Fading memory model for airway smooth muscle dynamic response

This work presents the application of a fading memory model to describe the behavior of contracted airway smooth muscle (ASM) for two biophysical cases: finite duration length steps and longitudinal sinusoidal oscillations. The model parameters were initially determined from literature data on transient step length change response and subsequently the model was applied to the two cases. Results were compared with previously published experimental data on ASM oscillations. The model confirms a trend observed in the experimental data which shows that: (i) the value of tissue length change is the most important factor to determine the degree of cross-bridge detachment and (ii) a strong correlation exists between increasing frequency and declining stiffness until a certain frequency (∼25 Hz) beyond which frequency dependence is negligible. Although the model was not intended to simulate biophysical events individually, the data could be explained by cross-bridge cycling rates. As the frequency increases, cross-bridge reattachment becomes less likely, until no further cross-bridge attachment is possible.     

A mathematical model of airway and pulmonary arteriole smooth muscle

Airway hyperresponsiveness is a major characteristic of asthma and is believed to result from the excessive contraction of airway smooth muscle cells (SMCs). However, the identification of the mechanisms responsible for airway hyperresponsiveness is hindered by our limited understanding of how calcium (Ca²⁺), myosin light chain kinase (MLCK), and myosin light chain phosphatase (MLCP) interact to regulate airway SMC contraction. In this work, we present a modified Hai-Murphy cross-bridge model of SMC contraction that incorporates Ca²⁺ regulation of MLCK and MLCP. A comparative fit of the model simulations to experimental data predicts 1), that airway and arteriole SMC contraction is initiated by fast activation by Ca²⁺ of MLCK; 2), that airway SMC, but not arteriole SMC, is inhibited by a slower activation by Ca²⁺ of MLCP; and 3), that the presence of a contractile agonist inhibits MLCP to enhance the Ca²⁺ sensitivity of airway and arteriole SMCs. The implication of these findings is that murine airway SMCs exploit a Ca²⁺-dependent mechanism to favor a default state of relaxation. The rate of SMC relaxation is determined principally by the rate of release of the latch-bridge state, which is predicted to be faster in airway than in arteriole. In addition, the model also predicts that oscillations in calcium concentration, commonly observed during agonist-induced smooth muscle contraction, cause a significantly greater contraction than an elevated steady calcium concentration.     

Studies of the mechanism of passive anaphylaxis in human airway smooth muscle

This investigation was carried out to study allergic contraction of passively sensitized human airway smooth muscle in response to specific antigen challenge. We attempted to determine the role played by histamine, slow reaction substances (SRSs), and cyclooxygenase products in the mediation of this response in tracheal smooth muscle. Tissues were passively sensitized with serum from ragweed-sensitive patients (15 h, 4 degrees C). Subsequent challenge with ragweed antigen produced a slowly developing contraction. The peak contraction to a dose producing a maximal response was 37 +/- 6% of the carbachol maximum. Mepyramine (5 X 10(-6) M) did not alter the contraction. Methylprednisolone (2 X 10(-5) M) attenuated the response to antigen but had no significant effect on the contractile response to arachidonic acid. Indomethacin (5.6-28 X 10(-6) M) enhanced the peak antigen-induced contractions by 25 +/- 11% whereas 5,8,11,14-eicosatetraynoic acid (6.4 X 10(-5) M) selectively attenuated the antigen-induced contraction by 86 +/- 12%. Nordihydroguarietic acid (6-12 X 10(-6) M) attenuated both the antigen plus arachidonate induced responses. FPL-55712 (1-2 X 10(-6) M) antagonized the contractions to antigen. Compound 48/80 and goat antihuman immunoglobulin E produced similar slowly developing contractions in sensitized and in some nonsensitized tissues. These responses, except for an early component of the response to 48/80, were independent of histamine and were reversed by FPL-55712. These findings suggest that arachidonic acid metabolites mediate (slow reacting substances) and modulate (prostaglandins) allergic contraction of human airway smooth muscle while any histamine released contributes little or nothing to the contraction in the larger airways.     

Empirical model for dynamic force-length behavior of airway smooth muscle.

