Efficiency of platelet adhesion to fibrinogen depends on both cell activation and flow

The kinetics of adhesion of platelets to fibrinogen (Fg) immobilized on pol ystyrene latex beads (Fg-beads) was determined in suspensions undergoing Co uette flow at well-defined homogeneous shear rates. The efficiency of plate let adhesion to Fg-beads was compared for ADP-activated versus "resting" pl atelets. The effects of the shear rate (100-2000 s(-1)), Fg density on the beads (24-2882 Fg/mu m(2)), the concentration of ADP used to activate the p latelets, and the presence of soluble fibrinogen were assessed. "Resting" p latelets did not specifically adhere to Fg-beads at levels detectable with our methodology. The apparent efficiency of platelet adhesion to Fg-beads r eadily correlated with the proportion of platelets "quantally" activated by doses of ADP, i.e., only ADP-activated platelets appeared to adhere to Fg- beads, with a maximal adhesion efficiency of 6-10% at shear rates of 100-30 0 s(-1), decreasing with increasing shear rates up to 2000 s(-1). The adhes ion efficiency was found to decrease by only threefold when decreasing the density of fg at the surface of the beads by 100-fold, with only moderate d ecreases in the presence of physiologic concentrations of soluble Fg. These adhesive interactions were also compared using activated GPIIbIIIa-coated beads. Our studies provide novel model particles for studying platelet adhe sion relevant to hemostasis and thrombosis, and show how the state of activ ation of the platelet and the local flow conditions regulate Fg-dependent a dhesion.