Effect of C-fiber-mediated, ozone-induced rapid shallow breathing on airway epithelial injury in rats

Abstract

We examined the relationship between C-fiber-mediated, ozone-induced rapid shallow breathing and airway epithelial cell injury at different airway sites within the lower respiratory tract of conscious Wistar rats (n = 24). We combined an acute 8-h ozone inhalation with vagal perineural capsaicin treatment, a selective C-fiber conduction block, and 5-bromo-2'-deoxyuridine (BrdU) labeling as an index of epithelial injury. Vehicle-treated rats that inhaled ozone developed a rapid shallow breathing pattern during ozone inhalation, whereas the capsaicin-treated rats that inhaled ozone showed no changes in respiratory frequency. In vehicle-treated, ozone-exposed rats that developed rapid shallow breathing, a progressive increase in BrdU-labeling density (no. of BrdU-labeled cells/mm(2) airway) was observed starting at the bifurcation of the left main stem bronchi (central airway) and going down either a short or long airway path. In vehicle-treated, ozone-exposed rats, terminal bronchioles supplied by short and long airway paths had a similar degree of BrdU-labeling density that was significantly (P < 0.05) greater than the BrdU-labeling density of the proximal airways that supply them. In contrast, the attenuation of rapid shallow breathing produced by capsaicin treatment resulted in a significantly reduced BrdU-labeling density in the terminal bronchioles supplied by short airway paths compared with the terminal bronchioles supplied by long airway paths. Our data indicate that ozone-induced rapid shallow breathing protects large conducting airways while producing a more even distribution of injury to terminal bronchioles.

Activation of Neurokinin-1 Receptors during Ozone Inhalation Contributes to Epithelial Injury and Repair

“…It has been proposed that this distribution of ozone-induced lesions is the result of numerous factors, including the distribution of ozone uptake at different airway sites, and the sensitivity of different cell types to ozoneinduced injury (33). Previous work in our laboratory has shown that the pattern of distribution of ozone-induced airway injury is in part dependent upon the development of rapid, shallow breathing during exposure, which tends to protect the large conducting airways, but produces a more even distribution of injury in terminal bronchioles (31). Subsequently, Alfaro and colleagues (32) demonstrated that rapid, shallow breathing results in a redistribution of ozone uptake, in part explaining the findings of Schelegle and coworkers.…”

“…In brief, whole-body plethysmography was used to measure respiratory frequency and estimate VT as previously described (31).…”

“…To quantify the number of proliferating epithelial cells, the left lung lobes were dissected to expose the airways, and whole mounts were stained for BrdU as previously described (31). Stained whole mounts of duodenum served as positive control tissue.…”

“…Counts were expressed as number of positive cells per average surface area of counting field (no./mm 2 ). Surface area of airways was estimated by measuring heights, widths, and angles of all the airway sites in rats, followed by trigonometric calculations, as described previously (31).…”

“…SR140333 has been demonstrated to be a potent and selective NK-1 receptor inhibitor in rats, where it competitively inhibits substance P activity (30). We labeled airway epithelium with ethidium homodimer (a marker of necrotic cells) and 5-bromo-29-deoxyuridine (BrdU; a marker of epithelial proliferation) and sampled sites along short and long airway paths using previously described techniques (31). BrdU has been used reliably and extensively in our laboratory, and is an excellent cumulative marker of epithelial proliferation.…”