Thrombin receptor occupancy modulates aggregation efficiency and platelet surface expression of vWF and thrombospondin at low thrombin concentrations

Previous studies evaluating requirements for occupancy of thrombin receptors in normal platelet secretion and aggregation, using the thrombin antagonists hirudin and PPACK (D-Phe-Pro-Arg-chloromethylketone), have suggested that at low thrombin activating concentrations (0.025-0.13 U/ml), occupancy was required only in the first 45-60 s following activation. In our study, we differentiate between thrombin receptor occupancy requirements for surface expression of secreted adhesive proteins, for activation of GPIIb-IIIa receptors, and for aggregation of washed platelets (WP) in laminar shear flow. Platelets activated with 0.05 U/ml thrombin for 10 min to allow maximal secretion (hereafter referred to as "pre-activated platelets"), then sheared, showed a 50-70% decrease in platelet counts after 60 s of shear. Treatment of pre-activated platelets with hirudin or PPACK produced a 65% reduction of capture efficiencies, alphaG (reflecting experimental/theoretical initial rates of aggregation), as well as a 30-40% decrease in the surface expression of von Willebrand factor (vWF) and thrombospondin (TSP). However, alpha-granule membrane P-selectin expression and numbers of activated GPIIb-IIIa receptors were comparable for treated and non-treated platelets. No significant difference in any of the parameters tested was observed when platelets were similarly pre-activated with 0.2 U/ml thrombin, due to treatment with thrombin antagonists. Binding of soluble FITC-vWF (GRGDSP-sensitive) to pre-activated, thrombin antagonist treated platelets, was greatly reduced (> or =80%). Soluble Fg was shown to bind to antagonist-treated pre-activated platelets, but could not significantly enhance platelet aggregation. Although occupancy of thrombin receptors by catalytically active thrombin is required transiently for secretion and activation of platelets, there is a further requirement for thrombin occupancy at low thrombin concentrations, for optimizing initial rates of platelet aggregation, surface expression of vWF and TSP, and activated GPIIb-IIIa ligand recognition.