Incubation of human washed platelets with 9,11-epithio-11,12-methano-thromboxane A2 (STA2), a stable analogue of thromboxane A2, caused the activation of protein kinase C and myosin light chain (MLC) kinase to the same extents as those induced by thrombin as judged by measuring the phosphorylation of a 40-kilodalton protein and MLC, respectively. However, STA2 stimulated much less phosphoinositide turnover than thrombin. Furthermore, the doses of STA2 necessary for protein kinase C activation and phosphoinositide turnover were higher than those necessary for MLC kinase activation, although the doses of thrombin necessary for these three reactions were nearly the same. These results suggest that protein kinase C may be activated at the Ca2+ concentrations higher than those required for MLC kinase activation by the action of STA2, presumably due to the inability of this agonist to produce diacylglycerol in an amount enough to increase the affinity of the enzyme for Ca2+.
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