Renal vasodilating and diuretic actions of a selective endothelin ETB receptor agonist, IRL1620

Pharmacological Potential of Endothelin Receptors Agonists and Antagonists

Endothelins are potent predominantly vasoconstricting agents that act as local autocrine and paracrine mediators. Endothelin-1 is the most potent and sustained vasoconstrictor and pressor substance yet identified. Abnormalities of the endothelin system occur in a range of diseases associated with vasoconstriction, vasospasm, and vascular hypertrophy. ET receptor antagonists were until recently regarded as drugs of great promise in patients with congestive heart failure, pulmonary hypertension and others. The aim of this article is a survey of compounds that affect the endothelin receptors and clinical trials with these agents.

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.

Contrasting effects of intervention with ETA and ETB receptor antagonists in hypertension induced by angiotensin II and high-salt diet

Endothelin (ET) receptor antagonists are antihypertensive and renoprotective in angiotensin II (AngII)-induced hypertension if administered when AngII infusion commences, but their effects on established hypertension are poorly understood. We therefore tested the effects of intervening with an ETA (ABT-627) or ETB (A-192621) receptor antagonist after establishing hypertension with AngII (65 ng/min s.c.) plus 8% NaCl diet (AngII-HS) in rats. Prior to administration of ABT-627, AngII-HS and AngII-HS plus ABT-627 groups displayed robust hypertension (mean arterial pressure (MAP), 170 +/- 5 and 165 +/- 5 mm Hg versus 110 +/- 3 mm Hg in normal salt control rats at day 7, P < 0.05). Administering ABT-627 from day 8 of AngII-HS treatment prevented further rises in MAP (168 +/- 5 and 191 +/- 3 mm Hg at day 13 in AngII-HS plus ABT-627 and AngII-HS, P < 0.001), without blunting the significant increases in urinary protein (19-fold), albumin (25-fold), or MCP-1 excretion (6- to 8-fold) or the reduction in creatinine clearance. Administering A-192621 from day 8 mildly exacerbated AngII-HS induced hypertension (P < 0.05 for AngII-HS versus AngII-HS plus A-192621 on days 11 and 12 only) and reduced plasma nitrite/nitrate concentration (P < 0.05), without affecting proteinuria, albuminuria, or creatinine clearance. These results confirm the importance of ETA receptor signaling in maintaining AngII-HS hypertension and suggest that including ETB receptor blockade in therapeutic approaches to treating hypertension would be ineffective or even counterproductive.

Regulation of the epithelial Na+ channel by endothelin-1 in rat collecting duct

We used patch-clamp electrophysiology to investigate regulation of the epithelial Na+ channel (ENaC) by endothelin-1 (ET-1) in isolated, split-open rat collecting ducts. ET-1 significantly decreases ENaC open probability by about threefold within 5 min. ET-1 decreases ENaC activity through basolateral membrane ETB but not ETA receptors. In rat collecting duct, we find no role for phospholipase C or protein kinase C in the rapid response of ENaC to ET-1. ET-1, although, does activate src family tyrosine kinases and their downstream MAPK1/2 effector cascade in renal principal cells. Both src kinases and MAPK1/2 signaling are necessary for ET-1-dependent decreases in ENaC open probability in the split-open collecting duct. We conclude that ET-1 in a physiologically relevant manner rapidly suppresses ENaC activity in native, mammalian principal cells. These findings may provide a potential mechanism for the natriuresis observed in vivo in response to ET-1, as well as a potential cause for the salt-sensitive hypertension found in animals with impaired endothelin signaling.

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

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

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

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

Role of endothelin in cardiovascular disease

Endothelins are a family of peptides, which comprises endothelin-1 (ET-1), endothelin-2 (ET-2) and endothelin-3 (ET-3), each containing 21 amino-acids. ET-1 is a peptide secreted mostly by vascular endothelial cells, the predominant isoform expressed in vasculature and the most potent vasoconstrictor currently known. ET-1 also has inotropic, chemotactic and mitogenic properties. In addition, it influences salt and water homeostasis through its effects on the renin-angiotensin-aldosterone system (RAAS), vasopressin and atrial natriuretic peptide and stimulates the sympathetic nervous system. The overall action of endothelin is to increase blood pressure and vascular tone. Therefore, endothelin antagonists may play an important role in the treatment of cardiac, vascular and renal diseases associated with regional or systemic vasoconstriction and cell proliferation, such as essential hypertension, pulmonary hypertension, chronic heart failure and chronic renal failure. Long-term anti-endothelin therapy may improve symptoms and favourably alter the progression of heart failure. Endothelin appears to participate in induction and progression of sclerotic renal changes, leading to progression to end-stage renal disease. Anti-endothelin therapy might offer additional benefits in the prevention of progression of chronic renal failure in addition to the known benefits of RAAS inhibition. Clinical trials have demonstrated potentially important benefits of endothelin antagonists for patients with essential hypertension, pulmonary hypertension and heart failure. Further studies are necessary to determine the role of anti-endothelin therapy in the treatment of cardiovascular diseases and determine the different roles of selective receptor antagonism vs. mixed ET(A/B)-receptor antagonism in human diseases.

