Physiology and pathophysiology of the aquaporin-2 water channel.

Aquaporins are integral membrane proteins, which function as specialized water channels to facilitate the passage of water through the cell membrane. In mammals six different aquaporins have been identified up to now, four of which (aquaporin-1 to aquaporin-4) are expressed in the kidney. Because of its importance for normal water homeostasis and its involvement in many water balance disorders, aquaporin-2, the predominant vasopressin-regulated water channel of the renal collecting duct, is discussed in detail.