Nervous control of mucin secretion into human bronchi

Lung innervation in the eye of a cytokine storm: neuroimmune interactions and COVID-19

COVID-19 is an infectious disease caused by the coronavirus SARS-CoV-2, which was first reported in Wuhan, China, in December 2019 and has caused a global pandemic. Acute respiratory distress syndrome (ARDS) is a common feature of severe forms of COVID-19 and can lead to respiratory failure, especially in older individuals. The increasing recognition of the neurotropic potential of SARS-CoV-2 has sparked interest in the role of the nervous system in respiratory failure in people with COVID-19. However, the neuroimmune interactions in the lung in the context of ARDS are poorly understood. In this Perspectives article, we propose the concept of the neuroimmune unit as a critical determinant of lung function in the context of COVID-19, inflammatory conditions and ageing, focusing particularly on the involvement of the vagus nerve. We discuss approaches such as neurostimulation and pharmacological neuromodulation to reduce tissue inflammation with the aim of preventing respiratory failure.

Acute respiratory distress syndrome is a common occurrence in COVID-19, an infectious disease caused by the coronavirus SARS-CoV-2. In this article, the authors consider how lung innervation might crosstalk with the immune system to modulate lung function and influence outcomes in COVID-19.

Effect of pyridostigmine on in vivo and in vitro respiratory muscle of mdx mice

The current work was conducted to verify the contribution of neuromuscular transmission defects at the neuromuscular junction to Duchenne Muscular Dystrophy disease progression and respiratory dysfunction. We tested pyridostigmine and pyridostigmine encapsulated in liposomes (liposomal PYR), an acetylcholinesterase inhibitor to improve muscular contraction on respiratory muscle function in mdx mice at different ages. We evaluated in vivo with the whole-body plethysmography, the ventilatory response to hypercapnia, and measured in vitro diaphragm strength in each group. Compared to C57BL10 mice, only 17 and 22 month-old mdx presented blunted ventilatory response, under normocapnia and hypercapnia. Free pyridostigmine (1mg/kg) was toxic to mdx mice, unlike liposomal PYR, which did not show any side effect, confirming that the encapsulation in liposomes is effective in reducing the toxic effects of this drug. Treatment with liposomal PYR, either acute or chronic, did not show any beneficial effect on respiratory function of this DMD experimental model. The encapsulation in liposomes is effective to abolish toxic effects of drugs.

Control of Neurotransmission by NaV1.7 in Human, Guinea Pig, and Mouse Airway Parasympathetic Nerves

Little is known about the neuronal voltage-gated sodium channels (NaVs) that control neurotransmission in the parasympathetic nervous system. We evaluated the expression of the α subunits of each of the nine NaVs in human, guinea pig, and mouse airway parasympathetic ganglia. We combined this information with a pharmacological analysis of selective NaV blockers on parasympathetic contractions of isolated airway smooth muscle. As would be expected from previous studies, tetrodotoxin potently blocked the parasympathetic responses in the airways of each species. Gene expression analysis showed that that NaV 1.7 was virtually the only tetrodotoxin-sensitive NaV1 gene expressed in guinea pig and human airway parasympathetic ganglia, where mouse ganglia expressed NaV1.1, 1.3, and 1.7. Using selective pharmacological blockers supported the gene expression results, showing that blocking NaV1.7 alone can abolish the responses in guinea pig and human bronchi, but not in mouse airways. To block the responses in mouse airways requires that NaV1.7 along with NaV1.1 and/or NaV1.3 is blocked. These results may suggest novel indications for NaV1.7-blocking drugs, in which there is an overactive parasympathetic drive, such as in asthma. The data also raise the potential concern of antiparasympathetic side effects for systemic NaV1.7 blockers.

Tiotropium for the treatment of asthma: a drug safety evaluation

INTRODUCTION

Tiotropium, a once-daily long-acting anticholinergic bronchodilator, has recently been approved for use in the treatment of asthma in a number of countries, including the EU and the USA, and was incorporated into the 2015 update of the Global Initiative for Asthma treatment guidelines. Here we review safety data from published clinical trials to help inform the use of tiotropium in the treatment of patients with asthma.

AREAS COVERED

Safety data from recently published clinical trials, which compared tiotropium with placebo or an active control, were reviewed. Trials included children, adolescents, and adults across severities of symptomatic asthma, and assessed tiotropium delivered via the Respimat and HandiHaler devices.

EXPERT OPINION

Based on the reviewed scientific evidence, tiotropium is a safe and well-tolerated long-acting anticholinergic bronchodilator for use in the treatment of asthma. In the trials assessed, the safety of tiotropium was found to be comparable with that of placebo and alternative therapeutic options, including a doubling in the dose of inhaled corticosteroids and the long-acting β2-agonist salmeterol.

