Studies on the Activation of Phosphorylase in Skeletal Muscle by Contraction and by Epinephrine

Abstract

Abstract The activation of phosphorylase during muscle contraction and in the presence of epinephrine was studied in isolated frog sartorius and rat diaphragm, and in rat gastrocnemius in vivo. In all three tissues, epinephrine-induced conversion of phosphorylase b to a was accompanied by conversion of phosphorylase b kinase to the activated form (as evidenced by increased pH 6.8 to 8.2 activity ratios), and by increased tissue levels of cyclic adenosine 3',5'-monophosphate. In contrast, when glycogenolysis was facilitated by muscle contraction (as induced by electrical stimulation), phosphorylase b to a conversion took place in all three tissues under conditions in which there was no detectable conversion of phosphorylase b kinase to the activated form and no increase in tissue levels of cyclic 3',5'-AMP. It is concluded that the mechanism of activation of phosphorylase by muscle contraction is basically different from that involving adrenergic amines. Activation of the enzyme during muscle contraction is not accompanied by increased cyclic 3',5'-AMP levels nor conversion of phosphorylase kinase to the activated form. The possibility that calcium ion may regulate phosphorylase kinase activity during muscle contraction is discussed.