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

Mechanism of protective actions of sparsentan in the kidney: lessons from studies in models of chronic kidney disease

Simultaneous inhibition of angiotensin II AT1 and endothelin ETA receptors has emerged as a promising approach for treatment of chronic progressive kidney disease. This therapeutic approach has been advanced by the introduction of sparsentan, the first dual AT1 and ETA receptor antagonist. Sparsentan is a single molecule with high affinity for both receptors. It is US Food and Drug Administration approved for immunoglobulin A nephropathy (IgAN) and is currently being developed as a treatment for rare kidney diseases, such as focal segmental glomerulosclerosis. Clinical studies have demonstrated the efficacy and safety of sparsentan in these conditions. In parallel with clinical development, studies have been conducted to elucidate the mechanisms of action of sparsentan and its position in the context of published evidence characterizing the nephroprotective effects of dual ETA and AT1 receptor inhibition. This review summarizes this evidence, documenting beneficial anti-inflammatory, antifibrotic, and hemodynamic actions of sparsentan in the kidney and protective actions in glomerular endothelial cells, mesangial cells, the tubulointerstitium, and podocytes, thus providing the rationale for the use of sparsentan as therapy for focal segmental glomerulosclerosis and IgAN and suggesting potential benefits in other renal diseases, such as Alport syndrome.

Renal Autocrine and Paracrine Signaling: A Story of Self-protection

Autocrine and paracrine signaling in the kidney adds an extra level of diversity and complexity to renal physiology. The extensive scientific production on the topic precludes easy understanding of the fundamental purpose of the vast number of molecules and systems that influence the renal function. This systematic review provides the broader pen strokes for a collected image of renal paracrine signaling. First, we recapitulate the essence of each paracrine system one by one. Thereafter the single components are merged into an overarching physiological concept. The presented survey shows that despite the diversity in the web of paracrine factors, the collected effect on renal function may not be complicated after all. In essence, paracrine activation provides an intelligent system that perceives minor perturbations and reacts with a coordinated and integrated tissue response that relieves the work load from the renal epithelia and favors diuresis and natriuresis. We suggest that the overall function of paracrine signaling is reno-protection and argue that renal paracrine signaling and self-regulation are two sides of the same coin. Thus local paracrine signaling is an intrinsic function of the kidney, and the overall renal effect of changes in blood pressure, volume load, and systemic hormones will always be tinted by its paracrine status.

A novel neuroregenerative approach using ET(B) receptor agonist, IRL-1620, to treat CNS disorders

Endothelin B (ET(B)) receptors present in abundance the central nervous system (CNS) have been shown to have significant implications in its development and neurogenesis. We have targeted ET(B) receptors stimulation using a highly specific agonist, IRL-1620, to treat CNS disorders. In a rat model of cerebral ischemia intravenous administration IRL-1620 significantly reduced infarct volume and improved neurological and motor functions compared to control. This improvement, in part, is due to an increase in neuroregeneration. We also investigated the role of IRL-1620 in animal models of Alzheimer's disease (AD). IRL-1620 improved learning and memory, reduced oxidative stress and increased VEGF and NGF in Abeta treated rats. IRL-1620 also improved learning and memory in an aged APP/PS1 transgenic mouse model of AD. These promising findings prompted us to initiate human studies. Successful chemistry, manufacturing and control along with mice, rat and dog toxicological studies led to completion of a human Phase I study in healthy volunteers. We found that a dose of 0.6 microg/kg of IRL-1620 can be safely administered, three times every four hours, without any adverse effect. A Phase II clinical study with IRL-1620 has been initiated in patients with cerebral ischemia and mild to moderate AD.

Endothelin and the renal microcirculation

Endothelin (ET) is one of the most potent renal vasoconstrictors. Endothelin plays an essential role in the regulation of renal blood flow, glomerular filtration, sodium and water transport, and acid-base balance. ET-1, ET-2, and ET-3 are the three distinct endothelin isoforms comprising the endothelin family. ET-1 is the major physiologically relevant peptide and exerts its biological activity through two G-protein-coupled receptors: ET(A) and ET(B). Both ET(A) and ET(B) are expressed by the renal vasculature. Although ET(A) are expressed mainly by vascular smooth muscle cells, ET(B) are expressed by both renal endothelial and vascular smooth muscle cells. Activation of the endothelin system, or overexpression of downstream endothelin signaling pathways, has been implicated in several pathophysiological conditions including hypertension, acute kidney injury, diabetic nephropathy, and immune nephritis. In this review, we focus on the effects of endothelin on the renal microvasculature, and update recent findings on endothelin in the regulation of renal hemodynamics.

