Nitric oxide-dependent modulation of smooth-muscle tone by airway parasympathetic nerves

The differential roles of the two NO-GC isoforms in adjusting airway reactivity

The enzyme, nitric oxide-sensitive guanylyl cyclase (NO-GC), is activated by binding NO to its prosthetic heme group and catalyzes the formation of cGMP. The NO-GC is primarily known to mediate vascular smooth muscle relaxation in the lung, and inhaled NO has been successfully used as a selective pulmonary vasodilator. In comparison, NO-GC’s impact on the regulation of airway tone is less acknowledged and, most importantly, little is known about the issue that NO-GC signaling is accomplished by two isoforms: NO-GC1 and NO-GC2, implying the existence of distinct “cGMP pools.” Herein, we investigated the functional role of the NO-GC isoforms in respiration by measuring lung function parameters of isoform-specific knockout (KO) mice using noninvasive and invasive techniques. Our data revealed the participation and ongoing influence of NO-GC1-derived cGMP in the regulation of airway tone by showing that respiratory resistance was enhanced in NO-GC1-KOs and increased more pronouncedly after the challenge with the bronchoconstrictor methacholine. The tissue resistance and stiffness of NO-GC1-KOs were also higher because of narrowed airways that cause tissue distortion. Contrariwise, NO-GC2-KOs displayed reduced tissue elasticity, elastic recoil, and airway reactivity to methacholine, which did not even increase in an ovalbumin model of asthma that induced hyperresponsiveness in NO-GC1-KOs. In addition, conscious NO-GC2-KOs showed a higher breathing rate with a shorter duration of inspiration and expiration time, which remained faster even in the presence of bronchoconstrictors that slow down breathing. Thus, we provide evidence of two distinct NO/cGMP pathways in airways, accomplished by either NO-GC1 or NO-GC2, adjusting differentially the airway reactivity.

Airway Sensory Nerve Plasticity in Asthma and Chronic Cough

Airway sensory nerves detect a wide variety of chemical and mechanical stimuli, and relay signals to circuits within the brainstem that regulate breathing, cough, and bronchoconstriction. Recent advances in histological methods, single cell PCR analysis and transgenic mouse models have illuminated a remarkable degree of sensory nerve heterogeneity and have enabled an unprecedented ability to test the functional role of specific neuronal populations in healthy and diseased lungs. This review focuses on how neuronal plasticity contributes to development of two of the most common airway diseases, asthma and chronic cough, and discusses the therapeutic implications of emerging treatments that target airway sensory nerves.

Vagal Afferent Innervation of the Airways in Health and Disease

Vagal sensory neurons constitute the major afferent supply to the airways and lungs. Subsets of afferents are defined by their embryological origin, molecular profile, neurochemistry, functionality, and anatomical organization, and collectively these nerves are essential for the regulation of respiratory physiology and pulmonary defense through local responses and centrally mediated neural pathways. Mechanical and chemical activation of airway afferents depends on a myriad of ionic and receptor-mediated signaling, much of which has yet to be fully explored. Alterations in the sensitivity and neurochemical phenotype of vagal afferent nerves and/or the neural pathways that they innervate occur in a wide variety of pulmonary diseases, and as such, understanding the mechanisms of vagal sensory function and dysfunction may reveal novel therapeutic targets. In this comprehensive review we discuss historical and state-of-the-art concepts in airway sensory neurobiology and explore mechanisms underlying how vagal sensory pathways become dysfunctional in pathological conditions.

Protective effect of zingerone on increased vascular contractility in diabetic rat aorta

The aim of the present study was to investigate the effect and possible mechanism of action of zingerone, the main constituent of ginger, on vascular reactivity in isolated aorta from diabetic rats. The results show that incubation of aortae with zingerone alleviates the exaggerated vasoconstriction of diabetic aortae to phenylephrine, as well as the impaired relaxatory response to acetylcholine in a concentration-dependent manner. Furthermore, Zingerone directly relax phenylephrine-precontracted aortae. The vasorelaxatory response is significantly attenuated by the nitric oxide synthase inhibitor Nω-nitro-l-arginine methyl ester hydrochloride and the guanylate cyclase inhibitor methylene blue but no effect of either the potassium channels blocker tetraethylammonium chloride, or the cyclooxygenase inhibitor indomethacin was observed. Zingerone had no effect on advanced glycation end product formation as well. In conclusion, zingerone ameliorates enhanced vascular contraction in diabetic aortae which may be mediated by its vasodilator effect through NO- and guanylate cyclase stimulation.

