Bronchoprotective and bronchodilatory effects of deep inspiration in rabbits subjected to bronchial challenge.

The effect of deep inspiration (DI) on airway responsiveness differs in asthmatic and normal human subjects. The mechanism for the effects of DI on airway responsiveness in vivo has not been identified. To elucidate potential mechanisms, we compared the effects of DI imposed before or during induced bronchoconstriction on the airway response to methacholine (MCh) in rabbits. The changes in airway resistance in response to intravenous MCh were continuously monitored. DI depressed the maximum response to MCh when imposed before or during the MCh challenge; however, the inhibitory effect of DI was greater when imposed during bronchoconstriction. Because immature rabbits have greater airway reactivity than mature rabbits, we compared the effects of DI on their airway responses. No differences were observed. Our results suggest that the mechanisms by which DI inhibits airway responsiveness do not depend on prior activation of airway smooth muscle (ASM). These results are consistent with the possibility that reorganization of the contractile apparatus caused by stretch of ASM during DI contributes to depression of the airway response.     

Mechanical response to methacholine and deep inspiration in supine men.

The effects of supine posture on airway responses to inhaled methacholine and deep inspiration (DI) were studied in seven male volunteers. On a control day, subjects were in a seated position during both methacholine inhalation and lung function measurements. On a second occasion, the whole procedure was repeated with the subjects lying supine for the entire duration of the study. On a third occasion, methacholine was inhaled from the seated position and measurements were taken in a supine position. Finally, on a fourth occasion, methacholine was inhaled from the supine position and measurements were taken in the seated position. Going from sitting to supine position, the functional residual capacity decreased by approximately 1 liter in all subjects. When lung function measurements (pulmonary resistance, dynamic elastance, residual volume, and maximal flows) were taken in supine position, the response to methacholine was greater than at control, and this was associated with a greater dyspnea and a faster recovery of dynamic elastance after DI. By contrast, when methacholine was inhaled in supine position but measurements were taken in sitting position, the response to methacholine was similar to control day. These findings document the potential of the decrease in the operational lung volumes in eliciting or sustaining airflow obstruction in nocturnal asthma. It is speculated that the exaggerated response to methacholine in the supine posture may variably contribute to airway smooth muscle adaptation to short length, airway wall edema, and faster airway renarrowing after a large inflation.     

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.     

Detection of allergen-induced airway hyperresponsiveness in isolated mouse lungs

Airway hyperresponsiveness (AHR) is a hallmark of bronchial asthma. Important features of this exaggerated response to bronchoconstrictive stimuli have mostly been investigated in vivo in intact animals or in vitro in isolated tracheal or bronchial tissues. Both approaches have important advantages but also certain limitations. Therefore, the aim of our study was to develop an ex vivo model of isolated lungs from sensitized mice for the investigation of airway responsiveness (AR). BALB/c mice were sensitized by intraperitoneal ovalbumin (Ova) and subsequently challenged by Ova inhalation. In vivo AR was measured in unrestrained animals by whole body plethysmography after stimulation with aerosolized methacholine (MCh) with determination of enhanced pause (P(enh)). Twenty-four hours after each P(enh) measurement, airway resistance was continuously registered in isolated, perfused, and ventilated lungs on stimulation with inhaled or intravascular MCh or nebulized Ova. In a subset of experiments, in vivo AR was additionally measured in orotracheally intubated, spontaneously breathing mice 24 h after P(enh) measurement, and lungs were isolated further 24 h later. Isolated lungs of allergen-sensitized and -challenged mice showed increased AR after MCh inhalation or infusion as well as after specific provocation with aerosolized allergen. AR was increased on days 2 and 5 after Ova challenge and had returned to baseline on day 9. AHR in isolated lungs after aerosolized or intravascular MCh strongly correlated with in vivo AR. Pretreatment of isolated lungs with the beta(2)-agonist fenoterol diminished AR. In conclusion, this model provides new opportunities to investigate mechanisms of AHR as well as pharmacological interventions on an intact organ level.     

