An in vitro study examining the duration between deep inspirations on the rate of renarrowing

Force adaptation through the intravenous route in naïve mice

Aim of the study: Force adaptation is a process whereby the contractile capacity of the airway smooth muscle increases during a sustained contraction (aka tone). Tone also increases the response to a nebulized challenge with methacholine in vivo, presumably through force adaptation. Yet, due to its patchy pattern of deposition, nebulized methacholine often spurs small airway narrowing heterogeneity and closure, two important enhancers of the methacholine response. This raises the possibility that the potentiating effect of tone on the methacholine response is not due to force adaptation but by furthering heterogeneity and closure. Herein, methacholine was delivered homogenously through the intravenous (i.v.) route. Materials and Methods: Female and male BALB/c mice were subjected to one of two i.v. methacholine challenges, each of the same cumulative dose but starting by a 20-min period either with or without tone induced by serial i.v. boluses. Changes in respiratory mechanics were monitored throughout by oscillometry, and the response after the final dose was compared between the two challenges to assess the effect of tone. Results: For the elastance of the respiratory system (Ers), tone potentiated the methacholine response by 64 and 405% in females (37.4 ± 10.7 vs. 61.5 ± 15.1 cmH2O/mL; p = 0.01) and males (33.0 ± 14.3 vs. 166.7 ± 60.6 cmH2O/mL; p = 0.0004), respectively. For the resistance of the respiratory system (Rrs), tone potentiated the methacholine response by 129 and 225% in females (9.7 ± 3.5 vs. 22.2 ± 4.3 cmH2O·s/mL; p = 0.0003) and males (10.7 ± 3.1 vs. 34.7 ± 7.9 cmH2O·s/mL; p < 0.0001), respectively. Conclusions: As previously reported with nebulized challenges, tone increases the response to i.v. methacholine in both sexes; albeit sexual dimorphisms were obvious regarding the relative resistive versus elastic nature of this potentiation. This represents further support that tone increases the lung response to methacholine through force adaptation.

In mice of both sexes, repeated contractions of smooth muscle in vivo greatly enhance the response of peripheral airways to methacholine

BALB/c mice from both sexes underwent one of two nebulized methacholine challenges that were preceded by a period of 20 min either with or without tone induced by repeated contractions of the airway smooth muscle. Impedance was monitored throughout and the constant phase model was used to dissociate the impact of tone on conducting airways (R_N - Newtonian resistance) versus the lung periphery (G and H - tissue resistance and elastance). The effect of tone on smooth muscle contractility was also tested on excised tracheas. While tone markedly potentiated the methacholine-induced gains in H and G in both sexes, the gain in R_N was only potentiated in males. The contractility of female and male tracheas was also potentiated by tone. Inversely, the methacholine-induced gain in hysteresivity (G/H) was mitigated by tone in both sexes. Therefore, the tone-induced muscle hypercontractility impacts predominantly the lung periphery in vivo, but also promotes further airway narrowing in males while protecting against narrowing heterogeneity in both sexes.

Nocturnal bilevel positive airway pressure for the treatment of asthma

Nocturnal worsening of asthma may be due to reduced lung volumes and fewer sigh breaths, which have been shown to increase airway resistance and bronchoreactivity. We hypothesized that mimicking deep inspiration using nocturnal mechanical support would improve symptoms in patients with asthma. Subjects with asthma underwent usual care and bilevel positive airway pressure (PAP) therapy for 4 weeks, separated by 4 weeks, and methacholine challenge (PC20) and subjective assessments. 13 patients with asthma alone and 8 with asthma + OSA completed the protocol. Change in bronchoreactivity (ratio of Post/Pre PC20) was not significantly different during usual care and bilevel PAP [0.86 (IQR 0.19, 1.82) vs 0.94 (IQR 0.56, 2.5), p = 0.88], nor was the change in Asthma Control Test different: 0.1 ± 2.2 vs. -0.2 ± 2.9, p = 0.79, respectively. Bilevel PAP therapy for four weeks did not improve subjective or objective measures of asthma severity in patients with asthma or those with asthma and OSA, although there was heterogeneity in response.

