VASOPRESSIN AND NOREPINEPHRINE STIMULATION OF INOSITOL PHOSPHATE ACCUMULATION IN RAT HEPATOCYTES ARE MODIFIED DIFFERENTLY BY PROTEIN KINASE-C AND PROTEIN KINASE-A

Rat hepatocytes were maintained in primary monolayer culture for 24 h in the presence of serum. Treatment of hepatocytes with 1 μM 4β-phorbol 12β-myristate 13α-acetate (PMA) for 5-15 min increased membrane-associated protein kinase C activity and concomitantly decreased soluble activity. Membrane protein kinase C activity returned to basal values within 1 h then decreased by more than 50% within 2 h. Prolonged (2-18 h) incubation with PMA did not further decrease protein kinase C activity. Pretreatment of hepatocytes with PMA for 5-15 min had little effect on the subsequent actions of 100 nM vasopressin but abolished the stimulation of inositol phosphate accumulation by 3 nM vasopressin and 20 μM norepinephrine. Long-term exposure (2-18 h) of hepatocytes to 1 μM PMA actually enhanced the effects of vasopressin and 20 μM norepinephrine. The stimulation by norepinephrine (20 μM) of inositol phosphate accumulation was abolished by the α1-adrenergic antagonist prazosin (1 μM), whereas the β-adrenergic antagonist propranolol (30 μM) had little effect. Addition of 8Br-cAMP (100 μM) or glucagon (10 nM) for 5 min or 8 h had no significant effect alone, but enhanced the subsequent vasopressin stimulation of inositol phosphate accumulation. There was no effect of 8Br-cAMP or glucagon on norepinephrine stimulation of phosphoinositide breakdown. These data indicate that the stimulation of phospholipase C activity in rat hepatocytes by 3 nM vasopressin is enhanced by cyclic AMP-dependent kinase but inhibited by protein kinase C. In contrast, down regulation of protein kinase C markedly enhanced the maximal phosphoinositide response due to both vasopressin and norepinephrine. © 1990.