Endothelin systems in the brain: involvement in pathophysiological responses of damaged nerve tissues

In addition to their potent vasoconstriction effects, endothelins (ETs) show multiple actions in various tissues including the brain. The brain contains high levels of ETs, and their production is stimulated in many brain disorders. Accumulating evidence indicates that activation of brain ET receptors is involved in several pathophysiological responses in damaged brains. In this article, the roles of brain ET systems in relation to brain disorders are reviewed. In the acute phase of stroke, prolonged vasospasm of cerebral arteries and brain edema occur, both of which aggravate brain damage. Studies using ET antagonists show that activation of ETA receptors in the brain vascular smooth muscle induces vasospasm after stroke. Brain edema is induced by increased activity of vascular permeability factors, such as vascular endothelial growth factor and matrix metalloproteinases. Activation of ETB receptors stimulates astrocytic production of these permeability factors. Increases in reactive astrocytes are observed in neurodegenerative diseases and in the chronic phase of stroke, where they facilitate the repair of damaged nerve tissues by releasing neurotrophic factors. ETs promote the induction of reactive astrocytes through ETB receptors. ETs also stimulate the production of astrocytic neurotrophic factors. Recent studies have shown high expression of ETB receptors in neural progenitors. Activation of ETB receptors in neural progenitors promotes their proliferation and migration, suggesting roles for ETB receptors in neurogenesis. Much effort has been invested in the pursuit of novel drugs to induce protection or repair of damaged nerve tissues. From these studies, the pharmacological significance of brain ET systems as a possible target of neuroprotective drugs is anticipated.

Endothelin Receptors in Gastrointestinal Smooth Muscle

Endothelins (ETs) are a family of peptides with 21-amino-acid residues. ET-1 was identified as a potent vasoconstrictor produced by vascular endothelial cells. Three distinct isoforms of ET, i.e. ET-1, ET-2 and ET-3, have been found to exist in a variety of tissues. ET was later found to cause contraction as well as relaxation of smooth muscle in many physiologic systems. In the gastrointestinal tract, ET causes contraction and/or relaxation of the esophagus, stomach, ileum and colon. In the hepatobiliary system, ET causes contraction of the portal vein, hepatic stellate cells, gallbladder and common bile duct. In mammalian species, two classes of ET receptors, ET(A) and ET(B), have been cloned. ET(A) receptors have higher affinities for ET-1 and ET-2 than ET-3, while ET(B) receptors have the same affinities for ET-1, ET-2 and ET-3. In the gastrointestinal system, ET causes smooth muscle contraction through interaction with ET(A) receptors, ET(B) receptors or both ET(A) and ET(B) receptors, depending on the tissues and species. In addition to contraction, ET causes smooth muscle relaxation through interaction with ET(A) receptors or ET(B) receptors. At the present time, there are no studies showing that ET causes smooth muscle relaxation through interaction with both ET(A) and ET(B) subtypes. ET induces contraction in most of the non-sphincter muscle except the fundus of the stomach. On the other hand, ET causes relaxation and contraction in the lower esophageal and internal anal sphincters. ET may play an important role in the control of human gastrointestinal motility and portal vein pressure.

Endothelin receptors in the detached retina of the pig

Endothelin-1 (ET-1) is a potent vasoconstrictor that causes hypoperfusion of the neurosensory retina. We investigated immunohistochemically the expression of the receptors for ET-1, ET(A) and ET(B), in control and locally detached retinas of the pig. Immunoreactivity for ET(A) was expressed in the innermost retinal layers and in the outer plexiform layer in control retinas, and was additionally strongly expressed by retinal blood vessels at 7 days after detachment of the sensory retina from the pigment epithelium. Immunoreactivity for ET(B) was expressed by the innermost retinal layers, by ganglion cell somata, and by Müller glial cells in the control tissue, and was not altered in its expression after detachment. The vascular expression of ET(A) may suggest a hypoperfusion of the retina after detachment.

Receptor subtypes mediating the inotropic effects and Ca(2+) signaling induced by endothelin-1 through crosstalk with norepinephrine in canine ventricular myocardium

In canine ventricular myocardium, endothelin-1 (ET-1) alone induced only a weak transient negative inotropic effect (NIE). However, ET-1 induced a marked sustained positive inotropic effect (PIE) subsequent to a transient NIE in the presence of norepinephrine (NE) at low concentrations (0.1 - 1 nM) and elicited a pronounced sustained NIE in the presence of NE at high concentrations (around 100 nM). Thus, the extent of beta-adrenoceptor stimulation induced by NE played a crucial role in determining the characteristics of the inotropic effects of ET-1. The characteristics of ET receptor subtypes involved in contractile regulation and Ca(2+) signaling induced by ET-1 were determined. The ET-1-induced transient NIE and decrease in Ca(2+) transients were abolished by the selective ET(A)-receptor antagonist FR319317, but not by the selective ET(B)-receptor antagonist BQ-788. The sustained PIE and the increase in Ca(2+) transients induced by ET-1 were abolished by FR319317, but not inhibited by BQ-788. In contrast, the sustained NIE of ET-1 was abolished by the non-selective ET antagonist TAK-044, markedly attenuated by FR319317, and partially inhibited by BQ-788. ET-1 alone elicited a PIE in the presence of BQ-788, which indicates that the activation of ET(B)-receptors counteracts the development of the PIE of ET-1. The current findings indicate that both ET(A) and ET(B) receptors are involved in the regulation of Ca(2+) signaling and contractility in canine ventricular myocardium.

Intracerebroventricular administration of anti-endothelin-1 IgG selectively upregulates endothelin-A and kappa opioid receptors

Endothelin (ET) type A receptor antagonists enhance morphine-induced antinociception and restore morphine analgesia in morphine tolerant rats [Peptides 23 (2002) 1837; Peptides 24 (2003) 553]. These studies suggest that the central ET and opioid systems functionally interact. To explore this idea further, we determined the effect of i.c.v. administration of anti-ET-1 IgG (rabbit) on brain opioid receptor and ET receptor expression. Three days after implanting cannula into the lateral ventricle, male Sprague-Dawley rats were administered 10 microl (i.c.v.) of either control rabbit IgG (2.5 microg/microl) or anti-ET IgG (2.5 microg/microl) on day 1, day 3, and day 5. On day 6, animals were killed and the caudate and hippocampus collected. Anti-ET IgG had no significant effect on expression, measured by Western blots, of mu, delta or ET-B receptors, but increased kappa opioid (59%) and ET-A (33%) receptor protein expression in the caudate. [35S]-GTP-gamma-S binding assays demonstrated that anti-ET IgG decreased [D-Ala2-MePhe4, Gly-ol5]enkephalin efficacy, but not potency in the caudate. Control experiments showed that there was no detectable rabbit IgG in caudate and hippocampal samples. These results suggest that ET in the CSF negatively regulates kappa opioid and ET-A receptors in certain brain regions. These findings support the hypothesis that CSF neuropeptides have regulatory effects and further demonstrate a link between ET and the opioid receptor system.

ET – phone the pain clinic

Recent findings by Khodorova et al. demonstrate that the vasoconstrictor endothelin-1 plays an important role in certain nociceptive behaviors in an animal model of pain, through activation of sensory neurons. Endothelin-1 might also have the unexpected capacity to release an opioid from surrounding keratinocytes and thereby inhibit the pain response. Such results suggest that, in the periphery, there are important interactions between sensory nerve terminals and surrounding cells, and that glia and keratinocytes could modulate the perception of environmental stimuli to a greater extent than previously considered.

Differential expression of endothelin receptors in regenerating spinal motor neurons in mice

On day 4 after sciatic nerve crush injury, expression and localization of endothelin receptors ET(A) and ET(B) in the lumbar spinal cord were examined. Immunohistochemical staining with antibodies to ET(A) and ET(B) receptors showed cytoplasmic distribution of ET(A) receptors in motor neurons, whereas ET(B) receptors were localized in the perinuclear region. On the injured side of the lumbar spinal cord, when compared to contralateral, results demonstrated an up-regulation of ET(B) and a down-regulation of ET(A) receptors expression at the level of both mRNA and protein. These results suggest that ET(B) receptors may play a role in the regeneration of axotomized motor neurons.

