The effect of an alpha-adrenergic receptor blocking drug on histamine sensitivity in bronchial asthma

Effect of Inhaled Alpha-Blocker on Airflow Obstruction in Asthma1

A controlled study of the effect on airflow obstruction of 30 mg of the alpha-blocker thymoxamine given by inhalation is reported in 10 patients with chronic stable asthma. No significant change in peak expiratory flow rate was seen with either thymoxamine or placebo in any patient. Alpha-adrenergic receptors are therefore unlikely to contribute significantly to resting bronchomotor tone in chronic stable asthma.

Alpha(1L)-, but not alpha(1H)-, adrenoceptor antagonist prevents allergic bronchoconstriction in guinea pigs in vivo

alpha-Adrenoceptors have been classified into alpha(1)- and alpha(2)-adrenoceptors. Recently, the alpha(1)-adrenoceptors were divided into two subtypes: alpha(1L) with low affinity and alpha(1H) with high affinity for prazosin. Little is known concerning the role of each subtype of alpha(1)-adrenoceptor in asthma. We investigated the effects of specific antagonists of alpha(1)- and alpha(2)-, alpha(1H)-, alpha(1L)-, and alpha(2)-adrenoceptors, namely moxisylyte, prazosin, 3-[N-[2-(4-hydroxy-2-isopropyl-5-methylphenoxy) ethyl]-N-methylaminomethyl]-4-methoxy-2, 5, 6-trimethylphenol hemifumarate (JTH-601), and yohimbine, respectively, on antigen-induced airway reactions in guinea pigs. Fifteen minutes after intravenous administration of moxisylyte (0.01, 0.1 or 1 mg/kg), prazosin (0.01, 0.1, 1 or 10 mg/kg), JTH-601 (1, 3, 6 or 10 mg/kg) or yohimbine (0.1 or 1 mg/kg), passively sensitized and artificially ventilated animals received an aerosolized antigen challenge. Bronchial responsiveness to inhaled methacholine was assessed as the dose of methacholine required to produce a 200% increase in the pressure at the airway opening (PC(200)) in non-sensitized animals. JTH-601 and moxisylyte, but not prazosin or yohimbine, dose dependently inhibited antigen-induced bronchoconstriction. None of the tested drugs altered PC(200). JTH-601 significantly reduced leukotriene C(4) levels in bronchoalveolar lavage fluid obtained 5 min after antigen challenge, but prazosin did not. These results indicate that prevention of antigen-induced bronchoconstriction by blockade of alpha-adrenoceptors is due to the inhibition of mediator release via alpha(1L)-adrenoceptor antagonism.

Adrenoceptors in airway smooth muscle

This review examines the roles and functional significance of alpha and beta-adrenoceptor subtypes in airway smooth muscle, with emphasis on human airway function and the influence of asthma. Specifically, we have examined the distribution of beta-adrenoceptors in lung and the influence of age, the epithelium, respiratory viruses and inflammation associated with asthma on airway smooth muscle beta-adrenoceptor function. Sites of action, beta 2-selectivity, efficacy and tolerance are also examined in relation to the use of beta 2-agonists in man. In addition, alpha-adrenoceptor function in airway smooth muscle has been reviewed, with some emphasis on comparing observations made in airway smooth muscle with those in animal models.

Alpha-adrenergic blockers

alpha-Adrenergic blockers are important drugs in the treatment of hypertension and other cardiovascular and noncardiovascular disorders. The ability to selectively block alpha-receptor subtypes provides a greater margin of safety and efficacy for these drugs.

Thymoxamine: lack of antihistaminic effects in clinical doses in man

The purpose of this study was to investigate whether the alpha 1-adrenoceptor antagonist thymoxamine possesses antihistaminic activity in clinical doses in man, as has been reported on the guinea pig ileum in vitro. Five normal subjects were given on three separate occasions intravenous infusions of thymoxamine (0.15 mg kg-1 loading dose followed by 0.15 mg kg-1 h-1), chlorpheniramine (1.5 mg loading dose followed by 1.5 mg h-1) and normal saline (placebo). Intravenous bolus doses of histamine (1 and 2 micrograms kg-1) were given after pretreatment with propranolol 10 mg to block the beta-adrenoceptor agonist effects of the catecholamines released by the histamine injections. Histamine caused a dose-dependent reduction of FEV1 and FVC that was antagonised by chlorpheniramine but not by thymoxamine, suggesting that thymoxamine has no antihistaminic activity in the doses used in man. Thymoxamine caused a small enhancement of the bronchoconstrictor effect of the lower dose of histamine. The relatively selective action of thymoxamine makes it a suitable agent for the investigation of alpha 1-adrenoceptors.

