Molecular mechanisms of expression and regulation of water channel proteins aquaporins in the exocrine gland cells

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Localization of aquaporin 5 (AQP5) in the rat Brunner's gland and its regulation by vasoactive intestinal polypeptide (VIP) were in vestigated. Treatment of slices of duodenum with VIP in vitro for 2 or 7 min significantly increased amount of AQP5 in the duodenal apical membrane fraction. Intravenous injection of rats with VIP (40 ug/kg body weight) for 2 or 7 min provoked strong dilation of the lumen and increased the staining intensity of AQP5 in the apical and lateral membranes,whereas thae staining inside of the cells was appreciably reduced. AQP1 was also found to be localized in the apical and basolateral membranes of Brunner's gland; VIP however did not provoke any significant change in the AQP1 level in the apical membrane as judged from the results of the in vitro and vivo experiments. These results suggest that VIP induced the exocytosis of granule contents and simultanenously caused translocation of AQP5 to the apical membrane in the Brunner's gland.The expression level of aquapoin (AQP)5 in the submandibular gland (SMG) was found to be different among individual rats of the Sprague-Dawley (SD) straim. Such differences were observed for AQP5 but not for AQP1 and consequently the SD strain was divided into two groups, one expressing a high level of AQP5 and the other a low one. Breeding between brother and sister was repeated for two times within high expressers and low expressers to obtain the third generation progenies (F2); the AQP5 level of the SMG in the third generation (F2rats) from high expressers was significantly higher than the F2 from low expressers. Our present study suggests the existence of genetic variation in the expression of a water channel protein, AQP5, in rats.