Endothelin receptors in human coronary artery and aorta

Human microvascular reactivity: a review of vasomodulating stimuli and non-invasive imaging assessment

The microvasculature serves an imperative function in regulating perfusion and nutrient exchange throughout the body, adaptively altering blood flow to preserve hemodynamic and metabolic homeostasis. Its normal functioning is vital to tissue health, whereas its dysfunction is present in many chronic conditions, including diabetes, heart disease, and cognitive decline. As microvascular dysfunction often appears early in disease progression, its detection can offer early diagnostic information. To detect microvascular dysfunction, one uses imaging to probe the microvasculature's ability to react to a stimulus, also known as microvascular reactivity (MVR). An assessment of MVR requires an integrated understanding of vascular physiology, techniques for stimulating reactivity, and available imaging methods to capture the dynamic response. Practical considerations, including compatibility between the selected stimulus and imaging approach, likewise require attention. In this review, we provide a comprehensive foundation necessary for informed imaging of MVR, with a particular focus on the challenging endeavor of assessing microvascular function in deep tissues.

Metabolism and Disposition of a Novel Selective α7 Neuronal Acetylcholine Receptor Agonist ABT-126 in Humans: Characterization of the Major Roles for Flavin-Containing Monooxygenases and UDP-Glucuronosyl Transferase 1A4 and 2B10 in Catalysis

Mass balance, metabolism, and excretion of ABT-126, an α7 neuronal acetylcholine receptor agonist, were characterized in healthy male subjects (n = 4) after a single 100-mg (100 μCi) oral dose. The total recovery of the administered radioactivity was 94.0% (±2.09%), with 81.5% (±10.2%) in urine and 12.4% (±9.3%) in feces. Metabolite profiling indicated that ABT-126 had been extensively metabolized, with 6.6% of the dose remaining as unchanged parent drug in urine. Parent drug accounted for 12.2% of the administered radioactivity in feces. The primary metabolic transformations of ABT-126 involved aza-adamantane N-oxidation (M1, 50.3% in urine) and aza-adamantane N-glucuronidation (M11, 19.9% in urine). M1 and M11 were also major circulating metabolites, accounting for 32.6% and 36.6% of the drug-related material in plasma, respectively. These results demonstrated that ABT-126 is eliminated primarily by hepatic metabolism, followed by urinary excretion. Enzymatic studies suggested that M1 formation is mediated primarily by human liver flavin-containing monooxygenase (FMO)3 and, to a lesser extent, by human kidney FMO1; M11 is generated mainly by human uridine 5'-diphospho-glucuronosyltransferase (UGT) 1A4, whereas UGT 2B10 also contributes to ABT-126 glucuronidation. Species-dependent formation of M11 was observed in hepatocytes; M11 was formed in human and monkey hepatocytes, but not in rat and dog hepatocytes, suggesting that monkeys constitute an appropriate model for predicting the fate of compounds undergoing significant N-glucuronidation. M1 and M11 are not expected to have clinically relevant on- or off-target pharmacologic activities. In summary, this study characterized ABT-126 metabolites in the circulation and excreta and the primary elimination pathways of ABT-126 in humans.

SCF-KIT signaling induces endothelin-3 synthesis and secretion: Thereby activates and regulates endothelin-B-receptor for generating temporally- and spatially-precise nitric oxide to modulate SCF- and or KIT-expressing cell functions

We demonstrate that SCF-KIT signaling induces synthesis and secretion of endothelin-3 (ET3) in human umbilical vein endothelial cells and melanoma cells in vitro, gastrointestinal stromal tumors, human sun-exposed skin, and myenteric plexus of human colon post-fasting in vivo. This is the first report of a physiological mechanism of ET3 induction. Integrating our finding with supporting data from literature leads us to discover a previously unreported pathway of nitric oxide (NO) generation derived from physiological endothelial NO synthase (eNOS) or neuronal NOS (nNOS) activation (referred to as the KIT-ET3-NO pathway). It involves: (1) SCF-expressing cells communicate with neighboring KIT-expressing cells directly or indirectly (cleaved soluble SCF). (2) SCF-KIT signaling induces timely local ET3 synthesis and secretion. (3) ET3 binds to ETBR on both sides of intercellular space. (4) ET3-binding-initiated-ETBR activation increases cytosolic Ca²⁺, activates cell-specific eNOS or nNOS. (5) Temporally- and spatially-precise NO generation. NO diffuses into neighboring cells, thus acts in both SCF- and KIT-expressing cells. (6) NO modulates diverse cell-specific functions by NO/cGMP pathway, controlling transcriptional factors, or other mechanisms. We demonstrate the critical physiological role of the KIT-ET3-NO pathway in fulfilling high demand (exceeding basal level) of endothelium-dependent NO generation for coping with atherosclerosis, pregnancy, and aging. The KIT-ET3-NO pathway most likely also play critical roles in other cell functions that involve dual requirement of SCF-KIT signaling and NO. New strategies (e.g. enhancing the KIT-ET3-NO pathway) to harness the benefit of endogenous eNOS and nNOS activation and precise NO generation for correcting pathophysiology and restoring functions warrant investigation.

Granulosa cell endothelin-2 expression is fundamental for ovulatory follicle rupture

Ovulation is dependent upon numerous factors mediating follicular growth, vascularization, and ultimately oocyte release via follicle rupture. Endothelin-2 (EDN2) is a potent vasoconstrictor that is transiently produced prior to follicle rupture by granulosa cells of periovulatory follicles and induces ovarian contraction. To determine the role of Edn2 expression, surgical transplant and novel conditional knockout mice were super-ovulated and analyzed. Conditional knockout mice utilized a new iCre driven by the Esr2 promoter to selectively remove Edn2. Follicle rupture and fertility were significantly impaired in the absence of ovarian Edn2 expression. When ovaries of Edn2KO mice were transplanted in wild type recipients, significantly more corpora lutea containing un-ovulated oocytes were present after hormonal stimulation (1.0 vs. 5.4, p = 0.010). Following selective ablation of Edn2 in granulosa cells, Esr2-Edn2KO dams had reduced oocytes ovulated (3.8 vs. 16.4 oocytes/ovary) and smaller litters (4.29 ± l.02 vs. 8.50 pups/dam). However, the number of pregnancies per pairing was not different and the reproductive axis remained intact. Esr2-Edn2KO ovaries had a higher percentage of antral follicles and fewer corpora lutea; follicles progressed to the antral stage but many were unable to rupture. Conditional loss of endothelin receptor A in granulosa cells also decreased ovulation but did not affect fecundity. These data demonstrate that EDN2-induced intraovarian contraction is a critical trigger of normal ovulation and subsequent fecundity.

Up-regulation of the Endothelin Receptor A in Placental Tissue From First Trimester Delayed Miscarriages

OBJECTIVE

This study tested the hypothesis that the endothelin (ET)/ET receptor (ETR) system in biologic fluids and in the human placenta is altered in delayed miscarriages as compared to apparently normal early pregnancies (reference group).

METHODS

Immunoreactive ET (irET) concentrations were measured in plasma, urine, and cervical smears from 57 pregnant women in the weeks 6 to 14 of gestation (46 delayed miscarriages, 11 references) with radioimmunoassay (RIA). ET-1, ETR-A, and ETR-B mRNA, and ETR protein expression were measured in placental tissue of 45 early pregnancies (31 delayed miscarriages, 14 references) using semiquantitative reverse-transcription polymerase chain reaction (RT-PCR) and immunoblotting, respectively.

RESULTS

irET levels in plasma, urine, and cervical smears did not differ between groups. Two prevailing ETR-A and ETR-B proteins were found at 45 and 55 kd, and were distributed similarly in delayed miscarriages and references. ETR-A protein and mRNA levels were 54% (P = .009) and threefold (P = .021) higher, respectively, in delayed miscarriages versus references. There was no difference in placental ETR-B and ET-1 mRNA levels between groups.

