Distribution and functional role of renal ET receptor subtypes in normotensive and hypertensive rats

Renal Microcirculation Injury as the Main Cause of Ischemic Acute Kidney Injury Development

Acute kidney injury (AKI) can result from multiple factors. The main cause is reduced renal perfusion. Kidneys are susceptible to ischemia due to the anatomy of microcirculation that wraps around the renal tubules-peritubular capillary (PTC) network. Cortical and medullary superficial tubules have a large share in transport and require the supply of oxygen for ATP production, while it is the cortex that receives almost 100% of the blood flowing through the kidneys and the medulla only accounts for 5-10% of it. This difference makes the tubules present in the superficial layer of the medulla very susceptible to ischemia. Impaired blood flow causes damage to the endothelium, with an increase in its prothrombotic and pro-adhesive properties. This causes congestion in the microcirculation of the renal medulla. The next stage is the migration of pericytes with the disintegration of these vessels. The phenomenon of destruction of small vessels is called peritubular rarefaction, attributed as the main cause of further irreversible changes in the damaged kidney leading to the development of chronic kidney disease. In this article, we will present the characteristic structure of renal microcirculation, its regulation, and the mechanism of damage in acute ischemia, and we will try to find methods of prevention with particular emphasis on the inhibition of the renin-angiotensin-aldosterone system.

Heterotrimeric Gα12/13 proteins in kidney injury and disease

Gα₁₂ and Gα₁₃ are ubiquitous members of the heterotrimeric guanine nucleotide-binding protein (G protein) family that play central and integrative roles in the regulation of signal transduction cascades within various cell types in the kidney. Gα₁₂/Gα₁₃ proteins enable the kidney to adapt to an ever-changing environment by transducing stimuli from cell surface receptors and accessory proteins to effector systems. Therefore, perturbations in Gα₁₂/Gα₁₃ levels or their activity can contribute to the pathogenesis of various renal diseases, including renal cancer. This review will highlight and discuss the complex and expanding roles of Gα₁₂/Gα₁₃ proteins on distinct renal pathologies, with emphasis on more recently reported findings. Deciphering how the different Gα₁₂/Gα₁₃ interaction networks participate in the onset and development of renal diseases may lead to the discovery of new therapeutic strategies.

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.

The macro- and microcirculation of the kidney

Acute kidney injury (AKI) remains one of the main causes of morbidity and mortality in the intensive care medicine today. Its pathophysiology and progress to chronic kidney disease is still under investigation. In addition, the lack of techniques to adequately monitor renal function and microcirculation at the bedside makes its therapeutic resolution challenging. In this article, we review current concepts related to renal hemodynamics compromise as being the event underlying AKI. In doing so, we discuss the physiology of the renal circulation and the effects of alterations in systemic hemodynamics that lead to renal injury specifically in the context of reperfusion injury and sepsis. The ultimate key culprit of AKI leading to failure is the dysfunction of the renal microcirculation. The cellular and subcellular components of the renal microcirculation are discussed and how their injury contributes to AKI is described.

Endothelin receptor-specific control of endoplasmic reticulum stress and apoptosis in the kidney

Endothelin-1 (ET-1) promotes renal damage during cardiovascular disease; yet, the molecular mechanisms involved remain unknown. Endoplasmic reticulum (ER) stress, triggered by unfolded protein accumulation in the ER, contributes to apoptosis and organ injury. These studies aimed to determine whether the ET-1 system promotes renal ER stress development in response to tunicamycin. ET_B deficient (ET_B def) or transgenic control (TG-con) rats were used in the presence or absence of ET_A receptor antagonism. Tunicamycin treatment similarly increased cortical ER stress markers in both rat genotypes; however, only ET_B def rats showed a 14-24 fold increase from baseline for medullary GRP78, sXBP-1, and CHOP. Pre-treatment of TG-con rats with the ET_A blocker ABT-627 for 1 week prior to tunicamycin injection significantly reduced the ER stress response in cortex and medulla, and also inhibited renal apoptosis. Pre-treatment with ABT-627 failed to decrease renal ER stress and apoptosis in ET_B def rats. In conclusion, the ET-1 system is important for the development of tunicamycin-induced renal ER stress and apoptosis. ET_A receptor activation induces renal ER stress genes and apoptosis, while functional activation of the ET_B receptor has protective effects. These results highlight targeting the ET_A receptor as a therapeutic approach against ER stress-induced kidney injury.

Endothelin and the renal microcirculation

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

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.

Factors regulating the renal circulation in spontaneously hypertensive rats

Hypertension is one of the leading causes of health morbidity and mortality which are linked to many life threatening diseases such as stroke heart problems and renal dysfunction.

The integrity of renal microcirculation is crucial to maintaining the clearance and the excretory function in the normotensive and hypertensive conditions. Furthermore, any alteration in the renal function is involved in the pathophysiology of hypertension.

The aim of this review is to provide a brief discussion of some factors that regulate renal haemodynamics in spontaneously hypertensive rats, an animal model of hypertension, and how these factors are linked to the disease.

Renal and systemic effects of endothelin-1 in diabetic-hypertensive rats

The Cohen-Rosenthal Diabetic Hypertensive rat (CRDH) is a unique animal model in which genetic hypertension and diabetes developed after crossbreeding of Cohen diabetic rats sensitive substrain (CDR) and spontaneously hypertensive rats (SHR). The present study examined: 1) The acute effects of ET-1 on the systemic and renal hemodynamics in CRDH rats, CDR, and SHR; 2) The expression of ET-1 and its receptors in the renal tissue of CRDH rats. Intravenous injection of ET-1 (1.0 nmol/kg) into anesthetized SHR rats resulted in a significant immediate depressor response (mean arterial pressure (MAP) decreased from 165 ± 3 to 124 ± 12 mmHg, p < 0.0001) followed by a minor hypertensive phase (MAP increased to 170 ± 2 mmHg). Simultaneously, the administration of ET-1 caused a significant decrease in renal blood flow (RBF) from 5.8 ± 0.9 ml/min to 3.2 ± 0.5 ml/min (p = 0.026). These responses were blunted in CRDH rats and CDR. Analysis of intra-renal blood flow by laser-Doppler in CRDH rats revealed that ET-1 injection caused a decrease in cortical blood flow (Δ = -12 ± 2.9%). However, in contrast to its well-known renal medullary vasodilatory effect, ET-1 produced a significant decline in the medulla blood flow (Δ = -17.5 ± 3.4%) (p = 0.0125). These findings suggest that CDR and CRDH rats have reduced sensitivity to vascular and renal action of ET-1. Furthermore, in the CRDH rats, the expected ET-1-induced medullary vasodilatation was abolished and even reversed into prolonged vasoconstriction.

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.

Renal endothelin system and excretory function in Wistar-Kyoto and Long-Evans rats

AIM

The role of the kidney endothelin system in the renal regulation of fluid and electrolyte excretion was investigated in Wistar-Kyoto (WKY) and Long-Evans (LE) rats in which we found previously marked differences in the renal excretory responses to endothelin A receptor blockade.

METHODS

The selective endothelin A and B receptor antagonists BQ-123 (16.4 nmol kg(-1) min(-1)) and BQ-788 (25 nmol kg(-1) min(-1)) were infused i.v. for 50 min in conscious chronically instrumented WKY and LE rats and their renal function and renal endothelin system were studied.

