A novel bronchial ring bioassay for the evaluation of small airway smooth muscle function in mice

Advances in our understanding of murine airway physiology have been hindered by the lack of suitable, ex vivo, small airway bioassay systems. In this study, we introduce a novel small murine airway bioassay system that permits the physiological and pharmacological study of intrapulmonary bronchial smooth muscle via a bronchial ring (BR) preparation utilizing BR segments as small as 200 microm in diameter. Using this ex vivo BR bioassay, we characterized small airway smooth muscle contraction and relaxation in the presence and absence of bronchial epithelium. In control BRs, the application of mechanical stretch is followed by spontaneous bronchial smooth muscle relaxation. BRs pretreated with methacholine (MCh) partially attenuate this stretch-induced relaxation by as much as 42% compared with control. MCh elicited a dose-dependent bronchial constriction with a maximal tension (E(max)) of 8.7 +/- 0.2 mN at an EC(50) of 0.33 +/- 0.02 microM. In the presence of nifedipine, ryanodine, 2-aminoethoxydiphenyl borate, and SKF-96365, E(max) to MCh was significantly reduced. In epithelium-denuded BRs, MCh-induced contraction was significantly enhanced to 11.4 +/- 1.0 mN with an EC(50) of 0.16 +/- 0.04 microM (P < 0.01). Substance P relaxed MCh-precontracted BR by 62.1%; however, this bronchial relaxation effect was completely lost in epithelium-denuded BRs. Papaverine virtually abolished MCh-induced constriction in both epithelium-intact and epithelium-denuded bronchial smooth muscle. In conclusion, this study introduces a novel murine small airway BR bioassay that allows for the physiological study of smooth muscle airway contractile responses that may aid in our understanding of the pathophysiology of asthma.     

Effect of resting smooth muscle length on contractile response in resistance airways

We studied the effect of resting smooth muscle length on the contractile response of the major resistance airways (generations 0-5) in 18 mongrel dogs in vivo using tantalum bronchography. Dose-response curves to 10(-10) to 10(-7) mol/kg methacholine (MCh) were generated [at functional residual capacity (FRC)] by repeated intravenous bolus administration using tantalum bronchography after each dose. Airway constriction varied substantially with dose-equivalent stimulation and varied sequentially from trachea (8.8 +/- 2.2% change in airway diam) to fifth-generation bronchus (49.8 +/- 3.0%; P less than 0.001). Length-tension curves were generated for each airway to determine the airway diameter (i.e., resting in situ smooth muscle length) at which maximal constriction was elicited using bolus intravenous injection of 10(-8) mol/kg MCh. A Frank-Starling relationship was obtained for each airway; the transpulmonary pressure at which maximal constriction was elicited increased progressively from 2.50 +/- 1.12 cmH2O for trachea (approximately FRC) to 18.3 +/- 1.05 cmH2O for fifth-generation airways (approximately 50% TLC) (P less than 0.001). A similar relationship was obtained when change in airway diameter was plotted as a function of airway radius. We demonstrate substantial heterogeneity in the lung volumes at which maximal constriction is elicited and in distribution of parasympathomimetic constriction within the first few generations of resistance bronchi. Our data also suggest that lung hyperinflation may lead to augmented airway contractile responses by shifting resting smooth muscle length toward optimum resting smooth muscle length.     

Effect of bronchial smooth muscle contraction on lung compliance.

Lung compliance is generally considered to represent a blend of surface and tissue forces, and changes in compliance in vivo are commonly used to indicate changes in surface forces. There are, however, theoretical arguments that would allow contraction of airway smooth muscle to affect substantially the elasticity of the lung. In the present study we evaluated the role of conducting airway contraction on lung compliance in vivo by infusing methacholine (MCh) at a constant rate into the bronchial circulation. With a steady-state MCh infusion of 2.4 micrograms/min into the bronchial perfusate (perfusate concentration = 0.7 microM), there was an approximate doubling of lung resistance and a 50% fall in dynamic compliance. There were also significant decreases in chord compliance measured from the quasi-static pressure-volume curves and in total lung capacity and residual volume. When the same infusion rate was administered into the pulmonary artery, no changes in lung mechanics were observed. These results indicate that the conducting airways may have a major role in regulating lung elasticity. This linkage between airway contraction and lung compliance may account for the common observation that pharmacological challenges given to the lung usually result in similar changes in lung compliance and airway conductance. Our results also suggest the possibility that the lung tissue resistance, which dominates the measurement of lung resistance in many species, might in fact reflect the physical properties of conducting airways.     

