PATHOPHYSIOLOGY OF THE AQUAPORIN WATER CHANNELS

Discovery of aquaporin water channel proteins has provided insight int o the molecular mechanism of membrane water permeability. The distribu tion of known mammalian aquaporins predicts roles in physiology and di sease. Aquaporin-1 mediates proximal tubule fluid reabsorption, secret ion of aqueous humor and cerebrospinal fluid, and lung water homeostas is. Aquaporin-2 mediates vasopressin-dependent renal collecting duct w ater permeability; mutations or downregulation can cause nephrogenic d iabetes insipidus. Aquaporin-3 in the basolateral membrane of the coll ecting duct provides an exit pathway for reabsorbed water. Aquaporin-4 is abundant in brain and probably participates in reabsorption of cer ebrospinal fluid osmoregulation, and regulation of brain edema. Aquapo rin-5 mediates fluid secretion in salivary and lacrimal glands and is abundant in alveolar epithelium of the lung. Specific regulation of me mbrane water permeability will likely prove important to understanding edema formation and fluid balance in both normal physiology and disea se.