Inhaled prazosin in asthma

Epinephrine evokes shortening of human airway smooth muscle cells following β2 adrenergic receptor desensitization

Epinephrine (EPI), an endogenous catecholamine involved in the body's fight-or-flight responses to stress, activates α1-adrenergic receptors (α1ARs) expressed on various organs to evoke a wide range of physiological functions, including vasoconstriction. In the smooth muscle of human bronchi, however, the functional role of EPI on α1ARs remains controversial. Classically, evidence suggests that EPI promotes bronchodilation by stimulating β2-adrenergic receptors (β2ARs). Conventionally, the selective β2AR agonism of EPI was thought to be, in part, due to a predominance of β2ARs and/or a sparse, or lack of α1AR activity in human airway smooth muscle (HASM) cells. Surprisingly, we find that HASM cells express a high abundance of ADRA1B (the α1AR subtype B) and identify a spontaneous "switch-like" activation of α1ARs that evokes intracellular calcium, myosin light chain phosphorylation, and HASM cell shortening. The switch-like responses, and related EPI-induced biochemical and mechanical signals, emerged upon pharmacological inhibition of β2ARs and/or under experimental conditions that induce β2AR tachyphylaxis. EPI-induced procontractile effects were abrogated by an α1AR antagonist, doxazosin mesylate (DM). These data collectively uncover a previously unrecognized feed-forward mechanism driving bronchospasm via two distinct classes of G protein-coupled receptors (GPCRs) and provide a basis for reexamining α1AR inhibition for the management of stress/exercise-induced asthma and/or β2-agonist insensitivity in patients with difficult-to-control, disease subtypes.

The pharmacologic treatment of uncomplicated arterial hypertension in patients with airway dysfunction

Because many antihypertensive drugs can affect airway function, the treatment of hypertension in patients with airway dysfunction is complex. For example, the worsening or precipitation of asthma by beta-adrenoceptor antagonists is well-recognized, but beta(1)-adrenoceptor blockers that exert mild beta(2)-agonist effects, and those that modulate the endogenous production of nitric oxide, affect airway function to a lesser extent. Therapy with selective alpha(1)-blockers is not contraindicated in cases of chronic airway obstruction. Conversely, alpha(2)-agonists must not be given to asthmatic subjects because they can adversely affect the bronchi. Calcium channel blockers do not exert severe side effects on the airways. Angiotensin-converting enzyme inhibitors may cause cough and exacerbate or even induce asthma; however, angiotensin II type I (AT(1)) antagonists do not cause cough. 5-Hydroxytryptamine modifiers such as urapidil are a treatment option for patients with chronic airway obstruction. In patients with airway dysfunction, we suggest treatment with thiazide diuretics as the initial drug choice, and calcium channel blockers if the response is poor. In the case of no response, calcium channel blockers alone must be used. However, there is no strict rule because individual patients may respond differently to individual drugs and drug combinations. Consequently, it is important to adopt a flexible approach. For patients who are unresponsive to the aforementioned drugs, AT(1) receptor antagonists, newer beta(1)-adrenoceptor-blocking agents with ancillary properties (eg, celiprolol or nebivolol), and/or vasodilators can be considered.

Effects of inhaled glaucine on pulmonary responses to antigen in sensitized guinea pigs

The alkaloid (S)-(+)-1,2,9,10-tetramethoxyaporphine (glaucine) is a phosphodiesterase 4 inhibitor with bronchodilator and anti-inflammatory activity in vitro. In this study, we examined the in vivo effects of glaucine on an animal model of asthma. In ovalbumin sensitized guinea pigs, inhaled glaucine (10 mg ml(-1), 3 min) inhibited the acute bronchoconstriction produced by aerosol antigen (antigen response was 256+/-42 and 95+/-14 cm H(2)O l(-1) s(-1) in control and glaucine-treated animals, respectively; P<0.05). Pretreatment with glaucine (10 mg ml(-1), 10 min inhalation, 30 min pre- and 3 h post-antigen exposure) markedly reduced airway hyperreactivity to histamine, eosinophil lung accumulation, and increased eosinophil peroxidase activity in bronchoalveolar lavage fluid 24 h after exposure of conscious guinea pigs to aerosol antigen. In addition, inhaled glaucine (5-10 mg ml(-1), 3 min) inhibited the microvascular leakage produced after inhaled antigen at all airway levels. These data support the potential interest of phosphodiesterase 4 inhibitors in asthma treatment.

