Mechanisms of big endothelin-1-induced diuresis and natriuresis : role of ET(B) receptors

G protein-coupled estrogen receptor 1 regulates renal endothelin-1 signaling system in a sex-specific manner

Demographic studies reveal lower prevalence of hypertension among premenopausal females compared to age-matched males. The kidney plays a central role in the maintenance of sodium (Na⁺) homeostasis and consequently blood pressure. Renal endothelin-1 (ET-1) is a pro-natriuretic peptide that contributes to sex differences in blood pressure regulation and Na⁺ homeostasis. We recently showed that activation of renal medullary G protein-coupled estrogen receptor 1 (GPER1) promotes ET-1-dependent natriuresis in female, but not male, rats. We hypothesized that GPER1 upregulates the renal ET-1 signaling system in females, but not males. To test our hypothesis, we determined the effect of GPER1 deletion on ET-1 and its downstream effectors in the renal cortex, outer and inner medulla obtained from 12-16-week-old female and male mice. GPER1 knockout (KO) mice and wildtype (WT) littermates were implanted with telemetry transmitters for blood pressure assessment, and we used metabolic cages to determine urinary Na⁺ excretion. GPER1 deletion did not significantly affect 24-h mean arterial pressure (MAP) nor urinary Na⁺ excretion. However, GPER1 deletion decreased urinary ET-1 excretion in females but not males. Of note, female WT mice had greater urinary ET-1 excretion than male WT littermates, whereas no sex differences were observed in GPER1 KO mice. GPER1 deletion increased inner medullary ET-1 peptide content in both sexes but increased outer medullary ET-1 content in females only. Cortical ET-1 content increased in response to GPER1 deletion in both sexes. Furthermore, GPER1 deletion notably increased inner medullary ET receptor A (ET_A) and decreased outer medullary ET receptor B (ET_B) mRNA expression in male, but not female, mice. We conclude that GPER1 is required for greater ET-1 excretion in females. Our data suggest that GPER1 is an upstream regulator of renal medullary ET-1 production and ET receptor expression in a sex-specific manner. Overall, our study identifies the role of GPER1 as a sex-specific upstream regulator of the renal ET-1 system.

Cardioprotective Effects of Mebudipine in a Rat Model of Doxorubicin-Induced Heart Failure

Background

Mebudipine, a dihydropyridine calcium-channel blocker (CCB), shows greater time- and voltage-dependent inhibitory effects than nifedipine. Its significant negative chronotropic effects without having considerable negative inotropic properties may make it a suitable candidate for the pharmacotherapy of heart failure (HF). This study aimed to investigate the possible beneficial action of mebudipine in a rat model of HF.

Methods

The present study carried out in the Department of Pharmacology at the Iran University of Medical Sciences during the years of 2009-2011. An experimental model of HF was induced in male Wistar rats using doxorubicin (DOX). The rats were divided into five groups with seven animals in each group: normal control group, DOX-induced HF control groups, and treatment groups. The animals were administered DOX for 15 days. A consistent deterioration occurred after a four-week rest period. The animals were then treated with intraperitoneal mebudipine (0.5 mg/kg) and intraperitoneal amlodipine (0.35 mg/kg), as well as an equal volume of distilled water for 15 days. The plasma levels of big endothelin-1 (BET-1), creatine kinase-myocardial band (CK-MB), lactate dehydrogenase (LDH), aspartate aminotransferase (AST), and alanine aminotransferase (ALT), as well as the clinical status (heart rate and blood pressure), were assessed before and after treatment. Statistical analysis was performed with SPSS software using parametric and nonparametric ANOVA.

Results

Mebudipine and amlodipine reversed the increased plasma BET-1 values in the treated animals when compared with the HF control group (0.103 and 0.112 vs 0.231 pg/mL, respectively). The increased plasma levels of AST, ALT, CK-MB, and LDH were also reversed in the HF animals that received mebudipine or amlodipine.

Conclusion

The administration of mebudipine to HF animals, akin to amlodipine, palliated the clinical and biochemical signs of the disease in the present study. The abstract was presented in the Iranian Congress of Physiology and Pharmacology as a poster and published in the Scientific Information Database as a supplement (2015; Vol 22).

Role of GRK4 in the regulation of the renal ETB receptor in hypertension

The endothelin receptor type B (ETBR) regulates water and electrolyte balance and blood pressure, in part, by inhibiting renal sodium transport. Our preliminary study found that the ETBR-mediated diuresis and natriuresis are impaired in hypertension with unknown mechanism. Persistently increased activity of G protein-coupled receptor kinase 4 (GRK4), caused by increased expression or genetic variants (eg, GRKγ142V), impairs the ability of the kidney to excrete a sodium load, in part, by impairing renal dopamine D₁ receptor function through persistent phosphorylation. Our present study found that although renal ETBR expression was not different between Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHRs), renal ETBR phosphorylation was higher in SHRs. The role of hyper-phosphorylation in impaired ETBR-function was supported by results in human (h) GRK4γ transgenic mice. Stimulation of ETBR by BQ3020-induced natriuresis in human (h) GRK4γ wild-type (WT) mice. However, in hGRK4γ 142V transgenic mice, the renal ETBR was hyperphosphorylated and ETBR-mediated natriuresis and diuresis were not evident. There were co-localization and co-immunoprecipitation of ETBR and GRK4 in renal proximal tubule (RPT) cells from both WKY and SHRs but was greater in the latter than the former group. SiRNA-mediated downregulation of GRK4 expression, recovered the impaired inhibitory effect of ETBR on Na⁺ -K⁺ -ATPase activity in RPT cells from SHR. In vivo downregulation of renal GRK4 expression, via ultrasound-targeted microbubble destruction, decreased ETBR phosphorylation and restored ETBR-mediated natriuresis and diuresis in SHRs. This study provides a mechanism by which GRK4, via regulation of renal ETBR function, participates in the pathogenesis of hypertension.

