Airway stability and heterogeneity in the constricted lung.

The effect of bronchoconstriction on airway resistance is known to be spatially heterogeneous and dependent on tidal volume. We present a model of a single terminal airway that explains these features. The model describes a feedback between flow and airway resistance mediated by parenchymal interdependence and the mechanics of activated smooth muscle. The pressure-tidal volume relationship for a constricted terminal airway is computed and shown to be sigmoidal. Constricted terminal airways are predicted to have two stable states: one effectively open and one nearly closed. We argue that the heterogeneity of whole lung constriction is a consequence of this behavior. Airways are partitioned between the two states to accommodate total flow, and changes in tidal volume and end-expiratory pressure affect the number of airways in each state. Quantitative predictions for whole lung resistance and elastance agree with data from previously published studies on lung impedance.     

Effect of bronchomotor tone on work rate of breathing

We studied the optimal airway caliber for minimizing the work rate of breathing in the lung (W) with different bronchomotor tones in six normal subjects. The inhalation of methacholine contracted airway smooth muscle, and the inhalation of salbutamol relaxed it. To calculate W at a given alveolar ventilation (VA), anatomical dead space (VDanat), pulmonary resistance (RL), and dynamic compliance were measured simultaneously, breath by breath, during various breathing maneuvers. VDanat increased and RL decreased with both increased breathing frequency and tidal volume, even at a given airway tone. This suggests that the airway caliber varied even at a given bronchomotor tone. The minimum W at a given VA increased in constricted airways, but there was no significant difference between control airways after saline inhalation and relaxed airways. It has been suggested that airway smooth muscle tones at both control and relaxed conditions bring W to a minimum and that the airway smooth muscle tone existing in the control state acts to keep the airway caliber optimal in order to minimize the W and stabilize the airway mechanics.     

Complex airway behavior and paradoxical responses to bronchoprovocation.

Heterogeneity of airway constriction and regional ventilation in asthma are commonly studied under the paradigm that each airway's response is independent from other airways. However, some paradoxical effects and contradictions in recent experimental and theoretical findings suggest that considering interactions among serial and parallel airways may be necessary. To examine airway behavior in a bronchial tree with 12 generations, we used an integrative model of bronchoconstriction, including for each airway the effects of pressure, tethering forces, and smooth muscle forces modulated by tidal stretching during breathing. We introduced a relative smooth muscle activation factor (T(r)) to simulate increasing and decreasing levels of activation. At low levels of T(r), the model exhibited uniform ventilation and homogeneous airway narrowing. But as T(r) reached a critical level, the airway behavior suddenly changed to a dual response with a combination of constriction and dilation. Ventilation decreased dramatically in a group of terminal units but increased in the rest. A local increase of T(r) in a single central airway resulted in full closure, while no central airway closed under global elevation of T(r). Lung volume affected the response to both local and global stimulation. Compared with imaging data for local and global stimuli, as well as for the time course of airway lumen caliber during bronchoconstriction recovery, the model predictions were similar. The results illustrate the relevance of dynamic interactions among serial and parallel pathways in airway interdependence, which may be critical for the understanding of pathological conditions in asthma.     

Tissue strains induced in airways due to mechanical ventilation

Better understanding of the stress/strain environment in airway tissues is very important in order to avoid lung injuries for patients undergoing mechanical ventilation for treatment of respiratory problems. Airway tissue strains responsible for stressing the lung's fiber network and rupturing the lung due to compliant airways are very difficult to measure experimentally. A computational model that incorporates the heterogeneity of the airways was developed to study the effects of airway tissue material properties on strain distributions within each layer of the airway wall. The geometry and boundary conditions of the tissue strain analysis were obtained from the organ-level analysis model. Two sets of airway tissue properties (heterogeneous and homogeneous) were considered in order to estimate the strain levels induced within the tissue. The simulation results showed that the homogeneous model overestimated the maximum strain in the mucosa layer and underestimated the maximum strain in the smooth muscle and cartilage layers. The results of strain levels obtained from the tissue analysis are very important because these strains at the cellular-level can create inflammatory responses, thus damaging the airway tissues.     

