Airway irritation and cough evoked by acid: from human to ion channel

Stimulatory effect of methylglyoxal on capsaicin-sensitive lung vagal afferents in rats: role of TRPA1

Methylglyoxal (MG), a reactive metabolic byproduct of glycolysis, is a causative of painful diabetic neuropathy. Patients with diabetes are associated with more frequent severe asthma exacerbation. Stimulation of capsaicin-sensitive lung vagal (CSLV) afferents may contribute to the pathogenesis of hyperreactive airway diseases such as asthma. However, the possibility of the stimulatory effect of MG on CSLV afferents and the underlying mechanisms remain unknown. Our results showed that intravenous injection of MG (25 mg/kg, MG25) in anesthetized, spontaneously breathing rats elicited pulmonary chemoreflexes characterized by apnea, bradycardia, and hypotension. The MG-induced apneic response was reproducible and dose dependent. MG25 no longer evoked these reflex responses after perineural capsaicin treatment of both cervical vagi to block C-fibers' conduction, suggesting that the reflexes were mediated through the stimulation of CSLV afferents. Pretreatment with HC030031 [an antagonist of transient receptor potential ankyrin subtype 1 protein (TRPA1)] or AP18 (another TRPA1 antagonist), but not their vehicle, markedly attenuated the apneic response induced by MG25. Consistently, electrophysiological results showed that pretreatment with HC030031 largely attenuated the intense discharge in CSLV afferents induced by injection of MG25 in open-chest and artificially ventilated rats. In isolated CSLV neurons, the perfusion of MG evoked an abrupt and pronounced increase in calcium transients in a concentration-dependent manner. This stimulatory effect on CSLV neurons was also abolished by HC030031 treatment but not by its vehicle. In conclusion, these results suggest that MG exerts a stimulatory effect on CSLV afferents, inducing pulmonary chemoreflexes, and such stimulation is mediated through the TRPA1 activation.NEW & NOTEWORTHY Methylglyoxal (MG) is implicated in the development of painful diabetic neuropathy. A retrospective cohort study revealed an increased incidence of asthma exacerbations in patients with diabetes. This study demonstrated that elevated circulating MG levels stimulate capsaicin-sensitive lung vagal afferents via activation of TRPA1, which in turn triggers respiratory reflexes. These findings provide new information for understanding the pathogenic mechanism of diabetes-associated hyperreactive airway diseases and potential therapy.

Chemical Elements, Flavor Chemicals, and Nicotine in Unused and Used Electronic Cigarettes Aged 5-10 Years and Effects of pH

The concentrations of elements/metals, nicotine, flavor chemicals and acids were compared in the e-liquids of unused and used first-generation electronic cigarettes (ECs) that were stored for 5-10 years. Metal analysis was performed using inductively coupled plasma optical emission spectroscopy; nicotine and flavor chemical analyses were performed using gas chromatography/mass spectroscopy. Of the 22 elements analyzed, 10 (aluminum, chromium, copper, iron, lead, nickel, selenium, silicon, tin, zinc) were often found in the e-liquids. Five elements had the highest average concentrations: copper (1161.6 mg/L), zinc (295.8 mg/L), tin (287.6 mg/L), nickel (71.1 mg/L), and lead (50.3 mg/L). Nicotine concentrations were always lower than label concentrations indicated. Of the 181 flavor chemicals analyzed, 11 were detected in at least one sample, with hydroxyacetone being present in all samples. In used products, some flavor chemicals appeared to be by-products of heating. E-liquids with the highest concentrations of acids and the lowest pH levels also had the highest concentrations of elements/metals. Metal concentrations in e-liquids increased after use in some products, and some metal concentrations, such as nickel, were high enough to be a health concern. Leachates from discarded ECs could contribute toxic metals/chemicals to the environment, supporting the need for better regulation of atomizer design, composition, and disposal.