An empirical mathematical model that describes the relation between force and length for dynamic loading of maximally activated airway smooth muscle is described. The model consists of three first-order, ordinary differential equations: one for muscle shortening, one for lengthening, and a third that describes the evolution of an internal variable that depends on muscle history. The model fits data on the dynamic force-length behavior of maximally activated trachealis muscle for a range of amplitudes and rates of shortening and lengthening. The muscle model is incorporated into a model for an intact airway tethered to the surrounding parenchyma. As an example of its use, the model airway is subjected to the loading that occurs during a deep breath. After the breath, the rate of muscle shortening is determined by the interaction between muscle dynamics and the elastic load that is imposed by interdependence forces.     

A maturational model for the study of airway smooth muscle adaptation to mechanical oscillation

It has been shown that mechanical stretches imposed on airway smooth muscle (ASM) by deep inspiration reduce the subsequent contractile response of the ASM. This passive maneuver of lengthening and retraction of the muscle is beneficial in normal subjects to counteract bronchospasm. However, it is detrimental to hyperresponsive airways because it triggers further bronchoconstriction. Although the exact mechanisms for this contrary response by normal and hyperresponsive airways are unclear, it has been suggested that the phenomenon is related to changes in ASM adaptability to mechanical oscillation. Healthy immature airways of both human and animal exhibit hyperresponsiveness, but whether the adaptative properties of hyperresponsive airway differ from normal is still unknown. In this article, we review the phenomenon of ASM adaptation to mechanical oscillation and its relevance and implication to airway hyperresponsiveness. We demonstrate that the age-specific expression of ASM adaptation is prominent using an established maturational animal model developed in our laboratory. Our data on immature ASM showed potentiated contractile force shortly after a length oscillation compared with the maximum force generated before oscillation. Several potential mechanisms such as myogenic response, changes in actin polymerization, or changes in the quantity of the cytoskeletal regulatory proteins plectin and vimentin, which may underlie this age-specific force potentiation, are discussed. We suggest a working model of the structure of smooth muscle associated with force transmission, which may help to elucidate the mechanisms responsible for the age-specific expression of smooth muscle adaptation. It is important to study the maturational profile of ASM adaptation as it could contribute to juvenile hyperresponsiveness.     

Neurokinin-neurotrophin interactions in airway smooth muscle

Neurally derived tachykinins such as substance P (SP) play a key role in modulating airway contractility (especially with inflammation). Separately, the neurotrophin brain-derived neurotrophic factor (BDNF; potentially derived from nerves as well as airway smooth muscle; ASM) and its tropomyosin-related kinase receptor, TrkB, are involved in enhanced airway contractility. In this study, we hypothesized that neurokinins and neurotrophins are linked in enhancing intracellular Ca(2+) concentration ([Ca(2+)](i)) regulation in ASM. In rat ASM cells, 24 h exposure to 10 nM SP significantly increased BDNF and TrkB expression (P < 0.05). Furthermore, [Ca(2+)](i) responses to 1 μM ACh as well as BDNF (30 min) effects on [Ca(2+)](i) regulation were enhanced by prior SP exposure, largely via increased Ca(2+) influx (P < 0.05). The enhancing effect of SP on BDNF signaling was blunted by the neurokinin-2 receptor antagonist MEN-10376 (1 μM, P < 0.05) to a greater extent than the neurokinin-1 receptor antagonist RP-67580 (5 nM). Chelation of extracellular BDNF (chimeric TrkB-F(c); 1 μg/ml), as well as tyrosine kinase inhibition (100 nM K252a), substantially blunted SP effects (P < 0.05). Overnight (24 h) exposure of ASM cells to 50% oxygen increased BDNF and TrkB expression and potentiated both SP- and BDNF-induced enhancement of [Ca(2+)](i) (P < 0.05). These results suggest a novel interaction between SP and BDNF in regulating agonist-induced [Ca(2+)](i) regulation in ASM. The autocrine mechanism we present here represents a new area in the development of bronchoconstrictive reflex response and airway hyperreactive disorders.     

