Airway smooth muscle (ASM) is the major effector of excessive airway narrowing in asthma. Changes in some of the mechanical properties of ASM could contribute to excessive narrowing and have not been systematically studied in human ASM from nonasthmatic and asthmatic subjects.Human ASM strips (eight asthmatic and six nonasthmatic) were studied at in situ length and force was normalised to maximal force induced by electric field stimulation (EFS). Measurements included: passive and active force versus length before and after length adaptation, the forcevelocity relationship, maximal shortening and force recovery after length oscillation. Force was converted to stress by dividing by cross-sectional area of muscle.The only functional differences were that the asthmatic tissue was stiffer at longer lengths (p,0.05) and oscillatory strain reduced isometric force in response to EFS by 19% as opposed to 36% in nonasthmatics (p,0.01).The mechanical properties of human ASM from asthmatic and nonasthmatic subjects are comparable except for increased passive stiffness and attenuated decline in force generation after an oscillatory perturbation. These data may relate to reduced bronchodilation induced by a deep inspiration in asthmatic subjects.KEYWORDS: Airway hyperresponsiveness, airway mechanics, asthma, force-velocity relationships, length-tension relationships, smooth muscle A sthma is characterised by exaggerated airway narrowing caused by airway smooth muscle (ASM) shortening. However, it is unclear whether there is a fundamental phenotypic change in the ASM itself or if the nonmuscle components of the airway wall or surrounding lung parenchyma are primary contributors to this airway hyperresponsiveness (AHR) [1,2]. A major hurdle to a clear understanding of ASM contractile function in disease has been the limited data. Of the 12 studies in which ASM mechanical properties have been compared in asthmatic and nonasthmatic tissue, seven have demonstrated no differences [3][4][5][6][7][8][9], while five have shown increases in force, shortening or agonist sensitivity [10][11][12][13][14].We have previously demonstrated that ASM cell bundles carefully dissected from the tracheas of nonasthmatic subjects whose lungs were donated for medical research provide a valuable, high quality tissue preparation for study of the mechanical properties of ASM [15].We showed that the mechanical properties of nonasthmatic ASM were similar to those measured in other mammalian models. This is in contrast to previous studies which suggested that human ASM produced less force per unit area and shortened less than the ASM of other mammals [16].The purpose of this study was to re-evaluate a series of hypotheses related to ASM mechanics that have been suggested as possible defects in asthmatic ASM function and potential contributors to AHR. These include determining whether asthmatic ASM produces more stress (force per unit cross-sectional area of muscle) than nonasthmatic ASM [17,18]; whether the length-tension relationship of asthmat...

Lung inflammation and airway hyperresponsiveness (AHR) are hallmarks of asthma, but their interrelationship is unclear. Excessive shortening of airway smooth muscle (ASM) in response to bronchoconstrictors is likely an important determinant of AHR. Hypercontractility of ASM could stem from a change in the intrinsic properties of the muscle, or it could be due to extrinsic factors such as chronic exposure of the muscle to inflammatory mediators in the airways. The latter could be the link between lung inflammation and AHR. The present study was designed to examine the influence of chronic exposure to a contractile agonist on the force-generating capacity of ASM. Force generation in response to electric field stimulation (EFS) was measured in ovine trachealis with or without a basal tone induced by acetylcholine (ACh). While the tone was maintained, the EFS-induced force decreased transiently but increased over time to reach a plateau in approximately 50 minutes. The total force (ACh tone + EFS force) increased monotonically and in proportion to ACh concentration. The results indicate that the muscle adapted to the basal tone and regained its contractile ability in response to a second stimulus (EFS) over time. Analysis suggests that this is due to a cytoskeletal transformation that allows the cytoskeleton to bear force, thus freeing up actomyosin crossbridges to generate more force. Force adaptation in ASM as a consequence of prolonged exposure to the many spasmogens found in asthmatic airways could be a mechanism contributing to AHR seen in asthma.

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.

Muscle contraction underlies many essential functions such as breathing, heart beating, locomotion, regulation of blood pressure, and airway resistance. Active shortening of muscle is the result of cycling of myosin cross-bridges that leads to sliding of myosin filaments relative to actin filaments. In this study, we have developed a computer program that allows us to alter the rates of transitions between any cross-bridge-states in a stochastic cycle. The cross-bridge states within the cycle are divided into six attached (between myosin cross-bridges and actin filaments) states and one detached state. The population of cross-bridges in each of the states is determined by the transition rates throughout the cycle; differential equations describing the transitions are set up as a cyclic matrix. A method for rapidly obtaining steady-state exact solutions for the cyclic matrix has been developed to reduce computation time and avoid the divergence problem associated with numerical solutions. In the seven-state model, two power strokes are assumed for each cross-bridge cycle, one before the release of inorganic phosphate, and one after. The characteristic hyperbolic force-velocity relationship observed in muscle contraction can be reproduced by the model. Deviation from the single hyperbolic behavior at low velocities can be mimicked by allowing the rate of cross-bridge-attachment to vary with velocity. The effects of [ATP], [ADP], and [P(i)] are simulated by changing transition rates between specific states. The model has revealed new insights on how the force-velocity characteristics are related to the state transitions in the cross-bridge cycle.