Could an increase in airway smooth muscle shortening velocity cause airway hyperresponsiveness?

Airway hyperresponsiveness (AHR) is a characteristic feature of asthma. It has been proposed that an increase in the shortening velocity of airway smooth muscle (ASM) could contribute to AHR. To address this possibility, we tested whether an increase in the isotonic shortening velocity of ASM is associated with an increase in the rate and total amount of shortening when ASM is subjected to an oscillating load, as occurs during breathing. Experiments were performed in vitro using 27 rat tracheal ASM strips supramaximally stimulated with methacholine. Isotonic velocity at 20% isometric force (Fiso) was measured, and then the load on the muscle was varied sinusoidally (0.33 ± 0.25 Fiso, 1.2 Hz) for 20 min, while muscle length was measured. A large amplitude oscillation was applied every 4 min to simulate a deep breath. We found that: 1) ASM strips with a higher isotonic velocity shortened more quickly during the force oscillations, both initially (P < 0.001) and after the simulated deep breaths (P = 0.002); 2) ASM strips with a higher isotonic velocity exhibited a greater total shortening during the force oscillation protocol (P < 0.005); and 3) the effect of an increase in isotonic velocity was at least comparable in magnitude to the effect of a proportional increase in ASM force-generating capacity. A cross-bridge model showed that an increase in the total amount of shortening with increased isotonic velocity could be explained by a change in either the cycling rate of phosphorylated cross bridges or the rate of myosin light chain phosphorylation. We conclude that, if asthma involves an increase in ASM velocity, this could be an important factor in the associated AHR.     

Increased in vitro responses of tracheal smooth muscle from hyperresponsive guinea pigs

Repeated aerosol antigen challenge of previously sensitized guinea pigs induces airway hyperresponsiveness to inhaled acetylcholine. To determine the mechanism producing these airway changes and assuming that changes in the trachealis muscle reflect changes in muscle of the entire tracheobronchial tree, we examined the in vitro smooth muscle mechanics and morphometric parameters of tracheae from guinea pigs demonstrating hyperresponsiveness in vivo vs. tracheae from control guinea pigs. No differences between these groups were found in luminal volume at zero transmural pressure, passive pressure-volume characteristics, or area of airway wall. Smooth muscle areas were slightly less in tracheae from hyperresponsive guinea pigs. Tracheae from hyperresponsive guinea pigs had both significantly increased isovolumetric force generation and isobaric shortening compared with tracheae from controls when evaluated over the range of transmural pressures from -40 to 40 cmH2O. We conclude that the in vivo airway hyperresponsiveness induced with repeated antigen challenge is associated with both increased force generation and shortening of tracheal smooth muscle without increased muscle mass, suggesting enhanced contractile activity.     

Parenchymal interdependence and airway response to methacholine in excised dog lobes

The objective of this investigation was to determine the minimum transpulmonary pressure (PL) at which the forces of interdependence between the airways and the lung parenchyma can prevent airway closure in response to maximal stimulation of the airways in excised canine lobes. We first present an analysis of the relationship between PL and the transmural pressure (Ptm) that airway smooth muscle must generate to close the airways. This analysis predicts that airway closure can occur at PL less than or equal to 10 cmH2O with maximal airway stimulation. We tested this prediction in eight excised canine lobes by nebulizing 50% methacholine into the airways while the lobe was held at constant PL values ranging from 25 to 5 cmH2O. Airway closure was assessed by comparing changes in alveolar pressure (measured by an alveolar capsule technique) and pressure at the airway opening during low-amplitude oscillations in lobar volume. Airway closure occurred in two of the eight lobes at PL = 10 cmH2O; in an additional five it occurred at PL = 7.5 cmH2O. We conclude that the forces of parenchymal interdependence per se are not sufficient to prevent airway closure at PL less than or equal to 7.5 cmH2O in excised canine lobes.     

Responsiveness of the isolated airway during simulated deep inspirations: effect of airway smooth muscle stiffness and strain.