Airway constriction measured from tantalum bronchograms in conscious mice in response to methacholine

A single-projection X-ray technique showed an increase in functional residual capacity (FRC) in conscious mice in response to aerosolized methacholine (MCh) with little change in airway resistance (Raw) measured using barometric plethysmography (Lai-Fook SJ, Houtz PK, Lai Y-L. J Appl Physiol 104: 521-533, 2008). The increase in FRC presumably prevented airway constriction by offsetting airway contractility. We sought a more direct measure of airway constriction. Anesthetized Balb/c mice were intubated with a 22-G catheter, and tantalum dust was insufflated into the lungs to produce a well-defined bronchogram. After overnight recovery, the conscious mouse was placed in a sealed box, and bronchograms were taken at maximum and minimum points of the box pressure cycle before (control) and after 1-min exposures to 25, 50, and 100 mg/ml MCh aerosol. After overnight recovery, each mouse was studied under both room and body temperature box air conditions to correct for gas compression effects on the control tidal volume (Vt) and to determine Vt and Raw with MCh. Airway diameter (D), FRC, and Vt were measured from the X-ray images. Compared with control, D decreased by 24%, frequency decreased by 35%, FRC increased by 120%, and Raw doubled, to reach limiting values with 100 mg/ml MCh. Vt was unchanged with MCh. The limiting D occurred near zero airway elastic recoil, where the maximal contractility was relatively small. The conscious mouse adapted to MCh by breathing at a higher lung volume and reduced frequency to reach a limit in constriction.     

Effects of airway pressure on bronchial blood flow

We studied the effects of increased airway pressure caused by increasing levels of positive end-expiratory pressure (PEEP) on bronchial arterial pressure-flow relationships. In eight alpha-chloralose-anesthetized mechanically ventilated sheep (23-27 kg), the common bronchial artery, the bronchial branch of the bronchoesophageal artery, was cannulated and perfused with a pump. The control bronchial blood flow (avg 12 +/- 1 ml/min or 0.48 ml X min-1 X kg-1) was set to maintain mean bronchial arterial pressure equal to systemic blood pressure. Pressure-flow curves of the bronchial circulation were measured by making step changes in bronchial blood flow, and changes in these curves were analyzed with measurements of the pressure at zero flow and the slope of the linearized curve. The zero-flow pressure represents the effective downstream pressure, and the slope represents the resistance through the bronchial vasculature. At a constant bronchial arterial pressure of 100 mmHg, an 8 mmHg increase in mean airway pressure caused a 40% reduction in bronchial blood flow. Under constant flow conditions, increases in mean airway pressure with the application of PEEP caused substantial increases in bronchial arterial pressure, averaging 4.6 mmHg for every millimeters of mercury increase in mean airway pressure. However, bronchial arterial pressure at zero flow increased approximately one-for-one with increases in mean airway pressure. Thus the acute sensitivity of the bronchial artery to changes in mean airway pressure results primarily from changes in bronchovascular resistance and not downstream pressure.     

Effects of salbutamol and Ro-20-1724 on airway and parenchymal mechanics in rats.