Alterations in endothelin receptors following hemorrhage and resuscitation by centhaquin

Endothelin-1 (ET-1) acts on ET(A) and ET(B) receptors and has been implicated in hemorrhagic shock (shock). We determined effect of shock and resuscitation by hypertonic saline (saline) or centhaquin on ET(A) and ET(B) receptor expression. Rats were anesthetized, a pressure catheter was placed in the left femoral artery; blood was withdrawn from the right femoral artery to bring mean arterial pressure (MAP) to 35 mm Hg for 30 min, resuscitation was performed and 90 min later sacrificed to collect samples for biochemical estimations. Resuscitation with centhaquin decreased blood lactate and increased MAP. Protein levels of ET(A) or ET(B) receptor were unaltered in the brain, heart, lung and liver following shock or resuscitation. In the abdominal aorta, shock produced an increase (140 %) in ET(A) expression which was attenuated by saline and centhaquin; ET(B) expression was unaltered following shock but was increased (79 %) by centhaquin. In renal medulla, ET(A) expression was unaltered following shock, but was decreased (-61 %) by centhaquin; shock produced a decrease (-34 %) in ET(B) expression which was completely attenuated by centhaquin and not saline. Shock induced changes in ET(A) and ET(B) receptors in the aorta and renal medulla are reversed by centhaquin and may be contributing to its efficacy.

First-in-Man Demonstration of Direct Endothelin-Mediated Natriuresis and Diuresis

Endothelin (ET) receptor antagonists are potentially novel therapeutic agents in chronic kidney disease and resistant hypertension, but their use is complicated by sodium and water retention. In animal studies, this side effect arises from ETB receptor blockade in the renal tubule. Previous attempts to determine whether this mechanism operates in humans have been confounded by the hemodynamic consequences of ET receptor stimulation/blockade. We aimed to determine the effects of ET signaling on salt transport in the human nephron by administering subpressor doses of the ET-1 precursor, big ET-1. We conducted a 2-phase randomized, double-blind, placebo-controlled crossover study in 10 healthy volunteers. After sodium restriction, subjects received either intravenous placebo or big ET-1, in escalating dose (≤300 pmol/min). This increased plasma concentration and urinary excretion of ET-1. Big ET-1 reduced heart rate (≈8 beats/min) but did not otherwise affect systemic hemodynamics or glomerular filtration rate. Big ET-1 increased the fractional excretion of sodium (from 0.5 to 1.0%). It also increased free water clearance and tended to increase the abundance of the sodium-potassium-chloride cotransporter (NKCC2) in urinary extracellular vesicles. Our protocol induced modest increases in circulating and urinary ET-1. Sodium and water excretion increased in the absence of significant hemodynamic perturbation, supporting a direct action of ET-1 on the renal tubule. Our data also suggest that sodium reabsorption is stimulated by ET-1 in the thick ascending limb and suppressed in the distal renal tubule. Fluid retention associated with ET receptor antagonist therapy may be circumvented by coprescribing potassium-sparing diuretics.

Endothelin-1 mediated high glucose-induced epithelial-mesenchymal transition in renal tubular cells

OBJECTIVE

The pathogenesis of interstitial fibrosis in diabetic nephropathy (DN) is an intractable problem without good therapy. Emerging evidence suggests that epithelial-mesenchymal transition (EMT) is an important mechanism for tubular epithelial cells undergoing profibrotic change in DN. Endothelin-1 (ET-1) is an important cytokine which can cause fibrogenesis and is reportedly involved in DN. However, the role of ET-1 in EMT in DN is unknown. The present study was designed to investigate the role of ET-1 in high glucose-induced EMT and the signaling pathway mediating the effect of ET-1 in renal tubular cells.

METHOD

Tubular epithelial cells (NRK52E) were treated with normal glucose (d-glucose 5.6mmol/L, NG), high glucose (30mmol/L, HG), high osmotic (d-glucose 5.6mmol/L+d-mannitol 24.4mmol/L), HG+ETA antagonist BQ123 (2μg/ml), ET-1, ET-1+ hypoxia inducible factor (HIF)-1α siRNA, CoCl2 (100μmol/L), CoCl2+HIF-1α siRNA or CoCl2+BQ123. The supernatant level of ET-1 was measured by ELISA and the expression of vimentin, E-cadherin and HIF-1α was detected by RT-PCR and western blot.

RESULT

The ET-1 level increased markedly in the supernatant of NRK52E incubated with HG. In NRK52E induced with HG or ET-1, the expression of vimentin was upregulated, whereas the expression of E-cadherin was downregulated. BQ123 attenuated HG- and CoCl2-induced EMT while HIF-1α siRNA did not affect ET-1 induced EMT.

CONCLUSIONS

High glucose induced ET-1 production that mediated the EMT induced by high glucose in renal tubular epithelial cells, and HIF-1α acted as the upstream signal to regulate ET-1.

Endothelin antagonists in hypertension and kidney disease

The endothelin (ET) system seems to play a pivotal role in hypertension and in proteinuric kidney disease, including the micro- and macro-vascular complications of diabetes. Endothelin-1 (ET-1) is a multifunctional peptide that primarily acts as a potent vasoconstrictor with direct effects on systemic vasculature and the kidney. ET-1 and ET receptors are expressed in the vascular smooth muscle cells, endothelial cells, fibroblasts and macrophages in systemic vasculature and arterioles of the kidney, and are associated with collagen accumulation, inflammation, extracellular matrix remodeling, and renal fibrosis. Experimental evidence and recent clinical studies suggest that endothelin receptor blockade, in particular selective ETAR blockade, holds promise in the treatment of hypertension, proteinuria, and diabetes. Concomitant blockade of the ETB receptor is not usually beneficial and may lead to vasoconstriction and salt and water retention. The side-effect profile of ET receptor antagonists and relatively poor antagonist selectivity for ETA receptor are limitations that need to be addressed. This review will discuss what is currently known about the endothelin system, the role of ET-1 in the pathogenesis of hypertension and kidney disease, and summarize literature on the therapeutic potential of endothelin system antagonism.

