Endothelin receptor subtypes in human and guinea-pig pulmonary tissues

Endothelin-1 in Health and Disease

Discovered almost 40 years ago, the potent vasoconstrictor peptide endothelin-1 (ET-1) has a wide range of roles both physiologically and pathologically. In recent years, there has been a focus on the contribution of ET-1 to disease. This has led to the development of various ET receptor antagonists, some of which are approved for the treatment of pulmonary arterial hypertension, while clinical trials for other diseases have been numerous yet, for the most part, unsuccessful. However, given the vast physiological impact of ET-1, it is both surprising and disappointing that therapeutics targeting the ET-1 pathway remain limited. Strategies aimed at the pathways influencing the synthesis and release of ET-1 could provide new therapeutic avenues, yet research using cultured cells in vitro has had little follow up in intact ex vivo and in vivo preparations. This article summarises what is currently known about the synthesis, storage and release of ET-1 as well as the role of ET-1 in several diseases including cardiovascular diseases, COVID-19 and chronic pain. Unravelling the ET-1 pathway and identifying therapeutic targets has the potential to treat many diseases whether through disease prevention, slowing disease progression or reversing pathology.

Orthogonal Peptide-Templated Labeling Elucidates Lateral ETA R/ETB R Proximity and Reveals Altered Downstream Signaling

Fine‐tuning of G protein‐coupled receptor (GPCR) signaling is important to maintain cellular homeostasis. Recent studies demonstrated that lateral GPCR interactions in the cell membrane can impact signaling profiles. Here, we report on a one‐step labeling method of multiple membrane‐embedded GPCRs. Based on short peptide tags, complementary probes transfer the cargo (e. g. a fluorescent dye) by an acyl transfer reaction with high spatial and temporal resolution within 5 min. We applied this approach to four receptors of the cardiovascular system: the endothelin receptor A and B (ET_AR and ET_BR), angiotensin II receptor type 1, and apelin. Wild type‐like G protein activation after N‐terminal modification was demonstrated for all receptor species. Using FRET‐competent dyes, a constitutive proximity between hetero‐receptors was limited to ET_AR/ET_BR. Further, we demonstrate, that ET_AR expression regulates the signaling of co‐expressed ET_BR. Our orthogonal peptide‐templated labeling of different GPCRs provides novel insight into the regulation of GPCR signaling.

The ring size of monocyclic ET-1 controls selectivity and signaling efficiency at both endothelin receptor subtypes

Cardiovascular diseases (CVDs) like hypertension are a major cause for death worldwide. In the cardiovascular tissue, the endothelin system-consisting of the receptor subtypes A (ET_A R) and B (ET_B R) and the mixed agonist endothelin 1 (ET-1)-is a major key player in the regulation of vascular tone and blood pressure. Tight control of this system is required to maintain homeostasis; otherwise, the endothelin system can cause severe CVDs like pulmonary artery hypertension. The high sequence homology between both receptor subtypes limits the development of novel and selective ligands. Identification of small differences in receptor-ligand interactions and determination of selectivity constraints are crucial to fine-tune ligand properties and subsequent signaling events. Here, we report on novel ET-1 analogs and their detailed pharmacological characterization. We generated simplified ET-1-derived monocyclic peptides to provide an accessible synthesis route. By detailed in vitro characterization, we demonstrated that both G protein signaling and the subsequent arrestin recruitment of activated ET_B R remain intact, whereas activation of the ET_A R depends on the intramolecular ring size. Increasing of the intramolecular ring structure reduces activity at the ET_A R and shifts the peptide toward ET_B R selectivity. All ET-1 analogs displayed efficient ET_B R-mediated signaling by G protein activation and arrestin 3 recruitment. Our study provides in-depth characterization of the ET-1/ET_A R and ET-1/ET_B R interactions, which has the potential for future development of endothelin-based drugs for CVD treatment. By identification of Lys⁹ for selective labeling, novel analogs for peptide-mediated shuttling by ET-1 are proposed.

Functional estimation of endothelin-1 receptor antagonism by bosentan, macitentan and ambrisentan in human pulmonary and radial arteries in vitro

BACKGROUND

Endothelin receptor antagonists are approved for pulmonary arterial hypertension. Development of selective ET_A-receptor antagonists over mixed or dual receptor antagonists has depended on a range of receptor binding assays, second messenger assays and functional blood vessel assays. This study compared the 3 clinically-approved endothelin receptor antagonists in assays of human isolated pulmonary and radial arteries in vitro.

METHODS

Human isolated pulmonary (i.d. 5.5mm) and human radial (i.d. 3.23mm) artery ring segments were mounted in organ baths for isometric force measurement. Single concentration-contraction curves to endothelin-1 were constructed in the absence or presence of bosentan (1-10µM), macitentan (0.03-0.3µM) or ambrisentan (0.1-1µM).

RESULTS

All 3 endothelin antagonists caused competitive rightward shifts in the endothelin-1 concentration-response curves in both arteries. The Clark plot and analysis gave the following pK_B values: bosentan, pulmonary artery 6.28±0.13 and radial artery 6.04±0.10; macitentan, pulmonary artery 8.02±0.13 and radial artery 7.49±0.08; and ambrisentan, pulmonary artery 7.38±0.13 and radial artery 6.96±0.10.

CONCLUSIONS

Noting the maximum plasma levels attained from recommended oral doses of each antagonist in volunteers, the pK_B findings here show that there would be significant antagonism of endothelin-1 contraction in the pulmonary and radial arteries at therapeutic plasma levels. This functional assay confirms in human tissue that much higher plasma concentrations of endothelin-1 receptor antagonists are required to be effective than those predicted from binding or other biochemical assays.

Endothelin

The endothelins comprise three structurally similar 21-amino acid peptides. Endothelin-1 and -2 activate two G-protein coupled receptors, ETA and ETB, with equal affinity, whereas endothelin-3 has a lower affinity for the ETA subtype. Genes encoding the peptides are present only among vertebrates. The ligand-receptor signaling pathway is a vertebrate innovation and may reflect the evolution of endothelin-1 as the most potent vasoconstrictor in the human cardiovascular system with remarkably long lasting action. Highly selective peptide ETA and ETB antagonists and ETB agonists together with radiolabeled analogs have accurately delineated endothelin pharmacology in humans and animal models, although surprisingly no ETA agonist has been discovered. ET antagonists (bosentan, ambrisentan) have revolutionized the treatment of pulmonary arterial hypertension, with the next generation of antagonists exhibiting improved efficacy (macitentan). Clinical trials continue to explore new applications, particularly in renal failure and for reducing proteinuria in diabetic nephropathy. Translational studies suggest a potential benefit of ETB agonists in chemotherapy and neuroprotection. However, demonstrating clinical efficacy of combined inhibitors of the endothelin converting enzyme and neutral endopeptidase has proved elusive. Over 28 genetic modifications have been made to the ET system in mice through global or cell-specific knockouts, knock ins, or alterations in gene expression of endothelin ligands or their target receptors. These studies have identified key roles for the endothelin isoforms and new therapeutic targets in development, fluid-electrolyte homeostasis, and cardiovascular and neuronal function. For the future, novel pharmacological strategies are emerging via small molecule epigenetic modulators, biologicals such as ETB monoclonal antibodies and the potential of signaling pathway biased agonists and antagonists.

Hypoxic Pulmonary Vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Differences in endothelin receptor types in the vasculature of Bothrops jararaca (Viperidae) and Oxyrhopus guibei (Colubridae) snakes

Endothelins (ETs) are vasoactive peptides evolutionary well conserved that exert their effects through two specific receptors (ET(A) and ET(B)) widely distributed in all vertebrates. In snakes, the presence and function of endothelins and their receptors are still scarcely described. We have recently demonstrated the presence of ET(A) and ET(B2) receptors in the snake Bothrops jararaca (Bj). In the present work we showed that distinctively from Bj, the vascular contraction induced by endothelin in Oxyrhopus guibei (Og) snake is mediated only by ET(A) receptors. Selective ET(B) agonists (SRTX-c and IRL(1620)) and antagonists (IRL(1038) and BQ(788)) were ineffective in Og preparations of isolated aorta. We also showed that ET-1 response on Og arterial blood pressure was monophasic hypertensive as opposed to biphasic (hypotension followed by hypertension) in Bj. Furthermore, we characterized the relaxing properties of endothelin receptor ET(B1) in pre-contracted aorta preparations. We showed that IRL(1620) induced relaxation of pre-contracted Bj aorta but was ineffective in relaxing Og preparations. IRL(1620) relaxing effect on Bj aorta was abolished by l-NAME, indicating involvement of NO release, and was reduced by selective ET(B) antagonists. Our findings suggest that Og snake has a more primitive spectrum of ET receptors (only ET(A) receptor) than Bj (presence of ET(A), ET(B1) and ET(B2) receptors).

Inhalation of the ETAreceptor antagonist LU-135252 selectively attenuates hypoxic pulmonary vasoconstriction

Endogenous endothelin (ET)-1 modulates hypoxic pulmonary vasoconstriction (HPV). Accordingly, intravenously applied ETAreceptor antagonists reduce HPV, but this is accompanied by systemic vasodilation. We hypothesized that inhalation of an ETAreceptor antagonist might act selectively on the pulmonary vasculature and investigated the effects of aerosolized LU-135252 in an experimental model of HPV. Sixteen piglets (weight: 25 ± 1 kg) were anesthetized and mechanically ventilated at an inspiratory oxygen fraction (FiO2) of 0.3. After 1 h of hypoxia at FiO20.15, animals were randomly assigned either to receive aerosolized LU-135252 as bolus (0.3 mg/kg for 20 min; n = 8, LU group), or to receive aerosolized saline ( n = 8, controls). In all animals, hypoxia significantly increased mean pulmonary arterial pressure (32 ± 1 vs. 23 ± 1 mmHg; P < 0.01; means ± SE) and increased arterial plasma ET-1 (0.52 ± 0.04 vs. 0.37 ± 0.05 fmol/ml; P < 0.01) compared with mild hyperoxia at FiO20.3. Inhalation of LU-135252 induced a significant and sustained decrease in mean pulmonary arterial pressure compared with controls (LU group: 27 ± 1 mmHg; controls: 32 ± 1 mmHg; values at 4 h of hypoxia; P < 0.01). In parallel, mean systemic arterial pressure and cardiac output remained stable and were not significantly different from control values. Consequently, in our experimental model of HPV, the inhaled ETAreceptor antagonist LU-135252 induced selective pulmonary vasodilation without adverse systemic hemodynamic effects.