Airway closure on imaging relates to airway hyperresponsiveness and peripheral airway disease in asthma

The regional pattern and extent of airway closure measured by three-dimensional ventilation imaging may relate to airway hyperresponsiveness (AHR) and peripheral airways disease in asthmatic subjects. We hypothesized that asthmatic airways are predisposed to closure during bronchoconstriction in the presence of ventilation heterogeneity and AHR. Fourteen asthmatic subjects (6 women) underwent combined ventilation single photon emission computed tomography/computed tomography scans before and after methacholine challenge. Regional airway closure was determined by complete loss of ventilation following methacholine challenge. Peripheral airway disease was measured by multiple-breath nitrogen washout from which S(cond) (index of peripheral conductive airway abnormality) was derived. Relationships between airway closure and lung function were examined by multiple-linear regression. Forced expiratory volume in 1 s was 87.5 ± 15.8% predicted, and seven subjects had AHR. Methacholine challenge decreased forced expiratory volume in 1 s by 23 ± 5% and increased nonventilated volume from 16 ± 4 to 29 ± 13% of computed tomography lung volume. The increase in airway closure measured by nonventilated volume correlated independently with both S(cond) (partial R(2) = 0.22) and with AHR (partial R(2) = 0.38). The extent of airway closure induced by methacholine inhalation in asthmatic subjects is greater with increasing peripheral airways disease, as measured by ventilation heterogeneity, and with worse AHR.     

Smooth muscle development during postnatal growth of distal bronchioles in infant rhesus monkeys.

Development of smooth muscle in conducting airways begins early in fetal life. Whereas the pattern and regulation of smooth muscle differentiation are well-defined, the impact of airway growth on the process is not. To evaluate the transformations in organization during postnatal growth, smooth muscle bundle organization (size, abundance, and orientation) was mapped in five generations of distal airways of infant rhesus monkeys (5 days and 1, 2, 3, and 6 mo old). On the basis of direct measurement of the bronchiole proximal to the terminal bronchiole, length increased by 2-fold, diameter by 1.35-fold, and surface area by 2.8-fold between 5 days and 6 mo of age. Smooth muscle bundle size was greater in proximal bronchioles than in respiratory bronchioles and did not change with age. However, relative bundle size decreased in proportion to airway size as the airways grew. Relative bundle abundance was constant regardless of airway generation or age. The distribution of smooth muscle bundle orientation changed with age in each airway generation, and there were significant changes in the terminal and respiratory bronchioles. We conclude that smooth muscle undergoes marked organizational changes as airways grow during postnatal development.     

In vivo loads on airway smooth muscle: the role of noncontractile airway structures.

The degree of airway smooth muscle contraction and shortening that occurs in vivo is modified by many factors, including those that influence the degree of muscle activation, the resting muscle length, and the loads against which the muscle contracts. Canine trachealis muscle will shorten up to 70% of starting length from optimal length in vitro but will only shorten by around 30% in vivo. This limitation of shortening may be a result of the muscle shortening against an elastic load such as could be applied by tracheal cartilage. Limitation of airway smooth muscle shortening in smaller airways may be the result of contraction against an elastic load, such as could be applied by lung parenchymal recoil. Measurement of the elastic loads applied by the tracheal cartilage to the trachealis muscle and by lung parenchymal recoil to smooth muscle of smaller airways were performed in canine preparations. In both experiments the calculated elastic loads applied by the cartilage and the parenchymal recoil explained in part the limitation of maximal active shortening and airway narrowing observed. We conclude that the elastic loads provided by surrounding structures are important in determining the degree of airway smooth muscle shortening and the resultant airway narrowing.     

