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RENAL HEMODYNAMICS AND CARDIORENAL INTEGRATION
1Cardiorenal Research Laboratory, Mayo Clinic and Mayo Clinic College of Medicine, Rochester, Minnesota; and 2Department of Cardiology, Helios-Klinikum, Erfurt, Germany
Submitted 9 August 2006 ; accepted in final form 21 October 2006
| ABSTRACT |
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hormone; enzymatic degradation product; cardiorenal regulation; BNP 332
Mature BNP 132 is cleaved from the 108 amino acid prohormone BNP, presumably by the serine protease corin (24). Subsequently, BNP 132 either binds to the NPR-A or the natriuretic peptide clearance receptor or is enzymatically degraded. While ANP is a substrate for degradation by neutral endopeptidase 24.11, BNP appears to be relatively resistant (4, 9, 14). Most recently, Brandt et al. (4) reported that the ubiquitous aminopeptidase dipeptidyl peptidase IV (DPP4; CD26; EC 3.4.14.5 [EC] ) cleaves BNP 132 to produce BNP 332 (Fig. 1). Importantly, the specificity constant of BNP 132 was comparable to those reported for the DPP4 substrates glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide, for which cleavage by DPP4 in vivo has been reported (4, 10). Truncation of BNP 132 to BNP 332 is also consistent with the presence of BNP 332 in human plasma reported by Shimizu et al. (20) who had sought to define additional molecular forms of the BNP system in human heart failure. However, what remains unclear is the biological activity of BNP 332. Specifically, it is undefined in vivo if removal of two NH2-terminal amino acids from BNP 132 enhances, attenuates, or has no effect on biological activity in the overall control of cardiorenal function. In addition to being of physiological importance, this question has now gained important clinical significance as DPP4 activity has been reported to be increased in cardiovascular disease states and DPP4 inhibitors are in clinical development for the treatment of diabetes mellitus, as several peptides involved in glucose homeostasis are also substrates of DPP4 as mentioned above (7, 10, 11, 15).
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| MATERIALS AND METHODS |
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Dogs were maintained on a sodium-controlled diet (Hills I/d diet; Hills Pet Nutrition, Topeka, KS). On the evening before the acute experiment, they were fasted and given access to water ad libitum. On the day of the acute study, animals were anesthetized with pentobarbital and fentanyl, intubated, and mechanically ventilated with 5 l/min supplemental oxygen. A flow-directed balloon-tipped thermodilution catheter was inserted via the right external jugular vein for hemodynamic measurements. The femoral vein was cannulated for continuous infusions, and the femoral artery was cannulated for mean arterial pressure measurements and blood sampling. Pressures were recorded and analyzed digitally (Sonometrics, London, ON, Canada). Via a left lateral flank incision the ureter was cannulated for urine sampling, and the renal artery was equipped with a flow probe (Carolina Medical Electronics, King, NC). Cardiac output was measured by thermodilution (cardiac output model 9510-A computer; American Edwards Laboratories, Irvine, CA).
The study protocol started with the administration of a weight-adjusted inulin bolus. Continuous inulin and saline infusions at a rate of 1 ml/min each were started. After 60 min of equilibration, a baseline clearance was done. All clearances lasted 30 min and consisted of urine collection, blood sampling, and hemodynamic measurements. After the baseline clearance, the saline infusion was replaced with an infusion of synthetic human BNP 132 (Phoenix Peptide, Belmont, CA; diluted in saline) at a concentration of 30 ng·kg1·min1 (infusion rate 1 ml/min). After a lead-in period of 15 min, a 30-min clearance was done. Thereafter, BNP 132 was replaced with a saline infusion (1 ml/min), and after a washout period of 60 min, a postinfusion clearance was done. Thereafter, the saline infusion was replaced with an infusion of synthetic human BNP 332 (synthesized by the Peptide Synthesis Facility, Mayo Clinic Rochester, MN; diluted in saline) on an equimolar basis compared with the BNP 132 infusion (i.e., 28.39 ng·kg1·min1, infusion rate 1 ml/min). After a 15-min lead-in period, a 30-min clearance was done. In half of the studies, the sequence of BNP 132 and BNP 332 infusions was reversed to compensate for a possible carryover effect. Volume loss was replaced with saline.
