Anatomical localization and pharmacological activity of mature endothelins and their precursors in human vascular tissue

Endothelin-1 axes in the framework of predictive, preventive and personalised (3P) medicine

Endothelin-1 (ET-1) is involved in the regulation of a myriad of processes highly relevant for physical and mental well-being; female and male health; in the modulation of senses, pain, stress reactions and drug sensitivity as well as healing processes, amongst others. Shifted ET-1 homeostasis may influence and predict the development and progression of suboptimal health conditions, metabolic impairments with cascading complications, ageing and related pathologies, cardiovascular diseases, neurodegenerative pathologies, aggressive malignancies, modulating, therefore, individual outcomes of both non-communicable and infectious diseases such as COVID-19. This article provides an in-depth analysis of the involvement of ET-1 and related regulatory pathways in physiological and pathophysiological processes and estimates its capacity as a predictor of ageing and related pathologies,a sensor of lifestyle quality and progression of suboptimal health conditions to diseases for their targeted preventionand as a potent target for cost-effective treatments tailored to the person.

Endothelins and Sarafotoxins: Peptides of Similar Structure and Different Function

Endothelins are endogenous vasoactive peptides that are considered among the most potent vasoconstrictor substances known. In addition to their vascular effects, endothelins and their receptors have been shown to be present in many organs and share plenty physiological and pathophysiological functions. Sarafotoxins are natural substances from the venom of snakes genus Atractaspis, structurally and pharmacologically near to endothelins. The current minireview focuses on the chemical and molecular aspects of endothelins and sarafotoxins, and their receptors in physiological and pathophysiological processes.

Pharmacological Potential of Endothelin Receptors Agonists and Antagonists

Endothelins are potent predominantly vasoconstricting agents that act as local autocrine and paracrine mediators. Endothelin-1 is the most potent and sustained vasoconstrictor and pressor substance yet identified. Abnormalities of the endothelin system occur in a range of diseases associated with vasoconstriction, vasospasm, and vascular hypertrophy. ET receptor antagonists were until recently regarded as drugs of great promise in patients with congestive heart failure, pulmonary hypertension and others. The aim of this article is a survey of compounds that affect the endothelin receptors and clinical trials with these agents.

Air Pollution-Induced Vascular Dysfunction: Potential Role of Endothelin-1 (ET-1) System

Exposure to air pollution negatively impacts cardiovascular health. Studies show that increased exposure to a number of airborne pollutants increases the risk for cardiovascular disease progression, myocardial events, and cardiovascular mortality. A hypothesized mechanism linking air pollution and cardiovascular disease is the development of systemic inflammation and endothelium dysfunction, the latter of which can result from an imbalance of vasoactive factors within the vasculature. Endothelin-1 (ET-1) is a potent peptide vasoconstrictor that plays a significant role in regulating vascular homeostasis. It has been reported that the production and function of ET-1 and its receptors are upregulated in a number of disease states associated with endothelium dysfunction including hypertension and atherosclerosis. This mini-review surveys epidemiological and experimental air pollution studies focused on ET-1 dysregulation as a plausible mechanism underlying the development of cardiovascular disease. Although alterations in ET-1 system components are observed in some studies, there remains a need for future research to clarify whether these specific changes are compensatory or causally related to vascular injury and dysfunction. Moreover, further research may test the efficacy of selective ET-1 pharmacological interventions (e.g., ETA receptor inhibitors) to determine whether these treatments could impede the deleterious impact of air pollution exposure on cardiovascular health.

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.

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@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.

Current state of endothelin receptor antagonism in hypertension and pulmonary hypertension

Endothelin 1 (ET-1), a potent vasoconstrictive substance, was discovered in 1988 by Yanagisawa and colleagues, and since then, a quarter of a century has passed. Understanding the biology of ET-1 has rapidly developed by characterizing the components of its receptors and processing enzymes. Numerous studies have revealed not only physiological but also various pathophysiological roles of the ET system. At first, ET-1 was the attractive and promising target for the treatment of hypertension owing to its potent vasoconstrictive nature and a variety of ET receptor antagonists (ERAs) were studied. However, the clinical application to treat hypertension was disappointing because of the side effects, including liver toxicity and fluid retention. On the other hand, ERAs have been established as orphan drugs for the treatment of pulmonary arterial hypertension and improved the prognosis of patients. Furthermore, multipotency of the ET system in the pathogenesis of multiple diseases has led to the development of translational research not only in the field of hypertension but in a variety of fields. Furthermore, a range of studies are ongoing to apply ERAs to clinical situations. In this article, we review the pathophysiological roles of the ET system in hypertension and pulmonary hypertension and the potential of ET receptor antagonism for the treatment of these diseases.

Role of endothelin in uteroplacental circulation and fetal vascular function

Endothelins are 21-amino acid peptides involved in vascular homeostasis. Three types of peptide have been identified, with endothelin-1 (ET-1) being the most potent vasoconstrictor currently known. Two endothelin receptor subtypes are found in various tissues, including the brain, heart, blood vessel, lung, and placenta. The ETA-receptor is associated with vasoconstriction in vascular smooth muscle. Conversely, the ETB-receptor can elicit a vasoconstrictor effect in vascular smooth muscle and a vasodilator effect via its action in endothelial cells. Both receptors play a key role in maintaining circulatory homeostasis and vascular function. Changes in ET-1 expression are found in various disease states, and overexpression of ET-1 is observed in hypertension and preeclampsia in pregnancy. Placental localization of ET-1 implies a key role in regulating the uteroplacental circulation. Additionally, ET-1 is important in the fetal circulation and is involved in the pulmonary circulation and closure of the ductus arteriosus after birth, as well as fetal growth constriction in utero. ET receptor antagonists and nitric oxide donors may provide therapeutic potential in treating conditions associated with overexpression of ET and hypertension.

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.

Effect of Adenotonsillectomy on Endothelin-1 and C-Reactive Protein Levels in Children with Sleep-Disordered Breathing

Objective. This study aimed to evaluate the influence of adenotonsillectomy on the plasma concentration of endothelin-1 (ET-1) and C-reactive protein (CRP) in children with sleep-disordered breathing (SDB). The relationship between quality of life and ET-1 levels was also evaluated.

Setting. Tertiary referral center.

Study Design. Before-and-after case series.

Methods. Fasting blood samples for ET-1 and high-sensitivity CRP were drawn preoperatively in all patients and at 3 to 4 months postoperatively. The Obstructive Sleep Apnea–18 (OSA-18) survey and Brouilette symptom score were completed by each child’s parents during the same time periods.

