Mechanisms of histamine-induced contraction of canine airway smooth muscle

Airway smooth muscle function in asthma

Known to have affected around 340 million people across the world in 2018, asthma is a prevalent chronic inflammatory disease of the airways. The symptoms such as wheezing, dyspnea, chest tightness, and cough reflect episodes of reversible airway obstruction. Asthma is a heterogeneous disease that varies in clinical presentation, severity, and pathobiology, but consistently features airway hyperresponsiveness (AHR)—excessive airway narrowing due to an exaggerated response of the airways to various stimuli. Airway smooth muscle (ASM) is the major effector of exaggerated airway narrowing and AHR and many factors may contribute to its altered function in asthma. These include genetic predispositions, early life exposure to viruses, pollutants and allergens that lead to chronic exposure to inflammatory cells and mediators, altered innervation, airway structural cell remodeling, and airway mechanical stress. Early studies aiming to address the dysfunctional nature of ASM in the etiology and pathogenesis of asthma have been inconclusive due to the methodological limitations in assessing the intrapulmonary airways, the site of asthma. The study of the trachealis, although convenient, has been misleading as it has shown no alterations in asthma and it is not as exposed to inflammatory cells as intrapulmonary ASM. Furthermore, the cartilage rings offer protection against stress and strain of repeated contractions. More recent strategies that allow for the isolation of viable intrapulmonary ASM tissue reveal significant mechanical differences between asthmatic and non-asthmatic tissues. This review will thus summarize the latest techniques used to study ASM mechanics within its environment and in isolation, identify the potential causes of the discrepancy between the ASM of the extra- and intrapulmonary airways, and address future directions that may lead to an improved understanding of ASM hypercontractility in asthma.

Natural Antispasmodics: Source, Stereochemical Configuration, and Biological Activity

Natural products with antispasmodic activity have been used in traditional medicine to alleviate different illnesses since the remote past. We searched the literature and compiled the antispasmodic activity of 248 natural compounds isolated from terrestrial plants. In this review, we summarized all the natural products reported with antispasmodic activity until the end of 2017. We also provided chemical information about their extraction as well as the model used to test their activities. Results showed that members of the Lamiaceae and Asteraceae families had the highest number of isolated compounds with antispasmodic activity. Moreover, monoterpenoids, flavonoids, triterpenes, and alkaloids were the chemical groups with the highest number of antispasmodic compounds. Lastly, a structural comparison of natural versus synthetic compounds was discussed.

Pharmacological characterisation of the interaction between glycopyrronium bromide and indacaterol fumarate in human isolated bronchi, small airways and bronchial epithelial cells

BACKGROUND

Nowadays, there is a considerable gap in knowledge concerning the mechanism(s) by which long-acting β2-agonists (LABAs) and long-acting muscarinic antagonists (LAMAs) interact to induce bronchodilation. This study aimed to characterise the pharmacological interaction between glycopyrronium bromide and indacaterol fumarate and to identify the mechanism(s) leading to the bronchorelaxant effect of this interaction.

METHODS

The effects of glycopyrronium plus indacaterol on the contractile tone of medium and small human isolated bronchi were evaluated, and acetylcholine and cAMP concentrations were quantified. The interaction was assessed by Bliss Independence approach.

RESULTS

Glycopyrronium plus indacaterol synergistically inhibited the bronchial tone (medium bronchi, +32.51 % ± 7.86 %; small bronchi, +28.46 % ± 5.35 %; P < 0.05 vs. additive effect). The maximal effect was reached 140 min post-administration. A significant (P < 0.05) synergistic effect was observed during 9 h post-administration on the cholinergic tone, but not on the histaminergic contractility. Co-administration of glycopyrronium and indacaterol reduced the release of acetylcholine from the epithelium but not from bronchi, and enhanced cAMP levels in bronchi and epithelial cells (P < 0.05 vs. control), an effect that was inhibited by the selective KCa(++) channel blocker iberiotoxin. The role of cAMP-dependent pathway was confirmed by the synergistic effect elicited by the adenylate cyclase activator forskolin on glycopyrronium (P < 0.05 vs. additive effect), but not on indacaterol (P > 0.05 vs. additive effect), with regard of the bronchial relaxant response and cAMP increase.

