The inhibition of platelet prothrombinase activity by prostacyclin

Prostacyclin is able to inhibit the development of platelet prothrombinase activity. This inhibition, which also occurs with dibutyryl cAMP, is presumably due to the ability of prostacyclin to prevent the formation of a negatively charged phospholipid surface at the exterior half of the platelet membrane. Generation of this procoagulant surface, as induced by platelet activation with collagen plus thrombin, does not depend on thromboxane A2 formation

On the clot-promoting activity of human platelets in a one-stage prothrombinase assay

The procoagulant activity of activated platelets in a one-stage prothrombinase assay is reevaluated. It is shown that the apparent procoagulant activity of platelets activated by ADP or collagen can be explained by minor cell lysis accompanying platelet activation. The reduction in clotting time observed with thrombin activated platelets can be explained by a combined effect of minor cell lysis and release and activation of factor V from the platelets. Platelets stimulated by ionophore A23187 or by the combined action of collagen plus thrombin show a much shorter clotting time than can be accounted for by minor platelet lysis or release and activation of factor V from the platelets. The results with this clotting assay essentially confirm previous observations [Bevers et al.: Eur. J. Biochem. 122:429–436, 1982] using a spectrophotometric method with highly purified coagulation factors and a chromogenic substrate to measure the rate of thrombin formation with activated platelets