Final answer:
The increase in water permeability of the distal tubule and collecting ducts due to hormone stimulus is mediated by an increase in aquaporin-2 activity, as ADH prompts aquaporins to move to the plasma membrane, raising water reabsorption.
Step-by-step explanation:
The hormone-induced increases in water permeability of the distal tubule and collecting ducts is mediated by an increase in aquaporin-2 activity. Antidiuretic hormone (ADH) causes aquaporins to move to the collecting duct plasma membrane, which increases water reabsorption. Specifically, when stimulated by ADH, aquaporin channels are inserted into the apical membrane of principal cells, leading to a dramatic rise in water flow out of the tubule and into the bloodstream. This results in the reabsorption of more water into the blood, thereby concentrating the urine and regulating blood osmolarity and blood pressure.