Endothelial, Not Hemodynamic, Differences Are Responsible for Preferential Leukocyte Rolling in Rat Mesenteric Venules

At the onset of the inflammatory process, leukocytes roll along venular but not arteriolar walls before they firmly attach and emigrate. To test whether differences in hydrodynamic flow conditions are responsible for the preferential occurrence of leukocyte rolling in venules, we varied wall shear rate, γw, between 30 and 2,000 sec by selective micro-occlusion of side branches in venules and arterioles (diameter, 20–37 μm) of the exposed mesentery of anesthetized rats. In venules, 39% (range, 6–77%) of all passing leukocytes were found interacting with the endothelium (rolling), whereas this fraction was only 0.6% in arterioles. The fraction of rolling leukocytes in venules decreased from 49±13% at γw,400 secʼ (N = 12). Mean leukocyte rolling velocity in venules increased with γw, but the most frequent rolling velocity class was 20–40 μm/sec at all shear rates. In arterioles, even prolonged (up to 90 minutes) conditions of reduced flow (γw,