Endothelin-stimulated secretion of natriuretic peptides by rat atrial myocytes is mediated by endothelin A receptors

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

Simple Summary

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

Abstract

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

Endothelin receptor B, a candidate gene from human studies at high altitude, improves cardiac tolerance to hypoxia in genetically engineered heterozygote mice

To better understand human adaptation to stress, and in particular to hypoxia, we took advantage of one of nature's experiments at high altitude (HA) and studied Ethiopians, a population that is well-adapted to HA hypoxic stress. Using whole-genome sequencing, we discovered that EDNRB (Endothelin receptor type B) is a candidate gene involved in HA adaptation. To test whether EDNRB plays a critical role in hypoxia tolerance and adaptation, we generated EdnrB knockout mice and found that when EdnrB (-/+) heterozygote mice are treated with lower levels of oxygen (O2), they tolerate various levels of hypoxia (even extreme hypoxia, e.g., 5% O2) very well. For example, they maintain ejection fraction, cardiac contractility, and cardiac output in severe hypoxia. Furthermore, O2 delivery to vital organs was significantly higher and blood lactate was lower in EdnrB (-/+) compared with wild type in hypoxia. Tissue hypoxia in brain, heart, and kidney was lower in EdnrB (-/+) mice as well. These data demonstrate that a lower level of EDNRB significantly improves cardiac performance and tissue perfusion under various levels of hypoxia. Transcriptomic profiling of left ventricles revealed three specific genes [natriuretic peptide type A (Nppa), sarcolipin (Sln), and myosin light polypeptide 4 (Myl4)] that were oppositely expressed (q < 0.05) between EdnrB (-/+) and wild type. Functions related to these gene networks were consistent with a better cardiac contractility and performance. We conclude that EDNRB plays a key role in hypoxia tolerance and that a lower level of EDNRB contributes, at least in part, to HA adaptation in humans.

Ghrelin and its relation with N-terminal brain natriuretic peptide, endothelin-1 and nitric oxide in patients with idiopathic pulmonary hypertension

OBJECTIVES

To investigate ghrelin levels in patients with idiopathic pulmonary arterial hypertension (IPAH) and the association of ghrelin with N-terminal brain natriuretic peptide (N-BNP), endothelin-1 (ET-1) and nitric oxide (NO).

METHODS

Plasma ghrelin, N-BNP, ET-1 and NO were measured, and echocardiography was performed in 20 IPAH patients and in 20 control subjects matched for age, sex and body mass index.

RESULTS

Plasma ghrelin and NT-proBNP levels were significantly higher in IPAH patients compared with values in control subjects (p < 0.05). In IPAH patients, ghrelin levels correlated positively with N-BNP (r = 0.616, p = 0.004), NO (r = 0.464, p = 0.039), right ventricle diameter (RVD; r = 0.485, p = 0.030) and pulmonary arterial systolic pressure (PASP; r = 0.591, p = 0.006). N-BNP levels correlated positively with RVD (r = 0.551, p = 0.012) and ET-1 (r = 0.451, p = 0.046).

CONCLUSIONS

Plasma ghrelin levels were elevated in IPAH. Increased ghrelin levels correlated positively with N-BNP, PASP, RVD and NO, and N-BNP levels correlated positively with RVD and ET-1. Pulmonary vascular pathology is a complex imbalance of opposing forces. Ghrelin may not only provide a novel prognostic biomarker for IPAH but also be a potential new therapeutic strategy.

Ouabain stimulates atrial natriuretic peptide secretion via the endothelin-1/ET(B) receptor-mediated pathway in beating rabbit atria

AIMS

Ouabain has been reported to increase the secretion of atrial natriuretic peptide (ANP) in vitro. However, the mechanism by which ouabain increases ANP secretion is not well known. Therefore, the purpose of the present study was to investigate the underlying mechanism of ouabain-stimulated ANP secretion.

MAIN METHODS

A perfused beating rabbit atrial model was used. The ANP and ET-1 levels in the atrial perfusates were measured by radioimmunoassays.

KEY FINDINGS

Ouabain (1.0, 3.0 and 6.0 μmol/L) significantly increased atrial ANP secretion in a dose-dependent manner, while the endothelin (ET)-1 levels were increased by the higher doses (3.0 and 6.0 μmol/L) of ouabain. Ouabain-increased atrial ET-1 release was blocked by PD98059 (30.0 μmol/L), an inhibitor of mitogen-activated protein kinase (MAPK). Nifedipine (1.0 μmol/L), an inhibitor of L-type Ca(2+) channels, completely abolished ouabain-increased ANP secretion without changing the ouabain-induced atrial dynamics. KB-R7943 (3.0 μmol/L), an inhibitor of Na(+)-Ca(2+) exchangers, completely blocked the effects of ouabain-increased atrial dynamics, but did not modulate ouabain-increased ANP secretion. ET-1 significantly stimulated atrial ANP release in a dose-dependent manner. The effects of ET-1 and ouabain on ANP secretion were completely blocked by BQ788 (0.3 μmol/L), an inhibitor of ET-1 type B (ET(B)) receptors, but not by BQ123 (0.3 μM), an inhibitor of ET-1 type A receptors. Ouabain-increased atrial ANP secretion was blocked by PD98059 and indomethacin (30.0 μmol/L), an inhibitor of cyclooxygenase.

SIGNIFICANCE

Ouabain significantly stimulated atrial ANP secretion via an ET-1-ET(B) receptor-mediated pathway involving MAPK signaling pathway activation and prostaglandin formation.

