To determine whether altered airway smooth muscle contractility contributes to airway hyperreactivity resulting from hyperoxic exposure, in vitro contractile responses of airways to two physiological constrictors, acetylcholine (10(-9) to 10(-4) M) and histamine (10(-8) to 10(-4) M), were examined. Extrathoracic trachea, intrathoracic trachea, and bronchus from 1- to 2-day-old (newborn) guinea pigs exposed to 85% oxygen for 84 h were compared with tissues obtained from newborns reared in room air. Responses in the presence and absence of aspirin (ASA; 10(-3) M) were compared. Hyperoxic exposure did not affect the histology of the airway epithelia. Contractile responses to acetylcholine and histamine were similar. Without ASA, maximal tensions generated were higher in both extrathoracic and intrathoracic trachea obtained from hyperoxia-exposed neonates than in trachea from newborns reared in room air. ASA caused maximal tensions of trachea from newborns reared in room air to increase but did not affect the already increased contractility of trachea from hyperoxia-exposed animals; the tensions achieved in hyperoxic tissues with and without ASA were similar to the hyperactive responses induced by ASA in tissues from animals reared in room air. Bronchi showed responses similar to those seen in tracheal segments. Thus, despite no apparent histological effect on the airway epithelium, hyperoxic exposure seems to increase airway smooth muscle contractility, is nonspecific for different constricting agents, and shows no regional differences in airway reactivity.

The purpose of this study was to investigate the ontogeny of guinea pig airway smooth muscle (ASM) responses and the epithelial modulation of these responses. Paired tracheal rings from fetal, newborn, and adult guinea pigs were studied. One of each pair was denuded of airway epithelium (AE) by gentle rubbing. Isometric tension was measured in rings mounted in organ baths filled with Krebs' solution. Cumulative dose-response curves were generated by adding either acetylcholine (ACh) or histamine over a concentration range of 10(-8)-10(-4) M. Significant agent-specific, age-related differences in maximal contraction were seen for both ACh and histamine in intact tissues (Ach: for fetus 66.7 +/- 6.2 x 10(-2) g/mg wet wt, for newborn 51.4 +/- 6.2, for adult 29.3 +/- 2.6; histamine: for fetus 46.1 +/- 5.1, for newborn 72.9 +/- 6.0, for adult 25.3 +/- 3.2). Similar differences in sensitivity to both agents were observed (EC50 with ACh: for fetus 0.80 +/- 0.11 x 10(-6) M; for newborn 0.85 +/- 0.26 x 10(-6) M; for adult 1.7 +/- 0.20 x 10(-6) M; EC50 with histamine; for fetus 1.88 +/- 0.50 x 10(-6) M; for newborn 1.34 +/- 0.16 x 10(-6) M; for adult 3.78 +/- 0.75 x 10(-6) M). Removal of AE caused a significant decrease in maximal responses to ACh in fetal tissue, a smaller, insignificant one for newborn and a nonsignificant alteration for adult tissues. Age-related sensitivity difference was abolished with removal of AE to ACh but not to histamine.(ABSTRACT TRUNCATED AT 250 WORDS)

We have previously found an age-dependent relaxing effect of furosemide in normal fetal, newborn, and adult guinea pig airways with fetal trachea exhibiting the greatest relaxation and adult tissue the least. This study was designed to expand upon this finding by determining if in vivo hyperoxic exposure would influence in vitro airway relaxation mediated by the loop diuretics, furosemide and ethacrynic acid, and the P2-adrenoceptor agonist, salbutamol. Newborn guinea pigs were raised in >95% FiO, until ill; controls in room air. Isometric relaxation to 3 X lop5 M furosemide, 3 X 10-% ethacrynic acid, or 1 0~' -1 0~" M salbutamol was recorded in 3 X lop" M histamine-constricted airway rings. Ethacrynic acid, like furosemide, relaxed newborn Recent evidence suggests that furosemide modulates airway reactivity in vivo. Inhaled furosemide attenuates exercise-(I), allergen-(2), and cold air-(3, 4) induced bronchoconstriction in asthmatic adults, and cold air-induced bronchospasm in asthmatic children (5). Furthermore, systemic furosemide decreases airway resistance in infants with bronchopulmonary dysplasia (6-10). Although the mechanisms of furosemide's pulmonary effects have yet to be fully established, most researchers conclude that these effects are unrelated to diuresis. Various mechanisms have been espoused, including inhibition of pulmonary nerves ( l l ) , increased pulmonary venous capacitance (12), increased efferent pulmonary lymph flow (13) Medical Center; University of Hawaii Leahi Trust; and National Institutes of Health Grant guinea pig airways. Hyperoxia did not alter the contractile effect of 3 X 1 0 -M histamine but did significantly decrease the relaxing effect of furosemide, ethacrynic acid, and salbutamol. Loop diuretic mediated airway relaxation was accentuated in HEPES buffer when compared with Krebs, whereas salbutamolmediated relaxation was unaffected. These results suggest that hyperoxia nonspecifically decreases airway responsiveness to the relaxing agents studied. (Pediatr Res 38: 280-285, 1995) Abbreviations ORT, optimal resting tension FiO,, fraction of inspired oxygen increased oncotic pressure leading to decreased efferent pulmonary transcapillary flow (12,14). Stevens et al. (15) found that furosemide directly relaxes normal isolated guinea pig airways, suggesting another possible mechanism.Furosemide, bumetanide, and ethacrynic acid bind to a Na+-K+-Cl-cotransporter in the thick ascending loop of Henle (16,17). This transporter is also functional in other tissues (18). The possible role of this cotransporter in vascular smooth muscle function was first hypothesized by Deth and coworkers (19) who found evidence for a functional Na+-K+-C1-cotransporter in vascular smooth muscle by demonstrating that: 1) '%b uptake (a marker for Kt transport) is inhibited by furosemide in the presence of ouabain (to inhibit Na-KATPase) and is ~a + -, K+-, and C1--(substrate) dependent; and 2) uptake of 4 5~a in rat aorta is reduced by furosemide. They further suggest that HEPES buffer increases...

