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

Abstract

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.

Reduced NHE3-mediated Na+ absorption increases survival and decreases the incidence of intestinal obstructions in cystic fibrosis mice

“…cholangiocytes (40) and pancreatic ducts (35), where NHE3 appears to serve absorptive functions that counter CFTRmediated secretion. It is therefore possible that inhibition of absorption might also prevent or reduce some of the other pathologies that occur in the bile and pancreatic ducts.…”

Physiology of Cholangiocytes

“…An additional mechanism that may contribute to secretin-induced HCO 3 − -rich choleresis is the Na + /H + exchanger NHE3, which, as described earlier, is present on the cholangiocyte apical plasma membrane (201). NHE3 stimulates absorption of fluid from the biliary lumen, thus counterbalancing fluid secretion in resting bile duct epithelia (183).…”

“…8), and these are individually reviewed in detail below. Na + /H + exchangers-An NHE ortholog, NHE3/SLC9A3, has been identified on the rat and mouse cholangiocyte apical plasma membrane (201). Functional studies in IBDUs from NHE3/SLC9A3 knockout mice and from normal rats and mice treated with an NHE inhibitor, 5-(N-ethyl-N-isopropyl)-amiloride, have shown that NHE3/SLC9A3 is involved in cholangiocyte fluid absorption and counterbalancing fluid secretion (201).…”

“…Na + /H + exchangers-An NHE ortholog, NHE3/SLC9A3, has been identified on the rat and mouse cholangiocyte apical plasma membrane (201). Functional studies in IBDUs from NHE3/SLC9A3 knockout mice and from normal rats and mice treated with an NHE inhibitor, 5-(N-ethyl-N-isopropyl)-amiloride, have shown that NHE3/SLC9A3 is involved in cholangiocyte fluid absorption and counterbalancing fluid secretion (201). In addition to these functions, NHE3/SLC9A3 may also be a site for regulation of cholangiocyte HCO 3 − and fluid secretion, presumably through upregulation of CFTR via protein-protein interactions (201).…”

“…Functional studies in IBDUs from NHE3/SLC9A3 knockout mice and from normal rats and mice treated with an NHE inhibitor, 5-(N-ethyl-N-isopropyl)-amiloride, have shown that NHE3/SLC9A3 is involved in cholangiocyte fluid absorption and counterbalancing fluid secretion (201). In addition to these functions, NHE3/SLC9A3 may also be a site for regulation of cholangiocyte HCO 3 − and fluid secretion, presumably through upregulation of CFTR via protein-protein interactions (201). The presence of another ortholog, NHE2/SLC9A2, on the rat cholangiocyte apical plasma membrane has also been reported (176,252).…”