Results. The mean ET-1 level decreased from 3.51 ± 0.93 fmol/mL to 2.67 ± 0.69 fmol/mL postoperatively ( P < .01). OSA-18 survey scores and Brouilette symptom scores also decreased in the postoperative period ( P < .01). When comparing moderate and severe cases to mild cases according to Brouilette scores, ET-1 levels were significantly higher in moderate and severe cases ( P < .01). There was a significant correlation between ET-1 and the OSA-18 survey scale ( r = 0.442; P = .001). Although CRP levels decreased from 0.63 ± 1.19 mg/dL to 0.31 ± 0.23 mg/dL postoperatively, this was not statistically significant.

Conclusion. Adenotonsillectomy effectively lowered plasma ET-1 levels in children with SDB and thus may have reduced their related risk for cardiovascular disease. In addition, adenotonsillectomy improved quality of life in this group.

Mechanisms of sympathetic activation and blood pressure elevation by intermittent hypoxia

Sleep disordered breathing with recurrent apneas is one of the most frequently encountered breathing disorder in adult humans and preterm infants. Recurrent apnea patients exhibit several co-morbidities including hypertension and persistent sympathetic activation. Intermittent hypoxia (IH) resulting from apneas appears to be the primary stimulus for evoking autonomic changes. The purpose of this article is to briefly review the effects of IH on chemo- and baro-reflexes and circulating vasoactive hormones and their contribution to sympathetic activation and blood pressures. Sleep apnea patients and IH-treated rodents exhibit exaggerated arterial chemo-reflex. Studies on rodent models demonstrated that IH leads to hyperactive carotid body response to hypoxia. On the other hand, baro-reflex function is attenuated in patients with sleep apnea and in IH-treated rodents. Circulating vasoactive hormone levels are elevated in sleep apnea patients and in rodent models of IH. Thus, persistent sympathetic activation and hypertension associated with sleep apneas seems to be due to a combination of altered chemo- and baro-reflexes resulting in sympathetic activation and action of elevated circulating levels of vasoactive hormones on vasculature.

Cardiac toxicity of high-dose cyclophosphamide and melphalan in patients with multiple myeloma treated with tandem autologous hematopoietic stem cell transplantation

Tandem autologous hematopoetic stem cell transplantation (HSCT) is an effective treatment in patients with multiple myeloma (MM). Patients receive high-dose cyclophosphamide (CY) followed by two myeloablative dosages of melphalan (MEL). Cardiotoxicity treatment related data are scanty. In 30 patients with MM chemotherapy was followed by high-dose CY (cycle CY), and two autologous tandem HSCT treatments with MEL (cycles MEL I and MEL II). During each 15-day treatment troponin I (TnI), brain natriuretic peptide (BNP) and endothelin-1 (ET-1) were controlled at six time points. All patients underwent conventional and tissue Doppler echocardiography prior to CY therapy (Eho 0), before cycle MEL I (Eho 1), before cycle MEL II (Eho 2), and 3 months after the completion of therapy (Eho 3). None of the patients developed clinical signs of heart failure. The peak TnI concentrations were noted at days 8, 11, and 15 during all three chemotherapy cycles. During all three cycles there was a significant increase in baseline BNP concentrations and BNP levels measured at day 1 after treatment with CY and MEL (CY: P = 0.0001, MEL I: P = 0.001, MEL II: P = 0.001). The highest BNP concentration occurred during CY treatment (0.517 +/- 0.391 microg/L). During cycles MEL I and MEL II we noted the peak BNP concentrations at day 4 following chemotherapy (MEL I 0.376 +/- 0.418 microg/L; MEL II 0.363 +/- 0.379 microg/L). During all three cycles the highest ET-1 levels occurred at day 1 after chemotherapy (CY 1.146 +/- 1.313 ng/L; MEL I 1.054 +/- 2.242 ng/L; MEL II 0.618 +/- 0.539 ng/L). A significant increase in ET-1 concentrations relative to the basal values occurred only in cycle MEL II (P = 0.003). The duration of wave a in the Doppler pulmonary vein flow increased significantly (Eho 0/Eho 1: P = 0.008, Eho 0/Eho 3: P = 0.026). There was a significant decrease in the A/a ratio in flow velocities during chemotherapy (Eho 0/Eho 1: P = 0.002, Eho 0/Eho 3: P < 0.0001). Early diastolic tissue Doppler velocities (Em) decreased significantly during individual cycles of chemotherapy (P = 0.006). A significant post-treatment increase in the incidence of mitral regurgitation was observed (Eho 0/Eho 3: P = 0.003). Treatment of MM patients with tandem autologous HSCT is cardiotoxic. Our patients did not develop clinically overt heart failure or myocardial necrosis. Increased plasma levels of BNP and ET-1 were compatible with transient neurohormonal activation of heart failure. Doppler echocardiography studies revealed worsening of left ventricular diastolic function and occurrence of functional mitral regurgitation.

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).

Enzymatic pathways involved in the generation of endothelin-1(1-31) from exogenous big endothelin-1 in the rabbit aorta

We investigated whether blood vessels contribute to the production of ET-1(1-31) from exogenous big endothelin-1 (BigET-1) in the rabbit and assessed which enzymes are involved in this process. Vascular reactivity experiments, using standard muscle bath procedures, showed that BigET-1 induces contraction in endothelium-intact rabbit aortic rings. Preincubation of the rings with phosphoramidon, CGS35066 or thiorphan reduced BigET-1-induced contraction. Conversely, chymostatin did not affect BigET-1-induced contraction. Thiorphan and phosphoramidon, but not CGS35066 or chymostatin, reduced ET-1(1-31)-induced contraction. None of the enzymatic inhibitors affected the contraction afforded by ET-1.BQ123-, but not BQ788-, selective antagonists for ET(A) and ET(B) receptors, respectively, produced concentration-dependent rightward displacements of the ET-1(1-31) and ET-1 concentration-response curves. By the use of enzymatic assays, we found that the aorta, as well as the heart, lung, kidney and liver, possess a chymase-like activity. Enzyme immunoassays detected significant levels of Ir-ET-1(1-31) in bathing medium of aortas after the addition of BigET-1 (30 nM). Neither thiorphan nor chymostatin altered the levels of Ir-ET-1(1-31). Conversely, the levels of Ir-ET-1(1-31) were increased in the presence of phosphoramidon. This marked increase of the 31-amino-acid peptide was abolished when phosphoramidon and chymostatin were added simultaneously. The major new finding of the present work is that the rabbit aorta generates ET-1(1-31) from exogenously administered BigET-1. Additionally, by measuring the production of ET-1(1-31), we showed that a chymase-like enzyme is involved in this process when ECE and NEP are inhibited by phosphoramidon. Our results also suggest that ET-1(1-31) is an alternate intermediate in the production of ET-1 following BigET-1 administration. Finally, we showed that NEP is the predominant enzymatic pathway involved in the cleavage of ET-1(1-31) to a bioactive metabolite that will act on ET(A) receptors to induce contraction in the rabbit aorta.