Failure of enhancement by labetalol of bronchopulmonary effects of histamine in guinea-pigs: independence of alpha-adrenoceptor antagonism

The effect of labetalol on histamine-induced bronchoconstriction was studied in anaesthetized guinea-pigs. Unlike propranolol (1 mg kg-1), the same dose of labetalol did not enhance histamine-induced bronchoconstriction. To determine whether the absence of enhancement of the respiratory effects of histamine by labetalol was due to its alpha 1-blocking properties or to its partial agonist activity at beta 2-adrenoceptors, the effects of propranolol plus prazosin and of propranolol plus labetalol on histamine-induced bronchoconstriction were examined. In both cases, the bronchoconstrictor effects of histamine were enhanced to the same extent as with propranolol alone. These data support the hypothesis that the non impairment of respiratory mechanics by labetalol is not due to antagonism at alpha-adrenoceptors and may be mediated by its partial agonist activity at beta 2-adrenoceptors.

Influence of circulating alpha adrenoceptor agonists on lung function in patients with exercise induced asthma and healthy subjects

The influence of circulating noradrenaline (in this context primarily a non-selective alpha agonist) and the alpha 1 selective agonist phenylephrine on bronchial tone, blood pressure, and heart rate was studied in eight patients with exercise induced asthma and eight age and sex matched controls. All subjects refrained from taking treatment for at least one week before the trial. The agonists were infused intravenously in stepwise increasing doses of 0.04, 0.085, 0.17, and 0.34 micrograms/kg a minute for noradrenaline and 0.5, 1.0, 2.0, and 4.0 micrograms/kg a minute for phenylephrine. At the highest dose the plasma concentration of noradrenaline was about 30 nmol/l, resembling the concentrations found during intense exercise, and that of phenylephrine was about 400 nmol/l. Both agonists caused dose dependent and similar increases in blood pressure in the two groups. Despite clearcut cardiovascular effects (systolic and diastolic blood pressure increased by about 40-50/25-30 mm Hg), neither agonist altered lung function, as assessed by measurements of specific airway compliance (sGaw), peak expiratory flow (PEF), or end expiratory flow rate, in either group. It is concluded that circulating alpha agonists, whether alpha 1 selective (phenylephrine) or non-selective (noradrenaline), fail to alter basal bronchial tone in patients with exercise induced asthma or in healthy subjects.

Bronchial responsiveness to methacholine during airway cooling in normal subjects

In order to investigate the effects of airway cooling on bronchial responsiveness in normal subjects, we measured bronchial responsiveness to inhaled methacholine with and without the inhalation of cold air. Two out of seven subjects showed an increase in baseline respiratory resistance (Rrs) during cooling of the airway but the other five subjects showed little change in their baseline Rrs. All subjects increased bronchial responsiveness to methacholine. Additionally, the threshold dose of methacholine decreased to one-third of the control dose with cooling of the airway. We speculate that airway cooling increased bronchial responsiveness to methacholine in normal subjects presumably due to increased vagal tone, increased alpha-adrenergic activity and/or a release of chemical mediators.

The role of alpha and beta adrenoceptors in airway hyperresponsiveness to histamine

