Methacholine causes reflex bronchoconstriction

To determine whether methacholine causes vagally mediated reflexconstriction of airway smooth muscle, we administered methacholine tosheep either via the bronchial artery or as an aerosol via tracheostomyinto the lower airways. We then measured the contraction of anisolated, in situ segment of trachealis smooth muscle and determinedthe effect of vagotomy on the trachealis response. Administeringmethacholine to the subcarinal airways via the bronchial artery(0.5-10.0 µg/ml) caused dose-dependent bronchoconstriction andcontraction of the tracheal segment. At the highest methacholine concentration delivered, trachealis smooth muscle tension increased anaverage of 186% over baseline. Aerosolized methacholine (5-7 breaths of 100 mg/ml) increased trachealis tension by 58% and airwaysresistance by 183%. As the bronchial circulation in the sheep does notsupply the trachea, we postulated that the trachealis contraction wascaused by a reflex response to methacholine in the lower airways.Bilateral vagotomy essentially eliminated the trachealis response andthe airways resistance change after lower airways challenge (either viathe bronchial artery or via aerosol) with methacholine. We concludethat 1) methacholine causes asubstantial reflex contraction of airway smooth muscle and2) the assumption may not be validthat a response to methacholine in humans or experimental animalsrepresents solely the direct effect on smooth muscle.

     

Intrinsic and antigen-induced airway hyperresponsiveness are the result of diverse physiological mechanisms.

Airway hyperresponsiveness (AHR) is a defining feature of asthma. We have previously shown, in mice sensitized and challenged with antigen, that AHR is attributable to normal airway smooth muscle contraction with exaggerated airway closure. In the present study we sought to determine if the same was true for mice known to have intrinsic AHR, the genetic strain of mice, A/J. We found that A/J mice have AHR characterized by minimal increase in elastance following aerosolized methacholine challenge compared with mice (BALB/c) that have been antigen sensitized and challenged [concentration that evokes 50% change in elastance (PC(50)): 22.9 +/- 5.7 mg/ml for A/J vs. 3.3 +/- 0.4 mg/ml for antigen-challenged and -sensitized mice; P < 0.004]. Similar results were found when intravenous methacholine was used (PC(30) 0.22 +/- 0.08 mg/ml for A/J vs. 0.03 +/- 0.004 mg/ml for antigen-challenged and -sensitized mice). Computational model analysis revealed that the AHR in A/J mice is dominated by exaggerated airway smooth muscle contraction and that when the route of methacholine administration was changed to intravenous, central airway constriction dominates. Absorption atelectasis was used to provide evidence of the lack of airway closure in A/J mice. Bronchoconstriction during ventilation with 100% oxygen resulted in a mean 9.8% loss of visible lung area in A/J mice compared with 28% in antigen-sensitized and -challenged mice (P < 0.02). We conclude that the physiology of AHR depends on the mouse model used and the route of bronchial agonist administration.     

Inhalation of adenosine 5'-monophosphate increases methacholine airway responsiveness

Airway hyperresponsiveness is a characteristic feature in asthmatic subjects, but the mechanism of the hyperresponsiveness is not known. The purpose of this study was to investigate whether methacholine airway responsiveness was increased 24 h after inhalation of adenosine 5'-monophosphate (AMP). Ten atopic asthmatic subjects and six atopic normal subjects were studied on 4 study days. On the 1st day, a methacholine inhalation test was performed, followed within 48 h by an AMP inhalation test. Seven days later the second AMP test was performed, and 24 h later the methacholine inhalation test was repeated. Response was measured using partial flow-volume curves, and the concentration required to cause a 40% fall in the partial flow-volume curve (PC40) was calculated. The geometric mean methacholine PC40 fell from 1.36 mg/ml on day 1 (before AMP inhalation) to 0.71 mg/ml on day 4 (24 h after AMP inhalation, P less than 0.01). There was no change in the mean PC40 for adenosine on the 2 study days (5.82 and 7.06 mg/ml, P greater than 0.1). These findings suggest that adenosine release may contribute to the increase in airway responsiveness after allergen challenge.     

