Serial natriuretic response to atrial peptide in preascitic bile duct ligated dogs

We determined if nine precirrhotic unanaesthetized dogs with chronic bile duct ligation (CBDL) responded uniformly to atrial natriuretic peptide (ANF) by infusing this peptide sequentially over 8-12 weeks at 175 ng.kg-1.min-1 and observing the natriuretic response. ANF was administered every 2 weeks post-CBDL until the 8th week and given again during the cirrhotic phase with ascites present (10-12 weeks post-CBDL). Sodium balance studies were conducted at similar time intervals. During the control period and at weeks, 2, 6, and 8 post-CBDL all dogs responded to ANF with a significant change in sodium excretion (delta UNaV, 50-240 mu equiv./min). At these times, all dogs were in sodium balance. At week 4 and during the ascitic period, heterogeneity of response to ANF was observed. In the former interval, five dogs responded (delta UNaV,75-230 mu equiv./min) and four did not, while in the latter interval, five dogs responded (delta UNaV, 50-240 mu equiv./min) and three did not (one dog died). In both time periods, there was severe urinary sodium retention (daily UNaV, 11 +/- 3 and 2 +/- 1 mequiv./day, respectively) while the dogs were ingesting 45 mequiv.Na+/day. The heterogeneity of natriuretic response was not correlated to plasma immunoreactive ANF, renin, or aldosterone levels. Plasma volume was significantly expanded from control during both intervals. We conclude that there is transient sodium retention during the 4th week post-CBDL, and that this period is associated with the heterogeneity of natriuretic response to ANF, despite the absence of ascites or edema.     

Role of atrial natriuretic peptide and urinary cGMP in the natriuretic and diuretic response to central hypervolemia in normal human subjects

The response of plasma atrial natriuretic peptide (ANP) and urinary cGMP excretion to central hypervolemia induced by water immersion was assessed twice in five healthy male subjects, once while immersed in water to the neck for 3 h and again on a control day. Plasma ANP and urinary cGMP were measured by radioimmunoassay. Compared with the control day, overall change in plasma ANP on the immersion day was significant (p less than 0.05). In response to water immersion, plasma ANP increased from a base-line level of 13.2 +/- 3.1 (mean +/- SEM) to 24.2 +/- 5.5 pg/mL by 0.5 h of immersion and was sustained at that level throughout the immersion period. Plasma ANP returned to the base-line level at 1 h postimmersion. Urinary cGMP excretion increased significantly by 1 h of immersion and was sustained at that level throughout water immersion and 1 h postimmersion (p less than 0.05). During water immersion urine flow, urinary sodium and potassium excretion, free water clearance, and osmolar clearance increased while plasma renin activity, serum aldosterone, and blood pressure fell; all changes were significant (p less than 0.05). Creatinine clearance and hematocrit did not show any significant changes. These data suggest that an increase in plasma ANP may contribute to the natriuretic and diuretic response to central hypervolemia, and that the measurement of urinary cGMP may be a valuable marker of ANP biological responsiveness.     

Increase in atrial natriuretic peptide in response to physical exercise

Circulating atrial natriuretic peptide (ANP) level was determined during physical exercise to investigate the correlation between changes in ANP level and heart rate increases. Six subjects exercised at a work level of 75% VO2max for 30 min, two also performed two successive exercises at 75% VO2max while two more exercised for longer at 55% VO2max. Plasma ANP levels and heart rate increased in all the exercising subjects. At the end of the exercise, the ANP level fell immediately, suggesting an immediate reduction in ANP secretion by the heart. Pre-exercise values were reached after 30 min. Successive exercises gave the same heart rate related ANP patterns without previous secretory episodes having any effect. These results lead to the conclusion that ANP intervenes in the cardiovascular adjustments to exercise.     

Modification of the renal response to endopeptidase inhibition and atrial natriuretic peptide infusion in normal dogs.

