James W. Paterson

Influence of the epithelium on responsiveness of guinea‐pig isolated trachea to contractile and relaxant agonists

1 The potency (pD2) and maximal contractile effect (Emax) of histamine, acetylcholine, carbachol and K+ were assessed from cumulative concentration‐effect curves in guinea‐pig isolated tracheal ring preparations with and without an intact epithelium. 2 Estimates of Emax were not significantly different in epithelium‐denuded preparations compared with those measured in intact preparations; pD2 values for acetylcholine, carbachol and K+ were not significantly altered. In contrast, the potency of histamine was significantly increased by about 4 fold in preparations devoid of epithelial cells. 3 Estimates of potency and Emax were also determined for the smooth muscle relaxants isoprenaline, forskolin and theophylline (which increase intracellular cyclic AMP) and for nitroglycerin (which increases cyclic GMP) in both intact and epithelium‐stripped tracheal rings. The pD2 values for these relaxants were not significantly altered by the removal of the epithelium. However, with the exception of nitroglycerin, Emax values for these relaxants were significantly lower in stripped than in intact tracheal rings that had been maximally precontracted with carbachol. 4 The autoradiographic localisation of binding sites for the non‐selective β‐adrenoceptor ligand [125I]‐iodocyanopindolol (I‐CYP) showed that the epithelium of the guinea‐pig trachea had a 75 ± 16% greater density of β‐adrenoceptors than the smooth muscle. Removing the epithelium did not significantly alter either the density of smooth muscle binding sites or the affinity of I‐CYP binding. It was concluded that the reduced functional response of guinea‐pig trachea to isoprenaline was probably not due to smooth muscle β‐adrenoceptor dysfunction. 5 Results indicate that the epithelium plays an important role in the modulation of responsiveness of guinea‐pig trachea to histamine and relaxants that mediate their effects by selectively increasing intracellular cyclic AMP levels.

Effect of steroids on β‐adrenoceptor‐mediated relaxation of pig bronchus

1 Progesterone, testosterone (40 μm), cortisol and cortisol hemisuccinate (80 μm) caused 6–8 fold potentiations of (±)‐isoprenaline (Iso)‐induced relaxations of pig bronchus while several other steroids caused smaller potentiations or had no effect. 2 17β‐Oestradiol (40 μm) increased the potency of Iso, (−)‐adrenaline (Adr) and (−)‐noradrenaline (NA) by 10.6, 2.3 and 2.6 fold respectively but had no significant effect on the potency of fenoterol (Fen). 3 Inhibition of catechol‐O‐methyl transferase (COMT) with U‐0521 (30 μm) caused a 6 fold increase in the potency of Iso but failed to alter the potency of Adr, NA or Fen. The extraneuronal uptake inhibitor normetanephrine (50 μm) caused significant 2 fold increases in the potency of Iso and Adr but did not potentiate the responses to NA or Fen. 4 In preparations where the potency of Iso had already been increased by U‐0521 (30 μm) or by normetanephrine, 17β‐oestradiol produced no significant further increase in potency. These results indicate that steroid‐induced increases in the potency of catecholamines in pig bronchus can be explained in terms of inhibition of COMT or extraneuronal uptake or both.

Co‐axial bioassay of a smooth muscle relaxant factor released from guinea‐pig tracheal epithelium