Alpha smooth muscle actin expression in developing and adult human lung

Abstract. Myofibroblast-like cells containing smooth muscle actin have been identified in lung injury and repair. These cells differ from typical smooth muscle cells by architectural configuration, location and lack of smooth muscle myosin. Their progenitors are unknown. We hypothesized that these cells might have a developmental analog critical to lung morphogenesis. Lung tissue from developing and adult human lungs was studied using a highly specific monoclonal antibody directed against alpha smooth muscle actin (ASMA). Cells im-munoreactive for ASMA (ASMA cells) were identified prenatally in the form of smooth muscle investing the developing vasculature and airway structures. ASMA was not expressed in undifferentiated mesenchymal cells at any prenatal stage. Late in development, ASMA cells within the lung acinus increased proportionally to terminal airway and vascular complexity. In the early postnatal period, the specific distribution of ASMA cells within inflated lung became clearer, and three populations were identified: (1) typical smooth muscle investing the large airways and blood vessels; (2) small clusters of cells with in the acinus distributed at the tips of septa protruding into the alveolar duct; (3) individual cells within the alveolar sac sparsely distributed near the junctions of individual alveoli, frequently in association with small blood vessels. We conclude that ASMA cells appear only in developing small and large airways and pulmonary vessels and that they may play a critical role in branching morphogenesis during development.     

Inhibition of p21 Activated Kinase (PAK) Reduces Airway Responsiveness In Vivo and In Vitro in Murine and Human Airways

The p21-activated protein kinases (Paks) have been implicated in the regulation of smooth muscle contractility, but the physiologic effects of Pak activation on airway reactivity in vivo are unknown. A mouse model with a genetic deletion of Pak1 (Pak1(-/-)) was used to determine the role of Pak in the response of the airways in vivo to challenge with inhaled or intravenous acetylcholine (ACh). Pulmonary resistance was measured in anesthetized mechanically ventilated Pak1(-/-) and wild type mice. Pak1(-/-) mice exhibited lower airway reactivity to ACh compared with wild type mice. Tracheal segments dissected from Pak1(-/-) mice and studied in vitro also exhibited reduced responsiveness to ACh compared with tracheas from wild type mice. Morphometric assessment and pulmonary function analysis revealed no differences in the structure of the airways or lung parenchyma, suggesting that that the reduced airway responsiveness did not result from structural abnormalities in the lungs or airways due to Pak1 deletion. Inhalation of the small molecule synthetic Pak1 inhibitor, IPA3, also significantly reduced in vivo airway responsiveness to ACh and 5-hydroxytryptamine (5-Ht) in wild type mice. IPA3 inhibited the contractility of isolated human bronchial tissues to ACh, confirming that this inhibitor is also effective in human airway smooth muscle tissue. The results demonstrate that Pak is a critical component of the contractile activation process in airway smooth muscle, and suggest that Pak inhibition could provide a novel strategy for reducing airway hyperresponsiveness.     

Airway closure with high PEEP in vivo.

When airway smooth muscle is contracted in vitro, the airway lumen continues to narrow with increasing concentrations of agonist until complete airway closure occurs. Although there remains some controversy regarding whether airways can close in vivo, recent work has clearly demonstrated that, if the airway is sufficiently stimulated with contractile agonists, complete closure of even large cartilaginous conducting airways can readily occur with the lung at functional residual capacity (Brown RH and Mitzner W. J Appl Physiol 85: 2012-2017, 1998). This result suggests that the tethering of airways in situ by parenchymal attachments is small at functional residual capacity. However, at lung volumes above functional residual capacity, the outward tethering of airways should increase, because both the parenchymal shear modulus and tethering forces increase in proportion to the transpulmonary pressure. In the present study, we tested whether we could prevent airway closure in vivo by increasing lung volume with positive end-expiratory pressure (PEEP). Airway smooth muscle was stimulated with increasing methacholine doses delivered directly to airway smooth muscle at three levels of PEEP (0, 6, and 10 cmH(2)O). Our results show that increased lung volume shifted the airway methacholine dose-response curve to the right, but, in many airways in most animals, airway closure still occurred even at the highest levels of PEEP.     

