Neutral endopeptidase inhibition potentiates the renal actions of atrial natriuretic factor

Prevention against renal damage in rats with subtotal nephrectomy by sacubitril/valsartan (LCZ696), a dual-acting angiotensin receptor-neprilysin inhibitor

Although patients with chronic kidney disease (CKD) are at increased risk for end‐stage renal disease and cardiovascular events, adequate drug therapies for preventing the deterioration of these conditions are still not established. This study was undertaken to evaluate a preventive effect of an angiotensin receptor‐neprilysin inhibitor sacubitril/valsartan (LCZ696), which is converted to sacubitril and valsartan in the body, against the progression of renal disease in rats with subtotal nephrectomy, an animal model of human CKD. Mean survival time after subtotal nephrectomy was about 100 days in Wistar rats with vehicle. LCZ696‐(30 mg/kg) and valsartan‐(15 mg/kg) prolonged the survival of these animals, and the effect of LCZ696 on survival was significantly greater than that of valsartan. Renoprotective effects of LCZ696 judged by serum creatinine and urinary protein excretions were larger than those of valsartan. Cardioprotective effects judged by cardiac left ventricular mass, fractional shortening, and fibrosis of LCZ696 and valsartan were not detected under the present condition. Thus, the renoprotective effect of LCZ696 was stronger than that of valsartan in rats with subtotal nephrectomy. This study provides the idea that, compared to valsartan, LCZ696 is more effective for the treatment of human CKD.

Combination of drugs acting on the natriuretic system and the renin-angiotensin system in heart failure

Conventional diuretic agents are very effective agents in relieving volume overload and congestive symptoms in chronic heart failure (CHF). However, they are associated with activation of the renin-angiotensin system (RAS) and the sympathetic nervous system and a reduction in glomerular filtration rate, all of which have been associated with adverse outcomes in CHF. Therefore, there is an increasing interest in drugs that target the natriuretic system without neurohormonal activation and deterioration of renal function. In this review, we will discuss the underlying rationale and evidence behind currently pursued strategies that target the natriuretic system. This includes the administration of natriuretic peptides (NPs) and strategies that potentiate the NP system, such as neutral endopeptidase inhibition. We will also highlight some potentially important interactions of these strategies with drugs that target the RAS.

New Pharmacological Strategies to Increase cGMP

The intracellular nucleotide cyclic guanosine monophosphate (cGMP) is found in many human organ tissues. Its concentration increases in response to the activation of receptor enzymes called guanylyl cyclases (GCs). Different ligands bind GCs, generating the second messenger cGMP, which in turn leads to a variety of biological actions. A deficit or dysfunction of this pathway at the cardiac, vascular, and renal levels manifests in cardiovascular diseases such as heart failure, arterial hypertension, and pulmonary arterial hypertension. An impairment of the cGMP pathway also may be involved in the pathogenesis of obesity as well as dementia. Therefore, agents enhancing the generation of cGMP for the treatment of these conditions have been intensively studied. Some have already been approved, and others are currently under investigation. This review discusses the potential of novel drugs directly or indirectly targeting cGMP as well as the progress of research to date.

Natriuretic and antialdosterone actions of chronic oral NEP inhibition during progressive congestive heart failure

BACKGROUND

Neutral endopeptidase (NEP) degrades atrial natriuretic peptide (ANP) that via cyclic guanosine monophosphate (cGMP) is natriuretic and aldosterone-inhibiting. We hypothesized that chronic oral NEP inhibition (NEPI), initiated in early experimental congestive heart failure (CHF), would delay onset of decreases in sodium excretion during the progression of CHF and, in the severe phase, suppress aldosterone activation and reduce the magnitude of sodium retention. We also hypothesized that chronic NEPI during progressive CHF (PCHF) would improve the natriuretic response to acute volume expansion.

METHODS

In a novel canine model that progresses over 38 days from early to moderate and finally severe CHF, we defined the actions of chronic NEPI (candoxatril, 10 mg/kg, orally, twice a day) upon cardiorenal and neurohumoral function as well as the clinical well being of treated and untreated dogs in CHF.

RESULTS

From baseline through the moderate phase of CHF, NEPI maintained sodium excretion. In contrast, in moderate CHF, sodium excretion was reduced compared to the early phase in the controls. In severe CHF, sodium excretion was higher with NEPI compared to control. Chronic NEPI also resulted in lower plasma aldosterone as compared to controls. In severe CHF, the natriuretic response to acute saline volume expansion was enhanced with oral NEPI as compared to control.

CONCLUSION

This study supports the conclusion that chronic oral NEPI delays the onset of reduction in sodium excretion during the transition from early to severe CHF in this model of PCHF. This therapeutic strategy also improved the natriuretic response to acute volume expansion in severe CHF while enhancing ANP and suppressing aldosterone activation. Thus, these studies demonstrated a selective renal and adrenal action of chronic NEPI in heart failure indicating a therapeutic potential.

RENAL RESPONSES TO ATRIAL NATRIURETIC PEPTIDE ARE PRESERVED IN BILATERAL URETERAL OBSTRUCTION AND AUGMENTED BY NEUTRAL ENDOPEPTIDASE INHIBITION

PURPOSE

Atrial natriuretic peptide (ANP) contributes to post-obstructive diuresis in bilateral ureteral obstruction (BUO). In this study we examined the activity of neutral endopeptidase (NEP), an enzyme responsible for degradation of ANP, in the kidney in rats subjected to BUO for 24 hours.

MATERIALS AND METHODS

Renal function was examined by the clearance method in sham operated rats and BUO rats after obstruction release. Renal responses to an intravenous bolus injection of ANP (5 microg/kg) were studied in sham operated and BUO rats with or without pretreatment with intravenous phosphoramidon (100 microg/kg per minute), a NEP inhibitor.

