Evidence indicating that the glucagon‐induced increase in cytoplasmic free Ca2+ concentration in hepatocytes is mediated by an increase in cyclic AMP concentration

The mechanism whereby glucagon causes an increase in the concentration of cytoplasmic free Ca2+, [Ca2+]c, in isolated hepatocytes has been investigated. There have been proposals of cyclic-AMP-dependent and cyclic-AMP-independent mechanisms. In this work, the inactivation of pyruvate kinase was used as an indicator of increases in the activity of cyclic-AMP-dependent protein kinase, A-kinase. [Ca2+]c was measured using the fluorescent probe indo-1. The decrease in activity of pyruvate kinase caused by an increase in [Ca2+]c alone, i.e. mediated by mechanisms not involving cyclic AMP and exemplified by the effect of vasopressin, was of minimal significance under the conditions of the enzyme assay. Studies of the effects of a wide range of glucagon concentrations indicate that any increase in [Ca2+]c caused by glucagon was always associated with a decrease in pyruvate kinase activity. A similar relationship was obtained if glucagon-receptor occupancy was circumvented by using the 8-bromo-derivative of cyclic AMP to activate the A-kinase. It was also found that the cyclic AMP phosphodiesterase inhibitor isobutylmethylxanthine could potentiate the ability of glucagon to increase [Ca2+]c: no such potentiation was observed when vasopressin was used to raise [Ca2+]c. Together these data indicate that an increase in cyclic AMP concentration, sufficiently great to activate A-kinase, is a mechanism that mediates the glucagon-induced increase in [Ca2+]c.