Influence of alpha-adrenoceptor blockade on antigen- and propranolol-induced bronchoconstriction in guinea-pigs in vivo

1. Beta-adrenoceptor antagonists, such as propranolol, can provoke severe bronchoconstriction only in asthmatic subjects. Recently, we developed a guinea-pig model of propranolol-induced bronchoconstriction (PIB) and the purpose of this study was to investigate the role of alpha-adrenergic nerve pathways in this reaction. 2. Phentolamine administered after an antigen challenge did not inhibit PIB; however, its administration before the antigen challenge significantly inhibited the antigen-induced bronchoconstriction and also bronchoconstriction induced by methacholine inhalation. 3. We conclude that the alpha-adrenergic nerve system is not involved in the development of PIB following allergic reaction in our guinea-pig model.

Bronchodilator and anti-inflammatory activities of glaucine: In vitro studies in human airway smooth muscle and polymorphonuclear leukocytes

1. Selective phosphodiesterase 4 (PDE4) inhibitors are of potential interest in the treatment of asthma. We examined the effects of the alkaloid S-(+)-glaucine, a PDE4 inhibitor, on human isolated bronchus and granulocyte function. 2. Glaucine selectively inhibited PDE4 from human bronchus and polymorphonuclear leukocytes (PMN) in a non-competitive manner (Ki=3.4 microM). Glaucine displaced [3H]-rolipram from its high-affinity binding sites in rat brain cortex membranes (IC50 approximately 100 microM). 3. Glaucine inhibited the spontaneous and histamine-induced tone in human isolated bronchus (pD2 approximately 4.5). Glaucine (10 microM) did not potentiate the isoprenaline-induced relaxation but augmented cyclic AMP accumulation by isoprenaline. The glaucine-induced relaxation was resistant to H-89, a protein kinase A inhibitor. Glaucine depressed the contractile responses to Ca2+ (pD'2 approximately 3.62) and reduced the sustained rise of [Ca2+]i produced by histamine in cultured human airway smooth muscle cells (-log IC50 approximately 4.3). 4. Glaucine augmented cyclic AMP levels in human polymorphonuclear leukocytes challenged with N-formyl-Met-Leu-Phe (FMLP) or isoprenaline, and inhibited FMLP-induced superoxide generation, elastase release, leukotriene B4 production, [Ca2+]i signal and platelet aggregation as well as opsonized zymosan-, phorbol myristate acetate-, and A23187-induced superoxide release. The inhibitory effect of glaucine on superoxide generation by FMLP was reduced by H-89. 5. In conclusion, Ca2+ channel antagonism by glaucine appears mainly responsible for the relaxant effect of glaucine in human isolated bronchus while PDE4 inhibition contributes to the inhibitory effects of glaucine in human granulocytes. The very low PDE4/binding site ratio found for glaucine makes this compound attractive for further structure-activity studies.

Spectral analysis of heart rate variability in bronchial asthma

Sympathetic and parasympathetic activity was evaluated in ten healthy controls, nine asymptomatic, untreated asthmatic subjects and ten asthmatic patients during treatment for acute asthma, by measurement of the variation in resting heart rate using frequency spectrum analysis. Heart rate was recorded by ECG and respiratory rate by impedance plethysmography. Spectral density of the beat-to-beat heart rate was measured within the low frequency band 0.04 to 0.10 Hz (low frequency power) modulated by sympathetic and parasympathetic activity, and within a 0.12 Hz band width at the respiratory frequency mode (respiratory frequency power) modulated by parasympathetic activity. Acute asthmatics had higher heart rates than either of the other two groups; this was probably related to the effects of beta-adrenoceptor agonist medication. Sympathetically mediated heart rate variability (normalized low frequency power) was significantly lower in both asymptomatic (p less than 0.002) and acute (p less than 0.02) asthma subjects compared to controls. This is consistent with altered sympathetic/parasympathetic regulation of heart rate in subjects with bronchial asthma.

