von Willebrand factor-mediated platelet adhesion in flowing blood is influenced by hematocrit.

We have studied the influence of hematocrit on the capture and rolling of platelets on von Willebrand factor (VWF) as well as the biomechanical properties of the interaction. When fluorescently-labeled platelets in whole blood (in the presence of anti-aIIbb3 MoAb LJ-CP8 to avoid platelet stable arrest) were perfused onto immobilized VWF through rectangular flow chambers, platelets formed numerous rolling attachments. The characterization is based on information obtained from a tailored-specific image analysis method applied to continuous sequences of microscopical images. Platelet movement on the surface, the duration of each arrest (lifetime), the frequency of this movement, and the distance traveled before its resumes the velocity of a non-interacting cell were measured. Our data indicated that hematocrit influenced not just the rate of capture from the flowing stream, enhancing platelet diffusivity to the wall, but also the ability to sustain interactions with the surface. Possibly due to the augmented intercellular collisions, as the hematocrit was increased from 5% to 40% the frequency of attachment increased 1.5 times. The removal rate constant was faster at high hematocrit (29.5 s-1 at 40% Hct and 21.1 s-1 at 5% Hct). Variation of hydrodynamic forces was used to sample the on-rate and bond resistance, ultimately resulting in different association and dissociation constants. The number of adhering platelets revealed a bell-shaped dependence on the wall shear rate, also affected by the hematocrit. Increasing hematocrit, platelets showed a faster saltatory movement, i.e., stayed on the surface for shorter durations, traveling longer distances. In rapid flow conditions (3000 s-1), increasing the hematocrit from 5% to 40%, the mean translational velocity increased exponentially (from 0.03 to 0.21 µm/s). Our findings support the concept of transport-enhancing capability of red cells but also suggest their influence on platelet rolling on immobilized VWF before adhering firmly to form thrombi.