Inhibition of intrarenal neutral endopeptidase 24:11 (NEP) increases the natriuretic response to infused atrial natriuretic peptide (ANP). In various models of canine heart failure, angiotensin and kinins have been shown to modulate ANP and (or) NEP activity. In the present study, we examined possible modulators of NEP activity in normal dogs by infusing various agents into the left renal artery (or by denervating the left kidney) and comparing the response of this kidney with that of the contralateral one following the combined intravenous infusion of Squibb 28603 (a potent NEP inhibitor) and ANP (75 ng.kg-1.min-1). Four dogs received angiotensin (1.5 ng.kg-1.min-1) into the left renal artery, 8 dogs received saralasin (5 micrograms/min), 5 dogs received noradrenaline (2 micrograms/min), and 6 dogs received bradykinin (3 micrograms/min). Five dogs underwent left renal denervation. Angiotensin inhibited sodium excretion following the NEP inhibitor alone and after the NEP inhibitor plus ANP. Saralasin augmented the natriuretic response. None of the other protocols influenced sodium excretion. We conclude that angiotensin may modulate either the enzymatic degradation of ANP or influence its renal tubular effects.     

Age differences in renal response to atrial natriuretic peptide in rabbits

Plasma levels of atrial natriuretic peptide (ANP) and the effect of exogenous ANP on renal function have been studied in newborn and adult rabbits. In order to investigate an age difference in responsiveness to ANP, we studied the renal effects of alpha-human ANP (1-28) administered at the same dose per kg body weight in adult and neonatal rabbits. Plasma basal ANP levels were similar in 18 newborn (4- to 11-day-old) compared to 7 adult rabbits (150 +/- 16 and 151 +/- 28 pg/ml, resp.). Eleven newborn and 11 adult rabbits were anesthetized and mechanically ventilated. After a control period, each animal received an hANP loading dose (3 micrograms/kg i.v.), followed by an infusion of 0.3 micrograms/kg/min. Blood gases remained stable throughout the experiment in both groups. Mean blood pressure decreased in newborn (28.5 +/- 0.8 to 26.2 +/- 1.0 mmHg) and adult (92 +/- 3 to 84 +/- 3 mmHg) animals. Percent hANP-induced changes in renal functions in newborn and adult rabbits were, respectively: urine flow rate: -21 +/- 4% and +57 +/- 8%; urinary sodium excretion: +4 +/- 7% and +81 +/- 11%; glomerular filtration rate (GFR): -19 +/- 4% and -4 +/- 6%; renal blood flow (RBF): -22 +/- 4% and -11 +/- 5%. As expected, diuresis and natriuresis increased in adult rabbits. Failure of hANP to increase natriuresis and diuresis in newborn rabbits could be related to the marked decrease in GFR, receptor immaturity and/or interactions with other hormonal systems.     

Plasma atrial natriuretic peptide levels in children with cardiac diseases: correlation with cGMP levels and haemodynamic parameters

In children with various forms of cardiac diseases (aged 2 months to 16 years) significantly higher plasma atrial natriuretic peptide (ANP; range 36-680, median 247 pg/ml) and cyclic 3'5'-guanosine monophosphate (cGMP; range 0.2-46, median 8.2 pmol/ml) levels were found than in control children (p less than 0.0001). In control children (aged 4 months to 17 years) plasma ANP and cGMP levels were measured in the range of 2.4-98 pg/ml and of 0.2-2.8 pmol/ml, respectively. There was a linear correlation between the two parameters in children with cardiac diseases (r = 0.62, p less than 0.01). Children with elevated mean right atrial pressure (i.e., greater than 6 mm Hg) showed significantly higher plasma ANP levels than children with normal atrial pressure (p less than 0.01). However, there was only a weak linear correlation between mean right atrial pressure and plasma ANP levels (r = 0.48, p less than 0.01). Plasma ANP levels from right atrium, pulmonary artery, left atrium and left ventricle were significantly higher than those from vena cava (p less than 0.05). Analysis of ANP-like immunoreactive material by high performance liquid chromatography suggested that alpha-ANP is the major form of circulating ANP in blood of children with cardiac diseases.     

