Vasopressin and norepinephrine stimulation of inositol phosphate accumulation in rat hepatocytes are modified differently by protein f1nase 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 microM 4 beta-phorbol 12 beta-myristate 13 alpha-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 microM norepinephrine. Long-term exposure (2-18 h) of hepatocytes to 1 microM PMA actually enhanced the effects of vasopressin and 20 microM norepinephrine. The stimulation by norepinephrine (20 microM) of inositol phosphate accumulation was abolished by the alpha 1-adrenergic antagonist prazosin (1 microM), whereas the beta-adrenergic antagonist propranolol (30 microM) had little effect. Addition of 8Br-cAMP (100 microM) 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.
Pittner
R A
RA
Department of Biochemistry, University of Tennessee, Memphis 88163.
Fain
J N
JN
D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002478:Cells, Cultured; D005934:Glucagon / Q000494:pharmacology; D007295:Inositol Phosphates / Q000378:metabolism*; D008099:Liver / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D009638:Norepinephrine / Q000494:pharmacology*; D011493:Protein Kinase C / Q000378:metabolism; D011494:Protein Kinases / Q000378:metabolism*; D051381:Rats; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D014667:Vasopressins / Q000494:pharmacology*
D007295:Inositol Phosphates; D014667:Vasopressins; D000001:Calcimycin; D005934:Glucagon; D011494:Protein Kinases; D011493:Protein Kinase C; D013755:Tetradecanoylphorbol Acetate; D009638:Norepinephrine