BACKGROUND: The direct effect of bronchoconstriction on cough receptor sensitivity is unknown, and the antitussive effect of beta 2 adrenergic agonists in man has been controversial. This study was designed to throw light on these questions. METHODS: The threshold of the cough response to inhaled capsaicin, a stimulant acting on C fibre endings, and tartaric acid, a chemostimulant, was measured before and 10 minutes after inhalation of methacholine, which caused a nearly 20% fall in forced expiratory volume in one second (FEV1), in 14 normal subjects (study 1), and also before and 30 minutes after inhalation of procaterol (30 micrograms), placebo, and saline in eight normal subjects (study 2). Progressively increasing concentrations of capsaicin and tartaric acid solutions were inhaled for 15 seconds by mouth tidal breathing at one minute intervals and cough threshold was defined as the lowest concentration of capsaicin and tartaric acid that elicited five or more coughs. RESULTS: In study 1 the geometric mean values of the cough threshold of response to capsaicin and tartaric acid before methacholine callenge, 2.98 (GSE 1.30) micrograms/ml and 46.6 (1.22) mg/ml, were not significantly different from those of the response to methacholine inhalation, 3.45 (1.33) micrograms/ml and 32.9 (1.37) mg/ml. In study 2 the geometric mean value of the cough threshold of response to capsaicin before inhalation of procaterol (4.61 (GSE 1.84) micrograms/ml) was not different from that after inhalation of procaterol (4.61 (GSE 1.84) micrograms/ml), which had significant bronchodilator effects. The cough threshold was not altered by placebo or saline. CONCLUSIONS: These findings suggest that muscarinic receptor stimulation, bronchoconstriction, beta 2 receptor stimulation, or bronchodilation might have no direct effect on the sensitivity of the cough receptors in normal subjects.
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