INCREASED EXPRESSION OF PROCOAGULANT ACTIVITY ON THE SURFACE OF HUMANPLATELETS EXPOSED TO HEAVY-METAL COMPOUNDS

One of the essential roles for platelets in haemostasis is in the pote ntiation of blood clotting due to the contribution of anionic phosphol ipid from the surface of the cells, as an essential cofactor to the pr oteolytic reactions of coagulation (platelet procoagulant activity). O nly a limited number of agonists are known to initiate platelet procoa gulant activity. In this study the rate of thrombin formation on the p latelet surface was observed to increase in a dose-dependent manner up on treatment of washed platelets with heavy-metal compounds. Unlike th e immediate increase observed upon treatment of platelets with calcium ionophore, A23187, the change due to these agents was progressive, ap proaching a maximum after 10 min. The maximum-fold acceleration of the rate of thrombin formation compared with control platelets was calcul ated for HgCl2 (56-fold), AgNO3 (42-fold) phenylmercuriacetate (24-fol d) and thimerosal (14-fold), compared with 70-fold observed for calciu m ionophore. The increase in procoagulant activity due to HgCl2 coinci ded with a large increase in intracellular calcium and phosphorylation of 22 and 45 kDa proteins. It is considered that the mechanism respon sible for the increase in procoagulant activity is exposure of anionic phospholipids. This was detected by a 2-fold increase in the binding of I-125-annexin V upon addition of HgCl2, compared with resting plate lets (3-fold on treatment of platelets with calcium ionophore). In con trast to the generation of activity by A23187 and other known agonists of this reaction, heavy-metal compounds appeared to cause little or n o release of microparticles from the platelet surface. Since HgCl2 did not cause aggregation of platelets or significant release of serotini n, these findings may give further support to the need for exposure an d ligation of glycoprotein IIb:IIIa for vesiculization to occur. Treat ment of platelets with heavy metals may constitute a new approach to i nvestigating the early changes in the cell membrane which lead to incr eased expression of anionic phospholipid.