Role of NO and cytochrome P-450-derived eicosanoids in ET-1-induced changes in intrarenal hemodynamics in rats

Endothelin-1 (ET-1) produces potent renal effects that we have previously shown to be dependent on cytochrome P-450 (CYP450) metabolites of aracidonic acid (24) This study evaluated the role of these metabolites in the effects produced by ET-1 on renal blood flow (RBF), cortical blood flow (CBF), medullary blood flow (MBF), and mean arterial blood pressure (MBP). ET-1 (20-200 pmol/kg) increased MBP, renal vascular resistance (RVR), and MBF but reduced CBF and RBF in a dose-dependent manner. The decreases in CBF and RBF, and increases in MBP and RVR were blunted by BMS-182874, an ET(A) receptor antagonist or BQ-788, an ET(B) receptor antagonist. Similarly, indomethacin, an inhibitor of cyclooxygenase activity, or 12,12-dibromododecenoic acid (DBDD), a CYP450-dependent inhibitor of production of 20-hydroxyeicosatetraenoic acid (20-HETE), blunted these effects. ET-3 elicited dose-related reduction in CBF and increase in MBF. Indomethacin accentuated the reduction in CBF and attenuated the increase in MBF, as did DBDD. ET-1-induced increase in MBF was attenuated by BQ-788, N(omega)-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide (NO) synthesis, indomethacin, or DBDD. DBDD inhibited the hemodynamic effects of L-NAME. Miconazole, the inhibitor of CYP450-dependent epoxygenase activity, was without effect. These results indicate that hemodynamic changes produced by ET-1 are mediated by vasoconstrictor prostanoids and/or prostanoid-like substances, possibly, 20-HETE via activation of ET(A) and ET(B) receptors. However, the increase in MBF is mediated by vasodilator prostanoids or by NO via ET(B) receptor activation.

Increased renal medullary endothelin-1 synthesis in prehypertensive DOCA- and salt-treated rats

To investigate the role of renal endothelin-1 (ET-1) synthesis in water-sodium homeostasis, we measured mRNA expressions, protein levels, enzyme activity, and receptor binding of the renal ET-1 system in a DOCA- and salt-treated rat model. Male Wistar rats were divided into control and DOCA- and salt-treated (DOCA-Salt) groups. The DOCA-Salt group received 25 mg/kg body wt DOCA and was maintained on 1% NaCl drinking water. Rats were killed on days 1, 2, 4, and 10 of the experiment. Urinary ET-1-like immunoreactivity significantly increased from the second day in the DOCA-Salt group and correlated well with the urinary sodium excretion rate (r = 0.81, P < 0.001). Renal endothelin-converting enzyme (ECE) activity, ET-1, and ECE-1 mRNA expressions were significantly increased in the renal medullary area of DOCA-Salt rats. In situ hybridization and immunohistochemical studies showed that the increase in ET-1 synthesis was mainly localized in the inner medullary collecting ducts. The maximum binding of endothelin B receptor also increased from the second day in the renal medulla of the DOCA-Salt group. Our results suggest that renal medullary synthesized ET-1 may be a natriuretic factor and may participate in the intrarenal regulation of water and salt homeostasis in prehypertensive DOCA-and salt-treated rats.

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.

