Bradykinin causes airway hyperresponsiveness and enhances maximal airway narrowing. Role of microvascular leakage and airway edema

Inundation of asthma target research: Untangling asthma riddles

Asthma is an inveterate inflammatory disorder, delineated by the airway inflammation, bronchial hyperresponsiveness (BHR) and airway wall remodeling. Although, asthma is a vague term, and is recognized as heterogenous entity encompassing different phenotypes. Targeting single mediator or receptor did not prove much clinical significant, as asthma is complex disease involving myriad inflammatory mediators. Asthma may probably involve a large number of different types of molecular and cellular components interacting through complex pathophysiological pathways. This review covers the past, present, and future therapeutic approaches and pathophysiological mechanisms of asthma. Furthermore, review describe importance of targeting several mediators/modulators and receptor antagonists involved in the physiopathology of asthma. Novel targets for asthma research include Galectins, Immunological targets, K ⁺ Channels, Kinases and Transcription Factors, Toll-like receptors, Selectins and Transient receptor potential channels. But recent developments in asthma research are very promising, these include Bitter taste receptors (TAS2R) abated airway obstruction in mouse model of asthma and Calcium-sensing receptor obliterate inflammation and in bronchial hyperresponsiveness allergic asthma. All these progresses in asthma targets, and asthma phenotypes exploration are auspicious in untangling of asthma riddles.

Inducible disruption of autophagy in the lung causes airway hyper-responsiveness

Autophagy is a highly conserved process primarily known for its role in cellular adaptation to nutritional stress. This bulk protein degradation pathway relocates nutrients during starvation. Recent studies, however, have revealed essential roles of autophagy in various organs under normal conditions. Especially, autophagy is now recognized as the pathway responsible for the elimination of damaged proteins resulting from environmental stress. Lungs are constantly exposed to high oxygen tension and environmental chemicals. To investigate the importance of autophagy in lung physiology, we used an inducible system to ablate Atg7 expression, which is a protein essential for autophagy, in the respiratory epithelial cells of adult mice. We found that Atg7 deficiency caused swelling of bronchiolar epithelial cells and accumulation of p62, which links substrate proteins to the autophagy machinery. Bronchiolar epithelial cells, isolated by micro-dissection of lung tissues, had elevated expression of cytoprotective genes that are typically activated by Nrf2. Interestingly, Atg7-deficient lungs displayed hyper-responsiveness to cholinergic stimuli without apparent inflammatory signs. Swollen bronchiolar epithelial cells may have lead to mechanical airway constriction and lowered the threshold for the increase of airway resistance. This study demonstrates the critical role of autophagy in the lungs for the maintenance of pulmonary homeostasis.

Oxidative and nitrative stress in bronchial asthma

There has been a marked increase in the global prevalence, morbidity, and mortality of asthma, and its associated economic burden has also grown over the last 40 years. Approximately 300 million people worldwide currently have asthma, and its prevalence increases by 50% every decade. Airway inflammation is the most proximate cause of the recurrent episodes of airflow limitation in asthma. Recent research has revealed that numerous biologically active proinflammatory mediators are responsible for the pathogenesis of asthma. Among these mediators, there is increasing evidence that endogenous or exogenous reactive oxygen species (ROS) and reactive nitrogen species (RNS) are responsible for the airway inflammation of asthma. Many reports have shown that there is an excessive production of ROS and RNS in the airways of asthmatic individuals compared with healthy subjects. Excessively produced ROS and RNS have been reported to lead to airway inflammation, airway hyper-responsiveness, airway microvascular hyperpermeability, tissue injury, and remodeling in animal models and human studies. Although human lungs have a potent antioxidant system, excessive oxidative and nitrative stress leads to an imbalance of oxidants/antioxidants. This review describes the rapidly accruing data linking oxidative and nitrative events to the pathogenesis of bronchial asthma.

