Impaired sodium excretion, decreased glomerular filtration rate and elevated blood pressure in endothelin receptor type B deficient rats

Beneficial effects of time and energy restriction diets on the development of experimental acute kidney injury in Rat: Bax/Bcl-2 and histopathological evaluation

People's lifestyles and, especially, their eating habits affect their health and the functioning of the organs in their bodies, including the kidneys. One's diet influences the cells' responses to stressful conditions such as acute kidney injury (AKI). This study aims to determine the preconditioning effects of four different diets: energy restriction (ER) diet, time restriction (TR) eating, intermittent fasting (IF), and high-fat diet (HF) on histopathological indices of the kidney as well as the molecules involved in apoptosis during AKI. Adult male rats underwent ER, TR, IF, and HF diets for eight weeks. Then, AKI was induced, and renal function indices, histopathological indices, and molecules involved in apoptosis were measured. In animals with AKI, urinary albumin excretion, serum urea, creatinine and, Bax/Bcl-2 ratio increased in the kidney, while renal eGFR decreased. ER and TR diets improved renal parameters and prevented an increase in the Bax/Bcl-2 ratio. The IF diet improved renal parameters but had no effect on the Bax/Bcl-2 ratio. On the other hand, the HF diet worsened renal function and increased the Bax/Bcl-2 ratio. Histopathological examination also showed improved kidney conditions in the ER and TR groups and more damage in the HF group. This study demonstrated that ER and TR diets have renoprotective effects on AKI and possibly cause the resistance of kidney cells to damage by reducing the Bax/Bcl-2 ratio and improving apoptotic conditions.

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.

Association between estimated glomerular filtration rate and sodium excretion in urine of African descendants in Brazil: a population-based study

INTRODUCTION

Excessive salt intake is a risk factor for the development of chronic kidney disease (CKD). Objective: To evaluate the association between estimated glomerular filtration rate (eGFR) and sodium excretion in urine samples of Brazilians of African ancestry.

METHODS

Cross-sectional, population-based study of 1,211 Brazilians of African ancestry living in Alcântara City, Maranhão, Brazil. Demographic, nutritional, clinical, and laboratory data were analyzed. The urinary excretion of sodium was estimated using the Kawasaki equation. Calculations of eGFR were based on the Chronic Kidney Disease Epidemiology Collaboration equation. Multivariate linear-regression model was used to identify the relationship between sodium excretion and eGFR.

RESULTS

Mean age was 37.5±11.7 years and 52.8% were women. Mean urinary excretion of sodium was 204.6±15.3 mmol/day and eGFR was 111.8±15.3 mL/min/1.73m2. According to multivariate linear regression, GFR was independently correlated with sodium excretion (β=0.11; p<0.001), age (β=-0.67; p<0.001), female sex (β=-0.20; p<0.001), and body mass index (BMI; β=-0.09; p<0.001).

CONCLUSIONS

The present study showed that age, female sex, BMI, and correlated negatively with eGFR. Sodium excretion was the only variable that showed a positive correlation with eGFR, indicating that high levels of urinary sodium excretion may contribute to hyperfiltration with potentially harmful consequences.

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-Converting Enzyme/Neutral Endopeptidase Inhibitor SLV338 Prevents Hypertensive Cardiac Remodeling in a Blood Pressure–Independent Manner

Hypertensive heart disease is a major contributor to cardiovascular mortality. Endothelin is a potent vasoconstrictive and profibrotic mediator produced by the endothelin-converting enzyme (ECE), whereas natriuretic peptides, degraded by the neutral endopeptidase (NEP), have diuretic, vasodilatory, and antifibrotic properties. Thus, combined ECE/NEP inhibition may halt hypertensive cardiac remodeling. This study examined effects of SLV338, a novel ECE/NEP inhibitor, on cardiac protection in experimental renovascular hypertension (2-kidney, 1-clip [2K1C]). Male rats were allocated to 5 groups: sham-operated rats, untreated animals with 2K1C, 2K1C animals treated with oral SLV338 (30 and 100 mg/kg per day), and 2K1C animals treated with oral losartan (20 mg/kg per day). Treatment duration was 12 weeks. Blood pressure was assessed every 4 weeks. At study end, hearts were taken for histology/computer-aided histomorphometry/immunohistochemistry. Pharmacological properties of SLV338 are described. SLV338 is a dual ECE/NEP inhibitor, as demonstrated both in vitro and in vivo. In the 2K1C study, losartan lowered blood pressure by ≤46 mm Hg, whereas both dosages of SLV338 had no effect. However, SLV338 (both dosages) completely normalized cardiac interstitial fibrosis, perivascular fibrosis, myocyte diameter, and media:lumen ratio of cardiac arteries, as did losartan. Cardiac transforming growth factor-β1 expression was significantly enhanced in untreated 2K1C rats versus controls, whereas treatment with SLV338 and losartan prevented this effect. Taken together, dual ECE/NEP inhibitor SLV338 prevents cardiac remodeling to the same extent as losartan, but in a blood pressure–independent manner, in a rat model of renovascular hypertension. This effect is at least partially mediated via suppression of cardiac transforming growth factor-β1 expression.