The therapeutic potential of PD156707 and related butenolide endothelin antagonists

Plasma concentrations of the peptide endothelin (ET) are elevated in several cardiovascular diseases. Animal studies suggest that activation of ET receptors may contribute to the increase in vascular resistance and remodelling of cardiovascular tissues that are characteristic of these pathologies. Antagonists of these receptors may therefore have important clinical potential. PD156707 (Parke-Davis) is one of a series of novel, orally-active butenolide endothelin antagonists and is highly selective for the ETA receptor. In man, this subtype mediates the profound vasoconstrictor effects of the ET peptides, and blockade of the ETA receptor may therefore produce beneficial vasodilatation. The advantage of selective ETA receptor antagonism is that it leaves unaffected vascular ETB receptors, which mediate vasorelaxation, and non-vascular ETB receptors, particularly in the lung and kidneys, which act to clear ET from the plasma. PD156707 exhibits subnanomolar affinity and greater than 1000-fold selectivity for human ETA receptors and potently inhibits ET-1-mediated vasoconstriction in human isolated blood vessels. In rats, PD156707 has good oral bioavailability (41%) and a relatively short terminal t1/2 of approximately 1 h. Structural analogues of PD156707 that have comparable selectivity and potency for the ETA receptor are reported to have even better oral bioavailability and longer plasma t1/2 values. Preclinical studies with PD156707 indicate efficacy in animal models of congestive heart failure (CHF), pulmonary hypertension (PH) and cerebral ischaemia. We await data from clinical trials to confirm the therapeutic potential of the ETA-selective butenolide antagonists in man.

Acute cardiopulmonary effects of a dual-endothelin receptor antagonist on oleic acid-induced pulmonary arterial hypertension in dogs

The objective of this study was to evaluate the cardiopulmonary effects of a dual-endothelin (ET) receptor antagonist, Tezosentan, on oleic acid (OA)-induced acute lung injury with pulmonary arterial hypertension in dogs. Twelve pentobarbital-anesthetized dogs with intravenous OA-induced acute lung injury (ALI) were divided into 2 groups. The control group (n=6) received saline treatment, whereas the treatment group (n=6) received the ET receptor antagonist, Tezosentan (1 mg/kg intravenous [i.v.]+1 mg/kg/h i.v. infusion). Cardiopulmonary parameters were monitored continuously for 1 hour. OA administration resulted in a significant increase in mean pulmonary arterial pressure (MPAP) and pulmonary vascular resistance (PVR) and a decrease in mean systemic arterial pressure (MSAP), systemic vascular resistance (SVR), and cardiac output (CO) in all dogs. Tezosentan treatment markedly attenuated the pulmonary hypertension, with a 32% decrease in MPAP (from 23 +/- 2 mm Hg to 15 +/- 2 mm Hg; P<.01) and a 22% decrease in PVR (from 860 +/- 105 dyn.s.cm(-5) to 670 +/- 96 dyn.s.cm(-5); P<.01) at the end of study. MSAP and SVR were unchanged after Tezosentan treatment, and there was an increase in cardiac output and a decline in peak inspiratory pressure (PIP) in the Tezosentan group compared with the control group. These results indicate that the dual-ET receptor antagonist, Tezosentan, can attenuate the pulmonary hypertension induced by OA. Thus, dual-ET receptor antagonists such as Tezosentan may be useful in the management of acute pulmonary arterial hypertension, complicating the course of OA-induced lung injury.

Effects of endothelin-1 on epithelial ion transport in human airways

Endothelin-1 (ET-1) exerts many biological effects in airways, including bronchoconstriction, airway mucus secretion, cell proliferation, and inflammation. We investigated the effect of ET-1 on Na absorption and Cl secretion in human bronchial epithelial cells. Addition of 10(-7) M ET-1 had no effect on the inhibition of the short circuit current (Isc) induced by amiloride, a Na channel blocker. Addition of 10(-7) M ET-1 to the apical bath in the presence of amiloride increased Isc in cultured human bronchial epithelial cells studied in Ussing chambers. No effect was observed when ET-1 was added to basolateral bath, indicating that the involved ET-1 receptors are likely present only in the apical membrane of the cells. Use of Cl-free solutions and bumetanide reduced the ET-1-induced increases in Isc, indicating that ET-1 stimulates Cl secretion. The ET-1-induced increase in Isc was prevented by exposure to the ETB receptor antagonist BQ-788 but not to the ETA receptor antagonist BQ-123. ET-1 did not raise intracellular Ca levels, but increased the intracellular concentration of cAMP. These findings indicate that ET-1 is a Cl secretagogue in human airways and acts presumably through apically located ETB receptors and activation of the cAMP pathway.

Spasmogenic action of endothelin-1 on isolated equine pulmonary artery and bronchus

REASONS FOR PERFORMING STUDY

There is currently little published information about the effects of endothelin-1 (ET-1), a potent endogenous spasmogen of vascular and airway smooth muscle, on pulmonary vasculature and airways or which ET receptor subtypes mediate ET-1-induced vasoconstrictive and bronchoconstrictive action in the horse.

OBJECTIVES

To investigate the effect of endothelin-1 (ET-1) on smooth muscle from isolated equine pulmonary artery and bronchus. In addition, the roles of ETA and ETB receptors in ET-1 mediated contraction in these tissues were assessed.

METHODS

The force generation of ring segments from pulmonary arteries or third-generation airways (obtained from horses subjected to euthanasia for orthopaedic reasons) were studied in an organ bath at 37 degrees C in response to exogenous endothelin and selective endothelin A (BQ123) or B receptor (BQ788) antagonists.

RESULTS

ET-1 produced concentration-dependent contractions of the equine pulmonary artery and bronchus. The threshold for contraction was 10(-10) and 10(-9) mol/l ET-1 for pulmonary artery and bronchus, respectively. The maximal contraction induced by the highest ET-1 concentration (10(-7) mol/l) was 173 and 194% of the contraction obtained with 100 mmol/l KCl in pulmonary artery and bronchus, respectively. ET-1 potency was 25 times greater in equine pulmonary artery than in equine bronchus (concentration of ET-1 producing 50% of maximal contraction [EC50] = 5.6 10(-9) mol/l and 2.2 10(-8) mol/l, respectively). In pulmonary artery, ET-1 induced contractions were significantly inhibited by the ETA receptor antagonist BQ123 (1 micromol/l; dose-response curve to ET-1 was shifted to the right by 5.4-fold), but not by the ETB antagonist BQ788. In bronchus, dose-responses curves to ET-1 were shifted to the right by BQ123 (1 micromol/l; 2.5-fold), but not by BQ788 (1 micromol/l). In the presence of both antagonists, the dose-response curve to ET-1 was shifted to the right by 4.5-fold.

CONCLUSIONS

These functional studies demonstrate that ET-1 is a potent spasmogen of equine third generation pulmonary artery and bronchus, and that contractions are mediated via ETA receptors in the former and both ETA and ETB receptors in the latter.

POTENTIAL CLINICAL RELEVANCE

Endothelin receptor antagonists may have potential for treating equine pulmonary hypertension or bronchoconstriction.

Endothelin-induced vascular and bronchial effects in pig airways: role in acute allergic responses

The effects of endothelin (ET) agonists on airway mechanics and bronchial blood flow were studied as well as the effects of mixed ET-receptor antagonist bosentan on allergen-induced airway reactions in the pig. ET agonists [ET-1, ET-3, and the ET(B) receptor-selective agonist Sarafotoxin 6c (Sf6c)] were given as intravenous injections (0.4-200 pmol/kg) to eight anesthetized pigs. Bosentan (10 mg/kg iv) was then administered, and the injections were repeated. Only Sf6c caused a significant increase in airway resistance, and this response was blocked by bosentan. Sf6c and ET-1 (200 and 400 pmol/kg, respectively) were also given as aerosols to five pigs. Sf6c, but not ET-1, caused bronchoconstriction via this route. All agonists (intravenous) caused increases in bronchial vascular conductance, an effect that was blocked by an NO-synthase inhibitor (N(G)-nitro-L-arginine) but unaffected by a cyxlooxygenase inhibitor (diclofenac). Fourteen pigs were sensitized with ascaris suum antigen. Under anesthesia, eight pigs were pretreated with bosentan, and six pigs were controls. They were all challenged with allergen aerosol resulting in acute bronchoconstriction and elevation of ET-1 in bronchoalveolar lavage fluid. Bosentan did not affect the maximal acute airway obstruction but markedly increased baseline bronchial vascular conductance, suggesting a basal vascular tone regulated by ETs. In conclusion, ETs induce bronchoconstriction primarily via the ET(B) receptor in the pig. However, ETs are probably not involved in the allergen-induced acute bronchoconstriction in this model.

Role of endothelin-1 in lung disease

Endothelin-1 (ET-1) is a 21 amino acid peptide with diverse biological activity that has been implicated in numerous diseases. ET-1 is a potent mitogen regulator of smooth muscle tone, and inflammatory mediator that may play a key role in diseases of the airways, pulmonary circulation, and inflammatory lung diseases, both acute and chronic. This review will focus on the biology of ET-1 and its role in lung disease.