Potential mechanisms of improvement of airway hyperresponsiveness by inhaled corticosteroid therapy in asthmatic patients

OBJECTIVE

Recent clinical trials with administration of IL-5 antibodies to asthmatic patients have revealed reduction of eosinophilia but unaltered airway hyperresponsiveness (AHR). In contrast, inhaled corticosteroid (ICS) therapy eliminates both eosinophilia and AHR. This study was designed to examine the mechanisms by which ICS improves airway hyperresponsiveness in asthmatic patients.

METHODS

Clinical variables of asthma involving vascular permeability and IL-5 levels were examined in 23 asthmatic patients and 11 normal control subjects. After the first sputum induction, inhaled beclomethasone dipropionate (BDP 800 microg/day) was administered to asthmatic patients for 8 weeks, and sputum induction was repeated.

RESULTS

IL-5 levels in induced sputum and airway vascular permeability index were significantly higher in asthmatic patients. IL-5 was positively correlated with percentage of eosinophils in induced sputum, and negatively correlated with FEV1, but not correlated with PC20 methacholine. After BDP therapy, eosinophils, ECP, and IL-5 levels were significantly decreased to the same levels as in normal subjects. Conversely, PC20 methacholine and airway vascular permeability did not improve to the same levels as in normal subjects. Increase in PC20 methacholine from before to after BDP therapy was significantly correlated with decrease in airway permeability index, but not with decrease in IL-5 level.

CONCLUSION

Our results suggest a clear dissociation between IL-5 and AHR. ICS therapy improves AHR at least in part through decrease in airway vascular permeability.

Inflamatory mechanisms in bronchial asthma and COPD

Both bronchial asthma and chronic obstructive pulmonary disease (COPD) are recognized as inflammatory diseases, although the inflammatory process for each disease is different. In this review, I describe some inflammatory molecules that seem to be involved in the inflammatory process in each disease.

Species differences in bradykinin receptor-mediated responses of the airways

1. Bradykinin (BK) is a nine amino acid peptide (Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg) formed from the plasma precursor kininogen during inflammation and tissue injury. The actions of BK are mediated by G protein-coupled cell surface receptors, designated B1 and B2. 2. BK has a plethora of effects in the airways including bronchoconstriction, bronchodilation, stimulation of cholinergic and sensory nerves, mucus secretion, cough and oedema resulting from promotion of microvascular leakage. These airway effects are mediated in the main by the B2 receptor subtype. 3. BK acts mainly indirectly, primarily through airway nerve activation, but also by the release of prostanoids, thromboxanes and nitric oxide (NO). 4. Airway responses to BK have been studied in detail in guinea-pigs, mice, sheep and rats. This review describes the effects of BK in these species and draws comparison with its effects in normal humans and patients with respiratory diseases. 5. Despite its many and varied effects in the airways of animals and man, the exact contribution of BK to airways disease remains unclear.

Changes in indices of airway hyperresponsiveness during one year of treatment with inhaled corticosteroids in patients with asthma

We analyzed the changes in indices of airway hyperresponsiveness, including hypersensitivity and hyperreactivity, during one year of treatment with inhaled corticosteroids. We then investigated on which of them the inhaled corticosteroids had a primary effect. Fifty outpatients with asthma were recruited and treated with inhaled beclomethasone dipropionate. They underwent bronchoprovocation tests on the initial visit and at 3, 6, and 12 months. The dose of methacholine required to produce a 20% fall in the forced expiratory volume in 1 second (PD20-FEV1) was measured to evaluate airway hypersensitivity. A relatively novel index, the percent change in the forced vital capacity (deltaFVC%) at the PD20-FEV1, was assessed as a marker of airway hyperreactivity. PD20-FEV1 and deltaFVC% were assumed to indicate the horizontal shift of the dose-response curve and the vertical change in the maximal response plateau, respectively. Log(PD20-FEV1) and deltaFVC% continued to improve throughout the year (p < 0.001 and p = 0.002, respectively). Log(PD20-FEV1) improved significantly at the 3-month evaluation (p < 0.001), and deltaFVC% improved at the 6-month evaluation (p = 0.012). Log(PD20-FEV1) had no or weak relationships with deltaFVC% at all evaluation points. In conclusion, inhaled corticosteroids continued not only to reverse the leftward shift of the curve, but also to restore the plateau. Furthermore, their effect was reflected primarily by the former rather than the latter: They should be followed separately to examine how much airway inflammation is reduced.