Airway Gland Structure and Function

Submucosal glands contribute to airway surface liquid (ASL), a film that protects all airway surfaces. Glandular mucus comprises electrolytes, water, the gel-forming mucin MUC5B, and hundreds of different proteins with diverse protective functions. Gland volume per unit area of mucosal surface correlates positively with impaction rate of inhaled particles. In human main bronchi, the volume of the glands is ∼ 50 times that of surface goblet cells, but the glands diminish in size and frequency distally. ASL and its trapped particles are removed from the airways by mucociliary transport. Airway glands have a tubuloacinar structure, with a single terminal duct, a nonciliated collecting duct, then branching secretory tubules lined with mucous cells and ending in serous acini. They allow for a massive increase in numbers of mucus-producing cells without replacing surface ciliated cells. Active secretion of Cl(-) and HCO3 (-) by serous cells produces most of the fluid of gland secretions. Glands are densely innervated by tonically active, mutually excitatory airway intrinsic neurons. Most gland mucus is secreted constitutively in vivo, with large, transient increases produced by emergency reflex drive from the vagus. Elevations of [cAMP]i and [Ca(2+)]i coordinate electrolyte and macromolecular secretion and probably occur together for baseline activity in vivo, with cholinergic elevation of [Ca(2+)]i being mainly responsive for transient increases in secretion. Altered submucosal gland function contributes to the pathology of all obstructive diseases, but is an early stage of pathogenesis only in cystic fibrosis.

Tiotropium attenuates IL-13-induced goblet cell metaplasia of human airway epithelial cells

BACKGROUND

It has been shown that acetylcholine is both a neurotransmitter and acts as a local mediator, produced by airway cells including epithelial cells. In vivo studies have demonstrated an indirect role for acetylcholine in epithelial cell differentiation. Here, we aimed to investigate direct effects of endogenous non-neuronal acetylcholine on epithelial cell differentiation.

METHODS

Human airway epithelial cells from healthy donors were cultured at an air-liquid interface (ALI). Cells were exposed to the muscarinic antagonist tiotropium (10 nM), interleukin (IL)-13 (1, 2 and 5 ng/mL), or a combination of IL-13 and tiotropium, during or after differentiation at the ALI.

RESULTS

Human airway epithelial cells expressed all components of the non-neuronal cholinergic system, suggesting acetylcholine production. Tiotropium had no effects on epithelial cell differentiation after air exposure. Differentiation into goblet cells was barely induced after air exposure. Therefore, IL-13 (1 ng/mL) was used to induce goblet cell metaplasia. IL-13 induced MUC5AC-positive cells (5-fold) and goblet cells (14-fold), as assessed by histochemistry, and MUC5AC gene expression (105-fold). These effects were partly prevented by tiotropium (47-92%). Goblet cell metaplasia was induced by IL-13 in a dose-dependent manner, which was inhibited by tiotropium. In addition, tiotropium reversed goblet cell metaplasia induced by 2 weeks of IL-13 exposure. IL-13 decreased forkhead box protein A2 (FoxA2) expression (1.6-fold) and increased FoxA3 (3.6-fold) and SAM-pointed domain-containing ETS transcription factor (SPDEF) (5.2-fold) expression. Tiotropium prevented the effects on FoxA2 and FoxA3, but not on SPDEF.

CONCLUSIONS

We demonstrate that tiotropium has no effects on epithelial cell differentiation after air exposure, but inhibits and reverses IL-13-induced goblet cell metaplasia, possibly via FoxA2 and FoxA3. This indicates that non-neuronal acetylcholine contributes to goblet cell differentiation by a direct effect on epithelial cells.

The impact of asthma exacerbations and preventive strategies

OBJECTIVE

To review the pathophysiologic mechanisms underlying asthma exacerbations, the impact of exacerbations, and both current and future treatment strategies to establish asthma control and reduce the risk of future exacerbations.

RESEARCH DESIGN AND METHODS

Relevant adult data were identified via PubMed, with additional references obtained by reviewing bibliographies from selected articles.

