Epinephrine regulation of skeletal muscle glycogen metabolism. Studies utilizing the perfused rat hindlimb preparation

Studies of rat skeletal muscle glycogen metabolism carried out in a perfused hindlimb system indicated that epinephrine activates phosphorylase via the cascade of phosphorylation reactions classically linked to the β-adrenergic receptor/adenylate cyclase system. The β blocker propranolol completely blocked the effects of epinephrine on cAMP, cAMP-dependent protein kinase, phosphorylase, and glucose-6-P, whereas the α blocker phentolamine was totally ineffective. Omission of glucose from the perfusion medium did not modify the effects of epinephrine. Glycogen synthase activity in control perfused and nonperfused muscle was largely glucose-6-P-dependent (-glucose-6-P/+glucose-6-P activity ratios of 0.1 and 0.2, respectively). Epinephrine perfusion caused a small decrease in the enzyme's activity ratio (0.1 to 0.05) and a large increase in its K(a) for glucose-6-P (0.3 to 1.5 mM). This increase in glucose-6-P dependency correlated in time with protein kinase activation and was totally blocked by propranolol and unaffected by phentolamine. Comparison of the kinetics of glycogen synthase in extracts of control and epinephrine-perfused muscle with the kinetics of purified rat skeletal muscle glycogen synthase a phosphorylated to various degrees by cAMP-dependent protein kinase indicated that the enzyme was already substantially phosphorylated in control muscle and that epinephrine treatment caused further phosphorylation of synthase, presumably via camp-dependent protein kinase. These data provide a basis for speculation about in vivo regulation of the enzyme.