Human airway smooth muscle in culture

We describe a method for culturing human airway smooth muscle. Cells were enzymatically and mechanically dispersed from strips of smooth muscle harvested from surgically removed lobar bronchi, and were seeded on to dishes containing Dulbecco's modified Eagle's medium supplemented with 10% fetal bovine serum. After 14-21 days confluent monolayers of cells formed, which were subcultured and identified as smooth muscle by positive immunocytochemical staining for actin and myosin. The retention of functional plasmalemmal receptors and of intracellular signal transduction pathways in cell culture was demonstrated in 45Ca-labelled monolayers by the stimulation of efflux of intracellularly stored 45Ca in response to extracellularly applied 10 microM carbachol or 10 microM histamine. Human airway smooth muscle in cell culture provides a novel preparation for investigating the physiology and pathophysiology of the human airways.     

Plasticity in canine airway smooth muscle

The large volume changes of some hollow viscera require a greater length range for the smooth muscle of their walls than can be accommodated by a fixed array of sliding filaments. A possible explanation is that smooth muscles adapt to length changes by forming variable numbers of contractile units in series. To test for such plasticity we examined the muscle length dependence of shortening velocity and compliance, both of which will vary directly with the number of thick filaments in series. Dog tracheal smooth muscle was studied because its cells are arrayed in long, straight, parallel bundles that span the length of the preparation. In experiments where muscle length was changed, both compliance and velocity showed a strong dependence on muscle length, varying by 1.7-fold and 2.2-fold, respectively, over a threefold range of length. The variation in isometric force was substantially less, ranging from a 1.2- to 1.3-fold in two series of experiments where length was varied by twofold to an insignificant 4% variation in a third series where a threefold length range was studied. Tetanic force was below its steady level after both stretches and releases, and increased to a steady level with 5-6 tetani at 5 min intervals. These results suggest strongly that the number of contractile units in series varies directly with the adapted muscle length. Temporary force depression after a length change would occur if the change transiently moved the filaments from their optimum overlap. The relative length independence of the adapted force is explained by the reforming of the filament lattice to produce optimum force development, with commensurate changes of velocity and compliance.     

Relaxation of canine airway smooth muscle

Relaxation of airway smooth muscle is an inadequately understood yet critical process that, if impaired, may have significant implications for asthma. Here we explore why relaxation is an important process to consider, how it may determine airway hyperresponsiveness, and some of the factors that influence relaxation of the airway smooth muscle. These include mechanical and biochemical factors such as deep inspirations or large amplitude oscillation of the muscle, plastic properties of the muscle, the load the muscle experiences, calcium, phosphorylation of the myosin light chain, cytoskeletal proteins, and sensitization.     

Excitation-contraction coupling in airway smooth muscle

Excitation-contraction (EC) coupling in striated muscles is mediated by the cardiac or skeletal muscle isoform of voltage-dependent L-type Ca(2+) channel (Ca(v)1.2 and Ca(v)1.1, respectively) that senses a depolarization of the cell membrane, and in response, activates its corresponding isoform of intracellular Ca(2+) release channel/ryanodine receptor (RyR) to release stored Ca(2+), thereby initiating muscle contraction. Specifically, in cardiac muscle following cell membrane depolarization, Ca(v)1.2 activates cardiac RyR (RyR2) through an influx of extracellular Ca(2+). In contrast, in skeletal muscle, Ca(v)1.1 activates skeletal muscle RyR (RyR1) through a direct physical coupling that negates the need for extracellular Ca(2+). Since airway smooth muscle (ASM) expresses Ca(v)1.2 and all three RyR isoforms, we examined whether a cardiac muscle type of EC coupling also mediates contraction in this tissue. We found that the sustained contractions of rat ASM preparations induced by depolarization with KCl were indeed partially reversed ( approximately 40%) by 200 mum ryanodine, thus indicating a functional coupling of L-type channels and RyRs in ASM. However, KCl still caused transient ASM contractions and stored Ca(2+) release in cultured ASM cells without extracellular Ca(2+). Further analyses of rat ASM indicated that this tissue expresses as many as four L-type channel isoforms, including Ca(v)1.1. Moreover, Ca(v)1.1 and RyR1 in rat ASM cells have a similar distribution near the cell membrane in rat ASM cells and thus may be directly coupled as in skeletal muscle. Collectively, our data implicate that EC-coupling mechanisms in striated muscles may also broadly transduce diverse smooth muscle functions.     

