Evidence for endothelin involvement in the response to high salt

Endothelin receptor B is required for the blood pressure-lowering effect of G protein-coupled estrogen receptor 1 in ovariectomized rats

Activation of G protein-coupled estrogen receptor 1 (GPER1) elicits antihypertensive actions in different animal models. The endothelin-1 signaling system plays a fundamental role in blood pressure regulation. Lack of functional endothelin receptor B (ETB) evokes hypertension and salt sensitivity. GPER1 and ETB interact to promote urinary sodium excretion in female rats. We hypothesized that activation of GPER1 protects against hypertension and salt sensitivity induced by ETB antagonism in female rats. Female Sprague-Dawley rats were implanted with radiotelemetry. Animals were then subjected to ovariectomy and simultaneously implanted with minipumps to deliver either the GPER1 agonist G1 or its corresponding vehicle. Two weeks post surgery, we initiated treatment of rats with the ETB antagonist A-192621. Animals were maintained on a normal-salt diet and then challenged with a high-salt diet for an additional 5 days. Assessment of mean arterial blood pressure revealed an increase in vehicle-treated, but not G1-treated, rats in response to ovariectomy. A-192621 increased blood pressure in normal-salt diet-fed vehicle- and G1-treated rats. G1 improved the circadian blood pressure rhythms that were disrupted in A-192621-treated ovariectomized rats. Thus, although systemic GPER1 activation did not protect ovariectomized rats from hypertension and salt sensitivity induced by ETB antagonism, it maintained circadian blood pressure rhythms. Functional ETB is required to elicit the antihypertensive actions of GPER1. Additional studies are needed to improve our understanding of the interaction between G protein-coupled receptors in regulating circadian blood pressure rhythm.NEW & NOTEWORTHY Systemic G protein-coupled estrogen receptor 1 (GPER1) activation in rats prevents the increase in blood pressure evoked by ovariectomy. Blockade of endothelin receptor B negates the blood pressure-lowering impact of GPER1 in ovariectomized rats. Endothelin receptor B plays an important role in mediating the blood pressure-lowering action of GPER1 activation in female rats.

Taming resistant hypertension: The promise of novel pharmacologic approaches and renal denervation

Resistant hypertension is associated with an exceedingly high cardiovascular risk and there remains an unmet therapeutic need driven by pathophysiologic pathways unaddressed by guideline-recommended therapy. While spironolactone is widely considered as the preferable fourth-line drug, its broad application is limited by its side effect profile, especially off-target steroid receptor-mediated effects and hyperkalaemia in at-risk subpopulations. Recent landmark trials have reported promising safety and efficacy results for a number of novel compounds targeting relevant pathophysiologic pathways that remain unopposed by contemporary drugs. These include the dual endothelin receptor antagonist, aprocitentan, the aldosterone synthase inhibitor, baxdrostat and the nonsteroidal mineralocorticoid receptor antagonist finerenone. Furthermore, the evidence base for consideration of catheter-based renal denervation as a safe and effective adjunct therapeutic approach across the clinical spectrum of hypertension has been further substantiated. This review will summarise the recently published evidence on novel antihypertensive drugs and renal denervation in the context of resistant hypertension.

Endothelin mediates sex-differences in acclimation to high salt diet in rats

INTRODUCTION

Current understanding of sodium (Na+) handling is based on studies done primarily in males. Contrary to the gradual increase in high salt (HS) induced natriuresis over 3-5 days in males, female Sprague Dawley (SD) rats have a robust natriuresis after 1 day of HS. Renal endothelin-1 (ET-1) signaling, through ET receptor A and B, is an important natriuretic pathway and was implicated in our previous dietary salt acclimation studies, however, the contribution of ET receptors to sex-differences in acclimation to dietary Na+ challenges has yet to be clarified. We hypothesized that ET receptors mediate the augmented natriuretic capacity of female rats in response to a HS diet.

METHODS

To test our hypothesis, male and female SD rats were implanted with telemeters and randomly assigned to treatment with A-182086, a dual ETA and ETB receptor antagonist, or control. 24-h urine samples were collected and assessed for electrolytes and ET-1. Studies were performed on a normal salt (NS, 0.3% NaCl) diet and after challenging rats with HS (4% NaCl) diet for 1 day.

RESULTS

We found that A-182086 increased blood pressure in male and female SD rats fed either diet. Importantly, A-182086 eliminated sex-differences in natriuresis on NS and HS. In particular, A-182086 promotes HS-induced natriuresis in male rats rather than attenuating the natriuretic capacity of females. Further, the sex-difference in urinary ET-1 excretion in NS-fed rats was eliminated by A-182086.

CONCLUSION

In conclusion, ET receptors are crucial for mediating sex-difference in the natriuretic capacity primarily through their actions in male rats.

Aldosterone: Renal Action and Physiological Effects

Aldosterone exerts profound effects on renal and cardiovascular physiology. In the kidney, aldosterone acts to preserve electrolyte and acid-base balance in response to changes in dietary sodium (Na+ ) or potassium (K+ ) intake. These physiological actions, principally through activation of mineralocorticoid receptors (MRs), have important effects particularly in patients with renal and cardiovascular disease as demonstrated by multiple clinical trials. Multiple factors, be they genetic, humoral, dietary, or otherwise, can play a role in influencing the rate of aldosterone synthesis and secretion from the adrenal cortex. Normally, aldosterone secretion and action respond to dietary Na+ intake. In the kidney, the distal nephron and collecting duct are the main targets of aldosterone and MR action, which stimulates Na+ absorption in part via the epithelial Na+ channel (ENaC), the principal channel responsible for the fine-tuning of Na+ balance. Our understanding of the regulatory factors that allow aldosterone, via multiple signaling pathways, to function properly clearly implicates this hormone as central to many pathophysiological effects that become dysfunctional in disease states. Numerous pathologies that affect blood pressure (BP), electrolyte balance, and overall cardiovascular health are due to abnormal secretion of aldosterone, mutations in MR, ENaC, or effectors and modulators of their action. Study of the mechanisms of these pathologies has allowed researchers and clinicians to create novel dietary and pharmacological targets to improve human health. This article covers the regulation of aldosterone synthesis and secretion, receptors, effector molecules, and signaling pathways that modulate its action in the kidney. We also consider the role of aldosterone in disease and the benefit of mineralocorticoid antagonists. © 2023 American Physiological Society. Compr Physiol 13:4409-4491, 2023.

Dietary sodium restriction prevents vascular endothelial growth factor inhibitor-induced hypertension

BACKGROUND

Vascular endothelial growth factor inhibitors (VEGFIs) are effective anticancer agents which often induce hypertension. VEGFI-induced hypertension is sodium-sensitive in animal studies. Therefore, the efficacy of dietary sodium restriction (DSR) to prevent VEGFI-induced hypertension in cancer patients was studied.

METHODS

Cancer patients with VEGFI-induced hypertension (day mean >135/85 mmHg or a rise in systolic and/or diastolic BP ≥ 20 mmHg) were treated with DSR (aiming at <4 g salt/day). The primary endpoint was the difference in daytime mean arterial blood pressure (MAP) increase between the treatment cycle with and without DSR.

RESULTS

During the first VEGFI treatment cycle without DSR, mean daytime MAP increased from 95 to 110 mmHg. During the subsequent treatment cycle with DSR, mean daytime MAP increased from 94 to 102 mmHg. Therefore, DSR attenuated the increase in mean daytime MAP by 7 mmHg (95% CI 1.3-12.0, P = 0.009). DSR prevented the rise in the endothelin-1/renin ratio that normally accompanies VEGFI-induced hypertension (P = 0.020) and prevented the onset of proteinuria: 0.15 (0.10-0.25) g/24 h with DSR versus 0.19 (0.11-0.32) g/24 h without DSR; P = 0.005.

DISCUSSION

DSR significantly attenuated VEGFI induced BP rise and proteinuria and thus is an effective non-pharmacological intervention.

Knockout of the Circadian Clock Protein PER1 (Period1) Exacerbates Hypertension and Increases Kidney Injury in Dahl Salt-Sensitive Rats

BACKGROUND

Circadian rhythms play an essential role in physiological function. The molecular clock that underlies circadian physiological function consists of a core group of transcription factors, including the protein PER1 (Period1). Studies in mice show that PER1 plays a role in the regulation of blood pressure and renal sodium handling; however, the results are dependent on the strain being studied. Using male Dahl salt-sensitive (SS) rats with global knockout of PER1 (SSPer1-/-), we aim to test the hypothesis that PER1 plays a key role in the regulation of salt-sensitive blood pressure.

