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HORMONES

Raymond Oliver A. Cruz, MD

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WHAT ARE HORMONES?

n Hormones are chemical substances,present in very low concentrations in the

blood, that regulate the metabolism of atleast one specific organ or tissue

n The site of secretion is distant from the

target organn

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WHAT ARE GLANDS?

n A gland is a group of cells specialized tosecrete hormones into the bloodstream

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WHAT ARE RECEPTORS?

n Receptors are proteins or glycoproteinsthat specifically binds a hormone and

produces a biologic action after suchbinding

n Have high specificity

n

May be found on the plasma membrane orinside the nucleus of the cell

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Comparison of Receptors with

Transport Proteins

Feature Receptors Transport Proteins

Concentration Very low (thousands/cell) Very high (billions/ L)

Binding affinity High (pmol/L to nmol/Lrange)

Low ( mol/L range)Binding specificity

Very high LowSaturability

Yes NoReversibility

Yes YesSignal

transduction

Yes No

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n Feedback mechanisms are usually themeans by which hormones are regulated

n Before we discuss the individualhormones, we will first look into howthese hormones work

n

n

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HORMONES CAN AFFECT ANY

OF THE FOLLOWING STEPS:

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n Hormones can be thought of as signals,and receptors are signal detectors

n Each component serves as a link in thecommunication process betweenextracellular events and chemical

changes within the cell

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General Features of HormoneClasses

Group I Group II

Types Steroids,iodothyronines,calcitriol, retinoids

Polypeptides, proteins, glycoproteins,catecholaminesSolubility Lipophilic Hydrophilic

Transportproteins

Yes NoPlasma half-life Long (hours to days) Short (minutes)Receptor Intracellular Plasma membrane

Mediator Receptor-hormonecomplex cAMP, cGMP, Ca2+

, metabolites ofcomplex phosphoinositols, kinasecascades

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Classification of Hormones by

Mechanism of ActionI. Hormones that bind to intracellular receptors Androgens Calcitriol (1,25[OH]2-D3)

Estrogens Glucocorticoids Mineralocorticoids Progestins

Retinoic acid Thyroid hormones (T3 and T4)

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Classification of Hormones by

Mechanism of Action

2-Adrenergiccatecholamines -AdrenergiccatecholaminesAdrenocorticotropic

hormone (ACTH)Antidiuretic

hormone (ADH,Vasopressin)CalcitoninChorionic gonadotropin,

human (hCG)

Corticotropin-releasing hormone(CRH)Follicle-stimulating hormone (FSH)GlucagonLipotropinLuteinizing hormone (LH)

Melanocyte-stimulatinghormone (MSH)Parathyroid hormone (PTH)SomatostatinThyroid-stimulating hormone (TSH)

II. Hormones that bind to cell surface receptorsA. The second messenger is cAMP

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Classification of Hormones by

Mechanism of Action

Atrial natriuretic factor (ANF) Nitric oxide (NO)

II. Hormones that bind to cell surface receptorsB. The second messenger is cGMP

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Classification of Hormones by

Mechanism of Action

Acetylcholine (muscarinic) 1-Adrenergic catecholamines

Angiotensin II Antidiuretic hormone(vasopressin) Cholecystokinin

Gastrin Gonadotropin-releasing hormone (GnRH) Oxytocin Platelet-derived growth factor Substance P Thyrotropin-releasing hormone (TRH)

II. Hormones that bind to cell surface receptorsC. The second messenger is calcium or

phosphatidylinositols (or both)

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Classification of Hormones by

Mechanism of Action

Adiponectin Chorionic somatomammotropin Epidermal growth factor Erythropoietin Fibroblast growth factor Growth hormone Insulin Insulin-like growth factors I and II Leptin Nerve growth factor Platelet-derived growth factor

Prolactin

II. Hormones that bind to cell surface receptorsD. The second messenger is a kinase or

phosphatase cascade

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SPECIFICITY AND SELECTIVITY

A cell may have no hormone receptors (1), have onereceptor (2+5+6), have receptors for several hormones

(3), or have a receptor but no hormone in the vicinity (4).

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SIGNAL TRANSDUCTION BY

INTRACELLULAR RECEPTORSn Utilized by molecules such as steroid

hormones, Vitamin D, retinoic acid, and

thyroxinen Act through intracellular receptors located

in the cytosol or the nucleus

n

Why do you think these classes ofhormones can enter the cell? Ans: They are all lipid-soluble

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TRE- Thyroid response element GRE- glucocorticoid response elementHSP- heat shock protein - transcribed gene

Repressorremoved

+

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SIGNAL TRANSDUCTION BY

INTRACELLULAR RECEPTORSn The effects of agonists acting through

intracellular receptors are not immediate

n For example, hours to days are required toachieve the therapeutic effects ofsteroids for the treatment of asthma

n

How do steroids help in the treatment ofasthma?