RESULTS

In BUO rats natriuresis and diuresis occurred despite a marked decrease in the glomerular filtration rate (GFR). ANP administration increased GFR and induced marked natriuresis and diuresis in sham operated and BUO rats. Inhibition of ANP degradation by phosphoramidon induced natriuresis and diuresis, and accentuated these renal responses to ANP.

CONCLUSIONS

Renal responses to ANP and renal NEP activity were preserved in 24-hour BUO. NEP inhibition to attenuate ANP degradation augmented responses to ANP in increasing GFR, natriuresis and diuresis. These findings provide the theoretical potential for facilitating the recovery of GFR after BUO release by inhibiting ANP degradation by pharmacological means.

Neutral endopeptidase expression in mesangial cells

In the kidney, neutral endopeptidase (NEP) is implicated in the metabolism of several peptides involved in blood pressure and sodium homeostasis control, such as the atrial natriuretic peptide, bradykinin and angiotensin I. Due to its physiological importance in the modulation of pressor responses, the presence of NEP in mouse mesangial cells has been investigated, since these cells control glomerular function and are able to synthesise components of the renin-angiotensin system. A NEP-like activity (NEP-like) that cleaves the fluorogenic substrates Abz-BKQ-EDDnp and Abz-DRRL-EDDnp was purified from mesangial cell lysate by ion-exchange, followed by gel filtration chromatography. The enzyme was able to hydrolyse bradykinin at the G4-F5 peptide bond and was inhibited by thiorphan. A pH study established that enzyme activity was maximal at pH 7.5 and the determined K(m) was 4.86 M using Abz-DRRL-EDDnp as substrate. NEP-like was recognised by monoclonal anti-NEP and had a molecular mass of 95 kDa. The purified enzyme was sequenced and showed similarity with human, rat, mouse and rabbit NEPs. We isolated, for the first time, NEP-like from mesangial cells. This enzyme could have an important role in the renal physiology by its action upon different peptides that are able to alter renal haemodynamics.

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.

Natriuretic peptide receptors and neutral endopeptidase in mediating the renal actions of a new therapeutic synthetic natriuretic peptide dendroaspis natriuretic peptide

OBJECTIVES

The objectives of the current study were to define for the first time the roles of the natriuretic peptide (NP) receptors and neutral endopeptidase (NEP) in mediating and modulating the renal actions of Dendroaspis natriuretic peptide (DNP), a new therapeutic synthetic NP.

BACKGROUND

Recent reports have advanced the therapeutic potential of a newly described synthetic NP called DNP. Dendroaspis natriuretic peptide is a 38-amino acid peptide recently isolated from the venom of Dendroaspis augusticeps (the green mamba snake).

METHODS

Synthetic DNP was administered intra-renally at 5 ng/kg/min to 11 normal anesthetized dogs, 5 of which received the NP receptor antagonist HS-142-1 (3 mg/kg intravenous bolus) while the remaining 6 dogs received an infusion of the NEP inhibitor, candoxatrilat (8 and 80 microg/kg/min) (Pfizer, Sandwich United Kingdom).

RESULTS

Intra-renal DNP resulted in marked natriuresis associated with increased urinary cyclic guanosine monophosphate excretion (UcGMPV), glomerular filtration rate (GFR), and renal blood flow (RBF) and decreased distal fractional sodium reabsorption (FNaR) compared with baseline. HS-142-1 attenuated the natriuretic response to DNP, resulting in decreased UcGMPV, GFR, and RBF and increased distal FNaR. In contrast, low and high doses of NEP inhibitor did not potentiate the renal actions of DNP.

CONCLUSIONS

We report that the NP receptor blockade attenuated the renal actions of synthetic DNP and that the NEP inhibitor did not alter the renal response to DNP. This latter finding is a unique property of synthetic DNP, as distinguished from other known NPs, supporting its potential as a therapeutic agent.

Vasopeptidase inhibitors, neutral endopeptidase inhibitors, and dual inhibitors of angiotensin-converting enzyme and neutral endopeptidase

Vasopeptidase inhibitors represent a new class of cardiovascular drugs. They function as a combined angiotensin-converting enzyme (ACE) inhibitor and neutral endopeptidase (NEP) inhibitor, the latter of which potentiates the actions of atrial natriuretic peptide (ANP) by minimizing its degradation in the circulation. The consequence of such dual inhibition is a synergistic reduction of vasoconstriction and enhancement of vasodilation, thereby serving to more effectively reduce blood pressure. Furthermore, inhibition of the renin-angiotensin-aldosterone system (RAAS) prevents physiologic compensatory responses in vivo seen with NEP inhibition alone. Vasopeptidase inhibitors have also shown to potentiate bradykinin and adrenomedullin, which additionally contribute to cardiovascular regulation. The most extensively researched and promising agents within the class of VP inhibitors is omapatrilat, a mercaptoacyl derivative of a bicyclic thiazepinone dipeptide. It is a single molecule with equal potency and affinity for ACE and NEP inhibition. Although ACE inhibition tends to more selectively benefit high-renin models of hypertension, vasopeptidase inhibition has been shown to be equally efficacious in low-, normal-, and high-renin models. Contrary to NEP inhibition alone, omapatrilat has also demonstrated the ability to significantly reduce blood pressure in spontaneously hypertensive rats, the equivalent of essential hypertension in humans. Studies also suggest that omapatrilat has cardioprotective properties, especially in the setting of congestive heart failure. More specifically, animal models have demonstrated omapatrilat to be more effective than ACE inhibition alone in remodeling the heart and improving its contractile function. Human studies have documented the efficacy of omapatrilat in the treatment of both hypertension and, to a lesser extent, heart failure. Safety concerns (specifically angioedema) are currently being addressed before the widespread utilization of this promising new agent.

