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 basolateral Cl(-) uptake pathway facilitating apical Cl(-) secretion. To determine if cholangiocytes possess similar mechanisms independent of CO2/HCO, we assessed Cl(-)-dependent secretion in rat liver isolated polarized bile duct units (IBDUs) by using videomicroscopy. Without CO2/HCO, 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 transport. In contrast, FSK-stimulated secretion was inhibited approximately 85% by K(+) channel inhibitor Ba(2+) and blocked completely by bumetanide plus Ba(2+). IBDU Na(+)-K(+)-2Cl(-) cotransport activity was assessed by recording intracellular pH during NH4Cl exposure. Bumetanide inhibited initial acidification rates due to NH entry in the presence and absence of CO2/HCO. In contrast, when stimulated by FSK, a 35% increase in Na(+)-K(+)-2Cl(-) cotransport activity occurred without CO2/HCO. These data suggest a cellular model of HCO-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. Bumetanide-sensitive Na(+)-K(+)-2Cl(-) cotransport, Cl(-)/HCO exchange, and Ba(2+)-sensitive K(+) channels are important components of stimulated fluid secretion in intrahepatic bile duct epithelium.
Singh
S K
SK
Liver Center, Department of Internal Medicine, Yale University School of Medicine, New Haven, Connecticut 06520-8019, USA. satish.singh@bmc.org
Mennone
A
A
Gigliozzi
A
A
Fraioli
F
F
Boyer
J L
JL
D000643:Ammonium Chloride / Q000378:metabolism; D000818:Animals; D001464:Barium / Q000494:pharmacology; D001639:Bicarbonates / Q000378:metabolism; D001653:Bile Ducts, Intrahepatic / Q000378:metabolism*; D002034:Bumetanide / Q000494:pharmacology; D002245:Carbon Dioxide / Q000502:physiology; D002352:Carrier Proteins / Q000378:metabolism; D016764:Cell Polarity; D002478:Cells, Cultured; D002712:Chlorides / Q000502:physiology*; D005576:Colforsin / Q000494:pharmacology; D004847:Epithelial Cells / Q000187:drug effects / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D017136:Ion Transport / Q000187:drug effects; D008297:Male; D008954:Models, Biological; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000502:physiology; D028021:Sodium-Potassium-Chloride Symporters
D001639:Bicarbonates; D002352:Carrier Proteins; D002712:Chlorides; D028021:Sodium-Potassium-Chloride Symporters; D000643:Ammonium Chloride; D002034:Bumetanide; D002245:Carbon Dioxide; D005576:Colforsin; D001464:Barium; D012964:Sodium