Characterization of glucagon and catecholamine effects on isolated sheep hepatocytes

The purpose of this study was to characterize the glycogenolytic response to catecholamines and glucagon in isolated sheep hepatocytes. In this species, epinephrine appeared to exert its action on hepatic glycogenolysis by altering the cytosolic concentrations of both adenosine 3',5'-cyclic monophosphate (cAMP) and Ca2+. In contrast to results obtained in rat hepatocytes, glucagon failed to induce a rise in free cytosolic Ca2+ in sheep liver. Experiments on isolated hepatocytes or on liver plasma membranes showed that in sheep, glucagon was more efficient than epinephrine in promoting the production of cAMP. In the presence of glucagon or epinephrine, the activation of the glycogen phosphorylase a always appeared greater in sheep than in rat liver cells, whereas the variations in cellular cAMP were quite limited in sheep. The alpha 1- and beta-agonists (phenylephrine and isoproterenol) were alone as efficient as epinephrine in promoting phosphorylase a activation in sheep hepatocytes. All these results indicate the existence in sheep liver of a glycogen phosphorylase highly responsive to hormones.