Airway effects of purine nucleosides and nucleotides and release with bronchial provocation in asthma

Adenosine, AMP, and ADP all caused similar concentration-related bronchoconstriction when inhaled by patients with asthma, whereas the adenosine hydrolysis product inosine had no effect. Geometric mean provocation concentrations of adenosine AMP and ADP causing a 20% fall in forced expiratory volume in 1 s (PCf20) were 2.34, 4.27, and 2.19 mumol/ml and 40% fall in specific airway conductance (PCs40) 3.16, 5.01, and 2.0 mumol/ml. Bronchoconstriction was rapid in onset, reaching a maximum 2-5 min after a single inhalation of AMP. In 31 asthmatic subjects a positive correlation was established between airway responsiveness to histamine, as an index of non-specific responsiveness, and airway reactivity to adenosine (PCf20, r = 0.60; PCs40, r = 0.64; P less than 0.01). Following bronchial provocation with allergen in nine subjects, plasma levels of adenosine increased from a mean base line of 5.4 +/- 0.9 to 9.6 +/- 2.0 ng/ml at 15 min (P less than 0.01) in parallel with a fall in forced expiratory volume in 1 s. With methacholine provocation bronchoconstriction reached maximum 2-5 min postchallenge being followed by, but not accompanied by, significant increases in plasma levels of adenosine. These data suggest that adenosine is a specific bronchoconstrictor that may contribute to airflow obstruction in asthma.     

Inhaled PAF fails to induce airway hyperresponsiveness to methacholine in normal human subjects

The effects of three increasing doses of platelet-activating factor (PAF) on airway caliber and methacholine bronchial responsiveness were studied. On separate occasions nine normal subjects inhaled a single cumulative provocation concentration of methacholine (control) causing a 40% fall (PC40 Vp30) in maximum expiratory flow rate at 70% of base-line vital capacity below total lung capacity during a partial forced expiratory maneuver or 100 or 200 micrograms PAF, and seven subjects inhaled a further dose of 400 micrograms PAF. Methacholine responsiveness was measured before, at 3 and 7 h, then on days 1, 2, 3, 4, 7, 10, and 14 after each challenge. The maximum falls in Vp30 appeared dose dependent, but a significant difference between the magnitude of the responses was only observed between the 400- and 100-micrograms PAF dose (P less than 0.05). During the control period repeated methacholine challenges resulted in a progressive increase in cumulative provocation concentration of an agonist causing a 20% fall in forced expiratory volume in 1 s from base line, reaching significance on days 1 and 2 (2.44- and 2.4-fold of base line, respectively, P less than 0.01) before returning to base line on day 7. No difference was seen in methacholine responsiveness after any of the three doses of PAF compared with that after the control. We conclude that PAF causes dose-dependent bronchoconstriction but does not change airways responsiveness to methacholine and that repeated high-dose methacholine challenge leads to loss of responsiveness to this agonist.     

Ozone-induced changes in muscarinic bronchial reactivity by different testing methods

We examined the effect of ozone (O3) on muscarinic bronchial reactivity in the guinea pig and compared reactivity determined by two different routes of agonist delivery. Reactivity before and from 4 h to 2 days after O3 exposure (3.0 ppm, 2 h) was determined by measuring specific airway resistance upon administration of intravenous acetylcholine and/or aerosolized methacholine challenge in 34 unanesthetized, spontaneously breathing animals. Before exposure, we observed more gradual and reproducible results to intravenous agonist. After exposure, hyperreactivity to parenteral agonist occurred consistently, but not to inhaled agonist. Hyperreactivity demonstrable by either route was similar in magnitude and time course within 14 h of exposure. Two days later, hyperreactivity to inhaled agonist had remitted; that to intravenous drug persisted. Our results indicate that variability in the occurrence and time course of O3-induced hyperreactivity to inhaled agonist may be a consequence of the technique employed. The consistent occurrence of hyperreactivity after O3 to parenteral agonist suggests mechanisms other than airway mucosal hyperpermeability are responsible for this hyperreactivity.     

