Involvement of thromboxane A(2) in airway mucous cells in asthma-related cough

Pharmacology of Bradykinin-Evoked Coughing in Guinea Pigs

Bradykinin has been implicated as a mediator of the acute pathophysiological and inflammatory consequences of respiratory tract infections and in exacerbations of chronic diseases such as asthma. Bradykinin may also be a trigger for the coughing associated with these and other conditions. We have thus set out to evaluate the pharmacology of bradykinin-evoked coughing in guinea pigs. When inhaled, bradykinin induced paroxysmal coughing that was abolished by the bradykinin B2 receptor antagonist HOE 140. These cough responses rapidly desensitized, consistent with reports of B2 receptor desensitization. Bradykinin-evoked cough was potentiated by inhibition of both neutral endopeptidase and angiotensin-converting enzyme (with thiorphan and captopril, respectively), but was largely unaffected by muscarinic or thromboxane receptor blockade (atropine and ICI 192605), cyclooxygenase, or nitric oxide synthase inhibition (meclofenamic acid and N(G)-nitro-L-arginine). Calcium influx studies in bronchopulmonary vagal afferent neurons dissociated from vagal sensory ganglia indicated that the tachykinin-containing C-fibers arising from the jugular ganglia mediate bradykinin-evoked coughing. Also implicating the jugular C-fibers was the observation that simultaneous blockade of neurokinin2 (NK2; SR48968) and NK3 (SR142801 or SB223412) receptors nearly abolished the bradykinin-evoked cough responses. The data suggest that bradykinin induces coughing in guinea pigs by activating B2 receptors on bronchopulmonary C-fibers. We speculate that therapeutics targeting the actions of bradykinin may prove useful in the treatment of cough.

Blocking of thromboxane A₂ receptor attenuates airway mucus hyperproduction induced by cigarette smoke

Cigarette smoking is one of the risk factors for chronic obstructive pulmonary disease (COPD). In this study, we investigated the effects of thromboxane A2 (TxA2) receptor antagonists on airway mucus production induced by cigarette smoke. Rats were exposed to cigarette smoke 1h/day, 6 days/week for 4 weeks. Seratrodast (2, 5, 10mg/kg day) was administered intragastrically prior to smoke exposure. Thromboxane B2 (TxB2) in the bronchoalveolar lavage fluid and lung tissues was determined by enzyme immunoassay. Airway mucus production was determined by alcin-blue/periodic acid sthiff (AB-PAS) staining, Muc5ac immunohistochemical staining, and RT-PCR. The phosphorylation of ERK and p38 was evaluated by Western blotting. Seratrodast reduced the overproduction of TxB2 in both bronchoalveolar lavage fluid and lung tissues. Cigarette smoke exposure markedly increased AB/PAS-stained goblet cells and rat Muc5ac expression in the airway, which was significantly attenuated by seratrodast administration. The induced phosphorylation of ERK and p38 was also attenuated by seratrodast. TxA2 receptor antagonist could reduce Muc5ac production induced by cigarette smoke in vivo, possibly through the mitogen-activated protein kinases (MAPK) signaling pathway.

International Union of Basic and Clinical Pharmacology. LXXXIII: classification of prostanoid receptors, updating 15 years of progress

It is now more than 15 years since the molecular structures of the major prostanoid receptors were elucidated. Since then, substantial progress has been achieved with respect to distribution and function, signal transduction mechanisms, and the design of agonists and antagonists (http://www.iuphar-db.org/DATABASE/FamilyIntroductionForward?familyId=58). This review systematically details these advances. More recent developments in prostanoid receptor research are included. The DP(2) receptor, also termed CRTH2, has little structural resemblance to DP(1) and other receptors described in the original prostanoid receptor classification. DP(2) receptors are more closely related to chemoattractant receptors. Prostanoid receptors have also been found to heterodimerize with other prostanoid receptor subtypes and nonprostanoids. This may extend signal transduction pathways and create new ligand recognition sites: prostacyclin/thromboxane A(2) heterodimeric receptors for 8-epi-prostaglandin E(2), wild-type/alternative (alt4) heterodimers for the prostaglandin FP receptor for bimatoprost and the prostamides. It is anticipated that the 15 years of research progress described herein will lead to novel therapeutic entities.

