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

Tetrahydroxystilbene glucoside attenuated homocysteine-upregulated endothelin receptors in vascular smooth muscle cells via the ERK1 /2 /NF-κB signaling pathway

2,3,5,4'-Tetrahydrostilbene-2-o-β-d-glucoside (TSG) is the main active component of Polygonum multiflorum Thunb. It has effects on hypertension. However, the mechanism is unclear. Current research is devoted to exploring the mechanism of TSG improving HHcy-induced hypertension. The mice received a subcutaneous injection of Hcy in the presence or absence of TSG for 4 weeks. Blood pressure (BP) was measured using a noninvasive tail-cuff plethysmography method. Levels of plasma Hcy and endothelin-1 were measured using ELISA. Rat SMA without endothelium was cultured in a serum-free medium in the presence or absence of TSG with or without Hcy. The contractile response to sarafotoxin 6c or endothein-1 was studied using a sensitive myography. The levels of protein were detected using Western blotting. The results showed that TSG lowered HHcy-elevated BP and decreased levels of plasma Hcy and endothelin-1 in mice. Furthermore, the results showed that TSG inhibited Hcy-upregulated ET receptor expression and ET receptor-mediated contractile responses as well as the levels of p-ERK_1/2 and p-p65 in SMA. In vivo results further validate the in vitro results. In conclusion, TSG can decrease the levels of plasma Hcy and ET-1 and downregulate Hcy-upregulated ET receptors in VSMCs by inhibiting the ERK_1/2 /NF-κB/ET_B2 pathway to lower the BP.

The roles of endothelin and its receptors in cigarette smoke-associated pulmonary hypertension with chronic lung disease

Chronic exposure to cigarette smoke is the major risk factor for the development of pulmonary hypertension (PH) with chronic lung disease (i.e. PH group III). The pathogenesis of smoke-associated PH group III in chronic obstructive pulmonary disease (COPD) involves cigarette smoke exposure-induced damage to lung tissue and dysfunction of pulmonary system with increased synthesis and release of endothelin-1 (ET-1), hypoxia, inflammation, pulmonary vascular remodeling. Many studies have demonstrated that cigarette smoke exposure induces activation of mitogen-activated protein kinase (MAPK) signal pathway that leads to up-regulation of ET-1 and its receptors with the receptor-mediated enhanced contraction, proliferation of pulmonary vascular smooth muscle cells, pulmonary vascular remodeling, elevated pulmonary arterial pressure and finally PH group III. This mini-review article aims to summarize the current state of understanding on the roles of cigarette smoke-induced up-regulation of ET-1 and its receptors in the development of PH group III. Understanding the underlying molecular mechanisms that cigarette smoke exposure leads to PH group III may provide a novel strategy for the treatment.

Endothelin

The endothelins comprise three structurally similar 21-amino acid peptides. Endothelin-1 and -2 activate two G-protein coupled receptors, ETA and ETB, with equal affinity, whereas endothelin-3 has a lower affinity for the ETA subtype. Genes encoding the peptides are present only among vertebrates. The ligand-receptor signaling pathway is a vertebrate innovation and may reflect the evolution of endothelin-1 as the most potent vasoconstrictor in the human cardiovascular system with remarkably long lasting action. Highly selective peptide ETA and ETB antagonists and ETB agonists together with radiolabeled analogs have accurately delineated endothelin pharmacology in humans and animal models, although surprisingly no ETA agonist has been discovered. ET antagonists (bosentan, ambrisentan) have revolutionized the treatment of pulmonary arterial hypertension, with the next generation of antagonists exhibiting improved efficacy (macitentan). Clinical trials continue to explore new applications, particularly in renal failure and for reducing proteinuria in diabetic nephropathy. Translational studies suggest a potential benefit of ETB agonists in chemotherapy and neuroprotection. However, demonstrating clinical efficacy of combined inhibitors of the endothelin converting enzyme and neutral endopeptidase has proved elusive. Over 28 genetic modifications have been made to the ET system in mice through global or cell-specific knockouts, knock ins, or alterations in gene expression of endothelin ligands or their target receptors. These studies have identified key roles for the endothelin isoforms and new therapeutic targets in development, fluid-electrolyte homeostasis, and cardiovascular and neuronal function. For the future, novel pharmacological strategies are emerging via small molecule epigenetic modulators, biologicals such as ETB monoclonal antibodies and the potential of signaling pathway biased agonists and antagonists.

Endothelin@25 - new agonists, antagonists, inhibitors and emerging research frontiers: IUPHAR Review 12

Since the discovery of endothelin (ET)-1 in 1988, the main components of the signalling pathway have become established, comprising three structurally similar endogenous 21-amino acid peptides, ET-1, ET-2 and ET-3, that activate two GPCRs, ETA and ETB . Our aim in this review is to highlight the recent progress in ET research. The ET-like domain peptide, corresponding to prepro-ET-193-166 , has been proposed to be co-synthesized and released with ET-1, to modulate the actions of the peptide. ET-1 remains the most potent vasoconstrictor in the human cardiovascular system with a particularly long-lasting action. To date, the major therapeutic strategy to block the unwanted actions of ET in disease, principally in pulmonary arterial hypertension, has been to use antagonists that are selective for the ETA receptor (ambrisentan) or that block both receptor subtypes (bosentan). Macitentan represents the next generation of antagonists, being more potent than bosentan, with longer receptor occupancy and it is converted to an active metabolite; properties contributing to greater pharmacodynamic and pharmacokinetic efficacy. A second strategy is now being more widely tested in clinical trials and uses combined inhibitors of ET-converting enzyme and neutral endopeptidase such as SLV306 (daglutril). A third strategy based on activating the ETB receptor, has led to the renaissance of the modified peptide agonist IRL1620 as a clinical candidate in delivering anti-tumour drugs and as a pharmacological tool to investigate experimental pathophysiological conditions. Finally, we discuss biased signalling, epigenetic regulation and targeting with monoclonal antibodies as prospective new areas for ET research.

Vasopressin Aggravates Cardiopulmonary Bypass-Induced Gastric Mucosal Ischemia

Background/Aim: Upper gastrointestinal bleeding (UGIB) is one of the most frequent gastrointestinal complications after cardiac surgery with cardiopulmonary bypass (CPB). Endothelin expression and microcirculatory dysfunction have been shown to be involved in UGIB. The aim of this study was to analyze the effect of vasopressin during CPB on the gastric mucosal microcirculation and the involvement of the endothelin system. Methods: Eighteen pigs were randomized into three groups (n = 6 each): group I = sham, group II = CPB (1-hour CPB) and group III = CPB + vasopressin (1-hour CPB and vasopressin administration during CPB to maintain baseline arterial pressure). All animals were observed for a further 90 min after termination of CPB. Systemic hemodynamics as well as blood flow and oxygen saturation of the gastric mucosa were measured continuously. At the end of the experiment, the gastric mucosal expressions of endothelin-1 (ET-1) and its receptor subtypes A (ETA) and B (ETB) were determined by polymerase chain reaction. Gastric mucosal injury, apoptotic cell death and leukocytic infiltration were determined by histology and immunohistochemical analyses of cleaved caspase-3 and myeloperoxidase. Results: CPB decreased gastric microvascular perfusion, which was associated with an increased expression of ET-1 and ETA. Vasopressin aggravated the CPB-associated malperfusion, whereas it completely abrogated the upregulation of ET-1 and ETA. Interestingly, vasopressin did not induce gastric mucosal morphologic injury, leukocytic infiltration or apoptotic cell death. Conclusion: Vasopressin aggravates CPB-associated microvascular malperfusion of the gastric mucosa but does not induce gastric mucosal injury.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Endothelial endothelin B receptor-mediated prevention of cerebrovascular remodeling is attenuated in diabetes because of up-regulation of smooth muscle endothelin receptors

