In vitro studies on the effect of activated protein C on platelet activation and thrombin generation

The effect of activated protein C (APC) on agonist-induced platelet activation and on thrombin generation after intrinsic (IA) and extrinsic (EA) activation of the coagulation system was studied by flow cytometry and by measuring levels of prothrombin fragment F1+2. In platelet activation studies blood drawn from healthy volunteers was anticoagulated with 10 micrograms/ml APC and incubated at 37 degrees C either with saline, recombinant tissue factor (r-TF), arachidonic acid (AA), ADP or collagen. At definite times aliquots were taken and processed for flow studies. Platelet activation was measured using fluorescent monoclonal antibodies to platelet surface receptors GPIIIa (CD-61) and P-selectin (CD-62). Flow cytometric analysis showed platelet activation after all agonists used. APC did not influence AA-, ADP- and collagen-induced platelet activation but completely inhibited activation of platelets induced by r-TF. The effect of APC on r-TF-mediated platelet activation was concentration-dependent in the range of 0.5 to 20 micrograms/ml showing an increase in CD-62 expression at lower concentrations. In citrated and APC-anticoagulated blood the generation of thrombin was studied after IA and EA. At 10 and 20 micrograms/ml APC effectively prevented blood clotting which rapidly occurred especially after EA. The amount of thrombin generated via the extrinsic pathway was reduced by APC whereas after IA F1+2 levels measured in the presence of APC were still strongly increased. These results indicate that small amounts of thrombin generated by r-TF are sufficient to activate platelets as well as blood coagulation. APC exerts strong concentration-dependent anticoagulant actions and effectively prevents activation of platelets.