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Antidiuretic Hormone (ADH) / Vasopressin - Complete Guide

Quick Orientation

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PART 1: COMPLETE TEACHING NOTES - ADH

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1. Nomenclature & Basic Facts

• Chemical class: Peptide hormone (nonapeptide)
• Synthesized by: Magnocellular neurons of the supraoptic (SON) and paraventricular (PVN) nuclei of the hypothalamus
• Stored and released from: Posterior pituitary (neurohypophysis)

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2. Chemistry & Structure

• Nonapeptide (9 amino acids)
• Has a 6-amino acid ring closed by a disulfide bond (Cys 1-6) + a 3-amino acid side chain
• Position 8 residue = Arginine in humans (hence "arginine vasopressin")
• In pigs: Lysine at position 8 (lysine vasopressin)

• Modified at position 1 (desamino) + D-amino acid at position 8
• Antidiuretic:pressor ratio = 4000:1 (vs. natural vasopressin)
• Half-life: 1.5-2.5 hours (vs. ~15 min for native vasopressin)

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3. Synthesis

1. Gene on chromosome 20 encodes pre-propressophysin
2. Cleavage → propressophysin (vasopressin + neurophysin II + copeptin)
3. Packaged into neurosecretory granules, transported via axonal flow down the pituitary stalk
4. Stored in Herring bodies in the posterior pituitary nerve terminals

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4. Regulation of Secretion

A. Osmotic Regulation (PRIMARY stimulus)

• Osmoreceptors in the anterior hypothalamus (organum vasculosum of lamina terminalis - OVLT)
• Threshold plasma osmolality: ~280-285 mOsm/kg
• A rise of as little as 1-2% in plasma osmolality triggers ADH release
• Response: graded increase in ADH secretion → urine concentration

B. Volume/Pressure Regulation (SECONDARY stimulus)

• High-pressure baroreceptors: carotid sinus, aortic arch
• Low-pressure baroreceptors: atria (cardiac), pulmonary vasculature
• A >10% fall in blood pressure or volume is needed to trigger ADH (less sensitive than osmoreceptors)
• Signals via vagus and glossopharyngeal nerves → nucleus tractus solitarius → hypothalamus

C. Factors STIMULATING ADH secretion

D. Factors INHIBITING ADH secretion

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5. Mechanism of Action

V1 Receptors (Vascular)

• Location: Vascular smooth muscle, liver, platelets
• Coupling: Gq → Phospholipase C → IP3 + DAG → ↑Ca²⁺
• Effect: Vasoconstriction (at high ADH concentrations), glycogenolysis in liver, platelet aggregation
• Clinical use: Vasopressin as vasopressor in septic shock

V2 Receptors (Renal)

• Location: Principal cells of collecting duct (and thick ascending limb)
• Coupling: Gs → Adenylyl cyclase → ↑cAMP → PKA activation
• Effect: Phosphorylation and insertion of Aquaporin-2 (AQP-2) water channels into luminal membrane of collecting duct cells
• Result: Water moves from tubule lumen → cell → blood via AQP-3 and AQP-4 on basolateral side
• Net: Concentrated urine, water retention

Extrarenal V2 receptors:

• On vascular endothelium: release of von Willebrand factor (vWF) and Factor VIII
• Clinical: Desmopressin used in hemophilia A and von Willebrand disease

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6. Renal Physiology of ADH

Countercurrent Mechanism + ADH:

1. Loop of Henle creates a hypertonic medullary interstitium (up to 1200 mOsm/kg)
2. Collecting duct is normally impermeable to water
3. When ADH binds V2 receptor → cAMP → PKA → AQP-2 insertion
4. Water flows from tubule lumen (hypotonic) to hypertonic interstitium
5. Result: Urine can be concentrated from 50 mOsm/kg (dilute) to 1200 mOsm/kg (concentrated)
6. Urine volume range: 0.4 L/day (max ADH) to 20 L/day (no ADH)

Sites of ADH action in kidney:

• Collecting duct (principal cells) - primary site
• Thick ascending limb - enhances NaCl reabsorption (maintains medullary gradient)
• Inner medullary collecting duct - urea transporter (UT-A1) insertion → adds to medullary hypertonicity

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7. Disorders of ADH

A. SIADH - Syndrome of Inappropriate ADH Secretion

• Euvolemic hyponatremia (Na⁺ < 135 mEq/L)
• Decreased plasma osmolality (<280 mOsm/kg)
• Urine osmolality inappropriately high (>100 mOsm/kg)
• Urine Na⁺ elevated (>20 mEq/L) - because kidneys are functioning normally
• Normal adrenal and thyroid function (these must be excluded)