We investigated the effects of a selective beta(2)-agonist, salbutamol, and of phosphodiesterase type 4 inhibition with 4-(3-butoxy-4-methoxy benzyl)-2-imidazolidinone (Ro-20-1724) on the airway and parenchymal mechanics during steady-state constriction induced by MCh administered as an aerosol or intravenously (iv). The wave-tube technique was used to measure the lung input impedance (ZL) between 0.5 and 20 Hz in 31 anesthetized, paralyzed, open-chest adult Brown Norway rats. To separate the airway and parenchymal responses, a model containing an airway resistance (Raw) and inertance (Iaw), and a parenchymal damping (G) and elastance (H), was fitted to ZL spectra under control conditions, during steady-state constriction, and after either salbutamol or Ro-20-1724 delivery. In the Brown Norway rat, the response to iv MCh infusion was seen in Raw and G, whereas continuous aerosolized MCh challenge produced increases in G and H only. Both salbutamol, administered either as an aerosol or iv, and Ro-20-1724 significantly reversed the increases in Raw and G when MCh was administered iv. During the MCh aerosol challenge, Ro-20-1724 significantly reversed the increases in G and H, whereas salbutamol had no effect. These results suggest that, after MCh-induced changes in lung function, salbutamol increases the airway caliber. Ro-20-1724 is effective in reversing the airway narrowings, and it may also decrease the parenchymal constriction.     

Effects of edema on small airway narrowing

Wagner, Elizabeth M. Effects of edema on small airwaynarrowing. J. Appl. Physiol. 83(3):784-791, 1997.Numerous mediators of inflammation have beendemonstrated to cause airway microvascular fluid and proteinextravasation. That fluid extravasation results in airway wall edemaleading to airway narrowing and enhanced reactivity has not beenconfirmed. In anesthetized, ventilated sheep(n = 30), airway vascularfluid extravasation was induced by infusing bradykinin(10⁶ M) through acannulated, blood-perfused bronchial artery. Airway wall edema andluminal narrowing were determined morphometrically. Airway reactivityto methacholine (MCh; 10 µg/ml, intrabronchial artery) was determinedby measuring conducting airway resistance (Raw) by forced oscillation.Raw measurements were made and lung lobes were excised and quick frozenbefore or after a 1-h bradykinin infusion. In 10 airways per lobe(range 0.2- to 2.0-mm relaxed diameter), wall area occupied 32 ± 2% (SE) of the total normalized airway area(n = 9). Bradykinin infusion increasedwall area to 42 ± 5% (P = 0.02);luminal area decreased by <5%; and smooth muscle perimeter, ameasure of smooth muscle constriction, was not altered(n = 5). Raw showed nochange from baseline (1.4 ± 0.4 cmH₂O · l¹ · s)after bradykinin infusion (n = 10).During MCh challenge, Raw increased by 3.2 ± 04 cmH₂O · l¹ · s,and this change did not differ after administration of bradykinin. MChchallenge caused similar decreases in smooth muscle perimeter (10%)and luminal area (72 vs. 68%) before and after bradykinin infusion.However, the time constant of recovery of Raw from MCh constriction wasincreased from control (40 ± 3 s) to 57 ± 10 s after bradykinininfusion (P = 0.03). When lung lobeswere excised at the same time after MCh challenge was terminated(n = 5), luminal area was greaterbefore bradykinin infusion than after (86 vs. 78%;P = 0.007), as was smooth muscleperimeter. The results of this study demonstrate that airway wall edemalimits relaxation after induced constriction rather than enhancingconstriction.

     

