Author + information
- Received January 14, 2000
- Revision received April 23, 2000
- Accepted June 19, 2000
- Published online November 1, 2000.
- Horng H Chen, MB, BCh* (, )
- J.Aaron Grantham, MD,
- John A Schirger, MD,
- Michihisa Jougasaki, MD, PhD,
- Margaret M Redfield, MD and
- John C Burnett Jr., MD
- ↵*Reprint requests and correspondence:
Dr. Horng H. Chen, Cardiorenal Research Laboratory, Guggenheim 915, Mayo Clinic and Foundation, 200 First Street Southwest, Rochester, Minnesota 55905
The objective of this investigation was to define for the first time the cardiorenal and humoral actions of repeated short-term administration of subcutaneous (SQ) brain natriuretic peptide (BNP) administration during the evolution of experimental heart failure.
The rationale of this study was based on BNP as a vasodilating, natriuretic, renin-inhibiting and lusitropic peptide of cardiac origin.
First, we defined the cardiorenal and humoral responses to acute low and high dose (5 μg/kg or 25 μg/kg) of SQ BNP in experimental heart failure to establish the acute efficacy of an SQ delivery. Second, we characterized the response to 10 days of repeated short-term administration of BNP during the evolution of experimental heart failure produced by rapid ventricular pacing.
Plasma BNP and 3′,5′-cyclic guanosine monophosphate rapidly increased and peaked at 30 min after acute SQ BNP administration with increases in urinary sodium excretion, urine flow and renal blood flow in association with reductions in cardiac filling pressures. After 10 days of repeated short-term administration of SQ BNP, cardiac output was increased and systemic vascular resistance and pulmonary capillary wedge pressure were decreased, as compared with untreated dogs with heart failure.
This study demonstrated for the first time that repeated short-term administration of SQ BNP administration for 10 days during the evolution of left ventricular dysfunction in a canine model results in an improvement in cardiovascular hemodynamics. This investigation supports a potential novel strategy for the chronic administration of BNP in the therapeutics of heart failure.
Brain natriuretic peptide (BNP) is a 32-amino acid peptide with natriuretic, renin-inhibiting (1), vasodilating and lusitropic properties (2). Originally isolated from porcine brain, BNP was subsequently found to be synthesized and secreted in greatest abundance in cardiomyocytes (3–5). Brain natriuretic peptide is structurally similar to atrial natriuretic peptide (ANP) but genetically distinct. Both are of myocardial cell origin and play a role in the control of sodium excretion and arterial blood pressure. Specifically, studies have established that BNP, like ANP, binds to the natriuretic peptide receptor which, via 3′,5′-cyclic guanosine monophosphate (cGMP), mediates its biological actions (6,7). Despite these similarities, BNP has emerged as the more biologically potent peptide based upon its greater natriuretic actions, its decreased susceptibility for degradation by neutral endopeptidase 24.11 and its enhanced ability to augment cGMP (8,9). Circulating BNP is also superior to ANP as a diagnostic serum marker for altered ventricular structure and function and for identification of patients with asymptomatic left ventricular dysfunction (10,11).
Recognizing the greater cardiorenal potency of BNP compared with ANP, intravenous BNP infusion in humans with congestive heart failure (CHF) has recently been pursued as a therapeutic approach to acutely decompensated heart failure. These investigations have demonstrated that acute intravenous BNP decreases pulmonary artery pressure (PAP), pulmonary capillary wedge pressure (PCWP), right atrial pressure (RAP) and mean arterial pressure (MAP) and increases cardiac index, urine volume and urine sodium excretion (UNaV) without associated neurohumoral activation (12–14). Acute BNP infusion in normal humans reduces isovolumic relaxation time of the left ventricle and improves transmitral Doppler flow profiles, suggesting that this peptide may enhance left ventricular diastolic relaxation (15). Indeed, such lusitropic properties were recently reported in experimental studies in dogs with and without heart failure (2). While such beneficial cardiorenal and endocrine actions of acute intravenous BNP administration in CHF are now well established, the effects of chronic BNP administration in CHF remain undefined, as does a strategy for more long-term BNP administration.
