Role of calcium fluxes in the action of glucagon on glucose metabolism in rat hepatocytes.

The aim of the present study was to assess the role of calcium fluxes in the action of glucagon on glycogenolysis and gluconeogenesis in isolated rat hepatocytes. Calcium influx was blocked by two ways: by use of the compound tetramethrin and by reduction of extracellular calcium to 1 microM. The minimal concentration of tetramethrin that inhibited glucagon-mediated calcium entry was 7.5 x 10(-7) M. In the presence of 7.5 x 10(-7) M tetramethrin, glucagon-induced glycogenolysis was markedly attenuated when glucagon concentration was 10(-9) M or higher. In contrast, tetramethrin had no effect on glucogenolysis evoked by lower concentrations of glucagon. Similarly, tetramethrin greatly reduced gluconeogenesis induced by high concentrations of glucagon without affecting the effect of low concentrations of glucagon. The same results were obtained in the presence of 1 microM extracellular calcium. To abolish glucagon-induced elevation of cytoplasmic free calcium concentration, we heavily loaded quin2 into hepatocytes. In these cells, glycogenolysis evoked by low concentrations of glucagon was completely abolished. Glycogenolysis caused by high concentrations of glucagon was markedly inhibited. These results indicate that glucagon action on hepatic glucose metabolism is mediated by two different mechanisms, which depend on concentrations of glucagon.