Sarafotoxin S6c: an agonist which distinguishes between endothelin receptor subtypes

Tetrahydroxystilbene glucoside attenuated homocysteine-upregulated endothelin receptors in vascular smooth muscle cells via the ERK1 /2 /NF-κB signaling pathway

2,3,5,4'-Tetrahydrostilbene-2-o-β-d-glucoside (TSG) is the main active component of Polygonum multiflorum Thunb. It has effects on hypertension. However, the mechanism is unclear. Current research is devoted to exploring the mechanism of TSG improving HHcy-induced hypertension. The mice received a subcutaneous injection of Hcy in the presence or absence of TSG for 4 weeks. Blood pressure (BP) was measured using a noninvasive tail-cuff plethysmography method. Levels of plasma Hcy and endothelin-1 were measured using ELISA. Rat SMA without endothelium was cultured in a serum-free medium in the presence or absence of TSG with or without Hcy. The contractile response to sarafotoxin 6c or endothein-1 was studied using a sensitive myography. The levels of protein were detected using Western blotting. The results showed that TSG lowered HHcy-elevated BP and decreased levels of plasma Hcy and endothelin-1 in mice. Furthermore, the results showed that TSG inhibited Hcy-upregulated ET receptor expression and ET receptor-mediated contractile responses as well as the levels of p-ERK_1/2 and p-p65 in SMA. In vivo results further validate the in vitro results. In conclusion, TSG can decrease the levels of plasma Hcy and ET-1 and downregulate Hcy-upregulated ET receptors in VSMCs by inhibiting the ERK_1/2 /NF-κB/ET_B2 pathway to lower the BP.

Endothelin and the Cardiovascular System: The Long Journey and Where We Are Going

Simple Summary

In this review, we describe the basic functions of endothelin and related molecules, including their receptors and enzymes. Furthermore, we discuss the important role of endothelin in several cardiovascular diseases, the relevant clinical evidence for targeting the endothelin pathway, and the scope of endothelin-targeting treatments in the future. We highlight the present uses of endothelin receptor antagonists and the advancements in the development of future treatment options, thereby providing an overview of endothelin research over the years and its future scope.

Abstract

Endothelin was first discovered more than 30 years ago as a potent vasoconstrictor. In subsequent years, three isoforms, two canonical receptors, and two converting enzymes were identified, and their basic functions were elucidated by numerous preclinical and clinical studies. Over the years, the endothelin system has been found to be critical in the pathogenesis of several cardiovascular diseases, including hypertension, pulmonary arterial hypertension, heart failure, and coronary artery disease. In this review, we summarize the current knowledge on endothelin and its role in cardiovascular diseases. Furthermore, we discuss how endothelin-targeting therapies, such as endothelin receptor antagonists, have been employed to treat cardiovascular diseases with varying degrees of success. Lastly, we provide a glimpse of what could be in store for endothelin-targeting treatment options for cardiovascular diseases in the future.

Inhibitors of the MAPK/ NF-κB pathway attenuate the upregulation of the ETB receptor mediated by high glucose in vascular smooth muscle cells

BACKGROUND

Increased vascular smooth muscle cell (VSMC) endothelin type B (ET_B) receptor expression is involved in cardiovascular diseases. High glucose (HG) in diabetes is closely related to cardiovascular complications. Although diabetes upregulates VSMC endothelin subtype B (ET_B) receptors, its mechanism is still unclear. Our aim is to investigate the mechanism of HG-induced ET_B receptors in VSMCs.

METHODS

Rat superior mesenteric arteries (SMAs) without endothelium were cultured in medium without serum for 24 h. HG with or without mitogen-activated protein kinase (MAPK) signaling pathway inhibitors and downstream nuclear factor-kappaB (NF-κB) inhibitors was coincubated with SMAs. A sensitive myograph detected the contractile responses to sarafotoxin 6c. Western blotting and immunofluorescence staining were used to determine protein expression.

RESULTS

HG promoted the expression of VSMC ET_B receptors in rat SMAs and enhanced the ET_B receptor-induced contractile response. The results showed that HG increased vascular smooth muscle cell (VSMC) ET_B receptor expression and ET_B receptor-induced contractile responses in rat SMAs. Both extracellular signal-related kinase 1 and 2 (ERK1/2) inhibitors (U0126) and P38 inhibitors (SB203580) significantly inhibited HG-increased VSMC ET_B receptors. However, a C-jun terminal kinase (p-JNK) inhibitor (SP600125) did not affect HG- upregulated VSMC ET_B receptors. Further study showed that NF-κB using an IκB kinase inhibitor (wedelolactone) also significantly inhibited HG-increased VSMC ET_B receptors.

CONCLUSION

In conclusion, HG upregulated the VSMC ET_B receptor by activating the ERK1/2- or P38- NF-κB signaling pathway.

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.

Toxinology provides multidirectional and multidimensional opportunities: A personal perspective

In nature, toxins have evolved as weapons to capture and subdue the prey or to counter predators or competitors. When they are inadvertently injected into humans, they cause symptoms ranging from mild discomfort to debilitation and death. Toxinology is the science of studying venoms and toxins that are produced by a wide variety of organisms. In the past, the structure, function and mechanisms of most abundant and/or most toxic components were characterized to understand and to develop strategies to neutralize their toxicity. With recent technical advances, we are able to evaluate and determine the toxin profiles using transcriptomes of venom glands and proteomes of tiny amounts of venom. Enormous amounts of data from these studies have opened tremendous opportunities in many directions of basic and applied research. The lower costs for profiling venoms will further fuel the expansion of toxin database, which in turn will provide greater exciting and bright opportunities in toxin research.

Activation of neuronal endothelin B receptors mediates pressor response through alpha-1 adrenergic receptors

Abnormalities in activity of the endothelin (ET) system have been widely reported in a number of cardiovascular disease states such as hypertension and heart failure. Although the vascular responses to ET are well established, the interaction between ET and other important modulators of blood pressure, such as the sympathetic nervous system, are less understood. Previous reports implicate ET signaling through ET type B (ET_B) receptors in increasing neuronal activity. Therefore, we hypothesized that activation of ET_B receptors on sympathetic nerves would increase blood pressure through an adrenergic‐mediated mechanism. Thus, we used anesthetized ET_B‐deficient rats, which only express functional ET_B receptors on adrenergic neurons, and genetic controls, which express functional ET_B receptors in vascular tissue and kidney epithelium. We determined the pressor response to the selective ET_B receptor agonist sarafotoxin c (S6c). Separate groups of rats were treated with the α₁‐adrenergic receptor antagonist prazosin or the β‐adrenergic receptor antagonist propranolol to elucidate the role of adrenergic signaling in mediating the blood pressure response. We observed a dose‐dependent pressor response to S6c in ET_B‐deficient rats that was reversed by prazosin treatment and augmented by propranolol. In genetic control rats, the effects of S6c on sympathetic neurons were mostly masked by the direct activity of ET_B receptor activation on the vasculature. Heart rate was mostly unaffected by S6c across all groups and treatments. These results suggest that ET_B activation on sympathetic neurons causes an increase in blood pressure mediated through α₁‐adrenergic receptor signaling.

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.

Endothelin receptors and their antagonists

All three members of the endothelin (ET) family of peptides, ET-1, ET-2, and ET-3, are expressed in the human kidney, with ET-1 being the predominant isoform. ET-1 and ET-2 bind to two G-protein–coupled receptors, ET_A and ET_B, whereas at physiological concentrations ET-3 has little affinity for the ET_A receptor. The human kidney is unusual among the peripheral organs in expressing a high density of ET_B. The renal vascular endothelium only expresses the ET_B subtype and ET-1 acts in an autocrine or paracrine manner to release vasodilators. Endothelial ET_B in kidney, as well as liver and lungs, also has a critical role in scavenging ET-1 from the plasma. The third major function is ET-1 activation of ET_B in in the nephron to reduce salt and water re-absorption. In contrast, ET_A predominate on smooth muscle, causing vasoconstriction and mediating many of the pathophysiological actions of ET-1. The role of the two receptors has been delineated using highly selective ET_A (BQ123, TAK-044) and ET_B (BQ788) peptide antagonists. Nonpeptide antagonists, bosentan, macitentan, and ambrisentan, that are either mixed ET_A/ET_B antagonists or display ET_A selectivity, have been approved for clinical use but to date are limited to pulmonary hypertension. Ambrisentan is in clinical trials in patients with type 2 diabetic nephropathy. This review summarizes ET-receptor antagonism in the human kidney, and considers the relative merits of selective versus nonselective antagonism in renal disease.

