Role of fibrinogen binding in thrombin-induced platelet procoagulant activity.

(A) Inhibition of thrombin- or TRAP-induced fibrinogen binding to gel-filtered platelets by antifibrinogen Ab detected by fibrinogen-FITC binding. Gel-filtered platelets were incubated with fibrinogen-FITC (100 μg/mL), GPRP (1.25 μmol/L), and IV.3 (20 μg/mL) and antifibrinogen Ab (100 μg-1 mg/mL). Platelets were activated with 1 U/mL thrombin or with 50 μmol/L TRAP; ▴ indicates 25 000 thrombin-activated platelets/μL; ▵, 25 000 TRAP-activated platelets/μL. (B) The influence of antifibrinogen Ab on thrombin-induced platelet procoagulant activity detected by annexin V-FITC binding; gel-filtered platelets were preincubated with IV.3 (20 μg/mL) for 5 minutes, were incubated with the antifibrinogen Ab (100 μg-1 mg/mL) for 10 minutes and activated with thrombin (1 U/mL); ● indicates 25 000 thrombin-activated platelets/μL. The fluorescence of 5000 platelets per measuring point was determined by flow cytometry. Data are the mean of 3 experiments using platelets from different donors.