RESULTS

Without effects on glomerular filtration rate or renal blood flow, BQ-123 and BQ-788 decreased by more than 50% (P < 0.01) both urine flow rate and electrolyte excretion in WKY rats but only urine flow rate (P < 0.05) in LE rats. Endothelin-1 content, preproET-1/GPDH mRNA ratio, B(max) and K(d) of total endothelin receptors in renal cortex did not differ between the two strains. In contrast, plasma endothelin-1 concentration (0.58 +/- 0.04 vs. 1.05 +/- 0.01 femtomol mL(-1); P < 0.01), renal papillary ET-1 concentration (68 +/- 5 vs. 478 +/- 62 fmol mg(-1) protein; P < 0.01) and preproET-1/GPDH mRNA ratio (0.65 +/- 0.09 vs. 0.88 +/- 0.05; P < 0.05) as well as total endothelin receptor number in renal papilla (B(max) 5.3 +/- 0.4 vs. and 9.0 +/- 1.2 pmol mg(-1) protein; P < 0.05) were markedly lower in LE than in WKY rats. In vitro studies showed that in both strains ET(B) receptors on renal cortical membranes amounted between 65% and 67% and on papillary membranes between 85% and 88%.

CONCLUSION

The present data show that the selective ET(A) or ET(B) receptor blockade differentially affects tubular water and salt handling, which becomes apparent in conditions of low renal papillary endothelin receptor number and tissue endothelin-1 concentration.

Positron emission tomography using 18F-labelled endothelin-1 reveals prevention of binding to cardiac receptors owing to tissue-specific clearance by ET B receptors in vivo

Our aim was to synthesise an (18)F analogue of endothelin-1 (ET-1), to dynamically image ET receptors in vivo by positron emission tomography (PET) and to elucidate the function of the ET(B) subtype as a clearing receptor in organs expressing high densities including kidney and lung.[(18)F]-ET-1 was characterised in vitro and bound with a single subnanomolar affinity (K(D)=0.43+/-0.05 nM, B(max)=27.8+/-2.1 fmol mg(-1) protein) to human left ventricle (n=4). The in vivo distribution of [(18)F]-ET-1 in anaesthetised rats was measured using a dedicated small animal PET scanner (microPET) and ex vivo analysis. Dynamic PET data demonstrated that high levels of radioligand accumulated rapidly in the lung, kidney and liver, consistent with receptor binding. The in vivo distribution correlated with the anatomical localisation of receptors detected in vitro using [(125)I]-ET-1. However, the receptor density visualised in the heart was unexpectedly low compared with that predicted from the in vitro measurements.[(18)F]-ET-1 binding in lungs could not be displaced by the ET(B) selective antagonist BQ788, in agreement with the proposed internalisation of ET-1 by ET(B) receptors. In contrast, infusion of BQ788 prior to injecting [(18)F]-ET-1 significantly reduce the amount of radioligand visualised in the ET(B) rich lung and kidney by 85% (P< 0.05, n=3) and 55% (P<0.05, n=3), respectively. Under conditions of ET(B) receptor blockade, the heart could be visualised by microPET imaging.These results suggest that clearance by ET(B) receptors in the lung and kidney prevents binding of ET-1 to receptors in the heart.

Evidence against the hypothesis that endothelial endothelin B receptor expression is regulated by relaxin and pregnancy

The endothelial endothelin B (ET(B)) receptor subtype is critical for renal vasodilation induced by relaxin in nonpregnant rats and during pregnancy (the latter via endogenous circulating relaxin). Here we tested whether expression of vascular ET(B) receptor protein is regulated by relaxin. Small renal arteries were harvested from virgin and midterm pregnant rats as well as nonpregnant rats that were administered recombinant human relaxin (rhRLX) at 4 mug/h or vehicle for 5 d or 4-6 h. Small renal arteries dissected from additional virgin rats were incubated in vitro with rhRLX or vehicle for 3 h at 37 C. ET(B) expression was also evaluated in cultured human endothelial cells: aortic, coronary, umbilical vein, and dermal microvascular endothelial cells. Cells were incubated for 4, 8, or 24 h with rhRLX (5, 1, or 0.1 ng/ml) or vehicle. ET(B) protein expression in arteries and cells was evaluated by Western analysis. No regulation of ET(B) expression was observed in small renal arteries in any of the experimental protocols, nor was there an increase in the vasorelaxation response to ET-3 in small renal arteries incubated in vitro with rhRLX. rhRLX only sporadically altered ET(B) expression in human coronary artery endothelial cells and human umbilical vein endothelial cells at certain time points or doses, and no regulation was observed in human aortic endothelial cells or human dermal microvascular endothelial cells. These results suggest that regulation of ET(B) receptor protein has little or no role in relaxin stimulation of the endothelial ET(B)/nitric oxide vasodilatory pathway.

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

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

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.

Renal endothelin receptor type B upregulation in rats with low or high renin hypertension

OBJECTIVES

To evaluate the role of endothelin-1 (ET-1) in hypertension, we investigated density and distribution of ETA and ETB receptors in hearts and kidneys of deoxycorticosterone acetate (DOCA)-salt and 1 kidney -- 1 clip (1K1C) hypertensive rats.

METHODS

Five groups of uninephrectomized Wistar rats were put on a low salt diet. Three groups of rats drank tap water and two groups received saline. One group of each regimen received DOCA subcutaneously and two corresponding groups without DOCA served as controls. The fifth group of rats had the renal artery clipped to induce 1K1C hypertension. At 6 weeks, mean arterial pressure (MAP) was recorded and membrane binding assays using 125I-ET-1 were carried out.

RESULTS

MAP was increased from control 122 +/- 3 to 155 +/- 6 and 218 +/- 11 mmHg in DOCA-salt and 1K1C rats, respectively, and cardiac weight index was increased. ETA receptors were predominantly expressed in the heart, whereas ETB receptors were predominant in the kidney. In the kidneys, the density of the ETB receptor subtype was upregulated in DOCA-salt and 1K1C rats from 160 +/- 8 to 217 +/- 12 and 190 +/- 2 fmol/mg (P < 0.05), respectively, and ETA tended to be downregulated (P = 0.057). Plasma renin activity was decreased in DOCA-salt rats from 17 +/- 3 to 0.17 +/- 0.01 ng/ml per h and increased in 1K1C rats on low salt diet to 30 +/- 5 ng/ml per h.

CONCLUSIONS

Since ETB is the predominant endothelin receptor in the kidneys, upregulation of the ETB receptor mediating vasodilation and downregulation of the ETA receptor mediating vasoconstriction would be compatible with a mainly renal counter-regulatory effect of endothelin-1 to hypertension. Both low and high renin models of hypertension may be affected.

Dual constrictor and dilator actions of ET(B) receptors in the rat renal microcirculation: interactions with ET(A) receptors

The vascular actions of endothelin-1 (ET-1) reflect the combination of vasoconstrictor ET(A) and ET(B) receptors on smooth muscle cells and vasodilator ET(B) receptors on endothelial cells. The present study investigated the contribution of ET receptor subtypes using a comprehensive battery of agonists and antagonists infused directly into the renal artery of anesthetized rats to evaluate the actions of each receptor class alone and their interactions. ET-1 (5 pmol) reduced renal blood flow (RBF) 25+/-1%. ET(A) antagonist BQ-123 attenuated this response to a 15+/-1% decrease in RBF (P < 0.01), indicating net constriction by ET(B) receptors. Combined receptor blockade (BQ-123+BQ-788) resulted in a renal vasoconstriction of 7+/-1% (P = 0.001 vs. BQ-123), supporting a constrictor action of ET(B) receptors. In marked contrast, the ET(B) antagonist BQ-788 enhanced the ET-1 RBF response to 60+/-5% (P < 0.001), suggesting ET(B)-mediated net dilation. Consistent with ET(A) blockade, the ET(B) agonist sarafotoxin 6C (S6C) produced vasoconstriction, reducing RBF by 23+/-5%. Dose-response curves for ET-1 and S6C showed similar degrees of constriction between 0.2 and 100 pmol. Both antagonists (BQ-123, BQ-788) were equally effective at threefold lower than the standard doses, suggesting complete inhibition. We conclude that ET(B) receptors alone exert a net constrictor effect but cause a net dilator influence when costimulated with ET(A) receptors. Such opposing actions indicate more complex than additive interaction between receptor subtypes. Model analysis suggests ET(A)-mediated constriction is appreciably greater without than with costimulation of ET(B) receptors. Possible explanations include ET-1 clearance by ET(B) receptors and/or a dilator ET(B) receptor function that counteracts constriction.

