Endogenous Natriuretic Peptides Participate in Renal and Humoral Actions of Acute Vasopeptidase Inhibition in Experimental Mild Heart Failure
Mild heart failure is characterized by increases in atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) in the absence of activation of the renin-angiotensin-aldosterone system (RAAS). Vasopeptidase (VP) inhibitors are novel molecules that coinhibit neutral endopeptidase 24.11, which degrades the natriuretic peptides (NPs) and ACE. In a well-characterized canine model of mild heart failure produced by ventricular pacing at 180 bpm for 10 days, we defined the renal and humoral actions of acute VP inhibition with omapatrilat (OMA, n=6) and acute ACE inhibition (n=5) alone with fosinoprilat. We also sought to determine whether the NPs participate in the renal actions of acute VP inhibition by the administration of OMA together with an intrarenal administration of the NP receptor antagonist HS-142-1 (n=5). OMA resulted in a greater natriuretic response than did ACE inhibition in association with increases in plasma cGMP, ANP, BNP, urinary cGMP, urinary ANP excretion, and glomerular filtration rate (P<0.05 for OMA versus ACE inhibition). Plasma renin activity was increased only in the group subjected to ACE inhibition. Administration of intrarenal HS-142-1 attenuated the renal properties of OMA in association with a decrease in urinary cGMP excretion despite similar increases in plasma ANP and BNP. This study provides new insight into a unique new pharmacological agent that has beneficial renal actions in experimental mild heart failure beyond the actions that are observed with ACE inhibition alone and that are linked to the NP system.
Mild heart failure (HF) is the initial chronic compensated phase of left ventricular dysfunction, which is characterized by sodium balance and increases in the cardiac natriuretic peptides (NPs), atrial NP (ANP), and brain NP (BNP), without activation of the renin-angiotensin-aldosterone system (RAAS).1–3 The biological role of NPs in mild HF is underscored by reports that NP receptor antagonism in experimental mild HF results in RAAS activation and sodium retention, consistent with a role for NPs in mild HF to maintain sodium balance and suppress the RAAS.4 Such a role is further supported by Lopez et al,5 who demonstrated that genetic deletion of the receptor for ANP and BNP results in an impaired renal response to volume loading.
The only recommended therapy for mild HF is the use of ACE inhibition (ACEI), which is based on the Studies of Left Ventricular Dysfunction (SOLVD) trial and the Survival and Ventricular Enlargement (SAVE) study.6,7 A major conclusion of the SOLVD trial was the need to identify new therapeutic strategies in mild HF based in part on the suboptimal impact of ACEI on mortality.7 The SOLVD trial stated that “while ACEI was well tolerated by patients with asymptomatic left ventricular dysfunction (ALVD) and reduced the incidence of HF and related hospitalizations, the lack of a significant effect on overall mortality emphasizes the need to explore more effective means or additional means of treating ALVD.”7
Vasopeptidase (VP) inhibition is an innovative pharmacological strategy that via a single molecule simultaneously inhibits the activity of neutral endopeptidase 24.11 (NEP) and ACE. NEP is a metalloprotease that is localized in greatest abundance in the kidney and degrades the NPs and other vasoactive peptides, such as bradykinin and adrenomedullin.8,9 Omapatrilat (OMA) is the most clinically advanced and is a mercaptoacyl derivative of a bicyclic thiazepinone dipeptide with a molecular weight of 408.5. OMA inhibits both NEP and ACE with Ki values of 8.9 and 6.0 nmol/L, respectively.10 Studies have demonstrated greater blood pressure–reducing properties in hypertension by OMA than by conventional antihypertensive agents.11 In the IMPRESS trial in symptomatic HF, OMA was superior to ACEI in reducing a combined end point of worsening HF and hospitalization compared with ACEI alone.12 In IMPRESS, the only significant biological action of OMA therapy versus ACEI was lower serum creatinine and blood urea nitrogen consistent with greater renoprotection. Importantly, a recent reevaluation of the SOLVD data reported that improved renal function is a powerful predictor of survival in asymptomatic and symptomatic HF.13 To date, however, the specific renal actions of VP inhibition remain unclear, and the contributing role of NPs in the renal mechanism of VP inhibition in HF remains untested.
Therefore, we hypothesized that acute simultaneous inhibition of ACE and NEP would result in more favorable cardiorenal and humoral responses with OMA than with acute ACEI alone. We also hypothesized that the renal actions of OMA would involve the endogenous NPs. Therefore, our objective was to define the renal and humoral actions of acute VP inhibition with OMA in a large animal model of experimental mild HF and to compare such actions with the actions of ACEI alone. We also administered systemic OMA and an intrarenal NP receptor antagonist to determine the role of the endogenous NPs in mediating the renal actions of acute VP inhibition.