Upregulation of endothelin B receptors in kidneys of DOCA-salt hypertensive rats

Experiments were designed to elucidate the role of endothelin B receptors (ET(B)) on arterial pressure and renal function in deoxycorticosterone acetate (DOCA)-salt hypertensive rats. Male Sprague-Dawley rats underwent uninephrectomy and were treated with either DOCA and salt (0.9% NaCl to drink) or placebo. DOCA-salt rats given the ET(B)-selective antagonist, A-192621, for 1 wk (10 mg. kg(-1). day(-1) in the food) had significantly greater systolic arterial pressure compared with untreated DOCA-salt rats (208 +/- 7 vs. 182 +/- 4 mmHg) whereas pressure in placebo rats was unchanged. In DOCA-salt, but not placebo rats, A-192621 significantly decreased sodium and water excretion along with parallel decreases in food and water intake. To determine whether the response in DOCA-salt rats was due to increased expression of ET(B) receptors, endothelin receptor binding was performed by using membranes from renal medulla. Maximum binding (B(max)) of [(125)I]ET-1, [(125)I]ET-3, and [(125)I]IRL-1620 increased from 227 +/- 42, 146 +/- 28, and 21 +/- 1 fmol/mg protein, respectively, in placebo rats to 335 +/- 27, 300 +/- 38, and 61 +/- 6 fmol/mg protein, respectively, in DOCA-salt hypertensive rats. The fraction of receptors that are the ET(B) subtype was significantly increased in DOCA-salt (0.88 +/- 0.07) compared with placebo (0.64 +/- 0.01). The difference between [(125)I]ET-3 and [(125)I]IRL-1620 binding is consistent with possible ET(B) receptor subtypes in the kidney. These results indicate that ET(B) receptors in the renal medulla are up-regulated in the DOCA-salt hypertensive rat and may serve to maintain a lower arterial pressure by promoting salt and water excretion.

Renal effects of endothelin in anesthetized rabbits

Intrarenal arterial infusion of endothelin-1 (1, 3 and 10 ng/kg per min) reduced renal blood flow, urine flow rate and urinary Na+ excretion without affecting fractional Na+ excretion in anesthetized rabbits. An endothelin ET(A) receptor antagonist (R)2-[(R)-2-[(S)-2-[[1-(hexahydro-1H-azepinyl)]carbonyl]amino-4-me thyl-pentanoyl]amino-3-[3-(1-methyl-1H-indolyl)]propionyl]amino-3-(2-pyr idyl)propionic acid (FR139317, 1 microg/kg per min) attenuated the endothelin-1 (1 ng/kg per min)-induced renal responses. An endothelin ET(B) receptor antagonist N-cis 2,6-dimetylpiperidinocarbonyl-L-gamma-metylleucyl-D-1-met hoxycarbonyltryptophanyl-D-norleucine (BQ-788, 1 microg/kg per min) potentiated the endothelin-1-induced changes in renal blood flow, urine flow rate and urinary Na+ excretion. A nitric oxide (NO) synthase inhibitor Nomega-nitro-L-arginine methyl ester (L-NAME, 50 microg/kg per min) also potentiated the endothelin-1-induced reductions in urine flow rate and urinary Na+ excretion but not the reduction in renal blood flow. Endothelin-1 reduced fractional Na+ excretion in the presence of BQ-788 or L-NAME. A spontaneous NO donor 1-hydroxy-2-oxo-3-(N-methyl-3-aminopropyl)-3-methyl-1-triazene (30 ng/kg per min) slightly attenuated the antinatriuresis but not the vasoconstriction induced by endothelin-1. These results suggest that in the rabbit kidney in vivo endothelin ET(A) receptors mediate endothelin-1-evoked vasoconstriction and tubular Na+ reabsorption, that the concomitant stimulation of endothelin ET(B) receptors by endothelin-1 counteracts both the ET(A) receptor-mediated vascular and tubular actions, and that the tubular action, but not the vascular action, of endothelin-1 is also susceptible to changes in renal NO level.