In vivo, breathing movements, including tidal and deep inspirations (DIs), exert a number of beneficial effects on respiratory system responsiveness in healthy humans that are diminished or lost in asthma, possibly as a result of reduced distension (strain) of airway smooth muscle (ASM). We used bronchial segments from pigs to assess airway responsiveness under static conditions and during simulated tidal volume oscillations with and without DI and to determine the roles of airway stiffness and ASM strain on responsiveness. To simulate airway dilations during breathing, we cycled the luminal volume of liquid-filled segments. Volume oscillations (15 cycles/min) were set so that, in relaxed airways, they produced a transmural pressure increase of approximately 5-10 cmH(2)O for tidal maneuvers and approximately 5-30 cmH(2)O for DIs. ACh dose-response curves (10(-7)-3 x 10(-3) M) were constructed under static and dynamic conditions, and maximal response and sensitivity were determined. Airway stiffness was measured from tidal trough-to-peak pressure and volume cycles. ASM strain produced by DI was estimated from luminal volume, airway length, and inner wall area. DIs produced substantial ( approximately 40-50%) dilation, reflected by a decrease in maximal response (P < 0.001) and sensitivity (P < 0.05). However, the magnitude of bronchodilation decreased significantly in proportion to airway stiffening caused by contractile activation and an associated reduction in ASM strain. Tidal oscillations, in comparison, had little effect on responsiveness. We conclude that DI regulates airway responsiveness at the airway level, but this is limited by airway stiffness due to reduced ASM strain.     

Isovolumetric and isobaric rabbit tracheal contraction in vitro

Isovolumetric and isobaric tracheal smooth muscle (TSM) contraction were studied in vitro in a preparation of the whole rabbit trachea. Eight tracheae from New Zealand White rabbits were excised and mounted at a fixed length in an organ bath. Electrical field stimulation (EFS) was performed in isovolumetric and isobaric conditions at varying transmural pressures (TMP). Supramaximal stimulation with methacholine was done at 0 TMP. Active change in pressure (delta P) with EFS showed a peak at 3.1 +/- 1.06 cmH2O TMP during inflation and at 4.1 +/- 1.18 cmH2O TMP during deflation (mean +/- SE). Active delta P decreased at higher or lower TMP. Active change in volume with EFS showed a peak at 3.2 +/- 1.26 cmH2O TMP during inflation and at 1.8 +/- 0.98 cmH2O TMP during deflation. A decrease in response was also observed at higher and lower TMP. From these data, we concluded that TSM is at optimal length (Lmax) at TMP of 2-3 cmH2O. Maximal TSM shortening with supramaximal stimulation with methacholine was 32% Lmax. This figure is considerably smaller than the 80% shortening found in unloaded strips of TSM. We conclude that rabbit TSM length is close to Lmax at TMP similar to those found at functional residual capacity and that the loads that the muscle has to overcome probably contribute to the limited shortening observed in situ.     

Intraluminal pressure oscillation enhances subsequent airway contraction in isolated bronchial segments.

A period of deep inspiration in humans has been shown to attenuate subsequent bronchoconstriction, a phenomenon termed bronchoprotection. The bronchoprotective effect of deep inspiration may be caused though a depression in the force production of airway smooth muscle (ASM). We determined the response of whole airway segments and isolated ASM to a period of cyclic stretches. Isovolumetric contraction to electrical field stimulation (EFS) was assessed in porcine bronchial segments before and after intraluminal pressure oscillation from 5 to 25 cmH(2)O for 10 min at 0.5 Hz. Morphometry showed that this pressure oscillation stretched ASM length by 21%. After pressure oscillation, the response to EFS was not reduced but instead was modestly enhanced (P < 0.01). Airway responses to EFS returned to preoscillation levels 10 min after the end of oscillation. The increase in EFS response after pressure oscillation was not altered by the addition of indomethacin. In a separate experiment, we assessed isometric force in isolated ASM strips before and after length oscillation. The amplitude, frequency, and duration of length oscillation were similar to those induced in bronchial segments. In contrast to bronchial segments, length oscillation of ASM produced a significant depression in isometric force induced by EFS (P < 0.01). These results suggest that the response of ASM to length oscillation is modified by the airway wall. They also suggest that the phenomenon of bronchoprotection reported in some in vivo studies may not be an intrinsic property of the airway.     

