Radioimmunoassay of Platelet Factor 4 and β‐Thromboglobulin: Development and Application to Studies of Platelet Release in Relation to Fibrinopeptide A Generation

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Abstract
Platelet and fibrinogen survival and turnover studies [in humans] have shown that platelet activation and fibrin formation may occur to different degrees in different thrombotic disorders. More direct evidence of differential involvement of platelet activation and fibrin formation should be provided by specifically measuring the products of these reactions, i.e., released platelet proteins and fibrinopeptide A. Two platelet proteins, platelet factor 4 (PF4) and .beta.-thromboglobulin (.beta.TG), were isolated and characterized, and sensitive and specific radioimmunoassays were developed to measure them. These assays were employed, along with the radioimmunoassay for fibrinopeptide A (FPA), to study the release of PF4 and .beta.TG in relation to FPA cleavage. PF4 and .beta.TG were released by ADP and collagen with time course and concentration dependence similar to that of [14C]serotonin release. FPA was not cleaved from fibrinogen during ADP or collagen-induced platelet release. Thrombin caused release of PF4 and .beta.TG as well as cleavage of FPA. Cleavage of FPA occurred with concentrations of thrombin about 100 times less than did release of PF4 and .beta.TG, and release of [14C]serotonin required still higher thrombin concentrations. Release of [14C]serotonin and platelet proteins was similar as a function of time. Sodium citrate inhibited platelet release induced by thrombin.