Procoagulant activity on platelets adhered to collagen or plasma clot

In a new 2-stage assay of platelet procoagulant activity (PCA), we first su bjected gel-filtered platelets to adhesion on collagen (as a model of prima ry hemostasis) or plasma clots (as a model of preformed thrombus) for 30 mi nutes, and then the adherent platelets were supplemented with pooled, repti lase-treated, diluted plasma. Defibrinated plasma provided coagulation fact ors for assembly on platelet membranes without uncontrolled binding of thro mbin to fibrin(ogen). Platelet adhesion to both surfaces showed modest indi vidual variation, which increased at platelet densities that allowed aggreg ation. However, adhesion-induced PCA varied individually and surface-indepe ndently >3-fold, suggesting a uniform platelet procoagulant mechanism. Perm anently adhered platelets showed markedly enhanced PCA when compared with t he platelet pool in suspension, even after strong activation. The rate of t hrombin generation induced by clot-adherent platelets was markedly faster t han on collagen-adherent platelets during the initial phase of coagulation, whereas collagen-induced PCA proceeded slowly, strongly promoted by tissue thromboplastin. Therefore at 10 minutes, after adjustment for adhered plat elets, collagen supported soluble thrombin formation as much as 5 times tha t of the thrombin-retaining clots. Activation of platelets by their firm ad hesion was accompanied by formation of microparticles, representing about o ne third of the total soluble PCA. Collagen-adhered platelets provide solub le thrombin and microparticles, whereas the preformed clot serves to locali ze and accelerate hemostasis at the injury site, with the contribution of r etained thrombin and microparticles.