G-protein coupled receptors regulating cough

Cough is a protective mechanism but can occur excessively in disease. Cough can be modulated by a range of GPCRs which can be either inhibitory or excitatory. Prostaglandin E2 and bradykinin can activate airway sensory nerves via EP3 and B2 receptors receptively and have both been shown to mediate their effects though TRPV1 and TRPA1 receptors. Activation of the β2-adrenoceptor and cannabinoid CB2 receptors can inhibit sensory nerves and prevent cough. It is currently thought that activation of the β2-adrenoceptor causes c-AMP dependent activation of PKA; however, recent research has suggested that the pathway involves PKG-mediated opening of the BKCa channel leading to hyperpolarization.

Cough and Asthma

Cough is the most common complaint for which patients seek medical attention. Cough variant asthma (CVA) is a form of asthma, which presents solely with cough. CVA is one of the most common causes of chronic cough. More importantly, 30 to 40% of adult patients with CVA, unless adequately treated, may progress to classic asthma. CVA shares a number of pathophysiological features with classic asthma such as atopy, airway hyper-responsiveness, eosinophilic airway inflammation and various features of airway remodeling. Inhaled corticosteroids remain the most important form of treatment of CVA as they improve cough and reduce the risk of progression to classic asthma most likely through their prevention of airway remodeling and chronic airflow obstruction.

Encoding of the cough reflex in anesthetized guinea pigs

We have previously described the physiological and morphological properties of the cough receptors and their sites of termination in the airways and centrally in the nucleus tractus solitarius (nTS). In the present study, we have addressed the hypothesis that the primary central synapses of the cough receptors subserve an essential role in the encoding of cough. We found that cough requires sustained, high-frequency (≥8-Hz) afferent nerve activation. We also found evidence for processes that both facilitate (summation, sensitization) and inhibit the initiation of cough. Sensitization of cough occurs with repetitive subthreshold activation of the cough receptors or by coincident activation of C-fibers and/or nTS neurokinin receptor activation. Desensitization of cough evoked by repetitive and/or continuous afferent nerve activation has a rapid onset (<60 s) and does not differentiate between tussive stimuli, suggesting a central nervous system-dependent process. The cough reflex can also be actively inhibited upon activation of other airway afferent nerve subtypes, including slowly adapting receptors and pulmonary C-fibers. The sensitization and desensitization of cough are likely attributable to the prominent, primary, and unique role of N-methyl-d-aspartate receptor-dependent signaling at the central synapses of the cough receptors. These attributes may have direct relevance to the presentation of cough in disease and for the effectiveness of antitussive therapies.

Induced sputum concentrations of mucin in patients with asthma and chronic cough

BACKGROUND

Mucus hypersecretion is an important pathophysiologic index of airway disease. Measurement of secreted mucin in sputum has been reported in asthma, but not in chronic cough with or without increased sputum production.

METHODS

We studied 49 patients with classic asthma (CA), 39 with cough-variant asthma (CVA), nine and five with chronic cough associated with sinobronchial syndrome (SBS) and gastroesophageal reflux disease (GERD), respectively, and 11 healthy controls. Seventeen patients with CA, but none from the other groups, were taking antiinflammatory medications. Mucin levels in induced sputum supernatants were measured by enzyme-linked immunosorbent assay, which detects airway mucin, probably including MUC5AC and MUC5B.

RESULTS

Mucin levels were higher in patients with CA (674.2 +/- 548.8 microg/mL) and SBS (638.4 +/- 650.7 microg/mL) than in controls (212.0 +/- 167.1 microg/mL) (P = .0037 and .044). They were also higher in patients with CA than in those with CVA (350.4 +/- 374.0 microg/mL) and GERD (134.3 +/- 93.1 microg/mL) (P = .0016 and 0.015), but results did not differ between the latter groups and controls. When the four disease groups were combined, patients with frequent sputum production had greater mucin levels than those with occasional (P = .0023) or no sputum production (P < .0001). Patients with CA showed negative correlations of mucin levels with respiratory resistance indices on impulse oscillation and with airway sensitivity to methacholine.