RESULTS

Asthma exacerbations or 'attacks' are acute episodes of progressive worsening of symptoms which occur in patients with all degrees of asthma severity and are an important cause of morbidity and mortality. For patients, these asthma attacks constitute a considerable part of the disease burden in terms of both personal suffering and economic impact. Exacerbations are characterized in part by decreases in expiratory flow or lung function. The pathophysiologic mechanism underlying these changes is likely to be different depending on the specific asthma phenotype. Asthma exacerbations are commonly initiated by upper respiratory tract infections and/or environmental allergens, although there are other known factors which increase the risk of a patient developing exacerbations, such as cigarette smoking. Establishing asthma control and reducing the risk of future exacerbations is the main goal of asthma treatment. Inhaled corticosteroids alone or in combination with long-acting β2-agonists, in addition to other step-up strategies such as leukotriene receptor antagonists and theophylline, are recommended. The anti-immunoglobulin E monoclonal antibody omalizumab should also be considered in difficult-to-treat allergic asthma.

CONCLUSIONS

Despite the currently available treatments, many patients with asthma remain symptomatic and experience exacerbations regardless of disease severity. New therapies, including long-acting anticholinergics, anti-cytokines, and chemoattractant receptor-homologous molecules, are under investigation with some promising results. In addition to increased education and use of self-management plans, these novel therapies are essential to help improve asthma control and reduce exacerbation risk.

Is there a rationale and role for long-acting anticholinergic bronchodilators in asthma?

Despite current guidelines and the range of available treatments, over a half of patients with asthma continue to suffer from poor symptomatic control and remain at risk of future worsening. Although a number of non-pharmacological measures are crucial for good clinical management of asthma, new therapeutic controller medications will have a role in the future management of the disease. Several long-acting anticholinergic bronchodilators are under investigation or are available for the treatment of respiratory diseases, including tiotropium bromide, aclidinium bromide, glycopyrronium bromide, glycopyrrolate and umeclidinium bromide, although none is yet licensed for the treatment of asthma. A recent Phase III investigation demonstrated that the once-daily long-acting anticholinergic bronchodilator tiotropium bromide improves lung function and reduces the risk of exacerbation in patients with symptomatic asthma, despite the use of inhaled corticosteroids (ICS) and long-acting β₂-agonists (LABAs). This has prompted the question of what the rationale is for long-acting anticholinergic bronchodilators in asthma. Bronchial smooth muscle contraction is the primary cause of reversible airway narrowing in asthma, and the baseline level of contraction is predominantly set by the level of ‘cholinergic tone’. Patients with asthma have increased bronchial smooth muscle tone and mucus hypersecretion, possibly as a result of elevated cholinergic activity, which anticholinergic compounds are known to reduce. Further, anticholinergic compounds may also have anti-inflammatory properties. Thus, evidence suggests that long-acting anticholinergic bronchodilators might offer benefits for the maintenance of asthma control, such as in patients failing to gain control on ICS and a LABA, or those with frequent exacerbations.

Autonomic neural control of intrathoracic airways

Autonomic neural control of the intrathoracic airways aids in optimizing air flow and gas exchange. In addition, and perhaps more importantly, the autonomic nervous system contributes to host defense of the respiratory tract. These functions are accomplished by tightly regulating airway caliber, blood flow, and secretions. Although both the sympathetic and parasympathetic branches of the autonomic nervous system innervate the airways, it is the later that dominates, especially with respect to control of airway smooth muscle and secretions. Parasympathetic tone in the airways is regulated by reflex activity often initiated by activation of airway stretch receptors and polymodal nociceptors. This review discusses the preganglionic, ganglionic, and postganglionic mechanisms of airway autonomic innervation. Additionally, it provides a brief overview of how dysregulation of the airway autonomic nervous system may contribute to respiratory diseases.

Feasibility of percutaneous vagus nerve stimulation for the treatment of acute asthma exacerbations

OBJECTIVES

This study assessed the feasibility of an investigational vagus nerve stimulation (VNS) device for treating acute asthma exacerbations in patients not responding to at least 1 hour of initial standard care therapy.

METHODS

This was a prospective, nonrandomized study of patients treated in the ED for moderate to severe acute asthma (forced expiratory volume in 1 second [FEV(1)] 25% to 70% of predicted). Treatment entailed percutaneous placement of an electrode near the right carotid sheath and 60 minutes of VNS and continued standard care. VNS voltage was adjusted to perceived improvement, muscle twitching, or adverse events (AEs). All AEs, vital signs, FEV(1), perceived work of breathing (WOB), and final disposition were recorded.

