Endocrine Part 1

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Biochemical Aspect of Endocrine Glands- I.

Biomedical Importance:

1. Survival depends on- ability to adapt to a constantly changing

environment.2. Necessity of intercellular communication.

Definition: Hormone: (Greek - arouse to activity).

Is a substance that is synthesized in one organ and transported by

the circulatory system to act on another tissue. (endocrine)

Overview of Hormonal cascade & Important Endocrine glands:

Environmental/ internal signal

CNS

Limbic System

Hypothalamus

Ant. Pituitary

Target gland

Final hormone

Systemic effects

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Nature of Hormones

Hormones can be divided into five major classes:

(1) amino acid derivatives- (dopamine, catecholamine, and thyroid

hormone)

(2) small neuropeptides- ( gonadotropin-releasing hormone (GnRH),

thyrotropin-releasing hormone (TRH), somatostatin, and vasopressin)

(3) large proteins- (luteinizing hormone (LH), and PTH)

(4) steroid hormones- (cortisol and estrogen that are synthesized from

cholesterol-based precursors)

(5) vitamin derivatives- (retinoids (vitamin A) and vitamin D).

A variety ofpeptide growth factors, most of which act locally,

share actions with hormones.

As a rule, amino acid derivatives and peptide hormones

interact with cell-surface membrane receptors.

Steroids, thyroid hormones, vitamin D, and retinoids are lipid-

soluble and interact with intracellular nuclear receptors.

Classification:

General Features of Hormone Classes

Group I Group II

Types Steroids, iodothyronines,calcitriol, retinoids

Polypeptides, proteins, glycoproteins, catecholamines

Solubility Lipophilic HydrophilicTransport

proteinsYes No

Plasma half-life

Long (hours to days) Short (minutes)

Receptor Intracellular Plasma membrane

Mediator Receptor-hormone complex cAMP, cGMP, Ca2+, metabolites of complexphosphinositols, kinase cascades

RECEPTOR SPECIFICITY & CROSS TALK

Hormone Synthesis & Processing.

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Secretion, Transport & Degradation.

Classification of Hormones byMechanism of Action

I. Hormones that bind to intracellular receptors

Steroid hormonesRetinoic acidThyroid hormones (T3 and T4 )

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

Adrenergic catecholamines (2, )TSH,FSH, LH, MSH, ACTH, PTH

ADHCalcitoninCRHGlucagonSomatostatin

B. The second messenger is cGMP

Atrial natriuretic factorNitric oxide

C. The second messenger is calcium orphosphatidylinositols (or both)

1 -Adrenergic catecholaminesAngiotensin IIAntidiuretic hormone (vasopressin)Gastrin, CCKGnRHOxytocinThyrotropin-releasing hormone (TRH)

D. The second messenger is a kinase orphosphatase cascade

AdiponectinChorionic somatomammotropinEGF, FGF,IGF 1 & 2, GHErythropoietinLeptinProlactin

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Hormone Action & Signal transduction:

a. A hormonereceptor interaction results in generation of an

intracellular signal:

b. Regulate the activity of a select set of genes,c. Affect the activity of specific proteins.

d. The signal can influence the location of proteins in the cell and

e. Affect general processes such as protein synthesis

I. Intracellular Receptors:-

(Steroid and thyroid hormone("superfamily") Receptors)

Composed of a single polypeptide chain that has, in the

simplest analysis, three distinct domains:

The amino-terminus: In most cases, this region is involved in

activating or stimulating transcription by interacting with other

Membrane Receptor Families and Signaling Pathways

Receptors Effectors Signaling Pathways

G ProteinCoupled Seven-Transmembrane (GPCR)

