The MPTP regulatory component, CypD, is required for normal platelet activation. Platelets isolated from CypD^+/+ or CypD^−/− mice were left unstimulated or stimulated with thrombin (0.5 U/mL) plus convulxin (Cvx) (250 ng/mL) for 5 minutes. (A) Platelet Δψ_m was assessed by flow cytometry using the cationic dye TMRM (n = 3). (B-D) Percentages of (B) annexin V^high, (C) fibrinogen^high, and (D) P-selectin^high platelets (n = 4) are shown; ^#P < .05 compared with unstimulated CypD^+/+ platelets; *P < .05 for comparison between CypD^+/+ and CypD^−/− platelets. (E) Fibrinogen (Fbg) and JON/A binding are shown. Platelets were stimulated with the indicated agonist(s) and examined using FITC-labeled antifibrinogen and PE-labeled JON/A. Gates are drawn to highlight activated platelets with high levels of JON/A binding and moderate fibrinogen binding (A1), and activated platelets with low levels of JON/A binding and high-level fibrinogen binding (A2) (n = 4). Error bars represent SE.