Endothelin receptor subtypes and stimulation of aldosterone secretion

Aldosterone and cardiovascular diseases

Aldosterone's role in the kidney and its pathophysiologic actions in hypertension are well known. However, its role or that of its receptor [minieralocorticoid receptor (MR)] in other cardiovascular (CV) disease are less well described. To identify their potential roles in six CV conditions (heart failure, myocardial infarction, atrial fibrillation, stroke, atherosclerosis, and thrombosis), we assessed these associations in the following four areas: (i) mechanistic studies in rodents and humans; (ii) pre-clinical studies of MR antagonists; (iii) clinical trials of MR antagonists; and (iv) genetics. The data were acquired from an online search of the National Library of Medicine using the PubMed search engine from January 2011 through June 2021. There were 3702 publications identified with 200 publications meeting our inclusion and exclusion criteria. Data strongly supported an association between heart failure and dysregulated aldosterone/MR. This association is not surprising given aldosterone/MR's prominent role in regulating sodium/volume homeostasis. Atrial fibrillation and myocardial infarction are also associated with dysregulated aldosterone/MR, but less strongly. For the most part, the data were insufficient to determine whether there was a relationship between atherosclerosis, stroke, or thrombosis and aldosterone/MR dysregulation. This review clearly documented an expanding role for aldosterone/MR's dysregulation in CV diseases beyond hypertension. How expansive it might be is limited by the currently available data. It is anticipated that with an increased focus on aldosterone/MR's potential roles in these diseases, additional clinical and pre-clinical data will clarify these relationships, thereby, opening approaches to use modulators of aldosterone/MR's action to more precisely treat these CV conditions.

Aldosterone breakthrough from a pharmacological perspective

Aldosterone (Aldo) breakthrough is a well-known phenomenon that occurs in patients with long-term renin-angiotensin aldosterone system (RAAS) blockade using inhibitors of renin or angiotensin converting enzyme or angiotensin II type 1 receptor blockers. The blockade of the mineralocorticoid receptor (MR), an Aldo binding receptor, is effective in managing patients with resistant hypertension, defined as uncontrollable blood pressure despite the concurrent use of three antihypertensive drugs. In other words, MR inhibitors are not used as first-line antihypertensive drugs in most guidelines for hypertension management. Aldo breakthrough puts hypertensive patients at higher risk of cardiovascular disease and worsens future outcomes. This review discusses Aldo secretion and the mechanism of Aldo breakthrough, dependent or independent of the RAAS, with consideration of the pharmacological aspects of this phenomenon, as well as hypothetical views.

Androgens influence microvascular dilation in PCOS through ET-A and ET-B receptors

Hyperandrogenism and vascular dysfunction often coexist in women with polycystic ovary syndrome (PCOS). We hypothesized that testosterone compromises cutaneous microvascular dilation in women with PCOS via the endothelin-1 ET-B subtype receptor. To control and isolate testosterone's effects on microvascular dilation, we administered a gonadotropin-releasing hormone antagonist (GnRHant) for 11 days in obese, otherwise healthy women [controls, 22.0 (4) yr, 36.0 (3.2) kg/m(2)] or women with PCOS [23 (4) yr, 35.4 (1.3) kg/m(2)], adding testosterone (T; 2.5 mg/day) on days 8-11. Using laser Doppler flowmetry and cutaneous microdialysis, we measured changes in skin microcirculatory responsiveness (ΔCVC) to local heating while perfusing ET-A (BQ-123) and ET-B (BQ-788) receptor antagonists under three experimental conditions: baseline (BL; prehormone intervention), GnRHant (day 4 of administration), and T administration. At BL, ET-A receptor inhibition enhanced heat-induced vasodilation in both groups [ΔCVC control 2.03 (0.65), PCOS 2.10 (0.25), AU/mmHg, P < 0.05]; ET-B receptor inhibition reduced vasodilation in controls only [ΔCVC 0.98 (0.39), 1.41 (0.45) AU/mmHg for controls, PCOS] compared with saline [ΔCVC controls 1.27 (0.48), PCOS 1.31 (0.13) AU/mmHg]. GnRHant enhanced vasodilation in PCOS [saline ΔCVC 1.69 (0.23) AU/mmHg vs. BL, P < 0.05] and abolished the ET-A effect in both groups, a response reasserted with T in controls. ET-B receptor inhibition reduced heat-induced vasodilation in both groups during GnRHant and T [ΔCVC, controls: 0.95 (0.21) vs. 0.51 (13); PCOS: 1.27 (0.23) vs. 0.84 (0.27); for GnRHant vs. T, P < 0.05]. These data demonstrate that androgen suppression improves microvascular dilation in PCOS via ET-A and ET-B receptors.

Endothelium-derived steroidogenic factor enhances angiotensin II-stimulated aldosterone release by bovine zona glomerulosa cells

Endothelium-derived steroidogenic factor (EDSF) is an endothelial peptide that stimulates aldosterone release from bovine adrenal zona glomerulosa (ZG) cells. The regulation of aldosterone release by combinations of EDSF and angiotensin II (AII) or EDSF and ACTH was investigated. Endothelial cells (ECs) and EC-conditioned media (ECCM) increased aldosterone release from ZG cells, an activity attributed to EDSF. AII (10(-12) to 10(-8) M) and ACTH (10(-12) to 10(-9) M) also stimulated the release of aldosterone from ZG cells. The stimulation by AII, but not ACTH, was greatly enhanced when ZG cells were coincubated with ECs. AII was metabolized by ECs to peptides identified by mass spectrometry as angiotensin (1-7) and angiotensin IV. There was very little metabolism of AII by ZG cells. Neither of these two AII metabolites altered aldosterone release from ZG cells, so they could not account for the enhanced response with ECs. AII-induced aldosterone release from ZG cells was enhanced by ECCM but not cell-free conditioned medium. This enhanced response was not due to increased EDSF release from ECs by AII. The synergistic effect of EDSF and AII was apparent when AII was added during or after the generation of ECCM and not observed when the AII component of the enhancement was blocked by the AII antagonist, losartan. These studies indicate that EDSF enhances the steroidogenic effect of AII. In the adrenal gland, ECs are in close anatomical relationship with ZG cells and may sensitize ZG cells to the steroidogenic action of AII by releasing EDSF.

