Active release of human platelet factor VIII-related antigen by adenosine diphosphate, collagen, and thrombin.

Platelet Factor VIII-related antigen (VIIIR:Ag) represents a significant proportion of the total circulating VIIIR:Ag pool. However, its participation in the events of primary hemostasis has not been shown. We now report that platelet-contained VIIIR:Ag is released from platelets by collagen, ADP and thrombin. The concentrations of these agonists, required for VIIIR:Ag release, are the same or lower than those required for release of serotonin, lysosomal enzymes, or fibrinogen. This release has the features of an energy-dependent secretory response because it is blocked by the metabolic inhibitors, antimycin A and 2-deoxy-D-glucose. The electrophoretic characteristics of the VIIIR:Ag released by collagen and ADP are similar to those of plasma VIIIR:Ag. However, thrombin-released platelet VIIIR:Ag differs from that of plasma in that the less anodal forms are relatively depleted. These differences do not appear to be the result of proteolytic degradation of platelet-derived VIIIR:Ag, but may reflect interactions between specific molecular forms of VIIIR:Ag and the platelet membrane. These studies suggest mechanisms by which platelet-contained VIIIR:Ag may contribute to the primary events of hemostasis.