Endothelin-1 receptor subtypes expression and binding in a perfused rat model of myocardial infarction

Endothelin-1 (ET-1) pathophysiologic actions are mediated via binding with two receptor subtypes, ET(A) and ET(B). Release of ET-1 from endocardial endothelial cells and cardiac myocytes can modulate heart tissue necrosis and alterations. This study investigates the remodeling processes in Sprague-Dawley rats of myocardial infarction (MI) induced by ligating the left anterior descending coronary artery. Histological studies were done on cell type distribution using cell specific markers and Western blot analysis to localize ET-1 receptor subtypes and assess their expression post-MI. In addition, the binding kinetics of ET-1 with its receptors in heart perfusion, inlet via the aortic lumen and effluent outlet via the right atrium, between two animal model-subgroups were done: (1) sham-operated, and sham-operated-CHAPS (3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulfonate)-treated; and (2) MI-operated, and MI-operated-CHAPS-treated. Effluent ET-1 concentration was plotted vs. time using a physical model for 1:1 ligand-receptor binding at coronary endothelium and myocytes. First order impulse function was used to calculate the affinity constants. In MI hearts, fluorescence activity increased for ET(A) vs. ET(B) across areas of the muscle compared to normal hearts. Western blotting showed upregulation of ET(A) and ET(B) receptors in MI compared with normal hearts. Results of ET-1 binding affinity post-MI indicated drastic reduction in spite the upregulation of ET(B) on coronary endothelium. Furthermore, substantial affinity increase was observed between ET-1 binding with ET(A) at the myocyte site. These findings stipulate that during 1 month post-MI some biochemical and hormonal effects could alter ET-1 receptor subtype(s) regulation and pharmacodynamics thus predisposing to cardiac hypertrophy and mitogenesis.

International Union of Pharmacology. XXIX. Update on endothelin receptor nomenclature

In mammals, the endothelin (ET) family comprises three endogenous isoforms, ET-1, ET-2, and ET-3. ET-1 is the principal isoform in the human cardiovascular system and remains the most potent and long-lasting constrictor of human vessels discovered. In humans, endothelins mediate their actions via only two receptor types that have been cloned and classified as the ET(A) and ET(B) receptors in the first NC-IUPHAR (International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification) report on nomenclature in 1994. This report was compiled before the discovery of the majority of endothelin receptor antagonists (particularly nonpeptides) currently used in the characterization of receptors and now updated in the present review. Endothelin receptors continue to be classified according to their rank order of potency for the three endogenous isoforms of endothelin. A selective ET(A) receptor agonist has not been discovered, but highly selective antagonists include peptides (BQ123, cyclo-[D-Asp-L-Pro-D-Val-L-Leu-D-Trp-]; FR139317, N- [(hexahydro-1-azepinyl)carbonyl]L-Leu(1-Me)D-Trp-3 (2-pyridyl)-D-Ala) and the generally more potent nonpeptides, such as PD156707, SB234551, L754142, A127722, and TBC11251. Sarafotoxin S6c, BQ3020 ([Ala(11,15)]Ac-ET-1((6-21))), and IRL1620 [Suc-(Glu(9), Ala(11,15))-ET-1((8-21))] are widely used synthetic ET(B) receptor agonists. A limited number of peptide (BQ788) and nonpeptide (A192621) ET(B) antagonists have also been developed. They are generally less potent than ET(A) antagonists and display lower selectivity (usually only 1 to 2 orders of magnitude) for the ET(B) receptor. Radioligands highly selective for either ET(A) ((125)I-PD151242, (125)I-PD164333, and (3)H-BQ123) or ET(B) receptors ((125)I-BQ3020 and (125)I-IRL1620) have further consolidated classification into only these two types, with no strong molecular or pharmacological evidence to support the existence of further receptors in mammals.

Evidence for two endothelin Et(A) receptor subtypes in rabbit arteriolar smooth muscle

1. Effects of endothelin-1 (Et-1) were studied on membrane currents in choroidal arteriolar smooth muscle by using perforated patch-clamp recordings. 2. Et-1 (10 nM) activated oscillatory Ca(2+)-activated Cl(-)-currents (I(Cl(Ca))) which could not be reversed by washing out. 3. Currents through L-type Ca(2+) channels were resolved in a divalent free medium (I(Ca(L)Na)). Et-1 reduced I(Ca(L)Na) by 75 +/- 7% within 30 s and this effect faded over 5 min, when the depression remained constant. On washing out Et-1, I(Ca(L)Na) almost completely recovered within 10 s. 4. BQ123 (1 microM), a peptide Et(A) receptor blocker, prevented the activation of I(Cl(Ca)), but failed to inhibit I(Cl(Ca)) transients once they had been initiated. In contrast, BQ123 not only prevented but also reversed the inhibition of I(Ca(L)Na) by Et-1. BQ788 (1 microM), an Et(B) receptor antagonist, did not prevent the activation of I(Cl(Ca)) or the inhibition of I(Ca(L)Na) by Et-1. 5. ABT-627 (10 nM), a non-peptide Et(A) receptor antagonist also blocked the activation of I(Cl(Ca)). However, on I(Ca(L)Na), ABT-627 (10 nM) mimicked the action of Et-1 an effect blocked by BQ123 suggesting that ABT-627 acted as an agonist. 6. The data are consistent with choroidal arteriolar smooth muscle cells having two types of Et(A) receptor, one where BQ123 is an antagonist and ABT-627 an agonist, where ligands dissociate freely and this receptor is coupled to inhibition of L-type Ca(2+) channels. In the other, BQ123 and ABT-627 are both antagonists and with Et-1 the receptor converts to a high affinity state producing the classical irreversible activation I(Cl(Ca)).

Enhanced expression of endothelin receptor subtypes in cirrhotic rat liver

BACKGROUND/AIMS

A number of vasoactive substances have been implicated as potential mediators of intrahepatic portal hypertension. Endothelin (ET)-1 has been suggested to be involved in the regulation of hepatic microcirculation and development of portal hypertension. The aim of this study was to clarify the localization of two subtypes of ET receptors, ET A (ETAR) and B receptors (ETBR), in normal rat liver, and how the receptor expressions are altered in CCl4-induced cirrhotic rat liver.

METHODS

Liver specimens were examined immunohistochemically after reacting with anti-ETAR and anti-ETBR rabbit polyclonal antibodies. Immunogold staining was also performed using the same antibodies, and examined under light and electron microscopy.

RESULTS

In normal rat liver, immunohistochemistry revealed expression of ETAR and ETBR on the hepatic sinusoidal lining cells. By immunogold electron microscopy, electron-dense gold particles indicating the presence of ETARs were localized mainly on hepatic stellate cells (HSCs) and to a lesser extent on sinusoidal endothelial cells (SECs), while ETBRs were expressed equally intensely on HSCs and SECs. In cirrhotic animals, both ETAR and ETBR increased significantly on HSCs, while there were no significant increases in either receptor on SECs.

CONCLUSIONS

In the normal state, HSCs possess both ETARs and ETBRs, while SECs mainly possess ETBRs. In cirrhosis, endothelins may exert more intense effects on HSCs via the enhanced ETARs and ETBRs, causing an increase in hepatic sinusoidal microvascular tone.

New developments in heart failure: role of endothelin and the use of endothelin receptor antagonists

Despite conventional therapy, there is still much room for improvement in the prognosis of patients with chronic systolic heart failure. Evidence supports a role for endothelin-1 (ET-1), a potent vasoconstrictor, in the pathophysiology of heart failure. Given its potentially deleterious effects, the optimal treatment of heart failure may need to include efforts directed toward antagonizing this hormone. In support of this notion, the use of ET receptor antagonists produces a number of beneficial effects in heart failure, including both improvements in hemodynamics and reductions in the levels of other vasoconstricting neurohormones. There are at least 2 receptors for ET-1 (the ET-A and ET-B receptor), and the effects of ET-1 binding differ depending on the receptor involved. It is still unclear whether blockade of the ET-A receptor alone or the combined blockade of both the ET-A and ET-B receptors will be most efficacious as a therapeutic strategy. Long-term benefits have been achieved with the use of a mixed ET-A/B receptor antagonist, when added to standard triple-drug therapy, in patients with severe heart failure. We await the results of ongoing trials to determine if these agents will fulfill the promise of adding substantial incremental benefit to the treatment of the disease.