Brain natriuretic peptide is cosecreted with atrial natriuretic peptide from porcine cardiocytes

Using primary cultures of atrial cardiocytes from neonatal pig, the secretion brain natriuretic peptide (BNP) and atrial natriuretic peptide (ANP)-like immunoreactivities (LI) was studied in vitro. Porcine cardiocytes time-dependently secreted both BNP-LI and ANP-LI into medium under a serum-free condition, although the amount of BNP-LI secreted was about one-third that of ANP-LI. Phorbol ester and Ca2+ ionophore had less stimulatory effects on secretion of BNP-LI than that of ANP-LI. Reverse-phase HPLC of the conditioned medium revealed a single major BNP-LI component corresponding to synthetic porcine BNP(1-26). These data suggest that a small molecular weight form BNP, possibly BNP(1-26), is cosecreted with ANP from porcine cardiocytes.     

Placental barrier to atrial natriuretic peptide in rats

Transplacental passage of 125I-labelled synthetic rat atrial natriuretic peptide (ANP) was investigated in 20-day pregnant rats under pentobarbitone anesthesia. Although significant quantities of radioactivity were detected in the fetal plasma after maternal injections and in the maternal plasma after fetal injections of 125I-labelled synthetic ANP, no fraction of the placentally transferred radioactivity was due to intact ANP. Despite a rapid maternal and fetal metabolism of ANP, both maternal and fetal plasma radioactivity remained relatively stable for at least 3 h and less than 10% of the injected radioactivity was excreted in the maternal urine during a 90-min period. It is concluded that ANP is not transported in either direction across the placenta in rats.     

Linear alpha-human atrial natriuretic peptide analogs display receptor binding activity and inhibit alpha-hANP-induced cGMP accumulation

We have synthesized a series of [Cys(R)7,23]alpha-hANP analogs, in which the two Cys residues were modified with various alkyl groups(R); i.e., R=Acm, Pe, Qe, Cam, Me, Ae, Bzl, Cm, Ocam and sulfo. The Acm-, Cam-, and Me-analogs exhibited binding activity as potent as alpha-hANP in rat vascular smooth muscle cells (VSMC). Binding activity of the analogs decreased progressively as the bulkiness of the R group increased. None of the analogs caused accumulation of cGMP in VSMC and vasorelaxant activity in rat aorta. Acm-, Cam- and Me-analogs substantially antagonized alpha-hANP-induced cGMP accumulation, but did not antagonize vasorelaxation induced by alpha-hANP in vitro.     

Evaluation of atrial natriuretic peptide and brain natriuretic peptide in atrial granules of rats with experimental congestive heart failure.

The natriuretic peptides are believed to play an important role in the pathophysiology of congestive heart failure (CHF). We utilized a quantitative cytomorphometric method, using double immunocytochemical labeling, to assess the characteristics of atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) in atrial granules in an experimental model of rats with CHF induced by aortocaval fistula. Rats with CHF were further divided into decompensated (sodium-retaining) and compensated (sodium-excreting) subgroups and compared with a sham-operated control group. A total of 947 granules in myocytes in the right atrium were analyzed, using electron microscopy and a computerized analysis system. Decompensated CHF was associated with alterations in the modal nature of granule content packing, as depicted by moving bin analysis, and in the granule density of both peptides. In control rats, the mean density of gold particles attached to both peptides was 347.0 +/- 103.6 and 306.3 +/- 89.9 gold particles/microm2 for ANP and BNP, respectively. Similar mean density was revealed in the compensated rats (390.6 +/- 81.0 and 351.3 +/- 62.1 gold particles/microm2 for ANP and BNP, respectively). However, in rats with decompensated CHF, a significant decrease in the mean density of gold particles was observed (141.6 +/- 67.3 and 158.0 +/- 71.2 gold particles/microm2 for ANP and BNP, respectively; p<0.05 compared with compensated rats, for both ANP and BNP). The ANP:BNP ratio did not differ between groups. These findings indicate that the development of decompensated CHF in rats with aortocaval fistula is associated with a marked decrease in the density of both peptides in atrial granules, as well as in alterations in the quantal nature of granule formation. The data further suggest that both peptides, ANP and BNP, may be regulated in the atrium by a common secretory mechanism in CHF.     

