Reduced beta 2-adrenoceptor responsiveness in exercise-induced asthma

Beta₂-agonists for exercise-induced asthma

BACKGROUND

It is well known that physical exercise can trigger asthma symptoms and can induce bronchial obstruction in people without clinical asthma. International guidelines on asthma management recommend the use of beta2-agonists at any stage of the disease. At present, however, no consensus has been reached about the efficacy and safety of beta2-agonists in the pretreatment of exercise-induced asthma and exercise-induced bronchoconstriction. For the purpose of the present review, both of these conditions are referred to by the acronymous EIA, independently from the presence of an underlying chronic clinical disease.

OBJECTIVES

To assess the effects of inhaled short- and long-acting beta2-agonists, compared with placebo, in the pretreatment of children and adults with exercise-induced asthma (or exercise-induced bronchoconstriction).

SEARCH METHODS

Trials were identified by electronic searching of the Cochrane Airways Group Specialised Register of Trials and by handsearching of respiratory journals and meetings. Searches are current as of August 2013.

SELECTION CRITERIA

We included randomised, double-blind, placebo-controlled trials of any study design, published in full text, that assessed the effects of inhaled beta2-agonists on EIA in adults and children. We excluded studies that did not clearly state diagnostic criteria for EIA.

DATA COLLECTION AND ANALYSIS

We used standard methodological procedures as expected by The Cochrane Collaboration.

MAIN RESULTS

We included 53 trials consisting of 1139 participants. Forty-eight studies used a cross-over design, and five were performed in accordance with a parallel-group design. Forty-five studies addressed the effect of a single beta2-agonist administration, and eight focused on long-term treatment. We addressed these two different intervention regimens as different comparisons.Among primary outcomes for short-term administration, data on maximum fall in forced expiratory volume in 1 second (FEV1) showed a significant protective effect for both short-acting beta-agonists (SABA) and long-acting beta-agonists (LABA) compared with placebo, with a mean difference of -17.67% (95% confidence interval (CI) -19.51% to -15.84%, P = 0.00001, 799 participants from 72 studies). The subgroup analysis of studies performed in adults compared with those performed in children showed high heterogeneity confined to children, despite the comparable mean bronchoprotective effect.Secondary outcomes on other pulmonary function parameters confirmed a more positive and protective effect of beta2-agonists on EIA compared with placebo. Occurrence of side effects was not significantly different between beta2-agonists and placebo.Overall evaluation of the included long-term studies suggests a beta2-agonist bronchoprotective effect for the first dose of treatment. However, long-term use of both SABA and LABA induced the onset of tolerance and decreased the duration of drug effect, even after a short treatment period.

AUTHORS' CONCLUSIONS

Evidence of low to moderate quality shows that beta2-agonists, both SABA and LABA, when administered in a single dose, are effective and safe in preventing EIA.Long-term regular administration of inhaled beta2-agonists induces tolerance and lacks sufficient safety data. This finding appears to be of particular clinical relevance in view of the potential for prolonged regular use of beta2-agonists as monotherapy in the pretreatment of EIA, despite the warnings of drug agencies (FDA, EMA) regarding LABA.

Genetic polymorphism of the beta-2 adrenergic receptor in atopic and non-atopic subjects

To investigate a possible genetic basis for reported differences in beta-2 receptor expression in atopic subjects, DNA from 42 atopic children (22 asthmatics and 22 with allergic rhinitis) and 30 non-atopic subjects was Southern blotted and Ban-1 restriction fragment polymorphisms (RFLPS) were studied using a 2.6 kb probe of the human beta-2 receptor gene. Two alleles 3.1 kb and 2.9 kb were identified. Homozygotes and heterozygotes for the two alleles were found with equal frequency in the atopic patients who had asthma and in those who had allergic rhinitis only. The gene frequencies for the upper and lower alleles were 0.45 and 0.55 respectively. Our studies do not provide evidence for an association between a particular polymorphic form of the human beta-2 receptor gene and atopy.