Reduced Mechanical Stretch Induces Enhanced Endothelin B Receptor-Mediated Contractility via Activation of Focal Adhesion Kinase and Extracellular Regulated Kinase 1/2 in Cerebral Arteries from Rat

Cerebral ischaemia results in enhanced endothelin B (ETB ) receptor-mediated contraction and receptor protein expression in the affected cerebrovascular smooth muscle cells (SMC). Organ culture of cerebral arteries is a method to induce similar alterations in ETB receptor expression. We suggest that rapid and sustained reduction in wall tension/stretch is a possible trigger mechanism for this vascular remodelling. Isolated rat middle cerebral artery (MCA) segments were incubated in a wire myograph with or without mechanical stretch, prior to assessment of their contractile response to the selective ETB receptor agonist sarafotoxin 6c. The involvement of extracellular regulated kinase (ERK) 1/2 and focal adhesion kinase (FAK) was studied by their specific inhibitors U0126 and PF-228, respectively. Compared with their stretched counterparts, unstretched MCA segments showed a significantly increased ETB receptor-mediated contractile response after 12 hr of incubation, which was attenuated by either U0126 or PF-228. The functionally increased ETB -mediated contractility could be attributed to two different mechanisms: (i) a difference in ETB receptor localization from primarily endothelial expression to SMC expression and (ii) an increased calcium sensitivity of the SMCs due to an increased expression of the calcium channel transient receptor potential canonical 1. Collectively, our results present a possible mechanism linking lack of vessel wall stretch/tension to changes in ETB receptor-mediated contractility via triggering of an early mechanosensitive signalling pathway involving ERK1/2 and FAK signalling. A mechanism likely to be an initiating factor for the increased ETB receptor-mediated contractility found after cerebral ischaemia.

Endothelin@25 - new agonists, antagonists, inhibitors and emerging research frontiers: IUPHAR Review 12

Since the discovery of endothelin (ET)-1 in 1988, the main components of the signalling pathway have become established, comprising three structurally similar endogenous 21-amino acid peptides, ET-1, ET-2 and ET-3, that activate two GPCRs, ETA and ETB . Our aim in this review is to highlight the recent progress in ET research. The ET-like domain peptide, corresponding to prepro-ET-193-166 , has been proposed to be co-synthesized and released with ET-1, to modulate the actions of the peptide. ET-1 remains the most potent vasoconstrictor in the human cardiovascular system with a particularly long-lasting action. To date, the major therapeutic strategy to block the unwanted actions of ET in disease, principally in pulmonary arterial hypertension, has been to use antagonists that are selective for the ETA receptor (ambrisentan) or that block both receptor subtypes (bosentan). Macitentan represents the next generation of antagonists, being more potent than bosentan, with longer receptor occupancy and it is converted to an active metabolite; properties contributing to greater pharmacodynamic and pharmacokinetic efficacy. A second strategy is now being more widely tested in clinical trials and uses combined inhibitors of ET-converting enzyme and neutral endopeptidase such as SLV306 (daglutril). A third strategy based on activating the ETB receptor, has led to the renaissance of the modified peptide agonist IRL1620 as a clinical candidate in delivering anti-tumour drugs and as a pharmacological tool to investigate experimental pathophysiological conditions. Finally, we discuss biased signalling, epigenetic regulation and targeting with monoclonal antibodies as prospective new areas for ET research.

Endothelin-1/endothelin A receptor-mediated biased signaling is a new player in modulating human ovarian cancer cell tumorigenesis

The endothelin-1 (ET-1)/endothelin A receptor (ETAR, a G protein-coupled receptor) axis confers pleiotropic effects on both tumor cells and the tumor microenvironment, modulating chemo-resistance and other tumor-associated processes by activating Gαq- and β-arrestin-mediated pathways. While the precise mechanisms by which these effects occur remain to be elucidated, interference with ETAR signaling has emerged as a promising antitumor strategy in many cancers including ovarian cancer (OC). However, current clinical approaches using ETAR antagonists in the absence of a detailed knowledge of downstream signaling have resulted in multiple adverse side effects and limited therapeutic efficacy. To maximize the safety and efficacy of ETAR-targeted OC therapy, we investigated the role of other G protein subunits such as Gαs in the ETAR-mediated ovarian oncogenic signaling. In HEY (human metastatic OC) cells where the ET-1/ETAR axis is well-characterized, Gαs signaling inhibits ETAR-mediated OC cell migration, wound healing, proliferation and colony formation on soft agar while inducing OC cell apoptosis. Mechanistically, ET-1/ETAR is coupled to Gαs/cAMP signaling in the same ovarian carcinoma-derived cell line. Gαs/cAMP/PKA activation inhibits ETAR-mediated β-arrestin activation of angiogenic/metastatic Calcrl and Icam2 expression. Consistent with our findings, Gαs overexpression is associated with improved survival in OC patients in the analysis of the Cancer Genome Atlas data. In conclusion, our results indicate a novel function for Gαs signaling in ET-1/ETAR-mediated OC oncogenesis and may provide a rationale for a biased signaling mechanism, which selectively activates Gαs-coupled tumor suppressive pathways while blocking Gαq-/β-arrestin-mediated oncogenic pathways, to improve the targeting of the ETAR axis in OC.

Adaptive regulation of endothelin receptor type-A and type-B in vascular smooth muscle cells during pregnancy in rats

Normal pregnancy is associated with systemic vasodilation and decreased vascular contraction, partly due to increased release of endothelium-derived vasodilator substances. Endothelin-1 (ET-1) is an endothelium-derived vasoconstrictor acting via endothelin receptor type A (ETA R) and possibly type B (ETB R) in vascular smooth muscle cells (VSMCs), with additional vasodilator effects via endothelial ETB R. However, the role of ET-1 receptor subtypes in the regulation of vascular function during pregnancy is unclear. We investigated whether the decreased vascular contraction during pregnancy reflects changes in the expression/activity of ETAR and ETBR. Contraction was measured in single aortic VSMCs isolated from virgin, mid-pregnant (mid-Preg, day 12), and late-Preg (day 19) Sprague-Dawley rats, and the mRNA expression, protein amount, tissue and cellular distribution of ETAR and ETBR were examined using RT-PCR, Western blots, immunohistochemistry, and immunofluorescence. Phenylephrine (Phe, 10(-5)  M), KCl (51 mM), and ET-1 (10(-6)  M) caused VSMC contraction that was in late-Preg < mid-Preg and virgin rats. In VSMCs treated with ETB R antagonist BQ788, ET-1 caused significant contraction that was still in late-Preg < mid-Preg and virgin rats. In VSMCs treated with the ETAR antagonist BQ123, ET-1 caused a small contraction; and the ETBR agonists IRL-1620 and sarafotoxin 6c (S6c) caused similar contraction that was in late-Preg < mid-Preg and virgin rats. RT-PCR revealed similar ETAR, but greater ETBR mRNA expression in pregnant versus virgin rats. Western blots revealed similar ETAR, and greater protein amount of ETBR in endothelium-intact vessels, but reduced ETBR in endothelium-denuded vessels of pregnant versus virgin rats. Immunohistochemistry revealed prominent ETBR staining in the intima, but reduced ETAR and ETBR in the aortic media of pregnant rats. Immunofluorescence signal for ETAR and ETBR was less in VSMCs of pregnant versus virgin rats. The pregnancy-associated decrease in ETAR- and ETBR-mediated VSMC contraction appears to involve downregulation of ETAR and ETBR expression/activity in VSM, and may play a role in the adaptive vasodilation during pregnancy.

