Extrarenal ETBPlays a Significant Role in Controlling Cardiovascular Responses to High Dietary Sodium in Rats

Endothelin B receptors impair baroreflex function and increase blood pressure variability during high salt diet

Baroreflex function is an integral component maintaining consistent blood pressure. Hypertension is often associated with baroreflex dysfunction, and environmental risk factors such as high salt diet exacerbate hypertension in subjects with baroreflex dysfunction. However, the interactions between high salt diet, baroreflex dysfunction, and hypertension are incompletely understood. The endothelin system is another potent mediator of blood pressure control especially in response to a high salt diet. We hypothesized that the endothelin B (ETB) receptor activation on adrenergic nerves decreases baroreflex sensitivity. We utilized male ETB receptor deficient (ETB-def) rats that express functional ETB receptors only on adrenergic nerves and transgenic (TG) controls to evaluate baroreflex function during normal (0.49% NaCl) and high (4.0% NaCl) salt diets. In conscious rats equipped with telemetry, ETB-def rats had an increased lability of systolic blood pressure (SBP) compared to TG controls as indicated by higher standard deviation (SD) of SBP under both normal (10.2 ± 0.6 vs. 12.4 ± 0.9 mmHg, respectively, p = 0.0001) and high (11.7 ± 0.6 vs. 16.1 ± 1.0 mmHg, p = 0.0001) salt diets. In anesthetized preparations, ETB-def rats displayed reduced heart rate (p genotype = 0.0167) and renal sympathetic nerve (p genotype = 0.0022) baroreflex sensitivity. We then gave male Sprague-Dawley rats the selective ETB receptor antagonist, A-192621 (10 mg/kg/day), to block ETB receptors. Following ETB receptor antagonism, even though SBP increased (131 ± 7 before vs. 152 ± 8 mmHg after, p < 0.0001), the lability (standard deviation) of SBP decreased (9.3 ± 2.0 vs. 7.1 ± 1.1 mmHg, p = 0.0155). These data support our hypothesis that ETB receptors on adrenergic nerves contribute to baroreflex dysfunction.

Renal denervation attenuates hypertension but not salt sensitivity in ETB receptor-deficient rats

Hypertension is a prevalent pathology that increases risk for numerous cardiovascular diseases. Because the etiology of hypertension varies across patients, specific and effective therapeutic approaches are needed. The role of renal sympathetic nerves is established in numerous forms of hypertension, but their contribution to salt sensitivity and interaction with factors such as endothelin-1 are poorly understood. Rats deficient of functional ET_B receptors (ET_B-def) on all tissues except sympathetic nerves are hypertensive and exhibit salt-sensitive increases in blood pressure. We hypothesized that renal sympathetic nerves contribute to hypertension and salt sensitivity in ET_B-def rats. The hypothesis was tested through bilateral renal sympathetic nerve denervation and measuring blood pressure during normal salt (0.49% NaCl) and high-salt (4.0% NaCl) diets. Denervation reduced mean arterial pressure in ET_B-def rats compared with sham-operated controls by 12 ± 3 (SE) mmHg; however, denervation did not affect the increase in blood pressure after 2 wk of high-salt diet (+19 ± 3 vs. +16 ± 3 mmHg relative to normal salt diet; denervated vs. sham, respectively). Denervation reduced cardiac sympathetic-to-parasympathetic tone [low frequency-high frequency (LF/HF)] during normal salt diet and vasomotor LF/HF tone during high-salt diet in ET_B-def rats. We conclude that the renal sympathetic nerves contribute to the hypertension but not to salt sensitivity of ET_B-def rats.

Association of oral endothelin receptor antagonists with risks of cardiovascular events and mortality: meta-analysis of randomized controlled trials

BACKGROUND

Endothelin receptor antagonists (ERAs) are widely used in a variety of disorders, including pulmonary artery hypertension, systemic sclerosis, diabetic and kidney diseases, and several tumors. However, reported adverse events, especially increased risks of cardiovascular disease (CVD) morbidity and mortality, have cast doubt on their potential clinical application. Therefore, we conducted this meta-analysis to confirm whether ERAs increased CVD risk and mortality.

METHODS

We systematically searched PubMed (1966-2015), EMBASE (1974-2015), ClinicalTrials.gov, and the Cochrane Controlled Clinical Trials Register Database for randomized controlled trials published between Jan 1, 1990 and Mar 18, 2015. Inclusion criteria included a study duration of more than 3 weeks, the use of a randomized control group receiving an oral ERA or placebo, and the availability of outcome data for cardiovascular events and all-cause death.

