The role of thrombin in ADP-induced platelet aggregation and release: a critical evaluation.

The role of thrombin in ADP-induced aggregation and release in vitro was critically examined. The addition of heparin or hirudin to citrated platelet rich plasma did not prevent aggregation or release. The addition of citrate to heparinized plasma restored secondary aggregation and release. Hirudin did not prevent irreversible aggregation. These results are incompatible with the hypothesis that thrombin is a primary and necessary mediator of platelet aggregation (Ardlie & Han, 1974; Han & Ardlie, 1974a, b, c). This hypothesis is based in part on the assumption that EDTA enhances the elution of clotting factors from platelets; we found no enhanced elution of factors II, V, X, VIII, IX or XI when platelets were washed in EDTA.