The Role of Platelets in the Contact Phase of Blood Coagulation

Summary: Freshly collected intact platelets were shown to have an activity important in the initiation of intrinsic coagulation. They enhance the reactions involving factors XII and XI to form contact activation product in the absence of calcium. This activity, termed contact product forming activity (CPFA), is specifically and rapidly stimulated by‘physiological’concentrations of ADP in platelet suspensions not stirred and not undergoing platelet aggregation. The stimulation of CPFA by ADP was shown to be reversible. A phospholipid mixture (Folch) with PF3 activity was inactive in all test systems for CPFA. Although independent of platelet aggregation, the stimulation of CPFA by ADP was accompanied by a reversible change in platelet shape characteristic of the response of platelets to ADP. The possible significance of CPFA is discussed.