Airway responses to methacholine in asymptomatic nonatopic cigarette smokers

We prospectively performed methacholine bronchoprovocation challenges on 46 young smokers to examine the effects of cigarette smoking on airway responsiveness. The subjects, ages 18-35 yr, had no past or present history or physical examination findings of asthma or other lung diseases, rhinitis, allergic diseases, or respiratory infections; were skin test negative to 29 common aeroallergens; and had base-line pulmonary function values greater than 80% predicted. Sixteen of 46 (35%) subjects had a 20% or greater drop in forced expiratory volume in 1 s at a provocative methacholine concentration less than or equal to 25 mg/ml. The degree of methacholine responsiveness was not dependent upon base-line pulmonary function values or the amount of cigarettes consumed, and there was no association between the amount of cigarettes consumed and base-line pulmonary function values. These data suggest that many young asymptomatic nonatopic smokers have increased airway responsiveness to inhaled methacholine without clinically significant hyperreactive airway disease.     

Protease-activated receptor 2 mediates eosinophil infiltration and hyperreactivity in allergic inflammation of the airway

Trypsin and mast cell tryptase can signal to epithelial cells, myocytes, and nerve fibers of the respiratory tract by cleaving proteinase-activated receptor 2 (PAR2). Since tryptase inhibitors are under development to treat asthma, a precise understanding of the contribution of PAR2 to airway inflammation is required. We examined the role of PAR2 in allergic inflammation of the airway by comparing OVA-sensitized and -challenged mice lacking or overexpressing PAR2. In wild-type mice, immunoreactive PAR2 was detected in airway epithelial cells and myocytes, and intranasal administration of a PAR2 agonist stimulated macrophage infiltration into bronchoalveolar lavage fluid. OVA challenge of immunized wild-type mice stimulated infiltration of leukocytes into bronchoalveolar lavage and induced airway hyperreactivity to inhaled methacholine. Compared with wild-type animals, eosinophil infiltration was inhibited by 73% in mice lacking PAR2 and increased by 88% in mice overexpressing PAR2. Similarly, compared with wild-type animals, airway hyperreactivity to inhaled methacholine (40 micro g/ml) was diminished 38% in mice lacking PAR2 and increased by 52% in mice overexpressing PAR2. PAR2 deletion also reduced IgE levels to OVA sensitization by 4-fold compared with those of wild-type animals. Thus, PAR2 contributes to the development of immunity and to allergic inflammation of the airway. Our results support the proposal that tryptase inhibitors and PAR2 antagonists may be useful therapies for inflammatory airway disease.     

Volume history and effect on airway reactivity in infants and adults.

Volume history is an important determinant of airway responsiveness. In healthy adults undergoing airway challenge, deep inspiration (DI) provides bronchodilating and bronchoprotective effects; however, the effectiveness of DI is limited in asthmatic adults. We hypothesized that, when assessed under similar conditions, healthy infants have heightened airway reactivity compared with healthy adults and that the effectiveness of DI is limited in infants. We compared the effect of DI on reactivity by using full (DI) vs. partial (no DI) forced-expiratory maneuvers on 2 days in supine, healthy nonasthmatic infants (21) and adults (10). Reactivity was assessed by methacholine doses that decreased forced expiratory flow after exhalation of 75% forced vital capacity during a full maneuver and maximal expiratory flow at functional residual capacity during a partial maneuver by 30% from baseline. Reactivity in adults increased when DI was absent, whereas infants' reactivity was unchanged. Infants were more reactive than adults in the presence of DI; however, adult and infant reactivity was similar in its absence. Our findings indicate that healthy infants are more reactive than adults and, like asthmatic adults, do not benefit from DI; this difference may be an important characteristic of airway hyperreactivity.     

