Relationship between secretion of platelet Factor 4 and thrombin generation during in vitro blood clotting

We have studied the effects of both impaired prothrombin activation and direct inhibition of thrombin on the platelet release reaction in clotting blood to determine the role of thrombin in this process. In blood from two patients with congenital Factor V deficiency, prothrombin activation during spontaneous in vitro clotting was delayed and decreased. Secretion of platelet Factor 4 was also delayed and was detected only after thrombin formation was initiated. Addition of a small amount of normal plasma to the patients' blood in vitro corrected the abnormalities in both thrombin formation and the platelet release reaction in parallel fashion. A delay in the onset of secretion of platelet Factor 4 was also observed when thrombin generated in normal blood during spontaneous in vitro clotting was inhibited by either purified hirudin or anti-thrombin Fab. These observations suggest that thrombin is the essential stimulus for platelet secretion during in vitro blood clotting. The effect of inhibitors of the platelet release reaction on prothrombin activation during in vitro blood clotting was also studied. When either prostacyclin or the combination of prostaglandin E(1) and N(6)O(2')-dibutyryl cyclic AMP was added, secretion of platelet Factor 4 was inhibited 85-95%. We were unable to detect any inhibition of initiation of prothrombin activation or inhibition of that part of thrombin generation associated with clotting. These results indicate either that the platelet release reaction may not be required for the initiation of prothrombin activation or only a very limited amount of secretion may be necessary for normal generation of thrombin to occur.