Physiology of endothelin and the kidney

Since its discovery in 1988 as an endothelial cell-derived peptide that exerts the most potent vasoconstriction of any known endogenous compound, endothelin (ET) has emerged as an important regulator of renal physiology and pathophysiology. This review focuses on how the ET system impacts renal function in health; it is apparent that ET regulates multiple aspects of kidney function. These include modulation of glomerular filtration rate and renal blood flow, control of renin release, and regulation of transport of sodium, water, protons, and bicarbonate. These effects are exerted through ET interactions with almost every cell type in the kidney, including mesangial cells, podocytes, endothelium, vascular smooth muscle, every section of the nephron, and renal nerves. In addition, while not the subject of the current review, ET can also indirectly affect renal function through modulation of extrarenal systems, including the vasculature, nervous system, adrenal gland, circulating hormones, and the heart. As will become apparent, these pleiotropic effects of ET are of fundamental physiologic importance in the control of renal function in health. In addition, to help put these effects into perspective, we will also discuss, albeit to a relatively limited extent, how alterations in the ET system can contribute to hypertension and kidney disease.

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.

Changes in endothelin receptor type B and neuronal nitric oxide synthase in puromycin aminonucleoside-induced nephrotic syndrome

The collecting duct endothelin (ET) system, which involves ET-1 and its two receptors, may play a role in the regulation of renal sodium in association with the nitric oxide synthase (NOS) system. We determined whether sodium retention is associated with changes in the endothelin and NOS systems at different stages (i.e., a sodium retaining stage and a compensatory stage) of nephrotic syndromes. On day 7 after puromycin aminonucleoside (PAN) injection, urinary sodium excretion was decreased, ascites had developed, and there was a positive sodium balance. ET-1 mRNA expression was increased in the inner medulla of the kidney, whereas protein expression of ET receptor type B (ET(B)R) was unchanged. The expression of neuronal NOS (nNOS) was decreased in the inner medulla. On day 14, urinary sodium excretion was unchanged compared with controls. The expression of ET(B)R increased, while nNOS expression in the inner medulla was comparable to controls. These findings suggest that decreased nNOS plays a role in the development of sodium retention in the nephrotic syndrome. Recovery of nNOS and increased renal ET(B)R synthesis may promote sodium excretion in later stages of the nephrotic syndrome (on day 14).

Endothelial dysfunction: a strategic target in the treatment of hypertension?

Endothelial dysfunction is a common feature of hypertension, and it results from the imbalanced release of endothelium-derived relaxing factors (EDRFs; in particular, nitric oxide) and endothelium-derived contracting factors (EDCFs; angiotensin II, endothelins, uridine adenosine tetraphosphate, and cyclooxygenase-derived EDCFs). Thus, drugs that increase EDRFs (using direct nitric oxide releasing compounds, tetrahydrobiopterin, or L-arginine supplementation) or decrease EDCF release or actions (using cyclooxygenase inhibitor or thromboxane A2/prostanoid receptor antagonists) would prevent the dysfunction. Many conventional antihypertensive drugs, including angiotensin-converting enzyme inhibitors, calcium channel blockers, and third-generation beta-blockers, possess the ability to reverse endothelial dysfunction. Their use is attractive, as they can address arterial blood pressure and vascular tone simultaneously. The severity of endothelial dysfunction correlates with the development of coronary artery disease and predicts future cardiovascular events. Thus, endothelial dysfunction needs to be considered as a strategic target in the treatment of hypertension.

Regulation of vascular contractility and blood pressure by the E2F2 transcription factor

BACKGROUND

Recent studies have identified a polymorphism in the endothelin-converting enzyme (ECE)-1b promoter (-338C/A) that is strongly associated with hypertension in women. The polymorphism is located in a consensus binding sequence for the E2F family of transcription factors. E2F proteins are crucially involved in cell-cycle regulation, but their roles in cardiovascular function are poorly understood. Here, we investigated the potential role of E2F2 in blood pressure regulation.

METHODS AND RESULTS

Tail-cuff measurements of systolic and diastolic blood pressures were significantly higher in E2F2-null (E2F2(-/-)) mice than in their wild-type littermates, and in ex vivo ring assays, aortas from the E2F2(-/-) mice exhibited significantly greater contractility in response to big endothelin-1. Big endothelin-1 is activated by ECE-1, and mRNA levels of ECE-1b, the repressive ECE-1 isoform, were significantly lower in E2F2(-/-) mice than in wild-type mice. In endothelial cells, chromatin immunoprecipitation assays confirmed that E2F2 binds the ECE-1b promoter, and promoter-reporter assays indicated that E2F2 activates ECE-1b transcription. Furthermore, loss or downregulation of E2F2 led to a decline in ECE-1b levels, to higher levels of the membranous ECE-1 isoforms (ie, ECE-1a, -1c, and -1d), and to deregulated ECE-1 activity. Finally, Sam68 coimmunoprecipitated with E2F2, occupied the ECE-1b promoter (chromatin immunoprecipitation), and repressed E2F2-mediated ECE-1b promoter activity (promoter-reporter assays).

CONCLUSIONS

Our results identify a cell-cycle-independent mechanism by which E2F2 regulates endothelial function, arterial contractility, and blood pressure.