Structure and function of the cerebrovasculature is critical for ischemic stroke outcome. We showed that diabetes causes cerebrovascular remodeling by activation of the endothelin A (ET(A)) receptors. The goal of this study was to test the hypotheses that vasculoprotective endothelial ET(B) receptors are decreased and pharmacological inhibition of the ET(B) receptor augments vascular remodeling of middle cerebral arteries (MCAs) in type 2 diabetes. MCA structure, matrix metalloprotease (MMP) activity, and matrix proteins as well as ET(A) and ET(B) receptor profiles were assessed in control Wistar and diabetic Goto-Kakizaki rats treated with vehicle, the ET(B) receptor antagonist (2R,3R,4S)-4-(1,3-benzodioxol-5-yl)-1-[2-[(2,6-diethylphenyl)amino]-2-oxoethyl]-2-(4-propoxyphenyl)pyrrolidine-3-carboxylic acid (A192621) (30 mg/kg/day), or the dual ET receptor antagonist bosentan (100 mg/kg/day) for 4 weeks. Diabetes increased vascular smooth muscle (VSM) ET(A) and ET(B) receptors; the increase was prevented by chronic bosentan treatment. MCA wall thickness was increased in diabetes, and this was associated with increased MMP-2 activity and collagen deposition but reduced MMP-13 activity. Because of up-regulation of VSM ET receptors in diabetes, selective ET(B) receptor antagonism with A192621 blunts this response, and combined ET(A) and ET(B) receptor blockade with bosentan completely prevents this response. On the other hand, A192621 treatment augmented remodeling in control animals, indicating a physiological protective role for this receptor subtype. Attenuation of changes in ET receptor profile with bosentan treatment suggests that ET-1 has a positive feedback on the expression of its receptors in the cerebrovasculature. These results emphasize that ET receptor antagonism may yield different results in healthy and diseased states.

Endothelin receptors: what's new and what do we need to know?

Receptors are at the heart of how a molecule transmits a signal to a cell. Two receptor classes for endothelin (ET) are recognized, the ET(A) and ET(B) receptors. Intriguing questions have arisen in the field of ET receptor pharmacology, physiology, and function. For example, a host of pharmacological studies support the interaction of the ET(A) and ET(B) receptor in tissues (veins, arteries, bronchus, arterioles, esophagus), but yet few have been able to demonstrate direct ET(A)/ET(B) receptor interaction. Have we modeled this interaction wrong? Do we have a truly selective ET(A) receptor agonist such that we could selectively stimulate this important receptor? What can we learn from the recent phylogenic studies of the ET receptor family? Have we adequately addressed the number of biological molecules with which ET can interact to exert a biological effect? Recent mass spectrometry studies in our laboratory suggest that ET-1 interacts with other hereto unrecognized proteins. Biased ligands (ligands at the same receptor that elicit distinct signaling responses) have been discovered for other receptors. Do these exist for ET receptors and can we take advantage of this possibility in drug design? These and other questions will be posed in this minireview on topics on ET receptors.

Endothelin receptor selectivity in chronic renal failure

Chronic kidney diseases are increasing worldwide at an alarming rate, and they are emerging as a major public health problem. Treatments that slow the progression of chronic kidney disease are needed. Endothelin-1 (ET-1) is a potent vasoconstrictor with proinflammatory, mitogenic and profibrotic effects that is closely involved in both normal renal physiology and pathology. Increasing evidence suggests that ET-1 and its cognate receptors are involved in a variety of progressive renal disorders to the extent that renal ET-1 expression correlates with disease severity and renal function impairment. Endothelin receptor antagonists have been used in renoprotection studies owing to their capacity of improving renal hemodynamics and reducing proteinuria. Whether selective ET(A) or non-selective ET(A)/ET(B) receptor antagonists are preferable is still a matter of debate. As angiotensin II blockers are not invariably effective in retarding disease progression when treatment is started late in the course of the disease, it is foreseeable that an ET-1 antagonist in addition to angiotensin-converting enzyme inhibitors could represent a combined treatment for progressive nephropathies. The focus of this review is to examine the role endothelin-1 plays in kidney diseases and to determine the ideal setting for antagonizing its biological activity in chronic nephropathies.

Endothelin receptors and pain

The endogenous endothelin (ET) peptides participate in a remarkable variety of pain-relatedprocesses. Pain that is elevated by inflammation, by skin incision, by cancer, during a Sickle Cell Disease crisis and by treatments that mimic neuropathic and inflammatory pain and are all reduced by local administration of antagonists of endothelin receptors. Many effects of endogenously released endothelin are simulated by acute, local subcutaneous administration of endothelin, which at very high concentrations causes pain and at lower concentrations sensitizes the nocifensive reactions to mechanical, thermal and chemical stimuli.

PERSPECTIVE

In this paper we review the biochemistry, second messenger pathways and hetero-receptor coupling that are activated by ET receptors, the cellular physiological responses to ET receptor activation, and the contribution to pain of such mechanisms occurring in the periphery and the CNS. Our goal is to frame the subject of endothelin and pain for a broad readership, and to present the generally accepted as well as the disputed concepts, including important unanswered questions.

Effect of chronic and selective endothelin receptor antagonism on microvascular function in type 2 diabetes

Vascular dysfunction, which presents either as an increased response to vasoconstrictors or an impaired relaxation to dilator agents, results in worsened cardiovascular outcomes in diabetes. We have established that the mesenteric circulation in Type 2 diabetes is hyperreactive to the potent vasoconstrictor endothelin-1 (ET-1) and displays increased nitric oxide-dependent vasodilation. The current study examined the individual and/or the relative roles of the ET receptors governing vascular function in the Goto-Kakizaki rat, a mildly hyperglycemic, normotensive, and nonobese model of Type 2 diabetes. Diabetic and control rats received an antagonist to either the ET type A (ETA; atrasentan; 5 mg x kg(-1) x day(-1)) or type B (ET(B); A-192621; 15 or 30 mg x kg(-1) x day(-1)) receptors for 4 wk. Third-order mesenteric arteries were isolated, and vascular function was assessed with a wire myograph. Maximum response to ET-1 was increased in diabetes and attenuated by ETA antagonism. ETB blockade with 15 mg/kg A-192621 augmented vasoconstriction in controls, whereas it had no further effect on ET-1 hyperreactivity in diabetes. The higher dose of A-192621 showed an ETA-like effect and decreased vasoconstriction in diabetes. Maximum relaxation to acetylcholine (ACh) was similar across groups and treatments. ETB antagonism at either dose had no effect on vasorelaxation in control rats, whereas in diabetes the dose-response curve to ACh was shifted to the right, indicating a decreased relaxation at 15 mg/kg A-192621. These results suggest that ETA receptor blockade attenuates vascular dysfunction and that ETB receptor antagonism exhibits differential effects depending on the dose of the antagonists and the disease state.

