Toluene diisocyanate increases airway responsiveness to substance P and decreases airway neutral endopeptidase

Broadening COVID-19 Interventions to Drug Innovation: Neprilysin Pathway as a Friend, Foe, or Promising Molecular Target?

The coronavirus disease 2019 (COVID-19) pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus is anticipated to transition to an endemic state as vaccines are providing relief in some, but not all, countries. Drug discovery for COVID-19 can offer another tool in the fight against the pandemic. Additionally, COVID-19 impacts multiple organs that call for a systems medicine approach to planetary health and therapeutics innovation. In this context, innovation for drugs that prevent and treat COVID-19 is timely and much needed. As the virus variants emerge under different ecological conditions and contexts in the long haul, a broad array of vaccine and drug options will be necessary. This expert review article argues for a need to expand the COVID-19 interventions, including and beyond vaccines, to stimulate discovery and development of novel medicines against SARS-CoV-2 infection. The Renin-Angiotensin-Aldosterone System (RAAS) is known to play a major role in SARS-CoV-2 infection. Neprilysin (NEP) and angiotensin-converting enzyme (ACE) have emerged as the pharmaceutical targets of interest in the search for therapeutic interventions against COVID-19. While the NEP/ACE inhibitors offer promise for repurposing against COVID-19, they may display a multitude of effects in different organ systems, some beneficial, and others adverse, in modulating the inflammation responses in the course of COVID-19. This expert review offers an analysis and discussion to deepen our present understanding of the pathophysiological function of neprilysin in multiple organs, and the possible effects of NEP inhibitor-induced inflammatory responses in COVID-19-infected patients.

Mini review: Neural mechanisms underlying airway hyperresponsiveness

Neural changes underly hyperresponsiveness in asthma and other airway diseases. Afferent sensory nerves, nerves within the brainstem, and efferent parasympathetic nerves all contribute to airway hyperresponsiveness. Inflammation plays a critical role in these nerve changes. Chronic inflammation and pre-natal exposures lead to increased airway innervation and structural changes. Acute inflammation leads to shifts in neurotransmitter expression of afferent nerves and dysfunction of M₂ muscarinic receptors on efferent nerve endings. Eosinophils and macrophages drive these changes through release of inflammatory mediators. Novel tools, including optogenetics, two photon microscopy, and optical clearing and whole mount microscopy, allow for improved studies of the structure and function of airway nerves and airway hyperresponsiveness.

Pathogenesis and disease mechanisms of occupational asthma

Occupational asthma (OA) is one of the most common forms of work-related lung disease in all industrialized nations. The clinical management of patients with OA depends on an understanding of the multifactorial pathogenetic mechanisms that can contribute to this disease. This article discusses the various immunologic and nonimmunologic mechanisms and genetic susceptibility factors that drive the inflammatory processes of OA.

Toluene diisocyanate caused electrophysiological disturbances in the upper airways wall

OBJECTIVES

Toluene diisocyanate (TDI) due to its widespread use in industry is one of the most common and well-known causes of occupational asthma and Reactive Airways Dysfunction Syndrome (RADS). In this study the impact of TDI on the electrophysiological properties of the airways wall, particularly on the mechanisms of absorption of sodium ions and chloride ions secretion was evaluated.

MATERIALS AND METHODS

Isolated rabbit tracheal wall (from outbred stock animals) was mounted in an apparatus for electrophysiological experiments by means of Ussing method and was mechanically stimulated by the jet flux of specified fluid directed onto the mucosal surface of the tissue from a peristaltic pump. The measured parameters were: transepithelial potential difference under control conditions (PD, mV), after mechanical stimulation (dPD or physiological reaction of hyperpolarization, mV) and electric resistance (R, Omega cm2). When TDI (0.035 mM) was added to stimulation fluid, only the immediate reaction was identified and when it was added to incubation fluid and other experimental fluids, the late (post-incubation) reaction was determined. The experiments involving the inhibition of Na+ by amiloride and Cl- by bumetanide were also performed.

RESULTS

A series of functional tests for 72 pieces of tracheal wall from 36 animals were performed. It has been shown that short-term exposure to TDI significantly changed the course of reactions to mechanical stimulation. Also after incubation in the presence of TDI, the reactions to mechanical stimulation were changed in relation to control conditions.

CONCLUSIONS

The immediate reaction of the isolated rabbit tracheal wall after exposure to TDI depends on the duration of exposure and on the physiological condition of the tissue in respect of sodium and chloride ion transport.

Epidemiologic evidence for asthma and exposure to air toxics: linkages between occupational, indoor, and community air pollution research

Outdoor ambient air pollutant exposures in communities are relevant to the acute exacerbation and possibly the onset of asthma. However, the complexity of pollutant mixtures and etiologic heterogeneity of asthma has made it difficult to identify causal components in those mixtures. Occupational exposures associated with asthma may yield clues to causal components in ambient air pollution because such exposures are often identifiable as single-chemical agents (e.g., metal compounds). However, translating occupational to community exposure-response relationships is limited. Of the air toxics found to cause occupational asthma, only formaldehyde has been frequently investigated in epidemiologic studies of allergic respiratory responses to indoor air, where general consistency can be shown despite lower ambient exposures. The specific volatile organic compounds (VOCs) identified in association with occupational asthma are generally not the same as those in studies showing respiratory effects of VOC mixtures on nonoccupational adult and pediatric asthma. In addition, experimental evidence indicates that airborne polycyclic aromatic hydrocarbon (PAH) exposures linked to diesel exhaust particles (DEPs) have proinflammatory effects on airways, but there is insufficient supporting evidence from the occupational literature of effects of DEPs on asthma or lung function. In contrast, nonoccupational epidemiologic studies have frequently shown associations between allergic responses or asthma with exposures to ambient air pollutant mixtures with PAH components, including black smoke, high home or school traffic density (particularly truck traffic), and environmental tobacco smoke. Other particle-phase and gaseous co-pollutants are likely causal in these associations as well. Epidemiologic research on the relationship of both asthma onset and exacerbation to air pollution is needed to disentangle effects of air toxics from monitored criteria air pollutants such as particle mass. Community studies should focus on air toxics expected to have adverse respiratory effects based on biological mechanisms, particularly irritant and immunological pathways to asthma onset and exacerbation.