Antigen challenge of sensitized rats increases airway responsiveness to methacholine

We measured airway responsiveness to methacholine (MCh) of highly inbred rats before and after six inhalational challenges with antigen. Ten Brown-Norway rats (130-216 g) that were actively sensitized to ovalbumin (OA) received six challenges with OA at 5-day intervals beginning 19 days after sensitization. An aerosol of OA (5% wt/vol) was inhaled for 1, 2, 5, and 10 min or until pulmonary resistance (RL) increased by at least 50%. Challenges with aerosolized MCh were performed immediately before and 14 days after sensitization, 2 days after the 3rd OA exposure, and 2, 7, 12, and 17 days after the 6th OA challenge. Four unsensitized rats underwent inhalational challenges with MCh over an equivalent time period. Responsiveness to MCh was calculated as the concentration of MCh required to increase RL to 200% of the control value (EC200RL). Seven out of 10 rats in the experimental group reacted to the first OA challenge with an immediate increase in RL of greater than 50% of control (range 70-550%). Three animals were unreactive to OA. Base-line EC200RL for all rats undergoing sensitization was 2.13 mg/ml (geometric mean), and it did not change significantly after sensitization (2.05 mg/ml). However, EC200RL of the rats that reacted to OA (n = 7) decreased significantly after 3 (1.11 mg/ml; P less than 0.005) and 6 OA exposures (0.96 mg/ml; P less than 0.005). The increase in responsiveness to inhaled MCh was present 17 days after the last OA exposure (EC200RL = 1.40 mg/ml; P less than 0.05). EC200RL of neither the unreactive sensitized rats (n = 3) nor the control rats (n = 4) changed after OA challenges.(ABSTRACT TRUNCATED AT 250 WORDS)     

Partitioning of pulmonary resistance during constriction in the dog: effects of volume history

We assessed the relative changes in airways and lung tissue with bronchoconstriction, and the changes in each during and following a deep inhalation (DI). We partitioned pulmonary resistance (RL) into airway (Raw) and tissue (Vtis) components using alveolar capsules in 10 anesthetized, paralyzed, and open-chested dogs ventilated sinusoidally with 350-ml breaths at 1 Hz. We made measurements before and during bronchoconstriction induced by vagal stimulation or inhalation of histamine or prostaglandin F2 alpha (PGF2 alpha), each of which decreased dynamic compliance by approximately 40%. With histamine and PGF2 alpha the rise in RL was predominantly due to Vtis. With vagal stimulation there was a relatively greater increase in Raw than Vtis. At higher lung volumes, Vtis increases offset falls in Raw, producing higher RL at these volumes before and during constriction with PGF2 alpha and histamine. During constriction with vagal stimulation, the fall in Raw with inflation overrode the rise in Vtis, resulting in a lower RL at the higher compared with the lower lung volume. The changes seen after a DI in the control and constricted states were due to alterations in tissue properties, both viscous and elastic. However, the relative hysteresis of the airways and parenchyma were equal, since Raw, our index of airway size, was unchanged after a DI.     

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.     

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.     

Assessment of heterogeneous airway constriction in dogs: a structure-function analysis

Obstructive lung diseases are often characterized by heterogeneous patterns of bronchoconstriction, although specific relationships between structural heterogeneity and lung function have yet to be established. We measured respiratory input impedance (Zrs) in eight anesthetized dogs using broadband forced oscillations at baseline and during intravenous methacholine (MCh) infusion. We also obtained high-resolution computed tomographic (HRCT) scans in 4 dogs and identified 20-30 individual airway segments in each animal. The Zrs spectra and HRCT images were obtained before and 5 min following a deep inspiration (DI) to 35 cmH(2)O. Each Zrs spectrum was fitted with two different models of the respiratory system: 1) a lumped airways model consisting of a single airway compartment, and 2) a distributed airways model incorporating a continuous distribution of airway resistances. For the latter, we found that the mean level and spread of airway resistances increased with MCh dose. Whereas a DI had no effect on average airway resistance during MCh infusion, it did increase the level of airway heterogeneity. At baseline and low-to-moderate doses of MCh, the lumped airways model was statistically more appropriate to describe Zrs in the majority of dogs. At the highest doses of MCh, the distributed airways model provided a superior fit in half of the dogs. There was a significant correlation between heterogeneity assessed with inverse modeling and the standard deviation of airway diameters obtained from HRCT. These data demonstrate that increases in airway heterogeneity as assessed with forced oscillations and inverse modeling can be linked to specific structural alterations in airway diameters.     