The present study was conducted in 3 groups of anesthetized male mongrel dogs (18 to 23 kg) with pacing-induced mild HF in accordance with the Animal Welfare Act and with the approval of the Mayo Clinic Animal Care and Use Committee. The 3 groups consisted of the following: (1) acute VP inhibition with OMA (n=6), (2) acute ACEI alone (n=5), and (3) acute VP inhibition with OMA plus intrarenal administration of an NP antagonist (n=5).
Pacing-Induced Mild HF
Mild HF was produced by ventricular pacing at 180 bpm for 10 days. In previous studies by Redfield et al,14 this resulted in a canine model of mild HF with a cardiorenal and neurohumoral profile that mimics human mild HF.
On the day of the experiment, dogs were anesthetized with sodium pentobarbital, intubated, and mechanically ventilated (Harvard respirator, Amersham). A flow-directed balloon-tipped thermodilution catheter (Criticath, Ohmeda) was inserted for cardiac hemodynamic measurement. The femoral artery was cannulated for blood pressure monitoring and blood sampling. The femoral vein was also cannulated for infusions. The left kidney was exposed via a flank incision, and the ureter was cannulated for urine collection. An electromagnetic flow probe was placed to measure renal blood flow (RBF). In the third group only, a curved 22-gauge needle was inserted into the renal artery for the administration of the intrarenal NP receptor antagonist HS-142-1 (HS).
A 30-minute baseline clearance was performed after a 60-minute equilibration period. Midway through the clearance, cardiac hemodynamics were measured, and arterial blood was drawn for hormonal and electrolyte analysis. After the 30-minute baseline clearance, group 1 received an intravenous bolus of the VP inhibitor OMA (Bristol-Myers Squibb) at 1 μmol/kg administered over 20 minutes. Group 2 received an intravenous bolus of the ACE inhibitor fosinoprilat (Bristol-Myers Squibb) at 1 μmol/kg administered over 20 minutes. Group 3 received an intravenous bolus of the VP inhibitor OMA at 1 μmol/kg administered over 20 minutes plus an intrarenal administration of the NP receptor antagonist HS (Tokyo Research Laboratories, Kyowa Hakko Kogyo Co) at 0.5 mg/kg into the left renal artery. After a 15-minute lead-in period, three 30-minute clearances were performed, followed by two 60-minute clearances in all 3 groups.
To ensure that the differential response between the OMA and ACEI groups was not because of insufficient dosage of ACE inhibitor, we increased the ACE inhibitor dosage by ×10 (to 10 μmol/kg) in 2 dogs and observed no further changes in any cardiovascular, renal, or humoral parameters. We chose to administer the NP receptor antagonist HS intrarenally so as to accurately define its renal effects in the absence of any systemic cardiovascular changes.
Hormone and Electrolyte Analysis
After plasma extraction, plasma and urine ANP, BNP, plasma renin activity, angiotensin II, and cGMP were measured by radioimmunoassay.15–20 Urinary and plasma inulin were measured by the anthrone method. Urinary and plasma lithium were determined by flame emission spectrophotometry (model 357, Instrumentation Laboratory).
Results of the quantitative studies were expressed as mean±SEM. Data were assessed by repeated-measures 1-way ANOVA for comparisons within groups, and 2-way ANOVA was used for comparison between groups, with the use of GraphPad Prism software. Statistical significance was accepted at P<0.05.
An expanded Methods section can be found in an online data supplement available at http://www.hypertensionaha.org.
Renal, Cardiovascular, and Humoral Actions of Acute OMA Compared With Acute ACEI
Figure 1 illustrates the renal response to acute OMA compared with acute ACEI in experimental mild HF. Compared with baseline, acute OMA increased urinary sodium excretion (UNaV) and glomerular filtration rate (GFR) and decreased distal fractional sodium reabsorption (DFNaR). Acute ACEI increased UNaV and decreased DFNaR, with no significant change in GFR compared with baseline. When compared with ACEI, acute OMA resulted in a greater increase in UNaV and GFR and a greater decrease in DFNaR.