Age-related changes in contractile responses of rabbit lower urinary tract to endothelin

PURPOSE

As there are significant amounts of endothelin (ET) receptors in the mammalian urinary tract, we investigated the pharmacological properties and localization of ET receptors in the rabbit lower urinary tract as a function of age.

MATERIALS AND METHODS

The characteristics of ET receptors in bladder dome, trigone and urethra of 6 weeks and 6 months old male rabbits were determined using muscle bath and autoradiographic techniques.

RESULTS

ET-1 produces significant contractile responses in smooth muscle strips from bladder dome, trigone, and urethra in both 6 weeks and 6 months old rabbits. Although there was no significant difference in the maximum contractile response of urethral muscle strips to ET-1 between 6 weeks and 6 months old rabbits, the maximum responses to ET-1 were higher in both bladder dome and trigone of 6 weeks than 6 months old rabbits. A selective ETA receptor antagonist, BQ 610, shifted the concentration response curve to ET-1 to the right without decreasing maximal contractile responses in all regions from both age groups, whereas a selective ETB receptor antagonist, IRL 1038, had no significant effect on the contractile response in these tissues. Autoradiographic studies indicate that both ET receptor subtypes are expressed in bladder dome, trigone, and urethra with the ETA subtype being located only in the smooth muscle layers and the ETB subtype being located in both the urothelial and smooth muscle layers.

CONCLUSION

Our data indicate the presence of region- and age-dependent differences in the contractile properties of ET receptors in the male rabbit lower urinary tract. Although both ETA and ETB receptor subtypes are present in the smooth muscle layers, the ETA receptor is the sub-type that is primarily involved in the mediation of contractions.

Endothelin receptor subtype antagonist activity of S-0139 in various isolated rabbit and canine arteries

Vascular responses to endothelin peptides have been proposed to be mainly mediated via subtypes of the endothelin receptor, endothelin ET(A1), endothelin ET(B1), and endothelin ET(B2). The antagonist activity of 27-O-3-[2-(3-carboxy-acryloylamino)-5-hydroxyphenyl]acryloyloxy myricerone, sodium salt (S-0139) at these endothelin receptor subtypes was evaluated using isolated rabbit femoral, pulmonary, and mesenteric arteries. S-0139 competitively antagonized the endothelin-1-induced contraction mediated by the endothelin ET(A1) receptor in endothelium-denuded rabbit femoral arteries with a pA(2) value of 8.6+/-0.1. Endothelin ET(B2) receptor-mediated contraction induced by sarafotoxin S6c in endothelium-denuded rabbit pulmonary arteries was also inhibited by S-0139 with a pA(2) value of 5.6+/-0. 1. The pA(2) value of S-0139 for the endothelin ET(B1) receptor, evaluated from the endothelin-3-induced relaxant response in endothelium-intact rabbit mesenteric arteries, was 6.2+/-0.2. In isolated canine basilar, coronary, mesenteric and renal arteries, endothelin-1 caused concentration-dependent contractions with EC(50) values of 0.49+/-0.07, 0.61+/-0.25, 0.92+/-0.21 and 1.18+/-0.24 nM, respectively. S-0139 antagonized the endothelin-1-induced contraction in these arteries with pA(2) values of 8.0+/-0.1, 7. 6+/-0.2, 7.6+/-0.2 and 7.6+/-0.1, respectively. These results suggest that S-0139 is a potent and selective endothelin ET(A1) receptor antagonist, and that the contractions induced by endothelin-1 in canine basilar, coronary, mesenteric and renal arteries are mediated mainly via the endothelin ET(A1) receptor subtype.

ET-receptor subtypes: roles in regional renal vascular actions of exogenous and endogenous endothelins in anesthetized rabbits

The roles of endothelin (ET)-receptor subtypes, in the regional renal vascular effects of exogenous and endogenous ETs, were examined in pentobarbitone-anesthetized rabbits. The effects of renal arterial infusion of ET-1 (0.05-12.8 ng/kg/min) and the ET(B)-agonist [Ala1,3,11,15]-ET-1 (12.5-800 ng/kg/min) were compared. We then tested the effects of the ET(A)-antagonist BQ610 and the ET(B)-antagonist BQ788 (both 200 microg/kg plus 100 microg/kg/h, i.v.) on basal hemodynamics and on responses to renal arterial ET-1. Both ET-1 and [Ala1,3,11,15]-ET-1 dose-dependently reduced total renal blood flow (RBF) and cortical blood flow (CBF), but not medullary blood flow (MBF). ET-1 was 34-fold more potent than [Ala1,3,11,15-ET-1. BQ610 reduced mean arterial pressure (MAP; 14%), and increased RBF (21%) and CBF (12%), but not MBF. BQ788 increased MAP (13%), and reduced RBF (29%) and CBF (15%) but not MBF. Coadministration of both agents increased RBF (18%) and CBF (9%), without significantly affecting MAP. Neither antagonist (alone or combined) significantly affected responses to renal arterial ET-1. We conclude that the predominant renal vascular effects of exogenous and endogenous ETs are cortical vasoconstriction, but not at vascular sites controlling MBF. ET(A)-receptors contribute to the renal vasoconstrictor effects of endogenous ETs. ET(B2)-like receptors appear to contribute to the vasoconstrictor effects of [Ala1,3,11,15]-ET-1.

Characterization of a novel porcine endothelin(B) receptor splice variant

Screening of porcine cerebellum cDNA library with porcine endothelin(B) (ET(B)) receptor cDNA revealed a novel ET(B) receptor cDNA that is distinctly different from the wild-type ET(B) receptor in length and the amino acid sequence at the C-terminal end. This sequence appears to represent alternate splicing of the carboxy terminal end of ET(B) receptor, resulting in a polypeptide of 429 amino acids in length, which is 14 amino acids shorter than the wild-type porcine ET(B) receptor. Characterization of the wild-type and alternately spliced ET(B) receptors expressed in COS cells revealed that both receptors displayed very similar binding [apparent dissociation constant (K(d)) and maximum binding (B(max)) for (125)I-ET-1 were 71 pM and 1.6 pmol/mg protein for wild-type and 81 pM and 1.2 pmol/mg protein for splice variant ET(B) receptors] as well as functional properties. These data suggest that the differences in the amino acids at the C-terminal end had no effect on binding or functional coupling of these alternately spliced ET(B) receptors.

Characterization of endothelin receptors in the anesthetized ferret: a novel model for investigating the functional ET(B) receptor subtypes

The pharmacology of endothelin (ET)-1, big ET-1, ET-3, and S6c were characterized in the anesthetized ferret to assess whether this species would provide a new and suitable nonrodent model to be used in characterization of endothelin antagonists. Unlike other species such as dog, rabbit, and rat, the ferret exhibited a dose-dependent pressor response to both ET-1 and big ET-1 with no preceding vasodilatory response. The median effective concentration (ED50) values were 0.047+/-0.009 and 0.469+/-0.003 nmol/kg for ET-1 and big ET-1, respectively. ET-3 and S6c, however, were found to elicit a transient vasodilatory response preceding the pressor response, with ED50 values of 0.23+/-0.09 and 0.18+/-0.03 nmol/kg, respectively. The rank potency of the agonists for the pressor response was found to be ET-1 > S6c > big ET-1 > ET-3. The ET(A)-specific antagonist BQ-123 was shown to block only partially the ET-1 and big ET-1 pressor response with median antagonistic dose (AD50) of 0.24+/-0.11 and 0.015+/-0.005 mg/kg, i.v., respectively, and blockade of the ET(A) receptor did not uncover an ET(B)-induced vasodilation. The dual ET(A/B) antagonist L-754,142 completely antagonized the ET-1 and big ET-1 pressor responses with AD50 values of 0.195+/-0.063 and 0.019+/-0.006 mg/kg, respectively. The ET(B) antagonist BQ-788 blocked the depressor response of S6c entirely but was unable to antagonize the pressor response completely. BQ-123 was shown to antagonize the S6c pressor response partially, suggesting a possible interaction between the ET(A) and ET(B) receptors in the ferret. The unexpected absence of an ET-1-mediated depressor response but the presence of ET-3 and S6c vasodilation in this species supports the theory that there may be subtypes of the ET(B) receptor. These studies demonstrate that the anesthetized ferret provides a suitable model for assessing the physiological potencies of the endothelins and may provide a tool for further understanding of the diversity of the ET(B) receptor.

