Relationship between Platelet Secretion and Prothrombin Cleavage in Native Whole Blood

To determine the relationship between platelet secretion and prothrombin conversion in whole blood, the release of platelet factor 4 and the generation of a X_a-specific cleavage product of prothrombin, fragment 1 + 2, were measured during the coagulation of whole blood. There was a parallel increase in the concentration of the two proteins. Over the first 5 min of incubation, platelet factor 4 concentration increased 6 ng/ml per min, and after 6-7 min, the rate of release increased to 750 ng/ml per min. Over the initial 5-7 min of incubation, fragment 1 + 2 concentration increased 1.5 pmol/ml per min with a subsequent increase of 45 pmol/ml per min. Incubation with 10 μM prostaglandin E₁ or 15 μM prostaglandin I₂ inhibited secretion of platelet factor 4 and delayed the onset of the rapid phase of fragment 1 + 2 generation by 8 min, while stimulation of platelet secretion with 1 μg/ml collagen suspension enhanced production of fragment 1 + 2. The addition of either 10 μM epinephrine or 100 ng/ml collagen suspension to whole blood did not affect either platelet factor 4 release or fragment 1 + 2 generation, although the combination of 3 μM epinephrine and 100 ng/ml collagen suspension enhanced platelet release and prothrombin cleavage.

The relationship between platelet factor 4 release and prothrombin cleavage was also studied in Factor VIII-deficient blood. When 0.001 U/ml factor VIII activity was present, <80 ng/ml platelet factor 4 were released, and no fragment 1 + 2 was generated after 30 min of incubation. The addition of 0.008-0.08 U/ml Factor VIII activity progressively increased platelet factor 4 release and prothrombin cleavage. Platelet factor 4 release was normal at 0.08 U/ml Factor VIII activity, whereas prothrombin cleavage was still delayed. Very little thrombin, the amount generated by the cleavage of 3-5 nM fragment 1 + 2, was needed to induce release of platelet factor 4.

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