Key roles of endothelin-1 and p38 MAPK in the regulation of atrial stretch response

Mechanisms regulating stretch response in the left ventricle are investigated in detail but not well understood in atrial myocardium. Hypertrophic growth of atrial myocardium contributes to the pathogenesis of atrial fibrillation. In this study, we sought to elucidate mechanisms of stretch-induced activation of key signaling pathways and hypertrophy-associated genes in rat atria. Stretching of isolated atria induced a rapid increase in phosphorylation of p38 MAPK and ERK and induced a p38 MAPK-dependent increase in DNA binding activity of transcription factors Elk-1 and GATA-4. Inhibition of the ERK pathway had no effect on the cardiac transcription factors studied. Stretch-induced increase in atrial contractile function was substantially enhanced by inhibition of p38 MAPK. p38 MAPK also regulated stretch-induced increase in c-fos, β-myosin heavy chain, B-type natriuretic peptide mRNA levels, and atrial natriuretic peptide secretion in isolated atria. Various autocrine/paracrine factors are known to mediate the stretch response in the left ventricle. Stretching of isolated atria resulted in a robust increase in endothelin-1 (ET-1) mRNA levels, while apelin and adrenomedullin signaling cascades were downregulated. Administration of mixed ET(A/B) receptor antagonist bosentan attenuated the stretch-induced activation of GATA-4 in isolated atria, whereas ANG II receptor type-1 antagonist CV-11974 had no effect. Moreover, analysis of RNA from intact atrial and ventricular myocardium revealed significantly higher mRNA levels of ET(A) receptor and ET converting enzyme-1 in atrial compared with ventricular myocardium. In conclusion, our findings identify the local ET-1 system and p38 MAPK as key regulators of load-induced hypertrophic response in isolated rat atria.

Neutralization of transforming growth factor-beta attenuates hypertension and prevents renal injury in uremic rats

OBJECTIVE

We investigate the role of transforming growth factor-beta (TGF-beta) in hypertension and renal failure progression in uremic rats, and whether it modulates the endothelin (ET) system.

DESIGN

Following renal mass reduction, uremic rats (Nx) received the pan-specific TGF-beta neutralizing antibody 1D11 (0.5 mg/kg, three times/week), the isotype control antibody 13C4 or the AT1 antagonist losartan (10 mg/kg per day) for 6 weeks.

RESULTS

Before treatment, the blood pressure was higher in Nx rats and increased further over time in Nx+13C4 rats. At the end of the study, Nx+13C4 rats exhibited increased serum creatinine, proteinuria and renal expression and excretion of TGF-beta1 and ET-1. ET-1 concentrations were greater in vascular and renal tissues, whereas the ETB receptor expression was reduced. Renal injuries were comprised of blood vessel hypertrophy, glomerular sclerosis, tubular atrophy and interstitial fibrosis, which was associated with increased alpha-smooth muscle actin expression. Treatment of uremic rats with the 1D11 antibody attenuated the increase in blood pressure and the decline in renal function. Losartan normalized the blood pressure and significantly attenuated the increase in serum creatinine and proteinuria. However, both treatments prevented renal TGF-beta1 and ET-1 overexpression, and prevented all renal histological injuries. The 1D11 antibody only improved ETB receptor expression.

CONCLUSIONS

Neutralization of TGF-beta attenuates hypertension and renal failure progression in uremic animals, in part, by preventing renal injury processes. These effects may be related to the modulation of the ET system, preventing renal ET-1 overproduction and the reduction of ETB receptor expression. Our data also suggest that TGF-beta1 is involved, at least in part, in the pathological effects related to angiotensin II in chronic renal failure.

The Role of Endothelin in Mediating Ischemia/Hypoxia-Induced Atrial Natriuretic Peptide Release

The aim of the present study was to investigate the putative role of endothelin (ET) in mediating ischemia/hypoxia-induced ANP release utilizing exogenous ET-1 or ET receptor antagonists (BQ-123 or Bosentan). Isolated rat hearts with non-distended atria were perfused using a Langendorff apparatus and heart rate maintained constant via atrial pacing. Global ischemia was induced either by direct reduction in perfusion or by infusion of exogenous ET-1 (5 x 10(-10) M) for 30 minutes. Perfusion with the ET receptor antagonists, BQ-123 (10(-6) M) or Bosentan (10(-5) M) was initiated 10 minutes before onset of ischemia. Moderate or severe ischemia was induced by reduction (52-61% and 70-82%, respectively) in perfusate flow. Thirty minutes of ischemia/hypoxia (5% O2) was followed by 30 minutes of reperfusion/re-oxygenation. Both moderate and severe ischemia increased ANP release. BQ-123 and Bosentan did not affect basal or ischemia-induced ANP release. Exogenous ET-1 perfusion induced a late increase in ANP release (P < 0.01) that did not exceed the increase in ANP release associated with equivalent direct flow reduction. Hypoxia induced an 8-fold increase in ANP release rate. The ANP release rate returned toward basal levels after re-oxygenation. Bosentan, but not BQ-123, significantly attenuated (P < 0.01) hypoxia-induced ANP release. In conclusion, in this system, ANP release is stimulated by moderate (or severe) ischemia and severe hypoxia independent of change in atrial distension; endogenous ET does not mediate basal and ischemia-induced ANP release; and hypoxia-induced ANP release is partially modulated via interaction with endogenous ET.

The endothelin system and its role in acute myocardial infarction

Immediately after an acute myocardial infarction (AMI) or in models of ischemia-reperfusion injury, cardiac endothelin (ET) system is markedly activated, and plasma levels of ET are increased. In the heart, expression of the main components of the ET system (ET-1 peptide, both receptor subtypes ETA and ETB, though not endothelin converting enzyme) are increased both at the gene level and protein level, in the viable myocardium, and--even more substantially--in the necrotic area. Despite these conspicuous abnormalities, the role of ET in this setting remains unclear. In the absence of human data, most short-term studies in animals (in terms of hours to up to 8 days post-AMI) and in the reperfused ischemic heart, have found beneficial effects of ET receptor blockade on survival rate, incidence of arrhythmias, cardiac function, and morphology. In contrast, many studies in which a long-term ET inhibition was started immediately post-infarction and the late effects were examined in animals with ensuing chronic heart failure (14-100 days postinfarction), adverse effects were also observed, such as scar thinning, further ventricular dilation, or even a worse survival rate. It appears that the ET system plays a dual role during the early post-AMI period. At present, it is not clear whether the short-term beneficial effects or long-term adverse effects of ET receptor blockade would prevail. Acute use of short-acting ET receptor antagonists in patients with AMI complicated by an acute heart failure is an attractive possibility that also remains to be investigated.

