Adenosine-induced Bronchoconstriction Research Articles

A novel method for studying airway hyperresponsiveness in allergic guinea pigs in vivo using the PreciseInhale system for delivery of dry powder aerosols

Inhaled adenosine receptor agonists induce bronchoconstriction and inflammation in asthma and are used as bronchial challenge agents for the diagnosis of asthma and in respiratory drug development. Recently developed dry powder aerosols of adenosine have several advantages over nebulised adenosine 5′-monophosphate (AMP) as bronchial challenge agents. However, reverse translation of this bronchial challenge technique to pre-clinical drug development is limited by the difficulty of administering powder aerosols to animals. The aim of the current study was to develop methods for delivering powder aerosols of adenosine receptor agonists to sensitised guinea pigs (as a model of allergic asthma) and evaluate their effect as challenge agents for the measurement of airway responsiveness. The PreciseInhale system delivered micronised AMP and adenosine powders, with mass median aerodynamic diameters of 1.81 and 3.21 μm and deposition fractions of 31 and 48% in the lungs, respectively. Bronchoconstrictor responses in passively sensitised, anaesthetised, spontaneously breathing guinea pigs were compared to responses to nebulised and intravenously administered AMP and adenosine. AMP- and adenosine-induced bronchoconstriction following all routes of administration with the magnitude of response ranking intravenous > dry powder > nebulisation, probably reflecting differences in exposure to the adenosine agonists delivered by the different routes. In conclusion, the PreciseInhale system delivered AMP and adenosine dry powder aerosols accurately into the lungs, suggesting this method can be used to investigate drug effects on airway responsiveness.

The novel inhaled glucocorticoid receptor agonist GW870086X protects against adenosine-induced bronchoconstriction in asthma

The novel inhaled glucocorticoid receptor agonist GW870086X protects against adenosine-induced bronchoconstriction in asthma

Regadenoson use in patients with chronic obstructive pulmonary disease: the state of current knowledge

Stress testing is challenging in patients with chronic obstructive pulmonary disease (COPD). Functional capacity is generally decreased in this patient population, limiting patients’ ability to achieve physiologic stress through exercise. Additionally, due to emphysematous changes, COPD patients tend to have poor acoustic windows that impair the quality and therefore diagnostic accuracy of stress echocardiography techniques. Pharmacologic stress myocardial perfusion imaging (MPI) testing is also problematic, particularly due to the concern for adenosine-induced bronchoconstriction with conventional vasodilator stress agents. Regadenoson, a selective A2A adenosine receptor agonist, has gained popularity due to its ease of administration and improved patient experience in the general population. The literature describing the experience with regadenoson in COPD patients, though limited, is rapidly growing and reassuring. This review summarizes the pharmacology and clinical application of this novel stress agent and presents the available data on the safety and tolerability of its use in COPD patients.

Effects of adenosine A2A receptor low affinity agonist Regadenoson in patients with obstructive pulmonary disease, at risk for adenosine-induced bronchoconstriction mediated via A2B and/or A3 adenosine receptors.

The pharmaceutical Regadenoson (Lexiscan TM ) Produced by Astellas Inc. Is currently in use as a pharmacological stress agent for myocardial perfusion imaging within the field of Nuclear Cardiology. Regadenoson is a selective A 2A adenosine receptor agonist, and also has a low affinity for adenosine receptors which cause bronchoconstriction in patients with reactive airways mediated via A 2B and A3 adenosine receptors. In a study of 42 patients with mild to moderate asthma and/or moderate to severe COPD who had a positive adenosine monophosphate test, Regadenoson proved to be relatively safe and well tolerated by most patients (90%). 8 Regadenoson is a low affinity agonist (Ki H 1.3 µM) for the A2A adenosine receptor, with at least 10-fold lower affinity for the A1 adenosine receptor (Ki > 16.5 µM), and weak, if any, affinity for the A2B and A3 adenosine receptors. 2,9 Activation of the A2A adenosine receptor by regadenoson produces coronary vasodilation 8 and increases coronary blood flow (CBF). It

Safety of regadenoson, a selective adenosine A 2A agonist, in patients with chronic obstructive pulmonary disease: A randomized, double-blind, placebo-controlled trial (RegCOPD trial)

