Cholangiocytes absorb and secrete fluid, modifying primary canalicular bile. In several Cl−-secreting epithelia, Na+-K+-2Cl− cotransport is a basolateral Cl− uptake pathway facilitating apical Cl− secretion. To determine if cholangiocytes possess similar mechanisms independent of CO2/HCO[Formula: see text], we assessed Cl−-dependent secretion in rat liver isolated polarized bile duct units (IBDUs) by using videomicroscopy. Without CO2/HCO[Formula: see text], forskolin (FSK) stimulated secretion entirely dependent on Na+ and Cl−and inhibited by Na+-K+-2Cl−inhibitor bumetanide. Carbonic anhydrase inhibitor ethoxyzolamide had no effect on FSK-stimulated secretion, indicating negligible endogenous CO2/HCO[Formula: see text] transport. In contrast, FSK-stimulated secretion was inhibited ∼85% by K+ channel inhibitor Ba2+ and blocked completely by bumetanide plus Ba2+. IBDU Na+-K+-2Cl− cotransport activity was assessed by recording intracellular pH during NH4Cl exposure. Bumetanide inhibited initial acidification rates due to NH[Formula: see text] entry in the presence and absence of CO2/HCO[Formula: see text]. In contrast, when stimulated by FSK, a 35% increase in Na+-K+-2Cl− cotransport activity occurred without CO2/HCO[Formula: see text]. These data suggest a cellular model of HCO[Formula: see text]-independent secretion in which Na+-K+-2Cl−cotransport maintains high intracellular Cl−concentration. Intracellular cAMP concentration increases activate basolateral K+ conductance, raises apical Cl−permeability, and causes transcellular Cl− movement into the lumen. Polarized IBDU cholangiocytes are capable of vectorial Cl−-dependent fluid secretion independent of HCO[Formula: see text]. Bumetanide-sensitive Na+-K+-2Cl− cotransport, Cl−/HCO[Formula: see text] exchange, and Ba2+-sensitive K+ channels are important components of stimulated fluid secretion in intrahepatic bile duct epithelium.
Intrahepatic Bile Duct Epithelium