This study was designed to define the cardiorenal and endocrine actions of repeated short-term administration of BNP administered subcutaneously (SQ) for 10 days during the evolution of experimental heart failure in a canine model. First, we defined the cardiorenal and humoral responses to acute low- and high-dose SQ BNP in experimental heart failure to establish the efficacy of this delivery route. Second, we defined the cardiorenal and humoral responses to repeated short-term administration of SQ BNP for 10 days during the evolution of experimental heart failure based on the findings of the acute studies. We hypothesized that repeated short-term administration of SQ BNP during the evolution of experimental heart failure would result in improved cardiovascular hemodynamics when compared with untreated heart failure.
Studies were conducted in three groups of anesthetized male mongrel dogs (18 to 23 kg) with chronic heart failure produced by rapid ventricular pacing at 180 beats/min for 10 days on a fixed sodium diet (16). The three groups consisted of an acute low dose BNP group (n = 5), an acute high dose BNP group (n = 7) and a chronic BNP group (n = 6). Studies were performed in accordance with the Animal Welfare Act and with approval of the Mayo Clinic Institutional Animal Care and Use Committee.
Model of pacing-induced chronic heart failure
All dogs underwent implantation of a programmable cardiac pacemaker (Medtronic, Minneapolis, Minnesota). Under pentobarbital sodium anesthesia (30 mg/kg, intravenous) and artificial ventilation (Harvard respirator, Harvard Apparatus, Millis, Massachusetts) with 5 L/min supplemental oxygen, a left lateral thoracotomy and pericardiotomy were performed. With the heart exposed, a screw-in epicardial pacemaker lead was implanted into the right ventricle. The pacemaker generator was implanted SQ into the left chest wall and connected to the pacemaker lead. Dogs received pre- and postoperative prophylactic antibiotic treatment with 225 mg clindamycin SQ and 400,000 U procaine penicillin G plus 500 mg dihydrostreptomycin intramuscularly (Combiotic, Pfizer, Inc., New York, New York). Postoperative prophylactic antibiotic was continued through the first 2 postoperative days. Dogs were fed a fixed sodium diet (58 mEq/day, Hill’s ID) and allowed water ad lib. All dogs were walked daily. Appetite, activity, body temperature and condition of surgical skin sites were documented. After a 14 day postoperative recovery period, the pacemaker was turned on at 180 beats/min.
Acute low- and high-dose SQ BNP
On day 11 of rapid ventricular pacing at 180 beats/min, acute low dose SQ BNP (5 μg/kg, n = 5) or acute high-dose SQ BNP (25 μg/kg, n = 7) was administered to two separate groups of dogs with heart failure. On the night before the acute experiment, animals were fasted and given 300 mg of lithium carbonate for assessment of renal tubular function. On the day of the acute experiment, dogs were anesthetized with sodium pentobarbital (15 mg/kg, intravenous), intubated and mechanically ventilated with supplemental oxygen (Harvard respirator, Amersham, Massachusetts) at 20 cycles per minute. A flow-directed balloon-tipped thermodilution catheter (Ohmeda, Criticath, Madison, Wisconsin) was advanced into the pulmonary artery via the external jugular vein for cardiac hemodynamic measurement. The femoral artery was cannulated for blood pressure monitoring and blood sampling. The femoral vein was also cannulated for inulin and normal saline infusion. The left kidney was exposed via a flank incision, and the ureter was cannulated for urine collection. A calibrated electromagnetic flow probe was placed around the renal artery to measure renal blood flow (RBF).
The experiment began after a 60 min equilibration period, with a 30 min baseline urinary clearance. After the 30 min baseline urinary clearance, canine BNP 5 μg/kg (acute low-dose group) or 25 μg/kg (acute high-dose group) dissolved in 1 ml of normal saline was given SQ in the left hind limb. After a 15 min lead-in period, two 30 min clearance periods were carried out followed by four 60 min clearances for a total duration of 5 h after administration of SQ BNP. Cardiovascular parameters measured during the acute experiment included MAP, RAP, PAP, cardiac output (CO) and PCWP. Cardiac output was determined by thermodilution in triplicate and averaged (Cardiac Output model 9510-A computer, American Edwards Laboratories, Irvine, California). Mean arterial pressure was assessed via direct measurement from the femoral arterial catheter. Systemic vascular resistance (SVR) was calculated as (SVR = [MAP − RAP]/CO). Inulin was administered intravenously at the start of the equilibration period as a calculated bolus followed by a 1 mL/min continuous infusion to achieve plasma levels of 40 to 60 mg/dl. Glomerular filtration rate (GFR) was measured by inulin clearance. Renal vascular resistance (RVR) was calculated as (RVR = [MAP − RAP]/RBF).