Effect of luminal atrial natriuretic peptide on chloride reabsorption in mouse cortical thick ascending limb: inhibition by endothelin

Insofar as neutral endopeptidase inhibition has afforded evidence for a tubular luminal action of atrial natriuretic peptide (ANP), the present study was undertaken to investigate a possible effect of the peptide on chloride reabsorption (JCl) in thick ascending limb (TAL). Luminal addition of ANP to in vitro microperfused cortical TAL (CTAL) significantly decreased JCl with a threshold and a maximum concentration of 10(-12) M and 10(-9) M, respectively. A similar effect of 10(-9) M ANP was observed in medullary TAL (MTAL). The effect of luminal ANP was significantly reduced by HS-142-1, a specific inhibitor of guanylyl cyclase receptor, and by H-8, a protein kinase G inhibitor, but was not affected by the protein kinase C inhibitor bisindolylmaleimide I. Unexpectedly, the effect of ANP was not additive with that of endothelin (ET), a peptide that was previously shown to decrease JCl in TAL through a calcium-independent, protein kinase C-mediated pathway. Indeed, ET-1 (10(-8) M in the lumen) significantly decreased JCl and prevented a further effect of ANP on the same tubule. Similarly, the decrease of JCl induced by simultaneous addition of ET and ANP was not higher than that obtained with each agent alone. Conversely, the inhibitory effect of ANP was enhanced in the presence of cyclic guanosine monophosphate (cGMP; 10(-6) M in the lumen). ET-1 significantly attenuated the ANP-stimulated generation of cGMP in microdissected CTAL and failed to prevent a further decrease of JCl promoted by a permeant cGMP analogue. It is concluded that luminal ANP decreased Cl reabsorption in mouse CTAL and MTAL. This effect was abrogated by ET-1 as a result of the inhibition of ANP-stimulated cGMP generation.

Renal response to acute neutral endopeptidase inhibition in mild and severe experimental heart failure

BACKGROUND

Neutral endopeptidase 24.11 (NEP) is a metalloprotease that is localized in the greatest abundance in the kidney and degrades natriuretic peptides, such as atrial natriuretic peptide (ANP). Mild congestive heart failure (CHF) is characterized by increases in circulating ANP without activation of the renin-angiotensin-aldosterone system (RAAS) or sodium retention. In contrast, severe CHF is characterized by sodium retention and coactivation of both ANP and the RAAS.

METHODS AND RESULTS

We defined the acute cardiorenal actions of the NEP inhibitor candoxatrilat (8 microg. kg(-1). min(-1)) in 4 groups of anesthetized dogs (normal, n=8; mild CHF, n=6; severe CHF, n=5; and severe CHF with chronic AT(1) receptor antagonism, n=5). Mild CHF was produced by rapid ventricular pacing at 180 bpm for 10 days and severe CHF at 245 bpm for 10 days. In mild CHF, urinary sodium excretion and glomerular filtration rate were greatest in response to acute NEP inhibition compared with the response in either control animals or those with severe CHF. Furthermore, an increase in glomerular filtration rate was observed only in mild CHF in association with increases in renal blood flow and decreases in renal vascular resistance and distal tubular sodium reabsorption. Urinary ANP and cGMP excretion, markers for renal biological actions of ANP, were greatest in mild CHF. The renal actions observed in mild CHF were attenuated in severe CHF and not restored by chronic AT(1) receptor antagonism.

CONCLUSIONS

The results of the present study demonstrate that acute NEP inhibition in mild CHF results in marked increases in renal hemodynamics and sodium excretion that exceed that observed in control animals and severe CHF. These studies underscore the potential therapeutic role for NEP inhibition to enhance renal function in mild CHF, an important phase of CHF that is marked by selective activation of endogenous ANP in the absence of an activated RAAS.

Renal effects of 2-mercaptoacetyl-L-leucyl-L-phenylalanine, a novel selective inhibitor of neutral endopeptidase 24.11 (Neprilysin): comparison with SQ 28,603

The effects of 2-mercaptoacetyl-L-leucyl-L-phenylalanine (MA-LF) on the activity of neutral endopeptidase and on renal hemodynamics and excretory function were investigated in experiments in vitro and in vivo. In vitro studies showed that the compound effectively inhibited purified bovine kidney neutral endopeptidase (Ki = 0.012 microM), while having slight influence on the activity of angiotensin I converting enzyme (Ki = 0.14 microM). In experiments on normal anesthetized rats (thiobutabarbital sodium salt, 100 mg/kg), IV administration of MA-LF (20 and 60 mg/kg) produced a dose-dependent increase in absolute rate and fractional excretion of sodium (+324% and +299%, respectively) and urinary flow rate (+261%), but did not change renal and systemic hemodynamics. Renal excretory effects of the new compound were comparable to those of the selective neutral endopeptidase inhibitor SQ 28,603. These results demonstrate that MA-LF is a potent neutral endopeptidase inhibitor with prominent natriuretic and diuretic properties.

