Activation of protein kinase and glycogen phosphorylase in isolated rat liver cells by glucagon and catecholamines

Abstract

In liver cells isolated from fed female rats, glucagon (290nM) increased adenosine 3':5'-monophosphate (cyclic AMP) content and decreased cyclic AMP binding 30 s after addition of hormones. Both returned to control values after 10 min. Glucagon also stimulated cyclic AMP-independent protein kinase activity at 30 s and decreased protein kinase activity assayed in the presence of 2 muM cyclic AMP at 1 min. Glucagon increased the levels of glycogen phosphorylase a, but there was no change in total glycogen phosphorylase activity. Glucagon increased glycogen phosphorylase a at concentrations considerably less than those required to affect cyclic AMP and protein kinase. The phosphodiesterase inhibitor, 1-methyl-3-isobutyl xanthine, potentiated the action of glucagon on all variables, but did not increase the maximuM activation of glycogen phosphorylase. Epinephrine (1muM) decreased cyclic AMP binding and increased glycogen phosphorylase a after a 1-min incubation with cells. Although 0.1 muM epinephrine stimulated phosphorylase a, a concentration of 10 muM was required to increase protein kinase activity. 1-Methyl-3-isobutyl xanthine (0.1 mM) potentiated the action of epinephrine on cyclic AMP and protein kinase. (-)-Propranolol (10muM) completely abolished the changes in cyclic AMP binding and protein kinase due to epinephrine (1muM) in the presence of 0.1mM 1-methyl-3-isobutyl xanthine, yet inhibited the increase in phosphorylase a by only 14 per cent. Phenylephrine (0.1muM) increased glycogen phosphorylase a, although concentrations as great as 10 muM failed to affect cyclic AMP binding or protein kinase in the absence of phosphodiesterase inhibitor. Isoproterenol (0.1muM) stimulated phosphorylase and decreased cyclic AMP binding, but only a concentration of 10muM increased protein kinase. 1-Methyl-3-isobutyl xanthine potentiated the action of isoproterenol on cyclic AMP binding and protein kinase, and propranolol reduced the augmentation of glucose release and glycogen phosphorylase activity due to isoproterenol. These data indicate that both alpha- and beta-adrenergic agents are capable of stimulating glycogenolysis and glycogen phosphorylase a in isolated rat liver cells. Low concentrations of glucagon and beta-adrenergic agonists stimulate glycogen phosphorylase without any detectable increase in cyclic AMP or protein kinase activity. The effects of alpha-adrenergic agents appear to be completely independent of changes in cyclic AMP protein kinase activity.