The effect of aggregation and release on platelet prothrombin-converting activity.
Platelets provide a procoagulant activity for the conversion of prothrombin to thrombin during normal hemostatis. This activity designated as platelet prothrombin-converting activity (PPCA) was monitored as rate of thrombin production in a two-stage assay using gel-filtered bovine platelets, factor Xa, and prothrombin. Expression of PPCA was not associated with ADP-induced release or platelet shape change but was associated with aggregation. Release of the contents of dense bodies, measured by release of 14C-5-hydroxytryptamine, was not required for expression of PPCA during platelet aggregation. During the PPCA assay, 5-hydroxytrypamine was released, but only after onset of thrombin production. Furthermore, the release of 5-hydroxytryptamine was retarded during the assay by the addition of 2 mM theophylline and 100 nM prostaglandin E1 without a comparable reduction in PPCA. In addition, 125I-factor-Xa was bound in greater amounts to platelets (aspirin-treated) after ADP-induced aggregation (without detectable release) than to unactivated control platelets. Finally, the PPCA of the ADP-activated platelets was saturated with respect to factors Xa and Va at less than 1 nM concentrations, indicating that the aggregation induced by ADP leads to the exposure of specific procoagulant sites by some process other than dense body secretion.
D000818:Animals; D001792:Blood Platelets / Q000378:metabolism / Q000502:physiology*; D002417:Cattle; D005170:Factor X / Q000378:metabolism*; D010974:Platelet Aggregation; D011516:Prothrombin / Q000378:metabolism*; D012701:Serotonin / Q000378:metabolism; D013917:Thrombin / Q000378:metabolism
D012701:Serotonin; D011516:Prothrombin; D005170:Factor X; D013917:Thrombin