• S - Small cell lung cancer (SCLC) - most common malignancy (ectopic ADH)
• I - Intracranial pathology (meningitis, trauma, SAH, tumors)
• A - Alveolar/pulmonary disease (pneumonia, TB, abscess)
• D - Drugs (cyclophosphamide, vincristine, carbamazepine, SSRIs, opioids)
• H - Hypothyroidism, HIV

• Mild/chronic: Free water restriction (mainstay)
• Severe symptomatic hyponatremia: Hypertonic saline (3% NaCl) - correct Na⁺ slowly (max 8-10 mEq/L/24h to avoid osmotic demyelination)
• Vasopressin receptor antagonists (Vaptans - tolvaptan, conivaptan): for euvolemic and hypervolemic hyponatremia
• Demeclocycline: antagonizes ADH action at collecting duct (older therapy)

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B. Diabetes Insipidus (DI)

Central DI (CDI)

• Cause: Deficiency in synthesis or secretion of ADH
• Causes: Tumors (craniopharyngioma, metastases), trauma, neurosurgery, granulomas (sarcoidosis, histiocytosis X), autoimmune, idiopathic
• Urine: Dilute (low osmolality)
• Plasma: Hypertonic (high Na⁺, high osmolality)
• Response to desmopressin: YES - urine concentrates promptly (confirms CDI)
• Treatment: Desmopressin (DDAVP) - intranasal 10-40 mcg in 2-3 doses, oral 0.1-0.2 mg TDS, or SC 1-4 mcg q12h

Nephrogenic DI (NDI)

• Cause: Kidneys fail to respond to ADH (end-organ resistance)
• Causes:
  • Genetic: V2 receptor mutations (X-linked), AQP-2 gene mutations (autosomal)
  • Drugs: Lithium (most common acquired cause), demeclocycline, tetracyclines, amphotericin B
  • Metabolic: Hypercalcemia, hypokalemia
  • Renal disease: chronic pyelonephritis, medullary cystic disease
• Response to desmopressin: NO response (distinguishes from central DI)
• Treatment: Correct underlying cause + low-sodium diet + thiazide diuretics (paradoxical - cause mild volume depletion → proximal tubular reabsorption compensates → less delivery to collecting duct → less polyuria)

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8. Pharmacological Uses of ADH Analogues

Desmopressin (DDAVP)

• Central diabetes insipidus
• Primary nocturnal enuresis
• Hemophilia A (mild-moderate) - releases Factor VIII
• von Willebrand disease (type 1)
• Uremic bleeding

Vasopressin (Natural)

• Vasopressor in septic shock (V1 action)
• Variceal bleeding - causes splanchnic vasoconstriction (V1)
• Cardiac arrest (ACLS) - was used but now largely removed from guidelines

Vaptans (V2 receptor ANTAGONISTS)

• Tolvaptan: oral, selective V2 antagonist
• Conivaptan: IV, V1+V2 antagonist
• Use: Euvolemic and hypervolemic hyponatremia (SIADH, heart failure, cirrhosis)

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9. Laboratory Evaluation of ADH Disorders

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PART 2: MODEL LONG ANSWER QUESTION - 25 MARKS (MUHS FORMAT)

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Question: Write a detailed note on Antidiuretic Hormone (ADH). Include its synthesis, regulation of secretion, mechanism of action, physiological effects, and clinical importance with special reference to SIADH and Diabetes Insipidus. (25 marks)

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ANSWER

Introduction (2 marks)

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Chemistry and Synthesis (3 marks)

• A 6-membered ring formed by a disulfide bond between cysteine residues at positions 1 and 6
• A 3-amino acid side chain with arginine at position 8 (hence arginine vasopressin)

1. Hypothalamic neurons (supraoptic and paraventricular nuclei) synthesize pre-propressophysin
2. Signal peptide cleavage yields propressophysin (ADH + neurophysin II + copeptin)
3. Packaged into neurosecretory granules and transported by axonal flow down the pituitary stalk
4. Stored in nerve terminals (Herring bodies) of the posterior pituitary
5. Released by calcium-dependent exocytosis in response to stimuli

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Regulation of Secretion (4 marks)

• Osmoreceptors in the anterior hypothalamus (OVLT and subfornical organ) detect plasma hypertonicity
• Threshold: plasma osmolality > 280-285 mOsm/kg
• A 1-2% rise in osmolality triggers proportional ADH release
• Produces graded antidiuresis

• High-pressure receptors in carotid sinus and aortic arch sense arterial pressure
• Low-pressure receptors in cardiac atria and pulmonary vessels sense venous return
• A >10% fall in blood pressure or ECF volume is required to trigger significant ADH release
• Afferent signals travel via vagus and glossopharyngeal nerves to NTS → hypothalamus
• Explains elevated ADH in heart failure, cirrhosis, nephrotic syndrome (perceived low ECV)