Right atrial pressure (RAP), pulmonary capillary wedge pressure (PCWP), and pulmonary artery pressure (PAP) responses to acute OMA compared with ACEI are illustrated in Figure 2. Acute OMA resulted in sustained reduction in RAP, PCWP, and PAP compared with baseline. With acute ACEI, there was a transient reduction in RAP, with no change in PCWP and PAP compared with baseline. There was a greater reduction in RAP with OMA compared with ACEI; however, there were no statistically significant differences in PCWP and PAP when OMA was compared with ACEI. There was also a transient increase in both cardiac output (CO) and RBF and a decrease in mean arterial pressure (MAP) with both OMA and ACEI compared with baseline (Table 1); these values returned to baseline by 60 minutes in both groups. There were no statistically significant differences in the change in CO, RBF, or MAP when OMA was compared with ACEI.
Acute OMA increased plasma ANP (from 144±15 [baseline] to 259±24 [peak] pg/mL, P<0.05), plasma BNP (from 101±18 [baseline] to 163±35 [peak] pg/mL, P<0.05), plasma cGMP (from 18±3 [baseline] to 25±2 [peak] pmol/mL, P<0.05), and urinary cGMP excretion (from 1239±330 [baseline] to 2173±485 [peak] pmol/min, P<0.05). Acute ACEI did not result in any increase in plasma ANP, plasma BNP, plasma cGMP, or urinary cGMP. Therefore, the increases in plasma ANP, BNP, cGMP, and urinary cGMP excretion were greater with OMA than with ACEI alone (P<0.05 for OMA versus ACEI, 2-way ANOVA). Despite a similar reduction of plasma angiotensin II by both OMA (from 95±34 pg/mL at baseline to 34±9 pg/mL, P<0.05) and ACEI (from 92±43 pg/mL at baseline to 36±7 pg/mL, P<0.05) (P=NS for OMA versus ACEI), plasma renin activity remained unchanged in the OMA group (from 9±1 ng · mL−1 · h−1 at baseline to 10±2 ng · mL−1 · h−1, P=NS) but was increased in the ACEI group (from 8±3 [baseline] to 15±1 [peak] ng · mL−1 · h−1, P<0.05), which was also statistically different from OMA (P<0.05 for OMA versus ACEI).
Renal and Humoral Actions of Acute OMA Compared With Acute OMA Plus Intrarenal HS
Table 2 reports the renal properties of acute OMA and acute OMA plus intrarenal HS administration. Compared with the administration of acute OMA alone, the administration of intrarenal HS with acute OMA decreased UNaV and GFR with an increase in DFNaR. Although intrarenal administration of the NP receptor antagonist HS attenuated the renal response to OMA, the increases in plasma ANP (from 131±28 [baseline] to 451±177 [peak] pg/mL, P<0.05) and BNP (from 32±4 [baseline] to 105±33 [peak] pg/mL, P<0.05) remained intact and were similar to those observed with acute OMA alone (P=NS for OMA versus OMA plus intrarenal HS). However, acute OMA plus intrarenal HS was associated with a lower concentration of plasma cGMP (from 26±4 pmol/mL at baseline to 11±4 pmol/mL, P<0.05) and urinary cGMP excretion (from 1615±345 pmol/min at baseline to 860±231 pmol/min, P<0.05) compared with acute OMA alone. Specifically, OMA alone increased plasma cGMP (from 18±3 [baseline] to 25±2 [peak] pmol/mL, P<0.05) and urinary cGMP excretion (from 1239±330 [baseline] to 2173±485 [peak] pmol/min, P<0.05). These responses in plasma cGMP and urinary cGMP excretion were different when the 2 groups were compared (P<0.05 for acute OMA plus intrarenal HS versus acute OMA alone). Last, plasma renin activity increased (from 7±1 [baseline] to 17±2 [peak] ng · mL−1 · h−1, P<0.05) with OMA plus intrarenal HS compared the OMA alone (from 9±1 [baseline] to 10±2 [peak] ng · mL−1 · h−1, P=NS) (P<0.05 for OMA plus intrarenal HS versus OMA). These renal and humoral responses observed with the intrarenal administration of HS and OMA occurred in the absence of differences in CO, MAP, and RBF compared with the group administered OMA alone (P=NS for acute OMA plus intrarenal HS versus acute OMA alone).
Our present findings demonstrate that acute VP inhibition in mild HF results in increases in UNaV and GFR and reductions in distal tubular reabsorption of sodium that were greater than those with ACEI. These renal actions were in part secondary to the cardiac NPs, as demonstrated by the increased plasma and urinary ANP, BNP, and cGMP (their second messenger) and by the observation that inhibition of the renal NP receptors by HS attenuated these renal hemodynamic and tubular responses. Unlike ACEI, there was no activation of renin with VP inhibition. The present study provides new insight into a new pharmacological strategy that has beneficial actions in mild HF beyond those observed with ACEI alone and that functions in part via the endogenous cardiac NPs.
