Expression and localization of aquaporin-1 on the apical membrane of enterocytes in the small intestine of bottlenose dolphins.

The small and large intestines are primary sites for water intake in mammals. To reveal how water is absorbed in the intestines of cetaceans, histological and molecular-biological studies were performed on the small intestine of the bottlenose dolphin, Tursiops truncatus. In histological studies using fresh specimens, obvious villi and deep crypts of Lieberkühn, lined by abundant enterocytes with microvilli and goblet cells, were observed in the mucosa. Expressions and immunolocalizations of aquaporin-1 (AQP1), a member of the water-selective channel termed AQP, were also investigated in the intestine. By reverse transcriptional polymerase chain reaction and rapid amplification of cDNA ends using RNA extracted from the dolphins' small intestines, the full length of mRNA for AQP1 was sequenced. The deductive amino acid sequence for an open reading frame showed high homologies with other mammals' AQP1, and water permeability of the protein was certified by cRNA injection to Xenopus oocytes. Immunohistochemistry showed AQP1 distribution on the apical membrane of the enterocytes, especially in the crypts. These data suggest that AQP1 is a channel protein responsible for water absorption in the small intestine of dolphins.