Molecular forms of atrial natriuretic factor in normal and failing human myocardium

N-terminal pro atrial natriuretic peptide as a prognostic marker of cardiac resynchronization therapy recipients

Background

Although the dynamic changes of atrial natriuretic peptide (ANP) expressions in a failing heart are well-documented, the clinical implications of detailed measurements of each ANP molecular form processed from proANP remain unclear.

Methods

Patients screening was conducted on patients who were eligible for cardiac resynchronization therapy (CRT) between 2014 and 2019 in our institution. Blood samples and echocardiographic parameters were collected on the day before and six months after implantation. Total ANP, proANP, and N-terminal fragment of proANP (NT-proANP) were examined as predictive biomarkers for cardiac death, left ventricular assist device implantation, and heart failure hospitalization following CRT implantation.

Results

A total of 86 subjects (mean age 70 years, 64 males) who underwent successful CRT implantation were enrolled. Plasma levels of total ANP, proANP, and NT-proANP were not normally distributed [25.8 pM (interquartile range: 11.1-53.1), 2.2 pM (1.0-5.4), and 4.1 nM (2.4-7.1), respectively]. Over a median follow-up of 2.7 years, 31 patients (2 deaths and 29 heart failure hospitalizations) reached the endpoints. Among the different ANP forms, only NT-proANP emerged as an independent predictor of the composite outcome (adjusted odds ratio of 2.542 in those with levels above vs. below the median, 95 % confidence interval 1.151-5.615, p = 0.021). NT-proANP levels were associated with left atrial volume and left diastolic functional parameters and decreased in response to echocardiographic improvements at six months post-implantation (16 ± 44 % decrease in responders vs 18 ± 60 % increase in non-responders, p = 0.005).

Conclusion

Pre-implantation NT-proANP levels could serve as a predictive factor for clinical outcomes in recipients of CRT.

The Strengths and Weaknesses of Left Atrial Appendage Ligation or Exclusion (LARIAT, AtriaClip, Surgical Suture)

Left atrial appendage (LAA) epicardial exclusion has been associated with addressing 2 potential deleterious consequences attributed to the LAA, namely, thrombus formation and an arrhythmogenic contributor in advanced forms of atrial fibrillation. With more than 60 years of history, the surgical exclusion of the LAA has been firmly established. Numerous approaches have been used for surgical LAA exclusion including surgical resections, suture ligation, cutting and non-cutting staples, and surgical clips. Additionally, a percutaneous epicardial LAA ligation approach has been developed. A discussion of the various epicardial LAA exclusion approaches and their efficacy will be discussed, along with the salient beneficial affects on LAA thrombus formation, LAA electrical isolation and neuroendocrine homeostasis.

Nonthrombogenic Roles of the Left Atrial Appendage: JACC Review Topic of the Week

The atrial appendage (LAA) is a well-established source of cardioembolism in patients with atrial fibrillation. Therefore, research involving the LAA has largely focused on its thrombogenic attribute and the utility of its exclusion in stroke prevention. However, recent studies have highlighted several novel functions of the LAA that may have important therapeutic implications. In this paper, we provide a concise overview of the LAA anatomy and summarize the emerging data on its nonthrombogenic roles.

The Strengths and Weaknesses of Left Atrial Appendage Ligation or Exclusion (LARIAT, AtriaClip, Surgical Suture)

Left atrial appendage (LAA) epicardial exclusion has been associated with addressing 2 potential deleterious consequences attributed to the LAA, namely, thrombus formation and an arrhythmogenic contributor in advanced forms of atrial fibrillation. With more than 60 years of history, the surgical exclusion of the LAA has been firmly established. Numerous approaches have been used for surgical LAA exclusion including surgical resections, suture ligation, cutting and non-cutting staples, and surgical clips. Additionally, a percutaneous epicardial LAA ligation approach has been developed. A discussion of the various epicardial LAA exclusion approaches and their efficacy will be discussed, along with the salient beneficial affects on LAA thrombus formation, LAA electrical isolation and neuroendocrine homeostasis.