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.

Competitive metabolism of L-arginine: arginase as a therapeutic target in asthma

Exhaled breath nitric oxide (NO) is an accepted asthma biomarker. Lung concentrations of NO and its amino acid precursor, L-arginine, are regulated by the relative expressions of the NO synthase (NOS) and arginase isoforms. Increased expression of arginase I and NOS2 occurs in murine models of allergic asthma and in biopsies of asthmatic airways. Although clinical trials involving the inhibition of NO-producing enzymes have shown mixed results, small molecule arginase inhibitors have shown potential as a therapeutic intervention in animal and cell culture models. Their transition to clinical trials is hampered by concerns regarding their safety and potential toxicity. In this review, we discuss the paradigm of arginase and NOS competition for their substrate L-arginine in the asthmatic airway. We address the functional role of L-arginine in inflammation and the potential role of arginase inhibitors as therapeutics.

Autonomic neural control of the airways

The airways and lungs are innervated by both sympathetic and parasympathetic nerves. Cholinergic parasympathetic innervation is well conserved in the airways while the distribution of noncholinergic parasympathetic and adrenergic sympathetic nerves varies considerably amongst species. Autonomic nerve function is regulated primarily through reflexes initiated upon bronchopulmonary vagal afferent nerves. Central regulation of autonomic tone is poorly described but some key elements have been defined.

Neuronal modulation of airway and vascular tone and their influence on nonspecific airways responsiveness in asthma

The autonomic nervous system provides both cholinergic and noncholinergic neural inputs to end organs within the airways, which includes the airway and vascular smooth muscle. Heightened responsiveness of the airways to bronchoconstrictive agents is a hallmark feature of reactive airways diseases. The mechanisms underpinning airways hyperreactivity still largely remain unresolved. In this paper we summarize the substantial body of evidence that implicates dysfunction of the autonomic nerves that innervate smooth muscle in the airways and associated vasculature as a prominent cause of airways hyperresponsiveness in asthma.

Low voltage vagal nerve stimulation reduces bronchoconstriction in guinea pigs through catecholamine release

OBJECTIVE

  Electrical stimulation of the vagus nerve at relatively high voltages (e.g., >10 V) can induce bronchoconstriction. However, low voltage (≤2 V) vagus nerve stimulation (VNS) can attenuate histamine-invoked bronchoconstriction. Here, we identify the mechanism for this inhibition.

METHODS

  In urethanea-nesthetized guinea pigs, bipolar electrodes were attached to both vagus nerves and changes in pulmonary inflation pressure were recorded in response to i.v. histamine and during VNS. The attenuation of the histamine response by low-voltage VNS was then examined in the presence of pharmacologic inhibitors or nerve ligation.

RESULTS

  Low-voltage VNS attenuated histamine-induced bronchoconstriction (4.4 ± 0.3 vs. 3.2 ± 0.2 cm H(2) O, p < 0.01) and remained effective following administration of a nitric oxide synthase inhibitor, NG-nitro-L-arginine methyl ester, and after sympathetic nerve depletion with guanethidine, but not after the β-adrenoceptor antagonist propranolol. Nerve ligation caudal to the electrodes did not block the inhibition but cephalic nerve ligation did. Low-voltage VNS increased circulating epinephrine and norepinephrine without but not with cephalic nerve ligation.

CONCLUSION

  These results indicate that low-voltage VNS attenuates histamine-induced bronchoconstriction via activation of afferent nerves, resulting in a systemic increase in catecholamines likely arising from the adrenal medulla.

Force oscillations simulating breathing maneuvers do not prevent force adaptation

Airway inflammation in patients with asthma exposes the airway smooth muscle (ASM) to a variety of spasmogens. These spasmogens increase ASM tone, which can lead to force adaptation. Length oscillations of ASM, which occur in vivo due to breathing maneuvers, can attenuate force adaptation. However, in the presence of tone, the force oscillations required to achieve these length oscillations may be unphysiologic (i.e., magnitude greater than the ones achieved due to the swings in transpulmonary pressure required for breathing). In the present study, we applied force oscillations simulating the tension oscillations experienced by the wall of a fourth-generation airway during tidal breathing with or without deep inspirations (DI) to ASM. The goal was to investigate whether force adaptation occurs in conditions mimicking breathing maneuvers. Tone was induced by carbachol (average, 20 nM), and the force-generating capacity of the ASM was assessed at 5-minute intervals before and after carbachol administration using electrical field stimulations (EFS). The results show that force oscillations applied before the introduction of tone had a small effect on the force produced by EFS (declined to 96.8% [P > 0.05] and 92.3% [P < 0.05] with and without DI, respectively). The tone induced by carbachol transiently decreased after a DI and declined significantly (P < 0.05) due to tidal breathing oscillations (25%). These force oscillations did not prevent force adaptation (gain of force of 11.2 ± 2.2 versus 13.5 ± 2.7 and 11.2 ± 3.0% in static versus dynamic conditions with or without DI, respectively). The lack of effect of simulated breathing maneuvers on force adaptation suggests that this gain in ASM force may occur in vivo and could contribute to the development of airway hyperresponsiveness.

