Molecular Mechanisms for the Mechanical Modulation of Airway Responsiveness

The smooth muscle of the airways is exposed to continuously changing mechanical forces during normal breathing. The mechanical oscillations that occur during breathing have profound effects on airway tone and airway responsiveness both in experimental animals and humans in vivo and in isolated airway tissues in vitro. Experimental evidence suggests that alterations in the contractile and mechanical properties of airway smooth muscle tissues caused by mechanical perturbations result from adaptive changes in the organization of the cytoskeletal architecture of the smooth muscle cell. The cytoskeleton is a dynamic structure that undergoes rapid reorganization in response to external mechanical and pharmacologic stimuli. Contractile stimulation initiates the assembly of cytoskeletal/extracellular matrix adhesion complex proteins into large macromolecular signaling complexes (adhesomes) that undergo activation to mediate the polymerization and reorganization of a submembranous network of actin filaments at the cortex of the cell. Cortical actin polymerization is catalyzed by Neuronal-Wiskott–Aldrich syndrome protein (N-WASP) and the Arp2/3 complex, which are activated by pathways regulated by paxillin and the small GTPase, cdc42. These processes create a strong and rigid cytoskeletal framework that may serve to strengthen the membrane for the transmission of force generated by the contractile apparatus to the extracellular matrix, and to enable the adaptation of smooth muscle cells to mechanical stresses. This model for the regulation of airway smooth muscle function can provide novel perspectives to explain the normal physiologic behavior of the airways and pathophysiologic properties of the airways in asthma.

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Airway smooth muscle tone increases airway responsiveness in healthy young adults

AJP Lung Cellular and Molecular Physiology ◽

10.1152/ajplung.00400.2016 ◽

2017 ◽

Vol 312 (3) ◽

pp. L348-L357 ◽

Cited By ~ 7

Author(s):

Morgan Gazzola ◽

Katherine Lortie ◽

Cyndi Henry ◽

Samuel Mailhot-Larouche ◽

David G. Chapman ◽

...

Keyword(s):

Smooth Muscle ◽

Airway Smooth Muscle ◽

Muscle Tone ◽

Airway Responsiveness ◽

High Dose ◽

Forced Oscillation Technique ◽

Single High Dose ◽

Healthy Humans

Force adaptation, a process whereby sustained spasmogenic activation (viz., tone) of airway smooth muscle (ASM) increases its contractile capacity, has been reported in isolated ASM tissues in vitro, as well as in mice in vivo. The objective of the present study was to assess the effect of tone on airway responsiveness in humans. Ten healthy volunteers underwent methacholine challenge on two occasions. One challenge consisted of six serial doses of saline followed by a single high dose of methacholine. The other consisted of six low doses of methacholine 5 min apart followed by a higher dose. The cumulative dose was identical for both challenges. After both methacholine challenges, subjects took a deep inspiration (DI) to total lung capacity as another way to probe ASM mechanics. Responses to methacholine and the DI were measured using a multifrequency forced oscillation technique. Compared with a single high dose, the challenge preceded by tone led to an elevated response measured by respiratory system resistance (Rrs) and reactance at 5 Hz. However, there was no difference in the increase in Rrs at 19 Hz, suggesting a predominant effect on smaller airways. Increased tone also reduced the efficacy of DI, measured by an attenuated maximal dilation during the DI and an increased renarrowing post-DI. We conclude that ASM tone increases small airway responsiveness to inhaled methacholine and reduces the effectiveness of DI in healthy humans. This suggests that force adaptation may contribute to airway hyperresponsiveness and the reduced bronchodilatory effect of DI in asthma.

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Chronic inflation of ferret lungs with CPAP reduces airway smooth muscle contractility in vivo and in vitro

Journal of Applied Physiology ◽

10.1152/japplphysiol.00241.2007 ◽

2008 ◽

Vol 104 (3) ◽

pp. 610-615 ◽

Cited By ~ 31

Author(s):

Z. Xue ◽

L. Zhang ◽

Y. Liu ◽

S. J. Gunst ◽

R. S. Tepper

Keyword(s):