The effect of a specific alpha 1-adrenoceptor antagonist, prazosin, on histamine-induced bronchoconstriction was compared to a beta 2-adrenoceptor agonist, salbutamol, in 16 subjects with nonspecific bronchial hyperresponsiveness whose PC20H ranged from 0.10 to 5.12 mg/ml. PC20H was calculated from a histamine inhalation test performed before and after 0.5 mg of prazosin by dry powder inhalation and 200 mcg of salbutamol by pressurized aerosol. PC20H was also measured in six subjects before and after placebo (20 mg lactose) by dry powder inhalation in a randomized double-blind study with prazosin. Mean (+/- SE) PC20H before and after placebo was 1.77 (0.32) and 1.57 (0.38) mg/ml, respectively, an 0.89-fold change. Mean (+/- SE) PC20H before and after prazosin for the 16 subjects was 1.92 (0.34) and 3.10 (0.72) mg/ml, a 1.51-fold change (p less than 0.001), and PC20H before and after salbutamol was 2.08 (0.33) and 9.54 (2.51) mg/ml, a 4.08-fold change (p less than 0.001). There was a positive correlation between the prazosin and salbutamol responses (r = 0.55, p less than 0.05). A dose response for salbutamol was performed in eight subjects, and PC20H was determined by use of increasing doses of salbutamol until PC20H was more than 16 mg/ml. The dose of salbutamol required varied widely between subjects and did not relate to baseline PC20H. The results suggest a role for alpha-adrenoceptors in addition to beta-adrenoceptors in histamine-induced bronchoconstriction.

Propranolol vs metoprolol vs labetalol: a comparative study in essential hypertension

In a double blind, within patient investigation of twenty-four patients (nineteen males and five females) with a mean age of 46.3 years (SD 10.9 years) with mild to moderate essential hypertension a comparison between equipotent beta-blocking doses of propranolol, metoprolol and labetalol was carried out. Blood pressure and pulse rate were measured in lying, sitting and standing positions and before, during and after isometric and dynamic exercise. Peak expiratory flow was recorded before and during dynamic exercise. All the active treatments were better than placebo in reducing blood pressure and heart rate. Comparing the effects of treatment, labetalol lowered sitting diastolic pressure significantly more than propranolol and standing diastolic pressure than both propranolol and metoprolol. Metoprolol and propranolol were more effective in reducing heart rate. Propranolol significantly reduced peak flow rate compared to labetalol. During the exercise, both isometric and dynamic, the heart rate and the blood pressure, both systolic and diastolic, of the treated patients were lower than those on placebo. There was little difference between the drugs in the influence on blood pressure, but metoprolol and propranolol were significantly more effective than labetalol in lowering the heart rate.

Comparison between an alpha-adrenergic antagonist and a beta 2-adrenergic agonist in bronchial asthma

Fifteen patients suffering from asthma received inhalations of phentolamine, albuterol (salbutamol), a combination of phentolamine and albuterol, and placebo, in a single-blind fashion; the changes in the pulmonary function tests were recorded over a three-hour period. Three patients responded to phentolamine with marked bronchodilatation, whereas severe bronchoconstriction was induced by the drug in two patients. Five patients improved more with phentolamine than with placebo, while all patients improved more markedly with albuterol and still more following inhalation of the combination of both drugs. As a group, there were no statistically significant differences between the responses to phentolamine compared with placebo, or between albuterol alone compared with the combination of both drugs. We concluded that both alpha-antagonist and beta 2-agonist agents act in the same direction in most patients, the beta 2-agonist being the dominant. These results do not offer convincing proof that enhanced alpha-adrenergic activity is the main bronchoconstrictor mechanism even in those few with good response to phentolamine, who also showed very good responsiveness to albuterol. The mechanism of phentolamine-induced bronchoconstriction was discussed, but in the light of presently accepted theories, we were unable to evolve a reasonable explanation.

The modulation of adrenergic airway tone in normal and ascaris-sensitive beagle dogs

The airway effects of adrenergic receptor stimulation/inhibition were investigated in sets of normal (N) and natively ascaris-sensitized (S) beagle dogs. In one group, the effect of beta-adrenergic stimulation/alpha-adrenergic inhibition in airway conductance (Gaw) and dynamic lung compliance (CDYN) were observed. For both N and S dogs, thymoxamine pretreatment served to enhance an isoproterenol inhibition of PGF2 alpha-induced Gaw decreases. No effect was observed in CDYN. In another set of animals, the effects of i.v. administration of norepinephrine (NE), epinephrine (EPI) and phenylephrine (PE), before and after pretreatment with propranolol/indomethacin were observed. For N dogs, all three alpha-adrenergic agents failed to produce bronchospasm; however, propranolol/indomethacin pretreatment significantly potentiated the effects of all three alpha-agents. No significant changes were observed in the CDYN response either prior to or after pretreatment with propranolol/indomethacin. In contrast, for S dogs, all three alpha-agonists produced moderate bronchoconstriction with or without propranolol/indomethacin pretreatment. In addition, the Gaw response to i.v. PE was statistically greater in S dogs than that observed in the N group. Finally, only i.v. PE produced significant decreases in the CDYN response in the S group of animals. These data suggest the presence of alpha-adrenergic influence in the canine airways and elaboration of this action appears to occur best in propranolol/indomethacin-pretreated animals.

