Rac1 regulates shear-induced PS exposure independent of caspase-induced PS exposure. (A-C) WT and Rac1^−/− platelets were pretreated with 0.1% DMSO or 20 µM Q-VD-OPh, stimulated with 0.1 or 1 µM ABT-737, and analyzed using flow cytometry to detect PS exposure. (A) Representative histograms of Annexin V binding. (B) Quantification of total PS positive events (mean ± SEM, n = 3). (C) Percentage of platelets that bound Annexin V (Annexin V⁺) (mean ± SEM, n = 3). (D-E) Jurkat cells were treated with 0.1% DMSO, 20 µM Q-VD-OPh, or 100 µM NSC23766, stimulated with 0.1 or 1 µg/mL anti-FAS, and analyzed via flow cytometry for PS exposure. (D) Representative histograms of Annexin V binding. (E) Quantification of Annexin V-positive events (fold increase relative to the baseline; mean ± SEM, n = 7). (F-G) Washed WT and Rac1^−/− mouse platelets treated with 0.1% DMSO, 20 µM Q-VD-OPh, or 40 µM Z-DEVD-FMK (in DMSO) were stimulated with increasing doses of thrombin, subjected to shear, and analyzed for (F) PS exposure and (G) MV release (mean ± SEM, n = 4). The response of thrombin-stimulated WT platelets treated with DMSO was defined as 100% for each thrombin dose. For all data, *, **, and *** represent statistical significance of P < .05, .01, and .001, respectively.