Agonist of 5-HT1A/7 receptors but not that of 5-HT2 receptors disinhibits tracheobronchial-projecting airway vagal preganglionic neurons of rats

The vagus nerves supply the major cholinergic tone to airway smooth muscles physiologically and play critical roles in the genesis of airway hyperreactivity under some pathological conditions. Postganglionic airway cholinergic tone relies largely on the ongoing activity of medullary airway vagal preganglionic neurons (AVPNs), of which the tracheobronchial-projecting ones are primarily located in the external formation of the nucleus ambiguus (eNA). AVPNs are regulated by 5-HT, and 5-HT(1A/7) and 5-HT(2) receptors have been indicated to be involved. But the mechanisms at synaptic level are unknown. In the present study, tracheobronchial-projecting AVPNs (T-AVPNs) were retrogradely labeled from the trachea wall; fluorescently labeled T-AVPNs in the eNA were recorded with whole-cell voltage patch clamp; and the effects of 5-HT(1A/7) receptor agonist (±)-8-Hydroxy-2-(dipropylamino) tetralin hydrobromide (8-OH-DPAT) (1 μmol L(-1)) and 5-HT(2) receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (10 μmol L(-1)) on the synaptic inputs were examined. 8-OH-DPAT significantly inhibited the GABAergic and glycinergic spontaneous inhibitory postsynaptic currents (sIPSCs) of T-AVPNs in both the frequency and amplitude but had no effect on the GABAergic and glycinergic miniature inhibitory postsynaptic currents (mIPSCs). The 8-OH-DPAT inhibition of the GABAergic and glycinergic sIPSCs was prevented by 5-HT(1A/7) receptor antagonist N-[2-[4-(2-Methoxyphenyl)-1-piperazinyl] ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate salt (WAY-100635) (1 μmol L(-1)). 8-OH-DPAT had no effect on the glutamatergic spontaneous excitatory postsynaptic currents (sEPSCs) and caused no alterations in the baseline current and input resistance of T-AVPNs. DOI had no effect on any types of the synaptic inputs of T-AVPNs. These results suggest that 5-HT(1A/7) receptor agonist causes "disinhibition" of T-AVPNs, which might, in part, account for the reflex increase of airway resistance.

Characterization of the vagal motor neurons projecting to the Guinea pig airways and esophagus

Distinct parasympathetic postganglionic neurons mediate contractions and relaxations of the guinea pig airways. We set out to characterize the vagal inputs that regulate contractile and relaxant airway parasympathetic postganglionic neurons. Single and dual retrograde neuronal tracing from the airways and esophagus revealed that distinct, but intermingled, subsets of neurons in the compact formation of the nucleus ambiguus (nAmb) innervate these two tissues. Tracheal and esophageal neurons identified in the nAmb were cholinergic. Esophageal projecting neurons also preferentially (greater than 70%) expressed the neuropeptide CGRP, but could not otherwise be distinguished immunohistochemically from tracheal projecting preganglionic neurons. Few tracheal or esophageal neurons were located in the dorsal motor nucleus of the vagus. Electrical stimulation of the vagi in vitro elicited stimulus dependent tracheal and esophageal contractions and tracheal relaxations. The voltage required to evoke tracheal smooth muscle relaxation was significantly higher than that required for evoking either tracheal contractions or esophageal longitudinal striated muscle contractions. Together our data support the hypothesis that distinct vagal preganglionic pathways regulate airway contractile and relaxant postganglionic neurons. The relaxant preganglionic neurons can also be differentiated from the vagal motor neurons that innervate the esophageal striated muscle.