CONCLUSIONS

Sputum mucin levels differ in various respiratory conditions when compared with controls, primarily reflecting the degree of sputum production. Airway mucin might possibly exert protective effects in asthma, at least between exacerbations, but this issue needs to be further clarified by future studies.

Sensory nerves and airway irritability

The lung, like many other organs, is innervated by a variety of sensory nerves and by nerves of the parasympathetic and sympathetic nervous systems that regulate the function of cells within the respiratory tract. Activation of sensory nerves by both mechanical and chemical stimuli elicits a number of defensive reflexes, including cough, altered breathing pattern, and altered autonomic drive, which are important for normal lung homeostasis. However, diseases that afflict the lung are associated with altered reflexes, resulting in a variety of symptoms, including increased cough, dyspnea, airways obstruction, and bronchial hyperresponsiveness. This review summarizes the current knowledge concerning the physiological role of different sensory nerve subtypes that innervate the lung, the factors which lead to their activation, and pharmacological approaches that have been used to interrogate the function of these nerves. This information may potentially facilitate the identification of novel drug targets for the treatment of respiratory disorders such as cough, asthma, and chronic obstructive pulmonary disease.

Prostanoid receptor antagonists: development strategies and therapeutic applications

Identification of the primary products of cyclo-oxygenase (COX)/prostaglandin synthase(s), which occurred between 1958 and 1976, was followed by a classification system for prostanoid receptors (DP, EP(1), EP(2) ...) based mainly on the pharmacological actions of natural and synthetic agonists and a few antagonists. The design of potent selective antagonists was rapid for certain prostanoid receptors (EP(1), TP), slow for others (FP, IP) and has yet to be achieved in certain cases (EP(2)). While some antagonists are structurally related to the natural agonist, most recent compounds are 'non-prostanoid' (often acyl-sulphonamides) and have emerged from high-throughput screening of compound libraries, made possible by the development of (functional) assays involving single recombinant prostanoid receptors. Selective antagonists have been crucial to defining the roles of PGD(2) (acting on DP(1) and DP(2) receptors) and PGE(2) (on EP(1) and EP(4) receptors) in various inflammatory conditions; there are clear opportunities for therapeutic intervention. The vast endeavour on TP (thromboxane) antagonists is considered in relation to their limited pharmaceutical success in the cardiovascular area. Correspondingly, the clinical utility of IP (prostacyclin) antagonists is assessed in relation to the cloud hanging over the long-term safety of selective COX-2 inhibitors. Aspirin apart, COX inhibitors broadly suppress all prostanoid pathways, while high selectivity has been a major goal in receptor antagonist development; more targeted therapy may require an intermediate position with defined antagonist selectivity profiles. This review is intended to provide overviews of each antagonist class (including prostamide antagonists), covering major development strategies and current and potential clinical usage.

Eosinophilic airway disorders associated with chronic cough

Chronic cough is a major clinical problem. The causes of chronic cough can be categorized into eosinophilic and noneosinophilic disorders, the former being comprised of asthma, cough variant asthma (CVA), atopic cough (AC) and non-asthmatic eosinophilic bronchitis (NAEB). Cough is one of the major symptoms of asthma. Cough in asthma can be classified into three categories; 1) CVA: asthma presenting solely with coughing, 2) cough-predominant asthma: asthma predominantly presenting with coughing but also with dyspnea and/or wheezing, and 3) cough remaining after treatment with inhaled corticosteroid (ICS) and beta2-agonists in patients with classical asthma, despite control of other symptoms. There may be two subtypes in the last category; one is cough responsive to anti-mediator drugs such as leukotriene receptor antagonists and histamine H1 receptor antagonists, and the other is cough due to co-morbid conditions such as gastroesophageal reflux. CVA is one of the commonest causes of chronic isolated cough. It shares a number of pathophysiological features with classical asthma with wheezing such as atopy, airway hyperresponsiveness (AHR), eosinophilic airway inflammation and various features of airway remodeling. One third of adult patients may develop wheezing and progress to classical asthma. As established in classical asthma, ICS is considered the first-line treatment, which improves cough and may also reduce the risk of progression to classical asthma. AC proposed by Fujimura et al. presents with bronchodilator-resistant dry cough associated with an atopic constitution. It involves eosinophilic tracheobronchitis and cough hypersensitivity and responds to ICS treatment, while lacking in AHR and variable airflow obstruction. These features are shared by non-asthmatic eosinophilic bronchitis (NAEB). However, atopic cough does not involve bronchoalveolar eosinophilia, has no evidence of airway remodeling, and rarely progresses to classical asthma, unlike CVA and NAEB. Histamine H1 antagonists are effective in atopic cough, but their efficacy in NAEB is unknown. AHR of NAEB may improve with ICS within the normal range. Taken together, NAEB significantly overlaps with atopic cough, but might also include milder cases of CVA with very modest AHR. The similarity and difference of these related entities presenting with chronic cough and characterized by airway eosinophilia will be discussed.