METHODS

The model was generated using CRISPR/Cas9 and was characterized using radiotelemetry and measures of renal function and circadian rhythm.

RESULTS

SSPer1-/- rats had similar mean arterial pressure when fed a normal 0.4% NaCl diet but developed augmented hypertension after three weeks on a high-salt (4% NaCl) diet. Despite being maintained on a normal 12:12 light:dark cycle, SSPer1-/- rats exhibited desynchrony mean arterial pressure rhythms on a high-salt diet, as evidenced by increased variability in the time of peak mean arterial pressure. SSPer1-/- rats excrete less sodium after three weeks on the high-salt diet. Furthermore, SSPer1-/- rats exhibited decreased creatinine clearance, a measurement of renal function, as well as increased signs of kidney tissue damage. SSPer1-/- rats also exhibited higher plasma aldosterone levels.

CONCLUSIONS

Altogether, our findings demonstrate that loss of PER1 in Dahl SS rats causes an array of deleterious effects, including exacerbation of the development of salt-sensitive hypertension and renal damage.

Sex differences in apoptosis do not contribute to sex differences in blood pressure or renal T cells in spontaneously hypertensive rats

Apoptosis is a physiological and anti-inflammatory form of cell death that is indispensable for normal physiology and homeostasis. Several studies have reported aberrant activation of apoptosis in various tissues at the onset of hypertension. However, the functional significance of apoptosis during essential hypertension remains largely undefined. The current study was designed to test the hypothesis that apoptosis contributes to sex differences in blood pressure and the T cell profile in spontaneously hypertensive rats (SHR). Apoptosis was measured in kidney, aorta and spleen of 13-week-old adult hypertensive male and female SHR. Female SHR had greater renal and aortic apoptosis compared to age-matched males; apoptosis in the spleen was comparable between the sexes. Based on well-established sex differences in hypertension, we tested the hypothesis that greater apoptosis in female SHR contributes to the lower BP and pro-inflammatory profile compared to males. Male and female SHR were randomized to receive vehicle or ZVAD-FMK, a cell permeable pan-caspase inhibitor, in established hypertension from 13 to 15 weeks of age or at the onset of hypertension from 6 to 12 weeks or age. Treatment with ZVAD-FMK lowered renal apoptosis in both studies, yet neither BP nor renal T cells were altered in either male or female SHR. These results suggest that apoptosis does not contribute to the control or maintenance of BP in male or female SHR or sex differences in renal T cells.

Sex Differences in Diurnal Sodium Handling During Diet-Induced Obesity in Rats

BACKGROUND

Emerging evidence over the past several years suggests that diurnal control of sodium excretion is sex dependent and involves the renal endothelin system. Given recent awareness of disruptions of circadian function in obesity, we determined whether diet-induced obesity impairs renal handling of an acute salt load at different times of day and whether this varies by sex and is associated with renal endothelin dysfunction.

METHODS

Male and female Sprague-Dawley rats were placed on a high-fat diet for 8 weeks before assessing renal sodium handling and blood pressure.

RESULTS

Male, but not female, rats on high fat had a significantly reduced natriuretic response to acute NaCl injection at the beginning of their active period that was associated with lower endothelin 1 (ET-1) excretion, lower ET-1 mRNA expression in the cortex and outer medulla as well as lower ET_B receptor expression in the outer medulla of the high-fat rats. Obese males also had significantly higher blood pressure (telemetry) that was exacerbated by adding high salt to the diet during the last 2 weeks. While female rats developed hypertension with a high-fat diet, they were not salt sensitive and ET-1 excretion was unchanged.

CONCLUSIONS

These data identify diet-induced obesity as a sex-specific disruptive factor for maintaining proper sodium handling. Although high-fat diets induce hypertension in both sexes, these data reveal that males are at greater risk of salt-dependent hypertension and further suggest that females have more redundant systems that can be productive against salt-sensitive hypertension in at least some circumstances.

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.

Endothelin-1 in the pathophysiology of obesity and insulin resistance

The association between plasma endothelin-1 (ET-1) and obesity has been documented for decades, yet the contribution of ET-1 to risk factors associated with obesity is not fully understood. In 1994, one of first papers to document this association also noted a positive correlation between plasma insulin and ET-1, suggesting a potential contribution of ET-1 to the development of insulin resistance. Both endogenous receptors for ET-1, ETA and ETB are present in all insulin-sensitive tissues including adipose, liver and muscle, and ET-1 actions within these tissues suggest that ET-1 may be playing a role in the pathogenesis of insulin resistance. Further, antagonists for ET-1 receptors are clinically approved making these sites attractive therapeutic targets. This review focuses on known mechanisms through which ET-1 affects plasma lipid profiles and insulin signalling in these metabolically important tissues and also identifies gaps in our understanding of ET-1 in obesity-related pathophysiology.

Functional Interaction of Endothelin Receptors in Mediating Natriuresis Evoked by G Protein-Coupled Estrogen Receptor 1

The G protein–coupled estrogen receptor 1 (GPER1) mediates rapid estrogenic signaling. We recently reported that activation of GPER1 in the renal medulla evokes endothelin-1–dependent natriuresis in female, but not male, rats. However, the involvement of the ET receptors, ET_A and ET_B, underlying GPER1 natriuretic action remain unclear. In this study, we used genetic and pharmacologic methods to identify the contributions of ET_A and ET_B in mediating this female-specific natriuretic effect of renal medullary GPER1. Infusion of the GPER1-selective agonist G1 (5 pmol/kg per minute) into the renal medulla for 40 minutes increased Na⁺ excretion and urine flow in anesthetized female ET_B-deficient (ET_B def) rats and littermate controls but did not affect blood pressure or urinary K⁺ excretion in either group. Pretreatment with the selective ET_A inhibitor ABT-627 (5 mg/kg, intravenous) abolished G1-induced natriuresis in ET_B def rats. To further isolate the effects of inhibiting either receptor alone, we conducted the same experiments in anesthetized female Sprague-Dawley (SD) rats pretreated or not with ABT-627 and/or the selective ET_B inhibitor A-192621 (10 mg/kg, intravenous). Neither antagonism of ET_A nor antagonism of ET_B receptor alone affected the G1-induced increase in Na⁺ excretion and urine flow in SD rats. However, simultaneous antagonism of both receptors completely abolished these effects. These data suggest that ET_A and ET_B receptors can mediate the natriuretic and diuretic response to renal medullary GPER1 activation in female rats.

Blood pressure and albuminuria in a female mouse model of systemic lupus erythematosus: impact of long-term high salt consumption

Hypertension and kidney involvement are common in patients with autoimmune disease. Sodium intake is linked to hypertension in both human and animal studies. Evidence suggests that dietary salt may be an important environmental factor that promotes autoimmune activity. Therefore, we hypothesized that a long-term high-salt diet would accelerate the progression of autoimmunity, hypertension, and albuminuria during systemic lupus erythematosus (SLE), an autoimmune disease that predominantly affects young women and has a high prevalence of hypertension and renal disease. To test this hypothesis, an established experimental model of SLE (female NZBWF1 mice) that develops hypertension and renal disease was used. SLE mice were fed a high-salt (4% NaCl) or normal (0.4% NaCl) diet for 24 wk beginning at 10 wk of age and ending at 34 wk of age, a time by which female NZBWF1 mice typically have hypertension and exhibit signs of renal disease. Plasma anti-dsDNA autoantibodies were measured as an indicator of active SLE disease, and urinary albumin was monitored longitudinally as a marker of renal disease. Arterial pressure was measured in conscious, freely moving mice at 34 wk of age. Urinary endothelin-1 (ET-1) excretion, renal endothelin A and B receptor protein expression, and renal mRNA expression of NOS1, NOS2, NOX2, MCP-1, TNF-α, serum- and glucocorticoid-regulated kinase 1, and interleukin-2 (IL-2) were assessed to determine the impact on gene products commonly altered by a high-salt diet. SLE mice fed a high-salt diet had increased circulating autoantibodies, but the high-salt diet did not significantly affect albuminuria or arterial pressure. Urinary ET-1 excretion was increased, whereas renal endothelin A receptor and IL-2 expression were decreased in response to a high-salt diet. These data suggest that a chronic high-salt diet may not accelerate cardiovascular and renal consequences commonly associated with SLE.