Intraspinal application of endothelin results in focal ischemic injury of spinal gray matter and restricts the differentiation of engrafted neural stem cells

Previous data have shown that pluripotent stem cells engrafted into the contused spinal cord differentiate only along an astrocytic lineage. The unknown restrictive cues appear to be quite rigid as even neuronal-restricted precursors fail to differentiate to the mature potential they exhibit in vitro after similar grafting into the contused spinal cord. It has been hypothesized that this potent lineage restriction is, in part, the result of the significant loss of both gray and white matter observed following spinal contusion, which elicits a massive acute inflammatory response and is manifested chronically by dramatic cystic cavitation. To evaluate the gray matter component, we developed a clinically relevant model of focal gray matter ischemic injury using the potent vasoconstrictor endothelin (ET-1) and characterized the differentiation of pluripotent stem cells transplanted into this atraumatic vascular SCI. Results demonstrate that low dose ET-1 microinjection into cervical spinal gray matter results in an inflammatory response that is temporally comparable to that observed following traumatic SCI, as well as chronic gray matter loss, but without significant cystic cavitation or white matter degeneration. However, despite the preservation of host spinal parenchyma, no elaboration of neuronal phenotypes was observed from engrafted stem or precursor cells. These results suggest that a common pathologic component responsible for this lineage restriction exists between contusive SCI and ET-1 mediated focal ischemic SCI.

The endothelin receptor antagonist TAK-044 in the treatment of reperfusion injury in organ transplantation

Ischaemia and reperfusion are complex interrelated events that involve the vascular endothelium. The endothelium produces endothelin (ET), the most potent vasoconstrictor identified to date. During ischaemia, endothelial transcription of ET is increased, leading to strong and sustained vasoconstriction. Ischaemia and reperfusion are indispensable parts of solid organ transplantation. Various experimental data discussed in this review show an efficacy of the mixed ET(A/B) antagonist TAK-044 in the treatment of reperfusion injury. The overall safety and tolerability of TAK-044 in humans seems to be excellent. Large clinical randomised trials on the use of TAK-044 in the treatment of reperfusion injury in different clinical situations are missing, nevertheless, the future for ET antagonists is promising.

Elevation of plasmatic endothelin in patients with heart failure

BACKGROUND

Endothelin 1 is an autocrine and paracrine factor with vasoconstrictive, mitogenic, and inotropic activities in vascular and cardiac muscles. Its elevation has been reported in patients with chronic heart failure and its production may be conditioned by activation of other neurohumoral factors that are stimulated by the disease.

METHODS

The objective of this study was to correlate level of endothelin (ET) with echocardiographic, clinical, and biochemical markers and to determine its role as an independent marker of severity. We included patients with congestive heart failure in whom echocardiographic evaluation had been done and serum markers measured. Serum endothelin 1 levels were determined by radioimmunoassay (RIA). Correlation between endothelin concentration, echocardiographic parameters, potentially confounding factors, and severity of heart failure was made.

RESULTS

Patients with symptomatic heart failure and longer time of evolution had higher levels of endothelin unrelated to levels of troponin T, tumor necrosis factor, and atrial natriuretic peptide. There were no differences in levels of endothelin, independently of the etiology of cardiac failure.

CONCLUSIONS

These results support the idea that endothelin plays an important independent role in the physiopathology of heart failure. It may be a severity marker and an attractive therapeutic target.

Role of endothelin in cardiovascular disease

Endothelins are a family of peptides, which comprises endothelin-1 (ET-1), endothelin-2 (ET-2) and endothelin-3 (ET-3), each containing 21 amino-acids. ET-1 is a peptide secreted mostly by vascular endothelial cells, the predominant isoform expressed in vasculature and the most potent vasoconstrictor currently known. ET-1 also has inotropic, chemotactic and mitogenic properties. In addition, it influences salt and water homeostasis through its effects on the renin-angiotensin-aldosterone system (RAAS), vasopressin and atrial natriuretic peptide and stimulates the sympathetic nervous system. The overall action of endothelin is to increase blood pressure and vascular tone. Therefore, endothelin antagonists may play an important role in the treatment of cardiac, vascular and renal diseases associated with regional or systemic vasoconstriction and cell proliferation, such as essential hypertension, pulmonary hypertension, chronic heart failure and chronic renal failure. Long-term anti-endothelin therapy may improve symptoms and favourably alter the progression of heart failure. Endothelin appears to participate in induction and progression of sclerotic renal changes, leading to progression to end-stage renal disease. Anti-endothelin therapy might offer additional benefits in the prevention of progression of chronic renal failure in addition to the known benefits of RAAS inhibition. Clinical trials have demonstrated potentially important benefits of endothelin antagonists for patients with essential hypertension, pulmonary hypertension and heart failure. Further studies are necessary to determine the role of anti-endothelin therapy in the treatment of cardiovascular diseases and determine the different roles of selective receptor antagonism vs. mixed ET(A/B)-receptor antagonism in human diseases.

The chemical syntheses and bioactivities of novel peptide-based endothelin antagonists

Endothelin antagonists, novel tripeptides containing a series of unnatural amino acids, were synthesized and characterized. A linear peptide BQ-485, perhydroazepin-1-yl-L-leucyl (1)-D-tryptophanyl (2)-D-tryptophan (3), was selected as the parent compound. The introduction of D-Phe derivatives into these peptidic ET antagonists resulted in potent activity against the contraction of rat aortic smooth muscles induced by ET-1 (10 nM) which activated the ET receptors. Among these compounds, 15 tripeptides had high enough antagonistic activity at the level of 10(-7) mol/L (IC50). The activity of three compounds was 10(-6) mol/L (IC50). These HIM-CO-Leu-D-Trp-D-Phe(-R)-OH compounds as ETA antagonists may provide a tool for the development of therapeutic agents in the treatment of putative ET-1-related disorders.

Endothelin system: the double-edged sword in health and disease

The endothelin system consists of two G-protein-coupled receptors, three peptide ligands, and two activating peptidases. Its pharmacological complexity is reflected by the diverse expression pattern of endothelin system components, which have a variety of physiological and pathophysiological roles. In the vessels, the endothelin system has a basal vasoconstricting role and participates in the development of diseases such as hypertension, atherosclerosis, and vasospasm after subarachnoid hemorrhage. In the heart, the endothelin system affects inotropy and chronotropy, and it mediates cardiac hypertrophy and remodeling in congestive heart failure. In the lungs, the endothelin system regulates the tone of airways and blood vessels, and it is involved in the development of pulmonary hypertension. In the kidney, it controls water and sodium excretion and acid-base balance, and it participates in acute and chronic renal failure. In the brain, the endothelin system modulates cardiorespiratory centers and the release of hormones. More advanced functional analysis of the endothelin system awaits not only additional pharmacological studies using highly specific endothelin antagonists but also the generation of genetically altered rodent models with conditional loss-of-function and gain-of-function manipulations.

