Airway smooth muscle dynamics: a common pathway of airway obstruction in asthma.

Excessive airway obstruction is the cause of symptoms and abnormal lung function in asthma. As airway smooth muscle (ASM) is the effecter controlling airway calibre, it is suspected that dysfunction of ASM contributes to the pathophysiology of asthma. However, the precise role of ASM in the series of events leading to asthmatic symptoms is not clear. It is not certain whether, in asthma, there is a change in the intrinsic properties of ASM, a change in the structure and mechanical properties of the noncontractile components of the airway wall, or a change in the interdependence of the airway wall with the surrounding lung parenchyma. All these potential changes could result from acute or chronic airway inflammation and associated tissue repair and remodelling. Anti-inflammatory therapy, however, does not "cure" asthma, and airway hyperresponsiveness can persist in asthmatics, even in the absence of airway inflammation. This is perhaps because the therapy does not directly address a fundamental abnormality of asthma, that of exaggerated airway narrowing due to excessive shortening of ASM. In the present study, a central role for airway smooth muscle in the pathogenesis of airway hyperresponsiveness in asthma is explored.

Maturational changes in airway smooth muscle shortening and relaxation. Implications for asthma.

Greater airway responsiveness in healthy juveniles is considered a factor in the higher asthma prevalence at a young age compared with adults. Several studies on the contractile response of airway smooth muscle (ASM) from birth to adulthood have addressed the hypothesis that a maturation of ASM plays a role in juvenile airway hyperresponsiveness. Maturation of distinct ASM properties, i.e. force generation, shortening, and relaxation, has been reported, although the majority of the studies have focused on maturation of maximum force and/or sensitivity to contractile agonists. However, in most animal species maturation of the ability to generate force does not correlate with maturation of airway responsiveness. Ontogenesis of ASM shortening has been less extensively studied and the existing reports emphasize an increase during maturation of tissue passive forces opposing shortening. ASM spontaneous relaxation has been very minimally investigated. We have recently demonstrated that the ability of ASM to spontaneously relax during stimulation is sharply reduced in juvenile airway tissue. It remains to be determined the role of these ASM properties in the onset of childhood asthma and whether specific alterations are induced by the occurrence of obstructive airway diseases in young individuals.

Reducing agents inhibit the contractile response of isolated guinea-pig main bronchi.

BACKGROUND: Oxidants are involved in many respiratory disorders, including asthma and chronic obstructive pulmonary diseases. Reduced glutathione (GSH), one of the most important antioxidant compounds against oxidant free radicals, is particularly abundant in the respiratory epithelial lining fluid, where its concentration is increased in inflammatory disorders. OBJECTIVE: We hypothesized that reducing agents may have a direct effect on airway smooth muscle. Therefore, we studied the effects of GSH on airway smooth muscle contractility in guinea-pig main bronchi. In parallel, we evaluated superoxide anion generation associated with in vitro bronchial smooth muscle contraction. METHODS: Guinea-pig main bronchi were mounted in organ baths filled with Krebs-Henseleit solution. Concentration-response curves to acetylcholine (Ach) (10(-9)-10(-3) M), carbachol (10(-9)-10(-4) M), or histamine (10(-9)-10(-3) M) were performed in the presence or absence of either reduced or oxidized glutathione (GSSG) (10(-5)-10(-3) M). We also evaluated the effects of GSH and GSSG on allergen-induced contraction in main bronchi obtained from ovalbumin-sensitized guinea-pig. Superoxide dismutase (SOD)-inhibited cytochrome c reduction kinetics was performed to evaluate superoxide anion (O2-) production during Ach-induced contraction. RESULTS: Reduced but not oxidized glutathione significantly decreased smooth muscle contraction induced by Ach, carbachol, and histamine. Similarly, only the reduced form of glutathione attenuated the bronchoconstriction induced by allergen exposure in bronchi from sensitized animals. Finally, SOD-inhibited cytochrome c reduction kinetics demonstrated increased O2- production following bronchial smooth muscle contraction. This production was not affected by epithelium removal. CONCLUSION: Our findings show that GSH decreases bronchial smooth muscle contraction to different stimuli and that oxidant free radicals are produced during bronchial smooth muscle contraction. We suggest that oxidants are involved in the mechanisms of bronchoconstriction and that reducing agents could be a possible therapeutic option for airway obstruction sustained by bronchospasm.