Patients with reactive airways are at risk for adenosine-induced bronchoconstriction, mediated via A(2B) and/or A(3) adenosine receptors. In this randomized, double-blind, placebo-controlled crossover trial, we examined the safety of regadenoson, a selective adenosine A(2A) receptor agonist, in patients with moderate chronic obstructive pulmonary disease (COPD) (n = 38) and patients with severe COPD (n = 11) with a baseline mean forced expiratory volume in 1 second (FEV(1)) of 1.74 +/- 0.50 L and 1.0 +/- 0.35 L, respectively, 37% of whom had dyspnea during activities of daily living. Patients receiving glucocorticoids or oxygen and those with pretreatment wheezing were included. Short-acting bronchodilators were withheld for at least 8 hours before treatment. No differences emerged between regadenoson and placebo on multiple lung function parameters, including repeated FEV(1) and forced vital capacity, respiratory rate, pulmonary examinations, and oxygen saturation. The mean maximum decline in FEV(1) was 0.11 +/- 0.02 L and 0.12 +/- 0.02 L (P = .55) in patients after regadenoson and placebo, respectively, and new-onset wheezing was observed in 6% and 12%, respectively (P = .33). No patient required acute treatment with bronchodilators or oxygen. This pilot study showed the overall safety of regadenoson in 49 compromised outpatients with clinically stable moderate and severe chronic obstructive pulmonary disease.

Safety of regadenoson, an adenosine A 2A receptor agonist for myocardial perfusion imaging, in mild asthma and moderate asthma patients: A randomized, double-blind, placebo-controlled trial

Patients with reactive airways are at risk for adenosine-induced bronchoconstriction, mediated via A(2B) and/or A(3) adenosine receptors. To examine the effects of regadenoson, a selective adenosine A(2A) receptor agonist, on airway resistance, we conducted a randomized, double-blind, placebo-controlled crossover trial in asthmatic patients with a positive adenosine monophosphate challenge test. The mean ratio of the forced expiratory volume in 1 second (FEV(1)) at each tested time point relative to the baseline FEV(1) was significantly higher after treatment with regadenoson compared with placebo from 10 to 60 minutes after treatment. One patient had a substantial but asymptomatic FEV(1) reduction (-36.2%) after regadenoson that reversed spontaneously. The most common adverse events with regadenoson were tachycardia (66%), dizziness (53%), headache (45%), and dyspnea (34%). The mean heart rate significantly increased with regadenoson (maximum of +10.4 beats/min) versus placebo. In this pilot safety study of 48 patients with mild or moderate asthma who had bronchial reactivity to adenosine monophosphate, regadenoson was safe and well tolerated.

A role for neural pathways in adenosine-induced bronchoconstriction

adenosine is a widely distributed purine nucleoside that serves many roles in homeostasis and the cellar response to stress. As an extracellular signaling molecule, adenosine engages P1 purinergic receptors, known as the adenosine receptors, to regulate a vast array of cellular responses ([9][1], [

Involvement of A1adenosine receptors and neural pathways in adenosine-induced bronchoconstriction in mice

High levels of adenosine can be measured from the lungs of asthmatics, and it is well recognized that aerosolized 5'AMP, the precursor of adenosine, elicits robust bronchoconstriction in patients with this disease. Characterization of mice with elevated adenosine levels secondary to the loss of adenosine deaminase (ADA) expression, the primary metabolic enzyme for adenosine, further support a role for this ubiquitous mediator in the pathogenesis of asthma. To begin to identify pathways by which adenosine can alter airway tone, we examined adenosine-induced bronchoconstriction in four mouse lines, each lacking one of the receptors for this nucleoside. We show, using direct measures of airway mechanics, that adenosine can increase airway resistance and that this increase in resistance is mediated by binding the A(1) receptor. Further examination of this response using pharmacologically, surgically, and genetically manipulated mice supports a model in which adenosine-induced bronchoconstriction occurs indirectly through the activation of sensory neurons.

Role of Adenosine Receptors in the Treatment of Asthma and Chronic Obstructive Pulmonary Disease

Adenosine is a naturally occurring purine nucleoside with a ubiquitous presence in human tissue, where it plays a key role in many biological processes such as energy generation and protein metabolism. It has been shown that adenosine induces bronchoconstriction in asthmatic and chronic obstructive pulmonary disease (COPD) patients, but not in normal airways. Four different G-protein-coupled adenosine receptors have been described, namely adenosine A(1), A(2A), A(2B) and A(3) receptors. The main mechanism of adenosine-induced bronchoconstriction appears to involve the release of inflammatory mediators from mast cells via activation of the A(2B) receptor. However, adenosine can also act on A(1), A(2A) and A(3) receptors. In recent years there has been an increasing interest in the role of adenosine receptors in asthma and COPD, since it is now clear that they play an important role in the pathophysiology of asthma and COPD. Adenosine receptors are involved in the production and release of a variety of mediators from inflammatory and structural cells. A therapeutic potential for adenosine receptor modulation has even been anticipated. This review focuses on the role of adenosine and adenosine receptors in the treatment of asthma and COPD.