Aldosterone modulates steroid receptor binding to the endothelin-1 gene (edn1)

Aldosterone and endothelin-1 (ET-1) act on collecting duct cells of the kidney and are important regulators of renal sodium transport and cardiovascular physiology. We recently identified the ET-1 gene (edn1) as a novel aldosterone-induced transcript. However, aldosterone action on edn1 has not been characterized at the present time. In this report, we show that aldosterone stimulated edn1 mRNA in acutely isolated rat inner medullary collecting duct cells ex vivo and ET-1 peptide in rat inner medulla in vivo. Aldosterone induction of edn1 mRNA occurred in cortical, outer medullary, and inner medullary collecting duct cells in vitro. Inspection of the edn1 promoter revealed two putative hormone response elements. Levels of heterogeneous nuclear RNA synthesis demonstrated that edn1 mRNA stimulation occurred at the level of transcription. RNA knockdowns corroborated pharmacological studies and demonstrated both mineralocorticoid receptor and glucocorticoid receptor participated in this response. Aldosterone resulted in dose-dependent nuclear translocation and binding of mineralocorticoid receptor and glucocorticoid receptor to the edn1 hormone response elements. Hormone receptors mediated the association of chromatin remodeling complexes, histone modification, and RNA polymerase II at the edn1 promoter. Direct interaction between aldosterone and ET-1 has important implications for renal and cardiovascular function.

Endothelin-1, big endothelin-1, and nitric oxide in patients with chronic renal disease and hypertension

The complex pathogenesis of chronic renal disease (CRD) depends on endothelin (ET) axis (ETs and ET receptors) and nitric oxide (NO) because of their vasoactive effects and their role in general modulation of vascular homeostasis. Various renal cells synthesize ETs and NO that play a significant role in renal hemodynamics as well as in water and salt excretion via urine. ET-1 is a strong vasoconstrictor. Besides its vasoactive effects, ET-1 modulates mitosis and apoptosis in a cell type-dependent manner, and may play an important role in CRD pathogenesis. The aims of this study were to emphasize the role and interactions of ET-1, Big ET-1, and NO in CRD. Concentrations of these vasoactive molecules were measured in plasma/serum and/or urine of 57 patients with diabetic nephropathy (subgroup 1), arterial hypertension (subgroup 2) or CRD with chronic renal insufficiency (subgroup 3), and in healthy control subjects (n=18). In comparison with control group, urine concentration of Big ET-1 was significantly increased (13.13 pmol/L vs. 11.34 pmol/L; P<0.001) in CRD patients, whereas plasma and urine concentrations of ET-1 did not differ significantly. NO concentrations were also significantly increased in CRD patients (serum, 72.55 micromol/L; P<0.001, and urine 141.74 micromol/L; P<0.05) as compared to control group. Study results indicated that Big ET-1 and NO could be useful diagnostic parameters in CRD for their diagnostic sensitivity and diagnostic specificity (Big ET-1 in urine: 56.1 and 88.9%, and NO in serum: 66.7 and 83.3%, respectively). In addition, Big ET-1 may prove useful in the differential diagnosis of diabetic nephropathy (78.6% diagnostic sensitivity and 88.9% diagnostic specificity).

Physiology and pathophysiology of the vasopressin-regulated renal water reabsorption

To prevent dehydration, terrestrial animals and humans have developed a sensitive and versatile system to maintain their water homeostasis. In states of hypernatremia or hypovolemia, the antidiuretic hormone vasopressin (AVP) is released from the pituitary and binds its type-2 receptor in renal principal cells. This triggers an intracellular cAMP signaling cascade, which phosphorylates aquaporin-2 (AQP2) and targets the channel to the apical plasma membrane. Driven by an osmotic gradient, pro-urinary water then passes the membrane through AQP2 and leaves the cell on the basolateral side via AQP3 and AQP4 water channels. When water homeostasis is restored, AVP levels decline, and AQP2 is internalized from the plasma membrane, leaving the plasma membrane watertight again. The action of AVP is counterbalanced by several hormones like prostaglandin E2, bradykinin, dopamine, endothelin-1, acetylcholine, epidermal growth factor, and purines. Moreover, AQP2 is strongly involved in the pathophysiology of disorders characterized by renal concentrating defects, as well as conditions associated with severe water retention. This review focuses on our recent increase in understanding of the molecular mechanisms underlying AVP-regulated renal water transport in both health and disease.

Low NaCl intake elevates renal medullary endothelin-1 and endothelin A (ETA) receptor mRNA but not the sensitivity of renal Na+ excretion to ETA receptor blockade in rats

AIMS

This study was performed to investigate the effects of NaCl intake on renal mRNA expression of pre-pro-endothelin-1 (ET-1), endothelin A (ET(A)) and endothelin B (ET(B)) receptors as well as on renal ET-1 content in rats. We further tested for NaCl intake-dependent differences in the contribution of the ET system to renal sodium handling.

METHODS

Male Sprague-Dawley rats with telemetric devices were randomized to 0.15%, 0.60% and 1.80% NaCl diets with or without losartan. Renal sodium balance and arterial pressure were monitored. Renal blood flow and fractional sodium excretion (FENa) were measured in response to acute infusion of ET(A) and ET(B) blockers into the inner stripe of the outer renal medulla.

RESULTS

Medullary pre-pro-ET-1, ET(A) and ET(B) receptor mRNA was 50%, 81% and 33% higher in rats on 0.15% vs. 1.80% NaCl. Losartan reduced medullary gene expression in rats on 0.15% NaCl. Medullary ET-1 content was 983 +/- 88 and 479 +/- 42 ng mg(-1) protein in rats on 0.15% and 1.80% NaCl (P < 0.001). Chronic ET(A) receptor blocker treatment reduced arterial pressure by 8-10 mmHg in rats on 0.15% vs. 1.80% NaCl without affecting renal sodium balances. Acute medullary ET(A) or ET(B) receptor blockade did not alter medullary blood flow and FENa in animals on either diet.

CONCLUSION

In rats renal medullary ET-1 content and mRNA expression of three ET system components are inversely related to NaCl intake. Higher expression levels on low NaCl intake are AT(1) receptor dependent but are not associated with increased sensitivity of renal sodium handling to ET(A) receptor blockade.