Effect of chronic endothelin receptor antagonism on cerebrovascular function in type 2 diabetes

Diabetes increases the risk of stroke and contributes to poor clinical outcomes in this patient population. Myogenic tone of the cerebral vasculature, including basilar arteries, plays a key role in controlling cerebral blood flow. Increased myogenic tone is ameliorated with ET receptor antagonism in Type 1 diabetes. However, the role of endothelin-1 (ET-1) and its receptors in cerebrovascular dysfunction in Type 2 diabetes, a common comorbidity in stroke patients, remains poorly elucidated. Therefore, we hypothesized that 1) cerebrovascular dysfunction occurs in the Goto-Kakizaki (GK) model of Type 2 diabetes, and 2) pharmacological antagonism of ETA receptors ameliorates, while ETB receptor blockade augments vascular dysfunction. GK or control rats were treated with antagonists to either ETA (atrasentan, 5 mg.kg(-1).day(-1)) or ETB (A-192621, 15 or 30 mg.kg(-1).day(-1)) receptors for 4 wk and vascular function of basilar arteries was assessed using a wire myograph. GK rats exhibited increased sensitivity to ET-1. ET(A) receptor antagonism caused a rightward shift, indicating decreased sensitivity in diabetes, while it increased sensitivity to ET-1 in control rats. Endothelium-dependent relaxation was impaired in diabetes. ETA receptor blockade restored relaxation to control values in the GK animals with no significant effect in Wistar rats and ETB blockade with 30 mg.kg(-1).day(-1) A-192621 caused paradoxical constriction in diabetes. These studies demonstrate that cerebrovascular dysfunction occurs and may contribute to altered regulation of myogenic tone and cerebral blood flow in diabetes. While ETA receptors mediate vascular dysfunction, ETB receptors display differential effects. These results underscore the importance of ETA/ETB receptor balance and interactions in cerebrovascular dysfunction in diabetes.

The endothelin system as a therapeutic target in cardiovascular disease: great expectations or bleak house?

There is considerable evidence that the potent vasoconstrictor endothelin-1 (ET-1) contributes to the pathogenesis of a variety of cardiovascular diseases. As such, pharmacological manipulation of the ET system might represent a promising therapeutic goal. Many clinical trials have assessed the potential of ET receptor antagonists in cardiovascular disease, the most positive of which have resulted in the licensing of the mixed ET receptor antagonist bosentan, and the selective ET(A) receptor antagonists, sitaxsentan and ambrisentan, for the treatment of pulmonary arterial hypertension (PAH). In contrast, despite encouraging data from in vitro and animal studies, outcomes in human heart failure have been disappointing, perhaps illustrating the risk of extrapolating preclinical work to man. Many further potential applications of these compounds, including resistant hypertension, chronic kidney disease, connective tissue disease and sub-arachnoid haemorrhage are currently being investigated in the clinic. Furthermore, experience from previous studies should enable improved trial design and scope remains for development of improved compounds and alternative therapeutic strategies. Although ET-converting enzyme inhibitors may represent one such alternative, there have been relatively few suitable compounds developed, and consequently, clinical experience with these agents remains extremely limited. Recent advances, together with an increased understanding of the biology of the ET system provided by improved experimental tools (including cell-specific transgenic deletion of ET receptors), should allow further targeting of clinical trials to diseases in which ET is involved and allow the therapeutic potential for targeting the ET system in cardiovascular disease to be fully realized.

Endothelin

In humans, the endothelins (ETs) comprise a family of three 21-amino-acid peptides, ET-1, ET-2 and ET-3. ET-1 is synthesised from a biologically inactive precursor, Big ET-1, by an unusual hydrolysis of the Trp21 -Val22 bond by the endothelin converting enzyme (ECE-1). In humans, there are four isoforms (ECE-1a-d) derived from a single gene by the action of alternative promoters. Structurally, they differ only in the amino acid sequence of the extreme N-terminus. A second enzyme, ECE-2, also exists as four isoforms and differs from ECE-1 in requiring an acidic pH for optimal activity. Human chymase can also cleave Big ET-1 to ET-1, which is cleaved, in turn, to the mature peptide as an alternative pathway. ET-1 is the principal isoform in the human cardiovascular system and remains one of the most potent constrictors of human vessels discovered. ET-1 is unusual in being released from a dual secretory pathway. The peptide is continuously released from vascular endothelial cells by the constitutive pathway, producing intense constriction of the underlying smooth muscle and contributing to the maintenance of endogenous vascular tone. ET-1 is also released from endothelial cell-specific storage granules (Weibel-Palade bodies) in response to external stimuli. ETs mediate their action by activating two G protein-coupled receptor sub-types, ETA and ET(B). Two therapeutic strategies have emerged to oppose the actions of ET-1, namely inhibition of the synthetic enzyme by combined ECE/neutral endopeptidase inhibitors such as SLV306, and receptor antagonists such as bosentan. The ET system is up-regulated in atherosclerosis, and ET antagonists may be of benefit in reducing blood pressure in essential hypertension. Bosentan, the first ET antagonist approved for clinical use, represents a significant new therapeutic strategy in the treatment of pulmonary arterial hypertension (PAH).

Disruption of the kinin B1 receptor gene affects potentiating effect of captopril on BK-induced contraction in mice stomach fundus

A transgenic mouse model, deficient in kinin B(1) receptor (B(1)(-/-)) was used to evaluate the role of B(2) receptor in the smooth muscle stomach fundus. The results showed that the potency of bradykinin (BK) to induce contraction in the gastric tissue was maintained whereas the efficacy was markedly reduced. The angiotensin converting enzyme (ACE) inhibitor captopril potentiated BK-induced effect in wild type (WT) but not in B(1)(-/-) fundus. However, ACE activity detected by the convertion of Ang I to Ang II was inhibited by captopril in both types of gastric tissues. Taking into account the hypothesis that captopril and ACE bind to the B(2) receptor, we suggest that this complex was not formed in the stomach deficient in B(1) receptor. Therefore, our finding strongly support the hypothesis that in smooth muscles that constitutively express the kinin B(1) and B(2) receptors, an interaction between captopril and ACE, B(1) and B(2) receptors should occur forming a complex protein interaction for the potentiating effect of ACE on kinin receptors.

Angiotensin-II type 1 receptor-mediated hypertension in D4 dopamine receptor-deficient mice

Dopamine receptors are important in systemic blood pressure regulation. D4 receptors are expressed in the kidney and brain, but their role in cardiovascular regulation is unknown. In pentobarbital-anesthetized mice, systolic and diastolic blood pressures were elevated in sixth-generation D4 receptor-deficient (D4(-/-)) mice and in tenth-generation D4(-/-) mice compared with D4 wild-type (D4(+/+)) littermates. The conscious blood pressures measured via a chronic arterial (femoral) catheter or telemetry (carotid) were also higher in D4(-/-) mice than in D4 littermates. Basal renal and plasma renin concentrations were similar in the 2 mouse strains. The protein expression of angiotensin II type 1 receptor was increased in homogenates of kidney (330+/-53%, n=5) and brain (272+/-69%, n=5) of D4(-/-) mice relative to D4(+/+) mice (kidney: 100+/-12%, n=5; brain: 100+/-32%, n=5). The expression of the receptor in renal membrane was also increased in D4(-/-) mice (289+/-28%, n=8) relative to D4(+/+) mice (100+/-14%, n=10). In contrast, the expression in the heart was similar in the 2 strains. Bolus intravenous injection of angiotensin II type 1 receptor antagonist losartan initially decreased mean arterial pressures to a similar degree in D4(-/-) and D4(+/+) littermates. However, the hypotensive effect of losartan dissipated after 10 minutes in D4(+/+) mice, whereas the effect persisted for >45 minutes in D4(-/-) mice. We conclude that the absence of the D(4) receptor increases blood pressure, possibly via increased angiotensin II type 1 receptor expression.

