Important Regulatory Role of Activated Platelet-Derived Procoagulant Activity in the Propagation of Thrombi Formed Under Arterial Blood Flow Conditions

Background: The functional links between the activation of platelets and the coagulation system have not been clarified. Methods and Results: Immobilized collagen fibrils were perfused with human blood containing fluoresceinated platelets in the presence of various concentrations of thrombin inhibitor. Coagulant activity around platelet thrombi was detected using a FITC-conjugated antibody against the fibrin monomer complex (F-405). Intra-cytosolic calcium ion concentrations ([Ca2+]i) in individual platelets and the volume of thrombi were detected with an ultrafast confocal laser microscope equipped with a piezo-motor control unit. The volume of platelet thrombi formed after 8 min of blood perfusion in the presence of 10, 25, 50, and 100 μmol/L argatroban was 7.69±0.46 μm3, 6.61±1.96 μm3, 3.63±1.54 μm3, and 1.67±0.75 μm3, respectively. There was a positive correlation between the volume of platelet thrombi and the amount of fibrin monomer complex produced around them. The [Ca2+]i of the platelets forming the thrombi oscillated between a minimum of 92.0±57.4 nmol/L, 120.1±68.1 nmol/L, and a maximum of 217.6±131.5 nmol/L, 367.6±189.1 nmol/L, respectively, in the presence of 100 and 10 μmol/L argatroban. Conclusions: The results suggest a crucial role of coagulant activity in both the generation of fibrin and the growth of platelet thrombi. (Circ J 2009; 73: 540 - 548)