Correlation between neutral endopetidase (NEP) 3.4.24.11 in serum and the degree of the bronchial hyperreactivity

Neutral endopeptidase (NEP) is described in airways as the major degrading enzyme of tachykinins such as neurokinin A (NKA) and substance P (SP). Due to its localization and mode of action NEP may play a role in the pathophysiology of bronchial reactivity (BR) especially under the aspect of neurogenic inflammation. Serum NEP concentrations were measured by ELISA to investigate if there is a correlation between serum NEP and the degree of bronchial reactivity expressed by PC20-FEV1 histamine(.). PC20-FEV1 histamine was determined in 31 asthmatic patients [age 31.9+/-1.3 years (mean+/-SEM) FEV1=92.1+/-2.4% (mean+/-SEM) 16 females/15 males]. Prior to the histamine challenge blood samples were obtained and stored at -70 degrees C until determination using ELISA. A significant correlation between serum NEP and the PC20-FEV1 (n=31, r=0.49, P<0.01) was found. The results suggest that serum NEP is modulating neuropeptide-induced effects in the pathophysiology of airway responsiveness.

Human neutral brush border endopeptidase EC 3.4.24.11 in urine, its isolation, characterisation and activity in renal diseases

Human neutral brush border endopeptidase (NEP) was purified from the urine of patients suffering from acute toxic tubulointerstitial nephropathy. An enzyme preparation with specific activity of 102 Ug(-1) protein was obtained. The urinary activities of neutral endopeptidase and alanine aminopeptidase were measured in patients with renal disease and in 30 control patients, resulting in a reference range from 0.1 to 0.7 Ug(-1) creatinine and 1.4-14.1 Ug(-1) creatinine, respectively. Urine enzyme activities were highest in patients with acute tubulotoxic renal diseases. Neutral endopeptidase and alanine aminopeptidase activities were found to be 6.5- and 10-fold higher than the upper value of the reference range, respectively. Smaller increases in the rate of excretion of these enzymes (2.5- and 3.5-fold), respectively, were observed in patients suffering from acute tubular insufficiency and even lower increases, 2- and 1.5-fold, respectively, were observed in patients with chronic renal diseases. In diabetics and kidney transplant patients the enzyme excretion rates were within the reference range. Assay of both transmembrane metalloproteinases in urine may prove valuable in serving as markers for renal toxicity. Together with beta-NAG these enzymes could be employed as differentiation markers between acute and chronic tubular insufficiency.

The link between met-enkephalin-induced down-regulation of APN activity and the release of superoxide anion

We have previously shown that methionine-enkephalin (MENK) differentially alters the production of superoxide anion (O(2)(-)) from neutrophils of different donors. This effect could be due to variable activity of proteolytic enzymes involved in the degradation of this neuropeptide. In this study, we investigated the possible association between the effect of MENK on O(2)(-)release and the two neutrophil associated hydrolytic enzymes that participate in enkephalin degradation; aminopeptidase N (APN) and neutral endopeptidase (NEP). We have demonstrated that APN but not NEP activity was down-regulated by MENK. This might be due to internalization, since APN down-regulation was observed only with intact neutrophils and not with the respective membranes. Preincubation of neutrophils with inhibitory anti CD13 MoAb (WM15) abbrogated the suppressive effect of MENK (10(-12), 10(-10)and 10(-8)M). These facts, show that in the periphery (as well as the brain) the dominant role in MENK hydrolysis can be attributed to APN. Also, they further support the idea of the link between the membrane associated CD13 and binding of the ligand to the opioid receptor.

Toluene diisocyanate enhances substance P in sensory neurons innervating the nasal mucosa

Inhalation of irritants, such as toluene diisocyanate (TDI), stimulates substance P (SP) release from peripheral processes of sensory neurons innervating the airways. The purpose of this study was to determine if TDI inhalation affects intraneuronal levels of SP and preprotachykinin (PPT) messenger RNA (mRNA) in the sensory neurons of the trigeminal ganglion (TG) which innervate the nasal epithelium. The nasal cavity of Fisher-344 rats was instilled with rhodamine-labeled latex microspheres. Ten days later, the rats were exposed to 60 ppb of 2,4-2,6-TDI vapor for 2 h. The TG were removed 1, 12, 24, 48, 72, and 96 h after TDI treatment and prepared for SP immunocytochemistry and PPT in situ hybridization. SP nerve fiber density in nasal epithelium was significantly increased 12, 24, and 48 h after TDI exposure. The proportion of microsphere-labeled cell bodies expressing high levels of SP immunoreactivity was decreased at 24 h but was increased above controls at 48 and 72 h. The proportion of microsphere-labeled cell bodies expressing high levels of PPT mRNA was increased above control levels at 24 and 48 h. The percentage of leukocytes observed in nasal lavage fluid was significantly increased 12, 24, 48, and 72 h after inhalation. These studies indicate that SP production in TG neurons projecting to the nasal epithelium is transiently increased after TDI exposure, suggesting that TDI inhalation not only causes SP release but also increased intraneuronal neuropeptide levels. Increased neuronal SP levels may be involved in maintaining neurogenic inflammation or the development of airway hyperresponsiveness.