Aberrant ETB receptor regulation of AT1 receptors in immortalized renal proximal tubule cells of spontaneously hypertensive rats

BACKGROUND

The renin-angiotensin and endothelin systems interact to regulate blood pressure, in part, by affecting sodium transport in the kidney. Because angiotensin II type 1 (AT(1)) receptor activation increases ETB receptor expression in renal proximal tubule cells from Wistar-Kyoto (WKY) rat, we hypothesize that ETB receptor activation may also regulate AT(1) receptor expression. Furthermore, ETB receptor regulation of the AT(1) receptor may be different in the WKY and spontaneously hypertensive rat (SHR).

METHOD

AT(1) and ETB receptors were studied in immortalized renal proximal tubule cells from WKY and SHRs, using immunoblotting, confocal microscopic colocalization, and immunoprecipitation.

RESULTS

In WKY renal proximal tubule cells, an ETB receptor agonist, BQ3020, decreased AT(1) receptor protein in a time- and concentration-dependent manner [median effective concentration (EC(50)) = 3.2 x 10(-10) mol/L, t(1/2)= 15 hours]. The inhibitory effect of BQ3020 (10(-8) mol/L/24 hours) on AT(1) receptor protein was blocked by an ETB receptor antagonist (BQ788). However, BQ3020 (10(-8) mol/L/24 hours) increased ETB receptor protein in WKY renal proximal tubule cells. In contrast, in SHR renal proximal tubule cells, BQ3020 (10(-8) mol/L/24 hours) no longer affected AT(1) or ETB receptor protein. AT(1)/ETB receptors colocalized and coimmunoprecipitated in WKY and SHRs. BQ3020 (10(-8) mol/L/15 minutes) treatment had no effect on AT(1)/ETB coimmunoprecipitation in WKY but decreased it in SHRs. BQ3020 (10(-8) mol/L/15 minutes) treatment increased AT(1) receptor phosphorylation in WKY, but decreased it in SHRs.

CONCLUSION

ETB receptors regulate AT(1) receptors by direct physical receptor interaction and receptor expression. An impaired ETB receptor regulation of the AT(1) receptor may participate in the pathogenesis of high blood pressure in the SHR.

Concomitant antagonism of endothelial and vascular smooth muscle cell ETB receptors for endothelin induces hypertension in the hamster

In the vascular system, endothelin (ET) type B (ET(B)) receptors for ET-1 are located on endothelial and on venous and arterial smooth muscle cells. In the present study, we investigated the hemodynamic effects of chronic ET(B) receptor blockade at low and high doses in the Syrian Golden hamster. After 16 days of gavage with A-192621 (0.5 or 30 mg.kg(-1).day(-1)), a selective ET(B) receptor antagonist, hamsters were anesthetized with a mixture of ketamine and xylazine (87 and 13 mg/kg im, respectively), and basal mean arterial blood pressure (MAP) and pressor responses to exogenous ET-1 were evaluated. The lower dose of A-192621 (0.5 mg.kg(-1).day(-1)) did not modify basal MAP, whereas the higher dose (30 mg.kg(-1).day(-1)) increased MAP and plasma ET levels. Radio-telemetry recordings confirmed the increase in MAP induced by the higher dose of A-192621 in conscious hamsters. On the other hand, although the lower dose of A-192621 was devoid of intrinsic pressor effects, it markedly reduced the transient hypotensive phase induced by intravenously injected IRL-1620, a selective ET(B) receptor agonist. Finally, A-192621 (0.5 mg.kg(-1).day(-1)) alone or A-192621 (30 mg.kg(-1).day(-1)) + atrasentan (6 mg.kg(-1).day(-1)), a selective ET(A) receptor antagonist, potentiated the pressor response to exogenous ET-1. Our results suggest that, in the hamster, ET(B) receptors on vascular smooth muscle cells are importantly involved in the clearance of endogenous ET-1, whereas the same receptor type on the endothelium is solely involved in the vasodilatory responses to the pressor peptide. Blockade of endothelial and vascular smooth muscle cell ET(B) receptors triggers a marked potentiation of ET(A)-dependent increases in systemic resistance.

Diabetes-related changes in contractile responses of stomach fundus to endothelin-1 in streptozotocin-induced diabetic rats

The contractile response of the stomach fundus to endothelin-1 (ET-1) was examined in streptozotocin (STZ)-induced diabetic rats. In STZ-diabetic rats (versus age-matched control rats) (a) ET-1 caused a longer-lasting contraction of stomach fundus strips, and (b) in the dose-response curve, the ET-1-induced contraction was significantly greater for a given concentration (3 x 10(-7) to 10(-7) M). Although repeated application of ET-1 led to desensitization, the desensitization was less pronounced in STZ-diabetic rats than in the controls. The density of the binding sites for [(125)I]-ET-1 was increased in the diabetic stomach fundus (versus the controls), but Kd values were similar between the two groups. The ET(B) receptor mRNA expression level was significantly increased in the diabetic stomach fundus. These results suggest that the diabetes-related enhancement of the ET-1-induced contraction of the stomach fundus may be due to an increase in the ET(B) receptor population.

Gq/G13signaling by ET-1 in smooth muscle: MYPT1 phosphorylation via ETAand CPI-17 dephosphorylation via ETB

We analyzed the signaling pathways initiated by endothelin receptors ETAand ETBin intestinal circular and longitudinal smooth muscle cells. The response to endothelin-1 (ET-1) consisted of two phases in both cell types. The initial, transient phase of contraction and phosphorylation of 20-kDa myosin light chain (MLC20) was mediated additively by ETAand ETBreceptors and initiated by Gαq-, Ca2+/calmodulin-dependent activation of MLC kinase. In contrast, the sustained phase was mediated selectively by ETAreceptors via a pathway involving sequential activation of Gα13, RhoA, and Rho kinase, resulting in phosphorylation of MYPT1 at Thr696and phosphorylation of MLC20. Although PKC was activated, CPI-17 was not phosphorylated and hence did not contribute to inhibition of MLC phosphatase. The absence of CPI-17 phosphorylation by PKC reflected active dephosphorylation of CPI-17 by protein phosphatase 2A (PP2A). PP2A was activated via a pathway involving ETB-dependent stimulation of p38 MAPK activity. CPI-17 phosphorylation was unmasked in the presence of the ETBantagonist BQ-788, but not the ETAantagonist BQ-123, and in the presence of a low concentration of okadaic acid, which selectively inactivates PP2A. The resultant phosphorylation of CPI-17 was blocked by bisindolylmaleimide, providing direct confirmation that it was PKC dependent. We conclude that the two phases of the intestinal smooth muscle response to ET-1 involve distinct receptors, G proteins, and signaling pathways. The sustained response is mediated via selective ETA-dependent phosphorylation of MYPT1. In contrast, ETBinitiates an inhibitory pathway involving p38 MAPK-dependent activation of PP2A that causes dephosphorylation of CPI-17.