Role of endothelium in vasomotor responses to endothelin system and protein kinase C activation in porcine retinal arterioles

PURPOSE

Endothelial cells synthesize vasodilator nitric oxide (NO) and vasoconstrictor endothelin-1 (ET-1) from NO synthase (eNOS) and endothelin-converting enzyme-1 (ECE-1), respectively. Protein kinase C (PKC) and Rho kinase (ROCK) are major signaling molecules mediating vasoconstriction. Although endothelial cells express eNOS, ECE-1, endothelin B (ET(B)) receptors, PKC, and ROCK, their influences on ET-1-induced vasoconstriction remain elusive. We studied whether these endothelial signaling molecules modulate retinal arteriolar constriction to ET-1.

METHODS

Porcine retinal arterioles were isolated and pressurized for vasomotor study, under conditions with intact or denuded endothelium, using videomicroscopic techniques.

RESULTS

Retinal arterioles developed similar resting tone (≈45% of maximum diameter) with or without endothelium. Endothelial denudation attenuated vasoconstriction to ET-1 precursor, big ET-1, by almost equal to 50%, but did not affect vasoconstrictions to ET-1, ET(B) agonist sarafotoxin S6c, or PKC activator phorbol-12, 13-dibutyrate (PDBu). The ROCK inhibitor H-1152 caused vasodilation, and abolished vasoconstrictions to ET-1 and PDBu independent of endothelium. With L-type voltage-operated calcium channel (L-VOCC) blocker nifedipine, PDBu-induced vasoconstriction was abolished and converted to NO-mediated vasodilation in the presence of endothelium. The ET-1-induced vasoconstriction was unaffected by NO released from endothelium during flow elevation.

CONCLUSIONS

Endothelial and smooth muscle ECE-1 contribute equally to synthesis of vasoactive ET-1 in retinal arterioles, with nominal role of endothelial ETB receptors in vasoconstriction to ET-1. The PKC activation leads to endothelium-dependent NO-mediated vasodilation when smooth muscle contraction is ablated by L-VOCC blockade. Endothelial cells and NO appear to have modest roles in modulating ROCK-dependent vasoconstriction, and are insufficient to counteract smooth muscle contractions to ET-1 and PKC activation.

Vascular endothelin receptor type B: structure, function and dysregulation in vascular disease

Endothelin-1 (ET-1) is a major regulator of vascular function, acting via both endothelin receptor type A (ET(A)R) and type B (ET(B)R). Although the role of ET(A)R in vascular smooth muscle (VSM) contraction has been studied, little is known about ET(B)R. ET(B)R is a G-protein coupled receptor with a molecular mass of ~50 kDa and 442 amino acids arranged in seven transmembrane domains. Alternative splice variants of ET(B)R and heterodimerization and cross-talk with ET(A)R may affect the receptor function. ET(B)R has been identified in numerous blood vessels with substantial effects in the systemic, renal, pulmonary, coronary and cerebral circulation. ET(B)R in the endothelium mediates the release of relaxing factors such as nitric oxide, prostacyclin and endothelium-derived hyperpolarizing factor, and could also play a role in ET-1 clearance. ET(B)R in VSM mediates increases in [Ca(2+)](i), protein kinase C, mitogen-activated protein kinase and other pathways of VSM contraction and cell growth. ET-1/ET(A)R signaling has been associated with salt-sensitive hypertension (HTN) and pulmonary arterial hypertension (PAH), and ET(A)R antagonists have shown some benefits in these conditions. In search for other pathogenetic factors and more effective approaches, the role of alterations in endothelial ET(B)R and VSM ET(B)R in vascular dysfunction, and the potential benefits of modulators of ET(B)R in treatment of HTN and PAH are being examined. Combined ET(A)R/ET(B)R antagonists could be more efficacious in the management of conditions involving upregulation of ET(A)R and ET(B)R in VSM. Combined ET(A)R antagonist with ET(B)R agonist may need to be evaluated in conditions associated with decreased endothelial ET(B)R expression/activity.

Selective targeting of G-protein-coupled receptor subtypes with venom peptides

The G-protein-coupled receptor (GPCR) family is one of the largest gene superfamilies with approx. 370 members responding to endogenous ligands in humans and a roughly equal amount of receptors sensitive to external stimuli from the surrounding. A number of receptors from this superfamily are well recognized targets for medical treatment of various disease conditions, whereas for many others the potential medical benefit of interference is still obscure. A general problem associated with GPCR research and therapeutics is the insufficient specificity of available ligands to differentiate between closely homologous receptor subtypes. In this context, venom peptides could make a significant contribution to the development of more specific drugs. Venoms from certain animals specialized in biochemical hunting contain a mixture of molecules that are directed towards a variety of membrane proteins. Peptide toxins isolated from these mixtures usually exhibit high specificity for their targets. Muscarinic toxins found from mamba snakes attracted much attention during the 1990s. These are 65-66 amino acid long peptides with a structural three-finger folding similar to the α-neurotoxins and they target the muscarinic acetylcholine receptors in a subtype-selective manner. Recently, several members of the three-finger toxins from mamba snakes as well as conotoxins from marine cone snails have been shown to selectively interact with subtypes of adrenergic receptors. In this review, we will discuss the GPCR-directed peptide toxins found from different venoms and how some of these can be useful in exploring specific roles of receptor subtypes.

Endothelin-mediated calcium responses in supraoptic nucleus astrocytes influence magnocellular neurosecretory firing activity

In addition to their peripheral vasoactive effects, accumulating evidence supports an important role for endothelins (ETs) in the regulation of the hypothalamic magnocellular neurosecretory system, which produces and releases the neurohormones vasopressin (VP) and oxytocin (OT). Still, the precise cellular substrates, loci and mechanisms underlying the actions of ETs on the magnocellular system are poorly understood. In the present study, we combined patch-clamp electrophysiology, confocal Ca(2+) imaging and immunohistochemistry to study the actions of ETs on supraoptic nucleus (SON) magnocellular neurosecretory neurones and astrocytes. Our studies show that ET-1 evoked rises in [Ca(2+) ](i) levels in SON astrocytes (but not neurones), an effect largely mediated by the activation of ET(B) receptors and mobilisation of thapsigargin-sensitive Ca(2+) stores. The presence of ET(B) receptors in SON astrocytes was also verified immunohistochemically. ET(B) receptor activation either increased (75%) or decreased (25%) SON firing activity, both in VP and putative OT neurones, and these effects were prevented when slices were preincubated in glutamate receptor blockers or nitric oxide synthase blockers, respectively. Moreover, ET(B) -mediated effects in SON neurones were also prevented by a gliotoxin compound, and when changes in [Ca(2+) ](i) were prevented with bath-applied BAPTA-AM or thapsigargin. Conversely, intracellular Ca(2+) chelation in the recorded SON neurones failed to block ET(B) -mediated effects. In summary, our results indicate that ET(B) receptor activation in SON astrocytes induces the mobilisation of [Ca(2+) ](i) , likely resulting in the activation of glutamate and nitric oxide signalling pathways, evoking in turn excitatory and inhibitory SON neuronal responses, respectively. Taken together, our study supports an important role for astrocytes in mediating the actions of ETs on the magnocellular neurosecretory system.

