Role of ADP in aggregation of VAMP-8^−/− platelets. Wild-type (A) and VAMP-8^−/− (B) platelets were incubated with the indicated concentrations of thrombin and percentage of light transmission was recorded. Panel C represents the mean percentage of aggregation versus thrombin concentration with SD indicated (3 aggregations/condition, n = 8 mice). Thrombin (D: 15 mU/mL; E: 100 mU/mL) was used to stimulate WT (black line) or VAMP-8^−/− platelets (gray line). Aggregation (upper trace) and release of ATP (lower trace) were monitored. (F) Aggregations were measured for VAMP-8^−/− platelets stimulated with 15 mU/mL thrombin and wild-type platelets stimulated with 15 mU/mL thrombin plus 1 U/mL apyrase. (G) Aggregations were measured for VAMP-8^−/− platelets stimulated with 15 mU/mL thrombin alone, 15 mU/mL thrombin plus 2 μM ADP, or 2 μM ADP alone. All aggregation measurements were performed with constant stirring.