Role of neutral endopeptidase in the metabolism of endothelin

The vascular endothelin system in hypertension--recent patents and discoveries

The discovery of endothelin two decades ago has now evolved into an intricate vascular endothelin (ET) system. Several ET isoforms, receptors, signaling pathways, agonists, antagonists, and clinical applications have been identified and documented in first-rate patents. The role of ET as one of the most potent endothelium-derived vasoconstricting factors is now complemented by a newly discovered role in vascular relaxation. ET synthesis is initiated by the transcription of ET genes in endothelial cells and the generation of the gene products preproET and big ET, which are further cleaved by specific ET converting enzymes into ET-1, -2, -3 and -4 isoforms. ET isoforms bind with different affinities to ET(A) and ET(B2) receptors in vascular smooth muscle, and stimulate [Ca(2+)](i), protein kinase C, mitogen-activated protein kinase and other signaling mechanisms of smooth muscle contraction, growth and proliferation. ET also binds to endothelial ET(B1) receptors, which mediate the release of vasodilator substances such as nitric oxide, prostacyclin and endothelium-derived hyperpolarizing factor. Endothelial ET(B1) receptors may also function in ET re-uptake and clearance. Although the effects of ET on vascular function and growth are well-recognized, the role of ET and its receptors in the regulation of blood pressure and in the pathogenesis of hypertension is not clearly established. Salt-dependent hypertension in experimental animals and some forms of moderate to severe hypertension in human may show elevated levels of plasma or vascular ET; however, other forms of hypertension show normal ET levels. The currently available ET receptor antagonists reduce blood pressure in some forms of experimental hypertension. Careful examination of recent patents may identify more effective and specific modulators of the vascular ET system for clinical use in human hypertension.

Glomerular abundance of nephrin and podocin in experimental nephrotic syndrome: different effects of antiproteinuric therapies

Nephrotic syndrome (NS) is a clinical state characterized by massive proteinuria, hypoalbuminemia, and eventual edema formation. Although the mechanisms underlying this phenomenon are not yet fully clarified, it is well accepted that nephrin and podocin are involved in the development of proteinuria. The effects of early treatment with various antiproteinuric therapies on proteinuria and glomerular staining of nephrin and podocin in rats with experimental NS have not been previously studied. Proteinuria and glomerular nephrin and podocin immunofluorescence were examined in rat kidneys with adriamycin-induced NS and the effects of antiproteinuric drug therapies during 5 wk with enalapril, losartan, alone or in combination, omapatrilat, and mycophenolate mofetil on these parameters were assessed. Injection of adriamycin caused a significant increase in daily (from 21.8 ± 1.4 to 983.1 ± 45.8 mg/day, P < 0.01) and cumulative protein excretion (from negligible values to 22,490 ± 931 mg, P < 0.001) during 5 wk. Early treatment with enalapril significantly decreased the daily (641.7 ± 82.4 mg/day, P < 0.0023) and cumulative proteinuria (15,727 ± 2,204 mg, P < 0.001). A similar effect, although to a lesser extent, was obtained after omapatrilat treatment: cumulative proteinuria was reduced to 18,706 ± 1,042 mg, P < 0.001. In contrast, losartan treatment did not significantly influence the cumulative proteinuria that remained comparable (20,351 ± 1,360 mg, P > 0.05) to that observed in untreated NS rats. Unexpectedly, when losartan was given in combination with enalapril, it abolished the beneficial effects of the latter. Pretreatment with mycophenolate mofetil exerted a moderate antiproteinuric effect, which appeared only during the last week of the experimental treatment. Nephrotic rats exhibited severe disruption of slit diaphragm structure as seen by rapid and profound loss of nephrin and podocin. Beneficial effects of enalapril, omapatrilat, and mycophenolate mofetil paralleled the preservation of nephrin, as determined immunohistochemically, and enabled prediction of significant antiproteinuric responses. Enalapril alone or in combination with losartan resulted in significant preservation of podocin. Pretreatment with enalapril, and to a lesser extent omapatrilat, is superior to losartan in reducing proteinuria in NS rats. A combination of ACE inhibitors with ANG II receptor blockers does not provide any advantageous antiproteinuric therapy in these animals. Nephrin loss is an indication of proteinuria in NS and the antiproteinuric effects of ACE inhibitors, vasopeptidase inhibitors, and mycophenolate mofetil attenuate this reduction. Not all the drugs which restore podocin reduce urinary protein in NS.

The role of endothelin-1 in myocarditis and inflammatory cardiomyopathy: old lessons and new insights

Endothelin-1 has emerged as an important participant in the pathophysiology of a variety of cardiovascular diseases, where it may act on endocrine, paracrine and autocrine bases. Here we review its regulated biosynthesis, receptor-mediated signaling, and functional consequences in the heart, with particular emphasis on cardiac development and disease. Exploring published data employing molecular genetic mouse models of endothelin dysregulation, we highlight its heretofore underappreciated role as a pro-inflammatory cytokine. We also present novel micro-array data from one such mouse model, which implicate the specific downstream pathways that may mediate endothelin-1's effects.Key words: endothelin-1, cardiac development, inflammation, transgenic mice, gene expression profiling.

Renal effects of omapatrilat and captopril in salt-loaded, nitric oxide-deficient rats

Inhibition of nitric oxide synthases causes systemic hypertension and renal injury in rats. Our objective was to examine whether omapatrilat, a vasopeptidase inhibitor that inhibits both angiotensin-converting enzyme (ACE) and neutral endopeptidase, could induce better regression of renal injury than ACE inhibitor alone. Ten groups of rats were studied. They were fed either a normal (0.8% NaCl) or a high (4% NaCl) sodium diet. Eight of these groups received NG-nitro-L-arginine methyl ester (L-NAME, 20 mg x kg(-1) x d(-1)) in their drinking water. After 4 weeks, 1 group on each diet was killed and considered the L-NAME group, whereas the others received L-NAME alone, captopril (200 mg x kg(-1) x d(-1)) plus L-NAME, or omapatrilat (80 mg x kg(-1) x d(-1)) plus L-NAME for 4 additional weeks. In rats receiving L-NAME alone for 8 weeks, the mortality rate was approximately 90%, irrespective of the diet. In contrast, all rats survived in the captopril and the omapatrilat groups. In rats fed a normal-sodium diet, captopril and omapatrilat normalized systolic blood pressure and induced a complete regression of renal injury. Creatinine clearance and proteinuria were also normalized. In the high-sodium-diet groups, both treatments were less efficient: blood pressure remained elevated, and the regression of renal fibrosis was only partial. Although proteinuria decreased significantly with captopril or omapatrilat, creatinine clearance remained lower than in the controls. These results demonstrate that, in nitric oxide-deficient rats fed a normal-sodium diet, ACE and vasopeptidase inhibitors exhibit a marked renoprotective effect, whereas these treatments are less efficient in rats fed a high-sodium diet.

Omapatrilat induces profound renal vasodilation but does not affect coronary hemodynamics

BACKGROUND

Omapatrilat has potent enzymatic inhibitory effects on the angiotensin-converting enzyme and neutral endopeptidase. The prolonged effects of this inhibition on systemic and regional hemodynamics, cardiovascular mass, and hydroxyproline concentration in spontaneously hypertensive rats were studied. The contribution of endogenous bradykinin on the cardiovascular actions of omapatrilat in this genetic model of hypertension was also investigated.

METHODS AND RESULTS

Systemic and regional hemodynamics (radionuclide-labeled microspheres), left and right ventricular and aortic masses, and hydroxyproline concentration were determined in 35-week-old spontaneously hypertensive rats after 12 weeks of treatment with omapatrilat (40 mg/kg/day), with and without the bradykinin receptor antagonist icatibant (500 microg/kg/day). Omapatrilat decreased mean arterial pressure, reducing total peripheral resistance as well as decreased left ventricular and aortic mass indices. It also induced a profound renovasodilation associated with a decrease renal vascular resistance that markedly increased renal blood flow. Coronary hemodynamics and left ventricular hydroxyproline concentration remained unaltered. Concomitant blockade of bradykinin receptors partially attenuated the hypotensive effect of omapatrilat and its effect on aortic mass; and icatibant did not influence the renovasodilation.

CONCLUSION

Omapatrilat produced profoundly beneficial effects on systemic and renal hemodynamics, as well as on left ventricular and aortic masses, without any effect on coronary hemodynamics. These effects of omapatrilat on arterial pressure and aortic mass, but not on renal hemodynamics and left ventricular mass, may have been at least partially mediated through the action of bradykinin.