Sympathetic nerve-dependent regulation of mucosal vascular tone modifies airway smooth muscle reactivity

The airways contain a dense subepithelial microvascular plexus that is involved in the supply and clearance of substances to and from the airway wall. We set out to test the hypothesis that airway smooth muscle reactivity to bronchoconstricting agents may be dependent on airway mucosal blood flow. Immunohistochemical staining identified vasoconstrictor and vasodilator nerve fibers associated with subepithelial blood vessels in the guinea pig airways. Intravital microscopy of the tracheal mucosal microvasculature in anesthetized guinea pigs revealed that blockade of α-adrenergic receptors increased baseline arteriole diameter by ~40%, whereas the α-adrenergic receptor agonist phenylephrine produced a modest (5%) vasoconstriction in excess of the baseline tone. In subsequent in vivo experiments, tracheal contractions evoked by topically applied histamine were significantly reduced (P < 0.05) and enhanced by α-adrenergic receptor blockade and activation, respectively. α-Adrenergic ligands produced similar significant (P < 0.05) effects on airway smooth muscle contractions evoked by topically administered capsaicin, intravenously administered neurokinin A, inhaled histamine, and topically administered antigen in sensitized animals. These responses were independent of any direct effect of α-adrenergic ligands on the airway smooth muscle tone. The data suggest that changes in blood flow in the vessels supplying the airways regulate the reactivity of the underlying airway smooth muscle to locally released and exogenously administered agents by regulating their clearance. We speculate that changes in mucosal vascular function or changes in neuronal regulation of the airway vasculature may contribute to airways responsiveness in disease.

Innervation of tracheal parasympathetic ganglia by esophageal cholinergic neurons: evidence from anatomic and functional studies in guinea pigs

In the present study, we describe a subset of nerve fibers, characterized by their immunoreactivity for the calcium-binding protein calretinin, that are densely and selectively associated with cholinergic postganglionic neurons in the guinea pig tracheal ganglia. Retrograde neuronal tracing with cholera toxin B, combined with immunohistochemical analyses, showed that these nerve fibers do not originate from sensory neurons in the nodose, jugular, or dorsal root ganglia or from motor neurons in the nucleus ambiguus, dorsal motor nucleus of the vagus nerve, spinal cord, stellate ganglia, or superior cervical ganglia. Calretinin-immunoreactive nerve fibers disappeared from tracheal segments after 48 h in organotypic culture, indicating that the fibers were of extrinsic origin. However, calretinin-positive nerve fibers persisted in tracheal ganglia when tracheae were cocultured with the adjacent esophagus intact. Immunohistochemical analysis of the esophagus revealed a population of cholinergic neurons in the esophageal myenteric plexus that coexpressed calretinin. In functional studies, electrical stimulation of the esophagus in vitro evoked measurable contractions of the trachea. These contractions were not altered by prior organotypic culture of the trachea and esophagus to remove the extrinsic innervation to the airways but were significantly (P < 0.05) inhibited by the ganglionic blocker hexamethonium or by physical disruption of the tissue connecting the trachea and esophagus. These data suggest that a subset of esophageal neurons, characterized by the expression of calretinin and acetylcholine, provide a previously unrecognized excitatory input to tracheal cholinergic ganglia in guinea pigs.

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.

The NADPH oxidase inhibitor diphenyleneiodonium is also a potent inhibitor of cholinesterases and the internal Ca(2+) pump

BACKGROUND AND PURPOSE

Diphenyleneiodonium (DPI) is often used as an NADPH oxidase inhibitor, but is increasingly being found to have unrelated side effects. We investigated its effects on smooth muscle contractions and the related mechanisms.

EXPERIMENTAL APPROACH

We studied isometric contractions in smooth muscle strips from bovine trachea. Cholinesterase activity was measured using a spectrophotometric assay; internal Ca(2+) pump activity was assessed by Ca(2+) uptake into smooth muscle microsomes.

KEY RESULTS

Contractions to acetylcholine were markedly enhanced by DPI (10(-4) M), whereas those to carbachol (CCh) were not, suggesting a possible inhibition of cholinesterase. DPI markedly suppressed contractions evoked by CCh, KCl and 5-HT, and also unmasked phasic activity in otherwise sustained responses. Direct biochemical assays confirmed that DPI was a potent inhibitor of acetylcholinesterase and butyrylcholinesterase (IC(50) approximately 8 x 10(-6) M and 6 x 10(-7) M, respectively), following a readily reversible, mixed non-competitive type of inhibition. The inhibitory effects of DPI on CCh contractions were not mimicked by another NADPH oxidase inhibitor (apocynin), nor the Src inhibitors PP1 or PP2, ruling out an action through the NADPH oxidase signalling pathway. Several features of the DPI-mediated suppression of agonist-evoked responses (i.e. suppression of peak magnitudes and unmasking of phasic activity) are similar to those of cyclopiazonic acid, an inhibitor of the internal Ca(2+) pump. Direct measurement of microsomal Ca(2+) uptake revealed that DPI modestly inhibits the internal Ca(2+) pump.

