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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
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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
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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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