Adverse effects of pneumoperitoneum on renal function: involvement of the endothelin and nitric oxide systems

Increased intra-abdominal pressure (IAP) during laparoscopy adversely affects kidney function. The mechanism underlying this phenomenon is largely unknown. This study was designed to investigate the involvement of endothelin (ET)-1 and nitric oxide (NO) systems in IAP-induced renal dysfunction. Rats were subjected to IAP of 14 mmHg for 1 h, followed by a deflation for 60 min (recovery). Four additional groups were pretreated with 1) ABT-627, an ET(A) antagonist; 2) A-192621, an ET(B) antagonist; 3) nitroglycerine; and 4) N(G)-nitro-L-arginine methyl ester, a NO synthase inhibitor, before IAP. Urine flow rate (V), absolute Na+ excretion (U(Na)V), glomerular filtration rate (GFR), and renal plasma flow (RPF) were determined. Significant reductions in kidney function and hemodynamics were observed when IAP was applied. V decreased from 8.1 +/- 1.0 to 5.8 +/- 0.5 microl/min, U(Na)V from 1.08 +/- 0.31 to 0.43 +/- 0.10 microeq/min, GFR from 1.84 +/- 0.12 to 1.05 +/- 0.06 ml/min (-46.9 +/- 2.7% from baseline), and RPF from 8.62 +/- 0.87 to 3.82 +/- 0.16 ml/min (-54 +/- 3.5% from baseline). When the animals were pretreated with either ABT-627 or A-192621, given alone or combined, the adverse effects of IAP on GFR, RPF, V, and U(Na)V were significantly augmented. When the animals were pretreated with nitroglycerine, the adverse effects of pneumoperitoneum on GFR and RPF were substantially improved. In contrast, pretreatment with N(G)-nitro-L-arginine methyl ester remarkably aggravated pneumoperitoneum-induced renal dysfunction. In conclusion, decreased renal excretory function and hypofiltration are induced by increased IAP. These effects are related to impairment of renal hemodynamics and could be partially ameliorated by pretreatment with nitroglycerine and aggravated by NO and ET blockade.

Renal parenchymal hypoxia, hypoxia adaptation, and the pathogenesis of radiocontrast nephropathy

BACKGROUND AND OBJECTIVES

Renal parenchymal Po(2) declines after the administration of iodinated radiocontrast agents, reaching critically low levels of approximately 10 mmHg in medullary structures.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

In this review, the causes of renal parenchymal hypoxia and its potential role in the pathogenesis of contrast nephropathy are appraised.

RESULTS

Commonly associated predisposing factors are associated with a propensity to enhance renal hypoxia. Indeed, animal models of radiocontrast nephropathy require the induction of such predisposing factors, mimicking clinical scenarios that lead to contrast nephropathy in high-risk individuals. In these models, in association with medullary hypoxic damage, a transient local cellular hypoxia response is noted, initiated at least in part by hypoxia-inducible factors. Some predisposing conditions that are distinguished by chronically aggravated medullary hypoxia, such as tubulointerstitial disease and diabetes, are characterized by a priori upregulation of hypoxia-inducible factors, which seems to confer tolerance against radiocontrast-related hypoxic tubular damage. Renal dysfunction under such circumstances likely reflects to some extent altered intrarenal hemodynamics, rather than acute tubular injury.

CONCLUSIONS

Real-time, noninvasive novel methods may help to differentiate between evolving tubular damage and altered hemodynamics and in the design of appropriate preventive interventions.

Role of Endothelins in Hypertension

There are three peptides of endothelial origin, called endothelins (ETs), having different receptors that mediate a potent vasoconstrictor effect and also a mild vasodilation. Their renal effects are characterized by natriuresis in spite of the renal vasoconstriction. This effect, along with the stimulation of ETs by high sodium intake, suggests that ETs may be responsible for maintaining sodium balance when the renin angiotensin system is depressed. ET is activated in deoxycorticosterone acetate (DOCA) salt hypertension models and salt-sensitive hypertension. In humans, the role of ET seems to be similar to that shown in experimental animals; in both, ET participates in the regulation of salt metabolism. Salt-sensitive patients exhibit a blunted renal ET-1 response during sodium load. The role of ETs in humans has been investigated with use of nonspecific ET receptor blockers that inhibit the vasoconstriction and vasodilator components of ET. However, the effects of ET blockade should be investigated with ETA receptor blockers that mediate vasoconstriction alone. Effects of ET blockade should also be evaluated with respect to stimulation of oxidative stress and tissue damage, important mechanisms responsible for tissue fibrosis.

The prognostic value of big endothelin-1 in more than 2,300 patients with heart failure enrolled in the Valsartan Heart Failure Trial (Val-HeFT)

BACKGROUND

Endothelin is elevated in heart failure and contributes to neurohormonal activation, hemodynamic deterioration, and cardiovascular remodeling. Here, we examined its prognostic value in a large population of patients with chronic heart failure.

METHODS AND RESULTS

Big endothelin-1 (Big ET-1) and 4 other neurohormones were measured at study entry in 2359 patients enrolled in the Valsartan Heart Failure Trial (Val-HeFT) and their concentrations related to outcome over a median follow-up of 23 months. Baseline concentration of Big ET-1 (median 0.80 pmol/L) was proportional to severity of disease (New York Heart Association class, left ventricular structure and function). High circulating concentrations of brain natriuretic peptide (BNP), creatinine and bilirubin, advanced New York Heart Association class, elevated body mass index, and the presence of atrial fibrillation were independently associated to higher concentrations of Big ET-1. Big ET-1 (ranking second just behind BNP among neurohormonal factors) was an independent predictor of outcome defined as all-cause mortality (hazard ratio 1.49, 95% CI 1.20-1.84, P = .0003) or the combined endpoint of mortality and morbidity (hazard ratio 1.43, 95% CI 1.20-1.69, P < .0001) and provided incremental prognostic value compared with BNP.

CONCLUSIONS

In a large population of patients with symptomatic heart failure, the circulating concentration of Big ET-1, a precursor of the paracrine and bioactive peptide ET-1, was an independent marker of mortality and morbidity. In this setting, BNP remained the strongest neurohormonal prognostic factor.