RESULTS

A total of 33 trials met the inclusion criteria. There were 8098 cases in the ERA group and 5074 cases in the placebo group. Compared with the control group, the risk ratio (RR) for all-cause death in the ERA group was 0.983 [95% confidence interval (CI), 0.883 to 1.094, P = 0.754]. The summary RR for cardiovascular events was 1.651 in the ERA group (95% CI, 1.164 to 2.34, P = 0.005). The pooled results showed that ERAs treatment could lead to more edema, anemia, and abnormal transaminase levels. Also, there was an increased proportion of discontinued therapy in the ERA treatment because of side effects (RR = 1.322, 95% CI, 1.036 to 1.686, P = 0.025). There were no significant differences in the experienced episodes of headache and dyspnea between the active therapy and control groups.

CONCLUSIONS

ERAs therapy is not significantly associated with increased all-cause death, but there are more cardiovascular events and edema or fluid retention, anemia, and liver enzymes disorder. Large clinical randomized controlled studies are needed to further confirm the safety of the clinical application of ERAs.

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.

Antidiuretic effects of the endothelin receptor antagonist avosentan

Several clinical studies have investigated the potential benefits of endothelin receptor antagonism in chronic pathologies such as diabetic kidney disease. However, fluid retention and edema have been identified as major side effects of endothelin receptor antagonists. In the present study we hypothesized that avosentan which was described as a predominant ET_A receptor antagonist would produce fluid retention at high concentrations where non-specific blockade of ET_B receptors may occur. Incremental doses of the predominant ET_A receptor antagonist SPP301 (0.003; 0.03; 3 mg/kg) were administered intravenously to anesthetized Sprague-Dawley rats undergoing saline diuresis. Diuresis, glomerular filtration rate, and blood pressure (BP) were monitored. SPP301 decreased urine output (5.6; 34.8; 58.8% decrease from vehicle) and fractional excretion of water (5.7; 31.7; 56.4% decrease from vehicle) in a concentration-dependent manner. Glomerular filtration rate was unchanged while BP was reduced by 10 mmHg only by the highest dose of SPP301. Administration of the ET_B selective receptor antagonist BQ-788 (3 mg/kg) following SPP301 3 mg/kg did not further decrease urine output or water excretion and was without effect on glomerular filtration rate. These data indicate that increasing concentrations of SPP301 may also block ET_B receptors and cause antidiuresis. This effect could explain why fluid retention and edema occur during treatment with predominant ET_A receptor blockers.

Regulation of blood pressure and salt homeostasis by endothelin

Endothelin (ET) peptides and their receptors are intimately involved in the physiological control of systemic blood pressure and body Na homeostasis, exerting these effects through alterations in a host of circulating and local factors. Hormonal systems affected by ET include natriuretic peptides, aldosterone, catecholamines, and angiotensin. ET also directly regulates cardiac output, central and peripheral nervous system activity, renal Na and water excretion, systemic vascular resistance, and venous capacitance. ET regulation of these systems is often complex, sometimes involving opposing actions depending on which receptor isoform is activated, which cells are affected, and what other prevailing factors exist. A detailed understanding of this system is important; disordered regulation of the ET system is strongly associated with hypertension and dysregulated extracellular fluid volume homeostasis. In addition, ET receptor antagonists are being increasingly used for the treatment of a variety of diseases; while demonstrating benefit, these agents also have adverse effects on fluid retention that may substantially limit their clinical utility. This review provides a detailed analysis of how the ET system is involved in the control of blood pressure and Na homeostasis, focusing primarily on physiological regulation with some discussion of the role of the ET system in hypertension.

Down-regulation of FKBP12.6 and SERCA2a contributes to acute heart failure in septic shock and is related to an up-regulated endothelin signalling pathway

Acute heart failure (AHF) critically affects morbidity and mortality in patients suffering from septic shock. It is hypothesized that AHF is linked to down-regulation of FKBP12.6 (calstabin 2) and SERCA2a (sarco/endoplasmic reticulum Ca2+ ATPase 2a), which may be mediated by an activated endothelin (ET) system in the myocardium. The aim of the study was to test whether an attenuation of septic AHF can be achieved by a novel dual endothelin receptor antagonist, CPU0213, in association with up-regulation of FKBP12.6 and SERCA2a in rats. AHF in septic shock was produced by faeces leak from a surgically punctured caecum for 72 h in rats. CPU0213 (30 mg kg−1, s.c., every 12 h, for 3 days) was administered to rats 8 h after the operation. In the untreated model group, survival rate markedly decreased (P &lt; 0.01), and the cardiac performance was seriously compromised (P &lt; 0.01) relative to control. The AHF was characteristically associated with down-regulated mRNA and protein expressions of FKBP12.6, SERCA2a and PLB (phospholamban). Elevated ET-1 and mRNA abundances of the preproET-1, ECE (endothelin converting enzyme) and ETA and ETB receptors in the left ventricular tissue (P &lt; 0.01) were found. All abnormalities were reversed significantly following CPU0213 administration. In conclusion, septic AHF is attributed to down-regulation of FKBP12.6 and SERCA2a, which is related to an activated ET system. An endothelin receptor antagonism of CPU0213 significantly improves the cardiac performance by blocking both ETA and ETB 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.