Smooth Muscle ◽

Mechanical Stress ◽

Chronic Stress ◽

Airway Smooth Muscle ◽

Structural Changes ◽

Airway Responsiveness ◽

Wall Structure ◽

Airway Wall

The mechanical stress imposed on the lungs during breathing is an important modulator of airway responsiveness in vivo. Our recent study demonstrated that continuous positive airway pressure applied to the lungs of nonanesthetized, tracheotomized rabbits for 4 days decreased lower respiratory system responsiveness to challenge with ACh (Xue Z, Zhang L, Ramchandani R, Liu Y, Antony VB, Gunst SJ, Tepper RS. J. Appl Physiol 99: 677–682, 2005). In addition, airway segments excised from the lungs of these animals and studied in vitro exhibited reduced contractility. However, the mechanism for this reduction in contractility was not determined. The stress-induced decrease in airway responsiveness could have resulted from alterations in the excitation-contraction coupling mechanisms of the smooth muscle cells, or it might reflect changes in the structure and/or composition of the airway wall tissues. In the present study, we assessed the effect of prolonged chronic stress of the lungs in vivo on airway smooth muscle force generation, myosin light chain phosphorylation, and airway wall structure. To enhance the potential development of stress-induced structural changes, we applied mechanical stress for a prolonged period of time of 2–3 wk. Our results demonstrate a direct connection between the decreased airway responsiveness caused by chronic mechanical stress of the lungs in vivo and a persistent decrease in contractile protein activation in the airway smooth muscle isolated from those lungs. The chronic stress also caused an increase in airway size but no detectable changes in the composition of the airway wall.

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Pharmacological characterization of airway smooth muscle responses to antigen in ascaris-sensitive dogs

Canadian Journal of Physiology and Pharmacology ◽

10.1139/y86-231 ◽

1986 ◽

Vol 64 (11) ◽

pp. 1361-1367 ◽

Cited By ~ 3

Author(s):

M. S. Kannan ◽

C. Davis ◽

A. Sankaranarayanan ◽

A. R. C. Ladenius ◽

L. Kannan

Keyword(s):

Smooth Muscle ◽

Airway Smooth Muscle ◽

Smooth Muscle Contraction ◽

Airway Responsiveness ◽

Antigen Challenge ◽

Minor Role ◽

Pharmacological Characterization ◽

A Minor

The dog model of ascaris airway sensitivity was chosen because of its frequency and its immunologic similarity to the human atopic asthmatic state. We studied the mediators of the antigen-induced airway response in vitro and the alterations in the in vivo and in vitro responsiveness to spasmogens evoked by antigen challenge. A myogenic basis of altered reactivity was suggested by the following: (i) tetrodotoxin-insensitive spontaneous active tone; (ii) phasic contractions of airway smooth muscle; and (iii) responsiveness to leukotrienes C4 and D4. The pharmacologic characteristics of the antigen-induced airway smooth muscle contraction in vitro were similar to those induced by arachidonic acid and the leukotrienes only in some respects but were clearly different from those induced by compound 48/80. This suggested a predominant role for arachidonate lipoxygenase products. Histamine apeared to play a minor role in the antigen response. Comparisons were made between antigen-induced responses of actively and passively sensitized airways tissues. In the latter, histamine release appeared to contribute to the initial antigen-induced contraction and, unlike in actively sensitized airways, the responses were easily densensitized to repeated challenge. Alterations of airway responsiveness were demonstrated in vivo to acetylcholine and 5-HT following antigen challenge of highly ascaris-sensitive dogs. In vitro studies of passively sensitized muscle showed selectively enhanced response to 5-HT following antigen challenge. These studies suport the presence of altered myogenic properties of airway smooth muscle and nonspecific increased airway responsiveness in this animal model.

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Elastase alters contractility and promotes an inflammatory synthetic phenotype in airway smooth muscle tissues

AJP Lung Cellular and Molecular Physiology ◽

10.1152/ajplung.00334.2017 ◽

2018 ◽

Vol 314 (4) ◽

pp. L626-L634 ◽

Cited By ~ 7

Author(s):

Angelia D. Lockett ◽

Yidi Wu ◽

Susan J. Gunst

Keyword(s):