Distribution of bronchoconstrictor responses in isolated-perfused rat lung

We studied the effects of bronchoconstrictor stimuli administered selectively through isolated-perfused preparations of the bronchial and pulmonary circulations of 80 Sprague-Dawley rats. Dose-related contraction was elicited with infusion of acetylcholine (ACh), histamine, and serotonin (5-HT). Bolus infusion of 10(-5) mol ACh caused a 3.5-fold increase in pulmonary resistance (RL) after infusion into the pulmonary circulation (PC) and a 2.5-fold increase in the bronchial circulation (BC) (P less than 0.05 vs. control) that was blocked selectively in each circulation with atropine. Administration of 10(-5) mol 5-HT into the BC caused only a 45% increase in RL; the same dose of 5-HT caused a 5.1-fold increase in RL in the PC. A biphasic (increase at lower doses/decrease at higher doses) change in RL was elicited by histamine that was converted to dose-related constriction after H2-receptor blockade with cimetidine in both BC and PC. Response to exogenous ACh remained viable for greater than 5 h. Infusion of the mast cell degranulating agent, compound 48/80 (48/80), caused increase in RL that corresponded to quantitative recovery of histamine in the perfusates of both BC and PC. Histamine concentration in the perfusate increased from 47.2 +/- 31.8 (base line) to 624 +/- 60.1 ng/ml (2-fold increase in RL) in the BC and from 38.3 +/- 17.7 (base line) to 294.4 +/- 38.1 ng/ml (50% increase in RL) in the PC (P less than 0.001 vs. baseline concentration) after a 0.1-mg/ml dose of 48/80.(ABSTRACT TRUNCATED AT 250 WORDS)     

Blood flow distribution within the airway wall.

Altered perfusion of the bronchial mucosal plexus relative to the adventitial plexus may contribute to geometric changes in the airway wall and lumen. We studied bronchial perfusion distribution in sheep by using fluorescent microspheres at baseline and during intrabronchial artery challenge with methacholine chloride (MCh; n = 7). Additionally, we measured airway resistance (Raw) during MCh with control or increased perfusion (n = 9). Raw with MCh was significantly greater for high than control flow. Microspheres in histological sections lodged predominantly in the mucosa (60%), and this was not altered by MCh. However, more microspheres lodged in airways >1-mm in diameter during MCh and increased perfusion than MCh and control flow. In airways < or =1 mm in diameter, fewer microspheres lodged during control than increased flow. If the number of microspheres represents regional agonist access to airway smooth muscle, then the differences observed in Raw can be explained by the distribution of agonist. During challenge, there was greater MCh delivery to larger airways during increased flow and less delivery to smaller airways during control flow. The results demonstrate the effects of axial perfusion distribution on Raw.     

Effects of tidal volume stretch on airway constriction in vivo.

Tidal stresses are thought to be involved in maintaining airway patency in vivo. The present study examined the effects of normal stresses exerted by the lung parenchyma during tidal ventilation on recovery from agonist-induced airway constriction. In seven anesthetized dogs, one lung was selectively ventilated with a Univent endotracheal tube (Vitaid, Lewiston, NY). Airway tone was increased either transiently (intravenous bolus) or continuously (intravenous infusion) with methacholine (MCh). During one-lung ventilation, changes in the airway size of both lungs were measured for up to 40 min during recovery from constriction by using high-resolution computed tomography. After recovery to baseline, the alternate lung was ventilated, and the protocol was repeated. The absence of tidal stresses led to an attenuated recovery from either transient or steady-state airway constriction. The effectiveness or lack thereof of normal tidal stress in stabilizing airway size may be one factor that contributes to the lack of reversal with tidal breathing and deep inspiration seen in asthmatic subjects.     

Partitioning of airway responses to inhaled methacholine in the rat

We measured the changes in upper and lower airway resistance after inhalation of aerosols of methacholine (MCh) in doubling concentrations (16, 32, 64, and 128 mg/ml) in 11 anesthetized nonintubated spontaneously breathing rats. Upper airway resistance (Ru) increased from a control value of 0.48 +/- 0.04 cmH2O X ml-1 X s (mean +/- SE) to 0.85 +/- 0.15 after 128 mg/ml MCh, whereas lower airway resistance (Rlo) increased from 0.11 +/- 0.03 to 0.21 +/- 0.04. However, there was no correlation between the magnitudes of the changes in Ru and Rlo. In a further seven anesthetized spontaneously breathing rats aerosols of MCh were delivered into the lower airways via a tracheostomy and resulted in increases in Rlo from a control value of 0.20 +/- 0.03 to 0.66 +/- 0.12 after 128 mg/ml MCh. Ru also increased to approximately double its control value. We conclude that inhaled MCh causes narrowing of both Ru and Rlo in the anesthetized rat, the changes in Ru and Rlo are not correlated, and changes in Ru can occur when MCh deposition occurs only in the lower airways.     