Maintaining nitric oxide-induced airway relaxation with superoxide dismutase

BACKGROUND: We have previously shown that the protective effect of inhaled nitric oxide (iNO) against methacholine-induced bronchoconstriction is negated in airways subjected to hyperosmotic stress. In this study, hypothesizing that the impaired efficiency of iNO was caused by release of reactive oxygen radicals, we examined the effect of the radical scavenging enzyme superoxide dismutase (SOD). METHODS: Hemodynamic and respiratory measurements were performed on anesthetized rabbits after (1) inhalation of methacholine (MCh), (2) iNO (80ppm), followed by MCh, (3) inhalation of hypertonic saline (HS), followed by iNO and MCh and (4) pre-treatment with inhalation of SOD, followed by HS, iNO and MCh. We analyzed plasma for a marker of oxidative stress, 8-iso-prostaglandin (PG)F(2alpha) and for a marker of activation of COX-mediated inflammatory cascades, PGF(2alpha) metabolite. RESULTS: Pre-treatment with SOD restored the bronchoprotective response to iNO in hyperosmotic airways. No direct effect was seen by SOD treatment on levels of 8-iso-PGF(2alpha), but this marker of oxidative stress correlated positively with increased bronchoconstriction. Hyperosmotic challenge elevated levels of PGF(2alpha) metabolite, and pre-treatment with SOD protected against this activation of the inflammatory cascade. CONCLUSION: SOD pre-treatment restores the relaxant effects of iNO in hyperosmotically challenged airways by attenuating oxidative stress and activation of COX-mediated inflammatory cascades.     

Tidal midexpiratory flow as a measure of airway hyperresponsiveness in allergic mice

A method for the noninvasive measurement of airway responsiveness was validated in allergic BALB/c mice. With head-out body plethysmography and the decrease in tidal midexpiratory flow (EF(50)) as an indicator of airway obstruction, responses to inhaled methacholine (MCh) and the allergen ovalbumin were measured in conscious mice. Allergen-sensitized and -challenged mice developed airway hyperresponsiveness as measured by EF(50) to aerosolized MCh compared with that in control animals. This response was associated with increased allergen-specific IgE and IgG1 production, increased levels of interleukin-4 and interleukin-5 in bronchoalveolar lavage fluid and eosinophilic lung inflammation. Ovalbumin aerosol challenge elicited no acute bronchoconstriction but resulted in a significant decline in EF(50) baseline values 24 h after challenge in allergic mice. The decline in EF(50) to MCh challenge correlated closely with simultaneous decreases in pulmonary conductance and dynamic compliance. The decrease in EF(50) was partly inhibited by pretreatment with the inhaled beta(2)-agonist salbutamol. We conclude that measurement of EF(50) to inhaled bronchoconstrictors by head-out body plethysmography is a valid measure of airway hyperresponsiveness in mice.     

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)     