Plasma catecholamines--analytical challenges and physiological limitations

Catecholamines in plasma may be measured to assess sympathoadrenal activity. Numerous assay methodologies have been published, illustrating the fact that there are many analytical problems. Different methodologies are discussed briefly. A plea for better validation, especially with regard to specificity (which should not be confused with sensitivity or reproducibility), is made. Plasma NA is a frequently used marker for sympathetic nerve activity in humans, but the data obtained are often misinterpreted due to lack of appreciation of the physiological determinants of the NA concentration measured. NA overflow from an organ gives a good reflection of nerve activity in that organ. However, sympathetic nerve activity is highly differentiated, particularly during stress, and conventional plasma NA levels (usually forearm venous samples) cannot be taken as an indication of 'sympathetic tone' in the whole individual. NA is rapidly removed from plasma, resulting in meaningless net veno-arterial concentration differences over organs unless its removal from arterial plasma is taken into account. In the forearm, for example, 40-50% of catecholamines are removed during one passage; about half of the NA in a venous sample is derived from the arm and half from the rest of the body. Therefore, conventional venous sampling overemphasizes local (mainly skeletal muscle) nerve activity. Whole-body sympathetic nerve activity may be monitored in arterial or mixed venous (i.e. pulmonary arterial) samples, which reflect NA overflow from all organs in the body. NA levels are determined both by overflow to plasma and clearance from plasma. NA turnover studies with 3H-NA infusions may be needed to assess clearance, but the simpler concentration measurements usually yield adequate information if the sampling site is relevant. NA overflow from an organ can be assessed (using 3H-NA or ADR as a marker for NA extraction in the organ) and provides valuable information on local sympathetic activity. Mental stress elicits marked circulatory responses, with mainly cardiorenal sympathetic activation and minor elevations of conventional venous plasma NA levels, thus illustrating the differentiated firing pattern of the sympathetic nerves. Circulating ADR is less important than neurogenic mechanisms in the responses to stress. Concentration-effect studies for infused catecholamines may be used for receptor sensitivity studies in vivo, but reflexogenic contributions to responses need to be determined. However, prejunctional mechanisms cannot be assessed without knowledge of the nerve activity present; for example, ADR infusion leads to increased nerve activity. When correctly sampled, measured and interpreted, plasma catecholamines can yield very valuable information on sympathoadrenal activity.

Beta-receptor responsiveness after desipramine treatment

To examine whether a tricyclic antidepressant affects the functional response to a beta-receptor agonist in man, the response of heart rate, blood pressure, and plasma cAMP to isoproterenol was measured in 14 normal controls taking 75 mg desipramine daily. Desipramine significantly increased the bolus dose of isoproterenol needed to increase heart rate by 25 bpm at 14-30 days but not at 3-8 days. During infusions of isoproterenol, the increase in systolic blood pressure was blunted at both 3-8 days and 14-30 days, while the decrease in diastolic blood pressure was unaffected. Blood pressure findings were not affected by preadministration of bethanechol. In ten controls, isoproterenol infusions increased plasma cAMP, but this was unaffected by desipramine treatment. These findings suggest a decrease in the functional response of beta 1, but not beta 2, receptors after treatment with desipramine.

Influence of beta-adrenergic receptor function during terbutaline treatment on allergen sensitivity and bronchodilator response to terbutaline in asthmatic subjects