Endothelin-1 in chronic renal failure and hypertension

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

Gene expression profiles and transcription factors involved in parathyroid hormone signaling in osteoblasts revealed by microarray and bioinformatics

Parathyroid hormone (PTH) binds to its receptor PTH1R (parathyroid hormone 1 receptor) in osteoblastic cells to regulate bone remodeling and calcium homeostasis. While prolonged exposure to PTH causes increased bone resorption, intermittent injections of PTH have an anabolic effect on bone. The molecular mechanisms regulating these processes are still largely unknown. Here, we present our results on gene expression profile changes in the PTH-treated osteoblastic cell line, UMR 106-01, using DNA microarray analysis. A total of 125 known genes and 30 unknown expressed sequence tags (ESTs) were found to have at least 2-fold expression changes after PTH treatment at 4, 12, and 24 h. 14 genes were previously known to be PTH-regulated but many were unknown to be regulated by PTH prior to our experiments. Real-time reverse transcriptase-PCR confirmed that 90 and 50% of the genes are regulated more than 2-fold by PTH in UMR 106-01 and rat primary osteoblastic cells, respectively. Most genes belong to the following protein families: hormones, growth factors, and receptors; signal transduction pathway proteins; transcription factors; proteases; metabolic enzymes; structural and matrix proteins; transporters; etc. These results provide a comprehensive and deeper knowledge about PTH regulation of osteoblastic gene expression. Next, we designed a computational method to extract information about transcription factors likely involved in regulating these genes. These factors include those previously known to be involved in PTH signaling (AP-1 and the cAMP response element-binding protein), those that were identified by microarray data (C/EBP), and some novel transcription factors (AP-2, AP-4, SP1, FoxD3, etc.). Our results suggest that a reliable bioinformatics approach can be easily applied for other systems.

Increased expression of renal neutral endopeptidase in severe heart failure

The enzyme neutral endopeptidase (NEP; EC 3.4.24.11) cleaves several vasoactive peptides such as the atrial natriuretic peptide (ANP). ANP is a hormone of cardiac origin with diuretic and natriuretic actions. Despite elevated circulating levels of ANP, congestive heart failure (CHF) is characterized by progressive sodium and water retention. In order to elucidate the loss of natriuretic and diuretic properties of ANP in CHF we analyzed activity, protein concentrations, mRNA and immunostaining of NEP in kidneys of different models of severe CHF in the rat.CHF was induced by either aortocaval shunt, aortic banding or myocardial infarction in the rat. All models were defined by increased left ventricular end-diastolic pressure and decreased contractility. The diminished effectiveness of ANP was reflected by reduced cGMP/ANP ratio in animals with shunt or infarction. Renal NEP activity was increased in rats with aortocaval shunt (203 +/- 7%, p < 0.001), aortic banding (184 +/- 11%, p < 0.001) and infarction (149 +/- 10%, p < 0.005). Western blot analysis revealed a significant increase in renal NEP protein content in two models of CHF (shunt: 214 +/- 57%, p < 0.05; infarction: 310 +/- 53 %, p < 0.01). The elevated protein expression was paralleled by a threefold increase in renal NEP-mRNA level in the infarction model. The increased renal NEP protein expression and activity may lead to enhanced degradation of ANP and may contribute to the decreased renal response to ANP in heart failure. Thus, the capacity to counteract sodium and water retention, would be diminished. The increased renal NEP activity may therefore be a hitherto unknown factor in the progression of CHF.

Hormonal, renal, hemodynamic responses to acute neutral endopeptidase inhibition in heart transplant patients

We investigated the hemodynamic, renal, and hormonal responses to neutral endopeptidase (NEP) inhibition during a 6-h, double-blind, randomized, placebo-controlled study in seven chronic, stable heart transplant patients. Baseline characteristics were similar during both experiments, and no significant changes were observed after placebo. NEP inhibition increased circulating endothelin-1 (from 2.01 +/- 0.1 to 2.90 +/- 0.2 pmol/l; P < 0.01), atrial natriuretic peptide (ANP; from 21.5 +/- 2.7 to 29.6 +/- 3.7 pmol/l; P < 0.01), and the ANP second messenger cGMP. Noteworthy, systemic blood pressure did not increase. Renal plasma flow and glomerular filtration rate remained unmodified after NEP inhibition. Filtration fraction (33 +/- 13%), diuresis (196 +/- 62%), and natriuresis (315 +/- 105%) increased significantly in relation to ANP and cGMP. A strong inverse relationship was observed between excreted cGMP and sodium reabsorption (r = -0.71, P < 0.0001). Thus, despite significantly increasing endothelin-1, NEP inhibition did not adversely influence systemic or renal hemodynamics in transplant patients. ANP, possibly through a tubular action, enhances the natriuresis observed after NEP inhibition.

Endothelin antagonism with bosentan: current status and future perspectives

Endothelin receptor antagonists have been proposed for the treatment of a variety of disorders in which the endothelins may act as pathogenic mediators, such as hypertension, congestive heart failure, and cerebral vasospasm. Bosentan is a nonpeptide competitive antagonist, which can be a good tool for studying the endothelin system. It is specific for the endothelin system and blocks the actions of endothelin at both mammalian receptors (A and B). Bosentan has recently moved into Phase III clinical trial. This review will attempt to overview the experimental and clinical effects of bosentan.

Participation of renal and circulating endothelin in salt-sensitive essential hypertension

Salt sensitivity of blood pressure is a cardiovascular risk factor, independent of and in addition to hypertension. In essential hypertension, a conglomerate of clinical and biochemical characteristics defines a salt-sensitive phenotype. Despite extensive research on multiple natriuretic and antinatriuretic systems, there is no definitive answer yet about the major causes of salt-sensitivity, probably reflecting the complexity of salt-balance regulation. The endothelins, ubiquitous peptides first described as potent vasoconstrictors, also have vasodilator, natriuretic and antinatriuretic actions, depending on their site of generation and binding to different receptors. We review the available data on endothelin in salt-sensitive essential hypertension and conclude that abnormalities of renal endothelin may play a primary role. More importantly, the salt-sensitive patient may have blood pressure-dependency on endothelin in all states of salt balance, thus predicting that endothelin receptor blockers will have a major therapeutic role in salt-sensitive essential hypertension.

Toward an optimal joint recognition of the S1' subsites of endothelin converting enzyme-1 (ECE-1), angiotensin converting enzyme (ACE), and neutral endopeptidase (NEP)

The formation of vasoconstrictors (e.g., angiotensin II and endothelin) and the inactivation of vasodilators (e.g., bradykinin and atrial natriuretic) by membrane-bound zinc metallopeptidases are key mechanisms in the control of blood pressure and fluid homeostasis. The way in which these peptides modulate physiological functions has been intensively studied. With the aim to develop compounds that can jointly block the three metallopeptidases-neutral endopeptidase (NEP, neprilysin), angiotensin-converting enzyme (ACE), and endothelin-converting enzyme (ECE-1)-we studied the common structural specificity of the S1' subsites of these peptidases. Various mercaptoacyl amino acids of the general formula HS-CH2-CH(R1')CO-Trp-OH, possessing more or less constrained R1' side chains, were designed. The mercapto-acyl synthons contain one or two asymmetrical centers. The K(i) values of the separated stereoisomers of the most efficient inhibitors were used to determine the stereochemical preference of each enzyme. A guideline for the joint inhibition of the three peptidases was obtained with the (2R,3R) isomer of compound 13b. Its K(i) values on NEP, ACE, and ECE were 0.7, 43, and 26 nM, respectively.

Vascular effects of newer cardiovascular drugs: focus on nebivolol and ACE-inhibitors

Alterations in the function and structure of the blood vessel wall account for most clinical events in the coronary and cerebrovascular circulation such as myocardial infarction and stroke. Cardiovascular drugs may exert beneficial effects on the vascular wall both at the level of the endothelium and vascular smooth muscle cells. Therefore, endothelial mediators, in particular nitric oxide (NO) and endothelin (ET), are of special interest. Drugs can modulate the expression and actions of NO, a vasodilator with antiproliferative and antithrombotic properties, and of ET, a potent vasoconstrictor and proliferative mitogenic agent. The most successful drugs in this context are statins and angiotensin-converting enzyme (ACE)-inhibitors. While statins increase the expression of NO synthase. ACE-inhibitors increase the release of NO via bradykinin-mediated mechanisms. Antioxidant properties of drugs are also important, as oxidative stress is crucial in atherosclerotic vascular disease. These properties may explain part of the effects of calcium antagonists and ACE-inhibitors. Indeed, angiotensin II stimulates NAD(P)H oxidases responsible for the formation of superoxide, which inactivates NO. ACE-Inhibitors thus increase the bioavailability of NO. Newer cardiovascular drugs such as nebivolol are able to directly stimulate NO release from the endothelium both in isolated arteries and in the human forearm circulation. ET receptor antagonists may exert beneficial effects in the vessel wall by preventing the effects of ET at its receptors and by reducing ET production. In summary, cardiovascular drugs have important effects on the vessel wall, which may be clinically relevant for the prevention and treatment of cardiovascular disease.

