Cl--dependent secretory mechanisms in isolated rat bile duct epithelial units

Cholangiocytes absorb and secrete fluid, modifying primary canalicular bile . In several Cl--secreting epithelia, Na+-K+-2Cl(-) cotransport is a basola teral Cl- uptake pathway facilitating apical Cl- secretion. To determine if cholangiocytes possess similar mechanisms independent of CO2/HCO3-, we ass essed Cl- -dependent secretion in rat liver isolated polarized bile duct un its (IBDUs) by using videomicroscopy. Without CO2/HCO3-, forskolin (FSK) st imulated secretion entirely dependent on Na+ and Cl- and inhibited by Na(+) K(+)2Cl(-) inhibitor bumetanide. Carbonic anhydrase inhibitor ethoxyzolamid e had no effect on FSK-stimulated secretion, indicating negligible endogeno us CO2/HCO3- transport. In contrast, FSK-stimulated secretion was inhibited similar to5% by K+ channel inhibitor Ba2+ and blocked completely by bumeta nide plus Ba2+. IBDU Na+-K+-2Cl(-) cotransport activity was assessed by rec ording intracellular pH during NH4Cl exposure. Bumetanide inhibited initial acidification rates due to NH4+ entry in the presence and absence of CO2/H CO3-. In contrast, when stimulated by FSK, a 35% increase in Na+-K+-2Cl(-) cotransport activity occurred without CO2/HCO3-. These data suggest a cellu lar model of HCO3- -independent secretion in which Na+-K+-2Cl(-) cotranspor t maintains high intracellular Cl- concentration. Intracellular cAMP concen tration increases activate basolateral K+ conductance, raises apical Cl- pe rmeability, and causes transcellular Cl- movement into the lumen. Polarized IBDU cholangiocytes are capable of vectorial Cl--dependent fluid secretion independent of HCO3-. Bumetanide-sensitive Na+-K+-2Cl(-) cotransport, Cl-/ HCO3- exchange, and Ba2+ sensitive K+ channels are important components of stimulated fluid secretion in intrahepatic bile duct epithelium.