Endothelin upregulates the expression of vasopressin V2 mRNA in the inner medullary collecting duct of the rat

Recent studies in our laboratory have demonstrated that bosentan, a mixed endothelin ET(A)/ET(B) receptor antagonist, prevented the upregulation of the arginine vasopressin (AVP) V(2) receptor in the inner medullary collecting duct (IMCD) of cardiomyopathic hamsters. These results suggested that endothelin-1 (ET-1) is involved in the upregulation of AVP V(2) receptors. Studies were performed to detect the effect of ET-1 on the expression of AVP V(2) receptors and the ET receptor mediating these effects within the IMCD of the rat. Rat IMCD tissue was isolated and incubated with the following: ET-1, or ET-1 in combination with ET(A) and ET(B) receptor antagonists BQ-123 and BQ-788, respectively, and sarafotoxin c (S6c), an ET(B) receptor-specific agonist. Tissue samples were then analyzed using quantitative reverse-transcriptase polymerase chain reaction (RT-PCR) and Western blotting. ET-1 treatment resulted in increased V(2) mRNA from a control level of 186.8 +/- 15.0 amol/microg total RNA to 430.7 +/- 49.0 amol/microg total RNA (P <.003). ET-1/ET(A) treatment resulted in no significant decrease in V(2) mRNA expression 335.0 +/- 38.0 amol/microg total RNA. Whereas ET-1/ET(B), and ET-1/ET(B)/ET(A) treatment resulted in V(2) mRNA approaching control 256.0 +/- 15.0 amol/microg total RNA, and 215.6 +/- 42.3 amol/microg total RNA. However, ET-3 treatment produced no significant changes in V(2) receptor mRNA expression. Sarafotoxin treatment corroborated both the ET-1 and ET receptor antagonist data, demonstrating striking significant increases in V(2) receptor mRNA and protein expression. S6c treatment increased V(2) mRNA expression from a control level of 199 +/- 17.3 amol/microg total RNA to 284.3 +/- 42.1 amol/microg total RNA (P < 05). Western blotting revealed that changes in V(2) mRNA expression in the various treatment conditions were similar to changes in protein expression. Overall, these data indicate that in the IMCD ET-1 increases AVP V(2) receptor expression and these changes are mediated by the ET(B) receptor.

Historical review: Endothelin

Endothelin (ET) is a potent vasoconstrictive peptide that was isolated initially from the conditioned medium of cultured endothelial cells. In 1988, details of the isolation and identification, amino acid sequence, cDNA sequence and pharmacology of ET were published. Subsequently, ET isoforms, ET receptors and endothelin-converting enzyme (ECE) were cloned. Because ET was thought to be important in cardiovascular homeostasis, many investigators focused on the physiological and pathophysiological significance of ET. Accordingly, ET receptor antagonists and ECE inhibitors have been developed rapidly, mostly for the treatment of cardiovascular diseases. The field of molecular biology has provided valuable information about ET, including evidence that the ET system plays important roles in the early development of the neural crest and, thus, in the formation of organs. These results now present new avenues of ET research.

Endothelin-1-induced responses in isolated mouse vessels: the expression and function of receptor types

Mice have been increasingly used as models for investigating cardiovascular diseases. However, the responsiveness of mouse vasculature to endothelin (ET)-1 has not been clearly established. The goal of this study was to determine the role of ET receptors (ET(A) and ET(B)) in mouse vessels using isometric force measurements. Results showed that in the abdominal aorta ET-1 induced a concentration-dependent contraction (EC(50): 1.4 nM) with maximum reaching 89.5 +/- 4.9% (10 nM) of that induced by 60 mM K(+) [with nitric oxide synthase (NOS) inhibitor N(omega)-nitro-l-arginine methyl ester (l-NAME)]. However, in the thoracic aorta or the carotid artery, ET-1 was poorly effective. RT-PCR revealed that in the endothelium-denuded abdominal aorta, the PCR product for ET(B) receptors was very low compared with ET(A). Similarly in tissues treated with l-NAME, the ET(B) receptor-specific agonist sarafotoxin 6c (S6c; 100 nM) induced only a minimal contraction (<5%). Meanwhile, the ET(A) antagonist BQ-123 (1 microM) completely inhibited the maximum ET-1 (10 nM) contractile response. Furthermore, we found that in the abdominal aorta that had not been treated with l-NAME, ET-1-induced contraction significantly decreased. However, in such specimens, S6c was unable to induce any relaxation on phenylephrine-induced contraction. These results indicate that the role of ET receptors differs considerably among mouse vessels. In the abdominal aorta, ET(A) receptor mediates a potent vasoconstrictor response, whereas ET(B) has, if any, only a minimal functional presence. Also, our data suggest that ET-1 might involve a NOS-dependent vasodilation in the abdominal aorta, which remains to be further defined.

Dual constrictor and dilator actions of ET(B) receptors in the rat renal microcirculation: interactions with ET(A) receptors

The vascular actions of endothelin-1 (ET-1) reflect the combination of vasoconstrictor ET(A) and ET(B) receptors on smooth muscle cells and vasodilator ET(B) receptors on endothelial cells. The present study investigated the contribution of ET receptor subtypes using a comprehensive battery of agonists and antagonists infused directly into the renal artery of anesthetized rats to evaluate the actions of each receptor class alone and their interactions. ET-1 (5 pmol) reduced renal blood flow (RBF) 25+/-1%. ET(A) antagonist BQ-123 attenuated this response to a 15+/-1% decrease in RBF (P < 0.01), indicating net constriction by ET(B) receptors. Combined receptor blockade (BQ-123+BQ-788) resulted in a renal vasoconstriction of 7+/-1% (P = 0.001 vs. BQ-123), supporting a constrictor action of ET(B) receptors. In marked contrast, the ET(B) antagonist BQ-788 enhanced the ET-1 RBF response to 60+/-5% (P < 0.001), suggesting ET(B)-mediated net dilation. Consistent with ET(A) blockade, the ET(B) agonist sarafotoxin 6C (S6C) produced vasoconstriction, reducing RBF by 23+/-5%. Dose-response curves for ET-1 and S6C showed similar degrees of constriction between 0.2 and 100 pmol. Both antagonists (BQ-123, BQ-788) were equally effective at threefold lower than the standard doses, suggesting complete inhibition. We conclude that ET(B) receptors alone exert a net constrictor effect but cause a net dilator influence when costimulated with ET(A) receptors. Such opposing actions indicate more complex than additive interaction between receptor subtypes. Model analysis suggests ET(A)-mediated constriction is appreciably greater without than with costimulation of ET(B) receptors. Possible explanations include ET-1 clearance by ET(B) receptors and/or a dilator ET(B) receptor function that counteracts constriction.

Selective blockade of endothelin-B receptors exacerbates ischemic brain damage in the rat

BACKGROUND AND PURPOSE

Endothelins act through 2 receptors, namely, ET(A) and ET(B). In the cerebral circulation, ET(A) mediates marked and prolonged vasoconstriction, and its blockade increases cerebral blood flow (CBF) and reduces ischemic brain damage. However, the role of ET(B) receptors remains unclear. In this study we examined, in rats, the kinetics of expression of ET(B) and the effects of ET(B) blockade on changes in CBF and brain damage after focal cerebral ischemia and N-methyl-D-aspartate (NMDA)-induced excitotoxic injury.

METHODS

Rats were subjected to transient (60 minutes) focal cerebral ischemia or cortical injection of NMDA. The selective ET(B) antagonist BQ-788 was injected intracerebroventricularly 30 minutes before and 30 minutes after the onset of ischemia. Cortical perfusion was monitored by laser-Doppler flowmetry. The volume of infarction or NMDA-induced cortical lesion was assessed at 24 hours after the insult. The reverse transcription-polymerase chain reaction technique was used to assess ET(B) expression.

RESULTS

Cerebral ischemia failed to alter the expression of ET(B) mRNA in both acute and chronic stages. Administration of BQ-788 resulted in an increase in infarction volume (178%; P<0.05) accompanied by a decrease in residual CBF (-26.7% versus control; P<0.01). In these animals we found a positive correlation between the volume of infarction and the severity of the decrease in CBF. NMDA-induced cortical lesions were not affected by the administration of BQ-788.

