Aquaporins (AQPs) are a large family of integral membrane water transport
channel proteins that facilitate the transport of water through the cell membrane.
This function is conserved in animals, plants and bacteria. At least
ten isoforms of aquaporin have been identified in mammals, designated
AQP0 through AQP9. Aquaporins are widely distributed and it is not uncommon
for more than one type of AQP to be present in the same cell. Although
most aquaporins are only permeable to water, AQP3, AQP7 and AQP9 are
permeable to urea and glycerol. AQP2 is the only water channel that is activated
by vasopressin to enhance water reabsorption in the kidney collecting
duct. Aquaporins are involved in renal water absorption, generation of pulmonary
secretions, lacrimation, and the secretion and reabsorption of cerebrospinal
fluid and aqueous humor. In the lung, the 27-kDa AQP5 is responsible
for the majority of water transport across the apical membrane of type I
alveolar epithelial cells. The gene encoding human AQP5 maps to chromosome
12q13.