Time course of release from activated platelets. Human platelets (1.2 × 10⁹/mL) were prepared as described and were stimulated with thrombin (0.3 U/mL, A), convulxin (0.3 μg/mL, B), PAR1-agonist (50μM, C), or PAR4-agonist (500μM, D) for the indicated times. Thrombin stimulation was stopped with hirudin (0.6 U/mL) followed by centrifugation; the rest of the reactions were stopped by centrifugation, and the releasates were recovered. Release from the 3 classes of granules was measured using the marker cargo molecules: [³H]-serotonin for dense granules (■), PF4 for α-granules (▴), and β-hexosaminidase for lysosomes (▾). Percent release was calculated using the equation [(Releasate)/(Pellet + Releasate)] × 100. Each time point was measured in triplicate, and the averages and SDs are indicated.