CONCLUSIONS

Our results suggest that the ET(B) antagonist BQ-788 induces deleterious effects that are mediated by the reduction of residual blood flow after ischemia and argue that the optimal therapeutic strategy in stroke would be to target the use of selective ET(A) antagonists and not mixed ET(A)/ET(B) antagonists.

The extracellular N terminus of the endothelin B (ETB) receptor is cleaved by a metalloprotease in an agonist-dependent process

The extracellular N terminus of the endothelin B (ET(B)) receptor is susceptible to limited proteolysis (cleavage at R64 downward arrow S65), but the regulation and the functional consequences of the proteolysis remain elusive. We analyzed the ET(B) receptor or an ET(B)-GFP fusion protein stably or transiently expressed in HEK293 cells. After incubation of cells at 4 degrees C, only the full-length ET(B) receptor was detected at the cell surface. However, when cells were incubated at 37 degrees C, N-terminal cleavage was observed, provided endothelin 1 was present during the incubation. Cleavage was not inhibited by internalization inhibitors (sucrose, phenylarsine oxide). However, in cells incubated with both internalization inhibitors and metalloprotease inhibitors (batimastat, inhibitor of TNFalpha-convertase) or metal chelators (EDTA, phenanthroline), the cleavage was blocked, indicating that metalloproteases cleave the agonist-occupied ET(B) receptor at the cell surface. Functional analysis of a mutant ET(B) receptor lacking the first 64 amino acids ([Delta2-64]ET(B) receptor) revealed normal functional properties, but a 15-fold reduced cell surface expression. The results suggest a role of the N-terminal proteolysis in the regulation of cell surface expression of the ET(B) receptor. This is the first example of a multispanning membrane protein, which is cleaved by a metalloprotease, but retains its functional activity and overall structure.

Function of the endothelinB receptor in cardiovascular physiology and pathophysiology

One of the two receptors by which the potent vasoactive effects of endothelin (ET)-1 are mediated is the ET(B) receptor (ET(BR)), which is found in several tissues, but, more importantly from a cardiovascular point of view, on the endothelial cell. The endothelial cell also has the unique capability of releasing ET-1, as well as other factors, such as the endothelial-derived relaxing factors and prostacyclin, which counteract the myotropic effects of the peptide. The secretory and contractile responses to ET-1 rely on G-protein-coupled ET(BR)s, as well as ET(A)-G-protein-coupled receptor-like proteins. The mitogenic properties of ET-1 via ET(A) receptors (ET(AR)s) coupled to mitogen-activated protein kinases and tyrosine kinases on the vascular smooth muscle may occur in conjunction with the anti-apoptotic characteristics of the endothelial ET(BR)s. Interestingly, most of the relevant antagonists and agonists for both ET(AR)s and ET(BR)s have been developed by the pharmaceutical industry. This highlights the therapeutical potential of compounds that act on ET receptors. In normal as well as in physiopathological conditions, the ET(BR) plays an important role in the control of vascular tone, and must be taken into account when using ET receptor antagonists for the treatment of cardiovascular diseases. For the management of congestive heart failure, renal failure and primary pulmonary hypertension, the most recent literature supports the use of selective ET(AR) antagonists rather than mixed antagonists of ET(AR)s and ET(BR)s. Nonetheless, validation of this view will have to await the first clinical trials comparing the actions of ET(A) to mixed ET(A)/ET(B) receptor antagonists.

Contribution of endothelin to pulmonary vascular tone under normoxic and hypoxic conditions

The contribution of endothelin to resting pulmonary vascular tone and hypoxic pulmonary vasoconstriction in humans is unknown. We studied the hemodynamic effects of BQ-123, an endothelin type A receptor antagonist, on healthy volunteers exposed to normoxia and hypoxia. Hemodynamics were measured at room air and after 15 min of exposure to hypoxia (arterial PO(2) 99.8 +/- 1.8 and 49.4 +/- 0.4 mmHg, respectively). Measurements were then repeated in the presence of BQ-123. BQ-123 decreased pulmonary vascular resistance (PVR) 26% and systemic vascular resistance (SVR) 21%, whereas it increased cardiac output (CO) 22% (all P < 0.05). Hypoxia raised CO 28% and PVR 95%, whereas it reduced SVR 23% (all P < 0.01). During BQ-123 infusion, hypoxia increased CO 29% and PVR 97% and decreased SVR 22% (all P < 0.01). The pulmonary vasoconstrictive response to hypoxia was similar in the absence and presence of BQ-123 [P = not significant (NS)]. In vehicle-treated control subjects, hypoxic pulmonary vasoconstriction did not change with repeated exposure to hypoxia (P = NS). Endothelin contributes to basal pulmonary and systemic vascular tone during normoxia, but does not mediate the additional pulmonary vasoconstriction induced by acute hypoxia.

M(3)-receptor knockout mice: muscarinic receptor function in atria, stomach fundus, urinary bladder, and trachea

Negative chronotropic and smooth muscle contractile responses to the nonselective muscarinic agonist carbamylcholine were compared in isolated tissues from M(3)-muscarinic receptor knockout and wild-type mice. Carbamylcholine (10(-8)-3.0 x 10(-5) M) induced a concentration-dependent decrease in atrial rate that was similar in atria from M(3)-receptor knockout and wild-type mice, indicating that M(3) receptors were not involved in muscarinic receptor-mediated atrial rate decreases. In contrast, the M(3) receptor was a major muscarinic receptor involved in smooth muscle contraction of stomach fundus, urinary bladder, and trachea, although differences existed in the extent of M(3)-receptor involvement among the tissues. Contraction to carbamylcholine was virtually abolished in urinary bladder from M(3)-receptor knockout mice, suggesting that contraction was predominantly due to M(3)-receptor activation. However, approximately 50-60% maximal contraction to carbamylcholine occurred in stomach fundus and trachea from M(3)-receptor knockout mice, indicating that contraction in these tissues was also due to M(2)-receptor activation. High concentrations of carbamylcholine relaxed the stomach fundus from M(3)-receptor knockout mice by M(1)-receptor activation. Thus M(3)-receptor knockout mice provided unambiguous evidence that M(3) receptors 1) play no role in carbamylcholine-induced atrial rate reduction, 2) are the predominant receptor mediating carbamylcholine-induced urinary bladder contractility, and 3) share contractile responsibility with M(2) receptors in mouse stomach fundus and trachea.

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

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

SRC family kinases mediate epithelial Na+ channel inhibition by endothelin

The epithelial Na(+) channel (ENaC) is implicated in the pathogenesis of salt-sensitive hypertension. Recent evidence from animal models suggests that the vasoactive peptide, endothelin (ET-1), may be an important negative regulator of ENaC in vivo. We investigated the signaling pathway involved in endothelin-mediated ENaC inhibition. Experiments were performed in NIH 3T3 cells stably expressing genes for the three (alpha, beta, and gamma) ENaC subunits. In whole cell patch clamp experiments, we found that ET-1 treatment induced a dose-dependent decrease in amiloride-sensitive currents. Using receptor-specific antagonists, we determined that the effects of ET-1 were attributed to activation of the ET(B) receptor. Moreover, the inhibitory effect of ET-1 on ENaC could be completely blocked when cells were pretreated with the selective Src family kinase inhibitor, PP2. Further studies revealed that basal Src family kinase activity strongly regulates ENaC whole cell currents and single channel gating. These results suggest that Src family kinases lie in a signaling pathway activated by ET-1 and are components of a novel negative regulatory cascade resulting in ENaC inhibition.