Aldosterone biosynthesis, regulation, and classical mechanism of action

Circulating aldosterone is principally made in the glomerulosa zone of the adrenal cortex by a series of enzyme steps leading to the conversion of cholesterol to aldosterone. Uniquely, aldosterone's production is regulated at two critical enzyme steps: (1) early in its biosynthetic pathway (the conversion of cholesterol to pregnenolone cholesterol side chain cleavage enzyme) and (2) late (the conversion of corticosterone to aldosterone by aldosterone synthase). A variety of factors modify aldosterone secretion--the most important are angiotensin II (AngII), the end-product of the renin-angiotensin system (RAS), and potassium. However ACTH, neural mediators and natriuretic factors also contribute at least over the short run. Aldosterone's classical epithelial effect is to increase the transport of sodium across the cell in exchange for potassium and hydrogen ions. Although still controversial, there is an increasing body of data that supports the hypothesis that aldosterone can be synthesized in tissues outside of the adrenal cortex, specifically in the heart and the vasculature. Aldosterone's biosynthesis appears to be regulated in these tissues similar to what occurs in the adrenal cortex. The role of this extra adrenal aldosterone production in health and disease is as of yet undetermined.

Control of aldosterone secretion: a model for convergence in cellular signaling pathways

Aldosterone secretion by glomerulosa cells is stimulated by angiotensin II (ANG II), extracellular K(+), corticotrophin, and several paracrine factors. Electrophysiological, fluorimetric, and molecular biological techniques have significantly clarified the molecular action of these stimuli. The steroidogenic effect of corticotrophin is mediated by adenylyl cyclase, whereas potassium activates voltage-operated Ca(2+) channels. ANG II, bound to AT(1) receptors, acts through the inositol 1,4,5-trisphosphate (IP(3))-Ca(2+)/calmodulin system. All three types of IP(3) receptors are coexpressed, rendering a complex control of Ca(2+) release possible. Ca(2+) release is followed by both capacitative and voltage-activated Ca(2+) influx. ANG II inhibits the background K(+) channel TASK and Na(+)-K(+)-ATPase, and the ensuing depolarization activates T-type (Ca(v)3.2) Ca(2+) channels. Activation of protein kinase C by diacylglycerol (DAG) inhibits aldosterone production, whereas the arachidonate released from DAG in ANG II-stimulated cells is converted by lipoxygenase to 12-hydroxyeicosatetraenoic acid, which may also induce Ca(2+) signaling. Feedback effects and cross-talk of signal-transducing pathways sensitize glomerulosa cells to low-intensity stimuli, such as physiological elevations of [K(+)] (< or =1 mM), ANG II, and ACTH. Ca(2+) signaling is also modified by cell swelling, as well as receptor desensitization, resensitization, and downregulation. Long-term regulation of glomerulosa cells involves cell growth and proliferation and induction of steroidogenic enzymes. Ca(2+), receptor, and nonreceptor tyrosine kinases and mitogen-activated kinases participate in these processes. Ca(2+)- and cAMP-dependent phosphorylation induce the transfer of the steroid precursor cholesterol from the cytoplasm to the inner mitochondrial membrane. Ca(2+) signaling, transferred into the mitochondria, stimulates the reduction of pyridine nucleotides.

BQ-788, a selective endothelin ET(B) receptor antagonist

We describe characteristics of a selective endothelin (ET) ET(B) receptor antagonist, BQ-788 [N-cis-2,6-dimethylpiperidinocarbonyl-L-gamma-methylleucyl-D-1-methoxycarbonyltryptophanyl-D-norleucine], which is widely used to demonstrate the role of endogenous or exogenous ETs in vitro and in vivo. In vitro, BQ-788 potently and competitively inhibited (125)I-labeled ET-1 binding to ET(B) receptors in human Girrardi heart cells (hGH) with an IC(50) of 1.2 nM, but only poorly inhibited the binding to ET A receptors in human neuroblastoma cell line SK-N-MC cells (IC(50), 1300 nM). In isolated rabbit pulmonary arteries, BQ-788 showed no agonistic activity up to 10 microM and competitively inhibited the vasoconstriction induced by an ET(B)-selective agonist (pA(2), 8.4). BQ-788 also inhibited several bioactivities of ET-1, such as bronchoconstriction, cell proliferation, and clearance of perfused ET-1. Thus, it is confirmed that BQ-788 is a potent, selective ET(B) receptor antagonist. In vivo, in conscious rats, BQ-788, 3 mg/kg/h, i.v., completely inhibited a pharmacological dose of ET-1- or sarafotoxin6c (S6c) (0.5 nmol/kg, i.v.)-induced ET(B) receptor-mediated depressor, but not pressor responses. Furthermore, BQ-788 markedly increased the plasma concentration of ET-1, which is considered an index of potential ET(B) receptor blockade in vivo. In Dahl salt-sensitive hypertensive (DS) rats, BQ-788, 3 mg/kg/h, i.v., increased blood pressure by about 20 mm Hg. It is reported that BQ-788 also inhibited ET-1-induced bronchoconstriction, tumor growth and lipopolysaccharide-induced organ failure. These data suggest that BQ-788 is a good tool for demonstrating the role of ET-1 and ET(B) receptor subtypes in physiological and/or pathophysiological conditions.

Endothelin A receptor antagonists in congestive heart failure: blocking the beast while leaving the beauty untouched?

