Platelet von Willebrand's antigen II: active release by aggregating agents and a marker of platelet release reaction in vivo.

von Willebrand's antigen II (vW AgII), a plasma and platelet antigen immunologically and biochemically distinct from factor-VIII-related antigen (VIIIR:Ag), is decreased in von Willebrand's disease. vW AgII is released from platelets during aggregation and clotting. The release of platelet vW AgII was studied in washed platelets following aggregation by thrombin, collagen, and adenosine diphosphate (ADP). Total platelet vW AgII was 3.39 U/10(9) platelets. Thrombin and collagen yielded release of 85% and 86% of platelet vW AgII, respectively, at the highest concentrations. Release of platelet vW AgII was correlated with the release of 5-hydroxytryptamine (5HT). Release of vW AgII by collagen and thrombin was inhibited by metabolic inhibitors. In addition, collagen release of vW AgII was inhibited by aspirin. In clinical syndromes associated with intravascular platelet destruction, marked elevations of plasma vW AgII were noted. Thus, vW AgII is released by a metabolically active process from platelets during aggregation. In addition, vW AgII appears to be a marker of intravascular platelet destruction.