CONCLUSIONS AND IMPLICATIONS

DPI inhibits cholinesterase activity and the internal Ca(2+) pump in tracheal smooth muscle.

Acid-induced modulation of airway basal tone and contractility: Role of acid-sensing ion channels (ASICs) and TRPV1 receptor

The role of extracellular acidosis in inflammatory airway diseases is not well known. One consequence of tissue acidification is the stimulation of sensory nerves via the polymodal H(+)-gated transmembrane channels ASICs and TRPV1 receptor. The present study investigated the effect of acidosis on airway basal tone and responsiveness in the guinea pig. Acidosis (pH 6.8, 10 min, 37 degrees C) significantly decreased the basal tone of tracheal rings (p<0.01 vs. paired control). Moreover, pH fall raised the maximal contraction of tracheal rings to acetylcholine (p<0.05 vs. paired control). The pH-induced relaxation of airway basal tone was inhibited by pretreatments with ASIC1a or ASIC3/ASIC2a inhibitors (0.5 mM ibuprofen, 0.1 mM gadolinium), nitric oxide synthase inhibitor (1 mM L-NAME), and guanylate cyclase inhibitor (1 microM ODQ). In contrast, the pH-induced relaxation of airway basal tone was not modified by epithelium removal or pretreatments with a TRPV1 antagonist (1 microM capsazepine), a combination of NK(1,2,3) receptor antagonists (0.1 microM each), a blocker of voltage-sensitive Na(+) channels (1 microM tetrodotoxin), a cyclooxygenase inhibitor with no activity on ASICs (1 microM indomethacin) or ASIC3 and ASIC3/ASIC2b inhibitors (10 nM diclofenac, 1 microM aspirin). Furthermore, acid-induced hyperresponsiveness to acetylcholine was inhibited by epithelium removal, capsazepine, NK(1,2,3) receptor antagonists, tetrodotoxin, amiloride, ibuprofen and diclofenac. In summary, the initial pH-induced airway relaxation seems to be independent of sensory nerves, suggesting a regulation of airway basal tone mediated by smooth muscle ASICs. Conversely, the pH-induced hyperresponsiveness involves sensory nerves-dependent ASICs and TRPV1, and an unknown epithelial component in response to acidosis.

Reflex regulation of airway smooth muscle tone

Autonomic nerves in most mammalian species mediate both contractions and relaxations of airway smooth muscle. Cholinergic-parasympathetic nerves mediate contractions, whereas adrenergic-sympathetic and/or noncholinergic parasympathetic nerves mediate relaxations. Sympathetic-adrenergic innervation of human airway smooth muscle is sparse or nonexistent based on histological analyses and plays little or no role in regulating airway caliber. Rather, in humans and in many other species, postganglionic noncholinergic parasympathetic nerves provide the only relaxant innervation of airway smooth muscle. These noncholinergic nerves are anatomically and physiologically distinct from the postganglionic cholinergic parasympathetic nerves and differentially regulated by reflexes. Although bronchopulmonary vagal afferent nerves provide the primary afferent input regulating airway autonomic nerve activity, extrapulmonary afferent nerves, both vagal and nonvagal, can also reflexively regulate autonomic tone in airway smooth muscle. Reflexes result in either an enhanced activity in one or more of the autonomic efferent pathways, or a withdrawal of baseline cholinergic tone. These parallel excitatory and inhibitory afferent and efferent pathways add complexity to autonomic control of airway caliber. Dysfunction or dysregulation of these afferent and efferent nerves likely contributes to the pathogenesis of obstructive airways diseases and may account for the pulmonary symptoms associated with extrapulmonary disorders, including gastroesophageal reflux disease, cardiovascular disease, and rhinosinusitis.