RESULTS

Twenty-five subjects were enrolled. There were no serious AEs and no significant changes in vital signs. No subject required terminating VNS. One patient had minor bleeding from the procedure, and one had a hematoma and withdrew prior to VNS. AEs related to VNS were temporary and included cough (1 of 24), swallowing difficulty (2 of 24), voice change (2 of 24), and muscle twitching (14 of 24). These resolved when VNS ended. The FEV(1) improved at 15 minutes (median = 15.8%, 95% confidence interval [CI] = 9.3% to 22.4%), 30 minutes (median = 21.3%, 95% CI = 8.1% to 36.5%), and 60 minutes (median = 27.5%, 95% CI = 11.3% to 43.5%). WOB improved at 15 minutes (median = 53.9%, 95% CI = 33.7% to 73.9%), 30 minutes (median = 69.1%, 95% CI = 56.4% to 81.8%), and 60 minutes (median = 81.0%, 95% CI = 68.5% to 93.5%).

CONCLUSIONS

Percutaneous VNS did not result in serious AEs and was associated with improvements in FEV(1) and perceived dyspnea. Percutaneous VNS appears to be feasible for use in the treatment of moderate to severe acute asthma in patients unresponsive to initial standard care treatment.

Muscarinic receptor antagonists: effects on pulmonary function

In healthy lungs, muscarinic receptors control smooth muscle tone, mucus secretion, vasodilation, and inflammation. In chronic obstructive pulmonary disease (COPD) and asthma, cholinergic mechanisms contribute to increased bronchoconstriction and mucus secretion that limit airflow. This chapter reviews neuronal and nonneuronal sources of acetylcholine in the lung and the expression and role of M₁, M₂, and M₃ muscarinic receptor subtypes in lung physiology. It also discusses the evidence for and against the role of parasympathetic nerves in asthma, and the current use and therapeutic potential of muscarinic receptor antagonists in COPD and asthma.

Neurogenic inflammation in lung disease: burnt out?

Neurogenic inflammation results from activation of sensory nerves which, acting in an 'efferent' manner, release sensory neuropeptides to induce a wide variety of physiological and immunological responses. This process is easy to demonstrate experimentally in the airways of small laboratory animal species but in human airways is equivocal and, at best, minor compared with cholinergic neural control. Nevertheless, sensory neuropeptides (calcitonin gene-related peptide and the tachykinins, substance P and neurokinin A) induce airway responses in both laboratory animals and humans which suggest a potential for sensory-efferent control of human airways. In addition, there is indirect evidence for an increased 'expression' of sensory nerves and tachykinin receptors in asthma and bronchitis, which indicates that neurogenic inflammation contributes to pathophysiology of these airway conditions. In contrast, clinical trials using different classes of drugs to inhibit sensory nerve responses have failed to resolve whether neurogenic inflammation is involved in asthma, although there are concerns about the relevance of some of these studies. In contrast to their involvement in airway neurogenic inflammation, sensory nerves may be important in initiating protective reflexes, including coughing and sneezing, acting via their afferent pathways. Thus, although flickering, the concept of neurogenic inflammation in lung disease is not yet burnt out. However, it needs the rekindling of interest which re-evaluation as a protective process may bring, together with data from more appropriate clinical studies in asthma and chronic bronchitis.

Anatomy and Neurophysiology of the Cough Reflex

OBJECTIVES

To describe the anatomy and neurophysiology of the cough reflex.

METHODS

A review of the literature was carried out using PubMed and the ISI Web of Knowledge from 1951 to 2004. Most of the referenced studies were carried out in animals

CONCLUSIONS

Studies carried out in animals provide suggestive but inconclusive evidence that C-fibers and rapidly adapting receptors (RARs) arising from the vagus nerves mediate coughing. Recent studies also have suggested that a vagal afferent nerve subtype that is not readily classified as a RAR or a C-fiber may play an important role in regulating cough. Afferent nerves innervating other viscera, as well as somatosensory nerves innervating the chest wall, diaphragm, and abdominal musculature also likely play a less essential but important accessory role in regulating cough. The responsiveness and morphology of the airway vagal afferent nerve subtypes and the extrapulmonary afferent nerves that regulate coughing are described.

The airway cholinergic system: physiology and pharmacology

The present review summarizes the current knowledge of the cholinergic systems in the airways with special emphasis on the role of acetylcholine both as neurotransmitter in ganglia and postganglionic parasympathetic nerves and as non-neuronal paracrine mediator. The different cholinoceptors, various nicotinic and muscarinic receptors, as well as their signalling mechanisms are presented. The complex ganglionic and prejunctional mechanisms controlling the release of acetylcholine are explained, and it is discussed whether changes in transmitter release could be involved in airway dysfunctions. The effects of acetylcholine on different target cells, smooth muscles, nerves, surface epithelial and secretory cells as well as mast cells are described in detail, including the receptor subtypes involved in signal transmission.

