Inhibition of Glycogenolysis and Glycogen Phosphorylase by Insulin and Proinsulin in Rat Hepatocyte Cultures

The inhibitory action of insulin and proinsulin on basal and glucagon-activated glycogenolysis was studied in cultured rat hepatocytes containing [14C]glycogen. Insulin or proinsulin given as sole hormones in the presence of 5 mM glucose decreased basal release of [14C]glucose from [14C]glycogen to 20%. Half-maximal effective concentration of insulin was approximately 0.15 nM and of proinsulin was approximately 5 nM. Inhibition of [14C]lactate release from [14C]glycogen required slightly higher hormone concentrations with a similar difference in potency for insulin and proinsulin. The glucagon-stimulated release of [14C]glucose was completely blocked by insulin or proinsulin with half-maximal effective concentrations of approximately 0.2 and approximately 8 nM, respectively. In contrast, release of [14C]lactate in the presence of glucagon was increased slightly by insulin and proinsulin. Basal and glucagon-activated phosphorylase activity was inhibited by approximately 50% in a dose-dependent manner by both hormones, with differences in potency similar to those for the inhibition of glycogenolysis. These data point to a direct regulatory role of insulin in the control of hepatic glycogen breakdown even when acting as sole hormone. The results do not support the notion of a preferential inhibitory potency of proinsulin on hepatic glycogenolysis.