Endothelin as a regulator of cardiovascular function in health and disease

The endothelins are a family of endothelium-derived peptides that possess characteristically sustained vasoconstrictor properties. Endothelin-1 appears to be the predominant member of the family generated by vascular endothelial cells. In addition to its direct vascular effects, endothelin-1 has inotropic and mitogenic properties, influences homeostasis of salt and water, alters central and peripheral sympathetic activity and stimulates the renin-angiotensin-aldosterone system. Studies with endothelin receptor antagonists have indicated that endothelin-1 probably has complex opposing vascular effects mediated through vascular smooth muscle and endothelial ET(A) and ET(B)receptors. Endogenous generation of endothelin-1 appears to contribute to maintenance of basal vascular tone and blood pressure through activation of vascular smooth muscle ET(A)receptors. At the same time, endogenous endothelin-1 acts through endothelial ET(B) receptors to stimulate formation of nitric oxide tonically and to oppose vasoconstriction. In view of the multiple cardiovascular actions of endothelin-1, there has been much interest in its contribution to the pathophysiology of hypertension. Results of most studies suggest that generation of, or sensitivity to, endothelin-1 is no greater in hypertensive than it is in normotensive subjects. Nonetheless, the deleterious vascular effects of endogenous endothelin-1 may be accentuated by reduced generation of nitric oxide caused by hypertensive endothelial dysfunction. It also appears likely that endothelin participates in the adverse cardiac and vascular remodelling of hypertension, as well as in hypertensive renal damage. Irrespective of whether vascular endothelin activity is increased in hypertension, anti-endothelin agents do produce vasodilatation and lower blood pressure in hypertensive humans. There is more persuasive evidence for increased endothelin-1 activity in secondary forms of hypertension, including pre-eclampsia and renal hypertension. Endothelin-1 also appears to play an important role in pulmonary hypertension, both primary and secondary to diseases such as chronic heart failure. The hypotensive effects of endothelin converting enzyme inhibitors and endothelin receptor antagonists should be useful in the treatment of hypertension and related diseases. Development of such agents will increase knowledge of the physiological and pathological roles of the endothelins, and should generate drugs with novel benefits.

Alterations in basal protein kinase C activity modulate renal afferent arteriolar myogenic reactivity

Myogenic vasoconstriction of the renal afferent arteriole contributes to the autoregulation of renal blood flow, glomerular filtration rate, and glomerular capillary pressure (PGC). The reactivity of the afferent arteriole to pressure and the efficiency of PGC control are subject to physiological and pathophysiological alterations, but the determinants of the myogenic response of this vessel are largely unknown. We used the in vitro perfused hydronephrotic rat kidney to investigate the role of protein kinase C (PKC) in the control of this response. Inhibition of PKC by 1 microM chelerythrine attenuated myogenic reactivity but did not affect the afferent arteriole vasoconstrictor response to KCl (35 mM)-induced depolarization. Low concentrations of phorbol ester (10 nM phorbol 12-myristate 13-acetate) and low levels of ANG II or endothelin-1 (3 pM) potentiated myogenic vasoconstriction without affecting basal afferent arteriolar diameters. These actions were blocked by 1 microM chelerythrine, suggesting a PKC-dependent mechanism. Finally, although PKC inhibition attenuated basal myogenic responses, full reactivity to pressure was restored by 1 mM 4-aminopyridine, a pharmacological inhibitor of delayed rectifier K channels, which are known to be modulated by PKC. The ability of 4-aminopyridine to circumvent the effects of PKC inhibition militates against a direct role of PKC in myogenic signaling. We interpret these observations as indicating that basal PKC activity is an important determinant of myogenic reactivity in the renal afferent arteriole. However, PKC activation does not appear to play an obligate role in myogenic signaling in this vessel. We suggest that basal PKC activity directly modulates voltage-gated K channel activity, thereby indirectly affecting myogenic reactivity.

Effects of FR139317 on renal responses to acute nitric oxide blockade in anaesthetized rats

1. Effects of FR139317, an endothelin ETA receptor antagonist, on renal haemodynamic and excretory responses to acute nitric oxide (NO) blockade were examined using anaesthetized rats. 2. Intrarenal arterial infusion of NG-nitro-L-arginine (NOARG), the NO synthase inhibitor, at a rate of 40 micrograms/kg per min, produced a significant decrease in renal blood flow, with no change in systemic blood pressure. There were significant decreases in urine flow and urinary excretion of sodium during infusion of NOARG. In animals pretreated with FR139317, similar renal responses to the NOARG infusion were observed. 3. These results suggest that an action of endothelin-1 via ETA receptors does not greatly contribute to the renal haemodynamic and excretory responses to acute blockade of renal NO production.

SB 234551, a novel endothelin--A receptor antagonist, unmasks endothelin-induced renal vasodilatation in the dog

Infusion of endothelin-1 (ET-1) into conscious, chronically instrumented dogs (10 ng/kg.min i.v.) resulted in a significant increase in mean arterial pressure and significant reductions in renal plasma flow, glomerular filtration rate, and sodium excretion. Intravenous infusion of SB 209670 (30 micrograms/kg.min, i.v.) abolished these responses, whereas infusion of SB 234551 (30 micrograms/kg.min, i.v.) resulted in significant increases in renal plasma flow and urinary sodium excretion. These data indicate that SB 234551 can unmask ETB receptor-induced renal vasodilatation and inhibition of sodium reabsorption.