Inhibition of voltage-gated K(+) currents by endothelin-1 in human pulmonary arterial myocytes

Recent studies demonstrate that endothelin-1 (ET-1) constricts human pulmonary arteries (PA). In this study, we examined possible mechanisms by which ET-1 might constrict human PA. In smooth muscle cells freshly isolated from these arteries, whole cell patch-clamp techniques were used to examine voltage-gated K(+) (K(V)) currents. K(V) currents were isolated by addition of 100 nM charybdotoxin and were identified by current characteristics and inhibition by 4-aminopyridine (10 mM). ET-1 (10(-8) M) caused significant inhibition of K(V) current. Staurosporine (1 nM), a protein kinase C (PKC) inhibitor, abolished the effect of ET-1. Rings of human intrapulmonary arteries (0.8-2 mm OD) were suspended in tissue baths for isometric tension recording. ET-1-induced contraction was maximal at 10(-8) M, equal to that induced by K(V) channel inhibition with 4-aminopyridine, and attenuated by PKC inhibitors. These data suggest that ET-1 constricts human PA, possibly because of myocyte depolarization via PKC-dependent inhibition of K(V). Our results are consistent with data we reported previously in the rat, suggesting similar mechanisms may be operative in both species.

Pathogenic role of endothelin 1 in hemodynamic dysfunction in experimental acute pulmonary thromboembolism

The plasma endothelin-1 (ET-1) level is elevated in patients with acute pulmonary thromboembolism (APE). Whether ET-1 is a pathogenic mediator or a simple marker of APE is not known. We investigated the role of ET-1 in hemodynamic dysfunction in APE through evaluating the effects of ET(A) receptor antagonist in an experimental APE model. We also examined ET-1 expression in embolized lungs. In a canine autologous blood clot pulmonary embolism model, ET(A) receptor antagonist ZD2574 (10 mg/kg, intravenous; ZD2574 group; n = 6) or vehicle (control group; n = 5) was administered. Hemodynamic and gas exchange parameters and plasma levels of ET-1 were serially measured. Prepro-ET-1 mRNA expression and the distribution of ET-1 peptide in lung tissues were also examined. With ZD2574 pulmonary arterial pressure and pulmonary vascular resistance significantly decreased, and were lower compared with the control group. The decrease in cardiac output was also less in the ZD2574 group. Plasma ET-1 levels increased after embolization. Prepro-ET-1 mRNA expression increased in embolized lungs and ET-1 peptide expression also increased in embolized lungs, particularly in the muscular pulmonary arteries, compared with normal lungs. These findings suggest that ET-1 partially contributes to hemodynamic derangements of APE, and that ET(A) receptor antagonists might constitute a useful therapeutic tool for APE.

Endothelin receptor mediating contraction of isolated bovine coronary artery

We studied endothelin (ET) receptors and their subtypes on isolated bovine coronary arteries. Endothelin receptors that mediated contraction of isolated bovine coronary artery were characterized by the use of antagonists and agonists. Contractions induced by the nonselective agonist ET-1 (10-10-10-7 M) were not affected by the removal of the endothelium (pEC50: 8.52, maximal contraction: 105% of that induced by 60 mM KCl). BQ-123 (3 x 10-7 M) antagonized contractions of endothelium-denuded coronary rings induced by low concentrations of ET-1 (10-10 or 10-9 M), but potentiated the contractions induced by higher concentrations of ET-1 (3 x 10-8 and 10-7 M). BQ-788 (10-6 M) potentiated contractions induced by ET-1 (3 x 10-10 and 10-7 M). In the presence of BQ-788 (10-6 M), BQ-123 (3 x 10-8-3 x 10-6 M) concentration - dependently inhibited contractions induced by ET-1 (3 x 10-10 and 10-7 M) (pA2: 6.61). Sarafotoxin S6b (10-9-3 x 10-7 M) evoked contractions in the denuded coronary artery (pEC50: 8.49, maximal contraction: 139% of 60 mM KCl). The BQ-123 caused a concentration-dependent rightward shift of contractions induced by sarafotoxin S6b (pA2: 7.89). The present study indicates that ET-1 and sarafotoxin S6b contract the isolated bovine coronary artery by stimulating ETA receptors on smooth muscle cells, and that ETB receptors might suppress the ET-1-induced contractions.

Endothelin-1-induced airway and parenchymal mechanical responses in guinea-pigs: the roles of ETA and ETB receptors

Endothelin-1 (ET-1) has been shown to have a constrictor effect on the airways and parenchyma; however, the roles of the ETA and ETB receptors in the ET-1-induced changes in the airway and tissue compartments have not been fully explored. Low-frequency pulmonary impedance (ZL) was measured in anaesthetized, paralysed, open-chest guinea-pigs. ZL spectra were fitted by a model to estimate airway resistance (Raw) and inertance (Iaw), and coefficients of tissue damping (G) and elastance (H), and hysteresivity (eta = G/H). Two successive doses of ET-1 (0.05 and 0.2 nmol x kg(-1)) each evoked significant dose-related increases in Raw, G, H and eta. Pretreatment with 20 nmol x kg(-1) BQ-610 (a highly selective ETA receptor antagonist) resulted in a significantly decreased elevation only in H after the lower dose of ET-1. However, all parameters changed significantly less on the administration of ET-1 after pretreatment with 80 nmol-kg(-1) BQ-610, with 20 nmol x kg(-1) ETR-P1/fl (a novel ETA receptor antagonist) or with 20 nmol x kg(-1) IRL 1038 (an ETB receptor antagonist). The results of the separate assessments of the airway and tissue mechanics demonstrate that endothelin-1 induces airway and parenchymal constriction via stimulation of both receptor types in both compartments.

Endothelin-1-induced potentiation of adrenergic responses in the rabbit pulmonary artery: role of thromboxane A(2)

To examine whether low concentrations of endothelin-1 potentiate the vasoconstrictor response to adrenergic stimulation, we recorded the isometric response of rings of rabbit pulmonary artery to electrical stimulation and noradrenaline. Endothelin-1 (10(-10) M) potentiated the contractions induced by electrical stimulation and noradrenaline. The endothelin ET(B) receptor antagonist (2,6-dimethylpiperidinecarbonyl-gamma-methyl-Leu-N(in)-[Methoxycarbonyl]-D-Trp-D-Nle) (BQ-788, 10(-6) M), but not the endothelin ET(A) receptor antagonist cyclo(D-Asp-Pro-D-Val-Leu-D-TRP) (BQ-123, 10(-6) M), inhibited the potentiating effects of endothelin-1. Pretreatment with the cyclooxygenase inhibitor indomethacin, the thromboxane synthase inhibitor furegrelate and the thromboxane receptor antagonist [1S-[1alpha,2alpha(Z),3alpha,4alpha]]-7-[3-[[[[(1-oxoheptyl)amino]acetyl]amino] methyl]-7-oxabicyclo-[2.2.1]hept-2-yl]-5-heptenoic acid (SQ-30741) (all at 10(-5) M) prevented the potentiation induced by endothelin-1 on adrenergic stimulation. The Ca(2+) channel antagonist nifedipine (10(-6) M) did not affect the potentiation induced by endothelin-1. The results indicate that endothelin-1 potentiates the responses to electrical stimulation and noradrenaline by activating endothelin ET(B) receptors. This potentiation depends on the production of cyclooxygenase-generated factors, probably thromboxane A(2).

Human urotensin-II is a potent spasmogen of primate airway smooth muscle

The contractile profile of human urotensin-II (hU-II) was examined in primate airway and pulmonary vascular tissues. hU-II contracted tissues from different airway regions with similar potencies (pD(2)s from 8.6 to 9.2). However, there were regional differences in the efficacy of hU-II, with a progressive increase in the maximum contraction from trachea to smaller airway regions (from 9 to 41% of the contraction to 10 microM carbachol). hU-II potently contracted pulmonary artery tissues from different regions with similar potencies and efficacies: pD(2)s=8.7 to 9.3 and maximal contractions=79 to 86% of 60 mM KCl. hU-II potently contracted pulmonary vein preparations taken proximal to the atria, but had no effect in tissues from distal to the atria. This is the first report describing the contractile activity of hU-II in airways and suggests that the potential pathophysiological role of this peptide in lung diseases warrants investigation.

Impact of endothelin-1 in endotoxin-induced pulmonary vascular reactions

OBJECTIVES

Elevated endothelin-1 (ET-1) levels have been detected during sepsis. The aim of the study was to examine the role of thromboxane A2 (TXA2) and ET-1 in pulmonary vascular reactions after endotoxin (LPS) challenge.

DESIGN

Prospective experimental study in rabbits.

SETTING

Experimental laboratory in a university teaching hospital.

SUBJECTS

Twenty-four adult rabbits of either sex.

INTERVENTIONS

Experiments were performed on 30 isolated and ventilated rabbit lungs, which were perfused with a saline solution containing 10% autologous blood.

MEASUREMENTS AND MAIN RESULTS

Pulmonary arterial pressure and lung weight gain were continuously registered. Perfusate samples were drawn intermittently to determine ET-1, TXA2, and prostacyclin (PGI2) concentrations. LPS isolated from Escherichia coli (0.5 mg/mL; n = 6) was added to the perfusate. A marked pulmonary arterial pressure increase followed by massive edema formation after 60 mins was observed after LPS injection. At the same time, elevated TXA2 and PGI2 levels in the perfusate were measured. ET-1 was detected 30 mins after LPS infusion (13.4+/-2.6 fmol/L). Pretreatment with the ET(A) receptor antagonist LU135252 (10(-6) M; n = 6) almost completely suppressed the pressure reaction after endotoxin injection (p < .01 at 50 and 60 mins) and reduced edema formation (p < .05). The cyclooxygenase inhibitor diclofenac (10 microg/mL; n = 6) was as effective as LU135252 in preventing vascular reactions after LPS injection.

CONCLUSIONS

Pretreatment with the ET(A) receptor antagonist LU135252 and the cyclooxygenase inhibitor diclofenac reduced pulmonary vascular reactions after LPS challenge. Based on the current data, we conclude that the pulmonary arterial pressure increase and edema formation after LPS injection are related to an ET-1- and TXA2-dependent mechanism.

