Effect of OPG on leukocyte trafficking in rat mesenteric postcapillary venules. Traffic of leukocytes was monitored at various time intervals after 4 hours of intraperitoneal administration of 1-10 ng/mL of OPG, and of the control vehicle. Measurements were performed on 4 to 6 different segments of unbranched venules (25- to 40-μm diameter, 200-μm length). (A) Values of rolling leukocyte flux defined as number of cells that become visible as bright spheres if they travel inside the venules more slowly than red blood cells. (B) Numbers of leukocytes stably adherent to the same site of postcapillary vascular endothelium of rat mesentery. (C) Leukocytes labeled in vivo with AO were made visible by fluorescence epi-illumination and appeared as bright spheres. One of 4 experiments with similar results is shown. Magnification, 100×.