Furosemide Differentially Relaxes Airway and Vascular Smooth Muscle in Fetal, Newborn, and Adult Guinea Pigs

Abstract

Furosemide, an inhibitor of Cl-dependent Na+,K+ cotransport, is the most frequently used diuretic in newborns. Recently, furosemide was also demonstrated to decrease bronchial hyper-responsiveness in adults, although little is known about the direct effect of furosemide on smooth muscle of immature animals. This in vitro study was designed to determine the action of furosemide on airway and vascular smooth muscle during ontogeny. Extrathoracic trachea (ET), main stem bronchi, main pulmonary artery, and thoracic aorta ring segments from fetal, newborn, and adult Hartley albino guinea pigs were suspended in HEPES solution for measurement of isometric tension. Furosemide (30 or 300 microM) was administered after preconstriction with an ED35-70 concentration of histamine or acetylcholine for airway and ED40-100 concentration of norepinephrine for vessels. Furosemide (30 microM) caused significant relaxation of airway smooth muscle at all ages. After histamine-induced preconstriction, fetal airway segments exhibited greatest relaxation (183 +/- 28%), with newborn airway demonstrating 123 +/- 15% relaxation and modest relaxation seen in adults (40 +/- 4%). This pattern was similar for both ET and bronchus and appeared greater for histamine compared with ACh preconstriction. Epithelial removal slightly enhanced relaxation. Furosemide also relaxed pulmonary artery segments, but at a 10-fold higher concentration. In striking contrast to the pattern seen in airway, adult pulmonary artery relaxed more than newborn and newborn, more than fetus. Cyclooxygenase blockade and endothelium removal did not change pulmonary artery relaxation. Furosemide did not significantly relax aorta after NE preconstriction. Taken together, these results suggest that furosemide may be more effective in relaxing airway compared with vascular smooth muscle, and the ontogeny of these responses indicates a greater efficacy and selectivity in airways of immature animals.

Acute Hyperoxic Injury Attenuates the Relaxing Effects of “Loop” Diuretics and Salbutamol on Large Airways of Newborn Guinea Pigs

“…Tjhe dose of furosernide was chosen to approximate the serum level achieved in a newborn infant given a 1-mg/kg i.v. dose of furosemide (15). Preliminary experiments with ethacrynic acid, a more potent diuretic than furosemide, demonstrated quantitatively similar relaxation with a dose equal to 1/10 that of furosemide.…”

“…Pleural contents were removed en masse and placed in cold HEPES buffer solution (pH = 7.4): 140 mM NaCl, 4.5 mM KCl, 10 mM D-glucose, 1.5 mM CaC1,-2H,O, 1 mM MgC1,-6H20, 5 mM HEPES. HEPES buffer was used because it has been shown to accentuate the relaxing effect of furosemide on vascular (19) and airway smooth muscle (15). HEPES-filled organ baths were mainta:ined at 37OC and bubbled with 100% 0,.…”

“…ORT of rings of extrathoracic trachea, mainstem bronchus, aorta, and pulmonary artery were determined as previously described (15,20,25) except that a higher concentration of norepinephrine (6 X 10W6 M) was used for vascular tissues. ORT is the tension due to passive stretch at which each ring responds maximally to a constrictor.…”

“…(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 opinions or assertions contained herein are the private views of the authors and are increases the furosemide-sensitive portion of 8%b uptake and not to be considered as official or as reflecting thc vicws of the Department of thc Army accentuates the relaxing effect of fW0semide (19 Krebs buffer (15), suggesting that airway relaxation responses in HEPES compared with Krebs buffer may likewise be used to evaluate airway cotransporter activity. However, patients with normal airways are rarely treated in the clinical setting.…”