Transmembrane electrical potential difference regulates Na⁺/HCO₃^- cotransport and intracellular pH in hepatocytes

We have examined the hypothesis that a regulatory interplay between pH-regulated plasma membrane K⁺ conductance (g_k+) and electrogenic Na⁺/HCO₃^- cotransport contributes importantly to regulation of intracellular pH (pH_i) in hepatocytes. In individual cells, membrane depolarization produced by transient exposure to 50 mM K⁺ caused a reversible increase in pH_i in the presence, but not absence, of HCO₃^-, consistent with voltage-dependent HCO₃^- influx. In the absence of HCO₃^-, intracellular alkalinization and acidification produced by NH₄Cl exposure and withdrawal produced membrane hyperpolarization and depolarization, respectively, as expected for pH_i-induced changes in g_k+. By contrast, in the presence of HCO₃^-, NH₄Cl exposure and withdrawal produced a decrease in apparent buffering capacity and changes in membrane potential difference consistent with compensatory regulation of electrogenic Na⁺/HCO₃^- cotransport. Moreover, the rate of pH_i and potential difference recovery was several-fold greater in the presence as compared with the absence of HCO₃^-. Finally, continuous exposure to 10% CO₂ in the presence of HCO₃^- produced intracellular acidification, and the rate of pH_i recovery from intracellular acidosis was inhibited by Ba²⁺, which blocks pH_i-induced changes in g_K+, and by 4-acetamido-4′-isothiocyanatostilbene-2,2′-disulfonic acid, which inhibits Na⁺/HCO₃^- cotransport. These findings suggest that in hepatocytes, changes in transmembrane electrical potential difference, mediated by pH-sensitive g_K+, play a central role in regulation of pH_i through effects on electrogenic Na⁺/HCO₃^- cotransport.