Diurnal Control of Blood Pressure Is Uncoupled From Sodium Excretion

The diurnal rhythms of sodium handling and blood pressure are thought to be regulated by clock genes, such as Bmal1. However, little is known about the regulation of these factors by Bmal1, especially in rats. Using a novel whole-body Bmal1 knockout rat model (Bmal1-/-), we hypothesized that time of day regulation of sodium excretion is dependent on Bmal1. Using telemetry to continuously record mean arterial pressure, we observed that male and female Bmal1-/- rats had significantly reduced mean arterial pressure over the course of 24 hours compared with littermate controls. The circadian mean arterial pressure pattern remained intact in both sexes of Bmal1-/- rats, which is in contrast to the Bmal1-/- mouse model. Male Bmal1-/- rats had no significant difference in baseline sodium excretion between 12-hour active and inactive periods, indicating a lack of diurnal control independent of maintained mean arterial pressure rhythms. Female Bmal1-/- rats, however, had significantly greater sodium excretion during the active versus inactive period similar to controls. Thus, we observed a clear dissociation between circadian blood pressure and control of sodium excretion that is sex dependent. These findings are consistent with a more robust ability of females to maintain control of sodium excretion, and furthermore, demonstrate a novel role for Bmal1 in control of diurnal blood pressure independent of sodium excretion.

Necrosis Contributes to the Development of Hypertension in Male, but Not Female, Spontaneously Hypertensive Rats

Necrosis is a pathological form of cell death that induces an inflammatory response, and immune cell activation contributes to the development and maintenance of hypertension. Necrosis was measured in kidney, spleen, and aorta of 12- to 13-week-old male and female SHRs (spontaneously hypertensive rats); male SHRs had greater renal necrotic cell death than female SHRs. Because male SHRs have a higher blood pressure (BP) and a more proinflammatory T-cell profile than female SHRs, the current studies tested the hypothesis that greater necrotic cell death in male SHRs exacerbates increases in BP and contributes to the proinflammatory T-cell profile. Male and female SHRs were randomized to receive vehicle or Necrox-5-a cell permeable inhibitor of necrosis-from 6 to 12 weeks of age or from 11 to 13 weeks of age. In both studies, Necrox-5 decreased renal necrosis and abolished the sex difference. Treatment with Necrox-5 beginning at 6 weeks of age attenuated maturation-induced increases in BP in male SHR; BP in female SHR was not altered by Necrox-5 treatment. Necrox-5 decreased proinflammatory renal T cells in both sexes, although sex differences were maintained. Administration of Necrox-5 for 2 weeks in SHR with established hypertension resulted in a small but significant decrease in BP in males with no effect in females. These results suggest that greater necrotic cell death in male SHR exacerbates maturation-induced increases in BP with age contributing to sex differences in BP. Moreover, although necrosis is proinflammatory, it is unlikely to explain sex differences in the renal T-cell profile.

Tauroursodeoxycholic acid (TUDCA) abolishes chronic high salt-induced renal injury and inflammation

AIM

Chronic high salt intake exaggerates renal injury and inflammation, especially with the loss of functional ET_B receptors. Tauroursodeoxycholic acid (TUDCA) is a chemical chaperone and bile salt that is approved for the treatment of hepatic diseases. Our aim was to determine whether TUDCA is reno-protective in a model of ET_B receptor deficiency with chronic high salt-induced renal injury and inflammation.

METHODS

ET_B -deficient and transgenic control rats were placed on normal (0.8% NaCl) or high salt (8% NaCl) diet for 3 weeks, receiving TUDCA (400 mg/kg/d; ip) or vehicle. Histological and biochemical markers of kidney injury, renal cell death and renal inflammation were assessed.

RESULTS

In ET_B -deficient rats, high salt diet significantly increased glomerular and proximal tubular histological injury, proteinuria, albuminuria, excretion of tubular injury markers KIM-1 and NGAL, renal cortical cell death and renal CD4⁺ T cell numbers. TUDCA treatment increased proximal tubule megalin expression as well as prevented high salt diet-induced glomerular and tubular damage in ET_B -deficient rats, as indicated by reduced kidney injury markers, decreased glomerular permeability and proximal tubule brush border restoration, as well as reduced renal inflammation. However, TUDCA had no significant effect on blood pressure.

CONCLUSIONS

TUDCA protects against the development of glomerular and proximal tubular damage, decreases renal cell death and inflammation in the renal cortex in rats with ET_B receptor dysfunction on a chronic high salt diet. These results highlight the potential use of TUDCA as a preventive tool against chronic high salt induced renal damage.

Thick Ascending Limb Sodium Transport in the Pathogenesis of Hypertension

The thick ascending limb plays a key role in maintaining water and electrolyte balance. The importance of this segment in regulating blood pressure is evidenced by the effect of loop diuretics or local genetic defects on this parameter. Hormones and factors produced by thick ascending limbs have both autocrine and paracrine effects, which can extend prohypertensive signaling to other structures of the nephron. In this review, we discuss the role of the thick ascending limb in the development of hypertension, not as a sole participant, but one that works within the rich biological context of the renal medulla. We first provide an overview of the basic physiology of the segment and the anatomical considerations necessary to understand its relationship with other renal structures. We explore the physiopathological changes in thick ascending limbs occurring in both genetic and induced animal models of hypertension. We then discuss the racial differences and genetic defects that affect blood pressure in humans through changes in thick ascending limb transport rates. Throughout the text, we scrutinize methodologies and discuss the limitations of research techniques that, when overlooked, can lead investigators to make erroneous conclusions. Thus, in addition to advancing an understanding of the basic mechanisms of physiology, the ultimate goal of this work is to understand our research tools, to make better use of them, and to contextualize research data. Future advances in renal hypertension research will require not only collection of new experimental data, but also integration of our current knowledge.

Sex-Specific Contributions of Endothelin to Hypertension

PURPOSE OF REVIEW

Men and women differ in the prevalence, pathophysiology and control rate of hypertension in an age-dependent manner. The renal endothelin system plays a central role in sex differences in blood pressure regulation by control of sodium excretion and vascular function. Improving our understanding of the sex differences in the endothelin system, especially in regard to blood pressure regulation and sodium homeostasis, will fill a significant gap in our knowledge and may identify sex-specific therapeutic targets for management of hypertension.

RECENT FINDINGS

The current review will highlight evidence for the potential role for endothelin system in the pathophysiology of hypertension within three female populations: (i) postmenopausal women, (ii) women suffering from preeclampsia, or (iii) pulmonary arterial hypertension. Clinical trials that specifically address cardiovascular and renal diseases in females under different hormonal status are limited. Studies of the modulatory role of gonadal hormones and sex-specific mechanisms on critically important systems involved, such as endothelin, are needed to establish new clinical practice guidelines based on systematic evidence.

Sodium intake but not renal nerves attenuates renal venous pressure-induced changes in renal hemodynamics in rats

Increased central venous pressure and renal venous pressure (RVP) are associated with worsening of renal function in acute exacerbation of congestive heart failure. We tested whether an acute isolated elevation of RVP in one kidney leads to ipsilateral renal vasoconstriction and decreased glomerular filtration rate (GFR) and whether this depends on dietary salt intake or activation of renal nerves. Male Lewis rats received a normal (1% NaCl, NS) or high-salt (6% NaCl) diet for ≥14 days before the acute experiment. Rats were then randomized into the following three groups: time control and RVP elevation to either 10 or 20 mmHg to assess heart rate, renal blood flow (RBF), and GFR. To increase RVP, the left renal vein was partially occluded for 120 min. To determine the role of renal nerves, surgical denervation was conducted in rats on both diets. Renal sympathetic nerve activity (RSNA) was additionally recorded in a separate group of rats. Increasing RVP to 20 mmHg decreased ipsilateral RBF (7.5 ± 0.4 to 4.1 ± 0.7 ml/min, P < 0.001), renal vascular conductance (0.082 ± 0.006 to 0.060 ± 0.011 ml·min^-1·mmHg^-1, P < 0.05), and GFR (1.28 ± 0.08 to 0.40 ± 0.13 ml/min, P < 0.05) in NS rats. The reduction was abolished by high-salt diet but not by renal denervation. Furthermore, a major increase of RVP (1.6 ± 0.8 to 24.7 ± 1.2 mmHg) immediately suppressed RSNA and decreased heart rate ( P < 0.05), which points to suppression of both local and systemic sympathetic activity. Taken together, acute elevated RVP induces renal vasoconstriction and decreased GFR, which is more likely to be mediated via the renin-angiotensin system than via renal nerves.