Mechanisms of occupational asthma

BACKGROUND

The pathogenesis and the pathologic alterations of occupational asthma are similar to those of nonoccupational asthma. Occupational asthma may therefore represent a useful model of "human asthma" to investigate mechanisms and pathophysiology of asthma in general. In an occupational setting the cause and onset of asthma may be easily identified, and the natural history may be examined in follow-up studies. The mechanisms involved in occupational asthma include genetic predisposition, immunologically mediated responses, as well as nonspecific airway inflammation. In particular, high molecular weight (eg, grain dust, flour) and some low molecular weight sensitizers (eg, acid anhydrides and platinum halide salts) have been shown to induce occupational asthma through an immunoglobulin E (IgE)-dependent mechanism, while cell-dependent immunologic mechanisms are likely to be more relevant for occupational asthma induced by other low molecular weight sensitizers (eg, toluene diisocyanate and plicatic acid contained in western red cedar). The pathology of the airway mucosa of occupational asthma is remarkably similar to the pathology of nonoccupational asthma, ie, characterized by infiltration and accumulation of eosinophils, mast cells, and activated lymphocytes along with subepithelial fibrosis. In this article, the most relevant mechanisms are discussed with particular reference to the similarities and discrepancies between occupational and nonoccupational asthma.

Repeated challenge with dinitrobenzene sulphonic acid in dinitrofluorobenzene-sensitized mice results in vascular hyperpermeability in the trachea: a role for tachykinins

1. This study investigates the role of tachykinins in a repeated challenge with dinitrobenzene sulphonic acid (DNS) on the tracheal vascular permeability in dinitrofluorobenzene (DNFB)-sensitized mice. 2. DNFB-contact sensitization was followed by an intranasal (i.n.) challenge with DNS. A second challenge with DNS was administered 24 h after the first challenge. To assess changes in tracheal vascular permeability, Evans blue dye accumulation in tracheal tissue was measured. 3. A repeated challenge with DNS in DNFB-sensitized mice led to a 2.8 fold increase in tracheal vascular permeability when compared to DNFB-sensitized and vehicle-challenged mice or a 2.5 fold increase when compared to DNFB-sensitized single DNS-challenged mice (P<0.001, ANOVA). 4. RP67580 (10-9 mol mouse-1 i.v.) reduced the increased tracheal vascular permeability induced by a second exposure to DNS in DNFB-sensitized mice completely when injected 15 min before the second challenge (P<0.001, ANOVA). 5. The increased tracheal vascular permeability response induced by the second exposure to DNS could be mimicked with i.n. application of capsaicin (10-10 mol mouse-1) or substance P (SP) (10-12 mol mouse-1) to DNFB-sensitized and single DNS-challenged mice. 6. These results suggest that both tachykinin NK1 receptors and sensory nerves are involved in the development of vascular hyperpermeability changes found in the trachea of DNFB-sensitized mice after a repeated DNS-challenge.

A possible involvement of oxidative lung injury in endotoxin-induced bronchial hyperresponsiveness to substance P in guinea pigs

Reactive oxygen-derived free radical species have been implicated in the pathogenesis and pathophysiology of inflammatory lung diseases. In a guinea pig model of aerosolized endotoxin-induced bronchial hyperresponsiveness to substance P, a possible involvement of oxidative lung injury was assessed by measuring the changes in membrane-bound neutral endopeptidase activity in the airway tissues and the level of lipid peroxides in the plasma. Vehicle-treated animals developed a neutrophilic airway inflammation, bronchial hyperresponsiveness to substance P associated with neutral endopeptidase hypoactivity, and elevation of lipid peroxides at 18 to 24 h after an exposure to endotoxin (75 microgram/ml, 40 min). A nonselective phosphodiesterase inhibitor, aminophylline, and selective phosphodiesterase isoenzyme inhibitors, SDZ-ISQ-844 (type III/IV) and SDZ-MKS-492 (type III), attenuated the neutrophilic airway inflammation induced by endotoxin. Aminophylline, SDZ-MKS-492, and a superoxide anion-generating NADPH-oxidase inhibitor apocynin inhibited bronchial hyperresponsiveness to substance P with attenuation of neutral endopeptidase inactivation induced by endotoxin. SDZ-ISQ-844, SDZ-MKS-492, and apocynin attenuated the elevation of lipid peroxides. The generation of hypochlorite (OCl-) from whole blood leukocytes was attenuated by aminophylline, SDZ-ISQ-844, SDZ-MKS-492, and apocynin at 1 to 2 h after exposure. These results suggest that reactive oxygen-derived free radical species-mediated oxidative lung injury may play an important role in endotoxin-induced bronchial hyperresponsiveness to substance P, and that phosphodiesterase isoenzyme inhibitors may be potentially useful as anti-inflammatory drugs.

Effect of recombinant neutral endopeptidase (EC 3.4.24.11) on neuropeptide-mediated nasal fluid secretion and plasma exudation in the rat

The nasal mucosa harbors sensory nerves containing neuropeptides such as substance P (SP), which are released by capsaicin. The neuropeptides are degraded by peptidases, e.g., neutral endopeptidase (NEP) that is present in the nasal mucosa. We studied the effect of enzymatically active recombinant NEP (rNEP) on neuropeptide-evoked secretion of nasal fluid and plasma exudation in rats. rNEP administered intranasally (i.n.) reduced the capsaicin-evoked nasal fluid secretion but failed to reduce the secretion evoked by SP (exogenous) under the experimental conditions used. rNEP reduced the increase in nasal plasma exudation evoked by capsaicin (endogenous neuropeptides). Because rNEP reduced neuropeptide-mediated nasal fluid secretion and plasma exudation in the rat, we suggest that peptidase activity in the nasal mucosa will determine the magnitude of the response to locally released neuropeptides.

