Biliary epithelial cells (cholangiocytes) modulate bile fluidity and alkalinity absorbing and/or secreting fluid and electrolytes, particularly HCO3 and Cl-. Mechanisms responsible for transepithelial H+/HCO3 secretion in human cholangiocytes are largely unknown. Human cholangiocytes isolated by enzymatic digestion and immunomagnetic purification from normal liver tissue obtained from reduced grafts used for pediatric liver transplantation were cultured in the presence of human hepatocyte growth factor. Maintenance of cholangiocyte phenotypic features was assessed using markers such as cytokeratin 19, γ-glutamyltranspeptidase, vimentin, factor VIII-related antigen, desmin, epithelial membrane antigen (EMA), and human epithelial antigen (HEA) 125. Intracellular pH (pHi) transients were measured microfluorimetrically 2'7'-Bis(2-carboxyethyl)-5,6, carboxyfluorescein- acetossimethylester (BCECF). In the absence of HCO3, pHi recovery from an intracellular acid load (ammonia pre-pulse technique) was Na+-dependent and amiloride-inhibitable. No Na+-independent recovery was recorded even after stimulation with agents raising intracellular cyclic adenosine monophosphate (cAMP) concentrations. In the presence of HCO3, recovery from an intracellular acid load required Na+, but was only partly inhibited by amiloride. In these conditions H+ extrusion was inhibited by 4,4,- diisothiocyan atostilben-2,2,-disulfonic acid (DIDS) and by intracellular Cl- depletion. Acute removal of extracellular Cl-induced a phi alkalinization that was inhibited by DIDS. phi recovery from an intracellular alkaline load (isohydric CO2 changes) was Cl-dependent and DIDS- inhibitable. Administration of agents raising intracellular cAMP concentrations increased both Na+-dependent and Na+-independent Cl-/HCO3 exchange activity. Stimulation of Cl-/HCO3/- exchange activity was not prevented by the Cl- channel inhibitor 5'-nitro-2(2)-phenylpropyl-amino- benzoate(NPPB). In conclusion, human cholangiocytes possess two acid extruders (Na+/H+ exchanger and Na+-dependent Cl-/HCO3 exchange) and an acid loader (Cl-/HCO3/- exchange), whereas no evidence was found for cAMP activated H+-ATPase. Bicarbonate influx is thus mainly mediated by Na- dependent Cl-/HCO3- exchange, whereas Na+:HCO3 cotransport is not active in the physiological range of pHi. Stimulation of Na+-independent Cl- /HCO3/- exchanger by cAMP does not require activation of Cl- conductances. These mechanisms may underlay hormone-regulated biliary HCO3 secretion in the human biliary tree.
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