CONCLUSION

Neither irET nor ET-1 mRNA levels differ between delayed miscarriages and normal early pregnancies. Pregnancies at risk for miscarriage cannot be identified by measurement of ET in plasma, urine, or cervical smears. Within the ET/ETR system, ETR-A is selectively up-regulated in placental tissue of delayed miscarriages as compared to normal pregnancies. ETR protein processing is similar in both groups.

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.

Evidence for biased agonists and antagonists at the endothelin receptors

Biased ligands represent a new strategy for the development of more effective and better tolerated drugs. To date there has been a paucity of research exploring the potential of ligands that exhibit either G protein or β-arrestin pathway selectivity at the endothelin receptors. Re-analysis of data may allow researchers to determine whether there is existing evidence that the endogenous ET peptides or currently available agonists and antagonists exhibit pathway bias in a particular physiological or disease setting and this is explored in the review. An alternative to molecules that bind at the orthosteric site of the ET receptors are cell penetrating peptides that interact with a segment of an intracellular loop of the receptor to modify signalling behaviour. One such peptide IC2B has been shown to have efficacy in a model of pulmonary arterial hypertension. Finally, understanding the molecular pathways that contribute to disease is critical to determining whether biased ligands will provide clinical benefit. The role of ET_A signalling in ovarian cancer has been delineated in some detail and this has led to the suggestion that the development of ET_A G protein biased agonists or β-arrestin biased antagonists should be explored.

Cell-free synthesis of isotopically labelled peptide ligands for the functional characterization of G protein-coupled receptors

Cell‐free systems exploit the transcription and translation machinery of cells from different origins to produce proteins in a defined chemical environment. Due to its open nature, cell‐free protein production is a versatile tool to introduce specific labels such as heavy isotopes, non‐natural amino acids and tags into the protein while avoiding cell toxicity. In particular, radiolabelled peptides and proteins are valuable tools for the functional characterization of protein–protein interactions and for studying binding kinetics. In this study we evaluated cell‐free protein production for the generation of radiolabelled ligands for G protein‐coupled receptors (GPCRs). These receptors are seven‐transmembrane‐domain receptors activated by a plethora of extracellular stimuli including peptide ligands. Many GPCR peptide ligands contain disulphide bonds and are thus inherently difficult to produce in bacterial expression hosts or in Escherichia coli‐based cell‐free systems. Here, we established an adapted E. coli‐based cell‐free translation system for the production of disulphide bond‐containing GPCR peptide ligands and specifically introduce tritium labels for detection. The bacterial oxidoreductase DsbA is used as a chaperone to favour the formation of disulphide bonds and to enhance the yield of correctly folded proteins and peptides. We demonstrate the correct folding and formation of disulphide bonds and show high‐affinity ligand binding of the produced radio peptide ligands to the respective receptors. Thus, our system allows the fast, cost‐effective and reliable synthesis of custom GPCR peptide ligands for functional and structural studies.

Endothelin-1 impairs coronary arteriolar dilation: Role of p38 kinase-mediated superoxide production from NADPH oxidase

Elevated levels of endothelin-1 (ET-1), a potent vasoactive peptide, are implicated as a risk factor for cardiovascular diseases by exerting vasoconstriction. The aim of this study was to address whether ET-1, at sub-vasomotor concentrations, elicits adverse effects on coronary microvascular function. Porcine coronary arterioles (50-100μm) were isolated, cannulated and pressurized without flow for in vitro study. Diameter changes were recorded using a videomicrometer. Arterioles developed basal tone (60±3μm) and dilated to the endothelium-dependent nitric oxide (NO)-mediated vasodilators serotonin (1nmol/L to 0.1μmol/L) and adenosine (1nmol/L to 10μmol/L). Treating the vessels with a clinically relevant sub-vasomotor concentration of ET-1 (10pmol/L, 60min) significantly attenuated arteriolar dilations to adenosine and serotonin but not to endothelium-independent vasodilator sodium nitroprusside. The arteriolar wall contains ETA receptors and the adverse effect of ET-1 was prevented by ETA receptor antagonist BQ123, the superoxide scavenger Tempol, the NADPH oxidase inhibitors apocynin and VAS2870, the NOX2-based NADPH oxidase inhibitor gp91 ds-tat, or the p38 kinase inhibitor SB203580. However, ETB receptor antagonist BQ788, H2O2 scavenger catalase, scrambled gp91 ds-tat, or inhibitors of xanthine oxidase (allopurinol), PKC (Gö 6983), Rho kinase (Y27632), and c-Jun N-terminal kinase (SP600125) did not protect the vessel. Immunohistochemical staining showed that ET-1 elicited Tempol-, apocynin- and SB203580-sensitive superoxide productions in the arteriolar wall. Our results indicate that exposure of coronary arterioles to a pathophysiological, sub-vasomotor concentration of ET-1 leads to vascular dysfunction by impairing endothelium-dependent NO-mediated dilation via p38 kinase-mediated production of superoxide from NADPH oxidase following ETA receptor activation.

Generation and characterization of an endothelin-2 iCre mouse

A novel transgenic mouse line that expresses codon-improved Cre recombinase (iCre) under regulation of the Endothelin-2 gene (edn2) promoter was developed for the conditional deletion of genes in Endothelin-2 lineage cells and for the spatial and temporal localization of Endothelin-2 expression. Endothelin-2 (EDN2, ET-2, previously VIC) is a transcriptionally regulated 21 amino acid peptide implicated in vascular homeostasis, and more recently in female reproduction, gastrointestinal function, immunology, and cancer pathogenesis that acts through membrane receptors and G-protein signaling. A cassette (edn2-iCre) was constructed that contained iCre, a polyadenylation sequence, and a neomycin selection marker in front of the endogenous start codon of the edn2 gene in a mouse genome BAC clone. The cassette was introduced into the C57BL/6 genome by pronuclear injection, and two lines of edn2-iCre positive mice were produced. The edn2-iCre mice were bred with ROSA26-lacZ and Ai9 reporter mice to visualize areas of functional iCre expression. Strong expression was seen in the periovulatory ovary, stomach and small intestine, and colon. Uniquely, we report punctate expression in the corneal epithelium, the liver, the lung, the pituitary, the uterus, and the heart. In the embryo, expression is localized in developing hair follicles and the dermis. Therefore, edn2-iCre mice will serve as a novel line for conditional gene deletion in these tissues.

Endothelin-2, the forgotten isoform: emerging role in the cardiovascular system, ovarian development, immunology and cancer

Endothelin-2 [ET-2; also known as vasoactive intestinal contractor (VIC), in rodents] differs from endothelin-1 (ET-1) by only two amino acids, and unlike the third isoform, endothelin-3 (ET-3), it has the same affinity as ET-1 for both ET(A) and ET(B) receptors. It is often assumed that ET-2 would mimic the actions of the more abundant ET-1 and current pharmacological interventions used to inhibit the ET system would also block the actions of ET-2. These assumptions have focused research on ET-1 with ET-2 studied in much less detail. Recent research suggests that our understanding of the ET family requires re-evaluation. Although ET-2 is very similar in structure as well as pharmacology to ET-1, and may co-exist in the same tissue compartments, there is converging evidence for an important and distinct ET-2 pathway. Specifically is has been demonstrated that ET-2 has a key role in ovarian physiology, with ET-2-mediated contraction proposed as a final signal facilitating ovulation. Furthermore, ET-2 may also have a pathophysiological role in heart failure, immunology and cancer. Comparison of ET-2 versus ET-1 mRNA expression suggests this may be accomplished at the level of gene expression but differences may also exist in peptide synthesis by enzymes such as endothelin converting enzymes (ECEs) and chymase, which may allow the two pathways to be distinguished pharmacologically and become separate drug targets. LINKED ARTICLES This article is part of a themed section on Endothelin. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.168.issue-1.