Chronic ETA antagonist reverses hypertension and impairment of structure and function of peripheral small arteries in aortic stiffening

Arterial stiffness may contribute to the pathogenesis of hypertension. The goal of this study is to elucidate the role of Endothelin-1 (ET-1) in aortic stiffening-induced hypertension through ETA receptor activation. An increase in aortic stiffness was created by use of a non-constrictive restraint, NCR on the abdominal aortic surface. A group of rats underwent aortic NCR or sham operation for 12 weeks and were then treated with ETA receptor antagonist BQ-123 for 3 weeks. We found that 12 weeks of aortic NCR significantly increased pulse and mean pressure and altered peripheral flow pattern, accompanied by an increased serum ET-1 level (p < 0.05). The increase in aortic stiffness (evidenced by an elevated pulse wave velocity) caused hypertrophic structural remodeling and decreased arterial compliance, along with an impaired endothelial function in peripheral small arteries. BQ-123 treatment only partially attenuated peripheral arterial hypertrophy and restored arterial compliance, but completely recovered endothelium function, and consequently restored local flow and lowered blood pressure. Our findings underscore the hemodynamic coupling between aortic stiffening and peripheral arterial vessels and flow dynamics through an ETA-dependent mechanism. ETA receptor blockade may have therapeutic potential for improving peripheral vessel structure and function in the treatment of aortic stiffness-induced hypertension.

Diurnal pattern in skin Na+ and water content is associated with salt-sensitive hypertension in ETB receptor-deficient rats

Impairment in the ability of the skin to properly store Na⁺ nonosmotically (without water) has recently been hypothesized as contributing to salt-sensitive hypertension. Our laboratory has shown that endothelial production of endothelin-1 (ET-1) is crucial to skin Na⁺ handling. Furthermore, it is well established that loss of endothelin type B receptor (ET_B) receptor function impairs Na⁺ excretion by the kidney. Thus we hypothesized that rats lacking functional ET_B receptors (ET_B-def) will have a reduced capacity of the skin to store Na⁺ during chronic high-salt (HS) intake. We observed that ET_B-def rats exhibited salt-sensitive hypertension with an approximate doubling in the diurnal amplitude of mean arterial pressure compared with genetic control rats on a HS diet. Two weeks of HS diet significantly increased skin Na⁺ content relative to water; however, there was no significant difference between control and ET_B-def rats. Interestingly, HS intake led to a 19% increase in skin Na⁺ and 16% increase in water content (relative to dry wt.) during the active phase (zeitgeber time 16) versus inactive phase (zeitgeber time 4, P < 0.05) in ET_B-def rats. There was no significant circadian variation in total skin Na⁺ or water content of control rats fed normal or HS. These data indicate that ET_B receptors have little influence on the ability to store Na⁺ nonosmotically in the skin during long-term HS intake but, rather, appear to regulate diurnal rhythms in skin Na⁺ content and circadian blood pressure rhythms associated with a HS diet.

Metabolic and Biochemical Stressors in Diabetic Cardiomyopathy

Diabetic cardiomyopathy (DCM) or diabetes-induced cardiac dysfunction is a direct consequence of uncontrolled metabolic syndrome and is widespread in US population and worldwide. Despite of the heterogeneous and distinct features of DCM, the clinical relevance of DCM is now becoming established. DCM progresses to pathological cardiac remodeling with the higher risk of heart attack and subsequent heart failure in diabetic patients. In this review, we emphasize lipid substrate quality and the phenotypic, metabolic, and biochemical stressors of DCM in the rodent and human pathophysiology. We discuss lipoxygenase signaling in the inflammatory pathway with multiple contributing and confounding factors leading to DCM. Additionally, emerging biochemical pathways are emphasized to make progress toward therapeutic advancement to treat DCM.

Long-Term Endothelin-A Receptor Antagonism Provides Robust Renal Protection in Humanized Sickle Cell Disease Mice

Sickle cell disease (SCD)-associated nephropathy is a major source of morbidity and mortality in patients because of the lack of efficacious treatments targeting renal manifestations of the disease. Here, we describe a long-term treatment strategy with the selective endothelin-A receptor (ET_A) antagonist, ambrisentan, designed to interfere with the development of nephropathy in a humanized mouse model of SCD. Ambrisentan preserved GFR at the level of nondisease controls and prevented the development of proteinuria, albuminuria, and nephrinuria. Microscopy studies demonstrated prevention of podocyte loss and structural alterations, the absence of vascular congestion, and attenuation of glomerulosclerosis in treated mice. Studies in isolated glomeruli showed that treatment reduced inflammation and oxidative stress. At the level of renal tubules, ambrisentan treatment prevented the increased excretion of urinary tubular injury biomarkers. Additionally, the treatment strategy prevented tubular brush border loss, diminished tubular iron deposition, blocked the development of interstitial fibrosis, and prevented immune cell infiltration. Furthermore, the prevention of albuminuria in treated mice was associated with preservation of cortical megalin expression. In a separate series of identical experiments, combined ET_A and ET_B receptor antagonism provided only some of the protection observed with ambrisentan, highlighting the importance of exclusively targeting the ET_A receptor in SCD. Our results demonstrate that ambrisentan treatment provides robust protection from diverse renal pathologies in SCD mice, and suggest that long-term ET_A receptor antagonism may provide a strategy for the prevention of renal complications of SCD.

Loss of endothelin B receptor function impairs sodium excretion in a time- and sex-dependent manner

Recent studies suggested a direct link between circadian rhythms and regulation of sodium excretion. Endothelin-1 (ET-1) regulates sodium balance by promoting natriuresis through the endothelin B receptor (ET_B) in response to increased salt in the diet, but the effect that the time of day has on this natriuretic response is not known. Therefore, this study was designed to test the hypothesis that ET_B receptor activation contributes to the diurnal control of sodium excretion and that sex differences contribute to this control as well. Twelve-hour urine collections were used to measure sodium excretion. On day 3 of the experiment, a NaCl load (900 μeq) was given by oral gavage either at Zeitgeber time [ZT] 0 (inactive period) or ZT12 (active period) to examine the natriuretic response to the acute salt load. Male and female ET_B-deficient (ET_B def) rats showed an impaired natriuretic response to a salt load at ZT0 compared with their respective transgenic controls (Tg cont). Male ET_B def rats showed a delayed natriuretic response to a salt load given at ZT12 compared with male Tg cont, a contrast to the prompt response shown by female ET_B def rats. Treatment with ABT-627, an ET_A receptor antagonist, improved the natriuretic response seen within the first 12 h of a ZT0 salt load in both sexes. These findings demonstrate that diurnal excretion of an acute salt load 1) requires ET-1 and the ET_B receptor, 2) is more evident in male vs. female rats, and 3) is opposed by the ET_A receptor.

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, sex, and pregnancy: unique considerations for blood pressure control in females

Endothelin-1 (ET-1) is a potent vasoconstrictor, and dysregulation of the endothelin (ET) system has been implicated in the development of hypertension. Sex differences in the ET system have been identified in ET receptor expression and activation, levels of ET-1, and downstream mediators of the ET system. More specifically, males have greater ET-1/ETA receptor activation, whereas females exhibit greater ETB receptor activation. These differences have been suggested to contribute to the sex differences observed in blood pressure control, with greater ETB receptor activation in females potentially acting as an important pathway contributing to the lower prevalence of hypertension in young females compared with age-matched males. This hypothesis is further supported by studies in pregnancy; the role of the ET system is enhanced during pregnancy, with dysregulation of the ET system resulting in preeclampsia. Further research is necessary to elucidate the relative roles of the ET system in blood pressure control in both sexes and to further explore the potential benefits of pharmacological ET blockade in women.

High salt intake increases endothelin B receptor function in the renal medulla of rats

AIMS

Endothelin (ET)-1 promotes natriuresis via the endothelin B receptor (ETB) within the renal medulla. In male rats, direct interstitial infusion of ET-1 into the renal medulla has no effect on renal sodium and water excretion but is associated with endothelin A receptor (ETA)-dependent reductions in medullary blood flow. Loss of ETB function leads to salt-sensitive hypertension. We hypothesized that HS intake would increase the natriuretic and diuretic response to renal medullary infusion of ET peptides.