Acid-sensing ion channel 3: An analgesic target

Acid-sensing ion channel 3 (ASIC3) belongs to the epithelial sodium channel/degenerin (ENaC/DEG) superfamily. There are 7 different ASIC subunits encoded by 5 different genes. Most ASIC subunits form trimeric ion channels that upon activation by extracellular protons mediate a transient inward current inducing cellular excitability. ASIC subunits exhibit differential tissue expression and biophysical properties, and the ability of subunits to form homo- and heteromeric trimers further increases the complexity of currents measured and their pharmacological properties. ASIC3 is of particular interest, not only because it exhibits high expression in sensory neurones, but also because upon activation it does not fully inactivate: a transient current is followed by a sustained current that persists during a period of extracellular acidity, i.e. ASIC3 can encode prolonged acidosis as a nociceptive signal. Furthermore, certain mediators sensitize ASIC3 enabling smaller proton concentrations to activate it and other mediators can directly activate the channel at neutral pH. Moreover, there is a plethora of evidence using transgenic mouse models and pharmacology, which supports ASIC3 as being a potential target for development of analgesics. This review will focus on current understanding of ASIC3 function to provide an overview of how ASIC3 contributes to physiology and pathophysiology, examining the mechanisms by which it can be modulated, and highlighting gaps in current understanding and future research directions.

Mechanisms underlying the stimulatory effect of inhaled sulfur dioxide on vagal bronchopulmonary C-fibres

KEY POINTS

Brief inhalation of SO₂ of concentration >500 p.p.m. triggered a pronounced stimulatory effect on vagal bronchopulmonary C-fibres in anaesthetized rats. This stimulatory effect was drastically diminished by a pretreatment with NaHCO₃ that raised the baseline arterial pH, suggesting a possible involvement of acidification of airway fluid and/or tissue generated by inhaled SO₂ . The stimulation was completely abolished by pretreatment with antagonists of both acid-sensing ion channels and transient receptor potential vanilloid type-1 receptors, indicating that this effect was caused by acid activation of these cation channels expressed in airway sensory nerves. This conclusion was further supported by the results obtained from studies in isolated rat vagal bronchopulmonary sensory neurones and also in the cough response to SO₂ inhalation challenge in awake mice. These results provide new insight into the underlying mechanism of harmful irritant effects in the respiratory tract caused by accidental exposure to a high concentration of SO₂ .

ABSTRACT

Inhalation of sulfur dioxide (SO₂ ) triggers coughs and reflex bronchoconstriction, and stimulation of vagal bronchopulmonary C-fibres is primarily responsible. However, the mechanism underlying this stimulatory effect is not yet fully understood. In this study, we tested the hypothesis that the C-fibre stimulation was caused by SO₂ -induced local tissue acidosis in the lung and airways. Single-unit activities of bronchopulmonary C-fibres in response to inhalation challenges of SO₂ (500-1500 p.p.m., 10 breaths) were measured in anaesthetized rats. Inhalation of SO₂ reproducibly induced a pronounced and sustained stimulation (lasting for 15-60 s) of pulmonary C-fibres in a concentration-dependent manner. This stimulatory effect was significantly attenuated by an increase in arterial pH generated by infusion of sodium bicarbonate (NaHCO₃ ), and completely abrogated by a combined pretreatment with amiloride (an antagonist of acid-sensing ion channels, ASICs) and AMG8910 (a selective antagonist of the transient receptor potential vanilloid type-1 receptor, TRPV1). Furthermore, in isolated rat vagal pulmonary sensory neurones, perfusion of an aqueous solution of SO₂ evoked a transient increase in the intracellular Ca²⁺ concentration; this response was also markedly diminished by a pretreatment with amiloride and AMG8910. In addition, inhalation of SO₂ consistently evoked coughs in awake mice; responses were significantly smaller in TRPV1^-/- mice than in wild-type mice, and almost completely abolished after a pretreatment with amiloride in TRPV1^-/- mice. These results suggested that the stimulatory effect of inhaled SO₂ on bronchopulmonary C-fibres was generated by acidification of fluid and/or tissue in the lung and airways, which activated both ASICs and TRPV1 expressed in these sensory nerves.

Sex-specific airway hyperreactivity and sex-specific transcriptome remodeling in neonatal piglets challenged with intra-airway acid