The endothelin system in human saphenous vein graft disease

Saphenous vein graft stenosis is a significant clinical complication for coronary artery bypass patients. Endothelin-1, a peptide synthesised by vascular endothelial cells, is the most potent known vasoconstrictor and has mitogenic properties. Recent advances in our knowledge of endothelin-1 synthesis and endothelin receptor expression and function in normal and atherosclerotic human saphenous vein imply a role for the peptide in the progression of vein graft failure. Manipulation of the endothelin system, by selective receptor antagonism or inhibition of the specific endothelin-converting enzymes may, therefore, represent a novel therapeutic target for treating vein graft disease.

Endothelins: vasoactive modulators of renal function in health and disease

Vasoactive autocoids with directly opposing actions on the renal vasculature, glomerular function, and in salt and water homeostasis have been demonstrated in the kidney. In the renal cortex, endothelin (ET)-1 and angiotensin-II cause vasoconstriction, decreasing renal blood flow, and glomerular filtration rate, whereas bradykinin and atrial natriuretic peptide cause vasodilation and increase glomerular capillary permeability. ET-1 causes vasoconstriction of the afferent and efferent arteries and outer medullary descending vasa recta, thereby decreasing vasa recta and papillary blood flow, while bradykinin has the opposite effect. ET-1 stimulates cell proliferation, increasing the expression of several genes, including collagenase, prostaglandin endoperoxidase synthase, and platelet-derived growth factor. ET-1 promotes natriuresis via the ET-B receptor, causing down-regulation of the epithelial Na(+) channel in the renal tubule. Thus, ETs affect three major aspects of renal physiology: vascular and mesangial tone, Na(+) and water excretion, and cell proliferation and matrix formation.

Regulation of endothelin-1 production in cultured rat vascular smooth muscle cells

Endothelin-1 (ET-1) is secreted from all rat vascular smooth muscle cells (VSMCs) examined, in addition to endothelial cells (ECs). An average secretion rate of ET-1 from rat VSMCs was determined to be 10% that excreted from ECs. We examined the effects of 22 substances on ET-1 secretion from VSMCs and compared them with those from ECs. Transforming growth factor-beta1 (TGF-beta), acidic and basic fibroblast growth factors, epidermal growth factor, angiotensin II, and adrenaline stimulated ET-1 secretion from VSMCs, whereas forskolin, thrombin, and platelet-derived growth factor-BB reduced it. Only TGF-beta and phorbol ester elicited consistent effects on ET-1 secretion from VSMCs and ECs. Regulation of ET-1 and adrenomedullin secretion from VSMCs was distinctly different. These data suggest that ET- 1 production in VSMCs is regulated by a mechanism separate from that in ECs and from adrenomedullin production in VSMCs. Chromatographic analysis showed immunoreactive ET-1 secreted from VSMCs was mainly composed of big ET- 1, whereas approximately 90% of that from ECs was ET-1. By TGF-beta stimulation of VSMCs, the ratio of big ET-1 to ET-1 was further increased. Because big ET-1 is converted into ET-1 only on the surface of the ECs in the culture system, big ET-1 secreted from the VSMCs may function as a mediator transmitting a signal from VSMCs to ECs in vivo.

Endothelial dysfunction and hypertensive vasoconstriction

A change in endothelial function is a common phenomenon in patients with essential hypertension and in animals with hypertension, whether primary or induced by a salt-rich diet. In hypertensive subjects, there may be a change in the synthesis, or the effect, of nitric oxide. Nevertheless, hypertensive vasoconstriction is at present associated, above all, with the degradation of this mediator by free radicals, such as the superoxide anion, released in the dysfunctional vascular endothelium. These radicals are also formed when hypoxanthine is turned into xanthine, and when the latter becomes uric acid, both having been catalysed by the enzyme xanthine oxidase. In physiological conditions, the concentration of superoxide radicals remains low within the organism as a result of its reaction with the superoxide dismutase enzyme. However, in pathological situations, such as arterial hypertension, there may be an increase in the production of these radicals or a deficiency of the superoxide dismutase enzyme. In hypertensive patients, the release of vasoconstrictor peroxides derived from the activity of cyclo-oxygenase in the endothelium and the vascular smooth muscle is also important. The excess free radicals released by the dysfunctional endothelium also stimulate the synthesis of these contracting agents. Moreover, it should not be forgotten that endothelin-1, which is similarly synthesized and released in the vascular endothelium, is the most powerful known endogenous vasoconstrictor. This peptide would therefore play a prominent part in some forms of hypertension. Although no changes in endothelin plasma levels have been found in essential hypertension, there may be an increase in its local concentration. It should be borne in mind that endothelin could strengthen the effect of other vasoconstrictors. Moreover, it may also provoke the release of free radicals and of cyclo-oxygenase-derived vasoconstrictor factors. The latest theories therefore indicate that the increase in vasoconstriction, which characterizes arterial hypertension, is associated with a greater production of free radicals. At the present time, antioxidant agents and xanthine oxydase-inhibiting compounds are being used to treat hypertension and other pathologies linked to endothelial dysfunction. In addition, it is thought that the therapeutic benefit of some anti-hypertensive drugs, such as calcium antagonists and angiotensin-converting enzyme inhibitors, could be in part due to the inhibition of the production of free radicals that they provoke.

The stimulatory effect of endothelin-1 on frog adrenocortical cells is mediated through both the phospholipase C and the adenylyl cyclase transduction pathways

We have previously shown that endothelin-1 (ET-1) stimulates corticosterone and aldosterone secretion by the frog adrenal gland through activation of ET(A) receptors. In the present study, we have investigated the transduction pathways involved in the corticotropic action of ET-1. Exposure of frog adrenal explants to ET-1 provoked a time- and dose-dependent increase in inositol phosphate production and a parallel decrease in membrane polyphosphoinositide content. Incubation of adrenal explants with ET-1 also induced a dose-related increase of cAMP formation. The selective ET(A) receptor antagonist BQ-485 totally abolished the stimulatory effects of ET-1 on both inositol phosphate and cAMP production. In contrast, the selective ET(B) receptor agonist IRL 1620 did not significantly modify polyphosphoinositide hydrolysis or cAMP formation. Administration of the phospholipase C inhibitor U-73122 or the protein kinase A inhibitor H-89 to perifused frog adrenal slices significantly reduced the stimulatory effect of ET-1 on corticosterone and aldosterone secretion. Concomitant administration of the two inhibitors almost completely suppressed the corticotropic effect of ET-1. Taken together, these data indicate that, in the frog adrenal gland, the stimulatory effect of ET-1 on corticosteroid secretion is mediated through activation of both the phospholipase C and the adenylyl cyclase transduction pathways.