Pathophysiology of the endothelin system - lessons from genetically manipulated animal models

Shortly after discovery of ET-1 in 1988, the entire endothelin system was characterized. The endothelin system consists of the three peptides ET-1, ET-2 and ET-3, their G-protein-coupled receptors endothelin receptor A and B (ETRA and ETRB) and the two endothelin-converting enzymes (ECE-1 and ECE-2). Genetically modified animal models are an important tool in biomedical research. Here we describe the key findings obtained from genetically modified animal models either over-expressing compounds of the ET system or lacking these compounds (knockout mice). Results from the different transgenic and knockout models disclose that the ET system plays a major role in embryonic development. Two ET system-dependent neural crest-driven developmental pathways become obvious: one of them being an ET-1/ETAR axis, responsible for cardio-renal function and development as well as cranial development; the other seems to be an ET-3/ETBR mediated signalling pathway. Mutations within this axis are associated with disruptions in epidermal melanocytes and enteric neurons. These findings led to the discovery of similar findings in humans with Hirschsprung disease. In adult life the ET system is most important in the cardiovascular system and plays a role in fibrotic remodelling of the heart, lung and kidney as well as in the regulation of water and salt excretion.

Endothelin-1 and hypertension: from bench to bedside

Endothelin-1 (ET-1) exerts a wide range of biologic effects that can influence systemic blood pressure. Recent studies indicate that increased activity of the ET system in the vasculature, with resultant activation of primarily ET A receptors, can contribute to hypertension. In contrast, decreased production of ET-1 in the renal medulla, and reduced activation of collecting duct ET B receptors, can also elevate systemic blood pressure. Both ET A and combined A/B receptor blockers reduce blood pressure in hypertensive patients. Several important questions remain with respect to the ET system in hypertension, including how ET receptor antagonists will interact with other antihypertensive agents, which receptor subtypes should be targeted, and what the effect of ET blockade will be on hypertension-related end-organ damage as opposed to blood pressure alone.

[Endothelin-1 production and its involvement in cardiovascular diseases]

Endothelin (ET) has been implicated in the pathogenesis of several cardiovascular disorders because of its powerful vasoconstrictor and growth-promoting properties. The ET family consists of three isoforms, ET-1, ET-2 and ET-3. ET-1 appears to be the predominant member of the family generated by vascular endothelial cells. In view of the multiple cardiovascular actions of ET-1, there has been much interest in its contribution to the pathophysiology of hypertension and arteriosclerosis. We have been investigating the roles of ET(A) and ET(B) receptors in ET-1-related cardiovascular diseases using subtype-selective ET receptor antagonists and ET(B) receptor-deficient animals. Our studies have demonstrated that ET-1 overproduction and ET(A)-mediated ET-1 actions seem to play a crucial role in the development of several types of hypertensive and post-ischemic diseases. On the other hand, ET-1 biosynthesis and release are regulated at the transcriptional level, and various endogenous substances are known to stimulate ET-1 gene expression by DNA binding of transcription factors. We and others have recently demonstrated that nuclear factor-kappaB (NF-kappaB), a transcription factor with a pivotal role in inducing genes involved in immune, inflammatory and stress responses, is responsible for endothelial ET-1 production. In in vivo studies, agents that can inhibit the NF-kappaB activation improved the development of ET-1-related cardiovascular diseases. Thus, NF-kappaB inhibition may be a pertinent treatment for ET-1 related diseases.

Pathophysiological Mechanisms of Salt-Dependent Hypertension

Changes in salt intake are associated in general with corresponding changes in arterial blood pressure. An exaggerated increment in blood pressure driven by a salt load is characteristic of salt-sensitive hypertension, a condition affecting more than two thirds of individuals with essential hypertension who are older than 60 years. In the last decade, significant insight was gained about the role of the kidney in the increment in blood pressure induced by sodium retention. The present review focuses on the pathophysiological characteristics of the blood pressure increase driven by expansion of extracellular fluid and the increment in plasma sodium concentration. In addition, we discuss systemic and renal conditions that result in decreased urinary sodium excretion and were implicated in the development of salt-sensitive hypertension.