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TRANSDUCTION BY CELL-

SURFACE RECEPTORSn Initiated by ligand binding to receptors

located in the plasma membrane

n In contrast to intracellular receptors, cellsurface receptors do not regulate geneexpression directly

n

Do they also need to be lipophilic to beeffective?

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G-PROTEIN COMPLEX

E can be adenylyl cyclase, Ca2+, Na+, or Cl channels, or it could be a K+ channel,phospholipase C, or cGMP phosphodiesterase

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n There is a large family of G proteins, andthese are part of the superfamily of

GTPasesn The G protein family is classified

according to sequence homology intofour subfamilies (See table 43-3 of

Harpers 26th ed.)n Some G proteins are also involved in

olfaction and vision

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3 GENERALCLASSES OF CELL-

SURFACERECEPTORS

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1. NEUROTRANSMITTER RECEPTORS

LINKED TO ION CHANNELSn The receptor and ion channel are part of a

single multimolecular complex

n Seen in nerve and muscle (GABA,acetylcholine)

n The binding of a neurotransmitter causes

a rapid opening of ion channels that areselectively permeable to certain ions(remember your action potential inphysiology?)

n

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2. CATALYTIC RECEPTORS

n Have an inherent enzymatic activity aspart of their structure

n In most cases the enzyme activity is atyrosine-specific protein kinase

n For example, the binding of insulin to its

receptor activates the intrinsic tyrosinekinase activity to initiate insulins effects

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3. RECEPTORS INVOLVINGSECOND MESSENGERS

n Many receptors signal their recognition ofa bound hormone neurotransmitter by

initiating a series of reactions thatultimately result in a specific intracellularresponse

n These intracellular messengers serve toamplify signals to initiate its effects

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INTRACELLULAR

MESSENGER SYSTEMSn Second messenger molecules

n Intervene between the original message

(the hormone) and the ultimate effect onthe cell

n Are the hormones that utilize second

messengers usually lipophilic or not? Ans: They are usually not lipophilic, so

they use these 2nd messengers to initiatetheir effects on cell function

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ADENYLATE CYCLASE SYSTEM

n A membrane-bound enzyme that convertsATP to 3, 5-adenosine monophosphate

(also calles cyclic AMP or cAMP)n

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PKA exists in an inactive form as an R2C2heterotetramer consisting of two regulatory andtwo catalytic subunits.cAMP generated by the action of adenylyl cyclasebinds to the regulatory (R) subunit of PKA. Thisresults in dissociation of the regulatory and

catalytic subunits and activation of the latter.

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The active catalytic subunits phosphorylate anumber of target proteins on serine and threonineresidues.Phosphatases remove phosphate from theseresidues and thus terminate the physiologicresponse.A phosphodiesterase can also terminate theresponse by converting cAMP to 5'-AMP.

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Subclassification of Group II.A

Hormones.Hormones That StimulateAdenylyl Cyclase (HS)

ACTH GlucagonADH hCG -Adrenergics LHCalcitonin LPH

CRH MSHFSH PTHTSH

Hormones That InhibitAdenylyl Cyclase (HI)

Acetylcholine 2-Adrenergics

Angiotensin II

Somatostatin

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n In prokaryotic cells, cAMP binds to aspecific protein called catabolite

regulatory protein (CRP) that bindsdirectly to DNA and influences geneexpression.

n In eukaryotic cells, cAMP binds to a

protein kinase called protein kinase A(PKA) that is a heterotetramericmolecule consisting of two regulatorysubunits (R) and two catalytic subunits

(C).