Extracellular Matrix ◽

Smooth Muscle ◽

Airway Smooth Muscle ◽

Inflammatory Cells ◽

Contractile Force ◽

Airway Hyperreactivity ◽

Inflammatory Phenotype ◽

Increased Sensitivity

Neutrophil elastase is secreted by inflammatory cells during airway inflammation and can elicit airway hyperreactivity in vivo. Elastase can degrade multiple components of the extracellular matrix. We hypothesized that elastase might disrupt the connections between airway smooth muscle (ASM) cells and the extracellular matrix and that this might have direct effects on ASM tissue responsiveness and inflammation. The effect of elastase treatment on ASM contractility was assessed in vitro in isolated strips of canine tracheal smooth muscle by stimulation of tissues with cumulatively increasing concentrations of acetylcholine (ACh) and measurement of contractile force. Elastase treatment potentiated contractile responses to ACh at low concentrations but suppressed the maximal contractile force generated by the tissues without affecting the phosphorylation of myosin regulatory light chain (RLC). Elastase also promoted the secretion of eotaxin and the activation of Akt in ASM tissues and decreased expression of smooth muscle myosin heavy chain, consistent with promotion of a synthetic inflammatory phenotype. As the degradation of matrix proteins can alter integrin engagement, we evaluated the effect of elastase on the assembly and activation of integrin-associated adhesion junction complexes in ASM tissues. Elastase led to talin cleavage, reduced talin binding to vinculin, and suppressed activation of the adhesome proteins paxillin, focal adhesion kinase, and vinculin, indicating that elastase causes the disassembly of adhesion junction complexes and the inactivation of adhesome signaling proteins. We conclude that elastase promotes an inflammatory phenotype and increased sensitivity to ACh in ASM tissues by disrupting signaling pathways mediated by integrin-associated adhesion complexes.

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Inhibition of dihydropyridine-sensitive calcium entry in hypoxic relaxation of airway smooth muscle

AJP Lung Cellular and Molecular Physiology ◽

10.1152/ajplung.1995.268.2.l201 ◽

1995 ◽

Vol 268 (2) ◽

pp. L201-L206 ◽

Cited By ~ 2

Author(s):

C. Vannier ◽

T. L. Croxton ◽

L. S. Farley ◽

C. A. Hirshman

Keyword(s):

Smooth Muscle ◽

Airway Smooth Muscle ◽

Muscle Tone ◽

Bay K 8644 ◽

O2 Concentration ◽

Voltage Dependent ◽

Progressive Hypoxia ◽

Carbamyl Choline

Hypoxia dilates airways in vivo and reduces active tension of airway smooth muscle in vitro. To determine whether hypoxia impairs Ca2+ entry through voltage-dependent channels (VDC), we tested the ability of dihydropyridines to modulate hypoxia-induced relaxation of KCl- and carbamyl choline (carbachol)-contracted porcine bronchi. Carbachol- or KCl-contracted bronchial rings were exposed to progressive hypoxia in the presence or absence of 1 microM BAY K 8644 (an L-type-channel agonist). In separate experiments, rings were contracted with carbachol or KCl, treated with nifedipine (a VDC antagonist), and finally exposed to hypoxia. BAY K 8644 prevented hypoxia-induced relaxation in KCl-contracted bronchi. Nifedipine (10(-5) M) totally relaxed KCl- contracted bronchi. Carbachol-contracted bronchi were only partially relaxed by nifedipine but were completely relaxed when the O2 concentration of the gas was reduced from 95 to 0%. These data indicate that hypoxia can reduce airway smooth muscle tone by limiting entry of Ca2+ through a dihydropyridine-sensitive pathway, but that other mechanisms also contribute to hypoxia-induced relaxation of carbachol-contracted bronchi.

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Development and maintenance of force and stiffness in airway smooth muscle

Canadian Journal of Physiology and Pharmacology ◽

10.1139/cjpp-2014-0404 ◽

2015 ◽

Vol 93 (3) ◽

pp. 163-169 ◽

Cited By ~ 3

Author(s):

Bo Lan ◽

Brandon A. Norris ◽

Jeffrey C.-Y. Liu ◽

Peter D. Paré ◽

Chun Y. Seow ◽

...

Keyword(s):

Extracellular Matrix ◽

Smooth Muscle ◽

Solid State ◽

Airway Smooth Muscle ◽

Deep Inspiration ◽

Contractile Apparatus ◽

Tidal Breathing ◽

Actomyosin Interaction ◽

Filament Network

Airway smooth muscle (ASM) plays a central role in the excessive narrowing of the airway that characterizes the primary functional impairment in asthma. This phenomenon is known as airway hyper-responsiveness (AHR). Emerging evidence suggests that the development and maintenance of ASM force involves dynamic reorganization of the subcellular filament network in both the cytoskeleton and the contractile apparatus. In this review, evidence is presented to support the view that regulation of ASM contraction extends beyond the classical actomyosin interaction and involves processes within the cytoskeleton and at the interfaces between the cytoskeleton, the contractile apparatus, and the extracellular matrix. These processes are initiated when the muscle is activated, and collectively they cause the cytoskeleton and the contractile apparatus to undergo structural transformation, resulting in a more connected and solid state that allows force generated by the contractile apparatus to be transmitted to the extracellular domain. Solidification of the cytoskeleton also serves to stiffen the muscle and hence the airway. Oscillatory strain from tidal breathing and deep inspiration is believed to be the counter balance that prevents hypercontraction and stiffening of ASM in vivo. Dysregulation of this balance could lead to AHR seen in asthma.

