Inhibition of Thrombin-, Epinephrine- and Collagen-Induced Platelet Aggregation 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 (0.05 NIH U/ml final conc.), and inhibition was complete with physiological concentrations of α₁-acid GP (1.0-1.5 g/1 final conc.). The inhibitory effect seemed to occur immediately on thrombin addition, thus 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 cone.) reduced the combined inhibitory effect of heparin and antithrombin III on thrombin-induced platelet aggregation, thus consistent with the previous findings using heparin thrombin clotting time.

Snyder and 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, α₁-acid GP may possibly act as an inhibitor of the release reaction though fairly high concentrations (10 mg/ml final cone.) was needed for complete inhibition.