Tolvaptan is a vaptane class diuretic. Its mechanism and the clinical scenario where it is specifically superior to furosemide involve which of the following?

• A It blocks aldosterone receptors in the collecting duct, treating hyperaldosteronism-driven edema
• B It is a selective V2 vasopressin receptor antagonist (aquaretic), increasing free water excretion without significant electrolyte loss — used for euvolemic or hypervolemic hyponatremia ✓
• C It inhibits Na-K-2Cl cotransporter in the loop of Henle, producing greater natriuresis than furosemide
• D It blocks aquaporin-2 channels directly in the collecting duct, preventing water reabsorption irrespective of ADH levels

Explanation

Tolvaptan is a selective, oral V2 receptor antagonist (vasopressin-2 receptor in the renal collecting duct). Blocking V2 prevents ADH-induced insertion of aquaporin-2 water channels, resulting in excretion of electrolyte-free water (aquaresis) rather than the sodium-and-water diuresis of loop diuretics. This selectively raises serum sodium without causing hypokalemia or further electrolyte imbalance — making it specifically indicated for SIADH and hypervolemic hyponatremia (heart failure, cirrhosis, SIADH). Furosemide removes sodium-rich urine and can worsen hyponatremia if free water intake is maintained. Note: hepatotoxicity limits tolvaptan use in autosomal dominant polycystic kidney disease (ADPKD) beyond 18 months.

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

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