Differential effects of phorbol ester on phenylephrine and vasopressin-induced Ca2+ mobilization in isolated hepatocytes.
Receptor-mediated breakdown of PtdIns(4,5)P2 produces two cellular signals, Ins(1,4,5)P3, which can release intracellular Ca2+, and diacylglycerol, which activates a Ca2+- and phospholipid-dependent protein kinase (protein kinase C). This study assesses the significance of protein kinase C in relation to phenylephrine- and vasopressin-induced Ca2+ mobilization in hepatocytes. Phorbol ester (4 beta-phorbol-12-myristate-13-acetate), which can directly activate protein kinase C, had no effect either on Ca2+ efflux from the cell (measured with arsenazo III) or on Ca2+ influx (measured with Quin-2), processes which are inhibited and stimulated, respectively, by both phenylephrine and vasopressin. No evidence of synergism between phorbol ester pretreatment of hepatocytes and the Ca2+ ionophore (ionomycin)-mediated effects on the increase of cytosolic free Ca2+ and phosphorylase activation could be obtained. These findings suggest that protein kinase C is not obligatorily involved in the regulation of hepatocyte Ca2+ fluxes. Pretreatment of hepatocytes with phorbol ester (PMA) or 1-oleoyl-2-acetylglycerol totally inhibited the effects of phenylephrine in elevating the cytosolic free Ca2+; half-maximal inhibitory effects occurred at PMA and 1-oleoyl-2-acetylglycerol concentrations of 1 ng/ml and 12 micrograms/ml, respectively. In contrast, pretreatment with PMA had a much smaller effect on Ca2+ mobilization induced by vasopressin. These observations suggest that protein kinase C may be involved in "down-regulation" of the alpha 1-receptor in hepatocytes and may thus exert a negative influence on the Ca2+-signalling pathway.
D000634:Aminoquinolines / Q000378:metabolism; D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D004075:Diglycerides / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004357:Drug Synergism; D004987:Ethers / Q000494:pharmacology; D015759:Ionomycin; D008099:Liver / Q000187:drug effects* / Q000378:metabolism; D008297:Male; D010656:Phenylephrine / Q000494:pharmacology*; D010704:Phorbols / Q000494:pharmacology*; D006005:Phosphorylases / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology*; D014667:Vasopressins / Q000494:pharmacology*
D000634:Aminoquinolines; D004075:Diglycerides; D004987:Ethers; D010704:Phorbols; D014667:Vasopressins; D010656:Phenylephrine; D000001:Calcimycin; D015759:Ionomycin; C038237:1-oleoyl-2-acetylglycerol; D006005:Phosphorylases; D013755:Tetradecanoylphorbol Acetate; C035739:Quin2; D002118:Calcium