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.     

IL-10 gene knockout attenuates allergen-induced airway hyperresponsiveness in C57BL/6 mice

Intratracheal administration of interleukin-10 (IL-10) has been reported to inhibit allergic inflammation but augment airway hyperresponsiveness (AHR). In the present study, airway and smooth muscle responsiveness to methacholine (MCh) were compared in wild-type (WT) and IL-10-deficient (IL-10-KO) mice to investigate the role of endogenous IL-10 in AHR development. Naive WT and IL-10-KO mice exhibited similar dose-dependent increases in airway resistance (Raw) to intravenous MCh. Sensitization and challenge with ragweed (RW) induced a twofold increase in responsiveness to intravenous MCh in WT mice, but hyperresponsiveness was not observed in similarly treated IL-10-KO mice. Likewise, tracheal rings from RW-sensitized and -challenged WT mice exhibited a fourfold greater responsiveness to MCh than IL-10-KO tracheal preparations. Measurements of airway constriction by whole body plethysmography further supported the Raw and tracheal ring data (i.e., AHR was not observed in the absence of IL-10). Interestingly, factors previously implicated in the development of AHR, including IL-4, IL-5, IL-13, IgA, IgG1, IgE, eosinophilia, and lymphocyte recruitment to the airways, were upregulated in the IL-10-KO mice. Treatment with recombinant murine IL-10 at the time of allergen challenge reduced the magnitude of inflammation but reinstated AHR development in IL-10-KO mice. Adoptive transfer of mononuclear splenocytes to IL-10-sufficient severe combined immunodeficient mice indicated that lymphocytes were an important source of the IL-10 impacting AHR development. These results provide evidence that IL-10 expression promotes the development of allergen-induced smooth muscle hyperresponsiveness.     

Bronchial blood flow affects recovery from constriction in dog lung periphery

We investigated the effect of eliminating the bronchial circulation on recovery time from intravenous histamine challenge in canine lung periphery. Results from animals with intact bronchial circulations were compared with a second group in which the left lower lobe was isolated in situ. The pulmonary artery to this lobe was perfused and a bronchoscope was wedged in a small airway, which provided an index of resistance to airflow through the collateral system. The lobe was challenged with intravenous histamine, and the time constant of recovery (tau) from bronchoconstriction was measured. With or without pulmonary blood flow, elimination of the bronchial circulation increased tau 44.4 and 48.5%, respectively. This increase was similar to that found by stopping pulmonary blood flow alone (56.5%). Histamine challenges were also performed in sympathectomized or vagotomized animals with intact bronchial circulations. Neither of these conditions increased tau. We conclude that blood flow through the bronchial circulation affects the recovery time from intravenous histamine challenge in the lung periphery to a degree similar to that of the pulmonary circulation.     

Effects of rapid saline infusion on lung mechanics and airway responsiveness in humans.

Lung mechanics and airway responsiveness to methacholine (MCh) were studied in seven volunteers before and after a 20-min intravenous infusion of saline. Data were compared with those of a time point-matched control study. The following parameters were measured: 1-s forced expiratory volume, forced vital capacity, flows at 40% of control forced vital capacity on maximal (Vm(40)) and partial (Vp(40)) forced expiratory maneuvers, lung volumes, lung elastic recoil, lung resistance (Rl), dynamic elastance (Edyn), and within-breath resistance of respiratory system (Rrs). Rl and Edyn were measured during tidal breathing before and for 2 min after a deep inhalation and also at different lung volumes above and below functional residual capacity. Rrs was measured at functional residual capacity and at total lung capacity. Before MCh, saline infusion caused significant decrements of forced expiratory volume in 1 s, Vm(40), and Vp(40), but insignificantly affected lung volumes, elastic recoil, Rl, Edyn, and Rrs at any lung volume. Furthermore, saline infusion was associated with an increased response to MCh, which was not associated with significant changes in the ratio of Vm(40) to Vp(40). In conclusion, mild airflow obstruction and enhanced airway responsiveness were observed after saline, but this was not apparently due to altered elastic properties of the lung or inability of the airways to dilate with deep inhalation. It is speculated that it was likely the result of airway wall edema encroaching on the bronchial lumen.     

