughthe Na+/Ca2+ exchanger, so changes in

muscle intracellular sodium or in sodium exchangeo which systems can affect relaxation. The Na+/H+respect antiport may also alter the contractile statusare the of the cell by regulating intracellular pH.d chan- Activation of Na+/K+ ATPase reduces intra-causes cellular sodium causing an increase in calciumsmooth efflux via Na+/Ca2+ exchange. There is evid-

ence that activation of Na+/K+ ATPase is im-portant for drugs which act through cAMPsuch as [ agonists, but not those that act

iaxi-K) through cGMP. Na+/K+ ATPase activity isseveral increased by protein kinase A but not proteinby cal- kinase G in canine airway smooth muscle mem-Maxi- brane particulates,29 and inhibition of Na+/CAMP K+ ATPase with ouabain inhibits the relaxantmuscle responses to drugs that act through cAMP suchted by as [ agonists, but not those that act through)bit tra- cGMP such as nitroprusside.2829 Activators ofine and Na+/K+ ATPase or Na+/Ca2'+ exchange wouldcharyb- therefore be expected to act as airway smooth' guinea muscle relaxants. No such agents are currently94 95 and available.airway Amiloride, a non-specific inhibitor ofsodium

actional channels and transport processes includingr these Na+ entry channels, Na+/H+ and Na+/Ca2+dotoxin exchange, relaxes airway smooth muscle tone

voltage- and inhibits receptor-operated contraction ininhibit- canine and bovine trachealis and the response*. I AA-1nAenaline- to antigen in guinea pig airways in vitro.l_,chea is Further studies suggest that this effect may be

due to inhibition of Na+ entry channels'06 orprotein kinase C'07 rather than inhibition ofNa+/H+ exchange or Na+/Ca2" exchange. In-

s haled amiloride caused no bronchodilatation,)onse to however, in one study of normal and asthmatictivators subjects.'08

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Ainvay smooth muscle relaxation

MiscellaneousThese are several other post-receptor siteswhere drugs could modify the relaxant pro-cesses in airway smooth muscle including gua-nosine nucleotide regulatory proteins (Gproteins), calcium uptake into intracellularstores, and enzymes regulating the contractileprocess such as calmodulin, protein kinase C,and myosin light chain kinase. Few agentswhich affect these processes have been studied.Calmodulin antagonists have been shown toinhibit contraction in response to antigen inguinea pig trachea,85"05 but in general theseagents lack specificity. The putative proteinkinase C inhibitors H-7 and staurosporine in-hibit contractile responses to spasmogens inguinea pig"0 and bovine'07 airway smoothmuscle, but the drugs also lack specificity andhave other effects such as inhibition of myosinlight chain kinase.

Lithium, which inhibits myoinositol phos-phate leading to depletion of membrane phos-pholipid pools, causes relaxation and protectsagainst histamine-induced contraction inguinea pig airways in vitro."' Lithium alsoreduces the airway response to histamine inasthmatic subjects."2Some anaesthetic agents such as the ben-

zodiazepines, halothane, and ketamine havebeen shown to relax airway smooth muscle invarious animal species.'"""' This may berelevant for the treatment of acute asthma inintensive care units.

ConclusionThe pharmacology of airway smooth muscle iscomplex due to the wide range of effects ofreceptors and ion channels on airway smoothmuscle cells and the complex post-receptormechanisms involved in the contractile andrelaxant responses. This provides a wide rangeof potential sites for pharmacological agents toproduce relaxation of airway smooth muscle.Of the established smooth muscle relaxant

drugs, P2 adrenergic agonists are currently themost useful. Advances are being made in sev-eral areas, however. Modifications to the struc-ture of 12 adrenergic agonists have producedagents with a longer duration of action. Iden-tification of different isoenzymes of phospho-diesterase and the development of selectiveisoenzyme inhibitors may produce drugs whichcan exert tissue selectivity by acting an iso-enzymes present in airway smooth muscle tis-sue but no other tissues. This might preventsome of the undesirable side effects seen withnon-selective phosphodiesterase inhibitorssuch as theophylline.

Ion channel inhibitors or activators haveproved disappointing therapeutic agents todate. Although the calcium channel antagonistshave relaxant properties in vitro, they have notproved useful in vivo. The potassium channelactivators have additional properties to in-hibition of calcium entry via voltage-dependentcalcium channels but greater specificity for air-way smooth muscle is needed if they are to beuseful relaxant agents.A number of other steps in the contractile

and relaxant responses are open to targeting

by pharmacological agents, including mod-ulation ofG proteins, the inositol phospholipidcascade and calcium release mechanisms, cal-modulin and protein kinase C. There is alsopotential for drugs which facilitate calcium ex-trusion across the cell membrane by activatingsodium-calcium exchange, sodium potassiumATPase, or calcium magnesium ATPase. Air-way smooth muscle pharmacology is an excitingand developing area for which the future holdsmuch promise.

We thank Hilary Hughes for typing the manuscript and DanielDuthie for work on the graphics.

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