Altered mechanism of glucagon-mediated hepatic glycogenolysis during long-term starvation in the rat

Abstract

The effect of long-term starvation on glucagon-mediated hepatic glycogenolysis was investigated in the rat in vivo. Following glucagon (50 microgram/kg i.v.) fed rats showed rapid phosphorylase activation but no change in synthase-I activities. In contrast, rats fasted 72 hr (long-term fasting) showed rapid synthase inactivation but no significant phosphorylase activation. Rats fasted 24 hr (short-term fasting) demonstrated coordinated inactivation of synthase and activation of phosphorylase. Hepatic cyclic AMP responses were greater in fasted rats. Hepatic glycogen concentrations in rats fasted 72 hr were approximately 30% of fed levels. After glucagon, comparable decrements in hepatic glycogen and increments in plasma glucose concentrations were seen in fed and 72-hr groups. The diminished responsiveness of the hepatic phosphorylase system in rats fasted 72 hr was not attributable to altered cyclic AMP-dependent protein kinase or phosphorylase kinase activities. However, the diminished responsiveness could be ascribed to diminished total phosphorylase with nearly complete activation in the basal state. In fed and fasted rats, synthase decrements after glucagon correlated closely with basal levels of synthase-I. Thus, it is proposed that the enzymatic mechanism of glucagon-mediated hepatic glycogenolysis differs in fed and fasted rats. It is also proposed that partial hepatic glycogen reaccumulation during long-term fasting could be physiologically important for glucose homeostasis.