Endothelin as a causative factor of blunted volume reflex in diabetic rats

1. The study investigated whether endothelin (ET) contributed to the diabetes-associated alterations in volume reflex and characterised the receptor subtype that might be involved. The influence of renal sympathetic nerves on these aspects of ET was also examined. 2. Groups of nondiabetic and streptozotocin-induced diabetic rats were subjected to an acute isotonic saline volume expansion (VE), 10% body wt in the presence and absence of ET antagonists. 3. Cumulative urine sodium excretion (CuU(Na)V) after VE in diabetic rats reached values of 116+/-10 in the denervated and 74+/-6 micromol min(-1) g kidney wt(-1) in the innervated kidneys, which were both less (both P<0.001) than those achieved in the nondiabetic rats, at 267+/-9 in the denervated and 183+/-10 micromol min(-1) g kidney wt(-1) in the innervated kidney, respectively. 4. Diabetic rats pretreated with a nonselective ET(A)/ET(B) antagonist had an enhanced CuU(Na)V in the denervated kidneys by 37% (P<0.01) compared to that of untreated diabetic rats. At both doses of SB209670 these increments were less than the values obtained previously in nondiabetic rats (both P<0.01). The ET(A)/ET(B) antagonist had no meaningful effect on CuU(Na)V in the innervated kidneys of the diabetic rats, whereas previous studies in nondiabetic rats showed the response to be depressed. The CuU(Na)V responses to VE in diabetic rats given the selective ET(A) antagonist were not different from those observed in untreated diabetic rats, irrespective of whether or not the renal nerves were present. In nondiabetic rats, the ET(A) antagonist had an action similar to the mixed antagonist. 5. These findings demonstrate that activation of ET(B) receptors contributes to the depressed ability to excrete a saline load in diabetes mellitus, but its impact is obscured by the influence of the renal nerves.

Unique endothelin receptor binding in kidneys of ETB receptor deficient rats

Gariepy and colleagues (Gariepy CE, Williams SC, Richardson JA, Hammer RE, and Yanagisawa M. J Clin Invest 102: 1092-1101, 1998.) developed rescued spotting-lethal rats that carry a naturally occurring deletion of the endothelin (ET) type B receptor gene resulting in a lack of functional renal ETB receptor expression. It has been shown that rats homozygous (sl/sl) for the deletion have elevated plasma ET-1 levels; thus, the purpose of this study was to determine whether this deletion would result in a downregulation of ETA receptors in renal tissue. ET-1 and ET-3 binding experiments were performed with cortex, outer medullary, and inner medullary membranes of heterozygous (sl/+) and sl/sl ETB receptor-deficient rats. 125I-labeled ET-1 binding in sl/sl cortex and outer medulla was significantly lower than cortex and outer medulla from sl/+ rats. In contrast to sl/+ rats, [125I]ET-3 binding was not detected in the cortex and outer medulla of sl/sl rats, indicating a lack of ETB receptor expression. The inner medulla of sl/+ rats also demonstrated an abundance of ETB receptors. Surprisingly, however, we also observed significant [125I]ET-3 binding in the sl/sl inner medulla. Furthermore, ET-3 binding in the inner medulla could be blocked with an ETA receptor antagonist in sl/sl rats but not in tissue from sl/+ rats. These studies indicate that rats deficient in ETB receptors have decreased renal cortical and outer medullary ETA receptor number, most likely in response to elevated plasma ET-1 levels. In addition, homozygous ETB-deficient rats express a novel inner medullary ET-3 binding site.

Endotelin ETA and ETB receptor antagonism during cold preservation in renal transplantation

BACKGROUND

In this study, we investigated the effects of selective endothelin (ET) receptor antagonism during different periods of cold ischemia on glomerular and tubular function and long-term survival in renal transplantation.

METHODS

Left renal transplantation was performed in Lewis rats after 2 hr of cold ischemia without (n=8) and with (n=6) ETA receptor antagonism and after 16 hr of cold ischemia without treatment (n=6), with ETA receptor antagonism (n=8) and with ETB receptor antagonism (n=6). A control group (n=8) underwent right nephrectomy and left renal denervation. The ETA and ETB receptor antagonists (BQ-610 and A-192621, respectively) were added to the preservation solution (EuroCollins). After transplantation, renal glomerular and tubular functions were monitored for up to 60 days or death.

RESULTS

All animals in the control and 2-hr groups survived the follow-up protocol, with early postoperative recovery of glomerular and tubular function while the entire untreated 16-hr group died between day 3-6 postoperatively. BQ-610 treatment had no measurable effect on the renal function in the 2-hr group, however, it improved glomerular and tubular functions and led to 50% long-term survival (60 days) in the 16-hr group. A-192621 treatment had no effect on long-term survival or renal parameters.

CONCLUSION

ETA receptor antagonism had protective renal effects after prolonged ischemic preservation in renal transplantation while ETB receptor antagonism had not.

Renal nerves and endothelins interaction in the control of renal excretory function in conscious Long-Evans rats

The role of renal nerves and endothelins, acting at ET(A) receptors, in the regulation of renal excretory function was investigated in male Long-Evans rats. Catheters were placed in the femoral vein for fluid and drug infusion, in the femoral artery for blood pressure recording as well as in the bladder for urine collection. Infusion of 16.4 nmol/kg/min of the ET(A) receptor antagonist BQ-123 for 50 min was performed in freely moving, intact and renal denervated rats. As a result of BQ- 123 infusion, urine flow rate diminished (P < 0.02) and Uosm increased (P < 0.05) in the intact rats, but not in the renal denervated rats. Bilateral renal denervation itself as well as ET(A) receptor inhibition in both intact and renal denervated rats did not change the mean arterial pressure, heart rate, or the excretion of sodium, potassium and chloride. The data obtained suggest an interrelationship between renal nerves and endothelin-A receptors in the regulation of renal water excretion.