A nonlinear model of passive muscle viscosity

The material properties of passive skeletal muscle are critical to proper function and are frequently a target for therapeutic and interventional strategies. Investigations into the passive viscoelasticity of muscle have primarily focused on characterizing the elastic behavior, largely neglecting the viscous component. However, viscosity is a sizeable contributor to muscle stress and extensibility during passive stretch and thus there is a need for characterization of the viscous as well as the elastic components of muscle viscoelasticity. Single mouse muscle fibers were subjected to incremental stress relaxation tests to characterize the dependence of passive muscle stress on time, strain and strain rate. A model was then developed to describe fiber viscoelasticity incorporating the observed nonlinearities. The results of this model were compared with two commonly used linear viscoelastic models in their ability to represent fiber stress relaxation and strain rate sensitivity. The viscous component of mouse muscle fiber stress was not linear as is typically assumed, but rather a more complex function of time, strain and strain rate. The model developed here, which incorporates these nonlinearities, was better able to represent the stress relaxation behavior of fibers under the conditions tested than commonly used models with linear viscosity. It presents a new tool to investigate the changes in muscle viscous stresses with age, injury and disuse.     

Selected contribution: effect of chronic passive length change on airway smooth muscle length-tension relationship.

The ability of rabbit trachealis to undergo plastic adaptation to chronic shortening or lengthening was assessed by setting the muscle preparations at three lengths for 24 h in relaxed state: a reference length in which applied force was approximately 1-2% of maximal active force (P(o)) and lengths considerably shorter and longer than the reference. Passive and active length-tension (L-T) curves for the preparations were then obtained by electrical field stimulation at progressively increasing muscle length. Classically shaped L-T curves were obtained with a distinct optimal length (L(o)) at which P(o) developed; however, both the active and passive L-T curves were shifted, whereas P(o) remained unchanged. L(o) was 72% and 148% that of the reference preparations for the passively shortened and lengthened muscles, respectively. The results suggest that chronic narrowing of the airways could induce a shift in the L-T relationship of smooth muscle, resulting in a maintained potential for maximal force production.     

Molecular targets for steroids in airway vascular smooth muscle

The actions of norepinephrine (NE) released from airway sympathetic nerves are partially terminated by the extraneuronal catecholamine uptake. Because various steroid hormones inhibit extraneuronal uptake, it could be responsible for the airway vasoconstriction caused by inhaled glucocorticosteroids (GSs) in vivo. Using bronchial arteries obtained from donor lungs rejected for transplantation, we showed that a plasma membrane-associated transporter is responsible for NE uptake by airway vascular smooth muscle. We identified this transporter, namely the extraneuronal monoamine transporter (EMT), by demonstrating its function and mRNA expression. Furthermore, we showed that the rapid, nongenomic inhibitory GS effect on EMT is likely mediated through the activation of specific K+ channels in the plasma membrane. We believe that our studies identified new molecular targets for GSs in modulating noradrenergic control of airway vascular tone.     

The rigor configuration of smooth muscle heavy meromyosin trapped by a zero-length cross-linker