Defining the affinity and receptor sub-type selectivity of four classes of endothelin antagonists in clinically relevant human cardiovascular tissues

AIMS

We have compared the endothelin receptor subtype affinity (K(D)) and selectivity of four structural classes of antagonists (peptide, sulphonamide-based, carboxylic acid-based, myceric acid-based) in human cardiovascular tissues to determine whether these are predicted by values reported for human cloned receptors. Additionally, affinities (K(B)) for these antagonists, determined in ET-1-mediated vasoconstriction assays in human blood vessels, were used to identify discrepancies between K(B) and K(D) determined in the same tissues.

MAIN METHODS

Competition binding experiments were carried out in sections of human left ventricle, coronary artery and homogenates of saphenous vein to determine K(D) values for structurally different ET(A)-selective (FR139317, BMS 182874, S97-139, sitaxentan, ambrisentan) and mixed (PD142893, Ro462005, bosentan, L-749329, SB209670) antagonists. Schild-derived values of antagonist affinity were obtained in vascular functional studies.

KEY FINDINGS

When compared with previously reported data in human cloned endothelin receptors, those antagonists reported to be ET(A)-selective exhibited even greater ET(A) selectivity in human ventricle (BMS 182874, sitaxentan, ambrisentan) that expressed both receptor subtypes. Those antagonists reported to have <100 fold selectivity in cloned receptors (PD142893, Ro-462005, bosentan, SB209670, L-749329) did not distinguish between receptor subtypes in human left ventricle. For antagonists where we determined affinity in vascular functional and binding assays (Ro462005, bosentan, BMS 182874, L-749329, SB209670) there was no correlation between the degree of discrepancy in K(B) and K(D) and structural class.

SIGNIFICANCE

For an antagonist to retain ET(A)-selectivity in vivo it may be necessary to identify those compounds that have at least 1000 fold ET(A):ET(B) selectivity in in vitro assays.

Endothelin-1 and norepinephrine overflow from cardiac sympathetic nerve endings in myocardial ischemia

In protracted myocardial ischemia, sympathetic activation with carrier-mediated excessive norepinephrine (NE) release from its nerve endings due to reversal of NE transporter in an outward direction is a prominent cause of arrhythmias and cardiac dysfunction. Endothelin-1 (ET-1) and its receptors are intimately involved in the regulation of this carrier-mediated NE overflow in protracted myocardial ischemia. The ET-1 system is often complex, sometimes involving opposing actions depending on which receptor subtype is activated, which cells are affected, and whether stimuli are endogenously generated or exogenously applied. Therefore, a detailed understanding of the ET-1 system is important for applying drugs acting on this system in clinical settings for the treatment of ischemic cardiac disease. This article provides a detailed analysis of how the ET-1 system is involved in the regulation of carrier-mediated NE release from sympathetic nerve endings in protracted myocardial ischemia.

Comparison of endothelin receptors in normal versus cirrhotic human liver and in the liver from endothelial cell-specific ETB knockout mice

AIMS

Endothelin (ET) antagonists show promise in animal models of cirrhosis and portal hypertension. The aim was to pharmacologically characterise the expression of endothelin receptors in human liver, hepatic artery and portal vein.

MAIN METHODS

Immunofluorescence staining, receptor autoradiography and competition binding assays were used to localise and quantify ET receptors on hepatic parenchyma, hepatic artery and portal vein in human cirrhotic or normal liver. Additional experiments were performed to determine the affinity and selectivity of ET antagonists for liver ET endothelin receptors. An endothelial cell ET(B) knockout murine model was used to examine the function of sinusoid endothelial ET(B) receptors.

KEY FINDINGS

ET(B) receptors predominated in normal human liver and displayed the highest ratio (ET(B):ET(A) 63:47) compared with other peripheral tissues. In two patients examined, liver ET(B) expression was up-regulated in cirrhosis (ET(B):ET(A) 83:17). Both sub-types localised to the media of normal portal vein but ET(B) receptors were downregulated fivefold in the media of cirrhotic portal vein. Sinusoid diameter was fourfold smaller in endothelial cell ET(B) knockout mice. The liver morphology of ET(B) knockout mice was markedly different to normal murine liver, with loss of the wide spread sinusoidal pattern. In the knockout mice, sinusoids were reduced in both number and absolute diameter, while large intrahepatic veins were congested with red blood cells.

SIGNIFICANCE

These data support a role for the ET system in cirrhosis of the liver and suggest that endothelial ET(B) blockade may cause sinusoidal constriction which may contribute to hepatotoxicity associated with some endothelin antagonists.

Endothelin-1 and diabetic complications: focus on the vasculature

Diabetes is not only an endocrine but also a vascular disease. Cardiovascular complications are the leading cause of morbidity and mortality associated with diabetes. Diabetes affects both large and small vessels and hence diabetic complications are broadly classified as microvascular (retinopathy, nephropathy and neuropathy) and macrovascular (heart disease, stroke and peripheral arterial disease) complications. Endothelial dysfunction, defined as an imbalance of endothelium-derived vasoconstrictor and vasodilator substances, is a common denominator in the pathogenesis and progression of both macro and microvascular complications. While the pathophysiology of diabetic complications is complex, endothelin-1 (ET-1), a potent vasoconstrictor with proliferative, profibrotic, and proinflammatory properties, may contribute to many facets of diabetic vascular disease. This review will focus on the effects of ET-1 on function and structure of microvessels (retina, skin and mesenteric arteries) and macrovessels (coronary and cerebral arteries) and also discuss the relative role(s) of endothelin A (ET(A)) and ET(B) receptors in mediating ET-1 actions.

Working under pressure: coronary arteries and the endothelin system

Endogenous endothelin-1-dependent (ET-1) tone in coronary arteries depends on the balance between ET(A) and ET(B) receptor-mediated effects and on parameters such as receptor distribution and endothelial integrity. Numerous studies highlight the striking functional interactions that exist between nitric oxide (NO) and ET-1 in the regulation of vascular tone. Many of the cardiovascular complications associated with cardiovascular risk factors and aging are initially attributable, at least in part, to endothelial dysfunction characterized by a dysregulation between NO and ET-1. The contribution of the imbalance between smooth muscle ET(A/B) and endothelial ET(B) receptors to this process is poorly understood. An increased contribution of ET-1 that is associated with a proportional decrease in that of NO accompanies the development of coronary endothelial dysfunction, coronary vasospasm, and atherosclerosis. These data form the basis for the rationale of testing therapeutic approaches counteracting ET-1-induced cardiovascular dysfunction. It remains to be determined whether the beneficial role of endothelial ET(B) receptors declines with age and risk factors for cardiovascular diseases, revealing smooth muscle ET(B) receptors with proconstricting and proinflammatory activities.

The cardiovascular physiology and pharmacology of endothelin-1

One year after the discovery in 1980 that the endothelium was obligatory for acetylcholine to relax isolated arteries, it was clearly shown that the endothelium could also promote contraction. In 1988, Dr Yanagisawa's group identified endothelin-1 (ET-1) as the first endothelium-derived contracting factor. The circulating levels of this short (21 amino acids) peptide were quickly determined in humans and it was reported that in most cardiovascular diseases, circulating levels of ET-1 were increased and ET-1 was then recognized as a likely mediator of pathological vasoconstriction in human. The discovery of two receptor subtypes in 1990, ET(A) and ET(B), permitted optimization of bosentan, which entered clinical development in 1993, and was offered to patients with pulmonary arterial hypertension in 2001. In this report, we discuss the physiological and pathophysiological role of endothelium-derived ET-1, the pharmacology of its two receptors, focusing on the regulation of the vascular tone and as much as possible in humans. The coronary bed will be used as a running example, but references to the pulmonary, cerebral, and renal circulation will also be made. Many of the cardiovascular complications associated with aging and cardiovascular risk factors are initially attributable, at least in part, to endothelial dysfunction, particularly dysregulation of the vascular function associated with an imbalance in the close interdependence of NO and ET-1, in which the implication of the ET(B) receptor may be central.