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

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

Pronatriuretic peptide is a sensitive marker of the endocrine function of teleost heart

We recently characterized a novel heart-specific hormone from salmon (salmon cardiac peptide, sCP). We have now prepared a recombinant plasmid expressing the NH(2)-terminal fragment of pro-sCP (NT-pro-sCP) and used it to set up a specific RIA for the peptide. Because of the sensitivity of the assay and the high circulating levels, NT-pro-sCP can be measured from as little as 2 microl of serum. This enables repeated sampling from the same animal in different experimental setups. Mechanical load increased the release of NT-pro-sCP from isolated perfused salmon ventricle, in parallel with sCP. Bolus injection of human endothelin-1 (ET-1; 1 microg) in the dorsal aorta of salmon resulted in an extensive increase of serum NT-pro-sCP (from 0.99 +/- 0.11 to 4.6 +/-1.5 nmol/l). The response was abolished by pretreatment with a specific type A ET (ET(A)) receptor antagonist (BQ-123) but not with a type B ET receptor antagonist (BQ-788). The NT-pro-sCP levels had a good correlation with those of sCP (r(2) = 0.75). Our results demonstrate the practical usefulness of circulating NT-pro-sCP as a marker of the endocrine function of salmon heart. They also suggest that ET-1 has an important role in regulating sCP release from teleost heart by an ET(A) receptor-mediated mechanism.

Natriuretic peptide gene expression in DOCA-salt hypertension after blockade of type B endothelin receptor

We investigated the effect of long-term in vivo blockade of the ET-1 receptor subtype B (ET(B)) with A-192621, a selective ET(B) antagonist, on atrial and ventricular natriuretic peptide (NP) gene expression in deoxycorticosterone acetate (DOCA)-salt hypertension. In this model, stimulation of the cardiac natriuretic peptide (NP) and the endothelin system and suppression of the renin-angiotensin system is observed. DOCA-salt induced significant hypertension, cardiac hypertrophy and increased NP plasma and left atrial and right and left ventricular NP gene expression. ET(B) blockade per se produced hypertension and left ventricular hypertrophy but induced little change on the levels of ventricular NP and only increased left atrial natriuretic factor (ANF) mRNA levels. Combined ET(B) blockade/DOCA-salt treatment worsened hypertension, increased left ventricular hypertrophy and induced right ventricular hypertrophy. All animals so treated had increased ventricular NP gene expression. Collagen III and beta-myosin heavy chain gene expression were enhanced in both the right and the left ventricle of DOCA-salt hypertensive rats. The results of this study suggest that the ET(B) receptor does not participate directly in the modulation of atrial or ventricular NP gene expression and that this receptor mediates a protective cardiovascular function. ET(B) blockade can induce significant ventricular hypertrophy without an increase in ANF or brain NP gene expression.

Therapeutic role of bosentan in hypertension: lessons from the model of perinephritic hypertension

Since its discovery in 1988, there has been increasing evidence that endothelin-1 (ET-1) plays an important role in the pathophysiology of hypertension and its related end-organ damages. First studies, using ET-1 administration in animals or in humans suspected this role by demonstrating the hypertensive properties of ET-1. The latter, due to stimulation of ET(A) receptors inducing sustained vasoconstriction have been reported to follow transient vasodilation linked with activation of an endothelial ET(B) receptor releasing nitric oxide (NO). In certain instances, ET(B) smooth-muscle receptors might also induce contraction. Cloning of these receptors helped to develop ET-1 receptor antagonists. As soon as one of them became available, bosentan, a dual (ET(A) and ET(B)) ET-1 receptor antagonist, we tested its effects in the canine model of perinephritic hypertension. Bosentan was found to exert striking hypotensive effects, due to peripheral vasodilation but without affecting cardiac function. In further experiments, we observed that effects of bosentan were additional to those of ACE inhibitors or angiotensin II antagonists. This opened new therapeutic perspectives and also suggested a proper role of ET-1 in hypertension, independent of the renin-angiotensin system. To explain this role, we demonstrated a real imbalance characterized by an impairment of the NO system in favor of the ET-1 pathway. Recent studies suggest that such an imbalance may also occur in human hypertension. Furthermore, the contribution of ET-1 to human hypertension appears more convincing since bosentan was shown to decrease blood pressure in hypertensive subjects. Finally, ET-1 receptor antagonists might be of therapeutic interest to prevent hypertension induced end-organ damages. Whether or not these compounds are able to prevent or to reverse target organ injuries in man remains to be investigated.

Characterization of effects of endothelin-1 on the L-type Ca2+ current in human atrial myocytes

The effects of endothelin-1 (ET-1) on the L-type Ca2+ current (I(Ca)) were examined in whole cell patch-clamped human atrial myocytes. Depending on the initial current density, ET-1 (10 nM) increased the amplitude of I(Ca) by 99 +/- 7% or decreased it by 33 +/- 2%. The stimulatory effect predominated on current of low density (2.3 +/- 0.2 pA/pF), whereas I(Ca) of higher density (5.8 +/- 0.3 pA/pF) was inhibited by ET-1. After I(Ca) stimulation by 1 microM isoproterenol, ET-1 always inhibited the current by 32 +/- 7% (P < 0.05), an effect that was suppressed by pretreating myocytes with pertussis toxin. Atrial natriuretic peptide (ANP) inhibited I(Ca) (41 +/- 3%) by reducing intracellular cAMP concentration. In ANP-treated myocytes, the stimulatory effect of ET-1 on I(Ca) predominated (52 +/- 7%). The inhibitory effect of ET-1 on I(Ca) was blocked by the ET(A) antagonist BQ-123, whereas the stimulatory effect was suppressed by the ET(B) agonist BQ-788. We conclude that ET-1 has opposite effects on I(Ca) depending on the baseline amplitude of current, and both subtype ET receptors are implicated in the signal transduction pathways.

