vagal pathways mediating respiratory reflexes evoked by ROS in the lungs of anesthetized rats. J Appl Physiol 94: 1987, 2003. First published January 10, 2003 10.1152/ japplphysiol.01047.2002We investigated the afferent vagal pathways mediating respiratory reflexes evoked by reactive oxygen species (ROS) in the lungs of anesthetized rats. Spontaneous inhalation of 0.2% aerosolized H 2O2 acutely evoked initial bradypnea followed by delayed tachypnea, which was frequently mixed with delayed augmented inspiration. The initial response was abolished after perivagal capsaicin treatment (PCT), but was prolonged during vagal cooling (VC) to 7°C; PCT and VC are known to differentially block the conduction of unmyelinated C and myelinated fibers, respectively. The delayed responses were eliminated during VC but emerged earlier after PCT. Vagotomy, catalase (an antioxidant for H 2O2), dimethylthiourea (an antioxidant for ⅐ OH), or deferoxamine (an antioxidant for ⅐ OH) largely or totally suppressed these reflexive responses, whereas sham nerve treatment, heat-inactivated catalase, saline vehicle, or iron-saturated deferoxamine failed to do so. These results suggest that 1) the H 2O2-evoked initial and delayed airway reflexes are antagonistic and may result from stimulation of lung C fibers and rapidly adapting receptors, respectively, and 2) the reflex effects of H 2O2 are, in part, due to the action of ⅐ OH on these afferents. vagal sensory receptors; oxygen radicals; reactive oxygen species; hydrogen peroxide LUNG DISEASES, SUCH AS ASTHMA (10), chronic obstructive pulmonary disease (12), endotoxin shock (33), and vascular microembolism (50), or inhalation of oxidant irritants, such as toxic smoke (35), cigarette smoke (35), and ozone (45), may cause increased pulmonary production of reactive oxygen species from endogenous and/or exogenous sources. The major reactive oxygen species are the superoxide anion radical, hydrogen peroxide (H 2 O 2 ), and the hydroxyl radical (⅐ OH) (5). The superoxide anion radical dismutates to form H 2 O 2 , which, in the presence of iron, can further react to form ⅐ OH, a more reactive oxygen radical, via the Fenton reaction (5). Vagal sensory receptors are known to play an important role in detecting the onset of pathophysiological conditions and are responsible for triggering defensive or protective airway reflexes (8, 29, 39).While abundant information suggests the importance of reactive oxygen species in producing pulmonary pathophysiological consequences (4), very few studies have investigated their role and mechanism in eliciting airway reflexes.The concept that reactive oxygen species may stimulate pulmonary sensory receptors and play a vital role in eliciting airway reflexes is considerably new (29). This concept is indirectly supported from findings that vagally mediated airway reflexes evoked by inhaled cigarette smoke (27), inhaled wood smoke (22), or pulmonary air embolism (6) and responses of vagal sensory receptors to the latter two insults (7,24,25) are greatly attenuated b...