Decreased PS exposure in GPIb-V-IX–deficient platelets after stimulation by single or dual agonists. Binding of annexin V to GPIbβ^+/+ and GPIbβ^−/− platelets was measured by flow cytometry in resting platelets or after stimulation with (A) thrombin (10nM; n = 9) or CRP (10 μg/mL; n = 3), (B) CRP (50 μg/mL) in combination with thrombin (1nM; n = 3) or PAR4 agonist peptide (AYPGKF, 1mM; n = 3), or the calcium ionophore A23187 (100μM; n = 3). Values represent the mean percentage (± SEM) of annexin V-positive cells. *P < .05 versus WT mice. (C) Prothrombinase activity generated by GPIbβ^+/+ and GPIbβ^−/− platelets after stimulation with CRP (10 μg/mL; n = 3), CRP (50 μg/mL) in combination with thrombin (1nM; n = 3) or the PAR4 agonist AYPGKF peptide (1mM; n = 3), or A23187 (100μM; n = 3). Thrombin generation, expressed as the total amount of thrombin formed after 15 minutes, was measured by the use of a chromogenic substrate after the addition of FXa, FVa, and prothrombin. The thrombin generated by resting platelets was subtracted, and values are the mean ± SEM.