Endothelin A receptor antagonists in congestive heart failure: blocking the beast while leaving the beauty untouched?

Congestive heart failure (CHF) is a disease process characterized by impaired left ventricular function, increased peripheral and pulmonary vascular resistance and reduced exercise tolerance and dyspnea. Thus, mediators involved in the control of myocardial function and vascular tone may be involved in its pathophysiology. The family of endothelins (ET) consists of four closely related peptides, ET-1, ET-2, ET-3, and ET-4, which cause vasoconstriction, cell proliferation, and myocardial effects through activation of ET(A) receptors. In contrast, endothelial ET(B) receptors mediate vasodilation via release of nitric oxide and prostacyclin. In addition, ET(B) receptors in the lung are a major pathway for the clearance of ET-1 from plasma. Thus, infusion of an ET(A) receptor antagonist into the brachial artery in healthy humans leads to vasodilation whereas infusion of an ET(B) receptor antagonist causes vasoconstriction. ET-1 plasma levels are elevated in CHF and correlate both with the hemodynamic severity and with symptoms. Plasma levels of ET-1 and its precursor, big ET-1, are strong independent predictors of death in patients after myocardial infarction and with CHF. ET-1 contributes to increased systemic and pulmonary vascular resistance, vascular dysfunction, myocardial ischemia, and renal impairment in CHF. Selective ET(A) as well as combined ET(A/B) receptor antagonists have been studied in patients with CHF showing impressive hemodynamic improvements (i.e. reduced peripheral vascular and pulmonary resistance as well as increased cardiac output). These results indicate that ET receptor antagonists indeed have a potential to improve hemodynamics, symptoms, and potentially prognosis of CHF which still carries a high mortality.

Vasopeptidase inhibitors

Vasopeptidase inhibitors are a new class of cardiovascular drug that simultaneously inhibit both neutral endopeptidase and angiotensin-converting enzyme (ACE). They increase the availability of peptides that have vasodilatory and other vascular effects; they also inhibit production of angiotensin II. In animal models vasopeptidase inhibitors decrease blood pressure in low, medium, and high renin forms of hypertension, and they also appear to confer benefits in models of heart failure and ischaemic heart disease. Studies in human hypertension show that these agents are effective in decreasing blood pressure regardless of race or age. Experience with omapatrilat, the most clinically advanced of these drugs, has shown it to be more effective than currently available ACE inhibitors or other widely used antihypertensive agents. Studies with omapatrilat in congestive heart failure have shown beneficial effects on haemodynamics and symptoms. The vasopeptidase inhibitors appear to have safety profiles similar to ACE inhibitors, though the frequency of side-effects such as angio-oedema and cough remains to be established. Large trials with clinical endpoints, some already in progress, are needed to establish the place of this class of drug beside that of established therapies in conditions such as hypertension, heart failure, ischaemic heart disease, and nephropathy.

Lysosomal multienzyme complex: biochemistry, genetics, and molecular pathophysiology

Lysosomal enzymes sialidase (alpha-neuraminidase), beta-galactosidase, and N-acetylaminogalacto-6-sulfate sulfatase are involved in the catabolism of glycolipids, glycoproteins, and oligosaccharides. Their functional activity in the cell depends on their association in a multienzyme complex with lysosomal carboxypeptidase, cathepsin A. We review the data suggesting that the integrity of the complex plays a crucial role at different stages of biogenesis of lysosomal enzymes, including intracellular sorting and proteolytic processing of their precursors. The complex plays a protective role for all components, extending their half-life in the lysosome from several hours to several days; and for sialidase, the association with cathepsin A is also necessary for the expression of enzymatic activity. The disintegration of the complex due to genetic mutations in its components results in their functional deficiency and causes severe metabolic disorders: sialidosis (mutations in sialidase), GM1-gangliosidosis and Morquio disease type B (mutations in beta-galactosidase), galactosialidosis (mutations in cathepsin A), and Morquio disease type A (mutations in N-acetylaminogalacto-6-sulfate sulfatase). The genetic, biochemical, and direct structural studies described here clarify the molecular pathogenic mechanisms of these disorders and suggest new diagnostic tools.

Effect of neutral endopeptidase inhibition on the actions of adrenomedullin and endothelin-1 in resistance arteries from patients with chronic heart failure

Adrenomedullin and endothelin are novel peptides that are produced in the blood vessel wall and have contrasting biologic actions. Both may play a pathophysiological role in atherosclerosis and chronic heart failure. It has also been suggested that both peptides may be metabolized by neutral endopeptidase and that pharmacological manipulation of this enzyme may be of therapeutic interest. We investigated the effect of thiorphan, a neutral endopeptidase inhibitor, on the vasodilator response to adrenomedullin and the vasoconstrictor response to endothelin in small resistance arteries taken from patients with heart failure caused by coronary heart disease. Small resistance arteries were dissected from gluteal biopsy samples and studied with wire myography. Thiorphan did not affect the vasodilator response to adrenomedullin in arteries preconstricted with norepinephrine. Maximal responses were 66% (SD 11%) and 72% (8%) in the absence and presence of thiorphan, respectively (n=8). The vasoconstrictor response to endothelin was also unaffected. The maximum vasoconstrictor responses in the absence and presence of thiorphan were 152% (11%) and 132% (12%), respectively (n=8). The values of corresponding -log concentrations of agonist required to effect a 50% response (pD(2)) were 8.52 (0.11) and 8.64 (0.15), respectively. We showed that the inhibition of neutral endopeptidase does not augment the vasodilator and vasoconstrictor activities of adrenomedullin and endothelin, respectively, in small resistance arteries from patients with chronic heart failure. This suggests that neutral endopeptidase inhibition, as a therapeutic strategy, will enhance neither the potentially desirable vascular actions of adrenomedullin nor the potentially unfavorable vascular effects of endothelin-1 in human cardiovascular disease states.

Mechanism of uterine vascular refractoriness to endothelin-1 in pregnant sheep

Endothelin-1 (ET-1) is a potent vasoconstrictor and produces marked pressor responses when given systemically. Studies in sheep have demonstrated that during pregnancy the uterine vasculature is refractory to exogenously administered ET-1. We hypothesize that this pregnancy-dependent refractoriness is due to an upregulation of local uterine metabolism of ET-1 and/or ET(B) receptors and/or downregulation of local uterine ET(A) receptors. To investigate these possibilities, 21 nonpregnant and 17 pregnant sheep were used. Dose-response curves to intravenous infusion of ET-1 and phenylephrine were generated for pregnant and nonpregnant sheep. ET-1 infused systemically demonstrated vasoconstriction in the systemic and renal vasculature of pregnant and nonpregnant animals and vasoconstriction in the uterine vasculature of nonpregnant animals. The pregnant animals showed no uterine vascular response to ET-1. In contrast, phenylephrine showed vasoconstriction in the systemic, renal, and uterine circulations in both pregnant and nonpregnant sheep. After experimentation, the animals were euthanized, and tissues were harvested for Western blot and activity analysis of neutral endopeptidase (NEP) or RT-PCR analysis of endothelin-converting enzyme (ECE) and ET(A) and ET(B) receptors. The content and activity of NEP in the uterine and renal vasculature of pregnant and nonpregnant animals were similar. RT-PCR demonstrated the presence of ECE in the uterine vasculature of pregnant and nonpregnant sheep. ET(A) receptor mRNA was significantly reduced in pregnant compared with nonpregnant sheep, whereas ET(B) receptor mRNA remained unchanged. We conclude that the uterine vascular refractoriness seen in the pregnant sheep is due to a downregulation of ET(A) receptors.