Interactions between vagal afferent nerve subtypes mediating cough

The cough reflex is initiated through activation of vagal afferent nerves. Rapidly adapting receptors fulfill all criteria for the afferents subserving the cough reflex. Bronchopulmonary C-fibres may also initiate cough when activated. C-fibre-mediated cough may depend upon ongoing or initiated activity in rapidly adapting receptors. The interaction between airways C-fibres and rapidly adapting receptors may occur at sites in the periphery or in the brainstem. C-fibre mediated cough must also overcome a coincident inhibitory effect of C-fibre activation on cough, an inhibitory effect that becomes prominent under general anesthaesia.

Motor control of airway goblet cells and glands

Activation of nerves increases airway mucus secretion. The mucus derives from submucosal glands and epithelial goblet cells. Depending upon species and airway level, innervation comprises parasympathetic (cholinergic), sympathetic (adrenergic) and 'sensory-efferent' pathways. In all species studied, cholinergic mechanisms predominate, particularly in human airways. Muscarinic M3 receptors on the secretory cells mediate the cholinergic response. Tachykinins (substance P and neurokinin A) mediate the sensory-efferent response, acting via tachykinin NK1 receptors. Endogenous mechanisms regulate the magnitude of neurogenic secretion, including enzymes (degrade neurotransmitters), nitric oxide (NO) and vasoactive intestinal peptide (VIP) (regulate stimulated secretion), and muscarinic M2 autoreceptors (inhibit acetylcholine release). Exogenous opioids also inhibit neurogenic secretion prejunctionally. Both VIP and opioids act by opening large conductance, calcium-activated potassium (BK(Ca)) channels. Present understanding of neural control of mucus secretion in animal airways requires translation into human data. This information should lead to rational development of drugs for bronchial diseases in which neurogenic mucus hypersecretion contributes to pathophysiology, including chronic bronchitis and asthma.

Nitric oxide inhibition of basal and neurogenic mucus secretion in ferret trachea in vitro

1. In order to examine the role of nitric oxide (NO) on airway mucus secretion we studied the effects of the nitric oxide synthase (NOS) inhibitor L-N(G)-monomethyl-L-arginine (L-NMMA), a novel nitric oxide donor, (+/-)-(E)-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexeneamide (FK409), and the NO precursor L-arginine on basal mucus secretion in the ferret trachea in vitro in Ussing chambers. We also determined the effects of these agents upon secretion induced by electrical stimulation of nerves or by acetylcholine (ACh). We used 35SO4 as a mucus marker. 2. L-NMMA (0.01-1 mM) increased basal output of 35SO4-labelled macromolecules with a maximal increase above baseline of 248% at 0.1 mM L-NMMA. L-Arginine (1 mM) alone had no significant effect on basal secretion but reversed the potentiating effect of L-NMMA on basal secretion. L-NMMA-induced increases in basal mucus secretion were sustained for at least 30 min in the continuing presence of the NOS inhibitor. In contrast to the potentiating effects of L-NMMA, FK409 (100 nM) reduced basal secretion by 60% (at 1 nM and at 10 nM it was without effect). 3. Electrical stimulation (50 V, 10 Hz, 0.5 ms for 5 min) increased 35SO4 output by 174%. L-NMMA (1 and 10 mM) present during stimulation of tracheal segments resulted in significant potentiations of 214% and 116%, respectively, of the neurogenic response. The potentiated response to 10 mM L-NMMA was reversed by L-arginine (1 mM). At this dose L-arginine had no effect itself on basal secretion. In contrast to the potentiating effects of L-NMMA on neurogenic secretion, FK409 at 10 nM and 100 nM inhibited the neurogenic response by 98% and 99%. 4. At all concentrations tested, neither L-NMMA (0.01 mM-1 mM) nor FK409 (1-100 mM) had any significant effect on ACh-induced mucus secretion. 5. These observations lead us to conclude that nitric oxide, derived from constitutive NO synthase, acts as an endogenous inhibitor of both basal and neurogenic mucus secretion in ferret trachea in vitro.

Nasal mucus clearance in rats: differences with sex and phase of the oestrous cycle

We studied the changes in airway mucus rheology and clearability, as well as in morphometric indices, between male rats and females in either the oestrous or dioestrous phases of the oestrous cycle. Three-month-old Wistar rats were studied and the phases of the oestrous cycle were determined based on analysis of vaginal smears stained by a modified Shorr's procedure. Nasal mucus samples were analysed by means of magnetic rheometry and determination of in vitro transport rate in the frog palate preparation. In situ clearance on the exposed nasal septum was also determined. The mucociliary velocity in situ was significantly affected by both sex and the oestrous cycle phase. In female rats, dioestrous phase clearance was significantly slower than the oestrous phase one. Clearance in male rats was faster than that of both the phases studied in females. Mucus rigidity of females in the oestrous phase was more rigid than that of females in the dioestrous phase and that of males. Mucus viscosity/elasticity ratio for deformations performed at high frequencies was greater for females in comparison with males. Cough clearability computed on the basis of rheological parameters was predicted to be more favourable in females. There were no significant differences between the three groups in mucociliary clearance in vitro. Morphometric studies of the nasal epithelium showed that epithelial and glandular volumes did not exhibit sex- or oestrous-phase-related differences, but the amount of epithelial acidic glycoproteins was lower in females in the oestrous phase in comparison with males and those in the dioestrous phase, the same trend being observed in the glands of the lamina propria.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurokinin receptors subserving airways secretion

