A 50 PERCENT REDUCTION OF PLATELET SURFACE GLYCOPROTEIN IB DOES NOT AFFECT PLATELET-ADHESION UNDER FLOW CONDITIONS

Glycoprotein (GP) Ib is an adhesion receptor on the platelet surface t hat binds to von Wirlebrand Factor (VWF). vWF becomes attached to coll agens and other adhesive proteins that become exposed when the vessel wall is damaged. Several investigators have shown that during cardiopu lmonary bypass (CPB) surgery and also during platelet activation in vi tro by thrombin or thrombin receptor activating peptide (TRAP) GPIb di sappears from the platelet surface. Such a disappearance is presumed t o lead to a decreased adhesive capacity. In the present study, we show that a 65% decrease in platelet surface expression of GPIb, due to st imulation of platelets in Orgaran anticoagulated whole blood with 15 m u mol/L TRAP, had no effect on platelet adhesion to both collagen type III and the extracellular matrix (ECM) of human umbilical vein endoth elial cells under flow conditions in a single-pass perfusion system. I n contrast to adhesion, ristocetin-induced platelet agglutination was highly dependent on the presence of GPIb. Immunoelectron microscopic s tudies showed that GPIb almost immediately returned to the platelet su rface once platelets had attached to collagen. In a subsequent series of experiments, we showed that when less than 50% of GPIb was blocked by an inhibitory monoclonal antibody against GPIb (6D1), platelet adhe sion under flow conditions remained unaffected. (C) 1998 by The Americ an Society of Hematology.