Identifying airways responsible for heterogeneous ventilation and mechanical dysfunction in asthma: an image functional modeling approach.

We present an image functional modeling approach, which synthesizes imaging and mechanical data with anatomically explicit computational models. This approach is utilized to identify the relative importance of small and large airways in the simultaneous deterioration of mechanical function and ventilation in asthma. Positron emission tomographic (PET) images provide the spatial distribution and relative extent of ventilation defects in asthmatic subjects postbronchoconstriction. We also measured lung resistance and elastance from 0.15 to 8 Hz. The first step in image functional modeling involves mapping ventilation three-dimensional images to the computational model and identifying the largest sized airways of the model that, if selectively constricted, could precisely match the size and anatomic location of ventilation defects imaged by PET. In data from six asthmatic subjects, these airways had diameters <2.39 mm and mostly <0.44 mm. After isolating and effectively closing airways in the model associated with these ventilation defects, we imposed constriction with various means and standard deviations to the remaining airways to match the measured lung resistance and elastance from the same subject. Our results show that matching both the degree of mechanical impairment and the size and location of the PET ventilation defects requires either constriction of airways <2.4 mm alone, or a simultaneous constriction of small and large airways, but not just large airways alone. Also, whereas larger airway constriction may contribute to mechanical dysfunction during asthma, degradation in ventilation function requires heterogeneous distribution of near closures confined to small airways.     

Effect of airways constriction on exhaled nitric oxide.

While airway constriction has been shown to affect exhaled nitric oxide (NO), the mechanisms and location of constricted airways most likely to affect exhaled NO remain obscure. We studied the effects of histamine-induced airway constriction and ventilation heterogeneity on exhaled NO at 50 ml/s (Fe(NO,50)) and combined this with model simulations of Fe(NO,50) changes due to constriction of airways at various depths of the lung model. In 20 normal subjects, histamine induced a 26 +/- 15(SD)% Fe(NO,50) decrease, a 9 +/- 6% forced expiratory volume in 1 s (FEV(1)) decrease, a 19 +/- 9% mean forced midexpiratory flow between 25% and 75% forced vital capacity (FEF(25-75)) decrease, and a 94 +/- 119% increase in conductive ventilation heterogeneity. There was a significant correlation of Fe(NO,50) decrease with FEF(25-75) decrease (P = 0.006) but not with FEV(1) decrease or with increased ventilation heterogeneity. Simulations confirmed the negligible effect of ventilation heterogeneity on Fe(NO,50) and showed that the histamine-induced Fe(NO,50) decrease was due to constriction, with associated reduction in NO flux, of airways located proximal to generation 15. The model also indicated that the most marked effect of airways constriction on Fe(NO,50) is situated in generations 10-15 and that airway constriction beyond generation 15 markedly increases Fe(NO,50) due to interference with the NO backdiffusion effect. These mechanical factors should be considered when interpreting exhaled NO in lung disease.     

Thromboxane A2 induces airway constriction through an M3 muscarinic acetylcholine receptor-dependent mechanism

Thromboxane A2 (TXA2) is a potent lipid mediator released by platelets and inflammatory cells and is capable of inducing vasoconstriction and bronchoconstriction. In the airways, it has been postulated that TXA2 causes airway constriction by direct activation of thromboxane prostanoid (TP) receptors on airway smooth muscle cells. Here we demonstrate that although TXA2 can mediate a dramatic increase in airway smooth muscle constriction and lung resistance, this response is largely dependent on vagal innervation of the airways and is highly sensitive to muscarinic acetylcholine receptor (mAChR) antagonists. Further analyses employing pharmacological and genetic strategies demonstrate that TP-dependent changes in lung resistance and airway smooth muscle tension require expression of the M2 mAChR subtype. These results raise the possibility that some of the beneficial actions of anticholinergic agents used in the treatment of asthma and chronic obstructive pulmonary disease result from limiting physiological changes mediated through the TP receptor. Furthermore, these findings demonstrate a unique pathway for TP regulation of homeostatic mechanisms in the airway and suggest a paradigm for the role of TXA2 in other organ systems.     