Endothelin system: the double-edged sword in health and disease

The endothelin system consists of two G-protein-coupled receptors, three peptide ligands, and two activating peptidases. Its pharmacological complexity is reflected by the diverse expression pattern of endothelin system components, which have a variety of physiological and pathophysiological roles. In the vessels, the endothelin system has a basal vasoconstricting role and participates in the development of diseases such as hypertension, atherosclerosis, and vasospasm after subarachnoid hemorrhage. In the heart, the endothelin system affects inotropy and chronotropy, and it mediates cardiac hypertrophy and remodeling in congestive heart failure. In the lungs, the endothelin system regulates the tone of airways and blood vessels, and it is involved in the development of pulmonary hypertension. In the kidney, it controls water and sodium excretion and acid-base balance, and it participates in acute and chronic renal failure. In the brain, the endothelin system modulates cardiorespiratory centers and the release of hormones. More advanced functional analysis of the endothelin system awaits not only additional pharmacological studies using highly specific endothelin antagonists but also the generation of genetically altered rodent models with conditional loss-of-function and gain-of-function manipulations.

Endothelin receptor antagonists in congestive heart failure: a new therapeutic principle for the future?

Congestive heart failure (CHF) is characterized by impaired left ventricular function, increased peripheral and pulmonary vascular resistance and reduced exercise tolerance and dyspnea. Thus, mediators involved in the control of myocardial function and vascular tone may be involved in its pathophysiology. The family of endothelins (ET) consists of four closely related peptides, ET-1, ET-2, ET-3 and ET-4, which cause vasoconstriction, cell proliferation and myocardial effects through activation of ETA receptors. In contrast, endothelial ETB receptors mediate vasodilation via release of nitric oxide and prostacyclin. In addition, ETB receptors in the lung are a major pathway for the clearance of ET-1 from plasma. Thus, infusion of an ETA-receptor antagonist into the brachial artery in healthy humans leads to vasodilation, whereas infusion of an ETB-receptor antagonist causes vasoconstriction. Endothelin-1 plasma levels are elevated in CHF and correlate both with hemodynamic severity and symptoms. Plasma levels of ET-1 and its precursor, big ET-1, are strong independent predictors of death after myocardial infarction as well as in CHF. Endothelin-1 contributes to increased systemic and pulmonary vascular resistance, vascular dysfunction, myocardial ischemia and renal impairment in CHF. Selective ETA, as well as combined ETA/B-receptor antagonists, have been studied in patients with CHF, and their use has shown impressive hemodynamic improvement (i.e., reduced peripheral vascular and pulmonary resistance as well as increased cardiac output). These results indicate that ET-receptor antagonists, indeed, have a potential to improve hemodynamics, symptoms and, potentially, prognosis in patients with CHF, which still carries a high mortality.

Endothelins: vasoactive modulators of renal function in health and disease

Vasoactive autocoids with directly opposing actions on the renal vasculature, glomerular function, and in salt and water homeostasis have been demonstrated in the kidney. In the renal cortex, endothelin (ET)-1 and angiotensin-II cause vasoconstriction, decreasing renal blood flow, and glomerular filtration rate, whereas bradykinin and atrial natriuretic peptide cause vasodilation and increase glomerular capillary permeability. ET-1 causes vasoconstriction of the afferent and efferent arteries and outer medullary descending vasa recta, thereby decreasing vasa recta and papillary blood flow, while bradykinin has the opposite effect. ET-1 stimulates cell proliferation, increasing the expression of several genes, including collagenase, prostaglandin endoperoxidase synthase, and platelet-derived growth factor. ET-1 promotes natriuresis via the ET-B receptor, causing down-regulation of the epithelial Na(+) channel in the renal tubule. Thus, ETs affect three major aspects of renal physiology: vascular and mesangial tone, Na(+) and water excretion, and cell proliferation and matrix formation.

Therapeutic potential for endothelin receptor antagonists in cardiovascular disorders

The endothelins are synthesized in vascular endothelial and smooth muscle cells, as well as in neural, renal, pulmonal, and inflammatory cells. These peptides are converted by endothelin-converting enzymes (ECE-1 and -2) from 'big endothelins' originating from large preproendothelin peptides cleaved by endopeptidases. Endothelin (ET)-1 has major influence on the function and structure of the vasculature as it favors vasoconstriction and cell proliferation through activation of specific ET(A) and ET(B) receptors on vascular smooth muscle cells. In contrast, ET(B )receptors on endothelial cells cause vasodilation via release of nitric oxide (NO) and prostacyclin. Additionally, ET(B) receptors in the lung are a major pathway for the clearance of ET-1 from plasma. Indeed, ET-1 contributes to the pathogenesis of important disorders as arterial hypertension, atherosclerosis, and heart failure. In patients with atherosclerotic vascular disease (as well as in many other disease states), ET-1 levels are elevated and correlate with the number of involved sites. In patients with acute myocardial infarction, they correlate with 1-year prognosis. ET receptor antagonists have been widely studied in experimental models of cardiovascular disease. In arterial hypertension, they prevent vascular and myocardial hypertrophy. Experimentally, ET receptor blockade also prevents endothelial dysfunction and structural vascular changes in atherosclerosis due to hypercholesterolemia. In experimental myocardial ischemia, treatment with an ET receptor antagonist reduced infarct size and prevented left ventricular remodeling after myocardial infarction. Most impressively, treatment with the selective ET(A) receptor antagonist BQ123 significantly improved survival in an experimental model of heart failure. In many clinical conditions, such as congestive heart failure, both mixed ET(A/B )as well as selective ET(A) receptor antagonism ameliorates the clinical status of patients, i.e. symptoms and hemodynamics. A randomized clinical trial showed that a mixed ET(A/B) receptor antagonist effectively lowered arterial blood pressure in patients with arterial hypertension. In patients with primary pulmonary hypertension or pulmonary hypertension related to scleroderma, treatment with a mixed ET(A/B) receptor antagonist resulted in an improvement in exercise capacity. ET receptor blockers thus hold the potential to improve the outcome in patients with various cardiovascular disorders. Randomized clinical trials are under way to evaluate the effects of ET receptor antagonism on morbidity and mortality.

Effect of losartan on angiotensin II-mediated endothelin and prostanoid excretion in humans

Angiotensin II (Ang II) stimulates renal prostanoid and vascular endothelin-1 (ET-1) release. Most known Ang II effects are mediated by AT1 receptors. Our aim was to determine whether AT1 receptor activation mediates Ang II-evoked renal prostanoid and ET-1 release. Eleven healthy men were randomized in a crossover, double-blind fashion to receive 100 mg/day of losartan or matching placebo, for 8 days. Blood and urine were sampled before and after a 2-h infusion of Ang II at a rate previously determined to increase mean arterial pressure (MAP) by 25 to 30 mm Hg in each subject. After a 14-day washout, subjects received the alternate treatment. Pretreatment with losartan had little effect on baseline MAP, but increased plasma renin activity, and virtually eliminated the pressor response to Ang II infusion. Angiotensin II significantly increased prostanoid excretion after placebo; the prostanoid response to Ang II was even greater after losartan. Plasma ET-1 was not altered by Ang II infusion, with or without losartan. In contrast, urine ET-1 excretion rate decreased to 40% of baseline after Ang II but not after losartan pretreatment; losartan alone had no effect. We conclude that Ang II decreases renal ET-1 synthesis and release through the AT1 receptor. In contrast, Angiotensin II-mediated renal prostanoid synthesis does not require activation of AT1 receptors. These findings indicate that AT1 receptor antagonists could provide renal protection through indirect mechanisms.

Sodium homeostasis in transplanted rats with a spontaneously hypertensive rat kidney

Recipients of a kidney from spontaneously hypertensive rats (SHR) but not from normotensive Wistar-Kyoto rats (WKY) develop posttransplantation hypertension. To investigate whether renal sodium retention precedes the development of posttransplantation hypertension in recipients of an SHR kidney on a standard sodium diet (0.6% NaCl), we transplanted SHR and WKY kidneys to SHR x WKY F1 hybrids, measured daily sodium balances during the first 12 days after removal of both native kidneys, and recorded mean arterial pressure (MAP) after 8 wk. Recipients of an SHR kidney (n = 12) retained more sodium than recipients of a WKY kidney (n = 12) (7.3 +/- 10 vs. 4.0 +/- 0.7 mmol, P < 0.05). MAP was 144 +/- 6 mmHg in recipients of an SHR kidney and 106 +/- 5 mmHg in recipients of a WKY kidney (P < 0.01). Modest sodium restriction (0.2% NaCl) in a further group of recipients of an SHR kidney (n = 10) did not prevent posttransplantation hypertension (MAP, 142 +/- 4 mmHg). Urinary endothelin and urodilatin excretion rates were similar in recipients of an SHR and a WKY kidney. Transient excess sodium retention after renal transplantation may contribute to posttransplantation hypertension in recipients of an SHR kidney.

