DIFFERENT SITES FOR FIBRINOGEN AND FIBRIN RECEPTORS ON PLATELETS

Abstract

Platelet stickiness and aggregation depend on availability of surface membrane glycoprotein Ilb-IIIa complex to bind fibrinogen. The development of isometric tension in platelet-rich clots is a manifestation of fibrin binding to the cells as well as platelet contractile activity. The possibility that fibrinogen and fibrin may bind to different portions of the GPIIb-IIIa complex has apparently not been considered. In order to determine whether the receptors for the fibrinogen and fibrin on the GPIIb-IIIa complex are identical, we investigated the effect of various monoclonal antibodies to the Ilb-IIIa complex and the tetrapeptide Arg-Gly-Asp-Ser (RGDS), a recognition site on fibrinogen for IIb-IIIa, on the development of isometric tension and ultrastructure of platelet-fibrin clots. Monoclonal antibodies A2A6 and 7E3 decreased the maximal tension, as well as the rate of tension development. Platelets and fibrin were oriented longitudinally in antibody treated clots, but the concentrations of fibrin and platelet aggregates determined by morphometry were significantly reduced. T10 and 10E5 increased tension, while AP2 and PAC1 had no substantial effect. Increasing concentrations of RGDS from 62.5 pM to 500 pM resulted in greater maximal tension and rate of tension development, reaching a five-fold increase when 500 pM RGDS was used. RGDS did not affect the Mg-stimulated platelet actomyosin ATPase activity. Morphometry revealed increased concentrations of oriented fibrin and platelet aggregates in RGDS-treated clots. Results of this study confirm the different topography of the epitopes on the Ilb-IIIa complex and provide evidence for different receptor sites for fibrinogen and fibrin on the IIb-IIIa complex