Characterization of the endothelin receptor subtype mediating epithelium-derived relaxant nitric oxide release from guinea-pig trachea

Bronchial epithelium as a target for innovative treatments in asthma

Increasing evidence of a critical role played by the bronchial epithelium in airway homeostasis is opening new therapeutic avenues. Its unique situation at the interface with the environment suggests that the subtle regulation orchestrated by the epithelium between tolerance and specific immune response might be impaired in asthma. Airway mucus is acting as a physical and a biological fluid between the environment and the epithelium, synergistically moved by the cilia. In asthma, excessive mucus production is a hallmark of airway remodeling. Since many years we tried to therapeutically target mucus hypersecretion, but actually this option is still not achieved. The present review discusses the dynamic processes regulating airway mucus production. Airway inflammation is central in current asthma management. Understanding of how the airway epithelium influences the TH2 paradigm in response to deleterious agents is improving. The multiple receptors expressed by the airway epithelium are the transducers of the biological signals induced by various invasive agents to develop the most adapted response. Airway remodeling is observed in severe chronic airway diseases and may result from ongoing disturbance of signal transduction and epithelial renewal. Chronic airway diseases such as asthma will require assessment of these epithelial abnormalities to identify phenotypic characteristics associated with predicting a clinical benefit for epithelial-directed therapies.

Characterization of endothelin receptors in the peripheral lung tissues of horses unaffected and affected with recurrent airway obstruction

The purpose of the study was to determine and compare the expression of endothelin (ET) receptors in the peripheral lungs of healthy horses and those affected with recurrent airway obstruction (RAO) using reverse transcriptase polymerase chain reaction (RT-PCR), real-time PCR, Western blot analysis, and immunohistochemical techniques. Two groups of horses (7 healthy and 7 RAO-affected) were selected from a pool of horses destined for euthanasia. The grouping of horses was based on the history, clinical scoring, and pulmonary function testing. After euthanasia, gross postmortem evaluation of the lungs was conducted, and lung samples were collected and either stored at -80 degrees C or fixed in zinc-formalin for 12 h. The RT-PCR was performed by using specific primers for ETA and ETB receptors, and beta-actin. To determine the relative gene expression real-time PCR was performed. To detect ET receptor protein expression, Western blotting and immunohistochemical studies were performed using polyclonal antibodies against ETA and ETB receptors and beta-actin. The ET receptor expression was determined by performing either densitometric analyses or scoring of immunostaining. Statistical analyses were performed to detect differences in receptor expression within and between the 2 groups. The results indicated that ET receptor expression, particularly ETB receptors, was significantly greater in the peripheral lungs of RAO-affected horses than in those of healthy horses. Clinical trials using ET receptor antagonists, particularly ETB antagonists might help in developing a therapeutic strategy to treat this career-ending disease.

Immunohistochemical determination of the expression of endothelin receptors in bronchial smooth muscle and epithelium of healthy horses and horses affected by summer pasture-associated obstructive pulmonary disease

OBJECTIVE

To immunohistochemically determine the expression of endothelin (ET) receptors in bronchial smooth muscle and epithelium of healthy horses and horses affected by summer pasture-associated obstructive pulmonary disease (SPAOPD).

SAMPLE POPULATION

Tissue specimens obtained from 8 healthy and 8 SPAOPD-affected horses.

PROCEDURE

Horses were examined and assigned to healthy and SPAOPD groups. Horses were then euthanatized, and tissue specimens containing bronchi of approximately 4 to 8 mm in diameter were immediately collected from all lung lobes, fixed in zinc-formalin solution for 12 hours, and embedded in paraffin. Polyclonal primary antibodies against ET-A or ET-B receptors at a dilution of 1:200 and biotinylated IgG secondary antibodies were applied to tissue sections, followed by the addition of an avidin-biotin immunoperoxidase complex. Photographs of the stained slides were digitally recorded and analyzed by use of image analysis software to determine the intensity of staining. Two-way ANOVA was used for statistical analysis.

RESULTS

The left diaphragmatic lung lobe of SPAOPD-affected horses had a significantly greater area of bronchial smooth muscle that immunostained for ET-A, compared with that for healthy horses. All lung lobes of SPAOPD-affected horses, except for the right diaphragmatic lobe, had significantly greater staining for ET-B receptors in bronchial smooth muscle, compared with results for healthy horses.

CONCLUSIONS AND CLINICAL RELEVANCE

This study revealed overexpression of ET-A and, in particular, ETB receptors in the bronchial smooth muscle of SPAOPD-affected horses, which suggested upregulation of these receptors. These findings improve our understanding of the role of ET-1 in the pathogenesis of SPAOPD.

Nitric oxide in health and disease of the respiratory system

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

Multiple roles of nitric oxide in the airways

Nitric oxide is endogenously released in the airways by nitric oxide synthase. Functionally, two isoforms of this enzyme exist: constitutive and inducible. The former seems to protect airways from excessive bronchoconstriction while the latter has a modulatory role in inflammatory disorders of the airways such as asthma. This review explores the physiological and pathophysiological role of endogenous nitric oxide in the airways, and the clinical aspects of monitoring nitric oxide in exhaled air of patients with respiratory disease.