Hemodynamic, renal, and endocrine responses to acute ET(A) blockade at different ANG II plasma levels

Angiotensin (ANG) II effects may be partly mediated by endothelin (ET)-1. This study analyses the hemodynamic, renal, and hormonal responses of acute ET(A) receptor antagonism (LU-135252) at two ANG II plasma levels in eight conscious dogs. Protocol 1 involved a 60-min baseline, followed by two doses of ANG II for 60 min each (4 and 20 ng. kg(-1). min(-1)), termed ANG II 4 (slightly increased) and ANG II 20 (pathophysiologically increased ANG II plasma concentration). Protocol 2 was the same as protocol 1 but included 15 mg/kg iv LU-135252 after the baseline period. Protocol 3 was a 3-h time control. ANG II without LU-135252 did not increase plasma big ET-1 and ET-1, whereas LU-135252 increased ET-1 transiently after injection. This transient ET-1 increase was not reflected in urinary ET-1 excretion. The ANG II induced decreases in sodium, water, and potassium excretion, glomerular filtration rate, and fractional sodium excretion were not different with and without LU-135252. Mean arterial pressure increased during ANG II and was not lower with LU-135252 (-6 mmHg, not significant). Most importantly, during ANG II 20 LU-135252 prevented the decrease in cardiac output. Simultaneously, systemic vascular resistance increased 40% less, pulmonary vascular resistance was maintained at baseline levels, and central venous and wedge pressure were lower. Because ANG II stimulated endothelin de novo synthesis should just have started after 2 h of ANG II infusion, there must be mechanisms other than blocking the coupling of de novo synthesized endothelins to the ET(A) receptors to explain the effects of acute ET(A) receptor inhibition in our setting.

Activation of cardiorenal and pulmonary tissue endothelin-1 in experimental heart failure

Endothelin-1 (ET-1) is a peptide that has been implicated in congestive heart failure (CHF). Although increased concentrations of circulating ET-1 have been repeatedly demonstrated, the activation of local ET-1 in target tissues of CHF remains poorly defined. Our objective was to characterize ET-1 tissue concentrations and gene expression of prepro ET-1 in myocardial, renal, and pulmonary tissue in rapid ventricular pacing-induced canine CHF. Progressive rapid ventricular pacing (38 days) resulted in impaired cardiovascular hemodynamics, increased atrial and left ventricular mass, decreased renal sodium excretion, and increased ET-1 plasma concentrations (all P < 0.05). Tissue analysis revealed significant increases in local ET-1 during CHF in left ventricular, renal, and pulmonary tissue, whereas a moderate increase in left atrial ET-1 did not reach statistical significance. In contrast, prepro-ET-1 gene expression was increased more than threefold in pulmonary tissue and more than twofold in left atrial myocardium with no increase in left ventricular or renal gene expression. The present studies demonstrate a differential pattern of ET-1 activation in cardiorenal and pulmonary tissue with a strong accumulation of ET-1 in kidney and lung during CHF. Although the observed increase in left ventricular and renal ET-1 in association with unaltered gene expression is consistent with increased uptake, pulmonary and atrial tissue may contribute to increased circulating and local ET-1 in CHF.

Effect of selective ET(A) receptor blockade on natriuretic peptide gene expression in DOCA-salt hypertension

To determine the role of endothelin-1 (ET-1) in the upregulation of atrial natriuretic factor (ANF) and brain natriuretic peptide (BNP) observed in deoxycorticosterone acetate (DOCA)-salt hypertension, the selective ET-1 type-A receptor (ET(A)) antagonist ABT-627 was chronically administered to normal controls and hypertensive rats. Chronic ET(A) blockade in DOCA-salt-treated rats prevented the increase in blood pressure and circulating natriuretic protein (NP) levels and partially prevented left ventricular hypertrophy. The changes observed in NP gene expression in the atria were not affected by ABT-627. In the ventricles, ABT-627 reduced NP gene expression. Rats receiving the ET(A) antagonist alone showed reduced left ventricular NP gene expression. ABT-627 did not affect ventricular collagen III gene expression but enhanced left ventricular alpha-myosin heavy chain expression. These findings suggest that in vivo, ventricular but not atrial NP production is regulated by ET-1. This difference in response between atrial and ventricular NP gene expression to ET(A) receptor blockade is similar to that observed by us after applying angiotensin-converting enzyme inhibitors in other hypertensive models. In general therefore, atrial NP gene expression may not be as sensitive to the endocrine environment as is ventricular NP gene expression.

Altered cardiac endothelin receptors and protein kinase C in deoxycorticosterone-salt hypertensive rats

The aim of the present study was to assess the status of ET-1 receptor subtypes (ET(A)and ET(B)) in ventricular myocytes and fibroblasts and to determine the role of PKC-dependent pathways in ET-1-stimulated cardiac cells in deoxycorticosterone acetate (DOCA)-salt hypertensive rats. Systolic blood pressure and relative heart to body weight were significantly increased in DOCA-salt rats. In unilaterally nephrectomized (Uni-Nx) control rats, more than 90% of cardiomyocyte ET receptors were of the ET(A)subtype, whereas in fibroblasts ET(A)and ET(B)receptors were present in a 1:3 ratio. In DOCA-salt rats, the density of the ET(A)receptor subtype was reduced by 31% in cardiomyocytes and in cardiac fibroblasts only ET(B)receptor density was decreased by 29%. Affinity was unchanged. The relative expression of immunoreactive PKC alpha, gamma and epsilon was significantly increased, whereas PKC delta was not altered in cardiac extracts of DOCA-salt rats. In cardiac fibroblasts from DOCA-salt rats PKC delta was significantly increased and PKC epsilon was not translocated after ET-1 stimulation. The hearts of DOCA-salt hypertensive rats are thus characterized by: (1) decreased density of cardiomyocyte ET(A)receptors and fibroblast ET(B)receptors; (2) cell-specific enhanced expression of some PKC isoenzymes (alpha, gamma, delta and epsilon); and (3) unresponsiveness of PKC epsilon to translocate in the presence of ET-1. Together with alterations of ET-1-induced Ca(2+)handling in cardiac myocytes and fibroblasts, which we previously reported, results from the present study indicate a marked modification of the cardiac ET-1 system of DOCA-salt hypertensive rats.