The natriuretic peptides in heart failure: diagnostic and therapeutic potentials

The natriuretic peptides are a group of structurally similar but genetically distinct peptides that have diverse actions in cardiovascular, renal, and endocrine homeostasis. Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are of myocardial cell origin and C-type natriuretic peptide (CNP) is of endothelial origin. ANP and BNP bind to the natriuretic peptide-A receptor (NPR-A), which, via 3',5'-cyclic guanosine monophosphate (cGMP), mediates natriuresis, vasodilation, renin inhibition, antimitogenesis, and lusitropic properties. CNP lacks natriuretic actions but possesses vasodilating and growth-inhibiting actions via the guanylyl cyclase-linked natriuretic peptide-B receptor (NPR-B). All three peptides are cleared by the natriuretic peptide-C receptor (NPR-C) and are degraded by the ectoenzyme neutral endopeptidase 24.11 (NEP), both of which are widely expressed in the kidneys, lungs, and the vascular wall. Congestive heart failure (CHF) represents a pathological state in which the activation of the natriuretic peptides exceeds those of all other states. In this brief review, we will attempt to provide an update on important issues regarding natriuretic peptides in CHF, with a focus on their functional importance as a beneficial humoral response in asymptomatic left ventricular dysfunction (LVD), the mechanisms of natriuretic peptide hyporesponsiveness in severe heart failure, the diagnostic and prognostic significance of the natriuretic peptides in CHF, and the therapeutic potential of the natriuretic peptides in this multiorgan syndrome.

Neutral endopeptidase inhibition potentiates the natriuretic actions of adrenomedullin

Adrenomedullin (ADM) is a potent renal vasodilating and natriuretic peptide possessing a six amino acid disulfide ring. Neutral endopeptidase 24.11 (NEP) is localized in greatest abundance in the kidney and cleaves endogenous peptides like atrial natriuretic peptide, which also possesses a disulfide ring. We hypothesized that NEP inhibition potentiates the natriuretic actions of exogenous ADM in anesthetized dogs (n = 6). We therefore investigated renal function in which one kidney received intrarenal infusion of ADM (1 ng . kg-1 . min-1) while the contralateral kidney served as control before and during the systemic infusion of a NEP inhibitor (Candoxatrilat, 8 microg . kg-1 . min-1; Pfizer). In response to ADM, glomerular filtration rate (GFR) in the ADM kidney did not change, whereas renal blood flow, urine flow (UV), and urinary sodium excretion (UNaV) increased from baseline. Proximal and distal fractional reabsorption of sodium decreased in the ADM-infused kidney. In response to systemic NEP inhibition, UNaV and UV increased further in the ADM kidney. Indeed, DeltaUNaV and DeltaUV were markedly greater in the ADM kidney compared with the control kidney. Plasma ADM was unchanged during ADM infusion but increased during NEP inhibition. In conclusion, the present investigation is the first to demonstrate that NEP inhibition potentiates the natriuretic and diuretic responses to intrarenal ADM. This potentiation occurs secondary to a decrease in tubular sodium reabsorption. Lastly, the increase in plasma ADM during systemic NEP inhibition supports the conclusion that ADM is a substrate for NEP.

Tissue-specific regulation of renal and cardiac atrial natriuretic factor gene expression in deoxycorticosterone acetate-salt rats

Atrial natriuretic factor (ANF) is expressed in several noncardiac tissues where it may have an autocrine or paracrine function. Such function may be expected of locally synthesized ANF in the renal parenchyma. Previous investigations of the existence of ANF mRNA in the renal parenchyma have yielded conflicting results. The investigations reported here were designed to detect and measure ANF mRNA in normal rats and in rats subjected to a deoxycorticosterone acetate (DOCA)-salt treatment schedule known to strongly activate cardiac ANF gene expression. The expression of the renal ANF gene was measured using a newly developed quantitative competitive reverse transcription-polymerase chain reaction (QC-RT-PCR). This method uses an internal competitor that serves as an internal standard and makes the procedure independent of measurement relative to housekeeping genes. It was found that renal ANF mRNA levels were 10(7) times lower than those found in left or right atria, but immunoreactive (ir) renal ANF concentration by specific radioimmunoassay was 10(4) times lower than that of atrial irANF levels. Reverse-phase high-performance liquid chromatography analysis revealed that more than 99% of renal irANF is processed ANF(99-126). This finding suggests that most of the irANF measured in kidney extracts likely originates from atrial sources. Left atrial ANF mRNA levels after 1 week of DOCA-salt treatment was significantly higher than that of control rats ([21.06+/-2.99] x 10(-l5) mol/microg total RNAversus [8.59 +/-1.26] x 10(-5) mol/microg total RNA, P<.05). However, renal ANF mRNA levels in DOCA-salt rats were significantly decreased compared with those of control rats ([1.64+/-0.34] x 10(-22) mol/microg total RNA versus [3.96+/-0.61]x 10(-22) mol/microg total RNA, P<.05). These results indicate that (1) renal ANF mRNA can be consistently and specifically demonstrated after reverse transcription and PCR amplification; (2) renal and cardiac ANF synthesis are regulated in a tissue-specific, opposite manner during DOCA-salt treatment; and (3) the finding that renal ANF mRNA is downregulated by DOCA-salt treatment together with previous findings suggest the need for further investigation into the role of renal ANF mRNA downregulation in the pathogenetic mechanism that leads to volume expansion and hypertension after chronic DOCA-salt treatment.

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.

Intrarenal determinants of sodium retention in mild heart failure: effects of angiotensin-converting enzyme inhibition

The onset and the mechanisms leading to Na+ retention in incipient congestive heart failure (CHF) have not been systematically investigated. To investigate renal Na+ handling in the early or mild stages of CHF, Na+ balance and renal clearances were assessed in 10 asymptomatic patients with idiopathic or ischemic dilated cardiomyopathy and mild heart failure (HF) off treatment (left ventricular ejection fraction, 29.7+/-2%) and in 10 matched normal subjects during a diet containing 100 mmol/d of NaCl and after 8 days of high salt intake (250 mmol/d). Six patients were studied again after 6 weeks of treatment with enalapril (5 mg/d P.O.). At the end of the high salt diet, in patients with mild HF the cumulative Na+ balance exceeded by 110 mmol that of normal subjects (F=3.86, P<.001). During high salt intake, renal plasma flow and glomerular filtration rate were similarly increased in both normal subjects and mild HF patients. In spite of comparable increases of filtered Na+ in the two groups, fractional excretion of Na+, fractional clearance of free water, and fractional excretion of K+ (indexes of distal delivery of Na+) increased in normal subjects and were reduced in patients with mild HF. During enalapril treatment, in the mild HF patients the cumulative Na+ balance was restored to normal; furthermore, enalapril significantly attenuated the abnormalities in the distal delivery of Na+. Our results indicate that a defective adaptation of Na+ reabsorption in the proximal nephron is associated with Na+ retention in response to increased salt intake in the early or mild stages of HF. These abnormalities of renal Na+ handling are largely reversed by enalapril.

