Plasminogen activation (A_{405/490 nm} ) by platelets. Thrombin dose dependence by a one-stage assay (A) and two-stage assay (B), and platelet concentration dependence (C) of plasminogen activation. (A) Platelets (3 × 10⁸/mL) were incubated at 37°C with 1 μmol/L plasminogen, 0.6 mmol/L S2403, and thrombin at 0 (○), 1 U/mL (•), 2.5 U/mL (▴), 5 U/mL (▪), 10 U/mL (▾), or 40 U/mL (♦). (B) Platelets (3 × 10⁸/mL) were incubated with plasminogen and thrombin at 0 (○), 1 U/mL (•), 5 U/mL (▪), and 10 U/mL (▾) for 0.5 hours at 37°C. Hirudin (10 μg/mL) was then added, and 10 minutes later S2403 (0.6 mmol/L) was added and the incubation continued. (C) Platelets at 1 × 10⁸/mL (circles), 2 × 10⁸/mL (triangles), and 3 × 10⁸/mL (squares) were incubated with plasminogen and S2403 in the absence (open symbols) or presence (closed symbols) of 5 U/mL thrombin. Samples were assayed for plasmin activity as described in Materials and Methods. The data shown in (A) are from six experiments, and in (B and C) from three experiments each.