The inhibitory action of insulin and proinsuiin on basal and glucagon-activated glycogenolysis was studied in cultured rat hepatocytes containing [¹⁴C]glycogen. Insulin or proinsuiin 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 ∼0.15 nM and of proinsuiin was ∼5 nM. Inhibition of [¹⁴C]lactate release from [¹⁴C]glycogen required slightly higher hormone concentrations with a similar difference in potency for insulin and proinsuiin.

The glucagon-stimulated release of [¹⁴C]glucose was completely blocked by insulin or proinsuiin with half-maximal effective concentrations of ∼0.2 and 8 nM, respectively. In contrast, release of [¹⁴C]lactate in the presence of glucagon was increased slightly by insulin and proinsuiin.

Basal and glucagon-activated phosphorylase activity was inhibited by ∼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 proinsuiin on hepatic glycogenolysis.