Atrial natriuretic factor plays a significant role in body fluid homeostasis

Genetic Ablation and Guanylyl Cyclase/Natriuretic Peptide Receptor-A: Impact on the Pathophysiology of Cardiovascular Dysfunction

Mice bearing targeted gene mutations that affect the functions of natriuretic peptides (NPs) and natriuretic peptide receptors (NPRs) have contributed important information on the pathogenesis of hypertension, kidney disease, and cardiovascular dysfunction. Studies of mice having both complete gene disruption and tissue-specific gene ablation have contributed to our understanding of hypertension and cardiovascular disorders. These phenomena are consistent with an oligogenic inheritance in which interactions among a few alleles may account for genetic susceptibility to hypertension, renal insufficiency, and congestive heart failure. In addition to gene knockouts conferring increased risks of hypertension, kidney disorders, and cardiovascular dysfunction, studies of gene duplications have identified mutations that protect against high blood pressure and cardiovascular events, thus generating the notion that certain alleles can confer resistance to hypertension and heart disease. This review focuses on the intriguing phenotypes of Npr1 gene disruption and gene duplication in mice, with emphasis on hypertension and cardiovascular events using mouse models carrying Npr1 gene knockout and/or gene duplication. It also describes how Npr1 gene targeting in mice has contributed to our knowledge of the roles of NPs and NPRs in dose-dependently regulating hypertension and cardiovascular events.

Implications of the natriuretic peptide system in the pathogenesis of heart failure: diagnostic and therapeutic importance

The natriuretic peptide family consists of at least 3 structurally similar peptides: atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP). Under normal conditions, ANP is synthesized by the atrium and released in response to atrial stretch. This peptide plays an important role in sodium and water homeostasis and is involved in cardiovascular function. In contrast, BNP is synthesized primarily by the ventricles, and its circulatory concentrations are significantly elevated in profound congestive heart failure (CHF). While both plasma levels of ANP and BNP have been found to be increased in patients with various heart diseases, the elevation in circulatory BNP correlates better than ANP with the severity of CHF. Therefore, plasma BNP has been suggested (and lately used) to aid in the accurate diagnosis of heart failure in patients admitted to the emergency room with symptoms of decompensated heart failure. Furthermore, circulatory BNP has been utilized as a prognostic marker in CHF as well as a hormone guide in the evaluation of the efficacy of the conventional treatment of this disease state. In light of the cardiovascular and renal effects of BNP, which most likely exceed those of ANP, the former has been used as a therapeutic agent for the treatment of patients with acute severe CHF. Intravenous infusion of BNP into patients with sustained ventricular dysfunction causes a balanced arterial and venous vasodilatation that has been shown to result in rapid reduction in ventricular filling pressure and reversal of heart failure symptoms, such as dyspnea and acute hemodynamic abnormalities. Thus, the goal of this article is to review the physiology and pathophysiology of natriuretic peptides and the potential use of their circulating levels for diagnosis and treatment of heart failure.

Natriuretic peptides and salt sensitivity: endocrine cardiorenal integration in heart failure

Mammalian hearts contain a family of peptides with potent natriuretic, diuretic, and vasorelaxant actions. In addition to atrial natruretic peptide (ANP) and brain natriuretic peptide, recent studies in humans and animals have suggested that the N-terminal ANP prohormone fragment 31-67 may represent another adaptive mechanism to achieve body fluid homeostasis. Furthermore, these investigations have also suggested that via different mechanisms of action on target organisms, the C-terminal hormone ANP 99-126 and pro-ANP 31-67 may coordinate and contribute to the regulation of hemodynamic and renal function in pathophysiologic situations, such as heart failure.

Practical implications of current natriuretic peptide research

Since the original discovery of atrial natriuretic peptide (ANP) nearly 20 years ago and the subsequent realisation of the existence of a family of natriuretic peptides, there has been considerable progress in the elucidation of the physiological and pathophysiological significance of these peptides. This review has examined two potentially important practical aspects arising from natriuretic peptide research - the significance of measurement of plasma levels of ANP and of brain natriuretic peptide BNP for cardiovascular disease and the therapeutic potential of targeting the natriuretic peptide system. Several situations where the measurement of plasma ANP and BNP may be of benefit in the overall assessment and prognosis of cardiac disease have been discussed. The measurement of plasma levels of these peptides appears to have limited value as a specific diagnostic tool and is unlikely to replace well-established procedures to assess cardiac function. Nevertheless, given the strong negative predictive value, the value of the measurement of plasma natriuretic peptides particularly BNPs, in people with suspected heart disease, rests on the evidence that a normal value indicates a low risk of cardiac impairment. Moreover, a consistently elevated plasma level of BNP after myocardial infarction is associated with a distinctly poor prognosis. In turn, this may help to select those with high plasma levels for subsequent detailed investigation of cardiac dysfunction. This may be an important option, especially where the facilities for the more invasive cardiological procedures are not available. Intriguingly, recent research also suggests the possibility that plasma levels of natriuretic peptides may have an important role in guiding more effective therapy for heart failure. The potent cardiovascular and renal effects of ANP and BNP provide an important therapeutic potential for hypertension and for conditions associated with volume overload. A number of approaches which have been used to enhance endogenous activity of these peptides have been highlighted. The use of the native peptides ANP and BNP may well be valuable in some circumstances, such as in critically ill individuals with congestive heart failure or renal failure. However, the limitations of the use of peptides, especially for long-term treatment, are obvious. In view of this, considerable effort has been devoted to the development of orally active agents to enhance endogenous natriuretic peptides by inhibition of breakdown by neutral endopeptidase. This research has led to the development of vasopeptidase inhibitors - dual inhibitors of both endopeptidase and angiotensin-converting enzyme - to enhance endogenous natriuretic peptide function on a background of reduced angiotensin II activity. The broad spectrum of action and the potentially important target-organ protection of these inhibitors offer potential benefits which may well go beyond existing treatment of hypertension and of conditions associated with overt volume overload.

