Relative Contribution of Ca2+-Dependent Mechanism in Glucagon-Induced Glucose Output from the Liver

Abstract Divalent cations are known to affect the activity of the cAMP-generating system. By observing the effects of the addition of cobalt (Co 2+ ) and the depletion of calcium (Ca 2+ ), this study tried to determine the relative contribution of Ca 2+ -dependent mechanism in glucagon-induced glucose output from the isolated perfused rat liver. Co 2+ (1 mM) completely suppressed glucose and cAMP output induced by 0.1 nM glucagon and partly suppressed those induced by 1 to 10 nM glucagon. Co 2+ (1–5 mM) did not inhibit 125 I-labeled glucagon binding to hepatic cell membrane. Phenylephrine- or angiotensin II-induced glucose output was not affected by 1 mM Co 2+ . Co 2+ (1 mM) inhibited a glucagon-induced increase in [Ca 2+ ] i in isolated rat hepatocytes but did not inhibit a phenylephrine-induced increase in [Ca 2+ ] i . The removal of Ca 2+ from the perfusion medium impaired phenylephrine- or angiotensin II-induced glucose output, but did not impair glucagon-induced glucose output. When glucagon-induced cAMP production was inhibited by Co 2+ , the glucose output produced by 1 to 10 nM glucagon was impaired further in the Ca 2+ -free perfusion. Addition of 0.1 mM IBMX increased the glucose output produced by 1 nM glucagon but did not increase that produced by 10 nM glucagon in the Co 2+ -containing Ca 2+ -free perfusion. These results suggest that Co 2+ inhibits the glucagon-responsive adenylyl cyclase system directly, resulting in impaired glucose output. Glucagon increases [Ca 2+ ] i through a mechanism different from that of phenylephrine. Glucagon (0.01–10 nM)-induced glucose output from the liver is derived mainly through a cAMP-dependent mechanism. Only when glucagon-induced cAMP production was inhibited by Co 2+ was the Ca 2+ dependency observed in high concentrations (≥1 nM) of glucagon-induced glucose output, and it approximated 30% of the glucose output produced by 10 nM glucagon.