Impaired Platelet Procoagulant Mechanisms in Patients with Bleeding Disorders

Activated platelets contribute to the arrest of bleeding by forming aggregates at sites of vascular injury and by providing a surface for assembling enzyme complexes involved in fibrin formation (platelet procoagulant activity; PCA). Impairment in the latter property of platelets has been observed in some disorders of hemostasis. In Scott syndrome, there is a defect in membrane vesiculation and in the surface expression of phosphatidylserine (PS), the phospholipid that is necessary for assembling the factor VIIIa/IXa (tenase) and factor Va/Xa (prothrombinase) complexes involved in thrombin formation. A family with an isolated defect in vesiculation, but normal prothrombinase activity, has also been reported. In the Quebec platelet disorder, overexpression of the fibrinolytic enzyme urokinase-type plasminogen activator results in the degradation of alpha-granule proteins, including factor V, and a specific abnormality in platelet factor V is the basis for the prothrombinase defect in platelet factor V-New York. The impaired prothrombinase activity in patients with delta-storage pool deficiency may be due to a failure to provide sufficient amounts of secreted adenine nucleotides which, when bound to P2 purinergic receptors, are necessary to maintain the intracellular Ca2+ levels that are required for the surface expression of PS. Platelet prothrombinase activity and thrombin potential in patients with Glanzmann thrombasthenia (GPIIb-IIIa deficiency) may be decreased, normal, or increased, depending on the experimental conditions, for reasons that are not currently clear. The most consistent platelet PCA abnormality in the Bernard-Soulier syndrome (GPIb-complex deficiency) is an abnormally short serum prothrombin time, associated with a defect in the process by which an interaction between fibrin, von Willebrand factor, and GPIb promotes PCA.