A possible role of atrial natriuretic peptide in ethanol-induced acute diuresis.

The acute effects of ethanol on plasma atrial natriuretic peptide levels were investigated in 4 clinically healthy males, aged 24-26 years, consumed either 750 ml of water as a control study, or the same beverage with 1 ml/kg alcohol added, which increased the plasma alcohol concentration to 99.12 +/- 15.10 mg/dl at 60 min. Plasma atrial natriuretic peptide levels were significantly higher in the alcohol study compared to the control study at each time point (10, 20, 30, 60, 120 min after drinking onset), and with a peak at 10 min. Atrial natriuretic peptide levels showed a positive significant correlation with plasma antidiuretic hormone in the control group, while no relationship was found between the two peptides in the alcohol study. Moreover, a significant correlation exists between plasma atrial natriuretic peptide levels and systolic arterial blood pressure, and heart rate, and between the variations in atrial natriuretic peptide values and the variations in plasma sodium, serum ethanol, and plasma osmolality in the alcohol study. Acute ethanol intake causes an increase in urinary volume, and a decrease in urinary potassium excretion and urinary osmolality, and no change in urinary sodium excretion. These data suggest that acute ethanol administration causes a rapid increase in plasma levels of atrial natriuretic peptide, which could be an important factor of ethanol-induced diuresis. The main mechanisms for increased atrial natriuretic peptide release from atria after acute ethanol ingestion seem to be atrial stretch, due to the increase in arterial blood pressure, in heart rate, in sympathetic tone, and in plasma osmolality, and to a direct secretory effect by antidiuretic hormone.     

Cardiovascular effects of intravenous and intracoronary administration of atrial natriuretic peptide in halothane anesthetized dogs

Cardiovascular actions of synthetic 1-28 human natriuretic peptides (hANP) were examined in dogs anesthetized with halothane. In seven closed-chest dogs a Swan-Ganz catheter was inserted for measurement of cardiac output. Intravenous infusion of increasing doses of hANP (0.1, 0.3, 0.9 microgram/kg/min) lowered mean aortic pressure without affecting heart rate significantly. Cardiac output and pulmonary wedge pressure were markedly decreased while total peripheral resistance was increased significantly. All these parameters returned to control levels after 1 hr of recovery with an 100-150ml of saline infusion to increase pulmonary capillary wedge pressure to the preinfusion value. Intracoronary infusion of hANP (0.05 and 0.1 microgram/kg/min) did not cause any significant changes in coronary flow and regional contraction. These results indicate that the hypotensive action of hANP is due to a decrease in cardiac output mediated by reduced preload but not by negative inotropic action.     

Response of cardiac mast cells to atrial natriuretic peptide

Previously, our laboratory demonstrated that cardiac mast cell degranulation induces adverse ventricular remodeling in response to chronic volume overload. The purpose of this study was to investigate whether atrial natriuretic peptide (ANP), which is known to be elevated in chronic volume overload, causes cardiac mast cell degranulation. Relative to control, ANP induced significant histamine release from peritoneal mast cells, whereas isolated cardiac mast cells were not responsive. Infusion of ANP (225 pg/ml) into blood-perfused isolated rat hearts produced minimal activation of cardiac mast cells, similar to that seen in the control group. ANP also did not increase matrix metalloproteinase-2 activity, reduce collagen volume fraction, or alter diastolic or systolic cardiac function compared with saline-treated controls. In a subsequent study to evaluate the effects of natriuretic peptide receptor antagonism on volume overload-induced ventricular remodeling, anantin was administered to rats with an aortocaval fistula. Comparable increases of myocardial MMP-2 activity in treated and untreated rats with an aortocaval fistula were associated with equivalent decreases in ventricular collagen (P < 0.05 vs. sham-operated controls). Cardiac functional parameters and left ventricular hypertrophy were unaffected by anantin. We conclude that ANP is not a cardiac mast cell secretagogue and is not responsible for the cardiac mast cell-mediated adverse ventricular remodeling in response to volume overload.     