Role of endothelin receptor subtypes in volume-stimulated ANF secretion

The role of endothelin (ET) receptors was tested in volume-stimulated atrial natriuretic factor (ANF) secretion in conscious rats. Mean ANF responses to slow infusions (3 x 3.3 ml/8 min) were dose dependently reduced (P < 0.05) by bosentan (nonselective ET-receptor antagonist) from 64.1 +/- 18.1 (SE) pg/ml (control) to 52.6 +/- 16.1 (0.033 mg bosentan/rat), 16.1 +/- 7.6 (0. 33 mg/rat), and 11.6 +/- 6.5 pg/ml (3.3 mg/rat). The ET-A-receptor antagonist BQ-123 (1 mg/rat) had no effect relative to DMSO controls, whereas the putative ET-B antagonist IRL-1038 (0.1 mg/rat) abolished the response. In a second protocol, BQ-123 (>/=0.5 mg/rat) nonsignificantly reduced the peak ANF response (106.1 +/- 23.0 pg/ml) to 74.0 +/- 20.5 pg/ml for slow infusions (3.5 ml/8.5 min) but reduced the peak response (425.3 +/- 58.1 pg/ml) for fast infusions (6.6 ml/1 min) by 49.9% (P < 0.001) and for 340 pmoles ET-1 (328.8 +/- 69.5 pg/ml) by 83.5% (P < 0.0001). BQ-123 abolished the ET-1-induced increase in arterial pressure (21.8 +/- 5.2 mmHg at 1 min). Changes in central venous pressure were similar for DMSO and BQ-123 (slow: 0.91 and 1.14 mmHg; fast: 4.50 and 4.13 mmHg). The results suggest 1) ET-B receptors mainly mediate the ANF secretion to slow volume expansions of <1.6%/min; and 2) ET-A receptors mainly mediate the ANF response to acute volume overloads.

Mechanical strain activates BNP gene transcription through a p38/NF-kappaB-dependent mechanism

Application of mechanical strain to neonatal rat ventricular myocytes in culture evokes changes in gene expression reminiscent of those that occur with hypertrophy in vivo, such as stimulation of brain natriuretic peptide (BNP) gene expression. Here, we show that a major component of strain-dependent BNP promoter activation results from stimulation of p38 mitogen-activated protein kinase (MAPK) in the cardiac myocyte. Strain increased p38 activity in a time-dependent fashion. The p38 inhibitor SB203580 led to a reduction of approximately 60% in strain-activated human BNP (hBNP) promoter activity. Cotransfection of wild-type p38 increased both basal and strain-dependent promoter activity, while cotransfection with MKK6AL, a dominant-negative inhibitor of p38 MAPK kinase, resulted in partial inhibition of either p38- or strain-activated hBNP promoter activity. p38 MAPK increased hBNP promoter activity through activation of the transcription factor NF-kappaB. Activation of the hBNP promoter by either p38 or strain was mediated by DNA elements present in the 5' flanking sequence of the gene. Mechanical strain promoted assembly of NF-kappaB components on these DNA elements in vitro. Thus, induction of the hBNP promoter by mechanical strain depends, at least in part, on stimulation of p38 and subsequent activation of NF-kappaB. This activation may play an important role in signaling the increased BNP gene expression that accompanies hemodynamic overload and cardiac hypertrophy in vivo.

Regulation of natriuretic peptide secretion by the heart

Secreted by the heart, more specifically by atrial cardiomyocytes under normal conditions but also by ventricular myocytes during cardiac hypertrophy, natriuretic peptides are now considered important hormones in the control of blood pressure and salt and water excretion. Studies on natriuretic peptide secretagogues and their mechanisms of action have been complicated by hemodynamic changes and contractions to which the atria are constantly subjected. It now appears that atrial stretch through mechano-sensitive ion channels, adrenergic stimulation via alpha 1A-adrenergic receptors, and endothelin via its ETA receptor subtype are major triggering agents of natriuretic peptide release. With several other stimuli, such as angiotensin II and beta-adrenergic agents, modulation of natriuretic peptide release appears to be linked to local generation of prostaglandins. In all cases, intracellular calcium homeostasis, controlled by several ion channels, is considered a key element in the regulation of natriuretic peptide secretion.

Heterogeneity and underlying mechanism for inotropic action of endothelin-1 in rat ventricular myocytes

1. To clarify the mechanisms underlying the positive inotropic action of endothelin-1 (ET-1), we investigated the effect of ET-1 on twitch cell shortening and the Ca2+ transient in rat isolated ventricular myocytes loaded with a fluorescent Ca2+ indicator indo-1. 2. There was a cell-to-cell heterogeneity in response to ET-1. ET-1 (100 nM) increased twitch cell shortening in only 6 of 14 cells (44%) and the increase in twitch cell shortening was always accompanied by an increase in the amplitude of the Ca2+ transient. 3. The ET(A)- and ET(B)-receptors antagonist TAK-044 (100 nM) almost reversed both the ET-1-induced increases in twitch cell shortening and in the Ca2+ transient. In the ET-1 non-responding cells, the amplitude of the Ca2+ transient never increased. 4. Intracellular pH slightly increased (approximately 0.08 unit) after 30 min perfusion of ET-1 in rat ventricular myocytes. However, ET-1 did not change the myofilament responsiveness to Ca2+, which was assessed by (1) the relationship between the Ca2+ transient amplitude and twitch cell shortening, and by (2) the Ca2+ transient-cell shortening phase plane diagram during negative staircase. 5. We concluded that there was a cell-to-cell heterogeneity in the positive inotropic effect of ET-1, and that the ET-receptor-mediated positive inotropic effect was mainly due to an increase in the Ca2+ transient amplitude rather than to an increase in myofilament responsiveness to Ca2+.

