Characterization of high-conductance anion channels in rat bile duct epithelial cells

We have utilized patch clamp recording techniques to identify a high- conductance anion channel in the plasma membrane of rat bile duct epithelial cells. Cells were isolated from the intrahepatic bile duct 2-6 wk after bile duct ligation. Channels were present in 27% (28/102) of excised patches, and, with 150 mM Cl^- in bath and pipette solutions, the slope conductance of the fully open level was ~364 ± 18 pS (n = 8) with current reversal = 0 ± 1 mV. Channel characteristics were not affected by substitution of K⁺ for Na⁺ in the pipette solution; but substitution of HCO₃/^-, gluconate, or increased NaCl caused a shift in the reversal potential toward the new equilibrium potential for Cl^-. The permeability ratios were P(HCO₃/^- )/P(Cl^-) = 0.51 ± 0.03 (n = 5), P(gluconate)/P(Cl^-) = 0.12 ± 0.04 (n = 7), and P(Na⁺)/P(Cl^-) = 0.11 ± 0.02 (n = 3). Current transitions to a subconductance level at 72% of the fully open level were present in most studies. Channel open probability was greatest near 0 mV and decreased rapidly outside of -20 to +20 mV because of voltage-dependent channel closure. The time course for current relaxation of summed single channel currents could be described by a single exponential with more rapid channel closure as the magnitude of the voltage step away from 0 mV increased. In the cell-attached configuration, the channel was rarely open (4/35, 11%) but opening could be induced by negative pipette pressure (5/14, 35%). Possible physiological roles for this channel are discussed.