ETA and ETB receptors are expressed in vascular adventitial fibroblasts

The adventitia has been recognized to play important roles in vascular oxidative stress, remodeling, and contraction. We recently demonstrated that adventitial fibroblasts are able to express endothelin (ET)-1 in response to ANG II. However, it is unclear whether ET-1 receptors are expressed in the adventitia. We therefore investigated the expression and roles of both ET(A) and ET(B) receptors in collagen synthesis and ET-1 clearance in adventitial fibroblasts. Adventitial fibroblasts were isolated and cultured from the mouse thoracic aorta by the explant method. Cultured cells were treated with ANG II (100 nmol/l) or ET-1 (10 pM) in the presence or absence of the ANG II type 1 receptor antagonist losartan (100 μM), the ET-1 receptor antagonists BQ-123 (ET(A) receptor, 1 μM) and BQ-788 (ET(B) receptor, 1 μM), and the ET(B) receptor agonist sarafotoxin 6C (100 nM). ET-1 peptide levels were determined by ELISA, whereas ET(A), ET(B), and collagen levels were determined by Western blot analysis. ANG II increased ET-1 peptide levels in a time-dependent manner. ANG II increased ET(A) and ET(B) receptor protein levels as well as collagen in a similar fashion. ANG II-induced collagen was reduced while in the presence of BQ-123, suggesting a role for the ET(A) receptor in the regulation of the extracellular matrix. ANG II treatment in the presence of BQ-788 significantly increased ET-1 peptide levels. Conversely, the ET(B) receptor agonist sarafotoxin 6C significantly decreased ET-1 peptide levels. These data implicate a role for the ET(B) receptor in the clearance of the ET-1 peptide. In conclusion, both ET(A) and ET(B) receptors are expressed in adventitial fibroblasts, which paves the ground for the biological significance of adventitial ET-1. The ET(A) receptor subtype mediates collagen I expression, whereas the ET(B) receptor subtype may play a protective role through increasing the clearance of the ET-1 peptide.

Tobacco-associated pulmonary vascular dysfunction in smokers: role of the ET-1 pathway

Pulmonary vascular remodeling and dysfunction associated to tobacco smoking might pave the way for the subsequent development of pulmonary hypertension. Its prognosis is dreadful and its underlying mechanisms are so far largely unknown in humans. To assess the potential role of endothelin-1 and its receptors in smokers' pulmonary artery vasoactive properties. Endothelium-dependent vasodilation to ACh was assessed in pulmonary vascular rings from 34 smokers and compared with that of 10 nonsmokers. The effects of ET-A (BQ 123) or ET-B (BQ 788) blockers and that of an ET-B activator (sarafotoxin) were evaluated. Endothelin-1 was quantitated by ELISA. Expression of its receptors was quantitated by Western blotting. Smokers exhibited an impaired pulmonary endothelium-dependent vasodilation compared with nonsmokers (P < 0.01). In the former group, 8 of 34 subjects exhibited a marked endothelial dysfunction (ED(+)) whereas 26 (ED(-)) (P < 10(-4)) displayed a vasorelaxation to ACh that was comparable to that of nonsmokers. In ED(+) subjects, ET-A was overexpressed (P < 0.05) and inversely correlated (P < 10(-2)) with the response to ACh. Sarafotoxin significantly improved vasodilation in all subjects (P < 10(-2)). In conclusion, tobacco smoking is associated to an impaired pulmonary vasorelaxation at least partly mediated by an ET-1/ET-A-dependent dysfunction.

Differential endothelin receptor expression and function in rat myometrial cells and leiomyoma ELT3 cells

Uterine leiomyoma are the most common benign tumors of the myometrium. We previously identified endothelin (ET)-1 as a proliferative and antiapoptotic factor in Eker rat-derived leiomyoma (ELT3) cells. A major role of ETB receptor in the prosurvival effect was revealed. Here we investigated, in ELT3 and myometrial cells, the respective contribution of ETA and ETB in the proliferative effect of ET-1. In myometrial cells, binding experiments show that ETA is almost exclusively expressed and stimulates phospholipase C (PLC) activity and ERK1/2 phosphorylation and proliferation. In ELT3 cells, ETB is expressed at about the same level as ETA, and the two receptors are differently coupled to Gi protein. The ETB agonist, sarafotoxin S6c, stimulates PLC activity 60% less than ET-1 but is as potent as ET-1 to increase ERK1/2 phosphorylation and induce proliferation. However, the ability of ETA to activate ERK1/2 is observed after ETB desensitization. Although ETA and ETB antagonists partially reduce ET-1 stimulated PLC activity, they are without effect on ET-1-induced ERK1/2 phosphorylation and proliferation. Only the simultaneous use of ETA and ETB antagonists reduces ET-1-triggered ERK1/2 activation. These unconventional properties of ETRs may reveal the existence of functional ETA-ETB heterodimers. Finally, treatment of ELT3 cells with ETB but not ETA-directed small interfering RNA reduces the proliferative effect of ET-1. All the data obtained in ELT3 cells strengthen the relation between ETB overexpression, which decreases the ETA to ETB ratio, and the ability of leiomyoma cells to highly proliferate and resist apoptosis.

Mice lacking the ADP ribosyl cyclase CD38 exhibit attenuated renal vasoconstriction to angiotensin II, endothelin-1, and norepinephrine

ADP ribosyl (ADPR) cyclases comprise a family of ectoenzymes recently shown to influence cytosolic Ca(2+) concentration in a variety of cell types. At least two ADPR cyclase family members have been identified in mammals: CD38 and CD157. We recently found reduced renal vascular reactivity to angiotensin II (ANG II), endothelin-1 (ET-1), and norepinephrine (NE) in the presence of the broad ADPR cyclase inhibitor nicotinamide. We hypothesized that CD38 mediates effects attributed to ADPR cyclase. We found expression of ADPR cyclases CD38 and CD157 mRNA in spleen, thymus, skin, and preglomerular arterioles of wild-type (WT) animals. Mice lacking CD38 showed decreased CD157 expression in most tissues tested. No difference in systolic or mean arterial pressure was observed between strains in either conscious or anesthetized states, whereas heart rate was reduced 10-20% in CD38-/- animals (P < 0.05). During anesthesia, CD38-/- mice had reduced basal renal blood flow (RBF) and urine excretion (P < 0.05). RBF responses to intravenous injection of ANG II, ET-1, and NE were attenuated approximately 50% in CD38-/- vs. WT mice (P < 0.01 for all). The systemic pressor response to ANG II was decreased in the absence of CD38 (P < 0.01), whereas that to NE was normal (P > 0.05); ET-1 was administered at a nonpressor dose. Nicotinamide effectively inhibited ANG II-induced renal vasoconstriction in WT mice (P < 0.001), but had no effect on renal responses to ANG II in CD38-/- mice (P > 0.5). Overall, our observations indicate the presence of two ADPR cyclase family members in renal preglomerular resistance arterioles and the importance of CD38 participation in acute vascular responses to all three vasoconstrictors in the renal microcirculation.

Functional and molecular characterization of the endothelin system in retinal arterioles

PURPOSE

Activation of the endothelin (ET) system has been implicated in the pathogenesis of retinal ischemic disease. Although ET-1, the predominant endogenous isoform of ET, has been shown to cause constriction of retinal vessels, the expression and functional significance of its synthesis and the involved specific ET receptors in retinal arterioles remain unknown. The authors examined the roles of ET(A) and ET(B) receptors and of endothelin-converting enzyme (ECE)-1 in ET-1-induced vasomotor responses of single retinal arterioles.

METHODS

To exclude systemic confounding effects, porcine retinal arterioles were isolated for vasoreactivity and molecular studies.

RESULTS

Isolated and pressurized retinal arterioles developed basal tone and constricted in a manner dependent on concentration to ET-1. ET-1 precursor big ET-1 elicited time-dependent vasoconstriction over 20 minutes, which was blocked by the ECE-1 inhibitor phosphoramidon. ET(A) receptor antagonist BQ123 inhibited most (approximately 90%) of vasoconstrictions to ET-1 and big ET-1. ET(B) receptor agonist sarafotoxin also elicited concentration-dependent constriction of retinal arterioles but with significantly less potency than ET-1. ET(B) receptor antagonist BQ788 abolished vasoconstriction to sarafotoxin but only slightly reduced responses to ET-1 and big ET-1. Protein and mRNA expressions of ET(A), ET(B), and ECE-1 were detected in retinal arterioles. Immunohistochemistry revealed ET(A) and ET(B) receptors predominantly in smooth muscle and ECE-1 predominantly in endothelium and smooth muscle.

