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1 Liver Center and Department of Medicine, Yale University School of Medicine, New Haven, Connecticut 06520; and 2 Department of Molecular Genetics, Biochemistry, and Microbiology, University of Cincinnati, Cincinnati, Ohio 45267-0524
Na+/H+ exchanger (NHE) isoforms play important
roles in intracellular pH regulation and in fluid absorption. The
isoform NHE3 has been localized to apical surfaces of epithelia and in
some tissues may facilitate the absorption of NaCl. To determine
whether the apical isoform NHE3 is present in cholangiocytes and to
examine whether it has a functional role in cholangiocyte fluid
secretion and absorption, immunocytochemical studies were performed in
rat liver with NHE3 antibodies and functional studies were obtained in
isolated bile duct units from wild-type and NHE3 (
/) mice after
stimulation with forskolin, using videomicroscopic techniques. Our
results indicate that NHE3 protein is present on the apical membranes
of rat cholangiocytes and on the canalicular membrane of hepatocytes.
Western blots also detect NHE3 protein in rat cholangiocytes and
isolated canalicular membranes. After stimulation with forskolin, duct
units from NHE3 (/) mice fail to absorb the secreted fluid from the
cholangiocyte lumen compared with control animals. Similar findings
were observed in isolated bile duct units from wild-type mice and rats
in the presence of the Na+/H+ exchanger
inhibitor 5-(N-ethyl-N-isopropyl)-amiloride. In
contrast, we could not demonstrate absorption of fluid from the
canalicular lumen of mouse or rat hepatocyte couplets after stimulation
of secretion with forskolin. These findings indicate that NHE3 is located on the apical membrane of rat cholangiocytes and that this NHE
isoform can function to absorb fluid from the lumens of isolated rat
and mouse cholangiocyte preparations.
bile secretion; bile duct epithelium; hepatocyte
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