Involvement of tachykinin NK1 receptors in plasma protein extravasation induced by tachykinins in the guinea pig upper airways

Plasma protein extravasation in the upper airways of anesthetized guinea pigs was measured with the FITC (Fluorescein isothiocyanate)-dextran technique. The effect of selective tachykinin (NK1 and NK2) receptor agonists and antagonists, capsaicin or antigen was studied. The tachykinin NK1 receptor agonist, [Sar9]substance P sulfone, induced an increase in FITC-dextran extravasation which was blocked by the nasal application (30-100 nmol/kg) of the tachykinin NK1 receptor antagonist FK888, but not by 1 micromol/kg of the tachykinin NK2 receptor antagonist, MEN10,627. The tachykinin NK2 receptor agonist, [betaAla8]neurokinin A-(4-10), had no effect on dye leakage. FK888 (30 nmol/kg intranasal) abolished the increase in the tracer recovery induced both by antigen and capsaicin. Conversely, the intranasal administration of MEN10,627 (0.1-1.0 micromol/kg) significantly reduced capsaicin-induced and only marginally inhibited antigen-induced increase in plasma protein extravasation. Pretreatment with the neutral endopeptidase inhibitor, phosphoramidon, increased the effect of all inflammatory agents. These findings show that the plasma extravasation of the upper airways induced by exogenous or endogenous tachykinins is primarily mediated by tachykinin NK1 receptors. This inflammatory response could be controlled by locally applied tachykinin NK1 receptor antagonist.

Role of tachykinins and neutral endopeptidase in toluene diisocyanate-induced bronchial hyperresponsiveness in guinea pigs

The role of tachykinins in toluene diisocyanate (TDI)-induced non-specific bronchial hyperreactivity (NSBH) in guinea pigs was investigated, and it was determined whether or not the activity of airway neutral endopeptidase (NEP) was inhibited in conditions where a bronchial hyperreactivity to acetylcholine (ACh) was observed. Exposures to 3 ppm TDI for 1 h, or to 0.029 ppm for 8 weeks caused a significant bronchial hyperreactivity to ACh. The depletion of tachykinins by a pretreatment with capsaicin (140 mg/kg) eliminated the TDI-induced airway hyperresponsiveness in both patterns of exposure to TDI. Capsaicin treatment had no effect on the response to ACh in guinea-pigs exposed to air (controls). Bronchial NEP activity determined by histoenzymology was significantly less 4 and 24 h after the end of a 1-h exposure to 3 ppm TDI than after exposure to air. Bronchial NEP activity evaluated 24 h after the end of a 48-h exposure to 0.116 ppm TDI, or a 1-week exposure to 0.050 ppm TDI was not significantly different from those of controls exposed to air, whereas in the same conditions of exposure a NSBH is observed in guinea-pigs. These data suggest that tachykinins released from C-fibers upon acute or repeated exposures to high or low concentrations of TDI, respectively, play an essential role in the observed bronchial hyperreactivity, and that the inhibition of NEP by TDI cannot completely account for the observed airway hyperreactivity.

Bronchial responsiveness and inflammation in guinea-pigs exposed to toluene diisocyanate: a study on single and repeated exposure

The question of whether or not toluene diisocyanate (TDI)-induced airway hyperresponsiveness in the guinea-pig is accompanied by neutrophil influx into bronchoalveolar lavage fluid (BALF) was addressed. Two modes of exposure were studied; (1) acute exposures where animals were exposed to 3 ppm TDI for 1 h and experiments were carried out 30 min, 4 h, 24 h, 48 h and 1 week after the TDI exposures; (2) subacute exposures where animals were exposed to 0.080 and 0.046 ppm TDI for 48 h 1 week, respectively, and experiments were carried out 24 h after the TDI exposures. The changes in airway responsiveness to increasing doses of intravenous acetylcholine (ACh) in anaesthetized and tracheotomized spontaneously breathing guinea-pigs were examined. In order to elucidate the possible relationships of airway responsiveness to cellular infiltration, bronchoalveolar lavage was performed in additional group of guinea-pigs exposed to the same conditions. After acute exposure to 3 ppm TDI, increased bronchial responsiveness was evident within 30 min, lasted 48 h, but had vanished 1 week after the exposure. An influx of neutrophils occurred into the BALF within 1 h after exposure. The influx of neutrophil into BALF lasted 48 h and vanished 1 week after the end of exposure. After 48 h of exposure to TDI at 0.080 ppm, or 0.046 ppm for 1 week, increased bronchial responsiveness was evident 24 h after the end of the both modes of exposure, but no influx of neutrophils was observed into the BALF. It was concluded that even though the neutrophil influx and hyperresponsiveness evolve in the same way after acute exposure to a high concentration of TDI (3 ppm), this is not the case after subchronic exposure to low concentrations of TDI, where a bronchial hyperresponsiveness is observed without detectable neutrophil influx.

Multiple chemical sensitivity multiorgan dysesthesia, multiple symptom complex, and multiple confusion: problems in diagnosing the patient presenting with unexplained multisystemic symptoms

Patients are presenting in increasing numbers with multiorgan symptoms allegedly resulting from exposure to environmental chemicals. Among the symptoms expressed by patients with alleged multiple chemical sensitivities (MCS) are profound fatigue, mental confusion, myalgia, depression, anxiety, dizziness, headache, insomnia, loss of appetite, and numbness of the extremities, all in the absence of objective physical signs. Diagnostic criteria to assess the effects of environmental agents on organ systems are sorely needed because patients with MCS often have no tissue pathology or physiological abnormalities, but often do have diagnosable psychiatric illnesses. In treating patients with MCS, the physician should first perform a complete history and physical examination, including a comprehensive evaluation of chemical exposure. If the findings strongly suggest the presence of disease related to particular organ systems, further diagnostic evaluation should be undertaken. If abnormal findings are absent, psychiatric advice may be useful. The physician should keep an open mind about MCS but must also remember that a cause-effect relationship between exposure to multiple chemicals and symptoms has not been established.

