Final answer:
The action of vasopressin, or antidiuretic hormone (ADH), involves inserting AQP-2 water channels into the collecting duct cells' apical membrane, increasing water reabsorption, concentrating urine, and regulating blood osmolarity.
Step-by-step explanation:
The mechanism of action of vasopressin (antidiuretic hormone, ADH) is primarily through the regulation of water balance by affecting the water permeability of the kidney tubules. When stimulated by ADH, vasopressin induces the insertion of aquaporin-2 (AQP-2) channels into the apical membrane of cells in the collecting ducts of the kidneys. This insertion allows water to be reabsorbed from the collected filtrate back into the bloodstream, thus concentrating the urine and reducing water excretion. In addition, vasopressin can also act as a vasoconstrictor, increasing blood pressure when needed.
Vasopressin's activation of V2 receptors triggers a signaling cascade that results in AQP-2 redistribution to the cell membrane. This process increases the water permeability of the collecting ducts. Through a negative feedback mechanism, the secretion of vasopressin is regulated to maintain fluid homeostasis and blood osmolarity.
The correct answer to the student's question is D) insertion of AQP-2 into the apical membrane of collecting duct cells, which increases water reabsorption and concentrates the urine.