Calcitonin gene-related peptide in secretory granules of serous cells in the rat tracheal epithelium

Histamine-induced changes in rat tracheal goblet cell mucin store and mucosal edema

The pathology of chronic asthma in human and mouse is characterized by inflammation and remodeling of airway tissues. As a result of repeated inflammatory insults to the lower airways, smooth muscle thickening, mucin secretion and airway hyperreactivity may develop. In ovalbumin (OVA)-sensitized mice with repeated challenges with OVA to the lower airways, the trachea and bronchi are characterized by goblet cell hyperplasia and mucus hypersecretion from goblet cells. Previous study reports that intravenous (i.v.) application of a high dose of capsaicin releases tachykinin from capsaicin-sensitive nerves, producing acute plasma leakage and mucosal edema formation and causing depletion of mucin granules in goblet cells that results in a reduction in the number and size of Alcian blue (AB)-positive goblet cells in the rat trachea within a few minute after capsaicin application. Histamine is an important non-neural mediator of asthma from mast cells. The present study investigated whether i.v. application of a high dose of histamine (18 μmol/ml/kg) could result in these acute changes and the similar time-course changes in rat trachea. The tracheal whole mounts stained with chloroacetate esterase reagent and AB and tracheal methacrylate sections stained with AB and periodic acid-Schiff reagent were used for evaluation of histological and cellular changes. At 5 min after histamine application, mucosal leaky venules were numerous and subepithelial edema ratio (% of length of edema along the mucosal epithelial circumference of tracheal cross section) was found to be 48.2 ± 4.9, which was greater (P < 0.01) than saline-treated rats. But, the number of AB-positive goblet cells, 2,030 ± 170/mm(2) of mucosal surface epithelium, was similar to saline-treated group (P > 0.05). One day later, edema ratio remained large and the number of AB-positive goblet cells was 1,140 ± 150/mm(2) epithelium, reduced to half the number of the group at 5 min after histamine (P < 0.01). It is suggested that mucus hypersecretion occurred at this time point. At 3 or 5 days after histamine, edema ratio gradually decreased. The number of AB-positive goblet cells continued to remain small on day 3. On day 5 after histamine, the number of AB-positive goblet cells restored to the level of rat group at 5 min after histamine application. At 7 days after histamine, edema ratio returned to the level of saline-treated group. It is concluded that degranulation and thinning of tracheal goblet cells and mucus hypersecretion lagged behind histamine-induced acute plasma leakage and edema, and restoration of mucin store in goblet cells was associated with remission of mucosal edema.

Immunohistochemical localization of vanilloid receptor subtype 1 (TRPV1) in the guinea pig respiratory system

Transient receptor potential vanilloid-1 (TRPV1) containing nerves are implicated in cough and bronchoconstriction although the significance of their documentation on non-neuronal cells is unclear. We have investigated the anatomical distribution and location of TRPV1 in an animal species often utilized in models of cough and airway inflammation. The distribution and localization of TRPV1 immunoreactivity in the lung was studied using confocal microscopy. Double labelling were carried out using the panaxonal marker, protein gene product 9.5 (PGP) and the neuropeptide substance P. TRPV1 was localized to fine axons within the epithelium of the trachea, however this represented only a fraction of the total axonal innervation of the epithelium. TRPV1 immunoreactive axons were also found in and around subepithelial regions of the airways, including smooth muscle and blood vessels and within the lower airways, found in the vicinity of bronchi and bronchioles, and in and around alveolar tissue. TRPV1 in the epithelium of the trachea was co-localized with substance P containing axons, although TRPV1 immunoreactive neuropeptide negative axons were also discernible. We found evidence for TRPV1 localization to axons throughout the respiratory tract. The distribution was heterogeneous and represented a fraction of the total neuronal innervation of the airways. No TRPV1 was found localized to airway epithelial cells. TRPV1 was often co-localized with the sensory neuropeptide substance P but there was evidence of TRPV1 positive neurones that did not express substance P. This suggests a role for TRPV1 in the airway that is independent of sensory neuropeptides.

Proximal airway mucous cells of ovalbumin-sensitized and -challenged Brown Norway rats accumulate the neuropeptide calcitonin gene-related peptide

Mucous cell hypersecretion and increased neuropeptide production play a role in the exacerbation of symptoms associated with asthma. The source of these neuropeptides have been confined to the contributions of small afferent nerves or possibly neuroendocrine cells. We tested the hypothesis that repeated exposure to allergen would alter the sources and abundance of neuropeptides in airways. Right middle lobes from rats (8 wk old) exposed to 2.5% ovalbumin (OVA) for five episodes (30 min each) or filtered air were inflation fixed with paraformaldehyde. The lobes were dissected to expose the airway tree, permeabilized with DMSO, and incubated in antibody to rat calcitonin gene-related peptide (CGRP), followed with a fluorochrome-labeled second antibody. CGRP-positive structures were imaged via confocal microscopy. Airways were later embedded in plastic and sectioned for cell identification. In animals challenged with OVA, CGRP-positive cells, not neuroendocrine or neuronal in origin (confirmed by a lack of protein gene product 9.5 signal), were recorded along the axial path. In section, this fluorescent signal was localized to granules within epithelial cells. Alcian blue/periodic acid-Schiff staining of these same sections positively identify these cells as mucous cells. Mucous cells of animals not challenged with OVA were not positive for CGRP. We conclude that episodic allergen exposure results in the accumulation of CGRP within mucous cells, creating a new source for the release of this neuropeptide within the airway.