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.

D3 dopamine receptor regulation of ETB receptors in renal proximal tubule cells from WKY and SHRs

BACKGROUND

The dopaminergic and endothelin systems, by regulating sodium transport in the renal proximal tubule (RPT), participate in the control of blood pressure. The D(3) and ETB receptors are expressed in RPTs, and D(3) receptor function in RPTs is impaired in spontaneously hypertensive rats (SHRs). Therefore, we tested the hypothesis that D(3) receptors can regulate ETB receptors, and that D(3) receptor regulation of ETB receptors in RPTs is impaired in SHRs.

METHODS

ETB receptor expression in RPT cells was measured by immunoblotting and reverse transcriptase-PCR and ETB receptor function by measuring Na(+)-K(+) ATPase activity. D(3)/ETB receptor interaction was studied by co-immunoprecipitation.

RESULTS

In Wistar-Kyoto (WKY) RPT cells, the D(3) receptor agonist, PD128907, increased ETB receptor protein expression, effects that were blocked by removal of calcium in the culture medium. The stimulatory effect of D(3) on ETB receptor mRNA and protein expression was also blocked by nicardipine. In contrast, in SHR RPT cells, PD128907 decreased ETB receptor expression. Basal D(3)/ETB receptor co-immunoprecipitation was three times greater in WKY than in SHRs. The absolute amount of D(3)/ETB receptor co-immunoprecipitation induced by a D(3) receptor agonist was also greater in WKY than in SHRs. Stimulation of ETB receptors decreased Na(+)-K(+) ATPase activity in WKY but not in SHR cells. Pretreatment with PD128907 augmented the inhibitory effect of BQ3020 on Na(+)-K(+) ATPase activity in WKY but not in SHR cells.

CONCLUSIONS

D(3) receptors regulate ETB receptors by physical receptor interaction and govern receptor expression and function. D(3) receptor regulation of ETB receptors is aberrant in RPT cells from SHRs.

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

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

Endothelin ET(B) receptors in arteries and veins: multiple actions in the vein

Endothelin receptors (ET(A) and ET(B)) mediate responses to ET-1. ET(B) receptor function seems to differ between a similarly sized arterial and venous pair, the rat vena cava (RVC) and rat thoracic aorta (RA). ET(B) receptors mediate RVC contraction directly, but it is unclear whether ET(B) receptors mediate contraction in RA. Because of these apparent differences in ET(B) receptor-mediated vascular contraction, we hypothesize that relaxant ET(B)-receptor mechanisms in RVC would be different from those in RA. RA and RVC rings were isolated from rats for measurement of isometric contraction. When contracted with prostaglandin F-2alpha (PGF-2alpha) (20 microM), the ET(B) receptor agonist sarafotoxin-6c (S6c) (100 nM) significantly relaxed RA and RVC. N(omega)-Nitro-L-arginine (LNNA) (100 microM) or endothelial denudation abolished relaxation to S6c in RA. By contrast, S6c-induced relaxation of RVC was attenuated but not abolished by LNNA and endothelial denudation. RVC (PGF-2alpha-contracted) relaxed to low concentrations of ET-1, whereas under the same conditions RA responded with contraction. ET-1-induced relaxation in RA was observed only with ET(A) receptor blockade. Vessels from dopamine-beta-hydroxylase-ET(B) transgenic rats, which lack functional ET(B) receptors in the vasculature, were also used. RVC (PGF-2alpha-contracted) from these rats did not relax to ET-1. Thus, although both RA and RVC possess endothelial relaxant ET(B) receptors, RA and RVC differ in that relaxant ET(B) receptors may also be present in smooth muscle of RVC. Moreover, the mechanisms of endothelial cell ET(B) receptor-mediated relaxation in RA and RVC are not the same.

