Summary
Aggregation of human and rabbit gel filtered platelets (GFP) has been studied in presence of Ca2+, activated factor X (Xa) and different plasma preparations. It was found that factor Xa by itself is not a platelet aggregating agent. However, the platelets aggregated immediately when platelet poor plasma (PPP) was added to a mixture of GFP, Ca2+ and factor Xa. Aggregation also occurred immediately when factor V-deficient plasma was substituted for PPP, but not when factor II-deficient plasma was used. In the absence of factor Xa, aggregation occurred on addition of factor V- or VIII-deficient plasma, but only after some delay. The platelet aggregation experiments and experiments with centrifugations and resuspensions of the platelets, clotting experiments, and gel filtration of platelet free plasma (PEP) led to the following conclusions : Factors II and X are totally removed from the platelets by gel filtration, factor V is closely associated with the platelet surface, and part of the factor VIII-activity in the plasma is eluted together with the GFP without being associated with the platelets. This factor VIII-activity belonged to an extremely large molecule or molecular complex with a Mw in the order of 2 - 5 · 107.
