Introduction: The lung response to methacholine is substantially greater in BALB/c than C57BL/6 mice. Whether the stiffness of the lung, which partially dictates the load impeding ASM shortening, contributes to this differing response is unknown. Untangling the determinants of the methacholine response in mice may hint on causes of hyperresponsiveness in lung diseases.

Objective: Examine whether differences in lung stiffness between 2 strains of mice coincide with their differing response to methacholine.

Methods: Respiratory mechanics, lung volume and the response to nebulized methacholine were measured with the flexiVent and compared between male BALB/c and C57BL/6 mice (n=6). Lung size was also measured by volume displacement. The isometric contraction of ASM (when the load imparted by the lung does not matter) was measured on excised tracheas. The auxotonic contraction of ASM (when the load imparted by the lung matters) was assessed by measuring small airway constriction in lung slices using the physioLens.

Results: Lung stiffness was greater in C57BL/6 than BALB/c mice based on 6 different flexiVent readouts: e.g., tissue elastance (H) assessed by oscillometry was 20.8 ± 2.6 vs. 26.8 ± 3.2 cmH₂O?s/mL in BALB/c and C57BL/6 mice, respectively (p=0.002). This was not due to a different lung volume (e.g., functional residual capacity & displaced volume). Tracheal contraction was not different between strains. However, airway constriction in lung slices was greater in BALB/c than C57BL/6 mice (59.4 ± 8.1 vs. 44.0 ± 4.7% of luminal narrowing, respectively; p=0.009).

Conclusion: These results suggest that the lower response to methacholine in C57BL/6 vs. BALB/c mice is not due to a weaker ASM but to a stiffer lung.