Substance P enhances antigen-evoked mediator release from human nasal mucosa

Exogenous substance P (10-80 nmol/ml) induced a dose-dependent increase in nasal symptoms in asymptomatic allergics with rhinitis (n = 15) and controls (n = 8), but did not release any mediators. However, comparing the antigen-evoked release of mediators into nasal secretions with that of a substance P-pretreated antigen challenge, we found a significant enhancement of kinins, TAME esterase activity (p < 0.05-0.01), and histamine (p < 0.001, NS) 10-20 min after antigen challenge. These results suggest 1) that substance P-induced increase in nasal congestion is mediated through direct neurokinin receptor activation independently of mast cell activation, and 2) that during the allergic reaction there is a substance P-mast cell interaction that enhances the mediator response to nasal allergen challenge.

Neutral endopeptidase (NEP) and its role in pathological pulmonary change with inhalation exposure to JP-8 jet fuel

Through a simulated flightline exposure protocol, Fischer 344 rats (F344) were subjected to an aerosol/vapor mix of the military jet fuel, JP-8. Previous studies with this model of lung injury have revealed significant increases in pulmonary resistance, increased alveolar clearance of 99mTcDTPA, and a decrease in bronchoalveolar lavage fluid (BALF) concentration of the neuropeptide substance P (SP). Exposures to JP-8 were nose-only and for one hour daily. Six groups of Fischer 344 rats were exposed for 7, 28, or 56 days at two JP-8 concentrations (low dose = 469-520 mg/m3/hr, high dose = 814-1263 mg/m3/hr). Exposed groups were matched with longitudinal controls. In response to JP-8 inhalation, exposure animals demonstrated a dose-dependent as well as duration-determined reduction in BALF SP concentration. Both JP-8 concentrations caused significant pathological changes in lower pulmonary structures.

Role of substance P and neurokinin A in toluene diisocyanate-induced increased airway responsiveness in rabbits

The aim of the present study was to examine the role of neuropeptides, especially substance P (SP) and neurokinin A (NKA), in toluene diisocyanate (TDI)-induced airway hyperresponsiveness (AHR) to acetylcholine aerosols. Thirty parts per billion of TDI in air administered over 4 hours caused a significant increase in the airway constrictive response to acetylcholine (ACH) aerosols in rabbits (DeltaRI: 245 +/- 30%, p < 0.005) without altering basic values of respiratory, cardiovascular or blood gas parameters. Inhalation of the aerosolized neuropeptides SP and NKA resulted in a similar increase in airway responsiveness (AR) to ACH as exposure to 30 ppb TDI. To determine whether neuropeptides contribute to TDI-induced AHR, we studied their effects after systemic treatment with capsaicin as well as after infusion of specific synthetic antagonists for SP and NK2 (NKA) receptors. CAPS treatment performed on 4 consecutive days as well as antagonists' infusion only moderately (p > 0.05) decreased airway responses to ACH. CAPS application prevented the TDI-induced increase in AR to ACH in all rabbits. The increase in airway resistance to ACH did not significantly change after TDI exposure (98 +/- 22% of the control response before TDI, p > 0.05). Simultaneous infusion of specific synthetic SP and NK2 receptor antagonists also abolished the TDI-induced increase in airway responses to ACH in all animals investigated (p > 0.05). The results of this study demonstrate that neuropeptides, especially the tachykinins SP and NKA, are important mediators in TDI-induced AHR in rabbits.

Neurokinin receptors subserving airways secretion

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

Hydrogen peroxide inhibits lung neutral endopeptidase

Oxidants have been implicated in the pathogenesis of many inflammatory airway diseases. Neutral endopeptidase (also called enkephalinase, EC 3.4.24.11) is a peptidase that is involved in the degradation of several proinflammatory peptides, such as tachykinins and kinins. Indirect evidence suggests that airway neutral endopeptidase is inactivated by oxidants. To determine whether hydrogen peroxide inactivates neutral endopeptidase, we studied the activity of this peptidase in washed crude preparations of membranes from guinea pig lungs. Washed crude membrane preparations were exposed to increasing concentrations of hydrogen peroxide (1.25-25 mM) in the presence or absence of two different concentrations of catalase (300 and 700 U/mL). Neutral endopeptidase activity was inhibited by hydrogen peroxide in a concentration-dependent fashion (p = .0001). Addition of catalase prevented, in a concentration-dependent fashion, the inhibition of neutral endopeptidase induced by hydrogen peroxide (p = .0001). Mannitol (40 mM) and L-methionine (20 mM) did not prevent inhibition of neutral endopeptidase induced by hydrogen peroxide (2.5 mM). It can be concluded that neutral endopeptidase is inactivated by hydrogen peroxide, an effect that is prevented by catalase. Hydrogen peroxide-induced inactivation of neutral endopeptidase is not mediated by spontaneous generation of either hydroxyl radical or hypochlorous acid in the membrane preparation. Our results suggest that neutral endopeptidase inactivation may occur in airway diseases associated with exposure to or production of oxidants.

Antibronchospastic activity of MEN10,627, a novel tachykinin NK2 receptor antagonist, in guinea-pig airways