MAIN METHODS

Male Sprague-Dawley (SD) rats were fed a normal (NS) or high (HS) salt diet for 7days. Rats were anesthetized and a catheter implanted in the renal medulla for interstitial infusion along with a ureteral catheter for urine collection. Medullary infusion of a low dose of ETB receptor agonist, sarafotoxin 6c (S6c; 0.15μg/kg/h), or ET-1 (0.45μg/kg/h) was used to determine changes in sodium excretion (UNaV).

KEY FINDINGS

In HS fed rats, intramedullary infusion of a low dose of S6c induced a significant increase in UNaV, roughly 2-fold over baseline, compared to no response to this low dose in NS fed rats. In HS fed rats, intramedullary infusion of ET-1 induced a significantly greater increase in UNaV compared to NS fed rats, although this increase was not different from the HS time control studies.

SIGNIFICANCE

We conclude that high salt intake enhances the diuretic and natriuretic effects of ETB receptor activation in vivo consistent with a role for the ETB receptor in maintaining fluid-electrolyte homeostasis.

Endothelin-1 as a master regulator of whole-body Na+ homeostasis

The current study was designed to determine whether vascular endothelial-derived endothelin-1 (ET-1) is important for skin Na(+) buffering. In control mice (C57BL/6J), plasma Na(+) and osmolarity were significantly elevated in animals on high- vs. low-salt (HS and LS, respectively) intake. The increased plasma Na(+) and osmolarity were associated with increased ET-1 mRNA in vascular tissue. There was no detectable difference in skin Na(+):H2O in HS fed mice (0.119 ± 0.005 mM vs. 0.127 ± 0.007 mM; LS vs. HS); however, skin Na(+):H2O was significantly increased by blockade of the endothelin type A receptor with ABT-627 (0.116 ± 0.006 mM vs. 0.137 ± 0.007 mM; LS vs. HS; half-maximal inhibitory concentration, 0.055 nM). ET-1 peptide content in skin tissue was increased in floxed control animals on HS (85.9 ± 0.9 pg/mg vs. 106.4 ± 6.8 pg/mg; P < 0.05), but not in vascular endothelial cell endothelin-1 knockout (VEET KO) mice (76.4 ± 5.7 pg/mg vs. 65.7 ± 7.9 pg/mg; LS vs. HS). VEET KO mice also had a significantly elevated skin Na(+):H2O (0.113 ± 0.007 mM vs. 0.137 ± 0.005 mM; LS vs. HS; P < 0.05). Finally, ET-1 production was elevated in response to increasing extracellular osmolarity in cultured human endothelial cells. These data support the hypothesis that increased extrarenal vascular ET-1 production in response to HS intake is mediated by increased extracellular osmolarity and plays a critical role in regulating skin storage of Na(+).

High salt diet increases the pressor response to stress in female, but not male ETB-receptor-deficient rats

Acute stress in both rodents and humans causes a transient rise in blood pressure associated with an increase in plasma endothelin-1 (ET-1). High salt (HS) intake also increases ET-1 production, and interestingly, blunts the pressor response to acute air jet stress in rats. We previously reported that female rats lacking functional ET_B receptors everywhere except sympathetic nerves (ET_B def) had a greater degree of hypertension in response to a HS diet compared to their male counterparts when measured by the tail cuff method. However, we now report that salt-induced hypertension is not different between sexes when measured by telemetry. Therefore, additional experiments were designed to test the hypothesis that female ET_B def rats are more sensitive to acute stress when on a HS diet. The pressor response, measured by telemetry, to acute air jet stress was similar between male transgenic control (Tg control) and ET_B def rats following chronic HS intake. In contrast, female ET_B def rats had a significantly greater pressor response (about twofold higher) than female or male Tg control or male ET_B def rats maintained on HS, a finding that cannot be explained by increased vascular reactivity to ET-1 in female rats as we observed that male ET_B def rats had a greater pressor response to i.v. infusion of ET-1 compared to females. Furthermore, HS feeding exacerbated the pressor response to ET-1 in both male and female ET_B def rats. Given our previous studies demonstrating that the ET_A receptor functions to reduce the pressor response to acute stress, these findings further support a role for the ET receptor system in the pressor response to acute stress and that female rats have reduced ET_A receptor activity when on a HS diet compared to males.

ET-1 increases reactive oxygen species following hypoxia and high-salt diet in the mouse glomerulus

AIM

This study was designed to determine whether ET-1 derived from endothelial cells contributes to oxidative stress in the glomerulus of mice subjected to a high-salt diet and/or hypoxia.

METHODS

C57BL6/J control mice or vascular endothelial cell ET-1 knockout (VEET KO) mice were subjected to 3-h exposure to hypoxia (8% O₂) and/or 2 weeks of high-salt diet (4% NaCl) prior to metabolic cage assessment of renal function and isolation of glomeruli for the determination of reactive oxygen species (ROS).

RESULTS

In control mice, hypoxia significantly increased urinary protein excretion during the initial 24 h, but only in animals on a high-salt diet. Hypoxia increased glomerular ET-1 mRNA expression in control, but not in vascular endothelial cell ET-1 knockout (VEET KO) mice. Under normoxic conditions, mice on a high-salt diet had approx. 150% higher glomerular ET-1 mRNA expression compared with a normal-salt diet (P < 0.05). High-salt diet administration significantly increased glomerular ROS production in flox control, but not in glomeruli isolated from VEET KO mice. In C57BL6/J mice, the ETA receptor-selective antagonist, ABT-627, significantly attenuated the increase in glomerular ROS production produced by high-salt diet. In addition, chronic infusion of C57BL6/J mice with a subpressor dose of ET-1 (osmotic pumps) significantly increased the levels of glomerular ROS that were prevented by ETA antagonist treatment.

CONCLUSION

These data suggest that both hypoxia and a high-salt diet increase glomerular ROS production via endothelial-derived ET-1-ETA receptor activation and provide a potential mechanism for ET-1-induced nephropathy.

Role of renal sensory nerves in physiological and pathophysiological conditions

Whether activation of afferent renal nerves contributes to the regulation of arterial pressure and sodium balance has been long overlooked. In normotensive rats, activating renal mechanosensory nerves decrease efferent renal sympathetic nerve activity (ERSNA) and increase urinary sodium excretion, an inhibitory renorenal reflex. There is an interaction between efferent and afferent renal nerves, whereby increases in ERSNA increase afferent renal nerve activity (ARNA), leading to decreases in ERSNA by activation of the renorenal reflexes to maintain low ERSNA to minimize sodium retention. High-sodium diet enhances the responsiveness of the renal sensory nerves, while low dietary sodium reduces the responsiveness of the renal sensory nerves, thus producing physiologically appropriate responses to maintain sodium balance. Increased renal ANG II reduces the responsiveness of the renal sensory nerves in physiological and pathophysiological conditions, including hypertension, congestive heart failure, and ischemia-induced acute renal failure. Impairment of inhibitory renorenal reflexes in these pathological states would contribute to the hypertension and sodium retention. When the inhibitory renorenal reflexes are suppressed, excitatory reflexes may prevail. Renal denervation reduces arterial pressure in experimental hypertension and in treatment-resistant hypertensive patients. The fall in arterial pressure is associated with a fall in muscle sympathetic nerve activity, suggesting that increased ARNA contributes to increased arterial pressure in these patients. Although removal of both renal sympathetic and afferent renal sensory nerves most likely contributes to the arterial pressure reduction initially, additional mechanisms may be involved in long-term arterial pressure reduction since sympathetic and sensory nerves reinnervate renal tissue in a similar time-dependent fashion following renal denervation.

Female sex hormones protect against salt-sensitive hypertension but not essential hypertension

Initial studies found that female Dahl salt-sensitive (DS) rats exhibit greater blood pressure (BP) salt sensitivity than female spontaneously hypertensive rats (SHR). On the basis of the central role played by NO in sodium excretion and BP control, we further tested the hypothesis that blunted increases in BP in female SHR will be accompanied by greater increases in renal inner medullary nitric oxide synthase (NOS) activity and expression in response to a high-salt (HS) diet compared with DS rats. Gonad-intact and ovariectomized (OVX) female SHR and DS rats were placed on normal salt (NS; 0.4% salt) or HS (4% salt) diet for 2 wk. OVX did not alter BP in SHR, and HS diet produced a modest increase in BP. OVX significantly increased BP in DS rats on NS; HS further increased BP in all DS rats, although OVX had a greater increase in BP. Renal inner medullary NOS activity, total NOS3 protein, and NOS3 phosphorylated on serine residue 1177 were not altered by salt or OVX in either strain. NOS1 protein expression, however, significantly increased with HS only in SHR, and this corresponded to an increase in urinary nitrate/nitrite excretion. SHR also exhibit greater NOS1 and NOS3 protein expression than DS rats. These data indicate that female sex hormones offer protection against HS-mediated elevations in BP in DS rats but not SHR. We propose that the relative resistance to HS-mediated increases in BP in SHR is related to greater NOS expression and the ability to increase NOS1 protein expression compared with DS rats.

