Comparison of thrombin-induced aggregation, ATP secretion, and calcium elevation and the inhibitory effect of MPαC between wild-type and Rac1^−/− platelets. Washed wild-type or Rac1^−/− platelets were pretreated with MPαC or control peptide MCsC and then analyzed for (A) aggregation, (B) ATP release, and (C-F) calcium mobilization induced by thrombin. (C,E) Typical plots of calcium mobilization. (D,F) Quantitative data from 3 experiments (mean ± SEM). **P < .01 (1-way ANOVA).