The renal circulation in normal pregnancy and preeclampsia: is there a place for relaxin?

During the first trimester of human pregnancy, the maternal systemic circulation undergoes remarkable vasodilation. The kidneys participate in this vasodilatory response resulting in marked increases in renal plasma flow (RPF) and glomerular filtration rate (GFR). Comparable circulatory adaptations are observed in conscious gravid rats. Administration of the corpus luteal hormone relaxin (RLN) to nonpregnant rats and humans elicits vasodilatory changes like those of pregnancy. Systemic and renal vasodilation are compromised in midterm pregnant rats by neutralization or elimination of circulating RLN and in women conceiving with donor eggs who lack a corpus luteum and circulating RLN. Although RLN exerts both rapid (minutes) and sustained (hours to days) vasodilatory actions through different molecular mechanisms, a final common pathway is endothelial nitric oxide. In preeclampsia (PE), maternal systemic and renal vasoconstriction leads to hypertension and modest reduction in GFR exceeding that of RPF. Elevated level of circulating soluble vascular endothelial growth factor receptor-1 arising from the placenta is implicated in the hypertension and disruption of glomerular fenestrae and barrier function, the former causing reduced Kf and the latter proteinuria. Additional pathogenic factors are discussed. Last, potential clinical ramifications include RLN replacement in women conceiving with donor eggs and its therapeutic use in PE. Another goal has been to apply knowledge gained from investigating circulatory adaptations in pregnancy toward identifying and developing novel therapeutic strategies for renal and cardiovascular disease in the nonpregnant population. So far, one candidate to emerge is RLN and its potential therapeutic use in heart failure.

Effects of relaxin on arterial dilation, remodeling, and mechanical properties

Administering relaxin to conscious rats and humans elicits systemic and renal vasodilation. The molecular mechanisms vary according to the duration of relaxin exposure-so-called "rapid" (within minutes) or "sustained" (hours to days) vasodilatory responses-both being endothelium-dependent. Rapid responses are mediated by G(αi/o) protein coupling to phosphoinositol-3 kinase/Akt (protein kinase B)-dependent phosphorylation and activation of nitric oxide synthase. Sustained responses are mediated by vascular endothelial and placental growth factors, as well as increases in arterial gelatinase activity. Thus, after hours or days of relaxin treatment, respectively, arterial MMP-9 or MMP-2 hydrolyze "big" endothelin (ET) at a gly-leu bond to form ET(1-32), which in turn activates the endothelial ET(B) receptor/nitric oxide vasodilatory pathway. Administration of relaxin to conscious rats also increases global systemic arterial compliance and passive compliance of select isolated blood vessels such as small renal arteries (SRA). The increase in SRA passive compliance is mediated by both geometric remodeling (outward) and compositional remodeling (decreased collagen). Relaxin-induced geometric remodeling has also been observed in brain parenchymal arteries, and this remodeling appears to be via the activation of peroxisome proliferator-activated receptor-γ. Given the vasodilatory and arterial remodeling properties of relaxin, the hormone may have therapeutic potential in the settings of abnormal pregnancies, heart failure, and pathologies associated with stiffening of arteries.

Modulatory Role of Nitric Oxide/cGMP System in Endothelin-1-Induced Signaling Responses in Vascular Smooth Muscle Cells

Nitric oxide (NO) is an important vasoprotective molecule that serves not only as a vasodilator but also exerts antihypertrophic and antiproliferative effects in vascular smooth muscle cells (VSMC). The precise mechanism by which the antihypertrophic and antiproliferative responses of NO are mediated remains obscure. However, recent studies have suggested that one of the mechanisms by which this may be achieved includes the attenuation of signal transduction pathways responsible for inducing the hypertrophic and proliferative program in VSMC. Endothelin-1 is a powerful vasoconstrictor peptide with mitogenic and growth stimulatory properties and exerts its effects by activating multiple signaling pathways which include ERK 1/2, PKB and Rho-ROCK. Both cGMP-dependent and independent events have been reported to mediate the effect of NO on these pathways leading to its vasoprotective response. This review briefly summarizes some key studies on the modulatory effect of NO on these signaling pathways and discusses the possible role of cGMP system in this process.