CONCLUSIONS

ET-1 elicits constriction of retinal arterioles predominantly through the activation of smooth muscle ET(A) receptors. Endogenous production of ET-1 from vascular ECE-1 is sufficient to evoke ET(A) receptor-dependent constriction in retinal arterioles.

Paracrine regulation of the resumption of oocyte meiosis by endothelin-1

Mammalian oocytes remain dormant in the diplotene stage of prophase I until the resumption of meiosis characterized by germinal vesicle breakdown (GVBD) following the preovulatory gonadotropin stimulation. Based on genome-wide analysis of peri-ovulatory DNA microarray to identify paracrine hormone-receptor pairs, we found increases in ovarian transcripts for endothelin-1 and endothelin receptor type A (EDNRA) in response to the preovulatory luteinizing hormone (LH)/human chorionic gonadotropin (hCG) stimulation. Immunohistochemical analyses demonstrated localization of EDNRA in granulosa and cumulus cells. In cultured preovulatory follicles, treatment with endothelin-1 promoted oocyte GVBD. The stimulatory effect of endothelin-1 was blocked by cotreatment with antagonists for the type A, but not related type B, receptor. The stimulatory effect of hCG on GVBD was partially blocked by the same antagonist. The endothelin-1 promotion of GVBD was found to be mediated by the MAPK/ERK pathway but not by the inhibitory G protein. Studies using cumulus-oocyte complexes and denuded oocytes demonstrated that the endothelin-1 actions are mediated by cumulus cells. Furthermore, intrabursal administration with endothelin-1 induced oocyte GVBD in preovulatory follicles. Our findings demonstrate a paracrine role of endothelin-1 in the induction of the resumption of meiosis and provide further understanding on the molecular mechanisms underlying the nuclear maturation of oocytes induced by the preovulatory LH surge.

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).

Renal medullary ETB receptors produce diuresis and natriuresis via NOS1

Endothelin-1 (ET-1) plays an important role in the regulation of salt and water excretion in the kidney. Considerable in vitro evidence suggests that the renal medullary ET(B) receptor mediates ET-1-induced inhibition of electrolyte reabsorption by stimulating nitric oxide (NO) production. The present study was conducted to test the hypothesis that NO synthase 1 (NOS1) and protein kinase G (PKG) mediate the diuretic and natriuretic effects of ET(B) receptor stimulation in vivo. Infusion of the ET(B) receptor agonist sarafotoxin S6c (S6c: 0.45 microg x kg(-1) x h(-1)) in the renal medulla of anesthetized, male Sprague-Dawley rats markedly increased the urine flow (UV) and urinary sodium excretion (UNaV) by 67 and 120%, respectively. This was associated with an increase in medullary cGMP content but did not affect blood pressure. In addition, S6c-induced diuretic and natriuretic responses were absent in ET(B) receptor-deficient rats. Coinfusion of N(G)-propyl-l-arginine (10 microg x kg(-1) x h(-1)), a selective NOS1 inhibitor, suppressed S6c-induced increases in UV, UNaV, and medullary cGMP concentrations. Rp-8-Br-PET-cGMPS (10 microg x kg(-1) x h(-1)) or RQIKIWFQNRRMKWKK-LRK(5)H-amide (18 microg x kg(-1) x h(-1)), a PKG inhibitor, also inhibited S6c-induced increases in UV and UNaV. These results demonstrate that renal medullary ET(B) receptor activation induces diuretic and natriuretic responses through a NOS1, cGMP, and PKG pathway.

Chronic Activation of Endothelin B Receptors

Endothelin (ET) exerts powerful pressor actions primarily through activation of the ET(A) receptor subtype. The ET(B) receptor (ET(B)R) subtype, on the other hand, is generally thought to initiate physiological actions that decrease arterial pressure. Such actions include clearing ET from the bloodstream, initiating endothelium-mediated vasodilation, and facilitating renal sodium and water excretion. The effect of long-term activation of the ET(B)R on arterial pressure, however, never has been directly tested. In this study we evaluated cardiovascular responses to chronic (5-day) activation of ET(B)R in male rats using continuous intravenous infusion of the selective agonist sarafotoxin 6c. Surprisingly, we found that sarafotoxin 6c caused a sustained increase in arterial pressure that rapidly reversed on termination of infusion. The hypertension was associated with increased renal excretion of sodium and water and decreased plasma volume. Alterations in daily sodium intake did not affect the magnitude of the hypertension. Hemodynamic studies revealed a decreased cardiac output and increased total peripheral resistance during sarafotoxin 6c infusion. Infusion of sarafotoxin 6c caused a small increase in plasma ET levels. Nevertheless, the hypertension was not affected by coadministration of a selective ET(A) receptor antagonist (atrasentan) but was completely prevented by treatment with a combined ET(A) receptor and ET(B)R antagonist (A186280). These experiments reveal for the first time that chronic activation of ET(B)R in rats causes sustained hypertension.

Expression of endothelin receptors in frog, chicken, mouse and human pigment cells

Several reports have shown the participation of vasoactive endothelins (ETs) in the regulation of vertebrate pigment cells. In the present study, we identified ET receptors in pigment cells of vertebrate species by RT-PCR assays, and compared the differential expression of the various subtypes in each species by quantitative PCR. RT-PCR was performed with specific primers for ETC, ETA(X) or ETA in Xenopus laevis melanophores, ETA or ETB(2) in chicken melanocytes, ETA or ETB in murine (B-16, S-91 or Melan-A) or human (SK-Mel 23 or SK-Mel 28) melanoma cells, and the products obtained were confirmed by cloning and sequencing. The results showed the presence of ETA(X), but not ETA mRNA, and confirmed the expression of ETC in X. laevis melanophores. ETA and ETB(2) mRNAs were also demonstrated in chicken melanocytes. ETA and ETB receptor were identified in S-91, B16 and Melan-A murine cells. In human melanoma cells, SK-Mel 23 and SK-Mel 28, we confirmed the presence of ETB mRNA, and also found ETA mRNA. The comparison between the two subtypes present in the pigment cell of each species and among species demonstrated that the expression of ETAs in chicken, mouse, and human melanocytes is negligible, as is the expression of ETA(X) in Xenopus melanophores. The relative expression, as determined by quantitative PCR, was as follows: chicken ETB>SK-Mel 23 ETB>S91 ETB>>>Xenopus ETC, suggesting that the endothelin system plays a major role in avian and mammalian pigment cell regulation, as compared to lower vertebrates. The phylogenetic analysis revealed that subtype A receptors were probably the most primitive ET receptors, directly deriving from the ancestral type; all the other receptors, B subtypes and C, originated from diverse derivative molecules.