Cardiovascular hemodynamics was measured at the start of each clearance. Arterial blood was collected in heparin and EDTA tubes and immediately placed on ice midway through each clearance. After centrifugation at 2,500 rpm at 4°C, plasma was decanted and stored at −20°C until analysis. Urine was collected on ice during the entire period of each clearance for assessment of urine volume, electrolytes and inulin. Urine collected for cGMP analysis was heated to more than 90°C before storage.
Chronic SQ BNP
In a third group of dogs, SQ BNP was administered at a dose of 5 μg/kg every 8 h throughout the 10 days of evolving heart failure. This dose was chosen based upon the favorable cardiorenal effects observed in the acute studies (see Results section). On day 11, complete cardiorenal and neurohumoral measurements were assessed 8 h after the previous dose following the same protocol used in the acute studies described in detail above.
Hormonal and electrolyte analysis
After extraction plasma ANP and BNP were measured by radioimmunoassay (RIA) as previously described (1,17). Plasma and urinary samples for cGMP were measured by RIA using the method of Steiner et al. (18). Plasma renin activity was determined by RIA using the methods of Haber et al. (19). Urinary and plasma inulin concentrations were measured by the anthrone method. Urinary and plasma lithium levels were determined by flame emission spectrophotometry (model 357, Instrumentation Laboratory, Wilmington, Massachusetts). Employing the lithium clearance (CLLi) technique, proximal and distal fractional reabsorption of sodium (PFNaR and DFNaR, respectively) were calculated utilizing the following equation: PFNaR=(1−[CLLi/GFR])×100,DFNaR=(CLLi−CLNa)/CLLi×100, where CLLi = (urine Li × urine flow)/plasma Li and CLNa = (urine Na × urine flow)/plasma Na.
Results are expressed as mean ± SEM. Two-way repeated measures analysis of variance was used for all comparisons between two groups and one-way repeated measures analysis of variance for comparisons within each group using GraphPad Prism software. Statistical significance was accepted as p < 0.05.
Acute low- and high-dose SQ BNP
The responses of plasma BNP and cGMP and urinary cGMP excretion (UcGMPV) in the acute low- and high-dose SQ BNP groups are illustrated in Figure 1. Plasma BNP and cGMP increased in response to SQ BNP and peaked at 30 min in both groups. Urinary cGMP excretion also increased in response to SQ BNP but peaked at 60 min in both groups. Plasma BNP, plasma cGMP and UcGMPV returned to baseline by 180 min after SQ administration of BNP in both groups. The increases in plasma BNP, plasma cGMP and UcGMPV were significantly higher in the acute high-dose group as compared with the acute low-dose group. Plasma ANP remained unchanged in both groups with the administration of SQ BNP.
The responses in RAP and PWCP to SQ BNP in both the acute low-dose and the acute high-dose groups are illustrated in Figure 2. Right atrial pressure, PCWP (Fig. 2) and PAP (Table 1)were significantly reduced in both groups after SQ BNP administration with the peak response at 60 min after administration. In the acute low-dose group, RAP, PWCP and PAP remained significantly reduced at 300 min, while these parameters in the acute high-dose group returned to baseline by 180 min. In the acute low-dose group, there were no significant changes in CO, MAP or SVR (Table 1). In contrast, in the acute high-dose group, BNP significantly increased CO at 30 min, which returned to baseline by 120 min, but subsequently declined below baseline at 240 and 300 min (Table 1). In addition, in the acute high-dose group, BNP decreased MAP at 30 min, which returned to baseline by 180 min in association with a decrease in SVR at 30 min, which returned to baseline at 120 min and subsequently increased significantly above baseline at 180 min to 300 min (Table 1). Of note, CO was significantly lower and SVR significantly higher in the high-dose group compared with the low-dose group at 300 min.
