Potentiation of the thrombin induced platelet release reaction by fibrin

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Incubation of clots formed of human washed platelets, fibrinogen and Bothrops marajoensis fibrinogen clotting enzyme (BM enzyme) for 30 min. resulted in the loss of 5.3% of 14 [C] serotonin radioactivity and 10.1% of PF 4 antigen to the suspending fluid. These clots did not retract. Incubation of platelet suspension (7 × 10 8/ml) with saline or with 0.025 units of thrombin per ml during the same period of time resulted in a loss of 2.0% 14[C] serotonin radioactivity and 8.5% PF 4 antigen or 2.0% 14[C] serotonin radioactivity and 9.2% PF 4 antigen, respectively. If fibrinogen was present in the platelet suspension incubated with 0.025 units of thrombin per ml it did not clot during 30 min. and it did not modify loss of platelet constituents. Incubation of clots formed of washed platelets, purified fibrinogen, 0.025 units of thrombin per ml and BM enzyme during 30 min. resulted in clot retraction and release of 15.3% 14[C] serotonin radioactivity and 46.0% PF 4 antigen. This experiment indicated that fibrin potentiates platelet release reaction induced by thrombin. Similar but smaller potentiation was observed during 3 min. incubation of platelets in an aggregometer. Thrombin also enhanced light transmission increase of platelet suspensions caused by polymerizing fibrin. In conclusion, trace amounts of thrombin used in these studies did not induce platelet aggregation or platelet release reaction. Formation of polymerizing fibrin by BM enzyme did not cause platelet release reaction. When the platelets were exposed to trace amounts of thrombin in the presence of polymerizing fibrin the release reaction occurred.