Platelet-associated coagulation factors: immunological detection and the effect of calcium

Normal platelets were examined by conventional coagulation methods and specific immunological techniques for the presence of platelet-associated coagulation factors. Platelets washed in buffer containing calcium chloride gave different results form those washed in the absence of calcium. Factor XI was not detected in washed platelets by any technique. Factor II and factor X were removed from platelets by washing in calcium-free buffer but not by calcium-containing buffer. Procoagulant factor VIII was readily removed or inactivated by washing in the presence or absence of calcium, 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 calcium in the washing buffer gave higher factor V assays and stronger immunological reactions than when calcium-free buffer was used. The results are consistent with the presence of calcium dependent binding sites in platelets which may have a role in the enhancement of factor V and phospholipid reactions.