Effect of EPCs on collagen-induced platelet aggregation. Whole blood was pre-incubated with different concentrations of EPCs in a 4-channel lumi-aggregometer under shear (1,000 rpm) at 37°C. Platelet aggregation was initiated by adding collagen (3 μg/mL) and then monitored for 5 minutes. Representative traces of whole blood platelet aggregation from A) WT and B) P-sel−/− mice. The mean data ± SEM of 5 independent experiments, summarizing the effects of different EPC concentrations on collagen-induced platelet aggregation in WT and P-sel−/− mice are presented in C and D, respectively.