Renal tubular effects of endothelin-B receptor signaling: its role in cardiovascular homeostasis and extracellular volume regulation

The role of the endothelin-B receptor in vascular homeostasis is controversial because the receptor has both pressor and depressor effects in vivo. One potential depressor mechanism of endothelin-B activation is through the promotion of natriuresis and diuresis in the renal tubule. Recent studies demonstrate that rodents genetically deficient for the endothelin-B exhibit sodium-dependent hypertension due to an absence of tonic inhibition of the epithelial sodium channel in the distal nephron. These studies suggest that the predominant role of endothelin-B receptors in the basal physiologic state may be to regulate renal sodium excretion relative to the level of oral salt intake.

Cell biology and molecular mechanisms of injury in ischemic acute renal failure

The pathogenesis of acute renal failure has been attributed to persistent vasoconstriction and leukocyte-endothelial interactions, resulting in inflammation and compromise of local blood flow to the outer medulla, the loss of tubular epithelial cell polarity with multiple functional sequelae, necrosis or apoptosis of epithelial cells, and the de-differentiation, migration and proliferation of surviving cells. In this paper, the authors present their views of pathophysiology of ischemic acute renal failure.

Structure of human neutral endopeptidase (Neprilysin) complexed with phosphoramidon

Neutral endopeptidase is a mammalian type II integral membrane zinc-containing endopeptidase, which degrades and inactivates a number of bioactive peptides. The range of substrates cleaved by neutral endopeptidase in vitro includes the enkephalins, substance P, endothelin, bradykinin and atrial natriuretic factor. Due to the physiological importance of neutral endopeptidase in the modulation of nociceptive and pressor responses there is considerable interest in inhibitors of this enzyme as novel analgesics and anti-hypertensive agents. Here we describe the crystal structure of the extracellular domain (residues 52-749) of human NEP complexed with the generic metalloproteinase inhibitor phosphoramidon at 2.1 A resolution. The structure reveals two multiply connected folding domains which embrace a large central cavity containing the active site. The inhibitor is bound to one side of this cavity and its binding mode provides a detailed understanding of the ligand-binding and specificity determinants.

Age-dependent hypertension in Mpv17-deficient mice, a transgenic model of glomerulosclerosis and inner ear disease

The mutant mouse strain Mpv17-/-, carries a retroviral germline integration that inactivates the Mpv17 gene. Mpv17-deficient mice develop progressive glomerulosclerosis and sensineural deafness at early age. Characteristic basement membrane alterations are found in both sites of pathology. Mpv17 is a peroxisomal protein involved in the metabolism of reactive oxygen species, yet its molecular function is unknown. Dysregulation of antioxidant enzymes and basal membrane components has been established in this model and successful therapeutic intervention with antioxidants prove the causal role of reactive oxygen species in the development of the disease phenotype. We here investigated if the Mpv17-/- mice might be hypertensive. Indeed, our study revealed that Mpv17-/- mice developed significant systemic hypertension and tachycardia between 4 weeks and 5 months of age, accompanied by polyuria and elevated natriuresis. Judging from serum and urine parameters, the hypertensive condition develops concomitantly with the renal disease. Biochemical and pharmacological studies that used the endothelin receptor antagonist bosentan and the angiotensin converting enzyme inhibitor cilazapril indicated no involvement of the endothelin and renin-angiotensin systems in this hypertension, suggesting a potential novel mechanism of blood pressure regulation in this new murine hypertension model. Thus, Mpv17-/- mice unravel an intriguing new association between a defect in reactive oxygen metabolism and the age-dependent development of hypertension.

Endothelin-1 urinary excretion, but not endothelin-1 plasma concentration, is increased in renovascular hypertension

Animal experiments have shown an increase in prepro-endothelin-1 (prepro-ET-1) mRNA expression in the clipped kidney but none in the aortic and mesenteric arteries in 2-kidney, 1-clip Goldblatt hypertensive rats. The present study was aimed at investigating whether plasma and renal endothelin-1 (ET-1) systems are differently activated in patients with renovascular hypertension (RH). The plasma concentration and urinary excretion of ET-1 were measured in 5 patients with RH (before and after successful renal angioplasty), in 7 patients with essential hypertension (EH), and in 8 normotensive control subjects. Immediately before renal angioplasty, plasma samples for ET-1 and plasma renin activity (PRA) measurements were withdrawn from the aorta and both renal veins. Unlike the PRA, the plasma ET-1 concentration did not significantly differ between the involved and the uninvolved sides. The urinary ET-1 excretion level (Fig 1) was markedly increased in patients with RH (30+/-4 ng/g urinary creatinine (UC) vs. 2.5+/-0.2 ng/g UC and 2.6+/-0.5 ng/g UC in control subjects and patients with EH, respectively; P<.001), whereas the plasma ET-1 concentration was normal (0.8+/-0.2 pg/mL vs. 0.65+/-0.3 pg/mL and 0.8+/-0.2 pg/mL in control subjects and EH, respectively, not significant). Renal angioplasty was followed in all patients by normalization of blood pressure and PRA. One week after angioplasty, urinary ET-1 decreased to one fourth of baseline (8.04+/-5.23 ng/g UC, P<.001 vs. values before angioplasty and P<.04 vs. control subjects) and normalized 1 month thereafter (3.13+/-1.62 ng/g UC, not significant vs. control subjects), whereas plasma ET-1 remained steady. The present findings clearly indicate that in patients with RH, urinary ET-1 excretion is increased, whereas plasma ET-1 concentration remains normal. Successful percutaneous transluminal renal angioplasty induced a notable reduction in ET-1 urinary excretion, whereas it did not affect ET-1 plasma concentration.

Direct actions of endothelin-1 on single vessel hydraulic permeability

BACKGROUND

There is evidence that endothelin-1 (ET-1) increases extravasation of fluid and protein into vascular beds. The present study was designed to determine the direct effects of ET-1 on hydraulic permeability (Lp) when microvascular hydraulic and oncotic pressures are controlled.

METHODS

Postcapillary venules in the rat mesentery were perfused in situ and paired measurements of Lp obtained by using the modified Landis micro-occlusion method. Lp measured after a 15-minute perfusion with Ringer's albumin solution (control) was compared with Lp after a subsequent 15-minute perfusion with one of three treatments: control (n = 4), 8 pM ET-1 (n = 6), or 80 pM ET-1 (n = 6).

RESULTS

Baseline L for all vessels averaged (+/- SE) 8.1 +/-0.8 x 10(-8) cm x sec(-10 x cm H2O(-1) and was not significantly different between groups. Perfusion with either control or 8 pM ET-1 did not significantly change the Lp of any of the vessels. Significant decreases in Lp of 40 to 60% were observed in venules perfused with 80 pM ET-1. The average Lp in this group was 9.9 +/- 1.4 during baseline and decreased to 5.0 +/- 0.7 during ET-1 perfusion (p = 0.003). Washout of 80 pM ET-1 for periods of up to 15 minutes did not return Lp to baseline values.

CONCLUSION

Low-dose ET-1 did not directly increase Lp in postcapillary venules. ET-1 at 80 pM, however, significantly decreased Lp. These data implicate factors other than a direct permeability-increasing effect in ET-1. At higher concentrations, ET-1 may have a protective effect on endothelial barrier function.

Down regulation of kidney neutral endopeptidase mRNA, protein and activity during acute renal failure: possible mechanism for ischemia-induced acute renal failure in rats?

Neutral endopeptidase (NEP, 24.11) is an ectoenzyme involved in the degradation of peptide hormones such as endothelin (ET), atrial natriuretic factor and enkephalins. The current study was designed to assess the involvement of NEP in ischemia-induced acute renal failure (ARF). In unilaterally nephrectomized Sprague-Dawley rats, the left renal artery was occluded for 30 min under pentobarbital anesthesia (40 mg/kg, i.p.) at 37 degree C. In addition to plasma creatinine levels, NEP activity was determined in renal cortical membranes at 0, 2, 5, and 24 h following reperfusion. Plasma creatinine levels significantly increased at 2, 5 and 24 h. There was a significant decrease in NEP activity as early as 2 h following reperfusion that was maintained up to 24 h (57.9 +/- 4%) with a concomitant loss of enzyme protein shown by Western analysis. Northern analysis of kidney cortical RNA, probed with an NEP cDNA, showed a 45% decrease in NEP mRNA level by the end of the ischemic period and decreased further during reperfusion. Thus, decrease in NEP mRNA levels preceded the changes in protein level, enzyme activity and plasma creatinine levels. These data, along with the reported increase in the tissue level of ET in kidney cortex, and the beneficial effect of ET antibody as well as ET receptor antagonist in ARF, suggest that down regulation of NEP, one of the mechanisms leading to increased tissue level of ET, may be a contributing factor to ARF.

