Control of rat liver glycogen synthetase and phosphorylase activities by glucose

Abstract

In the perfused rat liver, hyperglycemia causes a rapid inactivation of phosphorylase and a conversion of glycogen synthetase from a glucose-6- P dependent form to a glucose-6- P independent form. This effect is similar in the presence or in the absence of added insulin, but is absent following adrenalectomy and fasting when glycogen synthetase phosphatase activity is not detectable. Under conditions where phosphatase activity is low, glucose increases its activity. The effect of glucose occurs in the absence of detectable alterations in tissue concentration of adenosine 3′,5′-monophosphate, the primary intracellular regulator mediating hormonal control of the activity of these enzymes. Circulating glucose levels also directly modify the effects of glucagon on the activities of these enzymes supporting the possibility that glucose levels may be important in moderating the control of liver glycogen metabolism in vivo .