Nine asthmatic patients with an allergy to birch or timothy underwent bronchial allergen provocations on three different trial days, with intervals of 2 to 5 wk. Two weeks prior to one of the provocations, no medication was allowed. Before the other two provocations the patients had been on continuous treatment with oral terbutaline (7.5-mg slow-release pill bid) for 2 wk, which was discontinued 12 or 48 h before the allergen provocation. After allergen challenges, terbutaline was inhaled in increasing doses (0.5 mg, 1.0 mg, and 2.0 mg), and pulmonary function was measured after each dose. Before each allergen provocation, blood samples were drawn for measurements of catecholamine and terbutaline concentrations and for in vitro measurements of beta-adrenergic receptor function on lymphocytes (isoproterenol-induced accumulation of cyclic AMP). Beta-adrenergic receptor function on blood lymphocytes was impaired after the two treatment periods, compared with the drug-free period, and was significantly more depressed at 12 h than 48 h after dosing. The bronchial responsiveness to allergen, defined as PC20PEF (median values), was 1,700 biologic units (BU) after the period of no treatment and 220 BU and 445 BU at 12 and 48 h after discontinuation of the terbutaline treatment (p less than 0.1 after 48 h). Five of the nine patients exhibited increased bronchial responsiveness 48 h after treatment, compared to results without treatment. The responsiveness was similar on all occasions in three patients. The bronchodilator response to inhaled terbutaline after allergen-induced bronchoconstriction was attenuated (p less than 0.01) at both 12 and 48 h after terbutaline, compared to results without treatment, indicating desensitization also of the bronchial beta-adrenergic receptors. We conclude that the early bronchial responsiveness to allergen is increased following a period of continuous treatment with a beta-adrenergic receptor agonist in some asthmatic patients and that the capability of a beta-agonist to reverse allergen-induced bronchoconstriction is attenuated after beta-agonist treatment.

Beta-adrenoceptors in human alveolar macrophages isolated by elutriation

1. beta-adrenoceptors on human alveolar macrophages obtained by bronchoalveolar lavage (BAL) from healthy smoking volunteers (n = 26) were characterized by studying cyclic AMP (cAMP) accumulation in intact macrophages evoked by adrenaline or isoprenaline, with or without appropriate antagonists and by radioligand binding to macrophage membranes, using [125I]-iodopindolol (125IPIN) as beta-adrenoceptor ligand. 2. In a second study, cAMP responses of alveolar macrophages to isoprenaline and PGE1 and of peripheral blood lymphocytes to isoprenaline were compared in smoking and non-smoking healthy volunteers (n = 9 + 9), as our initial studies were performed in smokers, due to their higher cell yield. 3. BAL yielded 47 +/- 23 x 10(6) cells in smokers and 12 +/- 6 x 10(6) cells in non-smokers with a recovery of 82 +/- 8% in the elutriation step (means +/- s.d.). The cell preparation consisted of 99.2 +/- 0.8% macrophages and their viability (trypan blue exclusion) was 97.5 +/- 5.2%. 4. Isoprenaline or adrenaline increased cAMP accumulation approximately 40-fold with or without the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX, 10(-4) M), which enhanced basal and stimulated cAMP accumulation approximately five-fold. Peak responses were seen after 2 min. EC50s for isoprenaline and adrenaline were 3-5 x 10(-7) M. Phentolamine did not alter responses to adrenaline, indicating absence of inhibitory alpha 2-adrenoceptors. Propranolol inhibited isoprenaline induced cAMP accumulation stereoselectively; pD2-values were 8.2 for (-)-propranolol, 5.6 for atenolol and 7.5 for ICI 118,551, suggesting a predominance of beta 2-adrenoceptors. 5. Specific 125IPIN binding to macrophage membranes was rapid and saturable. Non-specific binding was determined in the presence of 1 microM (-)-propranolol. KD values were 71 +/- 7 pM and the density of specific binding sites was 36 +/- 3 fmol mg-1 protein (three experiments on a membrane pool from 10 subjects; r values for Scatchard analyses = 0.98 +/- 0.01). Similar values were obtained when 200 microM isoprenaline (+ GTP) was used to assess non-specific binding. Competition experiments again showed stereoselectivity for propranolol and a predominance of beta 2-adrenoceptors, as judged by the displacement of specific 125IPIN binding by atenolol and ICI 118,551. 6. Macrophages from smokers responded with less marked cAMP accumulation upon stimulation with isoprenaline or PGE1 than did cells from non-smokers (difference approximately 30%; P less than 0.05 for both agonists) in the presence of IBMX. Thus macrophages from smokers may produce less cAMP due to post-receptor changes in responsiveness.(ABSTRACT TRUNCATED AT 400 WORDS)