Sympathoadrenal reactivity in exercise-induced asthma

The possibility that sympathoadrenal activity is altered in asthma was examined in eight patients with a history of exercise-induced asthma (EIA), eight matched patients with nonexercise induced asthma (NEIA), and eight matched healthy control subjects. No medication was allowed for at least one week before examination. In a pretrial exercise test diagnosis of EIA was confirmed and each individual's work capacity (Vo2 max) was determined. The trial consisted of an orthostatic test and a standardized exercise test at 80 to 90 percent of VO2 max on a treadmill. The trial exercise test caused a decrease in FEV1 in EIA patients only, whereas measurements of Sgaw revealed a significant but less pronounced postexercise bronchoconstriction in NEIA-patients as well. Basal plasma catecholamine levels were similar in all groups. Noradrenaline and adrenaline levels were approximately doubled by the orthostatic test and increased approximately ten-fold following exercise, with no differences between the groups. Plasma cAMP levels were approximately doubled by the exercise test. In the EIA patients there was an inverse correlation between increases in plasma cAMP and decreases in Sgaw. Our study does not support earlier claims that exaggerated catecholamine response to exercise causes postexercise bronchoconstriction by alpha-adrenoceptor stimulation in EIA. Differences in study results appear to have methodologic explanations.

Prazosin, an alpha 1-adrenoceptor antagonist, partially inhibits exercise-induced asthma

The effect of prazosin, a potent and specific alpha 1-adrenoceptor antagonist given by inhalation (total nebulized 2 mg) was compared with placebo in a double-blind randomized study of 10 atopic asthmatic children. Prazosin significantly (p less than 0.01) reduced the severity of post-exercise bronchoconstriction (maximum fall in peak expiratory flow after exercise 21.4% +/- SEM 6.3% after prazosin compared with 42.5% +/- 7.3% after placebo). This protective action of prazosin suggests that activation of alpha 1-adrenoceptor may be involved in the pathogenesis of exercise-induced asthma either by facilitation of mast-cell mediator release or by direct contraction of bronchial smooth muscle. Prazosin did not significantly change resting bronchomotor tone or histamine-induced bronchoconstriction, suggesting no effect on bronchial smooth muscle contractility.

Adrenoceptors: molecular nature and role in atopic diseases

Since Szentivanyi proposed the idea that asthma and other atopic diseases are due to a beta adrenergic defect there has been much interest in the role of the adrenergic receptors in allergy. The radioactive ligand binding techniques developed within the last few years have greatly increased our knowledge concerning the molecular nature of the adrenoceptors and the events following receptor stimulation. The adrenoceptors have shown to be very dynamic structures. Their number and affinity may be altered due to various physiological and pharmacological stimuli. Their role in the pathogenesis of atopic diseases has not been definitely settled, but there seem to be a true beta adrenergic hyporesponsiveness and alpha hyperresponsiveness in asthma. This article briefly describes the radioligand binding technique and summarizes our present knowledge of the nature of the alpha and beta adrenoceptors and their possible role in atopic diseases.

Inhaled prazosin in asthma

Prazosin, a potent and selective alpha-adrenergic antagonist, was given by inhalation to nine asthmatic subjects aged 25-48 years (six with positive skin tests). Prazosin 0.5 mg, salbutamol 1 mg, or placebo were given by nebuliser in randomised double-blind fashion on separate days. Although all subjects showed a significant increase in FEV1, vital capacity, and maximum expiratory flow at 70% of total lung capacity after salbutamol, there was no significant difference between prazosin and placebo. This suggests that alpha-adrenergic receptors are not important in the control of bronchial tone in asthma. The weak bronchodilatation ascribed to alpha-antagonists in previous studies could be explained by other pharmacological actions of the drugs used.