Combined endothelin a blockade and chlorthalidone treatment in a rat model of metabolic syndrome

Experiments determined whether the combination of endothelin A (ETA) receptor antagonist [ABT-627, atrasentan; (2R,3R,4S)-4-(1,3-benzodioxol-5-yl)-1-[2-(dibutylamino)-2-oxoethyl]-2-(4-methoxyphenyl)pyrrolidine-3-carboxylic acid] and a thiazide diuretic (chlorthalidone) would be more effective at lowering blood pressure and reducing renal injury in a rodent model of metabolic syndrome compared with either treatment alone. Male Dahl salt-sensitive rats were fed a high-fat (36% fat), high-salt (4% NaCl) diet for 4 weeks. Separate groups of rats were then treated with vehicle (control), ABT-627 (ABT; 5 mg/kg per day, in drinking water), chlorthalidone (CLTD; 5 mg/kg per day, in drinking water), or both ABT plus CLTD. Mean arterial pressure (MAP) was recorded continuously by telemetry. After 4 weeks, both ABT and CLTD severely attenuated the development of hypertension, whereas the combination further reduced MAP compared with ABT alone. All treatments prevented proteinuria. CLTD and ABT plus CLTD significantly reduced nephrin (a podocyte injury marker) and kidney injury molecule-1 (a tubulointerstitial injury marker) excretion. ABT, with or without CLTD, significantly reduced plasma 8-oxo-2'-deoxyguanosine, a measure of DNA oxidation, whereas CLTD alone had no effect. All treatments suppressed the number of ED1(+) cells (macrophages) in the kidney. Plasma tumor necrosis factor receptors 1 and 2 were reduced only in the combined ABT and CLTD group. These results suggest that ABT and CLTD have antihypertensive and renal-protective effects in a model of metabolic syndrome that are maximally effective when both drugs are administered together. The findings support the hypothesis that combined ETA antagonist and diuretic treatment may provide therapeutic benefit for individuals with metabolic syndrome consuming a Western diet.

Endothelin ET(B) receptors contribute to sex differences in blood pressure elevation in angiotensin II hypertensive rats on a high-salt diet

Female rats are more resistant to blood pressure increases induced by high salt (HS) intake or angiotensin (Ang) II infusion. Because endothelin ET(B) receptors on endothelial and epithelial cells mediate tonic vasodilation and sodium excretion, we hypothesized that ET(B) receptors limit the hypertensive response and renal injury induced by HS diet alone or with chronic AngII infusion (AngII/HS) in female compared with male rats. A 4 week HS diet (4% NaCl) did not significantly change blood pressure (measured by telemetry) in either male or female Sprague-Dawley rats. Administration of the ET(B) receptor antagonist A-192621 (1, 3 and 10 mg/kg per day in food) during HS feeding caused a dose-dependent increase in blood pressure in both sexes. In AngII/HS rats, males had a larger increase in blood pressure than females. The increase in blood pressure produced by ET(B) receptor blockade in male AngII/HS rats was not significant. However, A-192621 treatment resulted in a significant further increase in blood pressure in female AngII/HS rats. Male rats had consistently higher protein excretion rates before and during AngII/HS, but this was not significantly affected by ET(B) receptor blockade in either sex. In conclusion, ET(B) receptors play a significantly greater beneficial role in protecting female compared with male rats against AngII-induced hypertension and may contribute to the sex differences in AngII-induced hypertension.

[Endothelin: From discovery to pharmacotherapeutic innovations]

OBJECTIVES

Endothelin (ET) is a major therapeutic target in cardiopulmonary diseases. The purpose of this review is to present the main concepts concerning ET biology, its pathophysiological roles and the major pharmacological and medical advances recently developed around the concept of ET receptor blockade.

METHODS

Analysis of PubMed database (keywords: endothelin, endothelin receptor antagonists, pulmonary hypertension, etc.), and of abstract originating from recent international meetings.

RESULTS

ET is a peptide produced by vascular endothelial cells as well as by many other tissues. Both its production and its effects are activated in pathological situations associated with endothelial dysfunction. ET is characterized by a strong tropism toward tissues because of its polarized release, the strong tissue receptor density and high affinity of the receptors for the peptide. ET exerts several vascular effects, including vasoconstriction, proliferation and hypertrophy, as well as non-vascular effects, notably stimulation of cardiac hypertrophy, tissue fibrosis and inflammation. Both vascular and non-vascular effects depend on the stimulation of two receptor subtypes, ETA and ETB. ET receptor antagonists (ERA) demonstrated beneficial effects in many different pre-clinical models of cardiovascular and pulmonary diseases, and constitute a first-line treatment of patients with pulmonary arterial hypertension (PAH). Recently, the targeted search for a novel ERA led to the development of macitentan which, compared to existing ERA, show optimized tissue penetration, increased receptor affinity and in vivo pharmacological efficacy in pre-clinical models, associated with a favorable profile, in terms of hepatic safety and drug interactions. The clinical efficacy of macitentan in the treatment of PAH was recently demonstrated in the SERAPHIN trial, which contrasts with previous PAH trials because of its long duration, the high number of patients enrolled, and its primary endpoint evaluating morbidity/mortality. Results show a significant reduction of the primary composite morbidity/mortality endpoint (taking into account both progression of PAH and death) by 30 and 45% with macitentan 3 and 10mg, respectively, compared to placebo, and confirm on the large scale the favorable tolerance profile, especially at the hepatic level.

CONCLUSION

The extensive knowledge on the complexity of the ET system allowed the synthesis of a new antagonist optimized, in terms of pharmacological efficacy and safety, which also show promising therapeutic effects in PAH patients, with demonstrated results in a prospective study using a composite primary endpoint of morbidity-mortality.

Endothelin antagonists for diabetic and non-diabetic chronic kidney disease

Numerous pre-clinical studies have implicated endothelin-1 in the pathogenesis of diabetic and non-diabetic chronic kidney disease (CKD). Renal endothelin-1 production is almost universally increased in kidney disease. The pathologic effects of endothelin-1, including vasoconstriction, proteinuria, inflammation, cellular injury and fibrosis, are likely mediated by the endothelin A (ETA) receptor. ETA antagonism alone, and/or combined ETA/B blockade, reduces CKD progression. Based on the strong pre-clinical data, several clinical trials using ETA antagonists were conducted. Small trials involving acute intravenous endothelin receptor blockade suggest that ETA, but not ETB, blockade exerts protective renal and vascular effects in CKD patients. A large phase 3 trial (ASCEND) examined the effects of avosentan, an endothelin receptor antagonist, on renal disease progression in diabetic nephropathy. Proteinuria was reduced after 3-6 months of treatment. However the study was terminated due to increased morbidity and mortality associated with avosentan-induced fluid retention. Several phase 2 trials using avosentan at lower doses than in ASCEND, atrasentan or sitaxsentan (the latter two being highly ETA-selective) showed reductions in proteinuria on top of renin-angiotensin system blockade. Infrequent and clinically insignificant fluid retention was observed at the most effective doses. Additional trials using ETA blockers are ongoing or being planned in patients with diabetic nephropathy or focal segmental glomerulosclerosis. Moving forward, such studies must be conducted with careful patient selection and attention to dosing in order to minimize adverse side effects. Nonetheless, there is cause for optimism that this class of agents will ultimately prove to be effective for the treatment of CKD.