The Quintiles Prize Lecture 2004. The identification of the adenosine A2B receptor as a novel therapeutic target in asthma.

Adenosine is a powerful bronchoconstrictor of asthmatic, but not normal, airways. In vitro studies on isolated human mast cells and basophils revealed that adenosine and selective analogues augmented inflammatory mediator release from mast cells by stimulating A(2) receptors. Pharmacological blockade of mast cell mediator release in vivo also attenuated adenosine-induced bronchoconstriction, as did theophylline, by adenosine A(2) receptor antagonism. Further in vitro studies revealed that the asthmatic response to adenosine is likely to be mediated via the A(2B) subtype which is selectively antagonised by enprofylline. Studies in animal models, especially mice, have shown a close synergistic interaction between adenosine, Th2 and airway remodelling responses. The recent description of A(2B) receptors on human airway smooth muscle cells that mediate cytokine and chemokine release and induce differentiation of fibroblasts into myofibroblasts strengthens the view that adenosine maybe more than an inflammatory mediator in asthma but also participates in airway wall remodelling in this disease. These data have provided a firm basis for developing adenosine A(2B) receptor antagonists as a new therapeutic approach to this disease.

Dose-dependent respiratory responses to inhaled corticosteroids in asthma

It is commonly recommended that the dose of an inhaled corticosteroid (ICS) be increased when there is evidence of increased asthma severity. However, the dose-dependent responses to such an increase in ICS dose are difficult to quantify because of the relatively slow onset of ICS effects in the airways. This study explored the use of sequential quadrupling dose regimens and a range of endpoints to assess the response to the ICS budesonide in 21 subjects with mild asthma. In a randomized protocol, subjects inhaled increasing doses of budesonide (100, 400, 1600 μg/day), each dose being given for 1 or 2 weeks, followed by several outcome assessments. The researchers found a significant dose-dependent decrease in bronchial hyperreactivity (BHR), as assessed by adenosine-induced bronchoconstriction, somewhat more pronounced when the increased budesonide dose was given for a 2-week period. Use of increased dosages of budesonide was followed by increases in FEV1 and morning peak expiratory flow rate (PEFR); however, these changes in expiratory flow rates were small and were more variable than the changes in BHR. Changes in the evening PEFR, symptom scores, and reliever use of inhaled β-agonists were small and not ICS dose–related. The authors concluded that through use of a determination of BHR assessed by adenosine inhalation challenge, a clear-cut response to quadrupling increases in the inhaled budesonide dose could be detected in individuals with mild asthma. It is noteworthy, however, that this same investigative group has recently reported that a doubling of the dose of inhaled budesonide with worsening asthma symptoms in a patient self-management program was not associated with a reduced frequency of subsequent acute asthma exacerbations.

Identification of A3 receptor- and mast cell-dependent and -independent components of adenosine-mediated airway responsiveness in mice.

Adenosine-induced bronchoconstriction is a well-recognized feature of atopic asthma. Adenosine acts through four different G protein-coupled receptors to produce a myriad of physiological effects. To examine the contribution of the A(3) adenosine receptor to adenosine-induced bronchoconstriction and to assess the contribution of mast cells to this process, we quantified airway responsiveness to aerosolized adenosine in wild-type, A(3) receptor-deficient, and mast cell-deficient mice. Compared with the robust airway responses elicited by adenosine in wild-type mice, both A(3)-deficient and mast cell-deficient mice exhibited a significantly attenuated response compared with their respective wild-type controls. Histological examination of the airways 4 h after adenosine exposure revealed extensive degranulation of airway mast cells as well as infiltration of neutrophils in wild-type mice, whereas these findings were much diminished in A(3)-deficient mice and were not different from those in PBS-treated controls. These data indicate that the airway responses to aerosolized adenosine in mice occur largely through A(3) receptor activation and that mast cells contribute significantly to these responses, but that activation of additional adenosine receptors on a cell type(s) other than mast cells also contributes to adenosine-induced airway responsiveness in mice. Finally, our findings indicate that adenosine exposure can result in A(3)-dependent airway inflammation, as reflected in neutrophil recruitment, as well as alterations in airway function.