Selective ET(A) receptor antagonism with ABT-627 attenuates all renal effects of endothelin in humans

++Endothelin (ET-1) acts as a potent vasoconstrictor in the human kidney, and this vasoconstriction could contribute to the ischemia seen in acute renal failure. In animal studies, the vasoactive properties of ET-1 are known to be ET(A) receptor-and/or ET(B) receptor-mediated; however, the receptor subtype involved in the human kidney remains to be defined. In a phase I, single-center, double-blind, randomized, three-period, crossover design, the effects of orally administered ABT-627, a selective ET(A) receptor antagonist, on renal hemodynamics during ET-1 infusion were evaluated. Two doses of ABT-627 (5 and 20 mg) were compared with placebo and nifedipine. For each dose level of ABT-627, a cohort of nine subjects was studied. A para-aminohippuric acid/inulin clearance test was performed once at the end of each 7-d treatment period. Infusion of ET-1 significantly decreased effective renal plasma flow, GFR, sodium excretion, and urine flow. Pretreatment with 20 mg of ABT-627 significantly decreased mean arterial pressure. In contrast, 7 d of treatment with both doses of ABT-627 did not affect baseline renal parameters. However, because mean arterial pressure decreased, a tendency toward a reduction of renal vascular resistance could indeed be demonstrated. Compared with placebo, both doses of ABT-627 were equally effective in blocking all renal effects caused by ET-1 infusion. In the model of exogenous ET-1 infusion, ABT-627 had a tendency to prevent ET-1-induced renal changes more effectively compared with nifedipine. The contribution of endogenous ET-1 and the ET(A) receptor in maintaining basal renal vascular tone in the human kidney is small. In addition, compared with placebo, selective ET(A) receptor antagonism with both doses of ABT-627 completely prevented all renal changes caused by ET-1 infusion.

Endothelin inhibits thick ascending limb chloride flux via ET(B) receptor-mediated NO release

Endothelin-1 (ET-1) inhibits transport in various nephron segments, and the thick ascending limb of the loop of Henle (TALH) expresses ET-1 receptors. In many tissues, activation of ET(B) receptors stimulates release of NO, and we recently reported that endogenous NO inhibits TALH chloride flux (J(Cl)). However, the relationship between ET-1 and NO in the control of nephron transport has not been extensively studied. We hypothesized that ET-1 decreases NaCl transport by cortical TALHs through activation of ET(B) receptors and release of NO. Exogenous ET-1 (1 nM) decreased J(Cl) from 118.3 +/- 15.0 to 62.7 +/- 13.6 pmol. mm(-1). min(-1) (48.3 +/- 8.2% reduction), whereas removal of ET-1 increased J(Cl) in a separate group of tubules from 87.6 +/- 10.7 to 115.2 +/- 10.3 pmol. mm(-1). min(-1) (34.5 +/- 6.2% increase). To determine whether NO mediates the inhibitory effects of ET-1 on J(Cl), we examined the effect of inhibiting of NO synthase (NOS) with N(G)-nitro-L-arginine methyl ester (L-NAME) on ET-1-induced changes in J(Cl). L-NAME (5 mM) completely prevented the ET-1-induced reduction in J(Cl), whereas D-NAME did not. L-NAME alone had no effect on J(Cl). These data suggest that the effects of ET-1 are mediated by NO. Blockade of ET(B) receptors with BQ-788 prevented the inhibitory effects of 1 nM ET-1. Activation of ET(B) receptors with sarafotoxin S6c mimicked the inhibitory effect of ET-1 on J(Cl) (from 120.7 +/- 12.6 to 75.4 +/- 13.3 pmol. mm(-1). min(-1)). In contrast, ET(A) receptor antagonism with BQ-610 did not prevent ET-1-mediated inhibition of TALH J(Cl) (from 96.5 +/- 10.4 to 69.5 +/- 8.6 pmol. mm(-1). min(-1)). Endothelin increased intracellular calcium from 96.9 +/- 14.0 to 191.4 +/- 11.9 nM, an increase of 110.8 +/- 26.1%. We conclude that exogenous endothelin indirectly decreases TALH J(Cl) by activating ET(B) receptors, increasing intracellular calcium concentration, and stimulating NO release. These data suggest that endothelin acts as a physiological regulator of TALH NO synthesis, thus inhibiting chloride transport and contributing to the natriuretic effects of ET-1 observed in vivo.

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

Endothelin-1 (ET-1) at high concentrations has marked antidiuretic and antinatriuretic activities, whereas its precursor, big endothelin-1 (big ET-1), has surprisingly potent diuretic and natriuretic actions. The mechanisms underlying the excretory effects of big ET-1 have not been fully elucidated. To explore these mechanisms, we examined the effects of a highly selective ET(B) antagonist (A-192621.1), a calcium channel blocker (verapamil), a nitric oxide synthase inhibitor (N-nitro-L-arginine methyl ester [L-NAME]), and a cyclooxygenase inhibitor (indomethacin) on the systemic and renal actions of big ET-1 in anesthetized rats. An intravenous bolus injection of incremental doses of big ET-1 (0.3, 1. 0, and 3.0 nmol/kg) produced a significant hypertensive effect that was dose dependent and prolonged (from 113+/-7 mm Hg to a maximum of 148+/-6 mm Hg). The administration of big ET-1 induced marked diuretic and natriuretic responses (urinary flow rate increased from 8.5+/-1 to 110+/-14 microL/min, and fractional excretion of sodium increased from 0.38+/-0.13% to 7.51+/-1.24%). Glomerular filtration rate and renal plasma flow significantly decreased only at the highest dose of big ET-1. Pretreatment with A-192621.1 (3 mg/kg plus 3 mg. kg(-1). h(-1)) significantly abolished the diuretic (17+/-5 microL/min to a maximum of 19+/-3 microL/min) and natriuretic (0. 29+/-0.1% to a maximum of 1.93+/-0.37%) responses induced by big ET-1. Moreover, A-192621.1 potentiated the decline in glomerular filtration rate and renal plasma flow and the increase in mean arterial blood pressure produced by the low doses of big ET-1. Similar to A-192621.1, pretreatment with a nitric oxide synthase inhibitor (L-NAME, 10 mg/kg plus 5 mg. kg(-1). h(-1)) significantly and comparably reduced the diuretic and natriuretic actions of big ET-1 and augmented the hypoperfusion/hypofiltration and systemic vasoconstriction induced by high doses of the peptide. Pretreatment with verapamil (2 mg. kg(-1). h(-1)) slightly inhibited the diuretic/natriuretic effects of the high-dose big ET-1 and completely prevented the increase in mean arterial blood pressure provoked by the peptide. Unlike verapamil and L-NAME, only indomethacin administration was associated with significant natriuretic/diuretic responses and did not influence the pressor effect and renal actions of big ET-1. Taken together, these results suggest that big ET-1-induced diuretic and natriuretic responses are mediated mainly by stimulation of nitric oxide production coupled to ET(B) receptor subtype activation.

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

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

Protective effects of endothelin receptor antagonists in dogs with aortic cross-clamping

Endothelin (ET) may play an important role in the pathogenesis of vasoconstriction and acute renal failure after aortic cross-clamping (ACC). However, the relative contribution of the ET(A) and ET(B) receptors to the physiopathology of ischemic acute renal failure is poorly understood. This study was carried out to evaluate the potential protective effect of a selective ET(A) antagonist versus combined ET(A)/ET(B) antagonist on altered systemic, pulmonary, and renal hemodynamics induced by cross-clamping the suprarenal aorta for 1 h, followed by 2-h reperfusion. Studies were performed in three groups of anesthetized mongrel dogs. After baseline measurements, treatment (3 mg/kg i.v. bolus + 3 mg/kg/h infusion) with either a selective ET(A) antagonist, Ro 61-1790 (n = 5), or a combined ET(A)/ET(B) antagonist, bosentan (n = 5) or vehicle (n = 8) was initiated 5 min before ACC. There were marked increases in total peripheral (TPR), pulmonary (PVR), and renal (RVR) vascular resistances, and significant decreases in cardiac output (CO) and glomerular filtration rate (GFR) and tubular sodium reabsorption after ACC in the vehicle group. Both Ro 61-1790 and bosentan prevented the marked increases in TPR, PRV, and RVR, and attenuated the declines in CO, GFR, and tubular sodium reabsorption. We concluded that the profound systemic, pulmonary, and renal vasoconstriction, as well as the impairments in glomerular and tubular functions associated with ACC, is mostly ET mediated and that the ET(A) receptor activation makes a major contribution to the ET-mediated impairments of hemodynamics and renal function after ACC.