-Adrenergic, LH, FSH, TSH Gs, adenylate cyclase

Stimulation of cyclic AMP production,protein kinase A

Glucagon PTH, PTHrP ACTH,MSH GHRH, CRH

Ca2+ channels

Calmodulin, Ca2+-dependent kinases

-Adrenergic, Somatostatin Gi

Inhibition of cyclic AMP production

Activation of K+, Ca2+ channels

TRH, GnRH Gq, G11

Phospholipase C, diacylglycerol, IP3, proteinkinase C, voltage-dependent Ca2+ channels

Receptor Tyrosine Kinase

nsulin, IGF-I Tyrosine kinases, IRS MAP kinases, PI 3-kinase; AKT, also knownas protein kinase B, PKB

EGF, NGF Tyrosine kinases, ras Raf, MAP kinases, RSK

Cytokine ReceptorLinked Kinase

GH, PRL JAK, tyrosine kinases STAT, MAP kinase, PI 3-kinase, IRS-1

Serine Kinase

Activin, TGF-, MIS Serine kinase Smads

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components of the transcriptional machinery. The sequence is highly

variable among different receptors.

DNA binding domain: Amino acids in this region are responsible

for binding of the receptor to specific sequences of DNA.

The carboxy-terminus or ligand-binding domain: This is the

region that binds hormone.

In addition to these three core domains, two other important

regions of the receptor protein are a nuclear localization

sequence, which targets the the protein to nucleus, and a

dimerization domain, which is responsible for latching two

receptors together in a form capable of binding DNA.

Hormone-Receptor Binding and Interactions with DNA

When hormone binds to receptor, a characteristic series of events occurs:

Receptor activation is the term used to describe conformational

changes in the receptor induced by binding hormone. The major

consequence of activation is that the receptor becomes competent

to bind DNA.

Activated receptors bind

to "hormone response

elements", which areshort specific sequences of DNA which are located in promoters of

hormone-responsive genes. In most cases, hormone-receptor

complexes bind DNA in pairs.

Transcription from those genes to which the receptor is bound is

affected. Most commonly, receptor binding stimulates transcription.

The hormone-receptor complex thus functions as a transcription

factor.

I. Cell-Surface receptors:-

a. cAMP Mediated.

Synthesised from ATP by adenylyl cyclase located on theinner side of the plasma membrane.

Adenylyl cyclase is activated by a range of signalingmolecules through the activation of adenylyl cyclase

stimulatory G (Gs)-protein-coupled receptors and inhibitedby agonists of adenylyl cyclase inhibitory G (Gi)-protein-coupled receptors.

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Liver adenylyl cyclase responds more strongly toglucagon, and muscle adenylyl cyclase responds morestrongly to adrenaline.

cAMP decomposition into AMP is catalyzed by the enzymephosphodiesterase.

cyclic AMP works by activating protein kinase A (PKA, a

heterotetrameric molecule consisting of two regulatorysubunits (R) and two catalytic subunits (C).)

a. cGMP Mediated.

Guanylate cyclase (GC) catalyzes cGMP synthesis. This

enzyme converts GTP to cGMP. In turn, peptide hormones

such as the atrial natriuretic factor activate membrane-

bound GC, while soluble GC is typically activated by nitric

oxide to stimulate cGMP synthesis.

cGMP is a common regulator ofion channelconductance,

glycogenolysis, and cellular apoptosis. It also relaxes

smooth muscle tissues & causes vasodilation to increase

blood flow.

a. Calcium/ Phosphatidylinositol.

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b. Kinase/ Phosphatase Cascade. (Insulin)

Pathologic Mechanisms of Endocrine disease:1. Hormone Excess.

2. Hormone deficiency.

3. Resistance.

(IGFBP, insulin-like growth factor binding protein; IRS 14, insulin receptor substrate isoforms 14; PI-3 kinase, phosphatidylinositol3-kinase; PTEN, phosphatase and tensin; PKD1, phosphoinositide-dependent kinase; PKB, protein kinase B; SGK, serum andglucocorticoid-regulated kinase; aPKC, atypical protein kinase C; p70S6K, p70 ribosomal protein S6 kinase; mTOR, mammalian targetof rapamycin; GRB2, growth factor receptor binding protein 2; mSOS, mammalian son of sevenless; MEK, MAP kinase kinase andERK kinase; MAP kinase, mitogen-activated protein kinase; mTOR, mammalian Target Of Rapamycin).

