Platelet phosphatidylserine exposure

Platelets are known to participate in all phases of haemostasis in vivo. Besides the obvious contribution in formation of the platelet plug, they provide the surface necessary for the assembly of the tenase and prothrombinase complexes, a process leading to the formation of thrombin, and ultimately the fibrin clot. The complex assembly involves the translocation of negatively charged phosphatidylserine (PS) to the platelet surface. This translocation is thought to be due to a protein called scramblase, which in the presence of elevated cellular calcium levels arranges a rapid, non-selective bidirectional movement of phospholipids over the membrane in activated platelets (1). The -carboxylated factors II, VII, IX and X all show a high affinity for calcium ions, and the conformational change induced by the calcium binding exposes hydrophobic residues thought to interact with the platelet surface (2, 3). Platelet activation also induces a conformational change in the GPIIb/IIIa complex, making it able to bind vWf and fibrinogen, thereby strengthening the clot and enabling platelet aggregation (4) and clot retraction (5). Studies with GPIIb/IIIa antagonists also implicate that this receptor as important for PS exposure and factor V binding to activated platelets (6). We have earlier reported that activation of platelets in unanticoagulated freshly drawn whole blood (subsequently termed native blood) shortens the coagulation time (7). In this work we have studied the exposure of phosphatidylserine on the surface Platelet phosphatidylserine exposure and procoagulant activity in clotting whole blood – different effects of collagen,TRAP and calcium ionophore A23187