Rolling on the venular endothelium is a critical step in the recruitment of leukocytes during the inflammatory response. P-selectin is a key mediator of leukocyte rolling, which is an early event in the inflammatory cascade; this rolling is likely to be directly regulated by both local fluid shear forces and P-selectin site densities in the microvasculature. However, neither the spatial pattern of P-selectin expression in postcapillary venules nor the effect of local expression patterns on rolling behavior in intact functional venules is known. We investigated the influence of local shear forces and the spatial distribution of endothelial P-selectin in intact blood perfused post capillary venules in anesthetized mice using intravital confocal microscopy, high temporal resolution particle tracking, and immunofluorescent labeling. We demonstrated a shear-dependent increase in average leukocyte rolling velocity that was attributable to a shear-dependent increase in the occurrence of transient leukocyte detachments from the endothelial surface: translational velocity during leukocyte contact with the vessel wall remained constant. P-selectin expression was not different in venules with characteristically different shear rates or diameters but varied significantly within individual venules. In postcapillary venules, regions of high P-selectin expression correlated with regions of slow leukocyte rolling. Thus the characteristically variable leukocyte rolling in vivo is a function of the spatial heterogeneity in P-selectin expression. The study shows how the local hydrodynamic forces and the nonuniform pattern of P-selectin expression affect the behavior of interacting leukocytes, providing direct evidence for the local variation of adhesion molecule expression as a mechanism for the regulation of leukocyte recruitment.

Variations in wall shear stress are independent of location through a venular convergence in contrast to arteriolar bifurcations. In addition, wall shear rate is significantly more variable than viscosity and, therefore, primarily accountable for wall shear stress variations in postcapillary venules.

Cerebral oxygen saturation measured by cerebral oximetry compares well to the measured SjvO2 in normal subjects, despite multiple physiological reasons for differences. The closer relationship of SjvO2 to rSO2 than SaO2 under the conditions of these experiments indicates that the measurement reflects primarily intracranial saturation. However, outcome studies under clinical conditions are needed to determine the clinical utility of cerebral oximetry.

Leukocytes are delivered to venules in excess of the capacity of the local endothelium to support interactions. Elevated shear forces increase leukocyte recruitment to the vessel wall, which correlates to elevated rolling flux. In contrast, leukocyte firm adhesion is primarily affected by the activation state of the tissue and not by hemodynamic factors. Overall, the capacity of endothelial cells to support leukocyte interactions primarily regulates leukocyte recruitment and is not limited by leukocyte supply.

We sought to characterize how adherent leukocytes at the vessel wall, and the presence of erythrocytes, alter the streamlines (paths) of blood flow in the postcapillary venules. We directly visualized blood flow and leukocyte-endothelial cell interactions in postcapillary venules located in the cremaster muscle of anesthetized mice. Fluid streamlines were visualized by perfusing the cremaster muscle tissue with 0.5-micron fluorescent beads suspended in either buffer or whole blood, to examine the effect that erythrocytes have on the directionality of flow. Acute inflammation was induced in some animals by pretreatment of the vessels with tumor necrosis factor-alpha. To quantify the flow direction, the average deflection angle was defined as a scalar metric. Tracer bead trajectories were measurably altered by the presence of systemic levels of hematocrit, determined in each animal to be about 45%. Deviation from undirectional flow was also found to: (i) decrease with increasing vessel diameter, and (ii) increase with the number of adherent leukocytes. Fluid streamlines in the presence or absence of leukocyte adhesion or red cells agreed qualitatively with those obtained from theoretical calculations of blood flow using multiparticle adhesive dynamics. The microscale characteristics of venular flow are significantly altered during inflammation or changes in local hematocrit.