Angiotensin-converting enzyme activity in ovine bronchial vasculature.

Angiotensin-converting enzyme (ACE) plays a major role in the metabolism of bradykinin, angiotensin, and neuropeptides, which are all implicated in inflammatory airway diseases. The activity of ACE, which is localized on the luminal surface of endothelial cells (EC), has been well documented in pulmonary EC; however, few data exist regarding the relative activity of ACE in the airway vasculature. Therefore, we measured ACE activity in cultured EC from the sheep bronchial artery and bronchial mucosa (microvascular) and compared it with pulmonary artery EC. The baseline level of total ACE activity (cellular plus secreted) was significantly greater in bronchial microvascular EC (1.24 +/- 0.24 mU/106 cells) compared with bronchial artery EC (0.59 +/- 0.15 mU/106 cells; P < 0.05) and comparable to pulmonary artery EC (1.12 +/- 0.14 mU/106 cells; P > 0.05). Measured ACE activity secreted into culture medium for each cell type was 64-74% of total activity and did not differ among the three EC types (P = 0.17). Hydrocortisone (10 microg/ml; 48-72 h) treatment resulted in a significant increase in ACE activity in bronchial EC. Likewise, TNF-alpha (0.1 ng/ml) treatment markedly increased ACE activity in all cell lysates (P < 0.05). We confirmed the importance of ACE activity in vivo since, at the highest dose of bradykinin studied (10-8 M), bronchial artery pressure at constant flow showed a greater decrease after captopril treatment (36% before vs. 60% after; P = 0.05). These results demonstrate high ACE expression of the bronchial microvasculature and suggest an important regulatory role for ACE in the metabolism of kinin peptides known to contribute to airway pathology.     

Neonatal calves develop airflow limitation due to chronic hypobaric hypoxia

Neonates and infants presenting with pulmonary hypertension and chronic hypoxia often exhibit airway obstruction. To investigate this association, we utilized a system in which neonatal calves are exposed to chronic hypobaric hypoxia and develop severe pulmonary hypertension. For the present study, one of each pair of six age-matched pairs of neonatal calves was continuously exposed to hypobaric hypoxia at 4,500 m (CH); the other remained at 1,500 m. At 2 wk of age, mean pulmonary arterial pressure (MPAP), dynamic lung compliance (Cdyn), resistance (RL), and static respiratory system compliance (Crs) were measured at 4,500 m in both CH and control calves exposed acutely to hypoxia (C). These measurements were repeated after cumulative administrations of nebulized methacholine (MCh). Tissues were removed for histological examination and assessment of bronchial ring contractility to MCh and KCl. After 2 wk of hypobaric hypoxia, MPAP (C 35 +/- 1.7 vs. CH 120 +/- 7 mmHg, P less than 0.001) and RL (C 2.64 +/- 0.16 vs CH 4.99 +/- 0.47 cmH2O.l-1s, P less than 0.001) increased. Cdyn (C 0.100 +/- 0.01 vs. CH 0.082 +/- 0.007 l/cmH2O) and Crs (CH 0.46 +/- 0.003 vs. C 0.59 +/- 0.009 l/cmH2O) were not significantly different. Compared with airways of C calves, airways of CH animals did not exhibit in vivo or in vitro MCh hyperresponsiveness; however, in vitro contractility to KCl of airways from CH animals was significantly increased. Histologically, airways from the CH calves showed increases in airway fibrous tissue and smooth muscle.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of lung volume, volume history, and methacholine on lung tissue viscance