The antibronchospastic activity against acetylcholine, antigen, histamine plus platelet-activating factor (PAF) or the selective tachykinin neurokinin (NK)1 and NK2 receptor agonists of the novel tachykinin NK2 receptor antagonist, MEN10,627 (cyclo(Met-Asp-Trp-Phe-Dap-Leu)cyclo(2 beta-5 beta)), was studied in anesthetized guinea-pigs. MEN10,627 (30-100 nmol/kg i.v.) reduced in a dose-dependent manner the bronchospasm induced by the tachykinin NK2 receptor agonist [beta Ala8]neurokinin A-(4-10) and the effect of the highest dose lasted up to 5 h from its administration. Conversely, airway constriction induced by the NK1 receptor agonist [Sar9]substance P sulfone or acetylcholine was unaffected by MEN10,627 up to a dose of 3 mumol/kg i.v. In animals sensitized with ovalbumin and pretreated with the endopeptidase inhibitor phosphoramidon, the aerosolized antigen produced a bronchospasm which was inhibited by MEN10,627 (30-100 nmol/kg i.v.) but not by the tachykinin NK1 receptor antagonist, (+/-)-CP96,345 ([2R,3R-cis- and [2S,3S)-cis-2-(diphenylmethyl)-N-[(2-methoxyphenyl)-methyl]-1- azabicyclo[2.2.2]octan-3-amine]) (3 mumol/kg i.v.). Both MEN10,627 (30-100 nmol/kg i.v.) and (+/-)-CP96,345 (30-300 nmol/kg i.v.) reduced the PAF-induced hyperresponsiveness to histamine, without affecting the hypotension induced by PAF or the bronchospasm induced by histamine in guinea-pigs not exposed to PAF, showing the involvement of both tachykinin NK1 and NK2 receptors in this model. In summary, MEN10,627 behaves as a potent, selective and long-lasting tachykinin NK2 receptor antagonist in vivo. Further, tachykinin NK2 receptors could be activated during allergic responses and in the development of airway hyperresponsiveness.

Multiple chemical sensitivities--chemical sensitivity as a symptom of airway inflammation

The term multiple chemical sensitivity confuses etiology with diagnosis. Chemical sensitivity is a symptom expressed by patients. The symptoms complex is also expressed by the majority of patients with asthma reactive airway dysfunction syndrome or rhinitis following a single acute exposure, called reactive upper airway dysfunction syndrome. The chemically sensitivity patient merits evaluation for upper airway and bronchial reactivity that may cause extra-airway symptomatology.

Distribution of, and a putative role for, the cell-surface neutral metallo-endopeptidases during mammalian craniofacial development

Endopeptidase-24.11 (neutral endopeptidase, neprilysin, ‘enkephalinase’, EC 3.4.24.11) and endopeptidase-24.18 (endopeptidase-2, meprin, EC 3.4.24.18) are cell-surface zinc-dependent metallo-endopeptidases able to cleave a variety of bioactive peptides including growth factors. We report the first study of the cellular and tissue distribution of both enzymes and of the mRNA for NEP during embryonic development in the rat. Endopeptidase-24.11 protein was first detected at E10 in the lining of the gut and, at E12, the enzyme was present on the notochord, medial and lateral nasal processes, otocyst, mesonephros, heart and neuroepithelium. In contrast, at this time endopeptidase-24.18 was present only on the apical surface of the neuroepithelial cells. By E14 and E16, NEP was also detected in a wide range of craniofacial structures, notably the palatal mesenchyme, the choroid plexus, tongue and perichondrium. The distribution of endopeptidase-24.18 at these stages was restricted to the inner ear, the nasal conchae, and ependymal layer of the brain ventricles and the choroid plexus. Although endopeptidase-24.11 had been detectable in the craniofacial vasculature at E12 and E14, this was no longer apparent at E16. Significantly, the distribution of endopeptidase-24.11 mRNA closely matched the immunolocalization of the protein at all stages investigated. In order to explore the functional role of these enzymes, inhibition studies were carried out using two selective inhibitors of endopeptidase-24.11, phosphoramidon and thiorphan. E9.5 and E10.5 embryos exposed to either inhibitor displayed a characteristic, asymmetric abnormality consisting of a spherical swelling, possibly associated with a haematoma, predominantly on the left side of the prosencephalon, and the severity of this defect appeared to be a dose-dependent phenomenon. This study suggests that these enzymes play previously unrecognized roles during mammalian embryonic development.

Tachykinins and kinins in antigen-evoked plasma extravasation in guinea-pig nasal mucosa

The plasma extravasation evoked by instillation of 5% ovalbumin in the nasal mucosa of sensitized guinea-pigs was potentiated by the neutral endopeptidase inhibitor, phosphoramidon, and was reduced by the tachykinin NK1 receptor antagonist, CP-96,345. The bradykinin B2 receptor antagonist, HOE 140, also reduced the plasma extravasation evoked by the antigen. The combination of HOE 140 and CP-96,345 did not increase further the inhibition caused by HOE 140 alone. Plasma extravasation evoked by instillation of capsaicin was abolished by CP-96,345. HOE 140 blocked and CP-96,345 markedly reduced plasma extravasation caused by instillation of bradykinin. Plasma extravasation evoked by instillation of substance P was not affected by HOE 140. We conclude that antigen challenge causes plasma extravasation in the nasal mucosa of sensitized guinea-pigs, an effect that is due in part to the release of tachykinins from sensory nerve endings. Our evidence suggests that tachykinin release in response to antigen is provoked mainly by the release of kinins.

Effects of chronic airway inflammation on the activity and enzymatic inactivation of neuropeptides in guinea pig lungs

The effects of airway inflammation induced by chronic antigen exposure on substance P (SP)-induced increases and vasoactive intestinal peptide (VIP)-induced decreases in airway opening pressure (Pao), and the recovery of intact and hydrolyzed radiopeptide were studied in tracheally perfused guinea pig lungs. SP (10(-6) mol/kg) induced a significantly greater increase in Pao in lungs from antigen-exposed (30 +/- 5 cm H2O) than saline-exposed animals (15 +/- 1 cm H2O, P < 0.05). Significantly more intact 3H-SP and significantly less 3H-SP 1-7, a neutral endopeptidase (NEP) hydrolysis product, were recovered from the lung effluent of antigen-exposed than saline-exposed animals (P < 0.05). Injection of VIP (10(-9) mol/kg) induced significantly more pulmonary relaxation in saline-exposed compared with antigen-exposed lungs (62 +/- 4%, P < 0.001). In contrast to effluent from saline-exposed animals, lung effluent from antigen-exposed lungs contained less intact VIP, increased amounts of a tryptic hydrolysis product, and no products consistent with the degradation of VIP by NEP. These data indicate that inflamed lungs are more sensitive to the contractile effects of SP because it is less efficiently degraded by NEP and are less sensitive to the relaxant effects of VIP because it is more efficiently degraded by a tryptic enzyme. Changes in airway protease activity occur with allergic inflammation and may contribute to airway hyperresponsiveness.

