Phenomenology and mechanism of the transient adsorption of fibrinogen from plasma (Vroman effect)

Additional data are presented on the transient adsorption of fibrinogen from blood plasma (the Vroman effect) reported previously (Brash, J. L., and ten Hove, P., Thromb. Haemostasis 51, 326, 1984; Horbett, T. A., Thromb. Haemostasis 51, 174, 1984). This effect is believed to result from rapid initial adsorption followed by displacement by other proteins. Adsorption was measured using radioiodine-labeled fibrinogen added to plasma as a tracer and surfaces were in the form of tubing. Plasma dilution and time were the main experimental variables. Initial adsorption was found to be diffusion-limited and diffusivities of about 1.5 × 10−7 cm2 s−1 were obtained independent of the surface. Various surfaces were compared on the basis of fibrinogen adsorption versus plasma concentration curves, all of which showed peaks at about 1% plasma concentration in accord with the Vroman effect. For 5-min adsorptions it was found that the height of the peaks was inversely related to the displacement “rate.” At 24 h the peak heights were in a different relative order and indeed one surface (a polyurethane) showed no peak. At these long times, the fibrinogen adsorption appears to be determined by a surface-solution equilibrium. None of the characteristics of the adsorption-concentration curves (peak height, displacement phase, “equilibrium” adsorption) were correlated with water contact angle of the different surfaces. Preadsorbed fibrinogen showed rates of displacement after exposure to plasma considerably slower than the rates observed “in situ.” This effect is tentatively attributed to orientational and/or conformational changes occurring as a function of residence time on the surface. Experiments on glass in which the plasma fibrinogen concentration was changed over a wide range (about one-eighth normal to twice normal) gave 5-min adsorption-concentration curves which differed in peak height and initial slope. The latter was proportional to fibrinogen concentration as expected. The peak heights did not show any such simple relationship but were used to demonstrate that about 50% of the glass surface may be covered with fibrinogen under normal conditions, again demonstrating the high surface activity of this protein.