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The Glands & Various Axes:

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The Anterior Pituitary

The Master Gland Produces 6 Hormones: PRL, GH, ACTH, TSH, FSH & LH Secreted in a pulsatile manner,(-Hypothalamic RF) . Specific responses in peripheral target tissues. Feedback control.

Hormone deficiency may be inherited or acquired. Diagnoses are often elusive Performing the correct laboratory diagnostic tests.

Anterior Pituitary Hormone Expression and Regulation

Cell Corticotrope Somatotrope Lactotrope Thyrotrope Gonadotrope

Fetal

appearance

6 weeks 8 weeks 12 weeks 12 weeks 12 weeks

Hormone POMC GH PRL TSH FSH LH

Protein Polypeptide Polypeptide Polypeptide Glycoprotein , , subunits

Glycoprotein , , subunits

Amino acids 266 (ACTH 139)

191 199 211 210, 204

Stimulators CRH, AVP, GHRH, ghrelin Estrogen,TRH, VIP

TRH GnRH, activins,estrogen

Inhibitors Glucocorticoids Somatostatin, IGF-I Dopamine T3, T4, dopamine,somatostatin,glucocorticoids

Sex steroids, inhibin

Target gland Adrenal Liver, other tissues Breast, other tissues

Thyroid Ovary, testis

Trophiceffect

Steroidproduction

IGF-I production,growth induction,

insulin antagonism

Milkproduction

T4 synthesis andsecretion

Sex steroidproduction, follicle

growth, germ cellmaturation

Normalrange

ACTH, 422pg/L

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Hormone Test Blood Samples Interpretation

Growthhormone

Insulin tolerance test:

Regular insulin (0.050.15 U/kgIV)

30, 0, 30, 60, 120 min forglucose and GH

Glucose < 40 mg/dL; GH should be >3g/L

GHRH test: 1 g/kg IV 0, 15, 30, 45, 60, 120 min for

GH

Normal response is GH >3 g/L

L-Arginine test: 30 g IV over30 min

0, 30, 60, 120 min for GH Normal response is GH >3 g/L

L-Dopa test: 500 mg PO 0, 30, 60, 120 min for GH Normal response is GH >3 g/L

Prolactin TRH test: 200500 g IV 0, 20, and 60 min for TSHand PRL

Normal prolactin is >2 g/L and increase>200% of baseline

ACTH Insulin tolerance test: regularinsulin (0.050.15 U/kg IV)

30, 0, 30, 60, 90 min forglucose and cortisol

Glucose 7g/dL or to

>20 g/dL

CRH test: 1g/kg ovine CRHIV at 8 A.M.

0, 15, 30, 60, 90, 120 min forACTH and cortisol

Basal ACTH increases 2- to 4-fold andpeaks at 20100 pg/mL

Cortisol levels >2025 g/dL

Metyrapone test: Metyrapone(30 mg/kg) at midnight

Plasma 11-deoxycortisol andcortisol at 8 A.M.; ACTHcan also be measured

Plasma cortisol should be 7.5g/dL or ACTH >75 pg/mL

Standard ACTH stimulationtest: ACTH 1-24 (cosyntropin),0.25 mg IM or IV

0, 30, 60 min for cortisol andaldosterone

Normal response is cortisol >21 g/dL andaldosterone response of >4 ng/dL above

baseline

Low-dose ACTH test: ACTH1-24 (cosyntropin), 1 g IV

0, 30, 60 min for cortisol Cortisol should be >21 g/dL

3-day ACTH stimulation testconsists of 0.25 mg ACTHgiven IV over 8 h each day

Cortisol >21 g/dL

TSH Basal thyroid function tests:T4, T3, TSH

Basal measurements Low free thyroid hormone levels in thesetting of TSH levels that are notappropriately increased indicate pituitaryinsufficiency