Renal function in high-output heart failure in rats: role of endogenous natriuretic peptides

The physiologic and pathophysiologic importance of natriuretic peptides (NP) has been imperfectly defined. The diminished renal responses to exogenous atrial NP in heart failure have led to the perception that the endogenous NP system might be less effective and thus contribute to renal sodium retention in heart failure. This study tests the hypothesis that in experimental heart failure, the renal responses to an acute volume load are still dependent on the NP system. The specific antagonist HS-142-1 was used to block the effects of NP in a model of high-output heart failure induced by an aortocaval shunt. Plasma cGMP levels and renal cGMP excretion were significantly lower in shunted and sham-operated rats receiving HS-142-1, compared with vehicle-treated controls, indicating effective blockade of guanylate cyclase-coupled receptors. Baseline sodium excretion and urine flow rate were lower in HS-142-1-treated sham-operated rats (15.2+/-1.1 microl/min versus 27.5+/-3.1 microl/min with vehicle, P < 0.001) and in HS-142-1-treated shunted rats (8.1+/-1.3 microl/min versus 19.9+/-2.3 microl/min with vehicle, P < 0.001). After an acute volume load, the diuretic and natriuretic responses were attenuated by HS-142-1 in control and shunted rats. The renal responses were reduced by HS-142-1 to a significantly greater extent in shunted rats than in control rats. HS-142-1 did not induce any significant systemic hemodynamic changes in either group, nor did it alter renal blood flow. However, the GFR in HS-142-1-treated shunted rats was lower than that in vehicle-treated shunted rats, both at baseline (0.6+/-0.3 ml/min versus 2.1+/-0.4 ml/min with vehicle, P < 0.05) and after an acute volume load (1.2+/-0.4 ml/min versus 2.6+/-0.4 ml/min with vehicle, P = 0.01), whereas no such effect was observed in control rats. These data indicate that the maintenance of basal renal function and the responses to acute volume loading are dependent on the NP system. The NP seem to be of particular importance for the maintenance of GFR in this model of experimental heart failure. These observations provide new insights into the importance of the renal NP system in heart failure.

Alterations in brain levels of atrial and C-type natriuretic peptides after chronic moderate ethanol consumption in spontaneously hypertensive rats

Atrial (ANP) and C-type (CNP) natriuretic peptides have been found in brain regions associated with fluid homeostasis and blood pressure. Since chronic moderate ethanol consumption has been shown to prevent the age-dependent increase in blood pressure in experimental animals, the objective of the present studies was to investigate the effect of ethanol (20% (v/v) for 8 months) on the total content and concentration of ANP and CNP in the brain of spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats. Ethanol increased the content and concentration of both ANP and CNP in the hypothalamus, pons and medulla of SHR rats. In contrast, in the WKY rats ethanol had no effect on the levels of ANP in any of the brain regions studies, but enhanced the concentration of CNP in the hypothalamus and medulla. Thus, ethanol induced changes in the content of natriuretic peptides in distinct brain regions associated with control of cardiovascular activity. Such changes may be partially responsible for the effect of chronic moderate ethanol consumption on blood pressure.

Effect of chronic ethanol consumption on the atrial natriuretic system of spontaneously hypertensive rats

There is a lot of discussion on the effects of ethanol (ETOH) on blood pressure (BP). It has been suggested that chronic moderate ETOH consumption prevents the development of age-dependent hypertension in humans and spontaneously hypertensive rats (SHR). However, the mechanism mediating this effect is unknown. In the present studies, we hypothesized the implication of atrial natriuretic peptide (ANP), a BP-lowering hormone, on the antihypertensive effect of moderate ETOH consumption. A 20% v/v solution of alcohol was given as drinking fluid to SHR and normotensive Wistar-Kyoto (WKY) rats for up to 32 weeks. This treatment prevented, at least in part, the age-dependent increase of BP in SHR and WKY rats. The lower BP was associated with significantly lower levels of circulating atrial natriuretic peptide in both groups. After chronic ETOH administration, total ANP content and concentration were higher in the left and right atria of SHR and WKY rats than in water-treated controls. Despite the ETOH-induced increase in atrial ANP content, there was no significant change in atrial ANP mRNA, suggesting decreased atrial release. Chronic ETOH treatment significantly reduced ANP mRNA in the ventricles of SHR but not of WKY rats. Correspondingly, ventricular ANP content and concentration were lowered by ETOH in SHR only. Chronic ETOH administration induced a significant increase of plasma arginine vasopressin and a significant decrease of plasma aldosterone in SHR but not in WKY rats. Thus, chronic ETOH treatment prevented the age-dependent elevation of BP in both SHR and WKY rats and altered the activity of heart ANP as well as of the aldosterone and plasma arginine vasopressin systems.

Myocardial circulatory and metabolic effects of atrial natriuretic peptide after coronary artery bypass grafting

The purpose of this study was to examine the effects of incremental infusion rates of human atrial natriuretic peptide (ANP), 25, 50, 100 ng.kg-1. min-1, on myocardial blood flow and metabolism (n = 10), and to compare the effects of ANP on these variables with those of equipotent infusion rates of sodium nitroprusside (SNP) (n = 9) 1-3 h after coronary artery bypass grafting (CABG). ANP induced a dose-dependent decrease in mean arterial blood pressure and systemic vascular resistance. There were no changes in cardiac index, heart rate, or cardiac filling pressures. ANP caused no changes in myocardial blood flow or its distribution, and caused no changes in myocardial oxygen extraction. Regional myocardial lactate uptake (RMLU) and extraction (RMLE) increased significantly (P < 0.05) at 50 ng.kg-1.min-1 (10.2 +/- 3.8 mumol/min and 8.2% +/- 3.0%, respectively) as compared to control (-1.1 +/- 3.0 mumol/min and -1.3% +/- 3.3%, respectively). RMLE and RMLU were significantly (P < 0.05) higher with ANP (5.7% +/- 2.5% and 6.8 +/- 3.7 mumol/min, respectively) compared to SNP (-1.5% +/- 2.1% and -0.1 +/- 3.7 mumol/min, respectively). We conclude that ANP has no dilatory effects on coronary vascular resistance vessels and thus lacks the potential to maldistribute flow, and that ANP improves myocardial lactate metabolism after CABG.

