Role of bronchopulmonary C-fiber afferents in the apneic response to cigarette smoke

Pharmacological and physiological response in Apoe-/- mice exposed to cigarette smoke or e-cigarette aerosols

OBJECTIVE

Electronic cigarettes (e-cigs) are popular nicotine delivery devices, yet the health effects remain unclear. To determine equivalent biomarkers, we characterized the immediate response in Apoe^-/- mice exposed to tank/box-mod e-cig (e-cig_tank), pod e-cig (e-cig_pod), or cig smoke.

MATERIALS AND METHODS

Reproducible puff profiles were generated for each aerosol and delivered to Apoe^-/- mice via a nose-only exposure system. Serum cotinine levels were quantified at various time points through ELISA and utilized to model cotinine pharmacokinetics. In addition, particle size measurements and mouse respiratory function were characterized to calculate particle dosimetry.

RESULTS AND DISCUSSION

Cig and e-cig_tank particles were lognormally distributed with similar count median diameters (cig: 178 ± 2, e-cig_tank: 200 ± 34nm), while e-cig_pod particles were bimodally distributed and smaller (116 ± 13 and 13.3 ± 0.4 nm). Minute volumes decreased with cig exposure (5.4 ± 2.7 mL/min) compared to baseline (90.8 ± 11.6 mL/min), and less so with e-cig_tank (45.2 ± 9.2 mL/min) and e-cig_pod exposures (58.6 ± 6.8 mL/min), due to periods of apnea in the cig exposed groups. Cotinine was absorbed and eliminated most rapidly in the e-cig_pod group (tmax = 14.5; t1/2' = 51.9 min), whereas cotinine was absorbed (cig: 50.4, e-cig_tank: 40.1 min) and eliminated (cig: 104.6, e-cig_tank: 94.1 min) similarly in the cig and e-cig_tank groups. For exposure times which equate the area under the cotinine-concentration curve, ∼6.4× (e-cig_tank) and 4.6× (e-cig_pod) more nicotine deposited in e-cig compared to cig exposed mice.

CONCLUSIONS

This study provides a basis for incorporating cotinine pharmacokinetics into preclinical exposure studies, allowing for longitudinal studies of structural and functional changes due to exposure.

Air Pollution-Induced Autonomic Modulation

Air pollutants pose a serious worldwide health hazard, causing respiratory and cardiovascular morbidity and mortality. Pollutants perturb the autonomic nervous system, whose function is critical to cardiopulmonary homeostasis. Recent studies suggest that pollutants can stimulate defensive sensory nerves within the cardiopulmonary system, thus providing a possible mechanism for pollutant-induced autonomic dysfunction. A better understanding of the mechanisms involved would likely improve the management and treatment of pollution-related disease.

Nociceptive pulmonary-cardiac reflexes are altered in the spontaneously hypertensive rat

KEY POINTS

We investigated the cardiovascular and respiratory responses of the normotensive Wistar-Kyoto (WKY) rat and the spontaneously hypertensive (SH) rat to inhalation and intravenous injection of the noxious stimuli allyl isothiocyanate (AITC). AITC inhalation evoked atropine-sensitive bradycardia in conscious WKY rats, and evoked atropine-sensitive bradycardia and atenolol-sensitive tachycardia with premature ventricular contractions (PVCs) in conscious SH rats. Intravenous injection of AITC evoked bradycardia but no tachycardia/PVCs in conscious SHs, while inhalation and injection of AITC caused similar bradypnoea in conscious SH and WKY rats. Anaesthesia (inhaled isoflurane) inhibited the cardiac reflexes evoked by inhaled AITC but not injected AITC. Data indicate the presence of a de novo nociceptive pulmonary-cardiac reflex triggering sympathoexcitation in SH rats, and this reflex is dependent on vagal afferents but is not due to steady state blood pressure or due to remodelling of vagal efferent function.