Mechanisms underlying the relaxation induced by isokaempferide from Amburana cearensis in the guinea-pig isolated trachea

The present study examines possible mechanisms by which the flavonoid isokaempferide (IKPF; 5,7,4'-trihydroxy-3-methoxyflavone) from Amburana cearensis, a Brazilian medicinal plant popularly used as bronchodilator, induces relaxation of guinea-pig isolated trachea. In the trachea (with intact epithelium) contracted by carbachol, IKPF (1-1000 microM) caused a graded relaxation, and the epithelium removal increased the sensitivity of the airway smooth muscle to IKPF (EC50, in intact tissue: 77.4 [54.8-109.2] microM; in denuded epithelium: 15.0 [11.3-20.1] microM). The IKPF-induced relaxation was inhibited in 41% by the nitric oxide (NO) synthase inhibitor L-NAME (100 microM); in 31% and 50% by the soluble guanylate cyclase (sGC) inhibitor ODQ (3 and 33 microM); by propranolol (31%) and also by capsaicin (37%). In the trachea pre-contracted by 40 mM KCl the pre-incubation with glibenclamide (33 microM) or iberiotoxin (IbTX, 0.1 microM), selective K(+) channel inhibitors, inhibited the IKPF-induced relaxation by 39% and 38%, respectively. On the other hand, 4-aminopyridine (100 microM), a nonselective K(+) channel antagonist, did not significantly influence the effect of IKPF, while IbTX induced a rightward displacement of the IKPF concentration-response curve. However, in muscle pre-contracted with 120 mM KCl the relaxant effect of IKPF was significantly reduced and not affected by glibenclamide. In conclusion, these results indicate a direct and epithelium-independent relaxant effect of IKPF on smooth muscle fibers. Although this IKPF relaxant action seems to be multi-mediated, it occurs via both Ca(2+) and ATP-sensitive K(+) channels, but some other possible mechanisms unrelated to K(+) channels cannot be excluded.

Exhaled nitric oxide in paediatric asthma

Assessment of airway function is difficult in young children with asthma, and in addition, only reflects the status of the disease at the time of the measurement. Thus, there is increasing interest in monitoring airway inflammation in asthma, which may provide a longer term assessment of disease activity. Most methods of assessing asthmatic inflammation are invasive, and are not feasible in the paediatric population. This review discusses exhaled nitric oxide as a marker of asthmatic inflammation, and compares it with other recognized markers. Exhaled nitric oxide has the potential to become a noninvasive method of assessing asthma control in the paediatric population.

Inducible nitric oxide synthase evoked nitric oxide counteracts capsaicin-induced airway smooth muscle contraction, but exacerbates plasma extravasation

The contribution of nitric oxide (NO) to capsaicin-evoked airway responses was investigated in rats. The measurement of plasma NO level, airway dynamics, airway smooth muscle electromyogram, and plasma extravasation by India ink and Evans blue leakage technique was adapted. Capsaicin-evoked hypotension, bronchoconstriction, trachea plasma extravasation as well as increases in plasma NO level in a dose-dependent manner. L-732138 (NK1 receptor antagonist) or SR-48968 (NK2 receptor antagonist) pretreatment reduced capsaicin-enhanced hypotension, bronchoconstriction, plasma extravasation, and plasma NO level. N(G)-nitro-L-Arginine methyl ester (L-NAME, 10 mg/kg, i.v.), a non-selective NO synthase (NOS) inhibitor, or aminoguanidine (10 mg/kg, i.v.), a selective inducible NOS (iNOS) inhibitor, reduced capsaicin-induced increases in plasma NO level and protected against capsaicin-induced plasma extravasation, whereas L-arginine (150 mg/kg, i.v.), a NO precursor, enhanced capsaicin-evoked plasma NO level and plasma extravasation. L-Arginine pretreatment ameliorated capsaicin-induced bronchoconstriction, whereas L-NAME and aminoguanidine exaggerated capsaicin-induced bronchoconstriction. In summary, NK1 and NK2 receptors and iNOS play a role in NO formation and on capsaicin-induced bronchoconstriction and plasma extravasation. NO generated by iNOS counteracts tachykinin-mediated bronchoconstriction, but exacerbates tachykinin-mediated plasma extravasation.