• Nausea (most potent non-osmotic stimulus), pain, stress, angiotensin II
• Drugs: opioids, nicotine, cyclophosphamide, carbamazepine

• Hypo-osmolality, volume overload, alcohol, ANP, glucocorticoids

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Mechanism of Action (5 marks)

• Located on: vascular smooth muscle, liver, platelets
• Signaling: Gq → Phospholipase C → IP3/DAG → ↑intracellular Ca²⁺
• Effects: Vasoconstriction, glycogenolysis, platelet aggregation
• Activated at high ADH concentrations (hence "vasopressin")

• Located on: Principal cells of renal collecting duct
• Signaling: Gs → Adenylyl cyclase → ↑cAMP → Protein kinase A (PKA)
• PKA phosphorylates and promotes insertion of Aquaporin-2 (AQP-2) water channels into the apical (luminal) membrane
• Water enters the cell through AQP-2 and exits the basolateral side via AQP-3 and AQP-4
• Net effect: Water reabsorbed from tubular lumen into blood → concentrated urine

• Vascular endothelium: releases von Willebrand factor and Factor VIII
• Basis for desmopressin use in hemophilia A

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Physiological Effects (3 marks)

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Disorders of ADH: SIADH (4 marks)

1. Plasma Na⁺ < 135 mEq/L (hyponatremia)
2. Plasma osmolality < 280 mOsm/kg
3. Urine osmolality > 100 mOsm/kg (inappropriately concentrated)
4. Urine Na⁺ > 20 mEq/L (renal sodium wasting)
5. Euvolemia (no edema, no dehydration)
6. Normal thyroid and adrenal function

• CNS: meningitis, encephalitis, SAH, head injury, brain tumors
• Pulmonary: SCLC (most common malignancy - ectopic ADH), pneumonia, TB, lung abscess
• Drugs: cyclophosphamide, vincristine, carbamazepine, SSRIs, opioids, chlorpropamide
• Other: Postoperative state, HIV, hypothyroidism

• Mild/chronic: Free water restriction (800-1000 mL/day)
• Severe/symptomatic: Hypertonic saline (3% NaCl) - IV slowly; correction rate max 8-10 mEq/L in 24 hours (to avoid osmotic demyelination syndrome / central pontine myelinolysis)
• Vasopressin antagonists (vaptans): Tolvaptan (oral, V2-selective), Conivaptan (IV, V1+V2); cause aquaresis (free water excretion without sodium loss)
• Demeclocycline: Inhibits ADH action at collecting duct (second-line)
• Treat underlying cause

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Disorders of ADH: Diabetes Insipidus (4 marks)

Central (Cranial) DI

• Cause: Deficient ADH synthesis or secretion
• Etiology: Trauma, neurosurgery, pituitary tumors, craniopharyngioma, granulomas (sarcoidosis), autoimmune hypophysitis, idiopathic
• Findings: Low plasma ADH, high plasma Na⁺/osmolality, very dilute urine (osmolality < 200 mOsm/kg)
• Water deprivation test: Urine fails to concentrate; ADH level low
• DDAVP test: Urine osmolality rises (responds to exogenous ADH) - CONFIRMS CDI
• Treatment: Desmopressin (DDAVP) - intranasal, oral, or SC

Nephrogenic DI

• Cause: End-organ resistance to ADH (kidneys cannot respond)
• Etiology: Lithium toxicity (most common acquired), hypercalcemia, hypokalemia, V2 receptor gene mutation (X-linked), AQP-2 gene mutation, chronic pyelonephritis
• Findings: ADH is normal or elevated, but kidneys fail to concentrate urine
• DDAVP test: NO response - CONFIRMS NDI
• Treatment: Correct underlying cause; low sodium diet + thiazide diuretic (paradoxical effect - mild volume contraction → ↑proximal reabsorption → ↓delivery to collecting duct → ↓urine volume)

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Clinical Pharmacology of ADH Analogues (Bonus/Extra Points)

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Conclusion (Students: write 3-4 lines)

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Marks Distribution Guide (MUHS 25-mark Q)

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Exam Tips for MUHS

1. Always draw a diagram of the V2 receptor - cAMP - PKA - AQP-2 pathway. Examiners love it.
2. Compare table of CDI vs NDI vs SIADH is a must-include.
3. DDAVP test (response vs no response) is a classic MCQ/short note discriminator.
4. Remember: Alcohol inhibits ADH - that is why drunk patients have polyuria.
5. Osmotic demyelination syndrome: Do NOT correct hyponatremia faster than 10 mEq/L/24h.
6. Lithium causes NDI - commonly tested in pharmacology papers.
7. SCLC + hyponatremia = always think SIADH first.

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