ABSTRACT

Inhalation of noxious irritants/pollutants activates airway nociceptive afferents resulting in reflex bradycardia in healthy animals. Nevertheless, noxious pollutants evoke sympathoexcitation (tachycardia, hypertension) in cardiovascular disease patients. We hypothesize that cardiovascular disease alters nociceptive pulmonary-cardiac reflexes. Here, we studied reflex responses to irritants in normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive (SH) rats. Inhaled allyl isothiocyanate (AITC) evoked atropine-sensitive bradycardia with atrial-ventricular (AV) block in conscious WKY rats, thus indicating a parasympathetic reflex. Conversely, inhaled AITC in conscious SH rats evoked complex brady-tachycardia with both AV block and premature ventricular contractions (PVCs). Atropine abolished the bradycardia and AV block, but the atropine-insensitive tachycardia and PVCs were abolished by the β₁ -adrenoceptor antagonist atenolol. The aberrant AITC-evoked reflex in SH rats was not reduced by acute blood pressure reduction by captopril. Surprisingly, intravenous AITC only evoked bradycardia in conscious SH and WKY rats. Furthermore, anaesthesia reduced the cardiac reflexes evoked by inhaled but not injected AITC. Nevertheless, anaesthesia had little effect on AITC-evoked respiratory reflexes. Such data suggest distinct differences in nociceptive reflex pathways dependent on cardiovascular disease, administration route and downstream effector. AITC-evoked tachycardia in decerebrate SH rats was abolished by vagotomy. Finally, there was no difference in the cardiac responses of WKY and SH rats to vagal efferent electrical stimulation. Our data suggest that AITC inhalation in SH rats evokes de novo adrenergic reflexes following vagal afferent activation. This aberrant reflex is independent of steady state hypertension and is not evoked by intravenous AITC. We conclude that pre-existing hypertension aberrantly shifts nociceptive pulmonary-cardiac reflexes towards sympathoexcitation.

Characterization of cardiovascular reflexes evoked by airway stimulation with allylisothiocyanate, capsaicin, and ATP in Sprague-Dawley rats

Acute inhalation of airborne pollutants alters cardiovascular function and evidence suggests that pollutant-induced activation of airway sensory nerves via the gating of ion channels is critical to these systemic responses. Here, we have investigated the effect of capsaicin [transient receptor potential (TRP) vanilloid 1 (TRPV1) agonist], AITC [TRP ankyrin 1 (TRPA1) agonist], and ATP (P2X2/3 agonist) on bronchopulmonary sensory activity and cardiovascular responses of conscious Sprague-Dawley (SD) rats. Single fiber recordings show that allyl isothiocyanate (AITC) and capsaicin selectively activate C fibers, whereas subpopulations of both A and C fibers are activated by stimulation of P2X2/3 receptors. Inhalation of the agonists by conscious rats caused significant bradycardia, atrioventricular (AV) block, and prolonged PR intervals, although ATP-induced responses were lesser than those evoked by AITC or capsaicin. Responses to AITC were inhibited by the TRP channel blocker ruthenium red and the muscarinic antagonist atropine. AITC inhalation also caused a biphasic blood pressure response: a brief hypertensive phase followed by a hypotensive phase. Atropine accentuated the hypertensive phase, while preventing the hypotension. AITC-evoked bradycardia was not abolished by terazosin, the α1-adrenoceptor inhibitor, which prevented the hypertensive response. Anesthetics had profound effects on AITC-evoked bradycardia and AV block, which was abolished by urethane, ketamine, and isoflurane. Nevertheless, AITC inhalation caused bradycardia and AV block in paralyzed and ventilated rats following precollicular decerebration. In conclusion, we provide evidence that activation of ion channels expressed on nociceptive airway sensory nerves causes significant cardiovascular effects in conscious SD rats via reflex modulation of the autonomic nervous system.