Mucus secretion can be induced in the airways by activation of nerves. The principal mechanism mediating neurogenic mucus secretion is cholinergic. However, a small but significant secretory response remains after adrenoceptor and cholinoceptor blockade. The identity of this nonadrenergic, noncholinergic (NANC) neural mechanism is unclear but includes an orthodromic pathway and a capsaicin-sensitive "sensory-efferent" (or "local effector") pathway. The orthodromic pathway comprises cholinergic nerves (and to a much lesser extent adrenergic nerves) in which neuropeptides, including vasoactive intestinal peptide (VIP) and neuropeptide tyrosine (NPY), are colocalised and coreleased with the classical neurotransmitter. Investigation of the contribution of the orthodromic neural pathway to neurogenic secretion awaits development of selective receptor antagonists for VIP and NPY. The neurotransmitters of the sensory-efferent neural pathway include calcitonin gene related peptide and the tachykinin receptor agonists indicates that the tachykinin NK1 receptor is ubiquitous for airway secretory processes, including mucus secretion and ion transport. Antagonist studies show that the great proportion of the NANC neural mucus secretory response is mediated via NK1 receptors, with little or no contribution from NK2 receptors. The relevance of the sensory-efferent neural pathway in health is equivocal, but it may have increasing importance in chronic inflammatory bronchial diseases associated with mucus hypersecretion, for example, asthma and chronic bronchitis, in which there is some evidence for the potential for increased sensory-efferent neural activity.

Galanin and somatostatin inhibition of neurokinin A and B induced airway mucus secretion in the rat

Neurokinin A and B are present in neurons situated in lung and NK-1 receptors have been described on tracheal submucosal gland cells. In the present study we compared the ability of substance P (SP), neurokinin A (NKA) and neurokinin B (NKB) to stimulate airway mucus secretion. Furthermore, we characterized the interaction of NKA and NKB with galanin and somatostatin. The rank order of the tachykinins to stimulate airway mucus secretion was SP > NKA > NKB suggesting that NK-1 receptors mediate these effects(EC50:SP: 50 nmol/l, NKA: 200 nmol/l, NKB: 400 nmol/l). Galanin and somatostatin were equally potent to inhibit NK-A and NK-B stimulated airway mucus release. These results suggest that NK-A and NK-B are potent stimulators of airway macromolecule secretion. Galanin and somatostatin potently inhibit these actions of the tachykinins. Therefore, airway mucus secretion is controlled by a complex network of several different mediators.

Galanin and somatostatin inhibition of substance P-induced airway mucus secretion in the rat

Substance P is present in several neurons innervating the lung. Tachykinin receptors are expressed on submucosal gland cells. Substance P is known to be a potent stimulator of airway mucus secretion. In the present study we characterized the effects of galanin and somatostatin on basal and substance P-induced mucus secretion. The stimulatory effect of substance P was concentration-dependent (100 pmol/l: 112%, 1 nmol/l: 120%, 10 nmol/l: 153%, 100 nmol/l: 223%, 1 mumol/l: 275%, 10 mumol/l: 172%) and was inhibited by galanin and somatostatin (1 mumol/l substance P: 277%; 1 mumol/l substance P + 1 mumol/l somatostatin: 190%, p < 0.01; 1 mumol/l substance P + 1 mumol/l galanin: 206%, p < 0.05). In the presence of lower concentrations of substance P 1 mumol/l somatostatin and 1 mumol/l galanin did not modify mucus secretion. Lower concentrations of galanin and somatostatin did not significantly change mucus secretion stimulated by 1 mumol/l substance P. Both, galanin and somatostatin at 1 mumol/l left basal airway mucus secretion unaltered. These data suggest that mucus secretion into airways is regulated by a complex network of peptidergic stimulators and inhibitors including substance P, somatostatin and galanin.