Effects of BQ-123 on systemic and renal hemodynamic responses to endothelin-1 in the rat split hydronephrotic kidney

OBJECTIVE

To assess the site of action of endothelin-1 in vessels of different sizes in the kidney in vivo and investigate the function of endothelin A (ET(A)) receptors in mediating renal and systemic vasoconstriction.

DESIGN

The luminal diameters of different vessels were measured and glomerular blood flow in cortical glomeruli was determined by intravital videomicroscopy in the split hydronephrotic kidney of anesthetized female Wistar rats.

METHODS

The rats were infused with endothelin-1 (40 pmol/kg per min) with or without pretreatment with the selective ET(A)-receptor antagonist BQ-123 (0.5 mg/kg). Aortic clamping was used to control renal blood pressure during the endothelin-1 infusion.

RESULTS

Exogenous endothelin-1 induced a significant rise (30+/-3%) in mean arterial pressure and a marked, long-lasting fall in glomerular blood flow (53+/-3%) related to reduction of the inner diameter of arcuate (-30%), interlobular arteries (-33%) and afferent arterioles (-17%). Aortic clamping to normalize renal blood pressure did not attenuate the vasoconstriction and reduction in glomerular blood flow. Pretreatment with BQ-123 significantly reduced both the endothelin-1-induced rise in mean arterial pressure (12+/-1%) and the fall in glomerular blood flow (-23+/-11%). BQ-123 blunted the response to endothelin-1 in arcuate (-12%), interlobular (-11%) and afferent vessels (-5%). Acetylcholine and nitroprusside completely reversed the vasoconstriction in BQ-123-pretreated animals.

CONCLUSIONS

BQ-123 largely prevented the hemodynamic effects of exogenously administered endothelin-1. Our direct in-vivo techniques showed that ET(A) receptors are, at least in part, involved in endothelin-1 -mediated vasoconstriction in the rat kidney, and support the hypothesis that ET(A) receptors may help to control arterial pressure in anesthetized rats.

Effects of nitric oxide synthase inhibition and endothelin ETA receptor blockade on haemodynamics in hypertensive rats

1. The objectives of the present study were to study regional differences in haemodynamics between spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats induced by the nitric oxide synthase (NOS) inhibitor NG-monomethyl-L-arginine (L-NMMA) and the endothelin ETA receptor antagonist BQ 123 in vivo in tissues known to be important for blood pressure (BP) regulation (heart, kidney and skeletal muscle). Furthermore, the effect of acetylcholine (ACh) infusion (2 micrograms/kg per min) was examined after L-NMMA or BQ 123. The microsphere method was used for determinations of cardiac index (CI) and regional haemodynamics. 2. NG-Monomethyl-L-arginine (20 mg/kg) increased BP (26-48%; P < 0.01) and reduced CI in both rat strains. BQ 123 (1 mg/kg) reduced BP slightly (-4 to 11%; P < 0.05). 3. NG-Monomethyl-L-arginine significantly increased myocardial and skeletal muscle vascular resistance in SHR only; however, in the kidney, L-NMMA reduced blood flow and increased vascular resistance in both rat strains. 4. BQ 123 induced minor changes in regional haemodynamics that were not significantly different between the two strains. 5. Acetylcholine following BQ 123 induced an increase in myocardial blood flow in WKY rats, but decreased blood flow in SHR. Acetylcholine following L-NMMA reduced myocardial blood flow in both strains. 6. Acetylcholine following BQ 123 induced renal vasodilation in WKY rats but, following L-NMMA, ACh did not induce renal vasodilation in either rat strain. In contrast, L-NMMA did not abolish the vasodilation of acetylcholine in skeletal muscle in WKY rats. 7. In conclusion, the contribution of nitric oxide to basal vessel tone was not impaired in the heart, skeletal muscle and kidney in SHR. Antagonism of ETA receptors caused similar haemodynamic responses in both rat strains in these organs. Furthermore, NOS inhibition, but not ETA blockade, blunted the expected ACh-induced vasodilation in the heart and kidney in WKY rats, but not in skeletal muscle in both strains.

Influence of contrast media on the response of rat renal arteries to endothelin and nitric oxide: influence of contrast media

RATIONALE AND OBJECTIVES

Contrast media (CM) such as diatrizoate meglumine (DTZ) or iohexol can cause renal vasoconstriction in vivo, and this may initiate CM-induced nephropathy. Endothelin-1 (ET-1), a vasoconstrictor, and nitric oxide, a vasodilator, are key modulators of renal circulation. We tested the hypothesis that CM enhances arterial responses to ET-1, or diminishes responses to nitric oxide.

METHODS

A video dimension analyzer continuously recorded changes in diameter of isolated, pressurized rat interlobar renal arteries (200-400 microm diameter) superfused with combinations of CM, ET-1, nitric oxide, and other vasoactive agents.

RESULTS

Superfusion of arteries with 3.3% DTZ, but not with 3.3% iohexol, enhanced their sensitivity to ET-1 by approximately twofold, as assessed by shifts in concentration-response curves. Both DTZ and iohexol decreased the sensitivity of arteries to nitric oxide by approximately threefold. Neither DTZ nor iohexol affected arterial sensitivity to other vasoconstrictors (phenylephrine, potassium) or vasodilators (forskolin, diltiazem).

CONCLUSIONS

Diatrizoate meglumine and iohexol may induce or augment renal vasoconstriction in part by causing selective alterations in arterial sensitivity to ET-1 and to nitric oxide.

Effects of endothelin receptor antagonism and angiotensin-converting enzyme inhibition on cardiac and renal remodeling in the rat

The effects of a nonpeptide, orally active mixed endothelin (ET) ETA/ETB receptor antagonist, SB 217242, and an angiotensin-converting enzyme (ACE) inhibitor, ramipril, were evaluated after inter-renal aortic banding in the rat. Separate sham, vehicle, and treatment groups were compared in each study. In vehicle-treated animals in the ramipril group, aortic banding for 4 weeks produced significant cardiac hypertrophy (247 +/- 5 mg/100 g bw vs. 305 +/- 11 mg/100 g bw; p < 0.001), right (upstream) renal hypertrophy (380 +/- 6 mg/100 g bw vs. 559 +/- 28 mg/100 g bw; p < 0.001), and significant left (downstream) renal atrophy (405 +/- 4 mg/100 g bw vs. 192 +/- 25 mg/100 g bw; p < 0.001). Continuous ramipril treatment (1 mg/kg p.o. once daily), begun 3 days before aortic banding, inhibited cardiac hypertrophy (305 +/- 11 mg/100 g bw vs. 266 +/- 7 mg/100 g bw; p < 0.05) but did not alter renal hypertrophy or atrophy. In a similarly designed study, SB 217242 (30 mg/kg p.o. b.i.d.) had no effect on the development of cardiac hypertrophy (298 +/- 7 mg/100 g bw vs. 310 +/- 12 mg/100 g bw) or renal hypertrophy (561 +/- 15 mg/100 g bw vs. 575 +/- 19 mg/100 g bw), but abolished the development of renal atrophy (158 +/- 16 mg/100 g bw vs. 395 +/- 19 mg/100 g bw; p < 0.001). [125I]ET-1 radioligand binding experiments indicated that the density of both ETA and ETB receptors was increased dramatically (three- to fourfold) in the atrophic kidney cortex compared to sham or hypertrophic kidneys. In situ hybridization studies indicate an upregulation of ETB receptor mRNA in the glomeruli of atrophic kidneys within 5 days of aortic banding. In conclusion, an angiotensin-dependent mechanism may mediate cardiac hypertrophy associated with aortic banding, whereas ET-dependent mechanisms may mediate an atrophic response in the hypoperfused kidney, perhaps through an interaction with upregulated ETA and/or ETB receptors.