Evidence and Challenges in Left Atrial Appendage Management

This review aimed to discuss the anatomical properties of the left atrial appendage (LAA), its relationship with atrial fibrillation (AF), effectiveness of LAA occlusion (LAAO), techniques, and new devices used to perform this procedure. An electronic search was performed to identify studies, in the English language, on LAA management. Searches were performed on PubMed Central, Scopus, and Medline from the dates of database inception to February 2020. For the assessed papers, data were extracted from the reviewed text, tables, and figures, by two independent authors. Anticoagulant therapy for patients with AF has proven beneficial and is highly recommended, but it is challenging for many patients to maintain optimal treatment. Surgery is the most cost-effective option; surgical methods include simple LAA resection, thoracoscopic surgery, and catheter treatment. Each procedure has its advantages and disadvantages, and many prospective studies have been conducted to evaluate various treatment methods. In managing the LAA, dissection of the LAA, such as changes in its shape and size due to remodeling during AF, changes in autonomic nerve function, and thrombosis, must be understood anatomically and physiologically. We believe that early treatment intervention for the LAA should be considered particularly in cases of recurrent AF.

Conclusion: SLET under artificial pneumothorax is feasible and safe in minimally invasive McKeown esophagectomy.

Surgical clip closure of the left atrial appendage

Atrial fibrillation (AF) is the most common atrial arrhythmia, but it is not a benign disease. AF is an important risk factor for thromboembolic events, causing significant morbidity and mortality. The left atrial appendage (LAA) plays an important role in thrombus formation, but the ideal management of the LAA remains a topic of debate. The increasing popularity of surgical epicardial ablation and hybrid endoepicardial ablation approaches, especially in patients with a more advanced diseased substrate, has increased interest in epicardial LAA management. Minimally invasive treatment options for the LAA offer a unique opportunity to close the LAA with a clip device. This review highlights morphologic, electrophysiologic, and surgical aspects of the LAA with regard to AF surgery, and aims to illustrate the importance of surgical clip closure of the LAA.

Modification of Serum Natriuretic Peptide Profile and Echocardiographic Parameters After Surgical Left Atrial Appendage Exclusion/Resection During Mitral Valve Surgery

BACKGROUND

Being a well-recognised source of cardiac embolism, the left atrial (LA) appendage (LAA) is frequently excluded during mitral valve (MV) surgery. However, the LAA is also a source of cardiac hormones and a new drug (sacubitril), which beneficially interferes with hormonal imbalance during heart failure, leads to re-evaluation of the LAA for the maintenance of adequate hormone production in the heart. We compared the effects of LAA surgical resection/exclusion in patients with MV replacement (MVR) on natriuretic peptides (NPs) and related enzymes versus similar patients, in whom the LAA was preserved. A comparison of clinical response was also carried out.

METHOD

Haemodynamically stable patients scheduled for MV surgery with or without elimination of the LAA were studied before and 3 months after surgery. Serum NPs, furin, corin, and neprilysin were determined. A transthoracic echocardiogram was also performed before and after surgery.

RESULTS

Patients in the LAA intervention group exhibited lower levels of atrial natriuretic peptide (ANP) 3 months after surgery than patients with intact LAAs. There were no differences in NP and related enzyme levels pre- or postsurgery. The echocardiograms indicated a similar decrease in the diameters and volumes of the LA, and normal pulmonary arterial pressure values, in both groups. The indexed LA volume showed a positive correlation with postoperative brain natriuretic peptide.

CONCLUSIONS

Surgical resection or exclusion of the LAA in patients with MVR promotes a decrease in ANP production at 3 months postsurgery. Echocardiography is useful when evaluating surgical replacement of the MV with elimination of the LAA.

A 79-year-old woman with atrial fibrillation and new onset of heart failure

As an alternative to oral anticoagulation, percutaneous left atrial appendage (LAA) closure is an increasingly performed procedure to prevent arterial embolism in patients with non‐valvular atrial fibrillation. Besides procedure‐related complications, residual leaks, device‐related thrombus formation, and dislocation of the LAA occluder have been observed during follow‐up. Heart failure as a consequence of interventional LAA closure has not been reported so far. This case report describes a 79‐year‐old lady with permanent non‐valvular atrial fibrillation presenting with New York Heart Association Class IV heart failure. Symptoms had started immediately after attempted LAA closure 11 months before. Transoesophageal echocardiography demonstrated two devices in the LAA, a large peri‐device leak, a mobile LAA thrombus, a right atrial appendage thrombus, and shunting via a patent foramen ovale. Under a maximally tolerated dose of heart failure medication and edoxaban, the patient remains without bleeding or embolism in New York Heart Association Class II. Because of its unique anatomical and endocrine properties, the LAA plays an important role in situations of pressure and volume overload. Interventional LAA closure interacts unfavourably with left atrial compliance and reservoir function. Atrial and brain natriuretic peptide secretion is known to be significantly reduced after LAA closure. Both mechanisms may result in the development of heart failure. Attempted LAA closure—instead of being the solution—may create new serious problems. Development of heart failure should be assessed, and a systematic search for late leaks after LAA closure should be performed in trials investigating safety and efficacy of this intervention.