Genetic decreases in atrial natriuretic peptide and salt-sensitive hypertension

To determine if defects in the atrial natriuretic peptide (ANP) system can cause hypertension, mice were generated with a disruption of the proANP gene. Homozygous mutants had no circulating or atrial ANP, and their blood pressures were elevated by 8 to 23 millimeters of mercury when they were fed standard (0.5 percent sodium chloride) and intermediate (2 percent sodium chloride) salt diets. On standard salt diets, heterozygotes had normal amounts of circulating ANP and normal blood pressures. However, on high (8 percent sodium chloride) salt diets they were hypertensive, with blood pressures elevated by 27 millimeters of mercury. These results demonstrate that genetically reduced production of ANP can lead to salt-sensitive hypertension.

Functional characterization of ribozymes expressed using U1 and T7 vectors for the intracellular cleavage of ANF mRNA

Hammerhead ribozymes targeted to various GUC or GUA sites on rat atrial natriuretic factor (ANF) mRNA were developed. The catalytic activity of ribozymes to four of these sites, synthesized by transcription off synthetic oligodeoxynucleotide duplexes, was studied in detail. In vitro, ribozyme-mediated cleavage was highly Mg(2+)-dependent, and at concentrations approaching those found intracellularly, the rate but not the extent of cleavage was markedly reduced. To test for cellular activity, synthetic genes encoding the ribozymes were cloned between the initiation and termination sequences of the U1snRNA gene or between the T7RNA polymerase promoter and terminator sequences in pSP64. Both constructs had defined initiation and termination sequences to minimize transcript size and for message stability. In vitro the addition of T7 or U1 terminator sequences had variable effects on catalytic activity, presumably due to structural interactions between the ribozyme and the added sequence. The ribozyme-encoding plasmids were cotransfected with an expression plasmid containing a rat ANF cDNA into COS-1 cells using a liposome method, which provided high-level transfection efficiency. Quantitation of ANF mRNA by RNase protection showed marked decreases in ANF transcript levels with both the U1- and the T7-expressed ribozymes directed at three of the four sites on ANF mRNA. With all constructs, target accessibility, determined in vitro, was a more important determinant of intracellular ANF mRNA cleavage than catalytic activity per se. ANF mRNA cleavage was not merely due to an antisense effect, since a mutant construct that was catalytically inactive but could still bind produced less cleavage than the corresponding wild-type ribozyme construct. These findings indicate that both U1 and T7 vector systems provide efficient ribozyme expression for the intracellular cleavage of target mRNA.

Increased plasma atrial natriuretic peptide levels after heart transplant: relation to ventricular expression and severity of rejection

To determine whether elevated plasma atrial natriuretic peptide (ANP) levels observed after cardiac transplant are related to ventricular ANP expression and/or the severity of rejection, 59 ambulatory patients with cardiac transplant underwent hemodynamic evaluation, endomyocardial biopsy, and plasma ANP sampling. Forty-two of the 59 patients had right ventricular (RV) biopsy specimens immunohistochemically stained for the presence of ANP. Plasma ANP levels were elevated (p < 0.0001) in transplant patients (172 +/- 12 pg/ml) compared to normal subjects (36 +/- 4 pg/ml). Sixty-four percent of transplant patients showed stainable RV ANP on endomyocardial biopsy. There was no significant difference in plasma ANP levels between patients with or without RV ANP. The degree of RV staining did not correlate with plasma ANP levels, degree of rejection, mean atrial or systemic pressures, or specific immunosuppressive regimen. Plasma ANP levels were higher in patients with moderate or severe rejection (237 +/- 17 pg/ml) compared to patients with mild or no rejection (163 +/- 12 pg/ml; p 0.03), but there was significant overlap of values. These data suggest that ventricular ANP secretion may account for some of the increase in plasma ANP levels in cardiac transplant patients. However, increased plasma ANP levels in some transplant patients who have no RV ANP and the lack of correlation between the amount of stainable RV ANP and plasma ANP levels suggest that other mechanisms are also likely responsible for plasma ANP elevations in this setting.

Renal effects of natriuretic peptide receptor blockade in cirrhotic rats with ascites

The aim of this study was to assess the effect of HS-142-1, a recently discovered specific antagonist of endogenous natriuretic peptides, on systemic hemodynamics, renal function, and the renin-aldosterone system in rats with cirrhosis and ascites. The study consisted of three protocols, each including 10 conscious control rats and 10 conscious rats with carbon-tetrachloride-induced cirrhosis with ascites. In protocol 1, HS-142-1 administration (by intravenous bolus of 20 mg.kg-1.body weight in all protocols) was not associated with significant changes in mean arterial pressure, heart rate, cardiac output or total peripheral resistance in the two groups of animals. In protocol 2, HS-142-1 induced a significant reduction in glomerular filtration rate (from 4.2 +/- 0.5 to 2.6 +/- 0.3 ml/min, p < 0.025) in control animals. A decrease in renal plasma flow and an increase in renal vascular resistance also occurred, but these changes were not statistically significant. In cirrhotic rats, HS-142-1 resulted in a significant decrease in renal plasma flow (from 10.9 +/- 0.7 to 4.3 +/- 0.6 ml/min, p < 0.001) and a significant increase in renal vascular resistance (from 6.0 +/- 0.6 to 16.3 +/- 2.7 mm Hg.min.ml-1, p < 0.025). Glomerular filtration rate decreased more in cirrhotic rats with ascites than in control rats (from 3.8 +/- 0.3 to 1.3 +/- 0.2 ml/min, p < 0.001). Changes in urine flow rate and urinary sodium excretion rate paralleled those of glomerular filtration rate in both groups of animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood pressure and endocrine responses to changes in dietary sodium intake in cardiac transplant recipients. Implications for the control of sodium balance

BACKGROUND

The role of cardiac extrinsic innervation in the regulation of sodium balance and blood pressure is controversial.