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n Inhibitors of phosphodiesterase, mostnotably methylated xanthine derivatives

such as caffeine, increase intracellularcAMP and mimic or prolong the actionsof hormones through this signal

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CALCIUM-PHOSPHATIDYLINOSITOL

SYSTEMn

Many receptors respondto hormones byactivating amembrane-boundphophodiesterase

called PhospholipaseC

n Activated PhospholipaseC subsequently

cleaves membrane-boundphophatidylinositol1,4,5-trisphosphate,releasing two

fragments (inositol-

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n Inositol 1,4,5-trisphosphat

e (IP3)binds toreceptorson theendoplasmic reticulum,causing arapidrelease of

calciumfromintracellularstores

n

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n Releasedcalcium

ions permitstheformation ofa calcium-

calmodulincomplexthatmediates a

wide varietyof effects

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n IP3 is a short-lived signal

and isinactivatedto inositol1,4-

bisphosphate andinositol 1-phosphate

n

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n

n

Diacylglycerol(DAG) activatesprotein kinase C,

an enzyme thatphosphorylatesproteins

Protein kinase C

requires calciumfor maximumactivity

DAG and IP3 actsynergistically tocause increasedphosphorylation ofproteins

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WHAT IS CALMODULIN?

n Calmodulin is a protein that acts as amediator of calcium-linked effects in the

celln It is structurally and functionally related to

troponin C, which mediates the role ofcalcium in skeletal and cardiac musclecontraction

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CYCLIC GUANOSINE

MONOPHOSPHATEn Analogous to the cAMP pathway

n Synthesized from GTP by a membrane-

bound form of guanylate cyclasen Activates cGMP-dependent protein kinase

or protein kinase G

n Action also terminated byphophodiesterase

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CYCLIC GUANOSINE

MONOPHOSPHATEn Unlike cAMP which affects a wide variety

of processes, cGMP functions as a

specialized messenger, being involvedin smooth muscle relaxation, plateletaggregation, and the visual system

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CYCLIC GUANOSINE

MONOPHOSPHATEn Nitroprusside, nitroglycerin, nitric oxide, sodium

nitrite, and sodium azide, all cause smoothmuscle relaxation and are potent vasodilators.

n These agents increase cGMP by activating thesoluble form of guanylyl cyclase, and inhibitorsof cGMP phosphodiesterase (the drugsildenafil [Viagra], for example) enhance andprolong these responses

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Insulin Transmits Signals by

Several Kinase Cascades

Tyrosine (Y)

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JAK-STAT signaling pathway

n Takes part in the regulation of cellularresponses to cytokines and growth

factorsn Cytokines are a group of proteinaceous

signalling compounds that, likehormones and neurotransmitters, areused extensively for inter-cellcommunication.

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JAK-STAT signaling pathway

n Employing Janus kinases (JAKs) andSignal Transducers and Activators of

Transcription (STATs), the pathwaytransduces the signal carried byextracellular polypeptides to the cellnucleus, where activated STAT proteins

modify gene expression

http://upload.wikimedia.org/wikipedia/commons/6/6e/Jakstat_pathway.svg

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n JAKs, which have tyrosine kinase activity, bind to somecell surface cytokine receptors

n The binding of the ligand to the receptor triggersactivation of JAKs. Jak-P, an active kinase,phosphorylates the receptor on tyrosine residues

n

The STAT proteins associate with the phosphorylatedreceptor and then are themselves phosphorylated byJak-P

n STAT dimerizes, translocates to the nucleus, binds tospecific DNA elements, and regulates transcription

http://upload.wikimedia.org/wikipedia/commons/6/6e/Jakstat_pathway.svg

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n The transcription factor NF- B (Nuclear Factor

kappa B) is a heterodimeric complex typicallycomposed of two subunits termed p50 andp65

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n Normally, NF- B is kept sequestered in the

cytoplasm in a transcriptionally inactive formby members of the inhibitor of NF- B (I B)family

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n Extracellular stimuli such as proinflammatory

cytokines, reactive oxygen species, andmitogens lead to activation of the I B kinasecomplex, IKK, which is a heterohexamericstructure consisting of , , and subunits.

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n IKK phosphorylates I B on two serine

residues, and this targets I B forubiquitination and subsequentdegradation by the proteasome

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n Following I B degradation, free NF- B can

now translocate to the nucleus, where it bindsto a number of gene promoters and activatestranscription, particularly of genes involved inthe inflammatory response

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n Glucocorticoid hormones aretherapeutically useful agents for thetreatment of a variety of inflammatoryand immune diseases

n Their anti-inflammatory andimmunomodulatory actions are

explained in part by the inhibition of NF- B and its subsequent actions.