Endothelium-derived endothelin-1

One year after the revelation by Dr. Furchgott in 1980 that the endothelium was obligatory for acetylcholine to relax isolated arteries, it was clearly shown that the endothelium could also promote contraction. In 1988, Dr. Yanagisawa's group identified endothelin-1 (ET-1) as the first endothelium-derived contracting factor. The circulating levels of this short (21-amino acid) peptide were quickly determined in humans, and it was reported that, in most cardiovascular diseases, circulating levels of ET-1 were increased, and ET-1 was then tagged as "a bad guy." The discovery of two receptor subtypes in 1990, ET(A) and ET(B), permitted optimization of the first dual ET-1 receptor antagonist in 1993 by Dr. Clozel's team, who entered clinical development with bosentan, which was offered to patients with pulmonary arterial hypertension in 2001. The revelation of Dr. Furchgott opened a Pandora's box with ET-1 as one of the actors. In this brief review, we will discuss the physiological and pathophysiological role of endothelium-derived ET-1 focusing on the regulation of the vascular tone, and as much as possible in humans. The coronary bed will be used as a running example in this review because it is the most susceptible to endothelial dysfunction, but references to the cerebral and renal circulation will also be made. Many of the cardiovascular complications associated with aging and cardiovascular risk factors are initially attributable, at least in part, to endothelial dysfunction, particularly dysregulation of the vascular function associated with an imbalance in the close interdependence of nitric oxide and ET-1.

Endothelin-1-mediated vasoconstriction at rest and during dynamic exercise in healthy humans

It is now generally accepted that alpha-adrenoreceptor-mediated vasoconstriction is attenuated during exercise, but the efficacy of nonadrenergic vasoconstrictor pathways during exercise remains unclear. Thus, in eight young (23 +/- 1 yr), healthy volunteers, we contrasted changes in leg blood flow (ultrasound Doppler) before and during intra-arterial infusion of the alpha(1)-adrenoreceptor agonist phenylephrine (PE) with that of the nonadrenergic endothelin A (ET(A))/ET(B) receptor agonist ET-1. Heart rate, arterial blood pressure, common femoral artery diameter, and mean blood velocity were measured at rest and during knee-extensor exercise at 20%, 40%, and 60% of maximal work rate (WR(max)). Drug infusion rates were adjusted for blood flow to maintain comparable doses across all subjects and conditions. At rest, PE infusion (8 ng x ml(-1) x min(-1)) provoked a rapid and significant decrease in leg blood flow (-51 +/- 3%) within 2.5 min. Resting ET-1 infusion (40 pg x ml(-1) x min(-1)) significantly decreased leg blood flow within 5 min, reaching a maximal vasoconstriction (-34 +/- 3%) after 25-30 min of continuous infusion. Compared with rest, an exercise intensity-dependent attenuation to PE-mediated vasoconstriction was observed (-18 +/- 5%, -7 +/- 2%, and -1 +/- 3% change in leg blood flow at 20%, 40%, and 60% of WR(max), respectively). Vasoconstriction in response to ET-1 was also blunted in an exercise intensity-dependent manner (-13 +/- 3%, -7 +/- 4%, and 2 +/- 3% change in leg blood flow at 20%, 40%, and 60% of WR(max), respectively). These findings support a significant contribution of ET-1 and alpha-adrenergic receptors in the regulation of skeletal muscle blood flow in the human leg at rest and suggest a similar, intensity-dependent "lysis" of peripheral ET and alpha-adrenergic vasoconstriction during dynamic exercise.

Regional differences in the functional and biochemical properties of endothelin receptor subtypes in the rabbit prostatic urethra

OBJECTIVE

To examine the regional differences in the functional (pharmacological) and biochemical properties of endothelin (ET) receptors in the rabbit prostatic urethra.

MATERIALS AND METHODS

The properties of ET receptors in 6-month-old male rabbit prostatic urethras were examined using isolated muscle-bath and radioligand receptor-binding techniques. Using plasma membrane suspensions, saturation and inhibition experiments with [(125)I]ET-1 and unlabelled agonists and antagonists (ET(A)-selective antagonist BQ123, and ET(B)-selective agonist sarafotoxin 6c, STX6c) were done to determine the ET receptor densities and their subtype specificities in the different regions of the urethra.

RESULTS

The ETs (ET-1 and ET-3) produced significant concentration-dependent contractile responses in the smooth muscle strips from the different regions of the urethra. Although the maximum contractile responses induced by ET-1 were similar in the different regions, the maximum contractile responses induced by ET-3 were greater in the distal region than in the proximal or middle regions, suggesting that the contractile response to ET-1 is more potent than that to ET-3 in all regions, and that there are region-specific differences in the responses to ET-3 but not ET-1. Moreover, the ET-3-induced contractile response was suppressed by BQ788 (a selective antagonist of the ET(B) receptor) suggesting that the ET(B) receptor subtype contributes to the contractile responses mediated by ET-3. The ET receptors were expressed in higher concentrations in the distal than in the proximal or middle regions. BQ123 and STX6c inhibited [(125)I]ET-1 binding in all regions with high and low affinity constants, indicating the presence of both ET(A) and ET(B) receptor subtypes. The proportions of high-affinity binding sites for BQ123, representing ET(A) receptors, were approximately 68%, 63% and 42% in the proximal, middle and distal regions, respectively. By contrast, the proportions of high-affinity binding sites for STX6c, representing ET(B) receptors, were approximately 27%, 35% and 52% in the proximal, middle, and distal regions, respectively. These data indicate the presence of regional differences in the densities and subtype specificities of ET receptor subtypes, and the existence of regional differences in the rabbit prostatic urethra.

CONCLUSION

The results suggest regional differences in ET(B) receptor subtypes that mediate contractile responses to ET-3, reflecting differences in the densities and specificities of the ET receptor subtypes in the rabbit prostatic urethra.

Heredity of Endothelin Secretion

Background— Endothelial dysfunction predisposes to vascular injury in association with hypertension. Endothelin (ET-1) is a potent vasoactive peptide that is synthesized and released by the vascular endothelium and is a marker of endothelial function. Chromogranin A (CHGA) regulates the storage and release of catecholamines and may have direct actions on the microvasculature. CHGA , a candidate gene for intermediate phenotypes that contribute to hypertension, shows a pattern of single nucleotide polymorphism variations that alter the expression and function of this gene both in vivo and in vitro.

Methods and Results— In a study of twins (n=238 pairs), plasma ET-1 was 58±5% ( P <0.0001) heritable. Plasma ET-1 was both correlated and associated with chromogranin fragment levels, and the 2 were influenced by shared genetic determination (pleiotropy [ρ G ]; for the CHGA precursor, ρ G =0.318±0.105; P =0.0032). We therefore hypothesized that variation in the CHGA gene may influence ET-1 secretion. Carriers of the CHGA promoter −988G, −462A, and −89A minor alleles showed significantly higher mean plasma ET-1 than their major allele homozygote counterparts ( P =0.02, P =0.006, P =0.03, respectively). Analysis of a linkage disequilibrium block that spans these 3 single nucleotide polymorphisms showed a significant association between the GATACA haplotype and plasma ET-1 ( P =0.0075). In cultured human umbilical vein endothelial cells, CHGA caused dose-dependent secretion of ET-1 over a brief (<1 hour) time course at relatively low concentrations of CHGA (10 to 100 nmol/L) with a threshold concentration (10 nmol/L) in the range found circulating in humans in vivo.