Role of autonomic reflex arcs in cardiovascular responses to air pollution exposure

The body responds to environmental stressors by triggering autonomic reflexes in the pulmonary receptors, baroreceptors, and chemoreceptors to maintain homeostasis. Numerous studies have shown that exposure to various gases and airborne particles can alter the functional outcome of these reflexes, particularly with respect to the cardiovascular system. Modulation of autonomic neural input to the heart and vasculature following direct activation of sensory nerves in the respiratory system, elicitation of oxidative stress and inflammation, or through other mechanisms is one of the primary ways that exposure to air pollution affects normal cardiovascular function. Any homeostatic process that utilizes the autonomic nervous system to regulate organ function might be affected. Thus, air pollution and other inhaled environmental irritants have the potential to alter both local airway function and baro- and chemoreflex responses, which modulate autonomic control of blood pressure and detect concentrations of key gases in the body. While each of these reflex pathways causes distinct responses, the systems are heavily integrated and communicate through overlapping regions of the brainstem to cause global effects. This short review summarizes the function of major pulmonary sensory receptors, baroreceptors, and carotid body chemoreceptors and discusses the impacts of air pollution exposure on these systems.

Sensory nerves in lung and airways

Sensory nerves innervating the lung and airways play an important role in regulating various cardiopulmonary functions and maintaining homeostasis under both healthy and disease conditions. Their activities conducted by both vagal and sympathetic afferents are also responsible for eliciting important defense reflexes that protect the lung and body from potential health-hazardous effects of airborne particulates and chemical irritants. This article reviews the morphology, transduction properties, reflex functions, and respiratory sensations of these receptors, focusing primarily on recent findings derived from using new technologies such as neural immunochemistry, isolated airway-nerve preparation, cultured airway neurons, patch-clamp electrophysiology, transgenic mice, and other cellular and molecular approaches. Studies of the signal transduction of mechanosensitive afferents have revealed a new concept of sensory unit and cellular mechanism of activation, and identified additional types of sensory receptors in the lung. Chemosensitive properties of these lung afferents are further characterized by the expression of specific ligand-gated ion channels on nerve terminals, ganglion origin, and responses to the action of various inflammatory cells, mediators, and cytokines during acute and chronic airway inflammation and injuries. Increasing interest and extensive investigations have been focused on uncovering the mechanisms underlying hypersensitivity of these airway afferents, and their role in the manifestation of various symptoms under pathophysiological conditions. Several important and challenging questions regarding these sensory nerves are discussed. Searching for these answers will be a critical step in developing the translational research and effective treatments of airway diseases.

Metalloelastase in lungs and alveolar macrophages is modulated by extracellular substance P in mice

Metalloelastase (MMP-12), mainly produced by macrophages, has been shown to play a key role in the pathogenesis of emphysema in animal models. Chronic cigarette smoke increases pulmonary MMP-12, which is closely correlated with an elevation of pulmonary substance P (SP). Because alveolar macrophages (AMs) contain the neurokinin-1 receptor (NK1R), we tested whether SP was able to trigger the upregulation of MMP-12 synthesis in AMs by acting on the NK1R. AMs isolated from bronchoalveolar lavage cells in C3H/HeN mice were cultured with control medium or SP that was coupled without or with NK1R antagonists (CP-99,994 or aprepitant) for 24 h. We found that SP significantly increased the mRNA of MMP-12 and NK1R by 11-fold and 82%, respectively, in AMs (P<0.05), and these responses were abolished by NK1R antagonists with little change in the cells' viability. Because pulmonary SP is primarily released by bronchopulmonary C-fibers (PCFs), we further asked whether destruction of PCFs would reduce SP and MMP-12. Two groups of mice were pretreated with vehicle and neonatal capsaicin (NCAP) to degenerate PCFs, respectively. Our results show that NCAP treatment significantly decreased mRNA and protein levels of SP associated with a reduction NK1R and MMP-12 in the lungs and AMs. These findings suggest that SP has a modulatory effect on pulmonary MMP-12 by acting on NK1R to trigger MMP-12 syntheses in the AMs.

