Platelet procoagulant activity is independent of aggregation

Platelet prothrombinase activity and aggregation were studied in parallel in the same platelet suspensions with the aim of determining the relationship between these two responses. Stimulation of platelets with thrombin (0.12 U/ml) plus collagen (20 mu g/ml) led to maximum thrombin generation when the aggregation intensity was still only 30%. Since the rate of thrombin formation then remained constant as 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 not appear to result in partial masking of any procoagulant surface. This was confirmed by the fact that thrombin generation was the same on aggregated platelets as on activated platelets prevented from aggregating by addition of RGDS. Finally, maximum 5-hydroxy-tryptamine release could be obtained in the absence of thrombin generation. Thus platelet procoagulant activity did not appear to be related to aggregation or secretion and thrombin formation on the surface of activated platelets was in fact two to three times more important in the absence of stirring than under the conditions of continuous stirring required for aggregation. In conclusion, these results suggest procoagulant activity and aggregation to be two independent platelet responses which occur in different physiological situations.