We investigated the role of peroxynitrite, which is formed by a rapid reaction between nitric oxide (NO) and superoxide anion (O 2), in the airway microvascular hyperpermeability during the late allergic response (LAR) in sensitized guinea pigs in vivo. The occurrence of LAR was assessed as a 100% increase in the transpulmonary pressure, which was monitored by the esophageal catheter technique. Airway microvascular permeability was assessed by Monastral blue dye trapping between the endothelium using an image analyzer. In the LAR phase (4 to 6 h after antigen inhalation), microvascular hyperpermeability and eosinophil infiltration within the airway wall were observed. NO production and xanthine oxidase (XO)/xanthine dehydrogenase activity, which are responsible for O 2 production, were enhanced during the LAR. Peroxynitrite formation assessed by nitrotyrosine immunostaining was also exaggerated at that time. The microvascular hyperpermeability during the LAR was largely reduced by NO synthase inhibitor (L-NAME, 72.7 % inhibition; p � 0.05), XO inhibitor (AHPP, 60.8 % inhibition; p � 0.05) and peroxynitrite scavenger (ebselen, 81.0 % inhibition; p � 0.05). L-NAME had a small but significant inhibitory effect on airway eosinophil accumulation, but AHPP and ebselen had no effect. These results suggest that excessive production of O 2 and NO occurs

Endogenous exhaled nitric oxide (NO) is increased during the late response to inhaled allergen in patients with asthma and may be bronchoprotective in asthma or have a deleterious effect when generated in excess under inflammatory conditions. To investigate this, we evaluated the effect of inhibiting endogenous NO production with nebulized N G-nitro-L-arginine methyl ester (L-NAME), a nonselective NO synthase (NOS) inhibitor, on early and late asthmatic responses to inhaled allergen in patients with mild allergic asthma. After a screening allergen challenge (AC), 22 male patients attended two visits conducted in a double-blind, randomized, placebo-controlled, crossover manner. Twelve patients demonstrating an early asthmatic response only (single responders) inhaled either L-NAME 170 mg or 0.9 % saline 20 min before AC, with exhaled NO and FEV 1 measured for 3 h. Ten patients demonstrating both early and late asthmatic responses (dual responders) were studied in a similar fashion but inhaled two further doses of L-NAME or placebo 3.5 and 7 h after the initial dose, with exhaled NO and FEV 1 measured for 10 h. L-NAME reduced exhaled NO levels by 77 � 5 % (p � 0.01) and 71 � 7 % (p � 0.01) in single and dual responders, respectively, but had no significant effect on early or late asthmatic responses. Following AC in single responders, the mean ( � SEM) maximum fall in FEV 1 after L-NAME and saline was 21.2 � 2.9 % and 23.8 � 3.0%, respectively, and in dual