Neural and hydroxyl radical mechanisms underlying laryngeal airway hyperreactivity induced by laryngeal acid-pepsin insult in anesthetized rats

Abstract

Laryngopharyngeal or gastroesophageal reflux is associated with laryngeal airway hyperreactivity (LAH), but neither the cause-effect relationship nor the underlying mechanism has been elucidated. Here we established a rat model with enhanced laryngeal reflex reactivity induced by laryngeal acid-pepsin insult and investigated the neural and hydroxyl radical (*OH) mechanisms involved. The laryngeal segments of 103 anesthetized rats were functionally isolated while animals breathed spontaneously. Ammonia vapor was delivered into the laryngeal segment to measure laryngeal reflex reactivity. We found that the laryngeal pH 5-pepsin treatment doubled the reflex apneic response to ammonia, whereas laryngeal pH 7.4-pepsin, pH 2-pepsin, and pH 5-denatured pepsin treatment had no effect. Histological examination revealed limited laryngeal inflammation and epithelial damage after pH 5-pepsin treatment and more severe damage after pH 2-pepsin treatment. In rats that had received the laryngeal pH 5-pepsin treatment, the apneic response to ammonia was abolished by either denervation or perineural capsaicin treatment (PCT; a procedure that selectively blocks capsaicin-sensitive afferent fibers) of the superior laryngeal nerves, but was unaffected by perineural sham treatment. LAH was prevented by laryngeal application of either dimethylthiourea (DMTU; a *OH scavenger) or deferoxamine (DEF; an antioxidant for *OH), but was unaltered by the DMTU vehicle or iron-saturated DEF (ineffective DEF). LAH reappeared after recovery from PCT, DMTU, or DEF treatment. We conclude that 1) laryngeal insult by pepsin at a weakly acidic pH, but not at acidic pH, can produce LAH; and 2) LAH is probably mediated through sensitization of the capsaicin-sensitive laryngeal afferent fibers by a *OH mechanism.

Sensitization by Pulmonary Reactive Oxygen Species of Rat Vagal Lung C-Fibers: The Roles of the TRPV1, TRPA1, and P2X Receptors

“…In these studies [14]–[16], these exaggerated responses could be prevented by pretreatment with antioxidants, indicating that there seemed to be an involvement of ROS in the sensitization of airway C-fibers and the development of airway hypersensitivity. However, questions remained as to whether the suppressive effects of the antioxidants in these studies resulted from a prevention of sensitization of the afferent fibers by the increased ROS or occurred because there was a removal of the basal function of ROS, which led to diminished afferent fiber sensitivity to these airway insults.…”

“…It has been previously shown that acute intermittent hypoxia [14] and laryngeal insult with acid and pepsin [15], [16] are able to potentiate the afferent and/or reflex responses of the airway C-fibers to stimulants in rats; the potentiating effect of the former and latter insults were found to be mediated through the TRPA1 and P2X receptors, respectively. In these studies [14]–[16], these exaggerated responses could be prevented by pretreatment with antioxidants, indicating that there seemed to be an involvement of ROS in the sensitization of airway C-fibers and the development of airway hypersensitivity.…”

“…Shen et al [14] reported that acute intermittent hypoxia augmented the afferent and reflex responses to stimulants of VLCFs in rats. Tsai et al [15], [16] demonstrated that laryngeal acid-pepsin insult augmented the reflex responses to stimulants of the superior laryngeal C-fibers in rats. The exaggerated reflex responses observed in these studies can be prevented by pretreatment with antioxidants, indicating the possible involvement of ROS in the sensitization of airway C-fibers and the development of airway hypersensitivity.…”

Hypersensitivity of lung vagal C fibers induced by acute intermittent hypoxia in rats: role of reactive oxygen species and TRPA1

“…ROS not only sensitize the carotid sinus nerves but also the capsaicin-sensitive afferent fibers. Recent studies demonstrated that laryngeal acidpepsin insult can evoke laryngeal airway hyperreactivity through sensitization of the capsaicin-sensitive laryngeal afferent fibers by ROS (54,55). This concept is supported by the present finding that both AIH-enhanced reflex apnea and LVCF sensitivity to capsaicin and ␣,␤-methylene-ATP were significantly prevented by dimethylthiourea (a hydroxyl radical scavenger) or N-acetyl-L-cysteine (an antioxidant), so ROS may be a causative factor in the development of afferent hypersensitivity.…”

Lesions in the Larynx of Wistar RccHanTM: WIST Rats

“…It is also a clinical issue in patients, whereby inconsistencies in histological patterns makes the diagnose difficult 26 . Different rat models investigating impacting factors on reflux laryngitis to mimic the human conditions were developed as there is the determination of the relationship between reflux and laryngeal hypersensitivity and the influence of pepsin 27 or the influence of bile reflux 28 …”