Regulation of membrane chloride currents in rat bile duct epithelial cells.

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This study examines the conductive properties of the plasma membrane of cells isolated from the intrahepatic portion of bile ducts. Membrane Cl- conductance was measured in single cells using whole-cell patch clamp recording techniques and in cells in short-term culture using 36Cl and 125I efflux. Separate Ca(2+)- and cAMP-dependent Cl- currents were identified. Ca(2+)-dependent Cl- currents showed outward rectification of the current-voltage relation, time-dependent activation at depolarizing potentials, and reversal near the equilibrium potential for Cl-. Ionomycin (2 microM) increased this current from 357 +/- 72 pA to 1,192 +/- 414 pA (at +80 mV) in 5:7 cells, and stimulated efflux of 125I > 36Cl in 15:15 studies. Ionomycin-stimulated efflux was inhibited by the Cl- channel blocker 4,4'-diisothiocyano-2,2'-stilbene disulfonic acid (DIDS) (150 microM). A separate cAMP-activated Cl- current showed linear current-voltage relations and no time dependence. Forskolin (10 microM) or cpt-cAMP (500 microM) increased this current from 189 +/- 50 pA to 784 +/- 196 pA (at +80 mV) in 11:16 cells, and stimulated efflux of 36Cl > 125I in 16:16 studies. cAMP-stimulated efflux was unaffected by DIDS. Because the cAMP-stimulated Cl- conductance resembles that associated with cystic fibrosis transmembrane conductance regulator (CFTR), a putative Cl- channel protein, the presence of CFTR in rat liver was examined by immunoblot analyses. CFTR was detected as a 150-165-kD protein in specimens with increased numbers of duct cells. Immunoperoxidase staining confirmed localization of CFTR to bile duct cells but not hepatocytes. These findings suggest that Ca(2+)- and cAMP-regulated Cl- channels may participate in control of fluid and electrolyte secretion by intrahepatic bile duct epithelial cells, and that the cAMP-regulated conductance is associated with endogenous expression of CFTR. Abnormal ductular secretion may contribute to the pathogenesis of cholestatic liver disease in cystic fibrosis.

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