The present findings extend previous experimental and clinical studies and define the cardiorenal and humoral actions of VP inhibition compared with ACEI alone in experimental mild HF. The mechanism of the enhanced natriuresis to VP inhibition was mediated by both a hemodynamic action with an increase in GFR and a tubular mechanism involving a decrease in sodium reabsorption at the terminal nephron. Thus, inhibition of renal NP degradation by OMA led to a decrease in sodium reabsorption (at a nephron site known to be responsive to NPs) together with enhanced GFR, with a greater urinary excretion of ANP and BNP as well as urinary cGMP, the second messenger for the NP and a marker for the renal actions of the NPs.21 Furthermore, intrarenal HS together with OMA attenuated the renal effects of OMA. This attenuation was associated with a decrease in urinary cGMP excretion despite similar increases in plasma ANP and BNP consistent with renal NP receptor inhibition. These findings suggest that the renal effects of acute VP inhibition with OMA are mediated in part via the potentiation of the endogenous NPs. Of importance as well, the lack of increase in plasma renin activity with OMA in contrast to ACEI alone is consistent with a renin-inhibiting action of the NP. However, because NEP and ACE may degrade other vasoactive peptides and NPs, such as adrenomedullin and bradykinin, dual NEP/ACE inhibition may result in the potentiation of several biological active peptides, which will require further investigation.9 Last, the increase in plasma ANP with OMA (despite reductions in cardiac filling pressures, and thus atrial stretch) underscores the importance of NEP inhibition in augmenting the NPs.
The concept that simultaneous inhibition of NEP and ACE emerges as a unique therapeutic strategy in the treatment of HF. Studies have now been published that demonstrate in human hypertension greater blood pressure–reducing properties with OMA than with ACE inhibitors, and in the IMPRESS trial, VP inhibition was reported to be superior to ACEI in reducing a combined end point of worsening HF and hospitalization. Most important, the unique feature of coinhibition of ACE and NEP, compared with the ACE inhibitor lisinopril, in this recent HF trial was the preservation of renal function.12 Specifically, renal function was preserved to a greater magnitude with OMA than with ACEI. This renoprotective action of VP inhibition may have relevance to long-term survival, inasmuch as recent reanalyses of the SOLVD trial by Dries et al13 and by Hillege et al22 have reported that the presence of a maintained GFR compared with impaired renal function is a strong predictor of survival beyond that observed with other surrogate end points. To date, no study has compared the cardiorenal and humoral actions of VP inhibition versus ACEI alone in experimental mild HF in which renal and plasma angiotensin II are not activated.9,20
The present study has important therapeutic implications. First, emphasis has been placed on initiating therapy early during the progression of HF with the use of ACE inhibitors in mild HF. Indeed, studies support the conclusion that early drug intervention may delay the onset of HF symptoms.7 Therefore, the present study supports the need to further explore the efficacy of VP inhibition in mild HF on the basis of enhanced cardiorenal and humoral actions. Indeed, these beneficial renal actions were unassociated with RAAS activation, which occurs commonly with the use of conventional diuretics. Based on the superiority of acute VP inhibition with OMA in mild HF compared with ACEI alone, further studies should now explore the potential efficacy of long-term VP inhibition in mild HF.
In conclusion, the present study reports for the first time the renal actions of OMA in experimental HF. We observed that acute VP inhibition with OMA in experimental mild HF results in a greater natriuretic and GFR response than does treatment with ACEI alone and is associated with reductions in cardiac filling pressures. These renal actions, which were localized to the glomerulus and terminal nephron, occur in part secondary to the NPs, as demonstrated by the use of the NP receptor inhibitor HS, which attenuated these renal responses. Furthermore, unlike ACEI alone, there was no activation of renin with OMA, which is consistent with the renin inhibitory actions of the NPs. The present study provides new insight into a unique new pharmacological strategy that has beneficial actions in mild HF beyond those observed with ACEI alone.
This research was supported by grants HL-36634 and HL-07111 from the National Institutes of Health, by the Miami Heart Research Institute, by the Mayo Foundation, by the Bruce and Ruth Rappaport Program in Vascular Biology, and by the Bristol Myers Squibb Research Institute. Dr Chen is the recipient of the General Mills Clinician Investigator Fellowship. The authors gratefully acknowledge the assistance of Denise M. Heublein and Sharon S. Sandberg.
- Received October 9, 2000.
- Revision received December 5, 2000.
- Accepted February 12, 2001.
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