METHODS AND RESULTS

We performed a double-blind study of endocrine and blood pressure responses to 5 days of low- (LS, 10 mmol/d) and 5 days of high- (350 mmol/d) sodium intake in 12 cardiac transplant recipients, 12 matched healthy subjects, and 12 matched subjects with untreated essential hypertension. In transplant recipients on low sodium, supine blood pressure was 137/94 +/- 8/4 (mean +/- SEM) mm Hg and plasma atrial natriuretic peptide (ANP) was 59.3 +/- 6.3 pg/mL; on high sodium, blood pressure was 148/97 +/- 5/3 mmHg (P < .05 for systolic pressure versus LS), and ANP was 94.3 +/- 10.6 pg/mL (P < .01 versus LS), respectively. Plasma ANP for those on each diet was significantly higher in the cardiac transplant recipients than in healthy or hypertensive controls; relative changes in plasma ANP in changing from low- to high-sodium diet were similar in each group. Urinary sodium excretion by the fifth day of each diet was similar in each group. Suppression of plasma renin activity and aldosterone by high-sodium diet was blunted in cardiac transplant recipients compared with healthy subjects (respectively, plasma renin activity: 1.41 +/- 0.30 versus 0.68 +/- 0.21 ng.mL-1 x h-1, P < .05; aldosterone: 391 +/- 35 versus 166 +/- 21 pmol/L, P < .05).

CONCLUSIONS

These results suggest that extensive denervation of the heart does not result in major abnormalities in regulation of large changes in sodium intake and that intact cardiac innervation is not required for plasma ANP responses to altered sodium intake. Blood pressure after cardiac transplantation is sensitive to reduced sodium intake.

Effect of monoclonal anti-ANP antibodies on the acute functional adaptation to unilateral nephrectomy

The role of endogenous atrial natriuretic peptide (ANP) in the immediate response of sodium excretion to unilateral nephrectomy (UNX) was investigated in anesthetized euvolemic rats through measurement of UNX-induced change in plasma ANP concentration and the response of the remaining kidney to UNX following administration of monoclonal anti-ANP antibodies. The circulating ANP levels almost tripled (from 23 +/- 4 to 66 +/- 13 fmol/ml, P < 0.01) within two minutes after UNX, whereas no change resulted from sham intervention. In the control group receiving vehicle injection, UNX resulted in a twofold increase in urinary sodium excretion (from 1.39 +/- 0.25 to 2.88 +/- 0.28 mumol/min, P < 0.01) related to a decrease in the fractional reabsorption of sodium at both proximal and distal sites (estimated from fractional excretion of lithium). Urinary excretion of cyclic guanosine 3'-5'-monophosphate (cGMP) increased as well, but glomerular filtration rate did not change. In addition, UNX was associated with a short-lived (< 20 min) rise in systemic arterial pressure and a transient fall in right atrial pressure. Administration of monoclonal anti-ANP antibodies totally prevented the UNX-associated natriuresis by blunting both proximal and distal tubular reabsorption of sodium, and suppressed the rise in urinary cGMP excretion following UNX. The duration of the post-UNX increase in arterial pressure was longer when compared to values observed in controls. These observations indicate that ANP release is stimulated after uninephrectomy.(ABSTRACT TRUNCATED AT 250 WORDS)

Nonlinear mixed effect modeling of the pharmacodynamics of natriuretic peptides in rats

Natriuretic peptides have not only natriuretic/diuretic but also hypotensive activities, and the decreased renal perfusion caused by the excessive hypotension is known to attenuate the diuretic actions. The present study was designed to examine the relationship between the dosing (intravenous constant infusion) rates and the diuretic actions of alpha-rat atrial natriuretic peptide (alpha-rANP) and rat brain natriuretic peptide (rBNP) in rats, and population (nonlinear mixed effect model) analysis was applied to these complicated diuretic actions. The intrinsic diuretic activities of alpha-rANP and rBNP could be analyzed, and the effects of blood pressure, heart rate, and also inhibition of degradation enzyme on the diuresis of natriuretic peptides were estimated simultaneously. The population analysis was useful for analyzing such pharmacodynamic data for which the individual analysis could not be applied easily.

Cardiovascular and renal effects of atrial natriuretic factor

Atrial natriuretic factor (ANF) reduces cardiac output and systemic arterial blood pressure. The reduction in systemic arterial blood pressure is not caused by dilation of arterial resistance vessels, since total peripheral vascular resistance often increases during infusion of ANF. The reduction in cardiac output with subsequent hypotension can be explained by a decrease in venous return. The decrease in venous return is not due to pooling of blood in the capacitance vessels, since ANF reduces venous compliance. Reduced venous return during infusion of ANF can be explained by a reduction in circulating blood volume and an increase in resistance to venous return. The reduction in circulating blood volume is due to increased urine output and to a shift of circulating fluid into the interstitial space. The increase in renal sodium and water excretion is mediated by an increase in glomerular filtration rate and reduced sodium and chloride reabsorption in the collecting ducts. ANF also inhibits the renin-angiotensin-aldosterone system. The plasma level of ANF may be a parameter for the severity of heart diseases with increased preload. In congestive heart failure and supraventricular tachycardia, the increase in plasma ANF concentration may augment sodium excretion, but anti-natriuretic factors, such as reduction in renal perfusion pressure, may override the natriuretic effect of ANF. Reduced sodium excretion during mechanical ventilation with positive end-expiratory pressure (PEEP) is partly due to a decrease in ANF secretion.