Congestive heart failure (CHF) is a disease process characterized by impaired left ventricular function, increased peripheral and pulmonary vascular resistance and reduced exercise tolerance and dyspnea. Thus, mediators involved in the control of myocardial function and vascular tone may be involved in its pathophysiology. The family of endothelins (ET) consists of four closely related peptides, ET-1, ET-2, ET-3, and ET-4, which cause vasoconstriction, cell proliferation, and myocardial effects through activation of ET(A) receptors. In contrast, endothelial ET(B) receptors mediate vasodilation via release of nitric oxide and prostacyclin. In addition, ET(B) receptors in the lung are a major pathway for the clearance of ET-1 from plasma. Thus, infusion of an ET(A) receptor antagonist into the brachial artery in healthy humans leads to vasodilation whereas infusion of an ET(B) receptor antagonist causes vasoconstriction. ET-1 plasma levels are elevated in CHF and correlate both with the hemodynamic severity and with symptoms. Plasma levels of ET-1 and its precursor, big ET-1, are strong independent predictors of death in patients after myocardial infarction and with CHF. ET-1 contributes to increased systemic and pulmonary vascular resistance, vascular dysfunction, myocardial ischemia, and renal impairment in CHF. Selective ET(A) as well as combined ET(A/B) receptor antagonists have been studied in patients with CHF showing impressive hemodynamic improvements (i.e. reduced peripheral vascular and pulmonary resistance as well as increased cardiac output). These results indicate that ET receptor antagonists indeed have a potential to improve hemodynamics, symptoms, and potentially prognosis of CHF which still carries a high mortality.

Endothelin receptor antagonists in congestive heart failure: a new therapeutic principle for the future?

Congestive heart failure (CHF) is characterized by impaired left ventricular function, increased peripheral and pulmonary vascular resistance and reduced exercise tolerance and dyspnea. Thus, mediators involved in the control of myocardial function and vascular tone may be involved in its pathophysiology. The family of endothelins (ET) consists of four closely related peptides, ET-1, ET-2, ET-3 and ET-4, which cause vasoconstriction, cell proliferation and myocardial effects through activation of ETA receptors. In contrast, endothelial ETB receptors mediate vasodilation via release of nitric oxide and prostacyclin. In addition, ETB receptors in the lung are a major pathway for the clearance of ET-1 from plasma. Thus, infusion of an ETA-receptor antagonist into the brachial artery in healthy humans leads to vasodilation, whereas infusion of an ETB-receptor antagonist causes vasoconstriction. Endothelin-1 plasma levels are elevated in CHF and correlate both with hemodynamic severity and symptoms. Plasma levels of ET-1 and its precursor, big ET-1, are strong independent predictors of death after myocardial infarction as well as in CHF. Endothelin-1 contributes to increased systemic and pulmonary vascular resistance, vascular dysfunction, myocardial ischemia and renal impairment in CHF. Selective ETA, as well as combined ETA/B-receptor antagonists, have been studied in patients with CHF, and their use has shown impressive hemodynamic improvement (i.e., reduced peripheral vascular and pulmonary resistance as well as increased cardiac output). These results indicate that ET-receptor antagonists, indeed, have a potential to improve hemodynamics, symptoms and, potentially, prognosis in patients with CHF, which still carries a high mortality.

Therapeutic potential for endothelin receptor antagonists in cardiovascular disorders

The endothelins are synthesized in vascular endothelial and smooth muscle cells, as well as in neural, renal, pulmonal, and inflammatory cells. These peptides are converted by endothelin-converting enzymes (ECE-1 and -2) from 'big endothelins' originating from large preproendothelin peptides cleaved by endopeptidases. Endothelin (ET)-1 has major influence on the function and structure of the vasculature as it favors vasoconstriction and cell proliferation through activation of specific ET(A) and ET(B) receptors on vascular smooth muscle cells. In contrast, ET(B )receptors on endothelial cells cause vasodilation via release of nitric oxide (NO) and prostacyclin. Additionally, ET(B) receptors in the lung are a major pathway for the clearance of ET-1 from plasma. Indeed, ET-1 contributes to the pathogenesis of important disorders as arterial hypertension, atherosclerosis, and heart failure. In patients with atherosclerotic vascular disease (as well as in many other disease states), ET-1 levels are elevated and correlate with the number of involved sites. In patients with acute myocardial infarction, they correlate with 1-year prognosis. ET receptor antagonists have been widely studied in experimental models of cardiovascular disease. In arterial hypertension, they prevent vascular and myocardial hypertrophy. Experimentally, ET receptor blockade also prevents endothelial dysfunction and structural vascular changes in atherosclerosis due to hypercholesterolemia. In experimental myocardial ischemia, treatment with an ET receptor antagonist reduced infarct size and prevented left ventricular remodeling after myocardial infarction. Most impressively, treatment with the selective ET(A) receptor antagonist BQ123 significantly improved survival in an experimental model of heart failure. In many clinical conditions, such as congestive heart failure, both mixed ET(A/B )as well as selective ET(A) receptor antagonism ameliorates the clinical status of patients, i.e. symptoms and hemodynamics. A randomized clinical trial showed that a mixed ET(A/B) receptor antagonist effectively lowered arterial blood pressure in patients with arterial hypertension. In patients with primary pulmonary hypertension or pulmonary hypertension related to scleroderma, treatment with a mixed ET(A/B) receptor antagonist resulted in an improvement in exercise capacity. ET receptor blockers thus hold the potential to improve the outcome in patients with various cardiovascular disorders. Randomized clinical trials are under way to evaluate the effects of ET receptor antagonism on morbidity and mortality.

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

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

Adrenal capillary endothelial cells stimulate aldosterone release through a protein that is distinct from endothelin

We tested the possibility that bovine adrenal capillary endothelial cells (ECs) stimulate aldosterone secretion from bovine zona glomerulosa (ZG) cells by the release of a transferable factor. In coincubations of ZG cells and ECs in serum-free medium, aldosterone release was stimulated approximately 17-fold, and the stimulation was related to the concentration of ECs. The maximal stimulation by ECs was 75% of the maximal response to ACTH. In contrast, adrenal pericytes and fibroblasts were without effect. ECs incubated alone without ZG cells did not produce aldosterone. Conditioned medium from ECs (EC-CM), but not adrenal fibroblasts, stimulated aldosterone release approximately 3-fold. The stimulation increased with the concentration of EC-CM and the duration of conditioning time. Steroidogenic activity in EC-CM was abolished by pronase treatment, indicating that the active factor was a protein. However, the activity in EC-CM was distinct from that of endothelin-1 (ET-1), an endothelial peptide that also stimulates aldosterone secretion, as it was not blocked by the ET(B) receptor antagonist PD-145065, it did not alter [125I]ET-1 binding to ZG cells, and its release occurred before the release of ET-1. Neither ECs nor EC-CM stimulated the production of cortisol from zona fasciculata cells. The activity of EC-CM was not blocked by an angiotensin II AT1 receptor antagonist or a bradykinin B2 receptor antagonist. EC-CM stimulated increased intracellular calcium in fura-2-loaded ZG cells, but did not increase the production of cAMP. Using gel filtration, this peptide had an approximate molecular mass of 3000 Da and migrated earlier than ET-1. This study demonstrates that ECs in vitro alter steroidogenesis through the release of a transferable substance and suggests the existence of an endothelium-derived steroidogenic factor that is produced by adrenal capillary ECs. This endothelium-derived steroidogenic factor may function in the adrenal gland as a paracrine regulator of aldosterone secretion.