Allergic airway hyperresponsiveness and eosinophil infiltration is reduced by a selective iNOS inhibitor, 1400W, in mice

Nitric oxide (NO) hyperproduction has been reported in asthmatic airways and may contribute to airway inflammatory responses. The purpose of this study was to examine the role of NO via inducible NO synthase (iNOS) in allergic airway inflammation using a selective iNOS inhibitor, N-[3-(aminomethyl)benzyl] acetamidine (1400W), in ovalbumin (OVA)-sensitized Balb/c mice. Sensitized animals were challenged with aerosolized 0.5% OVA for 1 h on two occasions 4 h apart. 1400W or the vehicle was administered by osmotic mini-pump from 2 h before to 24 h after OVA challenge. Twenty-four hours after OVA challenge, the vehicle-treated mice showed a significant airway hyperresponsiveness to intravenous methacholine (P<0.05) as well as an influx of eosinophils into the airways (P<0.05). iNOS immunoreactivity was obvious in the epithelial and, to a lesser extent, the infiltrated inflammatory cells. iNOS protein in the airway assessed by Western blotting also increased. Pretreatment with 1400W almost completely abolished the OVA-induced airway hyperresponsiveness and to a lesser extent eosinophil accumulation into the airways. These results suggest that NO synthesized by iNOS may participate in airway hyperresponsiveness and eosinophil infiltration into the airways after allergic reaction.

Effect of antagonists for NK(2)and B(2) receptors on antigen-induced airway responses in allergic rabbits

The effects of the tachykinin NK(2)receptor antagonist, MEN 11420 (300 nmol/kg) and the bradykinin B(2)receptor antagonist, CP 0597 (17.2 and 172 nmol/kg) were studied in a rabbit model of antigen-induced airway responses. Antigen inhalation induced acute bronchoconstriction, airway hyperresponsiveness to histamine, and pulmonary eosinophil infiltration in 3-month-old rabbits immunized with Alternaria tenuis antigen within 24 h of birth. Treatment with MEN 11420 significantly reduced the acute bronchoconstriction induced by antigen, in terms of lung resistance. Antigen-induced changes in dynamic compliance were unaffected. CP 0597 had no effect on antigen-induced changes in lung function. Neither MEN 11420 nor CP 0597 had a significant effect on the antigen-induced increase in airway responsiveness to inhaled histamine or the pulmonary eosinophil infiltration 24 h after antigen challenge. We conclude that blockade of the NK(2)receptor can alter acute airway responses to antigen, but not antigen-induced eosinophilia or hyperresponsiveness to histamine. We also conclude that bradykinin B(2)receptor-mediated responses do not play a role in airway responses to antigen.

Effect of the Chinese herbal medicine, Bakumondo-to, on airway hyperresponsiveness induced by ozone exposure in guinea-pigs

OBJECTIVE

Bakumondo-to (Maimendong tang) is a Chinese herbal medicine that has been used as an anti-tussive agent. However, the effects of Bakumondo-to on airway hyperresponsiveness are unknown. We examine whether Bakumondo-to can inhibit airway hyperresponsiveness induced by ozone.

METHODOLOGY

Measurements of airway responsiveness and plasma extravasation and bronchoalveolar lavage (BAL) were performed before and after ozone exposure (3 p.p.m., 2 h). Guinea-pigs were anaesthetized with pentobarbital sodium and mechanically ventilated. Airway responsiveness was determined by an inhalation of doubling concentration of histamine, and the concentration of histamine required to produce a 200% increase in R(L) (PC200) was calculated by log-linear interpolation. Plasma extravasation was evaluated by measuring the extravasation of Evans blue dye in the airway.