Endothelin receptor antagonists. Promising new agents in the management of cardiovascular disorders

Since its discovery in 1988, endothelin (ET) has been widely implicated in the pathophysiology of cardiovascular disease. ET antagonists have favourable effects in experimental models of these conditions and have proved useful in elucidating the role of the ET system. Orally acting ET antagonists appear very promising in clinical trials, particularly in patients with chronic heart failure and hypertension, but more information on the roles of the ET receptor subtypes in health and disease is required so that an informed choice can be made between the use of endothelin-A (ET-A) receptor-selective and nonselective receptor antagonists.

Pharmacological characterization of PABSA, an orally active and highly potent endothelin-receptor antagonist

The pharmacological characterization of a nonpeptide endothelin (ET)-receptor antagonist, PABSA [(R)-(--)-2-(benzo[1,3]dioxol-5-yl)-N-(4-isopropyl-phenylsulfon yl)-2-(6-methyl-2-propylpyridin-3-yloxy)-acetamide hydrochloride] was studied. PABSA competitively inhibited the binding of [125I]-ET-1 to A7r5 cells expressing ET(A) receptors and of [125I]-ET-3 to COS cells expressing porcine ET(B) receptors with Ki values of 0.11 and 25 nM, respectively. PABSA inhibited ET(A) receptor-mediated and ET(B) receptor-mediated vasocontraction and ET(B) receptor-mediated vasorelaxation in isolated rabbit vessels with K(b) values of 0.46, 94, and 26 nM, respectively. The antagonist potency of PABSA for ET(A) receptor-mediated vasocontraction was 63- and 87-fold more potent than those of BQ-123 and bosentan, respectively, and was similar to those of TAK-044 and SB209670. Oral administration of PABSA (1-10 mg/kg) caused dose-dependent inhibition of the pressor response to exogenous ET- 1 (0.1 nmol/kg) in conscious normotensive rats. PABSA (10-100 mg/kg, p.o.) reduced blood pressure in deoxycorticosterone acetate (DOCA)-salt hypertensive rats, spontaneously hypertensive rats (SHRs), and stroke-prone spontaneously hypertensive rats (SHRSPs). The hypotensive effect of PABSA was sustained for > or =24 h in these rats. These results suggest that PABSA is a highly potent ET(A)-receptor antagonist with weak ET(B)-receptor antagonist activity. Because PABSA has a long duration of action in vivo, this antagonist should be useful in the therapy of ET-related disease.

Central effects of endothelin and its antagonists on sympathetic and cardiovascular regulation in SHR-SP

Intracerebroventricular injections of endothelin-1 (ET-1) are reported to cause dose-related increases in sympathetic nerve activity and blood pressure in anesthetized normotensive rats. These studies were performed to determine the following: which endothelin receptor, A or B, is involved in mediating sympathetic and cardiovascular effects of ET-1 injected centrally; whether central endothelin tonically participates in blood pressure regulation in normotensive rats; and whether the altered endothelin system in the central nervous system contributes to blood pressure elevation in hypertensive rats. ET-1, ET-A antagonist (BQ-123), or ET-B antagonist (RES-701-1) was injected into the lateral cerebral ventricle (i.c.v.) of urethane-anesthetized normotensive Wistar and Wistar-Kyoto (WKY) rats, spontaneously hypertensive rats (SHRs), and stroke-prone SHRs (SHR-SPs). In Wistar rats, i.c.v. injections of ET-1 (1, 5, 10 pmol) consistently increased sympathetic nerve activity, thereby elevating blood pressure in a dose-related manner. The pressor responses induced by i.c.v. ET-1 were abolished after intravenous pretreatment with phentolamine. Neither ET-A nor ET-B antagonist, when injected centrally, altered basal levels of sympathetic nerve activity, heart rate, or blood pressure in Wistar rats. However, sympathetic activation and pressor responses induced by i.c.v. injection of endothelin were completely abolished after i.c.v. pretreatment with ET-A antagonist but were unaffected after pretreatment with ET-B antagonist. Although i.c.v. injections of ET-1 increased sympathetic nerve activity and blood pressure in WKY rats, SHRs, and SHR-SPs, the magnitudes of these responses did not differ among these three groups. In contrast, i.c.v. injections of ET-A antagonist decreased sympathetic nerve activity, blood pressure, and heart rate only in SHR-SPs, but not in WKY rats and SHRs. In addition, the depressor effects of i.c.v. ET-A antagonist in SHR-SPs were ascertained while these rats were awake. In summary, i.c.v. injections of ET-1 increased sympathetic nerve activity and blood pressure via ET-A receptors but not via ET-B receptors. Central ET might tonically activate sympathetic nerve activity to thereby contribute to blood pressure elevation in SHR-SPs, but not in WKY rats and SHRs.

Endothelins in health and disease: an overview

1. There is an ever increasing volume of evidence implicating endothelin-1 and its isoforms in a range of disease processes. These include asthma, pulmonary and essential systemic hypertension, cardiac failure and uterine dysfunction. 2. However, it is also important to realize that the endothelins play an obligatory role in normal cellular proliferation, repair and tissue development. 3. The present brief review focuses on some of the physiological and pathophysiological mediator roles of the endothelins and provides a sketch of the receptor systems and some of the signal transduction pathways that are now known to operate following receptor activation. 4. Importantly, it is now clear that the endothelins, their receptors and synthesis and degradation pathways offer potentially important therapeutic targets.

PD 142893, SB 209670, and BQ 788 selectively antagonize vascular endothelial versus vascular smooth muscle ET(B)-receptor activity in the rat

The purpose of this study was to determine whether vascular endothelial and vascular smooth-muscle endothelin ET(B) receptors could be quantitatively differentiated by PD 142893 (PD), SB 209670 (SB), and BQ 788 (BQ) in the same species by using closely matched experimental conditions. The isolated perfused rat kidney (vascular smooth muscle) and isolated perfused rat mesentery (vascular endothelium) were challenged with increasing bolus doses of sarafotoxin S6c in the absence and presence of antagonist. PD, SB, and BQ produced parallel concentration-dependent rightward shifts in the S6c dose-response curve in the kidney. PD and SB also produced parallel concentration-dependent rightward shifts in the S6c dose-response curve in the mesentery. In contrast, BQ produced an insurmountable antagonism. Schild-derived pA2 values for PD and SB were significantly greater for inhibiting endothelial versus smooth-muscle ET(B) receptors. Furthermore, PD and SB differed in their relative potency between the two assays. Because BQ produced an insurmountable antagonism in the mesentery, it was not possible quantitatively to compare the antagonist activity in the two assays. These results indicate that PD, SB, and BQ selectively antagonize endothelial ET(B)-receptor activity over smooth-muscle ET(B)-receptor activity.

Characterization of the endothelin receptor subtype mediating epithelium-derived relaxant nitric oxide release from guinea-pig trachea

1. The endothelin (ET) receptor subtype that mediates niric oxide (NO)-dependent airway relaxation in tracheal tube preparations precontracted with carbachol and pretreated with indomethacin was investigated. The release of NO induced by ET from guinea-pig trachea using a recently developed porphyrinic microsensor was also measured. 2. ET-1 (1 pM-100 nM) contracted tracheal tube preparations pretreated with the NO-synthase inhibitor, L-NMMA, and relaxed, in an epithelium-dependent manner, preparations pretreated with the inactive enantiomer D-NMMA. The effect of L-NMMA was reversed by L-Arg, but not by D-Arg. 3. The selective ET(B) receptor agonists, IRL 1620 or sarafotoxin S6c, both (1 pM-100 nM) contracted tracheal tube preparations in a similar manner either after treatment with D-NMMA or with L-NMMA. In the presence of the ET(A) receptor antagonist, FR139317 (10 microM), ET-1 administration resulted in a contraction that was similar after either L-NMMA or D-NMMA. In the presence of the ET(B) receptor antagonist, BQ788 (1 microM), ET-1 relaxed and contracted tracheas pretreated with D-NMMA and L-NMMA, respectively. 4. Exposure of tracheal segments to ET-1 (1-1000 nM) caused a concentration-dependent increase in NO release that was reduced by L-NMMA. IRL1620 (1 microM) did not cause any significant NO release. FR139317 (10 microM), but not, BQ788 (1 microM), inhibited the NO release induced by ET-1. 5. These results demonstrate that in the isolated guinea-pig trachea activation of ET(B) receptors results in a contractile response, whereas activation of ET(A) receptors cause both a contraction, and an epithelium-dependent relaxation that is mediated by NO release.