CONCLUSIONS

Glycopyrronium/indacaterol co-administration leads to a synergistic improvement of bronchodilation by increasing cAMP concentrations in both airway smooth muscle and bronchial epithelium, and by decreasing acetylcholine release from the epithelium.

Mitochondrial N-formyl peptides cause airway contraction and lung neutrophil infiltration via formyl peptide receptor activation

Respiratory failure is a common characteristic of systemic inflammatory response syndrome (SIRS) and sepsis. Trauma and severe blood loss cause the release of endogenous molecules known as damage-associated molecular patterns (DAMPs). Mitochondrial N-formyl peptides (F-MITs) are DAMPs that share similarities with bacterial N-formylated peptides, and are potent immune system activators. Recently, we observed that hemorrhagic shock-induced increases in plasma levels of F-MITs associated with lung damage, and that antagonism of formyl peptide receptors (FPR) ameliorated hemorrhagic shock-induced lung injury in rats. Corroborating these data, in the present study, it was observed that F-MITs expression is higher in plasma samples from trauma patients with SIRS or sepsis when compared to control trauma group. Therefore, to better understand the role of F-MITs in the regulation of lung and airway function, we studied the hypothesis that F-MITs lead to airway contraction and lung inflammation. We observed that F-MITs induced concentration-dependent contraction in trachea, bronchi and bronchioles. However, pre-treatment with mast cells degranulator or FPR antagonist decreased this response. Finally, intratracheal challenge with F-MITs increased neutrophil elastase expression in lung and inducible nitric oxide synthase and cell division control protein 42 expression in all airway segments. These data suggest that F-MITs could be a putative target to treat respiratory failure in trauma patients.

Effects of lung inflation on airway heterogeneity during histaminergic bronchoconstriction

Lung inflation has been shown to dilate airways by altering the mechanical equilibrium between opposing airway and parenchymal forces. However, it is not known how heterogeneously such dilation occurs throughout the airway tree. In six anesthetized dogs, we measured the diameters of five to six central airway segments using high-resolution computed tomography, along with respiratory input impedance (Zrs) during generalized aerosol histamine challenge, and local histamine challenge in which the agonist was instilled directly onto the epithelia of the imaged central airways. Airway diameters and Zrs were measured at 12 and 25 cmH2O. The Zrs spectra were fitted with a model that incorporated continuous distributions of airway resistances. Airway heterogeneity was quantified using the coefficient of variation for predefined airway distribution functions. Significant reductions in average central airway diameter were observed at 12 cmH2O for both aerosolized and local challenges, along with significant increases upon inflation to 25 cmH2O. No significant differences were observed for the coefficient of variation of airway diameters under any condition. Significant increases in effective airway resistance as measured by Zrs were observed only for the aerosolized challenge at 12 cmH2O, which was completely reversed upon inflation. We conclude that the lung periphery may be the most dominant contributor to increases in airway resistance and tissue elastance during bronchoconstriction induced by aerosolized histamine. However, isolated constriction of only a few central airway segments may also affect tissue stiffness via interdependence with their surrounding parenchyma.

A 'Good' muscle in a 'Bad' environment: the importance of airway smooth muscle force adaptation to airway hyperresponsiveness

Asthma is characterized by airway inflammation, with a consequent increase in spasmogens, and exaggerated airway narrowing in response to stimuli, termed airway hyperresponsiveness (AHR). The nature of any relationship between inflammation and AHR is less clear. Recent ex vivo data has suggested a novel mechanism by which inflammation may lead to AHR, in which increased basal ASM-tone, due to the presence of spasmogens in the airways, may "strengthen" the ASM and ultimately lead to exaggerated airway narrowing. This phenomenon was termed "force adaptation" [Bossé, Y., Chin, L.Y., Paré, P.D., Seow, C.Y., 2009. Adaptation of airway smooth muscle to basal tone: relevance to airway hyperresponsiveness. Am. J. Respir. Cell Mol. Biol. 40, 13-18]. However, it is unknown whether the magnitude of the effect of force adaptation ex vivo could contribute to exaggerated airway narrowing in vivo. Our aim was to utilize a computational model of ASM shortening in order to quantify the potential effect of force adaptation on airway narrowing when all other mechanical factors were kept constant. The shortening in the model is dictated by a balance between physiological loads and ASM force-generating capacity at different lengths. The results suggest that the magnitude of the effect of force adaptation on ASM shortening would lead to substantially more airway narrowing during bronchial challenge at any given airway generation. We speculate that the increased basal ASM-tone in asthma, due to the presence of inflammation-derived spasmogens, produces an increase in the force-generating capacity of ASM, predisposing to AHR during subsequent challenge.

