Two distinct pathways regulate phosphatidylserine exposure and platelet procoagulant activity. Washed Bak/Bax-deficient mouse platelets (A,B) or human platelets (C-E; 3.0 × 10⁸/mL) were resuspended in Tyrode buffer in the presence of BSA (5 mg/mL), then incubated with vehicle (DMSO), ABT-737 (737, 1 μM; 90-180 minutes), calcium ionophore A23187 (A23187; 1 μM; 20 minutes), or CRP (10 μg/mL) and thrombin (1 U/mL; CRP & Thr; 20 minutes). In some experiments, platelets were preincubated with Q-VD-Oph (QVD; 50 μM) or calpeptin (CP; 100 μg/mL) before treatment with ABT-737/agonist. (A,E) The level of phosphatidylserine exposure (PS) in Bak/Bax-deficient mouse platelets (A) and human platelets (E), as measured by Alexa 488–conjugated annexin V binding, quantified by flow cytometry as described in “Methods.” Histograms depict the means ± SEM (n = 3). (B-D) The ability of platelets to promote thrombin generation in Bak/Bax-deficient mouse platelets (B) or human platelets (C,D) was assessed as described in Figure 1. Line graphs are taken from 1 representative of 3 independent experiments. The histogram (D) represents the means ± SEM (n = 3; ^nsP > .05; ***P < .001) of 3 independent experiments.