SN3
SHORT NOTE
5M
★ High-yield
Write a short note on antidiuretic hormone.
Source and Nature
Antidiuretic hormone (ADH, vasopressin) = a nonapeptide that conserves body water by increasing water reabsorption in the collecting duct.
- Synthesised in the supraoptic (chiefly) and paraventricular nuclei of the hypothalamus
- Transported down axons bound to neurophysin
- Stored and released from the posterior pituitary (neurohypophysis)
- Half-life 16–20 minutes
Stimuli for Secretion
| ↑ ADH | ↓ ADH |
|---|---|
| ↑ Plasma osmolality (the most sensitive stimulus — a 1% rise suffices) | ↓ Plasma osmolality |
| ↓ Blood volume / BP (needs a 10–15% fall) | ↑ Blood volume (atrial stretch) |
| Pain, stress, trauma, surgery, nausea | Alcohol |
| Nicotine, morphine, angiotensin II | Cold, ↑ ANP |
- Osmoreceptors lie in the anterior hypothalamus (organum vasculosum); threshold osmolality 280 mOsm/kg
- Volume receptors are in the atria and great veins
Mechanism of Action
ADH binds V2 receptor on basolateral membrane of collecting duct → Gs protein → adenylyl cyclase → cAMP → Protein kinase A → Vesicles containing aquaporin-2 inserted into apical membrane → Water reabsorbed down the osmotic gradient
Actions
| Receptor | Site | Action |
|---|---|---|
| V1 | Vascular smooth muscle | Vasoconstriction (only at high, pharmacological concentration) |
| V2 | Collecting duct | Water reabsorption — the chief physiological action |
| V2 | Endothelium | ↑ Factor VIII and vWF release — basis of desmopressin use in mild haemophilia |
| V1 | Inner medullary collecting duct | ↑ Urea permeability |
Applied Aspects
| Disorder | Defect | Urine | Treatment |
|---|---|---|---|
| Central diabetes insipidus | ↓ ADH secretion | Large volume, dilute (up to 20 L/day) | Desmopressin |
| Nephrogenic diabetes insipidus | Tubule unresponsive (lithium, hypercalcaemia, hypokalaemia) | Large volume, dilute | Thiazides, amiloride; ADH is useless |
| SIADH | Excess ADH (small cell lung cancer, CNS disease, drugs) | Small volume, concentrated | Fluid restriction, tolvaptan |
- Water deprivation test distinguishes the two forms of diabetes insipidus; desmopressin concentrates the urine only in the central form
- SIADH causes dilutional hyponatraemia — correct slowly, or central pontine myelinolysis may result
- Alcohol inhibits ADH → diuresis and the dehydration of a hangover
All questions in Renal System
- Q1Define glomerular filtration rate. Describe the filtration barrier, the forces governing filtration, and the factors affecting GFR. Add a note on autoregulation. ★
- Q2Describe tubular reabsorption in the different parts of the nephron. Explain glucose reabsorption with transport maximum and renal threshold. ★
- Q3Describe the counter-current mechanism of urine concentration. Explain the role of the loop of Henle, vasa recta and ADH. ★
- Q4Describe the role of the kidney in the regulation of acid–base balance. Explain bicarbonate reabsorption, titratable acid and the ammonia mechanism. ★
- Q5Describe the nerve supply of the urinary bladder and the mechanism of micturition. Add a note on the abnormalities of micturition. ★
- SN1Define renal clearance. Describe inulin and creatinine clearance and their significance. ★
- SN2Describe the juxtaglomerular apparatus and the renin–angiotensin system. ★
- SN3Write a short note on antidiuretic hormone. ★
- SN4Classify diuretics according to their site of action and describe their mechanisms. ★
- SN5Enumerate the renal function tests and give their normal values. ★
- SN6Describe the principle and types of dialysis. Give its indications.
- SN7Enumerate the non-excretory functions of the kidney. ★