Airway smooth muscle relaxation is impaired in mice lacking the p47phox subunit of NAD(P)H oxidase

NAD(P)H oxidase is one of the critical enzymes mediating cellular production of reactive oxygen species and has a central role in airway smooth muscle (ASM) cell proliferation. Since reactive oxygen species also affect ASM contractile response, we hypothesized a regulatory role of NAD(P)H oxidase in ASM contractility. We therefore studied ASM function in wild-type mice (C57BL/6J) and mice deficient in a component (p47phox) of NAD(P)H oxidase. In histological sections of the trachea, we found that the area occupied by ASM was 17% more in p47(phox-/-) than in wild-type mice. After correcting for the difference in ASM content, we found that force generation did not vary between the two genotypes. Similarly, their ASM shortening velocity, maximal power, and sensitivity to acetylcholine, as well as airway responsiveness to methacholine in vivo, were not significantly different. The main finding of this study was a significantly reduced ASM relaxation in p47phox-/- compared with wild-type mice both during the stimulus and after the end of stimulation. The tension relaxation attained at the 20th second of electric field stimulation was, respectively, 17.6 +/- 2.4 and 9.2 +/- 2.3% in null and wild-type mice (P <0.01 by t-test). Similar significant differences were found in the rate of tension relaxation and the time required to reduce tension by one-half. Our data suggest that NAD(P)H oxidase may have a role in the structural arrangement and mechanical properties of the airway tissue. Most importantly, we report the first evidence that the p47phox subunit of NAD(P)H oxidase plays a role in ASM relaxation.     

Homeokinesis and short-term variability of human airway caliber.

We hypothesized that short-term variation in airway caliber could be quantified by frequency distributions of respiratory impedance (Zrs) measured at high frequency. We measured Zrs at 6 Hz by forced oscillations during quiet breathing for 15 min in 10 seated asthmatic patients and 6 normal subjects in upright and supine positions before and after methacholine (MCh). We plotted frequency distributions of Zrs and calculated means, skewness, kurtosis, and significance of differences between normal and log-normal frequency distributions. The data were close to, but usually significantly different from, a log-normal frequency distribution. Mean lnZrs in upright and supine positions was significantly less in normal subjects than in asthmatic patients, but not after MCh and MCh in the supine position. The lnZrs SD (a measure of variation), in the upright position and after MCh was significantly less in normal subjects than in asthmatic patients, but not in normal subjects in the supine position and after MCh in the supine position. We conclude that 1) the configuration of the normal tracheobronchial tree is continuously changing and that this change is exaggerated in asthma, 2) in normal lungs, control of airway caliber is homeokinetic, maintaining variation within acceptable limits, 3) normal airway smooth muscle (ASM) when activated and unloaded closely mimics asthmatic ASM, 4) in asthma, generalized airway narrowing results primarily from ASM activation, whereas ASM unloading by increasing shortening velocity allows faster caliber fluctuations, 5) activation moves ASM farther from thermodynamic equilibrium, and 6) asthma may be a low-entropy disease exhibiting not only generalized airway narrowing but also an increased appearance of statistically unlikely airway configurations.     

Airways in smooth muscle α-actin null mice experience a compensatory mechanism that modulates their contractile response

We hypothesized that ablation of smooth muscle α-actin (SM α-A), a contractile-cytoskeletal protein expressed in airway smooth muscle (ASM) cells, abolishes ASM shortening capacity and decreases lung stiffness. In both SM α-A knockout and wild-type (WT) mice, airway resistance (Raw) determined by the forced oscillation technique rose in response to intravenous methacholine (Mch). However, the slope of Raw (cmH(2)O·ml(-1)·s) vs. log(2) Mch dose (μg·kg(-1)·min(-1)) was lower (P = 0.007) in mutant (0.54 ± 0.14) than in WT mice (1.23 ± 0.19). RT-PCR analysis performed on lung tissues confirmed that mutant mice lacked SM α-A mRNA and showed that these mice had robust expressions of both SM γ-A mRNA and skeletal muscle (SKM) α-A mRNA, which were not expressed in WT mice, and an enhanced SM22 mRNA expression relative to that in WT mice. Compared with corresponding spontaneously breathing mice, mechanical ventilation-induced lung mechanical strain increased the expression of SM α-A mRNA in WT lungs; in mutant mice, it augmented the expressions of SM γ-A mRNA and SM22 mRNA and did not alter that of SKM α-A mRNA. In mutant mice, the expression of SM γ-A mRNA in the lung during spontaneous breathing and its enhanced expression following mechanical ventilation are consistent with the likely possibility that in the absence of SM α-A, SM γ-A underwent polymerization and interacted with smooth muscle myosin to produce ASM shortening during cholinergic stimulation. Thus our data are consistent with ASM in mutant mice experiencing compensatory mechanisms that modulated its contractile muscle capacity.     