Distribution of endothelin receptor subtypes ETA and ETB in the rat kidney

The endothelin (ET) receptor system is markedly involved in the regulation of renal function under both physiological and pathophysiological conditions. The present study determined the detailed cellular localization of both ET receptor subtypes, ET(A) and ET(B), in the vascular and tubular system of the rat kidney by immunofluorescence microscopy. In the vascular system we observed both ET(A) and ET(B) receptors in the media of interlobular arteries and afferent and efferent arterioles. In interlobar and arcuate arteries, only ET(A) receptors were present on vascular smooth muscle cells. ET(B) receptor immunoreactivity was sparse on endothelial cells of renal arteries, whereas there was strong labeling of peritubular and glomerular capillaries as well as vasa recta endothelium. ET(A) receptors were evident on glomerular mesangial cells and pericytes of descending vasa recta bundles. In the renal tubular system, ET(B) receptors were located in epithelial cells of proximal tubules and inner medullary collecting ducts, whereas ET(A) receptors were found in distal tubules and cortical collecting ducts. Distribution of ET(A) and ET(B) receptors in the vascular and tubular system of the rat kidney reported in the present study supports the concept that both ET receptor subtypes cooperate in mediating renal cortical vasoconstriction but exert differential and partially antagonistic effects on renal medullary function.

Blood pressure regulation by ETA and ETB receptors in conscious, telemetry-instrumented mice and role of ETA in hypertension produced by selective ETB blockade

The net contribution of endothelin type A (ET(A)) and type B (ET(B)) receptors in blood pressure regulation in humans and experimental animals, including the conscious mouse, remains undefined. Thus we assessed the role of ET(A) and ET(B) receptors in the control of basal blood pressure and also the role of ET(A) receptors in maintaining the hypertensive effects of systemic ET(B) blockade in telemetry-instrumented mice. Mean arterial pressure (MAP) and heart rate were recorded continuously from the carotid artery and daily (24 h) values determined. At baseline, MAP ranged from 99 +/- 1 to 101 +/- 1 mmHg and heart rate ranged between 547 +/- 15 and 567 +/- 19 beats/min (n = 6). Daily oral administration of the ET(B) selective antagonist A-192621 [10 mg/kg twice daily] increased MAP to 108 +/- 1 and 112 +/- 2 mmHg on days 1 and 5, respectively. Subsequent coadministration of the ET(A) selective antagonist atrasentan (5 mg/kg twice daily) in conjunction with A-192621 (10 mg/kg twice daily) decreased MAP to baseline values on day 6 (99 +/- 2 mmHg) and to below baseline on day 8 (89 +/- 3 mmHg). In a separate group of mice (n = 6) in which the treatment was reversed, systemic blockade of ET(B) receptors produced no hypertension in animals pretreated with atrasentan, underscoring the importance of ET(A) receptors to maintain the hypertension produced by ET(B) blockade. In a third group of mice (n = 10), ET(A) blockade alone (atrasentan; 5 mg/kg twice daily) produced an immediate and sustained decrease in MAP to values below baseline (baseline values = 101 +/- 2 to 103 +/- 2 mmHg; atrasentan decreased pressure to 95 +/- 2 mmHg). Thus these data suggest that ET(A) and ET(B) receptors play a physiologically relevant role in the regulation of basal blood pressure in normal, conscious mice. Furthermore, systemic ET(B) receptor blockade produces sustained hypertension in conscious telemetry-instrumented mice that is absent in mice pretreated with an ET(A) antagonist, suggesting that ET(A) receptors maintain the hypertension produced by ET(B) blockade.

Renal effects of human urotensin-II in rats with experimental congestive heart failure

BACKGROUND

Urotensin II (U-II) and its receptor GPR-14 are expressed in the kidney and the cardiovascular system of various mammalian species. Recent studies suggested that the U-II/GPR-14 system is upregulated in patients with congestive heart failure (CHF). However, the involvement of the peptide in the alterations of renal function in CHF remains unknown.

METHODS

The effects of incremental doses (1.0-100.0 nmol/kg) of human U-II (hU-II) on renal haemodynamic and clearance parameters were assessed in rats with an aorto-caval fistula, an experimental model of CHF, and sham controls. Additionally, the effects of pre-treatment with the nitric oxide (NO) synthase blocker, nitro-L-arginine methyl ester (L-NAME), and the cyclooxygenase inhibitor, indomethacin, on the renal haemodynamic response to hU-II were studied in CHF rats.

RESULTS

hU-II caused a decrease in mean arterial pressure in control and CHF rats. In controls, hU-II did not alter renal blood flow (RBF), and caused a minimal decrease (-12.5%) in renal vascular resistance (RVR). However, in CHF rats, the peptide induced a marked increase in RBF (+28%) and a decrease in RVR (-21.5%). These effects were attenuated by L-NAME, but not by indomethacin. Furthermore, hU-II caused a significant increase (+29%) in glomerular filtration rate (GFR) in CHF rats, whereas GFR tended to decrease in controls. Sodium excretion was not altered in control or in CHF rats in response to hU-II.

CONCLUSIONS

hU-II exerts an NO-dependent renal vasodilatation that is more pronounced in rats with CHF. The data further suggest that the U-II/GPR-14 system may be involved in the regulation of renal haemodynamics in CHF.

The potential of endothelin antagonism as a therapeutic approach

Endothelin (ET) is a pivotal physiological regulator of blood pressure through its effects on blood vessels, heart, lung and kidneys, and the ET system can be overactive in disorders such as pulmonary hypertension, heart failure and renal disease. Such observations stimulated interest among scientists and pharmaceutical companies that have set up high-throughput screens to search for antagonists of ET receptors. The emerging compounds have been tested in animals with exciting results, leading to great hope that such inhibitors could be translated into human drugs with desirable therapeutic activities and few side effects. This review will describe the most relevant results obtained in experimental animals in a wide variety of disease conditions and focus on the data of selected compounds that have been employed in clinical trials.

