When blood contacts foreign material surfaces, platelets usually adhere and form aggregates on those surfaces, generating mural thrombi. The mechanism of mural thrombogenesis is not completely understood, but one hypothesis states that the local release of certain platelet-active substances from the platelets composing an initial small thrombus stimulates additional platelet recruitment to that thrombus, resulting in growth of the cell aggregate. The purpose of this paper is to investigate the feasibility of this hypothesis. Concentration profiles of adenosine diphosphate (ADP), thromboxane A2 (TxA2), and thrombin were computed in the vicinity of growing model thrombi 10 and 20 micron long. Wall shear rates of 100, 500, and 1,500 s-1 were considered for blood flowing through a thin rectangular slit 200 micron wide coated with collagen, a predominant subendothelial protein. The local concentrations of ADP and TxA2 were marginally large enough to stimulate platelet activation individually, while local thrombin levels can be much greater than required for stimulation. Antithrombin III, a natural thrombin inhibitor, did not significantly reduce the thrombin concentrations, but antithrombin III accelerated by heparin greatly reduced the local thrombin concentrations. The reduced thrombin levels may, however, still be large enough to activate platelets.
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