Nitric oxide in health and disease of the respiratory system

During the past decade a plethora of studies have unravelled the multiple roles of nitric oxide (NO) in airway physiology and pathophysiology. In the respiratory tract, NO is produced by a wide variety of cell types and is generated via oxidation of l-arginine that is catalyzed by the enzyme NO synthase (NOS). NOS exists in three distinct isoforms: neuronal NOS (nNOS), inducible NOS (iNOS), and endothelial NOS (eNOS). NO derived from the constitutive isoforms of NOS (nNOS and eNOS) and other NO-adduct molecules (nitrosothiols) have been shown to be modulators of bronchomotor tone. On the other hand, NO derived from iNOS seems to be a proinflammatory mediator with immunomodulatory effects. The concentration of this molecule in exhaled air is abnormal in activated states of different inflammatory airway diseases, and its monitoring is potentially a major advance in the management of, e.g., asthma. Finally, the production of NO under oxidative stress conditions secondarily generates strong oxidizing agents (reactive nitrogen species) that may modulate the development of chronic inflammatory airway diseases and/or amplify the inflammatory response. The fundamental mechanisms driving the altered NO bioactivity under pathological conditions still need to be fully clarified, because their regulation provides a novel target in the prevention and treatment of chronic inflammatory diseases of the airways.

Nitric oxide and vasoactive intestinal peptide as co-transmitters of airway smooth-muscle relaxation: analysis in neuronal nitric oxide synthase knockout mice

BACKGROUND

Both vasoactive intestinal peptide (VIP) and nitric oxide (NO) relax airway smooth muscle and are potential co-transmitters of neurogenic airway relaxation. The availability of neuronal NO synthase (nNOS) knockout mice (nNOS-/-) provides a unique opportunity for evaluating NO.

OBJECTIVE

To evaluate the relative importance of NO, especially that generated by nNOS, and VIP as transmitters of the inhibitory nonadrenergic, noncholinergic (NANC) system.

STUDY DESIGN

In this study, we compared the neurogenic (tetrodotoxin-sensitive) NANC relaxation of tracheal segments from nNOS-/- mice and control wild-type mice (nNOS(+/+)), induced by electrical field stimulation (EFS). We also examined the tracheal contractile response to methacholine and its relaxant response to VIP.

RESULTS

EFS (at 60 V for 2 ms, at 10, 15, or 20 Hz) dose-dependently reduced tracheal tension, and the relaxations were consistently smaller (approximately 40%) in trachea from nNOS-/- mice than from control wild-type mice (p < 0.001). VIP (10(- 8) to 10(-6) mol/L) induced concentration-dependent relaxations that were approximately 50% smaller in nNOS-/- tracheas than in control tracheas. Methacholine induced concentration-dependent contractions that were consistently higher in the nNOS-/- tracheas relative to wild-type mice tracheas (p > 0.05).

CONCLUSION

Our data suggest that, in mouse trachea, NO is probably responsible for mediating a large (approximately 60%) component of neurogenic NANC relaxation, and a similar (approximately 50%) component of the relaxant effect of VIP. The results imply that NO contributes significantly to neurogenic relaxation of mouse airway smooth muscle, whether due to neurogenic stimulation or to the neuropeptide VIP.

An in vivo guinea pig preparation for studying the autonomic regulation of airway smooth muscle tone

The autonomic nervous system plays a primary role in regulating airway smooth muscle tone. Here, we describe the development of an in vivo guinea pig model that permits systematic studies of the autonomic control of airway smooth muscle. The model is based on preparations previously described and utilizes measurements of isometric tension in a perfused segment of extrathoracic guinea pig trachea in situ. It has the advantage that the autonomic innervation to the tracheal segment under study can be physiologically or pharmacologically isolated and studied independently from other mechanisms regulating airway smooth muscle tone. Initial experiments were conducted to optimize model conditions. Subsequent experiments were designed to highlight the usefulness of this preparation for studying parasympathetic regulation of airway caliber. The results of the study demonstrate the utility of this model for future studies into the neural regulation of bronchomotor tone and the mechanisms of airway obstruction and hyperreactivity associated with disease.

Acid reflux and asthma

The association between asthma and gastroesophageal reflux (GER) has been further delineated with recent clinical investigations. The prevalence of GER development in asthmatics is higher than in control populations. Furthermore, asthmatics with GER have a higher risk of asthma hospitalization. Asthma medications may be one of the promoting factors for GER development in asthmatics. Inhaled albuterol decreases lower esophageal sphincter (LES) pressure and esophageal contraction amplitude. Furthermore, oral prednisone results in increased esophageal acid contact times. Respiratory symptoms also correlate with esophageal acid events. The role of neurogenic inflammation in asthma-induced bronchoconstriction is also being further developed. Therapy of GER may improve asthma outcomes in selected asthmatics. Currently, there are no double-blind, placebo-controlled, multicenter trials reporting asthma outcomes with antireflux therapy.

Increased airway inflammatory cells in endurance athletes: what do they mean?