Regulation of intrarenal blood flow in experimental heart failure: role of endothelin and nitric oxide

Congestive heart failure(CHF) is associated with a marked decrease in cortical blood flow and preservation of medullary blood flow. In the present study we tested the hypothesis that changes in the endothelin (ET) and nitric oxide (NO) systems in the kidney may contribute to the altered intrarenal hemodynamics in rats with aortocaval fistula, an experimental model of CHF. Cortical and medullary blood flow were measured simultaneously by laser-Doppler flowmetry in controls and rats with compensated and decompensated CHF. As previously reported [K. Gurbanov, I. Rubinstein, A. Hoffman, Z. Abassi, O. S. Better, and J. Winaver. Am. J Physiol. 271 (Renal Fluid Electrolyte Physiol. 40): F1166-F1172, 1996], administration of ET-1 in control rats produced a sustained cortical vasoconstriction and a transient medullary vasodilatory response. In rats with decompensated CHF, cortical vasoconstriction was severely blunted, whereas ET-1-induced medullary vasodilation was significantly prolonged. This prolonged response was mimicked by IRL-1620, a specific ETB agonist, and partially abolished by NO synthase (NOS) blockade. In line with these findings, expression of ET-1, ETA and ETB receptors, and endothelial NOS (eNOS), assessed by RT-PCR, and eNOS immunoreactivity, assessed by Western blotting, was significantly higher in the medulla than in the cortex. Moreover, expression of ET-1 mRNA in the cortex and eNOS mRNA in the cortex and the medulla increased in proportion to the severity of heart failure. These findings indicate that CHF is associated with altered regulation of intrarenal blood flow, which reflects alterations in expression and activity of the ET and NO systems. It is further suggested that exaggerated NO activity in the medulla contributes to preservation of medullary blood flow in the face of cortical vasoconstriction in CHF.

Renal endothelin in FK506-induced nephrotoxicity in spontaneously hypertensive rats

FK506, a major immunosuppressive agent, often causes nephrotoxicity accompanied by renal vasoconstriction. It is recognized that endothelin (ET) plays a role in the cyclosporin A-induced nephrotoxicity, but the involvement of ET in the FK506-induced renal dysfunction is still poorly understood. We elicited nephrotoxicity by daily administration of FK506 in spontaneously hypertensive rats, and we examined the renal gene expression of ET and its receptors and the effects of an ET receptor antagonist on FK506-induced renal dysfunction. FK506 administration (4 mg/kg/day, i.m.) for 14 days induced nephrotoxicity, including a renal vasoconstriction and a decrease in glomerular filtration rate. The renal dysfunction was accompanied by an increase in ET-1 mRNA levels, while ET(B)-receptor mRNA was unaffected. Continuous administration of an ET(A)/ET(B) antagonist, TAK-044 (3 mg/day, s.c.), which effectively blocked systemic and renal vascular responses to exogenously administered ET-1, partially attenuated the FK506-induced renal vasoconstriction. However the reduced glomerular filtration rate were not affected by TAK-044. Thus, although enhanced gene expression of ET-1 in the kidney is involved in the renal vasoconstriction, ET does not play a major role in the FK506-induced renal dysfunction.

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.

Renal vascular effects of the selective endothelin receptor antagonists in anaesthetized rats

1. Endothelin (ET) is a potent vasoconstrictor peptide which has been shown to have an important role in the regulation of systemic and renal haemodynamics. In order to elucidate the role of endogenous ET in the kidney, we examined the effects of ET receptor antagonists on systemic and renal vasculature in normotensive anaesthetized rats. 2. Intravenous injection of a selective ETA receptor antagonist, FR139317 (0.5 mumol kg-1, for 20 min) induced a very small fall in blood pressure. Similarly, a non-selective ETA/ETB receptor antagonist, TAK-044 (12.5 mumol kg-1, for 20 min) slightly decreased blood pressure. A selective ETB receptor antagonist, BQ-788 (0.5 mumol kg-1, for 20 min) had no effect of blood pressure. 3. FR139317 and TAK-044 did not affect renal blood flow or calculated renal vascular resistance. In contrast, BQ-788 significantly reduced renal blood flow by 18.2 +/- 2.4% and increased renal vascular resistance. Furthermore, the renal vascular action of BQ-788 was not observed when combined with FR139317. 4. Pretreatment with a nitric oxide (NO) synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME, 37 mumol kg-1, i.v.) and a cyclo-oxygenase inhibitor ibuprofen (44 mumol kg-1, i.v.) completely abolished the BQ-788-mediated renal vasoconstriction. 5. These results indicate that activation of ETB receptors by endogenous ET acts as a physiological brake for the ETA-mediated renal vasoconstriction; this effect appears to be mediated by stimulation of NO and/or vasodilator prostaglandin(s) release.

