Aquaporin water channels in mammals

Ten aquaporins have been cloned from various mammalian tissues. They are grouped according to their structure and function. The first group consists of 7 aquaporins; AQP0, 1, 2, 4, 5, 6, and 8. These channel molecules selectively transport water and do not transport glycerol and urea. The second group consists of 3 aquaporins; AQP3, 7, and 9. They transport not only water, but also small nonionic molecules such as glycerol and urea. The extensive tissue distribution and physiologic regulation by dehydration and hormones of these aquaporins suggest that aquaporins have important functions in water and solute transport in the body. However, the recent studies of knockout animals and humans with defective mutations of aquaporins showed unexpectedly small phenotypic effects. It is possible that other, unidentified aquaporins may compensate for these deficiencies. The future challenge of research in aquaporins should be the identification of their physiologic significance, and the discovery of new members, which will expand the research area of water metabolism and deepen our understanding of the physiology and pathophysiology of water transport in our body.