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In vitro, in silico and in vivo study challenges the impact of bronchial thermoplasty on acute airway smooth muscle mass loss

European Respiratory Journal ◽

10.1183/13993003.01680-2017 ◽

2018 ◽

Vol 51 (5) ◽

pp. 1701680 ◽

Cited By ~ 15

Author(s):

Igor L. Chernyavsky ◽

Richard J. Russell ◽

Ruth M. Saunders ◽

Gavin E. Morris ◽

Rachid Berair ◽

...

Keyword(s):

Smooth Muscle ◽

Asthma Control ◽

Muscle Mass ◽

Airway Smooth Muscle ◽

In Silico ◽

Heat Distribution ◽

Airway Wall ◽

Epithelial Integrity

Bronchial thermoplasty is a treatment for asthma. It is currently unclear whether its histopathological impact is sufficiently explained by the proportion of airway wall that is exposed to temperatures necessary to affect cell survival.Airway smooth muscle and bronchial epithelial cells were exposed to media (37–70°C) for 10 s to mimic thermoplasty. In silico we developed a mathematical model of airway heat distribution post-thermoplasty. In vivo we determined airway smooth muscle mass and epithelial integrity pre- and post-thermoplasty in 14 patients with severe asthma.In vitro airway smooth muscle and epithelial cell number decreased significantly following the addition of media heated to ≥65°C. In silico simulations showed a heterogeneous heat distribution that was amplified in larger airways, with <10% of the airway wall heated to >60°C in airways with an inner radius of ∼4 mm. In vivo at 6 weeks post-thermoplasty, there was an improvement in asthma control (measured via Asthma Control Questionnaire-6; mean difference 0.7, 95% CI 0.1–1.3; p=0.03), airway smooth muscle mass decreased (absolute median reduction 5%, interquartile range (IQR) 0–10; p=0.03) and epithelial integrity increased (14%, IQR 6–29; p=0.007). Neither of the latter two outcomes was related to improved asthma control.Integrated in vitro and in silico modelling suggest that the reduction in airway smooth muscle post-thermoplasty cannot be fully explained by acute heating, and nor did this reduction confer a greater improvement in asthma control.

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

Journal of Engineering and Science in Medical Diagnostics and Therapy ◽

10.1115/1.4042309 ◽

2019 ◽

Vol 2 (1) ◽

Cited By ~ 1

Author(s):

Ynuk Bossé

Keyword(s):

Smooth Muscle ◽

Airway Smooth Muscle ◽

Total Lung Capacity ◽

In Vitro Studies ◽

Deep Inspiration ◽

Bronchodilator Effect ◽

Lung Capacity ◽

Residual 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.

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Adaptation to chronic length change in explanted airway smooth muscle

Journal of Applied Physiology ◽

10.1152/japplphysiol.01180.2002 ◽

2003 ◽

Vol 95 (1) ◽

pp. 448-453 ◽

Cited By ~ 33

Author(s):

Jahanbakhsh Naghshin ◽

Lu Wang ◽

Peter D. Paré ◽

Chun Y. Seow

Keyword(s):

Smooth Muscle ◽

Airway Smooth Muscle ◽

Culture Media ◽

Length Change ◽

Functional Change ◽

Muscle Stiffness ◽

Airway Narrowing

It has been shown that airway smooth muscle in vitro is able to maintain active force over a large length range by adaptation in the absence of periodic stimulations at 4°C (Wang L, Paré PD, and Seow CY. J Appl Physiol 90: 734–740, 2001). In this study, we show that such adaptation also takes place at body temperature and that long-term adaptation results in irreversible functional change in the muscle that could lead to airway hyperresponsiveness. Rabbit tracheal muscle explants were passively maintained at shortened and in situ length for 3 and 7–8 days in culture media; the length-tension relationship was then examined. The length associated with maximal force generation decreased by 10.5 ± 3.8% (SE) after 3 days and 37.7 ± 8.5% after 7 or 8 days of passive shortening. At day 3, the left shift in the length-tension curve due to adaptation at short lengths was reversible by readapting the muscle at a longer length. The shift was, however, not completely reversible after 7 days. The results suggest that long-term adaptation of airway smooth muscle could lead to increased muscle stiffness and force-generating ability at short lengths. Under in vivo condition, this could translate into resistance to stretch-induced relaxation and excessive airway narrowing.

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