Humoral determinants of Na+ excretion after intravenous NaCl loading in normal volunteers

1. Twelve healthy volunteers maintained on a 100 mmol/day Na+ diet, were given an intravenous infusion of 2L saline (0.9%) between 10.00 h on 2 study days at least 1 week apart. Urine collections (90 min) were made from 08.30 to 16.00 h. Either carbidopa 100 mg or indomethacin 50 mg was given orally at 07.45 h on one study day and placebo was given on the other (in random order). 2. On the placebo day, saline infusion caused significant decreases in plasma albumin concentration, plasma renin activity (PRA), plasma aldosterone concentration and urinary aldosterone excretion, with 2 to 3-fold increases in plasma atrial natriuretic peptide (ANP) concentration and urinary dopamine: noradrenaline ratio (DA:NA), whereas mean urinary kallikrein and prostaglandin E2 (PGE2) excretion rates were unchanged. Carbidopa decreased urinary DA:NA and indomethacin decreased urinary PGE2 excretion, compared with the placebo day. Excretion of sodium (Na+) decreased below baseline in two out of six carbidopa-treated subjects and in three out of six indomethacin-treated subjects, but showed little or no change in the remainder. 3. These preliminary observations suggest that some subjects in the early phase of natriuresis after an intravenous Na+ load can be identified as having prostaglandin-dependent or dopamine-dependent mechanisms for Na+ excretion.

Role of endogenous atrial natriuretic peptide in DOCA-salt hypertensive rats. Effects of a novel nonpeptide antagonist for atrial natriuretic peptide receptor

BACKGROUND

To explore roles of endogenous atrial natriuretic peptide (ANP) in blood pressure and volume regulation, we examined the effects of a newly developed ANP antagonist, HS-142-1 (HS) in deoxycorticosterone acetate (DOCA)-salt hypertensive rats.

METHODS AND RESULTS

We examined 1) the effects of HS on ANP- or brain natriuretic peptide (BNP)-induced reductions in renal vascular resistance (RVR) of rat isolated perfused kidneys, 2) the effects of HS on cyclic GMP (cGMP) production in rat cultured vascular smooth muscle cells pretreated with ANP or BNP, and 3) the renal and systemic effects of HS in DOCA-salt-treated rats and control rats. We found that 1) HS dose-dependently reversed ANP- or BNP-induced decreases in RVR; 2) ANP or BNP at 100 nM caused an eightfold increase in cGMP production. These increases in cGMP were inhibited by HS in a dose-dependent fashion, and 300 micrograms/ml HS decreased cGMP to the control level. HS alone did not influence RVR or cGMP production; and 3) DOCA-salt rats showed higher plasma concentrations of ANP (198 versus 75 pg/ml) and BNP (23.7 versus 2.7 pg/ml, each p < 0.01) than the control rats. Bolus administration of 8 mg/kg HS elevated blood pressure by 8% (p < 0.01). This rise in blood pressure was attributed to an increase in systemic vascular resistance (+14%, p < 0.05). Conversely, urinary excretion of sodium (-41%), glomerular filtration rate (-27%), and plasma (-77%) and urinary cGMP (-69%, each p < 0.01) were decreased by administration of 8 mg/kg HS. These effects were dose dependent in DOCA-salt rats but slight or negligible in the control rats.

CONCLUSIONS

These results suggest that endogenous ANP and BNP may be involved in the regulation of blood pressure and body fluid volume in DOCA-salt rats in which ANP and BNP secretion is augmented.

Tubular site of the natriuresis after unilateral nephrectomy in the rat

Unilateral nephrectomy (UNX) is followed by a prompt increase in sodium excretion from the remaining kidney. Recently, an important role for atrial natriuretic peptide (ANP) in mediating the UNX-associated natriuresis has been suggested. The present studies were undertaken to gain insight into the intrarenal mechanisms participating in the post-UNX natriuresis in circumstances in which the release or the action of endogenous ANP were suppressed by prior removal of the right atrial appendage and by administration of monoclonal anti-ANP antibodies, respectively. In anesthetized euvolemic untreated rats, UNX resulted in a twofold increase in urinary excretion of sodium (from 0.93 +/- 0.23 to 2.14 +/- 0.34 microE/min; p < 0.03), whereas glomerular filtration rate did not change significantly. Fractional excretion of lithium, an index of proximal tubular handling of sodium, increased from 30.7 +/- 3.4% to 39.4 +/- 4.0%, and fractional distal reabsorption of sodium decreased from 98.6 +/- 0.2% to 96.5% +/- 0.4% (both p < 0.006). Neither sham atrial appendectomy nor the administration of nonspecific antibodies affect the natriuretic response of the remaining kidney. The natriuretic response to UNX was abolished in right atrial appendectomized rats, as well as in rats receiving anti-ANP antibodies. Post-UNX changes in both proximal and distal tubular reabsorption of sodium were also suppressed in these animals. These observations indicate that ANP may be an important mediator of the natriuretic response to UNX and that the proximal and the distal part of the nephron contribute to the postnephrectomy natriuresis.