Endothelin as a regulator of cardiovascular function in health and disease

The endothelins are a family of endothelium-derived peptides that possess characteristically sustained vasoconstrictor properties. Endothelin-1 appears to be the predominant member of the family generated by vascular endothelial cells. In addition to its direct vascular effects, endothelin-1 has inotropic and mitogenic properties, influences homeostasis of salt and water, alters central and peripheral sympathetic activity and stimulates the renin-angiotensin-aldosterone system. Studies with endothelin receptor antagonists have indicated that endothelin-1 probably has complex opposing vascular effects mediated through vascular smooth muscle and endothelial ET(A) and ET(B)receptors. Endogenous generation of endothelin-1 appears to contribute to maintenance of basal vascular tone and blood pressure through activation of vascular smooth muscle ET(A)receptors. At the same time, endogenous endothelin-1 acts through endothelial ET(B) receptors to stimulate formation of nitric oxide tonically and to oppose vasoconstriction. In view of the multiple cardiovascular actions of endothelin-1, there has been much interest in its contribution to the pathophysiology of hypertension. Results of most studies suggest that generation of, or sensitivity to, endothelin-1 is no greater in hypertensive than it is in normotensive subjects. Nonetheless, the deleterious vascular effects of endogenous endothelin-1 may be accentuated by reduced generation of nitric oxide caused by hypertensive endothelial dysfunction. It also appears likely that endothelin participates in the adverse cardiac and vascular remodelling of hypertension, as well as in hypertensive renal damage. Irrespective of whether vascular endothelin activity is increased in hypertension, anti-endothelin agents do produce vasodilatation and lower blood pressure in hypertensive humans. There is more persuasive evidence for increased endothelin-1 activity in secondary forms of hypertension, including pre-eclampsia and renal hypertension. Endothelin-1 also appears to play an important role in pulmonary hypertension, both primary and secondary to diseases such as chronic heart failure. The hypotensive effects of endothelin converting enzyme inhibitors and endothelin receptor antagonists should be useful in the treatment of hypertension and related diseases. Development of such agents will increase knowledge of the physiological and pathological roles of the endothelins, and should generate drugs with novel benefits.

Endothelin-converting enzyme in the human vasculature: evidence for differential conversion of big endothelin-3 by endothelial and smooth-muscle cells

Our aim was to localize endothelin-converting enzyme (ECE) in human saphenous vein grafts and to quantify enzymic activity in cultured human endothelial and smooth-muscle cells. Immunoreactive ECE localized to the endothelium and infiltrating macrophages in vein grafts, but little or no immunoreactivity was detected within the media or proliferated smooth muscle of the occlusive lesion. Cultures of human umbilical vein endothelial cells were incubated with big endothelin-1 (ET-1) (10 nM) to measure extracellular conversion. After 2 h the concentration of mature peptide in the medium was increased by 162.7 +/- 21.6 pM (n = 3 +/- SEM) above basal. Permeabilization of the cells increased conversion to 1077.9 +/- 52.8 pM, suggesting that about 85% of ECE activity was located intracellularly. In both cases, activity was inhibited by phosphoramidon but not by thiorphan. In contrast, conversion of big ET-3 (10 nM) under the same conditions was not detected in either intact or permeabilized cells after 2 h. Big ET-3 and big ET-1 were converted by a phosphoramidon-sensitive/thiorphan-insensitive enzyme on the surface of confluent cultures of human umbilical vein smooth-muscle cells, with concentrations of the corresponding mature peptides increasing by 99.5 +/- 14.5 pM and 222.2 +/- 11.6 pM, respectively. These results suggest that smooth-muscle cells could be responsible for the synthesis of ET-3 present in plasma and for additional processing of big ET-1 released by endothelial cells.

Endothelin converting-enzyme-1 mRNA expression in human cardiovascular disease

Endothelin-1 converting-enzyme (ECE-1) cleaves the precursor, big-endothelin-1, to the active peptide endothelin-1. The aim of this study was to investigate whether ECE-1 mRNA expression is modified in human cardiovascular disease. Tissue samples from the left human atrium were analyzed for ECE-1 expression and related to different clinical parameters. A quantitative PCR assay (qPCR) with competitive and non-competitive standards was established. The ECE-1 measurements were normalized by a GAPDH qPCR. Patients who suffered from a myocardial infarction had elevated ECE-1 levels when compared to controls (5.81+/-0.76 vs. 3.20+/-0.51 fg ECE-1, ng GAPDH, p<0.05). Drug treatment with the beta-blocker metoprolol was associated with a decreased ECE-1 expression level (3.90+/-0.58 vs. 5.81+/-0.76 fg ECE-1, ng GAPDH, p<0.1). We conclude that the expression of ECE-1 is altered in the atrial tissue depending on the physiological status of the heart. This suggests a differential role of ECE-1 in cardiovascular diseases.

The endothelin system in cardiovascular physiology and pathophysiology

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

Raised endothelin 1 levels in patients with colorectal liver metastases

BACKGROUND

Endothelin 1 (ET-1), a vasoconstrictor peptide, has been implicated as a tumour growth stimulator and an angiogenesis factor.

METHODS

To assess the involvement of ET-1 in colorectal cancer, immunoelectron microscopy for ET-1 was performed in colorectal liver metastases and normal liver (n = 6). ET-1 plasma levels were measured by radioimmunoassay in patients with colorectal cancer, with (n = 18) and without (n = 12) liver metastases, and in controls (n = 22).

RESULTS

In normal liver, ET-1 was present in endothelial cells; in tumour, it was observed in endothelial cells, tumour cells and myofibroblasts. Mean(s.d.) plasma ET-1 levels were 2.75 (1.37) pg/ml in controls, 4.53(1.61) pg/ml in patients with colorectal liver metastases (P = 0.001) and 3.92(1.32) pg/ml in patients without metastases (P = 0.02).

CONCLUSION

ET-1 was present in various cell types within colorectal liver metastases and raised levels were found in the plasma of patients with colorectal cancer. ET-1 may not only modulate tumour vascular tone but also act on tumour growth and angiogenesis, both locally and systemically.

Evidence using immunoelectron microscopy for regulated and constitutive pathways in the transport and release of endothelin

We investigated the distribution of endothelin (ET)-like immunoreactivity in the human coronary artery and examined sites linked to the storage and release of intracellular proteins. Intense ET-like immunoreactivity was observed at the light-microscope level in luminal coronary artery endothelial cells. A low level of staining also was detected in the outer medial smooth-muscle layer and diffusely within the adventitia. Immunoelectron microscopy was used to determine the ultrastructural localisation of ET in the endothelium. Positive ET-like immunoreactive staining was detected in secretory vesicles at the ultrastructural level. Quantitative immunoelectron microscopy revealed that ET-like immunoreactivity was predominantly localised to the cytoplasmic matrix (including secretory vesicles) and Weibel-Palade bodies (endothelial cell-specific storage granules). Labelling was detected in 44% of Weibel-Palade body profiles positively identified by using antisera to von Willebrand factor and in cytoplasm surrounding these structures. A low level of immunoreactive staining was associated with mitochondria, whereas the cell nucleus and Golgi complex showed little or no positive staining. These findings indicate that ET is released from human coronary artery endothelial cells via two distinct secretory pathways. We propose that ET is continuously transported in and released from secretory vesicles by the constitutive secretory pathway. ET may also be stored in Weibel-Palade bodies and released after an appropriate stimulus by the regulated pathway. Positive immunoreactivity was also observed in plasmalemmal vesicles (50- to 60-nm diameter), indicating a role for these structures in endocytosis.