Contribution of endothelin to the acute pressor response of L-NAME in stroke-prone spontaneously hypertensive rats

In this study, we examined whether endothelin (ET) plays a role in the short-term increase in mean arterial pressure (MAP) after nitric oxide synthase (NOS) inhibition with N(omega)-nitro-L-arginine methyl ester (L-NAME) in stroke-prone spontaneously hypertensive rats (SHRSPs). Experiments were performed by using Inactin-anesthetized male SHRSPs that were pretreated with chlorisondamine to block reflex autonomic cardiovascular effects. Injection of L-NAME (10 mg/kg, i.v.), but not D-NAME, produced rapid and marked increases (74 +/- 3 mm Hg) in MAP that were sustained for >1 h. In SHRSPs that were treated with the ET(A/B) receptor antagonist, L-754,142 (15 mg/kg + 15 mg/kg/h), L-NAME increased MAP by 45 +/- 4 mm Hg (p < 0.0001 compared with L-NAME alone). L-754,142 blocked pressor responses to big ET-1 by >90% but was without effect on pressor responses to norepinephrine. Plasma levels of ET-1 averaged 5 +/- 1 pg/ml in animals given vehicle and were slightly increased in animals given either L-NAME alone (7 +/- 2 pg/ml) or L-754,142 alone (7 +/- 2 pg/ml) but increased markedly when L-NAME and L-754,142 were given together (114 +/- 18 pg/ml). This may relate to an effect of L-754,142 to block ET-receptor-mediated clearance of ET-1. We conclude that ET plays a role in the short-term pressor response after NOS inhibition in SHRSPs.

Nephroprotection of an ET(A)-receptor blocker (LU 135252) in salt-loaded uninephrectomized stroke-prone spontaneously hypertensive rats

The present study was designed to assess whether the orally active and highly specific endothelin A (ET(A)) receptor antagonist LU 135252 affects progressive renal dysfunction in a hypertensive rat model of renal damage, ie, the uninephrectomized (UNX) stroke-prone spontaneously hypertensive rat (SHRsp). The animals were examined on a normal salt (0.25%) diet and, to sensitize the kidney to hypertensive injury, also on a high salt (3%) diet. Stereological methods were used to quantify indices of glomerulosclerosis, vascular damage, and tubulointerstitial damage. Treatment with LU 135252 (100 mg/kg body wt) did not affect systolic blood pressure (BP) in animals on a normal salt diet during the whole period of the experiment (18 weeks) or in salt-loaded animals until week 10; subsequently, BP was slightly but significantly lower in salt-loaded UNX-SHRsp given LU 135252. Between weeks 6 and 12, 40% of the untreated UNX-SHRsp on a high salt diet, but none on a standard salt diet, died; such mortality was completely prevented by LU 135252. Indices of renal damage were more abnormal in salt-loaded UNX-SHRsp compared with UNX-SHRsp on a normal salt diet. Development of glomerulosclerosis and tubulointerstitial and vascular damage in UNX-SHRsp on high salt was completely prevented by LU 135252. The respective indices were no longer significantly different from those of salt-loaded sham-operated SHRsp controls. In the less severely damaged kidneys of UNX-SHRsp on normal salt, treatment with LU 135252 tended to ameliorate the indices, but the difference was not statistically significant. The results document a role of the ET system, specifically of ET(A) receptors, in the development of progressive renal injury in salt-loaded UNX-SHRsp. LU 135252 completely prevented death and renal damage resulting from salt loading.

[125I]endothelin-1 binding to renal brush border and basolateral membranes

[125I]Endothelin-1 bound with high affinity to a single site on both brush border membranes (Kd=192+/-26 pM. Bmax=314+/-49 fmol/mg) and basolateral membranes (Kd=94.7+/-3.4 pM, Bmax=612+/-107 fmol/mg) isolated from rat renal cortex. Competition binding experiments using subtype selective ligands revealed that the proportion of ET(B) to ET(A) receptors was 80:20 and 60:40 in the brush border membrane and the basolateral membrane, respectively. The results demonstrate that endothelin-1 binds to brush border membranes, and that endothelin ET(B) receptors may be involved in the previously described effects of endothelin-1 on brush border membrane Na+ transport.

Endothelin-A receptor antagonism attenuates the hypertension and renal injury in Dahl salt-sensitive rats

The aim of this study was to examine the role of endothelin-A (ET(A)) receptors in mediating the hypertension and renal injury associated with high salt intake in Dahl salt-sensitive (DS) rats. To achieve this goal, we examined the effects of chronic selective ET(A) antagonist (A-127722) treatment at a dose of 10 mg/kg/d on arterial pressure, renal function, and morphology in DS and Dahl salt-resistant (DR) rats placed on a high sodium (8% NaCl) diet (HSD) for 3 weeks. Placement of DS rats (n=13) on HSD for 3 weeks caused a progressive increase in systolic pressure (from 118+/-3 to 186+/-15 mm Hg). The increase in systolic pressure was significantly attenuated (from 125+/-4 to 167+/-12 mm Hg) in DS rats treated with A-127722 (n=13). Mean arterial pressure (MAP) measured directly at the end of the study was also significantly lower by 18 mm Hg (P<.02) in the DS rats treated with A-127722. The slope of the chronic pressure-natriuresis curve was shifted to the right in DS rats and to the left by chronic ET(A) receptor blockade in DS rats. The hypertension in DS rats was associated with marked proteinuria (from 4.1+/-1.1 to 74.3+/-5.3 mg/24 h/100 g body weight) that was significantly attenuated (from 5.7+/-1.2 to 55.2+/-6.5 mg/24 h/100 g body weight) in DS rats treated with A-127722. The percentage of glomeruli displaying fibrosis, hypercellularity, and hyalinization was also significantly reduced after treatment with A-127722 in DS rats. Arterial pressure, protein excretion, renal hemodynamics, and morphologic structure were not significantly changed in response to ET(A) receptor blockade in DR rats placed on HSD. These data indicate that endothelin-A receptor activation may play a role in the exacerbation of hypertension and development of renal injury in DS rats.