Nonlinear Compliance Modulates Dynamic Bronchoconstriction in a Multiscale Airway Model

The role of breathing and deep inspirations (DI) in modulating airway hyperresponsiveness remains poorly understood. In particular, DIs are potent bronchodilators of constricted airways in nonasthmatic subjects but not in asthmatic subjects. Additionally, length fluctuations (mimicking DIs) have been shown to reduce mean contractile force when applied to airway smooth muscle (ASM) cells and tissue strips. However, these observations are not recapitulated on application of transmural pressure (P_TM) oscillations (that mimic tidal breathing and DIs) in isolated intact airways. To shed light on this paradox, we have developed a biomechanical model of the intact airway, accounting for strain-stiffening due to collagen recruitment (a large component of the extracellular matrix (ECM)), and dynamic actomyosin-driven force generation by ASM cells. In agreement with intact airway studies, our model shows that P_TM fluctuations at particular mean transmural pressures can lead to only limited bronchodilation. However, our model predicts that moving the airway to a more compliant point on the static pressure-radius relationship (which may involve reducing mean P_TM), before applying pressure fluctuations, can generate greater bronchodilation. This difference arises from competition between passive strain-stiffening of ECM and force generation by ASM yielding a highly nonlinear relationship between effective airway stiffness and P_TM, which is modified by the presence of contractile agonist. Effectively, the airway at its most compliant may allow for greater strain to be transmitted to subcellular contractile machinery. The model predictions lead us to hypothesize that the maximum possible bronchodilation of an airway depends on its static compliance at the P_TM about which the fluctuations are applied. We suggest the design of additional experimental protocols to test this hypothesis.     

Regulatable stiffness in relaxed airway smooth muscle: a target for asthma treatment?

The airway smooth muscle (ASM) layer within the airway wall modulates airway diameter and distensibility. Even in the relaxed state, the ASM layer possesses finite stiffness and limits the extent of airway distension by the radial force generated by parenchymal tethers and transmural pressure. Airway stiffness has often been attributed to passive elements, such as the extracellular matrix in the lamina reticularis, adventitia, and the smooth muscle layer that cannot be rapidly modulated by drug intervention such as ASM relaxation by β-agonists. In this study, we describe a calcium-sensitive component of ASM stiffness mediated through the Rho-kinase signaling pathway. The stiffness of ovine tracheal smooth muscle was assessed in the relaxed state under the following conditions: 1) in physiological saline solution (Krebs solution) with normal calcium concentration; 2) in calcium-free Krebs with 2 mM EGTA; 3) in Krebs with calcium entry blocker (SKF-96365); 4) in Krebs with myosin light chain kinase inhibitor (ML-7); and 5) in Krebs with Rho-kinase inhibitor (Y-27632). It was found that a substantial portion of the passive stiffness could be abolished when intracellular calcium was removed; this calcium-sensitive stiffness appeared to stem from intracellular source and was not sensitive to ML-7 inhibition of myosin light chain phosphorylation, but was sensitive to Y-27632 inhibition of Rho kinase. The results suggest that airway stiffness can be readily modulated by targeting the calcium-sensitive component of the passive stiffness within the muscle layer.     

Isoproterenol attenuates myosin phosphorylation and contraction of tracheal muscle

The effects of isoproterenol on isometric force, unloaded shortening velocity, and myosin phosphorylation were examined in thin muscle bundles (0.1-0.2 mm diam) dissected from lamb tracheal smooth muscle. Methacholine (10(-6) M) induced rapid increases in isometric force and in phosphorylation of the 20,000-Da myosin light chain. Myosin phosphorylation remained elevated during steady-state maintenance of isometric force. The shortening velocity peaked at 15 s after stimulation with methacholine and then declined to approximately 45% of the maximal value by 3 min. Isoproterenol pretreatment inhibited methacholine-stimulated myosin light chain phosphorylation, shortening velocity, and force during the early stages of force generation. However, the inhibitory effect of isoproterenol on force and myosin phosphorylation is proportionally greater than that on shortening velocity. Isoproterenol pretreatment also caused a rightward non-parallel shift in the methacholine dose-response curves for both isometric tension and myosin light chain phosphorylation. These data demonstrate that isoproterenol attenuates the contractile properties of airway smooth muscles by affecting the rate and extent of myosin light chain phosphorylation, perhaps through a mechanism that involves the synergistic interaction of myosin light chain kinase phosphorylation and Ca2+ metabolism.     