Pulmonary and renal neutral endopeptidase EC 3.4.24.11 in rats with experimental heart failure

Congestive heart failure is characterized by avid sodium retention and a blunted renal response to exogenous and endogenous atrial natriuretic peptide. Inhibition of neutral endopeptidase EC 3.4.24.11, the main enzyme that degrades natriuretic peptides, produces a natriuretic response in different models of congestive heart failure. This raises the possibility that an increase in either the expression or activity of neutral endopeptidase is responsible for these phenomena. In the present study, we examined (1) the renal effects of SQ-28,603, a neutral endopeptidase inhibitor, in rats with moderate and severe congestive heart failure induced by an aortocaval fistula compared with sham controls, and (2) neutral endopeptidase expression and activity in the lungs and kidneys of these rats. Infusion of SQ-28,603 (40 mg/kg IV) induced a significant natriuretic response in normal rats and rats with moderate congestive heart failure. This response was blunted in rats with severe congestive heart failure. Surprisingly, renal neutral endopeptidase mRNA levels, assessed by quantitative reverse transcriptase-polymerase chain reaction; protein levels, assessed by Western blotting; and activity, assessed by gelatin gels, were comparable in all groups. Pulmonary neutral endopeptidase mRNA levels decreased by 45% in rats with severe congestive heart failure but not in rats with mild congestive heart failure. In addition, pulmonary neutral endopeptidase immunoreactivity levels and activity were significantly decreased in congestive heart failure in correlation with the severity of the disorder.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of neutral endopeptidase in dogs with evolving congestive heart failure

BACKGROUND

Recent studies suggest that neurohumoral mechanisms including decreased renal responses to increases in atrial natriuretic factor (ANF) play a central role in the progression from asymptomatic cardiac dysfunction to advanced congestive heart failure (CHF) with sodium retention, vasoconstriction, and reduced exercise tolerance. Recognizing that neutral endopeptidase 24.11 degrades ANF and may be enhanced in CHF, we hypothesized that chronic neutral endopeptidase inhibition (NEP-I) would potentiate renal responses to exogenous ANF and alter the temporal evolution of sodium retention in evolving CHF by potentiation of increased endogenous ANF.

METHODS AND RESULTS

We studied 13 conscious dogs with evolving CHF produced by rapid ventricular pacing at 250 beats per minute. Six of these dogs received NEP-I with candoxatril, 10 mg/kg PO BID, throughout evolving CHF. Responses to exogenous ANF, 10 micrograms/kg IV bolus, were assessed at baseline and after 6 days of CHF. Daily metabolic studies during evolving CHF with chronic NEP-I showed increased sodium excretion and renal cGMP generation consistent with enhanced renal activity of endogenous ANF compared with untreated controls. In addition, renal natriuretic and cGMP responses to exogenous ANF were intact in CHF with chronic NEP-I in contrast to markedly attenuated renal responses to exogenous ANF in untreated CHF. Despite enhanced ANF responsiveness and improved sodium balance in evolving CHF, a moderate degree of sodium retention was observed during chronic NEP-I in evolving CHF.

CONCLUSIONS

Enzymatic degradation by neutral endopeptidase limits local renal responses to increases in endogenous and exogenous ANF in CHF independent of changes in systemic hemodynamics or augmented plasma concentrations of ANF. The moderate sodium retention observed during evolving CHF despite chronic NEP-I probably reflects the antinatriuretic effects of hemodynamic and humoral factors independent of ANF activity.

Urinary neutral endopeptidase 24.11 activity: modulation by chronic salt loading

Neutral endopeptidase (NEP) 24.11 is a zinc-metallopeptidase involved in the metabolism of several biologically active peptides including enkephalin, atrial natriuretic peptide, bradykinin, and endothelin. The enzyme is found in abundant amounts in the brush border of renal proximal epithelial cells. A soluble form of NEP was previously identified in human urine with characteristics similar to the renal enzyme. The present study further characterized the excreted form of NEP activity in urine of normal rats using a sensitive two-stage enzymatic assay. The response of urinary NEP to known inhibitors such as phosphoramidon and thiorphan, and its dependence on pH and salt concentration was studied. In addition, we evaluated the effects of acute and chronic changes in salt balance, induced by i.v. saline infusion and drinking of saline solution, on urinary NEP and on the activity of the enzyme in isolated proximal tubules. Our findings demonstrated that abundant NEP activity was detected in the urine of normal rats. Furthermore, chronic salt loading, but not acute salt infusion, was associated with increased activity of NEP in urine and in isolated proximal tubules, suggesting that the enzyme may be regulated by salt balance. Finally, the data suggest that urinary NEP may be used as an index of enzyme activity in the kidney.

Atrial natriuretic peptide and its potential role in pharmacotherapy

Atrial natriuretic peptide (ANP) is a 28 amino-acid polypeptide secreted into the blood by atrial myocytes after atrial pressure and distension. Although its role in humans is not clear, it can produce a variety of physiologic effects including vasodilatation, natriuresis, and suppression of the renin-angiotensin-aldosterone axis. These actions are potentially useful in a variety of pathologic states such as hypertension and congestive heart failure, and diverse methods to augment the effects of ANP in these states have been devised. The results are exciting and, despite some problems, may lead to the pharmacologic use of enhancement of ANP actions in several clinical disorders.