RADS and RUDS--the toxic induction of asthma and rhinitis

Inhalation exposures can produce asthma and rhinitis by several mechanisms. Sensitization with the production of IgE specific for a substance can lead to symptoms on reexposure via mast cell degranulation and the release of inflammatory mediators. Some substances, known as environmental adjuvants, enhance the immune response to concomitant exposures with the environmental adjuvant. Respiratory irritants can lead to asthma and rhinitis through interaction with chemical irritant receptors in the airway, leading to release of substance P from sensory nerves and neurogenic inflammation. The reactive airways dysfunction syndrome is a chronic asthma-like syndrome resulting from a single acute exposure to a respiratory irritant, while the reactive upper-airways dysfunction syndrome is chronic rhinitis stemming from an irritant exposure. The dysregulation of neurogenic inflammation by chemical exposures may be an important mechanism in the toxic induction of reactive airways dysfunction syndrome and reactive upper-airways dysfunction syndrome and may play a role in understanding the sick building syndrome and the multiple chemical sensitivity syndrome.

Neutral endopeptidase activity and airway hyperresponsiveness to neurokinin A in asthmatic subjects in vivo

In a previous study we have shown that inhibition of the endogenous neuropeptide-degrading enzyme, neutral endopeptidase (NEP), potentiates airway narrowing to neurokinin A (NKA) in normal humans in vivo. In the present study, we tested the hypothesis that hyperresponsiveness to NKA in asthma is caused by a reduction in endogenous NEP activity. To that end, we used the NEP inhibitor, thiorphan, or placebo as inhaled pretreatment to NKA challenge in eight atopic asthmatic men, who were controlled by on-demand usage of beta 2-agonists alone. The dose of thiorphan pretreatment was obtained from pilot experiments in which 0.5 ml of a 2.5-mg/ml concentration appeared to be the maximally effective nebulized dose. Dose-response curves to inhaled NKA (1 to 125 micrograms/ml, 0.5 ml/dose) were recorded on 2 randomized days 1 wk apart, in a cross-over study. To detect any effects of thiorphan on bronchoconstriction per se, we also investigated the effect of thiorphan or placebo on the dose-response curve to inhaled methacholine in a separate set of experiments. The response was measured by FEV1 and by partial expiratory flow-volume curves (V40p). The position of the dose-response curves was expressed as the concentration causing a 20% fall in FEV1 (PC20FEV1) or a 40% fall in V40p (PC40V40p). Baseline FEV1 and V40p were not affected by either pretreatment (p > 0.06). PC20FEV1 and PC40V40p to NKA were significantly lower after thiorphan pretreatment as compared with placebo (mean difference +/- SEM: 2.3 +/- 0.6 and 1.6 +/- 0.5 doubling dose, respectively; p < 0.015).(ABSTRACT TRUNCATED AT 250 WORDS)

Sensory neuropeptide release by bradykinin: mechanisms and pathophysiological implications

Bradykinin (BK) and related kinins excite primary sensory neurons, thus leading to the activation of sensory impulses. More recently, both functional and neurochemical evidence have been accumulated that BK evokes release of neuropeptides, including calcitonin gene-related peptide and the tachykinins substance P and neurokinin A, from peripheral terminals of capsaicin-sensitive primary afferents. The present article will review the mechanisms and the pathophysiological implications of the ability of BK to release sensory neuropeptides at the peripheral level. An account of the clinical studies performed on this subject will be also given.

Neurogenic inflammation and sensitivity to environmental chemicals

Neurogenic inflammation as a pathway distinct from antigen-driven, immune-mediated inflammation may play a pivotal role in understanding a broad class of environmental health problems resulting from chemical exposures. Recent progress in understanding the mediators, triggers, and regulation of neurogenic inflammation is reviewed. Evidence for and speculations about a role for neurogenic inflammation in established disorders such as asthma, rhinitis, contact dermatitis, migraine headache, and rheumatoid arthritis are presented. The sick building syndrome and multiple chemical sensitivity syndrome have been defined as clinical entities in which exposure to chemical inhalants gives rise to disease. Current data on the existence of chemical irritant receptors in the airway and skin are discussed; neurogenic inflammation arising from stimulation of chemical irritant receptors is a possible model to explain many of the aspects of chemical sensitivities.

Endogenous tachykinins facilitate transmission through parasympathetic ganglia in guinea-pig trachea