Endothelin converting enzyme (ECE) activity in human vascular smooth muscle

1. We have characterized the human smooth muscle endothelin converting enzyme (ECE) present in the media of the endothelium-denuded human umbilical vein preparation. 2. Endothelin-1 (ET-1) and ET-2 were potent constrictors of umbilical vein with EC50 values of 9.2 nM and 29.6 nM, respectively. ET-1 was at least 30 times more potent than ET-3 suggesting the presence of constrictor ETA receptors. Little or no response was obtained to the ETB-selective agonist sarafotoxin 6c. These data suggest that endothelin-mediated vasoconstriction is via ETA receptors in this preparation. 3. Autoradiographical visualization of endothelin receptors with subtype selective ligands confirmed the predominance of the ETA receptor in the media of umbilical vein. High density of binding was obtained with the ETA selective [125I]-PD151242, with much lower levels detected with the ETB selective [125I]-BQ3020. 4. Big ET-1 (EC50 = 42.7 nM) and big ET-2(1-38) (EC50 = 99.0 nM) were less potent than ET-1 and ET-2, respectively. Big ET-2(1-38) was more potent than its isoform big ET-2(1-37) with concentration-response curves to big ET-2(1-37) incomplete at 300 nM. No response was obtained to big ET-3 at concentrations up to 700 nM. The C-terminal fragments, big ET-1(22-38) and big ET-2(22-38) were inactive. 5. Responses to ET-1 were unaffected by either the neutral endopeptidase (NEP) inhibitor thiorphan (10(-5) M) or by the dual NEP/ECE inhibitor phosphoramidon (10(-5) M). Big ET-1 was also unaffected by thiorphan but antagonized in a concentration-dependent manner by phosphoramidon (10(-5) M and 10(-4) M). 6. Addition of all four big endothelin peptides to human umbilical vein preparations resulted in detectable amounts of ET-IR in the bathing medium. Therefore, although big ET-3 was functionally inactive this reflects the low potency of ET-3 at the ETA receptor rather than the lack of ability of this smooth muscle ECE to convert big ET-3 to ET-3. 7. To conclude we have demonstrated the presence of a phosphoramidon-sensitive ECE on the smooth muscle layer of the human umbilical vein which can convert big ET-1, big ET-2(1-37), big ET-2(1-38) and big ET-3 to their mature biologically active forms. The precise subcellular localization of this enzyme and its physiological relevance remains to be determined.

Differential distribution of endothelin peptides and receptors in human adrenal gland

Sub-type selective ligands revealed a differential distribution of endothelin (ET) receptors within human adrenal glands. High densities of ETA receptors were localized, using [125I]-PD151242, to the smooth muscle layer of the arteries, smaller vessels within the capsular plexus and to the secretory cells of zona glomerulosa (KD = 139.8 +/- 39.7, Bmax = 69.7 +/- 9.1 fmol mg-1 protein, mean of 3 individuals+/-sem). ETB receptors were present in the medulla (KD = 145.2 +/- 16.4, Bmax = 75.5 +/- 12.3), zona glomerulosa (KD = 100.6 +/- 35.1, Bmax = 63.1 +/- 10.0), fasiculata (KD 145.1 +/- 16.2, Bmax = 67.9 +/- 6.9) and reticularis (KD = 118.2 +/- 18.6, Bmax = 71.9 +/- 6.5). ETB receptors were not detected within the smooth muscle of the vasculature. Messenger RNA encoding both sub-types was present in adrenals. ET-like immunoreactivity was localized to the cytoplasm of the endothelial cells from arteries supplying the gland and resistance vessels within the capsular plexus. Staining was also detected in these cells using anti-big ET-1 and less intensely with anti-big ET-2 antisera but not within cells within the cortex or medulla. Big ET-3-like immunoreactivity was localized to secretory cells of the medulla. Staining was not found using antiserum that could detect ET-3, suggesting further processing of big ET-3 may occur within the plasma, and that the adrenals could be a source of ET-3. The presence of ET-1 was confirmed by high performance liquid chromatography and radioimmunoassay although ET-3 was not detected. The results suggest that ET-1 is the predominant mature isoform, which is localized mainly to adrenal vasculature, particularly the capsular plexus, and may contribute to blood flow regulation in the gland.

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.

Characterization of the renal phenotype of transgenic rats expressing the human endothelin-2 gene

We have previously established a transgenic rat model termed TGR(hET-2)37 overexpressing the human endothelin-2 (ET-2) gene with high renal transgene expression. This renal overexpression is of pathophysiological interest because a long-term activated paracrine renal endothelin system has been implicated in chronic renal failure due to progressive glomerular injury. Therefore, our aim in the present study was to analyze renal transgene expression in detail and address the question of whether transgene expression causes phenotypic and functional changes in the kidney. We used reverse transcription-polymerase chain reaction and in situ hybridization techniques for transgene expression analysis. Tissue ET-2 concentrations were measured with a specific radioimmunoassay. For histological evaluation of renal tissue, all samples were subjected to hematoxylin-eosin and periodic acid-Schiff staining. Renal tissue ET-2 concentrations were significantly increased in TGR(hET-2)37 rats. Using in situ hybridization, we found that the human ET-2 gene was almost exclusively expressed within the glomeruli. The glomerular transgene expression resulted in a significantly increased glomerular injury score and likewise in a significantly increased protein excretion, whereas glomerular filtration rate was not altered. Blood pressure was similar in TGR(hET-2)37 rats and age-matched controls, suggesting that the local changes in the kidney were correlated with paracrine endothelin actions. In conclusion, our study revealed that the major renal expression site of the human ET-2 transgene in TGR(hET-2)37 rats was within the glomeruli and caused the development of glomerulo-sclerosis with significantly increased protein excretion that is independent of blood pressure. We suggest that TGR(hET-2)37 rats are a new monogenetic animal model for study of the paracrine renal endothelin system and its involvement in renal pathophysiology.