Chronic obstructive pulmonary disease: emerging therapies

Despite the high prevalence of and mortality from chronic obstructive pulmonary disease, extensive research on the underlying pathophysiology and specific therapeutics for this disease is, relatively, in its infancy. Several novel molecular targets are being investigated as potential treatments for the disease. The most exciting new class of compounds is the phosphodiesterase 4 inhibitors; Ariflo (SB 207499)-a member of this class, and the most advanced in development (Phase III)-was reported recently to have significant clinical efficacy in patients with chronic obstructive pulmonary disease. Phosphodiesterase 4 inhibitors, such as Ariflo, possibly represent the most important advance in pulmonary medicine in recent years.

ET-1-induced pulmonary vasoconstriction shifts from ET(A)- to ET(B)-receptor-mediated reaction after preconstriction

Endothelin-1 (ET-1) has been reported to induce pulmonary vasoconstriction via either ET(A) or ET(B) receptors, and vasorelaxation after ET-1 injection has been observed. Our study investigated the effects of ET-1 in isolated rabbit lungs, which were studied at basal tone (part I) and after preconstriction (U-46619; part II). Pulmonary arterial pressure (PAP) and lung weight gain were monitored continuously. In part I, ET-1 (10(-8) M; n = 6; control) was injected after pretreatment with the ET(A)-receptor antagonist BQ-123 (10(-6) M; n = 6) or the ET(B)-receptor antagonist BQ-788 (10(-6) M; n = 6). The same protocol was carried out in part II after elevation of pulmonary vascular tone. ET-1 induced an immediate PAP increase (DeltaPAP 4.3 +/- 0.4 mmHg at 10 min) that was attenuated by pretreatment with BQ-123 (P < 0.05 at 10 min and P < 0.01 thereafter) and that was more pronounced after BQ-788 (P < 0.01 at 10 min and P < 0.001 thereafter). In part II, ET-1 induced an immediate rise in PAP with a maximum after 5 min (DeltaPAP 6.3 +/- 1.4 mmHg), leveling off at DeltaPAP 3.2 +/- 0.2 mmHg after 15 min. Pretreatment with BQ-123 failed to attenuate the increase. BQ-788 significantly reduced the peak pressure at 5 min (0.75 +/- 0.4 mmHg; P < 0.001) as well as the plateau pressure thereafter (P < 0.01). We conclude that ET-1 administration causes pulmonary vasoconstriction independent of basal vascular tone, and, at normal vascular tone, the vasoconstriction seems to be mediated via ET(A) receptors. BQ-788 treatment resulted in even more pronounced vasoconstriction. After pulmonary preconstriction, ET(A) antagonism exerted no effects on PAP, whereas ET(B) antagonism blocked the PAP increase. Therefore, ET-1-induced pulmonary vasoconstriction is shifted from an ET(A)-related to an ET(B)-mediated mechanism after pulmonary vascular preconstriction.

Endothelins and asthma

In the decade since endothelin-1 (ET-1) and related endogenous peptides were first identified as vascular endothelium-derived spasmogens, with potential pathophysiological roles in vascular diseases, there has been a significant accumulation of evidence pointing to mediator roles in obstructive respiratory diseases such as asthma. Critical pieces of evidence for this concept include the fact that ET-1 is an extremely potent spasmogen in human and animal airway smooth muscle and that it is synthesised in and released from the bronchial epithelium. Importantly, symptomatic asthma involves a marked enhancement of these processes, whereas asthmatics treated with anti-inflammatory glucocorticoids exhibit reductions in these previously elevated indices. Despite this profile, a causal link between ET-1 and asthma has not been definitively established. This review attempts to bring together some of the evidence suggesting the potential mediator roles for ET-1 in this disease.

Suc-[Glu9,Ala11,15]-endothelin-1 (8-21), IRL 1620, identifies two populations of ET(B) receptors in guinea-pig bronchus

The pharmacological properties of endothelin receptors (ETR) were investigated in guinea-pig bronchus by comparing binding and functional results. In binding assays, both the ET(B) agonists, endothelin-3 (ET-3) and N-suc-[Glu9,Ala11,15]ET-1(8-21) (IRL 1620), and the antagonist, N-cis-2,6-dimethylpiperidinocarbonyl-L-gamma-methylleucyl-D- 1-methoxycarbonyltryptophanyl-D-norleucine (BQ 788), showed biphasic inhibition curves of [125I]-endothelin-1 (ET-1) binding to bronchus membranes prepared from intact or epithelium-deprived tissue. IRL 1620 did not completely displace specifically [125I]-ET-1 bound to these tissue preparations. In the presence of the ET(A)-selective antagonist, cyclo(-D-Trp-D-Asp-L-Pro-D-Val-L-Leu) (BQ 123, 1 microM), IRL 1620 displacement curves were shallow but a complete inhibition was reached at a concentration of 1 microM. Both curves were better represented by two-site models. In addition, BQ 788 competition curves became monophasic when binding experiments were performed in the presence of 1 microM BQ 123. The non-selective agonist, ET-1, and BQ 123 inhibited [125I]-ET binding to bronchus membranes in dose-dependent fashions with monophasic curves. The contracting activity of IRL 1620 (0.55 nM- 1.6 microM) was tested on multiple-ring bronchial preparations pretreated with peptidase and cyclo-oxygenase inhibitors. BQ 788 shifted IRL1620 concentration-response curves to the right while BQ 123 did not influence bronchial responsiveness. In addition, a potentiation of the maximal response to the agonist was observed in BQ 788 treated bronchial rings. This effect was abolished by tissue pretreatment with Nomega-nitro-L-argininemethylester (L-NAME) or epithelium removal but not by pretreatment with atropine or iberiotoxin. Our results demonstrate that guinea-pig bronchus contains two populations of ET(B) receptors with different affinities for the ET(B)-selective agonist, IRL 1620. One ET(B) receptor population appears to activate bronchial muscle contraction while another on epithelial cells causes muscle relaxation through the release of nitric oxide (NO).

Nitric oxide and endothelin-1 in coronary and pulmonary circulation

Since the discovery of the vasorelaxant properties of nitric oxide and the vasoconstrictor effect of endothelin-1, there have been many studies of the distribution and functional significance of these agents in various vascular beds. In the coronary and pulmonary circulation nitric oxide and endothelin-1 actions have been largely investigated in terms of an imbalance between the opposing effects of these vasoactive agents leading to pathophysiological conditions. This article review functional and immunocytochemical studies with emphasis on the ultrastructural localization of nitric oxide synthase and endothelin-1 in the coronary and pulmonary vascular beds. Localization of nitric oxide synthase (type III or I or II) has been shown in endothelial cells, smooth muscle, and perivascular nerves of the coronary and pulmonary vascular beds and in the neurons, nerve fibers, and the small granule-containing cells within cardiac ganglia. Endothelin-1 was mainly localized in subpopulations of coronary and pulmonary endothelial cells. These immunocytochemical studies provide information about the sources of nitric oxide and endothelin-1 that contribute to the vasomotor control of cardiac and pulmonary circulation under normal and pathophysiological conditions.

Effect of infused angiotensin II on the bronchoconstrictor activity of inhaled endothelin-1 in asthma

STUDY OBJECTIVES

Endothelin (ET)-1 is a potent bronchoconstrictor, and asthmatics demonstrate bronchial hyperresponsiveness to ET-1 given by inhalation. Angiotensin II (Ang II) is increased in plasma in acute severe asthma, causes bronchoconstriction in asthmatics, and potentiates contractions induced by ET-1 in bovine bronchial smooth muscle in vitro, and contractions induced by methacholine both in vitro and in vivo. We wished to examine any potentiation of the bronchoconstrictor activity of inhaled ET-1 by infused Ang II at subbronchoconstrictor doses.

DESIGN

Double-blind randomized placebo-controlled study.

SETTING

Asthma research unit in university hospital.

PATIENTS

Eight asthmatic subjects with baseline FEV1 88% predicted, bronchial hyperreactivity (geometric mean, concentration of methacholine producing 20% fall, methacholine PC20 2.5 mg/mL), and mean age 37.1 years.

INTERVENTIONS

We examined the effect of subbronchoconstrictor doses of infused Ang II (1 ng/kg/min and 2 ng/kg/min) or placebo on bronchoconstrictor responses to inhaled ET-1 (dose range, 0.96 to 15.36 nmol).

MEASUREMENTS

Oxygen saturation, noninvasive BP, and spirometric measurements were made throughout the study visits. Blood was sampled for plasma Ang II levels at baseline and before and after ET-1 inhalation.

RESULTS

Ang II infusion did not produce bronchoconstriction per se at either dose prior to ET-1 challenge. Bronchial challenge with inhaled ET-1 produced dose-dependent bronchoconstriction, but there was no difference in bronchial responsiveness to ET-1 comparing infusion of placebo with Ang II at 1 ng/kg/min or 2 ng/kg/min (geometric mean, concentration of ET-1 producing 15% fall, 5.34 nmol, 4.95 nmol, and 4.96 nmol, respectively) (analysis of variance, p > 0.05). There was an increase in systolic and diastolic BP at the higher dose of Ang II compared to placebo (mean 136/86 vs 117/75 mm Hg, respectively). Plasma Ang II was elevated following infusion of both doses of Ang II compared to placebo.

CONCLUSIONS

In contrast to the potentiating effect on methacholine-induced bronchoconstriction, Ang II at subbronchoconstrictor doses does not potentiate ET-1-induced bronchoconstriction in asthma.

Airway endothelin levels in asthma: influence of endobronchial hypertonic saline challenge

BACKGROUND

The pathophysiology of exercise-induced asthma is not well understood. Hypertonicity of the airway lining fluid resulting from loss of water due to hyperventilation is considered to play a role, but the precise mechanism by which hypertonicity can induce bronchoconstriction is unknown. Peptides of the endothelin (ET) family have potent smooth muscle contractile properties, and have been linked to airway narrowing in stable asthma. We postulated that ET release may contribute to the acute bronchoconstrictor response induced by a hypertonic stimulus.