Cough sensors. I. Physiological and pharmacological properties of the afferent nerves regulating cough

The afferent nerves regulating cough have been reasonably well defined. The selective effects of general anesthesia on C-fiber-dependent cough and the opposing effects of C-fiber subtypes in cough have led to some uncertainty about their regulation of this defensive reflex. But a role for C-fibers in cough seems almost certain, given the unique pharmacological properties of these unmyelinated vagal afferent nerves and the ability of many C-fiber-selective stimulants to evoke cough. The role of myelinated laryngeal, tracheal, and bronchial afferent nerve subtypes that can be activated by punctate mechanical stimuli, inhaled particulates, accumulated secretions, and acid has also been demonstrated. These "cough receptors" are distinct from the slowly and rapidly adapting intrapulmonary stretch receptors responding to lung inflation. Indeed, intrapulmonary rapidly and slowly adapting receptors and pulmonary C-fibers may play no role or a nonessential role in cough, or might even actively inhibit cough upon activation. A critical review of the studies of the afferent nerve subtypes most often implicated in cough is provided.

TP receptor-mediated release of eosinophil chemotactic activity from human bronchial smooth muscle cells

There are reports indicating that thromboxane A(2) receptors (TP receptors) may stimulate the eosinophil accumulation in the lower airways of asthmatics, however, the mechanisms behind such an effect remain unknown. We quantified the synthesis of eosinophil chemotactic activity and eosinophilic CC chemokines, including CCL5, CCL7, CCL8, CCL11, CCL13, CCL24, and CCL26 in primary cultures of human bronchial smooth muscle cells (BSMC) stimulated with a prostanoid TP receptor agonist, IBOP (10(-9)-10(-7) M). The activation of prostanoid TP receptors in BSMC induced the release of potent eosinophil chemoattractant(s) in the presence of interleukin (IL)-4. CCL11/eotaxin-1 was the only synthesis significantly increased by IBOP co-stimulated with IL-4, and pretreatment with an anti-CCL11 antibody abrogated the eosinophil chemotactic activity released from IBOP/IL-4-stimulated BSMC. The effect of IBOP was also completely blocked by pretreatment with a prostanoid TP receptor-specific antagonist, AA-2414. IBOP had no effect on the expression of IL-4 receptor-alpha, or on the IL-4-induced phosphorylation of STAT6 in BSMC. In conclusion, activation of prostanoid TP receptors in a Th2-dominant microenvironment might exacerbate the eosinophilic inflammation of the airways by synthesis and release of CCL11 from BSMC.

