Secretin induces the apical insertion of aquaporin-1 water channels in ratcholangiocytes

Aquaporin-1 (AQP1) water channels are present in the apical and basolateral plasma membrane domains of bile duct epithelial cells, or cholangiocytes, and mediate the transport of water in these cells. We previously reported t hat secretin, a hormone known to stimulate ductal bile secretion, increases cholangiocyte osmotic water permeability and stimulates the redistribution of AQP1 from an intracellular vesicular pool to the cholangiocyte plasma m embrane. Nevertheless, the target plasma membrane domain (i.e., basolateral or apical) for secretin-regulated trafficking of AQP1 in cholangiocytes is unknown, as is the functional significance of this process for the secreti on of ductal bile. In this study, we used primarily an in vivo model (i.e., rats with cholangiocyte hyperplasia induced by bile duct ligation) to addr ess these issues. AQP1 was quantitated by immunoblotting in apical and baso lateral plasma membranes prepared from cholangiocytes isolated from rats 20 min after intravenous infusion of secretin. Secretin increased bile flow ( 78%, P < 0.01) as well as the amount of AQP1 in the apical cholangiocyte pl asma membrane (127%, P < 0.05). In contrast, the amount of AQP1 in the baso lateral cholangiocyte membrane and the specific activity of an apical chola ngiocyte marker enzyme (i.e., gamma-glutamyltranspeptidase) were unaffected by secretin. Similar observations were made when freshly isolated cholangi ocytes were directly exposed to secretin. Immunohistochemistry for AQP1 in liver sections from secretin-treated rats showed intensified staining at th e apical region of cholangiocytes. Pretreatment of rats with colchicine (bu t not with its inactive analog beta-lumicolchicine) inhibited both the incr eases of AQP1 in the cholangiocyte plasma membrane (94%, P < 0.05) and the bile flow induced by secretin (54%, P < 0.05). Our results in vivo indicate that secretin induces the microtubule-dependent insertion of AQP1 exclusiv ely into the secretory pole (i.e., apical membrane domain) of rat cholangio cytes, a process that likely accounts for the ability of secretin to stimul ate ductal bile secretion.