EphB2 signaling regulates initial platelet activation. CRP-XL (0.5 µg/mL [A] or 1 µg/mL [B]) and thrombin-induced fibrinogen binding (0.1 U/mL [C] or 0.2 U/mL [D]) were measured in control and EphB2^LacZ mouse whole blood by flow cytometry. Similarly, CRP-XL (0.5 µg/mL [E] or 1 µg/mL [F]) and thrombin (0.1 U/ml [G] or 0.2 U/ml [H])-induced P-selectin exposure were measured in control and EphB2^LacZ mouse whole blood by flow cytometry. ATP release was measured as a marker for dense granule secretion in washed platelets upon stimulation with 1 µg/mL CRP-XL (I-J). The level of fibrinogen binding or P-selectin exposure or ATP release (at 5 minutes) obtained in control was taken as 100% to calculate the percentage of reduction. Data represent mean ± SD (n = 4) (Student t test: *P ≤ .05 and **P ≤ .01). (K) Rap1b-GTP was precipitated from control and EphB2^LacZ resting and stimulated platelets with CRP-XL (0.5 µg/mL) and analyzed by immunoblotting using a Rap1b antibody. The blot is representative of 3 separate experiments. ATP, adenosine triphosphate; GTP, guanosine triphosphate.