Pharmacological characterization of endothelin-induced contraction in the guinea-pig oesophageal muscularis mucosae

1. In the oesophageal muscularis mucosae, we examined the effects of endothelin-1 (ET-1), endothelin-2 (ET-2), endothelin-3 (ET-3) and sarafotoxin S6c (SX6c) as agonists, and FR139317, BQ-123 and RES-701-1 as endothelin receptor antagonists. 2. All of the endothelins produced tonic contractions which were frequently superimposed on rhythmic motility in a concentration-dependent manner. The order of potency (-log EC50) was ET-1 (8.61)=SX6c (8.65)>ET-2 (8.40)>ET-3 (8.18). 3. FR139317 (1-3 microM) and BQ-123 (1 microM) caused parallel rightward shifts of the concentration-response curve to ET-1, but at higher concentrations caused no further shift. RES-701-1 (3 microM) caused a rightward shift of the concentration-response curve to ET-1, while RES-701-1 (10 microM) had no additional effect. RES-701-1 (0.1-1 microM) concentration-dependently caused a rightward shift of the concentration-response curve to SX6c. The contraction to ET-1 (10 nM) in preparations desensitized to the actions of SX6c was greatly inhibited by pretreatment with FR139317 (10 microM). 4. Modulation of the Ca2+ concentration in the Krebs solution caused the concentration-response curve to ET-1 or SX6c to shift to the right and downward as external Ca2+ concentrations decreased. Verapamil (30 microM) abolished rhythmic motility induced by ET-1 or SX6c. Ni2+ (0.1 mM) weakly inhibited ET-1- or SX6c-induced tonic contraction. SK&F 96365 (60 microM) completely inhibited ET-1-induced contractions. 5. We conclude that there are two types of ET-receptors, excitatory ET(A)- and ET(B)-receptors in the oesophageal muscularis mucosae. These receptors mediate tonic contractions predominantly by opening receptor-operated Ca2+ channels (ROCs) and partly by opening T-type Ca2+ channels, and mediate rhythmic motility by opening L-type Ca2+ channels.

Endothelin ET(B1) receptor-mediated relaxation of rabbit basilar artery

This study tests whether endothelin receptor agonist-induced relaxation of the cerebral vasculature is mediated via endothelin ET(B1) receptor activation. Sarafotoxin S6c, an endothelin ET(B) receptor agonist, relaxed rabbit basilar artery constricted with serotonin in situ. BQ788 (N-cis-2,6-dimethylpiperidinocarbonyl L-gamma-MeLeu-D-Trp (COOCH3)-Nle), and RES-701-1 (Gly-Asn-Trp-His-Gly-Thr-Ala-Pro-Asp-Trp-Phe-Phe-Asn-Tyr-Tyr-Trp), endothelin ET(B1/B2) and endothelin ET(B1) receptor antagonists, respectively, prevented sarafotoxin S6c-induced relaxation. RES-701-1 was selective for the ET(B1) receptor, as the endothelin-1 constriction elicited in the presence of BQ610 (homopiperidenyl-CO-Leu-D-Trp (CHO)-D-Trp-OH), an endothelin ET(A) receptor antagonist, was enhanced by RES-701-1, and relaxed by BQ788. These results represent the first demonstration of the presence of endothelin ET(B1) receptors in the cerebral vasculature.

The characteristics of endothelin receptor subtypes on muscle contraction and neuro-transmission in rat vas deferens

We observed endothelin (ET)-induced contractile responses on prostatic and epididymal segments, as well as the facilitation of an electrically stimulated tone on prostatic segments of isolated rat vas deferens. In both segments, the selective ET(B)-receptor agonists, IRL 1620 and sarafotoxin S6c, produced only a small contraction or no contraction at a concentration of 1 microM. The rank order of contraction potencies (pD2 value) was ET-1 = ET-2 > ET-3 >> sarafotoxin S6c = IRL 1620. The maximum responses of ET-induced contractions in the prostatic segments were larger than those in the epididymal segments. The contractile response to ET-3 was antagonized by pretreatment for 30 min with BQ-123 (10 nM), a selective ET(A) receptor antagonist, and BQ-788 (1 microM), a selective ET(B) receptor antagonist. The contractile responses to ET-1 were antagonized by pretreatment with BQ-123 (10 microM), but not with BQ-788 (1 microM). The ET-3-induced facilitation on the twitch response to electrical stimulation in the prostatic segment of the vas deferens was antagonized by BQ-123 (0.1 microM) and BQ-788 (1 microM). The ET-1-induced facilitation was antagonized by pretreatment with BQ-123 (3 microM), but not with BQ-788 (10 microM). These results suggest that in rat vas deferens the ET(A) receptors are divided into BQ-123-sensitive ET(A1) and BQ-123-insensitive ET(A2) subtypes, and the production of a contractile response of smooth muscle as well as the facilitation of neurotransmission are accomplished through mediation by ET(A1)- and ET(A2)-subtypes.

Functional role for glycosylated subtypes of rat endothelin receptors

Glycosylation of endothelin (ET) receptors was found to occur in rat cerebellar and atrial membranes. Specifically, we investigated whether the ETA and ETB receptor subtypes differed in their sensitivity to deglycosylation treatment and whether the two affinity states (nanomolar and picomolar) observed in each receptor subtype reflect differences in glycosylation states. Pretreatment of cerebellar or atrial membranes with endoglycosidase H (endo H) caused a marked decrease in the number of maximal binding sites that bind ligand with nanomolar affinity, whereas ligand affinity remained the same. The picomolar-affinity binding sites were not affected by endo H. The use of specific antagonists indicated that the receptor subtype most likely to be influenced by glycosylation is ETA. We suggest that in both cerebellar and atrial membranes, the carbohydrate chains of the ETA receptor contribute to the binding of ligand to the nanomolar-affinity binding sites, but not to the picomolar-affinity binding sites.

Endothelin: receptor subtypes, signal transduction, regulation of Ca2+ transients and contractility in rabbit ventricular myocardium

Endothelin (ET) isopeptides, ET-1, ET-2 and ET-3, elicit a positive inotropic effect (PIE) in association with a negative lusitropic effect, essentially with identical efficacies and potencies in the isolated rabbit papillary muscle, but with different concentration-dependent properties. Pharmacological analysis indicates that the PIE of ET-1 is mediated by an ETA2 subtype that is less sensitive to BQ-123 and FR139317, whereas the PIE of ET-3 is mediated by an ETA1 subtype that is highly sensitive to these ETA antagonists. ETs increased the amplitude of intracellular Ca2+ transient (CaT) in indo-1 loaded rabbit ventricular myocytes, but the increase was much smaller than that produced by elevation of [Ca2+]o or isoproterenol for a given extent of PIE, an indication of increased myofibrillar Ca2+ sensitivity. ETs stimulate phosphoinositide (PI) hydrolysis, which leads to production of inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). Evidence for the role of IP3-induced Ca2+ release in cardiac E-C coupling is tenuous. Generation of IP3 induced by ET-1 was transient and returned to the baseline level when the PIE reached an elevated steady level. Protein kinase C (PKC) that is activated by DAG and also via other pathways triggered by ETs stimulates Na+-H+ exchanger to lead to an increased [Na+]i and alkalinization. The former may contribute to an increase in the amplitude of CaT through Na+-Ca2+ exchanger, and the latter, to an increase in myofibrillar Ca2+ sensitivity. A number of PKC inhibitors, such as staurosporine, H-7, calphostin C and chelerythrine, consistently and selectively inhibited the PIE of ET-3 without affecting the PIE of isoproterenol and Bay k 8644. The maximum inhibition was 20-30% of the total response. A Na+-H+ exchange inhibitor, [5-(N-ethyl-N-isopropyl) amiloride (EIPA)] or a Ca2+ antagonist, verapamil, could not completely inhibit the PIE of ET-3, but the combination of both inhibitors totally abolished the PIE of ET-3. These findings indicate that activation of PKC and subsequent activation of Na+-H+ exchanger and/or L-type Ca2+ channels may play a crucial role in the cardiac action of ET isopeptides in the rabbit ventricular myocardium.