Airway smooth muscle adapting in dynamic conditions is refractory to the bronchodilator effect of a deep inspiration

Airway smooth muscle (ASM) is continuously strained during breathing at tidal volume. Whether this tidal strain influences the magnitude of the bronchodilator response to a deep inspiration (DI) is not clearly defined. The present in vitro study examines the effect of tidal strain on the bronchodilator effect of DIs. ASM strips from sheep tracheas were mounted in organ baths and then subjected to stretches (30% strain), simulating DIs at varying time intervals. In between simulated DIs, the strips were either held at a fixed length (isometric) or oscillated continuously by 6% (length oscillations) to simulate tidal strain. The contractile state of the strips was also controlled by adding either methacholine or isoproterenol to activate or relax ASM, respectively. Although the time-dependent gain in force caused by methacholine was attenuated by length oscillations, part of the acquired force in the oscillating condition was preserved postsimulated DIs, which was not the case in the isometric condition. Consequently, the bronchodilator effect of simulated DIs (i.e., the decline in force postsimulated versus presimulated DIs) was attenuated in oscillating versus isometric conditions. These findings suggest that an ASM operating in a dynamic environment acquired adaptations that make it refractory to the decline in contractility inflicted by a larger strain simulating a DI.

Shortening of airway smooth muscle is modulated by prolonging the time without simulated deep inspirations in ovine tracheal strips

The shortening of airway smooth muscle (ASM) is greatly affected by time. This is because stimuli affecting ASM shortening, such as bronchoactive molecules or the strain inflicted by breathing maneuvers, not only alter quick biochemical processes regulating contraction but also slower processes that allow ASM to adapt to an ever-changing length. Little attention has been given to the effect of time on ASM shortening. The present study investigates the effect of changing the time interval between simulated deep inspirations (DIs) on ASM shortening and its responsiveness to simulated DIs. Excised tracheal strips from sheep were mounted in organ baths and either activated with methacholine or relaxed with isoproterenol. They were then subjected to simulated DIs by imposing swings in distending stress, emulating a transmural pressure from 5 to 30 cmH₂O. The simulated DIs were intercalated by 2, 5, 10, or 30 min. In between simulated DIs, the distending stress was either fixed or oscillating to simulate tidal breathing. The results show that although shortening was increased by prolonging the interval between simulated DIs, the bronchodilator effect of simulated DIs (i.e., the elongation of the strip post- vs. pre-DI) was not affected, and the rate of re-shortening post-simulated DIs was decreased. As the frequency with which DIs are taken increases upon bronchoconstriction, our results may be relevant to typical alterations observed in asthma, such as an increased rate of re-narrowing post-DI.NEW & NOTEWORTHY The frequency with which patients with asthma take deep inspirations (DIs) increases during bronchoconstriction. This in vitro study investigated the effect of changing the time interval between simulated DIs on airway smooth muscle shortening. The results demonstrated that decreasing the interval between simulated DIs not only decreases shortening, which may be protective against excessive airway narrowing, but also increases the rate of re-shortening post-simulated DIs, which may contribute to the increased rate of re-narrowing post-DI observed in asthma.

An in silico study examining the role of airway smooth muscle dynamics and airway compliance on the rate of airway re-narrowing after deep inspiration

Deep inspirations are a widely studied topic due to their varied effectiveness as a bronchodilator in asthmatic and non-asthmatic patients. Specifically, they are known to be effective at reversing bronchoconstriction in non-asthmatic patients but may fail to prevent bronchoconstriction in asthmatic patients. Inspired by a recent study on the effect of deep inspirations on the rate of re-narrowing of an isolated airway, we investigate whether the latch-bridge dynamics of smooth muscle cross-bridge theory, coupled with non-linear compliance of the airway wall, can account for the reported results: namely that only the rate of renarrowing after DI is sensitive to the interval between deep inspirations, while other measures are unaffected. We develop and present length- and pressure-controlled protocols which mimic both the experiments performed in the study, as well as simulate in vivo conditions respectively. Both protocols are simulated and show qualitative agreement with the results reported by the experiments, suggesting that latch-bridge dynamics coupled with airway wall non-compliance may be sufficient to explain these results. Moreover pressure- and length-controlled protocols show important differences which should be considered when designing in vitro experiments to mimic in vivo conditions.

Effect of Loading History on Airway Smooth Muscle Cell-Matrix Adhesions

Integrin-mediated adhesions between airway smooth muscle (ASM) cells and the extracellular matrix (ECM) regulate how contractile forces generated within the cell are transmitted to its external environment. Environmental cues are known to influence the formation, size, and survival of cell-matrix adhesions, but it is not yet known how they are affected by dynamic fluctuations associated with tidal breathing in the intact airway. Here, we develop two closely related theoretical models to study adhesion dynamics in response to oscillatory loading of the ECM, representing the dynamic environment of ASM cells in vivo. Using a discrete stochastic-elastic model, we simulate individual integrin binding and rupture events and observe two stable regimes in which either bond formation or bond rupture dominate, depending on the amplitude of the oscillatory loading. These regimes have either a high or low fraction of persistent adhesions, which could affect the level of strain transmission between contracted ASM cells and the airway tissue. For intermediate loading, we observe a region of bistability and hysteresis due to shared loading between existing bonds; the level of adhesion depends on the loading history. These findings are replicated in a related continuum model, which we use to investigate the effect of perturbations mimicking deep inspirations (DIs). Because of the bistability, a DI applied to the high adhesion state could either induce a permanent switch to a lower adhesion state or allow a return of the system to the high adhesion state. Transitions between states are further influenced by the frequency of oscillations, cytoskeletal or ECM stiffnesses, and binding affinities, which modify the magnitudes of the stable adhesion states as well as the region of bistability. These findings could explain (in part) the transient bronchodilatory effect of a DI observed in asthmatics compared to a more sustained effect in normal subjects.

