Regulation of hepatic glycogen phosphorylase and glycogen synthase by calcium and diacylglycerol

Abstract

Incubation of rat hepatocytes with angiotensin II (1 nM) produced a time-dependent accumulation of 1,2-diacylglycerol and inactivation of glycogen synthase with maximum effects at 10 min. The level of diacylglycerol then gradually declined and the activity of glycogen synthase I returned to control values at 30 min. In contrast, angiotensin II caused an increase in cytosolic Ca 2+ and an activation of glycogen phosphorylase which were rapid and transient, reaching maximum values in less than 2 min and then returning to control levels at 15 min. There were excellent correlations between the changes in glycogen synthase I and diacylglycerol levels and between the changes in phosphorylase α and cytosolic Ca 2+ in these time-course studies. However, there was no correlation between the changes in diacylglycerol and phosphorylase α or between the changes in cytosolic Ca 2+ and glycogen synthase I. Norepinephrine also caused a slow increase in diacylglycerol and inactivation of glycogen synthase, and a rapid increase in cytosolic free Ca 2+ and activation of glycogen phosphorylase. Addition of an α 1-adrenergic blocker (prazosin or phentolamine) caused rapid decreases in cytosolic free Ca 2+ and phosphorylase α, but only slowly reversed the inactivation of synthase and accumulation of diacylglycerol. The dose-response curves for norepinephrine and prazosin on glycogen synthase were well correlated with those on diacylglycerol. It is proposed that in liver cells, Ca 2+-mobilizing hormones regulate phosphorylase α through a Ca 2+-dependent mechanism and inactivate glycogen synthase through the generation of diacylglycerol, at least in part. The data provide additional support for the view that protein kinase C may be important in the regulation of glycogen synthase in liver.