Acute airway acidification is a potent stimulus of sensory nerves and occurs commonly with gastroesophageal reflux disease, cystic fibrosis, and asthma. In infants and adults, airway acidification can acutely precipitate asthma-like symptoms, and treatment-resistant asthma can be associated with gastroesophageal reflux disease. Airway protective behaviors, such as mucus secretion and airway smooth muscle contraction, are often exaggerated in asthma. These behaviors are manifested through activation of neural circuits. In some populations, the neural response to acid might be particularly important. For example, the immune response in infants is relatively immature compared with adults. Infants also have a high frequency of gastroesophageal reflux. Thus, in the current study, we compared the transcriptomes of an airway-nervous system circuit (e.g., tracheal epithelia, nodose ganglia, and brain stem) in neonatal piglets challenged with intra-airway acid. We hypothesized that the identification of parallel changes in the transcriptomes of two neutrally connected tissues might reveal the circuit response, and, hence, molecules important for the manifestation of asthma-like features. Intra-airway acid induced airway hyperreactivity and airway obstruction in male piglets. In contrast, female piglets displayed airway obstruction without airway hyperreactivity. Pairwise comparisons revealed parallel changes in genes directly implicated in airway hyperreactivity ( scn10a) in male acid-challenged piglets, whereas acid-challenged females exhibited parallel changes in genes associated with mild asthma ( stat 1 and isg15). These findings reveal sex-specific responses to acute airway acidification and highlight distinct molecules within a neural circuit that might be critical for the manifestation of asthma-like symptoms in pediatric populations.

Acid-Sensing Ion Channel 1a Contributes to Airway Hyperreactivity in Mice

Neurons innervating the airways contribute to airway hyperreactivity (AHR), a hallmark feature of asthma. Several observations suggested that acid-sensing ion channels (ASICs), neuronal cation channels activated by protons, might contribute to AHR. For example, ASICs are found in vagal sensory neurons that innervate airways, and asthmatic airways can become acidic. Moreover, airway acidification activates ASIC currents and depolarizes neurons innervating airways. We found ASIC1a protein in vagal ganglia neurons, but not airway epithelium or smooth muscle. We induced AHR by sensitizing mice to ovalbumin and found that ASIC1a^-/- mice failed to exhibit AHR despite a robust inflammatory response. Loss of ASIC1a also decreased bronchoalveolar lavage fluid levels of substance P, a sensory neuropeptide secreted from vagal sensory neurons that contributes to AHR. These findings suggest that ASIC1a is an important mediator of AHR and raise the possibility that inhibiting ASIC channels might be beneficial in asthma.

Differential regulation of ASICs and TRPV1 by zinc in rat bronchopulmonary sensory neurons

PURPOSE

Zinc has been known to act as a signaling molecule that regulates a variety of neuronal functions. In this study, we aimed to study the effect of zinc on two populations of acid-sensitive ion channels, acid-sensing ion channels (ASICs), and transient receptor potential vanilloid receptor-1 (TRPV1), in vagal bronchopulmonary sensory neurons.

METHODS

Rat vagal sensory neurons innervating lungs and airways were retrogradely labeled with a fluorescent tracer. Whole-cell perforated patch-clamp recordings were carried out in primarily cultured bronchopulmonary sensory neurons. The acid-evoked ASIC and TRPV1 currents were measured and compared between before and after the zinc pretreatment.

RESULTS

ASIC currents were induced by a pH drop from 7.4 to 6.8 or 6.5 in the presence of capsazepine (10 µM), a specific TRPV1 antagonist. Pretreatment with zinc (50 or 300 µM, 2 min) displayed different effects on the two distinct phenotypes of ASIC currents: a marked potentiation on ASIC channels with fast kinetics of activation and inactivation or no significant effect on ASIC currents with slow activation and inactivation. On the other hand, pretreatment with zinc significantly inhibited the acid (pH 5.5 or 5.3)-induced TRPV1 currents. The inhibition was abolished by intracellular chelation of zinc by TPEN (25 µM), indicating that intracellular accumulation of zinc was likely required for its inhibitory effect on TRPV1 channels.

CONCLUSIONS

Our study showed that zinc differentially regulates the activities of ASICs and TRPV1 channels in rat vagal bronchopulmonary sensory neurons.

Changing the paradigm for cough: does 'cough hypersensitivity' aid our understanding?

Chronic cough is a common reason for patients to seek medication attention. Over the last few decades, we have experienced significant clinical success by applying the paradigm of 'evaluating and treating the causes for chronic cough'. However, we still ask ourselves 'what underlies chronic cough. Indeed in a considerable proportion of patients cough is idiopathic, or unexplained despite vigorous evaluation. Commonly associated conditions such as rhinitis, eosinophilic bronchitis, asthma, or gastroesophageal acidic reflux may not be fundamental to cough, and thus may be triggers rather than causes. The cardinal feature of chronic cough is persistent upregulation the cough reflex, which may be driven by complex interactions between biologic, neurologic, immunologic, genetic, comorbid, and environmental factors. We suggest the new paradigm 'cough hypersensitivity syndrome' should finally bring us further advances in understanding and management of chronic cough.