Autonomic nervous system abnormalities and asthma

Autonomic nervous system function has been studied both in vitro and in vivo using a variety of methodologies. In asthmatic patients, beta-adrenergic hyporesponsiveness and alpha-adrenergic and cholinergic hyperresponsiveness can be frequently demonstrated. These observations have provided support for the beta blockade theory of asthma, advanced in the late 1960s by Andor Szentivanyi's experiments involving sensitized rodents. However, in addition to asthma, aberrations in autonomic nervous system function have been noted in other individuals including cystic fibrosis patients and their parents, patients with emphysema and bronchitis, and in patients (allergic rhinitis and atopic dermatitis) who have demonstrable IgE antibody responses to a variety of antigens. Thus, although these defects are not specific for asthma, it is noteworthy that these conditions share many clinical features; the ultimate phenotypic expression of these abnormalities may depend on both genetic and environmental factors that have yet to be defined.

Beta-adrenergic receptor responsiveness to isoprenaline in humans: concentration-effect, as compared with dose-effect evaluation and influence of autonomic reflexes

1. Different techniques of assessing beta-adrenoceptor sensitivity in vivo, by use of i.v. infusions or bolus injections of isoprenaline (ISO), were compared in healthy volunteers. The importance of autonomic reflexes for responses to ISO was evaluated by studying the influence of 'autonomic blockade' by atropine and clonidine, which antagonize muscarinic effects and reduce sympathetic activity, respectively. Estimates of in vivo responsiveness to ISO were compared with parameters reflecting beta 2-adrenoceptor function in vitro in lymphocytes. 2. Heart rate responses to infused ISO were not significantly altered by 'autonomic blockade' when evaluated from concentration-effect curves. When related to the infused dose of ISO, however, sensitivity was artefactually increased (P less than 0.05), as the plasma concentrations of ISO were 40% higher after atropine and clonidine. Heart rate responses to bolus injections of ISO were attenuated (P less than 0.05) by 'autonomic blockade', suggesting that facilitatory reflexes contribute to these non-steady state responses. Intersubject variations in heart rate responsiveness to ISO were greater than the intrasubject variability caused by counterregulatory reflexes. 3. 'Autonomic blockade' lowered venous plasma noradrenaline at rest. The noradrenaline response to ISO infusion was attenuated and the diastolic blood pressure response enhanced, indicating that a counterregulatory vasoconstrictor reflex normally is activated by ISO-induced vasodilatation. The plasma cyclic AMP response to ISO, on the other hand, was unaffected by atropine and clonidine and reflects beta 2-adrenoceptor responsiveness in vivo. 4. In vitro data for beta-adrenoceptor binding sites (Bmax;[125I]-IHYP binding) and cyclic AMP responses to ISO in lymphocytes correlated with DBP and noradrenaline responses to infused ISO. No correlations were found between in vitro data and heart rate, plasma cyclic AMP or plasma glycerol responses to infused ISO in vivo. 5. During prolonged ISO infusions (in six other healthy subjects) physiological responses reached greater than 90% of their steady state level after 8 min, but no definite steady state level could be defined for the plasma concentration of ISO during 40 min of infusion. 6. The ISO infusion test showed a good reproducibility, especially when repeated on the same day. Evaluation of plasma concentration-effect relationships increase the precision of the ISO infusion test as confounding inter- and intra-individual variations in ISO concentrations (as caused by e.g. autonomic blockade) will be taken into account.(ABSTRACT TRUNCATED AT 400 WORDS)