Randomised double-blind placebo-controlled study of the effect of inhibition of nitric oxide synthesis in bradykinin-induced asthma

Summary

Background

Bronchoconstriction induced by bradykinin is reduced by the release of nitric oxide (NO) in the airways of guineapigs. Inhaled NO is known to cause broncho-dilatation in asthmatic patients. To find out the role of endogenous NO in airway response to bradykinin in asthma, we examined the effect of the NO synthase inhibitor N^G-monomethyl-L-arginine (L-NMMA) on bronchoconstriction after bradykinin challenge in ten patients with mild asthma.

Methods

The study had a randomised, double-blind, placebo-controlled, cross-over design. Participants were studied during two phases, each consisting of 2 study days. After baseline measurements of forced expiratory volume in 1 s (FEV₁) participants inhaled an aerosol of L-NMMA or saline (placebo). After 5 min, saline and doubling doses of bradykinin (from 0·25 nmol) were inhaled until FEV₁ fell by at least 20% of the post-saline value. The effect of L-NMMA and placebo on airway response to doubling concentrations of methacholine (from 0·03 mg/mL) was then examined. We also assessed the effect of the inactive enantiomer of L-NMMA, D-NMMA, and placebo on bronchoconstriction after bradykinin or methacholine challenge in six of the participants.

Findings

The geometric mean of the provocative dose producing a 20% fall in FEV₁ to bradykinin was 138·0 nmol (range 48·2-475·2 nmol) after placebo and 11·2 nmol (range 0·9-51·3 nmol) after L-NMMA (p < 0·01). L-NMMA also caused a decrease in the provocative concentration of methacholine producing a 20% fall in FEV1 from 0·93 mg/mL (range 0·12-2·55 mg/mL) to 0·38 mg/mL (range 0·06-0·92 mg/mL; p < 0· 01). In contrast, D-NMMA did not affect airway response to bradykinin or methacholine.

Interpretation

The results suggest that bronchoconstriction after bradykinin inhalation is greatly inhibited by the formation of NO in airways of asthmatic patients and that NO could have a bronchoprotective role in asthma.

Introduction

The mechanism of the exaggerated airway response to bronchoconstrictive triggers in asthma is still obscure.¹ Increased concentrations or activity of various endogenous inflammatory mediators have been suggested as important determinants of the exaggerated response.² Another hypothesis is that impaired activity of bronchoprotective factors may contribute to the pathogenesis of asthma.³

Nitric oxide (NO) is derived from the guanidino nitrogen of L-arginine by the action of inducible or constitutive isoforms of NO synthase; it is produced by various cells in the respiratory tract, including epithelial cells and non-adrenergic non-cholinergic nerve cells.⁴ Relaxation of human airways induced by non-adrenergic non-cholinergic nerves is mediated by NO;⁵ this observation and the finding that inhaled NO causes bronchodilatation in asthmatic patients⁶ suggest that NO is a bronchoprotective factor.

Bradykinin is an endogenous mediator involved in airway inflammatory responses, including plasma protein extravasation, smooth muscle contraction, and mucus hypersecretion.⁷ Bradykinin inhaled via aerosols causes bronchoconstriction in asthma patients⁸ and kinin concentrations are increased in the bronchoalveolar lavage fluid of asthma patients challenged with allergen.⁹ These findings led to the hypothesis that bradykinin plays an important part in asthma.

In guineapigs the bronchoconstriction provoked by bradykinin inhalation is greatly increased when the NO synthase pathway is inhibited.¹⁰ This observation suggests that bradykinin releases NO which, in turn, relaxes the airways. There is also evidence that bradykinin relaxation of guineapig isolated trachea is mediated by NO released from the airway epithelium.11, 12

Little is known about the role of endogenous NO in modulating bronchoconstriction in asthmatic patients. To investigate whether endogenous NO influences the bronchoconstriction induced by bradykinin in asthma, we assessed the effect of inhaled NO synthase inhibitor N^G-monomethyl-L-arginine (L-NMMA) on airway narrowing induced by aerosolised bradykinin in individuals with mild asthma who were free of symptoms at the time of the study. We also tested the selectivity of NO synthase inhibition on bradykinin-induced bronchoconstriction by studying the effect of L-NMMA on methacholine-induced bronchoconstriction. The effect of D-NMMA, the inactive enantiomer of L-NMMA, on bradykinin-induced and methacholine-induced bronchoconstriction was also investigated. All participants were outpatients of our asthma clinic that were known to have asthma.