Acid-sensing by airway afferent nerves

Inhalation of acid aerosol or aspiration of acid solution evokes a stimulatory effect on airway C-fiber and Aδ afferents, which in turn causes airway irritation and triggers an array of defense reflex responses (e.g., cough, reflex bronchoconstriction, etc.). Tissue acidosis can also occur locally in the respiratory tract as a result of ischemia or inflammation, such as in the airways of asthmatic patients during exacerbation. The action of proton on the airway sensory neurons is generated by activation of two different current species: a transient (rapidly activating and inactivating) current mediated through the acid-sensing ion channels, and a slowly activating and sustained current mediated through the transient receptor potential vanilloid type 1 (TRPV1) receptor. In view of the recent findings that the expression and/or sensitivity of TRPV1 are up-regulated in the airway sensory nerves during chronic inflammatory reaction, the proton-evoked irritant effects on these nerves may play an important part in the manifestation of various symptoms associated with airway inflammatory diseases.

Transient receptor potential vanilloid 2 (TRPV2), a potential novel biomarker in childhood asthma

BACKGROUND

The presence of transient receptor potential vanilloid 2 (TRPV2) in human peripheral blood cells may suggest a role under pathological conditions. The aim of this study was to explore the relationship between the expression profile of TRPV2 gene and childhood asthma in the north of China. The effects of allergens exposure on the expression of TRPV2 gene were also investigated.

METHODS

Sixty asthmatics children confirmed by physician diagnosis and 60 healthy children as a control group were recruited. Serum total IgE and specific IgE were measured. Using quantitative Reverse Transcription Polymerase Chain Reaction (qRT-PCR), TRPV2 was detected in total RNA extracted from peripheral blood lymphocytes. Student's t-test and chi-square test were used to analyze the relationship between TRPV2 transcript and different parameter variables on susceptibility of childhood asthma. Multiple logistic regression was used to analyze the associations between TRPV2 gene and allergens.

RESULTS

The expression level of TRPV2 gene was increased 2.6 times in asthmatic children compared with controls (p < .01). The up-regulation of TRPV2 gene and sensitization to one of three the allergens-spring pollen, dust mite, and dog and cat hair-were correlated with childhood asthma. In addition, the hypersensitivity to spring pollen, cockroach, and dust mite and up-regulation of TRPV2 gene expression may be the risk factors for the childhood asthma in Beijing.

CONCLUSIONS

The increased expression of TRPV2 gene in peripheral lymphocytes is closely correlated with childhood asthma in the north of China. This study provides a potential new biomarker of childhood asthma and lays the basis for further clarification of the pathogenesis underlying asthma.

The influence of sour taste on Dysphagia in brain injury: blind study

Objective

To verify the influence of sour taste on swallowing and the presence of reflex cough when sour material was swallowed in patients with dysphagia secondary to brain injury.

Method

Fifty dysphagic brain injury patients who underwent videofluoroscopic swallowing study (VFSS) were recruited. The patients who had shown severe aspiration at 2 ml of liquid were excluded. The dysphagic patients were given 5 ml each of a sour tasting liquid (SOUR) and a thin liquid barium (LIQUID) in random order. An expert analyzed the result of VFSS by reviewing recorded videotapes. Analysis components consisted of the Penetration-Aspiration-Scale (PAS) score, oral transit time (OTT), pharyngeal transit time (PTT), pharyngeal delay time (PDT) and the reflex cough presence.

Results

The PAS score for SOUR was significantly lower than the one for LIQUID (p=0.03). The mean OTT for SOUR was significantly shortened compared to that for LIQUID (p=0.03). The mean PTT and PDT were also shortened in SOUR, although the differences were not statistically significant (p=0.26 and p=0.32, respectively). There was no significant difference between SOUR and LIQUID regarding the presence of reflex cough (p=1.00).

Conclusion

The sour taste could enhance sensorimotor feedback in the oropharynx, thus lowering the chances of penetration-aspiration caused by shortening of the oropharyngeal passage times. There was no significant difference in the presence of reflex cough produced between LIQUID and SOUR.