Maternal vasodilation in pregnancy: the emerging role of relaxin

Pregnancy is a unique physiological condition of profound maternal renal and systemic vasodilation. Our goal has been to unveil the reproductive hormones mediating this remarkable vasodilatory state and the underlying molecular mechanisms. In addition to advancing our knowledge of pregnancy physiology, reaching this goal may translate into therapeutics for pregnancy pathologies such as preeclampsia and for diseases associated with vasoconstriction and arterial stiffness in nonpregnant women and men. An emerging player is the 6 kDa corpus luteal hormone relaxin, which circulates during pregnancy. Relaxin administration to rats and humans induces systemic and renal vasodilation regardless of sex, thus mimicking the pregnant condition. Immunoneutralization or elimination of the source of circulating relaxin prevents renal and systemic vasodilation in midterm pregnant rats. Infertile women who become pregnant by donor eggs (IVF with embryo transfer) lack a corpus luteum and circulating relaxin, and they show a markedly subdued gestational increase in glomerular filtration rate. These data implicate relaxin as one of the vasodilatory reproductive hormones of pregnancy. There are different molecular mechanisms underlying the so-called rapid and sustained vasodilatory actions of relaxin. The former is mediated by Gα(i/o) protein coupling to phosphatidylinositol-3 kinase/Akt (protein kinase B)-dependent phosphorylation and activation of endothelial nitric oxide synthase, the latter by vascular endothelial and placental growth factors, and increases in arterial gelatinase(s) activity. The gelatinases, in turn, hydrolyze big endothelin (ET) at a gly-leu bond to form ET(1-32), which activates the endothelial ET(B) receptor/nitric oxide vasodilatory pathway.

The interaction between endothelin-1 and nitric oxide in the vasculature: new perspectives

Nitric oxide (NO) and endothelin-1 (ET-1) are natural counterparts in vascular function, and it is becoming increasingly clear that an imbalance between these two mediators is a characteristic of endothelial dysfunction and is important in the progression of vascular disease. Here, we review classical and more recent data that suggest that ET-1 should be regarded as an essential component of NO signaling. In particular, we review evidence of the role of ET-1 in models of acute and chronic NO synthase blockade. Furthermore, we discuss the possible mechanisms by which NO modulates ET-1 activity. On the basis of these studies, we suggest that NO tonically inhibits ET-1 function, and in conditions of diminished NO bioavailability, the deleterious effects of unmitigated ET-1 actions result in vasoconstriction and eventually lead to vascular remodeling and dysfunction.

ADP-ribosyl cyclase and ryanodine receptors mediate endothelin ETA and ETB receptor-induced renal vasoconstriction in vivo

ADP-ribosyl cyclase (ADPR cyclase) and ryanodine receptors (RyR) participate in calcium transduction in isolated afferent arterioles. We hypothesized that this signaling pathway is activated by ETA and ETB receptors in the renal vasculature to mediate vasoconstriction in vivo. To test this, we measured acute renal blood flow (RBF) responses to ET-1 in anesthetized rats and mice in the presence and absence of functional ADPR cyclase and/or RyR. Inhibitors of ADPR cyclase (nicotinamide) or RyR (ruthenium red) reduced RBF responses to ET-1 by 44% (P < 0.04 for both) in Sprague-Dawley rats. Mice lacking the predominant form of ADPR cyclase (CD38-/-) had RBF responses to ET-1 that were 47% weaker than those seen in wild-type mice (P = 0.01). Selective ETA receptor stimulation (ET-1+BQ788) produced decreases in RBF that were attenuated by 43 and 56% by nicotinamide or ruthenium red, respectively (P < 0.02 for both). ADPR cyclase or RyR inhibition also reduced vasoconstrictor effects of the ETB receptor agonist sarafotoxin 6c (S6c; 77 and 54%, respectively, P < 0.02 for both). ETB receptor stimulation by ET-1 + the ETA receptor antagonist BQ123 elicited responses that were attenuated by 59 and 60% by nicotinamide and ruthenium red, respectively (P < 0.01 for both). Nicotinamide attenuated RBF responses to S6c by 54% during inhibition of nitric oxide synthesis (P = 0.001). We conclude that in the renal microcirculation in vivo 1) ET-1-induced vasoconstriction is mediated by ADPR cyclase and RyR; 2) both ETA and ETB receptors activate this pathway; and 3) ADPR cyclase participates in ETB receptor signaling independently of NO.