Endothelial dysfunction in acute renal failure: role of circulating and tissue endothelin-1

The kidney is an important target and source of the potent vasoconstrictor and mitogen endothelin-1 (ET-1). However, its exact role in acute renal failure (ARF) remains to be determined. ARF was induced in male Wistar-Kyoto rats (n = 7) in a 2-kidney, 2-clip model of 30-min clamping. Twenty-four hours after clamp release, contractions to angiotensin I (Angl) and II, ET-1, and big ET-1 were studied in isolated aortic and renal artery rings. Endothelium-dependent and -independent relaxations were assessed by acetylcholine and sodium nitroprusside. ET-1 clearance, tissue uptake, plasma levels, and vascular and kidney content were investigated. In addition, ET(A) and Et(B) receptor mRNA expression was determined. Sham-operated animals served as controls (n = 7). In ARF, ET-1 plasma levels and tissue content of the renal artery, the aorta, and the kidney markedly increased (P<0.01). Plasma half-life of radiolabeled 125I-ET-1 was markedly prolonged, whereas 125I-ET-1 tissue uptake decreased in the kidney in ARF. Contractions to AngI and AngII were blunted (P<0.05) and those to KCl were unchanged, whereas vascular responses to big ET-1 and ET-1 were enhanced in the renal artery and also in the aorta in ARF (P<0.05 to 0.001). Correspondingly, ET(A) and Et(B) receptor mRNA expression significantly increased in both vascular beds. In addition, endothelium-dependent relaxation to acetylcholine was diminished and inversely correlated with vascular ET-1 protein levels in the renal artery (r = -0.827, P<0.001) and the aorta (r = -0.812, P<0.001). In conclusion, the present study demonstrates that increase of circulating and tissue ET-1 protein levels and ET(A) and Et(B) receptor gene expression occurs, which induces endothelial dysfunction and enhanced vasoconstriction in different vascular beds in ARF.

A three-dimensional construction of the active site (region 507-749) of human neutral endopeptidase (EC.3.4.24.11)

A three-dimensional model of the 507-749 region of neutral endopeptidase-24.11 (NEP; E.C.3.4.24.11) was constructed integrating the results of secondary structure predictions and sequence homologies with the bacterial endopeptidase thermolysin. Additional data were extracted from the structure of two other metalloproteases, astacin and stromelysin. The resulting model accounts for the main biological properties of NEP and has been used to describe the environment close to the zinc atom defining the catalytic site. The analysis of several thiol inhibitors, complexed in the model active site, revealed the presence of a large hydrophobic pocket at the S1' subsite level. This is supported by the nature of the constitutive amino acids. The computed energies of bound inhibitors correspond with the relative affinities of the stereoisomers of benzofused macrocycle derivatives of thiorphan. The model could be used to facilitate the design of new NEP inhibitors, as illustrated in the paper.

Urinary endothelin in adolescents and young adults with insulin-dependent diabetes mellitus: relation to urinary albumin, blood pressure, and other factors

Endothelin (ET) is a potent vasoconstrictive peptide that may play a role in vascular pathology in general and diabetic nephropathy in particular. The aim of this study was to investigate (1) alterations of urinary ET1 (UET1) in adolescents and young adults with insulin-dependent diabetes mellitus (IDDM) and (2) the relation of UET1 to other indices of diabetic nephropathy and to risk factors of diabetic angiopathy in general. In 130 IDDM subjects aged 15.2+/-4.9 years with a diabetes duration of 7.3+/-5.1 years, UET1 by radioimmunoassay, urinary albumin by nephelometry, plasma renin by immunoradiometric assay, hemoglobin A1c (HbA1c) by high-performance liquid chromatography, and routine biochemistry analyses were determined. Forty-eight controls, healthy siblings of the diabetics of comparable age, were similarly studied. Total 24-hour UET1 excretion was higher in diabetics than in controls (10,866+/-7,270 and 6,598+/-3,294 pg/24 h, respectively, P=.000). This difference was also noted if male and female diabetics were separately compared with controls. In diabetics with normoalbuminuria (<20 microg/min), total 24-hour UET1 excretion was also higher than in controls (P=.002). In diabetics but not in controls, 24-hour UET1 values were higher in males than in females (P=.018). In IDDM subjects, UET1 showed a linear relationship with age (P=.002), urinary albumin (P=.000), serum creatinine (P=.001), systolic blood pressure (P=.038), triglycerides (P=.003), and HbA1c (P=.041). Multiple regression analysis demonstrated that the variables interacting independently with UET1 were urinary albumin (P=.003) and serum creatinine (P=.038). UET1 is elevated early (in adolescence) in IDDM subjects, and it is positively correlated with the degree of albuminuria. These data suggest that the amount of UET1 possibly reflects the severity of diabetic renovascular damage. It may thus be speculated that UET1 could be used as another index of diabetic nephropathy or its progress.

Renal expression of transforming growth factor-beta inducible gene-h3 (beta ig-h3) in normal and diabetic rats

BACKGROUND

Transforming growth factor-beta (TGF-beta) has been implicated in the pathogenesis of a number of kidney diseases characterized by glomerulosclerosis and tubulointerstitial fibrosis. TGF-beta is secreted in a latent form requiring extracellular modification to become biologically active. TGF-beta inducible gene-h3 (beta ig-h3) is a recently identified TGF-beta-induced gene product. The present study sought to examine beta ig-h3 expression in normal and diabetic rats.

METHODS

Beta ig-h3, TGF-beta1 and alpha1 (IV) collagen gene expression were assessed by Northern blot analysis and in situ hybridization in 20 Sprague Dawley rats, randomly assigned to receive streptozotocin (diabetic, N = 11) or citrate buffer alone (control, N = 9) and sacrificed eight months later. The effect of exogenous TGF-beta1 on beta ig-h3 expression was also assessed in cultured proximal tubular cells.

RESULTS

In situ hybridization localized beta ig-h3 gene expression to the juxtaglomerular apparatus and the pars recta (S3 segment) of proximal tubules in both control and diabetic animals. Kidney TGF-beta 1, beta ig-h3 and alpha1 (IV) collagen mRNA from diabetic rats were increased two- to threefold compared with controls (P < 0.01). There was a significant correlation between TGF-beta1 and beta ig-h3 gene expression in kidneys from diabetic rats (r = 0.73, P = 0.01). In addition, beta ig-h3 mRNA increased in response to exogenous TGF-beta1 in a dose-dependent fashion in cultured proximal tubular cells.

CONCLUSION

These findings support the hypothesis that biologically active TGF-beta plays a pathogenetic role in diabetic kidney disease and suggest that beta ig-h3 may be a useful index of TGF-beta1 bioactivity in the kidney.

Dual inhibition of neutral endopeptidase and angiotensin-converting enzyme in rats with hypertension and diabetes mellitus

