In the collecting duct, which channel mediates water reabsorption in response to ADH?

• A. ENaC
• B. Aquaporin-3
• C. Aquaporin-4
• D. Aquaporin-2

Answer: (D)

Antidiuretic hormone controls water reabsorption in the collecting duct by increasing the water permeability of the apical membrane of principal cells. It does this by triggering the insertion of aquaporin-2 channels into the apical surface, allowing water to move from the tubular lumen into the cell and then out into the bloodstream via basolateral aquaporins. Without ADH, aquaporin-2 stays stored inside the cell and the collecting duct becomes relatively impermeable to water, producing dilute urine. ENaC handles sodium transport and is not the water channel regulated by ADH, while aquaporin-3 and aquaporin-4 are basolateral water channels that provide exit pathways but are not the channels inserted into the apical membrane in response to ADH.