The Strain on Airway Smooth Muscle During a Deep Inspiration to Total Lung Capacity

The deep inspiration (DI) maneuver entices a great deal of interest because of its ability to temporarily ease the flow of air into the lungs. This salutary effect of a DI is proposed to be mediated, at least partially, by momentarily increasing the operating length of airway smooth muscle (ASM). Concerningly, this premise is largely derived from a growing body of in vitro studies investigating the effect of stretching ASM by different magnitudes on its contractility. The relevance of these in vitro findings remains uncertain, as the real range of strains ASM undergoes in vivo during a DI is somewhat elusive. In order to understand the regulation of ASM contractility by a DI and to infer on its putative contribution to the bronchodilator effect of a DI, it is imperative that in vitro studies incorporate levels of strains that are physiologically relevant. This review summarizes the methods that may be used in vivo in humans to estimate the strain experienced by ASM during a DI from functional residual capacity (FRC) to total lung capacity (TLC). The strengths and limitations of each method, as well as the potential confounders, are also discussed. A rough estimated range of ASM strains is provided for the purpose of guiding future in vitro studies that aim at quantifying the regulatory effect of DI on ASM contractility. However, it is emphasized that, owing to the many limitations and confounders, more studies will be needed to reach conclusive statements.

Interval between simulated deep inspirations on the dynamics of airway smooth muscle contraction in guinea pig bronchi

A certain amount of time is required to achieve a maximal contraction from airway smooth muscle (ASM) and stretches of substantial magnitude, such as the ones imparted by deep inspirations (DIs), interfere with contraction. The duration of ASM contraction without interference may thus affect its shortening, its mechanical response to DIs and the overall toll it exerts on the respiratory system. In this study, the effect of changing the interval between DIs on the dynamics of ASM was examined in vitro. Isolated bronchi derived from guinea pigs were held isotonically and stimulated to both contract and relax, in a randomized order, in response to 10^-5 M of methacholine and 10^-6 M of isoproterenol, respectively. Interference to ASM was inflicted after 2, 5, 10 and 30 min in a randomized order, by imposing a stretch that simulated a DI. The shortening before the stretch, the stiffness before and during the stretch, the post-stretch elongation of ASM and the ensuing re-shortening were measured. These experiments were also performed in the presence of simulated tidal breathing achieved through force fluctuations. The results demonstrate that, with or without force fluctuations, increasing the interval between simulated DIs increased shortening and post-stretch elongation, but not stiffness and re-shortening. These time-dependent effects were not observed when ASM was held in the relaxed state. These findings may help understand to which extent ASM shortening and the regulatory effect of DI are affected by changing the interval between DIs. The potential consequences of these findings on airway narrowing are also discussed.

A Distribution-Moment Approximation for Coupled Dynamics of the Airway Wall and Airway Smooth Muscle

Asthma is fundamentally a disease of airway constriction. Due to a variety of experimental challenges, the dynamics of airways are poorly understood. Of specific interest is the narrowing of the airway due to forces produced by the airway smooth muscle wrapped around each airway. The interaction between the muscle and the airway wall is crucial for the airway constriction that occurs during an asthma attack. Although cross-bridge theory is a well-studied representation of complex smooth muscle dynamics, and these dynamics can be coupled to the airway wall, this comes at significant computational cost-even for isolated airways. Because many phenomena of interest in pulmonary physiology cannot be adequately understood by studying isolated airways, this presents a significant limitation. We present a distribution-moment approximation of this coupled system and study the validity of the approximation throughout the physiological range. We show that the distribution-moment approximation is valid in most conditions, and we explore the region of breakdown. These results show that in many situations, the distribution-moment approximation is a viable option that provides an orders-of-magnitude reduction in computational complexity; not only is this valuable for isolated airway studies, but it moreover offers the prospect that rich ASM dynamics might be incorporated into interacting airway models where previously this was precluded by computational cost.