PLATELET PROCOAGULANT ACTIVITY IS INDEPENDENT OF AGGREGATION

Platelet prothrombinase activity and aggregation were studied in paral lel in the same platelet suspensions with the aim of determining the r elationship between these two responses. Stimulation of platelets with thrombin (0.12 U/ml) plus collagen (20 mu g/ml) led to maximum thromb in generation when the aggregation intensity was still only 30%. Since the rate of thrombin formation then remained constant as aggregation increased up to its maximum intensity, the aggregation processes did n ot appear to result in partial masking of any procoagulant surface. Th is was confirmed by the fact that thrombin generation was the same on aggregated platelets as on activated platelets prevented from aggregat ing by addition of RGDS, Finally, maximum 5-hydroxy-tryptamine release could be obtained in the absence of thrombin generation. Thus platele t procoagulant activity did not appear to be related to aggregation or secretion and thrombin formation on the surface of activated platelet s was in fact two to three times more important in the absence of stir ring than under the conditions of continuous stirring required for agg regation. In conclusion, these results suggest procoagulant activity a nd aggregation to be two independent platelet responses which occur in different physiological situations.