Natriuretic peptides in human heart: Novel insight into their molecular forms, functions, and diagnostic use

Among the three natriuretic peptides, atrial/A-type natriuretic peptide (ANP) and brain/B-type natriuretic peptide (BNP) are primarily produced by, and secreted from, heart tissue. They maintain cardiovascular homeostasis by binding to natriuretic peptide receptor-A. Since plasma ANP and BNP concentrations, as well as expression, are elevated in response to increased body fluid volume and pressure load on the heart wall, these peptides are widely utilized as diagnostic biomarkers for evaluating heart failure. Regardless of their high utility, differences in their molecular forms between healthy and diseased subjects and how these relate to pathophysiology have not well been examined. Recent studies have shown that the circulating molecular forms of ANP and BNP are not uniform; bioactive α-ANP is the major ANP form, whereas the weakly active proBNP is the major BNP form. The relative ratios of the different molecular forms are altered under different pathophysiological conditions. These facts indicate that detailed measurements of each form may provide useful information on the pathophysiological state of heart tissue. Here, we revisit the relationship between the molecular forms of, and pathophysiological alterations in, human ANP and BNP and discuss the possible utility of the measurement of each of the molecular forms. The third peptide, C-type natriuretic peptide, activates natriuretic peptide receptor-B, but little is known about its production and function in the heart because of its extremely low levels. However, through recent studies, its role in the heart is gradually becoming clear. Here, we summarize its molecular forms, assay systems, and functions in the heart.

Cardiovascular Therapies Targeting Left Atrial Appendage

Left atrial appendage (LAA) closure has evolved as an effective strategy for stroke prevention in patients with atrial fibrillation who are considered suitable for oral anticoagulation. There is strong evidence based on randomized clinical trials with 1 percutaneous device, as well as a large registry experience with several devices, regarding the safety and efficacy of this strategy. In addition, there is encouraging data regarding the effect of epicardial LAA closure on decreasing arrhythmia burden and improvements in systemic homeostasis by neurohormonal modulation. However, there are several unresolved issues regarding optimal patient selection, device selection, management of periprocedural complications including device-related thrombus, residual leaks, and pericarditis. In this review, we summarize the rationale, evidence, optimal patient selection, and common challenges encountered with mechanical LAA exclusion.

Morphological changes evaluation of left atrial appendage in patients with ischaemic heart disease

BACKGROUND

Since the majority of morphological changes evaluation of myocardium in ischaemic heart disease was in animal model, we detected the importance to evaluate such changes in human patients to gain insights into the targets of cellular damage and to reconcile or refine those experiments.

METHODS

Tissue sections from left atrial appendage of the heart were carefully dissected from seventy five patients underwent conventional coronary artery bypass grafting at the cardiothoracic surgical department, Manchester Royal Infirmary. Tissue was fixed, sectioned, stained and six random sections were photographed and the images were assessed and quantified using Image Analyser Pro-Plus software, version 4.1. Arterioles, venules, intermediate sized vessels, and capillaries were directly counted within the highlighted area of myocardium under LM. Ultra-thin sections were imaged in a Tecnai 12 Biotwin transmission electron microscope at a magnification of ×4200 and photographed by a camera with a black and white film to quantify different structures of myocardium.

RESULTS

The arteriole wall to lumen ratio was significantly increased in ischaemic heart disease patients 18.57 ± 2.89 compared to controls 8.3 ± 1.57, (P < 0.01). The regression analysis between vascular density and cardiomyocyte size demonstrated a significant inverse correlation between transverse cardiomyocyte diameter and arteriole, capillary and total vessel density (P < 0.01, 0.04, 0.02), respectively. Lumen area of the distal myocardial capillary was significantly reduced in IHD patients compared to controls (P < 0.01).

CONCLUSION

These results elucidate the morphological changes in the myocardial microvasculature of patients with ischaemic heart disease and its pathological magnitudes.

