The activity of glycogen synthase phosphatase limits hepatic glycogen deposition in the adrenalectomized starved rat

Hepatocytes from adrenalectomized 48 h-starved rats responded to increasing glucose concentrations with a progressively more complete inactivation of phosphorylase. Yet no activation of glycogen synthase occurred, even in a K+-rich medium. Protein phosphatase activities in crude liver preparations were assayed with purified substrates. Adrenalectomy plus starvation decreased synthase phosphatase activity by about 90%, but hardly affected phosphorylase phosphatase activity. Synthase b present in liver extracts from adrenalectomized starved rats was rapidly and completely converted into the a form on addition of liver extract from a normal fed rat. Glycogen synthesis can be slowly re-induced by administration of either glucose or cortisol to the deficient rats. In these conditions there was a close correspondence between the initial recovery of synthase phosphatase activity and the amount of synthase a present in the liver. The latter parameter was strictly correlated with the measured rate of glycogen synthesis in vivo. The decreased activity of synthase phosphatase emerges thus as the single factor that limits hepatic glycogen deposition in the adrenalectomized starved rat.