Blood coagulation at the site of microvascular injury: effects of low-dose aspirin

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Abstract
The sequence of coagulant reactions in vivo following vascular injury is poorly characterized. Using quantitative immunoassays, the time courses were evaluated for activation of prothrombin, factor (F)V, FXIII, fibrinogen (Fbg) cleavage, and FVa inactivation in bleeding-time blood collected at 30-second intervals from 12 healthy subjects both before and after aspirin ingestion. Prothrombin decreased at a maximum rate of 14.2 ± 0.6 nM per second to 10% of initial values at the end of bleeding. Significant amounts of α-thrombin B chain appeared rapidly at 90 seconds of bleeding and increased at a maximum rate of 0.224 ± 0.03 nM per second to a peak value of 38 nM. Kinetics of prethrombin 2 generation was almost identical. Prothrombinase concentration reached a peak value of 22 pM at 150 seconds and then decreased to 9 pM at the end of bleeding. Prothrombin fragment 1.2 (F1.2) was produced explosively (0.673 ± 0.05 nM per second), whereas thrombin-antithrombin III (TAT) complexes were generated at a much slower rate (0.11 ± 0.008 nM per second;P = .002). FVa light chain was detectable 30 seconds later than the heavy chain (150 seconds) and was produced at a slightly slower rate (0.027 ± 0.001 nM per second) when compared with the heavy chain (0.032 ± 0.002 nM per second; P = .041). The 30 000 fragment (residues 307-506) of FVa heavy chain produced by activated protein C appeared as early as at 90 seconds and increased with time. Fbg was removed from the blood shed with a high rate of 0.047 ± 0.02 μM/s and became undetectable at approximately 180 seconds of bleeding. The velocity of FXIII activation correlated with thrombin B-chain formation. A 7-day aspirin administration (75 mg/d) resulted in significant reductions in maximum rates of (1) prothrombin removal (by 29%; P = .008); generation of α-thrombin B-chain (by 27.2%; P = .022), and prethrombin 2 (by 26%; P = .014); formation of F1.2 (by 31.4%;P = .009) and TAT (by 30.3%; P = 0.013); (2) release of FVa heavy chain (by 25%; P = .003) and FVa light chain (by 29.6%; P = .007); (3) Fbg depletion from solution (by 30.5%; P = .002); and (4) FXIII activation (by 28.6%; P = .003). Total amounts of the proteins studied, collected at every interval, also significantly decreased following aspirin ingestion. These results indicate that low-dose aspirin impairs thrombin generation and reactions catalyzed by this enzyme at the site of the injury.