Effect of Dietary Sodium Intake on the Expression of Endothelin-Converting Enzyme in the Renal Medulla

BACKGROUND/AIM

Endothelin-converting enzyme (ECE) catalyzes the generation of endothelin-1 (ET-1) from its inactive precursor big-ET-1. Previous studies suggested that the ET-1 system is involved in the regulation of sodium excretion by the kidney. In particular, ET-1 via the ET(B) receptor-mediated signaling has been shown to increase renal medullary blood flow and decrease sodium transport in the collecting duct, both acting to promote renal sodium excretion. The present study was designed to evaluate the possibility that alterations in dietary salt intake may regulate the ECE-1.

METHODS

Wistar rats were fed for 3 days either with a diet containing low salt (0.01% NaCl), normal salt (0.5% NaCl), or high salt intake, either by high salt diet (4% NaCl) or normal salt diet plus 0.9% saline drinking. The expression of and immunoreactive protein levels of ECE-1 in the renal medulla was studied by RT-PCR, Northern blotting and Western blotting techniques.

RESULTS

The expression of ECE-1 mRNA (by RT-PCR and Northern blotting), as well as the immunoreactive levels of ECE-1, were significantly higher in the renal medulla of rats exposed to high salt intake than in rats on normal salt diet.

CONCLUSION

The findings suggest that upregulation of ECE-1, leading to increased generation of ET-1 in the renal medulla, may be a compensatory mechanism promoting enhanced sodium excretion by the kidney in response to high salt intake.

Impaired response of the denervated kidney to endothelin receptor blockade in normotensive and spontaneously hypertensive rats

BACKGROUND

As yet, there are only limited data available on the exact role of endothelin (ET) acting through endothelin-A (ETA) receptors in renal sodium and water regulation and the potential functional implications of an interaction of the renal ET system with renal nerves in normotensive and spontaneously hypertensive rats.

METHODS

Experiments were carried out in 64 male conscious spontaneously hypertensive rats and in 56 normotensive Wistar-Kyoto (WKY) rats. Bilateral renal denervation (BRD) was performed in 32 spontaneously hypertensive rats and 28 WKY rats 7 days before the experiments. The ETA receptor antagonist, BQ-123 (16.4 nmol/kg x min intravenously) or the endothelin-B (ETB) receptor antagonist, BQ-788 (25 nmol/kg x min intravenously) were infused at a rate of 25 microL/min for 50 minutes.

RESULTS

Renal papillary ET-1 concentration in intact spontaneously hypertensive rats was 67.8% lower than in intact WKY rats (154 +/- 40 fmol/mg protein vs. 478 +/- 62 fmol/mg protein, P < 0.01). BRD decreased papillary ET-1 by 73.5% in WKY rats to 127 +/- 19 fmol/mg protein (P < 0.001), but had no effect in spontaneously hypertensive rats (122 +/- 37 fmol/mg protein). BRD, BQ-123, or BQ-788 did not affect glomerular filtration rate (GFR) or renal blood flow (RBF) in any of the groups. In intact WKY, BQ-123 decreased urine flow rate (V) from 4.65 +/- 0.44 microL/min.100 g body weight to 2.44 +/- 0.35 microL/min.100 g body weight (P < 0.01), urinary excretion of sodium (UNaV) from 238.2 +/- 27.4 to 100.2 +/- 17.0 (P < 0.01) and potassium (UKV) from 532.1 +/- 62.6 nmol/min.100 g body weight to 243.0 +/- 34.2 nmol/min.100 g body weight (P < 0.001), whereas BQ-788 decreased only V and UNaV. In renal denervated WKY, BQ-123 or BQ-788 did not alter V, UNaV, or UKV. In intact spontaneously hypertensive rats BQ-123 but not BQ-788 decreased V from 3.94 +/- 0.48 microL/min.100 g body weight to 2.55 +/- 0.44 microL/min.100 g body weight (P < 0.05). In renal denervated spontaneously hypertensive rats neither BQ-123 nor BQ-788 affected V, UNaV, or UKV.

CONCLUSION

An interaction between ET and renal nerves is involved in the control of renal function. Moreover, renal nerves participate in the regulation of ET-1 production within the kidney. Finally, decreased synthesis of ET-1 in the renal papilla of spontaneously hypertensive rats may contribute to development and/or maintenance of hypertension due to modulation of renal excretory function.

Endothelin-1 in chronic renal failure and hypertension

Investigation into the role of endothelin-1 (ET-1) in renal function has revealed two major direct actions leading to the control of extracellular volume and blood pressure. These are the regulation of renal hemodynamics and glomerular filtration rate and the modulation of sodium and water excretion. In the rat remnant kidney model of chronic renal failure, ET-1 production is increased in blood vessels and renal tissues. These changes are related to an increase in preproET-1 expression and correlate with the rise in blood pressure, the development of cardiovascular hypertrophy, and the degree of renal insufficiency and injury. Selective ETA receptor blockade prevents the progression of hypertension and the vascular and renal damage, supporting a role for ET-1 in chronic renal failure progression. The increase in ET-1 production can be associated with other local mediators, including angiotensin II, transforming growth factor-beta1 and nitric oxide, the local production of which is also altered in chronic renal failure. In human patients with essential hypertension, atherosclerosis, and nephrosclerosis, plasma ET-1 levels are increased compared with patients with uncomplicated essential hypertension. Similarly, plasma ET-1 concentrations are markedly increased in patients with end-stage renal disease undergoing dialysis, and this correlates with blood pressure, suggesting that ET-1 may contribute to hypertension in these patients. The treatment of anemia in patients with renal failure with human recombinant erythropoietin increases blood pressure by accentuating the underlying endothelial dysfunction and the elevated vascular ET-1 production. Overall, these results support a role for ET-1 in hypertension and the end-organ damage associated with chronic renal failure. ETA receptor blockade may then represent a potential target for the management of hypertension and cardiovascular and renal protection.

