The onset of liver glycogen synthesis in fasted-refed rats. Effects of streptozocin diabetes and of peripheral insulin replacement.

The mechanism of liver glycogen synthesis after refeeding has been investigated in diabetic rats, diabetic insulin-treated rats, and in control rats fasted for 48 h. The accumulation of liver glycogen was the same in diabetic rats and in control rats after 2 h of feeding, but did not proceed any further in the diabetic group during the next 2 h. Insulin-treated diabetic rats synthesized five times more hepatic glycogen than the control rats after 1 h of refeeding, but the amount accumulated at the end of the refeeding period was the same. Feeding resulted in a transient activation of glycogen synthase in untreated as well as in treated diabetic rats. In control rats, however, glycogen synthase was already partially in the active form before access to food, and the onset of glycogen synthesis occurred without further activation of the enzyme. A transient inactivation of phosphorylase was observed in all groups during the meal, but was very slight in the untreated diabetic rats in which phosphorylase a values were already reduced before the access to food. Peripheral glycemia was markedly increased upon refeeding in treated and untreated diabetic rats, but remained normal in control rats. Peripheral insulinemia was increased by feeding in the control rats and remained low in the diabetic rats and high in the insulin-treated diabetic rats. The results indicate that, in normal controls in contrast to diabetic rats, synthase activation is not a prerequisite for the initiation of glycogen synthesis after a meal; phosphorylase inactivation may be of major importance in normal controls, but also appears to play a role in the diabetic animals.(ABSTRACT TRUNCATED AT 250 WORDS)