Canine models of asthma and COPD

Dogs have been extensively used to model the important components of asthma and COPD. Many of the key features of human asthma such as reversible airflow obstruction, pulmonary inflammation, airway hyperresponsiveness and cough are demonstrated in dogs after provocation with antigen, following a period of hyperventilation with dry air or after inhalation of ozone. Furthermore, standard anti-asthma drugs such as beta-adrenergic agonists, corticosteroids and leukotriene inhibitors are effective in these models. The pathology and pathophysiology of chronic bronchitis and emphysema can also be demonstrated in dogs after exposure to cigarette smoke, following inhalation of sulfur dioxide and by intra-tracheal or aerosol administration of proteolytic enzymes such as papain. These canine models of COPD have been used to evaluate a variety of new methodologies and treatments before they are tested in humans. This review highlights some of the important features of these canine models and how they have increased our understanding of the pathology, pathophysiology and control of human asthma and COPD.

Acute ventilatory and circulatory reactions evoked by nicotine: are they excitatory or depressant?

Either excitatory or inhibitory cardio-respiratory responses induced by nicotine have been reported. We evaluated the joint and separate contributions of peripheral arterial chemoreceptors and pulmonary vagal afferences to nicotine-induced cardio-respiratory responses in 11 pentobarbitone-anaesthetized cats. Nicotine, given i.v. in doses of from 1 to 200 microg/kg, evoked dose-dependent transient increases in tidal volume (VT) and arterial blood pressure (BP), but the highest doses evoked brief apnoea, immediately followed by intense hyperventilation, as well as discrete early hypotension followed by late hypertension. Bilateral section of the aortic and carotid nerves abolished all hyperventilatory responses to nicotine, giving way to apnoea followed by few cycles of reduced VT and profound hypotension followed by slight hypertension in response to intermediate doses (50-100 microg/kg). Subsequent bilateral vagotomy (BV) suppressed apnoeic and hypotensive responses. In other cats initially subjected to BV, only increases in VT and BP were observed in response to nicotine, effects which were no longer observed after additional carotid and aortic deafferentation. These data suggest that excitatory effects of nicotine on respiration and BP are reflexes evoked by stimulation of peripheral arterial chemoreceptors, while inhibitory effects are also reflex responses but evoked from stimulation of pulmonary vagal afferences.

Role of aortic chemoreceptors in the hypertensive response to cigarette smoke

To determine the role of aortic chemoreceptors in the rise of systemic blood pressure (BP) seen with smoking cigarettes, their responses to injecting various doses of nicotine (Nic) into the left atrium and to a puff of cigarette smoke were studied in 31 cats. The activity of 71% (n = 36) of the chemoreceptor fibres was stimulated significantly by a puff of cigarette smoke (delivering approximately 10.0 micrograms kg-1 Nic). The mean threshold dose to stimulation by nicotine of these fibres was 8.0 micrograms kg-1. The activity of the remaining 29% (n = 15) was not stimulated; their mean threshold dose to stimulation by Nic was 24 micrograms kg-1. Stimulation of fibres lasted for about 14 +/- 0.2 sec and the rise in BP was seen for 20-60 sec. By using hexamethonium it was established that the stimulation was produced exclusively by Nic contained in cigarette smoke. It was concluded that aortic chemoreceptors must contribute to the reflex rise in BP produced by smoking cigarettes.

Vagal bronchopulmonary C-fibers and acute ventilatory response to inhaled irritants