Adrenergic and endothelin B receptor-dependent hypertension in dopamine receptor type-2 knockout mice

Polymorphism of the dopamine receptor type-2 (D(2)) gene is associated with essential hypertension. To assess whether D(2) receptors participate in regulation of blood pressure (BP), we studied mice in which the D(2) receptor was disrupted. In anesthetized mice, systolic and diastolic BPs (in millimeters of mercury) were higher in D(2) homozygous and heterozygous mutant mice than in D(2)+/+ littermates. BP after alpha-adrenergic blockade decreased to a greater extent in D(2)-/- mice than in D(2)+/+ mice. Epinephrine excretion was greater in D(2)-/- mice than in D(2)+/+ mice, and acute adrenalectomy decreased BP to a similar level in D(2)-/- and D(2)+/+ mice. An endothelin B (ET[B]) receptor blocker for both ET(B1) and ET(B2) receptors decreased, whereas a selective ET(B1) blocker increased, BP in D(2)-/- mice but not D(2)+/+ mice. ET(B) receptor expression was greater in D(2)-/- mice than in D(2)+/+ mice. In contrast, blockade of ET(A) and V(1) vasopressin receptors had no effect on BP in either D(2)-/- or D(2)+/+ mice. The hypotensive effect of an AT(1) antagonist was also similar in D(2)-/- and D(2)+/+ mice. Basal Na(+),K(+)-ATPase activities in renal cortex and medulla were higher in D(2)+/+ mice than in D(2)-/- mice. Urine flow and sodium excretion were higher in D(2)-/- mice than in D(2)+/+ mice before and after acute saline loading. Thus, complete loss of the D(2) receptor results in hypertension that is not due to impairment of sodium excretion. Instead, enhanced vascular reactivity in the D(2) mutant mice may be caused by increased sympathetic and ET(B) receptor activities.

Differential modulation of endothelin ligand-induced contraction in isolated tracheae from endothelin B (ET(B)) receptor knockout mice

The role of endothelin B (ET(B)) receptors in mediating ET ligand-induced contractions in mouse trachea was examined in ET(B) receptor knockout animals. Autoradiographic binding studies, using [(125)I]-ET-1, confirmed the presence of ET(A) receptors in tracheal and bronchial airway smooth muscle from wild-type (+/+) and homozygous recessive (-/-) ET(B) receptor knockout mice. In contrast, ET(B) receptors were not detected in airway tissues from (-/-) mice. In tracheae from (+/+) mice, the rank order of potencies of the ET ligands was sarafotoxin (Stx) S6c>ET-1>ET-3; Stx S6c had a lower efficacy than ET-1 or ET-3. In tissues from (-/-) mice there was no response to Stx S6c (up to 0.1 microM), whereas the maximum responses and potencies of ET-1 and ET-3 were similar to those in (+/+) tracheae. ET-3 concentration-response curve was biphasic in (+/+) tissues (via ET(A) and ET(B) receptor activation), and monophasic in (-/-) preparations (via stimulation of only ET(A) receptors). In (+/+) preparations SB 234551 (1 nM), an ET(A) receptor-selective antagonist, inhibited the secondary phase, but not the first phase, of the ET-3 concentration-response curve, whereas A192621 (100 nM), an ET(B) receptor-selective antagonist, had the opposite effect. In (-/-) tissues SB 234551 (1 nM), but not A192621 (100 nM), produced a rightward shift in ET-3 concentration-response curves. The results confirm the significant influence of both ET(A) and ET(B) receptors in mediating ET-1-induced contractions in mouse trachea. Furthermore, the data do not support the hypothesis of atypical ET(B) receptors. In this preparation ET-3 is not an ET(B) receptor-selective ligand, producing contractions via activation of both ET(A) and ET(B) receptors.

Evidence for vasoconstriction mediated by the endothelin-B receptor in domestic swine

Endothelin-1 (ET-1), a potent vasoactive and mitogenic peptide, has been implicated in a number of cardiovascular diseases, including congestive heart failure, neointimal hyperplasia associated with restenosis, and hypertension. The vasoconstriction induced by ET-1 is thought to be mediated mainly by its action on ET(A) receptors on vascular smooth muscle cells. Recent studies have indicated that vasoconstriction also may be mediated by stimulation of an ET(B)-receptor subtype. Increased use of the pig as a cardiovascular model prompted us to examine the receptor profile in this species using ABT-627, a potent, nonpeptide antagonist of the ET(A) receptor. The precursor to ET-1, big ET-1 (0.02 nmol/kg/min), was infused intravenously in domestic swine, resulting in a sustained increase in mean blood pressure of 38 +/- 3 mm Hg. After stabilization of the pressor response, ABT-627 (0.1-10 microg/kg/min) or vehicle was infused for 30 min. Whereas vehicle infusion had no appreciable effect, a dose-related reversal of the pressor response to big ET-1 (11-100%) was observed by the end of the ABT-627 infusion. Blood samples were assayed for plasma concentrations of ABT-627; peak levels ranged from 9 +/- 2 to 937 +/- 168 ng/ml. In a separate group of pigs, the highest dose of ABT-627 produced only a modest reversal of the hypertensive response to an infusion of angiotensin II (300 ng/kg/min). Additional results indicate that the vasoconstrictor effects produced by sarafotoxin 6C (0.03 and 0.3 nmol/kg), an agonist of the ET(B) receptor, are not blocked by treatment with ABT-627 (10 microg/kg/min). However, complete blockade of the S6C response could be achieved using the ET(B) antagonist, A-192621 (0.33 mg/kg/min). Our results define the dose-response relation for the ET(A)-receptor antagonist ABT-627 in the vasculature of the domestic pig and suggest the presence of an ET(B)-receptor subtype that mediates vasoconstriction in this species.

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.

Novel endothelin B receptor transcripts with the potential of generating a new receptor

Using RT-PCR and rapid amplification of 5' cDNA ends (5' RACE), we have cloned three previously unrecognized endothelin B receptor (EDNRB) transcripts from a human melanoma cell line. Three distinct types of cDNAs (EDNRBDelta1, Delta2 and Delta3) were identified. EDNRBDelta1 starts upstream of the published transcription start site of hEDNRB without splicing, whereas, EDNRBDelta2 and EDNRBDelta3 are alternatively spliced. EDNRBDelta1 and EDNRBDelta2 share the same transcription initiation site and are 560bp upstream of the conventional hEDNRB, whereas that of EDNRBDelta3 is 939bp upstream from that described for the conventional hEDNRB. Interestingly, many transcription factor motifs are detectable in the upstream regions of these transcription initiation sites. The predicted amino acid sequences reveal that EDNRBDelta1 and EDNRBDelta2 produce the same protein as the conventional hEDNRB, but EDNRBDelta3 would give rise to additional in-frame 89- or 83-amino-acid residues at the N-terminus. EDNRBDelta3 generates the same amino acid sequence at the C terminus, but utilizes the polyadenylation signal within the open reading frame, resulting in a shorter 3'UTR. These transcripts are widely expressed in human tissues, but their expression patterns vary between different tissues.

