ADH (vasopressin) is synthesized in the paraventricular and supraoptic nuclei of the hypothalamus and stored in the posterior pituitary. Its primary stimulus for secretion is:

• A Fall in plasma osmolality below 280 mOsm/kg
• B Increase in atrial natriuretic peptide (ANP)
• C Activation of carotid baroreceptors by high blood pressure
• D Rise in plasma osmolality above 280–285 mOsm/kg detected by hypothalamic osmoreceptors ✓

Explanation

Hypothalamic osmoreceptors (anterior hypothalamus, near but distinct from the supraoptic nucleus) sense plasma hyperosmolality above approximately 280–285 mOsm/kg and stimulate ADH release. The osmoreceptor system is highly sensitive; a 1–2% rise in osmolality doubles ADH secretion. A fall in osmolality below 280 mOsm/kg suppresses ADH. ANP inhibits ADH. Baroreceptor-mediated ADH release occurs with significant volume depletion (>8–10% fall) and is a lower-sensitivity but high-capacity override system.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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