It has been suggested that combined inhibition of angiotensin-converting enzyme (ACE) and neutral endopeptidase (NEP) may lower blood pressure more effectively than either treatment alone, independent of the degree of salt and volume status or the activity of the renin-angiotensin system. The effects of NEP inhibition in hypertension associated with diabetes mellitus are largely unknown. We therefore compared ACE inhibition, NEP inhibition, and dual NEP/ACE inhibition in diabetic hypertensive rats. Spontaneously hypertensive rats (SHR) aged 9 to 10 weeks were injected with either streptozotocin (45 mg/kg) or citrate buffer and randomized to receive either the ACE inhibitor captopril (25 mg/kg BID), the NEP inhibitor SCH 42495 (30 mg/kg BID), the dual NEP/ACE inhibitor S 21402 (25 or 50 mg/kg BID), or vehicle by gavage for 4 weeks. A group of diabetic SHR was also allocated to receive the combination of SCH 42495 (30 mg/kg BID) and captopril (25 mg/kg BID). The degree of renal NEP inhibition was determined by autoradiography, and plasma renin activity (PRA) was determined by radioimmunoassay. In diabetic SHR, the dual NEP/ACE inhibitor (50 mg/kg BID), as well as the combination of the NEP inhibitor and the ACE inhibitor, reduced systolic blood pressure more effectively than the ACE inhibitor (P<0.001) or the NEP inhibitor (P<0.001) alone. In nondiabetic SHR, the dual NEP/ACE inhibitor and the ACE inhibitor were equally effective, while the NEP inhibitor had only slight blood pressure lowering effects. Relative heart weight decreased in parallel to the changes in blood pressure. Renal NEP was clearly inhibited (70% to 92%; P<0.001) by both the NEP inhibitor and the dual NEP/ACE inhibitor. Both the ACE inhibitor and the dual NEP/ACE inhibitor increased PRA, but the stimulating effect of dual NEP/ACE inhibition on PRA was less than that observed with ACE inhibition alone (P<0.05). Albuminuria in diabetic SHR was lower during treatment with both the dual NEP/ACE inhibitor (50 mg/kg BID) and the combination of NEP inhibition and ACE inhibition compared with vehicle treatment (P<0.05). In conclusion, the present study shows that hypertension in SHR with streptozotocin-induced diabetes is modulated by natriuretic peptides and thus is sensitive to NEP inhibition. The increased efficacy of dual NEP/ACE inhibition on blood pressure in diabetic SHR, compared with ACE or NEP inhibition alone, suggests that this therapeutic approach may prove beneficial in the treatment of hypertension associated with diabetes mellitus and other forms of volume-dependent hypertension.

Urinary endothelin-like immunoreactivity in patients with cirrhosis

BACKGROUND/AIMS

To investigate a possible relationship between the renal production of endothelin and the presence of renal dysfunction and activation of vasoactive systems in cirrhosis, the urinary excretion and the circulating plasma levels of immunoreactive endothelin (irET) and the plasma levels of vasoactive hormones were measured in 19 healthy subjects, 12 cirrhotic patients without ascites and 39 patients with ascites and different degrees of renal dysfunction.

METHODS

The urinary excretion and the circulating levels of irET were assessed after 5 days on a 40 mEq sodium diet and off diuretics. Renal function parameters and the plasma levels of vasoactive hormones were also measured.

RESULTS

Patients with and without ascites had similar values of urinary irET as compared with healthy subjects (30+/-3, 31+/-3 and 29+/-2 ng/day, respectively, p>0.10). By contrast, patients with ascites had higher circulating levels of irET (15+/-1.2 pg/ml) than patients without ascites and healthy subjects (11+/-1.6 and 5+/-0.4 pg/ml, p<0.01). In patients with cirrhosis, no correlation was found between urinary irET and circulating irET. Moreover, urinary irET did not correlate with liver tests, serum and urine sodium, glomerular filtration rate or vasoactive substances. Patients with hepatorenal syndrome had similar urinary irET to patients with ascites without hepatorenal syndrome.

CONCLUSIONS

Urinary excretion of irET is not increased in cirrhotic patients with ascites and does not correlate with abnormalities in renal function.

Enhanced natriuretic response to neutral endopeptidase inhibition in patients with moderate chronic renal failure

Atrial natriuretic factor (ANF) has natriuretic, renin-suppressing and chronic hypotensive actions that may be utilized by inhibition of ANF degradation by neutral endopeptidase, E.C.24.11 (NEP). Three groups of 8 male patients [GFR 103 +/- 8 (Normal), 64 +/- 6 (Moderate CRF), and 16 +/- 2 ml/min (Severe CRF)] received 100 mg i.v. bolus of the NEP inhibitor candoxatrilat or placebo in random order in a double-blind crossover study. GFR (51CR-EDTA), ERPF (125I-hippuran). ANF (IRMA), urinary cGMP (RIA) and albumin (RIA) and sodium excretion and flow rate were measured hourly for two hours before and for seven hours after candoxatrilat administration. After candoxatrilat plasma ANF rose two- to threefold from baseline, and remained elevated for 5(N) and 7(M,S) hours (P < 0.01(N,S), P < 0.03(M)) associated with an immediate rise in urine cGMP excretion from 23.5(N), 25.4(M) and 10.4(S) nmol/hr (base) to 51.7(N), 73.8(M) and 27.5(S)(peak) lasting 7(N,M,S) hours (P < 0.01(N,M,S)). There was a marked natriuresis in all three groups, the cumulative sodium excretion at seven hours post-candoxatrilat being 104(N), 140(M), 102(S) mmol (P < 0.05(N,M,S)). This was greatest in those with moderate CRF (moderate CRF vs. normal, P = 0.036, moderate vs. severe CRF, P = 0.01, normal vs. severe CRF, P = 0.74). Following candoxatrilat there was a near doubling of the urine flow rate (P < 0.01(N,S), P < 0.02(M)). Urine albumin excretion increased in patients with renal failure (P < 0.01), but there was no change in GFR, ERPF or systemic blood pressure. We conclude that the marked natriuretic effects of acute NEP inhibition seen in normal subjects are enhanced in the presence of moderate CRF and sustained even in severe renal impairment.

Neutral endopeptidase regulates C-type natriuretic peptide metabolism but does not potentiate its bioactivity in vivo

C-type natriuretic peptide (CNP) is a newly described 22-amino acid peptide of endothelial and renal cell origin with selective cardiovascular actions. Recent in vitro studies have reported that CNP is the most susceptible of all natriuretic peptides to enzymatic degradation by neutral endopeptidase 24.11 (NEP). The present study was undertaken to define the role of NEP in total and regional CNP metabolism and the modulatory actions of NEP inhibition on the biological actions of CNP. CNP (10 ng x kg(-1) x min(-1)) followed by candoxatrilat (240 microg x kg(-1) bolus and 8 microg x kg(-1) x min(-1)), a potent and selective NEP inhibitor, was administered intravenously to a group of anesthetized mongrel dogs (group 1) to permit calculation of total metabolic clearance rate (MCR); results were compared with those in a group receiving vehicle infusion followed by candoxatrilat (group 2; both groups, n=7). NEP inhibition increased circulating CNP achieved by exogenous infusion and reduced total MCR in group 1. The regional CNP MCRs increased after CNP administration. While the pulmonary MCR did not change during concomitant candoxatrilat infusion, renal MCR was suppressed. Hemodynamic changes were not different between groups. A mild natriuretic and diuretic effect in association with an increase in circulating and urinary ANP levels was not different between groups. Urinary CNP excretion did not change with CNP infusion but markedly increased after NEP inhibition. We conclude that (1) circulating CNP achieved by exogenous CNP infusion is regulated by NEP in vivo, (2) regional MCRs are heterogeneous with NEP inhibition, (3) NEP inhibition does not potentiate acute cardiovascular actions of CNP, and (4) a mild natriuretic and diuretic effect observed with CNP and NEP inhibition may be ANP dependent.

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.

Urinary endothelin-1 excretion is enhanced by low-dose infusion of brain natriuretic peptide in normal humans

To evaluate the functional relationship between cardiac natriuretic peptides and endothelin-1 within the human kidney, we studied the effects exerted by infusion of brain natriuretic peptide on urinary endothelin-1 excretion. We studied twice in a single-blind manner five normal volunteers who received a constant infusion of 5% dextrose (250 mL/h) or human brain natriuretic peptide-32 at a dose of 4 pmol/kg per minute. Blood samples were drawn at intervals for measurement of hematocrit and concentrations of creatinine, electrolytes, brain natriuretic peptide, and endothelin-1. Urine was collected an intervals for measurement of flow rate and concentrations of creatinine, sodium, cGMP, and endothelin-1. Blood pressure and heart rate were measured every 15 minutes. Placebo administration did not change blood pressure, heart rate, or any of the other parameters measured in plasma and urine. As expected, brain natriuretic peptide infusion caused significant increases in its own plasma levels (basal versus peak levels [mean +/- SD], 1.45 +/- 0.20 versus 50.5 +/- 6.0 pmol/L, P < .01), in urinary cGMP (0.75 +/- 0.16 versus 1.92 +/- 0.81 fmol/min, P < .05), and in urinary sodium excretion (140.0 +/- 38.7 versus 624.2 +/- 181.6 mumol/min, P < .01). In addition, it caused an increase in urinary endothelin-1 excretion (4.32 +/- 2.11 versus 19.67 +/- 9.52 fmol/min, P < .05), without modifying plasma endothelin-1, blood pressure, heart rate, creatinine clearance, and urinary flow rate. Our data indicate that brain natriuretic peptide, at plasma levels comparable to those observed in patients with heart failure, causes a significant increase in urinary but not plasma endothelin-1, thus demonstrating a functional link between cardiac natriuretic peptides and renal release of endothelin-1.

