Effect of 17β-E2 on thrombin-dependent platelet aggregation and integrin α_IIbβ₃ activation. (A) Aggregation traces of gel-filtered platelets treated with 100 nM 17β-E2 (E2), 0.02 U/mL thrombin (Thr), 100 nM 17β-E2 for 30 seconds and then 0.02 U/mL thrombin (E2+Thr), 100 nM 17β-E2, and 10 μM ICI 182 780 for 30 seconds and then 0.02 U/mL thrombin (ICI+E2+Thr), 10 μM PP1 for 15 minutes, 100 nM 17β-E2 for 30 seconds and then 0.02 U/mL thrombin (PP1+E2+Thr). Identical results were obtained from 4 similar experiments performed with platelets from different donors. (B) Binding of FITC-labeled PAC-1, a fibrinogen mimetic antibody that recognizes the active form of integrin α_IIbβ₃ to resting platelets (Bas), and platelets treated as indicated in panel A. Incubation with PAC-1 was for 10 minutes at 37°C before fixation. Data are mean ± SEM of 7 independent experiments performed with platelets from different donors. * indicates that the difference in PAC-1 binding between the samples stimulated with thrombin in the presence or absence of 17β-E2 is statistically significant (P < .005).