Characterization of adenosine receptor(s) involved in adenosine-induced bronchoconstriction in an allergic mouse model.

We recently reported that adenosine caused bronchoconstriction and enhanced airway inflammation in an allergic mouse model. In this study, we further report the characterization of the subtype of adenosine receptor(s) involved in bronchoconstriction. 5'-(N-ethylcarboxamido)adenosine (NECA), a nonselective adenosine agonist, elicited bronchoconstriction in a dose-dependent manner. Little effects of N(6)-cyclopentyladenosine (A(1)-selective agonist) and 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (A(2A)-selective agonist) compared with NECA were observed in this model. 2-Chloro-N(6)-(3-iodobenzyl)-9-[5-(methylcarbamoyl)-beta-d-ribofuranosyl]adenosine, an A(3)-selective receptor agonist, produced a dose-dependent bronchoconstrictor response, which was blocked by selective A(3) antagonist 2,3-diethyl-4,5-dipropyl-6-phenylpyridine-3-thiocarboxylate-5-carboxylate (MRS1523). However, MRS1523 only partially inhibited NECA-induced bronchoconstriction. Neither selective A(1) nor A(2A) antagonists affected NECA-induced bronchoconstriction. Enprofylline, a relatively selective A(2B) receptor antagonist, blocked partly NECA-induced bronchoconstriction. Furthermore, a combination of enprofylline and MRS1523 completely abolished NECA-induced bronchoconstrictor response. Using RT-PCR, we found that all four adenosine receptor subtypes are expressed in control lungs. Allergen sensitization and challenge significantly increased transcript levels of the A(2B) and A(3) receptors, whereas the A(1) receptor message decreased. No change in transcript levels of A(2A) receptors was observed after allergen sensitization and challenge. These findings suggest that A(2B) and A(3) adenosine receptors play an important role in adenosine-induced bronchoconstriction in our allergic mouse model. Finally, whether the airway effects of the receptor agonists/antagonists are direct or indirect needs further investigations.

Adenosine-receptor subtypes: their relevance to adenosine-mediated responses in asthma and chronic obstructive pulmonary disease.

Adenosine administration by inhalation elicits concentration-related bronchoconstriction in subjects with asthma and chronic obstructive pulmonary disease (COPD). The mechanisms of adenosine-induced bronchoconstriction appear to involve a selective interaction with activated mast cells with subsequent release of preformed and newly-formed mediators. Further evidence linking adenosine signalling to asthma and COPD comes from the finding that many cell types that play important roles in the exacerbation of these conditions express adenosine receptors and demonstrate relevant effects through stimulation of these receptors. Therefore, blockade of these receptors may be a valuable approach to the treatment of asthma and chronic obstructive pulmonary disease. Promising adenosine-receptor targets for novel therapeutics of asthma and chronic obstructive pulmonary disease have recently been identified in a number of inflammatory cell types, including mast cells, eosinophils, lymphocytes, neutrophils, and macrophages. The recent characterisation of the A2B receptors indicates the human lung mast cell as one of the most strategic cellular targets.

Adenosine A2Breceptors as therapeutic targets

Recent evidence indicates that A 2B receptors mediate cellular functions with potential clinical relevance. Both A 2A and A 2B receptors mediate vasodilation, and the receptor type involved depends on the vascular bed and species studied. In some experimental models, A 2B -induced vasodilation is mediated by the endothelium, but it is unclear whether this vasodilation is due to nitric oxide or whether A 2B receptors also mediate direct vascular smooth muscle relaxation. A 2B receptors expressed in smooth muscle cells inhibit their growth, raising the possibility that these receptors play a role in the vascular remodeling process observed in hypertension and atherosclerosis. A 2B receptors are also expressed in neurons, and there are several examples of these receptors mediating neuroexcitatory actions, including potentiation of neurotransmitter release. The highest expression of A 2B receptors is found throughout the intestinal tract. During diarrheal processes, neutrophils recruited into intestinal crypts release a soluble neutrophil-derived secretagogue, which then increases intestinal secretion. It is now known that this neutrophil product is AMP, which is then converted to adenosine to activate A 2B receptors expressed in intestinal epithelium. It was also recently found that activation of human mast cells by adenosine is mediated by A 2B receptors. Mast cell activation is involved in adenosine-induced bronchoconstriction in asthmatics, suggesting that A 2B receptors are involved in this process. Our understanding of the functional role of A 2B receptors is hindered by the lack of selective agonist and antagonist of this receptor type. Recent studies suggest the feasibility of developing A 2B antagonists. Such agents may prove useful in the treatment of diarrheal diseases and in asthma.

