Endocrine Glands that function Organs That Function ... · the mammary gland alveoli and ducts).! • When the baby suckles, touch receptors located in the nipple send sensory signals

PSLY 4210/5210 !Comparative Animal Physiology !Spring 2010!!Endocrine Systems!

Topic 17 - 1!

Endocrine and Neurendocrine Systems!Main function is to maintain internal homeostasis in response to internal and external (environmental) changes.!Accomplished via chemical messengers that travel different distances to target cells and regulate their activity.! Short range: Neurons and local paracrine/autocrine

cells produce and release chemical messengers within micrometers of target cells!

 Long range: neurosecretory cells and non-neural endocrine cells make hormones: chemical messengers that travel via the circulatory system to target tissues throughout the body.!

Figure 15.1 Chemical signals act over short and long distances

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Endocrine Glands that function Primarily to Secrete Hormones!

Pituitary gland: antidiuretic hormone (ADH), oxytocin, and tropic hormones!Thyroid gland: thyroxin, triiodothyronine and calcitonin!Parathyroid glands: parathyroid hormone (PTH)!Adrenal glands: many corticosteroids (e.g., cortisol, aldosterone, and androgens), aned medullary hormones (e.g., epinephrine, norepinephrine)!Pancreatic islet cells: alpha cells secrete glucagon and beta cells secrete insulin !Ovaries and testes: estrogens, progesterone, and androgens !Pineal gland: melatonin!Thymus gland: thymosine!

Organs That Function Secondarily as Endocrine Glands!

Heart: secretes atrial natriuretic peptide (ANP)! Occurs when there is an expansion in blood volume. ANP causes

diuresis, increasing secretion of Na+ into the distal tubule and concentrating it in the urine. Water follows by osmosis, decreasing the blood volume.!

Kidney: secretes erythropoietin, which stimulates the bone marrow to produce more red blood cells!Liver: secretes somatomedin, a growth factor.!Skin: secrets 1,25-dihydroxyvitamin D3 which is responsible for calcium homeostasis!Gastrointestinal tract: secrets gastrin, CCk, and VIP.!Adipose tissue: secretes leptin, which plays a role in obesity!Hypothalamus: secretes several releasing and inhibiting hormones.!

Figure 15.6 Discrete glands probably evolved from diffusely distributed secretory cells Chemical Classification of Hormones!Steroid hormones! All based on cholesterol. Secreted by adrenal cortex,

testes, and ovaries! Examples: aldosterone, cortisol, testosterone,

progestrone! Nonpolar and hydrophobic (lipophilic), must be

carried in the plasma by a protein that will bind with the hormone and transport it to the target call!•  Once it reaches the target cell the hormone is released!•  It enters the cell and binds to receptors that are either within

the cytoplasm (that carry the hormone into the nucleus) or proceeds directly through the nuclear membrane.!

•  Once in the nucleus, the hormone binds to a domain on a chromosome, promoting transcription and protein synthesis.!


Topic 17 - 2!

Figure 15.2 Steroid hormones are derived from cholesterol Chemical Classification of Hormones!Protein and (glyco)peptide hormones! Secreted by all endocrine glands except adrenal

cortex, gonads and thyroid, e.g., insulin, LH, FSH! Generally present as a prohormone, they undergo

modification and become the active form of the hormone later.!

 Polar (hydrophilic): can’t cross the lipid bilayer of cell membrane and must bind to cell membrane receptors!•  Once the hormone binds to the receptor, several changes

occur in the cytoplasmic side of the cell membrane which then stimulates a second messenger (c-AMP , c-GMP, phospholipase C, Ca2+, or thyrosine kinase)!

•  These then activate cytoplasmic proteins.!

Figure 15.3 Peptide and protein hormones consist of assemblages of amino acids Chemical Classification of Hormones!Tyrosine derivatives! Examples: thyroid hormone, catecholamines! Thyroxine is nonpolar and crosses the cell

membrane easily!•  Very similar to steroids in size and is very water insoluble!•  Carried mainly by thyroxine-binding globulin (TBG:

which carries T4 more than T3)!•  Location of receptor proteins is in the nucleus of the target

cells. ! Catecholamines (dopamine, epinephrine, and

norepinephrine), however, usually have receptors on the target cell membrane!

