Effect of capsaicin inhalation on methacholine responsiveness in normal subjects

It has been reported that capsaicin stimulates bronchial C-fibers to release neuropeptides and enhances bronchial responsiveness in animal studies. This study was conducted to examine the effect of inhaled capsaicin on bronchial responsiveness in humans. A provocative concentration of methacholine producing a 20% fall in forced expiratory volume in one second (PC20-FEV1) was measured before and one hour after, one minute inhalation of 15.6 microM capsaicin solution (capsaicin day) or saline (saline day) in seven normal subjects whose PC20-FEV1 values were 40 mg/ml or less. Geometric mean value of PC20-FEV1 measured one hour after saline inhalation following pretreatment methacholine challenge was 89.3 (GSEM, 1.69) mg/ml which was significantly (p = 0.018) greater than that of the pre-treatment methacholine challenge [19.1 (GSEM, 1.37) mg/ml] on a saline day. However, PC20-FEV1 was not different before and one hour after inhalation of capsaicin [18.1 (GSEM, 1.40) and 23.1 (GSEM, 1.80) mg/ml] on a capsaicin day. The PC20-FEV1 value of the post-treatment methacholine test was significantly (p = 0.0277) lower after capsaicin inhalation than after saline inhalation while the value of pre-treatment methacholine provocation was not different between capsaicin and saline days. Change of the post-treatment PC20-FEV1 from the pre-treatment value shown as doubling concentrations was 0.35 +/- 0.60 on a capsaicin day which was significantly lower than that (2.23 +/- 0.47) on a saline day. These results confirm the decreased responses to repeated inhalation of methacholine (methacholine tachyphylaxis) in normal subjects and indicate that capsaicin inhalation reduces the methacholine tachyphylaxis.