Thrombin‐, Epinephrine‐ and Collagen‐Induced Platelet Aggregation Inhibited by α1‐Acid Glycoprotein

α1‐acid glycoprotein (α1‐acid GP) isolated from human plasma was found to inhibit thrombin‐induced aggregation of washed human platelets (final thrombin concentration 0.05 NIH U/ml), and inhibition was complete with physiological concentrations of the glycoprotein (1.0–1.5 g/l final conc.). The inhibitory effect seemed to occur immediately on thrombin addition, thus being similar to the effect of heparin previously observed. As opposed to heparin, however, α1‐acid GP did not affect spontaneous platelet aggregation. Furthermore, α1‐acid GP (in optimal concentrations) reduced the combined inhibitory effect of heparin and antithrombin III on thrombin‐induced platelet aggregation, thus being consistent with the previous findings using heparin thrombin clotting time. Snyder & Coodley (1976) found α1‐acid GP to inhibit platelet aggregation induced by epinephrine and adenosine diphosphate in platelet‐rich plasma. As we also found α1‐acid GP to inhibit collagen‐induced platelet aggregation, this acid glycoprotein may possibly act as an inhibitor of the release reaction although fairly high concentrations (10 mg/ml final conc.) were needed for complete inhibition.