BACKGROUND

Inflammatory cells are increased in the airways of endurance athletes, but their role in causing exercise-induced respiratory symptoms and bronchoconstriction, or their possible long-term consequences, are uncertain.

AIM

To put the results of athlete studies in perspective, by analysing the pathogenesis of airway cell changes and their impact on respiratory function.

RESULTS

Athletes of different endurance sports at rest showed increased airway neutrophils. Elite swimmers and skiers also showed large increases in airway eosinophils and lymphocytes, possibly related to chronic, exercise-related exposure to irritants or cold and dry air, respectively. Post-exercise studies reported variable responses of airway cells to exercise, but found no evidence of inflammatory cell activation in the airways, at variance with exercise-induced neutrophil activation in peripheral blood. The increase in airway inflammatory cells in athletes can result from hyperventilation-induced increase in airway osmolarity stimulating bronchial epithelial cells to release chemotactic factors. Hyperosmolarity may also inhibit activation of inflammatory cells by causing shedding of adhesion molecules, possibly explaining why airway inflammation appears 'frustrated' in athletes. Data on exhaled nitric oxide are few and variable, not allowing conclusions about its usefulness as a marker of airway inflammation in athletes, or its role in modulating bronchial responsiveness.

CONCLUSIONS

The acute and long-term effects of exercise on airway cells need further study. Airway inflammatory cells are increased but not activated in athletes, both at rest and after exercise, and airway inflammation appears to regress in athletes quitting competitions. Altogether, these findings do not clearly indicate that habitual intense exercise may be detrimental for respiratory health. Rather, airway changes may represent chronic adaptive responses to exercise hyperventilation. An improved understanding of the effects of exercise on the airways will likely have a clinical impact on sports medicine, and on the current approach to exercise-based rehabilitation in respiratory disease.

Synergistic interactions between airway afferent nerve subtypes mediating reflex bronchospasm in guinea pigs

The hypothesis that airway afferent nerve subtypes act synergistically to initiate reflex bronchospasm in guinea pigs was addressed. Laryngeal mucosal application of capsaicin or bradykinin or the epithelial lipoxygenase metabolite 15(S)-hydroxyeicosatetraenoic acid evoked slowly developing but pronounced and sustained increases in tracheal cholinergic tone in situ. These reflexes were reversed by atropine and prevented by vagotomy, trimethaphan, or laryngeal denervation. Central nervous system-acting neurokinin receptor antagonists also abolished the reflexes without altering baseline cholinergic tone. Baseline tone was, however, reversed by disrupting pulmonary afferent innervation while preserving the innervation of the trachea and larynx. Surprisingly, selective pulmonary denervation also prevented the laryngeal capsaicin-induced tracheal reflexes, suggesting that laryngeal C-fibers act synergistically with continuously active intrapulmonary mechanoreceptors to initiate reflex bronchospasm. Indeed, reflex bronchospasm evoked by histamine was markedly potentiated by bradykinin, an effect mimicked by intracerebroventricular, but not intravenous, substance P. These data, as well as anatomic evidence for afferent nerve subtype convergence in the commissural nucleus of the solitary tract, suggest that airway nociceptors and mechanoreceptors may act synergistically to regulate airway tone.

Evidence for differential reflex regulation of cholinergic and noncholinergic parasympathetic nerves innervating the airways

The hypothesis that cholinergic and nonadrenergic, noncholinergic parasympathetic nerves innervating the airways are subject to differential reflex regulation was addressed. Pronounced contractile and relaxant parasympathetic reflex responses could be evoked by intravenous histamine, laryngeal mucosal application of capsaicin, inhaled capsaicin, or electrical stimulation of the vagal afferent nerves projecting to the esophagus and abdominal viscera. These data suggest that activation of multiple vagal afferent nerve subtypes can initiate both cholinergic and noncholinergic parasympathetic reflexes in the airways. Conversely, hypoxia or activation of the diving response from the nose evoked only cholinergic contractile reflexes. All contractile and relaxant responses evoked by these stimuli were absent in vagotomized animals or in animals pretreated with the ganglionic blocker trimethaphan, confirming their reflex and parasympathetic nature. The data indicate that cholinergic and noncholinergic parasympathetic nerves regulating airway caliber in guinea pigs are comprised of two distinct parasympathetic pathways that are subject to differential reflex regulation. This previously unrecognized complexity of autonomic regulation of airway caliber has potentially important implications for the mechanisms of airways hyperresponsiveness.