aquaporin

An aquaporin is a microscopic “water gate” built into a cell’s outer wall. Cell membranes are oily and naturally repel water, so water cannot cross quickly on its own. Aquaporins are protein tunnels narrow enough to let water molecules slip through single file while blocking salts and other particles.

In the kidney, the key player is aquaporin-2 in the collecting duct. Normally it sits stored in little internal bubbles inside the cell, where it cannot move water. When vasopressin binds its V2 receptor, the resulting cyclic-AMP signal causes these bubbles to fuse with the membrane facing the urine, lining it with open water channels. Water then leaves the tubule, the urine concentrates, and the body keeps its water. Other aquaporins (such as types 3 and 4) sit on the opposite side of the cell to let that water continue into the bloodstream.

Because aquaporin-2 is the final step that vasopressin controls, problems here cause disease. Inherited defects in aquaporin-2 or in its V2 receptor produce a form of diabetes insipidus in which the kidney makes the channels poorly or cannot deploy them, so dilute urine pours out despite plenty of hormone.