Interaction of coagulation factor Xa with human platelets.

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Abstract
When human 125I-labeled Factor Xa is incubated with washed platelets, prothrombin, and Ca2+, a small amount of thrombin is formed which causes the platelet release reaction after a period of time that decreases as the Xa concentration is increased from 0.9 to 19 ng/ml. After a further lag period, the Xa binds reversibly to receptors on the platelet surface and rapid thrombin formation follows (3 units or 1 .mu.g of thrombin formed per min/ng of Xa bound to 108 platelets). When platelets are treated with either thrombin (0.5 units/ml) or calcium ionophore A23187 prior to addition of Xa, binding begins immediately. Thrombin formation occurs at the platelet surface at rates that correlate with the amount of Xa bound. Dibutyryl cyclic AMP inhibits the release reaction, Xa binding, and rate of thrombin generation in parallel. The platelet Xa receptor is distinct from the previously described thrombin receptor and appears to be a protein because treatment of platelets with thrombin at 50 units/ml destroys Xa binding sites. Specific receptors for Xa apparently appear on the platelet surface after the release reaction occurs. The bound Xa catalyzes thrombin formation 1000-fold faster than does Xa added to reactions in which phospholipids are substituted for platelets.