Extra-natriuretic effects of atrial peptide in humans

To evaluate extra-natriuretic effects of atrial natriuretic peptide (ANP), plasma ANP (pANP) levels were assessed in seven healthy men on low-sodium diet (80 mEq NaCl/day), in basal conditions and during stepwise infusion of human ANP (2, 4, 8 and 16 ng/min/kg). To determine the individual physiological (PHY) pANP level, we measured pANP in the same subjects after a high-salt diet (400 mEq NaCl/day), that is, in a physiological stimulation of ANP. We then compared the effects of the PHY levels of ANP to the effects of pharmacological (PHA) pANP levels. Neither PHY nor PHA pANP levels modified creatinine clearance or blood pressure. The progressive rise in pANP levels was associated with increases in urinary excretion of Na+, K+ and urea. ANP alone respectively accounted for 41%, 30% and 92% of the increase in natriuresis, kaliuresis and urea excretion that occurred after changing salt intake from 80 to 400 mEq/day. Pharmacological ANP levels raised CH2O and reduced UOsm. Interestingly, PHA levels were associated with significant decrease in serum K+ (from 4.5 +/- .1 to 4.0 +/- .1 mEq/liter) and plasma urea (from 31.9 +/- 5 to 24.2 +/- 4 mg/dl). The mean cumulative urinary potassium and urea losses corresponded to the theoretical body losses of potassium and urea; moreover, the individual cumulative urinary losses of potassium and urea significantly correlated with the corresponding decrement in their plasma levels. In conclusion, ANP has both physiological and pharmacological significance in the control of potassium and urea metabolism by decreasing plasma levels of K+ and urea through effects on the renal excretory function.

Atrial natriuretic factor in cats subjected to acute myocardial ischaemia

Eighteen anaesthetized open chest cats were subjected to 10, 30, or 50 min occlusion of the left anterior descending coronary artery (LAD). Heart rate, left ventricular end-diastolic (LVEDP) and systolic pressure (LVSP), and dp/dt were continuously recorded during the experiments. Prior to LAD-occlusion, and just before termination of the experiments, blood samples were collected from the left femoral artery for measurements of atrial natriuretic factor (ANF), catecholamines, electrolytes, urea, and creatinine. Simultaneously, biopsies were collected from the right auricular wall. The tissue was embedded in Lowicryl K4M, and ultrathin sections were incubated with anti-ANF antibodies and secondary antibodies conjugated to gold particles. The density of ANF-containing atrial-specific granules labelled with gold particles was morphometrically calculated. LVEDP increased significantly in all three time groups, and when pooling the pre- and postocclusion values, there was an increase from 5.1 +/- 0.4 to 10.3 +/- 1.2 mmHg (p < 0.05). The noradrenaline level increased from 0.93 +/- 0.18 to 2.34 +/- 0.75 nmol l-1 (p < 0.05) after LAD-occlusion. Similarly, the mean plasma level of ANF in the 18 cats increased from 57.6 +/- 11.9 to 98.9 +/- 22.6 pmol l-1 (p < 0.05). Atrial granular density appeared to decline after 10 min of occlusion (from 0.141 +/- 0.017 to 0.127 +/- 0.022 granules-1 microns 2 sarcoplasm), and after 30 min there was a significant decrease (0.080 +/- 0.012 granules/microns 2, p < 0.05). However, after 50 min occlusion the granular density was almost restored (0.133 +/- 0.017 granules/microns 2). Plasma ANF showed a positive linear correlation to LVEDP and to the noradrenaline level.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of changes in perfusion pressure and angiotensin II on the renal excretory responses to atrial natriuretic peptides

The renal actions of atriopeptin III were examined in anaesthetised rats at differing perfusion pressures before and following blockade of the renin-angiotensin system. At normal perfusion pressure 1000 ng kg-1 atriopeptin III caused reversible increases in glomerular filtration rate, of 20%, urine flow, absolute and fractional sodium excretions of 51-93%. Reduction of left renal perfusion pressure to 80 mm Hg decreased glomerular filtration rate by 30% and urine flow, absolute and fractional sodium excretions by 80% while atriopeptin III administration only minimally changed these variables. Concomitantly, right kidney perfusion pressure rose by 15 mm Hg which significantly increased fluid output, while the atriopeptin III induced diuresis and natriuresis were significantly larger. During infusion of captopril 900 micrograms kg-1 h-1 when pressures at the left and right kidneys had been reduced and elevated, respectively, atriopeptin III caused larger excretory responses in both kidneys which were greater than without captopril. These result suggested that the atriopeptin III mediated natriuresis and diuresis were directly proportional to perfusion pressure and attenuated by angiotensin II.

Plasma atrial natriuretic peptide and renin-aldosterone in patients with cirrhosis and ascites: basal levels, changes during daily activity and nocturnal diuresis

Measurements of plasma atrial natriuretic peptide concentrations at 8 AM showed raised levels in 21 patients with cirrhosis and ascites (10.5 +/- 0.8 pmol/L) compared with levels in 10 age-matched controls (4.1 +/- 0.64 pmol/L; p less than 0.0001). In eight patients and 10 controls, atrial natriuretic peptide, plasma renin activity, plasma aldosterone and urinary sodium excretion were measured every 4 hr for 24 hr. Subjects were mobile between 8 AM and 11 PM and supine from 11 PM to 8 AM. In controls, urinary sodium excretion was highest between 4 PM and 11 PM (19.34 +/- 3.74 mumol/min) and lowest between midnight and 8 AM (7.06 +/- 1.23 mumol/min; p less than 0.001). In patients, urinary sodium excretion was 0.63 +/- 0.14 mumol/min between 4 PM and midnight and 1.85 +/- 0.71 mumol/min (p less than 0.08) between midnight and 8 AM. In patients during the day, mean plasma atrial natriuretic peptide concentration did not change despite large individual variation, but large, sustained rises in plasma renin activity and plasma aldosterone were seen. Correlations were noted between atrial natriuretic peptide and urinary sodium excretion between midnight and 8 AM (r = 0.65; p less than 0.02) and 4 PM and midnight (r = 0.54; p less than 0.05) but not between 8 AM and 4 PM. Plasma renin activity dropped from 12.54 +/- 2.49 at midnight to 7.41 +/- 0.88 pmol/hr/ml at 8 AM (p less than 0.05); plasma aldosterone decreased from 1,032 +/- 101 to 798 +/- 56 pmol/L (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Inappropriately elevated levels of atrial natriuretic peptide may contribute to the pathophysiology of orthostatic hypotension