Conclusions— These results suggest that common, heritable variation in expression of the human CHGA gene influences endothelial ET-1 secretion in vivo, explained by a CHGA stimulus/ET-1 secretion coupling in endothelial cells in vitro. The findings document a previously unsuspected interaction between the sympathochromaffin system and the endothelium and suggest novel genetic and cell biological approaches to the prediction, diagnosis, and mechanism of endothelial dysfunction in human disease.

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).

Vascular endothelin in hypertension

Endothelins are powerful vasoconstrictor peptides that also play numerous other functions in many different organs. Endothelin-1 (ET-1) is the most abundant and important of this family of peptides in blood vessels. Production of ET-1 is increased in the endothelium and the kidney in salt-dependent models of hypertension (e.g.: DOCA-salt rats and Dahl salt-sensitive rats, in salt-loaded SHR-SP, in angiotensin II-infused and in diabetic rats). ET-1 elicits an inflammatory response by increasing oxidant stress in the vascular wall, which induces vascular remodeling and endothelial dysfunction found in the hypertensive models that exhibit an endothelin-mediated component. Endothelin receptor antagonism reduces blood pressure and vascular hypertrophic remodeling present in these hypertensive models. Patients with stage 2 hypertension have enhanced vascular expression of ET-1. Endothelin receptor antagonists lower blood pressure in hypertensive patients. They could become therapeutic agents for prevention of target organ damage in hypertension and in type 2 diabetes, chronic renal failure and congestive heart failure. Side effects of endothelin receptor blockers have prevented up to the present their development for these indications. New endothelin antagonists devoid of these side effects, or alternatively inhibitors of the endothelin converting enzymes that generate ET-1 may in the future become available to block the endothelin system. However, to date endothelin antagonists have been approved only for the treatment of primary pulmonary hypertension, a rapidly fatal condition in which the endothelin system plays an important role and endothelin antagonists exert favorable effects.

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.

Myocardial Protection with the Non-Selective Endothelin Receptor Antagonist L-753,037 Following Acute Coronary Artery Occlusion in the Dog

SUMMARY

Efficacy of a new, potent non-selective endothelin antagonist, l-753037, was examined in a model of canine coronary artery occlusion and reperfusion to assess whether blockade of both ETA and ETB receptors would enhance or reduce myocardial ischemic injury. Instrumented dogs were randomized to receive vehicle (n = 9) or l-753037 (0.1 microg/kg/min, n = 9) by intracoronary infusion 30 minutes before a 90-minute LCx coronary artery occlusion and through 4 hours of reperfusion. After 4 hours of reperfusion, plasma ET-1 levels rose significantly in both groups: 24 +/- 3 fmol/ml in vehicle animals (P < 0.01) versus 42 +/- 5 fmol/ml with l-753037 (P < 0.05). Treatment with l-753037 normalized total LCx flow and regional myocardial flow after 4 hours of reperfusion in all regions. LCx flow was reduced 16% from pre-occlusion baseline (P = 0.45) with treatment compared with 35% with vehicle (P < 0.01). Endocardial flow in the risk region returned to baseline values with l-753037 treatment but was reduced approximately 50% in vehicle animals. l-753037 treatment was associated with a 38% reduction in infarct size (24.1 +/- 3.9% AAR with l-753037 treatment versus 38.7 +/- 3.1% with vehicle, P < 0.01). Thus, a non-selective endothelin antagonist provides significant myocardial protection primarily by improving regional myocardial flow distribution following reperfusion and demonstrated no detrimental effects associated with blockade of the ETB receptor.

Increased endothelin-induced Ca2+ signaling, tyrosine phosphorylation, and coronary artery disease in diabetic dyslipidemic Swine are prevented by atorvastatin

Endothelin-1 (ET-1) signaling mechanisms have been implicated in the pathogenesis of excess coronary artery disease in diabetic dyslipidemia. We hypothesized that in diabetic dyslipidemia ET-1-induced coronary smooth muscle calcium (Ca2+m) and tyrosine phosphorylation would be increased, and the lipid lowering agent, atorvastatin, would inhibit these increases. Male Yucatan miniature swine groups were treated for 20 weeks: normal low-fat fed control, high-fat/cholesterol fed (hyperlipidemic), hyperlipidemic made diabetic with alloxan (diabetic dyslipidemic), and diabetic dyslipidemic treated with atorvastatin (atorvastatin-treated). Blood glucose values were 5-fold greater in diabetic dyslipidemic and atorvastatin-treated versus control and hyperlipidemic. Total and low-density lipoprotein (LDL) plasma cholesterol in hyperlipidemic, diabetic dyslipidemic, and atorvastatin-treated were approximately 5-fold greater than control. Intravascular ultrasound detectable coronary disease and hypertriglyceridemia were only observed in diabetic dyslipidemic and were abolished by atorvastatin. In freshly isolated cells, the Ca2+m response to ET-1 in diabetic dyslipidemic was greater than in control, hyperlipidemic, and atorvastatin-treated groups. Selective ET-1 receptor antagonists showed in the control group that the ETB subtype inhibits ETA regulation of Ca2+m. There was almost a complete switch of receptor subtype regulation of Ca2+m from largely ETA in control to an increased inhibitory interaction between ETA and ETB in hyperlipidemic and diabetic dyslipidemic groups, such that neither ETA nor ETB antagonist alone could block the ET-1-induced Ca2+m response. The inhibitory interaction was attenuated in the atorvastatin-treated group. In single cells, basal and ET-1-induced tyrosine phosphorylation in diabetic dyslipidemic were more than 3- and 6-fold greater, respectively, than in control, hyperlipidemic, and atorvastatin-treated. Attenuation by atorvastatin of coronary disease and ET-1-induced Ca2+m and tyrosine phosphorylation signaling with no change in cholesterol provides strong evidence for direct actions of atorvastatin and/or triglycerides on the vascular wall.

Developmental changes in the biochemical and functional properties of endothelin receptors in rabbit renal pelvis

OBJECTIVE

To examine the effect of age on the biochemical and functional properties, and regional distribution of endothelin (ET) receptors in the rabbit renal pelvis.

MATERIALS AND METHODS

The properties of ET receptors in 6-week-old and 6-month-old male rabbit renal pelves were examined using isolated muscle-bath and radioligand receptor-binding techniques.

RESULTS

ET-1 caused a significant increase in the contractile force in muscle strips from all regions of the renal pelvis from both age groups, with the following rank order: upper=middle>lower. The magnitude of the ET-1-induced contractile responses were similar in the lower pelvic regions in both ages, but the responses in the upper and middle regions were significantly greater in younger rabbits. ET-1 increased the frequency of spontaneous activity in a concentration-dependent manner in the upper and middle pelvic regions in both age groups, with significantly smaller ED50 values in the younger than in the older rabbits. In both age groups the lower pelvic region lacked spontaneous activity. The density of total ET receptors was higher in the upper and middle regions of the renal pelvis than in the lower renal pelvis of both ages, with the density in the upper and middle regions being greater in older than in younger rabbits. In all regions, ET subtype selective compounds inhibited [125I]ET-1, binding consistent with the predominance of the ETA receptor subtype, except in the lower region of the older rabbits, in which the densities of ETA and ETB subtypes were similar. In all regions, the younger renal pelvis contained a higher proportion of ETA receptors than in older tissues. Light microscopic autoradiographic data indicated the presence of ETA and ETB receptors in smooth muscle and epithelial cells, respectively.