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.

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.

High resolution localization of endothelin receptors in rat renal medulla

The cellular localization of endothelin receptors in the inner medulla of the rat kidney was investigated by using high resolution light and electron microscopic autoradiography, with the microwave irradiation fixation methods. Kidney slices were incubated with 125I-endothelin-1 alone or with selective ligands for the endothelin ETB and/or ETA receptors for light microscopic autoradiography. At the microscopic level, 125I-endothelin-1 was found to bind specifically to the glomeruli, arterioles and peritubular spaces in the cortex and vasa recta and surrounding tissues in the inner medulla. These bindings were also observed when the tissue slices were incubated in the presence of IRL1620 (ETB receptor agonist) or 97-139 (ETA receptor antagonist). Electron microscopic autoradiography using 125I-endothelin-1 in the inner medulla revealed silver grains over endothelial cells of the vasa recta and interstitial and collecting duct cells. No grains were detected over inner lining cells of the thin limbs of Henle's loop. These interstitial cells contained abundant microorganelles and lipid droplets, and had extensive cytoplasmic processes that closely related to the basement membranes of the vasa recta and loop of Henle. These findings demonstrate that type 1 interstitial cells are also primary sites for endothelin receptors as well as endothelial cells of the vasa recta and collecting duct cells in the inner medulla.

Selective blockade of endothelin receptor subtypes on systemic and renal vascular responses to endothelin-1 and IRL1620, a selective endothelin ETB-receptor agonist, in anesthetized rats

By using BQ-788 as a selective endothelin ETB-receptor antagonist and FR139317 as a selective endothelin ETA-receptor antagonist, we have characterized the receptor subtypes mediating the systemic and renal vascular effects of endothelin-1 and IRL1620, a selective endothelin ETB-receptor agonist (succinyl-[Glu9,Ala11,5]-endothelin-1(8-21)), in anesthetized rats. Bolus intravenous injection of endothelin-1 (0.5 nmol/kg) and IRL1620 (1.65 nmol/kg) produced a transient fall in systemic blood pressure followed by a sustained increase. The initial fall in blood pressure observed after endothelin-1 and IRL1620 administration was completely blocked by BQ-788 (0.5 mumol/kg, i.v.), whereas the pressor response was blocked by FR139317 (0.8 mumol/kg, i.v.). Renal blood flow was decreased and calculated renal vascular resistance was dramatically increased by endothelin-1 and IRL1620. The reduction of renal blood flow by endothelin-1 was significantly suppressed by FR139317 but potentiated by BQ-788. Both BQ-788 and FR139317 partially blocked the renal vasoconstriction by IRL1620. Pretreatment by BQ-788 itself decreased renal blood flow by 14.1%. These results indicate that the systemic depressor responses induced by endothelin-1 and IRL1620 are mediated through the endothelin ETB-receptor, and the pressor responses are mediated through the endothelin ETA-receptor. In the renal vasculature of anesthetized rats, it is suggested that vasoconstriction is mediated through both endothelin ETA- and ETB-receptors and that endothelin ETB-receptors may be also involved in vasodilating responses to endothelin peptides.

Endothelins: renal tubule synthesis and actions

1. Renal tubules and, in particular, the inner medullary collecting duct, produce endothelin and express cognate receptors. 2. Endothelins inhibit vasopressin-stimulated cAMP accumulation and water reabsorption in the collecting duct; endothelins may also inhibit sodium reabsorption in the proximal tubule and collecting duct. 3. Autocrine inhibition of sodium and water reabsorption in the inner medullary collecting duct by endothelin may play a role in maintaining extracellular fluid volume homeostasis. 4. Derangements in autocrine inhibition of sodium and water reabsorption in the inner medullary collecting duct by endothelin may be involved in the pathogenesis of the hypertensive state. 5. Nephron-derived endothelins may function in a paracrine manner to regulate interstitial, juxtaglomerular and vascular smooth muscle cell function.