, an exocrine-type water channel, was detected in the rat duodenum by Western blot analysis, and was localized by immunohistochemistry in the secretory granule membranes as well as in the apical and lateral aspects of the plasma membrane of Brunner's gland cells. Incubation of duodenal slices with vasoactive intestinal polypeptide (VIP) in vitro significantly increased the amount of AQP5 in the apical membrane fraction in a dose-and time-dependent manner with the amount reaching a plateau at 100 nM VIP and becoming near maximal after a 30-s incubation. Protein kinase inhibitors, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7, 50 M), and N-[2-(pbromocinnamylamino)ethyl]-5-isoquinolinesulfonamide (H-89; PKAspecific, 1 M) blocked this increase, but PKC-specific inhibitor calphostin C did not, implying the involvement of PKA but not PKC in this cellular event. Intravenous injection with VIP (40 g/kg body wt) provoked dilation of the lumen of the Brunner's gland at 2 and 7 min and increased the staining intensity of AQP5 in the apical and lateral membranes. AQP1 (both nonglycosylated and glycosylated forms) was also found to localize in the apical and basolateral membranes of cells 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 above in vitro and in vivo experiments. These results suggest that VIP induced the exocytosis of granule contents and simultaneously caused translocation of AQP5 but not of AQP1 to the apical membrane in Brunner's gland cells. aquaporin 5; water channel; translocation COMPLEMENTARY DNAS of members of the large family of aquaporins (AQPs) have been cloned recently from a variety of mammalian tissues (29). Proteins of this AQP family selectively transport water and other components, such as urea and glycerol (29). Recent investigations have uncovered the molecular basis of the water transport across the cell membrane; e.g., the AQP5 molecule, one of the exocrine-type AQPs, was demonstrated to be increased in concentration at the apical membrane of the salivary gland in response to the stimulation of muscarinic receptors or to an increase in the intracellular level of calcium ions (10,11,28). AQP1 in rat bile duct cells and AQP2 in rat renal collecting duct cells were reported to traffic from an intracellular location to the apical membrane in response to secretin and vasopressin, respectively (16,21,30).A recent study of ours (22) demonstrated AQP5 to be present in the duodenum, where it was localized in the apical and lateral membranes of the secretory cells of Brunner's gland. In the duodenum, epithelial cells and cells of Brunner's gland secrete bicarbonate and mucin, respectively, into the duodenal lumen, which secretion is thought to play an important role in mucosal protection against gastric acid. Mucin and water are known to combine together to create a viscoelastic gel that becomes infiltrated with bicarbonate, thus forming a physicochemical barrier to hydrogen ions ...