On the mechanism of the spermine-exerted inhibition on alpha-thrombin-induced platelet activation

Previous reports have shown that various amines inhibited platelet activati on, but no definitive conclusions on their action mechanism were drawn, We have further investigated the action of spermine on platelet responses evok ed by alpha-thrombin and other agonists, Spermine inhibited in a concentrat ion-dependent manner (1-10 mM), and more efficiently than spermidine and pu trescine, the a-thrombin-induced (1.5 nM) platelet activation. Spermine add ed at a concentration that inhibited completely aggregation only partially affected the thrombin-induced increase in cytosolic Ca2+ concentration, pro tein phosphorylation, and ATP secretion, The polyamine had little effect on the morphology of resting platelets, as measured by electron microscopy, t hrombin hydrolytic activity, and fibrinogen clotting capacity but decreased the thrombin binding to platelets and isolated glycocalicin, Spermine part ially inhibited the aggregation elicited by ADP, vasopressin, platelet-acti vating factor, thrombin receptor-activating peptide, fluoroaluminate, ionom ycin, and dioctanoylglycerol but did not affect the cytosolic Ca2+ increase induced by these agonists. The polyamine bound to both glycocalicin and pl atelets, and it inhibited the fibrinogen binding to stimulated platelets. T he amount of C-14-spermine bound to resting cells decreased in the presence of the glycoprotein GPIb-antibody LJIB1, whereas the polyamine bound to ac tivated platelets, which was higher than that tied to resting cells, was ma rkedly reduced by LJCP8 or decorsin, a GPIIb/IIIa antibody and antagonist-p eptide, respectively, These results indicate that spermine specifically inh ibits the thrombin binding to GPIb of resting platelets and the fibrinogen binding to GPIIb/IIIa (integrin alpha(IIb)beta(3)) of activated platelets, (C) 2000 Elsevier Science Ltd. All rights reserved.