Airway narrowing and internal structural constraints.

A computer model has been developed to simulate the movement restriction in the lamina propria-submucosa (L-S) layer (sandwiched by the basement membrane and the muscle layer) in a cartilage-free airway due to constriction of the smooth muscle layer. It is assumed that the basement membrane is inextensible; therefore, in the two-dimensional simulation, the perimeter outlining the membrane is a constant whether the airway is constricted or dilated. The cross-sectional area of the L-S layer is also assumed to be constant during the simulated airway narrowing. Folding of the mucosal membrane in constricted airways is assumed to be a consequence of the L-S area conservation and also due to tethering between the basement membrane and the muscle layer. The number of tethers determines the number of folds. The simulation indicates that the pressure in the L-S layer resulting from movement restriction can be a major force opposing muscle contraction and that the maximum shortening of the muscle layer is inversely proportional to the number of tethers (or folds) and the L-S layer thickness.     

The tumor necrosis factor family member LIGHT is a target for asthmatic airway remodeling

Individuals with chronic asthma show a progressive decline in lung function that is thought to be due to structural remodeling of the airways characterized by subepithelial fibrosis and smooth muscle hyperplasia. Here we show that the tumor necrosis factor (TNF) family member LIGHT is expressed on lung inflammatory cells after allergen exposure. Pharmacological inhibition of LIGHT using a fusion protein between the IgG Fc domain and lymphotoxin β receptor (LTβR) reduces lung fibrosis, smooth muscle hyperplasia and airway hyperresponsiveness in mouse models of chronic asthma, despite having little effect on airway eosinophilia. LIGHT-deficient mice also show a similar impairment in fibrosis and smooth muscle accumulation. Blockade of LIGHT suppresses expression of lung transforming growth factor-β (TGF-β) and interleukin-13 (IL-13), cytokines implicated in remodeling in humans, whereas exogenous administration of LIGHT to the airways induces fibrosis and smooth muscle hyperplasia, Thus, LIGHT may be targeted to prevent asthma-related airway remodeling.     

Effects of lung volume, bronchoconstriction, and cigarette smoke on morphometric airway dimensions

To examine the role of airway wall thickening in the bronchial hyperresponsiveness observed after exposure to cigarette smoke, we compared the airway dimensions of guinea pigs exposed to smoke (n = 7) or air (n = 7). After exposure the animals were anesthetized with urethan, pulmonary resistance was measured, and the lungs were removed, distended with Formalin, and fixed near functional residual capacity. The effects of lung inflation and bronchoconstriction on airway dimensions were studied separately by distending and fixing lungs with Formalin at total lung capacity (TLC) (n = 3), 50% TLC (n = 3), and 25% TLC (n = 3) or near residual volume after bronchoconstriction (n = 3). On transverse sections of extraparenchymal and intraparenchymal airways the following dimensions were measured: the internal area (Ai) and internal perimeter (Pi), defined by the epithelium, and the external area (Ae) and external perimeter (Pe), defined by the outer border of smooth muscle. Airway wall area (WA) was then calculated, WA = Ae - Ai. Ai, Pe, and Ae decreased with decreasing lung volume and after bronchoconstriction. However, WA and Pi did not change significantly with lung volume or after bronchoconstriction. After cigarette smoke exposure airway resistance was increased (P less than 0.05); however, there was no difference in WA between the smoke- and air-exposed groups when the airways were matched by Pi. We conclude that Pi and WA are constant despite changes in lung volume and smooth muscle tone and that airway hyperresponsiveness induced by cigarette smoke is not mediated by increased airway wall thickness.     