Experiments were carried out in anesthetized Sprague-Dawley rats to determine the role of vagal bronchopulmonary C-fiber afferents in regulating the respiratory responses to inhaled irritants. Spontaneous inhalation of 2 tidal breaths of a known airway irritant (sulfur dioxide, 0.5%; ammonia, 1%; cigarette smoke, 50%) into the lower airways invariably elicited an immediate and transient inhibitory effect on breathing, characterized by apnea or bradypnea and accompanied by bradycardia, which lasted for 3-8 breaths. A delayed hyperpnea was also induced by inhalation of cigarette smoke, but not by sulfur dioxide or ammonia. After perineural capsaicin treatment of both cervical vagi to selectively block the conduction of capsaicin-sensitive C-fibers, these inhaled irritants no longer evoked any inhibitory effect on breathing; conversely, an augmented inspiration was triggered within the first 3 breaths from the onset of cigarette smoke inhalation in > 85% of the rats studied, but after a delay of several breaths following inhalation of ammonia or sulfur dioxide in only 30% of the rats. The augmented breaths were completely abolished when both cervical vagi were cooled to 6-7 degrees C. Bilateral vagotomy eliminated all the immediate responses to these irritants. These results suggest that both vagal C-fiber endings and irritant receptors in the airways are activated by these inhaled irritants, but the more dominant and consistent inhibitory effect on breathing is elicited primarily by stimulation of C-fiber afferents.

Cigarette smoke-induced bronchoconstriction and release of tachykinins in guinea pig lungs

Two series of experiments were carried out to determine whether the release of tachykinins is involved in the bronchoconstriction induced by inhalation of cigarette smoke in guinea pigs. In the first series, cigarette smoke consistently induced bronchoconstriction (delta RL = +203% and delta Cdyn = -46%) in anesthetized guinea pigs, and the response was only partially blocked by bilateral cervical vagotomy. However, the smoke-induced bronchial constriction was completely abolished in animals receiving a systemic capsaicin pretreatment to destroy the tachykinin-containing C-fiber afferents. In the second series, the bronchoconstrictive effect of cigarette smoke was increased by approx. three times in isolated perfused guinea pig lungs when phosphoramidon (3 x 10(-6) M) was added to the perfusate to prevent the degradation of tachykinins after their release. Moreover, the enhanced bronchomotor response to smoke was accompanied by an overflow of neurokinin A-like immunoreactivity (LI) and calcitonin gene-related peptide -LI in the pulmonary effluent. These studies showed that cigarette smoke triggers the release of tachykinins in the lungs, which plays an important role in the smoke-induced bronchoconstrictive effect in guinea pigs.

Nicotine-induced airway smooth muscle contraction: neural mechanisms involving the airway epithelium. Functional and histologic studies in vitro

To assess the mechanism of and the role of the epithelium in nicotine-induced bronchoconstriction in vitro, we performed a combined functional and histologic study. Functional study: We suspended tracheal strips or rings from 16 ferrets (1124 +/- 561 g, mean +/- SD) in organ baths. Alternate tracheal strips had their epithelium removed. Dose-response curves to acetylcholine (ACh) and nicotine were established for pairs of tissues with and without epithelium, each pair receiving only one dose of nicotine. Nicotine induced brief muscle contractions not exceeding 25% of the ACh-induced maximum. Contractions were blocked by hexamethonium and 10(-7) M atropine and were abolished or inhibited strongly by tetrodotoxin (TTX), suggesting the involvement of nicotinic neuronal and muscarinic smooth muscle receptors. Removal of the epithelium strongly inhibited contractions at concentrations of nicotine greater than 3 x 10(-5) M which completely removed any dose-response effect. ACh-induced contractions were unchanged, demonstrating smooth muscle integrity. We suggest that the removal of the epithelium attenuates nicotine-induced bronchoconstriction through the removal of nerves running in or close to the epithelium. Histologic study: In tracheae from 15 ferrets (8 male, 7 female), mean weight (+/- SD) 1288 (+/- 470) g, we examined 4 techniques of epithelium removal: (1) gentle scraping with a scalpel blade moved backwards (away from the cutting edge), (2) moving a Q-tip through the unopened tracheal tube without lateral pressure, and (3, 4) stroking the mucosa of opened tracheal segments with a Q-tip, exerting (3) light or (4) moderate pressure. All methods were equally (97%-100%) efficient in removing the epithelium but differed in the amount of damage caused to the basement membrane and/or submucosal tissue. Method (2) caused less damage to the basement membrane than the other methods but still removed almost one-third of it. The study showed that complete removal of the epithelium is at the expense of the submucosa and that a given result of "epithelium removal" is also attributable to removal of the neighboring subepithelial structures.