When chicken gizzard heavy meromyosin (HMM) in its rigor complex with actin was reacted with the zero-length cross-linker 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide (EDC), HMM cross-linked with actin but also the two heads of the HMM molecule cross-linked to each other [Onishi, H., Maita, T., Matsuda, G., & Fujiwara, K. (1989) Biochemistry 28, 1898-1904, 1905-1912]. By ultracentrifugal fractionation of the EDC-treated acto-HMM in the presence of Mg-ATP, we obtained a preparation enriched for gizzard HMM with cross-linked heads. When HMM molecules in this preparation were rotary-shadowed and observed in an electron microscope, many head pairs were in contact with each other. The amount of HMM with cross-linked heads determined by electron microscopy was equal to that of the cross-linked NH2-terminal 24K tryptic fragments of HMM heavy chains determined by NaDodSO4 gel electrophoresis, indicating that this cross-linking is primarily responsible for the contact observed between two HMM heads. Most pairs of the contacted heads originated in the same HMM molecule, although a few pairs belonged to different HMM molecules. Cross-linking between the two heads of the same HMM molecule appeared to occur within the distal, more globular half of each head. However, the cross-linking sites were located at different positions within the globular portion. The actin-activated Mg-ATPase activity of the HMM sample treated with EDC in the presence of actin increased in a biphasic manner, depending on the concentration of F-actin, with two apparent association constants: 2.9 x 10(4) M-1 and one much less than 1 x 10(4) M-1. Since the apparent association constant obtained with the HMM control was similar to the latter value, the association constant for HMM molecules with cross-linked heads was identified to be the former value. The binding of HMM to actin was thus strengthened at least by a factor of 3 by the cross-linking between two HMM heads. These results suggest that HMM heads are trapped by treatment with EDC in the rigor complex configuration and that this configuration is retained even after the HMM has been released from actin. The EDC reactivity of rabbit skeletal muscle HMM, however, was different from that of chicken gizzard HMM. The treatment of acto-HMM complexes with EDC did not generate cross-linking between two skeletal muscle HMM heads.     

A comparison of preterm and adult airway smooth muscle mechanics

To determine whether airway smooth muscle undergoes a maturational change regarding force generation, length-tension relationships were determined in isolated trachealis strips from adult and preterm sheep. At the length of maximum force generation, passive active and total tensions of the adult muscle were 2.5 times greater than preterm values (P less than 0.001). KCl stimulation yielded a greater peak tension in the adult strips than in the preterm strips (P less than 0.01). Preterm strips required higher concentrations of KCl to initiate contractions and higher concentrations to reach peak tension. Acetylcholine- (ACh) induced contraction resulted in greater force development at each dose in the adult strips compared with preterm strips (P less than 0.001). The dose of ACh required to reach a half-maximal response was significantly less for the adult strips than for the preterm strips (P less than 0.005). These data demonstrate that both force generation and receptor sensitivity increase with age. This inability of immature smooth muscle to generate as much force as adult smooth muscle may help explain why very preterm neonates requiring intermittent positive-pressure ventilation are at risk for developing structural airway problems.     

Inspiratory rhythm in airway smooth muscle tone

In anesthetized paralyzed open-chested cats ventilated with low tidal volumes at high frequency, we recorded phrenic nerve activity, transpulmonary pressure (TPP), and either the tension in an upper tracheal segment or the impulse activity in a pulmonary branch of the vagus nerve. The TPP and upper tracheal segment tension fluctuated with respiration, with peak pressure and tension paralleling phrenic nerve activity. Increased end-tidal CO2 or stimulation of the carotid chemoreceptors with sodium cyanide increased both TPP and tracheal segment tension during the increased activity of the phrenic nerve. Lowering end-tidal CO2 or hyperinflating the lungs to achieve neural apnea (lack of phrenic activity) caused a decrease in TPP and tracheal segment tension and abolished the inspiratory fluctuations. During neural apnea produced by lowering end-tidal CO2, lung inflation caused no further decrease in tracheal segment tension and TPP. Likewise, stimulation of the cervical sympathetics, which caused a reduction in TPP and tracheal segment tension during normal breathing, caused no further reduction in these parameters when the stimulation occurred during neural apnea. During neural apnea the tracheal segment tension and TPP were the same as those following the transection of the vagi or the administration of atropine (0.5 mg/kg). Numerous fibers in the pulmonary branch of the vagus nerve fired in synchrony with the phrenic nerve. Only these fibers had activity which paralleled changes in TPP and tracheal tension. We propose that the major excitatory input to airway smooth muscle arises from cholinergic nerves that fire during inspiration, which have preganglionic cell bodies in the ventral respiratory group in the region of the nucleus ambiguus and are driven by the same pattern generators that drive the phrenic and inspiratory intercostal motoneurons.