Urinary protein profiling with surface-enhanced laser desorption/ionization time-of-flight mass spectrometry in ETB receptor-deficient rats

The pathways leading to salt-sensitive hypertension and renal damage in rescued ETB receptor-deficient (ETBRd) rats are still unknown. The objective of the study was therefore to identify modifications of urinary peptide and protein expression in ETBRd rats (n = 9) and wild-type controls (n = 6) using SDS - polyacrylamide gel electrophoresis (SDS-PAGE) and surface-enhanced laser desorption/ionization time-of-flight mass spectrometry (SELDI-TOF-MS) technology. Glomerular filtration rate, glomerulosclerosis, and tubulointerstitial fibrosis did not differ between the groups. ETBRd rats showed slightly higher blood pressure (p < 0.001), media/lumen ratio of intrarenal arteries (p < 0.01), and albuminuria (p < 0.01). SDS-PAGE confirmed albuminuria, but showed no differences in the urinary excretion of low molecular weight proteins (<60 kDa). SELDI-TOF-MS profiling revealed 9 proteomic features at molecular masses (Da) of 2720, 2980, 3130, 3345, 6466, 6682, 8550, 18 729, and 37 492, which were significantly elevated (p < 0.02) in urine of ETBRd rats. The results demonstrate that, independent of structural changes in the kidneys, ETB-receptor deficiency causes specific differences in urinary peptide and protein excretion. SELDI-TOF-MS may be a valuable tool for the characterization of urinary biomarkers helping to uncover the mechanism of ETBR action in the kidney.

Renal D3 dopamine receptor stimulation induces natriuresis by endothelin B receptor interactions

Dopaminergic and endothelin systems participate in the control blood pressure by regulating sodium transport in the renal proximal tubule. Disruption of either the endothelin B receptor (ETB) or D(3) dopamine receptor gene in mice produces hypertension. To examine whether these two receptors interact we studied the Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats by selectively infusing reagents into the right kidney of anesthetized rats. The D(3) receptor agonist (PD128907) caused natriuresis in WKY rats which was partially blocked by the ETB receptor antagonist. In contrast, PD128907 blunted sodium excretion in the SHRs. We found using laser confocal microscopy that the ETB receptor was mainly located in the cell membrane in control WKY cells. Treatment with the D(3) receptor antagonist caused its internalization into intracellular compartments that contained the D(3) receptors. Combined use of D(3) and ETB antagonists failed to internalize ETB receptors in cells from WKY rats. In contrast in SHR cells, ETB receptors were found mainly in internal compartments under basal condition and thus were likely prevented from interacting with the agonist-stimulated, membrane-bound D(3) receptors. Our studies suggest that D(3) receptors physically interact with proximal tubule ETB receptors and that the blunted natriuretic effect of dopamine in SHRs may be explained, in part, by abnormal D(3)/ETB receptor interactions.

Cell-type specific interaction of endothelin and the nitric oxide system: pattern of prepro-ET-1 expression in kidneys of L-NAME treated prepro-ET-1 promoter-lacZ-transgenic mice

Nitric oxide (NO) and endothelin-1 (ET-1) are known to play a major role in renal and vascular pathophysiology and exhibit a close interaction with ET-1, stimulating NO production; NO in turn inhibits ET-1 expression. Our objectives were (1) to establish a novel transgenic mouse model facilitating ET-1 expression assessment in vivo, (2) to validate this model by assessing prepro-ET-1 promoter activity in mice embryos by means of our novel model and comparing expression sites to well-established data on ET-1 in fetal development and (3) to investigate renal ET-NO interaction by assessing prepro-ET-1 promoter activity in different structures of the renal cortex in the setting of blocked NO synthases via L-NAME administration. We established transgenic mice carrying a lacZ reporter gene under control of the human prepro-ET-1 gene promoter sequence (8 kb of 5' sequences). Bluo-Gal staining of tissue sections revealed intracellular blue particles as indicators of prepro-ET-1 promoter activity. In mouse embryos, we detected high prepro-ET-1 promoter activity in the craniofacial region, as well as in bone and cartilage consistent with the literature. In order to investigate the interaction of ET-1 and NO in the kidney in vivo, transgenic mice at the age of 3-4 months were treated with a single dose of the NO synthase inhibitor L-NAME (25 mg (kg bw)(-1) i.p.) 12 h before kidney removal. Bluo-Gal staining of kidney sections revealed intracellular blue particles as indicators of prepro-ET-1 promoter activity in tubular and vascular endothelium and glomerular cells. Particle count was closely correlated to kidney tissue ET-1 content (R=0.918, P<0.001). Comparison of counts revealed an increase by 135+/-53% in L-NAME treated (n=12) compared to non-treated mice (n=10, P=0.001). Cell-type specific evaluation revealed an increase of 136+/-51% in tubular (P=0.001) and 105+/-41% in glomerular cells (P=0.046), but no significant increase in vascular endothelium. In conclusion, our study revealed a close interaction of renal endothelin and the NO system in a cell-type specific manner. Our new transgenic model provides a unique opportunity to analyse regulation of the ET system on a cellular level in vivo.