METHODS

Seven male asthmatic subjects underwent local endobronchial challenge with hypertonic (3.6%) saline and, as a control, isotonic (0.9%) saline aerosols in separate bronchopulmonary segments. Bronchoalveolar lavage (BAL) was performed at both sites during the phase of immediate bronchoconstriction. Concentrations of immunoreactive ET and of the mast cell products, histamine, tryptase and prostaglandin D2, in BAL fluid were measured.

RESULTS

Concentrations of ET in BAL fluid from the hypertonic saline-challenged sites were significantly lower than those in BAL fluid from sites exposed to isotonic saline (0.19 [0.11-1.24] fmol/mL vs. 0.40 [0.20-2.36] fmol/mL, P<0.05). Concentrations of histamine, tryptase, and prostaglandin D2 did not differ significantly between the two sites.

CONCLUSIONS

These findings do not support the hypothesis that ET release within the airway lumen is involved in the bronchoconstrictor response induced by hypertonic saline.

Selective activation of excitation-contraction coupling pathways by ET(A) and ET(B) receptors in guinea-pig tracheal smooth muscle

1. Signalling events responsible for endothelin(A) (ET(A)) and ET(B) receptor-induced contraction were examined in epithelium-denuded guinea-pig tracheal smooth muscle strips. Selective stimulation of each subtype was achieved by a combination of ET-1 (100 nM) and ET(A) and ET(B) receptor-selective antagonists, BQ-123 (10 microM) and BQ-788 (3 microM), respectively. 2. Both ET(A) and ET(B) receptors induced long-lasting contraction that was totally dependent on Ca2+ influx. Stimulation of ET(A) receptor induced both transient and sustained (Ca2+)i increases whereas that of ET(B) receptor induced only a sustained increase. Suppression of the transient (Ca2+)i increase by U73122 (3 microM) did not affect the ET(A)-induced sustained (Ca2+)i increase and tension development. Stimulation of ET(A) receptor, but not ET(B), induced phosphoinositide breakdown and protein kinase C (PKC). The activated PKC contributed to the contraction by increasing the Ca2+ sensitivity of the contractile apparatus. 3. Thus, ET(A) receptor is coupled both with phospholipase C/Ca2+/PKC signalling and Ca2+ influx pathways whereas ET(B) receptor was coupled only with the latter. 4. Stimulation of ET(B) receptor, but not ET(A), caused membrane depolarization measured with a fluorescent indicator, bis-(1,3 dibutylbarbituric acid)-trimethine oxonol. Both nifedipine (1 microM) and verapamil (10 microM) abolished ET(B)-induced Ca2+ influx and contraction, while they barely affected ET(A)-induced responses. 5. Therefore, the Ca2+ influx pathways activated by each subtype appeared to be completely different; ET(A) and ET(B) receptors opens voltage-independent Ca2+ channels and L-type voltage-dependent Ca2+ channels, respectively.

Haemodynamic influence and endothelin-1 plasma concentrations by selective or non-selective endothelin receptor antagonists in the pig in vivo

In the present study the haemodynamical effects of endothelin (ET)-receptor antagonism was evaluated using selective and non-selective ET(A)- and ET(B)-receptor blockade in normoxic pigs in vivo. In addition, the influence of the ET-antagonists on circulating plasma ET-1 levels was determined. BMS-182874 (10 and 30 mg kg(-1) i.v.), a selective ET(A)-receptor antagonist decreased the pulmonary and systemic vascular resistances. bosentan (10 and 30 mg(-1) i.v.), a non-selective ET(A)- and ET(B)-receptor antagonist caused principally similar effects as ETA-antagonism alone. No effects were observed by selective ET(B)-blockade using BQ-788 (30 microg kg(-1) i.v.). Of the three antagonists used only bosentan increased the circulating plasma ET-1 levels. It may therefore be concluded that ET contributes to basal systemic and pulmonary vascular tone through ET(A)-receptor activation. ET(B)-receptors are likely to cause the elevated plasma levels of ET-1 observed after bosentan administration. Furthermore, circulating plasma levels of ET-1 do not reflect the physiological effects of ET-1.

Modulation of ET-1-induced contraction of human bronchi by airway epithelium-dependent nitric oxide release via ET(A) receptor activation

1. The purpose of this work was to investigate whether endothelin-1 (ET-1) was able to induce the release of an inhibitory factor from the airway epithelium in isolated human bronchi and to identify this mediator as well as the endothelin receptor involved in this phenomenon. 2. In intact bronchi, ET-1 induced a concentration-dependent contraction (-logEC50 = 7.92+/-0.09, n = 18) which was potentiated by epithelium removal (-logEC50 = 8.65+/-0.11, n = 17). BQ-123 , an ET(A) receptor antagonist, induced a significant leftward shift of the ET-1 concentration-response curve (CRC). This leftward shift was abolished after epithelium removal. 3. L-NAME (3 x 10(-3) M), an inhibitor of nitric oxide (NO) synthase, induced a significant leftward shift of the ET-1 CRC, and abolished the potentiation by BQ-123 (10(-8) M) of ET-1-induced contraction. 4. In intact preparations, the ET(B) receptor antagonist BQ-788 induced only at 10(-5) M a slight rightward shift of the ET-1 CRC. In contrast, in epithelium-denuded bronchi or in intact preparations in the presence of L-NAME, BQ-788 displayed a non-competitive antagonism toward ET-1-induced contraction. 5. IRL 1620, a selective ET(B) receptor agonist, induced a contraction of the isolated bronchus (-logEC50=7.94+/-0.11, n= 19). This effect was not modified by epithelium removal or by BQ-123. BQ-788 exerted a competitive antagonism against IRL 1620 which was similar in the presence or absence of epithelium. 6. These results show that ET-1 exerts two opposite effects on the human airway smooth muscle. One is contractile via ETB-receptor activation, the other is inhibitory and responsible of NO release which counteracts via ETA-receptor activation the contraction.

The role of endothelin-1 as a mediator of the pressure response after air embolism in blood perfused lungs

OBJECTIVE

It is well known that lung embolism is associated with an increase in pulmonary vascular resistance. Since the mechanisms of pulmonary vascular reactions during embolism are still unclear, the aim of this study was to investigate the potential involvement of endothelin-1 (ET-1) and thromboxane A2 (TXA2) as mediators of the pulmonary artery pressure (PAP) increase after embolism using the selective ETA receptor antagonist LU135252 [1], the ETB receptor antagonist BQ788 [2], and the cyclooxygenase inhibitor diclofenac.

DESIGN

Prospective experimental study in rabbits.

SETTING

Experimental laboratory in a university teaching hospital.

SUBJECTS

36 adult rabbits of either sex.

INTERVENTIONS

The experiments were performed in 36 isolated and ventilated rabbit lungs which were perfused with a buffer solution containing 10% of autologous blood. Embolism was induced by the injection of 0.75 ml air into the pulmonary artery.

MEASUREMENTS AND RESULTS

PAP and lung weight, reflecting edema formation, were continuously recorded. Perfusate samples were drawn intermittently to determine TXA2 and ET-1 concentrations. Air injection resulted in an immediate increase in PAP up to 22.8 +/- 1.4 mm Hg at 2.5 min (control, n = 6), which was parallelled by an enhanced generation of TXA2. No relevant edema formation occurred during the observation period. Pretreatment with the ETA receptor antagonist LU135252 significantly reduced the pressure reaction after air embolism (p < 0.001) whereas the ETB receptor antagonist BQ788 (n = 6) was without marked effects. The administration of diclofenac (n = 6) did not alter the PAP increase 2.5 min after embolism, but significantly reduced the pressure reaction during the further observation period (p < 0.001). The application of LU135252 and diclofenac together (n = 6) also significantly reduced the PAP increase from 2.5 min during the total observation period (p < 0.001).

CONCLUSIONS

The acute pressure reaction after air embolism is mainly mediated via ET-1 by an ETA receptor related mechanism. TXA2 seems to maintain this reaction for a longer time.

L-NAME potentiates endothelin-stimulated thromboxane release from guinea pig lung

The bronchoconstrictor response after systemic administration of endothelins (ETs) in the guinea pig is indirectly mediated by thromboxane A2 (TxA2) release through ETB receptor activation. ETs also trigger the release of nitric oxide (NO) in endothelial cells by activation of ETB receptors. A growing body of evidence indicates that endogenous NO plays a key role in the regulation of pulmonary function. In this study we investigated the effect of an NO synthase inhibitor, L-NAME, on the release of TxA2 from the isolated, perfused guinea pig lung induced either by ET-1, the selective ETB receptor agonist IRL-1620, bradykinin (BK), or a TxA2-mimetic, U 46619. A 30 min intra-arterial (intra-arterial) infusion of L-NAME (300 microM) potentiated the TxA2 release with ET-1, IRL 1620, and BK (5, 50, and 50 nM, respectively) infused for 3 min (i.a.). U 46619 (10 nM) was ineffective as a stimulant of pulmonary eicosanoid release. Interestingly, L-NAME did not potentiate the release of prostacyclin (PGI2) triggered by ET-1, IRL 1620, or BK. Our results suggest a predominant role of ETB receptor activation in the release of TxA2. Furthermore, we suggest that NO in the guinea pig lung is a potent modulator of the TxA2 releasing activity of ET-1, IRL 1620, and BK, three agonists known to stimulate the release of NO.

The endothelin system in cardiovascular physiology and pathophysiology

Endothelin-1, a member of a novel family of regulatory peptides, is the most potent vasoconstrictor and pressor substance known. Endothelin-1 is a 21-amino-acid endothelium-derived peptide causing uniquely sustained vasoconstriction. In addition, endothelin-1 has pronounced effects on the coronary, renal and cerebral circulations, enhances responses to other vasoconstrictors, and is comitogenic. Recent studies have shown that the endothelins are essential for normal fetal development, and that endothelin-1 plays an important physiological role in the regulation of basal vascular tone and blood pressure in healthy humans. There is now also a wealth of evidence suggesting that endothelin-1 is a key mediator in a range of cardiovascular diseases associated with sustained vasoconstriction, such as chronic heart failure, and with vasospasm, such as subarachnoid haemorrhage. In addition, endothelin-1 appears to act in opposition to nitric oxide to promote the atherosclerotic process. There are a large number of oral and intravenously active endothelin antagonists entering clinical development and a number of clinical studies, particularly with endothelin receptor antagonists, are now under way. Such studies are beginning to define the role of the endothelins in cardiovascular disease and to confirm the potential of the endothelin system as an important new therapeutic target.