Interaction between Bronchoconstrictor Stimuli on Human Airway Smooth Muscle

In healthy human subjects, the simultaneous aerosol administration of histamine and methacholine results in a pronounced decrease in maximum flow rates on partial expiratory flow-volume (PEFV) curves. When given alone in the same concentrations, these drugs produced no or minimal decreases in flow rates. The results suggest an interaction of histamine and cholinergic stimuli on airway smooth muscle (ASM). This mechanism might explain many experiments where vagal blockade diminished or abolished ASM response to histamine and other stimuli, simply by interfering with histamine-cholinergic interaction at the ASM level. These findings confirm similar findings of animal in vitro experiments. The experiments clearly confirm the sensitivity and value of assessing drug effects prior to a deep breath. Flow-rate changes after a full inspiration, taken from the maximum expiratory flow-volume (MEFV) curve, show either no relationship to the concentration of inhaled methacholine or significantly less effect than that seen on the PEFV curve.     

Effects of lung inflation on pulmonary arterial blood volume in intact dogs.

The isolated effects of alterations of lung inflation and transmural pulmonary arterial pressure (pressure difference between intravascular and pleural pressure) on pulmonary arterial blood volume (Vpa) were investigated in anesthetized intact dogs. Using transvenous phrenic nerve stimulation, changes in transmural pulmonary arterial pressure (Ptm) at a fixed transpulmonary pressure (Ptp) were produced by the Mueller maneuver, and increases in Ptp at relatively constant Ptm by a quasi-Valsalva maneuver. Also, both Ptm and Ptp were allowed to change during open airway lung inflation. Vpa was determined during these three maneuvers by multiplying pulmonary blood flow by pulmonary arterial mean transit time obtained by an ether plethysmographic method. During open airway lung inflation, mean (plus or minus SD) Ptp increased by 7.2 (plus or minus 3.7) cmH2O and Ptm by 4.3 (plus or minus 3.4) cmH2O for a mean increase in Vpa by 26.2 (plus or minus 10.7) ml. A pulmonary arterial compliance term (Delta Vpa/Delta Ptm) calculated from the Mueller maneuver was 3.9 ml/cmH2O and an interdependence term (Delta Vpa/Delta Ptp) calculated from the quasi-Valsalva maneuver was 2.5 ml/cmH2O for a 19% increase in lung volume, and 1.2 ml/cmH2O for an increase in lung volume from 19% to 35%. These findings indicate that in normal anesthetized dogs near FRC for a given change in Ptp and Ptm the latter results in a greater increase of Vpa.     

Measurement of interrupter resistance in rabbits exposed to methacholine aerosols.

We studied the effect of increasing airway resistance on equilibration of airway and alveolar pressure during passive expiratory airflow interruption. In 10 anesthetized and paralyzed rabbits, airway and alveolar pressures were compared before and after airway resistance was increased with methacholine. In all studies, airway pressure rose to equilibrate with alveolar pressure immediately after the interruption (delta Pinit) regardless of increases in airway resistance. The pressures then remained equal during the interruption while gradually increasing to plateau (delta Pdiff). Before methacholine exposure, delta Pdiff was small (0.6 +/- 0.3 cmH2O). Steady-state resistance calculated from the sum of delta Pinit and delta Pdiff was similar to airway resistance calculated from delta Pinit alone. After methacholine, increased airway resistance was accompanied by increased delta Pdiff (2.0 +/- 0.5 cmH2O), causing disproportionate increase in steady-state resistance. delta Pdiff increases were equal in the airway and alveoli, implying resistive changes distal to the sampled alveoli. Thus increasing airway resistance did not delay pressure equilibration across airways. However, increases in airway resistance were accompanied by tissue resistive changes that were greater than the increases in airway resistance.     

Mechanics of left ventricular relaxation, early diastolic lengthening, and suction investigated in a mathematical model