Analysis of electrolytes and neurohormones. Electrolytes were measured by flame photometry (model IL943; Instrumentation Laboratory, Lexington, MA). Inulin was measured with the anthrone method (6). Glomerular filtration rate was assessed by inulin clearance. Plasma renin activity, angiotensin II, and aldosterone were determined by commercially available radioimmunoassays as described previously (12). Immunoreactivity for human BNP was measured with an immunoradiometric assay (Shionogi, Tokyo, Japan). cGMP was measured using a competitive RIA cGMP kit (PerkinElmer, Boston, MA).
Statistical analysis. Values are expressed as mean ± SE. For each peptide, changes from preinfusion levels were analyzed with paired t-test for normally distributed data. Peptides were compared with each other by analyzing the changes from the respective preinfusion clearance to the respective infusion clearance with paired t-test for normally distributed data. BNP values were log transformed before analysis. Wilcoxon signed rank test was used for data not normally distributed, specifically urinary sodium excretion and urinary cGMP excretion. Statistical significance was accepted at P < 0.05. Analyses were performed with GraphPad Prism 3.02 (GraphPad Software, San Diego, CA).
| RESULTS |
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Renal function. Both urine flow (Fig. 2C) and urinary sodium excretion (Fig. 2D) increased significantly compared with preinfusion levels but less so with BNP 332 compared with BNP 132 (P = 0.004 and P = 0.008 between peptides, respectively). The same was true for urinary potassium excretion and urinary cGMP excretion (P = 0.008; Fig. 3A). BNP 132 increased glomerular filtration rate, while BNP 332 tended to do so (P = 0.09) with no difference between peptides.
Humoral function. BNP immunoreactivity was not measured in two studies, one with BNP 132 and one with BNP 332 as the first peptide infusion, because the assay was not available. Immunoreactivity for human BNP was undetectable at baseline in five of the six studies for which data were available. BNP 332 infusion led to a significantly lesser increase in human BNP immunoreactivity than did BNP 132 infusion (33 ± 7% of the respective increase with BNP 132, P = 0.001; Fig. 3B). Similarly, BNP 332 led to a significantly lesser increase in plasma cGMP compared with BNP 132 (31 ± 7% of respective increase with BNP 132, P = 0.001; Fig. 3C). There were no differences between peptides with regard to changes in ANP, plasma renin activity, angiotensin II, aldosterone, sodium, and potassium. BNP 332 was associated with an increase in ANP and decreases in angiotensin II and plasma potassium, while BNP 132 decreased plasma renin activity, angiotensin II, and aldosterone. Hematocrit increased significantly with BNP 132 but remained unchanged with BNP 332, and this tended to be significant between peptides (P = 0.07).
| DISCUSSION |
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BNP 132 is known to have vasodilating, natriuretic, and renin suppressing actions, which were evident in our study. Importantly, BNP 332 had a significantly reduced ability to increase plasma cGMP, urine flow, urinary sodium excretion, and urinary cGMP excretion compared with BNP 132. Unlike BNP 132, it did not decrease mean arterial pressure, systemic or renal vascular resistance, and it did not increase renal blood flow. BNP 332 slightly increased plasma ANP, which was not statistically different from BNP 132, which also showed a trend for increasing ANP compared with preinfusion levels (P = 0.13). An increase in plasma ANP with BNP infusion has been reported earlier (5), and a possible explanation for this increase could be competition at the NPR-A receptor or at clearance mechanisms. Also consistent with previous reports is the increase in hematocrit with BNP 132 (16). This could be due to the diuretic effect of BNP 132 and to an increase in vascular permeability associated with NPR-A activation (18).
