UNSTIMULATED AND THROMBIN-STIMULATED PLATELETS BINDING TO IMMOBILIZEDFIBRINOGEN AND FIBRIN ON POLYSTYRENE SUPPORTS

The binding of unstimulated and thrombin-stimulated platelets was stud ied with immobilized fibrinogen and fibrin on polystyrene. The amount of fibrinogen bound to the polystyrene support was 2 mu g/tube, which represents 2.35 mu g/cm(2) Immobilized fibrin was obtained by adding t hrombin (5 nM) to immobilized fibrinogen. The number of unstimulated ( 111)Inplatelets bound to immobilized fibrinogen and fibrin was similar (3.2 +/- 0.3 +/- 10(6) and 3.1 +/- 0.4 x 10(6) platelets/mu g fibrin(o gen), respectively). The platelet binding steadily increased. In the f irst 2 min, the binding rate was 0.23 x 10(6) platelets mu g fibrinoge n/min. The binding rate then increased rapidly and saturation was reac hed at 10 min. The extent of the adhesion of resting platelets to immo bilized fibrinogen is about one half that of the same platelets stimul ated with thrombin. In thrombin-stimulated In-111-platelets, the bindi ng to immobilized fibrinogen and fibrin is time dependent, and saturat ion is reached at 5 min. The early rate of thrombin-stimulated platele t binding to fibrinogen is about twice that of binding to fibrin (1.25 and 0.74 x 10(6) platelets/mu g fibrin(ogen)/min, respectively). In s aturation conditions, 1 mu g fibrinogen binds 5.7 +/- 0.6 x 10(6) thro mbin-stimulated platelets and 1 mu g fibrin binds 4.6 +/- 0.5 x 10(6) thrombin-stimulated platelets. Our results indicate that the rate of p latelet aggregation is faster than fibrin formation, and the rate of f ibrinogen-platelet binding is faster than that of fibrin-platelet bind ing. Therefore, after thrombin stimulation, the binding of platelets t o fibrin must be secondary to the binding of platelets to fibrinogen.