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.