Differential effects of anticoagulants on the activation of platelets ex vivo

Because activation of platelets and of the coagulation system are interdependent mediators of thrombosis, platelet activation was characterized in whole blood in the presence of anticoagulants used to assess platelet function in vitro or as treatment for patients with occlusive arterial disease. Blood was anticoagulated alone or in combination with citrate, ethylenediaminetetraacetatic acid, corn trypsin inhibitor (CTI, an inhibitor of activated factor XII), heparin, enoxaparin, recombinant tick anticoagulant peptide (rTAP), or recombinant hirudin. Platelet activation in response to adenosine diphosphate (ADP) or collagen was detected by assay of P-selectin on the platelet surface delineated by flow cytometry. Although minimal activation was seen without ADP, the fraction of platelets expressing P-selectin in response to ADP was greatest in blood anticoagulated with citrate compared with CTI and all other anticoagulants. ADP-induced platelet activation was greater in blood anticoagulated with heparin compared with an equipotent anti-Xa concentration of enoxaparin. More variable results were seen with collagen, but platelet activation in the presence of citrate was greater than that with CTI. Interpretation of assays of inhibition of platelet activation by potentially therapeutic agents in vitro requires consideration of the effects of anticoagulants used. In addition, anticoagulants other than standard heparin may potentiate efficacy of antiplatelet drugs.