NO and NO-independent mechanisms mediate ETBreceptor buffering of ET-1-induced renal vasoconstriction in the rat

Vascular endothelin (ET) type B (ETB) receptors exert dilator and constrictor actions in a complex interaction with ETAreceptors. We aimed to clarify the presence and relative importance of nitric oxide (NO) and other mechanisms underlying the dilator effects of ETBreceptors in rat kidneys. Complete inhibition of NO production with Nω-nitro-l-arginine methyl ester (l-NAME, 25 mg/kg iv) enhanced the renal vasoconstriction elicited by ET-1 injected into the renal artery from −15 to −30%. Additional infusion of the NO donor nitroprusside (NP) into the renal artery did not reverse this effect (−29%) but effectively buffered ANG II-mediated vasoconstriction. Similarly, ET-1 responses were enhanced after a smaller intrarenal dose of l-NAME (−22 vs. −15%) and were unaffected by subsequent NP infusion (−21%). These results indicate that the responsiveness to ET-1 is buffered by ETBreceptor-stimulated phasic release of NO, rather than its static mean level. Infusion of the ETBreceptor antagonist BQ-788 into the renal artery further enhanced the ET-1 constrictor response to NP + l-NAME (−92 vs. −49%), revealing an NO-independent dilator component. In controls, vasoconstriction to ET-1 was unaffected by vehicle (−27 vs. −20%) and markedly enhanced by BQ-788 (−70%). The same pattern was observed when indomethacin (Indo) was used to inhibit cyclooxygenase (−20% for control, −22% with Indo, and −56% with ETBantagonist) or methylsulfonyl-6-(2-propargyloxyphenyl)-hexanamide (MS-PPOH) or miconazole + Indo was used to inhibit epoxygenase alone (−10% for control, −11% with MS-PPOH, and −35% with ETBantagonist) or in combination (−14% for control, −20% with Indo + miconazole, and −43% with ETBantagonist). We conclude that phasic release of NO, but not its static level, mediates part of the dilator effect of ETBreceptors and that an NO-independent mechanism, distinct from prostanoids and epoxyeicosatetraenoic acids, perhaps ETBreceptor clearance of ET-1, plays a major buffering role.

Emerging role of relaxin in renal and cardiovascular function

Although traditionally associated with reproductive processes, relaxin is emerging as an important player in renal and cardiovascular function. Much of our recently acquired understanding of relaxin in this new context has arisen from studies of maternal renal and cardiovascular adaptations to pregnancy in rats where the hormone is turning out to be an important mediator. First, we highlight the influence of relaxin on renal hemodynamics and glomerular filtration rate, as well as on other peripheral circulations. Second, we discuss the effect of relaxin on both the steady and pulsatile systemic arterial load, as well as on the heart, in particular, coronary blood flow. Third, we consider the impact of the hormone on cultured endothelial and vascular smooth muscle cells. Fourth, we address the interaction of relaxin with renal and cardiac disease, as well as its role in angiogenesis. Finally, in Perspectives, we point out several key research questions in need of investigation that relate to a potential autocrine/paracrine role of relaxin in renal and cardiovascular tissues. Furthermore, on the basis of its potent vasodilatory and matrix-degrading attributes, we speculate about the therapeutic potential of relaxin in renal and cardiovascular diseases.

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.