Involvement of nitric oxide in endothelin ETB receptor-mediated inhibitory actions on antidiuresis and norepinephrine overflow induced by stimulation of renal nerves in anesthetized dogs

We examined the effect of sarafotoxin S6c (S6c), a selective endothelin ETB-receptor agonist, on renal actions and norepinephrine (NE) overflow induced by renal nerve stimulation (RNS) in anesthetized dogs, with or without blockade of endogenous nitric oxide (NO) generation by NG-nitro-L-arginine (NOARG), a NO synthase inhibitor. RNS (0.5-2.0 Hz) produced significant decreases in urine flow, urinary and fractional excretion of sodium, and increased NE secretion rate, without affecting systemic and renal hemodynamics. When S6c (1 ng/kg/min) was infused intrarenally, there was a slight and transient increase in renal blood flow at 1-2 min after the start of the infusion, without any change in systemic hemodynamics and this response was followed by a gradual reduction. There was a significant increase in the basal level of urine flow with no effects on urinary and fractional excretion of sodium. In addition, S6c administration elicited an increase in urinary excretion of NO metabolites. NO2- and NO3-. During S6c infusion, RNS-induced antidiuretic action and increases in NE secretion rate were significantly attenuated. RNS during intrarenal arterial infusion of NOARG (40 micrograms/kg/min) led to potent reductions in urine formation and decreased renal blood flow and glomerular filtration rate. Simultaneously. NE secretion rate was markedly increased. In the presence of NOARG, S6c-induced suppressive actions on reductions in urine formation and increase in NE secretion rate in response to RNS were markedly attenuated. The peptide did not increase urinary excretion of NO metabolites. These findings suggest that ET functions as an inhibitory modulator of renal noradrenergic neurotransmission through ETB-receptor mechanisms, events that may be caused by NO production induced by the peptide.

Role of endothelin in mediating the attenuated renal hemodynamics in Dahl salt-sensitive hypertension

The aim of this study was to evaluate the role of endothelin (ET) in the hypertension associated with giving a high sodium diet in Dahl salt-sensitive (DS) rats. To achieve this goal, we examined the effects of intravenous infusion of the nonspecific ET(A)-ET(B) antagonist on arterial pressure and renal function in conscious, chronically instrumented DS and Dahl salt-resistant (DR) rats. After 3 weeks on a high sodium (8%) diet, mean arterial pressure (MAP) in DS rats (166+/-3 mm Hg) was significantly higher than in DR rats (124+/-3 mm Hg). Baseline glomerular filtration rate (GFR) and renal plasma flow (RPF) in DS rats (1.92+/-0.25 mL/min and 7.07+/-0.80 mL/min) were lower than in DR rats (2.52+/-0.21 mL/min and 7.98+/-0.85 mL/min), respectively. Renal vascular resistance was significantly higher in DS rats (32.78+/-5.88 mm Hg x mL(-1) x min(-1)) than in DR rats (24.60+/-5.04 mm Hg x mL(-1) x min(-1)). Intravenous infusion of the ET antagonist SB 209670 at a dose of 30 microg x kg(-1) x min(-1) for 75 minutes caused a significant decrease in MAP in DS rats (from 166+/-3 to 144+/-4 mm Hg). In contrast, the effect of the ET antagonism on MAP in DR rats was not significant. ET-antagonist infusion tended to improve GFR and RPF in DS but not in DR rats. To determine the renal effects of ET antagonism independent of the systemic hemodynamic responses, we examined the effects of the same ET antagonist in rats chronically implanted with a renal interstitial catheter. Arterial pressure in DS rats (181+/-5 mm Hg) was significantly higher than in DR rats (135+/-3 mm Hg). Renal interstitial infusion of SB 209670 at a dose of 200 ng x kg(-1) x min(-1) for 60 minutes caused no change in MAP in DS or DR rats. Intrarenal ET antagonism significantly increased GFR (25%), RPF (30%), urine flow (32%), and urinary sodium excretion (25%) in DS rats, while it had no significant effect in DR rats. Fractional excretion of sodium was not significantly changed by renal interstitial infusion of the ET antagonist in DS rats, indicating that improved renal excretory function in DS rats is most likely due to the associated improvement in renal hemodynamics. We conclude that ET may play a role in the attenuated renal hemodynamics and possibly the development of Dahl salt-sensitive hypertension.

Effects of sarafotoxin S6c on renal haemodynamics and urine formation in anaesthetized dogs

1. The effects of sarafotoxin S6c (S6c), a selective endothelin ETB receptor agonist, on renal haemodynamics and urine formation were examined in anaesthetized dogs. 2. Intrarenal arterial infusion of S6c at a rate of 1 or 5 ng/kg per min produced a transient increase in renal blood flow (RBF), with no change in systemic blood pressure and heart rate; RBF then decreased gradually to below the basal value. There were significant and dose-dependent increases in urine flow and free water clearance and decreases in urine osmolality during S6c infusion, whereas urinary excretion of sodium and glomerular filtration rate (GFR) remained unchanged. Simultaneously, S6c administration elicited a marked increase in urinary excretion of nitric oxide (NO) metabolites, NO2- and NO3- (UNOxV). 3. In dogs simultaneously administered S6c (5 ng/kg per min) and NG-nitro-L-arginine (NOARG; 40 micrograms/kg per min), a NO synthase inhibitor, the renal vasodilator effect of S6c was abolished and marked reductions in RBF and GFR were observed. The S6c-induced diuretic action was not affected by NOARG. In the presence of NOARG, there was a small amount of UNOxV at the basal level and the administration of S6c did not increase UNOxV. 4. These results suggest that an intrarenal arterial infusion of S6c enhances the production of NO in the kidney and that this enhancement contributes to the peptide-induced renal vasodilation. In contrast, it is unlikely that S6c-induced water diuresis is related to NO production stimulated by this peptide.

Endothelin-1 transgenic mice develop glomerulosclerosis, interstitial fibrosis, and renal cysts but not hypertension

The human endothelin-1 (ET-1) gene under the control of its natural promoter was transferred into the germline of mice. The transgene was expressed predominantly in the brain, lung, and kidney. Transgene expression was associated with a pathological phenotype manifested by signs such as age-dependent development of renal cysts, interstitial fibrosis of the kidneys, and glomerulosclerosis leading to a progressive decrease in glomerular filtration rate. This pathology developed in spite of only slightly elevated plasma and tissue ET-1 concentrations. Blood pressure was not affected even after the development of an impaired glomerular filtration rate. Therefore, these transgenic lines provide a new blood pressure-independent animal model of ET-1-induced renal pathology leading to renal fibrosis and fatal kidney disease.

Endothelins in the normal and diseased kidney

Endothelin-1 (ET-1) is a 21-amino acid peptide that potently modulates renal function. ET-1 is produced by, and binds to, most renal cell types. ET-1 exerts a wide range of biologic effects in the kidney, including constriction of most renal vessels, mesangial cell contraction, inhibition of sodium and water reabsorption by the nephron, enhancement of glomerular cell proliferation, and stimulation of extracellular matrix accumulation. ET-1 functions primarily as an autocrine or paracrine factor; its renal effects must be viewed in the context of its local production and actions. This is particularly important when comparing ET-1 biology in the nephron, where it promotes relative hypotension through increased salt and water excretion, with ET-1 effects in the vasculature, where it promotes relative hypertension through vasoconstriction. Numerous studies indicate that ET-1 is involved in the pathogenesis of a broad spectrum of renal diseases. These include those characterized by excessive renal vascular resistance, such as ischemic renal failure, cyclosporine (CyA) nephrotoxicity, radiocontrast nephropathy, endotoxemia, rhabdomyolysis, acute liver rejection, and others. ET-1 appears to play a role in cell proliferation in the setting of inflammatory glomerulonephritides. The peptide also may mediate, at least in part, excessive extracellular matrix accumulation and fibrosis occurring in chronic renal failure, diabetes mellitus, and other disorders. Deranged ET-1 production in the nephron may cause inappropriate sodium and water retention, thereby contributing to the development and/or maintenance of hypertension. Finally, impaired renal clearance of ET-1 may cause hypertension in patients with end-stage renal disease. Many ET-1 antagonists have been developed; however, their clinical usefulness has not yet been determined. Despite this, these agents hold great promise for the treatment of renal diseases; it is hoped that the next decade will witness their introduction into clinical practice.

