Laboratory Investigations Inhaled nitric oxide modulates leukocyte kinetics in the mesenteric venules of endotoxemic rats Rmi Nevire, MD; Serge Mordon, PhD; Xavier Marchal, PhD; Bruno Buys, ITA; Benoit Guery, MD, PhD; Daniel Mathieu, MD, PhD; Francis Wattel, MD; Claude Chopin, MD From the Services de Ranimation Mdicale, Hopital R. Salengro, (Drs. Nevire, Mordon, Marchal, Guery, Mathieu, Wattel, and Chopin and Mr. Buys), and the Centre Hospitalier et Universitaire (Drs. Nevire, Mathieu, and Wattell) Cedex, France. Objective: to determine whether inhaled nitric oxide (NO) would alter leukocyte kinetics in the septic microvasculature. Design: Randomized, controlled trial. Setting: Experimental laboratory. Subjects: Male Sprague Dawley rats. Interventions: Rats were treated with either saline or endotoxin (10 mg/kg, iv) and then allowed to breathe either air or air plus NO (10 ppm). Measurements and Main Results: After a 4-hr period, rolling, firm adhesion, and emigration of leukocytes and endothelial dysfunction were monitored in mesenteric venules by using intravital videomicroscopy. Compared with controls, endotoxemic rats exhibited a profound influx in mesenteric venule rolling leukocytes (55 17 vs. 70 19 leukocytes/min; p < .05), associated with a reduction of leukocyte rolling velocity (83 14 vs. 34 3 μ m/sec; p < .05). In endotoxemic rats, venular endothelium leukocyte firm adhesion (1.15 0.32 vs. 4.08 0.96 leukocytes/100 μm; p < .05) and emigration (0.84 0.47 vs. 4.23 1.2 leukocytes/100 μ m; p < .05) increased compared with controls. Inhaled NO had no effect on leukocyte kinetics in control rats. Inhaled NO significantly attenuated endotoxin-induced venular endothelium leukocyte adhesion (4.08 0.96 vs. 1.86 0.76 leukocytes/100 μ m; p < .05) and emigration (4.23 1.2 vs. 1.68 0.72 leukocytes/100 μm; p < .05). Compared with control rats, macromolecular (FITC- dextran) vascular leakage, expressed as the perivenular/intravenular fluorescence intensity ratio, increased in endotoxemic rats (0.56 0.02 vs. 0.81 0.05; p < .01). Endotoxin-induced macromolecular vascular leakage increases were partially prevented by inhaled NO (0.66 0.01 vs. 0.56 0.02; p < .05). Conclusion: These observations suggest that inhaled NO reduces leukocyte adhesion and the degree of vascular permeability dysfunction in mesenteric venule of endotoxemic rats. KEY WORDS: intravital microscopy; fluorescence; endotoxin; nitric oxide; leukocyte; leukocyte adherence; vascular permeability; mesentery; vascular shear rate; adhesion molecules The adhesion of leukocytes to the microvascular endothelium has been recognized as an important factor in the development of multiple organ dysfunction after septic insults (1-3). Adherent leukocytes induce direct organ injury as a result of the release of various mediators, such as tumor necrosis factor, leukotrienes, and reactive oxygen species, as well as indirect organ injury through microvascular