Clopidogrel preserves whole kidney autoregulatory behavior in ANG II-induced hypertension

This study tested the hypothesis that P2Y12 receptor blockade with clopidogrel preserves renal autoregulatory ability during ANG II-induced hypertension. Clopidogrel was administered orally to male Sprague-Dawley rats chronically infused with ANG II. After 14 days of treatment, whole kidney autoregulation of renal blood flow was assessed in vivo in pentobarbital-anesthetized rats using an ultrasonic flow probe placed around the left renal artery. In ANG II-vehicle-treated rats, decreasing arterial pressure over a range from 160 to 100 mmHg resulted in a 25 ± 5% decrease in renal blood flow, demonstrating a significant loss of autoregulation with an autoregulatory index of 0.66 ± 0.15. However, clopidogrel treatment preserved autoregulatory behavior in ANG II-treated rats to levels indistinguishable from normotensive sham-operated (sham) rats (autoregulatory index: 0.04 ± 0.14). Compared with normotensive sham-vehicle-treated rats, ANG II infusion increased renal CD3-positive T cell infiltration by 66 ± 6%, induced significant thickening of the preglomerular vessels and glomerular basement membrane and increased glomerular collagen I deposition, tubulointerstitial fibrosis, damage to the proximal tubular brush border, and protein excretion. Clopidogrel significantly reduced renal infiltration of T cells by 39 ± 9% and prevented interstitial artery thickening, ANG II-induced damage to the glomerular basement membrane, deposition of collagen type I, and tubulointerstitial fibrosis, despite the maintenance of hypertension. These data demonstrate that systemic P2Y12 receptor blockade with clopidogrel protects against impairment of autoregulatory behavior and renal vascular injury in ANG II-induced hypertension, possibly by reducing renal T cell infiltration.

Hypertension: physiology and pathophysiology

Despite major advances in understanding the pathophysiology of hypertension and availability of effective and safe antihypertensive drugs, suboptimal blood pressure (BP) control is still the most important risk factor for cardiovascular mortality and is globally responsible for more than 7 million deaths annually. Short-term and long-term BP regulation involve the integrated actions of multiple cardiovascular, renal, neural, endocrine, and local tissue control systems. Clinical and experimental observations strongly support a central role for the kidneys in the long-term regulation of BP, and abnormal renal-pressure natriuresis is present in all forms of chronic hypertension. Impaired renal-pressure natriuresis and chronic hypertension can be caused by intrarenal or extrarenal factors that reduce glomerular filtration rate or increase renal tubular reabsorption of salt and water; these factors include excessive activation of the renin-angiotensin-aldosterone and sympathetic nervous systems, increased formation of reactive oxygen species, endothelin, and inflammatory cytokines, or decreased synthesis of nitric oxide and various natriuretic factors. In human primary (essential) hypertension, the precise causes of impaired renal function are not completely understood, although excessive weight gain and dietary factors appear to play a major role since hypertension is rare in nonobese hunter-gathers living in nonindustrialized societies. Recent advances in genetics offer opportunities to discover gene-environment interactions that may also contribute to hypertension, although success thus far has been limited mainly to identification of rare monogenic forms of hypertension.

Distinct regulation of inner medullary collecting duct nitric oxide production from mice and rats

Nitric oxide (NO) and NO synthase 1 (NOS1) maintain sodium and water homeostasis. The NOS1α and NOS1β splice variants are expressed in the rat inner medulla, but only NOS1β is expressed in the mouse. Collecting duct NOS1 is necessary for blood pressure control. We hypothesized that NOS1 splice variant expression and NO production in the inner medullary collecting duct (IMCD) are regulated differently in mice and rats by high dietary sodium. Male C57blk/J6 mice and Sprague-Dawley rats were fed a 0.4% (normal salt; NS), or 4% (high salt; HS) NaCl diet for 2 or 7 days. Mean arterial pressure was not altered by HS, whereas urinary sodium excretion in mice and rats was increased significantly. Urinary excretion of nitrate/nitrite (NO(x)) and IMCD nitrite production were significantly greater in mice compared with rats on the HS diet. Western blotting indicated that only NOS1β and NOS3 were expressed in the mouse IMCD and that expression was unaffected by the HS diet at either time point. In contrast, NOS1α was detected in the IMCD of rats, in addition to NOS1β and NOS3. Feeding of the HS diet for 2 days increased NOS1α and NOS1β expression in the rat IMCD and 7 day feeding of the HS diet further increased NOS1β expression. Expression of NOS3 was unchanged by the HS diet at either time point. In conclusion, IMCD NO production in mice and rats is distinctly regulated under both NS and HS conditions, including expression of NOS1 splice variants.

Sex differences in ET-1 receptor expression and Ca2+ signaling in the IMCD

The inner medullary collecting duct (IMCD) is the nephron segment with the highest production of endothelin-1 (ET-1) and the greatest expression of ET-1 receptors that function to adjust Na(+) and water balance. We have reported that male rats have reduced natriuresis in response to direct intramedullary infusion of ET-1 compared with female rats. Our aim was to determine whether alterations of ET-1 receptor expression and downstream intracellular Ca(2+) signaling within the IMCD could account for these sex differences. IMCDs from male and female rats were isolated for radioligand binding or microdissected for intracellular Ca(2+) ([Ca(2+)]i) measurement by fluorescence imaging of fura-2 AM. IMCD from male and female rats had similar ETB expression (655 ± 201 vs. 567 ± 39 fmol/mg protein, respectively), whereas male rats had significantly higher ETA expression (436 ± 162 vs. 47 ± 29 fmol/mg protein, respectively; P < 0.05). The [Ca(2+)]i response to ET-1 was significantly greater in IMCDs from male compared with female rats (288 ± 52 vs. 118 ± 32 AUC, nM × 3 min, respectively; P < 0.05). In IMCDs from male rats, the [Ca(2+)]i response to ET-1 was significantly blunted by the ETA antagonist BQ-123 but not by the ETB antagonist BQ-788 (control: 137 ± 27; BQ-123: 53 ± 11; BQ-788: 84 ± 25 AUC, nM × 3 min; P < 0.05), consistent with greater ETA receptor function in male rats. These data demonstrate a sex difference in ETA receptor expression that results in differences in ET-1 Ca(2+) signaling in IMCD. Since activation of ETA receptors is thought to oppose ETB receptor activation, enhanced ETA function in male rats could limit the natriuretic effects of ETB receptor activation.

ET-1 actions in the kidney: evidence for sex differences

Hypertension and chronic kidney disease are more common in men than in premenopausal women at the same age. In animal models, females are relatively protected against genetic or pharmacological procedures that produce high blood pressure and renal injury. Overactivation or dysfunction of the endothelin (ET) system modulates the progression of hypertension or kidney diseases with the ET(A) receptor primarily mediating vasoconstriction, injury and anti-natriuresis, and ET(B) receptors having opposite effects. The purpose of this review is to examine the role of the ET system in the kidney with a focus on the inequality between the sexes associated with the susceptibility to and progression of hypertension and kidney diseases. In most animal models, males have higher renal ET-1 mRNA expression, greater ET(A) -mediated responses, including renal medullary vasoconstriction, and increased renal injury. These differences are reduced following gonadectomy suggesting a role for sex hormones, mainly testosterone. In contrast, females are relatively protected from high blood pressure and kidney damage via increased ET(B) versus ET(A) receptor function. Furthermore, ET(A) receptors may have a favourable effect on sodium excretion and reducing renal damage in females. In human studies, the genetic polymorphisms of the ET system are more associated with hypertension and renal injury in women. However, the knowledge of sex differences in the efficacy or adverse events of ET(A) antagonists in the treatment of hypertension and kidney disease is poorly described. Increased understanding how the ET system acts differently in the kidneys between sexes, especially with regard to receptor subtype function, could lead to better treatments for hypertension and renal disease. LINKED ARTICLES This article is part of a themed section on Endothelin. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.168.issue-1.

Pathophysiological roles of endothelin receptors in cardiovascular diseases

Endothelin (ET)-1 derived from endothelial cells has a much more important role in cardiovascular system regulation than the ET-2 and ET-3 isoforms. Numerous lines of evidence indicate that ET-1 possesses a number of biological activities leading to cardiovascular diseases (CVD) including hypertension and atherosclerosis. Physiological and pathophysiological responses to ET-1 in various tissues are mediated by interactions with ET(A)- and ET(B)-receptor subtypes. Both subtypes on vascular smooth muscle cells mediate vasoconstriction, whereas the ET(B)-receptor subtype on endothelial cells contributes to vasodilatation and ET-1 clearance. Although selective ET(A)- or nonselective ET(A)/ET(B)-receptor antagonisms have been assumed as potential strategies for the treatment of several CVD based on clinical and animal experiments, it remains unclear which antagonisms are suitable for individuals with CVD because upregulation of the nitric oxide system via the ET(B) receptor is responsible for vasoprotective effects such as vasodilatation and anti-cell proliferation. In this review, we have summarized the current understanding regarding the role of ET receptors, especially the ET(B) receptor, in CVD.