Endothelin-1 is increased overlying atherosclerotic plaques in human arteries

The distribution of Endothelin-1 (ET-1), a potent vasoactive peptide, within endothelium of human atherosclerotic arteries was examined using a novel en face immunohistochemical technique. The vast majority of endothelial cells were immunoreactive for ET-1. Staining intensity was increased in areas overlying atherosclerotic plaques, calcified media, fatty streaks and about flow dividers, compared with adjacent regions. Multinucleated 'giant' endothelial cells were more common in regions containing strong ET-1 staining than elsewhere. Clusters of leucocytes (probably monocytes) were frequently observed adhering to the endothelial monolayer but not neighbouring regions of denudation. Occasionally underlying macrophage/foam cells and smooth muscle cells were exposed to the surface and included in the en face (Häutchen) preparation. Smooth muscle cells did not stain for ET-1 while macrophages and the larger foam cells were positive for ET-1. These results support the hypothesis that expression of ET-1, at sites containing atheromatous disease, may be involved in the development of atherosclerosis.

Characterization of the endothelin receptor selective agonist, BQ3020 and antagonists BQ123, FR139317, BQ788, 50235, Ro462005 and bosentan in the heart

1. In this study we used ligand binding techniques to determine the affinity and selectivity of endothelin receptor agonists and antagonists in human left ventricle which expresses both ETA and ETB receptors, and compared these results with cardiovascular tissues from rat and porcine hearts. 2. The linear tripeptide antagonist, FR139317 competed for [125I]-ET-1 binding to human left ventricle with over 200,000 fold selectivity for the ETA receptor (KD ETA = 1.20 +/- 0.28 nM, KDETB = 287 +/- 93 microM). The ETA-selective non-peptide antagonist, 50235, competed with lower affinity and selectivity (KDETA = 162 +/- 61 nM, KDETB = 171 +/- 42 microM) in this tissue. BQ123 and FR139317 also showed high selectivity (greater than 20,000 fold) and affinity in rat (BQ123: KDETA = 1.18 +/- 0.16 nM, KDETB = 1370 +/- 1150 microM; FR139317: KDETA = 2.28 +/- 0.30 nM, KDETB = 292 +/- 114 microM) and pig heart (BQ123: KDETA = 0.52 +/- 0.05 nM, KDETB = 70.4 +/- 4.0 microM; FR139317: KDETA = 2.17 +/- 0.51 nM, KDETB = 47.1 +/- 5.7 microM) (n > or = 3 individuals +/- s.e.mean). 3. Although BQ3020 competed with over 1000 fold selectivity for the ETB subtype in human heart (KDETB = 1.38 +/- 0.72 nM, KDETA = 2.04 +/- 0.21 microM) the peptide inhibited only the binding of [125I]-ET-1 at concentrations greater than 100 nM in rat and porcine heart. This is in contrast to the data from the ETA-selective antagonists which indicated the presence of ETB sites in these tissues from animal hearts. 4. The peptide antagonist, BQ788, had a low, micromolar affinity (KD = 1.98 +/- 0.13 microM) using human left ventricle and no significant selectivity for the human ETB-subtype in this tissue. 5. The non-peptide ET antagonists, Ro462005 (KD = 50.3 +/- 9.5 microM) and bosentan (Ro470203; KD = 77.9 +/- 7.9 nM) competed monophasically for [125I]-ET-1 binding sites in human left ventricle. 6. The results show that the ETA antagonists, BQ123 and FR139317, are highly selective for ETA receptors in all cardiac tissues tested, whereas BQ788 has a low affinity and no selectivity in this human tissue. Further we showed that there are species differences in the binding of BQ3020 to the ETB receptors in the hearts derived from human, rat and pig.

Localization of endothelin peptides in human kidney

We investigated the synthesis and localization of endothelin isoforms in the human kidney using the reverse-transcriptase polymerase chain reaction (RT-PCR) and immunocytochemistry. PCR products corresponding to the expected size for mRNA encoding ET-1, ET-2 and ET-3 were found in homogenates of renal medulla, cortex and vessels from each of five individuals. Using four rabbit polyclonal antibodies to assess the distribution of mature ET, Big ET-1, Big ET-2 and Big ET-3 immunoreactivity in the human kidney, mature IR ET localized to the cytoplasm of endothelial cells lining intra-renal blood vessels including interlobular and arcuate arteries, arterioles and adjacent arcuate veins, all of which showed strongly positive staining. IR Big ET-1 co-localized with the mature peptide. No specific staining was detected within these anatomical regions when pre-immune sera were substituted or primary antibody omitted. Mature IR ET also localized to the cytoplasm of endothelial cells within the glomerulus. Other capillary endothelial cells did not stain, and other structures stained only faintly by comparison. IR Big ET-2 and Big ET-3 could not be detected. These results show that human kidney contains mRNA encoding all three peptide isoforms, but only mature ET and Big ET-1 peptides could be detected by immunocytochemical staining. This provides further evidence that ET-1 may function as a renal peptide in humans, as it is locally synthesized within the kidney.

Effects of oxidized low density lipoprotein on endothelin secretion by cultured endothelial cells and macrophages

The aim of this study was to determine how oxidized LDL affects endothelin secretion by endothelial cells, monocytes and macrophages. It was found that different degrees of oxidation of LDL had different effects on endothelin production. Extensively oxidized LDL inhibited endothelin secretion from cultured endothelial cells. It also attenuated endothelin secretion from phorbol ester-activated macrophages. The inhibitory effect on endothelin secretion required a substantial degree of LDL oxidation as reflected by an increase in absorbance at 234 nm (conjugated diene) of 0.7 AU with 125 nM LDL and a two- to three-fold increase in migration distance on electrophoresis. Oxidized LDL inhibited thymidine incorporation in porcine aortic endothelial cells, hence in these cells cytotoxicity may account for at least part of the inhibition of endothelin secretion. Acetyl LDL slightly increased basal endothelin release by endothelial cells, but native LDL or mildly oxidized LDL had no significant effect. Overall, the present findings argue against a stimulatory effect of oxidized LDL on endothelin release as contributing to increased vasoreactivity in atherosclerosis. In fact, the apparent inhibition of endothelin release by extensively oxidized LDL might tend to attenuate vasoreactivity near atherosclerotic lesions.

Endothelin in congestive heart failure

The endothelin (ET) family of peptides have potent vascular, cardiac and renal actions which may be of pathophysiological importance in congestive heart failure (CHF). In vivo studies with selective and non-selective ET receptor antagonists are required to clarify the role of ET in the pathophysiology of CHF and determine whether anti-ET drugs may be therapeutically useful in CHF. The impact of angiotensin converting enzyme (ACE) inhibitors on the management of CHF has been such that for any new treatment to be of value it will probably have to offer hemodynamic benefit over and above that already obtained with an ACE inhibitor; anti-ET agents seem to have this potential. The recent formal cloning and characterization of endothelin converting enzyme (ECE) should hasten the development of specific and selective ECE inhibitors and thus provide an alternative investigative, and perhaps therapeutic, tool. Morbidity and mortality from CHF remain unacceptably high even in patients receiving maximal medical therapy, including an ACE inhibitor. Blockade of either the generation (through ECE inhibition) or actions (through receptor blockade) of ET warrant further investigation as potential new therapeutic strategies.