ET(A) and ET(B) specific ligands synergistically antagonize endothelin-1 binding to an atypical endothelin receptor in primary rat astrocytes

Using a whole-cell binding procedure with long incubations at low temperature and subsequent acid stripping, we have characterized an atypical endothelin (ET) receptor in primary rat cortical astrocyte cultures. We found the following: (a) no competition for 125I-ET-1 binding by the ET(A) antagonists BQ-123 and LU 135252 or the ET(B) agonist IRL 1620; (b) weak competition by the ET(B) antagonist BQ-788 and by the predominant ET(B) ligand ET-3; (c) potent synergistic competition of ET(A) and ET(B) ligands in combination for 125I-ET-1 binding; (d) potent competition of ET-1 with any of the radioligands used, 125I-ET-1, 125I-IRL 1620, and [3H]BQ-123; (e) lack of competition of IRL 1620 and BQ-123 with the respective other radioligand; (f) shifting of the amount of acid-strippable 125I-ET-1 binding from 20 to 80% by ET(B) ligands and to 4% by ET(A) ligands; and (g) as a control, typical ET(A) and ET(B) binding characteristics of the RAT-1 fibroblast and the U373MG astrocytoma cell line, respectively, under our assay conditions. The unusual binding properties of astrocytic ET receptors described in this study appear to be the result of several binding sites in the receptor for different ET ligands or ligand epitopes.

Functional and binding characterization of endothelin receptors in human bronchus: evidence for a novel endothelin B receptor subtype?

Binding and functional studies were conducted to elucidate the receptor subtypes mediating contractions of human bronchus induced by endothelin (ET) receptor ligands. Binding experiments in human bronchial smooth muscle membrane preparations revealed the presence of ETA and ETB receptors in the ratio of approximately 40:60. In the presence of the combination of 1 microM BQ-123 (ETA receptor antagonist) and 1 microM S6c (ETB receptor agonist) or BQ-788 (ETB receptor antagonist) about 10 to 20% of [125I]-ET-1 binding remained. ET-1 (nonselective agonist), ET-3 (ETB receptor-preferring agonist), S6c, IRL 1620 or BQ-3020 (ETB receptor-selective agonists) potently contracted human bronchus. SB 209670 (10 microM) (ETA/ETB receptor antagonist) antagonized ET-1-induced contractions (pKB = 6.1), whereas, BQ-788 (3 microM), RES-701 (10 microM) or BQ-123 (3 microM) were without effect. The combination of BQ-788 (3 microM) and BQ-123 (3 microM) did not influence ET-1 concentration-response curves. Contractions elicited by IRL 1620 or BQ-3020, but not S6c or ET-3, were sensitive to inhibition by BQ-788 (0.03-3 microM). Based on the potent contractile effects of ETB receptor-selective agonists, and the lack of inhibitory effect of BQ-123, ET ligand-induced contractions in human bronchus appear to be mediated via an ETB receptor subtype(s). However, contractions induced by ET-1, ET-3 or S6c are not sensitive to classical ETB receptor antagonists such as BQ-788. Furthermore, a residual component (about 10-20%) of the binding of radiolabeled ET agonists is resistant to various ET ligands. Collectively, these data suggest the presence of a novel ETB receptor subtype which may mediate contraction induced by some ET ligands in human bronchus.

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

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

Endothelin-1-induced circulatory response in the rat: the role of ETA and ETB receptors

Production of the powerful vasoconstrictor endothelin-1 (ET1) is increased in a number of pathological conditions. This study was performed 1. to assess the effects of a twofold elevation of circulating ET1 on global hemodynamics and cardiac function, and 2. to determine the ET receptor subtypes that are responsible for this action. We have used the ETA receptor-selective antagonist BQ 610, the novel ETA receptor antagonist ETR-Pl/fl peptide and the specific ETB receptor antagonist IRL 1038 to investigate the role of these receptor subtypes in mediating circulatory changes induced by ET1 in anesthetized Wistar rats. ET1 infusion produced a significant rise in mean arterial pressure (MAP), elevated total peripheral resistance (TPR), and decreased cardiac output (CO). BQ 610 and ETR-Pl/fl pretreatment significantly attenuated the ET1-induced hemodynamic changes. Pretreatment with IRL 1038 had no effect on CO, but significantly reduced MAP and TPR elevation 20 min after ET1 infusion. These results suggest that ET1 may contribute to circulatory failure in conditions with increased ET1 production via a mechanism involving ETA receptors. ETB receptors, albeit to a lesser extent than ETA receptors, are also involved in mediating ET1-induced peripheral vasoconstriction in the rat.

Subtypes of endothelin receptors that mediate venous effects of endothelin-1 in anaesthetized rats

1. The subtypes of endothelin receptors that mediate the effects of endothelin-1 (ET-1) on mean arterial pressure (MAP), heart rate (HR), mean circulatory filling pressure (MCFP), arterial resistance (RA), cardiac output (CO) and venous resistance (RV) were characterized in 9 groups of pentobarbitone-anaesthetized rats via the injection of ET-1 in the absence and presence of bosentan (Ro 47-0203, ETA- and ETB-receptor antagonist), PD 142893 (ETA- and ETB-receptor antagonist) or FR 139317 (ETA-receptor antagonist), as well as injection of the ETB-receptor agonist, IRL 1620. 2. Cumulative i.v. bolus injections of ET-1 or IRL 1620 (0.5, 1 and 2 nmol kg-1) dose-dependently increased MAP (ET: by 22, 34 and 44; IRL: 8, 17 and 28 mmHg), RA (ET: 62, 108 and 162; IRL: 51, 63 and 86% over baseline), RV (ET: 70, 132 and 179; IRL: 81, 89 and 98% over baseline) and MCFP (ET: 1.1, 1.8 and 1.9; IRL: 0.9, 1.0 and 1.2 mmHg) and reduced CO (ET: -18, -35 and -44; IRL: -24; -26; -25% below baseline). Equimolar doses of ET-1 and IRL 1620 caused similar initial transient depressor responses. Saline did not modify any haemodynamic variables in the time-control group. 3. Bosentan (10 mg kg-1, i.v.) inhibited ET-induced increases in MAP, RV, RA and MCFP and decrease in CO. PD 142893 (22 mg kg-1, i.v.) abolished ET-induced changes on MAP, RV, RA and CO, but did not alter effects on MCFP. Bosentan alone did not cause haemodynamic changes, but PD 142893 alone elevated MCFP (0.9 +/- 0.3 mmHg at 1 h after injection) and did not alter other variables. Both antagonists abolished the initial depressor effects of ET-1. 4. FR 139317 (1 mg kg-1, i.v.) partially inhibited the increases in MAP, RV, RA and MCFP and decreases in CO elicited by ET-1, but did not alter the transient depressor response of ET-1. 5. The results show that both ETA- and ETB-receptors mediate the arterial and venous constrictor effects of ET-1. Bosentan is more efficacious than PD 142893 in inhibiting the venous effects of ET-1.