RESULTS

Ozone produced significant airway hyperresponsiveness and plasma extravasation, with an influx of neutrophils in BAL fluid. Bakumondo-to (400 mg/kg p.o.) significantly inhibited airway hyperresponsiveness, but had no effect on neutrophil influx or plasma extravasation.

CONCLUSIONS

We demonstrated that Bakumondo-to can attenuate airway hyperresponsiveness induced by ozone without affecting airway inflammation, which suggests that Bakumondo-to may act on the subsequent mechanisms after the induction of inflammation, such as mediator release.

Bradykinin B2 antagonist HOE 140 inhibits late allergic microvascular leakage in guinea pig airways

Because bradykinin has potent inflammatory actions, this molecule may be involved in the late allergic response (LAR). We investigated the role of the molecule in airway microvascular hyperpermeability during the LAR. Three weeks after ovalbumin (OVA) sensitization, animals were pretreated with bradykinin B2 receptor antagonist HOE 140 or vehicle for 30 min before the OVA inhalation challenge. The occurrence of LAR was judged by a two-fold increase in transpulmonary pressure (Ptp) from the baseline values. The microvascular permeability in the trachea was assessed by an index defined as the ratio of the area of vasculature labeled by the Monastral blue dye (area density percent). Significant microvascular hyperpermeability were observed during the LAR. The bradykinin concentrations in the bronchoalveolar lavage-fluid (BAL-f) were increased during the LAR. HOE 140 (0.1-10 mg/kg, s.c.) inhibited the airway microvascular hyperpermeability during the LAR dose-dependently. These findings suggest that bradykinin may play an important role in microvascular hyperpermeability during the LAR.

Role of peroxynitrite in airway microvascular hyperpermeability during late allergic phase in guinea pigs

We investigated the role of peroxynitrite, which is formed by a rapid reaction between nitric oxide (NO) and superoxide anion (O(2)(-)), in the airway microvascular hyperpermeability during the late allergic response (LAR) in sensitized guinea pigs in vivo. The occurrence of LAR was assessed as a 100% increase in the transpulmonary pressure, which was monitored by the esophageal catheter technique. Airway microvascular permeability was assessed by Monastral blue dye trapping between the endothelium using an image analyzer. In the LAR phase (4 to 6 h after antigen inhalation), microvascular hyperpermeability and eosinophil infiltration within the airway wall were observed. NO production and xanthine oxidase (XO)/xanthine dehydrogenase activity, which are responsible for O(2)(-) production, were enhanced during the LAR. Peroxynitrite formation assessed by nitrotyrosine immunostaining was also exaggerated at that time. The microvascular hyperpermeability during the LAR was largely reduced by NO synthase inhibitor (L-NAME, 72.7% inhibition; p < 0.05), XO inhibitor (AHPP, 60.8% inhibition; p < 0. 05) and peroxynitrite scavenger (ebselen, 81.0% inhibition; p < 0. 05). L-NAME had a small but significant inhibitory effect on airway eosinophil accumulation, but AHPP and ebselen had no effect. These results suggest that excessive production of O(2)(-) and NO occurs in the LAR. These two molecules appear to cause airway microvascular hyperpermeability via peroxynitrite formation.

