A patient with SIADH (syndrome of inappropriate ADH secretion) secondary to lung cancer has plasma Na+ of 118 mEq/L with neurological symptoms. Which drug selectively antagonizes the V2 vasopressin receptor in the collecting duct?
- A Conivaptan
- B Desmopressin
- C Furosemide
- D Tolvaptan ✓
Explanation
Tolvaptan is a selective, orally active V2-receptor antagonist (vaptans class). Vasopressin acting on V2 receptors inserts aquaporin-2 channels in the collecting duct, causing water reabsorption. V2 blockade by tolvaptan prevents aquaporin-2 insertion, producing electrolyte-free water diuresis (aquaresis) without natriuresis, raising serum sodium in hyponatremia. Conivaptan is a dual V1a/V2 antagonist available only IV. Desmopressin is a V2 agonist used for diabetes insipidus. Furosemide blocks NKCC2 in the loop of Henle and can help in SIADH but is not a vasopressin receptor antagonist. Tolvaptan is used for hyponatremia in SIADH and autosomal dominant polycystic kidney disease (ADPKD).
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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