Effects of the novel selective endothelin ET(A) receptor antagonist, SB 234551, on the cardiovascular responses to endotoxaemia in conscious rats

1. In conscious, freely moving, male, Long Evans rats, regional haemodynamic responses to exogenous endothelin-1 (ET-1; 25, 50 and 250 pmol kg(-1) i.v.) were assessed in the presence of vehicle, or the selective ET(A)-receptor antagonist, SB 234551. On the following day, the effects of SB 234551 on the haemodynamic responses to lipopolysaccharide (LPS) infusion (150 microg kg(-1) h(-1), i.v.) were determined. 2. When SB 234551 was given i.v. by primed infusion at a dose of 0.3 mg kg(-1) bolus, 0.3 mg kg(-1) h(-1) infusion, it caused selective inhibition of the vasoconstrictor effects of exogenous endothelin-1, whereas at a dose of 1 mg kg(-1), 1 mg kg(-1) h(-1), SB 234551 also inhibited some of the vasodilator effects of endothelin-1. 3. Infusion of LPS, in the presence of vehicle, caused a short-lived (1 - 2 h) hypotension, tachycardia, and vasodilatation in renal, superior mesenteric and hindquarters vascular beds. Thereafter, blood pressure, heart rate and mesenteric vascular conductance returned to baseline values, but renal vasodilatation persisted, and there was vasoconstriction in the hindquarters. 4. In the presence of SB 234551 (0.3 mg kg(-1), 0.3 mg kg(-1) h(-1)), the early (1 - 2 h) cardiovascular responses to LPS infusion were unaffected, but the subsequent recovery of mean arterial blood pressure was impaired, due to developing vasodilatation in the mesenteric and, to a lesser extent, hindquarters, vascular beds. SB 234551 had no effect on the renal haemodynamic responses to LPS infusion. 5. The results confirm an important, regionally-selective, vasoconstrictor role for endogenous endothelin in this model of endotoxaemia.

Cardiovascular effects of endothelin-1 and endothelin antagonists in conscious, hypertensive ((mRen-2)27) rats

SB 209670 is a potent antagonist of the vasoconstrictor (ET(A)- and ET(B)-receptor-mediated) and vasodilator (ET(B)-receptor-mediated) effects of endothelin, whereas SB 234551 is relatively selective for the constrictor (ET(A)-receptor-mediated) effects. Since we had previously found SB 209670 exerted antihypertensive, vasodilator effects in conscious, heterozygous, transgenic ((mRen-2)27) (abbreviated to TG) rats, here we compared the two antagonists in that model, and assessed their chronic effects on responses to exogenous endothelin-1. We did this to test our global hypothesis, namely, that SB 209670, but not SB 234551, would cause inhibition of the depressor effects of exogenous endothelin-1 in vivo, and that this differential effect would be associated with a more marked antihypertensive action of SB 234551 in TG rats. SB 209670 and SB 234551 (infused for 50 h) exerted similar, sustained, antihypertensive effects in TG rats. The antihypertensive effects of the antagonists occurred at times when the pressor effects of exogenous endothelin-1 were not significantly inhibited. Furthermore, SB 234551 did not exert a greater antihypertensive effect than SB 209670 at a time (i.e., 2 - 4 h) when the depressor effects of endothelin-1 were abolished by the latter, but not by the former (although this differential action was lost after 24 h infusion). The results caused us to reject the hypothesis that selective antagonism of the vasoconstrictor effects of endothelin-1 would result in SB 234551 exerting a greater antihypertensive effect than SB 209670 in TG rats.

Impact of gender and endothelin on renal vasodilation and hyperfiltration induced by relaxin in conscious rats

