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1 Center for Basic Research in Digestive Diseases, Departments of Internal Medicine and Biochemistry and Molecular Biology, Mayo Clinic and Foundation, Mayo Medical School, Rochester, Minnesota 55905; and 2 Departments of Biological Chemistry and Medicine, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205
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
that 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 membrane. 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 secretion 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 address these issues.
AQP1 was quantitated by immunoblotting in apical and basolateral 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 plasma membrane (127%,
P < 0.05). In contrast, the amount
of AQP1 in the basolateral cholangiocyte membrane and the specific
activity of an apical cholangiocyte marker enzyme (i.e.,
-glutamyltranspeptidase) were unaffected by secretin. Similar
observations were made when freshly isolated cholangiocytes were
directly exposed to secretin. Immunohistochemistry for AQP1 in liver
sections from secretin-treated rats showed intensified staining at the
apical region of cholangiocytes. Pretreatment of rats with colchicine
(but not with its inactive analog 
-lumicolchicine) inhibited both
the increases 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 exclusively into the secretory
pole (i.e., apical membrane domain) of rat cholangiocytes, a process
that likely accounts for the ability of secretin to stimulate ductal
bile secretion.
biliary epithelia; aquaporins; bile secretion
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