TRH test: 200500 g IV 0, 20, 60 min for TSH andPRL

TSH should increase by >5 mU/L unlessthyroid hormone levels are increased

LH, FSH LH, FSH, testosterone, estrogen Basal measurements Basal LH and FSH should be increased inpostmenopausal womenLow testosterone levels in the setting oflow LH and FSH indicate pituitaryinsufficiency

GnRH test: GnRH (100 g) IV 0, 30, 60 min for LH andFSH

In most adults, LH should increase by 10IU/L and FSH by 2 IU/L.

Multiplehormones Combined anterior pituitarytest: GHRH (1 g/kg),CRH (1g/kg), GnRH (100

g), TRH (200 g) are given IV

30, 0, 15, 30, 60, 90, 120min for GH, ACTH, cortisol,LH, FSH, and TSH

Combined or individual releasinghormone responses must be elevated in thecontext of basal target gland hormonevalues and may not be uniformlydiagnostic.

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Pituitary Adenomas: (e.g., acromegaly, prolactinomas, or Cushing's syndrome) .

When a pituitary adenoma is suspected based on MRI, initial hormonal evaluation usually includes

(1) basal PRL; (2) insulin-like growth factor (IGF) I; (3) 24-h urinary free cortisol (UFC) and/or overnightoral dexamethasone (1 mg) suppression test; (4) subunit, FSH, and LH; and (5) thyroid function tests.

Screening Tests for Functional Pituitary Adenomas

Test Comments

Acromegaly Serum IGF-I Interpret IGF-I relative to age- and sex-matchedcontrols

Oral glucose tolerance test with GHobtained at 0, 30, and 60 min

Normal subjects should suppress growth hormone to

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At high concentrations, AVP also causes contraction of smooth muscle in

blood vessels and in the gastrointestinal tract, induces glycogenolysis in the

liver, and potentiates adrenocorticotropic hormone (ACTH) release by

corticotropin-releasing factor. These effects are mediated by V1a or V1b

receptors that are coupled to phospholipase C.

1. Oxytocin.

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Thyroid

Table:Characteristics of Circulating T4 and T3

Hormone Property T4

T3

Serum concentrations

Total hormone 8 g/dL 0.14 g/dL

Fraction of total hormone in the free form 0.02% 0.3%Free (unbound) hormone 21 1012M

6 1012M

Serum half-life 7 d 0.75 d

Fraction directly from the thyroid 100% 20%

Production rate, including peripheral conversion 90 g/d 32 g/d

Intracellular hormone fraction 20% 70%

Relative metabolic potency 0.3 1

Receptor binding 1010M

1011M

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Investigation of primary hypothyroidism

Serum TSH is the investigation of choice; a high TSH level confirms primaryhypothyroidism. A low free T4 level confirms the hypothyroid state (and is

also essential to exclude TSH

deficiency if clinical hypothyroidism is strongly suspected and TSH is normal

or low).

Thyroid and other organ-specific antibodies may be present. Other

abnormalities include the following:

anaemia, which is usually normochromic and normocytic

in type but may be macrocytic (sometimes this is due toassociated pernicious anaemia) or microcytic (in women,

due to menorrhagia)

increased serum aspartate transferase levels, from muscle and/or liver

increased serum creatine kinase levels, with associated myopathy

hypercholesterolaemia and hypertriglyceridaemia

hyponatraemia due to an increase in ADH and impaired free water

clearance.

Hyperthyroidism:

Investigations

Serum TSH is suppressed in hyperthyroidism

(< 0.05 mU/L), except for the very rare instances of TSH

hypersecretion.

A raised free T4 or T3 confirms the diagnosis; T4 is

almost always raised but T3 is more sensitive as there

are occasional cases of isolated T3 toxicosis.