Figure 15.4 Amine hormones are derived from amino acids Storage Properties of Hormones !Steroids: limited storage of the hormone, usually produced “on demand”!Peptides, proteins and catecholamines: storage is also limited, ranging from one to several days!Thyroxine: usually stored for a much longer period in the thyroid gland, about several weeks in adults (less in children)! More active thyroxine synthesis in children made

them more vulnerable to radioactive 131I after the Chornobyl nuclear power plant exploded!

!!


Topic 17 - 3!

Secretion Mechanisms !Steroids: once synthesized, diffuse through the plasma membrane when needed.!Thyroid hormone: secreted by proteolysis of thyroglobulin; breaking it down releases thyroxine and triiodothyronine.!Peptides, proteins, catecholamines: generally stored in vesicles and are secreted by exocytosis.!

Figure 15.5 Snapshots of insulin synthesis, processing, and packaging

Plasma Protein Binding and Transport!Steroids: non-polar, hydrophobic, lipophilic, so rarely in free form in the circulation-need plasma proteins for transport. Their t1/2 in blood ranges over hours.!Thyroxine: also non-polar, hydrophobic, lipophilic, so needs plasma protein for transport. The t1/2 in blood ranges over several days.!Peptides, proteins: polar and hydrophilic-most do not need plasma proteins for transport. Their t1/2 in blood is a few minutes.!Catecholamines: also are not bound to plasma proteins. Their t1/2 in blood is a few seconds. !

Hormone Receptors and Action !Steroids: can pass through the cell membrane. Receptors located either with the cytoplasm or in the nucleus.! Receptor-hormone complex controls DNA transcription.!

Thyroxine: Receptors located either with the cytoplasm or in the nucleus.! Receptor-hormone complex controls transcription.!

Peptides, proteins, catecholamines: cannot pass through the plasma membrane, so their receptors are on the outside of the plasma membrane.!  In peptides and proteins, hormone binding triggers synthesis

of cytolic second messengers or triggers protein kinase activity.!

  In catecholamines, hormone binding causes change in membrane potential or triggers synthesis of cytolic second messengers.!

Hormonal Rhythm !Pattern of secretion of the hormone over time. Actual secretion is pulsatile (the hormone secretion starts and stops, over and over).!Circadian or diurnal rhythm: hormone is secreted every 24 hours. For example:!  Cortisol is secreted about 2 hours before awakening. Its secretion pattern

depends on the light:dark cycle (which is cued by the photoperiod).!  Growth hormone secretion, however, peaks during stages 3 and 4 of sleep,

whether at night or during the day. Its secretion pattern depends on what is known as the sleep:wake cycle, which is an endogenous (internal) rhythm.!

Ultradian Rhythm: hormone more frequently than once a day (often every ½, 1, or 2 hours). Examples include:! Hormones involved in menstruation or seasonal breeding

(LH, FSH).! Hormones involved in sex cell production (e.g., testosterone).!

Pituitary Gland (Hypophysis)!Connected to the hypothalamus by an infundibulum, structurally and functionally divided into 2 lobes:! Posterior pituitary or neurohypophysis (neural of the

pituitary; pars distalis)! Anterior hypophysis (adenohypophysis)!


Topic 17 - 4!

Figure 15.7 The vertebrate pituitary gland has two parts (Part 1) Posterior Pituitary Hormones!Does NOT have cells that produce OR store hormones. Neurosecretory cells of hypothalamus release hormones directly into posterior pituitary, which are then rapidly released into the systemic bloodstream:!Oxytocin:!  In females, it stimulates the contraction of the uterus during

labor, required for parturition (“childbirth”). Also stimulates milk release in a lactating female mammals (contractions of the mammary gland alveoli and ducts).!•  When the baby suckles, touch receptors located in the nipple send

sensory signals to the hypothalamus, oxytocin is released, and milk is ejected.!

•  When the baby stops suckling, stimulation ends and milk ejection stops.!

  In males, it is secreted during ejaculation, causing uterine contractions that aid movement of the sperm up the oviducts where they then fertilize the egg. !

