Aquaporin-6 expression in rat gastrointestinal tract is upregulated by feeding

Background: Some aquaporins have been identified and partially characterized in the gastrointestinal tract suggesting their involvement in fluid and small solutes movements. Here we studied the presence and localization of aquaporin-6 in some regions of the rat gastrointestinal tract, given its peculiar function as an ion selective channel. Materials and methods: The aquaporin-6 expression along the gastrointestinal tract was investigated by RT-PCR and immunoblotting, while its localization by in situ hybridization and immunohistochemistry. The effect of feeding on the aquaporin- 6 mRNA expression was investigated by quantitative real time RT-PCR. Results: RT-PCR and immunoblotting experiments showed that aquaporin-6 was expressed in all the investigated portions of the rat gastrointestinal tract. Immunoblotting analysis of brush border membrane vesicle preparations showed an intense signal for aquaporin-6 protein. The results of in situ hybridization experiments demonstrate that aquaporin-6 transcript is present in the isthmus, neck and basal regions of the stomach lining, and throughout the crypt-villus axis in both small and large intestine. In the latter regions, immunohistochemistry revealed strong aquaporin-6 labelling in the apical membrane of the surface epithelial cells, while weak or no labelling was observed in the crypt cells. In the stomach, an intense staining was observed in mucous neck cells and lower signal in principal cells and some parietal cells. In addition, jejunal mRNA expression was specifically stimulated by feeding. Conclusions: This study provides evidence for the aquaporin- 6 distribution throughout the gastrointestinal tract. Aquaporin- 6 localization at the apical membranes of the superficial epithelial cells and its upregulation by feeding in the small intestine suggest its direct involvement in the absorption of water and anions.