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)     

The effects of inhaled house dust mite on airway barrier function and sensitivity to inhaled methacholine in mice

Asthma is functionally characterized by increased airway sensitivity and reactivity. Multiple mechanisms are believed to underlie these functional disorders, including impairment of airway wall barrier function. One proposed mechanism of impaired barrier function is through the direct consequence of proteolytic properties of inhaled allergens, including house dust mite (HDM). Here, we have observed the direct effects of HDM on airway barrier function and response to nebulized or intravenous methacholine. HDM na?ve BALB/c mice were anesthetized, exposed to intranasal or intratracheal HDM (15 or 100 μg), and allowed to recover for 30 min or 2 h before methacholine challenge. A separate group of mice was exposed to intratracheal poly-L-lysine (PLL; 100 μg) for a duration of 30 min. This group served as a positive control for the presence of impaired barrier function and airway hypersensitivity. Negative control mice received saline challenges. Outcomes included assessment of lung mechanics in response to nebulized or intravenous methacholine as well as clearance of intratracheally instilled technetium-labeled ((99m)Tc) DTPA to evaluate airway epithelial barrier function. We found that PLL produced a leftward shift in the dose-response curve following nebulized but not intravenous methacholine challenge. This was associated with a significantly faster clearance of (99m)Tc-DTPA, indicating impairment in airway barrier function. However, HDM exposure did not produce changes in these outcomes when compared with saline-exposed mice. These findings suggest that direct impact on airway barrier function does not appear to be a mechanism by which HDM produces altered airway sensitivity in airway disease.     

Marked airway eosinophilia prevents development of airway hyper-responsiveness during an allergic response in IL-5 transgenic mice

Tissue eosinophilia probably plays important roles in the pathophysiology of bronchial asthma and allergic disorders; however, this concept was challenged recently by controversial results in mouse models of bronchial asthma treated with anti-IL-5 Ab and the failure of anti-IL-5 therapy in humans. We have now used a unique model, IL-5 transgenic (TG) mice, to address a fundamental question: is airway eosinophilia beneficial or detrimental in the allergic response? After sensitization and challenge with OVA, IL-5 TG mice showed a marked airway eosinophilia. Surprisingly, these IL-5 TG mice showed lower airway reactivity to methacholine. Immunohistochemical analysis of the lungs revealed a marked peribronchial infiltration of eosinophils, but no eosinophil degranulation. In vitro, mouse eosinophils from peritoneal lavage fluids did not produce superoxide anion, but did produce an anti-inflammatory and fibrotic cytokine, TGF-beta 1. Indeed, the TGF-beta 1 levels in bronchoalveolar lavage fluid specimens from IL-5 TG mice directly correlated with airway eosinophilia (r = 0.755). Furthermore, anti-IL-5 treatment of IL-5 TG mice decreased both airway eosinophilia and TGF-beta 1 levels in bronchoalveolar lavage fluids and increased airway reactivity. Thus, in mice, marked eosinophilia prevents the development of airway hyper-reactivity during an allergic response. Overall, the roles of eosinophils in asthma and in animal models need to be addressed carefully for their potentially detrimental and beneficial effects.     

Methacholine-induced airway reactivity of inbred rats

Dose-response curves to inhaled aerosolized methacholine chloride (MCh) were obtained in anesthetized spontaneously breathing rats. Thirty rats (10/strain), randomly selected from highly inbred ACI, Lewis (L), and Brown Norway (BN) strains and 40 rats (20/strain) from similarly inbred Wistar-Furth (WF) and Buffalo (Buf) strains were studied. Airway responses were quantitated from changes in pulmonary resistance (RL) and airway reactivity was calculated as the dose of MCh required to increase RL to 150% (ED150RL) and 200% (ED200RL) of base line. There were no statistically significant differences in ED150RL and ED200RL among the five rat strains. Large interindividual variability was present as evidenced by 128-fold differences in ED150RL and ED200RL between the least and most sensitive animal of the same strain. In contrast, seven animals studied repeatedly on different days had values of ED150RL that differed by an average of only 2.9-fold (range 1.6-5.3). Thirteen rats that were studied on two occasions separated by an interval of 3 mo showed no systematic changes in airway reactivity. We conclude that airway reactivity to inhaled methacholine in anesthetized nose-breathing rats is not strain related, and despite animals of a given strain being genetically identical, the variability in airway reactivity within strains suggests that environmental rather than genetic factors are the major determinants of that reactivity.     

Inhaled PAF stimulates leukotriene and thromboxane A2 production in humans.