Histamine-evoked acetylcholine release in sensitized tracheal preparation

The contractile response to histamine of tracheal muscle was studied in preparations from BSA-sensitized and non-sensitized guinea-pigs. Sensitization did not enhance the overall response to histamine. However, this response showed evidence of acetylcholine participation. In sensitized preparations, atropine (0.1 microM) caused a significant depression of the dose response to histamine (n = 11, p = 0.028), especially in the range 2-8 microM. Physostigmine (0.1 microM) significantly potentiated the effect of histamine (n = 8, p = 0.003), especially at greater than 4 microM histamine. The response to histamine of non-sensitized preparations was not altered by atropine (n = 11) or physostigmine (n = 8). The following agents did not discriminate between sensitized and non-sensitized preparations: Famotidine, an H2 antagonist; dimaprit, an H2 agonist; thioperamide, an H3 antagonist; alpha-methylhistamine, an H3 agonist; gallamine, an M2 antagonist, suggesting that muscarinic M2 receptor dysfunction alone is not sufficient to cause bronchial hyper-responsiveness. The results show that sensitization causes a change in the components of the contractile response to histamine rather than bronchial hyper-responsiveness to this agent.

Mechanisms of 5-hydroxytryptamine-induced contraction of isolated rat intrapulmonary bronchi

Previous studies in our laboratory and others suggested that activation of 5-HT2 receptors mediates 5-hydroxytryptamine (5-HT)-induced contraction of airway smooth muscle and that this response is dependent in part on endogenous acetylcholine (ACh). The purpose of the present study was to confirm a role for 5-HT2 receptors and endogenous ACh in 5-HT-induced contraction of rat bronchi. In this study, we examined the effects of 5-HT2 receptor antagonists (ketanserin and LY53857), acetylcholinesterase inhibitors (physostigmine and neostigmine), and a muscarinic receptor alkylating agent [propylbenzilylcholine mustard (PBCM)] on contractile responses evoked by 5-HT and the 5-HT2 receptor agonist, alpha-methyl-5-hydroxytryptamine (alpha-Me-5-HT). Concentration-response curves generated in isolated rat intrapulmonary bronchi in response to 5-HT and alpha-Me-5-HT were superimposable. Inhibition of acetylcholinesterase by physostigmine or neostigmine potentiated contractile responses elicited by 5-HT and alpha-Me-5-HT. Alkylation of muscarinic receptors with PBCM decreased maximal responses elicited by 5-HT or alpha-Me-5-HT in a concentration-dependent manner. Maximum contraction attained with exogenous ACh was decreased by PBCM in a concentration-dependent manner and, at the highest concentration evaluated, ACh-induced contractions were abolished. 5-Hydroxytryptamine-induced contraction was inhibited competitively by low concentrations of the 5-HT2-receptor selective antagonist, ketanserin; higher concentrations abolished contractile responses to the amine. The inhibition of 5-HT-induced contractile responses by another 5-HT2-receptor selective antagonist, LY53857, was non-competitive in nature. Together, the results suggest that 5-HT contracts rat airways directly by activating 5-HT2 receptors located on airway smooth muscle and indirectly by activation of 5-HT2 receptors on parasympathetic nerve endings to cause release of ACh. The potential physiological implication of these findings is that 5-HT released in inflammatory conditions such as asthma may play a role in causing bronchoconstriction by releasing ACh or by augmenting release of ACh from activated cholinergic nerves.