Chronic Activation of Endothelin B Receptors

Endothelin (ET) exerts powerful pressor actions primarily through activation of the ET(A) receptor subtype. The ET(B) receptor (ET(B)R) subtype, on the other hand, is generally thought to initiate physiological actions that decrease arterial pressure. Such actions include clearing ET from the bloodstream, initiating endothelium-mediated vasodilation, and facilitating renal sodium and water excretion. The effect of long-term activation of the ET(B)R on arterial pressure, however, never has been directly tested. In this study we evaluated cardiovascular responses to chronic (5-day) activation of ET(B)R in male rats using continuous intravenous infusion of the selective agonist sarafotoxin 6c. Surprisingly, we found that sarafotoxin 6c caused a sustained increase in arterial pressure that rapidly reversed on termination of infusion. The hypertension was associated with increased renal excretion of sodium and water and decreased plasma volume. Alterations in daily sodium intake did not affect the magnitude of the hypertension. Hemodynamic studies revealed a decreased cardiac output and increased total peripheral resistance during sarafotoxin 6c infusion. Infusion of sarafotoxin 6c caused a small increase in plasma ET levels. Nevertheless, the hypertension was not affected by coadministration of a selective ET(A) receptor antagonist (atrasentan) but was completely prevented by treatment with a combined ET(A) receptor and ET(B)R antagonist (A186280). These experiments reveal for the first time that chronic activation of ET(B)R in rats causes sustained hypertension.

Attenuated vasoconstrictor responses to endothelin in afferent arterioles during a high-salt diet

Endothelin-1 (ET-1) is increased in rats on a high-salt (HS) diet and participates in salt-dependent hypertension. Afferent arterioles (AA) are important for long-term blood pressure control, and therefore we hypothesized that a HS diet would alter their responsiveness to ET-1. Sprague-Dawley rats were fed either a normal-salt (NS; 0.66% NaCl) or HS (8%) diet for 1 wk. Diameters of AA were determined in response to increasing concentrations of big ET-1, ET-1, sarafotoxin 6c (S6c), or norepinephrine (NE), using the blood-perfused juxtamedullary nephron technique. ET-1 responses were also determined during blockade of endothelin type A (ETA) or type B (ETB) receptors with 10 nM ABT-627 or 30 nM A-192621, respectively. Expression of ETA and ETB receptors was determined in renal microvessels. Responses of AA to big ET-1, ET-1, and S6c were significantly attenuated during a HS diet (e.g., response to 10−10 M ET-1 in NS vs. HS rats: −52.5 ± 10.2 vs. +5.6 ± 11.3% of control diameter; P < 0.05), with no change in the responses to NE. ETB, but not ETA receptor blockade abolished the different response to ET-1 between the NS and HS groups. ETB receptor expression in renal microvessels was increased in response to HS (17.7 ± 2.4 vs. 6.6 ± 3.0% of β-actin, P = 0.02), whereas ETA receptor expression was unchanged. These results suggest that the reduced vasoconstrictor response of AA to endothelin peptides during a HS diet is mediated by increased vasodilatory function of endothelial ETB receptors. By preserving renal blood flow, this may be an important mechanism to restore sodium balance during a HS diet.

Pathogenesis of salt sensitivity of blood pressure

The effect of sodium in human blood pressure has been a contentious subject of considerable debate for decades. Nonetheless, it is generally conceded that there is heterogeneity to alterations in sodium and extracellular fluid volume in the blood pressure responses of normal and hypertensive humans. Although there are many forms of experimental and clinical hypertension that are clearly related to abnormalities of sodium handling and metabolism, it has been only the advent of the genetic revolution that has provided critical new insight into the mechanisms involved in many of these conditions. In this review, the clinical manifestations of salt sensitivity and several clinical syndromes associated with abnormal sodium metabolism are discussed, and factors that appear to be involved in many of the clinical abnormalities as well as relevant new insights derived from basic research are elucidated.

Early-onset endothelin receptor blockade in hypertensive heterozygous Ren-2 rats

Male heterozygous Ren-2 transgenic rats and Hannover Sprague-Dawley rats fed a normal or high-salt diet were either untreated or treated with the nonselective receptor ET(A)/ET(B) receptor blocker bosentan or the selective ET(A) receptor blocker, ABT-627, known as atrasentan. Survival rate was partly increased by bosentan and fully normalized by atrasentan. Bosentan did not significantly influence the course of hypertension in TGR, whereas atrasentan significantly decreased BP on both diets. Atrasentan substantially reduced proteinuria, cardiac hypertrophy, glomerulosclerosis and left ventricular ET-1 tissue concentration on both diets. Our data indicate that ET(A) receptor blockade is superior to nonselective blockade in attenuating hypertension, end-organ damage and improving survival rate.