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Evidence for three mechanisms for the inhibitionof NF- B by glucocorticoids has beenpresented:

(1)Glucocorticoids increase I B mRNA, whichleads to an increase of I B protein and moreefficient sequestration of NF- B in thecytoplasm

(2)The glucocorticoid receptor competes with NF- B for binding to coactivators

(3)The glucocorticoid receptor directly binds to thep65 subunit of NF- B and inhibits itsactivation

n

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n Incorrect regulation of NF- B has beenlinked to cancer, inflammatory and

autoimmune diseases, septic shock,viral infection and improper immunedevelopment

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HOW DO HORMONES AFFECTTRANSCRIPTION?

n

(1)Hormones allow the access of transcription factors toDNA

(2)Genes have regulatory regions, and transcription factorsbind to these to modulate the frequency oftranscription initiation

(3)Hormone-receptor complexes bind to hormone responseelements (HREs) to initiate transcription

(4)Other hormone-generated signals can modify thelocation, amount, or activity of transcription factorsand thereby influence binding to the regulatory orresponse element

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WHAT IS A HORMONERESPONSE ELEMENT?

n A hormone response element (HRE) is a shortsequence of DNA within the promoter of agene that is able to bind a specific hormone

receptor complex and therefore regulatetranscriptionn The sequence is most commonly a pair of

inverteD repeats separated by threenucleotides, which also indicates that the

receptor binds as a dimern A gene may have many different response

elements, allowing complex control to beexerted over the level and rate of transcription.

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n Hormone response elements resembleenhancer elements in that they are notstrictly dependent on position andlocation

n They generally are found within a fewhundred nucleotides upstream (5') of the

transcription initiation site, but they maybe located within the coding region ofthe gene

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The DNA Sequences of Several Hormone

Response Elements (HREs)

Hormone or Effector HRE DNA Sequence

Glucocorticoids GRE

Progestins PREMineralocorticoids MRE

Androgens ARE

Estrogens ERE

Thyroid hormone TRE

Retinoic acid RARE

Vitamin D VDRE

cAMP CRE

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n Members of thenuclear

receptor familyare divided intosix structuraldomains (AF)

n Domain A/B is alsocalled AF-1, orthe modulator

region, becauseit is involved inactivating

transcription

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n The C domainconsists of the

DNA-bindingdomain (DBD)

n The D region

contains thehinge, whichprovidesflexibility

between theDBD and theligand-binding

domain (LBD,

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n The LBD bindshormones or

metabolites withselectivity andthus specifies aparticularbiologicresponse

n The F region is

poorly defined

F ti l A t f th

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Functional Anatomy of theHypothalamus and Pituitary Gland

n The hypothalamus is a region of the brainthat controls an immense number of

bodily functionsn It is located in the middle of the base of

the brain, and encapsulates the ventralportion of the third ventricle.

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Optic chiasm

Hypthalamus

Pituitary Gland

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HYPOTHALAMIC HORMONES

n Controls the release of pituitaryhormones

n

Hypothalamic hormones are releasedfrom the hypothalamic nerve fiberendings around the capillaries of thehypothalamic-hypophysial system inthe pituitary stalk

n Released in a pulsatile manner

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PITUITARY

HORMONES

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GROWTH HORMONE (GH)

n Also called somatropin orsomatotropin

n

Synthesized in somatotropes, asubclass of the pituitary acidophiliccells

n

The genes for human growth hormoneare localized in the q22-24 region ofchromosome 17

n

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Disulfide bonds

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GROWTH HORMONE (GH)

n Peptides released by neurosecretorynuclei of the hypothalamus into the

portal venous blood surrounding thepituitary are the major controllers of GHsecretion by the somatotropes.

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GROWTH HORMONE (GH)

n Growth hormone releasing hormone(GHRH) from the arcuate nucleus of the

hypothalamus and ghrelin* promote GHsecretion, and somatostatin from theperiventricular nucleus inhibits it.

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Secretes GHRH

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n Ghrelin is a hormone produced by P/D1 cellslining the fundus of the human stomach thatstimulate appetite

n Ghrelin levels increase before meals anddecrease after meals

n It is considered the counterpart of the hormone

leptin, produced by adipose tissue, whichinduces satiation when present at higherlevels.

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n Much of the growth hormone in thecirculation is bound to a protein (growth

hormone binding protein, GHBP) whichis derived from the growth hormonereceptor

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n Growth hormone receptor is a member ofthe cytokine receptor superfamily.

n

One growth hormone molecule binds toone receptor and then recruits a secondreceptor to form a dimer through whichsignalling occurs.

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n The effects of growth hormone on thetissues of the body can generally be

described as anabolicn

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n Height growth in childhood is the best knowneffect of GH action, and appears to bestimulated by at least two mechanisms:

1. GH directly stimulates division andmultiplication of chondrocytes of cartilage.

These are the primary cells in the growingends (epiphyses) of children's long bones

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2. GH also stimulates production of insulin-likegrowth factor 1 (IGF1, formerly known assomatomedin C), a hormone homologous toproinsulin

The liver is a major target organ of GH forthis process, and is the principal site of IGF-1production

IGF-1 has growth-stimulating effects on awide variety of tissues. Additional IGF-1 is generated within target

tissues, making it apparently both an endocrineand an autocrine/paracrine hormone.