Serum-mediated relaxant response to toluene diisocyanate (TDI) in isolated guinea-pig bronchi.

The present study was designed to evaluate whether pre-incubation with serum, obtained from both control and toluene diisocyanate (TDI)-immunized guinea-pigs, modified the contractile response to TDI in isolated guinea-pig bronchial rings. Guinea-pigs were anaesthetized and the main bronchi dissected in two rings. Bronchial rings were incubated with normal or immune serum (100 microl ml(-1) for 2 h) and dose-response curves to TDI (0.03-1000 microM) were studied isometrically. Before serum incubation, in eight bronchial rings, epithelium was removed by rubbing the luminal surface gently with a gauze. In control rings, TDI produced a concentration-dependent contraction, whereas in rings pre-incubated with either normal or TDI-immune serum, it produced a concentration-dependent relaxation. Relaxation was 101.4 (SEM 17.4)% and 94.9 (SEM 21)% of the relaxation induced by isoproterenol (1 mM) respectively with normal and TDI-immune serum. Similarly to the pre-incubation with serum, pre-incubation with albumin produced a concentration-dependent relaxation to TDI. Serum-induced relaxant response to TDI was not affected by capsaicin desensitization, it was only partially inhibited by an NK1-tachykinin antagonist, whereas it was blocked by indomethacin. In bronchial rings without epithelium, pre-incubated with serum, TDI caused contraction at highest doses, while it still induced relaxation at the lowest doses. This study shows that one or more components of the serum modify the contractile response to TDI in isolated guinea-pig bronchi. In bronchial rings without epithelium serum was able to inhibit the contration induced by low doses of TDI.

Different ontogeny of rate of force generation and shortening velocity in guinea pig trachealis.

Juveniles of many species, including humans, display greater airway responsiveness than do adults. This may involve changes in airway smooth muscle function. In the present work we studied force production and shortening velocity in trachealis from 1-wk-old (1 wk), 3-wk-old (3 wk), and 3-mo-old (adult) guinea pigs. Strips were electrically stimulated (60 Hz, 18 V) at their optimal length (l(o)) to obtain maximum active stress (P(o)) and rate of stress generation. Then, force-velocity curves were elicited at 2.5 s from the onset of the stimulus. By applying a recently developed modification of Hill's equation for airway smooth muscle, the maximum shortening velocity at zero load (V(o)) and the value alpha. gamma/beta, an index of internal resistance to shortening (Rsi), were calculated (alpha, beta, and gamma are the constants of the equation). P(o) increased little with maturation, whereas the rate of stress generation increased significantly (0.40 +/- 0.03, 0.45 +/- 0.03, 0. 51 +/- 0.03 P(o)/s for 1 wk, 3 wk, and adult animals). V(o) slightly increased early with maturation to decrease significantly later (1. 79 +/- 0.67, 2.45 +/- 0.92, and 0.55 +/- 0.09 l(o)/s for 1 wk, 3 wk, and adult animals), whereas the Rsi showed an opposite trend (14.98 +/- 5.19, 8.99 +/- 3.01, and 32.07 +/- 5.54 mN. mm(-2). l(o)(-1). s for 1 wk, 3 wk, and adult animals). This early increase of force generation in combination with late increase of Rsi may explain the changes of V(o) with age. An elevated V(o) may contribute to the incidence of airway hyperresponsiveness in healthy juveniles.

Requirement for reactive oxygen species in serum-induced and platelet-derived growth factor-induced growth of airway smooth muscle.