Role of cAMP and calcium influx in endothelin-1-induced ANP release in rat cardiomyocytes

The mechanism of endothelin-1 (ET-1)-induced atrial natriuretic peptide (ANP) release was studied in neonatal rat ventricular cardiomyocytes. These cells expressed a single high-affinity class of ETA receptor (dissociation constant = 54 +/- 18 pM, n = 3), but no ETB receptors. Incubation of cardiomyocytes with ET-1 led to concentration-dependent ANP release and prostacyclin production. ET-1-induced ANP release was affected by neither protein kinase C (PKC) inhibition or downregulation nor by cyclooxygenase inhibition, indicating that ET-1-stimulated ANP secretion is not a PKC-mediated, prostaglandin-dependent process. Furthermore, ET-1 significantly stimulated adenosine 3',5'-cyclic monophosphate (cAMP) production and increased cytosolic calcium concentration in these preparations. Both ET-1-induced calcium influx and ANP release were decreased by the cAMP antagonist Rp-cAMPS, the Rp diastereoisomer of cAMP. Moreover, ET-1-induced ANP secretion was strongly inhibited in the presence of nifedipine as well as in the absence of extracellular calcium. Thus our results suggest that ET-1 stimulates ANP release in ventricular cardiomyocytes via an ETA receptor-mediated pathway involving cAMP formation and activation of a nifedipine-sensitive calcium channel.

Comparison of the effects of selective endothelin ETA and ETB receptor antagonists in congestive heart failure

OBJECTIVES

This study was designed 1) to determine the extent to which endogenous endothelin (ET) affects hemodynamic, hormonal and body fluid balance through ETA and ETB receptors in congestive heart failure (CHF); and 2) to assess the therapeutic benefits and adverse effects of ET receptor antagonists for ETA and ETB on cardiorenal and neurohormonal variables.

BACKGROUND

ET has two receptors, ETA and ETB, both of which are distributed in various tissues and cells. In vascular beds, ETA receptors mediate vasoconstriction, whereas ETB receptors mediate vasorelaxation. However, ETB receptors also exist in smooth muscle and mediate vasoconstriction.

METHODS

We administered either the ETA receptor antagonist FR139317 (FR [n = 8], 1 and 10 mg/kg body weight) or the ETB receptor antagonist RES-701-1 (RES [n = 8], 0.2 and 1.5 mg/kg) to dogs with CHF induced by rapid ventricular pacing. The effects of both antagonists on cardiorenal and hormonal functions were studied.

RESULTS

FR decreased cardiac pressures and the plasma atrial natriuretic peptide (ANP) level and increased cardiac output (CO). Urinary flow rate and urinary sodium excretion increased in association with an increase in the glomerular filtration rate and renal plasma flow (RPF). In contrast, RES increased cardiac pressures and decreased CO. It also decreased the plasma aldosterone level and RPF. Neither antagonist affected plasma norepinephrine levels.

CONCLUSIONS

Endogenous ETs increase cardiac pressures and the retention of body fluid through ETA receptors in CHF. The vasodilative action through ETB receptors is overall functionally more important than the constrictive action through ETB receptors. ETs may regulate the secretion of ANP and aldosterone. Our findings suggest that selective ETA receptor antagonists have potential therapeutic benefits affecting both hemodynamic variables and diuresis, whereas ETB receptor antagonists have adverse hemodynamic effects, with the possibility of preventing fluid retention through suppression of aldosterone secretion in dogs with CHF.

Tissue targeting of angiotensin peptides

Angiotensin II (Ang II) is an octapeptide generated by the sequential proteolytic action of renin and angiotensin converting enzyme on the glycoprotein angiotensinogen. While numerous mammalian tissues have been shown to express some or all of the components of the renin-angiotensin system (RAS), the function of most of these tissue RAS remains a matter of conjecture. To test for tissue-specific functions of Ang II and as an alternative to co-expressing all the components of RAS, we have engineered a fusion protein that leads to direct Ang II release within specific tissues. The angiotensin peptide is cleaved from the fusion protein within the secretory pathway by the ubiquitous endoprotease furin and is released from the cell by constitutive secretion. Direct injection of an expression vector encoding such a fusion protein into rat cardiac ventricles results in a highly localized expression of atrial natriuretic peptide mRNA (an angiotensin responsive marker of cardiac hypertrophy), demonstrating the utility of this approach for local targeting of mature peptides to tissues in animal models.

The role of endothelins in the paracrine control of the secretion and growth of the adrenal cortex

Endothelins (ETs) are a family of vasoactive peptides (ET-1, ET-2, and ET-3) mainly secreted by vascular endothelium and widely distributed in the various body systems, where they play major autocrine/paracrine regulatory functions, acting via two subtypes of receptors (ETA and ETB): Adrenal cortex synthesizes and releases ETS and expresses both ETA and ETB. Zona glomerulosa possesses both ETA and ETB, whereas zona fasciculata/reticularis is almost exclusively provided with ETB. ETS exert a strong mineralocorticoid and a less intense glucocorticoid secretagogue action, mainly via ETB receptors. ETS also appear to enhance the growth and steroidogenic capacity of zona glomerulosa and to stimulate its proliferative activity. This trophic action of ETS is likely to be mediated mainly by ETA receptors. The intraadrenal release of ETS undergoes a multiple regulation, with the rise in blood flow rate and the local release of nitric oxide being the main stimulatory factors. Data are also available that indicate that ETS may also have a role in the pathophysiology of primary aldosteronism caused by adrenal adenomas and carcinomas.

