A patient with central diabetes insipidus receives desmopressin and urine osmolality rises promptly. The cellular event in collecting duct principal cells responsible for this response is:

• A Transcriptional induction of aquaporin-3 channels over several days
• B Gq-mediated calcium release triggering contraction of duct smooth muscle
• C Gi-mediated fall in cAMP reducing water reabsorption
• D Gs-mediated increase in cAMP leading to insertion of aquaporin-2 channels in the apical membrane ✓

Explanation

The vasopressin V2 receptor on collecting duct principal cells is Gs coupled. Raised cAMP activates protein kinase D, which phosphorylates aquaporin-2 stored in intracellular vesicles, driving their fusion with the apical membrane and allowing water reabsorption within minutes. Desmopressin is a selective V2 agonist. V1 receptors use the Gq pathway and mediate vasoconstriction, not water handling, so option B is incorrect. The acute effect is vesicle trafficking, not new channel synthesis.

Reference: Harper's Illustrated Biochemistry, 31st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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