Reactive oxygen species have been recently identified as important mediators of mitogenic signaling in a number of cell types. We therefore explored their role in mediating mitogenesis of airway smooth muscle. The antioxidants catalase, N-acetylcysteine, and probucol significantly reduced proliferation in primary cultures of rat tracheal smooth muscle stimulated with fetal bovine serum or platelet-derived growth factor, without affecting cell viability or inducing apoptosis. N-Acetylcysteine also significantly reduced serum-stimulated elevation of c-Fos but did not prevent the normal mitogen-induced increase in c-fos mRNA. Fractionation of ribosomes by sucrose density centrifugation and subsequent dot-blot Northern analysis revealed that antioxidants reduced incorporation of c-fos mRNA into the heaviest polyribosomes, suggesting redox regulation of c-fos mRNA translation. Serum treatment of monolayers produced a small but reproducibly significant rise in superoxide dismutase-inhibitable reduction of ferricytochrome c by myocyte monolayers. Serum-induced ferricytochrome c reduction, cellular proliferation, and c-Fos elevation were decreased by the flavoprotein-dependent enzyme inhibitor dipheyleneiodonium. Growth responses to fetal bovine serum and superoxide dismutase-inhibitable reduction of ferricytochrome c were not different between cultured tracheal myocytes from wild-type versus gp91 phagocyte oxidase null mice. These results suggest that mitogen stimulation of airway smooth muscle induces signal transduction of cell proliferation that is in part dependent on generation of partially reduced oxygen species, generated by an NADH or NADPH oxidoreductase that is different from the oxidase in phagocytic cells.

Bronchopulmonary inflammation and airway smooth muscle hyperresponsiveness induced by nitrogen dioxide in guinea pigs.

We investigated whether acute exposure to nitrogen dioxide (NO2) causes major inflammatory responses (inflammatory cell recruitment, oedema and smooth muscle hyperresponsiveness) in guinea pig airways. Anaesthetised guinea pigs were exposed to 18 ppm NO2 or air for 4 h through a tracheal cannula. Bronchoalveolar lavage was performed and airway microvascular permeability and in vitro bronchial smooth muscle responsiveness were measured. Exposure to NO2 induced a significant increase in eosinophils and neutrophils in bronchoalveolar lavage fluid, microvascular leakage in the trachea and main bronchi (but not in peripheral airways), and a significant in vitro hyperresponsiveness to acetylcholine, electrical field stimulation, and neurokinin A, but not to histamine. Thus, this study shows that in vivo exposure to high concentrations of NO2 induces major inflammatory responses in guinea pig airways that mimic acute bronchitis induced by exposure to irritant gases in man.

Immunization and challenge with toluene diisocyanate decrease tachykinin and calcitonin gene-related peptide immunoreactivity in guinea pig central airways.

Toluene diisocyanate (TDI) is a potent sensitizer that causes occupational asthma in a significant proportion of subjects exposed. We used an animal model to investigate whether neuropeptide changes occur in the airways of immunized and TDI-challenged guinea pigs. Animals were immunized by weekly intradermal injections, challenged with TDI (5 to 20 ppb) after the third injection, and killed 6 h after exposure. Control guinea pigs received injections of saline. Lung tissue was processed immediately and analyzed for nerves using the streptavidin-biotin complex peroxidase method with antisera to the neural marker protein gene product 9.5 (PGP 9.5), substance P (SP), and calcitonin gene- related peptide (CGRP). We also quantified the inflammatory infiltrate in the submucosa of central airways, and we measured the serum level of specific IgG and IgG1. Specific antibodies against TDI were present only in immunized animals. Immunized as compared with nonimmunized animals had a significant increase in eosinophils in the submucosa of central airways, and a further increase was observed 6 h after TDI challenge. Immunization and TDI challenge did not modify the number of mononuclear cells in the submucosa of central airways in both nonimmunized and immunized animals. TDI exposure did not change the overall innervation in both nonimmunized and immunized animals, but the density of PGP 9.5-positive nerves was significantly different between nonimmunized and immunized TDI-challenged animals. The density of SP-, and CGRP-immunostained nerves was significantly lower in immunized TDI-challenged than in nonimmunized animals. TDI exposure significantly decreased the density of SP-positive nerves in nonimmunized animals. A negative relationship was found between the presence of airway inflammation, as indexed by eosinophil cell infiltration, and the density of PGP 9.5-, SP-, and CGRP-immunostained nerves. In conclusion, TDI produces airway inflammation and neuropeptides changes in the central airways of immunized guinea pigs 6 h after TDI challenge. These findings support an interaction between tachykinins, inflammatory (i.e., eosinophils) and possibly immune cells.