Effects of DP-1904, a thromboxane synthetase inhibitor, on the antigen-induced airway hyperresponsiveness and infiltration of inflammatory cells in guinea-pigs

The effect of DP-1904, a novel thromboxane (TX) synthetase inhibitor, on airway hyperresponsiveness was studied in actively sensitized guinea-pigs. Airway hyperresponsiveness to intravenous ACh was observed at 3 and 7 h after aerosolized antigen challenge. In the model, a significant correlation between increases of respiratory resistance and microvascular leakage was observed, corresponding to the elevation of TXB2 in bronchoalveolar lavage fluid (BALF) in the early phase. DP-1904, at doses of 3 mg/kg or higher given orally one hour prior to the antigen challenge, inhibited the TXB2 production and the development of airway hyperresponsiveness in the early phase. Further, DP-1904 significantly suppressed the accumulation of lymphocytes in BALF and airway hyperresponsiveness in the late phase, although it only slightly decreased the mobilization of eosinophils and neutrophils. The results suggest that TXA2 is possibly involved in the development of airway hyperresponsiveness, and DP-1904 prevented the airway hyperresponsiveness via inhibition of TXA2 production and regulation of inflammatory cells.

Inhibitory effect of NPC-17731 on BK-induced and antigen-induced airway reactions in guinea-pigs

BACKGROUND

Bradykinin (BK) has been suggested to act as a mediator in the airways in inflammatory conditions, such as asthma through the activation of B2-receptors. NPC-17731 (D-Arg0[Hyp3, D-HypE(trans-propyl)7, Oic8]BK) has potent antagonistic activity against B2-receptors without agonistic activity.

OBJECTIVE

We have evaluated the inhibitory effect of NPC-17731 against BK in guinea-pig airways. In addition, we have investigated the effects of NPC-17731 on antigen-induced airway responses.

METHODS

Bronchoconstriction was assessed as an increase in lung resistance (RL) and a decrease in dynamic compliance (Cdyn). Airway plasma leakage was assessed by extravasation of intravenously injected Evans blue dye. To estimate the effect of drugs on antigen-induced reactions, guinea-pigs were actively sensitized by exposure to aerosol ovalbumin (OA) twice and challenged by OA inhalation. Acute bronchoconstriction was measured for 15 min. Airway vascular leakage was measured at 10 min after the challenge. Assessment of airway hyperresponsiveness against acetylcholine and bronchoalveolar lavage were conducted at 18-24 h after the antigen-challenge.

RESULTS

NPC-17731 (0.3-30 microg/kg, i.v.) inhibited intravenously applied BK-induced bronchoconstriction in a dose-dependent manner. The 50% inhibitory doses (ID50) were 1.3 microg/kg for RL and 2.8 microg/kg for Cdyn. NPC-17731 (1-10 microg/kg, i.v.) inhibited BK-induced microvascular leakage in a dose-dependent manner (ID50 = 4.2 microg/kg). In addition, 10 microg/kg of NPC-17731 abolished the inhaled BK-induced bronchoconstriction. In the sensitized animals, 100 microg/kg NPC-17731 significantly reduced the airway microvascular leakage and the decrease in Cdyn induced by ovalbumin exposure (P < 0.05), but did not influence the increase in RL. NPC-17731 (100 microg/kg) inhibited the antigen-induced airway hyperresponsiveness and the increase in eosinophils in BAL fluids.

CONCLUSION

These results indicate that NPC-17731 is a potent BK antagonist in vivo and that BK may partially contribute to the antigen-induced airway responses in guinea-pigs.

Relationship between airway microvascular leakage, edema, and baseline airway functions

This study was designed to examine the relationship among microvascular leakage, edema, and baseline airway function. Microvascular leakage was induced in the airways of anesthetized, tracheostomized New Zealand White rabbits (n = 22) by using nebulized N-formyl-methionyl-leucyl-phenylalanine (10 mg) and was measured in the trachea by using the Evans blue dye technique. Airway wall thickness was assessed morphometrically in the right main bronchus after Formalin fixation at a pressure of 25 cmH2O. Areas calculated included the mucosal wall area, the adventitial wall area, the total wall area, and the percentage of total wall area consisting of blood vessels. A neutrophil count was also performed by analyzing numbers of cells in both the mucosal wall area and the adventitial wall area. Airway function was assessed before and 30 min after challenge with N-formyl-methionyl-leucyl-phenylalanine by determining airway resistance, functional residual capacity, specific airway resistance, and flow-volume and pressure-volume curves (after paralysis of the animals with suxamethonium). The concentration of Evans blue dye in tracheal tissue ranged from 31.3 to 131.2 micrograms. There was a significant correlation between this concentration and both the adventitial wall area (P < 0.01) and mucosal neutrophil numbers (P < 0.005). There was no correlation between Evans blue concentration and either blood vessel area or changes in respiratory physiology parameters before and after challenge. There was no significant difference between any respiratory physiology measurements before and after challenge. We conclude that an increase in microvascular leakage correlates with airway edema in the adventitia; however, these airway changes have no significant effect on airway elastic or resistive properties.