CONCLUSION

These data indicate the presence of regional differences in the density of ET receptors and in the contractile responses to ET-1 in rabbit renal pelvis, and that although older rabbit renal pelvis contains more total ET receptors than younger renal pelvis, the latter had a higher portion of the ETA receptor subtype and the younger tissues were more responsive to ET-1.

Endothelin receptor blockade and in-stent restenosis

The aim of the present study was to test whether oral dosing of an endothelin (ET) receptor antagonist was able to reduce restenosis in the model of stent-induced restenosis. After pigs underwent coronary artery catheterization they were randomly allocated either to controls or to treatment with the ET receptor antagonist BSF 208075. Thirty-seven pigs underwent percutaneous transluminal coronary angioplasty plus stent implantation; seven animals died of ventricular fibrillation due to procedure-related myocardial ischaemia. From the 30 surviving animals coronary arteries were sampled after 6 weeks of oral treatment with 10 mg/kg/day BSF 208075 or vehicle and histologically evaluated. Stent implantation had no effect on total coronary artery diameter, and media thickness was virtually identical in the two groups. However, neointimal thickness in the group treated with the ET receptor antagonist was significantly reduced, resulting in a significantly larger total coronary artery lumen in the treated group. As in the setting of stent implantation neither 'recoil' nor scar-related vascular remodelling can occur, this result allows the conclusion of a significant antiproliferative effect of ET receptor antagonism in pig coronary arteries.

Nitric oxide-mediated modulation of the endothelin-1 signalling pathway in the human cardiovascular system

1. We studied the ability of nitric oxide (NO) to physiologically antagonize endothelin-1 (ET-1) induced constrictions in human internal mammary artery (IMA). We also investigated the hypothesis that NO interacts directly with ET-receptor binding in human heart and aorta. 2. ET-1 potently contracted IMA (EC(50) 6.86 nM, 95% CI: 3.5 - 13.4 nM; n=12). The constrictor response to 10 nM ET-1 was fully reversed by the NO-donor diethylamine NONOate (DEA/NO; EC(50) 2.0 microM, 95% CI: 0.8 - 4.8 microM; n=5). The guanylate cyclase inhibitor ODQ (100 microM) reduced the response to DEA/NO but did not abolish it (E(MAX) 50.9+/-8.5% in the presence of ODQ; 113.0+/-8.4%, control). 3. The increase in cyclic GMP by 30 microM DEA/NO was abolished in the presence of 100 microM ODQ (n=6). 4. In saturation binding experiments the NO-donor Diethyltriamine NONOate (DETA/NO; 1 mM) caused a 90% reduction in maximum binding of [(125)I]-ET-1 in human heart, without affecting the affinity. This reduction in binding was abolished by haemoglobin. Pre-incubating either the radiolabel or the tissue with NO-donors did not reduce binding. A similar effect was observed in aortic smooth muscle. 5. We have shown that DEA/NO is able to reverse ET-1-induced contractions in the human vasculature. The binding studies suggest a direct interaction between NO and the ET receptor or receptor-ligand complex in human ventricular and aortic tissue. NO is released continuously in vivo, thus this apparent modification of ET-receptor binding may provide an additional mechanism by which NO counter-balances the effects of ET.

Binding characteristics of endothelin ET(A) receptors in normal and post-mortem rat lung

In control lung homogenates, optimal specific binding of [(125)I]endothelin-1 and minimal filter binding was achieved using 50 microg/ml bacitracin, 30 microM phenylmethylsulphonyl fluoride (PMSF) and 10 mM EDTA. In post-mortem tissue (8, 16, and 32 h), no significant changes were seen in ET(A) receptor affinity (K(d)) or number (B(max)): control and 32 h K(d) = 309 +/- 75, 225 +/- 32 pM and B(max) = 173 +/- 42, 185 +/- 17 fmol/mg protein, respectively. Autoradiographic binding sites for [(125)I]endothelin-1 were densely expressed on bronchiolar smooth muscle and parenchyma with moderate binding on epithelium and blood vessels. Histologic sections of post-mortem lung showed minimal deterioration of structures expressing ET(A) binding sites. Hence the ET(A) receptor is stable in the rat lung for up to 32 h post-mortem.

High density of endothelin binding sites in the hearts of infants and children

Endothelin (ET)-1 peripheral levels are high in children with respect to values of adults, but its pathophysiological significance remains to be established. In these conditions the interaction of ET-1 with its receptors may constitute a clue to the understanding of ET-1 function. Because a direct determination of ET binding sites in the heart of children is lacking, in this study we have attempted an assessment of the ET receptor status in cardiac tissue of infants (<1 year; 0.39 +/- 0.26 (SD) years, n=6) and children (1-14 years; 6.3 +/- 4.9 years, n=7) as well as an evaluation of the receptor modulation as a function of age, associated to the observed decrease of plasma ET levels between infants and children. ET-1 binding sites have also been evaluated in atrium and ventricle membranes of adult subjects recipient of cardiac transplantation (CHF) and of post-mortem cardiac specimens (autopsy) of non cardiac patients. Considering all the pediatric patients (infants +/- children) studied, an affinity constant (Kd) value of 38.2 +/- 6.1 (SEM) pM and a density (Bmax) value of 166.2 +/- 11.6 fmol/mg protein has been obtained for atrium. Similar values have been found in the ventricle. These values are significantly higher with respect to those obtained in adults: for atrial membranes, Kd = 22.2 +/- 9.7 and 11.6 +/- 1.8 pM; Bmax = 58.4 +/- 22.8 and 42.1 +/- 8.9 fmol/mg protein, respectively in explanted hearts and in post mortem specimens. No significant differences have been found in the binding parameters between infants and children, while, considering our results as a whole, a significant inverse correlation between Bmax and subject age (p<0.001) is suggested. The ET-A/ET-B ratio, evaluated by competition experiments with the specific ET-A antagonist BQ-123, was about 70:30 in pediatric patients, in both atrium and ventricle, without any difference between infants and children. Similar values for ET-A/ET-B ratio in adult CHF patients, in contrast to a reduction (significant only in ventricle) of the percent of ET-A subtype in autopsy, has been found. This is the first study concerning a direct evaluation of ET receptor status in children's hearts; the higher density of binding sites, associated to the elevation of plasma levels, could suggest a enhanced biological function of ET in children.

Blockade and reversal of endothelin-induced constriction in pial arteries from human brain

BACKGROUND AND PURPOSE

Substantial evidence now implicates endothelin (ET) in the pathophysiology of cerebrovascular disorders such as the delayed vasospasm associated with subarachnoid hemorrhage and ischemic stroke. We investigated the ET receptor subtypes mediating vasoconstriction in human pial arteries.

METHODS

ET receptors on human pial and intracerebral arteries were visualized with the use of autoradiography, and the subtypes mediating vasoconstriction were identified by means of wire myography.

RESULTS

ET-1 was more potent than ET-3 as a vasoconstrictor, indicating an ETA-mediated effect. Similarly, the selective ETB agonist sarafotoxin S6c had no effect on contractile action at concentrations up to 30 nmol/L. The nonpeptide ETA receptor antagonist PD156707 (3 to 30 nmol/L) caused a parallel rightward shift of the ET-1-induced response, yielding a pA2 of 9.2. Consistent with these results, PD156707 (30 nmol/L) fully reversed an established constriction in pial arteries induced by 1 nmol/L ET-1, while the selective ETB receptor antagonist BQ788 (1 micromol/L) had little effect. The calcium channel blocker nimodipine (0.3 to 3 micromol/L) significantly attenuated the maximum response to ET-1 in a concentration-dependent manner without changing potency. In agreement with the functional data, specific binding of [125I]PD151242 to ETA receptors was localized to the smooth muscle layer of pial and intracerebral blood vessels. In contrast, little or no [125I]BQ3020 binding to ETB receptors was detected.

