RENAL AQUAPORINS

Aquaporins (AQPs) are a newly recognized family of transmembrane prote ins that function as molecular water channels. At least four aquaporin s are expressed in the kidney where they mediate rapid water transport across water-permeable epithelia and play critical roles in urinary c oncentrating and diluting processes. AQP1 is constitutively expressed at extremely high levels in the proximal tubule and descending limb of Henle's loop. AQP2, -3 and -4 are expressed predominantly in the coll ecting duct system. AQP2 is the predominant water channel in the apica l plasma membrane and AQP3 and -4 are found in the basolateral plasma membrane. Short-term regulation of collecting duct water permeability by vasopressin is largely a consequence of regulated trafficking of AQ P2-containing vesicles to and from the apical plasma membrane.