Dehydroepiandrosterone and analogs inhibit DNA binding of AP-1 and airway smooth muscle proliferation.

The adrenal steroid dehydroepiandrosterone (DHEA) and its analogs reduce growth of immortalized and malignant cell lines. We therefore explored their effects on the growth of airway smooth muscle, whose hyperplasia may lead to fixed airways obstruction and enhanced airways hyperresponsiveness in severe chronic asthma. DHEA and its potent analog 16 alpha-bromoepiandrosterone dramatically reduced proliferation in primary cultures of rat tracheal smooth muscle stimulated with fetal bovine serum or platelet-derived growth factor. Growth inhibition was dose-dependent and could not be attributed to interference with glucose-6-phosphate dehydrogenase activity or cholesterol metabolism, as reported for immortalized or malignant cell lines, respectively. Expression of the early response gene c-fos remained intact, but DHEA and 16 alpha-bromoepiandrosterone decreased DNA binding of the transcription factor activator protein-1, a later response important for expression of genes that mediate DNA synthesis and cell cycle progression. These results suggest that the nonglucocorticoid steroid DHEA and its analogs may impair activation of secondary growth response genes in a fashion analogous to that reported for glucocorticoids and that they may prove useful for treatment of asthmatic airway remodeling in the human.

Isotonic smooth muscle response in human bronchi exposed in vitro to nitrogen dioxide.

Exposure to nitrogen dioxide (NO2), a common oxidant airborne pollutant, has been shown to cause reversible effects on lung function and airway responsiveness, in addition to airways inflammation. However, there have been conflicting reports concerning NO2-induced airway hyperresponsiveness. In the present study, we investigated the isotonic smooth muscle response in isolated human bronchi previously exposed in vitro to NO2. Bronchial segments were obtained from 12 patients who had undergone thoracotomy for lung cancer. Bronchial segments from each patient were exposed to air and to 2.5 parts per million (ppm) NO2 for 4 h. The contractile response of bronchial rings to acetylcholine, neurokinin A (NKA), and substance P was then studied under isotonic conditions. The response to NKA was also studied in rings, with or without epithelium, exposed either to air or 7 ppm NO2. No NO2-induced alteration of the bronchial smooth muscle isotonic response was found under any of the experimental conditions. We conclude that in vitro exposure to up to 7 ppm nitrogen dioxide does not cause alterations of the human bronchial smooth muscle shortening capacity.

In vivo exposure to nitrogen dioxide (NO2) induces a decrease in calcitonin gene-related peptide (CGRP) and tachykinin immunoreactivity in guinea-pig peripheral airways.

The mammalian respiratory tract is densely innervated by sensory and autonomic fibres. Subsets of the nerves contain bioactive regulatory peptides, such as substance P, calcitonin gene-related peptide (CGRP), and neurokinins. The sensory nervous system responds to inhaled irritants, resulting in a release of neuropeptides and, thus, a decrease in the peptide immunoreactivity of the fibres. We examined the effects of inhaled nitrogen dioxide (NO2), a well-known indoor and outdoor air pollutant, on pulmonary sensory neuropeptides. Guinea-pigs were exposed for 4 h to 18 parts per million (ppm) NO2 or to air (n = 5 each). At the end of the exposure, they were killed with urethane and their lungs were fixed in 1% paraformaldehyde in phosphate-buffered saline. Cryostat sections were stained with antisera to an anatomical nerve marker, protein gene product (PGP) 9.5, and to CGRP and tachykinins, utilizing the avidin-biotinylated peroxidase method. In the noncartilaginous airways (diameter < 250 microns) of NO2-exposed animals, less tachykinin- and CGRP-immunoreactive nerve fibres were found compared with controls. No change was seen in the total nerve fibre distribution (PGP 9.5). It is concluded that the peptidergic nerves of guinea-pig peripheral airways are a sensitive indicator of exposure to nitrogen dioxide.