Endothelin

In humans, the endothelins (ETs) comprise a family of three 21-amino-acid peptides, ET-1, ET-2 and ET-3. ET-1 is synthesised from a biologically inactive precursor, Big ET-1, by an unusual hydrolysis of the Trp21 -Val22 bond by the endothelin converting enzyme (ECE-1). In humans, there are four isoforms (ECE-1a-d) derived from a single gene by the action of alternative promoters. Structurally, they differ only in the amino acid sequence of the extreme N-terminus. A second enzyme, ECE-2, also exists as four isoforms and differs from ECE-1 in requiring an acidic pH for optimal activity. Human chymase can also cleave Big ET-1 to ET-1, which is cleaved, in turn, to the mature peptide as an alternative pathway. ET-1 is the principal isoform in the human cardiovascular system and remains one of the most potent constrictors of human vessels discovered. ET-1 is unusual in being released from a dual secretory pathway. The peptide is continuously released from vascular endothelial cells by the constitutive pathway, producing intense constriction of the underlying smooth muscle and contributing to the maintenance of endogenous vascular tone. ET-1 is also released from endothelial cell-specific storage granules (Weibel-Palade bodies) in response to external stimuli. ETs mediate their action by activating two G protein-coupled receptor sub-types, ETA and ET(B). Two therapeutic strategies have emerged to oppose the actions of ET-1, namely inhibition of the synthetic enzyme by combined ECE/neutral endopeptidase inhibitors such as SLV306, and receptor antagonists such as bosentan. The ET system is up-regulated in atherosclerosis, and ET antagonists may be of benefit in reducing blood pressure in essential hypertension. Bosentan, the first ET antagonist approved for clinical use, represents a significant new therapeutic strategy in the treatment of pulmonary arterial hypertension (PAH).

Chromatic effects of endothelin family peptides in non-innervated fish,Synbranchus marmoratus, melanophores

The biological activity of endothelins (ETs) in non-innervated Synbranchus marmoratus melanophores was demonstrated. These peptides induced a dose-dependent pigment aggregation (lightening skin) in these cells. However, they presented EC50's (effective concentration required to produce 50% of response) 26, 106 and 35 times higher than, respectively, the melanin concentrating hormone (MCH) EC50, and exhibited a characteristic temporal and dose-dependent autodessensibilization of the aggregative effect on the melanophores of this fish. The receptor characterization suggested the presence of the ET(B) subtype, since BQ-788 (selective antagonist of ET(B)) but not BQ-485 (selective antagonist of ET(A)) blocked the aggregative effect of the hormones. Confirming these data, sarafotoxin (SRTX) S6c, a toxin selective for ET(B), induced maximal aggregation of pigment granules. S6c presented an EC50 6.8 times higher than the MCH EC50, and 3.9, 15.6 and 5.1 times lower than the EC50's ETs, respectively. The melanotropic effect of SRTX S6b and vasoactive intestinal contractor (VIC) were demonstrated for the first time in this work. SRTX S6b induced a dose-dependent pigment aggregation and presented an EC50 2.54 and 17.2 times higher than the S6c and MCH EC50's, respectively. Compared to the ETs it was 1.53, 6.19 and 2.03 times lower, respectively.

Effect of YM598, a selective endothelin ETA receptor antagonist, on endothelin-1-induced bone formation

We investigated the effect of endothelin-1 on bone formation in vitro and in vivo, and the effect of YM598, a novel selective endothelin ET(A) receptor antagonist, on endothelin-1-induced responses. In in vitro studies, the effect of endothelin-1 on cellular responses was investigated by measuring intracellular Ca(2+) levels, cell growth and alkaline phosphatase activity in the mouse osteoblast-like cell line MC3T3-E1. In in vivo studies, the effect of endothelin-1 on bone morphogenetic protein-2-induced ectopic bone formation in rats was investigated. A carrier containing bone morphogenetic protein-2 with or without endothelin-1 was subcutaneously implanted over the thorax, and the tissue (carrier) calcium content 3 weeks after implantation was evaluated. The inhibitory effect of YM598 on these responses was also investigated. In the in vitro studies, endothelin-1 (10(-13) to 10(-6) M) significantly increased intracellular Ca(2+) concentration, DNA synthesis and cell number in a concentration-dependent manner, while significantly decreasing alkaline phosphatase activity. YM598 (10(-12) to 10(-4) M) significantly inhibited these increases, as well as the decrease in alkaline phosphatase activity, in a concentration-dependent manner. In the in vivo studies, the tissue calcium content 3 weeks after carrier implantation was significantly higher in the group that received both bone morphogenetic protein-2 and endothelin-1 than in the group receiving bone morphogenetic protein-2 alone. Chronically administered YM598 (1 mg/kg/day) marginally inhibited this endothelin-1-potentiated ectopic bone formation. These results suggest that endothelin-1 may induce bone formation via endothelin ET(A) receptors in vitro and in vivo.

Endothelin receptor antagonists

Endothelin receptor antagonists (ERAs) have been developed to block the effects of endothelin-1 (ET-1) in a variety of cardiovascular conditions. ET-1 is a powerful vasoconstrictor with mitogenic or co-mitogenic properties, which acts through the stimulation of 2 subtypes of receptors [endothelin receptor subtype A (ETA) and endothelin receptor subtype B (ETB) receptors]. Endogenous ET-1 is involved in a variety of conditions including systemic and pulmonary hypertension (PH), congestive heart failure (CHF), vascular remodeling (restenosis, atherosclerosis), renal failure, cancer, and cerebrovascular disease. The first dual ETA/ETB receptor blocker, bosentan, has already been approved by the Food and Drug Administration for the treatment of pulmonary arterial hypertension (PAH). Trials of endothelin receptor antagonists in heart failure have been completed with mixed results so far. Studies are ongoing on the effects of selective ETA antagonists or dual ETA/ETB antagonists in lung fibrosis, cancer, and subarachnoid hemorrhage. While non-peptidic ET-1 receptor antagonists suitable for oral intake with excellent bioavailability have become available, proven efficacy is limited to pulmonary hypertension, but it is possible that these agents might find a place in the treatment of several cardiovascular and non-cardiovascular diseases in the coming future.

Neurohormonal activation in severe scorpion envenomation: correlation with hemodynamics and circulating toxin

We studied the effects of scorpion (Androctonus australis hector) venom on hemodynamics and on the release of catecholamines, neuropeptide Y (NPY), endothelin-1 (ET-1) and atrial natriuretic peptide (ANP) in dog model of severe scorpion envenomation. Nine mongrel anesthetized dogs were submitted to mechanical ventilation through intubation and were administered intravenously purified dried scorpion venom (Androctonus autstralis) 0.05 mg/kg. Measurements including pulmonary artery catheter derived parameters, serum toxin levels and humoral variables were performed at baseline (before venom injection) and 5, 15, 30 and 60 min after venom injection. Humoral variables included: serum lactate, epinephrine (EP), norepinephrine (NE), NPY, ET-1 and ANP plasma concentrations. Scorpion venom caused rapid and transient increase of mean arterial pressure (MAP) and PAOP associated with a marked and sustained decline in cardiac output (-55% at 60 min; P < 0.001). Hemodynamic changes were associated with a rapid and significant increase of all measured hormones. The highest increase was for NE (28-fold) and EP (25-fold). MAP was closely correlated with NE and less significantly correlated with toxin levels. Similarly, significant correlation was observed between PAPO and ANP plasma levels. These findings support the implication of excessive catecholamines release in hemodynamic disturbances of severe SE and suggest that NPY and ET-1 could be involved in this process. Serum toxin does not appear to consistently contribute to these effects. Through its correlation with PAOP, ANP could be a reliable and useful marker of cardiac dysfunction in SE.

Positive inotropic and negative lusitropic effects of endothelin receptor agonism in vivo

The endothelin (ET) system is involved in the regulation of myocardial function in health as well as in several diseases, such as congestive heart failure, myocardial infarction, and septic myocardial depression. Conflicting results have been reported regarding the acute contractile properties of ET-1. We therefore investigated the effects of intracoronary infusions of ET-1 and of the selective ETBreceptor-selective agonist sarafotoxin 6c with increasing doses in anesthetized pigs. Myocardial effects were measured through analysis of the left ventricular pressure-volume relationship. ET-1 elicited increases in the myocardial contractile status (end-systolic elastance value of 0.94 ± 0.11 to 1.48 ± 0.23 and preload recruitable stroke work value of 68.7 ± 4.7 to 83.4 ± 7.2) that appear to be mediated through ETAreceptors, whereas impairment in left ventricular isovolumic relaxation (τ = 41.5 ± 1.4 to 58.1 ± 5.0 and t1/2= 23.0 ± 0.7 to 30.9 ± 2.6, where τ is the time constant for pressure decay and t1/2is the half-time for pressure decay) was ETBreceptor dependent. In addition, intravenous administration of ET-1 impaired ventricular relaxation but had no effect on contractility. Intracoronary sarafotoxin 6c administration caused impairments in left ventricular relaxation (τ from 43.3 ± 1.8 to 54.4 ± 3.4) as well as coronary vasoconstriction. In conclusion, ET-1 elicits positive inotropic and negative lusitropic myocardial effects in a pig model, possibly resulting from ETAand ETBreceptor activation, respectively.