CONCLUSIONS

These data indicate an important role for ETA receptors in ET-1-induced constriction of human pial arteries and suggest that ETA receptor antagonists may provide additional dilatory benefit in cerebrovascular disorders associated with raised ET levels.

The many aspects of endothelins in ischemia-reperfusion injury: emergence of a key mediator

The endothelins (ETs) are regulatory peptides, distributed in many organ systems and producing potent physiological effects. They are the most powerful vasoconstrictive substances known today. They also act as promitogens. Many data supporting pathophysiological roles for ETs are reported, especially regarding diseases related to the vascular system, such as hypertension, pulmonary hypertension, preeclampsia, ischemic heart diseases, renal failure, subarachnoidal hemorrhage, and cerebral ischemia. The development of drugs blocking ET binding to its receptors (antagonists) and the biosynthesis of ETs (ECE inhibitors) presently attracts great interest in terms of establishing new treatments for diseases in which ETs are believed to be involved. Here we review the evidence supporting a role for ETs in the various etiologies related to ischemia-reperfusion injury, such as is found in heart disease, cerebral ischemia, and organ transplantation.

Endothelin-1 activates phospholipases and channels at similar concentrations in porcine coronary arteries

Sensitivity of endothelin-1 (ET-1)-ion channel interactions has been proposed to exceed that of ET-1-phospholipase activation in vascular smooth muscle. We wanted to determine whether short-circuiting ion channels with staphylococcal alpha-toxin pores would shift the ET-1-force relation to the right as predicted from the above proposal. Medium size porcine coronary arteries (outer diameter 0.7-1.5 mm) were mounted on isometric force transducers. ET-1 concentration response curves were compared between intact rings and those subjected to alpha-toxin treatment with Ca buffered at 0.1 microM. The EC50 for treated rings (1.5 +/- 1.0 nM, n = 5 pigs) was similar to that for intact rings (1.9 +/- 0.4 nM). The Ca sensitivity of the alpha-toxin-treated rings (EC50 = 0.43 +/- 0.08 microM) was similar to that reported by other laboratories for intact and alpha-toxin-treated arteries and was shifted eightfold to the left by a high concentration of ET-1 (10 nM). Measurements of [32P]phosphatidic acid ([32P]PA) levels were used to evaluate phospholipase activity in intact arteries. The time courses for [32P]PA production and contraction were similar in response to high (100 nM) and to low (1 nM) ET-1. Significant increases in both steady-state contraction and [32P]PA occurred over a wide range of ET-1 concentrations tested (0.3-100 nM). Our findings support the concept that ET-1-phospholipase coupling is operative over the whole concentration range that induces contractile responses. It is suggested that both Ca entry and Ca sensitization processes are activated by ET-1 at low concentrations (<EC50) and that both processes contribute significantly to the integrated response.

Endothelin receptor subtypes and their functional relevance in human small coronary arteries

1. The potent constrictor peptide endothelin (ET) has been implicated in various cardiovascular disorders including myocardial infarction and atherosclerosis. We have investigated the nature of ET receptor subtypes present on human small coronary arteries. 2. Small coronary arteries were mounted in a wire-myograph for in vitro pharmacology. To investigate the ET receptor subtypes present in different segments of the coronary vascular tree, arteries were grouped according to internal diameter. Responses in arteries with small internal diameters (mean 316.7+/-7.9 microm; Group B) were compared to those in larger arteries (mean 586.2+/-23.1 microm; Group A). 3. ET-1 consistently and potently contracted arteries from Group A and B, with EC50 values of 1.7 (0.9-3.2) nM (n=15) and 2.3 (1.4-4.2) nM (n=14), respectively. No correlation was observed between ET-1 potency and internal diameter. The response to ET-1 was potently antagonized by the selective ET(A) receptor antagonist PD156707 in both Group A and Group B, yielding pA2 values of 8.60+/-0.12 (n=4-6) and 8.38+/-0.17 (n=4-6), respectively. Slopes from Schild regression were not significantly different from unity. 4. In contrast to ET-1, individual responses to ET-3 were variable. While all arteries from Group A responded to ET-3 (EC50 approximately 69 (23-210) nM) (n=12), no response was obtained in 5 of the 14 tested in Group B. Of those responding, many failed to reach a maximum at concentrations up to 1 microM. ET-1 was more potent than ET-3 in all arteries tested. A biphasic ET-3 response was observed in 8 arteries suggesting that a small ET(B) population was also present in some patients. The selective ET(B) receptor agonist sarafotoxin S6c had little or no effect up to 10 nM (n=4-6). 5. Responses to ET-1 and ET-3 were unaffected by removal of the endothelium in arteries from both groups suggesting a lack of functional, relaxant ET(B) receptors on endothelial cells (n=5). 6. Using autoradiography, specific high density binding of the non-selective, ET(A)/ET(B) ligand [125I]-ET-1 and selective ET(A) ligand [125I]-PD151242 was detected on the vascular smooth muscle layer of small intramyocardial coronary arteries (n=5). In contrast, little or no binding of the selective ET(B) receptor ligand [125I]-BQ3020 was observed (n=5). Similarly, [125I]-ET-1 binding to vascular smooth muscle was absent in the presence of the selective ET(A) receptor antagonist PD156707. 7. We conclude that human small epi- and intramyocardial coronary arteries express predominantly ET(A) receptors and it is these receptors which mediate ET-induced contractions. A constrictor ET(B) receptor population may exist in some patients. However, these receptors may have a limited role as contractions to ET-1 can be blocked fully by the selective ET(A) receptor antagonist PD156707.

Characterization of [125I]-PD164333, an ETA selective non-peptide radiolabelled antagonist, in normal and diseased human tissues

1 We have synthesized a new low molecular weight, non-peptide radioligand, [125I]-PD164333, an analogue of the orally active butenolide antagonists of the endothelin ETA receptor. 2 Analysis of saturation binding assays demonstrated that [125I]-PD164333 bound with high affinity to a single population of receptors (n > or = 3 individuals +/- s.e.mean) in human aorta (KD=0.26+/-0.08 nM; Bmax=8.8+/-3.95 fmol mg(-1) protein), left ventricle from the heart (KD=0.16+/-0.02 nM; Bmax=34.2+/-3.02 fmol mg(-1) protein) and kidney (KD=1.24+/-0.16 nM; Bmax=125.3+/-35.07 fmol mg(-1) protein). In each case Hill slopes were close to unity. 3 In kinetic experiments, the binding of [125I]-PD164333 to ETA receptors in sections of heart was time-dependent and rapid at 23 degrees C. The data were fitted to a one site model, with an association rate constant (K1 of 2.66+/-0.213x10(8) M(-1) min(-1), and a half-time for association of 11 min. The binding was reversible at 23 degrees C: analysis of the data indicated [125I]-PD164333 dissociated from a single site, with a dissociation rate constant of 0.0031+/-0.0004 min(-1), a half-time for dissociation of 216 min and a KD calculated from these kinetic data of 0.01 nM. 4 Unlabelled PD164333 inhibited the binding of [125I]-ET-1 to left ventricle (which expresses both subtypes) in a biphasic manner with a KDETA of 0.99+/-0.32 nM and KDETB of 2.41+/-0.22 microM, giving a selectivity of 2500 fold. ETA-selective ligands competed monophasically for [125I]-PD164333 binding in left ventricle, a one site fit was preferred to a two site model giving similar nanomolar affinities: BQ123, KD=3.93+/-0.18 nM; FR139317 KD=3.53+/-0.69 nM. In contrast, the ETB selective agonists, BQ3020 and sarafotoxin S6c (1 microM) did not inhibit binding. 5 In human isolated saphenous vein, unlabelled PD164333 was a functional antagonist, producing parallel rightward shifts of the endothelin-1 (ET-1) concentration-response curve (pA2=8.84) and a slope of unity. 6 In the human brain, autoradiography revealed high levels of [125I]-PD164333 binding to the pial arteries of the cerebral cortex and to the numerous smaller intercerebral vessels penetrating the underlying grey and white matter. Conduit and resistance vessels contributing to the control of blood pressure from the heart, kidney, lungs and adrenal also displayed high densities of binding. In diseased vessels, binding of [125I]-PD164333 was confined to the medial layer of both coronary arteries with advanced atherosclerotic lesions or occluded saphenous vein grafts. In contrast, little or no binding was detected in the proliferated smooth muscle of the intimal layer or occluded lesion. 7 These results show [125I]-PD164333 is a specific, high affinity, reversible non-peptide radioligand for human ETA receptors, which will facilitate the further characterization of this subtype, in vitro and in vivo.

