Airway smooth muscle tone curbs hyperresponsiveness in experimental asthma

Background & ObjectivesA sustained contraction of airway smooth muscle, hereinafter called tone, increases the response to methacholine in healthy mice and humans. However, the effect of tone in the context of an active inflammation remains to be investigated. The objective of the present study was to test the effect of tone on the in vivo response to methacholine in mice during an active inflammatory phase of experimental asthma. MethodsMale BALB/c mice were exposed once-daily to either intranasal saline or house dust mite for 10 consecutive days to induce experimental asthma. They then underwent one of two methacholine challenges 24 h after the last exposure. While the same cumulative dose was administered in both challenges, one was preceded by a 20-min period of tone induced by nebulizing low doses of methacholine at 5-min intervals. Respiratory mechanics were monitored before and throughout the methacholine challenge by oscillometry. Bronchoalveolar lavages (BAL) and histology were also performed. ResultsBAL inflammation and histological alterations were consistent with experimental asthma. In accordance with previous studies, tone potentiated the response to methacholine in control mice, mainly by stiffening the lung periphery. The lung was even stiffer upon methacholine challenge during an active phase of inflammation in mice with experimental asthma, but this was not further potentiated by tone. In fact, in mice with experimental asthma, tone mitigated hyperresponsiveness by preventing further airway narrowing and, more importantly, small airway narrowing heterogeneity and closure. ConclusionDuring an active inflammatory phase of experimental asthma, tone protects against hyperresponsiveness. Take-home messageThe effect of airway smooth muscle tone on the methacholine response was investigated in mice with or without experimental asthma. While tone potentiated the methacholine response in control mice, it mitigated hyperresponsiveness in experimental asthma. These results unveiled a protective role of the airway smooth muscle in experimental asthma.