PLASMA-MEMBRANE H+-HCO-3 TRANSPORT IN RAT HEPATOCYTES - A PRINCIPAL ROLE FOR NA+-COUPLED HCO-3 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).