Inflammatory events in the blood and airways of guinea pigs immunized to toluene diisocyanate.

Toluene diisocyanate (TDI)-induced asthma is a common cause of occupational lung disease. We used a model to investigate the course of bronchopulmonary inflammation following immunization with TDI. Guinea pigs were immunized by weekly intradermal injections and challenged with TDI 7 d after the third injection. The animals were killed at different times after challenge and prepared for histologic examination of central and peripheral airways, for immunohistochemical studies of T lymphocyte and eosinophil distribution, and for hematologic and serologic investigations. Specific IgG1 against TDI were present only in immunized animals. In immunized TDI-challenged animals there was a significant increase in the number of metachromatic cells (at 24 h) and a late increase of eosinophils (at 48 h) in the peripheral blood. Mast cells and eosinophils were also increased in the submucosa of central airways of immunized TDI-challenged animals. A similar pattern was observed in the animals' peripheral airways. Additionally, a significant increase of T-lymphocytes and eosinophils was found in the lamina propria at 6 h after exposure in immunized TDI-challenged animals as compared with control animals. In these immunized animals, TDI challenge caused a significant increase of eosinophils, T-lymphocytes, and CD4+ T cells. These findings indicate that intradermal injections of TDI induced a specific antibody response as well as an inflammatory process in both central and peripheral airways. T cells, particularly CD4+ T cells and eosinophils, are the key cells in the immunopathologic alterations induced by TDI in the guinea pig lung.

Effect of subchronic in vivo exposure to nitrogen dioxide on lung tissue inflammation, airway microvascular leakage, and in vitro bronchial muscle responsiveness in rats.

OBJECTIVES: In a previous study on bronchoalveolar lavage fluid from rats exposed in vivo for seven days to 10 ppm nitrogen dioxide (NO2), it has been shown that there is an influx of macrophages into the airways. The present study investigated the effect of seven day exposure to 10 ppm NO2, on: (a) lung tissue inflammation and morphology; (b) airway microvascular leakage; (c) in vitro contractile response of main bronchi. METHODS: Lung tissue was studied by light microscopy, after fixing the lungs by inflation with 4% formalin at a pressure of 20 cm H2O. Microvascular leakage was measured by extravasation of Evans blue dye in the larynx, trachea, main bronchi, and intrapulmonary airways. Smooth muscle responsiveness was evaluated by concentration-responses curves to acetylcholine (10(-9)-10(-3) M), serotonin (10(-9)-10(-4) M), and voltage-response curves (12-28 V) to electrical field stimulation. RESULTS: Histology showed an increased total inflammation at the level of respiratory bronchioles and alveoli. No influx of inflammatory cells was found in the main bronchi. A loss of cilia in the epithelium of small airways and ectasia of alveolar capillaries was also found. By contrast, no alterations to microvascular permeability or modification of bronchial smooth muscle responsiveness was found. CONCLUSIONS: Subchronic exposure to 10 ppm NO2 causes airway inflammation and structural damage, but does not cause any persistent alteration to microvascular permeability or bronchial smooth muscle responsiveness in rats.

Increased response to antigen and histamine release in smaller sensitized canine bronchi.

