Vasopressin's V2 receptor in the kidney mediates water reabsorption.

• A. True
• B. False
• C. It mediates vasoconstriction
• D. It is not involved in water reabsorption

Answer: (A)

The main idea is how vasopressin regulates water reabsorption in the kidney. Vasopressin acts on V2 receptors located on the collecting duct principal cells. When vasopressin binds these receptors, it activates a Gs protein that increases cyclic AMP, which then activates protein kinase A. This signaling causes aquaporin-2 channels to be inserted into the apical membrane of these cells, raising water permeability. Water can then move from the tubular lumen into the surrounding tissue, increasing water reabsorption and concentrating the urine. This mechanism is specifically linked to the V2 receptor; the V1 receptor mediates vasoconstriction rather than water reabsorption. So the statement is true.