Tachyphylaxis to inhaled histamine in asthmatic subjects

The bronchoconstriction induced by repeated histamine inhalation tests was studied in eight mild stable asthmatic subjects to determine whether histamine tachyphylaxis occurs in asthmatics. We also studied the specificity of histamine tachyphylaxis by examining for tachyphylaxis in response to inhaled acetylcholine in these subjects. We subsequently investigated whether indomethacin pretreatment inhibited histamine tachyphylaxis. Tachyphylaxis in response to inhaled histamine occurred in all subjects. The mean histamine provocative concentration causing a 20% fall in the forced expiratory volume in 1 s (PC20) increased from 3.04 +/- 1.9 (%SD), to 4.88 +/- 1.9, and to 6.53 +/- 2.2 mg/ml (P less than 0.0005) with successive inhalation tests. Tachyphylaxis was still present at 3 h (P less than 0.01), but not in all subjects at 6 h (P greater than 0.05). Tachyphylaxis, however, did not occur in response to inhaled acetylcholine. In addition, indomethacin pretreatment prevented histamine tachyphylaxis. Thus this study demonstrates that there is a histamine-specific mechanism that can partially protect the airways against repeated bronchoconstriction caused by histamine. This effect may occur through the release of inhibitory prostaglandins in the airway after histamine stimulation. Also when histamine inhalation tests are repeated on the same day, the tests should be separated by greater than 6 h to avoid tachyphylaxis.     

Effects of lung volume on maximal methacholine-induced bronchoconstriction in normal humans

We examined the effects of lung volume on the bronchoconstriction induced by inhaled aerosolized methacholine (MCh) in seven normal subjects. We constructed dose-response curves to MCh, using measurements of inspiratory pulmonary resistance (RL) during tidal breathing at functional residual capacity (FRC) and after a change in end-expiratory lung volume (EEV) to either FRC -0.5 liter (n = 5) or FRC +0.5 liter (n = 2). Aerosols of MCh were generated using a nebulizer with an output of 0.12 ml/min and administered for 2 min in progressively doubling concentrations from 1 to 256 mg/ml. After MCh, RL rose from a base-line value of 2.1 +/- 0.3 cmH2O. 1-1 X s (mean +/- SE; n = 7) to a maximum of 13.9 +/- 1.8. In five of the seven subjects a plateau response to MCh was obtained at FRC. There was no correlation between the concentration of MCh required to double RL and the maximum value of RL. The dose-response relationship to MCh was markedly altered by changing lung volume. The bronchoconstrictor response was enhanced at FRC - 0.5 liter; RL reached a maximum of 39.0 +/- 4.0 cmH2O X 1-1 X s. Conversely, at FRC + 0.5 liter the maximum value of RL was reduced in both subjects from 8.2 and 16.6 to 6.0 and 7.7 cmH2O X 1-1 X s, respectively. We conclude that lung volume is a major determinant of the bronchoconstrictor response to MCh in normal subjects. We suggest that changes in lung volume act to alter the forces of interdependence between airways and parenchyma that oppose airway smooth muscle contraction.     

Airway responsiveness to methacholine: effects of deep inhalations and airway inflammation.

We determined the dose-response curves to inhaled methacholine (MCh) in 16 asthmatic and 8 healthy subjects with prohibition of deep inhalations (DIs) and with 5 DIs taken after each MCh dose. Flow was measured on partial expiratory flow-volume curves at an absolute lung volume (plethysmographically determined) equal to 25% of control forced vital capacity (FVC). Airway inflammation was assessed in asthmatic subjects by analysis of induced sputum. Even when DIs were prohibited, the dose of MCh causing a 50% decrease in forced partial flow at 25% of control FVC (PD(50)MCh) was lower in asthmatic than in healthy subjects (P < 0.0001). In healthy but not in asthmatic subjects, repeated DIs significantly decreased the maximum response to MCh [from 90 +/- 4 to 62 +/- 8 (SD) % of control, P < 0.001], increased PD(50)MCh (P < 0.005), without affecting the dose causing 50% of maximal response. In asthmatic subjects, neither PD(50)MCh when DIs were prohibited nor changes in PD(50)MCh induced by DIs were significantly correlated with inflammatory cell numbers or percentages in sputum. We conclude that 1) even when DIs are prohibited, the responsiveness to MCh is greater in asthmatic than in healthy subjects; 2) repeated DIs reduce airway responsiveness in healthy but not in asthmatic subjects; and 3) neither airway hyperresponsiveness nor the inability of DIs to relax constricted airways in asthmatic subjects is related to the presence of inflammatory cells in the airways.     