Effects of endothelin-1 and nitric oxide on proliferation of cultured guinea pig bronchial smooth muscle cells

The proliferative effects of endothelin-1 (ET-1), both alone and in combination with epidermal growth factor (EGF), and the effect of nitric oxide (NO) on the cell proliferation were investigated in cultured guinea pig bronchial smooth muscle cells. ET-1 (10-100 nM) alone augmented cell proliferation, and was additive to the effect of EGF (0.48 nM) in a concentration-dependent manner. An ET(A) antagonist, BQ-123 (10 microM), reduced the cell-proliferative effect of ET-1, whereas an ET(B) antagonist, BQ-788 (10 microM), did not influence the effect. A NO donor, SIN-1 (10 nM-1 microM), reduced the cell-proliferative effect of ET-1 in a concentration-dependent manner. The effect of SIN-1 (1 microM) was partly, but significantly, reversed by a soluble guanylyl cyclase inhibitor, ODQ (1 microM). These results suggest that ET-1 acts not only as a co-mitogen with EGF but also as a mitogen alone, and that its action is mediated through activation of ET(A) receptors. Therefore, ET-1 may contribute to airway remodeling, a pathophysiological hallmark of asthma. In addition, NO, which is produced mainly in the airway epithelium and is partly mediated through cGMP-dependent pathway, may reduce the phenomenon.

Effect of a novel bifunctional endothelin receptor antagonist, IRL 3630A, on guinea pig respiratory mechanics

This study characterized the in vitro pharmacological properties of a newly developed endothelin receptor antagonist, N-butanesulfonyl-[N-(3, 5-dimethylbenzoyl)-N-methyl-3-[4-(5-isoxazolyl)-phenyl]-(D)- alanyl]-( L)-valineamide sodium salt (IRL 3630A), and its in vivo effects on respiratory mechanics were determined. IRL 3630A showed highly balanced affinities to human endothelin ET(A) and ET(B) receptors, giving apparent K(i) values of 1.5 and 1.2 nM, respectively. This compound also potently antagonized the endothelin-1-induced intracellular Ca(2+) increases in both embryonic bovine tracheal (EBTr) cells expressing endothelin ET(A) receptors and human Girardi heart (hGH) cells expressing endothelin ET(B) receptors. In guinea pig isolated tracheas having both endothelin ET(A) and ET(B) receptors, IRL 3630A greatly inhibited endothelin-1-induced contraction (pA(2)=7.1), which was partially or scarcely suppressed by the endothelin ET(A) receptor antagonist cyclo[-(D)-Trp-(D)-Asp-(L)-Pro-(D)-Val-(L)-Leu-] (BQ-123) or the endothelin ET(B) receptor antagonist N-(3, 5-dimethylbenzoyl)-N-methyl-3-(4-phenyl)-(D)-phenylalanyl-(L)-t ryptop han (IRL 2500), respectively. Bolus i.v. injections of IRL 3630A administered into anaesthetized guinea pigs at 10 and 30 microg/kg inhibited endothelin-1 (1.3 microg/kg)-induced changes in respiratory resistance and compliance in a dose dependent manner, whereas both sodium 2-benzo[1, 3]dioxol-5-yl-4-(4-methoxy-phenyl)-4-oxo-3-(3,4, 5-trimethoxy-benzyl)-but-2-enoate (an endothelin ET(A) receptor antagonist: PD 156707) and IRL 2500 at doses of up to 30 microg/kg did not affect endothelin-1-induced changes in respiratory mechanics, reflecting the in vitro results. IRL 3630A is thus an effective bifunctional endothelin receptor antagonist, and will be useful in clarifying the role of endothelin in pulmonary diseases such as bronchial asthma.

Hypoxia augments conversion of big-endothelin-1 and endothelin ET(B) receptor-mediated actions in rat lungs

We have examined the effect of endothelin-1, sarafotoxin-6C, big-endothelin-1 and other agents on perfused lungs from chronically hypoxic rats. Increases in pulmonary perfusion pressure induced by big-endothelin-1, endothelin-1, phenylephrine and potassium chloride were enhanced in hypoxic lungs, while the constrictor action of sarafotoxin-6C was not increased. When basal pulmonary perfusion pressure was raised, low doses of endothelin-1 and sarafotoxin-6C produced decreases in pulmonary perfusion pressure which were significantly greater in chronically hypoxic lungs, whereas responses to sodium nitroprusside were unchanged. Endothelin ET(B) receptor-mediated bronchoconstrictor responses were also potentiated in hypoxic lungs, whereas responses to carbachol were not. In hypoxic lungs, conversion of big-endothelin-1 to endothelin-1 was significantly increased. These data provide evidence for a generalised increase in vasomotor activity in chronically hypoxic lungs, and a more selective increase in endothelin ET(B) receptor-mediated vasodilator and bronchoconstrictor responses. Hypoxia also augments the conversion of big-endothelin-1 to endothelin-1.