Exercise-induced changes in neuropeptide Y, noradrenaline and endothelin-1 levels in young people with type I diabetes

In search of early signs of autonomic and vascular dysfunction in diabetic subjects, six young men with type I diabetes and six healthy subjects were investigated regarding arterial levels of noradrenaline (NA), neuropeptide Y- (NPY) and endothelin-1-(ET-1) like immunoreactivity (Li) during and after 1 h of exercise at 70% of peak oxygen uptake. Basal NA, NPY-Li and ET-1-Li levels did not differ between groups. NA and NPY-Li rose during exercise in diabetic subjects to only 60% of the control values (P < 0.05, interaction group x time P < 0.001). Disappearance rates for NA and NPY-Li did not differ between groups. Plasma ET-1-Li did not differ between groups during exercise. Values returned to basal levels within 5 min in the diabetic but not in the control group. In conclusion, diabetic subjects show lower NPY-Li and NA levels than control subjects during exercise but similar disappearance rates after exercise, indicating lower releases. Furthermore, plasma ET-1 levels did not differ between groups during exercise.

The correlation of plasma and urine endothelin-1 with the severity of nephropathy in Chinese patients with type 2 diabetes

To assess whether plasma and urinary endothelin-1 (ET-1) values are related to the severity of diabetic nephropathy, we measured plasma and urinary ET-1-like immunoreactivity (ET-1-LI) in 14 healthy subjects, and in 50 normoalbuminuric (group 1), 13 albuminuric (group 2), and 10 renally insufficient (group 3) patients with Type 2 diabetes. Plasma ET-1-LI values were significantly increased in group 3, and correlated positively with serum creatinine levels (r = 0.579, p < 0.01). Urinary ET-1-LI excretion in group 3 (49.3 +/- 7.3 pmol day-1) was significantly higher than that in healthy controls (27.0 +/- 1.1 pmol day-1) and in group 1 (32.2 +/- 2.2 pmol day-1), while that of group 2 (38.8 +/- 5.9 pmol day-1) was also higher than in healthy controls. A significant positive correlation between urinary ET-1-LI and serum creatinine values was also found (r = 0.297, p < 0.05). Trend analysis showed significant linear and quadratic trends in the elevation of plasma ET-1-LI and a significant linear trend in urinary ET-1-LI levels from healthy controls to groups 1, 2 and 3. Our results demonstrate that an increase in plasma and urine ET-1-LI correlates with the severity of diabetic nephropathy.

Natriuretic effect of non-pressor doses of endothelin-1 in conscious dogs

1. Renal, endocrine and haemodynamic responses to separate intravenous infusions of three doses of endothelin-1 (40, 400 and 4000 fmol kg-1 min-1) were investigated in conscious dogs. 2. Administration of 40, 400 and 4000 fmol kg-1 min-1 endothelin-1 for 120 min increased plasma endothelin-1 levels by two-, seven- and 250-fold, respectively. 3. The two low doses did not have measurable effects on mean arterial blood pressure and heart rate but tended to increase glomerular filtration rate. The high dose increased mean arterial blood pressure (MABP; from 104 +/- 4 to 138 +/- 4 mmHg, P < 0.05) and decreased heart rate (from 71 +/- 4 to 46 +/- 3 beats min-1, P < 0.05) as well as glomerular filtration rate (from 47 +/- 3 to 19 +/- 5 ml min-1, P < 0.05). 4. At rates of 40 and 400 fmol kg-1 min-1, endothelin-1 increased sodium excretion about five- and eightfold, respectively. Relative changes in fractional sodium excretion were very similar. The high dose was markedly antinatriuretic (reducing sodium excretion from 8.3 +/- 1.1 to 1.2 +/- 0.2 mumol min-1, P < 0.05). 5. Diuresis increased during the administration of the two lower doses, which did not change plasma atrial natriuretic peptide or vasopressin concentrations. Urine flow increased after termination of the infusion of the pressor dose despite elevated plasma vasopressin and subnormal glomerular filtration rate. 6. Infusion of endothelin-1 at 40 fmol kg-1 min-1 did not change the concentrations of angiotensin II and atrial natriuretic peptide in plasma. Infusion of 400 fmol kg-1 min-1 was associated with a decrease in plasma angiotensin II, while plasma atrial natriuretic peptide was unchanged. The high dose of endothelin-1 markedly increased plasma levels of both hormones. 7. It is concluded that endothelin-1 at low plasma concentrations increases sodium excretion while a higher pressor dose of endothelin-1 is antinatriuretic. However, increases in plasma endothelin-1 seem to elicit diuresis over a wide concentration range, although possibly by different mechanisms.

Effects of cyclosporin A on the synthesis, excretion, and metabolism of endothelin in the rat

Increasing evidence suggests that endothelin, a potent vasoconstrictor, is implicated in cyclosporin A (CsA)-induced nephrotoxicity. Increased levels of urinary and circulating endothelin have been described in CsA-treated humans and animals. The exact mechanisms by which CsA induces these increases are still unknown, and no data indicate whether these elevated levels reflect increased synthesis or decreased clearance of endothelin. In the present study, we investigated the effects of CsA administration (50 mg/kg per day i.p. for 6 days) to rats on plasma and urinary levels of endothelin; expression of endothelin-1 (ET-1), ET-3, and endothelin-converting enzyme in renal tissue; clearance of infused 125I-ET-1; and degradation of 125I-ET-1 by recombinant neutral endopeptidase. Rats given CsA for 6 days developed severe renal insufficiency, as shown by a 74% decrease in creatinine clearance rate (Ccr) (P < .006). Ccr was remarkably improved in CsA-treated rats that received bosentan, the combined antagonist of both endothelin A and endothelin B receptors. Urinary excretion of endothelin increased from an undetectable level to 31.7 +/- 6.0 pg/24 h (P < .001), and plasma levels of endothelin were unchanged (2.8 +/- 0.2 to 3.1 +/- 0.2 pg/mL). Reverse transcription followed by quantitative polymerase chain reaction revealed that ET-1 mRNA in the renal medulla increased by 59% (P < .006), whereas the expression of both ET-3 and endothelin-converting enzyme was unchanged. In other rats, neither acute nor chronic treatment with CsA affected either the clearance of 125I-ET-1 from the blood or the renal and pulmonary uptake of the peptide. Moreover, CsA did not affect the degradation of 125I-ET-1 by highly purified recombinant neutral endopeptidase, a well-known endothelinase. Taken together, these data suggest that the elevated urinary endothelin levels obtained after CsA treatment originate from the kidney and reflect increased renal synthesis of ET-1. Moreover, the production of endothelin appears to be regulated at the mRNA transcription level, and expressions of ET-1 and ET-3 are regulated independently.

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.

Molecular pharmacology of endothelin converting enzymes

A critical processing step in endothelin biosynthesis is the conversion of the intermediate "big endothelin" to its biologically active product catalysed by endothelin converting enzyme (ECE). In this commentary we discuss critically the cellular location, structure, and activity of the isoforms of ECE. The current evidence supporting a metallopeptidase ECE as the physiological regulator of endothelin production is described. Its sensitivity to inhibition by the fungal metabolite phosphoramidon and subsequent cloning of the enzyme indicate it to be a type II integral membrane protein homologous with neural endopeptidase-24.11 (E-24.11), the major neuropeptide-degrading ectoenzyme in brain and other tissues. Unlike E-24.11, however, ECE exists as a disulphide-linked dimer of subunit M(r) 120-130 kDa and is not inhibited by other E-24.11 inhibitors such as thiorphan. Alternative splicing produces two forms of ECE with distinct N-terminal tails. These isoforms of ECE-1 show similar specificity converting big endothelin-1 (ET-1) to ET-1 but big ET-2 and big ET-3 are converted much less efficiently. This suggests that additional forms of ECE remain to be isolated. Immunocytochemical studies indicate a predominant cell-surface location for ECE-1, like E-24.11. This is consistent with the conversion of exogenous big ET-1 when administered in vivo and the inhibition of this event by phosphoramidon. However, mature ET-1 can be detected in intracellular vesicles in endothelial cells, suggesting that some processing occurs in the constitutive secretory pathway. This may be mediated by ECE-2, a recently cloned member of the E-24.11/ECE family which has an acidic pH optimum. Selective inhibitors of ECE may have therapeutic applications in cardiovascular and renal medicine.