Effect of deep inspiration avoidance on ventilation heterogeneity and airway responsiveness in healthy adults

The mechanisms by which deep inspiration (DI) avoidance increases airway responsiveness in healthy subjects are not known. DI avoidance does not alter respiratory mechanics directly; however, computational modeling has predicted that DI avoidance would increase baseline ventilation heterogeneity. The aim was to determine if DI avoidance increased baseline ventilation heterogeneity and whether this correlated with the increase in airway responsiveness. Twelve healthy subjects had ventilation heterogeneity measured by multiple-breath nitrogen washout (MBNW) before and after 20 min of DI avoidance. This was followed by another 20-min period of DI avoidance before the inhalation of a single methacholine dose. The protocol was repeated on a separate day with the addition of five DIs at the end of each of the two periods of DI avoidance. Baseline ventilation heterogeneity in convection-dependent and diffusion-convection-dependent airways was calculated from MBNW. The response to methacholine was measured by the percent fall in forced expiratory volume in 1 s/forced vital capacity (FVC) (airway narrowing) and percent fall in FVC (airway closure). DI avoidance increased baseline diffusion-convection-dependent airways (P = 0.02) but did not affect convection-dependent airways (P = 0.9). DI avoidance increased both airway closure (P = 0.002) and airway narrowing (P = 0.02) during bronchial challenge. The increase in diffusion-convection-dependent airways due to DI avoidance did not correlate with the increase in either airway narrowing (r(s) = 0.14) or airway closure (r(s) = 0.12). These findings suggest that DI avoidance increases diffusion-convection-dependent ventilation heterogeneity that is not associated with the increase in airway responsiveness. We speculate that DI avoidance reduces surfactant release, which increases peripheral ventilation heterogeneity and also predisposes to peripheral airway closure.     

Effects of deep inhalation in asthma: relative airway and parenchymal hysteresis

Asthmatic subjects were screened for the effects or volume history on the degree of induced airway obstruction with methacholine by comparing isovolumic maximal expiratory flows (Vmax) from partial expiratory flow-volume curves (P) begun near functional residual capacity (FRC) followed by maximal expiratory flow-volume (M) maneuvers begun from total lung capacity (TLC). The isovolumic Vmax values from M and P maneuvers defined two groups: one had a high M/P ratio (high group), indicating a large degree of reversal with deep inhalation, another had a low M/P ratio (low group), indicating minimal reversal. No differences were found between groups. A more complete study was later performed in which we measured specific airway conductance (sGaw) and anatomical dead space (VD) as indices of airway size and hysteresis before and after deep inhalation. The area of quasi-static transpulmonary pressure (Ptp) volume (V) curves from FRC to TLC and back to FRC was measured as an index of parenchymal hysteresis. At base line both groups showed a decrease in both sGaw and VD after a deep inhalation (DI). After constriction neither group changed VD after DI, whereas sGaw increased significantly in the high group after DI. This suggests that dilation of airways with DI occurred peripheral to those contributing to VD in the high group. The areas of the Ptp-V curves were equal at base line; yet the increase in areas with constriction in the low group was much greater.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of airway anesthesia on ventilatory responses to graded dead spaces and CO2

Ventilatory response to graded external dead space (0.5, 1.0, 2.0, and 2.5 liters) with hyperoxia and CO2 steady-state inhalation (3, 5, 7, and 8% CO2 in O2) was studied before and after 4% lidocaine aerosol inhalation in nine healthy males. The mean ventilatory response (delta VE/delta PETCO2, where VE is minute ventilation and PETCO2 is end-tidal PCO2) to graded dead space before airway anesthesia was 10.2 +/- 4.6 (SD) l.min-1.Torr-1, which was significantly greater than the steady-state CO2 response (1.4 +/- 0.6 l.min-1.Torr-1, P less than 0.001). Dead-space loading produced greater oscillation in airway PCO2 than did CO2 gas loading. After airway anesthesia, ventilatory response to graded dead space decreased significantly, to 2.1 +/- 0.6 l.min-1.Torr-1 (P less than 0.01) but was still greater than that to CO2. The response to CO2 did not significantly differ (1.3 +/- 0.5 l.min-1.Torr-1). Tidal volume, mean inspiratory flow, respiratory frequency, inspiratory time, and expiratory time during dead-space breathing were also depressed after airway anesthesia, particularly during large dead-space loading. On the other hand, during CO2 inhalation, these respiratory variables did not significantly differ before and after airway anesthesia. These results suggest that in conscious humans vagal airway receptors play a role in the ventilatory response to graded dead space and control of the breathing pattern during dead-space loading by detecting the oscillation in airway PCO2. These receptors do not appear to contribute to the ventilatory response to inhaled CO2.     

