Tachyphylaxis to inhaled methacholine in normal but not asthmatic subjects

Methacholine inhalation tests measure airway responsiveness in asthmatic and normal subjects. Tachyphylaxis occurs with repeated methacholine inhalations in normal subjects. The purpose of this study was to examine the time course and mechanisms of methacholine tachyphylaxis in normal subjects and to determine whether this occurs in mildly asthmatic subjects. Fifteen normal and nine asthmatic subjects were studied on 2 study days, at least 48 h apart. Each day, two inhalation tests were carried out. On one day, subjects performed two methacholine inhalation tests 3 h apart. On another day, a saline inhalation test was followed 3 h later by a methacholine test. Results were expressed as the provocation concentration causing a 20% fall in forced expiratory volume in 1s (FEV₁), (PC₂₀). All normal subjects developed methacholine tachyphylaxis. The mean PC₂₀ increased from 47.3 mg/ml (%SE 1.34) to 115.6 (%SE 1.51) (P<0.0001) in a 3-h interval. This increase lasted for ≥6 h (P=0.012). Asthmatic subjects did not develop methacholine tachyphylaxis. Their mean methacholine PC₂₀s were 1.6 mg/ml (%SE 1.4) and 1.5 (%SE 1.4) (P=0.75) 3 h later. In two other series of experiments, normal subjects were pretreated with the cyclooxygenase inhibitors indomethacin (100 mg/day) or flurbiprofen (150 mg/day) or a placebo for 3 days before two methacholine tests 3 h apart. Both indomethacin and flurbiprofen significantly inhibited the development of methacholine tachyphylaxis. These results confirm that methacholine tachyphylaxis occurs in normal subjects, lasts ≥6 h, and may occur through the release of inhibitory prostaglandins. By contrast, methacholine tachyphylaxis does not occur in asthmatic subjects.