Taurocholate depolarizes rat hepatocytes in primary culture by increasing cell membrane Na⁺ conductance

Abstract

Rat hepatocytes in primary culture were impaled with conventional microelectrodes. Addition of 5–100 μmol/l taurocholate led to a slowly developing depolarization that was maximal at 50 μmol/l (10.5±1.5 mV, n=15) and not reversible. The effect was Na⁺ dependent and decreased in cells preincubated with 1 μmol/l taurocholate. Increasing external K⁺ tenfold depolarized the cells by 12.3±2.3 mV under control conditions and by 6.3±1.2 mV with 50 μmol/l taurocholate present (n=7). Depolarization by 1 mmol/l Ba²⁺ was 7.6±0.8 mV and 6.0±0.7 mV (n=9) before and after addition of taurocholate, respectively. Cable analysis and Na⁺ substitution experiments reveal that this apparent decrease in K⁺ conductance reflects an actual increase in Na⁺ conductance: in the presence of taurocholate, specific cell membrane resistance decreased from 2.8 to 2.3 kΩ · cm² · Na⁺ substitution by 95% depolarized cell membranes by 8.9±2.9 mV (n=9), probably due to indirect effects on K⁺ conductance via changes in cell pH. With taurocholate present, the same manoeuvre changed membrane voltages by −0.8±2.6 mV. When Na⁺ concentration was restored to 100% from solutions containing 5% Na⁺, cells hyperpolarized by 3.5±3.6 mV (n=7) under control conditions and depolarized by 4.4±2.9 mV in the presence of taurocholate, respectively. In Cl⁻ substitution experiments, there was no evidence for changes in Cl⁻ conductance by taurocholate. These results show that taurocholate-induced membrane depolarization is due to an increase in Na⁺ conductance probably via uptake of the bile acid.