Collecting duct-specific knockout of the endothelin B receptor causes hypertension and sodium retention

Collecting duct (CD)-derived endothelin-1 (ET-1) inhibits renal Na reabsorption and its deficiency increases blood pressure (BP). The role of CD endothelin B (ETB) receptors in mediating these effects is unknown. CD-specific knockout of the ETB receptor was achieved using an aquaporin-2 promoter-Cre recombinase transgene and the loxP-flanked ETB receptor gene (CD ETB KO). Systolic BP in mice with CD-specific knockout of the ETB receptor, ETA receptor (CD ETA KO) and ET-1 (CD ET-1 KO), and their respective controls were compared during normal- and high-salt diet. On a normal-sodium diet, CD ETB KO mice had elevated BP, which increased further during high salt feeding. However, the degree of hypertension in CD ETB KO mice and the further increase in BP during salt feeding were lower than that of CD ET-1 KO mice, whereas CD ETA KO mice were normotensive. CD ETB KO mice had impaired sodium excretion following acute sodium loading. Aldosterone and plasma renin activity were decreased in CD ETB KO mice on normal- and high-sodium diets, while plasma and urinary ET-1 levels did not differ from controls. In conclusion, the CD ETB receptor partially mediates the antihypertensive and natriuretic effects of ET-1. CD ETA and ETB receptors do not fully account for the antihypertensive and natriuretic effects of CD-derived ET-1, suggesting paracrine effects of this peptide.

Differential effects of endothelin on activation of renal mechanosensory nerves: stimulatory in high-sodium diet and inhibitory in low-sodium diet

Activation of renal mechanosensory nerves is enhanced by high and suppressed by low sodium dietary intake. Afferent renal denervation results in salt-sensitive hypertension, suggesting that activation of the afferent renal nerves contributes to water and sodium balance. Another model of salt-sensitive hypertension is the endothelin B receptor (ETBR)-deficient rat. ET and its receptors are present in sensory nerves. Therefore, we examined whether ET receptor blockade altered the responsiveness of the renal sensory nerves. In anesthetized rats fed high-sodium diet, renal pelvic administration of the ETBR antagonist BQ-788 reduced the afferent renal nerve activity (ARNA) response to increasing renal pelvic pressure 7.5 mmHg from 26+/-3 to 9+/-3% and the PGE2-mediated renal pelvic release of substance P from 9+/-1 to 3+/-1 pg/min. Conversely, in rats fed low-sodium diet, renal pelvic administration of the ETAR antagonist BQ-123 enhanced the ARNA response to increased renal pelvic pressure from 9+/-2 to 23+/-6% and the PGE2-mediated renal pelvic release of substance P from 0+/-0 to 6+/-1 pg/min. Adding the ETAR antagonist to ETBR-blocked renal pelvises restored the responsiveness of renal sensory nerves in rats fed a high-sodium diet. Adding the ETBR antagonist to ETAR-blocked pelvises suppressed the responsiveness of the renal sensory nerves in rats fed a low-sodium diet. In conclusion, activation of ETBR and ETAR contributes to the enhanced and suppressed responsiveness of renal sensory nerves in conditions of high- and low-sodium dietary intake, respectively. Impaired renorenal reflexes may contribute to the salt-sensitive hypertension in the ETBR-deficient rat.

The endothelin system and its antagonism in chronic kidney disease

The incidence of chronic kidney disease (CKD) is increasing worldwide. Cardiovascular disease (CVD) is strongly associated with CKD and constitutes one of its major causes of morbidity and mortality. Treatments that slow the progression of CKD and improve the cardiovascular risk profile of patients with CKD are needed. The endothelins (ET) are a family of related peptides, of which ET-1 is the most powerful endogenous vasoconstrictor and the predominant isoform in the cardiovascular and renal systems. The ET system has been widely implicated in both CVD and CKD. ET-1 contributes to the pathogenesis and maintenance of hypertension and arterial stiffness and more novel cardiovascular risk factors such as oxidative stress and inflammation. Through these, ET also contributes to endothelial dysfunction and atherosclerosis. By reversal of these effects, ET antagonists may reduce cardiovascular risk. In particular relation to the kidney, antagonism of the ET system may be of benefit in improving renal hemodynamics and reducing proteinuria. ET likely also is involved in progression of renal disease, and data are emerging to suggest a synergistic role for ET receptor antagonists with angiotensin-converting enzyme inhibitors in slowing CKD progression.