Identification of endothelin receptor subtypes in rat ciliary body using subtype-selective ligands

The endothelins are important vasoactive ocular peptides and there is some evidence that they may modulate intraocular pressure. We investigated the existence and localization of endothelin receptor subtypes using subtype selective ligands in rat ciliary body. Scatchard transformation of saturation binding experiments revealed that the KD and Bmax for [125I]ET-1 and [125I]ET-3 to membranes from ciliary body were 41.7+/-9 pM and 236+/-20 fmol mg-1 protein and 37. 8+/-0.4 pM and 160+/-2.0 fmol mg-1 protein, respectively. Competitive experiments in the presence of cyclic pentapeptide BQ123 (selective for ETA receptors) and BQ3020 (selective for ETB receptors), demonstrated the existence of ETA and ETB receptors in a ratio of 35:65. Cross-linking of [125I]ET-1 and [125I]ET-3 to ciliary body membranes resulted in the labeling of two bands with apparent molecular masses of 52 and 34 kDa, suggesting that ETA and ETB receptors have similar molecular mass. The 34 Kda band is a proteolytic degradation product of the 52 Kda band. Autoradiographic results show that specific [125I]ET-1 binding sites, displaced by BQ123 and BQ3020, are localized to the ciliary epithelium, supporting the idea that ETA and ETB subtype receptors exist in this tissue.

Endothelin contraction in pig coronary artery: receptor types and Ca(2+)-mobilization

Endothelin is one of the most potent vasoconstrictors known. It plays an important role in the regulation of vascular tone and in the development of many cardiovascular diseases. This study focuses on the receptor types and the Ca2+ mobilization responsible for endothelin-1 (ET-1) contraction in de-endothelialized pig coronary artery rings. ET-1 contracted the artery rings with an EC50 = 6.5 +/- 1 nM and a maximum contraction which was 98.6 +/- 9% of the contraction produced by 60 mM KCl. BQ123 (5 microM), an ETA antagonist, reversed 78 +/- 3% of the ET-1 contraction (50 nM). IRL1620, a selective ETB agonist, produced 23 +/- 3% of the total ET-1 contraction with an EC50 = 12.7 +/- 2 nM. More than 85% of the contraction due to 100 nM IRL 1620 was inhibited by 200 nMBQ788, an ETB antagonist. Therefore, approximately 80% of the ET-1 contraction in this artery occurred via ETA receptors, and the other 20% was mediated by ETB receptors. To assess the Ca2+ pools utilized during the ET-1 response, ET-1 contraction was also examined in medium containing an L-type Ca2+ channel blocker nitrendipine, and in Ca2+ free medium containing 0.2 mM EGTA. In Ca2+ containing medium the contraction elicited by ET-1 was 98.6 +/- 9% of the KCl contraction, however, in the presence 10 microM nitrendipine the ET-1 induced contraction was 54 +/- 7% of the KCl contraction, and in Ca(2+)-free medium it was 13 +/- 2%. Similarly, the IRL 1620 contractions in Ca2+ containing medium, in the presence of nitrendipine and in Ca(2+)-free medium were 22.4 +/- 3%, 12 +/- 3% and 11 +/- 2% of the KCl response respectively. Thus, both ETA and ETB contractions utilize extracellular Ca2+ pools via L-type Ca2+ channels and other undefined route(s), as well as intracellular Ca2+ pools. In the pig coronary artery smooth muscle, ET-1 contractions occur predominantly via ETA receptors, with ETB receptors using similar Ca2+ mobilization pathways, but the ETB receptors appear to use the intracellular Ca2+ stores to a greater extent.

Detection of endothelin receptors in human coronary artery vascular smooth muscle cells but not endothelial cells by using electron microscope autoradiography

Endothelin A (ET(A)) and ET(B) receptors located on vascular smooth-muscle cells mediate potent vasoconstriction in animal and human blood vessels. Although vascular endothelial ET(B) receptors mediate vasodilatation through release of nitric oxide, prostacyclin, or an endothelium-derived hyperpolarizing factor in animal vessels, there is less evidence to support ET-mediated endothelium-dependent relaxation in human arteries. We investigated the regional and subcellular localization of ET-receptor subtypes in human epicardial coronary arteries obtained from patients with congestive heart failure undergoing cardiac transplantation. Radioligand-binding studies revealed a predominance of the ET(A)-receptor subtype in the coronary artery. ET(A) and ET(B) receptors were located in the media and in perivascular structures by using macroautoradiography. Specific binding in the intimal layer was low. Autoradiography at the ultrastructural level revealed the presence of ET(A) and ET(B) receptors on medial vascular smooth-muscle cell plasmalemma and plasmalemmal vesicles with a predominance of the ET(A) subtype. ET receptors were not detected in either luminal endothelial cells of the coronary arteries or endothelial cells lining small adventitial blood vessels. This study is the first to identify the subcellular localization of ET receptors in human coronary artery vascular smooth-muscle cells. These receptors probably mediate the well-established vasoconstrictor response to ET-1. The inability to detect ET receptors on endothelial cells suggests a minor role for endothelial cells in ET-mediated vasodilatation of the coronary vasculature.

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.

Endothelin peptide and receptors in human atherosclerotic coronary artery and aorta

The aim of this study was to determine whether there is an alteration in the distribution or quantity of endothelin (ET) peptide or receptor subtypes in human atherosclerotic arteries. Levels of endogenous ET and big ET-1 detectable by radioimmunoassay in human aorta containing raised atherosclerotic plaques were significantly higher than those in histologically normal tissue (Student's t test, P < .01). Immunohistochemistry revealed ET-like immunoreactivity in endothelial cells lining the vessel lumen, neovascularization, recanalization of organized thrombus, and regions rich in macrophages. Little immunoreactivity was associated with vascular smooth muscle cells (VSMCs). Saturation binding assays with [125I]ET-1 indicated comparable affinities and maximal densities of receptors in the media of diseased and normal coronary arteries. Quantitative autoradiography with subtype-selective radioligands revealed similar small proportions of ETB receptors in the diseased and normal arterial media. In arteries with early and late disease, ETA receptors were localized to medial smooth muscle but were lacking in the VSMCs of the intimal layer, where ETB receptors were absent. ETB receptors were detected on perivascular nerves and lymphoid aggregates. In atherosclerotic arteries, microautoradiography localized ETB receptors to neovascularization and, interestingly, to macrophages. The results of this study indicate that there is an increase in ET and big ET-1 associated with fully developed atherosclerotic plaques. It is likely that this is derived from endothelial cells and macrophages but not VSMCs. ETA receptors predominate in the media of both normal and diseased arteries. ET receptors are deficient in intimal smooth muscle, and ETB receptors, where present, are found on endothelial cells and macrophages.

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.