A patient with syndrome of inappropriate antidiuretic hormone secretion (SIADH) is treated with tolvaptan. Tolvaptan corrects hyponatraemia by:

• A Selectively blocking V2 vasopressin receptors in the renal collecting duct, promoting free water excretion (aquaresis) without natriuresis ✓
• B Inhibiting aquaporin-2 channel transcription in collecting duct cells
• C Stimulating osmoreceptors to reduce ADH secretion from the posterior pituitary
• D Blocking V1 receptors in the systemic vasculature causing vasodilation and water redistribution

Explanation

Tolvaptan is a selective, oral V2 receptor antagonist (vaptans class). ADH/vasopressin acts on V2 receptors in renal collecting ducts to insert aquaporin-2 water channels, causing water reabsorption. Tolvaptan blocks V2 receptors, preventing aquaporin-2 insertion and resulting in excretion of pure electrolyte-free water (aquaresis) — raising serum sodium in euvolaemic and hypervolaemic hyponatraemia (SIADH, heart failure, cirrhosis). Unlike hypertonic saline, it does not risk hypernatraemia if monitored carefully.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.