Brain natriuretic peptide interacts with atrial natriuretic peptide receptor in cultured rat vascular smooth muscle cells

The effect of synthetic porcine brain natriuretic peptide (pBNP), a novel brain peptide with sequence homology to alpha-human atrial natriuretic peptide (hANP), on receptor binding and cGMP generation, was studied in cultured rat vascular smooth muscle cells (VSMC) and compared with that of alpha-hANP. 125I-pBNP bound to the cells in a time-dependent manner similar to that of 125I-alpha-hANP. Scatchard analysis indicated a single class of binding sites for pBNP with affinity and capacity identical to those of alpha-hANP. pBNP and alpha-hANP were almost equipotent in inhibiting the binding of either radioligand and stimulating intracellular cGMP generation. These data indicate that BNP and ANP interact with the same receptor sites to activate guanylate cyclase in rat VSMC.     

Urinary C-type natriuretic peptide excretion: a potential novel biomarker for renal fibrosis during aging

Renal aging is characterized by structural changes in the kidney including fibrosis, which contributes to the increased risk of kidney and cardiac failure in the elderly. Studies involving healthy kidney donors demonstrated subclinical age-related nephropathy on renal biopsy that was not detected by standard diagnostic tests. Thus there is a high-priority need for novel noninvasive biomarkers to detect the presence of preclinical age-associated renal structural and functional changes. C-type natriuretic peptide (CNP) possesses renoprotective properties and is present in the kidney; however, its modulation during aging remains undefined. We assessed circulating and urinary CNP in a Fischer rat model of experimental aging and also determined renal structural and functional adaptations to the aging process. Histological and electron microscopic analysis demonstrated significant renal fibrosis, glomerular basement membrane thickening, and mesangial matrix expansion with aging. While plasma CNP levels progressively declined with aging, urinary CNP excretion increased, along with the ratio of urinary to plasma CNP, which preceded significant elevations in proteinuria and blood pressure. Also, CNP immunoreactivity was increased in the distal and proximal tubules in both the aging rat and aging human kidneys. Our findings provide evidence that urinary CNP and its ratio to plasma CNP may represent a novel biomarker for early age-mediated renal structural alterations, particularly fibrosis. Thus urinary CNP could potentially aid in identifying subjects with preclinical structural changes before the onset of symptoms and disease, allowing for the initiation of strategies designed to prevent the progression of chronic kidney disease particularly in the aging population.     

Attenuated diuretic and natriuretic effects of atrial natriuretic peptide in rats with heart failure

Plasma levels of atrial natriuretic peptide (ANP) and renal responses to ANP were examined in rats with chronic cardiac failure produced by coronary artery ligation and in sham-operated controls. Plasma ANP levels were elevated in the rats with severe cardiac failure as compared with the controls (P less than 0.001). ANP injections at the doses of 1, 5, 25 and 50 micrograms/kg increased water and sodium excretion significantly at all but the lowest dose in the controls; only the two largest doses caused clear diuresis and natriuresis in the heart failure group. The diuretic and natriuretic effects of ANP were significantly weaker at the doses of 5 and 25 micrograms/kg in the rats with heart failure as compared with the controls. We conclude, that natriuretic and diuretic effects of ANP are attenuated in this chronic heart failure mode.     

Effects of atrial natriuretic peptide in the gut

The intestinal tract is a target organ for atrial natriuretic peptide (ANP), characterized by various biologic activities, immunoreactivity, as well as specific binding sites for ANP. A review of previous studies reveals that ANP is an important regulator of water and nutrient intake, which acts via multiple signaling pathways including activation of guanylyl cyclase to produce its biologic responses. As a regulator, the peptide locally controls hydrosaline balance and acute systemic effects. Therefore, ANP could also act as a local mediator or paracrine effector of intestinal function.