Hypoxic exposure time dependently modulates endothelin-induced contraction of pulmonary artery smooth muscle

Endothelins (ETs) have been implicated in the pathogenesis of hypoxia-induced pulmonary hypertension. We determined whether hypoxic exposure of rats (10% O2-90% N2, 1 atm, 1-48 days) altered contraction to ET in isolated segments of endothelium-denuded extralobar branch pulmonary artery (PA) and aorta. Hypoxic exposure increased hematocrit, right ventricular hypertrophy, and ET-1 plasma concentration. Hypoxia also caused a sustained decrease in PA but not in aorta sensitivity to ET-1. In comparison, hypoxic exposure throughout 12 days decreased time dependently the maximum contraction of PA to ET-1, BaCl2, and KCl. The hypoxia-induced decrease in maximum contraction of PA to ET-1 returned toward normal levels by 21 days and approximated control levels by 48 days. After 14 days of hypoxia, right ventricular hypertrophy correlated with decreased sensitivity of PA to ET-1. After 21 days of hypoxia, PA sensitivity to ET-2 and ET-3 was decreased, and sarafotoxin S6c-induced contraction was abolished. In conclusion, hypoxic exposure time dependently modulates the responsiveness of PA smooth muscle to ETs, BaCl2, and KCl. The hypoxia-induced changes in tissue responsiveness to ET-1 may be associated with increased plasma concentrations of this peptide.

Prostanoids counterbalance the bronchoconstrictor activity of endothelin-1 in pigs

In 12 anaesthetized spontaneously breathing pigs divided into two groups of six animals we evaluated the respiratory and haemodynamic responses to endothelin-1 (ET-1) administered by aerosol (200 pmol x kg(-1) in 1 ml of saline solution). In the first group (control group), the responses to ET-1 were evaluated before and after the blocking of endogenous nitric oxide (NO) by NG-nitro-L-arginine methyl ester (L-NAME 5 mg x kg(-1), i.v.). In the second group (indomethacin-pretreated group), the experimental protocol was similar to that of the control group, but the responses were evaluated after the blocking of endogenous prostanoids by indomethacin (3 mg x kg(-1), i.v.). Results show that in the control group ET-1 administered before and after L-NAME did not change compliance (Crs) or resistances (Rrs) of the respiratory system. In indomethacin-pretreated pigs, ET-1 significantly increased Rrs and decreased Crs. This constrictor effect appearing only during the block of arachidonic acid metabolites showed that ET-1 activity can be counterbalanced by a release of dilator prostanoids. In this group after L-NAME pretreatment ET-1 did not alter the mechanical properties of the respiratory system, suggesting an involvement of other bronchodilator mechanisms. In the control group, aerosol administered ET-1 increased mean systemic (MAP) and pulmonary (MPAP) arterial pressures, while when ET-1 was administered after L-NAME pretreatment, MPAP decreased. In the indomethacin-pretreated group, the peptide did not modify MAP, but caused an early decrease in MPAP when administered after L-NAME. Therefore, our results show that ET-1 caused a bronchoconstrictor effect only in indomethacin-pretreated pigs and suggest that the intrinsic constrictor activity of the peptide can be modulated especially by the release of dilator prostanoids.

Evaluation of bosentan, pinacidil and nitroprusside on human pulmonary arteries: comparison with rat pulmonary arteries

Bosentan (endothelin ETA/ETB antagonist), pinacidil (potassium channel opener) and nitroprusside (nitric oxide donor) were examined on isolated ring preparations of human intralobar pulmonary artery (3rd-5th generation, internal radius > or = 1 mm), rat main pulmonary artery (1st generation; internal radius > or = 1 mm) and rat intralobar pulmonary artery (3rd generation; internal radius 0.1-0.3 mm). The potency of endothelin-1 was the same in all three artery types. In human intralobar artery and rat main pulmonary artery, bosentan (3 and 10 microM) shifted the endothelin-1 concentration response curve to a higher concentration range (endothelin-1 concentration ratios, in human intralobar and rat main pulmonary artery, respectively: 3 microM bosentan, 4.5 and 8.1; 10 microM bosentan, 13.5 and 19.5), but caused no significant block of endothelin-1 in rat intralobar artery. The latter finding may be due to the reported presence of ETB receptors in rat intralobar arteries and the higher potency of bosentan on ETA than on ETB receptors. In contrast, the potencies of nitroprusside and pinacidil (relaxation of submaximal contractions to the thromboxane-mimetic, U46619) agreed on human and rat intralobar arteries but were 6 to 16-fold lower than on rat main pulmonary artery. We conclude that data obtained on pulmonary arteries from rats can be useful in predicting the effects of vasoactive drugs in human pulmonary arteries but selection of the most appropriate rat artery for study will depend on the drug group under investigation. For potassium channel openers and nitric oxide donors, good agreement between human and rat data will be found when using pulmonary arteries from the same anatomical location even though they differ markedly in size. In contrast, for endothelin antagonists, agreement is more likely to be found in arteries of comparable size, despite their different anatomical locations.

Endothelin-1-induced bronchoconstriction in asthma

Endothelin-1 (ET-1) has been indirectly implicated in the pathophysiology of asthma, and it is a potent bronchoconstrictor both in vitro and by inhalation in animal models in vivo. We examined the effect of inhaled ET-1 on airway tone in comparison with methacholine in eight asthmatics and five healthy volunteers in a double-blind randomized fashion. After a screening methacholine challenge each asthmatic had two ET-1 (doubling dose range, 0.96 to 15.36 nmol) and one methacholine (doubling dose range, 0.33 to 21.0 mumol) challenge, and normal subjects had a single ET-1 challenge. Inhalations were delivered using a dosimeter, and lung function measurements were made using constant-volume body plethysmography, with end points being a 35% fall in specific airway conductance (SGaw) and a 15% fall in FEV1. Samples for plasma ET-1 were taken before and after the inhalations, and pulse, blood pressure and oxygen saturation were monitored throughout the inhalations. All the asthmatic subjects displayed rapid-onset (< 5 min) dose-dependent bronchoconstriction to ET-1 across the dose range used, with mean (range) ET-1 PC35SGaw values of 5.15 (1.4 to 13.9) nmol, and 4.3 (1.2 to 8.3) nmol for the two ET-1 inhalations, and 0.42 (0.2 to 0.7) mumol for methacholine. Albuterol completely and rapidly reversed ET-1-induced bronchoconstriction, and in two patients not given albuterol, bronchoconstriction lasted 60 to 90 min. No significant bronchoconstriction was observed in any of the healthy volunteers across the ET-1 dose range used (mean PC35SGaw > 15.36 nmol). Oxygen saturation did not alter in either group, and plasma ET-1 did not change after ET-1 inhalation. Noninvasive blood pressure measurements revealed a fall in systolic blood pressure in normal subjects, with no change in asthmatics. Endothelin-1 is a potent bronchoconstrictor in asthma, with a bronchoconstrictor potency around 100 times that of methacholine in asthma. Asthmatics exhibit bronchial hyperreactivity to ET-1, and inhaled ET-1 can safely be given to asthmatics and normal subjects in the nebulized dose range 0.96 to 15.36 nmol.

ET-1-induced bronchoconstriction is mediated via ETB receptor in mice

Endothelin (ET)-1 is one of the most potent agonists of airway smooth muscle and can act via two different ET receptor subtypes, i.e., ETA and ETB. To determine the effects of ET-1 on in vivo pulmonary function and which ET receptors are involved in murine lungs, we investigated 1) the effects of ET and sarafotoxin S6c (S6c), a selective ETB agonist, on pulmonary function and 2) the effects of BQ-123 and BQ-788, specific ETA- and ETB-receptor antagonists, on ET-1-induced bronchoconstriction. ICR mice were anesthetized and mechanically ventilated (frequency = 2.5 Hz, tidal volume = 8 ml/kg, positive end-expiratory pressure = 3 cmH2O). Intravenous ET-1, ET-2, and ET-3 increased lung resistance similarly and equipotently, whereas S6c elicited a greater degree of bronchoconstriction. Mice were then pretreated with saline (Sal), BQ-123 (0.2, 1, and 5 mg/kg), or BQ-788 (0.2, 1, and 5 mg/kg) before administration of ET-1 (10(-7) mol/kg iv). No dose of BQ-123 blocked ET-1-induced constriction, whereas pretreatment with each dose of BQ-788 significantly inhibited ET-1-induced responses. There were significant differences in morphometrically assessed airway constriction between Sal and BQ-788 and between BQ-123 and BQ-788, whereas no significant difference was observed between Sal and BQ-123. There were no significant morphometric differences in the airway wall area among the three groups. These observations suggest that the ETB- but not ETA-receptor subtype may mediate the changes in murine pulmonary function in response to ET-1. In addition, the ETB-receptor antagonist reduces ET-1-induced airway narrowing by affecting airway smooth muscle contraction in mice.

Contraction of human airway smooth muscle by endothelin-1 and IRL 1620: effect of bosentan

We have examined whether 4-tert-butyl-N-[6-(2-hydroxy-ethoxy)-2,2'-bipyrimidin-4-yl]-benzen esulfonamide (bosentan; endothelin ET(A/B) receptor antagonist) and (R)2-[(R)-2-[(S)-2-[[1-(hexahydro-1H-azepinyl)]carbonyl] amino-4-methylpentanoil]amino-3-[3-(1-methyl-1H-indoyl)]prop ionyl]amino-3-(2-pyridyl) propionic acid (FR 139317; endothelin ET(A) receptor antagonist) inhibit contractions of human airway smooth muscle induced by endothelin-1 or Suc-[Glu9,Ala(11,15)]enthothelin-1-(8-21) (IRL 1620; endothelin ET(B) receptor agonist). Endothelin-1 and IRL 1620 were equipotent. Bosentan and FR 139317 (each 10 microM) produced a small shift in response curves to endothelin-1 (1.6- and 1.5-fold, respectively). However, bosentan was more potent against contractions elicited by IRL 1620 (10 microM, 11.2-fold shift) suggesting that these agonists exhibit different kinetic interactions with endothelin receptors or implying an interaction with a novel endothelin ET(B) receptor subtype in human airways.