1. Exogenous and endogenous tachykinins facilitate cholinergic nerve-induced bronchoconstriction in guinea-pig. Using a vagally innervated guinea-pig tracheal tube preparation we have investigated the involvement of endogenous capsaicin-sensitive neuropeptides in both pre- and postganglionic cholinergic neurotransmission. The effects of the neutral endopeptidase inhibitor (NEP), phosphoramidon, were investigated in this preparation either alone or in conjunction with sensory neuropeptide depletion by capsaicin pretreatment. The subtype of neurokinin receptor mediating this facilitatory effect of tachykinins has also been examined, by the use of selective tachykinin receptor agonists and a selective NK1 receptor antagonist. 2. Cholinergic contractions of the sealed Krebs filled tracheal tube preparation were recorded as increases in intraluminal pressure and were induced either by (i) pre-ganglionic vagus nerve stimulation (PGS), (ii) stimulation of postganglionic intramural nerves via transmural stimulating electrodes (TMS) in the presence of ganglion-blocking concentrations of hexamethonium and (iii) application of exogenous acetylcholine (ACh). 3. The effect of phosphoramidon, which inhibits the breakdown of tachykinins, was investigated on ACh-, PGS- and TMS-induced contractions. Phosphoramidon (1-10 microM) facilitated contractions of the trachea induced by PGS, in a concentration-dependent manner, but had no effect on contractions of the trachea induced either by TMS or exogenous ACh. 4. The facilitatory effect of phosphoramidon (10 microM) on PGS-induced contractions was abolished by pretreating guinea-pigs with capsaicin 7 +/- 2 days before the in vitro experiments. Capsaicin pretreatment did not significantly alter responses to the spasmogens, ACh or substance P. Depletion of sensory neuropeptides, by capsaicin pretreatment was confirmed by the lack of response to capsaicin (1 microM) in vitro. 5. The facilitatory effect of phosphoramidon (10 microM) on PGS-induced contractions was inhibited by the selective NK1 receptor antagonist, GR71251 (1 microM). When applied to the tissues during nerve stimulation,GR71251 caused a small, but significant, inhibition of PGS-induced contractions during low frequency stimulation. No significant effect of GR71251 on TMS-induced contractions was seen at any frequency. There was no significant effect of the NK1 receptor antagonist on contractions of the trachea induced by exogenous ACh.6. The selective NK1 receptor agonist, GR73632 facilitated contractions of the trachea induced by stimulation of both pre- and postganglionic cholinergic nerves, in a concentration-dependent manner, at concentrations that had no significant effect on basal tone (0.01-0.3 nM). The facilitatory effect ofGR73632 on both PGS- and TMS-induced contractions was antagonized by GR71251 (1 microM). In contrast, neurokinin A (1 - 10 nM), which preferentially stimulates NK2 receptors, facilitated contractions induced by both PGS and TMS, and caused a significant increase in basal tone of the trachea. The selective NK3 receptor agonist, senktide (30-300 mM), had no significant effect on nerve-induced contractions or basal tone of the trachea.7. These results suggest that there is release of endogenous tachykinins during vagus nerve stimulation,which can be depleted by capsaicin pretreatment and, which facilitate cholinergic nerve-induced contractions at the level of the parasympathetic ganglia. Facilitatory tachykinin receptors on the postganglionic nerve terminals can be demonstrated by exogenous agonists but do not appear to be activated by endogenous tachykinins under the stimulation conditions of these studies. These data suggest that NK1,receptors may be involved in mediating this facilitatory response to tachykinins but do not exclude an involvement of NK2 receptors. It appears unlikely, however, that NK3 receptors are involved.

Inhaled A23187 produces a preferential sensitization to substance P

We examined the effect of A23187-induced lung injury on airway responses to a variety of bronchoconstrictive aerosols in conscious guinea pigs. Guinea pigs were exposed to aerosolized A23187 or vehicle for 12 min or until labored breathing began. Animals were allowed to recover for 24 h, and then they were challenged with inhaled histamine, leukotriene D4 (LTD4), platelet-activating factor (PAF), or substance P. Eight minutes after start of the bronchoprovocative aerosol, the guinea pigs were killed and excised lung gas volume (ELGV) measurements were used as an index of in vivo airway obstruction. No differences in ELGV dose-response curves to LTD4 were seen in A23187- and vehicle-exposed animals. A23187 exposure produced small increases in both histamine and PAF sensitivity. However, A23187 caused a much more pronounced leftward shift in the dose-response to substance P. Coadministration of the neutral endopeptidase inhibitor, thiorphan, did not reduce the A23187-related airway responses to substance P. Histologic evaluation of A23187-treated lungs revealed peribronchiolar inflammation, bronchiolar epithelial injury, and mild alveolitis. We conclude that A23187 treatment produces differential airway responses to bronchoactive agents, with a preferential sensitization to substance P.

Neutral endopeptidase modulates endothelin-1-induced airway smooth muscle contraction in guinea-pig trachea

This study was designed to evaluate the role of neutral endopeptidase (NEP) in modulating the airway smooth muscle contraction induced by endothelin-1 in isolated segments of guinea-pig trachea. Endothelin-1 (10(-9)-10(-6) M) produced a concentration-dependent contraction that reached a maximum by 30 min. The NEP inhibitor leucine-thiorphan (10(-5) M) significantly increased the contractile response to endothelin-1. The addition of leucine-thiorphan to tracheal segments precontracted by 10(-9) and 10(-8) M endothelin-1 increased isometric tension by 181 +/- 65% (mean +/- 1 S.E.M.; P less than 0.05) and by 138 +/- 49% (P less than 0.05), respectively. In contrast, the kininase II inhibitor captopril and the peptidase inhibitors leupeptin and bestatin had no effect. Preincubation of endothelin-1 with 1 microgram recombinant human NEP decreased the contractile activity of endothelin-1 by 72 +/- 9%, whereas no effect was observed using heat-inactivated NEP. We conclude that NEP modulates endothelin-induced contraction of airway smooth muscle in the guinea-pig trachea.

Effect of neutral endopeptidase inhibitor on airway function and bronchial responsiveness in asthmatic subjects

We determined the effect of an inhibitor of neutral endopeptidase, acetorphan, on the skin responses to substance P and on the bronchostrictor effects of sodium metabisulphite aerosol in asthmatic subjects. One hour following ingestion of acetorphan (200 mg) or placebo tablets, cutaneous responses to substance P were performed in four subjects. In seven subjects, bronchial challenge with increasing concentrations of sodium metabisulphite solutions was performed and the concentration required to cause a 20% fall in baseline FEV1 determined (PC20). On the acetorphan day, there was a significant increase in the wheal and flare responses to substance P and to the diluent (0.9% NaCl) alone. However, there was no significant effect of acetorphan on the PC20 metabisulphite. We conclude that metabisulphite airway challenge in vivo may not invoke the release of endogenous neuropeptides. However, the degree of inhibition of neuropeptide breakdown by the oral dose of acetorphan used may not have been optimal.