Solution structure of a novel ETB receptor selective agonist ET1-21 [Cys(Acm)1,15, Aib3,11, Leu7] by nuclear magnetic resonance spectroscopy and molecular modelling

The solution structure of a biologically active modified linear endothelin-1 analogue, ET1-21[Cys(Acm)1,15, Aib3,11, Leu7], has been determined for the first time by two-dimensional nuclear magnetic resonance spectroscopy in a methanol-d3/water solvent mixture. Out of approximately one hundred linear peptide analogues tested by biological assay, this peptide, together with a dozen others, showed significant ETB selective agonist activity. Here we report the solution structure of an ETB selective agonist of a full-length, synthetic linear endothelin analogue. The calculated structures indicate that the peptide adopts an alpha-helical conformation between residues Ser5-His16, whilst both N- and C-termini show no preferred conformation. These results suggest that the disulphide bridges normally associated with endothelin and sarafotoxin peptides may not necessarily be important for either ETB receptor binding activity or the formation of a helical conformation in solution.

Mechanisms of action of endothelin-1 in rat adrenal

Displacement curves of 125I-Endothelim-1 (ET-1) binding to rat adrenal cells with unlabeled ET-1, and the ET-1 receptor-related peptides sarafotoxin and BQ-123, show that rat adrenal cortex possess, as its bovine counterpart, two different receptors to ET-1 named ET-A and ET-B. Binding of ET-1 to its rat adrenal receptors stimulates i) aldosterone production, in vivo and in vitro ii) calcium influx, which is mediated through voltage dependent- and receptor operated- calcium channels, iii) cholesterol uptake, iv) stimulation of Na+/K+-ATPase and iv) diacylglycerol production. While the last effect is mediated through ET-A receptors the others involve binding of ET-1 to ET-B receptors. Finally, ouabain potentiates the ET-1-mediated stimulation of aldosterone production, suggesting that the effect of the peptidic hormone on Na+/K+-ATPase could act as a negative feedback mechanism.

Endothelin-1 stimulates steroid secretion of human adrenocortical cells ex vivo via both ETA and ETB receptor subtypes

The role played by endothelins (ETs) and their receptor subtypes (ETA and ETB) in the regulation of steroid hormone secretion in human adrenal gland remains unclear. Therefore, we investigated the gene expression of ET-1 and its receptors in highly pure preparations of human adrenocortical cells and the effect of ET-1 on their secretory activity. Reverse transcription-PCR with primers specific for prepro-ET-1, ET-converting enzyme-1, ETA, and ETB complementary DNAs demonstrated the expression of all of these genes in human adrenocortical cells. ET-1 increased the secretion of aldosterone and cortisol by enhancing both earlier and late steps of their synthesis. The secretory response to ET-1 was partially (60%) inhibited by BQ-123 and BQ-788, which are selective antagonists of the ETA and ETB receptors, respectively. When added together, the two antagonists suppressed the secretagogue effect of ET-1. Collectively, these findings suggest that ET-1, acting via both ETA and ETB receptors, may exert an autocrine/paracrine regulation of the function of the human adrenal cortex.

Primary Hyperaldosteronism

OBJECTIVE

To characterize the syndrome of primary aldosteronism and summarize diagnostic and therapeutic strategies.

METHODS

We review the mechanisms of action of aldosterone and outline features that distinguish the major subtypes of aldosteronism.

RESULTS

The state of aldosterone excess should be suspected in every patient manifesting hypertension and hypokalemia. The documentation of low renin activity and high plasma aldosterone concentration in such patients suggests the presence of primary aldosteronism. Lack of appropriate suppression of plasma aldosterone after saline infusion is thought to be the best maneuver for confirming primary aldosteronism. Nonetheless, a similar lack of aldosterone suppressibility after either oral salt loading for 3 days or oral administration of a single 25-mg dose of captopril may help achieve the same purpose. Once primary aldosteronism has been diagnosed, the distinction between two major subtypes--unilateral adrenal adenoma or Conn's syndrome and bilateral idiopathic adrenal hyperplasia--is important because of the difference in management. Certain physiologic maneuvers, such as change of posture from supine to upright, oral administration of cyproheptadine, and radiologic localization with several techniques including iodocholesterol scanning and adrenal venous sampling, will almost always help distinguish unilateral adenoma from bilateral hyperplasia.

CONCLUSION

The distinction between adrenal adenoma and adrenal hyperplasia is critical because of the varied approach to treatment. Most patients with bilateral adrenal hyperplasia are managed medically with an aldosterone antagonist such as spironolactone, whereas most unilateral adenomas are resected after correction of hypertension and hypokalemia with appropriate medical therapy.

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.

Molecular pharmacology and pathophysiological significance of endothelin

Since the discovery of the most potent vasoconstrictor peptide, endothelin, in 1988, explosive investigations have rapidly clarified much of the basic pharmacological, biochemical and molecular biological features of endothelin, including the presence and structure of isopeptides and their genes (endothelin-1, -2 and -3), regulation of gene expression, intracellular processing, specific endothelin converting enzyme (ECE), receptor subtypes (ETA and ETB), intracellular signal transduction following receptor activation, etc. ECE was recently cloned, and its structure was shown to be a single transmembrane protein with a short intracellular N-terminal and a long extracellular C-terminal that contains the catalytic domain and numerous N-glycosylation sites. In addition to acute contractile or secretory actions, endothelin has been shown to exert long-term proliferative actions on many cell types. In this case, intracellular signal transduction appears to converge to activation of mitogen-activated protein kinase. As a recent dramatic advance, a number of non-peptide and orally active receptor antagonists have been developed. They, as well as current peptide antagonists, markedly accelerated the pace of investigations into the true pathophysiological roles of endogenous endothelin-1 in mature animals; e.g., hypertension, pulmonary hypertension, acute renal failure, cerebral vasospasm, vascular thickening, cardiac hypertrophy, chronic heart failure, etc. Thus, the interference with the endothelin pathway by either ECE-inhibition or receptor blockade may provide an exciting prospect for the development of novel therapeutic drugs.

