Platelet Aggregation Induced In Vitro by Rabbit Plasma Clot-associated Thrombin, and Its Inhibition by Thrombin Inhibitors

Summary

The activation of rabbit platelets by rabbit plasma clots, and the inhibition of clot-associated thrombin by heparin:antithrombin III, recombinant hirudin (rHV2Lys47) and argatroban, a low molecular weight thrombin inhibitor, was studied.

Plasma clots caused the aggregation of platelets suspended in a plasma-free medium as assessed by single platelet counting, and by scanning electron microscopy (platelet aggregates present on the clot surface). Platelet aggregation, induced by clot-associated thrombin, was inhibited by argatroban with an IC₅₀ of 14 ± 3 nM compared to an IC₅₀ of 12 ± 2 nM when human thrombin in solution titrated to give the same decrease in the platelet count as plasma clots was used. rHV2Lys47 also inhibited aggregation induced by clot-associated thrombin with an IC₅₀ of 1.6 ± 0.4 nM compared to 1.6 ± 0.5 nM with thrombin in solution. Heparin was less active against clot-associated thrombin (IC₅₀ = 69 ± 9 mU/ml) than against thrombin in solution (IC₅₀ = 15 ± 5 mU/ml).

This study shows that plasma clot-bound thrombin activates platelets and that direct-acting thrombin inhibitors such as argatroban and rHV2Lys47 are more effective than heparin:antithrombin III in inhibiting this phenomenon.

• References

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