1. Overnight recumbent and upright plasma atrial natriuretic peptide (ANP) levels were markedly elevated (P less than 0.001) in patients with orthostatic hypotension (OH). 2. Overnight urinary clearance of ANP was significantly lower (P less than 0.01) in patients with OH, and was inversely correlated with plasma ANP levels (r = -0.94, P less than 0.01). The same negative correlation (r = -0.87, P less than 0.01) was seen in normal subjects. 3. Reduced urinary clearance of ANP may be associated with reduced filtered load and increased binding of ANP to the neutral endopeptidase 24.11 receptor binding sites in the proximal renal tubule. 4. ANP may be involved in the pathophysiology of orthostatic hypotension.

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.

Bilateral atrial appendectomy abolishes increased plasma atrial natriuretic peptide release and blunts sodium and water excretion during volume loading in conscious dogs

The atrial appendages contain most of the atrial natriuretic factor (ANF) in the mammalian heart, and atrial appendage mechanical function predicts ANF secretion during volume loading. To demonstrate the crucial role of the atrial appendages in ANF release, we first measured hemodynamics and changes in plasma ANF after injection of 1,000 ml i.v. normal saline in conscious dogs and again after bilateral atrial appendectomy; we next measured changes in renal function using infusions of atriopeptin 24 to achieve plasma levels corresponding to levels achieved during volume loading; and we lastly measured renal function during acute volume expansion and also after atrial appendectomy. Plasma ANF increased from 65 +/- 11 to 246 +/- 54 pg/ml after volume loading but did not increase after atrial appendectomy. Atrial appendectomy did not alter the tachycardia or hemodynamic effects of volume loading. Infusion of 10 ng/kg/min atriopeptin 24 increased plasma ANF from 50 +/- 9 to 234 +/- 54 pg/ml, increased urine output 34 +/- 10%, and increased sodium excretion 62 +/- 10% in dogs with intact atrial appendages. Renal function was compared in dogs before atrial appendectomy: 20, 40, and 60 minutes after volume loading, urine flow rate increased by 5.9 +/- 0.5, 6.9 +/- 0.4, and 4.4 +/- 0.8 ml/min, while sodium excretion increased by 717 +/- 60, 839 +/- 84, and 582 +/- 57 mueq/min. After atrial appendectomy urine flow rate increased 2.1 +/- 0.7, 2.7 +/- 0.7, and 2.0 +/- 0.6 ml/min, and sodium excretion increased only by 327 +/- 110, 324 +/- 77, and 340 +/- 92 mueq/min (p less than 0.01) during volume loading.(ABSTRACT TRUNCATED AT 250 WORDS)

HS-142-1, a novel nonpeptide atrial natriuretic peptide (ANP) antagonist, blocks ANP-induced renal responses through a specific interaction with guanylyl cyclase-linked receptors

HS-142-1, a novel microbial product, blocked 125I-labeled rat atrial natriuretic peptide (rANP) (= ANF(99-126)) binding to bovine adrenocortical membranes, where guanylyl cyclase-containing receptors are predominantly expressed. However, HS-142-1 only slightly inhibited [125I]rANP binding to bovine lung membranes where only a small portion of binding sites are coupled to guanylyl cyclase. Further, HS-142-1 only recognized the 135 kDa ANP receptor, which is considered to be the guanylyl cyclase-containing receptor based on the results obtained in affinity cross-linking studies with bovine adrenocortical and lung membranes. Under identical conditions, Atriopeptin I selectively recognized guanylyl cyclase-free receptors both in binding and affinity cross-linking experiments. When injected intravenously (1 mg/kg) to anesthetized rats, HS-142-1 abolished ANP-induced diuresis and natriuresis. These results suggest that HS-142-1 works in vivo through a specific interaction with the ANP functional receptor, and that HS-142-1 will be a powerful tool for understanding the physiological roles of ANP in distinction from its pharmacological effects.

Effects of HS-142-1, a novel non-peptide ANP antagonist, on diuresis and natriuresis induced by acute volume expansion in anesthetized rats

In this study we used HS-142-1, a novel non-peptide antagonist for the atrial natriuretic peptide (ANP) receptor, to clarify the possible physiological significance of ANP in acute hypervolemia. Substantial volume expansion in anesthetized rats induced a strong diuresis and natriuresis. These renal responses were significantly blocked by HS-142-1 at a dose of 3.0 mgkg-1 i.v. This observation suggests that ANP and its guanylyl cyclase-coupled receptor are, under the present conditions, physiologically involved that appears to be responsible for the renal responses in the volume homeostasis.

HS-142-1, A novel polysaccharide of microbial origin, specifically recognizes guanylyl cyclase-linked ANP receptor in rat glomeruli

HS-142-1, a novel polysaccharide, of microbial origin had been characterized as a specific antagonist of guanylyl cyclase-linked atrial natriuretic peptide (ANP) receptors (ANP-GC receptor) in bovine adrenal cortex. The effect of HS-142-1 on ANP receptors of rat glomeruli were examined. HS-142-1 blocked rat ANP (r-ANP)-stimulated cGMP production in a concentration-dependent manner, although it caused only slight inhibition in the specific binding of [125I]-rANP to the glomeruli where only a small portion of the binding sites are coupled to guanylyl cyclase. HS-142-1 recognized the 135K ANP receptor which is thought to be ANP-GC receptors but did not recognized 60K receptor, guanylyl cyclase-free type from affinity cross-linking studies with glomerular membranes. These results indicate that HS-142-1 is a specific antagonist for the ANP-GC receptor in rat glomeruli, and that it will be a powerful tool for understanding the physiological roles of ANP in renal responses.