Hypertension induced by blockade of ET(B) receptors in conscious nonhuman primates: role of ET(A) receptors

The role of endothelin-B (ET(B)) receptors in circulatory homeostasis is ambiguous, reflecting vasodilator and constrictor effects ascribed to the receptor and diuretic and natriuretic responses that could oppose the hypertensive effects of ET excess. With the use of conscious, telemetry-instrumented cynomolgus monkeys, we characterized the hypertension produced by ET(B) blockade and the role of ET(A) receptors in mediating this response. Mean arterial pressure (MAP) and heart rate (HR) were measured 24 h/day for 24 days under control conditions and during administration of the ET(B)-selective antagonist A-192621 (0.1, 1.0, and 10 mg/kg bid, 4 days/dose) followed by coadministration of the ET(A) antagonist atrasentan (5 mg/kg bid) + A-192621 (10 mg/kg bid) for another 4 days. High-dose ET(B) blockade increased MAP from 79 +/- 3 (control) to 87 +/- 3 and 89 +/- 3 mmHg on the first and fourth day, respectively; HR was unchanged, and plasma ET-1 concentration increased from 2.1 +/- 0.3 pg/ml (control) to 7.24 +/- 0.99 and 11.03 +/- 2.37 pg/ml. Atrasentan + A-192621 (10 mg/kg) decreased MAP from hypertensive levels (89 +/- 3) to 75 +/- 2 and 71 +/- 4 mmHg on the first and fourth day, respectively; plasma ET-1 and HR increased to 26.64 +/- 3.72 and 28.65 +/- 2.89 pg/ml and 113 +/- 5 (control) to 132 +/- 5 and 133 +/- 7 beats/min. Thus systemic ET(B) blockade produces a sustained hypertension in conscious nonhuman primates, which is mediated by ET(A) receptors. These data suggest an importance clearance function for ET(B) receptors, one that influences arterial pressure homeostasis indirectly by reducing plasma ET-1 levels and minimizing ET(A) activation.

Role of endothelin ET(B) receptors in the renal hemodynamic and excretory responses to big endothelin-1

We determined the role of endothelin ET(B) receptor in the renal hemodynamic and excretory responses to big endothelin-1, using A-192621, a selective endothelin ET(B) receptor antagonist and the spotting-lethal (sl) rat, which carries a naturally occurring deletion in the endothelin ET(B) receptor gene. An intravenous injection of big endothelin-1 produced a hypertensive effect, which is greater in wild-type (+/+) rats pretreated with A-192621 and in homozygous (sl/sl) rats. Big endothelin-1 markedly increased urine flow, urinary excretion of sodium and fractional excretion of sodium in wild-type rats treated with the vehicle. These excretory responses to big endothelin-1 were markedly reduced by pharmacological endothelin ET(B) receptor blockade. On the other hand, big endothelin-1 injection to the endothelin ET(B) receptor-deficient homozygous animals resulted in a small diuretic effect. When renal perfusion pressure was protected from big endothelin-1-induced hypertension by an aortic clamp, the excretory responses in vehicle-treated wild-type rats were markedly attenuated. In homozygous or A-192621-treated wild-type rats, there was a small but significant decreasing effect in urine flow. In addition, big endothelin-1 significantly elevated nitric oxide (NO) metabolite production in the kidney of wild-type rats but not in the homozygous rats. We suggest that the diuretic and natriuretic responses to big endothelin-1 consist of pressure-dependent and pressure-independent effects and that the increased NO production via the activation of endothelin ET(B) receptors in the kidney is closely related to the big endothelin-1-induced excretory responses.

Function of the endothelinB receptor in cardiovascular physiology and pathophysiology

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

The intrarenal endothelin system and hypertension

The kidney is both a source of endothelin (ET) generation and an important target organ of this peptide. The highest concentrations of ET-1 in the body exist in the renal medulla, where it mediates natriuretic and diuretic effects through the ET(B) receptor subtype. It is proposed that aberrations in the renal ET system may lead to sodium and water retention and subsequently to the development of hypertension.

Contrasting pharmacological ETB receptor blockade with genetic ETB deficiency in renal responses to big ET-1

Renal clearance studies were conducted to determine the role of ET(B) receptors in the renal response to big endothelin-1 (big ET-1). Two series of experiments were conducted on Inactin-anesthetized rats to contrast acute pharmacological blockade of ET(B) receptors vs. genetic ET(B) receptor deficiency. In the first series, Sprague-Dawley rats were given either ET(B)-selective antagonist, A-192621, or vehicle (0.9% NaCl) prior to infusion of big ET-1 (10 pmol.kg(-1).min(-1)) for 60 min. A-192621 significantly increased baseline mean arterial pressure (MAP; 102 +/- 4 vs. 141 +/- 6 mmHg, P < 0.05) and urine flow rate (0.5 +/- 0.1 vs. 1.3 +/- 0.2 microl/min, P < 0.05) without any effect on glomerular filtration rate (GFR) or effective renal plasma flow (ERPF). Big ET-1 significantly increased MAP in both groups but to a higher level in rats given antagonist (120 +/- 6 vs. 169 +/- 6 mmHg, P < 0.05). Big ET-1 increased urine flow in control rats but decreased in rats given antagonist. GFR and ERPF were decreased in rats given big ET-1, an effect that was exaggerated by ET(B) blockade. Another series of experiments examined the response to big ET-1 in rats lacking functional renal ET(B) receptors, known as spotting lethal (sl) rats. Surprisingly, rats heterozygous (sl/+) for ET(B) receptor deficiency had a significantly higher baseline MAP compared with homozygous (sl/sl) rats (134 +/- 6 vs. 112 +/- 7 mmHg, P < 0.05), although other variables were similar. Big ET-1 produced no significant change in MAP in either group. Urine flow, GFR, and ERPF were significantly decreased in both groups, although these changes were much larger in sl/sl rats. These experiments indicate that the ET(B) receptor plays an important role in limiting the renal hemodynamic response to big ET-1. Furthermore, the diuretic actions of big ET-1 require a functional ET(B) receptor.