HCO ⁻₃ -coupled Na⁺ influx is a major determinant of Na⁺ turnover and Na⁺/K⁺ pump activity in rat hepatocytes

Summary

Recent studies in hepatocytes indicate that Na⁺-coupled HCO ⁻₃ transport contributes importantly, to regulation of intracellular pH and membrane HCO ⁻₃ transport. However, the direction of net coupled Na⁺ and HCO ⁻₃ movement and the effect of HCO ⁻₃ on Na⁺ turnover and Na⁺/K⁺ pump activity are not known. In these studies, the effect of HCO ⁻₃ on Na⁺ influx and turnover were measured in primary rat hepatocyte cultures with²²Na⁺, and [Na⁺] _i was measured in single hepatocytes using the Na⁺-sensitive fluorochrome SBFI. Na⁺/K⁺ pump activity was measured in intact perfused rat liver and hepatocyte monolayers as Na⁺-dependent or ouabain-suppressible⁸⁶Rb uptake, and was measured in single hepatocytes as the effect of transient pump inhibition by removal of extracellular K⁺ on membrane potential difference (PD) and [Na⁺] _i . In hepatocyte monolayers, HCO ⁻₃ increased²²Na⁺ entry and turnover rates by 50–65%, without measurably altering²²Na⁺ pool size or cell volume, and HCO ⁻₃ also increased Na⁺/K⁺ pump activity by 70%. In single cells, exposure to HCO ⁻₃ produced an abrupt and sustained rise in [Na⁺] _i , from ≈8 to 12mm. Na⁺/K⁺ pump activity assessed in single cells by PD excursions during transient K⁺ removal increased ≃2.5-fold in the presence of HCO ⁻₃ , and the rise in [Na⁺] _i produced by inhibition of the Na⁺/K⁺ pump was similarly increased ≃2.5-fold in the presence of HCO ⁻₃ . In intact perfused rat liver, HCO ⁻₃ increased both Na⁺/K⁺ pump activity and O₂ consumption. These findings indicate that, in hepatocytes, net coupled Na⁺ and HCO ⁻₃ movement is inward and represents a major determinant of Na⁺ influx and Na⁺/K⁺ pump activity. About half of hepatic Na⁺/K⁺ pump activity appears dedicated to recycling Na⁺ entering in conjunction with HCO ⁻₃ to maintain [Na⁺] _i within the physiologic range.