Effect of transpulmonary pressure on airway diameter and responsiveness of immature and mature rabbits.

We previously demonstrated that airway responsiveness is greater in immature than in mature rabbits; however, it is not known whether there are maturational differences in the effect of transpulmonary pressure (Ptp) on airway size and airway responsiveness. The relationship between Ptp and airway diameter was assessed in excised lungs insufflated with tantalum powder. Diameters of comparable intraparenchymal airway segments were measured from radiographs obtained at Ptp between 0 and 20 cmH(2)O. At Ptp > 8 cmH(2)O, the diameters were near maximal in both groups. With diameter normalized to its maximal value, changing Ptp between 8 and 0 cmH(2)O resulted in a greater decline of airway caliber in immature than mature airways. The increases in lung resistance (RL) in vivo at Ptp of 8, 5, and 2 cmH(2)O were measured during challenge with intravenous methacholine (MCh: 0.001-0.5 mg/kg). At Ptp of 8 cmH(2)O, both groups had very small responses to MCh and the maximal fold increases in RL did not differ (1.93 +/- 0.29 vs. 2.23 +/- 0.19). At Ptp of 5 and 2 cmH(2)O, the fold increases in RL were greater for immature than mature animals (13.19 +/- 1.81 vs. 3.89 +/- 0.37) and (17.74 +/- 2.15 vs. 4.6 +/- 0.52), respectively. We conclude that immature rabbits have greater airway distensibility and this difference may contribute to greater airway narrowing in immature compared with mature rabbits.     

CXCR2 is essential for maximal neutrophil recruitment and methacholine responsiveness after ozone exposure

Ozone (O(3)), a common air pollutant, induces airway inflammation and airway hyperresponsiveness. In mice, the neutrophil chemokines KC and macrophage inflammatory protein-2 (MIP-2) are expressed in the lungs following O(3) exposure. The purpose of this study was to determine whether CXCR2, the receptor for these chemokines, is essential to O(3)-induced neutrophil recruitment, injury to lungs, and increases in respiratory system responsiveness to methacholine (MCh). O(3) exposure (1 ppm for 3 h) increased the number of neutrophils in the bronchoalveolar lavage fluid (BALF) of wild-type (BALB/c) and CXCR2-deficient mice. However, CXCR2-deficient mice had significantly fewer emigrated neutrophils than did wild-type mice. The numbers of neutrophils in the blood and concentrations of BALF KC and MIP-2 did not differ between genotypes. Together, these data suggest CXCR2 is essential for maximal chemokine-directed migration of neutrophils to the air spaces. In wild-type mice, O(3) exposure increased BALF epithelial cell numbers and total protein levels, two indirect measures of lung injury. In contrast, in CXCR2-deficient mice, the number of BALF epithelial cells was not increased by O(3) exposure. Responses to inhaled MCh were measured by whole body plethysmography using enhanced pause as the outcome indicator. O(3) exposure increased responses to inhaled MCh in both wild-type and CXCR2-deficient mice 3 h after O(3) exposure. However, at 24 h after exposure, responses to inhaled MCh were elevated in wild-type but not CXCR2-deficient mice. These results indicate CXCR2 is essential for maximal neutrophil recruitment, epithelial cell sloughing, and persistent increases in MCh responsiveness after an acute O(3) exposure.