ABSTRACT. We examined the ontogeny of relaxation function may vary depending on the site of binding within responses to three categories of calcium channel antagothe calcium channel. (Pediatr Res 29: 278-281, 1991) nists, represented by verapamil, diltiazem, and nifedipine, for both potential-operated (KCI-mediated) and receptorAbbreviations operated channels [norepinephrine (NE)-mediated] in rat thoracic aorta. Aortic rings from 2-to 3-d, 1-wk, and 12-wk-old Sprague Dawley rats were mounted in an organ bath, bathed in Krebs' solution, and connected to a forcedisplacement transducer to measure isometric tension. Endothelium intact vessels at optimal passive force were exposed to a single EDso of isotonic KC1 or NE, equilibrium contraction was measured, then vessels were washed and exposed for 30 min to 1 pM verapamil, 1 pM diltiazem, or 0.1 pM nifedipine, followed by another dose of KC1 or NE.Verapamil and diltiazem demonstrated significant (p c 0.05) age-related increases in effectiveness for blocking KCI-mediated contraction [(% reduction of control contraction f SEM) (Verapamil: 2-3 d, 67.7 f 4.2; 1 wk, 72.5 f 1.8; 12 wk, 89.5 f 1.0. Diltiazem: 2-3 d, 64.6 f 2.9; 1 wk, 73.5 f 3.0; 12 wk, 83.1 f: 1.81. Nifedipine was equally effective at all ages: 2-3 d, 85.6 f 1.3; 1 wk, 90.0 f 1.6; and 12 wk, 91.3 f 1.4. Verapamil and diltiazem also showed significant age-related increases in effectiveness for blocking NE-mediated contraction (Verapamil: 2-3 d, 6.2 f 3.9; 1 wk, 28.0 f 4.8; 12 wk, 44.1 f 6.0. Diltiazem: 2-3 d, 8.0 f 3.1; 1 wk, 20.5 f 3.9; 12 wk, 46.5 f 4.8). Again, nifedipine was equally effective at all ages: 2-3 d, 42.0 f 6.8; 1 wk, 35.8 f 3.9; and 12 wk, 37.5 f 3.2. In summary, for the categories of calcium channel antagonists that interact at the phenylalkylamine (verapamil) and benzothiazepine (diltiazem) binding sites, there were age-related increases in effectiveness for blocking both potentialoperated and receptor-operated channels. However, for nifedipine, which binds to the 1,4-dihydropyridine binding site, no maturational change was observed. These results suggest that the ontogeny of calcium channel antagonists' We have previously (8) demonstrated the ontogeny of the concentration-dependent response in thoracic aortic rings among 2-d-, 1-wk-, and 1 Zwk-old rats for both potential-mediated (KClinduced) and receptor-mediated (NE-induced) contraction. Thus, calcium channel function is present from 2 d of age with perhaps increasing physiologic function during development. However, many aspects of calcium channel function during ontogeny are unknown. We therefore designed the present study to examine one aspect of calcium channel function, viz. the ontogeny of responses to three categories of calcium channel antagonists for KCI-and NE-mediated contraction in rat aorta. The objectives were 3-fold: to determine if there were 1) agerelated differences in response to selective calcium channel antagonists; 2) differential effects among the three categories of calcium channel antagonists; and 3 ) differe...