Effect of fibronectin and von Willebrand factor on the adhesion of human fixed washed platelets to collagen immobilized beads

The adhesion of human fixed washed platelets (FWP) to collagen was measured using collagen immobilized beads. The addition of normal plasma or severe von Willebrand disease (VWD) plasma to FWP decreased the adhesion, suggesting the presence of some inhibitors of platelet adhesion in human plasma. Although the adhesion of FWP in severe VWD plasma was not different from that of FWP in normal plasma, the addition of purified von Willebrand factor (vWF, 1-2 mu/ml ristocetin cofactor) to FWP in buffer increased the FWP adhesion at higher flow rates, and the percent of adhesion in the absence of vWF was 10% (collagen 500 micrograms) and 30% (collagen 1,000 micrograms) of that in the presence of vWF at 10 ml/min. The enhancing effect of the vWF on FWP adhesion was also observed by pretreatment of the collagen column with vWF suggesting the important role of bound vWF to the collagen; adhesion 72% to the collagen column (1,600 micrograms) treated with vWF and 16% to the collagen column without the pretreatment at 10 ml/min. The promoting effect of vWF was also present in some commercial factor VIII preparations which had no large or intermediate multimers of vWF antigen. The adhesion of FWP was inhibited by fibronectin (FN) and the binding of ristocetin cofactor (vWF:RCo) to collagen fiber was also inhibited by FN; bound vWF:RCo to 50 micrograms/ml collagen in the absence or presence of 125 micrograms/ml FN were 60% and 8% respectively. It is suggested that vWF, even small multimer of vWF:Ag, is involved in the initial platelet-collagen interaction at high flow rates, while plasma FN acts as one of anti-adhesion factor.