We studied the Schultz-Dale response in vitro in large and small size branches from 3rd to 6th generation bronchi from ragweed-sensitized dogs. The response to electric field stimulation (EFS) increased after antigen from 65.56 +/- 8.11 to 78.6 +/- 9.0 mN/mm2 of smooth muscle (P < 0.01), but no topographical difference was observed. The response to ragweed (% of the response to EFS) was 158.3 +/- 12 and 67.1 +/- 11.7 in strips from small and large branches respectively (P < 0.01), while no difference was observed between generations; when clustering bronchi according to dimension, it was 129.9 +/- 13.4 in small and 71.9 +/- 19.8 in large bronchi (P < 0.01). Histamine released from small and large branches was 2.90 +/- 1.01 and 0.76 +/- 0.20 (ng/mg of tissue) respectively (P < 0.05); no difference was found between generations. In conclusion, in sensitized dogs a greater response to antigen, which involves a higher histamine release, occurs in small compared to large bronchi. We suggest that control of distribution of ventilation occurs mainly at small bronchi level, which becomes the elective tissue to study the Schultz-Dale response. Finally, the classification of bronchi into generations is inadequate to study allergic bronchospasm.

The Schultz-Dale response of sensitized canine bronchial smooth muscle.

We wished to determine why in vitro agonist dose-response curves show reduced leftward shift (1/2-1 log dose units) in sensitized canine airway smooth muscle compared to curves elicited in vivo (2-3 log dose unit). The Schultz-Dale response was studied in sensitized dog tracheal (TSM) and bronchial (BSM) smooth muscle. Sensitized TSM challenged with specific antigen showed greater mechanical response, but only on exposure to 300 micrograms/ml; BSM responded to concentrations of ragweed as low as 0.001 microgram/ml. This result resolved the problem cited at the outset. Control TSM and BSM showed no response. The response in BSM is mediated through histamine release, and to a smaller extent by acetylcholine. With challenge release of histamine and acetylcholine increased significantly in sensitized airway smooth muscle. Integrated contractile responses obtained with high and low concentrations of antigen showed a dose-response relationship. Increased sensitivity of BSM to antigen compared to TSM indicates the former is the preparation of choice for study of allergic bronchoconstriction.

Effect of oxidant air pollutants on the respiratory system: insights from experimental animal research.

In the present paper, we have reviewed experimental animal studies on the effects of the two most important oxidant airborne pollutants, nitrogen dioxide and ozone, on the respiratory system. The toxic effects depend on concentration and length of exposure, and are generally similar for both oxidants, with ozone operative at lower concentrations. High doses of both oxidants cause death due to lung oedema. Exposure to sublethal levels causes functional alterations such as airflow limitation and airway hyperresponsiveness to bronchoconstrictor stimuli. These effects, which are generally reversible, are associated with epithelial injury, oedema and airway and parenchymal infiltration by inflammatory cells. Loss of cilia of airway epithelium and necrosis of type I alveolar epithelial cells are the most prominent consequences at the epithelial level. Inflammation is characterized by early neutrophilic infiltration, followed by an increased number of mononuclear cells, predominantly alveolar macrophages. After long-term exposure, whilst nitrogen dioxide causes predominantly emphysema, ozone produces mainly pulmonary fibrosis. Biochemical effects include lipid peroxidation, increased antioxidant metabolism, and alteration of enzyme activity. Nitrogen dioxide and ozone may also alter the immunological response and reduce the defence against infections, increasing the susceptibility of exposed animals to infections.

In-vitro exposure of guinea pig main bronchi to 2.5 ppm of nitrogen dioxide does not alter airway smooth muscle response.

In order to investigate whether the oxidant airborne pollutant nitrogen dioxide (NO2) affects airway smooth muscle responsiveness, the contractile response of guinea pig main bronchi after in vitro exposure to 2.5 ppm of nitrogen dioxide was studied. Main bronchi were cannulated and exposed for 2 or 4 h to a constant flow of either NO2 or air. After exposure, bronchial rings were obtained and placed in a 37 degrees C jacketed organ bath filled with Krebs-Henseleit solution. Concentration-response curves were performed for acetylcholine (10(-9)-10(-3) M), substance P (10(-9)-10(-4) M), and neurokinin A (10(-10)-10(-5) M), and voltage-response curves (12-28 V) were performed for electrical field stimulation. There was no significant difference in either the smooth muscle maximal contractile response, or sensitivity between the bronchi exposed to NO2 and those exposed to air. We conclude that in vitro exposure to 2.5 ppm of NO2 does not alter airway smooth muscle responsiveness in guinea pigs.