Effects of edema on small airway narrowing

Numerous mediators of inflammation have been demonstrated to cause airway microvascular fluid and protein extravasation. That fluid extravasation results in airway wall edema leading to airway narrowing and enhanced reactivity has not been confirmed. In anesthetized, ventilated sheep (n = 30), airway vascular fluid extravasation was induced by infusing bradykinin (10(-6) M) through a cannulated, blood-perfused bronchial artery. Airway wall edema and luminal narrowing were determined morphometrically. Airway reactivity to methacholine (MCh; 10 microg/ml, intrabronchial artery) was determined by measuring conducting airway resistance (Raw) by forced oscillation. Raw measurements were made and lung lobes were excised and quick frozen before or after a 1-h bradykinin infusion. In 10 airways per lobe (range 0.2- to 2.0-mm relaxed diameter), wall area occupied 32 +/- 2% (SE) of the total normalized airway area (n = 9). Bradykinin infusion increased wall area to 42 +/- 5% (P = 0.02); luminal area decreased by <5%; and smooth muscle perimeter, a measure of smooth muscle constriction, was not altered (n = 5). Raw showed no change from baseline (1.4 +/- 0.4 cmH2O . l-1 . s) after bradykinin infusion (n = 10). During MCh challenge, Raw increased by 3.2 +/- 04 cmH2O . l-1 . s, and this change did not differ after administration of bradykinin. MCh challenge caused similar decreases in smooth muscle perimeter (10%) and luminal area (72 vs. 68%) before and after bradykinin infusion. However, the time constant of recovery of Raw from MCh constriction was increased from control (40 +/- 3 s) to 57 +/- 10 s after bradykinin infusion (P = 0.03). When lung lobes were excised at the same time after MCh challenge was terminated (n = 5), luminal area was greater before bradykinin infusion than after (86 vs. 78%; P = 0.007), as was smooth muscle perimeter. The results of this study demonstrate that airway wall edema limits relaxation after induced constriction rather than enhancing constriction.

Interaction between airway edema and lung inflation on responsiveness of individual airways in vivo

Interaction between airway edema and lung inflation on responsiveness of individual airways in vivo. J. Appl. Physiol. 83(2): 366-370, 1997.-Inflammatory changes and airway wall thickening are suggested to cause increased airway responsiveness in patients with asthma. In five sheep, the dose-response relationships of individual airways were measured at different lung volumes to methacholine (MCh) before and after wall thickening caused by the inflammatory mediator bradykinin via the bronchial artery. At 4 cmH2O transpulmonary pressure (Ptp), 5 microg/ml MCh constricted the airways to a maximum of 18 +/- 3%. At 30 cmH2O Ptp, MCh resulted in less constriction (to 31 +/- 5%). Bradykinin increased airway wall area at 4 and 30 cmH2O Ptp (159 +/- 6 and 152 +/- 4%, respectively; P < 0.0001). At 4 cmH2O Ptp, bradykinin decreased airway luminal area (13 +/- 2%; P < 0.01), and the dose-response curve was significantly lower (P = 0.02). At 30 cmH2O, postbradykinin, the maximal airway narrowing was not significantly different (26 +/- 5%; P = 0.76). Bradykinin produced substantial airway wall thickening and slight potentiation of the MCh-induced airway constriction at low lung volume. At high lung volume, bradykinin increased wall thickness but had no effect on the MCh-induced airway constriction. We conclude that inflammatory fluid leakage in the airways cannot be a primary cause of airway hyperresponsiveness.

