Altered Flow Changes Thrombin Generation Rate of Circulating Platelets

Shear stress affects platelet participation in coagulation. Many numerical models have been developed to describe coagulation kinetics. However, most of those models used rate constants determined under static conditions. Little is known about the effects of flow on coagulation rate constants. In the present study, platelets were exposed to constant or pulsatile shear stress/rate, with or without prothrombin, factor Xa, and factor Va. Thrombin generation was measured using a modified prothrombinase assay, and the overall thrombin generation rate was solved using typical Michaelis–Menten kinetics. Platelet surface P-selectin and phosphatidylserine (PS) expression was measured using flow cytometry. The results demonstrated that the concentration of factor Va had a dominant effect on thrombin generation rate under flow. In comparison, the expression of PS was less sensitive to altered flow. The lumped overall rate constant for prothrombin conversion to thrombin was significantly affected by the shear forces that were applied to the coagulation complex. Constant shear stress/rate induced faster thrombin generation compared to pulsatile shear stress/rate, but elevated shear stress/rate did not necessarily enhance thrombin generation. Therefore, the overall thrombin generation rate is dynamic and must be described as a function of shear stress/rate, shear exposure time and the immediate availability of coagulation proteins.