Role of sodium/hydrogen exchanger isoform NHE3 in fluid secretion and absorption in mouse and rat cholangiocytes

Na+/H+ exchanger (NHE) isoforms play important roles in intracellular pH re gulation and in fluid absorption. The isoform NHE3 has been localized to ap ical surfaces of epithelia and in some tissues may facilitate the absorptio n of NaCl. To determine whether the apical isoform NHE3 is present in chola ngiocytes and to examine whether it has a functional role in cholangiocyte fluid secretion and absorption, immunocytochemical studies were performed i n rat liver with NHE3 antibodies and functional studies were obtained in is olated bile duct units from wild-type and NHE3 (-/-) mice after stimulation with forskolin, using videomicroscopic techniques. Our results indicate th at NHE3 protein is present on the apical membranes of rat cholangiocytes an d on the canalicular membrane of hepatocytes. Western blots also detect NHE 3 protein in rat cholangiocytes and isolated canalicular membranes. After s timulation with forskolin, duct units from NHE3 (-/-) mice fail to absorb t he secreted fluid from the cholangiocyte lumen compared with control animal s. Similar findings were observed in isolated bile duct units from wild-typ e mice and rats in the presence of the Na+/H+ exchanger inhibitor 5-(N-ethy l-N-isopropyl) amiloride. In contrast, we could not demonstrate absorption of fluid from the canalicular lumen of mouse or rat hepatocyte couplets aft er stimulation of secretion with forskolin. These findings indicate that NH E3 is located on the apical membrane of rat cholangiocytes and that this NH E isoform can function to absorb fluid from the lumens of isolated rat and mouse cholangiocyte preparations.