Characterization of endothelin receptors in the human umbilical artery and vein

The aim of the present study was to characterize pharmacologically endothelin receptors that are present in human umbilical vessels. 2. Endothelin-1 (ET-1) and endothelin-2 (ET-2) are potent stimulants of both the human umbilical artery (pEC50 7.9 and 7.5) and vein (pEC50 8.1 and 8.0). Endothelin-3 (ET-3) is inactive on the artery but contracts the vein (pEC50 7.6). IRL1620 is inactive in both vessels. The order of potency of agonists is suggestive of a typical ET(A) receptor in the artery (ET-1 = ET-2 > > ET-3) and a mixture of ET(A) and ET(B) receptors in the vein (ET-1 = ET-2 > or = ET-3). 3. The selective ET(A) receptor antagonist, BQ123, competitively inhibits the effect of ET-1 in the human umbilical artery (pA2 6.9), while in the vein, only a mixture of BQ123 and BQ788 (a selective ET(B) antagonist) weakly displaces to the right of the cumulative concentration-response curve to ET-1. Contractions induced by ET-3 in the vein are inhibited by BQ788 (pA2 7.6), but not by BQ123. 4. Inhibition of Ca2+ channels by nifedipine (0.1 microM) is accompanied by a significant decrease of the maximal response to ET-1 by 40% in the artery and by 30% in the vein. The response of the vein to ET-3 is almost abolished by nifedipine. 5. The results indicate that: (i) endothelins contract the human isolated umbilical artery via stimulation of an ET(A) receptor type; (ii) the contraction induced by ET-1 in the vein is mediated by both ET(A) and ET(B) receptors, while ET-3 stimulates the ET(B) receptor; (iii) the contribution of Ca2+ channels to the contraction mediated by the ET(B) receptor appears to be more important than to that mediated by the ET(A) receptor.

Influence of respiratory tract viral infection on endothelin-1-induced potentiation of cholinergic nerve-mediated contraction in mouse trachea

1. This study examined the influence of respiratory tract infection with influenza A/PR-8/34 virus on endothelin receptor-mediated modulation of contraction induced by stimulation of cholinergic nerves in mouse isolated trachea. 2. The ETB receptor-selective agonist, sarafotoxin S6c (30 nM) induced large transient contractions (118 +/- 5% Cmax, n = 13; where Cmax is the contraction induced by 10 microM carbachol) of isolated tracheal segments from control mice. The peak contractile response to 30 nM sarafotoxin S6c was significantly lower in preparations from virus-inoculated mice at day 2 (57 +/- 8% Cmax, n = 3, P < 0.05) and 4 post-inoculation (90 +/- 8% Cmax, n = 9, P < 0.05), consistent with virus-induced attentuation of the ETB receptor-effector system linked to airway smooth muscle contraction. The mean peak contraction to 30 nM sarafotoxin S6c of preparations from virus-inoculated mice at day 8 post-inoculation (94 +/- 17% Cmax, n = 4) was not significantly different from that of control. 3. Electrical field stimulation (EFS; 90 V, 0.5 ms duration, 10 s train, 0.1-30 Hz) of preparations from control and virus-inoculated mice, caused contractions that were abolished by 0.1 microM atropine or 3 microM tetrodotoxin, indicating that these responses were mediated by neuronally released acetylcholine. Sarafotoxin S6c markedly potentiated contractions induced by a standard stimulus (0.3 Hz, every 3 min) in tracheal segments from control and virus-inoculated mice. In tracheal tissue from control mice, 30 nM sarafotoxin S6c significantly increased a standard EFS-induced contraction of 24 +/- 4% Cmax by a further 24 +/- 3% Cmax (i.e. 2 fold increase, n = 11). Sarafotoxin S6c (30 nM) also markedly potentiated standard EFS-induced contractions in preparations from virus-inoculated mice at day 2 (17 +/- 2% Cmax, n = 3), day 4 (17 +/- 5% Cmax, n = 9) and day 8 (26 +/- 5% Cmax, n = 4) post-inoculation. The level of potentiation of EFS-induced contractions in preparations from virus-inoculated mice was similar to that in tissue from control mice at days, 2, 4 and 8 post-inoculation. In contrast, sarafotoxin S6c (30 nM) did not enhance contractile responses of tracheal segments from control and virus-inoculated mice to exogenously applied acetylcholine (n = 3). 4. Endothelin-1 (1 nM) caused similar potentiations of standard EFS-induced contractions in tracheal segments from control (13 +/- 2% Cmax, n = 23) and virus-inoculated mice at day 2 (13 +/- 1% Cmax, n = 5), day 4 (16 +/- 5% Cmax, n = 6), and day 8 (13 +/- 3% Cmax, n = 8) post-inoculation. In contrast, 1 nM endothelin-1 did not enhance contractile responses of tracheal segments from control and virus-inoculated mice to exogenously applied acetylcholine (n = 4). Neither the ETA receptor-selective antagonist, BQ-123 (3 microM) nor the ETB receptor-selective antagonist, BQ-788 (1 microM) alone had any significant inhibitory effect on endothelin-1-induced potentiations of tracheal segments from control or virus-inoculated mice at days 2, 4 and 8 post-inoculation. However, simultaneous pre-incubation with BQ-123 (3 microM) and BQ-788 (1 microM) prevented endothelin-1-evoked potentiations, indicative of a role for both ETA and ETB receptors in this system. 5. These data clearly demonstrate that respiratory tract viral infection attenuated the function of the postjunctional ETB receptor-effector system linked directly to airway smooth muscle contraction. However, the function of prejunctional ETA and ETB receptor-effector systems linked to augmentation of cholinergic nerve-mediated airway smooth muscle contraction remained unaffected during respiratory tract viral infection in mice.

The induction of a biphasic bronchospasm by the ETB agonist, IRL 1620, due to thromboxane A2 generation and endothelin-1 release in guinea-pigs

1. IRL 1620 (0.01-0.1 mg kg-1, i.v.), a selective endothelin B (ETB) receptor agonist, induced a dose-dependent biphasic increase in total lung resistance and a decrease in dynamic compliance in anaesthetized and artificially ventilated guinea-pigs. After intravenous injection of IRL 1620 (0.03 mg kg-1), the first phase was observed within 2 min whereas the second phase started between 5 and 10 min after injection and was long lasting. 2. In order to characterize which endothelin receptors are involved in both phases of bronchoconstriction, we studied the effect of ETA and ETB receptor antagonists (BQ 123 and BQ 788, respectively). BQ 788 (0.1-1 mg kg-1, i.v.) inhibited, in a dose-dependent manner, both phases of bronchoconstriction. BQ 123 (3 mg kg-1, i.v.) markedly inhibited (by 76%) the second phase of bronchoconstriction but had no effect on the early component of the response. 3. The effect of atropine, neurokinin-I (NK1) and neurokinin-2 (NK2) receptor antagonists (SR140333 and SR48968, respectively) were tested to investigate the possible involvement of cholinergic and sensory nerve activation, respectively, in the response to IRL 1620. Likewise, the role of arachidonic acid metabolites (leukotriene D4 antagonist, ONO-1078 and thromboxane A2 (TXA2) inhibitor, OKY-046) in this response was also investigated. OKY-046 (1 mg kg-1, i.v.) and atropine (1 mg kg-1, i.v.) partially inhibited the first phase (by 80% and 20%, respectively) without affecting the late phase of bronchoconstriction. Neither ONO-1078 (1 mg kg-1, i.v.) nor the combination of SR140333 (0.2 mg kg-1, i.v.) and SR 48968 (0.2 mg kg-1, i.v.) modified IRL 1620-induced bronchoconstriction. 4. A low dose of IRL 1620 (0.005 mg kg-1, i.v.) induced a monophasic bronchoconstriction. Pretreatment by phosphoramidon (100 mumol kg-1, i.v.) restored the second phase of bronchoconstriction. In this condition, BQ 123 (3 mg kg-1, i.v.) was able to inhibit partially the second phase of bronchoconstriction. 5. These results suggest that both phases of bronchoconstriction induced by IRL 1620 were mediated primarily by ETB receptor activation, the first phase being a consequence of TXA2 and acetylcholine release. The inhibition by an ETA receptor antagonist and the restoration by a neutral endopeptidase (NEP) inhibitor of the second phase of bronchoconstriction suggests that primary activation of ETB receptors leads to autocrine/paracrine endothelin-1 (ET-1) release that would subsequently cause profound bronchoconstriction through both ETA and ETB receptor activation.