Evidence for the presence of function of the inhibitory M2 receptors in the rabbit airways and lungs

We investigated to determine whether or not the inhibitory M2-receptors function in the rabbit lung and heart. Rabbits were anesthetized, vagotomized, paralyzed and ventilated. Administration of gallamine, an M2-receptor antagonist, augmented an increase of PT produced by vagal stimulation with or without simultaneous administration of histamine and the increases were dose-dependent. Conversely, prior treatment with pilocarpine, an M2-receptor agonist, reduced these responses in a dose-dependent manner. The PT responses to histamine injection only were not significantly altered by administration of either gallamine or pilocarpine. The remaining bronchoconstrictor responses to the three stimuli in the presence of gallamine or pilocarpine were completely blocked by atropine. In another series of experiments, gallamine treatment enhanced bronchoconstriction evoked by vagal stimulation but reduced acetylcholine (ACh)-induced bronchoconstriction. These opposite responses were dose-dependent for gallamine. The results suggest that there are inhibitory M2-receptors in the parasympathetic nerves innervating the lungs in the rabbit. Furthermore, gallamine treatment that completely blocked bradycardia evoked by ACh administration reduced vagally-mediated bradycardia. This implies that gallamine appears to have an antagonistic action on muscarinic receptors in the rabbit heart.

Time course of histamine-induced bronchoconstriction and its adrenergic and H2 modulation

We characterized the complete time course of histamine-induced bronchoconstriction and its modulation via the release of endogenous catecholamines and by its actions on H2-receptors in anesthetized, tracheostomized, paralyzed, and artificially ventilated mongrel dogs. Respiratory resistance (R) and elastance (E) were estimated continuously with a recursive least squares estimator. Three protocols were followed in which multiple histamine bolus injections were given 1 h apart. We found that the time courses of E and R had consistent patterns (transient peak that returned to baseline within 1000 sec) even in cases of low mean arterial pressure (MAP). Indomethacin pre-treatments prevented tachyphylaxis to repeated i.v. challenges. beta-blockade produced a mild increase in baseline and a potentiation of the histamine-induced response in E and these effects were not altered with further alpha-or H2-blockade. Blockade of alpha-receptors increased the time to recovery in both E and MAP presumably by decreasing blood flow. Finally, we suggest that preventing the H2-receptor induced increase in bronchial blood flow may have increased the time to maximal E without affecting the recovery time.

Role of histamine in allergen-induced asthmatic reactions, bronchial hyperreactivity and inflammation in unrestrained guinea pigs

In a new model using conscious, unrestrained and ovalbumin-sensitized guinea pigs, we investigated the effects of the selective histamine H1 receptor antagonist, mepyramine, on the development of allergen-induced early and late asthmatic reactions, bronchial hyperreactivity and airway inflammation, having each animal as its own control. In guinea pigs responding to a first allergen exposure with an early as well as a late asthmatic reaction (82% of the animals) a second, identical, allergen provocation was performed, in the absence (control) or presence of 1 mg/ml mepyramine aerosol, inhaled for 10 min, 1 h before provocation. The mepyramine treatment significantly reduced both early and late asthmatic reactions and prevented the development of bronchial hyperreactivity to histamine and methacholine after both reactions. Examination of the bronchoalveolar lavage fluid 24 h after the second allergen provocation revealed a general reduction of inflammatory cells after mepyramine treatment. The results indicate that histamine, released during the early asthmatic reaction, contributes to the development of the late asthmatic reaction as well as of early and late bronchial hyperreactivity, possibly via an effect on airway inflammation.

Effect of atrial natriuretic peptide on bronchial tone in anesthetized rabbits

The effect of atrial natriuretic peptide (ANP) on histamine-induced bronchoconstriction was studied in vivo (in normoxic and in hypoxic rabbits) and in vitro. Thirty-two anesthetized rabbits, spontaneously breathing room air or 10% O2, received infusions of ANP (20, 40, or 80 ng/min/kg normoxia; 20 ng/min/kg hypoxia) or the vehicle for 100 min. After 75 min of ANP infusion, bronchoconstriction was induced inhaling histamine; respiratory resistance (Rrs) was measured prior to and until 20 min posthistamine. The results show that the histamine-induced increase in Rrs was significantly reduced by ANP 80 ng/kg/min in normoxia, and by ANP 20 ng/kg/min in hypoxia. In vitro, ANP had no effect on tracheal and bronchial smooth muscle precontracted with histamine or acetylcholine. These results show that ANP can decrease a histamine-induced bronchoconstriction in vivo but not in vitro, suggesting an indirect mechanism of action.