Platelet-activating factor (PAF) is a potent bronchoconstrictor in humans and has been implicated as an inflammatory mediator in asthma. This study was performed to evaluate whether PAF-induced bronchoconstriction in vivo could be mediated through the release of the bronchoconstrictor eicosanoids, thromboxane (Tx) A2 and the cysteinyl leukotrienes. Ten asthmatic subjects were studied on three occasions after bronchial challenges with aerosolized PAF, methacholine, or isotonic saline. PAF caused bronchoconstriction in all 10 subjects (mean maximal percent fall in specific airway conductance 48.2 +/- 4.6) and was matched by methacholine challenge. Saline caused no changes in specific airway conductance. Urinary leukotriene E4 was significantly elevated after inhaled PAF (366.0 +/- 66.9 ng/mmol creatinine, P less than 0.01) compared with methacholine (41.6 +/- 13.3) and saline (33.6 +/- 4.6). The major urinary TxA2 metabolite 2,3-dinor TxB2 was elevated after inhaled PAF (41.3 +/- 7.1 ng/mmol creatinine, P less than 0.01) compared with methacholine (14.0 +/- 2.7) and saline (17.1 +/- 3.9). Urinary 2,3-dinor 6-oxo-prostaglandin F1 alpha after PAF (22.2 +/- 1.4) was raised with respect to the methacholine challenge (13.9 +/- 1.8, P less than 0.02), although no significant increase was observed compared with the saline control (18.6 +/- 3.3). Inhaled PAF leads to the secondary generation of cysteinyl leukotrienes and TxA2, and it is possible that these mediate some of the acute effects of inhaled PAF in vivo.     

Relationships among airway reactivity, pupillary alpha-adrenergic and cholinergic responsiveness, and age

Healthy adult volunteers (n = 122), who denied personal history of lung disease or family history of cystic fibrosis or asthma, took no interfering medications, and had forced expiratory volume in 1 s greater than or equal to 80% predicted, underwent methacholine challenge and pupillary reactivity testing. Pupil diameter measured in dark and light test conditions declined with age (Pearson's r = -0.54 and -0.36). Pupillary alpha-adrenergic responsiveness (expressed as the concentration of phenylephrine required to dilate the pupil 1 mm) was significantly correlated with age. Older subjects required lower concentrations for dilation and therefore were more sensitive to phenylephrine. Pupillary cholinergic responsiveness (the concentration of carbachol required for 1-mm constriction) was not significantly correlated with age. Therefore the significantly smaller baseline pupil size in the elderly cannot be explained by failure of alpha-adrenergic receptor responses or by increased pupillary cholinergic responsiveness. We found no significant correlation of methacholine bronchial reactivity with age. In addition, there was no relation between airway reactivity and pupillary alpha-adrenergic or cholinergic responsiveness in this sample of healthy adults. These findings, taken with others in the literature, suggest that the contribution of alpha-adrenergic and cholinergic responsiveness to nonspecific airway reactivity in healthy persons is small, if it exists at all, and that there is no significant change in airway reactivity with age in healthy adults.     

Tachyphylaxis to inhaled aerosolized histamine in anesthetized dogs

Three consecutive dose-response curves to inhaled aerosolized histamine, separated by 1-h intervals, were obtained in 20 anesthetized mongrel dogs. In general, successive histamine dose-response curves shifted progressively rightward. Changes in pulmonary resistance (RL) and dynamic compliance (Cdyn) in response to low concentrations of histamine were reproducible, but responses to high concentrations (sufficient to at least double RL or decrease Cdyn by at least 30%) decreased on successive dose-response curves. The concentration of histamine required to double RL increased significantly (P less than 0.05) from 1.01 mg/ml on the first to 1.62 and 2.02 mg/ml on the second and third dose-response curves. In contrast, consecutive methacholine dose-response curves were not significantly different. Indomethacin pretreatment (5 mg/kg iv) prevented histamine tachyphylaxis, whereas atropine (4 mg iv) did not. However, indomethacin did not alter base-line pulmonary mechanics or histamine responsiveness as measured on the first dose-response curve. We conclude that tachyphylaxis to inhaled aerosolized histamine occurs in anesthetized dogs. Our results are consistent with an important role for endogenous prostaglandins in modulating the airway responses to repeated histamine exposures.     