The responses of UNaV and DFNaR to SQ BNP in both acute low- and high-dose groups are illustrated in Figure 2. Urine sodium excretion (Fig. 2) and urine flow (Table 1) peaked 30 min after SQ BNP administration and returned to baseline by 180 min in both groups. This was accompanied by a decrease in DFNaR (Fig. 2) and PFNaR (Table 1), which also returned to baseline by 180 min. The magnitude and duration of the natriuresis and diuresis were similar in both low- and high-dose groups. Renal blood flow increased 60 min after SQ BNP in the low-dose group together with a decrease in RVR, which was sustained at 300 min (Table 1). In contrast, in the high-dose group, RBF increased and RVR decreased at 30 min but returned to baseline by 180 min (Table 1). Of note, at 300 min RBF was significantly higher in the low-dose group compared with the high-dose group. There were no significant changes in GFR in either group during the entire protocol (Table 1). Plasma renin activity decreased with administration of SQ BNP in both groups and returned to baseline by 300 min (Table 1); however, this was not statistically significant (p = 0.07).
Chronic SQ BNP
Figure 3 illustrates CO, PCWP and SVR of the chronic SQ BNP group on day 11, 8 h after the previous dose of SQ BNP, as compared with untreated dogs with heart failure.
The parameters for the untreated group with heart failure were derived from the baseline measurements on day 11 of rapid ventricular pacing of the dogs used in the acute studies (n = 12). Chronic SQ BNP administration for 10 days resulted in a higher CO and lower PCWP and SVR as compared with untreated heart failure. There was a trend for RAP to be lower with chronic SQ BNP as compared with untreated heart failure (1.5 ± 0.4 vs. 1.8 ± 0.3 mm Hg, p > 0.05). Plasma BNP levels (71 ± 9 vs. 52 ± 11 pg/ml, p > 0.05) and renin activity (8.0 ± 2.0 vs. 6.5 ± 1.8 ng/ml/h, p > 0.05) were similar between the chronic SQ BNP group as compared with the group with heart failure that was untreated.
The objective of this investigation was to define for the first time the cardiorenal and humoral actions of repeated short-term SQ BNP administration for 10 days during the evolution of pacing-induced left ventricular dysfunction. We first demonstrated that acute low- and high-dose SQ BNP rapidly increased plasma BNP and its second messenger cGMP, resulting in significant natriuresis, renal vasodilatation with decreases in cardiac filling pressures without activation of plasma renin activity. We also observed that the decrease in cardiac filling pressures and renal vasodilatation was more sustained with low-dose as compared with high-dose SQ BNP. Most importantly, this investigation established that repeated short-term SQ BNP administration for 10 days during the evolution of pacing-induced left ventricular dysfunction improved CO and decreased PCWP and SVR in the absence of activation of plasma renin activity when compared with an untreated group with heart failure. This investigation supports the conclusion that repeated short-term SQ BNP administration during the evolution of pacing-induced left ventricular dysfunction has favorable hemodynamic effects.
Acute low- and high-dose SQ BNP administration
Within 30 min of SQ administration of either low- or high-dose SQ BNP, plasma BNP significantly increased. Thus, BNP was rapidly absorbed and resulted in activation of natriuretic peptide receptors, as demonstrated by parallel increases in both plasma and urinary cGMP, markers of tissue and renal biological activity of the natriuretic peptides (20). The increases in plasma BNP and cGMP persisted for 2 h, which is longer than the changes reported with bolus intravenous injection (21). If the duration of receptor stimulation is of importance for the biological actions of BNP, then the SQ route may be more functionally significant as compared with acute intravenous bolus injection in heart failure.
Low-dose acute SQ BNP resulted in sustained reductions in RAP and PCWP and a maintenance of CO, MAP and SVR. This is in contrast with high-dose SQ BNP, which resulted in an initial decrease in cardiac filling pressures, which returned to baseline by the third hour. This shortened response in cardiac filling pressures to high-dose SQ BNP was associated with an initial increase in CO and decrease in MAP and SVR. Cardiac output and SVR subsequently decreased and increased, respectively, when compared with baseline at the fourth hour. In contrast with the acute low-dose, high-dose BNP transiently decreased MAP. We speculate that this peripheral vasodilation may have resulted in activation of counterregulatory mechanisms other than the renin–angiotensin system, such as the sympathetic nervous system and endothelin, resulting in increased SVR and decreased CO, thereby offsetting the initial reductions in cardiac filling pressures. However, the exact mechanism of this biphasic hemodynamic response observed after acute high-dose BNP-induced vasodilation requires further investigation.