Identification and characterization of a neutral endopeptidase activity in Aplysia californica

Kidney plasma membranes of Aplysia californica were shown to contain an endopeptidase activity which cleaved [Leu]enkephalin (Tyr-Gly-Gly-Phe-Leu) and [Leu]enkephalinamide (Tyr-Gly-Gly-Phe-Leu-NH2) at the Gly3-Phe4 bond, as determined by reverse-phase h.p.l.c. analysis of metabolites. The optimal pH was shown to be 6.5. The bivalent cation chelating agent, 1,10-phenanthroline protected [Leu]enkephalin from degradation, suggesting that this enzyme is a metallopeptidase. The degradation of [Leu]enkephalin was also abolished by the neutral endopeptidase-24.11 inhibitors RB104 (2-[(3-iodo-4-hydroxyl)-phenylmethyl]-4-N-[3-(hydroxyamino-3-oxo-1- phenylmethyl)-propyl]amino-4-oxobutanoic acid), HABCO-Gly [(3-hydroxy-aminocarbonyl-2-benzyl-1-oxypropyl)glycine], phosphoramidon and thiorphan, with IC50 values of 1 nM, 1 microM, 20 microM and 30 microM respectively. By contrast, the angiotensin-converting enzyme inhibitor captopril and the serine proteinase inhibitor phenylmethanesulphonyl fluoride were without effect. Phase separation experiments using Triton X-114 showed that about 64% of the neutral endopeptidase activity in the Aplysia kidney membrane corresponds to an integral membrane protein. A specific radioiodinated inhibitor ([125I]RB104) was shown to bind the Aplysia endopeptidase with high affinity; the KD and Bmax. values were 21 +/- 5 pM and 20.3 +/- 5 fmol/mg of proteins respectively. This inhibitor was used to determine the molecular form of the enzyme, after separation of solubilized membrane proteins on SDS/PAGE and transfer on to nitrocellulose membranes. A single protein band with an apparent molecular mass of 140 kDa was observed. The labelling was abolished by specific neutral endopeptidase inhibitors. This study provides the first biochemical characterization of an endopeptidase with catalytic properties similar to those of neutral endopeptidase-24.11 in the mollusc Aplysia californica.

Renal response to ANP in normal dogs during extreme inhibition of distal Na+ transport

Atrial natriuretic peptide (ANP) is thought to exert its major effect within the cortical and inner medullary collecting ducts (CCD and IMCD) by inhibiting Na+ transport along conductive channels and electroneutral pathways. These transport routes are also thought to be inhibited by a combination of amiloride, thiazide and bradykinin. We tested the ability of normal dogs to respond to ANP when various combinations of these Na+ transport inhibitors were present. In 24 dogs ANP raised UNaV from 31 +/- 6 to 223 +/- 41 mu Eq/min (P < 0.05), a delta of 192 mu Eq/min. Bradykinin alone did not depress delta UNaV in response to an ANP infusion. In the presence of extreme natriuresis caused by amiloride and thiazide, the response to ANP was magnified, presumably due to augmented Na+ delivery to the CCD and IMCD. When distal delivery of Na+ to one kidney was controlled by aortic clamping in the presence of amiloride, thiazide and bradykinin. delta UNaV in response to ANP was depressed (48 vs. 168 mu Eq/min). We conclude that in the presence of extreme inhibition of Na+ transport within the collecting ducts, ANP can still cause a further natriuresis, probably in the absence of augmented distal Na+ delivery.

Characterization of neutral endopeptidase 24.11 in dog glomeruli

Neutral endopeptidase (NEP; also known as neprilysin and enkephalinase; EC 3.4.24.11) is a cell-surface metallopeptidase that is present in many mammalian tissues. It is particularly abundant on the brush-border membranes of the kidney proximal tubule. In this paper, the presence of NEP in purified glomeruli from dog kidney was assessed by measuring phosphoramidon- and thiorphan-sensitive [D-Ala2,Leu5]enkephalin-degrading activity. Using this assay, the Km and kcat. of the glomerular enzyme were found to be identical to those of the tubular enzyme. By Western blotting the apparent M(r) of the glomerular enzyme was found to be 104,000, compared with 94,000 for the tubular enzyme. This might be due to a different glycosylation pattern, since endoglycosidase F treatment of NEP obtained from both tissues yielded deglycosylated enzymes with similar electrophoretic mobilities. The glomerular enzyme also appears to be membrane-bound, since it was retained in the detergent-rich phase after phase separation with Triton X-114. Autoradiography experiments performed with RB104, a new highly selective and potent NEP inhibitor, showed that NEP was expressed in both glomeruli and proximal tubules. The presence in glomeruli of NEP and some other brush-border peptidases (dipeptidyl-dipeptidase IV, aminopeptidase N and angiotensin I-converting enzyme) suggests that cell-surface peptidases might play an important role as regulators of plasma-derived peptides in this part of the nephron.