1 The ability of guinea‐pig trachea to release an epithelium‐derived relaxant factor (EpDRF) was assessed in a co‐axial bioassay system. 2 Histamine (100 μm) and methacholine (25 μm) caused endothelium‐dependent relaxation of rat isolated aorta, presumably via the release of endothelium‐derived relaxant factor (EDRF). In contrast, endothelium‐denuded rat aorta did not relax in response to these agents. 3 EDRF release was detected in response to methacholine in a co‐axial bioassay system, consisting of intact rabbit aorta tube (EDRF donor) and endothelium‐denuded rat aorta strip (assay preparation). These results indicated the transfer of EDRF from a donor to an assay preparation, thereby validating the co‐axial bioassay method. 4 Substitution of endothelium‐intact rabbit aorta tube by epithelium‐intact guinea‐pig tracheal tube tissue in co‐axial assemblies, still allowed the assay preparation to relax in response to histamine or methacholine. Removal of the intact tracheal tube from the system, or removal of the epithelium from the donor tracheal tube in co‐axial preparations, abolished such relaxant responses. These observations are consistent with histamine‐ or methacholine‐induced release of an epithelium‐derived relaxant factor (EpDRF) from the trachea. 5 In the co‐axial assembly comprising intact guinea‐pig trachea and endothelium‐denuded rat aorta, histamine and methacholine produced concentration‐dependent, EpDRF‐induced aortic relaxation. Mean concentrations of histamine and methacholine producing 50% of the maximum relaxation (EC50) were 39.8 μm and 2.7 μm respectively. Histamine‐induced relaxation was inhibited in the presence of mepyramine (2 μm) and responses to methacholine were inhibited by atropine (0.1 μm). 6 Methylene blue (50 μm) had no effect on such relaxant responses, indicating that EpDRF does not activate guanylate cyclase. Furthermore, the cyclo‐oxygenase inhibitor indomethacin (5 μm), the cyclo‐oxygenase/lipoxygenase inhibitor BW 755C (150 μm) and the leukotriene receptor antagonist FPL 55712 (10 μm) each failed significantly to alter EpDRF‐mediated relaxation of vascular smooth muscle suggesting that EpDRF is not a prostanoid. Platelet activating factor (Paf) failed to cause relaxation of endothelium‐denuded rat aorta, indicating that this mediator was also not EpDRF. 7 EpDRF was also released from human bronchial segments. 8 This study provides direct evidence for the release of an EpDRF from non‐diseased airway tissue and further suggests that healthy airway reactivity to spasmogens is modulated by the release of an endogenous protective, spasmolytic substance. The bronchial hyperreactivity of asthma may be partly caused by attenuated production of such an inhibitory signal.

PHARMACOLOGICAL RESPONSES OF HUMAN AND PORCINE LUNG PARENCHYMA, BRONCHUS AND PULMONARY ARTERY

1 Responses of preparations of human and porcine isolated bronchus and pulmonary artery to carbachol (CCh), methacholine, histamine, 5‐hydroxytryptamine (5‐HT), (−)‐noradrenaline (NA), (−)adrenaline (Adr) and (±)‐isoprenaline (Iso) were compared with responses to the same agonists in isolated lung parenchyma strips. 2 All preparations from both human and porcine lung contracted in response to histamine and all, except preparations of porcine pulmonary artery, contracted in response to CCh. Human and porcine pulmonary artery and parenchyma strip contracted in response to NA while bronchial preparations invariably relaxed. Iso caused relaxation of human and porcine bronchus and parenchyma strip. Although 5‐HT was completely inactive in tissues isolated from pig lung, this amine was a powerful spasmogen in human pulmonary artery, relaxed human bronchus and caused variable responses in human parenchyma. 3 Results indicate that the pharmacological characteristics of human and porcine parenchyma strips may be explained in terms of responses of vascular or airways smooth muscle.

Clinical assessment of bronchodilator drugs delivered by aerosol.

A method is described for the early clinical evaluation of bronchodilator aerosols. It is simple to perform, and an accurate estimate of potency can be obtained in a small number of patients. Using this method, three new sympathomimetic drugs, Th 1165a, salmefamol, and rimiterol, have been tested. All proved to be active bronchodilators and were equipotent with isoprenaline or salbutamol.

Classification of β‐adrenoceptors in human isolated bronchus

1 (±)‐Isoprenaline (Iso), (−)−adrenaline (Ad), (−)−noradrenaline (NA), (±)‐phenylephrine (Phe) and the β2‐selective adrenoceptor agonist (±)‐fenoterol (Fen) caused a concentration‐dependent relaxation of human isolated bronchial preparations. Iso, Ad and NA caused complete relaxation of both spontaneous and carbachol‐induced bronchial tone. Fen, which was only tested in preparations where tone was induced with carbachol, also caused complete relaxation. However, Phe was a partial agonist in all preparations tested. 2 When relaxation responses to these amines were calculated as a % of their maximal effects, comparison of EC50 values showed that the order of potency was Iso > Ad = Fen > NA > Phe (92:27:25:1:0.2) in preparations with carbachol‐induced tone and Iso > Ad > NA > Phe (112:38:1:0.3) in preparations with spontaneous tone. 3 pA2 values determined for the β‐adrenoceptor antagonists propranolol (non‐selective), atenolol (β‐selective) and ICI‐118, 551 (β2‐selective), using Iso as an agonist were, 9.3, 5.3 and 9.1 respectively. 4 These results indicate that β2‐adrenoceptors mediate relaxation of human isolated bronchus to sympathomimetic amines in preparations obtained 4–14 h post‐mortem from non‐diseased lung. α‐Adrenoceptors were apparently sparse or absent in this tissue.