A1 adenosine receptor-mediated Ins(1,4,5)P3 generation in allergic rabbit airway smooth muscle.

The signal transduction pathway for A1 adenosine receptor in airway smooth muscle from allergic rabbits was studied by investigating the effect of the selective A1 adenosine-receptor agonist N6-cyclopentyladenosine (CPA) on tissue levels of inositol 1,4, 5-trisphosphate [Ins(1,4,5)P3] measured by protein binding assay. CPA caused a rapid, transient, and concentration-dependent elevation of Ins(1,4,5)P3 in airways from allergic rabbits. The agonist also produced a concentration-dependent contraction of the airway preparations from these animals. Both the Ins(1,4,5)P3 and contractile responses generated by CPA were attenuated by the phospholipase C (PLC) inhibitor U-73122, indicating the coupling of these responses to PLC. The CPA-induced Ins(1,4,5)P3 production observed in the allergic rabbit tissues was also inhibited by the adenosine-receptor antagonist 8-( p-sulfophenyl)-theophylline, suggesting that the effect was mediated by A1 adenosine receptors. On the other hand, the A2 adenosine-receptor agonist CGS-21680 was ineffective in altering the tissue concentration of Ins(1,4,5)P3, indicating that A2 adenosine receptors may not be involved in the activation of PLC in the allergic rabbit airway smooth muscle. In this preparation, the Gi-Go inhibitor pertussis toxin (PTX) attenuated the CPA-induced Ins(1,4,5)P3 accumulation, providing evidence that the generation of Ins(1,4,5)P3 by A1 adenosine-receptor stimulation is coupled to a PTX-sensitive G protein(s). The results suggest that activation of A1 adenosine receptors in allergic rabbit airway smooth muscle causes the production of Ins(1,4,5)P3 via a PTX-sensitive G protein-coupled PLC, and this signaling mechanism may be involved, at least in part, in the generation of contractile responses. It is hypothesized that this process may contribute to adenosine-induced bronchoconstriction in allergic asthma.

Characterization of adenosine receptors involved in adenosine-induced bronchoconstriction in allergic rabbits.

1. Recent work has suggested that adenosine may be involved in asthma via the activation of A1 receptors. However, the role of the recently cloned A3 receptor in airways is largely unknown. In the present study, we have investigated the role of the A3 receptor in adenosine-induced bronchoconstriction in allergic rabbits. 2. Aerosol challenge of antigen (Ag) immunized rabbits with the adenosine precursor, adenosine 5'-monophosphate (AMP), resulted in a dose-dependent fall in dynamic compliance (Cdyn). The maximum fall in Cdyn in these rabbits was significantly greater than that in litter matched, sham immunized animals (P < 0.05). However, there was no significant difference in the maximum increase in airways resistance (Rt) between Ag and sham immunized rabbits (P > 0.05). 3. Aerosol challenge of Ag immunized rabbits with cyclopentyl-adenosine (CPA) (A1-receptor agonist) elicited a dose-dependent fall in Cdyn in Ag immunized rabbits and the maximum fall in Cdyn in these rabbits was significantly greater than that observed in sham immunized rabbits (P < 0.05). Similarly, CPA induced dose-dependent increases in R1 in Ag immunized rabbits whereas sham immunized rabbits failed to respond to CPA within the same dose range. The maximum increase in RL in Ag immunized rabbits was significantly greater than that of sham immunized rabbits (P < 0.05). 4. Aerosol challenge of either Ag or sham immunized rabbits with the A3 agonist aminophenylethyladenosine (APNEA) did not elicit dose-dependent changes in either RL or Cdyn. Moreover, there was no significant difference in the maximum response, measured by either parameter, between the two animal groups (P > 0.05). 5. These data provide further evidence for a role of the A1 receptor in the airways, but do not support a role for the A3 receptor in adenosine-induced bronchoconstriction in the allergic rabbit.