In vitro exposure to nitrogen dioxide (NO2) does not alter bronchial smooth muscle responsiveness in ovalbumin-sensitized guinea-pigs.

The aim of this study was to investigate whether in vitro exposure to NO2 affects responsiveness in ovalbumin-sensitized guinea-pig bronchi. Twenty-three animals were sensitized by three weekly intraperitoneal injections of 1 mg ovalbumin in saline with Freund's adjuvant; twenty-one control guinea-pigs received the diluent alone. From each animal, the two main bronchi were obtained and cannulated, then exposed in vitro to a constant intraluminal flow of: (i) either air or 2.5 ppm NO2 with four spikes of 10 ppm NO2 for 2 h; (ii) either air or 10 ppm NO2 for 4 h. A bronchial ring obtained from each animal before exposure was kept in aerated Krebs-Henseleit solution. Rings from bronchi exposed to air, NO2, or kept in Krebs solution were studied isometrically. We performed overall and non-adrenergic non-cholinergic voltage-response curves to electrical field stimulation, concentration-response curves to acetylcholine and to neurokinin A, followed by administration of 10 mg/ml ovalbumin. We did not find any significant difference in bronchial smooth muscle responsiveness between nonexposed, air-exposed and NO2-exposed bronchi, as well as between bronchi from control and sensitized animals. We conclude that in vitro exposure to NO2 does not alter bronchial smooth muscle responsiveness to either specific or non-specific stimuli.

The effects of toluene diisocyanate and of capsaicin on human bronchial smooth muscle in vitro.

Toluene diisocyanate contracts guinea-pig bronchial smooth muscle through a mechanism involving capsaicin-sensitive sensory nerves. In the present study, we investigated the effects of toluene diisocyanate, capsaicin and tachykinins on isolated human bronchi. In 44 rings, toluene diisocyanate (0.3 mM) produced a relaxation which averaged 16.9 +/- 1.1%, in ten rings it produced a shortening that was 15.1 +/- 3.3% and in ten preparations it gave no response. A second administration of toluene diisocyanate (0.3 mM) always produced a relaxation (n = 13, 18.1 +/- 3.9%). Capsaicin (0.03 mM) produced shortening in 15 (35 +/- 6.6%) and relaxation in 11 preparations (41 +/- 6.8%), whereas a second administration caused shortening in nine (25.1 +/- 6.1%) and relaxation in 16 rings (36.4 +/- 4.9%). When toluene diisocyanate was given after two consecutive capsaicin administrations, we observed shortening in two rings (10.0 +/- 3.6%), relaxation in ten rings (15.9 +/- 3.6%), and no response in four preparations. To test the role of NK1 and NK2 receptors in these conflicting responses, we performed concentration-response curves to different tachykinins. Substance P, neurokinin A and neurokinin A-(4-10), a specific NK2 receptor agonist, gave a concentration-dependent shortening, with neurokinin A being the most effective and neurokinin A-(4-10) the least. The specific NK1 receptor agonist, [Sar9, Met(O2)11]substance P, produced both shortening and relaxation. We conclude that toluene diisocyanate and capsaicin may produce both shortening and relaxation in isolated human bronchi through NK1 receptors.

Mechanisms and pathology of occupational asthma.

Since the pathogenesis and the pathological features of occupational asthma are similar to those of nonoccupational asthma, the former represents a very useful model for the investigation of the pathogenesis of asthma in general. More than one mechanism may be operative in occupational asthma. Among the mechanisms proposed, immunological mechanisms and airway inflammation play an important role. There is evidence to confirm that T-lymphocyte activation and local accumulation in the bronchial wall of activated eosinophils occurs in asthma of diverse aetiology, i.e. immunoglobulin E (IgE)-mediated, occupational and intrinsic. Neurogenic pathways should be further investigated as a potential mechanism of modulation and amplification of airway inflammation in occupational asthma.