Role of nitric oxide and septide-insensitive NK(1) receptors in bronchoconstriction induced by aerosolised neurokinin A in guinea-pigs

The tachykinin, neurokinin A (NKA), contracts guinea-pig airways both in vitro and in vivo, preferentially activating smooth muscle NK(2) receptors, although smooth muscle NK(1) receptors may also contribute. In vitro evidence suggests that NKA activates epithelial NK(1) receptors, inducing the release of nitric oxide (NO) and subsequent smooth muscle relaxation. A number of selective NK(1) receptor agonists have been reported to activate both smooth muscle and epithelial NK(1) receptors, however septide appears only to activate smooth muscle NK(1) receptors. The aim of the present study was to investigate whether NKA-induced bronchoconstriction in guinea-pigs in vivo may be limited by NO release via NK(1) receptor activation, and whether selective NK(1) receptor agonists may activate this mechanism differently. Aerosolized NKA caused an increase in total pulmonary resistance (RL) that was markedly reduced by the NK(2) receptor antagonist, SR 48968, and abolished by the combination of SR 48968 and the NK(1) receptor antagonist, CP-99, 994. The increase in RL evoked by NKA was potentiated by pretreatment with the NO synthase (NOs) inhibitor, L-NAME, but not by the inactive enantiomer D-NAME. Potentiation by L-NAME of NKA-induced increase in RL was reversed by L-Arginine, but not by D-Arginine. Pretreatment with L-NAME did not affect the increase in RL induced by the selective NK(2) receptor agonist, [beta-Ala(8)]NKA(4-10), and by the selective NK(1) receptor agonist, septide, whereas it markedly potentiated the increase in RL caused by a different NK(1) selective agonist, [Sar(9),Met(O(2))(11)]SP. Dose-response curves showed that septide was a more potent bronchoconstrictor than [Sar(9),Met(O(2))(11)]SP to cause bronchoconstriction. Pretreatment with the NK(1) receptor antagonist, CP-96,994, abolished the ability of L-NAME to increase bronchoconstriction to aerosolized NKA. Bronchoconstriction to aerosolized NKA was increased by L-NAME, after pretreatment with the NK(3) receptor antagonist, SR 142801. The present study shows that in vivo bronchoconstriction in response to the aerosolized naturally occurring tachykinin, NKA, is limited by its own ability to release relaxant NO via NK(1) receptor activation. This receptor is apparently insensitive to septide, thus justifying, at least in part, the high potency of septide to cause bronchoconstriction in guinea-pigs.

Detection of nitric oxide release induced by bradykinin in guinea pig trachea and main bronchi using a porphyrinic microsensor

Indirect evidence using nitric oxide (NO) synthase (NOS) inhibitors suggests that in guinea-pig airways bradykinin releases bronchoprotective NO. In this study, using a recently developed electrochemical method of NO measurement based on a porphyrinic microsensor, we investigated whether bradykinin releases NO from guinea-pig airways and whether the epithelium is the main source of NO. Further, the Ca(2+)-dependence of bradykinin-induced NO release was assessed stimulating airway preparations with bradykinin in Ca(2+)-free conditions. We also studied the immunohistochemical distribution of the Ca(2+)- dependent constitutive isoforms of NOS (constitutive NOS [cNOS]: neuronal and endothelial [ecNOS]) in our preparations. The porphyrinic microsensor was placed in the bathing fluid onto the mucosal surface of tracheal or main bronchial segments. Addition of bradykinin vehicle (0.9% saline) did not cause any detectable change of the baseline signal. Addition of bradykinin caused an upward shift of the baseline that reached a maximum within 1 to 2 s. The amplitude of the response to bradykinin was concentration-dependent between the range 1 nM to 10 microM, with a maximum effect at 10 microM. Bradykinin-induced NO release was higher in tracheal than in main bronchial segments. The selective bradykinin B(2) receptor antagonist D-Arg(0)-[Hyp(3), Thi(5), D-Tic(7), Oic(8)]bradykinin (1 microM) inhibited NO release induced by a submaximum concentration of bradykinin (1 microM). The ability of bradykinin to release NO was markedly reduced in epithelium-denuded segments, and abolished in Ca(2+)-free conditions and after pretreatment with N(G)-monomethyl-L-arginine (100 microM), but not with N(G)-monomethyl-D-arginine. Both cNOS isoforms were present in trachea and main bronchi, ecNOS being the predominant isoform in the epithelium. The study shows that bradykinin via B(2) receptor activation caused a rapid and Ca(2+)-dependent release of NO, mainly, but not exclusively, derived from the epithelium. It also shows that both cNOS isoforms may be involved in bradykinin-evoked NO release.