Posterior Pituitary Hormones!Antidiuretic hormone (ADH), also known as arginine vasopressin (AVP)! Stimulates retention of water by the kidneys. High osmolarity

stimulates ADH: !•  When osmolarity is high, more water will leave the osmoreceptors,

so they will shrink. This shrinking will stimulate the hypothalamus to produce ADH, which is then released from the posterior pituitary.ADH acts on the collecting ducts of the kidney, closing water transport channels and thus decreasing water excretion. !

•  When there is an increase in blood volume, it is sensed by receptors in the left atrium. These send signals to the hypothalamus that stop it from producing ADH and thus increasing water excretion.!

 At high doses, causes ADH also causes vasoconstriction of blood vessels (and is therefore also known as arginine vasopressin, AVP)!

Other Hypothalamic Hormones!Control releases of anterior pituitary hormones:! Corticotropin releasing hormone (CRH) controls

release of ACTH.! Gonadotropin Releasing Hormone (GnRH) controls

release of LH and FSH.! Thyroid Releasing Hormone (TRH) controls release

of TSH.! Growth Hormone Releasing Hormone (GHRH)

promotes release of GH.! Somatostatin (SS) inhibits the secretion of GH

hormone. ! Prolactin is regulated by hypothalamic inhibitory

hormone known as Prolactin-inhibiting hormone (PIH).!

Figure 15.7 The vertebrate pituitary gland has two parts (Part 2) Anterior Pituitary Hormones!Secreted by pars distalis of adenohypophysis:!Thyroid-stimulating hormone (TSH) acts on the thyroid gland to produce T3 and T4.!Adrenocorticotropic hormone (ACTH) acts on the adrenal gland to secret glucocorticoids (e.g., cortisol), mineralocorticoids (e.g., aldosterone) and sex steroids.!Growth hormone (GH) acts all over the body, e.g.,! Stimulates the liver to produce IGF-I, which promotes cell

growth throughout the body.!  Increases mass of body, including bones, muscle, and adipose

tissue (anabolic on proteins increases protein synthesis).! Acts catabolically on carbohydrates and lipids: increases

blood concentration of glucose and free fatty acids. Also has an anti-insulin effect, stimulating carbohydrate breakdown.!


Topic 17 - 5!

Figure 15.7 The vertebrate pituitary gland has two parts (Part 3) Anterior Pituitary Hormones!Follicle-stimulating hormone (FSH) acts on gonads!  In females: during the first part of the menstrual cycle, it

stimulates growth of ovarian follicle.!  In males: FHS + testosterone induces maturation of

seminiferous tubules and stimulates beginning of sperm production. Stimulates Sertoli cells, which guide developing sperm cells through the stages of spermatogenesis and also act as phagocytes, consuming the residual cytoplasm during spermatogenesis. Translocation of germ cells from the base to the lumen of the seminiferous tubules occurs by conformational changes in the Sertoli cells.!

Luteinizing hormone (LH) also acts on gonads!  In females: stimulates ovulation and conversion of ovulated

ovarian follicle into a corpus luteum.!  In males: stimulates the secretion of male sex hormones

(mainly testosterone) from interstitial cells in testes.!

Figure 15.7 The vertebrate pituitary gland has two parts (Part 4) Figure 16.8 The human female reproductive system (Part 3)

Figure 16.9 A synoptic view of events in the human female reproductive cycle Figure 16.10 Hormonal control of estrogen production and secretion by an ovarian follicle


Topic 17 - 6!

Figure 16.11 The human male reproductive system (Part 2) Figure 16.11 The human male reproductive system (Part 3)

Anterior Pituitary Hormones!Melanin-stimulating hormone (MSH) stimulates melanocytes, causing darkening of the skin in fishes, amphibians, and reptiles.!  Darkening of the skin in these animals is#

temporary, and important in#thermoregulation (darker animals absorb#more heat).!

  The darkening occurs as granules of melanin#(a brownish-black pigment that absorbs UV#light) are spread through the branches of #specialized melanocytes (melanophores).!