Electrostatic charge activates inflammatory vanilloid (VR1) receptors

The pathophysiology of neurogenic inflammation culminates in the overt symptoms of tissue inflammation through a series of events which are initiated by the activation of vanilloid receptors (VR1). This study was designed to test the hypothesis that a sufficiently negative, electrostatic charge carried on a particulate matter (PM) particle, could acquire a cloud of protons sufficient to activate proton-sensitive VR1 receptors and acid-sensitive ionic channels (ASICs) pathways. For this, nanometer-sized, synthetic polystyrene micells (SPM) or those charged with chemical groups (e.g. diamino, carboxyl) were used. These chemical groups imparted either a net positive (i.e. diamino) or negative (i.e. carboxyl) charge on the SPM when suspended in a neutral ionic medium. The zeta potential, a measure of the SPM's electronegativity, was taken in both cell culture nutrient medium and in ultraviolet light-distilled water (UV-DW). In both vehicles, the rank order of electronegativity (most to least negative) was carboxyl > polystyrene > diamino-SPM. Individual types of SPM were exposed to human, immortalized bronchial-tracheal epithelial cells (i.e. BEAS-2B) and endpoints of biological activation (i.e. membrane depolarization, increases in intracellular calcium (i.e. [Ca(2+)](i)) levels, IL-6 release) were measured. Cells loaded with a fluorescent probe for membrane depolarization (3,3'-dihexyloxacarbocyanine iodide, DiOC-6-3) showed a positive reaction when exposed to carboxyl-SPM but not to diamino-SPM. BEAS-2B cells exposed to carboxyl-SPM responded with significant increases in [Ca(2+)](i), and IL-6 release relative to uncharged SPM or diamino-SPM. This IL-6 release could be reduced by pretreatment with antagonists to the VR1 receptor (i.e. capsazepine) or to acid-sensitive ionc channels (i.e. amiloride). Although both diamino and carboxyl-SPM groups stimulated increases in IL-6 transcript, only the more electronegatively charged carboxyl-SPM stimulated mRNA-VR1 receptor. These data suggest that measurable inflammatory changes can be stimulated in human epithelial target cells by the electrostatic charge carried on an inert particle. Further, these changes appear to be mediated through acid-sensitive VR1 receptors and ASICs.

Neurogenic inflammation and particulate matter (PM) air pollutants

Exposure to a class of airborne pollutants known as particulate matter (PM) is an environmental health risk of global proportions. PM is thought to initiate and/or exacerbate respiratory disorders, such as asthma and airway hyper-responsiveness and is epidemiologically associated with causing death in the elderly and those with pre-existing respiratory, or cardiopulmonary disease. Plausible mechanisms of action to explain PM inflammation and its susceptible sub-population component are lacking. This review describes a series of published studies which indicate that PM initiates airway inflammation through sensory neural pathways, specifically by activation of capsaicin-sensitive vanilloid (e.g. VRI) irritant receptors. These acid-sensitive receptors are located on the sensory C nerve fibers that innervate the airways as well as on various immune and non-immune airway target cells. The activation of these receptors results in the release of neuropeptides from the sensory terminals that innervate the airways. Their interactions with airway target cells, result in signs of inflammation (e.g. bronchoconstriction, vasodilation, histamine release, mucous secretion etc.). Our data have linked the activation of the VR1 receptors to the surface charge carried on the colloidal particulates which constitute PM pollution. Related studies have examined how genetic and non-genetic factors modify the sensitivity of these irritant receptors and enhance the inflammatory responsiveness to PM. In summary, this review proposes a mechanism by which neurogenic elements initiate and sustain PM-mediated airway inflammation. Although neurogenic influences have been appreciated in normal airway homeostasis, they have not, until now, been associated with PM toxicity. The sensitivity of the sensory nervous system to irritants and its interactions with pulmonary target tissues, should encourage neuroscientists to explore the relevance of neurogenic influences to toxic disorders involving other peripheral target systems.

