Regulation of membrane chloride currents in rat bile duct epithelial cells

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 Ca²⁺-and cAMP-dependent Cl^- currents were identified. Ca²⁺-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 μM) 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 μM). A separate cAMP-activated Cl^- current showed linear current-voltage relations and no time dependence. Forskolin (10 μM) or cpt-cAMP (500 μM) 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 Ca²⁺- 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.