Prolactin (PL) is important in water and salt balance in many vertebrates. Also acts on the breasts of mammals:!  It stimulates breast development during pregnancy (not during

puberty).!  In males, it facilitates reproductive function, however, too high levels

of PRL interfere with the secretion of LH and FSH which are needed for spermatogenesis.!

Figure 16.16 Mammary glands and lactation

Figure 15.8 The adrenal gland consists of an inner medulla and an outer cortex Figure 15.9 Both hormonal and neural mechanisms modulate the action of the HPA axis


Topic 17 - 7!

Figure 15.10 Interactions of insulin, glucagon, and epinephrine Figure 15.11 The mammalian stress response

Figure 15.12 The CNS and the immune system interact during the stress response Circadian Rhythms!Found in most organisms. All animals produce endogenous circadian rhythms! Internal mechanisms that operate on an

approximately 24 hour cycle (generally 23-25 hours)!

 Regulates the sleep/wake cycle.! Also regulates the frequency of eating and

drinking, sensory sensitivity, nervous sensitivity/synaptic excitability, body temperature, secretion of hormones, mitotic activity, volume of urination, and sensitivity to drugs and toxicants.!

Figure 14.12 Daily rhythm of several physiological functions in a human


Topic 17 - 8!

Endogenous Rhythms and External Cues!Endogenous rhythms remain remain consistent despite lack of environmental cues indicating the time of day. In the lack of such cues, the rhythm is known as free-running.!Rhythms also be entrained to an environmental cue (daylight, tides, temperature).! Setting the phase of an environmental rhythm is a

phase factor (zeitgeber: “time giver”).!According to Ashoff’s Rule, the period length of a diurnal organism shortens as light intensity increases (and vice-versa for a nocturnal organism).!  In humans, rhythms run faster in bright light conditions and

subjects have trouble sleeping.!  In constant darkness, humans have difficulty waking.!

Figure 14.13 Circadian rhythm of metabolic rate (O2 consumption) and motor activity for a chaffinch

Entrainment!Circadian period must be equal to or a multiple of the driver (environmental oscillation) for the two to be coupled.!Entrainment in many animals is due to extraocular photoreceptors (entrainment is still seen in enucleated, or in eyeless animals):!  Insects in eclosion (pupation) often emerge at the same time.!  Crayfish: 6th tail ganglion essential for setting the clock.!  Birds: have about 20 extraretinal photoreceptors in the brain.!  Photoreceptors known in amphibians and some reptiles!  Mammals: may have a few located throughout the body.!

Entrainment can occur without light:!  In Uta (a diurnal lizard) entrainment follows light:dark cycle,

regardless of temperature.!  In Coleonyx (a nocturnal lizard) entrainment follows temperature and

light:dark cycle.!

Figure 14.14 Activity rhythms of two nocturnal flying squirrels (Glaucomys volans)

Control of Circadian Rhythms!In all vertebrates except mammals: control is in the pineal. In mammals, a part of the hypothalamus, the suprachiasmatic nucleus (SCN) is the main control center of the circadian rhythms of sleep and temperature.! Damage to the SCN results in less consistent body rhythms

that are no longer synchronized to environmental patterns of light and dark.!

The SCN is genetically controlled and independently generates the circadian rhythms (a single cultured cell continues to produce rhythmic action potentials).! SCN regulates waking and sleeping by controlling activity

levels in other areas of the brain.! The SCN regulates the pineal gland, an endocrine gland

located posterior to the thalamus.! The pineal gland secretes melatonin, a hormone that increases

sleepiness.!

Figure 14.16 The paired suprachiasmatic nuclei in the brain together constitute the major circadian clock of mammals


Topic 17 - 9!

Control of Circadian Rhythms!Two types of genes are responsible for generating the circadian rhythm.!1.  Period - produce proteins called Per.!2.  Timeless - produce proteins called Tim.!

Per and Tim proteins increase the activity of certain kinds of neurons in the SCN that regulate sleep and waking.!Mutations in the Per gene result in odd circadian rhythms.!

Documents

Endocrine Glands that function Organs That Function ... · the mammary gland alveoli and ducts).! • When the baby suckles, touch receptors located in the nipple send sensory signals