Molecular mechanism of water absorption through aquaporin in the small intestine of bottlenose dolphins

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Generally in mammals, water is absorbed mainly through cell junction of the enterocyte (paracellular pathway) in these mammals, and transcellular pathway with aquaporin (AQP : water-selective channel) is thought not to be used. We hypothesized that cetaceans that cannot access fresh water might use transcellular pathway to maximize water absorption in intestine. To certify this hypothesis, some experiments were carried out using bottlenose dolphins. In this study, expression of AQP 1, 3, 4, 5, 6, 7, 8 and 11 were certified in the small intestine of the dolphins. Immunohistological studies revealed that AQP1 was distributed in the apical membrane of enterocytes in the intestine. Furthermore, data suggesting the localization of Na+/K+ ATPase and expression of Na+/glucose transporters in enterocytes were also obtained. These data suggest that AQP1 is a channel protein responsible for water absorption in the small intestine of dolphins, and animals like dolphins living in chronic water-deprived conditions need to absorb water by transcellular pathway, maybe in addition to paracellular pathways, to maximize water injection in intestines.