Which of the following is NOT an ADH receptor antagonist?

• A. Demeclocycline
• B. Conivaptan
• C. Tolvaptan
• D. Furosemide

Answer: (D)

ADH receptor antagonists work by blocking the action of antidiuretic hormone on the collecting ducts, either at the receptor or by counteracting the hormone’s effects, which promotes free-water excretion (aquaresis) without a large loss of electrolytes. Demeclocycline reduces the kidney’s response to ADH, conivaptan blocks V1a and V2 receptors, and tolvaptan selectively blocks V2 receptors. Furosemide, on the other hand, is a loop diuretic that inhibits the Na-K-2Cl cotransporter in the thick ascending limb of the loop of Henle. Its diuretic effect comes from blocking ion reabsorption, not from blocking ADH receptors, so it does not function as an ADH receptor antagonist. Therefore, it is not an ADH receptor antagonist.