Thromboxane causes airway hyperresponsiveness after cigarette smoke-induced neurogenic inflammation

We investigated the role of neurogenic inflammation and the subsequent mechanisms in cigarette smoke-induced airway hyperresponsiveness in guinea pigs. Exposure to cigarette smoke was carried out at tidal volume for 3 min. Airway responsiveness to histamine was determined before and after smoke exposure followed by bronchoalveolar lavage (BAL). Plasma extravasation was evaluated by measuring the extravasation of Evans blue dye in the airway. Cigarette smoke produced significant airway hyperresponsiveness and plasma extravasation, with an influx of neutrophils in BAL fluid. FK-224 (10 mg/kg i.v.), a tachykinin antagonist at NK1 and NK2 receptors, significantly inhibited these changes. The thromboxane (Tx) B2 concentration was increased in BAL fluid after smoke exposure and was significantly inhibited by FK-224. OKY-046 (10 mg/kg i.v.), a Tx synthase inhibitor, significantly inhibited airway hyperresponsiveness but had no effect on neutrophil influx or plasma extravasation. The results suggest that neurogenic inflammation and the subsequent generation of Tx in the airway are important in the development of the airway hyperresponsiveness induced by cigarette smoke.

Mechanisms of increased airway microvascular permeability: role in airway inflammation and obstruction

1. Airway inflammation is a signal feature of human asthma, as is bronchial obstruction and the resultant airflow limitation. An obligatory accompaniment to airway inflammation is increased airway microvascular permeability, which in turn is causally related to bronchial oedema. In this review, we have attempted to describe the mechanisms of increased airway microvascular permeability and its relationship to oedema, bronchial obstruction and the hyperreactivity to spasmogenic stimuli which are such common features of asthma. 2. It is now clear that bronchial obstruction in chronic asthma can involve bronchial wall oedema and swelling in addition to reversible, elevated airway smooth muscle tone, mucus hypersecretion and airway plugging and potentially permanent structural changes in airway architecture. Inflammatory mediators released in the airway wall in asthma including histamine, platelet-activating factor, leukotrienes and bradykinin are potent inducers of increased bronchial microvascular permeability and are thus promoters of bronchial oedema, airway wall swelling and reduction in luminal calibre. 3. The primary mechanism believed to underlie acute increases in microvascular permeability is contraction of post-capillary venular endothelial cells, resulting in the formation of gaps between otherwise tightly associated cells. Extravasated plasma distributes to the interstitial spaces in the airway wall, resulting in oedema and swelling, but may also traverse the epithelium and collect in the airway lumen. 4. Luminal plasma may compromise epithelial integrity and cilial function and thus reduce mucus clearance. Plasma proteins may also promote the production of viscous mucus and the formation of luminal mucus plugs. Together, these effects can result in or contribute to airway obstruction and hyper-responsiveness. 5. An understanding of such mechanisms can provide insight concerning novel and effective anti-asthma therapies.

Airway wall liquid. Sources and role as an amplifier of bronchoconstriction

Airway liquid balance in asthma is largely determined by active plasma exudation from tracheobronchial microvessels into the interstitial spaces of the mucosa, submucosa, and/or adventitia, and from there into the luminal space. This exuded plasma is rich in proteins and cell mediators capable of initiating several events, including activation of sensory neural pathways, plasma protein cleavage, inflammatory cell recruitment, and inhibition of surfactant function. It can act to amplify the bronchoconstrictor response by increasing mucosal and/or submucosal thickness, altering mechanical properties of airway wall compartments, decoupling the airway wall from parenchymal attachments, filling airway interstices, and by creating an additional inward force because of surface tension, resulting in further airway constriction and possibly closure and thereby significantly increasing airways resistance.