Interactions between endothelin-1 and atrial natriuretic peptide influence cultured chick cardiac myocyte contractility

We have previously shown that rat atrial natriuretic peptide (ANP) reduces the contractility of cultured, spontaneously beating chick embryo ventricular cells, an effect opposite to that of endothelin-1. Endothelin-1 has been described as a secretagogue for natriuretic peptides in vitro and in vivo. Natriuretic peptides can inhibit endothelin-1 secretion from cultured endothelial cells, suggesting a negative feedback mechanism between endothelial cells and cardiomyocytes. The aim of this study was to determine whether ANP attenuated the endothelin-1-induced increase in myocyte contractility. Using a video-microscopy system we studied the contractility of isolated cultured chick ventricular myocytes in response to endothelin-1, chicken natriuretic peptide (ChNP), and both. We also used Northern blot analysis to study the time course of ChNP expression in response to endothelin-1. Endothelin-1 (10(-8) M) increased chick cardiomyocyte contractility by 20-25% between 5 and 15 min (P < 0.05). Although ChNP (3 x 10(-7) M) did not significantly change the amplitude of contraction in basal conditions, it prevented the endothelin-1-induced increase in contractility (P < 0.05) when perfused prior to endothelin-1, and reversed it when perfused 5 min after endothelin-1 exposure (P < 0.05). Endothelin-1 significantly increased the accumulation of ChNP mRNA in chick ventricular myocytes as early as the 30 min after exposure (P < 0.05), with a maximal effect after 2 h of stimulation (P < 0.01); no effect was observed after 4 h. These data support an interaction between endothelin-1 and natriuretic peptides as autocrine/paracrine factors regulating the contractile function of chick cardiac myocytes, as well as their antagonistic effects on cardiac cell contractility. The early and transient expression of ChNP mRNA in response to endothelin-1 may be involved in this interaction.

Pharmacological characterization of cardiovascular responses induced by endothelin-1 in the perfused rat heart

The effects of the endothelin receptor antagonist TAK-044 (cyclo[D-alpha-aspartyl-3-[(4-phenylpiperazin-1-yl)carbonyl]-L-ala nyl-L-alpha-aspartyl-D-2-(2-thienyl)glycyl-L-leucyl-D-tryptophyl]+ ++disodiu m salt) and BQ-123 (cyclo[D-Asp-Pro-D-Val-Leu-D-Trp]) were studied in the rat heart to characterize the receptor subtypes responsible for the cardiovascular actions of endothelin-1. Endothelin-1 induced a transient decrease and subsequent increase in perfusion pressure in perfused rat hearts, and increased left ventricular developed pressure. TAK-044 diminished these endothelin-1-induced responses (100 pmol/heart) with IC50 values of 140, 57 and 1.3 nM, respectively. BQ-123 (1-30 mu M) partially inhibited the endothelin-1-induced hypertension (30-40%) in the rat heart, and failed to inhibit the hypotension. The positive inotropic effect of endothelin-1 was abolished by BQ-123. Neither indomethacin (10 mu M) nor Nomega-nitro-L-arginine methyl ester (100 mu M) attenuated the endothelin-1-induced hypotension. TAK-044 and BQ-123 attenuated the positive inotropic effect of endothelin-1 in rat papillary muscles. In rat cardiac membrane fractions, TAK-044 and BQ-123 inhibited [125I]endothelin-1 binding to endothelin ET(A) receptors with IC50 values of 0.39 +/- 0.6 and 36 +/- 9 nM, respectively, whereas only TAK-044 potently blocked the endothelin ET(B) receptor subtype (IC50 value: 370 +/- 180 nM). These results suggest that endothelin-1 modulates cardiovascular functions in the rat heart by activating both endothelin ET(A) and endothelin ET(B) receptors, all of which are sensitive to TAK-044.

Requirement for prostaglandin synthesis in secretion of atrial natriuretic factor from isolated rat heart

Release of atrial natriuretic factor (ANF) from the heart is primarily affected by myocyte stretch. In addition, ANF release can be modulated by a variety of hormones and neurotransmitters, but the mechanisms involved in such modulation are not completely understood. In the present study, we investigated the effect of inhibition of cyclooxygenase activity on release of ANF from the isolated, spontaneously beating rat heart: (1) during basal conditions; and (2) in response to arginine vasopressin (AVP), acetylcholine (ACh) and angiotensin II (ANG II), in order to determine if cardiac prostaglandin synthesis is involved in modulation of basal and hormone-mediated ANF secretion. Basal secretion in the time controls remained stable for the duration of the experiment. AVP, ACh and ANG II reduced basal secretion significantly by 58 +/- 4%, 51 +/- 6% and 26 +/- 8%, respectively, independently of concomitant changes in coronary flow and heart rate. Inhibition of cyclooxygenase with indomethacin (1 x 10(-5) M) decreased basal ANF release by 38 +/- 6%, indicating that basal secretion requires prostaglandin production. The effects of AVP, ACh and ANG II were maintained during perfusion with indomethacin, suggesting a common mechanism of action which operates via inhibition of cyclooxygenase. Based on our previous findings that the effects of indomethacin, AVP and ACh are overcome by inhibition of NO/EDRF synthesis, we suggest a common mechanism of action by means of which NO/EDRF mediates the effects of these agents by inhibiting cyclooxygenase activity.

Cardiac endothelin-1 content and receptor subtype in spontaneously hypertensive rats

Endothelin-1 (ET-1), a newly discovered peptide with potent vasoconstrictor and growth-promoting effects, has been implicated in high blood pressure and cardiac hypertrophy in the spontaneously hypertensive rat (SHR). In the present study, we measured plasma ET-1 levels and tissue ET-1 concentrations in the four cardiac chambers of 17- to 18-week-old SHR and their normotensive controls. Wistar Kyoto (WKY) rats. SHR had slightly but significantly higher plasma ET-1 levels than WKY. The ventricles had the highest ET-1 content and the atria in both strains had the highest ET-1 concentrations. ET receptor subtypes were analysed by radiogand binding with ET-1, BQ-123 and IRL 1620 in crude membrane preparations of the four cardiac chambers. No differences in receptor subtype densities or affinities were apparent between the two strains. ET(A) represented 75 to 85% of both ET receptors. Competition analysis revealed that in both strains left ventricular tissue had lower receptor densities and higher affinities than the atria. These results suggest that ET-1 and its receptor although contributing in the maintenance of high blood pressure may not be an important factor during stable cardiac hypertrophy in adult SHR. The differential distribution of ET-1 content and receptor densities favoring the atria in both strains suggest that this peptide may have a different physiological role in the atria from that in the ventricles.

