Am. J. Respir. Crit. Care Med., Vol 150, No. 2, Aug 1994, 381-387.
Nonbronchodilator effects of inhaled beta 2 agonists. Greater protection against adenosine monophosphate- than methacholine-induced bronchoconstriction in asthma
BJ O'Connor, RW Fuller and PJ Barnes
Department of Thoracic Medicine, National Heart and Lung Institute, London, United Kingdom.
There has been controversy about possible beneficial effects of beta
agonists on airway function in asthma, in addition to their effects on
airway smooth muscle. We compared the protective effects of terbutaline on
bronchoconstrictor responses to methacholine, which constricts smooth
muscle directly, adenosine 5'-monophosphate (AMP), which acts indirectly by
mast cell activation, and sodium metabisulfite (MBS), which stimulates
sensory nerves, in 15 mild asthmatic subjects in a randomized double-blind
study carried out in two phases. In the first phase 12 subjects inhaled two
doses of 0.5 and 2.5 mg terbutaline or placebo administered as a dry power
(Tubohaler) 20 min before challenge with methacholine and AMP. Each subject
received increasing doubling doses of methacholine and AMP nebulized from a
dosimeter. Challenges were terminated when FEV1 fell by 20% from baseline
(PC20). In the second phase 10 subjects (seven of whom had participated in
Phase 1) inhaled 0.5 mg terbutaline or placebo before similar challenge
with methacholine and MBS. In Phase 1 terbutaline inhibited the
bronchoconstrictor response to methacholine by 2.1 and 3.3 doubling doses
but caused a significantly greater inhibition of the response to AMP of 3.4
and 4.8 doubling doses after 0.5 and 2.5 mg, respectively. In the second
phase 0.5 mg terbutaline had equivalent effects on responses to both
methacholine and MBS of 2.6 and 2.2 doubling dilutions, respectively. This
effect on methacholine and MBS implies functional antagonism of airway
smooth muscle. The enhanced effect on AMP implies an additional non-smooth
muscle action that may involve suppression of airway mast cell function.
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Copyright © 1994 American Thoracic Society
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