PLATELET-ADHESION ONTO ARTIFICIAL SURFACES - INHIBITION BY BENZAMIDINE, PENTAMIDINE, AND PYRIDOXAL-5-PHOSPHATE AS DEMONSTRATED BY FLOW CYTOMETRIC QUANTIFICATION OF PLATELET-ADHESION TO MICROSPHERES

An appreciable effort is directed toward designing strategies to minim ize platelet interactions with artificial surfaces, because their reac tivity is thought to promote thrombus formation and lead to materials failure. Although platelet glycoprotein Ib/IX (GPIb/IX) and glycoprote in IIb/IIa (GPIIb/IIIa) receptors are thought to mediate adhesion, whe ther GPIIb/IIIa receptors are activated and how this might occur are l argely unknown and are the focus of this article. There are a few ways , other than thrombin generation, that blood contact with artificial s urfaces can lead to GPIIb/IIIa activation. Complement activation can l ead to products capable of activating platelets (C1q, C5b-9), and cont act between platelet CD32 (Fc gamma RII) receptors and immobilized imm unoglobulin G could also activate platelets. In this article the poten tial role of these processes was evaluated by using various inhibitors in a microsphere-based platelet adhesion immunoassay. Polystyrene mic rospheres (10 mu m) were incubated in platelet-rich plasma before flow cytometric analysis of beads for adherent platelets. The data elimina ted occupancy of the Fc gamma RII receptor (by use of IV.3 blocking an tibody), C5b-9 production (by use of sCR1), and the indirect action of factor XIIa on complement components (by use of corn trypsin inhibito r) as playing roles in supporting platelet adhesion. Agents directed a gainst the first complement component (benzamidine, pentamidine, pyrid oxal-5-phosphate) were effective inhibitors of platelet adhesion and w ere also demonstrated to inhibit SC5b-9 and C3d levels on the bead sur face after serum incubations. Because these agents are not highly spec ific, it can not be concluded that Clq is a mediator of adhesion. Thes e agents were also demonstrated to inhibit fluorescein isothiocyanate- fibrinogen binding to activated washed platelets, therefore indicating that fibrinogen receptor expression is a requirement for platelet adh esion.