Preactivation and deactivation of blood platelets.

Human blood anticoagulated with a serine protease inhibitor was centrifuged with an erythrocyte agglutinating agent in the bottom of the tube. Platelet rich plasma (PRP) obtained in this manner contained platelets with different ability to aggregate with ADP located in the various layers of the tube. Platelets in the midportion of the tube were most extensively aggregated by a standard dose of ADP (1.6 micro M) and mechanical stirring. Platelets in the lowest portion aggregated least, but still showed normal "shape change" when they were stimulated. The uppermost platelets in the tube also showed somewhat reduced aggregation compared with the midportion. These observations could not be explained by differences in platelet concentrations. On a descriptive basis the platelets in the midportion of the tube are designated preactivated and the lowermost platelets deactivated.