Summary
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-chloromethylke-tone), 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, αG (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, α-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 (≥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.