Effects of selective endothelin ET(A) receptor antagonists on endothelin-1-induced potentiation of cancer pain

In some diseases in which endothelin-1 production increases, e.g. prostate cancer, endothelin-1 is considered to be involved in the generation of pain. In the present study, we investigated the effects of a selective endothelin ET(A) receptor antagonist, (E)-N-[6-methoxy-5-(2-methoxyphenoxy)[2,2'-bipyrimidin]-4-yl]-2-phenylethenesulfonamide monopotassium salt (YM598), on the nociception potentiated by endothelin-1 in a cancer inoculation-induced pain model in mice, induced by inoculation of the androgen-independent human prostate cancer cell line PPC-1 into the hind paws of severe combined immunodeficiency (SCID) mice. No pain responses were observed in the sham-operated mice, whereas monophasic pain responses were observed in the PPC-1-inoculated mice. Endothelin-1 (1 to 10 pmol/paw) but not sarafotoxin S6c potentiated the pain response in prostate cancer-inoculated mice. Both YM598 and atrasentan (0.3 to 3 mg/kg, p.o.) significantly inhibited the endothelin-1 (10 pmol/paw)-induced potentiation of nociception in a dose-dependent manner. These results suggest that selective endothelin ET(A) receptor antagonists might relieve pain in patients with various diseases in which endothelin-1 production is increased, e.g. prostate cancer.

Interleukin-1beta attenuates endothelin B receptor-mediated airway contractions in a murine in vitro model of asthma: roles of endothelin converting enzyme and mitogen-activated protein kinase pathways

BACKGROUND

Asthma is a chronic airway disease, known to involve several inflammatory mediators. Little is known about how these mediators interact in order to produce or attenuate even basic features of the disease, like airway hyper-reactivity and remodelling. Endothelin-1 (ET-1) and IL-1beta are two mediators suggested to play important roles in the induction of airway inflammation.

OBJECTIVE

To investigate the interactions between ET-1 and IL-1beta, using a novel in vitro model of asthma, focusing on airway smooth muscle contractility.

METHODS

Isolated murine tracheal segments were cultured from 1 to 8 days in the absence and presence of IL-1beta. The subsequent contractile responses to sarafotoxin 6c (S6c) (selective agonist for ETB receptor) and sarafotoxin 6b (S6b) (ETA and ETB receptor agonist) were recorded by a myographs system. In all experiments, ETB receptors were desensitized before the contractile response to S6b was recorded. Thus, the response to S6b is only mediated by ETA receptors in the present study. The mRNA expressions for ET-1 and endothelin (ET) receptors were quantified by real-time PCR.

RESULTS

Organ culture in the presence of IL-1beta attenuated the maximal contraction induced by S6c, but not S6b. This reduction was concentration-dependent and was significant after 2, 4 and 8 days of culture. To investigate the mechanisms behind this, inhibitors for endothelin converting enzyme (ECE) phosphoramidon, c-JUN N-terminal kinase (JNK) SP600125, extracellular-signal-regulated kinase 1/2(ERK 1/2) PD98059 and p38 pathway SB203580 were used. Individually, SP600125 and PD98059, but not SB203580, could partly reverse the reduction induced by IL-1beta. An additional effect was obtained when SP600125 and PD98059 were combined. The mRNA expressions for ET-1 and ETB receptor were up- and down-regulated, respectively, by IL-1beta.

CONCLUSION

Presence of IL-1beta in the airways attenuate the contractile response mediated via ETB receptors, an effect dependent on ECE, JNK and ERK 1/2 pathways.

The Endothelin/Sarafotoxin-Induced Increase of the Proliferation of Undifferentiated and DMSO-Differentiated GEM-81 Goldfish Erythrophoroma Cells is Mediated by ETB Receptors

Endothelins (ETs) and sarafotoxins (SRTXs) have been reported to exert ET(B)-mediated effects on vertebrate pigment cells. GEM-81 cell line, a red pigment cell-derived cutaneous tumor of the teleost Carassius auratus, expresses ET(B) receptors and can be differentiated with 1.5% DMSO treatment, thus constituting an useful model to investigate ET and SRTX effects on cultured fish pigment cells. Our aim was to characterize the pharmacology and biological effects mediated by ET receptors in DMSO-differentiated and undifferentiated cells. ET subtype receptors and their respective Ki values in both cell types were determined by competitive binding assays using (125)I ET-1 and BQ-485 (an ET(A) antagonist) or BQ-788 (an ET(B) antagonist). BQ-788, but not BQ-485, significantly reduced (125)I-ET-1 binding in both cell types, with similar low (Ki > nM) affinities. To determine the proliferation effects of ETs/SRTXs, cells were treated for 72 h with the hormones, and counted in a hemocytometer. The proliferation assays were repeated for SRTX S6c in the presence or absence of BQ-788. The results demonstrated that, with the exception of ET-1 (biphasic effect) and ET-3 (no significant effect) in undifferentiated GEM-81 cells, all the tested hormones induced increases in the proliferation of both types of cells. The hormones were equipotent in DMSO-differentiated cells, which exhibited increased sensitivity to ETs, but not to SRTXs, as compared with undifferentiated cells. The BQ-788 antagonistic effect was also exerted on the proliferation responses to SRTX S6c. These results corroborate the long and important evolutionary history of the ET/SRTX receptor system in vertebrate pigment cells.

Developmental expression of endothelin receptors in postnatal swine mesenteric artery

Studies were conducted to determine whether endothelin (ET) ETA and ETB receptor protein and mRNA expression is developmentally regulated in the postnatal swine mesenteric circulation. To this end, Western blotting and real-time reverse PCR were performed on protein and total RNA isolated from the mesenteric artery harvested from 3-, 10-, and 30-d-old swine. Western blot analysis revealed that ETA and ETB receptor protein expression in the swine mesenteric artery decreased over the age range studied; thus, ETA and ETB receptor protein expression was significantly greater in the 3-d-old group then progressively declined over the first postnatal month. Similar to the Western data, real-time PCR analysis revealed that ETA and ETB receptor mRNA expression also decreased over the age range studied; thus, ETA and ETB receptor mRNA expression was significantly greater in the 3-d-old group then progressively declined over the first postnatal month. Immunohistochemistry localized the ETA receptor to the vascular smooth muscle and the ETB receptor to the endothelial cell layer. Additionally, we report a partial cDNA sequence for the swine ETB receptor. We conclude that ETA and ETB receptor protein and mRNA expression is developmentally regulated in the postnatal swine mesenteric artery, being expressed to a greater degree in younger animals.