Identification and characterization of a novel endothelin receptor that binds both ETA- and ETB-selective ligands

This study demonstrates the presence of a novel endothelin (ET) receptor subtype that displays high affinity for both ETA- and ETB-selective ligands. This subtype has been identified in canine spleen membranes using ETB-selective agonists ET-3, IRL-1620, sarafotoxin 6c (S6c) as well as ETA-selective antagonists BQ123 and related cyclic pentapeptides. Binding of 125I-ET-3 to canine spleen membranes was specific and saturable with an apparent dissociation constant of 130 pM and maximum binding (Bmax) of 240.0 fmol/mg protein. Although the apparent affinities obtained with 125I-ET-1 and 125I-ET-3 were comparable (90 and 130 pM, respectively), the maximum binding obtained with 125I-ET-3 was approximately 35% of that obtained with 125I-ET-1, which indicates that canine spleen possesses both ETA and ETB receptors in the ratio 65:35. Competition binding experiments using 125I-ET-3 and unlabeled ET-1, ET-3, S6c, and IRL-1620 suggested that although ET-1 and ET-3 displayed similar high affinity, S6c and IRL-1620 were 20-300-fold weaker than ET-1 and ET-3 in competing for 125I-ET-3 binding to canine spleen membranes. In addition, BQ123, an ETA-selective antagonist, displaced 125I-ET-3 binding from canine spleen with an IC50 value of 30 nM. Similar profiles were obtained with related cyclic pentapeptides. Electrophysiological studies performed on Xenopus laevis oocytes injected with canine spleen poly(A)+ RNA indicated that the ETB receptor present in these tissues is functional and displays the same pharmacology as that observed in binding studies using these membranes. As a comparison, both binding and functional studies were performed in canine lung and the data indicate that the ETB receptor present in this tissue is similar to that of the cloned human ETB receptor but different from that present in canine spleen. These observations were further confirmed by performing cross-linking experiments on these membranes. Although canine lung and cloned human ETB receptors displayed the same molecular weight bands with similar pharmacology, canine spleen ETB receptors displayed different molecular weight bands and different pharmacology. In addition, the ETB receptors present in canine spleen were also identified in canine bladder, monkey spleen and human spleen. Thus, the data presented in this manuscript provide evidence for the presence of a novel ETB receptor in different tissues as well as different species including human.

Analysis of two pharmacologically predicted endothelin B receptor subtypes by using the endothelin B receptor gene knockout mouse

1. This study was performed to clarify whether the endothelin (ET) receptor subtypes mediating two pharmacologically heterogeneous response to ETH receptor agonists in normal mice are the product(s) of a single ETB receptor gene. 2. Vasodilator responses to sarafotoxin S6c (S6c) in the thoracic aorta and contractile responses to ET-1 and IRL1620 in the stomach were examined in tissues from normal and ETB receptor gene knockout mice, in the absence and presence of an ETA receptor antagonist, BQ-123, or an ETA/ETB receptor antagonist, PD142893. 3. In the normal mouse aorta precontracted with phenylephrine, S6c (0.1-100 nM) caused concentration-dependent relaxations (pD2 = 8.4). BQ-123 had no effect on these responses. However, PD142893 almost abolished the relaxations induced by 0.1-300 nM S6c. 4. In aortae taken from ETB receptor gene knockout mice, S6c up to 1 microM failed to cause relaxations, confirming that ETB receptors are involved in mediating this response. 5. In normal mouse gastric fundus, 0.1 nM-1 microM ET-1, S6c or IRL1620 caused dose-dependent, BQ-123-insensitive contractions, which were much more resistant to PD142893 than S6c-induced relaxations of the aorta. The pD2 values for S6c in the absence and presence of PD142893 (10 microM) were 8.12 +/- 0.11 and 7.70 +/- 0.11, respectively. 6. In the gastric fundus of the ETB receptor gene knockout mouse, S6c and IRL1620 caused no contractions. ET-1 (0.1 nM-1 microM) caused contractions sensitive to both BQ-123 and PD142893, indicating that only ETA receptors mediate ET-1-induced contractions of the knockout mouse gastric fundus. 7. Since both the PD142893-sensitive vasodilator response of the aorta and the PD142893-resistant contractile response of the gastric fundus to S6c were completely absent in the ETB receptor gene knockout mouse, we conclude that the two pharmacologically heterogeneous responses to S6c are mediated by receptors derived from the same ETB receptor gene.

Coexpression studies with endothelin receptor subtypes indicate the existence of intracellular cross-talk between ET(A) and ET(B) receptors

Human Girardi heart cells expressing endothelin ET(B) receptors (GH(B) cells) were transfected with human ET(A) cDNA, and coexpression of ET(A) and ET(B) in the ratio of 4:6 was demonstrated by Scatchard analysis. [125I]Endothelin (ET)-1 binding to ET(A)-transfected GH cells (GH(AB) cells) was displaced by an ET(A) antagonist, BQ-123, in a biphasic manner. An ET(B) agonist, BQ-3020, and an ET(B) antagonist, BQ-788, inhibited [125I]ET-1 binding to GH(AB) cells in a monophasic manner with low affinities (IC50 = 2,800 and 890 nM, respectively); IC50 values for ET(B) receptors seemed to be as weak as those for ET(A) receptors. However, BQ-3020 and BQ-788 had a high affinity for ET(B) receptors in a binding experiment using [125I]ET-1 in the presence of 1 microM BQ-123, where ET(A) receptors are masked (IC50 = 0.49 and 0.89 nM, respectively). The ET(B)-mediated increase in intracellular calcium concentrations in GH(AB) cells was not affected by 0.1 microM BQ-788 alone but was inhibited significantly by the same concentration of BQ-788 in combination with 10 microM BQ-123. ET-1 suppressed forskolin-stimulated accumulation of cAMP through the activation of ET(A) and ET(B) in GH(AB) cells; 1 microM BQ-123 or BQ-788 inhibited the suppression by only 20%, whereas a mixture of BQ-123 and BQ-788 (1 microM each) completely inhibited the cAMP decrease. These findings suggest that the stimulation of ET(A) receptors with ET-1 results in a lowering of the affinity of BQ-3020 and BQ-788 for ET(B) receptors in GH(AB) cells. We conclude that there is intracellular cross-talk between ET(A) and ET(B) receptors in GH(AB) cells.

Experimental diabetes upregulates the expression of uretereral endothelin receptors

We investigated the binding characteristics of endothelin (ET) receptors in the ureters of rats with experimentally induced diabetes and diuresis. Receptor binding experiments demonstrated an upregulation in the expression of [125I]ET-1 binding sites in the diabetic rat ureter but not in the diuretic rat ureter. ET-1, ET-3, IRL 1620, and BQ 610 inhibited [125I]ET-binding to the rat ureter consistent with the predominance of ETA receptors in these tissues. The subtype specificity of ET receptors in ureteral tissues was confirmed with inhibition data obtained from cloned human ETA and ETB receptors.

Inhibitors of endothelin

With the advent of the first generation of both selective and nonselective endothelin antagonists being a relatively recent event, the manifold therapeutic potentials of these compounds are only now being explored clinically. Undoubtedly, numerous clinical utilities for these compounds will soon be realized.

Autocrine effects of endothelin-1 on leukocyte-endothelial interaction: stimulation of endothelin B receptor subtype reduces endothelial adhesiveness via a nitric oxide-dependent mechanism

The effects of endothelin-1 (ET-1) on P-selectin-mediated leukocyte endothelial interaction were examined in vitro. Adherence of autologous polymorphonuclear leukocytes (PMNs) to the endothelium was markedly enhanced by endothelial stimulation with either (2 U/mL) thrombin, (1 mumol/L) histamine, or (100 nmol/L) phorbol myristate acetate (PMA). In contrast, ET-1 alone (10 and 100 nmol/L) only slightly increased the number of adhering PMNs. The increased PMN adherence to thrombin- or histamine-stimulated endothelium, which was blocked by an anti-P-selectin monoclonal antibody, was also significantly attenuated by preincubation of coronary segments with (100 nmol/L) ET-1. We further investigated the mechanism of this anti-adherence action of ET-1 on thrombin-stimulated endothelial adhesiveness. Preincubation of coronary segments with a selective ETA receptor antagonist, BQ485 (1 mumol/L), had no effect on ET-1 inhibition of thrombin-induced PMN adherence. In contrast, preincubation with a selective ETB receptor antagonist, BQ788 (1 mumol/L) significantly reversed ET-1 inhibition of thrombin-induced PMN adherence, whereas the selective ETB receptor agonist BQ-3020 mimicked the inhibitory action of ET-1 on thrombin-induced PMN adherence. Furthermore, (100 mumol/L) N omega-nitro-L-arginine methyl ester (L-NAME), a nitric oxide synthase (NOS) inhibitor, significantly attenuated ET-1 inhibition of thrombin-stimulated PMN adherence. These results suggest that ET-1 may inhibit P-selectin-mediated leukocyte-endothelial interaction via ETB receptor stimulation and subsequent endothelial NO formation. This autocrine effect of ET-1 may be involved in pathophysiologic states such as early atherogenesis by preventing leukocyte-endothelial interaction in constricted blood vessels.

