Smooth muscle contraction can be divided into two phases: the initial contraction determines the amount of developed force and the second phase determines how well the force is maintained. The initial phase is primarily due to activation of actomyosin interaction and is relatively well understood, whereas the second phase remains poorly understood. Force maintenance in the sustained phase can be disrupted by strains applied to the muscle; the strain causes actomyosin cross-bridges to detach and also the cytoskeletal structure to disassemble in a process known as fluidization, for which the underlying mechanism is largely unknown. In the present study we investigated the ability of airway smooth muscle to maintain force after the initial phase of contraction. Specifically, we examined the roles of Rho-kinase and protein kinase C (PKC) in force maintenance. We found that for the same degree of initial force inhibition, Rho-kinase substantially reduced the muscle's ability to sustain force under static conditions, whereas inhibition of PKC had a minimal effect on sustaining force. Under oscillatory strain, Rho-kinase inhibition caused further decline in force, but again, PKC inhibition had a minimal effect. We also found that Rho-kinase inhibition led to a decrease in the myosin filament mass in the muscle cells, suggesting that one of the functions of Rho-kinase is to stabilize myosin filaments. The results also suggest that dissolution of myosin filaments may be one of the mechanisms underlying the phenomenon of fluidization. These findings can shed light on the mechanism underlying deep inspiration induced bronchodilation.

Airway smooth muscle (ASM) in vivo is constantly subjected to oscillatory strain due to tidal breathing and deep inspirations. ASM contractility is known to be adversely affected by strains, especially those of large amplitudes. Based on the cross-bridge model of contraction, it is likely that strain impairs force generation by disrupting actomyosin cross-bridge interaction. There is also evidence that strain modulates muscle stiffness and force through induction of cytoskeletal remodeling. However, the molecular mechanism by which strain alters smooth muscle function is not entirely clear. Here, we examine the response of ASM to iso-velocity stretches to probe the components within the muscle preparation that give rise to different features in the force response. We found in ASM that force response to a ramp stretch showed a biphasic feature, with the initial phase associated with greater muscle stiffness compared with that in the later phase, and that the transition between the phases occurred at a critical strain of ∼3.3%. Only strains with amplitudes greater than the critical strain could lead to reduction in force and stiffness of the muscle in the subsequent stretches. The initial-phase stiffness was found to be linearly related to the degree of muscle activation, suggesting that the stiffness stems mainly from attached cross bridges. Both phases were affected by the degree of muscle activation and by inhibitors of myosin light-chain kinase, PKC, and Rho-kinase. Different responses due to different interventions suggest that cross-bridge and cytoskeletal stiffness is regulated differently by the kinases.

It has long been known that airway smooth muscle (ASM) contraction contributes significantly to the reversible airflow obstruction that defines asthma. It has also been postulated that phenotypic changes in ASM contribute to the airway hyperresponsiveness (AHR) that is a characteristic feature of asthma. Although there is agreement that the mass of ASM surrounding the airways is significantly increased in asthmatic compared with non-asthmatic airways, it is still uncertain whether there are quantitative or qualitative changes in the level of expression of the genes and proteins involved in the canonical contractile pathway in ASM that could account for AHR. This review will summarize past attempts at quantifying gene expression changes in the ASM of asthmatic lungs as well as non-asthmatic ASM cells stimulated with various inflammatory cytokines. The lack of consistent findings in asthmatic samples coupled with the relative concordance of results from stimulated ASM cells suggests that changes to the contractility of ASM tissues in asthma may be dependent on the presence of an inflammatory environment surrounding the ASM layer. Removal of the ASM from this environment could explain why hypercontractility is rarely seen ex vivo.Key words: asthma, airway smooth muscle, gene expression, protein expression, cytokines. Résumé :On sait depuis longtemps que la contraction du muscle lisse respiratoire (MLR) contribue de manière significative à l'obstruction réversible du débit d'air qui définit l'asthme. Il a aussi été postulé que des changements phénotypiques dans le MLR contribue à l'hyperréactivité bronchique (HRB) qui caractérise l'asthme. Même si l'on s'accorde sur le fait que la masse de MLR qui entoure les voies respiratoires est significativement plus importante en situation d'asthme comparativement à l'absence d'asthme, on ne sait pas encore avec certitude si des changements quantitatifs ou qualitatifs du niveau d'expression de gènes et de protéines impliqués dans les voies standards de régulation de la contraction chez le MLR pourraient expliquer l'HRB. Cet article de revue résumera les tentatives passées de quantification de changements d'expression génique dans des cellules de MLR de poumons asthmatiques et non asthmatiques, stimulées à l'aide de différentes cytokines inflammatoires. L'absence de données cohérentes issues d'échantillons asthmatiques, couplée à la concordance relative des résultats obtenus à partir de cellules de MLR stimulées suggère que les changements de contractilité du MLR dans l'asthme peuvent être dépendants de la présence d'un environnement inflammatoire autour de la couche de MLR. Le fait de retirer le MLR de son environnement pourrait expliquer pourquoi l'hyper-contractilité est rarement observée ex vivo. [Traduit par la Rédaction] Mots-clés : asthme, muscle lisse respiratoire, expression génique, expression protéique, cytokines.