Airway receptors

There are many types of afferent receptor in the airways; at least five in the larynx: pressure, drive, cold, irritant and C-fibre; and at least four in the trachea and bronchi: slowly and rapidly adapting stretch receptors (SARs and RARs), C-fibre receptors, and those in neuroepithelial bodies (NEBs). Histologically enough sensory structures have been identified to account for the various patterns of afferent activity, but most correlations are poor. For the larynx, four or more sensory structures have not definitively been identified with afferent discharges and reflex responses. For the trachea and bronchi, only SARs have been clearly identified morphologically and physiologically. The reflexes and afferent discharges from RARs and C-fibre receptors are fairly clear, some at least of the sensory terminals lie in the epithelium, but receptor complexes have not been mapped out. Nerves in NEBs have been identified, but not their local and central reflex actions.

Neuropeptides and capsaicin stimulate the release of inflammatory cytokines in a human bronchial epithelial cell line

The role of neuropeptides in initiating and modulating airway inflammation was examined in a human bronchial epithelial cell line (i.e. BEAS-2B). At a range of concentrations, exposure of BEAS-2B cells to Substance P (SP) or calcitonin gene related protein resulted in immediate increases in intracellular calcium ([Ca(2+)](i)), the synthesis of the transcripts for the inflammatory cytokines, IL-6, IL-8 and TNFalpha after 2 h exposure, and the release of their proteins after 6 h exposure. Addition of thiorphan (100 nM), an inhibitor of neutral endopeptidase, enhanced the levels of SP-stimulated cytokine release. Stimulation of IL-6 by SP occurred in a conventional receptor-mediated manner as demonstrated by its differential release by fragments SP 4-11 and SP 1-4 and by the blockage of IL-6 release with the non-peptide, NK-1 receptor antagonist, CP-99 994. In addition to the direct stimulation of inflammatory cytokines, SP (0.5 microM), in combination with TNFalpha (25 units/ml), synergistically stimulated IL-6 release. BEAS-2B cells also responded to the botanical irritant, capsaicin (10 microM) with increases in [Ca(2+)](i) and IL-8 cytokine release after 4 h exposure. The IL-8 release was dependent on the presence of extracellular calcium. Capsaicin-stimulated increases of [Ca(2+)](i) and cytokine release could be reduced to control levels by pre-exposure to capsazepine, an antagonist of capsaicin (i.e. vanilloid) receptor(s) or by deletion of extracellular calcium from the exposure media. The present data indicate that the BEAS-2B human epithelial cell line expresses neuropeptide and capsaicin-sensitive pathways, whose activation results in immediate increases of [Ca(2+)](i) stimulation of inflammatory cytokine transcripts and the release of their cytokine proteins.

Hyperosmolar saline induces airway resistance changes and neuropeptide release: a comparison with the effect of capsaicin, potassium and histamine

BACKGROUND

Healthy subjects do not show any bronchoconstricting response to inhalation of hypertonic saline, in contrast to subjects with symptoms of asthma. There is evidence indicating that these airway reactions may be related to stimulation of sensory nerves.

MATERIALS AND METHODS

We investigated the effects of hyperosmolar solutions on the changes in airway resistance as well as on release of neuropeptides from an isolated and perfused guinea pig lung model.

RESULTS

We observed that hyperosmolar saline (HS), capsaicin, potassium and histamine induced different patterns of response in airway resistance and neuropeptide release. HS 3.6% induced a biphasic response in airway resistance. Initially a minor relaxation, 4 +/- 1 cmH2O mL-1 min (P < 0.05), followed by a contraction, 22 +/- 3 cmH2O mL-1 min (P < 0.01). This was associated with release of calcitonin gene-related peptide (CGRP) 7.7 +/- 1.9 fmol mL-1 g (P < 0.01), but not of neurokinin A (NKA), a known bronchoconstrictor. Mannitol, at the same osmolarity as HS 3.6%, did not elicit a change in airway resistance, CRGP or NKA release. Capsaicin at 10-6 mol L-1 and potassium at 70 mmol L-1 induced a profound increase in airway tone (50 +/- 9 and 42 +/- 8 cmH2O mL-1 min respectively; P < 0.01) and elevation of both CGRP (6.4 +/- 1.9 and 3.9 +/- 1.1 fmol mL-1 g respectively; P < 0.05) and NKA (3.3 +/- 1.0 and 1.0 +/- 0.2 fmol mL-1 respectively; P < 0.05). Histamine increased the airway resistance by 42 +/- 8 cmH2O mL-1 min (P < 0.01) but had no effect on either CGRP or NKA release.

CONCLUSIONS

In healthy guinea pigs, hyperosmolar saline 3.6% initially caused relaxation of the airways followed by contraction and induced release of CGRP-LI. This was not seen with mannitol at the same osmolarity as for the hyperosmolar saline.