'Sensory-efferent' neural control of mucus secretion: characterization using tachykinin receptor antagonists in ferret trachea in vitro

1. We characterized the tachykinin receptor(s) mediating 'sensory-efferent' neural control of release of 35SO4-labelled macromolecules (mucus) from ferret trachea in vitro in Ussing chambers using selective tachykinin antagonists. Secretion was induced by substance P (SP), neurokinin A (NKA), capsaicin, the NK1 tachykinin receptor agonist [Sar9, Met(O2)11]substance P ([Sar9]SP), or acetylcholine (ACh), or by electrical stimulation of nerves. Antagonists used were FK888 and L-668,169, selective for the NK1 receptor, SR 48968, selective for the NK2 receptor, and FK224, a dual antagonist at NK1 and NK2 receptors. The selectivity of FK888 and SR 48968 was examined on NKA-induced contraction of ferret tracheal smooth muscle in vitro. 2. SP (1 microM) increased mucus secretion by 695% above vehicle controls. FK888 (0.1 microM-30 microM) inhibited SP-induced secretion in a dose-dependent manner, with complete inhibition at 10 microM and an IC50 of 1 microM. L-668,169 (1 microM) also completely inhibited SP-induced secretion. 3. NKA (1 microM) significantly increased mucus secretion by 271% above baseline, a response which was completely inhibited by FK888 (10 microM) or L-668,169 (microM). Secretion induced by ACh (10 microM: 317% above baseline) was not inhibited by FK888 but was inhibited by atropine. Capsaicin (10 microM)-induced secretion (456% above vehicle controls) was significantly inhibited by FK888 and by L-668,169 (111% and 103% inhibition respectively). 4. Electrical stimulation (50 V, 10 Hz, 0.5 ms, 5 min) increased mucus output above baseline (increased by 12 to 26 fold), a response blocked by tetrodotoxin (0.1 microM). FK888 (10 microM) inhibited the increase in secretion due to electrical stimulation by 47%. Atropine, propranolol and phentolamine in combination(APP) inhibited the response to electrical stimulation by 48%. The remaining NANC response, i.e. in the presence of APP, was further reduced by 66% with FK888. FK224 (10 microM) inhibited neurally evoked secretion by 73%. SR 48968 (0.1 fLM) had no effect on electrically-stimulated or [Sar9]SP-induced secretion.5. NKA (10nM- 1O microM: in the presence of DMSO control vehicle) induced tracheal smooth muscle contraction in a concentration-dependent manner with a maximal contraction of 30% of the maximal response to ACh (10 mM) and an ECm of 0.3 JAM. SR 48968 (0.1 microM in DMSO) inhibited the NKA induced contraction whereas FK888 did not. Neither antagonist had any inhibitory effect on ACh induced contraction.6. We conclude that 'sensory-efferent' neurogenic mucus secretion in ferret trachea in vitro is mediated via tachykinin NK, receptors with no involvement of NK2 receptors. Potent and selective tachykinin antagonists may have therapeutic potential in bronchial diseases such as asthma and chronic bronchitis in which neurogenic mucus hypersecretion may be aetiologically important.

Influence of inhaled atropine on lung mucociliary function in humans

The influence of aerosolized atropine sulfate on lung airway mucus clearance was investigated in healthy human subjects who were nonsmokers. Mucus transport was measured with radiolabeled insoluble particles inhaled by mouth and deposited onto mucosal surfaces; subsequent retention of radiolabel was quantitated over a 4- to 5-h period by a noninvasive, posteriorly aligned, gamma camera. Placebo and atropine clearance tests were matched in a given subject for initial and for final (24-h postinhalation) deposition pattern of labeled aerosol at the onset and conclusion, respectively, of tracheobronchial particle clearance. In seven subjects mucociliary function was delayed after inhalation of 0.025 mg/kg body weight atropine sulfate as compared with placebo (0.9% NaCl). On the basis of the area under the activity versus time curves, retention times during atropine exceeded placebo times by more than 30% (p less than 0.01). At 90 min postatropine inhalation, the Vmax50 exceeded baseline values by 21% (p less than 0.01). Urine retention was present in one subject and xerostomia was present in all subjects after atropine. These data suggest that a single dose of atropine sulfate delivered topically to the airway surfaces delays the continuous flow of airway mucus in healthy subjects and that basal autonomic tone is an inherent factor for optimal secretion and/or removal of tracheobronchial secretions.

Neural control of airway mucus secretion

Neural mechanisms contribute to the control of secretion of mucus in the airways of a number of animal species including humans. The nerves involved are adrenergic, cholinergic and non-adrenergic, non cholinergic (NANC) and contribute to greater or lesser degrees, depending upon the species, to secretion from submucosal glands and epithelial goblet cells. Experimental studies implicate abnormalities in neural control in the pathophysiology of certain bronchial diseases in humans which are associated with mucus hypersecretion. New observations indicate a number of novel interventions with therapeutic potential for control of mucus in chronic bronchitis and asthma.

