Regulation of rat liver glycogen synthase. Roles of Ca2+, phosphorylase kinase, and phosphorylase a.

Abstract

MgATP slowed the rate of glycogen synthase activation by glucose in Sephadex filtrates of liver extracts from normal rats.The effect was diminished by ethylene glycol bis(P-aminoethyl ether)-N,N,iV',iV'-tetraacetic acid (EGTA) and restored by Ca2+ in a concentration-dependent manner.In filtrates from gsd/gsd rats, which are deficient in liver phosphorylase kinase, MgATP decreased glycogen synthase activity, but this effect was not altered by EGTA unless exogenous liver phosphorylase kinase was added.The effects of EGTA and Ca2+ were also absent in liver filtrates f r o m normal rats made deficient in phosphorylase b y passage over a phosphorylase antibody-Sepharose affinity column, but were restored by addition of phosphorylase 6. W h e n synthase phosphatase activity was inhibited by the addition of exogenous phosphorylase a or glycogen, no effect of EGTA or Ca2+ on glycogen synthase was seen.Addition of cAMP to liver filtrates from normal and gsd/gsd rats in the presence of MgATP caused large declines in glycogen synthase activity which were abolished b y the heat-stable inhibitor of CAMP-dependent protein kinase.In contrast, addition of EGTA or removal of phosphorylase did not modify the effect of cAMP although it reduced that of MgATP in normal filtrates.It is proposed that Ca2+-dependent control of glycogen synthase in rat liver filtrates can function indirectly through phosphorylase kinase which catalyzes the formation of phosphorylase a which inhibits synthase phosphatase and allows synthase kinase@) to phosphorylate and inactivate glycogen synthase.In contrast, CAMP-dependent control of glycogen synthase appears to involve direct phosphorylation of the enzyme by CAMP-dependent protein kinase with little, if any, contribution f r o m phosphorylase kinase.The proposed indirect mechanism could account, in part at least, f o r liver glycogen synthase inactivation induced in vivo by calcium-dependent agents such as al-adrenergic agonists, vasopressin, and the ionophore A23187.a-Adrenergic agonists, vasopressin, and angiotensin I1 activate glycogen phosphorylase and inactivate glycogen synthase