Endothelium-independent vascular relaxation mediating ETB receptor in rabbit mesenteric arteries

Vascular response mediating endothelin (ET)B receptor was studied using isolated rabbit mesenteric arteries. ET-1 (0.1-30 nM) caused a concentration-dependent contraction, whereas ET-3 >100 nM caused only weak contraction. Up to 1 microM of sarafotoxin S6c showed no contraction. In arteries precontracted with phenylephrine, ET-3 (0. 03-1 nM) caused a concentration-dependent relaxation, which was not affected by endothelium denudation. The ET-3-induced relaxation was antagonized by BQ-788 and PD-142893 but not by BQ-123 in the endothelium-denuded arteries. Treatment with indomethacin but not with NG-nitro-L-arginine methyl ester completely inhibited the relaxation. ET-3 stimulated the release of 6-keto-PGF1alpha and PGE2 from the endothelium-denuded arteries. ET-3 also significantly increased cAMP content but not cGMP content in the arteries. Radioligand-binding studies using serial sections of the artery revealed the expression of not only ETA but also ETB receptors in the smooth muscle layer of the arteries. These results suggest that ET-3 activates ETB receptor in smooth muscle cells of rabbit mesenteric artery, producing vasodilator prostaglandins from arachidonic acid probably via a catalysis of cyclooxygenase, which accumulates cAMP in subendothelial tissues and produces relaxations.

Transcriptional down-regulation of the rabbit pulmonary artery endothelin B receptor during phenotypic modulation

1. We confirmed that endothelium-independent contraction of the rabbit pulmonary artery (RPA) is mediated through both an endothelin A (ET(A)R) and endothelin B (ET(B2)R) receptor. 2. The response of endothelium-denuded RPA rings to endothelin-1 (ET-1, pD2 = 7.84 +/- 0.03) was only partially inhibited by BQ123 (10 microM), an ET(A)R antagonist. 3. Pretreatment with 1 nM sarafotoxin S6c (S6c), an ET(B)R agonist, desensitized the ET(B2)R and significantly attenuated the response to ET-3 (pD2 = 7.40 +/- 0.02 before, <6.50 after S6c). 4. Pretreatment with S6c had little effect on the response to ET-1, but BQ123 (10 microM) caused a parallel shift to the right of the residual ETAR-mediated response to ET-1 (pD2 = 7.84 +/- 0.03 before S6c, 7.93 +/- 0.03 after S6c, 6.81 +/- 0.05 after BQ123). 5. Binding of radiolabelled ET-1 to early passage cultures of RPA vascular smooth muscle cells (VSMC) displayed two patterns of competitive displacement characteristic of the ET(A)R (BQ123 pIC50 = 8.73 +/- 0.05) or ET(B2)R (S6c pIC50 = 10.15). 6. Competitive displacement experiments using membranes from late passage VSMC confirmed only the presence of the ET(A)R (ET-1 pIC50 = 9.3, BQ123 pIC50 = 8.0, S6c pIC50 < 6.0). 7. The ET(A)R was functionally active and coupled to rises in intracellular calcium which exhibited prolonged homologous desensitization. 8. Using a reverse transcriptase polymerase chain reaction for the rabbit ET(B2)R, we demonstrated the absence of mRNA expression in phenotypically modified VSMC. 9. We conclude that the ET(B2)R expressed by VSMC which mediates contraction of RPA is rapidly down-regulated at the transcriptional level during phenotypic modulation in vitro.

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.

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.

Endothelin B receptors on human endothelial and smooth-muscle cells show equivalent binding pharmacology

We have described the pharmacologic profiles of endothelin B receptors in human endothelial cells and vascular and nonvascular smooth-muscle cells. First, by amplifying endothelin B receptor numbers through the use of phosphoramidon and intact cell-binding techniques, we demonstrated the presence of these receptors in human umbilical vein endothelial cells (100% endothelin B receptors), human aortic smooth-muscle cells (22% endothelin B, 78% endothelin A receptors), and human bronchial smooth-muscle cells (55% endothelin B, 45% endothelin A receptors) by using [125I]-endothelin-1 radioligand binding. The typical binding profiles of the endothelin B receptors were established through competition binding curve analysis with endothelin-1, endothelin-3, sarafotoxin 6c, and the endothelin A receptor-selective antagonist BQ-123. In the presence of BQ-123, a diverse group of antagonists, including PD 142893, BQ-788, SB 209670, and Ro 47-0203, were used to probe for binding differences indicative of multiple endothelin B-receptor subtypes. The results indicate a rank order of potency for the antagonists of BQ-788 > SB 209670 > PD 142893 > Ro 47-0203 for each cell line, and that between any of these human cell lines, measurements of [125I]-endothelin-1-binding antagonism for each of the four test compounds differed by less than twofold. Although this study cannot discount the possibility of more than one endothelin B-receptor subtype in humans, it does indicate that these tissues express receptors that show equivalent binding pharmacology.

The role of endothelin in hypertension

The endothelins are 21-amino-acid peptides which may play a role in the pathogenesis of hypertension. There is increasing evidence that the endothelins have a central function in mediating end-organ damage in hypertension, and that important effects of endothelin in the pathogenesis of hypertension may be based on the interactions of the endothelins and the renin-angiotensin and the nitric oxide systems.

Pharmacology of endothelins in vascular circuits of normal or heterozygous endothelin-A or endothelin-B knockout transgenic mice

Endothelin-1 (ET-1; 0.001-1 nmol) and the ETB receptor agonist IRL-1620 (0.01-1 nmol) induced a dose-dependent vasoconstriction of the arterial and venous mesenteric circuits and of the kidney in normal mice. BQ-123 (10(-7) M) or BQ-788 (10(-7) M) abolished the vasoconstriction induced by ET-1 in the arterial mesenteric and renal vasculatures without affecting that of norepinephrine (NE). In the venous mesenteric vasculature, only BQ-123 reduced the response to ET-1 but not to NE. In other experiments we compared the mesenteric and renal vascular reactivities to ET-1 and IRL-1620 in ETA or ETB heterozygous knockout mice with those of the wild-type strain. We observed a significant reduction in vascular reactivity to ET-1 but not to IRL-1620 in the arterial mesenteric and renal but not the venous mesenteric circuits of ETA knockout mice. In contrast, there was a significant reduction in vascular reactivity to ET-1 and IRL-1620 in the arterial mesenteric and renal circuits of ETB knockout mice. In the venous mesenteric vasculature, only the vasoconstriction induced by IRL-1620 was significantly reduced in the same ETB knockout strain. Our results suggest that, in the mouse, arterial mesenteric and renal vasoconstriction to ET-1 is mediated by both subtypes of ET receptors, whereas venous mesenteric vasoconstriction appears to be mediated uniquely by the ETA receptor subtype. Knockout of only one allele of the ETA or ETB gene appears to be sufficient for reduction of the ET-1 or IRL-1620 vasoconstrictor effects in the mesenteric and renal vascular beds of the mouse.

A new family of orphan G protein-coupled receptors predominantly expressed in the brain

The cloning of a cDNA encoding a G protein-coupled receptor homologous to the endothelin type B receptor, but unable to bind endothelin, was recently reported and termed ET(B)R-LP. We report here the isolation of a human cDNA encoding a receptor that is highly related to ET(B)R-LP and which was therefore termed ET(B)R-LP-2. Comparison of the two amino acid sequences revealed 68% overall homology and 48% identity. As is the case for ET(B)R-LP, the new receptor is strongly expressed in the human central nervous system (e.g. in cerebellar Bergmann glia, cerebral cortex, internal capsule fibers). Membranes of HEK-293 cells stably expressing ET(B)R-LP-2 did not bind endothelin-1, endothelin-2, endothelin-3, bombesin, cholecystokinin-8 or gastrin-releasing peptide.