Studies on endothelin receptors in the zonae fasciculata/reticularis of the rat adrenal cortex: contrast with the zona glomerulosa

This study investigated the ET(A) and ET(B) receptor subtypes in rat adrenal cortex. The ET(A) antagonist, BQ-123, inhibited the zona glomerulosa (zg), but not the inner zone (iz) response to ET-1. RES-701-1, the ET(B) antagonist, abolished the iz response to ET-1, but had less effect on the zg. [125I]ET-1 binding studies revealed two receptor subtypes in both zones, with ET(A) predominating in the zg, and ET(B) in the iz. These data suggest that the ET(A) subtype is functionally more important in the zg while the ET(B) receptor is the major subtype in the inner zones.

Endothelin-1-induced incorporation of cholesterol into rat adrenals

The effect of endothelin-1 (ET-1) on cholesterol uptake by adrenal cortex was evaluated through several experimental approaches: infusion of ET-1 followed by measurement of endogenous cholesterol in excised adrenals; infusion of ET-1 followed by tritiated cholesterol incorporation into adrenal quarters in vitro; coinfusion of ET-1 with tritiated cholesterol-enriched serum and determination of adrenal-associated radioactivity; and tritiated cholesterol incorporation in incubations of adrenal cells. In all cases ET-1 increased cholesterol uptake. Subcellular fractionation showed an ET-1-mediated augmentation in mitochondrial fraction. This increase was mediated by the subpopulation B of adrenal receptors for ET-1. In addition, ET-1 also increased cytochrome P450-SCC (side-chain cleavage) activity.

Endothelin adrenocortical secretagogue effect is mediated by the B receptor in rats

We investigated the gene expression and localization of endothelin-1 (ET-1) receptor subtypes ET(A) and ET(B) in the rat adrenal cortex as well as their involvement in the corticosteroid secretagogue effect of ET-1 in vitro. Reverse transcription-polymerase chain reaction with primers specific for ET(A) and ET(B) cDNAs demonstrated the expression of both receptor genes in homogenates of adrenocortical tissue. However, in isolated zona glomerulosa and zona fasciculata cells, only ET(B) mRNA was detected. Autoradiographic examination of the selective displacement of 125I-ET-1 binding by BQ-123 and BQ-788 (specific ligands for ET(A) and ET(B), respectively) indicated that zona glomerulosa possesses both ET(A) and ET(B), whereas zona fasciculata is exclusively provided with ET(B). ET-1 enhanced in a concentration-dependent manner aldosterone and corticosterone secretions of dispersed zona glomerulosa and zona fasciculata cells, respectively. The ET(B) antagonist BQ-788 markedly reduced the secretory response of zona glomerulosa cells and completely suppressed that of zona fasciculata cells, whereas the ET(A) antagonist BQ-123 was ineffective. These findings indicate that in the rat, the adrenocortical secretagogue action of ET-1 is mediated by the ET(B) receptor subtype and that the ET(A) receptor is not directly involved in such an effect.

Endothelin-1 and its receptors A and B in human aldosterone-producing adenomas

Endothelin-1 stimulates aldosterone secretion by interacting with specific receptors. Accordingly, we wished to investigate endothelin-1, endothelin-A (ETA) receptor, and endothelin-B (ETB) receptor gene expression, localization, and properties in aldosterone-producing adenomas and in the normal human adrenal cortex. We carried out 125I-endothelin-1 displacement studies with cold endothelin-1, endothelin-3, the specific ETA antagonist BQ-123, and the specific ETB weak agonist sarafotoxin 6 C and coanalyzed data with the nonlinear iterative curve-fitting program LIGAND. We also studied gene expression with reverse transcription-polymerase chain reaction with specific primers for endothelin-1, ETA, and ETB complementary DNA. Normal adrenal cortices from consenting kidney cancer patients (n = 2) and aldosterone-producing adenomas (n = 4) were studied; for the latter, surrounding normal cortex and kidney biopsy tissue served as controls. To further localize the receptor subtypes, tissue sections were studied by autoradiography in the presence and absence of 500 nmol/L BQ-123, 100 nmol/L sarafotoxin 6 C, and 1 mumol/L cold endothelin-1. In all tissues examined, endothelin-1, ETA, and ETB messenger RNAs were easily detected. However, in aldosterone-producing adenomas, both receptors' genes were expressed at a higher level than in the kidney.(ABSTRACT TRUNCATED AT 250 WORDS)

Gene expression, localization, and characterization of endothelin A and B receptors in the human adrenal cortex

