Cooper R H, Coll K E, Williamson J R
J Biol Chem. 1985 Mar 25;260(6):3281-8.
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.
受体介导的磷脂酰肌醇-4,5-二磷酸（PtdIns(4,5)P2）分解产生两种细胞信号，即肌醇-1,4,5-三磷酸（Ins(1,4,5)P3），它能释放细胞内的钙离子（Ca2+），以及二酰基甘油，它能激活一种依赖钙离子和磷脂的蛋白激酶（蛋白激酶C）。本研究评估了蛋白激酶C在去甲肾上腺素和血管加压素诱导的肝细胞钙离子动员方面的意义。佛波酯（4β-佛波醇-12-肉豆蔻酸酯-13-乙酸酯）能直接激活蛋白激酶C，但对细胞内钙离子外流（用偶氮胂III测量）或钙离子内流（用喹啉-2测量）均无影响，而去甲肾上腺素和血管加压素分别抑制和刺激这两个过程。未发现用佛波酯预处理肝细胞与钙离子载体（离子霉素）介导的对细胞溶质游离钙离子增加和磷酸化酶激活的效应之间存在协同作用的证据。这些发现表明蛋白激酶C并非必然参与肝细胞钙离子通量的调节。用佛波酯（PMA）或1-油酰基-2-乙酰甘油预处理肝细胞完全抑制了去甲肾上腺素升高细胞溶质游离钙离子的效应；半最大抑制效应分别在PMA浓度为1 ng/ml和1-油酰基-2-乙酰甘油浓度为12 μg/ml时出现。相比之下，用PMA预处理对血管加压素诱导的钙离子动员影响较小。这些观察结果表明蛋白激酶C可能参与肝细胞α1受体的“下调”，从而可能对钙离子信号通路产生负面影响。
