Antidiuretic hormone acting on V2 receptors in the collecting duct increases water reabsorption. The mechanism linking cAMP generation to final water permeability is:
- A PKA-mediated phosphorylation of aquaporin-2, triggering its vesicular insertion into the apical membrane ✓
- B Direct binding of cAMP to aquaporin channels, opening them constitutively
- C cGMP-dependent kinase phosphorylation of aquaporin-1 on the basolateral membrane
- D Transcriptional induction of new aquaporin-3 within minutes of hormone binding
Correct answer: A. PKA-mediated phosphorylation of aquaporin-2, triggering its vesicular insertion into the apical membrane
Explanation
V2 receptors are Gs-coupled. Generated cAMP activates PKA, which phosphorylates preformed aquaporin-2 stored in cytoplasmic vesicles, promoting their fusion with the apical membrane of principal cells within minutes. This rapid trafficking effect explains why ADH acts quickly. Option D fails because acute ADH action does not require new protein synthesis; transcriptional effects are slower and secondary. Aquaporin-3 is basolateral and constitutively expressed, so option C misidentifies both the messenger and the channel.
Reference: Ganong's Review of Medical Physiology, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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