Abstract 2076 Pulmonary: Reactive Airway Diseases I Poster Symposium, Monday, 5/3

The ability of furosemide to relax airways remains controversial despite some evidence of clinical efficacy and direct in vitro relaxation in guinea pigs. Others have suggested that furosemide attenuates bronchoconstriction via vasodilatory effects on tracheal arterioles and venules, rather than by direct effects on airway smooth muscle. In this study, we examined the response of isolated human fetal airway to furosemide in order to validate the relaxation responses obtained in various animal models. Further, we examined isolated newborn mouse airway to localize the effects of furosemide to epithelial (luminal) or adventitial (abluminal) surfaces. Human fetal (11-16 wks gestation) trachea or mainstem bronchus ring segments (Central Laboratory for Human Embryology, Univ. of Washington, Seattle) were mounted on transducers for isometric tension measurement. Airway rings were hung in organ baths and equilibrated in HEPES solution, at 37°C, bubbled with 100% O2 for 1 hr. Optimal resting tensions were determined and tissues were constricted with acetylcholine (ACh) EC50-75, followed by the addition of 300 µM furosemide or saline control. Preliminary results showed that there was a two-fold (p<0.05, t-test) greater relaxation to furosemide (n=9) in human fetal airway compared to saline control (n=6). In the next series of experiments, tracheae (200-400 µm diameter) from 1-5 day old mice were isolated, cleaned, cannulated and mounted in a chamber (Living Systems®, Burlington, VT) that allowed for separation of luminal and abluminal environments. Tissues were equilibrated in HEPES buffer, at 37°C, bubbled with 100% O2 and monitored by videomicrometry for measurement of airway diameter changes under constant luminal flow. Tracheae were constricted with abluminal ACh EC50-75 then exposed to cumulative doses of furosemide (3-300 µM) applied to either the luminal or abluminal surfaces. There were no differences in percent increase in airway diameter between cumulative responses to intraluminal (n=4) and abluminal (n=7) furosemide (p=0.628, 2-way ANOVA, repeated measures). In summary, this is the first study to demonstrate a direct relaxing effect of furosemide on human airway segments. In addition, the equivalent relaxation of mouse airway to cumulative concentrations of luminal and abluminal furosemide, confirms previously reported results from single dose application (Pediatr Res 41:256A, 1997). Taken together, comparable efficacy of furosemide whether applied from the epithelial surface or from the adventitia suggests that systemic as well as inhaled administration may result in bronchodilation, if appropriate concentrations of furosemide reach the tissue.