Effects of insulin, glucagon and dexamethasone on pyruvate kinase in cultured hepatocytes

Abstract

Long-term (24-48 h) and short-term (10-30 min) regulation by hormones of hepatic pyruvate kinase activity was investigated in adult rat hepatocytes cultured under serum-free conditions. In the absence of hormones, pyruvate kinase total activity decreased to 83%, 67% and 39% of the initial level at 24, 48 and 72 h of culture. Insulin (100 nM) maintained total activity significantly above control levels throughout this period. In contrast, glucagon (100 nM) and dexamethasone (100 nM) accelerated the gradual decrease within 24 h (glucagon) or 48 h (dexamethasone) of culture. In these long-term experiments, activity at non-saturating concentrations of phosphoenolpyruvate was decreased by glucagon and dexamethasone but not directly modulated by insulin. However, insulin increased the cellular content of the pyruvate kinase activator fructose-1,6-diphosphate. In short-term experiments on cells cultured under serum- and hormone-free conditions for 48 h, both glucagon and dexamethasone independently caused a rapid, dose-dependent increase of the K0.5 for phosphoenolpyruvate within 10 min, while Vmax was not affected. Insulin inhibited this action of glucagon and dexamethasone and, in their absence, significantly increased substrate affinity for phosphoenolpyruvate within 30 min. Cellular fructose-1,6-diphosphate contents remained unchanged under these conditions. The data identify glucocorticoids and insulin - in addition to glucagon - as short-term regulators of the catalytic properties of pyruvate kinase. All three hormones are effective in the long-term control of total enzyme activity.