The endothelin system and its potential as a therapeutic target in cardiovascular disease

Endothelin (ET)-1, an endothelium-derived peptide, is the most potent vasoconstrictor agent described to date. ET-1 also has positive inotropic and chronotropic effects in the heart and is a co-mitogen in both cardiac and vascular myocytes. The major elements of the system involved in formation of ET-1 and its isopeptides, as well as the receptors mediating their effects, have been cloned and characterised. Antagonists of the ET receptors are now available, and selective inhibitors of the ET-converting enzymes are being developed. Early studies using receptor antagonists support the involvement of ET-1 in the pathophysiology of several cardiovascular diseases. The relative merits of ET-converting enzyme inhibitors and receptor antagonists for the treatment of cardiovascular disease are discussed.

Phosphoramidon inhibition of the in vivo conversion of big endothelin-1 to endothelin-1 in the human forearm

1. The vasoconstrictor peptide, endothelin-1 (ET-1) and a biologically inactive C-terminal fragment (CTF) are generated from an intermediate big ET-1 by a putative ET converting enzyme, sensitive to phosphoramidon. We have developed a procedure using selective solid-phase extraction and specific radioimmunoassays to measure the levels of immunoreactive (IR) big ET-1 and the products of conversion (ET-1 and CTF) in human plasma. These techniques have been used to determine the levels of the three peptides in venous plasma following local infusions of ET-1 and big ET-1, both alone and together with phosphoramidon. 2. Infusion of ET-1 into the brachial artery (5 pmol min-1) significantly increased (P < 0.05) IR ET levels from a basal level of 2.3 pM to 55.2 pM in plasma from the infused arm after 60 min of infusion. This corresponded with a marked decrease in forearm blood flow from a basal level of 2.6 ml dl-1 min-1 to 1.7 ml dl-1 min-1. The levels of IR big ET-1 and CTF were unchanged. Co-infusion of phosphoramidon (30 nmol min-1) with ET-1 had no significant effect on the plasma IR levels of ET, big ET-1, CTF, or blood flow. 3. Big ET-1 (50 pmol min-1) significantly increased (P < 0.05) venous concentrations of all three IR peptides after 60 min compared to basal (ET: from 2.2 to 7.7 pM, big ET-1; from 0 to 386.0 pM, CTF: from 0.2 to 37.0 pM). Forearm blood flow decreased significantly (P<0.05) from a basal level of 3.0 ml dl-1 min-1 to 1.6 ml dl-1 min-1.4. When phosphoramidon was co-infused with big ET-1, both the rise in IR ET and associated vasoconstriction were abolished. However, IR CTF was still detected, suggesting that either some conversion by phosphoramidon-insensitive enzyme(s) was occurring, and/or that CTF was being protected from further degradation by phosphoramidon.5. These data show that in the human forearm the activity of a phosphoramidon-sensitive ET converting enzyme is at least in part responsible for the vasoconstrictor properties of exogenous big ET-1. Furthermore, because measurable levels of newly synthesized ET-1 are likely to be rapidly reduced in the blood/plasma through receptor binding, assay of IR big ET-1 and CTF may be a more sensitive measure of ET-1 generation in disease.

Endothelin ETA and ETB mRNA and receptors expressed by smooth muscle in the human vasculature: majority of the ETA sub-type

1. We measured the ratio of ETA and ETB sub-types in the media (containing mainly smooth muscle) of human cardiac arteries (aorta, pulmonary and coronary), internal mammary arteries and saphenous veins. 2. In saturation experiments, [125I]-endothelin-1 ([125I]-ET-1) bound with high affinity to the media of each vessel (n = 3 individuals or homogenate preparations +/- s.e. mean): coronary artery, KD = 0.14 +/- 0.02 nM, Bmax = 71.0 +/- 21.0 fmol mg-1 protein; pulmonary artery, KD = 0.85 +/- 0.25 nM, Bmax = 15.2 +/- 10.3 fmol mg-1 protein; aorta, KD = 0.51 +/- 0.02 nM, Bmax = 9.4 +/- 4.4 fmol mg-1 protein; internal mammary artery. KD = 0.34 +/- 0.31 nM, Bmax = 2.0 +/- 0.5 fmol mg-1 protein and saphenous vein, KD = 0.28 +/- 0.05 nM, Bmax = 52.8 +/- 1.0 fmol mg-1 protein. In each vessel, over the concentration-range tested, Hill slopes were close to unity and a one site fit was preferred to a two site model. 3. In competition binding assays, the ETA selective ligand, BQ123 inhibited the binding of 0.1 nM [125I]-ET-1 to the media in a biphasic manner. In each case, a two site fit was preferred to a one or three site model: coronary artery, KDETA = 0.85 +/- 0.03 nM, KDETB = 7.58 +/- 2.27 microM, ratio = 89:11%; pulmonary artery, KDETA = 0.27 +/- 0.05 nM, KDETB = 24.60 +/- 5.34 microM, ratio = 92:8%; aorta, KDETA = 0.80 +/- 0.40 nM, KDETB = 2.67 +/- 2.60 microM ratio = 89:11%; saphenous vein, KDETA = 0.55 +/- 0.17 nM, KDETB = 14.4 +/- 0.26 microM, 85:15% (n = 3 individuals or homogenate preparations +/- s.e. mean). BQ123 showed up to 18000 fold selectivity for the ETA over the ETB sub-type. The ETA-selective ligand, [125I]-PD151242 labelled 85% of the receptors detected by a fixed concentration of [125I]-ET-1 in media of internal mammary artery, measured by quantitative autoradiography. In contrast, the density of ETB receptors detected with [125I]-BQ3020 was 7.0 +/- 1.5 amol mm-2, representing about 8% of [125I]-ET-1. 4. A single band corresponding to the expected position for mRNA encoding the ETA receptor (299 base pairs) was found in the media in each of the five vessels (n = 3 individuals) using reverse transcript as epolymerase chain reaction assays. A single band corresponding to the ETB sub-type (428 base pairs) was also always detected.5. 35S-labelled antisense probes to ETA and ETB hybridised to the media of epicardial coronary arteries as well as intramyocardial vessels, confirming the presence of mRNA encoding both sub-types in the vascular smooth muscle of the vessel wall.6 Although mRNA for both receptors was detected, competition binding using BQ123 demonstrated that the majority (at least 85%) of ET receptors present in smooth muscle are the ETA sub-type. These results provide further support for the hypothesis that the ETA sub-type is the receptor that must be blocked in humans to produce a beneficial vasodilatation in pathophysiological conditions where there is an increase in peptide concentration or receptor density.