Synergism between subthreshold concentrations of thrombin and 12-0-tetradecanoylphorbol 13-acetate in platelet activation

Summary A comparison was made between the time courses and interdependence of platelet aggregation, serotonin release, and cytosolic free Ca 2+ concentration in the same sample of platelets loaded with [ 14 c]-serotonin and Ca 2+ -sensitive photoprotein aequorin. In 100 μg/ml aspirin-treated platelets, neither 0.01 U/ml thrombin nor 50nM TPA, an active phorbol ester, induced significant aggregation, serotonin release, or a rise in the intracellular calcium concentration. However, when these two agents were added together, marked aggregation and release were observed without a change in the cytosolic free Ca 2+ concentration. No correlation was observed between the extent of the synergistic effects and time of preincubation with TPA. Potentiatory effects of protein kinase C on receptor-mediated agonists need to be considered in platelet activation.