Compelling evidence indicates that the endothelium-derived potent vasoconstrictor endothelin-1 (ET-1) stimulates aldosterone secretion by interacting with specific receptors. Although two different ET-1 receptors have been identified and cloned, the receptor subtype involved in mediating aldosterone secretion is still unknown. Accordingly, we wished to investigate whether the genes of ET-1 and of its receptors A and B are expressed in the normal human adrenal cortex. We designed specific primers for ET-1 and the ETA and ETB receptors genes and developed a reverse transcription polymerase chain reaction (RT-PCR) with chemiluminescent quantitation of the cDNA. In addition, we carried out 125I ET-1 displacement studies with cold ET-1, ET-3 and the specific ETA and ETB ligands BQ123 and sarafotoxin 6C. Localization of each receptor subtype was also investigated by autoradiography. Binding experiments were first individually analyzed by Scatchard and Hofstee plot and then coanalyzed by the nonlinear iterative curve fitting program Ligand. Histologically normal adrenal cortex tissue, obtained from kidney cancer patients (n = 7), and an aldosterone-producing adenoma (APA), which is histogenetically derived from the zona glomerulosa (ZG) cells, were studied. Results showed that the ET-1, ETA and ETB mRNA can be detected by RT-PCR in all adrenal cortices as well as in the APA. The best fitting of the 125I ET-1 displacement binding data was consistently provided by a two-site model both in the normal adrenal cortex (F = 22.1, P < 0.0001) and in the APA (F = 18.4, P < 0.0001). In the former the density (Bmax) of the ETA and ETB subtype was 2.6 +/- 0.5 pmol/mg protein (m +/- SEM) and 1.19 +/- 0.6, respectively. The dissociation constant (Kd) of ET-1, ET-3, S6C, and BQ-123 for each receptor subtype resulted to be within the range reported for human tissue for the ETA and ETB receptors. In the APA tissue the Bmax tended to be lower (1.33 and 0.8 pmol/mg protein, for the ETA and ETB, respectively) but the Kd were similar. Autoradiographic studies confirmed the presence of both receptor subtypes on the ZG as well as on APA cells. Thus, the genes of ET-1 and both its receptor subtypes ETA and ETB are actively transcribed in the human adrenal cortex. Furthermore, both receptor subtypes are translated into proteins in ZG and APA cells.

Endothelin-1 stimulation of aldosterone and zona glomerulosa ouabain-sensitive sodium/potassium-ATPase

Endothelin stimulates the cells of the zona glomerulosa of the adrenal gland and releases aldosterone. While it is a less potent aldosterone secretagogue than angiotensin II endothelin also potentiates the effects of angiotensin II on aldosterone biosynthesis. Two endothelin receptors have been cloned and are expressed in the adrenal zona glomerulosa. Intravenous infusion of endothelin at a rate of 80 ng/kg/min for 30 min into rats produced increases in blood pressure, adrenal content of aldosterone and stimulated the ouabain-sensitive sodium potassium ATPase in the zona glomerulosa, but not in the zona fasciculata, of the adrenal. The simultaneous infusion of the isopeptide specific endothelin receptor A (ETA) antagonist BQ-123 blocked the pressor effects of endothelin, but did not alter the increase in aldosterone content of the zona glomerulosa or the ouabain-sensitive sodium potassium ATPase activity. Infusion of Sarafotoxin 6b, an ETB agonist, also increased the aldosterone content of the adrenal and stimulated the ouabain-sensitive sodium potassium ATPase in the zona glomerulosa, further indicating that the effect of endothelin is probably mediated by ETB or isopeptide non-specific endothelin receptor. The mechanism by which endothelin stimulates the sodium potassium ATPase is unclear as is the relation between a stimulated sodium potassium ATPase and the potentiation of angiotensin II effect on the adrenal.

Biochemical and pharmacological profile of a potent and selective endothelin B-receptor antagonist, BQ-788

We describe the characteristics of a potent and selective endothelin (ET) B-receptor antagonist, BQ-788 [N-cis-2,6-dimethylpiperidinocarbonyl-L-gamma-methylleucyl-D -1- methoxycarbonyltryptophanyl-D-norleucine]. In vitro, this compound potently and competitively inhibits 125I-labeled endothelin 1 (ET-1) binding to ETB receptors on human Girardi heart cells (IC50, 1.2 nM) but only poorly inhibits the binding to ETA receptors on human neuroblastoma cell line SK-N-MC cells (IC50, 1300 nM). In isolated rabbit pulmonary arteries, BQ-788 shows no agonist activity up to 10 microM and competitively antagonizes the vasoconstriction induced by an ETB-selective agonist, BQ-3020 (pA2, 8.4). In rat, an ETA-selective antagonist, BQ-123 (1 mg/kg, i.v.), does not affect transient depressor response to ET-1 (0.3 nmol/kg, i.v.) but potently inhibits following sustained pressor response; vice versa, BQ-788 (1 mg/kg, i.v.) abolishes the depressor response, resulting in a rapid onset of apparently enhanced pressor response. Thus, being a potent and selective ETB receptor antagonist, BQ-788 may be considered as a powerful tool for investigating the role of ET in physiological and pathological processes.

Identification of a ligand-binding site of the human endothelin-A receptor and specific regions required for ligand selectivity

To investigate the ligand-binding site of the human endothelin-A-receptor subtype (ETA), we have produced various chimeric and mutated receptors in chinese hamster ovary cells. The substitution of Lys140 with Ile located in the C-terminus of the second transmembrane region caused a 13-fold reduction in affinity for endothelin-1 (ET-1) and 3.6-fold lower Bmax than those values for the original receptor. Correspondingly, the mutated ETA receptor with the Lys140-->Ile substitution failed to induce an increase in the intracellular calcium concentration in the presence of 1 nM ET-1. Thus, the Lys140 in the ETA receptor is important in ligand binding. ETA and ETB receptors possess the ET isopeptides selective and non-selective binding activities, respectively. Displacement experiments and the binding of 125I-ET-3 to various chimera receptors demonstrated that both the third and fourth extracellular regions, including the flanking transmembrane regions, are responsible for the ligand-binding selectivity of the ETA receptor.

Comparison of the cardiovascular and neural activity of endothelin-1, -2, -3 and respective proendothelins: effects of phosphoramidon and thiorphan