The effects of local anaesthetic agents upon mucus secretion in the feline trachea in vivo

The actions of lignocaine and tetrodotoxin (TTX) in a tracheal segment of the cat were tested on secretion of mucus macromolecules radiolabelled with 35S and 3H. Lignocaine, 4.3-43 mM, given into the segment, caused a concentration dependent increase of secretion of 3H-and 35S-labelled macromolecules. At 43 mM, lignocaine increased secretion: delta 3H = +433 +/- 191%, delta 35S = +327 +/- 34.5% (n = 8). This effect lessened over 15-45 min. Atropine (1 mg/kg) had little effect on these responses. All concentrations of lignocaine tested (4.3-43 mM) abolished the effect of vagus nerve stimulation on secretion and diminished the effect of a submaximal concentration of pilocarpine (5 microM) in the segment in a dose-dependent manner. TTX in the segment did not alter the resting secretion. At 50 microM it abolished, and at 10 microM diminished, vagal control of secretion without affecting the secretory response to pilocarpine. The study shows that lignocaine, in concentrations which block vagal control of secretion (greater than or equal to 4.3 mM), stimulates the release of mucus macromolecules. Resting secretion is unaltered by TTX, and so does not appear to be under neurogenic inhibition. Larger concentrations of lignocaine (greater than or equal to 13 mM) also diminish pilocarpine-induced secretion, whereas TTX may inhibit nervous control of mucus secretion selectively. The results suggest that clinical anaesthesia of the airways with lignocaine may stimulate mucus secretion.

Opioid inhibition of neurally mediated mucus secretion in human bronchi

Capsaicin, which induces release of neuropeptides such as substance P from sensory nerves, stimulated mucus secretion in surgically resected human bronchi in vitro. Pretreatment of the tissue with the opioid antagonist naloxone significantly enhanced secretion, possibly by blocking the inhibitory effect of opiate premedication before surgery. Capsaicin-induced mucus secretion was completely blocked by morphine, and this effect was reversed by naloxone. Thus, sensory nerve stimulation increases mucus secretion in human airways, which might contribute to the mucus hypersecretion seen after inhalation of irritants such as cigarette smoke. Secretion can be completely inhibited by opioid drugs, so they may represent a new therapeutic approach to airway hypersecretion in chronic bronchitis and asthma, in which axon reflex mechanisms have been implicated.

The functional significance of the sympathetic innervation of mucous glands in the bronchi of man

1. Pieces of human bronchi, from lung resected for carcinoma of the bronchus, were mounted in Ussing chambers and given [35S]sulphate as radiolabelled precursor of mucous glycoproteins (mucins). The release of 35S, bound to macromolecules, into the luminal half-chamber was used as an index of mucin secretion. 2. Noradrenaline, at concentrations of 1, 10 and 100 microM, was given into both halves of the Ussing chamber. At the lowest concentration, noradrenaline failed to change mucin output, but at the two higher concentrations it stimulated output. 3. In other experiments the sympathetic nerves in the bronchial wall were labelled with 5-hydroxydopamine and examined under the electron microscope. The distances between adrenergic nerve varicosities and submucosal glands were measured; some sympathetic nerve varicosities were seen within 1 microns of gland cells. 4. A simple mathematical model for the diffusion of noradrenaline was used to predict the concentrations of the transmitter likely to result at different distances from a nerve if one or more vesicles of noradrenaline were released. 5. The model predicts that the release of a single large vesicle of noradrenaline is likely to generate an effective concentration of transmitter provided that the nerve is within 1 micron of the target cell.

Neuropeptides in human airways: function and clinical implications

Several neuropeptides have now been localized to nerves in human airways and have marked effects on airway smooth muscle tone, bronchial blood flow, microvascular leakage, and airway secretions. There is mounting evidence that they may act as neurotransmitters of nonadrenergic, noncholinergic nerves and may be co-transmitters of classic autonomic nerves. Vasoactive intestinal peptide and the related peptide histidine methionine are potent relaxants of human airways in vitro, yet their effects in vivo are disappointing because of problems in delivery. Sensory neuropeptides such as substance P, neurokinins A and B, and calcitonin gene-related peptide may be involved in neurogenic inflammatory reactions in asthma. Although there have been no clinical benefits from these discoveries, in the future the development of agents that interfere with or mimic neuropeptide effects may offer novel therapeutic approaches to airway diseases such as asthma.