Cardiovascular and renal effects of endothelin B receptor selective agonists in spontaneously hypertensive rats

Renal effects of endothelin (ET)-3 have been described in normotensive but not spontaneously hypertensive rats (SHR). Infusion (170 ng/kg/min) of the ETB receptor selective agonists ET-3 and sarafotoxin S6c (SS6c) was used to investigate ETB receptor modulation of renal function in SHR. ET-3 decreased glomerular filtration rate (GFR) and renal plasma flow (RPF) by approximately 95% (0.1 +/- 0.01 and 0.1 +/- 0.02 ml/min, respectively) versus vehicle (1.3 +/- 0.08 and 3.6 +/- 0.23, respectively) in SHR. ET-3 exerted a biphasic effect on urine flow (UV); an initial increase and then a decrease (vehicle, 4.2 +/- 0.55; ET-3, 0.2 +/- 0.09 microliter/min). ET-3 increased mean arterial pressure (vehicle, 159 +/- 4.1; ET-3, 174 +/- 3.1 mm Hg). SS6c decreased GFR and RPF by approximately 60% (0.8 +/- 0.12 and 2.0 +/- 0.18 ml/min, respectively) versus vehicle (2.0 +/- 0.19 and 5.2 +/- 0.45, respectively). UV did not change. Depressor effects of SS6c were observed (vehicle, 154 +/- 1.5; SS6c, 127 +/- 3.1 mm Hg). The ETB receptor selective agonists ET-3 and SS6c markedly decreased GFR and RPF in SHR, suggesting that endogenous ET-3 may modulate renal function through ETB receptors in SHR.

Role of endothelin as a mitogen in experimental glomerulonephritis in rats

Recent studies have revealed that endothelin-1 (ET-1) is a potent mitogen for mesangial cells in vitro. To determine whether ET-1 exerts the mitogenic action on mesangial cells in vivo, we examined the glomerular expression of ET-1 and its receptors in a rat model of mesangial proliferative glomerulonephritis and assessed the effect of a specific endothelin A (ET(A)) receptor antagonist, FR139317, on mesangial cell proliferation in this model. The levels of preproET-1 mRNA expression and ET-1 protein production in glomeruli increased markedly on days 4 and 7 after disease induction, and the levels changed in concordance with the glomerular cell proliferation. In contrast, the level of ET(A) receptor mRNA initially decreased on day 1, and thereafter increased on days 4 and 7. Administration of FR139317 to rats with experimental glomerulonephritis induced a significant reduction in mesangial cell proliferation. In addition, in situ hybridization of preproET-1 mRNA and double-immunolabeling of ED-1 and OX-7 in a mirror image section revealed that the principal cell expressing ET-1 in glomeruli were infiltrating macrophages on day 1, and they were replaced by mesangial cells on day 4. These findings indicate that ET-1 functions as a potent mitogen for mesangial cells in vivo in an autocrine or paracrine fashion.

Paracrine renal endothelin system in rats with liver cirrhosis

1. Liver cirrhosis was induced in rats by CCl4 administration. We analysed the expression of endothelin receptor subtypes in the renal cortex and medulla using Scatchard analysis and receptor autoradiography, and measured plasma as well as renal-tissue endothelin-1 concentrations using a specific radioimmunoassay. Furthermore, we analysed the effects of the non-selective (A/B) endothelin receptor antagonist, bosentan (6 and 100 mg kg-1 day-1) on mean arterial blood pressure, water and sodium excretion and glomerular filtration rate. 2. Our study revealed an overexpression of the endothelin B receptor (ETB) in the renal medulla of rats with liver cirrhosis (Cir: 2775 +/- 299 fmol mg-1; Con: 1695 +/- 255 fmol mg-1; n = 8; means +/- s.d., P < 0.01), whereas the density of ETB in the cortex and the endothelin A receptor (ETA) in the cortex and medulla were similar in both cirrhotic and control rats. Receptor autoradiography showed that the upregulation of medullary ETB in cirrhotic rats was due to an upregulation of ETB in the inner medullary collecting duct cells. 3. The tissue endothelin-1 concentrations were increased in the renal medulla of cirrhotic rats (Cir: 271 +/- 68 pg g-1wet wt.; Con: 153 +/- 36 pg g-1 wet wt., n = 8; means +/- s.d., P < 0.01). 4. The glomerular filtration rate was slightly decreased in cirrhotic rats but not altered after bosentan treatment in either cirrhotic or control rats. Bosentan decreased sodium excretion to a similar extent in both cirrhotic and control rats, whereas water excretion was significantly reduced by both dosages of bosentan in cirrhotic rats only (Cir + vehicle: 12.5 +/- 0.62 m day-1, Cir + 6 mg kg-1 day-1 bosentan: 8.6 +/- 1.0 ml day-1; Cir + 100 mg kg-1 day-1 bosentan: 7.4 +/- 0.6 ml day-1; n = 10; means +/- s.e.mean). 5. We therefore suggest that the upregulation of the medullary ETB in cirrhotic rats is involved in the regulation of water excretion in rats with CCl4-induced liver cirrhosis.

Endothelin receptors in normal and diseased kidneys

1. Endothelin-1 (ET-1), the most potent vasoconstrictor yet identified, mediates a multitude of responses in various tissues including the kidney. The biological responses of ET-1 are mediated by specific cell surface receptors classified as ET(A) and ETB. Species differences are observed in the distribution as well as function of these ET receptors. 2. Involvement of ET has been demonstrated in a number of renal diseases, including ischaemia-induced acute renal failure, chronic renal failure, radiocontrast and cyclosporin-induced nephrotoxicity. ET antibodies as well as ET receptor antagonists have been shown to be beneficial in these disease models.

Decrease in endothelin-1 renal receptors during the 1st month of life in the rat

Endothelin-1 (Et1), like angiotensin II, is implicated in postnatal maturation and development. The present study was designed to identify Et1 receptors and subtype Et1 receptors present in rat kidney between 1 and 30 days of postnatal life. On day 1, high-affinity and high-density Et1 binding sites were identified in rat kidney. The dissociation constant and maximum binding for ET1 to membranes from whole kidney were 0.073 +/- 0.05 nM and 1,345.9 +/- 73 fmol/mg protein, respectively. On day 30, affinity and receptor density were markedly decreased. The dissociation constant and maximum binding were 0.147 +/- 0.021 nM (P < 0.01) and 633.2 +/- 56.4 fmol/mg protein (P < 0.001), respectively. Using BQ 123 (EtA-selective antagonist) and sarafotoxin S6c (EtB-selective agonist), the two Et1 receptor subtypes EtA and EtB were identified in 1- and 30-day-old rat kidney. BQ 123 selectively recognized EtA receptors with high affinity (2.9 +/- 0.44 on day 1 and 4.0 +/- 0.5 nM on day 30) and sarafotoxin S6c bound with higher affinity EtB receptors (0.871 +/- 0.14 on day 1 and 0.717 +/- 0.12 nM on day 30). Between birth and day 30, the EtA binding capacity was decreased (304 +/- 27 vs. 752 +/- 202 fmol/mg protein, P < 0.05), whereas EtB binding was not affected (514 +/- 87 vs. 656 +/- 171 fmol/mg protein, NS). The decrease in the total number of Et1 receptors during the 1st month of life may be due to the concomitant decrease in the number of EtA receptors. Increased Et1 receptor density in early postnatal life suggests an influence of Et1 on immature kidney circulation and/or kidney growth.