Endothelin

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

Endothelin ETA Receptor Blockade With Darusentan Increases Sodium and Potassium Excretion in Aging Rats

This study investigated whether intrarenal endothelin-1(ET-1) contributes to sodium excretion in aged rats. Metabolic function studies were performed in male Wistar rats (3 and 24 months) treated with placebo or the orally active ET(A) receptor antagonist darusentan (20 mg/kg/d) for 4 weeks. Mean arterial pressure was measured using an intra-arterial catheter. Electrolytes, aldosterone levels, renin activity, and angiotensin converting enzyme activity were determined in plasma, and mRNA expression of epithelial sodium channel (ENaC) and Na(+), K(+)-ATPase subunits was measured in the renal cortex and medulla. Aging was associated with a marked decrease in urinary excretion of sodium, chloride, and potassium (all P < 0.001) as well as renin activity (P < 0.05), but had no significant effect on gene expression of ENaC or Na(+), K(+)-ATPase subunits. In aged rats, darusentan treatment increased ion excretion (P < 0.05), reduced cortical gene expression of alphaENaC and alpha(1)-Na(+), K(+)-ATPase (both P < 0.05), and increased plasma aldosterone levels (P < 0.01). These data demonstrate a decrease of sodium and potassium excretion in aged rats, changes that are partly sensitive to ETA receptor blockade. Treatment with darusentan also reduced cortical expression of alphaENaC and alpha(1)-Na(+), K(+)-ATPase and increased plasma aldosterone levels independently of blood pressure, electrolytes, renin activity, or angiotensin converting enzyme activity. These findings may provide new pathogenetic links between aging and sodium sensitivity.

ETA receptor blockade induces tubular cell proliferation and cyst growth in rats with polycystic kidney disease

Tissue concentrations of ET-1 are markedly elevated in the kidneys of Han:Sprague-Dawley (Han:SPRD) rats, a model of human autosomal dominant polycystic kidney disease (ADPKD). This study analyzed whether disease progression might be attenuated by endothelin receptor antagonists. Heterozygous Han:SPRD rats received an ETA receptor antagonist (LU 135252), a combined ETA/ETB receptor antagonist (LU 224332), or placebo for 4 mo. Glomerulosclerosis, protein excretion, and GFR remained unchanged, whereas interstitial fibrosis was enhanced by both compounds. BP was not reduced by both compounds in Han:SPRD rats. Renal blood flow (RBF) decreased in ADPKD rats treated with the ETA receptor antagonist. Long-term ETA receptor blockade furthermore increased markedly the number of renal cysts (ADPKD rats, 390 +/- 119 [cysts/kidney section +/- SD]; LU 135252-treated APKD rats, 1084 +/- 314; P < 0.001), cyst surface area (ADPKD rats, 7.97 +/- 2.04 [% of total section surface +/- SD]; LU 135252-treated ADPKD rats, 33.83 +/- 10.03; P < 0.001), and cell proliferation of tubular cells (ADPKD rats, 42.2 +/- 17.3 [BrdU-positive cells/1000 cells]; LU 135252-treated ADPKD rats, 339.4 +/- 286.9; P < 0.001). The additional blockade of the ETB receptor attenuated these effects in Han:SPRD rats. Both endothelin receptor antagonists had no effect on BP, protein excretion, GFR, and kidney morphology in Sprague-Dawley rats without renal cysts. It is concluded that ETA receptor blockade enhances tubular cell proliferation, cyst number, and size and reduces RBF in Han:SPRD rats. This is of major clinical impact because endothelin receptor antagonists are upcoming clinically used drugs.