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.     

Lack of adrenomedullin on antigen-induced bronchoconstriction in guinea pigs in vivo

Antigen challenge can provoke acute bronchoconstriction, recognized as immediate asthmatic response (IAR), but the evolving events in this reaction are not well defined. Recently, a novel peptide, designated adrenomedullin, was isolated from human pheochromocytoma, and has been shown to have potent systemic and pulmonary vasodilator activity.The purpose of this study was to elucidate the influence of adrenomedullin in the development of IAR. Passively sensitized guinea pigs were anesthetized and treated with diphenhydramine hydrochloride, and then artificially ventilated. Ovalbumin was inhaled after an intravenous administration of adrenomedullin. Other studies were performed in naive guinea pigs to investigate the airway responses to inhaled methacholine or histamine after an intravenous administration of adrenomedullin. Antigen challenge caused bronchoconstriction in sensitized guinea pigs. Adrenomedullin did not inhibit the antigen-induced bronchoconstriction in sensitized guinea pigs or the dose-dependent responses to inhaled methacholine or histamine in naive animals in spite of its vasodilating effect. We conclude that an intravenous administration of adrenomedullin does not influence antigen-induced bronchoconstriction or bronchial responsiveness to inhaled methacholine or histamine in vivo.     

In vivo PAF-induced airway eosinophil accumulation reduces bronchial responsiveness to inhaled histamine

Chronic eosinophilic bronchitis and bronchial hyperresponsiveness have been considered to be the fundamental features of bronchial asthma. However, the role of airway eosinophils in bronchial responsiveness in vivo has not been fully discussed. The aim of this study was to investigate the direct effect of airway eosinophil accumulation on bronchial responsiveness in vivo. Guinea pigs were transnasally treated with platelet activating factor (PAF) or vehicle twice a week for a total of 3 weeks. Anesthetized guinea pigs were surgically cannulated and artificially ventilated 48 h after the last administration of PAF or vehicle. Ten minutes after the installation of artificial ventilation, ascending doses of histamine were inhaled. In a subsequent study, selective inhibitors of diamine oxidase and histamine N-methyltransferase were intravenously administered before the histamine inhalation in the PAF-treated animals. Next study was conducted 20 min after treatment with indomethacin in this study line. Finally, ascending doses of methacholine were inhaled in our animal model. Proportion of eosinophils and the number of nuclear segmentation in bronchoalveolar lavage fluid significantly increased in guinea pigs treated with PAF compared with vehicle and this finding was confirmed histologically. Nevertheless, bronchial responsiveness to inhaled histamine, but not methacholine, was significantly decreased by the PAF treatment. This bronchoprotective effect induced by PAF remained following aminoguanidine and histamine N-methyltransferase administration, but abolished by treatment of indomethacin. These results suggest that in vivo airway eosinophils may reduce nonspecific bronchial responsiveness through production of inhibitory or bronchoprotective prostanoids, but not through histaminase production.     

Antigen sensitization and methacholine responses in dogs with hyperreactive airways

Antigen sensitization was induced in six Basenji-Greyhound (BG) dogs by weekly aerosol exposure to Ascaris suum. The effects on airway responsiveness to inhaled methacholine were studied before and at least 2 wk following Ascaris sensitization. All dogs developed detectable serum levels of Ascaris-specific immunoglobulin E (IgE), and five out of six dogs developed airway responsiveness to antigen over the 4- to 6-mo period. This was accompanied by a decrease rather than an increase in airway responsiveness to inhaled methacholine. When dogs were challenged with methacholine 30 min after Ascaris antigen aerosol challenge, however, dogs reactive to Ascaris became hyperresponsive to methacholine. The magnitude of the response to antigen correlated (r = 0.85) inversely with the dose of methacholine increasing pulmonary resistance 200%. These data show that in BG dogs airway responsiveness to methacholine is increased by acute antigen exposure but that sensitization of previously unsensitized dogs does not increase nonspecific airway responsiveness.