Several mechanisms could account for the reduced biological actions of BNP 332 compared with BNP 132. First, it is possible that removal of two amino acids from the NH2 terminus resulted in altered ligand-receptor interactions. This would not be surprising as previous studies with ANP have reported that deletion of amino acids on either the COOH terminus or NH2 terminus can affect ANP bioactivity (19, 23). However, we recently reported that BNP 132 and equimolar BNP 332 stimulated cGMP generation to a similar degree in cultured human cardiac fibroblasts (8), which argues against reduced receptor affinity of BNP 332 as an explanation for the findings in the current study. Another possible mechanism could be that removal of the two NH2-terminal amino acids of BNP 132 results in a molecule that is highly susceptible to further degradation or clearance or both. This is indirectly supported by our observations regarding plasma BNP-immunoreactivity and cGMP during infusion of equimolar concentrations of BNP 332 and BNP 132. Specifically, the increase in BNP immunoreactivity for BNP 332 was only about 33% of that observed for BNP 132; this was paralleled by a cGMP increase with BNP 332 that was only 31% of that induced by BNP 132. Of note, since the Shionogi assay employed in this study uses antibodies directed against epitopes on the ring structure and the COOH terminus of BNP 132, it can be expected, and indeed it has been reported that BNP detection with this assay is not affected by changes that are restricted to the NH2 terminus (17, 20). Thus, the assay should detect BNP 132 and BNP 332 with similar affinity. One could then speculate that enzymatic degradation of BNP 132 is a stepwise process, with every cleavage rendering the remaining peptide subject to one or more different peptidases with appropriate amino acid sequence-specific cleavage sites. As the BNP immunoreactivity during the BNP 332 infusion was only
33% of that seen with BNP 132 with a commensurate reduction in plasma cGMP generation, we speculate that cleavage of the two NH2-terminal amino acids accelerates further enzymatic degradation and thus inactivation.
The current findings have clinical relevance for the pathophysiology and therapeutics of heart failure in which BNP concentrations are high in the plasma and in which BNP 132 has been used as a therapeutic agent. It has been reported that DPP4 activity is increased in plasma of hypertensive patients with increased pulmonary artery pressure (15) and in atrial tissue samples of patients with chronic persistent atrial fibrillation undergoing open heart surgery (11). If DPP4 activity is increased in cardiovascular disease states, then the availability of the active mature BNP 132 may be reduced and its cardiorenal protective actions attenuated as well. From a treatment perspective, DPP4 inhibitors are being developed and have been extensively investigated as a potential treatment in diabetes mellitus, as the incretins glucagon-like-peptide-1 (GLP-1), GLP-2, and glucose-dependent insulinotropic peptide are some of the many substrates of DPP4 (1, 7). Of note, DPP4 inhibition did not cause hypoglycemia in healthy male volunteers (2). Thus, the possible therapeutic use of DPP4 inhibitors in heart failure may be an area warranting further research. If indeed DPP4 inhibitors were able to increase the bioavailability of BNP 132, it would be important to investigate whether chronic endogenous BNP augmentation could delay the progression of heart failure in less-advanced stages and whether a DPP4 inhibitor could increase the efficacy of exogenous BNP 132 in decompensated heart failure. Importantly, as other enzymes as well as the natriuretic peptide C receptor are involved in the clearance of BNP 132, studies with actual DPP4 inhibition will be required to assess the impact on BNP levels. Furthermore, given potentially relevant species differences, the findings of the current study, in which synthetic human BNP 132 and BNP 332 were given to canines, need to be confirmed in humans.
In summary, in this study BNP 332, the product of BNP 132 cleaved by DPP4, has reduced renal actions compared with BNP 132 and lacks vasodilating properties. These findings provide new insights into the integrated cardiorenal physiology of the BNP system with possible therapeutic implications.
| GRANTS |
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| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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