Leukocyte margination and deformation in mesenteric venules of rat

White blood cell (WBC) marginaton and rolling were studied in venules (20-60 μm in diameter) in rat mesentery to examine the relationship between WBC rolling velocity (V(wbc)), flux (F(wbc), cells/min), and deformation; and red blood cell (RBC) velocity (V(rbc)), wall shear rate (γ̇), and microvessel hematocrit (H(micro)). For a range of 1.0 ≤ V(rbc) ≤ 5.0 mm/s, V(wbc) remained constant (≃50 μm/s). The volume flux of WBCs rolling along the venular wall (normalized to systemic WBC concentration) decreased nonlinearly from 30 to 5% of total luminal flux as γ̇ ranged from 50 to 800 s^-1 and was more strongly correlated with γ̇ compared with F(wbc) vs. γ̇. As indicated by the rolling volumetric flux, margination was weakly affected by H(micro), except for H(micro) >50% where margination increased. Deformation of WBCs was quantitated during their rolling contact with the endothelium (EC) in terms of their length and heigth. As γ̇ increased from 50 to 800 s^-1, WBCs elongation to 140% of their estimated undeformed diameter, and the contact area between WBC and EC increased 3.6-fold. These data suggest that with increased γ̇ rolling of WBCs along the EC decreases, since only the most deformable WBCs of the circulating population persist in maintaining contact with the endothelium. It is concluded that V(wbc) is invariant with elevations in γ̇, since the greater deformations of rolling WBCs at high shear result in greater adhesive forces attendant to increased WBC-EC contact area, which attenuate increases in V(wbc).