Question

1. The kidneys handle Na+ in different places of the nephron. Which statement is correct?

   		Water is filtered out of glomerular capillaries by bulk flow.
   		Water is actively reabsorbed from the proximal tubule, and Na+ follows down its diffusion gradient.
   		The permeability of the ascending limb of the loop of Henle is modified by vasopressin.
   		Vasopressin inserts pumps in the collecting duct membrane that move water against its concentration gradient.

Answer 1

D) Vasopressin inserts pumps in the collecting duct membrane that move water against its concentration gradient.

Explanation:

Aquaporins selectively conduct water molecules in and out of the cell, while preventing the passage of ions and other solutes. Also known as water channels, aquaporins are integral membrane pore proteins.
Aquaporin-2 (AQP2) is the vasopressin-regulated water-channel protein expressed at the connecting tubule and collecting duct, and plays a key role in urine concentration and body-water homeostasis through short-term and long-term regulation of collecting duct water permeability.

Vasopressin plays a key role in the modulation of water permeability in mammalian kidney. The ability of vasopressin to increase water transport is mediated by translocation of the specific vasopressin-sensitive water channel AQP2, from intracellular vesicles to the apical membrane. The initial mechanism of vasopressin action involves the hormone binding to the V2 receptor located on the basolateral cell surface of collecting duct principal cells.