Exaggerated airway narrowing in mice treated with intratracheal cationic protein.

Airway hyperresponsiveness in mice with allergic airway inflammation can be attributed entirely to exaggerated closure of peripheral airways (Wagers S, Lundblad LK, Ekman M, Irvin CG, and Bates JHT. J Appl Physiol 96: 2019-2027, 2004). However, clinical asthma can be characterized by hyperresponsiveness of the central airways as well as the lung periphery. We, therefore, sought to establish a complementary model of hyperresponsiveness in the mouse due to excessive narrowing of the airways. We treated mice with a tracheal instillation of the cationic protein poly-l-lysine (PLL), hypothesizing that this would reduce the barrier function of the epithelium and thereby render the underlying airway smooth muscle more accessible to aerosolized methacholine. The PLL-treated animals were hypersensitive to methacholine: they exhibited an exaggerated response to submaximal doses but had a maximal response that was similar to controls. With the aid of a computational model of the mouse lung, we conclude that the methacholine responsiveness of PLL-treated mice is fundamentally different in nature to the hyperresponsiveness that we found previously in mice with allergically inflamed lungs.     

Competition of NO synthases and arginase in the airways hyperreactivity

The competition between arginases and NO synthases (NOS) for their common substrate L-arginine can be important in the airways hyperreactivity. We investigated the effect of the simultaneous modulation of arginase and NOS activities in allergen-induced airways hyperreactivity. We analysed the response of tracheal and lung tissue smooth muscle to histamine or acetylcholine after administration N(ω)-nitro-L-arginine methyl ester (L-NAME), aminoguanidine (AG) and N(ω)-hydroxy-L-arginine (NOHA) in the combinations in in vitro conditions. The results show the decrease of ovalbumin-induced hyperreactivity after inhibition of arginase activity with NOHA. A supplementation of L-arginine caused favourable effect on the airway smooth muscle response. We found the airway reactivity decrease on the whole if we used the combination of NOS and arginase inhibitors. The inhibition of both types of enzymes caused more expressive effect in tracheal smooth muscles. We recorded the difference in the response to histamine or acetylcholine. The simultaneous inhibition of iNOS (with AG) and arginase (with NOHA) evoked the most expressive effect. Results show the importance of competition of both types enzymes - NOS and arginase for the balance of theirs activities in the control of airways bronchomotoric tone in the conditions of the airways hyperreactivity.     

Reduction in airway hyperresponsiveness to methacholine by the application of RF energy in dogs.

We delivered controlled radio frequency energy to the airways of anesthetized, ventilated dogs to examine the effect of this treatment on reducing airway narrowing caused by a known airway constrictor. The airways of 11 dogs were treated with a specially designed bronchial catheter in three of four lung regions. Treatments in each of the three treated lung regions were controlled to a different temperature (55, 65, and 75 degrees C); the untreated lung region served as a control. We measured airway responsiveness to local methacholine chloride (MCh) challenge before and after treatment and examined posttreatment histology to 3 yr. Treatments controlled to 65 degrees C as well as 75 degrees C persistently and significantly reduced airway responsiveness to local MCh challenge (P < or = 0.022). Airway responsiveness (mean percent decrease in airway diameter after MCh challenge) averaged from 6 mo to 3 yr posttreatment was 79 +/- 2.2% in control airways vs. 39 +/- 2.6% (P < or = 0.001) for airways treated at 65 degrees C, and 26 +/- 2.7% (P < or = 0.001) for airways treated at 75 degrees C. Treatment effects were confined to the airway wall and the immediate peribronchial region on histological examination. Airway responsiveness to local MCh challenge was inversely correlated to the extent of altered airway smooth muscle observed in histology (r = -0.54, P < 0.001). We conclude that the temperature-controlled application of radio frequency energy to the airways can reduce airway responsiveness to MCh for at least 3 yr in dogs by reducing airway smooth muscle contractility.