Loop Diuretics and In Vitro Relaxation of Human Fetal and Newborn Mouse Airways

Abstract

This study was designed to test the hypotheses that furosemide directly causes relaxation in human fetal airway and that delivery of loop diuretics to either the adventitial or epithelial surface of newborn mouse airway results in equivalent relaxation. Isometric tension changes were measured in human fetal (11-16 wk) trachea and mainstem bronchus rings exposed to furosemide (300 M) or saline after acetylcholine or leukotriene D 4 constriction. Significant decreases in isometric tension to furosemide were demonstrated after constriction with acetylcholine or leukotriene D 4 . To examine the site of effect and mimic aerosolized and systemic administration, furosemide (3-300 M) and bumetanide (0.3-30 M) were applied separately to epithelial and adventitial surfaces of newborn mouse airways. No differences in airway diameter changes to epithelial or adventitial furosemide or bumetanide were observed, but a 10-fold difference in potency was found. In summary, human fetal airway relaxed to furosemide when constricted with either neurotransmitter or inflammatory mediator in vitro. Further, no differences in relaxation to equimolar epithelial and adventitial furosemide were observed in isolated newborn mouse airway. Taken together, this provides evidence that furosemide has a direct, nonepithelial-dependent effect on airway smooth muscle tone. Systemically administered furosemide improves pulmonary mechanics and gas exchange in infants with chronic lung disease independent of its diuretic effects (1, 2). Studies demonstrating a reduction of bronchoconstrictive responses in pediatric (3) and adult asthmatics (4, 5) with aerosolized furosemide further support the notion that a nondiuretic effect may produce improvements in pulmonary function. Indeed, the in vitro airway relaxation to furosemide that is observed in animal models (6 -8) suggests that there may be a direct effect on airway smooth muscle.However, a bronchodilatory response to inhaled furosemide is not consistently observed in infants with chronic lung disease (9, 10). Although aerosolized administration of furosemide could minimize systemic effects while producing localized bronchodilation, it is not widely accepted that inhaled furosemide has therapeutic efficacy during acute bronchoconstriction. Taken together, it is presently unclear whether these inconsistent results are because of species differences or inadequate concentrations of drug that reach the airway with aerosolized treatment, or because furosemide in fact does not act via a mechanism of direct airway smooth muscle relaxation (11). Therefore, the potential relevance of previous in vitro animal findings requires validation by the study of human airway response (12), with the first objective to test the hypothesis that furosemide causes direct relaxation of human fetal airway.In addition to demonstrating a direct relaxing effect on airway smooth muscle, further support for the bronchodilatory response to furosemide via aerosolization would require evidence that mechanisms can be activated from th...

Unexpected extra‐renal effects of loop diuretics in the preterm neonate

“…The response was similar whether the diuretics were applied to the luminal (epithelial) surface or abluminal (adventitial) surface. These results provide additional evidence that there is a direct, non‐epithelial‐dependent effect of furosemide on airway smooth muscle tone and are consistent with the possibility that the Na + –K + –2Cl − cotransporter is present in the human airway smooth muscle cell and that it plays a role in regulating airway tone (61).…”

“…Iwamoto et al. (61) studied isolated human foetal airway and newborn mouse airways to test the hypothesis that furosemide and bumetanide, both loop diuretics, would cause direct relaxation of human foetal airway. They found that human airway relaxed following furosemide after preconstriction with either acetylcholine or the inflammatory mediator leukotriene D‐4.…”

Na-K-2Cl cotransporter inhibition impairs human lung cellular proliferation

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“…Over the past two decades, clinical and in vitro studies have confirmed airway smooth muscle relaxation responses to these sulfonamide compounds (2,9,11,18,24), but the exact mechanisms mediating their clinical benefits remain controversial. In addition, the widespread presence of this cotransporter protein in various tissues suggests that chronic, systemic administration of NKCC inhibitors may result in other effects that may or may not be beneficial.…”

NKCC1 as a regulator of vascular tone and a novel target for antihypertensive therapeutics

“…Thus, in asthmatic patients, treatment of hypertension with ␤-blockers is complicated by their actions as bronchoconstrictors. This is not the case with NKCC1 inhibitors that lead to airway smooth muscle relaxation (9).…”