Insulin regulation of pyruvate kinase activity in cultured rat hepatocytes, in the presence of vasopressin, ionophore A23187 or 4 beta-phorbol 12 beta-myristate 13 alpha-acetate

Antagonizing effects of phorbol 12-myristate 13-acetate on hormonally stimulated gluconeogenesis in isolated rat hepatocytes involve activity changes of pyruvate kinase

The tumor-promoting phorbol ester phorbol 12-myristate 13-acetate partially neutralized the stimulatory effects of epinephrine (alpha 1-adrenergic actions), glucagon, and dibutyryl-cAMP on gluconeogenesis in isolated hepatocytes of fasted rats, when lactate or dihydroxyacetone was used as the substrate. By constructing metabolic crossover plots and by comparing rates of lactate production from dihydroxyacetone with K0.5 values of extracted pyruvate kinase for phosphoenolpyruvate, we obtained evidence that phorbol ester actions on hormonally stimulated gluconeogenesis were accompanied by proportionate increases in activity of pyruvate kinase. Although purified pyruvate kinase from rat liver was a substrate for protein kinase C in vitro, phosphorylation was not accompanied by modulation of kinetic parameters. Furthermore, incubation of pyruvate kinase extracted from hormone-treated hepatocytes with protein kinase C revealed no activation of the prephosphorylated enzyme. This and the absence of effects of the phorbol ester on basal rates of gluconeogenesis and lactate production suggest that effects of protein kinase C on pyruvate kinase activity in hepatocytes may result from impairment of steps at the level of hormone-induced signal transduction.

Effects of insulin on inositol phosphate production in cultured rat hepatocytes

Addition of vasopressin (100 nM) to rat hepatocytes prelabelled with [3H]inositol stimulated the production of inositol phosphates in the presence of 20 mM Li+. Preincubation of hepatocytes with insulin (50 nM) or glucagon (10 nM) had no significant effect alone but enhanced the effects of vasopressin after a lag period of at least 1 min. The effects of insulin and glucagon appeared additive in this respect. Insulin also enhanced the norepinephrine-mediated stimulation of inositol phosphate accumulation. The enhancement by insulin of the effects of vasopressin required at least 0.5-5 nM insulin and did not involve changes in [3H]inositol lipid labelling or IP3 phosphatase activity. The effect of insulin appeared insensitive to prior treatment of hepatocytes with pertussis toxin (200 ng/ml for 18-24 h) or cholera toxin (100 ng/ml for 3-4 h). The glucagon enhancement of the effects of vasopressin was not affected by pertussis toxin but was mimicked by cholera toxin. The response of hepatocytes to vasopressin in the absence of Li+ was smaller and more transient. Under these conditions a 5 min prior incubation with insulin inhibited the stimulation by vasopressin of inositol phosphate accumulation. A similar inhibitory effect of prior insulin exposure on the transient activation by vasopressin of exogenous phosphatidylinositol 4,5-bisphosphate breakdown by hepatocyte homogenates was also seen. These data indicate that insulin, although having no effect on basal inositol phosphate accumulation, can either enhance or antagonise the effects of vasopressin in primary rat liver hepatocyte cultures depending on the experimental conditions.