Endothelin receptors: receptor classification, novel receptor antagonists, and potential therapeutic targets

The development of endothelin receptor antagonists has progressed rapidly since the initial discovery of endothelin. Highly potent, orally active nonpeptide endothelin receptor antagonists have been identified, and are being used as pharmacological tools to elucidate the role of endothelin in pathological disorders. Subtype selective endothelin receptor antagonists will also be useful in understanding the physiological and pathological roles of the different subtypes of the endothelin receptors. The selectivity profile for the ideal endothelin receptor antagonist is presently unknown, and it may actually be that the optimal profile for a compound may depend on the clinical indication. In the near future, data from clinical trials with endothelin receptor antagonists will become available and will help to establish the role of endothelin in the etiology of human disease, as well as to provide valuable information concerning the optimum endothelin receptor subtype selectivity for antagonists needed for therapeutic agents.

Evidence for the existence of endothelin-B receptor subtypes and their physiological roles in the rat

The physiological roles of endothelin-B (ETB) receptor subtypes in systemic and renal hemodynamics were assessed in conscious Sprague-Dawley rats. Mean arterial pressure, hindlimb flow, and renal blood flow were measured via an implanted catheter and pulsed Doppler flow probes. Bolus intravenous injections of sarafotoxin 6c (S6c), a selective ETB agonist, elicited transient dose-dependent vasodilation, followed by sustained vasoconstriction in the systemic bed, but only vasoconstriction in the renal bed. RES-701-1, a selective ETB antagonist, blocked the dilator and potentiated the constrictor effect; SB-209670, a mixed ET receptor antagonist, attenuated both responses to S6c. In follow-up studies, the role of endogenous ET was assessed by administration of the antagonists alone: RES-701-1, SB-209670, and the ETA-selective antagonist BQ-123. RES-701-1 unmasked a significant systemic and renal vasoconstriction, which was attenuated by SB-209670 but not by BQ-123. SB-209670 and BQ-123 had no effect on basal hemodynamic parameters. Data from radioligand binding experiments showed that RES-701-1 binds with high affinity to the cloned human ETB receptor but poorly to the ETB receptor predominant in the rat kidney. Collectively, the results indicate that 1) the vascular effects of ET in the rat are mediated by two ETB receptor subtypes: an RES-701-1-sensitive subtype, mediating vasodilation, and an RES-701-1-insensitive subtype, mediating vasoconstriction; 2) the predominant role of endogenous ET is vasodilation; and 3) the ETA receptor plays a negligible role in the control of vascular tone in the rat.

Localization of endothelin ETB receptors on the myenteric plexus of guinea-pig ileum and the receptor-mediated release of acetylcholine

1. The type of endothelin (ET) receptor located on the myenteric neurones of guinea-pig ileum was determined by receptor autoradiography and function of the receptor was examined by release experiments of acetylcholine (ACh) from the longitudinal muscle myenteric plexus (LM-MP) preparations. 2. Specific [125I]-ET-1 binding sites were distributed in muscle layers, myenteric and submucous plexuses, and mucosa layers. High-grain densities were detected in both myenteric and submucous plexuses. 3. Binding in the myenteric plexus was abolished by incubation with either IRL 1620 (endothelin ETB receptor agonist) or BQ 788 (endothelin ETB receptor antagonist), but not with BQ 123 (endothelin ETA receptor antagonist). The [125I]-IRL 1620 binding sites were evident in the myenteric plexus. Thus, the endothelin receptor located on the myenteric neurones is of the ETB type. 4. ET-1 (10(-10)-3 x 10(-8) M) and ET-3 (10(-10)-3 x 10(-8) M) evoked 3H outflow from LM-MP preparations of ileum preloaded with [3H]-choline, in a concentration-dependent manner. There was no significant difference between maximum amounts of ET-1-evoked and ET-3-evoked 3H outflow. 5. ET-1 and ET-3 evoked outflow of 3H was BQ 788-sensitive, but BQ 123-insensitive. Both evoked outflows of 3H were Ca(2+)-dependent and tetrodotoxin-sensitive. 6. These results indicate that the endothelin ETB receptor is located on the enteric cholinergic neurones and that stimulation evokes the release of ACh.

Endothelins contract guinea-pig pulmonary artery and enhance its adrenergic response via ET(A) receptors

1. This study has pharmacologically characterized endothelin (ET) receptor subtype(s) mediating contraction and enhancement of adrenergic contraction in guinea-pig pulmonary artery. Isometric tension of the isolated endothelium-denuded ring preparations was measured in the presence of indomethacin (10(-5) mol/L) and N(G)-nitro-L-arginine methyl ester (L-NAME; 3 x 10(4) mol/L) to exclude a mechanism via endothelium, cyclo-oxygenase-generated eicosanoids and nitric oxide. 2. In the additional presence of tetrodotoxin (TTX; 3 x 10(-7) mol/L), ET-1 (10(-11)-10(-7) mol/L) concentration-dependently contracted the preparations. The rank order of potency to contract the preparations among ET receptor agonists was ET-1, sarafotoxin (STX)6b > ET-3 > IRL 1620, STX 6c. BQ-123 (7 x 10(-7)-7 x 10(-6) mol/L) concentrations-dependently shifted the concentration-contraction curve for ET-1 to the right in a parallel manner. Pretreatment with STX 6c (3 x 10(-7) mol/L for 30 min) did not significantly desensitize contractions to ET-1, ET-3 or IRL 1620 (P > 0.05; t-test, 10 d.f). 3. ET-1 (10(-10)-10(-9) mol/L) and STX 6b (10(-9)-10(-8) mol/L) significantly enhanced the electrical field stimulation-induced contraction in a BQ-123-sensitive manner (P < 0.05: t-test, 24-38 d.f), while ET-3 (10(-11)-10(-8) mol/L) and STX 6c (10(-11)-10(-7) mol/L) did not affect contractions. ET-1 (10(-11) mol/L) significantly enhanced contractions to exogenous noradrenaline in the presence of TTX (3 x 10(-7) mol/L) (P < 0.05; t-test, 16 d.f.). 4. These data indicate that the BQ-123-sensitive ET(A) receptor mediates both contraction and enhancement of adrenergic contractions in the guinea-pig pulmonary artery.

The positive inotropic effect and the hydrolysis of phosphoinositide induced by endothelin-3 in rabbit ventricular myocardium: inhibition by a selective antagonist of ET(A) receptors, FR139317

Endothelin-3 (ET-3), an isopeptide of ET, had a concentration-dependent positive inotropic effect (PIE) on rabbit papillary muscle. The maximal inotropic response to ET-3 was 65% of the maximal response to isoproterenol. ET-1 elicited a PIE below 10(-9) M, namely, over a concentration range at which ET-3 did not elicit a PIE. The selective ET(A) antagonist FR139317 effectively antagonized the PIE of ET-3. FR139317 abolished the PIE induced by ET-1 (<10(-9)M) but did not inhibit the PIE induced by high concentrations of ET-1. FR139317 also antagonized the PIE of sarafotoxin S6c. ET-3 caused a time- and concentration-dependent increase in [3H]inositol phosphates (inositol monophosphate, inositol bisphosphate and inositol trisphosphate). FR139317 at 10(-5) M decreased the ET-3-induced increase in inositol phosphates by about 60%, whereas it attenuated the increase in [3H]IP1 induced by ET-1 (3 x 10(-8)M) by only 20%. Thus, in the presence of FR139317, the PIEs of ET-3 and ET-1 were partially dissociated from the PI hydrolysis that was induced by these isopeptides. FR139317 inhibited the specific binding of [125I]ET-1 and of [125I]ET-3, and it was apparent that FR139317 had a high-affinity and a low-affinity site for competing for specific binding with each ligand. These findings indicate that different subtypes of the ET receptor are involved in the induction of the PIEs of ET-3 and ET-1. The PIE of ET-3 may be mediated predominantly by ET(A1) receptors that are susceptible to FR139317 and BQ-123 and partially by ET(B) receptor that are inhibited by RES-701-1. Both ET(A1) and ET(A2) receptors may be responsible for the PIE of ET-1, depending on the concentration in rabbit ventricular myocardium.