Renal tubular responsiveness to atrial natriuretic peptide in sodium-retaining chronic caval dogs. A possible role for kinins and luminal actions of the peptide

60% of chronic caval dogs with ascites did not respond to atrial natriuretic peptide (ANP) (75 ng.kg-1.min-1) with a natriuresis (TIVC-NR; delta UNaV = 2 +/- 0.8 mu eq/min) whereas the remaining 40% responded normally (TIVC-R; delta UNaV = 216 +/- 50 mu eq/min). Since proximal tubule neutral endopeptidase 24:11 (NEP) destroys most of intrarenal luminal ANP and kinins, we attempted to convert TIVC-NR into TIVC-R by providing NEP inhibition with SQ 28603 at 30 mg/kg. This potent and specific NEP inhibitor produced a natriuresis when administered alone to nine TIVC-NR dogs (delta UNaV = 67 +/- 2 mu eq/min) and permitted a natriuresis in the presence of ANP (delta UNaV = 97 +/- 18 mu eq/min). A natriuretic response to ANP could also be induced in TIVC-NR dogs by providing renal arterial bradykinin or intravenous captopril, a kininase inhibitor. Urodilatin, a natriuretic peptide not destroyed by intrarenal NEP was without effect in TIVC-NR dogs but increased UNaV when given to TIVC-R and normal dogs. Providing bradykinin to TIVC-NR now permitted an increment in delta UNaV (62 mu eq/min) when urodilatin was reinfused. TIVC-R dogs could be converted into TIVC-NR by pretreating with a specific bradykinin antagonist before infusing ANP. We conclude that TIVC-NR dogs are deficient in intrarenal kinins but are converted to responding dogs after NEP inhibition because of increased kinin delivery to the inner medullary collecting duct.

Response to atrial natriuretic peptide, endopeptidase 24.11 inhibitor and C-ANP receptor ligand in the rat

1. The present studies compared the renal and hypotensive response to (a) exogenous atrial natriuretic peptide (ANP) (99-126), (b) an endopeptidase-24.11 inhibitor (candoxatrilat) and (c) an antagonist of ANP clearance receptors (SC 46542) in conscious rats. 2. Infusion of low-dose-ANP (100 ng kg-1 min-1) produced a gradual increase in urinary sodium and guanosine 3':5'-cyclic monophosphate (cyclic GMP) excretion without significant change in glomerular filtration rate (GFR) or fractional lithium clearance (FeLi). There was a significant fall in blood pressure. 3. Infusion of high-dose ANP (300 ng kg-1 min-1) produced a brisk, 3 fold increase in urinary sodium and cyclic GMP excretion along with a rise in GFR, but had no significant effect on FeLi compared to the control group. The renal response was accompanied by a pronounced fall in blood pressure. 4. Candoxatrilat or SC 46542, alone, had no significant effect on sodium excretion compared to control animals. Both compounds enhanced the natriuretic and cyclic GMP responses to a low-dose ANP infusion (100 ng kg-1 min-1) to levels similar to, or greater than, those observed with the high-dose ANP (300 ng kg-1 min-1). However, unlike high-dose ANP, these renal effects were not accompanied by a significant change in GFR and neither compound potentiated the hypotensive effect of the low-dose ANP infusion. Only candoxatrilat when given with ANP produced a marked rise in FeLi.5. Similarly, combined administration of candoxatrilat and SC 46542 (without exogenous ANP) induced an increase in sodium and cyclic GMP excretion comparable to high-dose ANP but did so without a significant increase in GFR and with a significantly smaller fall in blood pressure. Interestingly, there was no increase in FeLi with the combination of the two compounds, suggesting that the major contribution to sodium excretion came from SC 46542.6. Both candoxatrilat and SC 46542 increased sodium and cyclic GMP excretion in the rat A-V fistula model of heart failure, a model hyporesponsive to infusions of ANP, without significant change in blood pressure.7. These data show that candoxatrilat and SC 46542 do not simply reproduce the effects of an ANP infusion but preferentially enhance the natriuretic response to ANP. Inhibition of E-24.11 may potentiate a tubule action of ANP while the renal mechanism of action of the C-ANP receptor ligand needs further study. Both manipulations are of potential value in the management of heart failure.

Atrial natriuretic peptide clearance receptors in trout: effects of receptor inhibition in vivo

Inactivation of circulating atrial natriuretic peptides (ANP) by specialized clearance (C) receptors has been characterized in mammals but has not been examined in fish. In the present study arterial blood pressure, urine flow, and urine electrolytes were measured in chronically cannulated rainbow trout, Oncorhynchus mykiss, during infusion of the specific C receptor inhibitor, SC-46542. C receptor inhibition decreased blood pressure and pulse pressure, increased heart rate and urine flow, but did not affect urinary electrolyte concentrations. These responses are consistent with those produced by exogenous ANP administration and indicate that: (1) trout possess C-type receptors capable of ANP inactivation, and (2) ANP-like molecules are continuously released and metabolized by trout in vivo. Phosphoramidon, an inhibitor of neutral endopeptidase, did not enhance the SC-46542 response, indicating that C receptors predominate in ANP inactivation in these fish.

Atrial natriuretic factor: its (patho)physiological significance in humans

The first human studies using relatively high-doses of ANF revealed similar effects as observed in the preceding animal reports, including effects on systemic vasculature (blood pressure fall, decrease in intravascular volume), renal vasculature (rise in GFR, fall in renal blood flow), renal electrolyte excretion (rises in many electrolytes), and changes in release of a number of different hormones. Whether all these changes are the result of direct ANF effects or secondary to a (single) primary event of the hormone remains to be determined. Certainly, it has been proven that more physiological doses of ANF fail to induce short-term changes in many of these parameters leaving only a rise in hematocrit, natriuresis and an inhibition of the RAAS as important detectable ANF effects in humans. This leads us to hypothesize that ANF is a "natriuretic" hormone with physiological significance. The primary function in humans is to regulate sodium homeostasis in response to changes in intravascular volume (cardiac atrial stretch). Induction of excess renal sodium excretion and extracellular volume shift appear to be the effector mechanisms. The exact mechanism of the natriuresis in humans still needs to be resolved. It appears however, that possibly a small rise in GFR, a reduction in proximal and distal tubular sodium reabsorption, as well as an ensuing medullary washout, are of importance. The pathophysiological role of ANF in human disease is unclear. One may find elevated plasma irANF levels and/or decreased responses to exogenous ANF in some disease states. Whether these findings are secondary to the disease state rather than the cause of the disease remains to be resolved. Therapeutic applications for ANF, or drugs that intervene in its production or receptor-binding, seem to be multiple. Most important could be the antihypertensive effect, although areas such as congestive heart failure, renal failure, liver cirrhosis and the nephrotic syndrome cannot be excluded. Although the data that have been gathered to date allowed us to draw some careful conclusions as to the (patho)physiological role of ANF, the exact place of ANF in sodium homeostatic control must still be better defined. To achieve this, we will need more carefully designed low-dose ANF infusion, as well as ANF-breakdown inhibitor studies. Even more promising, however, is the potential area of studies open to us when ANF-receptor (ant)agonists become available for human use.