Clearance receptor-binding atrial natriuretic peptides inhibit mitogenesis and proliferation of rat aortic smooth muscle cells

The current studies were designed to explore the effects of C-receptor-binding atrial natriuretic peptide analogues on serum-induced mitogenesis in cultured rat aortic smooth muscle cells. To this end, rANF99-126 and a series of truncated (rANF103-126, rANF103-125), ring-deleted (des[Gln116, Ser117, Gly118, Leu119, Gly120]rANF102-121-NH2 (c-ANF) and linear des(Cys105, Cys121)rANF104-126 peptide analogues were used. The latter two peptides have been reported to be selective for the ANF-C receptor. In cells subcultured between passage 3 to 19, rANF99-126, rANF103-126, and rANF103-125 concentration-dependently (0.1-1000 nM) inhibited serum-induced (3H) thymidine incorporation with maximal inhibition observed at 1 microM for each peptide (approximately 40, 31 and 56%) respectively. Furthermore, des[Cys105, Cys121]rANF104-126 inhibited serum-induced (3H)thymidine incorporation concentration-dependently without altering basal or elevated cellular cAMP or cGMP levels. Moreover, the reduction in thymidine incorporation was associated with inhibition of serum-induced clonal cell proliferation. In contrast, c-ANF failed to inhibit serum-induced mitogenesis, yet at a concentration of 100 nM it antagonized the antimitogenic effects of des[Cys105, Cys121]rANF104-126 or rANF99-126 without having any effect on basal or elevated cellular cyclic nucleotide levels. We conclude that the antimitogenic effect of atrial peptides is mediated through interaction with the ANF-C receptor and may be independent of changes in cellular cyclic nucleotide levels.

On the blood-brain barrier to peptides: specific binding of atrial natriuretic peptide in vivo and in vitro

Using the intracarotid bolus injection technique, a saturable binding of [125I]atrial natriuretic peptide (ANP) was found in 8 blood-brain barrier (BBB)-protected rat brain regions as well as in the pineal gland, choroid plexus, neurointermediate and anterior lobes of the pituitary, i.e. structures lacking a BBB. The presence of specific ANP binding on the BBB, here shown for the first time by an in vivo approach, was evidenced concomitantly in vitro by incubation of isolated microvessels. A single-class high affinity binding without regional differences was obtained with Kd = 0.23 nM and Bmax = 120 fmol/mg protein. From that a density of 1,400 binding sites per endothelial cell was calculated, thought to be localized predominantly in the luminal membranes. In the in vivo study, the portion of the extracted peptide that, under the conditions used, may have crossed the BBB by passive diffusion amounted to less than 0.4% of the labeled ANP administered. ANP itself did not change the tightness of the BBB to the non-diffusible reference molecule [14C]inulin. In the BBB-free areas, ANP enhanced the inulin space by nearly 50%.

Hypotension in transgenic mice expressing atrial natriuretic factor fusion genes

Chronic regulation of the cardiovascular system by atrial natriuretic factor was investigated by generating transgenic mice with elevated hormone levels in the systemic circulation. A fusion gene comprising the mouse transthyretin promoter and mouse atrial natriuretic factor structural sequences was designed so as to target hormone expression to the liver. Hepatic expression of atrial natriuretic factor was detectable as early as embryonic day 15 in transgenic animals. In adult transgenic mice, plasma immunoreactive atrial natriuretic factor concentration was elevated at least eightfold as compared with nontransgenic littermates. The mean arterial pressure of conscious transgenic mice was 75.5 +/- 0.9 mm Hg, significantly less than that of nontransgenic siblings (103.9 +/- 2.0 mm Hg). This difference in mean arterial pressure was not accompanied by significant changes in several other physiological parameters, including heart rate, plasma and urinary electrolytes, water intake, and urine volume. This study demonstrates that a chronic elevation of plasma atrial natriuretic factor decreases arterial blood pressure without inducing diuresis and natriuresis in transgenic mice and also illustrates the value of the transgenic approach for the study of the cardiovascular system.

Maximizing the natriuretic effect of endogenous atriopeptin in a rat model of heart failure

The effect of pharmacological manipulation of atriopeptin (AP) activity on sodium excretion and blood pressure was examined in the rat aortovenocaval (A-V) fistula model of cardiac failure. Introduction of an A-V shunt led to a marked and sustained elevation of plasma AP immunoreactivity and urinary cGMP levels. Further elevation of plasma AP levels by infusion of exogenous peptide induced modest increases in urinary sodium and cGMP excretion and a decrease in blood pressure but these responses were significantly attenuated compared to sham-operated animals. In contrast, low-dose infusion of M + B 22948 (a cGMP phosphodiesterase inhibitor) or thiorphan [a neutral endopeptidase (membrane metallo-endopeptidase, EC 3.4.24.11) inhibitor] induced a natriuresis in A-V fistula rats, which exceeded that seen in control animals given these compounds and matched the peak natriuresis produced in sham-operated animals by high doses of AP. In the doses used, these compounds had little effect on blood pressure. The greater renal efficacy of M + B 22948 in A-V fistula rats is consistent with postreceptor facilitation of AP activity. The effect of thiorphan on sodium excretion was accompanied by a pronounced increase in urinary cGMP and AP immunoreactivity excretion (and was attenuated by anti-AP monoclonal antibody) but could not be explained solely in terms of an increase in circulating AP levels. It is proposed that thiorphan allows filtered AP to reach renal tubule sites that are normally inaccessible to the peptide and are thus protected from down-regulation by high circulating AP levels. The implication of these observations for patients in cardiac failure is the potential for using pharmacological agents to maximize the response to endogenous AP without compromising cardiac function.