Plasma membrane H⁺-HCO₃^- transport in rat hepatocytes: A principal role for Na⁺-coupled HCO₃^- transport

Na⁺-coupled HCO₃^- transport has been demonstrated in the basolateral membrane of hepatocytes, but there is uncertainty regarding its stoichiometry or capacity compared with other mechanisms of H⁺-HCO₃^- transport. After preincubation in medium free of Na⁺, either in the presence or absence of HCO₃^--CO₂, rat hepatocytes in primary culture were reexposed to Na⁺ or HCO₃^--CO₂ alone or in combination. Transporter electrogenicity was assessed by measuring membrane potential difference (PD), and the relative capacities of Na⁺-coupled HCO₃^- transport, Cl^--HCO₃^- exchange, and Na⁺-H⁺ exchange were assessed by measuring the magnitude and rate of change of intracellular pH (pH(i)) using BCECF. In the absence of Na⁺, exposure to HCO₃^- alone had no consistent effect on PD or pH(i). In the absence of HCO₃^-, reexposure to Na⁺ depolarized cells by 3 ± 1 mV and caused an amiloride-inhibitable increase in pH(i) of 0.031 ± 0.02 units/min. In the presence of HCO₃^-, reexposure to Na⁺ hyperpolarized cells by -14 ± 5 mV and increased pH(i) at a rate of 0.133 ± 0.11 units/min; both the hyperpolarization and alkalinization were inhibited by SITS but unaffected by amiloride. These changes in PD indicate that Na⁺-coupled HCO₃^- transport is electrogenic, consistent with coupling of more than one HCO₃^- to each Na⁺. Furthermore, SITS-inhibitable Na⁺-dependent alkalinization exceeds amiloride-inhibitable Na⁺-dependent alkalinization by an order of magnitude, suggesting that the transport capacity of Na⁺-coupled HCO₃^- transport exceeds that of Na⁺-H⁺ exchange. Because Na⁺-H⁺ exchange is inactive at physiological pH(i), Na⁺-coupled HCO₃^- transport may contribute importantly to membrane HCO₃^- flux and regulation of pH(i).