Afferent receptors in the airways and cough

The roles of airway rapidly adapting receptors (RARs) and of C-fibre receptors in the induction of cough are reviewed. It is concluded that, while there is substantial evidence that irritant receptors in the laryngeal wall and RARs in the tracheobronchial mucosa can cause cough, the evidence for such a similar direct role for C-fibre receptors is tenuous. Indeed there is accumulating evidence that the C-fibre receptors may cause apnoea and rapid shallow breathing, and also reflexly inhibit cough. However the C-fibre receptors may release tachykinins when stimulated, and these in turn may cause plasma extravasation from mucosal postcapillary venules. RARs are excited by increases in interstitial liquid volume, so C-fibre receptors may indirectly enhance cough via the RARs.

Effects of endotracheal intubation on airway neuropeptide content, arterial oxygenation and lung volumes in anaesthetized rats

BACKGROUND

General anaesthesia affects lung volume and pulmonary gas exchange. What role is played by mechanical stimulation by the endotracheal tube?

METHODS

We investigated the effects of intubation on arterial oxygenation and lung volume in rats.

RESULTS

Endotracheal intubation caused an increase in PA-aO2 and volume of trapped gas in the lung. This was accompanied by a reduction in neuropeptides and calcitonin gene-related peptide (CGRP) in trachea, bronchi and lung, and in vasoactive intestinal peptide (VIP) in the trachea. The increase in PA-aO2 and volume of trapped gas due to intubation was not altered in the animals given capsaicin, in which neuropeptide levels were reduced.

CONCLUSIONS

These data suggest that the decrease in CGRP and VIP content in the airway tissues may be one of the consequences, but not the cause, of impaired gas exchange by endotracheal intubation. The increase in volume of trapped gas in the lung is apparently not mediated by activation of capsaicin-sensitive sensory nerves.

Pulmonary Neuroendocrine Cells and Lung Development

Pulmonary neuroendocrine cells produce bioactive peptides such as gastrin-releasing peptide (GRP) at high levels in developing fetal lung. The role of GRP and other peptides in promoting branching morphogenesis, cell proliferation, and cell differentiation during lung organogenesis is reviewed. Possible roles for bioactive peptides derived from these cells in the pathophysiology of perinatal lung disorders are discussed.

A rhesus monkey model to characterize the role of gastrin-releasing peptide (GRP) in lung development. Evidence for stimulation of airway growth

Gastrin-releasing peptide (GRP) is developmentally expressed in human fetal lung and is a growth factor for normal and neoplastic lung but its role in normal lung development has yet to be clearly defined. In this study we have characterized the expression of GRP and its receptor in fetal rhesus monkey lung and determined the effects of bombesin on fetal lung development in vitro. By RNA blot analysis, GRP mRNA was first detectable in fetal monkey lung at 63 days gestation, reached highest levels at 80 days gestation, and then declined to near adult levels by 120 days gestation; a pattern closely paralleling GRP expression in human fetal lung. As in human lung, in situ hybridization localized GRP mRNA to neuroendocrine cells though during the canalicular phase of development (between 63-80 days gestation) GRP mRNA was present not only in classic pulmonary neuroendocrine cells, but also in cells of budding airways. Immunohistochemistry showed that bombesin-like immunoreactivity was present in neuroendocrine cells, but not in budding airways, suggesting that in budding airways either the GRP mRNA is not translated, is rapidly secreted, or a related, but different RNA is present. RNase protection analysis using a probe to the monkey GRP receptor demonstrated that the time course of receptor RNA expression closely paralleled the time course of GRP RNA expression. In situ hybridization showed that GRP receptors were primarily expressed in epithelial cells of the developing airways. Thus GRP would appear to be secreted from neuroendocrine cells to act on target cells in developing airways. This hypothesis was confirmed by organ culture of fetal monkey lung in the presence of bombesin and bombesin antagonists. Bombesin treatment at 1 and 10 nM significantly increased DNA synthesis in airway epithelial cells and significantly increased the number and size of airways in cultured fetal lung. In fact, culturing 60 d fetal lung for 5 d with 10 nM bombesin increased airway size and number nearly to that observed in cultured 80 d fetal lung. The effects of bombesin could be blocked by specific GRP receptor antagonists. Thus this study demonstrates that GRP receptors are expressed on airway epithelial cells in developing fetal lung and that the interaction of GRP with the GRP receptor stimulates airway development.

Epithelial factors: modulation of the airway smooth muscle tone

The airway epithelium is composed of a heterogeneous population of cells. This epithelial layer is not only a physical barrier but also a target responding to a variety of inflammatory mediators. These cells can respond by releasing contracting and relaxing factors to modulate airway responsiveness. They can also metabolize some of the inflammatory mediators. Epithelial damage is a consistent feature of some respiratory conditions, but whether or not such damage contributes to airway disease is for the moment unknown. This review summarizes the literature on the known and proposed roles of the epithelium in the modulation of the airway smooth muscle tone.