Muscarinic effect of atrial natriuretic peptide on rabbit airways

1. The aim of the present work was to investigate under which circumstances atrial natriuretic peptide (ANP) modulates airway resistance. 2. Of the six groups of rabbits (n = 5) studied, three received an infusion of ANP (80 ng min-1 kg-1 i.v.) for a period of 100 min, while the other three were infused with the vehicle. Before receiving the infusion of ANP or the vehicle, the animals were pretreated with atropine (0.5 mg kg-1 i.v.), propranolol (2 mg kg-1 i.v.) or not pretreated. After 75 min of infusion of ANP, bronchoconstriction was induced by inhalation of histamine. Respiratory resistance (Rrs) was measured before and 3, 5, 10, 15 and 20 min post-histamine challenge. 3. Following 75 min of ANP infusion, plasma ANP concentration increased from 153 +/- 52 (mean +/- s.e.mean) to 1441 +/- 203 pg ml-1 (P < 0.05) without affecting baseline Rrs. Control Rrs values (12.5-20.4 cmH2O l-1 s) were significantly increased following the inhalation of histamine (P < 0.001). By themselves, atropine, propranolol or ANP did not modify the histamine-induced increase in Rrs. However, when the animals were pretreated with atropine, ANP infusion significantly reduced the increase in Rrs induced by histamine (30 +/- 2 vs 51 +/- 6 cmH2O l-1 s; P < 0.05). 4. These data suggest that ANP has an indirect modulating effect on the airway smooth muscle and will decrease Rrs when muscarinic receptors are blocked.

Effects of calcium channel and H1-receptor blockers on the responses of slowly adapting pulmonary stretch receptors to histamine in vagotomized rabbits

We studied the effects of calcium channel antagonists (verapamil and nifedipine) and H1-receptor blockers (mequitazine) on changes in the slowly adapting pulmonary stretch receptors (SARs) located below the carina in response to right atrial injections of histamine (60 and 80 micrograms/kg) in anesthetized artificially ventilated rabbits with bilateral vagotomy. After histamine was injected into the right atrium, the SARs became more active during expiration but decreased their activity during inspiration. These changes were more pronounced by increasing the dosage of histamine. However, administration of histamine had no significant effect on tracheal pressure (PT). Verapamil treatment (1 mg/kg) did not alter the SAR response to histamine, whereas the responses of SARs to histamine at different dosages were significantly diminished by treatment with nifedipine (1 mg/kg). Mequitazine (1 mg/kg), a potent H1-receptor blocker, blocked completely all the responses of SAR activity to histamine. These results suggest that the effect of histamine 60-80 micrograms/kg on SAR activity is mediated by the activation of H1-receptors of the peripheral airway smooth muscle and that this activation, at least in part, involves the opening of calcium channels of the airway smooth muscle.

Cholinergic and H1-receptor influences of histamine on slowly adapting pulmonary stretch receptor activity in the rabbit