Chronic administration of the hormone relaxin elicits renal vasodilation that is dependent on nitric oxide (NO) in both conscious intact and ovariectomized female rats. Our first objective was to test whether the hormone, when administered to approximate serum concentrations found in midterm pregnant rats, induces renal vasodilation in males. Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) increased significantly, on average, by 33 and 49% over baseline, respectively, after 5 days of recombinant human relaxin (rhRLX) administration to 12 conscious male rats by subcutaneous osmotic minipump. There were also significant decreases in hematocrit, plasma osmolality, and sodium concentration. Another objective was to determine whether endogenous endothelin (ET; via the endothelial ET(B) receptor) mediates the NO-dependent renal vasodilation produced by relaxin. rhRLX or vehicle was administered to conscious female rats (n = 9 and 8 rats, respectively). On the fifth day, baseline GFR and ERPF were both increased, on average, by 20-30% in the rats administered rhRLX (P < 0.05 vs. vehicle). Next, the specific ET(B)-receptor antagonist RES-701-1 was infused intravenously over 4 h in both groups of rats. In response to RES-701-1, there was a significant decline in both GFR and ERPF in the rats receiving rhRLX such that renal function converged in the two groups of animals. We conclude 1) relaxin induces marked changes in the renal circulation and in osmoregulation regardless of gender and 2) relaxin-induced renal vasodilation and hyperfiltration are mediated by endothelin through the endothelial ET(B) receptor subtype and NO.

Influence of endothelins and sarafotoxin 6c and L-NAME on renal vasoconstriction in the anaesthetized rat

1. An investigation was performed in pentobarbitone anaesthetized rats to compare the renal vasoconstrictor actions of endothelin-1 (ET-1), endothelin-3 (ET-3) and sarafotoxin 6c and their dependency on NO production. 2. Intra-renal arterial infusion of ET-1 and ET-3, from 1 - 1000 ng had no effect on blood pressure, but reduced renal blood flow maximally by 82 and 81% with EC50 values of 510+/-18 and 1113+/-17 ng, respectively and correspondingly increased renal vascular resistance and decreased conductance. 3. Direct renal arterial administration of sarafotoxin 6c was without effect on blood pressure but caused a maximum reduction in renal blood flow of 56% at 300 ng and had an EC50 of 86+/-4 ng. 4. Administration of the selective ETA receptor antagonist FR139317 at 0.3 and 1.0 mg kg-1 had no effect on basal levels of blood pressure, renal vascular resistance or renal blood flow. The lower dose of FR139317 had no effect on the ET-1 dose-response curve for renal blood flow while at 1.0 mg kg-1, FR139317 reduced the EC50 to 363+/-32 ng (P<0.05). 5. Infusion of L-NAME, 10 microg kg-1 min-1 increased blood pressure by approximately 15%, increased renal vascular resistance and decreased renal blood flow by some 40%. The EC50 values for renal blood flow were reduced to 358+/-68 ng (P<0.05) for ET-1, 638+/-69 ng (P<0.05) for ET-3 and 55+/-10 ng (P<0.01) for sarafotoxin 6c. The maximal reduction in renal blood flow induced by sarafotoxin 6c was raised (P<0.01) from 56% to approximately 100% and renal vascular resistance increased when NO production was blocked. 6. These results showed that the vasoconstrictor actions of ET-1 and ET-3 on resistance vessels controlling renal blood flow are mediated via ETB rather than ETA receptors. Moreover, both ET-1 and ET-3 dependent vasoconstrictions are slightly attenuated by concomitant NO production. By contrast, sarafotoxin 6c appears much more potent at the renal resistance vasculature and is much more powerfully modulated by NO.

Cloning and characterization of a novel endothelin receptor subtype in the avian class

Endothelin 3 (EDN 3) and the endothelin receptor B (EDNRB) are involved in the development of neural crest and particularly of the melanocytes and the enteric nervous system. We reported previously that the avian EDNRB gene is expressed in the neural fold before crest cell migration and later on in all the neural crest derivatives except, at any developmental stage, in the melanocytic lineage. However, quail melanoblasts proliferate in response to EDN 3 stimulation in vitro. These observations prompted us to search for another type of endothelin receptor (EDNR). We report here the cloning by reverse transcriptase-PCR of an avian cDNA encoding a subtype of EDNR, which we have called EDNRB2, because its deduced amino acid sequence is more closely related to that of EDNRB than to either the mammalian EDNRA or to the Xenopus EDNRC. Its expression pattern differs from that of the "classical" avian EDNRB because it is strongly expressed in melanoblasts and melanocytes. EDNRB2 transcripts are also abundant in the liver and kidney. Our pharmacological studies showed that EDNRB2 binds with similar affinity to EDN 1, EDN 2, and EDN 3, further confirming that this receptor belongs to the B type, although it displays a low affinity for sarafotoxin-c, a known EDNRB-selective agonist.