Angiotensin inhibition potentiates the renal responses to neutral endopeptidase inhibition in dogs with congestive heart failure

The renal natriuretic actions of endogenous atrial natriuretic factor are enhanced by neutral endopeptidase inhibition (NEP-I). Recognizing that activation of the renin-angiotensin-aldosterone system in congestive heart failure (CHF) antagonizes the renal actions of atrial natriuretic factor, we hypothesized that angiotensin II antagonism with converting enzyme inhibition would potentiate the renal actions of NEP-I in CHF. To test this hypothesis, the renal responses to a specific NEP-I (SQ 28,603) were assessed in dogs with eight days of experimental CHF produced by rapid ventricular pacing. The renal natriuretic responses to NEP-I in experimental CHF were significant. In the same model of CHF, chronic angiotensin antagonism with converting enzyme inhibition potentiated both renal hemodynamic and excretory responses to NEP-I. The potentiated renal hemodynamic response included significant increases in glomerular filtration rate and filtration fraction. In the CHF group with angiotensin antagonism, an intrarenal infusion of low-dose angiotensin abolished the potentiated renal responses to NEP-I, supporting the concept that intrarenal angiotensin antagonism, rather than improved systemic hemodynamics or potentiation of other peptide systems, mediated the enhanced renal responses to NEP-I in the presence of converting enzyme inhibition.

Neutral endopeptidase inhibitors and atrial natriuretic peptide

A limited number of ectoenzymes appear to be involved in the inactivation of circulating-regulatory peptides. Neutral endopeptidase 24.11, a metallopeptidase, is known to inactivate atrial natriuretic peptide (ANP), a substance with diuretic, natriuretic, and vasodilatory effects. Synthetic inhibitors of endopeptidase 24.11, which can prolong the activity of ANP, are currently available. These agents are being evaluated as possible innovative therapies for patients with hypertension and congestive heart failure.

Role of endogenous atrial natriuretic peptide in regulating sodium excretion in spontaneously hypertensive rats. Effects of neutral endopeptidase inhibition

To explore whether pathophysiological plasma levels of atrial natriuretic peptide (ANP) actually involve sodium excretion in spontaneously hypertensive rats (SHR), we examined the in vivo and ex vivo effects of ANP and an endopeptidase inhibitor, thiorphan, on urinary sodium excretion and the elimination rate of ANP. We found the following: 1) The basal plasma ANP level was higher in 16-week-old SHR than in Wistar-Kyoto (WKY) rats (109 +/- 10 [SEM] versus 63 +/- 4 pg/ml, p less than 0.001). Thiorphan (30 mg/kg i.v.) significantly increased plasma ANP by 60% in both SHR and WKY rats. However, increases in urinary sodium excretion (+290% versus +130%, p less than 0.05) and cyclic GMP (+160% versus +60%, p less than 0.05) were greater in SHR than in WKY rats. Urinary excretion of ANP was markedly increased by thiorphan, and its increase was greater in SHR than in WKY rats. 2) The thiorphan-induced natriuresis was substantially attenuated by antiserum for ANP but not by a bradykinin receptor antagonist. 3) Isolated SHR kidneys excreted 50% less sodium than WKY rat kidneys at perfusion pressures of 100 and 160 mm Hg (p less than 0.05). Urinary sodium excretion was increased at the perfusate ANP level of 100 pg/ml, a concentration similar to the SHR plasma ANP (+70% at 160 mm Hg). 4) After bolus administration of ANP to the isolated kidney, the ANP concentration of the recirculating perfusate decreased rapidly in a log-linear fashion.(ABSTRACT TRUNCATED AT 250 WORDS)

Pulmonary and urinary clearance of atrial natriuretic factor in acute congestive heart failure in dogs

Atrial natriuretic factor (ANF) is a peptide hormone of cardiac origin elevated in acute congestive heart failure (CHF), which is degraded by the enzyme neutral endopeptidase 24.11 (NEP). This study was designed to investigate the pulmonary and urinary clearance of ANF before and after the initiation of acute experimental CHF in dogs, and to assess the contribution of enzymatic degradation to these clearances in CHF. This study demonstrated a significant clearance of plasma ANF across the pulmonary circulation at baseline, and a tendency for pulmonary clearance to decrease in CHF (1115 +/- 268 to 498 +/- 173 ml/min, NS). The pulmonary extraction of ANF present at baseline was not altered with acute CHF (36.0 +/- 7.8 to 34.9 +/- 12.1%, NS). NEP inhibition (NEPI) abolished both the clearance and extraction of plasma ANF across the lung in CHF. Similarly, significant urinary clearance of ANF was present at baseline, and in acute CHF the urinary clearance of ANF decreased (0.14 +/- 0.02 to 0.02 +/- 0.01 ml/min, P less than 0.05). NEPI prevented the decrease in the urinary clearance of ANF, and enhanced the renal response to endogenous ANF, independent of further increases in plasma ANF during CHF. This study supports an important role for NEP in the pulmonary and urinary metabolism of endogenous ANF during acute CHF.