Afferent impulses of slowly adapting pulmonary stretch receptors (SARs) were obtained by dissecting fine slips from the left vagus nerve (LVN) and by leaving the rest of the nerve intact. In the same SAR preparation, changes of the receptor activity in response to right atrial injections of histamine (10 and 60 micrograms/kg) were successively examined before and after atropine (1 mg/kg), partial vagal efferent ablation, and mequitazine (1 mg/kg) in 10 rabbits. Administration of histamine led to an increase in the SAR activity, and this effect became more pronounced by increasing the dose of histamine. Atropine treatment diminished the responses of SARs to histamine at different doses. Partial vagal efferent ablation produced by denervation of the rest of the intact LVN slightly reduced the response of SARs to histamine at 10 micrograms/kg but had no significant effect on the SAR response to 60 micrograms/kg histamine. In the absence of vagal afferent and efferent activities on the left side, mequitazine, a potent H1-receptor blocker, completely blocked low- and high-dose effects of histamine on SARs. We compared the responses of the receptor activity to aerosol histamine (1 and 4%) and to topical application of histamine (0.1 ml, 0.025% and 0.1%) in six SAR preparations. The magnitude and duration of increased SAR activity became more prominent by increasing the concentration of histamine. The firing pattern and discharge rate of SARs following aerosol or intratracheal administration of histamine were similar to those after intra-atrial histamine. In addition, we also examined the excitatory responses of SAR activity to right atrial injections of histamine at 10 and 60 micrograms/kg before and after topical administration of atropine (0.1 ml, 1%, n = 6) or mequitazine (0.1 ml, 1%, n = 6) in 12 SAR preparations. Intratracheal atropine diminished the response of SARs to 10 micrograms/kg of histamine but had no significant effect on the response of SARs to histamine at 60 micrograms/kg. All the responses of SARs to histamine were completely blocked by topical application of mequitazine. These results suggest that the change of SAR activity produced by histamine at 10 micrograms/kg occurs mainly as a result of the release of acethylcholine (ACh) via the vagovagal reflex and that the activation of H1-receptors of the airway smooth muscle contributes importantly to the response of SARs to histamine at 60 micrograms/kg.

The excitatory effect of dopamine on isolated canine tracheal smooth muscle

The effect of exogenous dopamine on canine tracheal smooth muscle has been studied in-vitro. Dopamine at concentrations over 10(-5)M induced contractions of tracheal muscle strips and repeated exposures resulted in desensitization (tachyphylaxis) of the muscle. The sensitivity of the response varied dramatically among muscle strips. At lower concentrations, dopamine caused neither muscle relaxation nor inhibition of contractions evoked by 10(-6)M acetylcholine. Both a dopaminergic antagonist, haloperidol (10(-5) and 10(-4)M), and an alpha-adrenoceptor antagonist, phentolamine (10(-7) to 10(-5)M), attenuated the contraction to 10(-3)M dopamine. The beta-adrenoceptor antagonist, propranolol (10(-8) to 10(-6)M), enhanced the contraction. However, the contraction could only be abolished by phentolamine at 10(-4) M. Thus, in canine tracheal smooth muscle, the contractile response to dopamine is predominantly through the activity of alpha-adrenoceptors and the role of dopaminergic receptors is vague. It is suggested that the weakness of the dopamine-induced contraction results from an antagonism between alpha- and beta-adrenoceptor effects and the dopamine tachyphylaxis may reflect a gradually decreased activation of the alpha-adrenoceptor mechanism in comparison with the beta-adrenoceptor mechanism.

The effect of N-formyl-methionyl-leucyl-phenyl-alanine on cholinergic neurotransmission and its modulation by enkephalinase in rabbit airway smooth muscle

N-formyl-methionyl-leucyl-phenylalanine (FMLP), a synthetic analogue of bacterial chemotactic peptide, may play a role in airway hyperresponsiveness, and is cleaved by neutral endopeptidase-24.11 (enkephalinase). To determine the effect of FMLP on parasympathetic contraction of airway smooth muscle and its modulation by endogenous enkephalinase, we studied isolated rabbit tracheal ring segments under isometric conditions in vitro. FMLP did not cause muscle contraction, but it potentiated the contractile response to electrical field stimulation (EFS) in a dose-dependent fashion, with the maximal increase from the baseline response being 59.8 +/- 6.2% (mean +/- S.E.M., P less than 0.001), an effect that was abolished by t-Boc-Phe-Leu-Phe-Leu-Phe, partially inhibited by pyrilamine, but not by phentolamine or [D-Pro2,D-Trp7,9]substance P. In contrast, the contractile response to administered acetylcholine was not affected by FMLP. Pretreatment of tissues with thiorphan, an enkephalinase inhibitor, further potentiated the effect of FMLP on the EFS-induced contraction. These results suggest that FMLP facilitates cholinergic neurotransmission in rabbit airway smooth muscle probably by increasing acetylcholine release, and that this effect may be modulated by enkephalinase in the airway.