1. In the anaesthetized, ganglion-blocked rat, intravenous boluses of endothelin-1, endothelin-2 and endothelin-3 induced a transient hypotensive effect followed by a potent long lasting pressor response (ED50 mmHg: 0.72 +/- 0.05, 1.8 +/- 0.2 and 2.7 +/- 0.3 nmol kg-1, respectively). The maximal effect for the three peptides was of a similar order of magnitude (delta MAP: 84 to 89 mmHg). Neither of these effects was influenced by phosphoramidon or thiorphan (10 mg kg-1, i.v.). 2. Intravenously administered big-endothelin-1 and -2 induced a transient (1-2 min) hypotension followed by a potent long lasting (> 25 min) vasopressor effect (ED50 mmHg: 1.8 +/- 0.2 and 6.7 +/- 0.4 nmol kg-1, respectively), with a similar maximal activity (delta MAP: 85 +/- 4 and 81 +/- 2.4 mmHg, respectively). The onset of the big-endothelin-1 vasopressor effect was more rapid (5-6 min) than that of big-endothelin-2 (10-13 min). Big-endothelin-3 was found to induce only a potent, long lasting (> 35 min) hypertension, with a maximal effect of 75 +/- 4.6 mmHg at 10 nmol kg-1 and an ED50 mmHg of 6.5 +/- 0.4 nmol kg-1. The onset of this effect was much slower (20-25 min) than that of the other proendothelins. Pressor responses induced by big-endothelin-1, -2 and -3 (3, 15 and 10 nmol kg-1, respectively) were markedly reduced (60, 80 and 100%) in the presence of phosphoramidon (10 mg kg-1, i.v.). Thiorphan (10 mg kg-1, i.v.) did not inhibit the effects of big-endothelin-1, -2 and -3. 3. In the electrically stimulated rat vas deferens, endothelin-I and -2 were found to be equipotent enhancers of the twitch response (EC100%: 4.0 +/- 0.4 nm and 7.9 +/- 4.8 nm, respectively), both about 3-4 fold as active as endothelin-3 (EC100%: 19 +/- 2.5 nM). Endothelin-1 and -3 showed a comparable maximalstimulatory effect (Emax: 296 +/- 30 and 262 +/- 24%) while endothelin-2 was less active (Emax: 194 +/- 30%).4. Big-endothelin-l and -2 were potent enhancers of the twitch response too (EC 100,%: 10.0 +/- 2.6 nM and 21.6 +/- 3.2 nM, respectively), with a comparable maximal stimulatory effect (Emax: 254 +/- 22 and 264 +/-24%). Big-endothelin-3 was found to be less potent (EC,100%: 275 +/- 21 nM), but retained a marked potentiating effect (Emax: 200 +/- 38%). Phosphoramidon, but not thiorphan, concentration-dependently(10 and 100 microM) reduced big-endothelin-1 (58 and 86% respectively) and big-endothelin-2 (21 and 56%)-mediated responses. Conversely, the big-endothelin-3 effect was reduced by phosphoramidon only at 100 microM (-70%), while thiorphan acts concentration-dependently (31 and 71% at 10 and 100 microM respectively); thus, in the rat vas deferens, big-endothelin-I and -2 were as potent as their corresponding endothelins, while big-endothelin-3 was about 20 times less potent than endothelin-3.5. The increasing effect of endothelin-2 (194 +/- 30% over baseline) was significantly enhanced by either 10 microM phosphoramidon (277 +/- 42%) or thiorphan (318 +/- 15%). The endothelin-I and endothelin-3-mediated twitch enhancement was not affected by the two protease inhibitors (10 microM).6. These results suggest that in vivo big-endothelin-1, -2 and -3, are processed through a similar phosphoramid on-sensitive enzymatic pathway although with different apparent affinity. This enzymatic process is probably attributable to a neutral endoprotease, distinct from neutral-endopeptidase 24.11(NEP). On the other hand, a NEP-like enzymatic activity may be involved, in the rat vas deferens, in the activation of big-endothelin-3 to endothelin-3 and in the metabolism of endothelin-2, but not of endothelin-I or endothelin-3.

Mechanisms of ET-1 potentiation of angiotensin II stimulation of aldosterone production

Endothelin-1 (ET-1) exerts the following two types of aldosterone-stimulating actions on glomerulosa cells: ET-1-mediated direct stimulation of aldosterone secretion (per se effect) and potentiation of the aldosterone secretion to angiotensin II (ANG II; potentiation effect). The role of Ca2+ and protein kinase C (PKC) systems in these two effects was investigated. Incubations of calf cultured adrenal zona glomerulosa cells in low-Ca2+ media or in the presence of the Ca2+ channel antagonist verapamil reduced the aldosterone secretion to ET-1. When cells were preincubated with ET-1 in a low-Ca2+ media or in the presence of the Ca2+ channel antagonist verapamil, washed, and incubated in media with normal Ca2+, ANG II showed potentiation of ANG II-stimulated aldosterone secretion. The PKC inhibitors H-7 and staurosporine did not decrease ET-1-stimulated aldosterone secretion, but they inhibited the potentiation effect of ET-1 on ANG II-mediated aldosterone secretion. Adrenocorticotropic hormone desensitization or prolonged phorbol ester stimulation of PKC resulting in desensitization also resulted in the abolition of the ET-1-mediated ANG II potentiation of aldosterone secretion. The PKC inhibitors did not affect ANG II-stimulated aldosterone secretion. We conclude that ET-1 exerts a direct stimulation of aldosterone secretion through a mechanism dependent on Ca2+ and potentiates ANG II-mediated aldosterone stimulation through a mechanism involving PKC.

In vivo stimulation of aldosterone biosynthesis by endothelin: loci of action and effects of doses and infusion rate

Infusion of endothelin-1 (ET-1) into rats increased adrenal mitochondrial synthesis of aldosterone from deoxycorticosterone and the adrenal cytosolic content of aldosterone. The dose-response relationships for these last two effects of ET-1 were found to be biphasic with a maximum (corresponding to 80 to 200% increase) at 50 to 80 ng ET-1/kg/min, and were also dependent on the infusion rate. Plasma aldosterone levels were also increased in a similar ratio. Previous infusion of the converting enzyme inhibitor enalapril did not affect the ET-1-induced increase in steroidogenesis. Finally, pregnenolene production was also increased in incubations of mitochondria from treated rats. These results indicate that ET-1 augments aldosteronogenesis by increasing the early as well as the late pathway. These effects were independent of the formation of angiotensin II. Isolated glomerulosa cells responded to ET-1 increasing aldosterone production in a dose-related fashion. These results confirm a direct effect of ET-1 on the adrenal gland in vivo.

