Platelet-Associated Coagulation Factors: Immunological Detection and the Effect of Calcium

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Abstract
Normal [human] platelets were examined by conventional coagulation methods and specific immunological techniques for the presence of platelet-associated coagulation factors. Platelets washed in buffer containing CaCl2 gave different results from those washed in the absence of Ca. Factor XI was not detected in washed platelets by any technique. Factor II and factor X were removed from platelets by washing in Ca-free buffer but not by Ca-containing buffer. Procoagulant factor VIII was readily removed or inactivated by washing in the presence or absence of Ca, but factor VIII related antigen was consistently demonstrated after multiple washes in either buffer. Factor V activity and antigen was detected in platelet suspensions after multiple washes although the presence of Ca in the washing buffer gave higher factor V assays and stronger immunological reactions than when Ca-free buffer was used. There apparently are Ca-dependent binding sites in platelets which may have a role in the enhancement of factor V and phospholipid reactions.