IGF-1 will also have stimulatory effects onosteoblast and chondrocyte activity to promotebone growth.

n

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GROWTH HORMONE (GH)

n GH secretion is also affected by negativefeedback from circulating concentrations

of GH and IGF-1n

EFFECTS OF GROWTH

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EFFECTS OF GROWTHHORMONE

n Stimulators of GH secretion include exercise,hypoglycemia, dietary protein, and estradiol.Inhibitors of GH secretion include dietarycarbohydrate and glucocorticoids.

n Increases the transport of amino acids into musclecells and also increases protein synthesis

n Antagonizes the effects of insulinn Decreases the peripheral utilization of glucose

n Increases hepatic glucose glucose production viagluconeogenesis

n

EFFECTS OF GROWTH

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EFFECTS OF GROWTHHORMONE

n In liver, GH increases liver glycogenn Promotes the release of free fatty acids and glycerol

from adipose tissuen

Increases oxidation of free fatty acids in the livern Promotes a positive calcium, magnesium, and

phosphate balancen Causes retention of sodium, potassium and chloride

ions

n Promotes the growth of long bonesn Stimulates lactogenesisn

Genetic deficiencies associated

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Genetic deficiencies associatedwith GHn

GH-deficient dwarfs lack the ability tosynthesize or secrete GH, and these short-statured individuals respond well to GHtherapy

n Pygmies lack the IGF-1 response to GH but

not its metabolic effects; thus in pygmiesthe deficiency is post-receptor in nature

n Laron dwarfs have normal or excess plasmaGH, but lack liver GH receptors and havelow levels of circulating IGF-1. The defectin these individuals is clearly related to aninability to respond to GH by theproduction of IGF-1

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n The production of excessive amountsof GH before epiphyseal closure of

the long bones leads to gigantismn When GH becomes excessive after

epiphyseal closure, acral bonegrowth leads to the characteristic

features of acromegalyn

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PROLACTINn Peptide hormone

synthesized andsecreted by lactotropecells in the anterior

pituitary glandn Prolactin is a single chain

polypeptide of 199 aminoacids with a molecularweight of about 24,000daltons

n The molecule is folded dueto the activity of threedisulfide bonds

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n The prolactin receptor- encoded by agene on Chromosome 5 - interacts with

the prolactin molecule as atransmembrane receptor

n A specific second messenger has not yetbeen identified.

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n Pituitary prolactinsecretion isregulated byneuroendocrineneurons in thehypothalamus,most importantly byneurosecretorydopamine neuronsof the arcuatenucleus, whichinhibit prolactinsecretion.

n

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n Prolactin has many effects, the most importantof which is to stimulate the mammary glands

to produce milk (lactation).n Increased serum concentrations of prolactin

during pregnancy cause enlargement of themammary glands of the breasts and increases

the production of milk.n However, the high levels of progesterone during

pregnancy act directly on the breasts to stopejection of milk. It is only when the levels of

this hormone fall after childbirth that milkejection is possible.

n Another effect recently discovered by the

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n Another effect, recently discovered by theUniversity of Paisley and the Technische

Hochschule Zrich, is to provide thebody with sexual gratification aftersexual acts

n The hormone represses the effect of

dopamine, which is responsible forsexual arousal, thus causing the male'srefractory period. The amount ofprolactin can be an indicator for the

amount of sexual satisfaction andrelaxation. Unusual high amounts aresuspected to be responsible forimpotence and loss of libido

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n The prolactin receptor, a member of thehematopoietin/cytokine receptor

superfamily, is ubiquitously expressedby cells in the immune system

n Certain subpopulations of lymphocytes

synthesize and secrete biologicallyactive prolactin, which suggests thatprolactin can act as an autocrine and/orparacrine factor to modulate theactivities of cells of the immune system.

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n Tumors of prolactin-secreting cellscause amenorrhea and galactorrhea

in womenn Excess PRL is associated with

gynecomastia and impotence in men

CHORIONIC

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CHORIONICSOMATOMAMMOTROPIN (CS;

PLACENTAL LACTOGEN)n a polypeptide placental hormone

n structure and function is similar to that of

human growth hormonen modifies the metabolic state of the mother

during pregnancy to facilitate the energysupply of the fetus

n HPL is an anti-insulin.

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