We investigated the determinants of ventricular early diastolic lengthening and mechanics of suction using a mathematical model of the left ventricle (LV). The model was based on a force balance between the force represented by LV pressure (LVP) and active and passive myocardial forces. The predicted lengthening velocity (e') from the model agreed well with measurements from 10 dogs during 5 different interventions (R = 0.69, P < 0.001). The model showed that e' was increased when relaxation rate and systolic shortening increased, when passive stiffness was decreased, and when the rate of fall of LVP during early filling was decreased relative to the rate of fall of active stress. We first defined suction as the work the myocardium performed to pull blood into the ventricle. This occurred when contractile active forces decayed below and became weaker than restoring forces, producing a negative LVP. An alternative definition of suction is filling during falling pressure, commonly believed to be caused by release of restoring forces. However, the model showed that this phenomenon also occurred when there had been no systolic compression below unstressed length and therefore in the absence of restoring forces. In conclusion, relaxation rate, LVP, systolic shortening, and passive stiffness were all independent determinants of e'. The model generated a suction effect seen as lengthening occurring during falling pressure. However, this was not equivalent with the myocardium performing pulling work on the blood, which was performed only when restoring forces were higher than remaining active fiber force, corresponding to a negative transmural pressure.     

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.     

Cyclic stretch enhances reorientation and differentiation of 3-D culture model of human airway smooth muscle

Activation of airway smooth muscle (ASM) cells plays a central role in the pathophysiology of asthma. Because ASM is an important therapeutic target in asthma, it is beneficial to develop bioengineered ASM models available for assessing physiological and biophysical properties of ASM cells. In the physiological condition in vivo, ASM cells are surrounded by extracellular matrix (ECM) and exposed to mechanical stresses such as cyclic stretch. We utilized a 3-D culture model of human ASM cells embedded in type-I collagen gel. We further examined the effects of cyclic mechanical stretch, which mimics tidal breathing, on cell orientation and expression of contractile proteins of ASM cells within the 3-D gel. ASM cells in type-I collagen exhibited a tissue-like structure with actin stress fiber formation and intracellular Ca²⁺ mobilization in response to methacholine. Uniaxial cyclic stretching enhanced alignment of nuclei and actin stress fibers of ASM cells. Moreover, expression of mRNAs for contractile proteins such as α-smooth muscle actin, calponin, myosin heavy chain 11, and transgelin of stretched ASM cells was significantly higher than that under the static condition. Our findings suggest that mechanical force and interaction with ECM affects development of the ASM tissue-like construct and differentiation to the contractile phenotype in a 3-D culture model.     

Airway responsiveness depends on the diffusion rate of methacholine across the airway wall

During methacholine challenge tests of airway responsiveness, it is invariably assumed that the administered dose of agonist is accurately reflected in the dose that eventually reaches the airway smooth muscle (ASM). However, agonist must traverse a variety of tissue obstacles to reach the ASM, during which the agonist is subjected to both enzymatic breakdown and removal by the bronchial and pulmonary circulations. This raises the possibility that a significant fraction of the deposited agonist may never actually make it to the ASM. To understand the nature of this effect, we measured the time course of changes in airway resistance elicited by various durations of methacholine aerosol in mice. We fit to these data a computational model of a dynamically contracting airway responding to agonist that diffuses through an airway compartment, thereby obtaining rate constants that reflect the diffusive barrier to methacholine. We found that these barriers can contribute significantly to the time course of airway narrowing, raising the important possibility that alterations in the diffusive barrier presented by the airway wall may play a role in pathologically altered airway responsiveness.     

Responses to negative pressure surrounding the neck in anesthetized animals

Continuous positive pressure applied at the nose has been shown to cause a decrease in upper airway resistance. The present study was designed to determine whether a similar positive transmural pressure gradient, generated by applying a negative pressure at the body surface around the neck, altered upper airway patency. Studies were performed in nine spontaneously breathing anesthetized supine dogs. Airflow was measured with a pneumotachograph mounted on an airtight muzzle placed over the nose and mouth of each animal. Upper airway pressure was measured as the differential pressure between the extrathoracic trachea and the inside of the muzzle. Upper airway resistance was monitored as an index of airway patency. Negative pressure (-2 to -20 cmH2O) was applied around the neck by using a cuirass extending from the jaw to the thorax. In each animal, increasingly negative pressures were transmitted to the airway wall in a progressive, although not linear, fashion. Decreasing the pressure produced a progressive fall in upper airway resistance, without causing a significant change in respiratory drive or respiratory timing. At -5 cmH2O pressure, there occurred a significant fall in upper airway resistance, comparable with the response of a single, intravenous injection of sodium cyanide (0.5-3.0 mg), a respiratory stimulant that produces substantial increases in respiratory drive. We conclude that upper airway resistance is influenced by the transmural pressure across the airway wall and that such a gradient can be accomplished by making the extraluminal pressure more negative.(ABSTRACT TRUNCATED AT 250 WORDS)