Comparison of endothelin B (ETB) receptors in rabbit isolated pulmonary artery and bronchus

1. To explore potential differences between endothelin (ET) receptors in airway versus vascular smooth muscle from the same species, the ETB receptors mediating contractions produced by ET-1, ET-3 and the selective ETB ligands, sarafotoxin S6c (S6c) and BQ-3020, in rabbit bronchus and pulmonary artery were investigated by use of peptide and non-peptide ET receptor antagonists. 2. In rabbit pulmonary artery SB 209670 (10 microM), a mixed ETA/ETB receptor antagonist, was a more potent antagonist of contractions produced by S6c (pKB = 7.7; n = 9; P < 0.05), than those elicited by ET-1 (pKB = 6.7; n = 6) or ET-3 (pKB = 6.7; n = 5). BQ-788 (10 microM), an ETB receptor antagonist, inhibited responses produced by ET-3 (pKB = 5.1; n = 8), BQ-3020 (pKB = 5.2; n = 4) or S6c (pKB = 6.2; n = 9; P < 0.05 compared to potency versus ET-3- or BQ-3020-induced contractions), but was without inhibitory effect on ET-1-induced contractions (n = 5). RES-701 (10 microM), another selective ETB receptor antagonist, was without effect on contractions produced by S6c (n = 4) or ET-1 (n = 4), and potentiated ET-3- (n = 5) or BQ-3020-induced responses (n = 4). 3. The combination of BQ-788 (10 microM) and BQ-123 (10 microM), an ETA-selective receptor antagonist, antagonized contractions produced by lower concentrations of ET-1 (1 and 3 nM) in rabbit pulmonary artery, but was without effect on responses elicited by higher concentrations of ET-1 (n = 5). The combination of RES-701 (10 microM) and BQ-123 (10 microM) potentiated responses elicited by ET-1, producing a 3.7 fold shift to the left in the agonist concentration-response curve (n = 5). 4. In rabbit bronchus SB 209670 (3 microM) had similar potency for antagonism of contractions produced by ET-1 (pKB = 6.3; n = 6), ET-3 (pKB = 6.5; n = 6) or S6c (pKB = 6.1; n = 8). BQ-788 (3 microM) was without effect on responses elicited by ET-1, ET-3 or S6c (n = 6) but antagonized BQ-3020-induced contractions (pKB = 6.4; n = 4). RES-701 (3 microM) was without effect on contractions produced by S6c (n = 6) or BQ-3020 (n = 4), and potentiated rather than antagonized ET-1- or ET-3-induced responses (n = 6), reflected by a significant (about 6 fold) shift to the left in ET-1 or ET-3 concentration-response curves. The combination of BQ-788 (3 microM) and BQ-123 (3 microM) was without effect on contractions produced by ET-1 in rabbit bronchus (n = 6). The combination of RES-701 (3 microM) and BQ-123 (3 microM) potentiated responses elicited by ET-1, producing a 5.2 fold shift to the left in the agonist concentration-response curve (n = 5). 5. BQ-123 (3 or 10 microM), an ETA-selective receptor antagonist, was without effect on ET-1, ET-3 or S6c concentration-response curves (n = 3-6) in rabbit pulmonary artery or rabbit bronchus. 6. These data indicate that contractions induced by ET-1, ET-3, S6c and BQ-3020 in rabbit pulmonary artery or rabbit bronchus appear to be mediated predominantly via stimulation of ETB receptors. However, the qualitative and quantitative differences in the relative profiles of the various structurally diverse peptide and non-peptide antagonists examined suggests that responses produced by the ET ligands may not be mediated by a homogeneous ETB receptor population. In addition, the results suggest that differences exist in the ETB receptors mediating contraction in pulmonary vascular versus airway tissues in the same species. These receptors are not very sensitive to the standard ETB receptor antagonists, BQ-788 and RES-701. Furthermore, the results also provide further evidence that the potencies of ET receptor antagonists depend upon the ET agonist.

Effects of Ro 47-0203 on endothelin-1 and sarafotoxin S6c-induced contractions of human bronchus and guinea-pig trachea

Endothelin exerts a variety of biological effects in the lung which indicate that this peptide may have a role in the pathophysiology of a number of pulmonary diseases. In this study, the endothelin receptors on the human bronchus were compared with those on the guinea-pig trachea using the novel, non-peptide antagonist Ro 47-0203 (4-tert-butyl-N-[6-(2-hydroxy-ethoxy)-5-(2-methoxy-phenoxy)-2,2' -bipyrimidin-4-yl]-benzenesulphonamide, non-selective for endothelin ETA over endothelin ETB receptor) and the peptide antagonist BQ123 (cyclo(-D-Val-Leu-D-Trp-D-Asp-Pro), endothelin ETA receptor selective). On the human bronchus and guinea-pig trachea, the concentration-effect curve for endothelin-1 was shifted to the right by Ro 47-0203 (100 microM) with concentration ratios of 28.2 +/- 6.8 and 39.5 +/- 13.9, respectively but lower concentrations of the antagonist had no effect. Although the concentration-effect curve for sarafotoxin S6c on the human bronchus was shifted to the right by Ro 47-0203 (30 and 100 microM, concentration ratio: 6.88 +/- 1.72 and 69.7 +/- 17.2, respectively), equivalent degrees of inhibition could be obtained on guinea-pig trachea with lower concentrations of antagonist (10 and 30 microM, concentration ratio: 6.90 +/- 1.58 and 75.8 +/- 14.1 respectively). The lack of effect of BQ123 (10 microM) and the high concentrations of Ro 47-0203 needed to show antagonism indicate that endothelin receptors on both tissues are probably the endothelin ETB subtype. Although the antagonism by Ro 47-0203 is not classically competitive, the greater effect of Ro 47-0203 against sarafotoxin S6c on the guinea-pig trachea may reflect a difference between the endothelin ETB receptors on these tissues.

Failure of BQ123, a more potent antagonist of sarafotoxin 6b than of endothelin-1, to distinguish between these agonists in binding experiments

1. In homogenates of human saphenous vein, [125I]-ET-1 and [125I]-S6b each labelled a single population of high affinity binding sites with K(D) values of 0.64 +/- 0.11 nM and 0.55 +/- 0.08 nM respectively. Hill slopes were close to one. However, the density of receptors labelled by [125I]-ET-1 was significantly greater than that by [125I]-S6b (187.6 +/- 23.0 compared to 91.7 +/- 23.6 fmol mg-1 protein, P < 0.02). 2. BQ123, an ET(A-)selective antagonist, inhibited specific [125I]-ET-1 and [125I]-S6b binding with equal affinity. BQ123 competed in a biphasic manner for both [125I]-ET-1 (0.1 nM) and [125I]-S6b (0.1 nM) with ET(A) K(D) values of 0.55 +/- 0.17 nM and 0.52 +/- 0.02 nM and ET(B) K(D) values of 14.4 +/- 2.60 microM and 11.2 +/- 0.31 microM respectively. S6b monophasically inhibited 0.1 nM [125I]-ET-1 (K(D) 1.16 +/- 0.9 nM) but competed for 0.25 nM [125I]-ET-1 in a biphasic manner (K(D) high affinity site 1.99 +/- 0.84 nM, K(D) low affinity site 0.68 +/- 0.63 microM, ratio 67% : 33%). 3. BQ123 antagonized the vasoconstrictor responses of ET-1 with a pK(B) value of 6.47 whereas BQ123 exhibited 50 fold higher affinity against S6b-mediated vasoconstriction with a pK(B) value of 8.18. Regression slopes were 0.80 +/- 0.13 and 1.08 +/- 0.11 respectively. 4. In desensitization experiments, S6b (300 nM) did not contract preparations which were no longer responsive to ET-1 whereas a small contraction to ET-1 (300 nM) was obtained in preparations rendered unresponsive to S6b. 5. Medial sections of non-diseased human aorta, which express only ET(A) receptors, were used to compare dissociation rates of the two agonists. The time course for the dissociation of [125I]-ET-1 and [125I]-S6b was similar with 20-30% of each ligand dissociating at 4 h. 6. These data suggest that whilst BQ123, in common with other endothelin antagonists, is a much more potent blocker of S6b contractile responses than of ET-1 contractile responses, this is not reflected by the equal affinity of BQ123 determined in competition binding experiments against both [125I]-ET-1 and [125I]-S6b. This discrepancy in antagonist potency is probably not due to a marked difference in the rate of dissociation of [125I]-ET-1 and [125I]-S6b from endothelin receptors. One possible explanation is that ET-1 is activating an additional population of receptors which may have lower affinity for BQ123. This is suggested by the discrepancy in receptor density identified by [125I]-ET-1 and [125I]-S6b.

Endothelin-1-induced potentiation of human airway smooth muscle proliferation: an ETA receptor-mediated phenomenon

1. In this study the mitogenic effects in human cultured tracheal smooth muscle cells of endothelin-1 (ET-1), ET-3, and sarafotoxin S6c (S6c), the ETB receptor-selective agonist, were explored either alone or in combination with the potent mitogen, epidermal growth factor (EGF). 2. In confluent, growth-arrested human airway smooth, neither ET-1 (0.01 nM-1 microM) nor ET-3 (0.001 nM-1 microM) or S6c (0.01 nM-1 microM) induced cell proliferation, as assessed by [3H]-thymidine incorporation. In contrast, EGF (1.6 pM-16 nM) produced concentration-dependent stimulation of DNA synthesis (EC50 of about 0.06 nM). The maximum increase of about 60 fold above control, elicited by 16 nM EGF, was similar to that obtained with 10% foetal bovine serum (FBS). EGF (0.16-16 nM) also produced a concentration-dependent increase in cell counts, whereas ET-1 (1-100 nM) was without effect on this index of mitogenesis. 3. ET-1 (1-100 nM) potentiated EGF-induced proliferation of human tracheal smooth muscle cells. For example, ET-1 (100 nM), which alone was without significant effect, increased by 3.0 to 3.5 fold the mitogenic influence of EGF (0.16 nM). The potentiating effect of ET-1 on EGF-induced proliferation was antagonized by BQ-123 (3 microM), the ETA receptor antagonist, but was unaffected by the ETB receptor antagonist BQ-788 (10 microM). 4. Neither ET-3 (1-100 nM) nor S6c (1-100 nM) influenced the mitogenic effects of EGF (0.16-1.6 nM). 5. [125I]-ET-1 binding studies revealed that on average the ratio of ETA to ETB receptors in human cultured tracheal smooth muscle cells was 35:65 ( +/- 3; n = 4), confirming the predominance of the ETB receptor subtype in human airway smooth muscle. 6. These data indicate that ET-1 alone does not induce significant human airway smooth muscle cell proliferation. However, it potently potentiated mitogenesis induced by EGF, apparently via an ETA receptor-mediated mechanism. These findings suggest that ET-1, a mediator detected in increased amounts in patients with acute asthma, may potentiate the proliferative effects of mitogens and contribute to the airway smooth muscle hyperplasia associated with chronic severe asthma.