Cardiopulmonary effects of clenbuterol in the horse

Clenbuterol, a bronchospasmolytic agent (beta 2 agonist) was studied in terms of its hemodynamic and airflow response in eight, healthy horses. Four animals were instrumented to record intrapleural pressure and air flow, these were used to compute pulmonary resistance, peak flow rates, and tidal volumes. Four animals were instrumented to record pulmonary arterial pressure, carotid arterial pressure, cardiac output, and arterial gas tensions. After control values were recorded, clenbuterol (0.8 microgram/kg) was intravenously administered to each horse in each experiment group. Following clenbuterol administration, non-elastic resistance of the lung or pulmonary resistance significantly decreased, 33.6% reduction at 10 min post-clenbuterol. Pulmonary resistance remained lowered during the entire procedure and showed no tendency of returning toward control values by 3 h post-clenbuterol. Within 30 sec following clenbuterol injection carotid arterial pressure decreased (mean pressure decrease 28.2%). Accompanying the change in arterial pressure, the heart rate drastically increased, 99.0%. Both changes were transient and returned to control ranges within 2 min. Clenbuterol appears to be effective in reducing non-elastic resistance of the lung, however intravenous administration to an animal with pre-existing cardiovascular or cardiopulmonary disease should be avoided.

Plasma catecholamines during exercise-induced bronchoconstriction in bronchial asthma

Plasma levels of adrenaline and noradrenaline during and after submaximal exercise in patients with bronchial asthma were investigated. Three groups were studied comprising 10 patients with exercise-induced bronchoconstriction (EIB), 10 asthmatic patients without EIB and four normal control subjects. Plasma catecholamines were measured at rest, at the end of exercise, and five and 15 minutes after exercise. Changes in airway resistance were assessed by measuring peak expiratory flow rate. Significant differences in catecholamine levels between reacting and non-reacting patients were found. In 10 patients developing EIB adrenaline and noradrenaline levels had risen significantly by the end of exercise and remained elevated up to the fifth minute of recovery. The rise in catecholamine levels in non-reacting asthmatics was insignificant. In control subjects noradrenaline had increased significantly by the end of exercise.

Investigation of occurrence of tolerance to bronchodilator drugs in chronically pretreated guinea-pigs

1 The actions of sympathomimetic amines on isolated preparations of atria, trachea and ileum were studied in vitro in guinea-pigs, which had been pretreated for 5 or 12 days, by subcutaneous injection, with adrenaline (5 mug/kg), salbutamol (0.5 mug/kg), salbutamol (0.5 mug/kg), methoxamine (250 mug/kg) or saline (0.9% w/V NaCl solution). 2 In the trachea, a decrease in sensitivity (tolerance) to the relaxant effect of adrenaline was induced by pretreatment, for 12 but not for 5 days, with adrenaline. In these animals, cross-tolerance to isoprenaline or salbutamol was not observed. Tolerance to the relaxant actions of adrenaline isoprenaline or salbutamol was not observed after pretreatment with salbutamol. 3 In the trachea, pretreatment with methoxamine or adrenaline for 12 days did not change the sensitivity to the alpha-adrenoceptor-mediated contractor action of methoxamine. 4 In the atria from those guinea-pigs pretreated with adrenaline or salbutamol, there was no reduced sensitivity to the beta-adrenoceptor agonist actions of adrenaline, isoprenaline or salbutamol. In animals pretreated with methoxamine or adrenaline, there was no observable tolerance or cross tolerance to methoxamine with respect to its alpha-adrenoceptor-mediated positive inotropic action in the atria and no unequivocal evidence of a reduced sensitivity to that action of adrenaline. 5 It was confirmed that the twitch-like contractions of the longitudinal muscle of the electrically stimulated ileum were inhibited by sympatomimetic amines acting on alpha- and beta-adrenoceptors. There was no reduced sensitivity to the inhibitory actions of noradrenaline or isoprenaline on the twitch of ileum isolated from animals pretreated with adrenaline, salbutamol or methoxamine for 5 or 12 days. 6 From our results on these three preparations from the same animals, it is concluded that generalizations regarding changes in sensitivity to sympathomimetic amines following their prolonged administration should not be made in any one species.