Correlations between pharmacological responses and structure of human lung parenchyma strips

1 Correlations were sought between responses of human lung parenchyma strip to 5‐hydroxytryptamine (5‐HT) and (‐)‐noradrenaline (NA) and the proportions of the three major, potentially contractile components within the strip, namely smooth muscle in airways proximal to alveolar ducts, vascular smooth muscle and contractile cells in alveolar septa. 2 After the isometric measurement of responses to 5‐HT or to NA, lung strips were processed for stereological examination at the light microscopic level. On average, approximately 46% of the total volume of the lung strip was tissue and the remainder was air space. Tissue contained blood vessels (16.8%), airways proximal to alveolar ducts (4.8%) and alveolar parenchyma (78.4%). 3 Human lung parenchyma strips relaxed, contracted or failed to respond to 5‐HT or NA. Results indicated that these agonists caused simultaneous contraction of blood vessels and relaxation of airways proximal to alveolar ducts. The size and type of responses to 5‐HT or NA was significantly correlated with the ratio of the volume of blood vessels and larger airways. 4 Conversely, the proportion of alveolar tissue in lung strips was not significantly correlated with responses to 5‐HT or NA.

Adverse Reactions to β 2 -Agonist Bronchodilators

Summaryβ2-Agonists are safe and effective bronchodilator drugs. Their major adverse effects of skeletal muscle tremor, tachycardia and various metabolic effects are mediated by β-adrenoceptor stimulation and are reversible. Skeletal muscle tremor is the most frequent dose-limiting side effect. It may be reduced by commencing treatment with a low dose and if it persists another β2-agonist may be tried. Other side effects such as cardiac arrhythmias and reduction in PaO2 are a serious potential problem in some susceptible asthmatics. However, they are infrequent or of a mild degree and are generally outweighed by the good control of asthma produced by β2-agonists.Side effects from β2-agonist therapy can be minimised by use of the inhaled route which selectively delivers the drug to the airways. Furthermore, selective tolerance develops to their side effects. The dose of a β2-agonist should be assessed on the basis of therapeutic effect and the level of tolerance to its side effects. Recommended doses of β2-agonists used for long term therapy do not cause clinically significant desensitisation of airway β-adrenoceptors, although this may become a relevant problem in patients who are regularly receiving very high doses.Intravenous β2-agonists have a place in the treatment of severe asthma not responding to nebuliser therapy. In this life-threatening situation with severe airflow obstruction, monitoring of heart rate, PaO2, plasma potassium and the electrocardiogram should be mandatory and supplemental oxygen given so that serious adverse effects are prevented.

α1‐Adrenoceptor function and autoradiographic distribution in human asthmatic lung

1 The autoradiographic distribution of α1‐adrenoceptors was investigated in non‐diseased and asthmatic human lung by use of [3H]‐prazosin (H‐PZ). To validate binding and autoradiographic methods, H‐PZ binding was also measured in rat heart. 2 Significant levels of specific H‐PZ binding were detected in sections of rat heart. This binding was associated with a single class of non‐interacting sites of high affinity (dissociation constant, Kd = 1.17 ± 0.26 nm). The maximum binding capacity (Bmax) was 59.5 ± 4.5 fmol mg−1 protein. 3 In sharp contrast, very low levels of specific H‐PZ binding were found in both human non‐diseased and asthmatic bronchus, although a high level of binding of [125I]‐iodocyanopindolol (I‐CYP, 50 pm) to β‐adrenoceptors was detected in these airways. Furthermore, very low levels of autoradiographic grains representing specific H‐PZ binding were found in all airway structures in human non‐diseased or asthmatic lung parenchyma. 4 Consistent with these data, the α‐adrenoceptor agonist phenylephrine failed to induce significant increases in tone in bronchi isolated from either non‐diseased or asthmatic human lung. Results indicate that asthma does not involve significant increases in airway α1‐adrenoceptor function.

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.