Evidence that Washed Human Platelets Possess Factor‐X Activator Activity

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Abstract
Summary. Human platelets, washed by repeated albumin density gradient centrifugation or by gel filtration, aggregate upon addition of normal human serum and calcium chloride, due to generation of thrombin from the small amount of residual prothrombin present in normal serum. The rate of thrombin formation is dependent on the factor V provided by the platelets as shown by a marked decrease in formation when platelets from a subject with congenital factor‐V deficiency are used instead of normal platelets. When phospholipid, factor V (bovine serum absorbed with barium sulphate), calcium chloride and fibrinogen were added to serum, no thrombin formation was observed, indicating that serum itself was devoid of factor Xa. In addition, when washed platelets and calcium chloride were mixed with a factor‐X deficient serum or with a highly purified prothrombin preparation devoid of factor X, hardly any thrombin generation occurred, indicating that the washed platelets did not supply factor Xa. Therefore activated factor X is probably generated in the mixture containing washed platelets, normal serum and calcium chloride. Activation of factor X via the intrinsic pathway is unlikely since (a) washed platelets and normal serum are devoid of factor VIII, and (b) washed platelets and/or serum obtained from subjects with factor‐VIII or factor‐IX deficiency or with factor‐VIII antibodies behaved in the same way as normal platelets or serum in the test system. Activation of factor X via the extrinsic pathway is also unlikely since (a) serum and platelets from a congenitally factor‐VII deficient subject behaved normally in the test system, and (b) rapid thrombin generation occurred when serum was replaced by purified prothrombin and purified factor X devoid of factor VII. These findings indicate that washed human platelets possess factor‐X activator activity. Platelet factor 3 is not strongly implicated in the generation of thrombin in this test system since prostaglandin El, which reduces its availability, had no influence on the reaction.