Modulators of the vascular endothelin receptor in blood pressure regulation and hypertension

Endothelin (ET) is one of the most investigated molecules in vascular biology. Since its discovery two decades ago, several ET isoforms, receptors, signaling pathways, agonists and antagonists have been identified. ET functions as a potent endothelium-derived vasoconstrictor, but could also play a role in vascular relaxation. In endothelial cells, preproET and big ET are cleaved by ET converting enzymes into ET-1, -2, -3 and -4. These ET isoforms bind with different affinities to ET(A) and ET(B) receptors in vascular smooth muscle (VSM), and in turn increase [Ca(2+)](i), protein kinase C and mitogen-activated protein kinase and other signaling pathways of VSM contraction and cell proliferation. ET also binds to endothelial ET(B) receptors and stimulates the release of nitric oxide, prostacyclin and endothelium-derived hyperpolarizing factor. ET, via endothelial ET(B) receptor, could also promote ET re-uptake and clearance. While the effects of ET on vascular reactivity and growth have been thoroughly examined, its role in the regulation of blood pressure and the pathogenesis of hypertension is not clearly established. Elevated plasma and vascular tissue levels of ET have been identified in salt-sensitive hypertension and in moderate to severe hypertension, and ET receptor antagonists have been shown to reduce blood pressure to variable extents in these forms of hypertension. The development of new pharmacological and genetic tools could lead to more effective and specific modulators of the vascular ET system for treatment of hypertension and related cardiovascular disease.

Loss of renal medullary endothelin B receptor function during salt deprivation is regulated by angiotensin II

We have recently demonstrated that chronic infusion of exogenous ANG II, which induces blood pressure elevation, attenuates renal medullary endothelin B (ET(B)) receptor function in rats. Moreover, this was associated with a reduction of ET(B) receptor expression in the renal inner medulla. The aim of this present work was to investigate the effect of a physiological increase in endogenous ANG II (low-salt diet) on the renal ET system, including ET(B) receptor function. We hypothesized that endogenous ANG II reduces renal medullary ET(B) receptor function during low-salt intake. Rats were placed on a low-salt diet (0.01-0.02% NaCl) for 2 wk to allow an increase in endogenous ANG II. In rats on normal-salt chow, the stimulation of renal medullary ET(B) receptor by ET(B) receptor agonist sarafotoxin 6c (S6c) causes an increase in water (3.6 ± 0.4 from baseline vs. 10.5 ± 1.3 μl/min following S6c infusion; P < 0.05) and sodium excretion (0.38 ± 0.06 vs. 1.23 ± 0.17 μmol/min; P < 0.05). The low-salt diet reduced the ET(B)-dependent diuresis (4.5 ± 0.5 vs. 6.1 ± 0.9 μl/min) and natriuresis (0.40 ± 0.11 vs. 0.46 ± 0.12 μmol/min) in response to acute intramedullary infusion of S6c. Chronic treatment with candesartan restored renal medullary ET(B) receptor function; urine flow was 7.1 ± 0.9 vs. 15.9 ± 1.7 μl/min (P < 0.05), and sodium excretion was 0.4 ± 0.1 vs. 1.1 ± 0.1 μmol/min (P < 0.05) before and after intramedullary S6c infusion, respectively. Receptor binding assays determined that the sodium-depleted diet resulted in a similar level of ET(B) receptor binding in renal inner medulla compared with rats on a normal-salt diet. Candesartan reduced renal inner medullary ET(B) receptor binding (1,414 ± 95 vs. 862 ± 50 fmol/mg; P < 0.05). We conclude that endogenous ANG II attenuates renal medullary ET(B) receptor function to conserve sodium during salt deprivation independently of receptor expression.

Role of 20-hydroxyeicosatetraenoic acid in mediating hypertension in response to chronic renal medullary endothelin type B receptor blockade

Background

The renal medullary endothelin (ET-1) system plays an important role in the control of sodium excretion and arterial pressure (AP) through the activation of renal medullary ET-B receptors. We have previously shown that blockade of endothelin type B receptors (ET-B) leads to salt-sensitive hypertension through mechanisms that are not fully understood. One possible mechanism is through a reduction in renal medullary production of 20-hydroxyeicosatetraenoic acid (20-HETE). 20-HETE, a metabolite of arachidonic acid, has natriuretic properties similar to ET-B activation. While these findings suggest a possible interaction between ET-B receptor activation and 20-HETE production, it is unknown whether blockade of medullary ET-B receptors in rats maintained on a high sodium intake leads to reductions in 20-HETE production.

Methodology/Principal Findings

The effect of increasing sodium intake from low (NS = .8%) to high (HS = 8%) on renal medullary production of 20-HETE in the presence and absence of renal medullary ET-B receptor antagonism was examined. Renal medullary blockade of ET-B receptors resulted in salt sensitive hypertension. In control rats, blood pressure rose from 112.8±2.4 mmHg (NS) to 120.7±9.3 mmHg (HS). In contrast, when treated with an ET-B receptor blocker, blood pressure was significantly elevated from 123.7±3.2 (NS) to 164.2±7.1 (HS). Furthermore, increasing sodium intake was associated with elevated medullary 20-HETE (5.6±.8 in NS vs. 14.3±3.7 pg/mg in HS), an effect that was completely abolished by renal medullary ET-B receptor blockade (4.9±.8 for NS and 4.5±.6 pg/mg for HS). Finally, the hypertensive response to intramedullary ET-B receptor blockade was blunted in rats pretreated with a specific 20-HETE synthesis inhibitor.

Conclusion

These data suggest that increases in renal medullary production of 20-HETE associated with elevating salt intake may be, in part, due to ET-B receptor activation within the renal medulla.

Immunosuppression with mycophenolate mofetil attenuates the development of hypertension and albuminuria in deoxycorticosterone acetate-salt hypertensive rats

1. The interplay between the immune and renin-angiotensin systems is emerging as a crucial factor in the development and progression of hypertension. The aim of the present study was to determine the involvement of immune cells in the hypertension and renal injury produced by a non-angiotensin II-dependent form of hypertension, namely deoxycorticosterone acetate (DOCA)-salt-induced hypertension, in rats. 2. Male Sprague-Dawley rats underwent uninephrectomy and received either a sustained-release pellet of DOCA s.c. and 0.9% NaCl (saline) to drink for 21 days or a placebo pellet and water to drink for 21 days. Additional groups of DOCA-salt- and placebo-treated rats were treated concurrently with the immune suppressant mycophenolate mofetil (MMF; 30 mg/kg per day). Rats were placed in metabolic cages for 24 h urine collection prior to and at weekly intervals during the 21 day experimental period. 3. Mycophenolate mofetil significantly attenuated the development of hypertension in DOCA-salt rats compared with untreated DOCA-salt hypertensive rats (mean arterial pressure by telemetry on Day 18,146 ± 7 vs 180 ± 3 mmHg, respectively; P < 0.001), as well as proteinuria (87 ± 27 vs 305 ± 63 mg/day, respectively, on Day 21) and albuminuria (51 ± 15 vs 247 ± 73 mg/day, respectively, on Day 21). Creatinine clearance was better preserved in MMF-treated DOCA-salt rats compared with untreated DOCA-salt rats (0.74 ± 0.07 vs 0.49 ± 0.09 mL/min, respectively; P < 0.05), but was still significantly reduced compared with that in the placebo group (1.15 ± 0.12 mL/min; P < 0.05). Finally, MMF treatment significantly attenuated the DOCA-salt-induced rise in renal cortical T-lymphocyte and macrophage infiltration (P < 0.05). 4. These data indicate that immune cells play a deleterious role in both the hypertension and renal injury associated with DOCA-salt hypertension.

Regulation of sodium transport in the proximal tubule by endothelin

Human essential hypertension and rodent genetic hypertension are associated with increased sodium transport in the renal proximal tubule and medullary thick ascending limb of Henle. The proximal tubule, which secretes endothelin (ET), expresses the ET(B) receptor. Low (nM) concentrations of ET, via the ET(B) receptor, inhibit sodium and water transport and ATP-driven drug secretion in the proximal tubule. In contrast, very low (pM) and high nM concentrations of ET increase renal proximal sodium transport, but the receptor involved remains to be determined. The natriuretic effect of ET(B) receptor stimulation is impaired in spontaneously hypertensive rats, due in part to a defective interaction with D(3) dopamine and angiotensin II type 1 receptors. Impaired ET(B) receptor function in the renal proximal tubule may be important in the pathogenesis of genetic hypertension.