Inotropic effects of ETB receptor stimulation and their modulation by endocardial endothelium, NO, and prostaglandins

Endothelin (ET)-1 acts on ETA and ETB receptors. The latter include ETB1 (endothelial) and ETB2 (muscular) subtypes, which mediate opposite effects on vascular tone. This study investigated, in rabbit papillary muscles ( n = 84), the myocardial effects of ETB stimulation. ET-1 (10−9 M) was given in the absence or presence of BQ-123 (ETA antagonist). The effects of IRL-1620 (ETB1 agonist, 10−10–10−6 M) or sarafotoxin S6c (ETB agonist, 10−10–10−6 M) were evaluated in muscles with intact or damaged endocardial endothelium (EE); intact EE, in the presence of NG-nitro-l-arginine (l-NNA); and intact EE, in the presence of indomethacin (Indo). Sarafotoxin S6c effects were also studied in the presence of BQ-788 (ETB2 antagonist). ET-1 alone increased 64 ± 18% active tension (AT) but decreased it by 4 ± 2% in the presence of BQ-123. In muscles with intact EE, sarafotoxin S6c alone did not significantly alter myocardial performance. Sarafotoxin S6c (10−6 M) increased, however, AT by 120 ± 27% when EE was damaged and by 39 ± 8% or 23 ± 6% in the presence of l-NNA or Indo, respectively. In the presence of BQ-788, sarafotoxin S6c decreased AT (21 ± 3% at 10−6 M) in muscles with intact EE, an effect that was abolished when EE was damaged. IRL-1620 also decreased AT (22 ± 3% at 10−6 M) in muscles with intact EE, an effect that was abolished when EE was damaged or in the presence of l-NNA or Indo. In conclusion, the ETB-mediated negative inotropic effect is presumably due to ETB1 stimulation, requires an intact EE, and is mediated by NO and prostaglandins, whereas the ETB-mediated positive inotropic effect, observed when EE was damaged or NO and prostaglandins synthesis inhibited, is presumably due to ETB2 stimulation.

Pharmacological characterization of YM598, an orally active and highly potent selective endothelin ET(A) receptor antagonist

We describe here the pharmacology of (E)-N-[6-methoxy-5-(2-methoxyphenoxy)[2,2'-bipyrimidin]-4-yl]-2-phenylethenesulfonamide monopotassium salt (YM598), a novel selective endothelin ET(A) receptor antagonist synthesized through the modification of the ET(A)/ET(B) non-selective antagonist, bosentan. YM598 inhibited [125I]endothelin-1 binding to cloned human endothelin ET(A) and ET(B) receptor, with K(i) of 0.697 and 569 nM, and inhibited endothelin-1-induced increases in intracellular Ca(2+) concentration in human and rat endothelin ET(A) receptor. YM598 also inhibited endothelin-1-induced vasoconstriction in isolated rat aorta with a pA(2) value of 7.6. In vivo, YM598 inhibited the pressor response to big endothelin-1, a precursor peptide of endothelin-1. DR(2) values of YM598 in pithed rats were 0.53 mg/kg, i.v. and 0.77 mg/kg, p.o., and its antagonism in conscious rats was maintained for more than 6.5 h at 1 mg/kg, p.o. In contrast, YM598 had no effect on the sarafotoxin S6c-induced depressor or pressor responses. YM598 showed not only superior antagonistic activity and higher-selectivity for endothelin ET(A) receptor in vitro, but at least a 30-fold higher potency in vivo than bosentan. In conclusion, YM598 is a potent and orally active selective endothelin ET(A) receptor antagonist.

Up-Regulation of Endothelin Receptor Function and mRNA Expression in Airway Smooth Muscle Cells Following Sephadex-Induced Airway Inflammation

The hypothesis that up-regulation of bronchial constrictor endothelin receptors in airway smooth muscle cells may contribute to hyperreactivity during airway inflammation was tested in the present study by quantitative endothelin receptor mRNA analysis and functional responses in ring segments of rat trachea and bronchi. Real time reverse transcription polymerase chain reaction was used to quantify endothelin receptor expression in rat airway smooth muscle cells following Sephadex-induced inflammation. Compared with controls, Sephadex-induced airway inflammation caused a significant increase (3.9 times P<0.05) of endothelin receptor type B mRNA expression in bronchial smooth muscle cells, but not in tracheal smooth muscle cells. Functional myograph studies of bronchial and tracheal ring segments without epithelium (mechanically denuded) revealed an increase of the maximum contractile effects of endothelin-1 (a dual agonist for both endothelin type A and B receptors) and sarafotoxin 6c (a selective agonist for endothelin B receptors) in bronchial smooth muscle cells in Sephadex-induced inflammation, but not in tracheal smooth muscle cells. The enhanced maximal responses of bronchial smooth muscle cells to endothelin-1 and sarafotoxin 6c in Sephadex-induced inflammation support our molecular findings and hence imply a role for endothelin B receptors in airway hyperreactivity during airway inflammation.

Cellular distribution of the endothelin system in the human brain

The vasoconstrictor endothelin-1 (ET-1) may also act as a neuropeptide. ET-1 is formed by the catalytic action of endothelin-converting enzyme-1 (ECE-1) on big ET-1 and its cellular actions are mediated via ET(A) and ET(B) receptors. Although localisation of these components in rodent brain has been extensively investigated, no single study has mapped their distribution in human brain. Here we describe the localisation of ET-1 mRNA, ET-1, ECE-1, ET(A) and ET(B) receptors within 24 human brain regions. In situ RT-PCR has previously detected ET-1 mRNA in 22 areas (excluding the post-central gyrus and pineal gland), and ET-1 immunoreactivity was visualised in cells of all regions. Using specific antibodies we have immunolocalised ECE-1 and ET(B) receptors in cells of 24 areas, and ET(A) receptors in nine regions (choroidal epithelial cells, neurones in the diencephalon, hippocampus, amygdaloid, dentate nucleus, Purkinje cells of the cerebellum, flocculo-nodular lobe and vermis). ET-1 mRNA, ET-1, ECE-1 and ET(B) receptors were observed in cortical pyramidal cells, neurones (brainstem, basal nuclei, thalamus, insula and claustrum, limbic region), cells in the anterior pituitary gland; nerve cell processes in the pars nervosa; pinealocytes and choroidal epithelial cells. Only ET-1 mRNA, ET-1, ECE-1, and ET(B) receptors were visualised in cerebral capillary endothelial cells. The presence of ET-1 mRNA, ECE-1 and ET-1 in 22 brain regions confirms ET expression and processing in human brain. The localisation of ET-1 and ET(B) receptors suggests receptor-mediated action akin to a neurotransmitter role for ET-1.

Both endothelin-A and endothelin-B receptors are present on adult rat cardiac ventricular myocytes

Endothelin-A (ET(A)) and endothelin-B (ET(B)) receptors have been demonstrated in intact heart and cardiac membranes. ET(A) receptors have been demonstrated on adult ventricular myocytes. The aim of the present study was to determine the presence of ET(B) and the relative contribution of this receptor subtype to total endothelin-1 (ET-1) binding on adult ventricular myocytes. Saturation binding experiments indicated that ET-1 bound to a single population of receptors (Kd = 0.52 +/- 0.13 nM, n = 4) with an apparent maximum binding (Bmax) of 2.10 +/- 0.25 sites (x 10(5))/cell (n = 4). Competition experiments using 40 pM [125I]ET-1 and nonradioactive ET-1 revealed a Ki of 660 +/- 71 pM (n = 10) and a Hill coefficient (nH) of 0.99 +/- 0.10 (n = 10). A selective ET(A) antagonist, BQ610, displaced 80% of the bound [125I]ET-1. No displacement was observed by concentrations of an ET(B)-selective antagonist, BQ788, up to 1.0 microM. However, in the presence of 1.0 microM BQ610, BQ788 inhibited the remaining [125I]ET-1 binding. Similarly, in the presence of 1.0 microM BQ788, BQ610 inhibited the remaining specific [125I]ET-1 binding. Binding of an ET(B1)-selective agonist, [125I]IRL-1620, confirmed the presence of ET(B). ET(B) bound to ET-1 irreversibly, whereas binding to ET(A) demonstrated both reversible and irreversible components, and BQ610 and BQ788 bound reversibly. Reducing the incubation temperature to 0 degrees C did not alter the irreversible component of ET-1 binding. Hence, both ET(A) and ET(B) receptors are present on intact adult rat ventricular myocytes, and the ratio of ET(A):ET(B) binding sites is 4:1. Both receptor subtypes bind to ET-1 by a two-step association involving the formation of a tight receptor-ligand complex; however, the kinetics of ET-1 binding to ET(A) versus ET(B) differ.