Effects of the prior activation of protein kinase C on human platelet activation induced by thrombin.
Effects of the prior activation of protein kinase C (PKC) on the responses induced by thrombin were studied using human blood platelets, in which PKC is abundantly expressed. At a concentration of 25 nM, 12-O-tetradecanoyl-phorbol 13-acetate (TPA) induced little aggregation or release by itself but selectively elicited PKC activation in a time-dependent manner, as monitored by 47-kDa protein phosphorylation. Increases in the intracellular Ca2+ concentration of human platelets caused by thrombin, at any concentration, were markedly inhibited by the prior addition of 25 nM TPA in a time-dependent manner. However, the effects of TPA on platelet aggregation and secretion induced by thrombin varied, depending upon the agonist concentration; the PKC activator markedly enhanced the aggregation and secretion induced by lower concentrations of thrombin but had a tendency to weakly inhibit those induced by higher concentrations of thrombin. TPA enhanced the ionomycin effect upon aggregation and release without obvious effects on the ionophore-induced Ca2+ mobilization, suggesting that PKC potentiates platelet function by increasing intracellular sensitivity to Ca2+. There was a good time-dependent correlation of the TPA effects among the three parameters, namely, the phosphorylation of the 47-kDa protein, inhibition of the [Ca2+]i increase induced by thrombin, and enhanced ionomycin effects in aggregation and release. It is most likely that the final responses of agonist-activated platelets depend upon the balance of two factors induced by PKC; inhibition of Ca2+ signals and enhancement of intracellular sensitivity to Ca2+.
Department of Laboratory Medicine, Yamanashi Medical College, Japan.
Y
Y
Yatomi
M
M
Higashihara
K
K
Kurokawa
Y
Y
Ozaki
S
S
Kume
D001792:Blood Platelets / Q000187:drug effects / Q000201:enzymology*; D004789:Enzyme Activation; D006801:Humans; D015539:Platelet Activation / Q000187:drug effects*; D011493:Protein Kinase C / Q000378:metabolism*; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D013917:Thrombin / Q000494:pharmacology*
D011493:Protein Kinase C; D013917:Thrombin; D013755:Tetradecanoylphorbol Acetate