Effect of a single breath of cigarette smoke on slowly adapting receptors in canine lungs

Inhalation of cigarette smoke has been shown to induce bronchoconstriction which should stimulate slowly adapting pulmonary stretch receptors (PSRs). To test this possibility, the activity of PSRs was recorded from fine afferent filaments of the vagus nerve before and after 120 ml of smoke generated from high-nicotine cigarettes was delivered into the lungs in a single breath in anesthetized, open-chest and artificially ventilated dogs. The base-line activity of PSRs did not change during the first two breaths following smoke delivery. However, PSR activity started to increase by the third breath (post-smoke), concomitant with an increase in tracheal (transpulmonary) pressure. Both the smoke-induced increase in tracheal pressure and the delayed effect on PSRs were prevented by a pretreatment with aerosolized isoproterenol, a bronchodilator, suggesting that the delayed response of PSRs to smoke was elicited by the change in bronchomotor tone. Although smoke evoked a delayed stimulation in the majority (61%) of the PSRs studied, it caused a mild delayed inhibition (24%) or had no effect (15%) in some of the receptors. The variable responses to smoke among PSRs are probably related to the smoke-induced heterogeneous changes of mechanical properties in the lungs and their different anatomic locations.

Inhibitory effect of gas phase cigarette smoke on breathing: role of hydroxyl radical

Inhaling cigarette smoke evokes immediate bradypnea in rats, resulting from stimulation of vagal bronchopulmonary C-fiber afferents by smoke constituent(s) other than nicotine. To determine the contribution of the gas phase of smoke to this irritant effect, the acute respiratory responses to both cigarette smoke and gas phase smoke were studied and compared in anesthetized Sprague-Dawley rats; smoke (6 ml, 50%) was generated by a machine from low-nicotine research cigarettes and the gas phase was obtained by passing the smoke through a glass-fiber Cambridge filter. Inhalation of gas phase smoke alone evoked a transient inhibitory effect on breathing, prolonging expiratory time (Te) to a peak of 159 +/- 6% of the base line; this response was very similar to that triggered by inhaling the unfiltered smoke (Te = 177 +/- 12%). The bradypnea started within 1-4 breaths after the onset of smoke inhalation, lasted for 3-5 breaths and was completely abolished by vagotomy. This inhibitory effect of gas phase smoke on breathing was also largely prevented after a pretreatment with either intravenous infusion or aerosol inhalation of a hydroxyl radical scavenger, dimethylthiourea. These results suggest that the gas phase is primarily responsible for eliciting the reflexogenic bradypneic response to cigarette smoke in anesthetized rats and that hydroxyl radicals released endogenously in the lungs may be involved.

The stimulatory effect of nicotine on vagal pulmonary C-fibers in dogs

Our recent studies suggested that a nicotine-induced stimulation of afferent vagal C-fibers in the lungs was involved in eliciting the immediate cardiorespiratory responses to inhaling cigarette smoke. To examine this possibility, afferent impulses were recorded from vagal pulmonary C-fibers in 16 anesthetized, open-chest and artificially ventilated dogs, before and after four separate doses of nicotine (2.5, 5, 10 and 20 micrograms/kg) were injected into the right atrium. The base-line activity did not change after injection of isotonic saline. In contrast, nicotine stimulated 24 of 29 C-fibers: a burst of discharge was evoked immediately (1-2 sec) after the injection and usually lasted 3-8 sec. The peak responses of these pulmonary C-fibers to nicotine injections showed a dose-dependent relationship. In 17 C-fibers tested, the responses evoked by right atrial injection of 10 micrograms/kg of nicotine were similar to those evoked by delivery into the lungs of a single breath of smoke generated from cigarette with a high-nicotine content. Based upon these results, we conclude that nicotine alone stimulates vagal pulmonary C-fibers in a dose-dependent manner and this stimulant action of nicotine may play a part in eliciting the immediate reflex cardiorespiratory responses to inhalation of cigarette smoke.