We examined the effects of lung volume change and volume history on lung resistance (RL) and its components before and during induced constriction. Eleven subjects, including three current and four former asthmatics, were studied. RL, airway resistance (Raw), and, by subtraction, tissue viscance (Vtis) were measured at different lung volumes before and after a deep inhalation and were repeated after methacholine (MCh) aerosols up to maximal levels of constriction. Vtis, which average 9% of RL at base line, was unchanged by MCh and was not changed after deep inhalation but increased directly with lung volume. MCh aerosols induced constriction by increasing Raw, which was reversed by deep inhalation in inverse proportion to responsiveness. such that the more responsive subjects reversed less after a deep breath. Responsiveness correlated directly with the degree of maximal constriction, as more responsive subjects constricted to a greater degree. These results indicate that in humans Vtis comprises a small fraction of overall RL, which is clearly volume-dependent but unchanged by MCh-induced constriction and unrelated to the degree of responsiveness of the subject.     

Effects of elastase-induced emphysema on airway responsiveness to methacholine in rats

We examined the effects of elastase-induced emphysema on lung volumes, pulmonary mechanics, and airway responses to inhaled methacholine (MCh) of nine male Brown Norway rats. Measurements were made before and weekly for 4 wk after elastase in five rats. In four rats measurements were made before and at 3 wk after elastase; in these same animals the effects of changes in end-expiratory lung volume on the airway responses to MCh were evaluated before and after elastase. Airway responses were determined from peak pulmonary resistance (RL) calculated after 30-s aerosolizations of saline and doubling concentrations of MCh from 1 to 64 mg/ml. Porcine pancreatic elastase (1 IU/g) was administered intratracheally. Before elastase RL rose from 0.20 +/- 0.02 cmH2O.ml-1.s (mean +/- SE; n = 9) to 0.57 +/- 0.06 after MCh (64 mg/ml). A plateau was observed in the concentration-response curve. Static compliance and the maximum increase in RL (delta RL64) were significantly correlated (r = 0.799, P less than 0.01). Three weeks after elastase the maximal airway response to MCh was enhanced and no plateau was observed; delta RL64 was 0.78 +/- 0.07 cmH2O.ml-1.s, significantly higher than control delta RL64 (0.36 +/- 0.7, P less than 0.05). Before elastase, increase of end-expiratory lung volume to functional residual capacity + 1.56 ml (+/- 0.08 ml) significantly reduced RL at 64 mg MCh/ml from 0.62 +/- 0.05 cmH2O.ml-1.s to 0.50 +/- 0.03, P less than 0.05.(ABSTRACT TRUNCATED AT 250 WORDS)     

Frequency dependence of respiratory system mechanics during induced constriction in a murine model of asthma.

Airway dysfunction in asthma is characterized by hyperresponsiveness, heterogeneously narrowed airways, and closure of airways. To test the hypothesis that airway constriction in ovalbumin (OVA)-sensitized OVA-intranasally challenged (OVA/OVA) mice produces mechanical responses that are similar to those reported in asthmatic subjects, respiratory system resistance (Rrs) and elastance (Edyn,rs) spectra were obtained in OVA/OVA and control mice during intravenous methacholine (MCh) infusions. In control mice, MCh at 1,700 microg x kg(-1) x min(-1) produced 1) a 495 and 928% increase of Rrs at 0.5 Hz and 19.75 Hz, respectively, 2) a 33% rise in Edyn,rs at 0.5 Hz, and 3) a mild frequency (f)-dependent increase of Edyn,rs. The same MCh dose in OVA/OVA mice produced 1) elevations of Rrs at 0.5 Hz and 19.75 Hz of 1,792 and 774%, respectively, 2) a 390% rise in Edyn,rs at 0.5 Hz, and 3) marked f-dependent increases of Edyn,rs. During constriction, the f dependence of mechanics in control mice was consistent with homogeneous airway narrowing; however, in OVA/OVA mice, f dependence was characteristic of heterogeneously narrowed airways, closure of airways, and airway shunting. These mechanisms amplify the pulmonary mechanical responses to constrictor stimuli at physiological breathing rates and have important roles in the pathophysiology of human asthma.