Antigen- and histamine H1 receptor-mediated relaxation of guinea pig isolated trachea

We have investigated the effect of challenge in vitro with specific antigen (ovalbumin) on actively sensitized guinea pig tracheal rings maximally precontracted with methacholine. Ovalbumin relaxed the trachea in a concentration-dependent fashion with a negative log ED50 value (g/ml) of 7.0 +/- 0.3. In 16 experiments, the maximum antigen-induced relaxation was 26 +/- 3% of complete relaxation induced by 10(-3) M papaverine (mean +/- S.E.M.). Antigen-induced relaxations were selectively antagonized by diphenhydramine. Similarly, histamine relaxed the precontracted tracheal smooth muscle with a negative log molar ED50 of about 4.5 and a maximum effect of 28 +/- 3% (mean +/- S.E.M., n = 20). Histamine-induced relaxations were antagonized by diphenhydramine and mepyramine but were unaffected by cimetidine, metiamide or burimamide. Dimaprit (10(-5)-10(-3) M) did not relax the precontracted trachea. Indomethacin significantly inhibited relaxation induced by both antigen and histamine. In contrast, phenidone or 5,8,11,14-eicosa-tetraynoic acid had no effect on relaxation but reversed the inhibition by indomethacin. Neither propranolol (10(-6) M) nor removing the tracheal epithelium inhibited histamine-induced relaxation. These results suggest that antigen-induced relaxation of guinea pig tracheal smooth muscle involves activation of histamine H1 receptors and can occur independently of arachidonic acid metabolism, beta-adrenoceptor activation or airway epithelium.

Combined bronchodilator protection against histamine-induced bronchoconstriction in man

Sixteen stable asthmatics had the protective effects of inhaled fenoterol (200 micrograms) and inhaled ipratropium bromide (60 micrograms) against standardized histamine inhalation tests at 1 h examined in a randomized double blind fashion. There was no significant difference in the baseline forced expired volume in 1 s (FEV1) for the two study days (P greater than 0.05). There was an increase in FEV1 at 1 h on the fenoterol and ipratropium day compared with the fenoterol day (0.26 versus 0.17 l; P less than 0.05). The geometric mean provocative concentration of histamine to cause a 20% fall in FEV1 (PC20) was 6.31 mg/ml after fenoterol and 8.51 mg/ml after fenoterol and ipratropium (P = 0.038). There was no significant relationship between bronchodilator effect of the bronchodilators and the increase in PC20 from pre-study values, r = 0.307 (P = 0.25) for fenoterol alone and r = 0.195 (P = 0.47) for fenoterol and ipratropium. The relationship between pre-study histamine responsiveness and the increase in PC20 caused by the bronchodilators just failed to reach statistical significance, r = -0.441 (P = 0.09) for fenoterol alone and r = -0.47 (P = 0.06) for fenoterol and ipratropium. The study has shown a greater right shift of histamine responsiveness for combined inhaled fenoterol and ipratropium compared with inhaled fenoterol alone in this group of asthmatics.

The effect of dopamine on tracheal smooth muscle

Dose-response curves to dopamine were obtained on guinea-pig, dog and human tracheal smooth muscle. Dopamine produced a relaxation of the guinea-pig tracheal chain, and this relaxation was completely blocked by propranolol. The potency of dopamine as a beta-agonist was 1/10 000 that of isoprenaline, 1/250 that of adrenaline and 1/50 that of noradrenaline. In human and dog tracheal smooth muscle, dopamine induced a contraction which could be entirely abolished by alpha-adrenoceptor antagonists. As an alpha-agonist, the potency of dopamine was 1/20 that of adrenaline and noradrenaline. Our data also show that a dopamine induced contraction is greatly potentiated if the smooth muscle specimen has been previously slightly contracted with histamine. We conclude that there are few, if any, specific dopaminergic receptors in the airways. Dopamine acts on both alpha- and beta-adrenoceptors, but in humans and dogs, its effect is predominantly on alpha-adrenoceptors.