Adenosine receptors involved in the bronchoconstrictor effect of adenosine

Adenosine may play a role in the airflow obstruction in asthma and chronic obstructive pulmonary disease. Although the mechanism of adenosine-induced bronchoconstriction has been well explored, little is known about the identity of adenosine-receptors involved. Studies on rabbit and rat airways suggest that multiple receptors (A 1 , A 2B , and A 3 receptors) may be involved in the bronchoconstrictor of adenosine. Mast cell mediators contribute to adenosine-induced bronchoconstriction. Studies on isolated mast cell lines have revealed the presence of A 2B and A 3 receptors. The in vitro contraction of human bronchi is mediated by an adenosine receptor (A 1 or A 2 ?). A 2 receptors have been demonstrated by ligand binding in human peripheral airways. The presence of the A 3 receptor has been shown by in situ hybridization in eosinophils and mesenchymal tissue. The development of potent and specific adenosine receptor agonists and antagonists will increase our knowledge on this issue and may offer new therapeutic approaches to obstructive airway diseases.

Adenosine receptor-mediated bronchoconstriction and bronchial hyperresponsiveness in allergic rabbit model

Recently, there has been an increasing interest in adenosine as a potential mediator of allergic asthma. In the present study, we have employed an allergic rabbit model of late-phase asthma to characterize the adenosine receptor subtype in the asthmatic airways. The allergic rabbit model has physiological characteristics and pharmacological sensitivity comparable to human asthma. Adenosine causes a dose-dependent bronchoconstriction in allergic, but not in normal, rabbits. This adenosine-induced bronchoconstriction is significantly inhibited by theophylline, an adenosine receptor antagonist, thus suggesting the involvement of adenosine receptor(s). 5'-(N-ethylcarboxyamido) adenosine (NECA), a nonselective agonist, 2-[p-(2-carboxyethyl)-phenethylamino]-5'-(N-ethylcarboxamido)-aden osine (CGS-21680), an A2-selective receptor agonist, and cyclopentyl adenosine (CPA), an A1-selective agonist, were used to characterize the adenosine receptor subtype in the airways of allergic rabbits, respectively. The PC50 (concentration of agonist in mg/ml required to reduce the dynamic compliance 50% from the baseline) values for adenosine, CGS-21680, NECA, and CPA were 9.25 +/- 0.86, 6.8 +/- 0.69, 3.90 +/- 0.27, and 1.45 +/- 0.26 mg/ml, respectively. Thus the potency profile for the bronchoconstrictor of adenosine in this model is a typical A1-receptor subtype (CPA > NECA > CGS-21680 > adenosine). Adenosine also induced further bronchial hyperresponsiveness (BHR) at 1 but not at 24 h following aerosol challenge.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine-induced bronchoconstriction in conscious hyperresponsive and sensitized guinea pigs.

Inhaled adenosine induces bronchoconstriction in asthmatic and allergic subjects but not in nonasthmatics. This study examined the responses of conscious guinea pigs in which antigen sensitization is induced by ovalbumin pretreatment and airway hyperresponsiveness to carbachol is induced by exposure to ozone and platelet-activating factor-acether (PAF). Airway responses to aerosol challenge with carbachol or adenosine were determined as the change in specific airway conductance (SGaw) measured by whole-body plethysmography. In untreated animals, carbachol (20 micrograms/ml, 60 s) induced a rapid fall in SGaw (peak, 18 +/- 5% at 5 min) indicative of bronchoconstriction, whereas adenosine (1 mg/ml, 60 s) caused an increase in SGaw (34 +/- 8% at 15 min). Animals pretreated with ovalbumin displayed similar responses to carbachol (14 +/- 5% at 5 min) as control animals and were therefore sensitized but not hyperresponsive. However, adenosine (1 mg/ml) caused a rapid bronchoconstriction, peaking at 20 min (25 +/- 5%). Exposure of animals to nebulized PAF-acether (10 micrograms/ml) for 60 s produced a bronchoconstriction, which peaked at 10 min (18 +/- 7%) and returned to basal levels by 60 min. Similarly, exposure to ozone (1.4 ppm) for 60 min caused bronchoconstriction (peak at 20 min, 19 +/- 6%), with recovery after 1 h. Both PAF- and ozone-exposed animals displayed significant hyperresponsiveness to carbachol administered 1 h from the end of the exposure period. The peak bronchoconstrictor responses before and after PAF exposure were 10 +/- 9 and 28 +/- 4%, and responses before and after ozone exposure were 22 +/- 5 and 61 +/- 9%.(ABSTRACT TRUNCATED AT 250 WORDS)