Inhibition Of Collagen And ADP-Induced Platelet Aggregation By Plasma Fibronectin
Thrombosis and Haemostasis
Platelets contain fibronectin a glycoprotein with an established affinity for collagen. This observation has led other investigators to postulate that fibronectin is the platelet collagen receptor. The much greater concentration of fibronectin in the plasma surrounding platelets, however, has led us to suggest that plasma fibronectin may bind to collagen and competitively inhibit the platelet- collagen interaction. Rat platelets were isolated by Stractan density gradient centrifugation and
... rifugation and aggregated with acid-solubilized rat tail tendon collagen (Type I) in a Payton 300B Aggregometer. Fibronectin was twice purified by affinity chromatography with gelatin linked to CNBr- activated Sepharose 4B. Simultaneous addition of 50 μg fibronectin and 25 μg collagen to platelets suspended in Tyrodes solution at 37°C resulted in a 2-fold increase in lag time and a 30% decrease in aggregation rate as compared to control values. When collagen was preincubated in Tyrodes solution for 12 minutes at 26°C without platelets to allow for prior fibrillogenesis, the addition of 50 μg fibronectin with the platelets resulted in <20% increase in lag time and a 20-30% decrease in aggregation rate. In a separate series of experiments, fibronectin was also found to inhibit ADP-induced aggregation. In this case, the initial rate of aggregation was comparable with and without fibronectin, but this maximal rate was maintained for a shorter period in the presence of fibronectin. Thus, fibronectin reduced the in vitro aggregation response to two different physiological stimuli. Our data supports previous studies which indicate that fibronectin reduces the reactivity of platelets with collagen and provides evidence of a role for fibronectin in modulating platelet responses in the absence of collagen.