INITIATION OF PLATELET-ADHESION BY ARREST ONTO FIBRINOGEN OR TRANSLOCATION ON VON-WILLEBRAND-FACTOR

We have identified two distinct mechanisms initiating the adhesion of flowing platelets to thrombogenic surfaces. The integrin alpha(IIb)bet a(3) promotes immediate arrest onto fibrinogen but is fully efficient only at wall shear rates below 600-900 s(-1), perhaps because of a rel atively slow rate of bond formation or low resistance to tensile stres s. In contrast, glycoprotein Ib alpha binding to immobilized von Wille brand factor (vWF) appears to have fast association and dissociation r ates as well as high resistance to tensile stress, supporting slow mov ement of platelets in continuous contact with the surface even at shea r rates in excess of 6000 s(-1). This eventually allows activated alph a(IIb)beta(3) to arrest platelets onto vWF under conditions not permis sive of direct binding to fibrinogen. The coupling of these different functions may be crucial for thrombogenesis.