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

Divalent cations are known to affect the activity of the cAMP-generating system. By observing the effects of the addition of cobalt (Co2+) and the depletion of calcium (Ca2+), this study tried to determine the relative contribution of Ca2+-dependent mechanism in glucagon-induced glucose output from the isolated perfused rat liver. Co2+(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. Co2+(1–5 mM) did not inhibit125I-labeled glucagon binding to hepatic cell membrane. Phenylephrine- or angiotensin II-induced glucose output was not affected by 1 mM Co2+. Co2+(1 mM) inhibited a glucagon-induced increase in [Ca2+]iin isolated rat hepatocytes but did not inhibit a phenylephrine-induced increase in [Ca2+]i. The removal of Ca2+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 Co2+, the glucose output produced by 1 to 10 nM glucagon was impaired further in the Ca2+-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 Co2+-containing Ca2+-free perfusion. These results suggest that Co2+inhibits the glucagon-responsive adenylyl cyclase system directly, resulting in impaired glucose output. Glucagon increases [Ca2+]ithrough 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 Co2+was the Ca2+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.