Human trachealis and main bronchi smooth muscle are normoresponsive in asthma

Abstract

Rationale: Airway smooth muscle (ASM) plays a key role in airway hyperresponsiveness (AHR) but it is unclear whether its contractility is intrinsically changed in asthma.

Objectives: To investigate whether key parameters of ASM contractility are altered in subjects with asthma.

Methods: Human trachea and main bronchi were dissected free of epithelium and connective tissues and suspended in a force-length measurement set-up. After equilibration each tissue underwent a series of protocols to assess its methacholine dose-response relationship, shortening velocity, and response to length oscillations equivalent to tidal breathing and deep inspirations.

Measurements and main results: Main bronchi and tracheal ASM were significantly hyposensitive in subjects with asthma compared with control subjects. Trachea and main bronchi did not show significant differences in reactivity to methacholine and unloaded tissue shortening velocity (Vmax) compared with control subjects. There were no significant differences in responses to deep inspiration, with or without superimposed tidal breathing oscillations. No significant correlations were found between age, body mass index, or sex and sensitivity, reactivity, or Vmax.

Conclusions: Our data show that, in contrast to some animal models of AHR, human tracheal and main bronchial smooth muscle contractility is not increased in asthma. Specifically, our results indicate that it is highly unlikely that ASM half-maximum effective concentration (EC50) or Vmax contribute to AHR in asthma, but, because of high variability, we cannot conclude whether or not asthmatic ASM is hyperreactive.

Keywords: airway hyperresponsiveness; airway smooth muscle mechanics; asthma; shortening velocity; smooth muscle.

Figure 1.

Traces from all airway smooth muscle tissue mechanics experiments, and a histology sample. Traces are the average of all tissues from subjects with asthma. (A) Trace of methacholine (MCh) dose–response protocol. (B) Trace of force–velocity protocol. (C and D) Traces of deep inspiration (DI) protocols. (E) Sample of a histology image of smooth muscle cross-section with Masson's trichrome staining. EFS = electrical field stimulation.

Figure 2.

Methacholine (MCh) dose–response curves. Triangles represent trachea and circles main bronchi tissues; solid symbols are control subjects and open symbols are subjects with asthma. (A) Absolute stress dose–response of main bronchi (MB) and trachea (T) in subjects with asthma and control subjects. (B) Dose–response curves normalized to maximum stress (σ_max). (C) σ_max derived from curve fits of the dose response. No significant differences were found. (D) EC₅₀ derived from dose–response curves. EC₅₀ showed significant differences with disease state (*P = 0.05) but not location. (E) Confidence interval of the difference of the means of pooled trachea and main bronchi data for EC₅₀ and σ_max in control subjects versus subjects with asthma. EC₅₀ = half-maximum effective concentration.

Figure 3.

(A) Electrical field stimulation force–velocity curves. Shortening velocity was measured at five force clamps, and Vmax was calculated using extrapolation of a Hill-curve curve fit. (B) Vmax for main bronchi (MB) and trachea (T) in asthma and control. Triangles represent trachea and circles main bronchi tissues; solid symbols are control subjects and open symbols are subjects with asthma. No significant differences were found. (C) Confidence interval of the difference of the means of pooled trachea and main bronchi data of control subjects versus subjects with asthma

Figure 4.

Deep inspiration (DI) response. All stresses are normalized to the average contractile stress over three electrical field stimulation (EFS) contractions prior to the first DI. (A and C) EFS contractile stress after a DI in passive, relaxed airway smooth muscle. (B and D) EFS contractile stress after a DI in methacholine 10⁻⁶ M contracted airway smooth muscle. C and D follow the same protocol as A and B but with a continuous superimposed length oscillation equivalent in amplitude and frequency to tidal breathing. Black squares are control subjects (n = 6) and gray squares subjects with asthma (n = 6). Two-way repeated measures analysis of variance showed statistically significant differences within the same group of subjects, but not between subjects with asthma and control subjects. Markers indicate significant differences: *different from force prior to DI; **same as * but also different from all subsequent EFS contractions; ⁺different from force at 20 and 25 minutes; ⁺⁺different from force at 25 minutes.

Figure 5.

Body mass index (BMI), age, and sex correlations for three contractility parameters. None of the parameters showed a significant correlation with BMI, age, or sex.