Increased endothelin-1 gene expression in the endothelium of coronary arteries and endocardium in the DOCA-salt hypertensive rat

Endothelin-1 (ET-1) is a potent vasoconstrictor and inotropic agent which may also induce cell hypertrophy. The role of ET-1 in ventricular hypertrophy in hypertension is unknown. We investigated ET-1 gene expression and immunoreactive ET-1 (ir-ET-1) concentration in the heart of deoxycorticosterone acetate (DOCA)-salt hypertensive rats. To identify the cellular sites of ET-1 production in the heart, we performed in situ hybridization histochemistry. DOCA-treated rats which underwent unilateral nephrectomy (Uni-Nx) or not, exhibited mild systolic blood pressure elevation and ventricular hypertrophy. Blood pressure elevation and cardiac hypertrophy were more severe in DOCA-salt hypertensive rats. Ventricular ET-1 mRNA was similar in Uni-Nx control and DOCA-treated rats by Northern blot analysis, whereas in DOCA-salt hypertensive rats it was significantly increased. Ir-ET-1 concentration was also enhanced in ventricles from DOCA-salt hypertensive rats compared with Uni-Nx control rats. In situ hybridization histochemistry using a 35S-labelled complementary RNA ET-1 probe demonstrated that the level of ET-1 mRNA transcripts was increased exclusively in endothelial cells of large epicardial and small intramyocardial coronary arteries and in areas of the endocardium, but not significantly in myocardial cells of either the atria or ventricles. Enhanced ET-1 production may contribute to vascular changes, both structural and functional, in the heart in this model of hypertension in the rat, but probably does not contribute to the severe cardiac hypertrophy found in DOCA-salt hypertensive rats.

Differentiation of embryonal carcinoma cells to a neural or cardiomyocyte lineage is associated with selective expression of endothelin receptors

Endothelins (ETs) were initially characterized as potent vasoactive peptides acting through at least two distinct receptors, ETA and ETB. Subsequently, their significant growth- and hypertrophy-promoting properties in cardiac and other cells were recognized. We investigated the expression of endothelin receptors during differentiation of a pluripotential embryonal carcinoma cell line (P19) to a cardiomyocyte or a neural lineage. These cells resemble those of the inner cell mass of the blastocyst, and their differentiation is believed to closely mimic critical events in early embryogenesis. Differentiation of P19 to a cardiomyocyte lineage, by aggregation and exposure to dimethyl sulfoxide resulted in induction of ETA receptors as demonstrated by radioligand binding studies, Northern blotting, and reporter gene analysis. Moreover, the P19 differentiated to a cardiac lineage responded to ET-1 with a 3-fold increase in the secretion of atrial natriuretic peptide. In contrast, differentiation to a neural lineage, by aggregation and exposure to retinoic acid, was associated with the induction of predominantly ETB. Therefore, selective differentiation of the P19 led to the differential expression of endothelin receptors in a pattern consistent with that observed in normal myocardial and neural tissue. The induction of endothelin receptors in a model system of early embryogenesis provides strong support for the critical role of this peptide/receptor family in differentiation and development. As well, this model system is well suited for the study of the mechanisms controlling endothelin receptor expression during differentiation.

The membranes of cultured rat brain astrocytes contain endothelin-converting enzyme activity

Both endothelins and their big-endothelin precursors were found capable of inducing the release of arachidonic acid from purified cultures of rat astrocytes. Their order of potency was as follows: big-endothelin-3 < big-endothelin-1 < endothelin-1 = endothelin-3. Mature endothelins induced the release of arachidonic acid in a rapid fashion. In contrast, much longer incubation times were required for big-endothelins to exert an effect, suggesting that their activity was dependent on their conversion. When big-endothelin-1 was added to the incubation medium of intact live astrocytes, it was converted into mature endothelin-1 in a time-dependent manner and the conversion was inhibited by phosphoramidon. This suggests that astrocytic endothelin-converting enzyme is (at least in part) an external membrane-bound metalloprotease. Some conversion of big-endothelin-3 into endothelin-3 also occurred. However, it was less efficient than the conversion of big-endothelin-1, which is compatible with the lower bioactivity of big-endothelin-3 vs. that of big-endothelin-1 in astrocytes.

Endothelin receptor antagonism

Following the original report by Yanagisawa et al. (1988) more than 7 years ago, compelling evidence that ET plays an important role in the local regulation of smooth muscle tone and cell growth has been reported. In addition, many studies point to a significant role for endothelin in nonvascular function. The investigation of the endothelin system has been greatly advanced in the last 2 to 3 years through significant advances in the development of potent and selective ET receptor antagonists. These agents have proven to be essential tools for elucidating the biological significance of the ET system, leading to the realization that antagonism of the ET system may have significant therapeutic potential. As emphasized in this review, the importance of chronic blockade of the ET system may be a critical aspect of future research in this exciting area. Confounding issues remain the lack of information about the role of the ETB receptor, the apparent pharmacological evidence for additional ET receptor subtypes, and species variation in the tissue distribution of ET isoforms and receptor subtypes. Along with the greater ability to understand the endothelin system provided by potent and selective pharmacological agents, is the important contribution of modern molecular biology techniques, highlighted by the insights gained from recent reports of results from ET gene disruption studies. Kurihara et al. (1994) found that ET-1-deficient homozygous mice die at birth of apparent respiratory failure secondary to severe craniofacial abnormalities. Subsequently, Yanagisawa's laboratory has presented and published a series of complementary gene disruption studies. First, Hosoda et al. (1994) demonstrated remarkably, that ETA receptor knockout mice bear morphological abnormalities nearly identical to ET-1 knockout mice. Second, they found that disruption of the ET-3 peptide and ETB receptor genes result in homozygous mice that share identical phenotypic traits (i.e., coloration changes and aganglionic megacolon) which are similar to a previously known natural mutation, the Piebald-Lethal mouse (Hosoda et al., 1994; Baynash et al., 1994). This phenotype has a human corollary known as Hirschsprung's Disease and it is now known that the disease, though multigenic, results from a missense mutation of the ETB receptor gene in some individuals (Puffenberger et al., 1994). Taken together these data indicate that the endothelin system is essential to correct embryonic neural crest development, a completely novel finding within the superfamily of guanine-protein-linked receptors.(ABSTRACT TRUNCATED AT 400 WORDS)