Differential effects of airway afferent nerve subtypes on cough and respiration in anesthetized guinea pigs

The hypothesis that respiratory reflexes, such as cough, reflect the net and often opposing effects of activation of multiple afferent nerve subpopulations throughout the airways was evaluated. Laryngeal and tracheal mucosal challenge with either citric acid or mechanical probing reliably evoked coughing in anesthetized guinea pigs. No other stimulus reliably evoked coughing in these animals, regardless of route of administration and despite some profound effects on respiration. Selectively activating vagal C-fibers arising from the nodose ganglia with either adenosine or 2-methyl-5-HT evoked only tachypnea. Selectively activating vagal afferents arising from the jugular ganglia induced respiratory slowing and apnea. Nasal afferent nerve activation by capsaicin, citric acid, hypertonic saline, or histamine evoked only respiratory slowing. Histamine, which activates intrapulmonary rapidly adapting receptors but not airway or lung C-fibers or tracheal bronchial cough receptors induced bronchospasm and tachypnea, but no coughing. The results indicate that the reflexes initiated by stimuli thought to be selective for some afferent nerve subtypes will likely depend on the net and potentially opposing effects of multiple afferent nerve subpopulations throughout the airways. The data also provide further evidence that the afferent nerves regulating cough in anesthetized guinea pigs are distinct from either C-fibers or intrapulmonary rapidly adapting receptors.

The cough reflex in animals: relevance to human cough research

All mammalian species studied cough or display some similar respiratory reflex upon aerosol challenge with tussigenic stimuli such as citric acid or capsaicin. Animals cough to the same stimuli that evoke coughing in humans, and therapeutic agents that display antitussive effects in human studies also prevent coughing in animals. The many invasive procedures and complementary in vitro studies possible in animals but not readily reproduced in human subjects, along with the proven predictive value of cough studies in animals, provide the rationale for animal modeling of human cough. The advantages and disadvantages of studying cough in animals are discussed.

Sensory neural targets for the treatment of cough

1. Cough is a primary defensive reflex that protects the airways from potentially harmful stimuli. 2. During many respiratory diseases, the cough reflex threshold is lowered and coughing becomes excessive. 3. Currently available therapeutics are mostly ineffective at suppressing excessive coughing. 4. In the present review, we describe the sensory neural pathways involved in cough, how these pathways may become dysfunctional in airway disease and the most recent advances that have been made in identifying future targets for cough suppression.

Encoding of the cough reflex

Coughing can be both voluntarily induced and involuntarily initiated by activation of vagal afferent nerves innervating the airways and lungs. Centrally, cough is regulated at the level of the brainstem through integration of vagal afferent nerve input by relay neurones in the nucleus tractus solitarius (nTS). Projections to and from the nTS add further complexity to cough regulation, as do the profound influences of psychological and social factors known to regulate cough. Peripherally, both neuronal and non-neuronal elements in the airways regulate the excitability of the vagal afferent nerve terminals regulating cough. These multiple levels of integration and encoding of the cough reflex may render this defensive respiratory response highly susceptible to modulation both by disease processes and through therapeutic intervention.

Plasticity of peripheral mechanisms of cough

The cough reflex pathway is characterized by a remarkable plasticity often resulting in a persistent and uncontrollable urge to cough during airway inflammation. In many instances cough becomes up regulated to the extent that ceases to fulfill its defensive role in protecting the airways. The exact mechanisms underlying this plasticity are unknown and likely involves a variety of factors influencing the function of the peripheral and central nervous system. This review outlines the evidence of increased cough sensitivity during airway disease. This is followed by a discussion of the peripheral mechanisms involved including the potential role of inflammatory mediators, neutrophins and changes in the airway mucosal structure. A greater understanding of the mechanisms leading to enhanced cough should lead to the development of more effective therapeutic strategies.

Prostanoids as pharmacological targets in COPD and asthma

COPD (Chronic Obstructive Pulmonary Disease) and bronchial asthma are two severe lung diseases which represent a major problem of world public health. Leukotrienes and prostanoids play an important role in the pathogenesis of pulmonary diseases. Prostanoids: prostaglandins (PGs) and thromboxane A2 (TXA2), the cyclooxygenase metabolites of arachidonic acid are implicated in the inflammatory cascade that occurs in asthmatic airways. Recently, the roles played by isoprostanes or prostaglandin-like compounds nonenzymatically generated via peroxidation of membrane phospholipids by reactive oxygen species, in particular F2-isoprostanes, in pulmonary pathophysiology have been highlighted. This article aims to provide an overview of the role of prostanoids and isoprostanes in the pathogenesis of COPD and asthma and to discuss the pharmacological strategies developed in prevention and/or treatment of these pathologies.