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.

Prazosin in the treatment of chronic asthma

The role of prazosin, an alpha 1 adrenoceptor blocker, was investigated in patients with chronic stable asthma who continued to have symptoms despite conventional treatment. Forty patients were entered into a double blind, placebo controlled, crossover trial to examine the effect of adding oral prazosin (2 mg twice daily) to previous medication for three weeks. Sixteen patients withdrew from the study. The remaining 24 patients showed no significant change in peak expiratory flow, FEV1, forced vital capacity (FVC), FEV1/FVC ratio, diary card symptom scores, or dose of beta sympathomimetic.

Increased peripheral alpha-adrenoceptor response in allergic asthmatics

The peripheral alpha-adrenoceptor reactivity, measured as dermal blanching response to iontophoretically administered phenylephrine, was studied in 28 allergic asthmatics, 13 patients with allergic rhinoconjunctivitis and 21 healthy controls. The blanching response, which has previously been demonstrated to be mediated via alpha 1-adrenoceptors, was found to be significantly increased in allergic asthmatics compared with controls. It is suggested that there is an increased peripheral alpha-adrenoceptor reactivity in allergic asthma.

The effects of the 5 HT2 antagonist ketanserin in adult atopic asthma

We have studied eight adult atopic asthmatic patients in a randomized double-blind, placebo-controlled, cross-over trial in order to examine the effects of the 5 HT2 antagonist ketanserin on airways function (FEV1, FVC, V50(chi), and V25(pi]. Ketanserin did not significantly change resting bronchomotor tone. Treadmill exercise caused a similar maximal percentage fall in FEV1 after both drug [16.8 (3.7)%] and placebo [19.2 (4.2)%]; ketanserin did not significantly attenuate bronchoconstriction, measured over a 20-min period after exercise. These results suggest that 5 HT2 receptors do not play a major role in the control of resting bronchomotor tone or in exercise-induced bronchoconstriction in adult atopic asthmatic patients.

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.

The site of the defect in asthma. Neurohumoral, mediator or smooth muscle?

The nature of the underlying defect in asthma is still unclear. This article discusses where the primary problem might lie, starting with the assumption that it is likely to be in neurohumoral control, bronchial smooth muscle or cellular dysfunction with increased release of mediators. The weight of the evidence suggests that the latter is most likely. If true, the question of why this occurs still remains.

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.

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.

The action of prazosin and propylene glycol on methoxamine-induced bronchoconstriction in asthmatic subjects

The effect of 1 mg inhaled prazosin on bronchoconstriction induced by methoxamine was investigated in seven asthmatic subjects. Prazosin caused significant inhibition of the methoxamine-induced bronchoconstriction in six of the seven patients. These findings suggest that methoxamine produces bronchoconstriction in asthmatic subjects via stimulation of alpha-adrenoceptors. In previous studies propylene glycol has been used as a vehicle for delivery of prazosin. This substance was found to cause significant inhibition of methoxamine effects and to shift the dose response curve to histamine to the right in four of seven patients.

Comparison of the effects of single doses of atenolol and labetalol on airways obstruction in patients with hypertension and asthma

1 The effects of atenolol (100 mg), a cardioselective beta-adrenoceptor blocking agent, and labetalol (300 mg), a combined alpha- and non-selective beta-adrenoceptor blocking agent given in single doses were examined in a double-blind placebo controlled study in 11 asthmatic patients with hypertension. Changes in forced expiratory volume in one second (FEV1) were measured. 2 There were no significant differences between the effects of the active drugs, approximately half the patients having higher FEV1 values during each of the two active treatments. When compared with placebo, labetalol significantly reduced the effect of inhaled salbutamol on FEV1. 3 Whilst beta-adrenoceptor blockings drugs cannot be recommended in patients with airways obstruction, should they inadvertantly be given, labetalol like atenolol may be considered relatively safer then pure non-selective beta-adrenoceptor blocking drugs.

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.