Subcutaneous administration of acute low- and high-dose BNP resulted in a rapid natriuresis and diuresis that persisted for 2 h. The natriuresis and diuresis were similar between the two groups despite the greater increase in plasma BNP and cGMP and UcGMPV in the high-dose group. The decrease in arterial pressure with high-dose BNP may account for the lack of greater natriuretic and diuretic actions in the high-dose group because it is known that reductions in renal perfusion pressure attenuate the renal responsiveness to ANP (22). The mechanism of BNP-mediated natriuresis in this study was related to a decrease in tubular reabsorption of sodium. This occurred at both the proximal and distal tubules, the latter a site rich in natriuretic peptide receptors. At the proximal tubule, increases in RBF may have altered peritubular capillary hydrostatic forces favoring a decrease in proximal tubular reabsorption of sodium. It should be noted that there was no associated activation of plasma renin activity with BNP-mediated natriuresis and diuresis, which supports the renin-inhibiting actions of BNP as compared with conventional diuretics that activate the renin–angiotensin system.
Repeated short-term SQ BNP administration
The most important finding of this study was that repeated short-term SQ BNP administration for 10 days during the evolution of heart failure in this canine model resulted in a higher CO and lower PCWP and SVR in the absence of activation of plasma renin activity, as compared with heart failure that was untreated. The dose used in the 10 day chronic study was based upon the finding that acute low-dose SQ BNP resulted in a prompt natriuresis and diuresis equal to that observed with high-dose BNP, together with more sustained reductions in cardiac filling pressures without a decrease in MAP. We interpret the current findings as support for the conclusion that repeated short-term SQ BNP administered during the evolution of experimental heart failure has important beneficial cardiovascular and humoral actions.
This study may have important therapeutic implications. To date, the long-term medical treatment of heart failure has been limited to the use of orally active pharmacological agents. Many of these agents, such as angiotensin-converting enzyme inhibitors, beta-adrenergic blocking agents and endothelin-receptor blockers, oppose the actions of vasoconstricting, growth-promoting and sodium-retaining neurohumoral systems. Attempts have recently been made to promote activation or restoration of endogenous vasodilating, natriuretic and antimitogenic systems, which function via the second messenger, cGMP, such as the nitric oxide system with oral L-arginine (23) and the natriuretic peptide system with oral neutral endopeptidase inhibitors (24). With the beneficial effects of repeated short-term SQ BNP administration, this study supports the conclusion that further investigations utilizing chronic SQ BNP as a therapeutic strategy for the management of heart failure are clearly warranted to establish the safety and effective dose in human heart failure.
This study established that acute SQ administration of BNP results in rapid peptide absorption with beneficial cardiorenal and neurohumoral responses in a canine model of chronic heart failure. Most importantly, the current investigation established that repeated short-term SQ BNP administration during the evolution of heart failure in this model improves CO and decreases PCWP and SVR in the absence of activation of plasma renin activity. This novel route of BNP administration provides a potentially novel therapeutic strategy that allows for the chronic administration of this peptide of cardiac origin in the management of CHF.
The authors gratefully acknowledge the assistance of Denise M. Heublein, Sharon S. Sandberg and Gail Harty.
☆ Supported by grants HL 36634 and HL 07111 from the National Institutes of Health, the Miami Heart Research Institute, the Mayo Foundation, Bruce and Ruth Rappaport Program in Vascular Biology, the National Kidney Foundation of Minnesota, Inc. and the General Mills Clinician Investigator Fellowship awarded to Dr. Horng Chen.
- atrial natriuretic peptide
- brain natriuretic peptide
- 3′,5′-cyclic guanosine monophosphate
- congestive heart failure
- lithium clearance
- cardiac output
- distal fractional reabsorption of sodium
- glomerular filtration rate
- mean arterial pressure
- pulmonary artery pressure
- pulmonary capillary wedge pressure
- proximal fractional reabsorption of sodium
- right atrial pressure
- renal blood flow
- renal vascular resistance
- subcutaneous or subcutaneously
- systematic vascular resistance
- urinary cGMP excretion
- urine sodium excretion
- Received January 14, 2000.
- Revision received April 23, 2000.
- Accepted June 19, 2000.
- American College of Cardiology
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