Activation of endothelin receptors by sarafotoxin regulates Ca2+ homeostasis in cerebellar astrocytes

We carried out experiments designed to investigate the effects of sarafotoxin-6B (SFTx) on [Ca2+]i in cerebellar astrocytes using the Ca2+ indicator fura-2. Both endothelin-1 and sarafotoxin-6B increased [Ca2+]i in individual cerebellar astrocytes in cell culture. The shape of the response was variable but usually consisted of an initial peak of [Ca2+]i followed by an extended plateau increase in [Ca2+]i. In Ca(2+)-free medium only the initial peak was observed. If Ca2+ was subsequently readmitted to the external medium a plateau was now formed. When external Ca2+ was removed during a plateau, [Ca2+]i rapidly declined; replacing the external Ca2+ reversed this decline. The plateau was also reversibly reduced by addition of Ni2+ (5 mM) to the external medium. Addition of 50 mM K+ produced a small increase in [Ca2+]i in most cells. This response was blocked by nimodipine. However, nimodipine only slightly blocked the plateau increase in [Ca2+]i that was formed following activation of endothelin receptors. Furthermore, perfusion of cells with 50 mM K+ during the plateau portion of a response to SFTx reduced [Ca2+]i. In some cells addition of a phorbol ester produced a sustained increase in [Ca2+]i that was blocked by nimodipine. In conclusion, activation of endothelin receptors by SFTx in cerebellar astrocytes produces both Ca2+ mobilization and Ca2+ influx. The pathway for Ca2+ influx is predominantly a non-voltage-dependent one, although some entry through a dihydropyridine-sensitive pathway also appears to occur. Furthermore, activation of protein kinase C in cerebellar astrocytes activates voltage-sensitive Ca2+ channels.

Characteristics of binding of endothelin-1 and endothelin-3 to rat hearts. Developmental changes in mechanical responses and receptor subtypes

Endothelin-1 (ET-1) and endothelin-3 (ET-3) produced positive inotropic effects on electrically stimulated left atria and increased the frequency of spontaneously beating right atria of adult rats. The potency of the inotropic effect of ET-1 was greater than that of ET-3, but the potencies of the chronotropic effects of ET-1 and ET-3 were not significantly different. In the neonatal atria, ET-1 and ET-3 also induced positive inotropic and chronotropic responses. ET-1 and ET-3 showed weak or no cardiotonic effects on the adult ventricles, whereas they caused marked positive inotropy in the neonatal ventricles. The characteristics of binding sites for ET-1 and ET-3 were very similar between the atria and the ventricles of the rat neonate. Saturation and competition binding experiments have shown that neonatal cardiac membranes from both atria and ventricle have two distinct binding sites for endothelin, that is, a low-affinity and a high-affinity site. ET-1 was found to bind to the low-affinity sites with a significantly lower Kd than ET-3, whereas the estimated Kd values for ET-1 and ET-3 at the high affinity sites were similar. In contrast, the binding sites in adult atria were different from those of the ventricles: only a single binding site for both ET-1 and ET-3 was detected. Adult atrial membranes, on the other hand, had two distinct binding sites similar to those of neonatal membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro autoradiographic endothelin-1 binding sites and sarafotoxin S6B binding sites in rat tissues

1. The distribution of binding sites for [125I]-labelled endothelin-1 ([125I]-ET-1) and [125I]-labelled sarafotoxin S6B ([125I]-SRT) was visualized in rat tissues using in vitro autoradiography. 2. A high density of endothelin-1 (ET-1) binding was found in the heart. In the kidney, ET-1 binding occurred in association with glomeruli, proximal tubules, the inner stripe and inner medulla. In the adrenal, a high density of ET-1 binding occurred in the medulla as well as the zona glomerulosa. 3. The binding affinity constant (KA) for ET-1 binding in these sites ranged from 1 to 10 x 10(9)/mol per litre. 4. Although sarafotoxin S6B (SRT) was 10-100-fold weaker than ET-1 in displacing [125I]-ET-1 from these sites, 1 mumol/L unlabelled SRT completely abolished [125I]-ET-1 binding in all sites. Other venom peptides did not affect [125I]-ET-1 binding. 5. The pattern of [125I]-SRT receptor binding in rat tissues by in vitro autoradiography was identical to that for ET-1 receptor binding, and both unlabelled SRT and unlabelled ET-1 fully competed with [125I]-SRT for binding. 6. These results provide evidence that SRT binds to the ET receptor in a range of rat tissues. The results suggest that there may be subclasses of ET receptors which can be distinguished by the relative potencies of ET-1 and SRT at various tissues.

Inhibition by atrial and brain natriuretic peptides of endothelin-1 secretion after stimulation with angiotensin II and thrombin of cultured human endothelial cells

We examined the inhibition by atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) of endothelin-1 secretion stimulated by angiotensin II (ANGII) and thrombin using cultured human umbilical-vein endothelial cells. ANGII and thrombin dose-dependently stimulated immunoreactive (ir) endothelin-1 secretion. Human ANP(1-28) and human BNP-32 both inhibited such secretion in a dose-dependent way. Inhibition of this secretion by ANP and BNP was paralleled by an increase in the level of cyclic guanosine 5'-monophosphate (GMP). The addition of a cyclic GMP analogue, 8-bromo cyclic GMP, reduced this stimulated secretion. Rat ANP(5-25) was weaker than human ANP(1-28) at inhibiting ir-endothelin-1 secretion and increasing cyclic GMP in the cells. ir-Endothelin-1 in the medium consisted of two components separated by high pressure liquid chromatography; the major one corresponded to endothelin-1(1-21) and the minor one corresponded to big endothelin-1(1-38). Treatment with ANP and BNP did not affect this profile. These findings suggest that human ANP and BNP inhibit endothelin-1 secretion stimulated by ANGII and thrombin in these cells through a cyclic GMP-dependent process. Taken together with endothelin stimulation of ANP and BNP secretion from the heart, our results suggest the existence of a cardiac-endothelium feedback.

Isolation of anti-endothelin receptor monoclonal antibodies for use in receptor characterization

Monoclonal antibodies reactive with endothelin (ET) receptors have been prepared by immunization of mice with rat lung membranes. Of four clones isolated, three clones preferentially recognized 32,000-dalton ET receptor and the other has a higher affinity for the 45,000-dalton receptor. The binding of 125I-ET-1 to detergent-solubilized ET receptors which were adsorbed to the antibodies was displaced by increasing concentrations of unlabeled ET isopeptides. These results demonstrate that the four clones specific for the receptor have the potential to be a useful tool in the characterization of ET receptors.