Development of Platelet Contractile Force as a Research and Clinical Measure of Platelet Function

Abstract

This article reviews work performed at the Medical College of Virginia of Virginia Commonwealth University during the development of a whole-blood assay of platelet function. The new assay is capable of assessing platelet function during clotting and thus allows measurement of the contribution of platelets to thrombin generation. Because platelets are monitored in the presence of thrombin, the test gages platelets under conditions of maximal activation. Three parameters are simultaneously assessed on one 700 microL sample of citrated whole blood. Platelet contractile force (PCF), the force produced by platelets during clot retraction, is directly measured as a function of time. This parameter is sensitive to platelet number, platelet metabolic status, glycoprotein IIb/IIIa status, and the presence of antithrombin activities. Clot elastic modulus (CEM), also measured as a function of time, is sensitive to fibrinogen concentration, platelet concentration, the rate of thrombin generation, the flexibility of red cells, and the production of force by platelets. The third parameter, the thrombin generation time (TGT) is determined from the PCF kinetics curve. Because PCF is absolutely thrombin dependent (no thrombin-no force), the initial upswing in PCF occurs at the moment of thrombin production. TGT is sensitive to clotting factor deficiencies, clotting factor inhibitors, and the presence of antithrombins, all of which prolong the TGT and are known to be hemophilic states. Treatment of hemophilic states with hemostatic agents shortens the TGT toward normal. TGT has been demonstrated to be shorter and PCF to be increased in coronary artery disease, diabetes mellitus, and several other thrombophilic states. Treatment of thrombophilic states with a variety of heparin and nonheparin anticoagulants prolongs the TGT toward normal. The combination of PCF, CEM, and TGT measured on the same sample may allow rapid assessment of global hemostasis and the response to a variety of procoagulant and anticoagulant medications.

Force field evolution during human blood platelet activation

“…We observe that contractile forces of platelets reach a steady state after 25 min (see Fig. 2), which is strikingly similar to the time reported on clot retraction (Carr and Zeckert, 1991;Carr, 2003;Carr et al, 2007). Therefore the contractile forces measured with TFM may reflect platelet function during primary hemostasis.…”

“…In a final step, the plateletfibrin clot retracts and thus solidifies the clot mass. This retraction is thought to bring the wound edges closer together and to re-establish normal blood flow by pulling the constricting clot mass against the vessel wall (Carr, 2003). Platelets, in particular, contribute to clot retraction by actively contracting and pulling on the fibrin links, a reaction which is believed to be achieved by myosin motors in combination with cytoskeletal actin fibers.…”

“…Traditionally, research on platelet contraction has been closely linked to clinical applications. Thus, a widespread method for studying platelet contractile forces and clot elasticity was developed by Carr et al as a clinical measure for platelet function (Carr, 2003;Carr et al, 2007;Carr and Zeckert, 1991). It is based on the idea that contracting blood samples and platelet-rich plasma samples between a fixed and a freely supported surface change the gap size in a force-dependent manner.…”

Interplay of Platelet Contractility and Elasticity of Fibrin/Erythrocytes in Blood Clot Retraction

“…Cross-linking results in the stabilization of the viscoelastic fibrin network, which has many fibers originating from platelet aggregates (6)(7)(8)(9). The activated platelets are able to generate contractile forces that are propagated through the cross-linked fibrin fibers (10,11), effectively resulting in a reduction of the clot volume (12,13). The volume reduction of the clot, or clot contraction, has been suggested to play a critical role in hemostasis (14), wound healing, and the restoration of blood flow past otherwise obstructive blood clots (15).…”

The Platelet Integrin αIIbβ3 Differentially Interacts with Fibrin Versus Fibrinogen

“…However, individual activated ␣IIb␤3 molecules also attach to fibrin fibers (2) such that aggregated platelets are major components of hemostatic blood clots and thrombi (3). Further, the interaction of ␣IIb␤3 with fibrin is involved in platelet-driven clot contraction (4,5). In vivo, blood clotting is catalyzed by thrombin, which simultaneously induces fibrinogen binding to ␣IIb␤3 and the conversion of fibrinogen to fibrin.…”