Duration of tachyphylaxis in response to methacholine in healthy non-asthmatic subjects

It is well recognized that bronchial responsiveness to methacholine is reduced after methacholine-induced bronchoconstriction in non-asthmatic subjects, but not in asthmatic subjects. However, it is unknown how long the methacholine tachyphylaxis lasts. The present study was conducted to elucidate duration of the methacholine tachyphylaxis in healthy non-asthmatic subjects. Measurements of methacholine responsiveness were repeated six times at intervals of 1 h and 1, 2, 3 and 7 days in eight healthy non-asthmatic young female subjects in whom methacholine concentrations causing a 20% fall in forced expiratory volume in 1 sec (FEV1; PC20-Meth) were 40 mg/mL or less. Geometric mean value (GSEM) of PC20-Meth was 10.6 (1.44) mg/mL at the first challenge, 63.4 (1.85) at the 1-h interval, 50.2 (1.67) at the 1-day interval, 49.7 (1.68) at the 2-day interval, 17.3 (1.51) at the 3-day interval and 13.1 (1.44) mg/mL at the 7-day interval. The values at intervals of 1 h to 3 days were significantly greater than the initial value. These results indicate that measurements of methacholine responsiveness should be separated by at least 7 days to avoid the tachyphylaxis when the tests are repeated in non-asthmatic subjects.