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.