A Test in Context: Critical Evaluation of Natriuretic Peptide Testing in Heart Failure

Circulating natriuretic peptide measurements have been used extensively over the past 15 years to diagnose and monitor patients with heart failure. We are still learning how complex the dynamics of natriuretic peptides can be in the interpretation of test results in individual patients. Although natriuretic peptide measurements are widely used in practice, there are questions regarding why these peptides may not necessarily track with blood volume or invasive hemodynamic measurements in individual patients. Interpretation of natriuretic peptide measurements will depend on many factors, including special patient populations, obesity, renal function, the state of congestion or decongestion, and whether patients are receiving specific therapies. Natriuretic peptide measurements have clearly revolutionized clinical care for patients with heart failure, but further research should provide insights to help use these measurements to individualize patient care beyond the current guidelines.

Left atrial appendage occlusion for stroke prevention

Atrial fibrillation (AF) remains the most common arrhythmia encountered in clinical practice. One of its more common deleterious effects is the development of thromboembolism leading to stroke. The left atrial appendage (LAA) has been shown to the site of the majority of thrombus formation leading to stroke. Anticoagulation with warfarin has been the treatment of choice for prevention of embolic events. Newer anticoagulants have been developed but they still have the potential side effect of causing major bleeding. Occlusion of the LAA has emerged as an alternative therapeutic approach to medical therapy. The aim of this article is to discuss in detail the role of the LAA in thromboembolism in AF, role of device and surgical therapies, and the current clinical data supporting their use. This is particularly timely in that there is now an approved LAA closure device approved in the US for stroke prevention in patients with nonvalvular AF.

Left atrial appendage closure for stroke prevention in atrial fibrillation: state of the art and current challenges

Atrial fibrillation (AF) is a major risk factor for disabling ischemic strokes. Anticoagulation is highly effective for stroke prevention in patients with AF, but a substantial number of patients are unable to sustain chronic therapy with warfarin. Most strokes in patients with AF are thought to arise from thrombus formation in the left atrial appendage (LAA); therefore, occlusion of the orifice of the LAA provides a theoretically appealing option for stroke prevention. Surgical exclusion of the LAA is increasingly performed in patients undergoing open-heart surgery, and thoracoscopic epicardial occlusion of the LAA has yielded promising early results. Percutaneous LAA occlusion devices have shown some success initial trials, but additional safety and efficacy data are required before this approach can be routinely considered. Here we discuss the LAA in relation to AF-related embolic stroke, and how LAA occlusion devices could be used in stroke prevention in patients who cannot tolerate chronic anticoagulant therapy.

Pathophysiology of volume overload in acute heart failure syndromes

The inability to effectively regulate volume status is a major consequence of acute heart failure syndromes (AHFS). A variety of pathophysiologic processes contribute to this impairment, most notably neurohormonal activation of the renin-angiotensin-aldosterone system, arginine vasopressin, and the sympathetic nervous system. As a result, addressing volume overload is one of the most challenging aspects of AHFS management. Neurohormonal activation leads to substantial changes in hemodynamics and myocardial remodeling, which further contribute to the severity of heart failure (HF) disease and thereby cyclically increase the risk of further neurohormonal activation. Pulmonary capillary wedge pressure is a dependable reflection of volume status and has been used as a surrogate marker in recent studies to assess disease progression in response to innovative HF treatment strategies. Future approaches to HF treatment should focus on the more accurate assessment and management of volume status in an effort to improve patient care.

Elimination of the Left Atrial Appendage To Prevent Stroke or Embolism?

The "elimination" of the left atrial appendage (LAA) seems to be an attractive alternative to oral anticoagulation in the treatment of atrial fibrillation, especially in patients with contraindications to oral anticoagulation therapy. The LAA, however, plays an important role in the maintenance and regulation of the cardiac function, in arterial hypertension, atrial fibrillation, coronary heart disease, valvular heart disease, and heart failure. Data, mainly from animal studies, indicate that elimination of the LAA may impede thirst in patients with hypovolemia, may impair hemodynamic response to volume or pressure overload, may decrease cardiac output, and may promote heart failure. It may have adverse effects in humans as well. Further studies on the hemodynamic and neurohumoral consequences of left atrial appendage elimination are required to advance our understanding of LAA physiology and pathophysiology.

The role of endothelins and their receptors in heart failure

Endothelin (ET) is a peptide composed of 21 amino acids, derived from a larger precursor, the big-endothelin, by action of the endothelin-converting enzyme (ECE) family; three isoforms of endothelin, named ET-1, ET-2 and ET-3, have been identified. Endothelin-1 is generated mainly by vascular endothelial cells and exerts various important biological actions, mediated by two receptor subtypes, ET-A and ET-B, belonging to the G protein-coupled family that have been identified in various human tissues such as the cardiac tissue. Endothelin-1 is a potent vasoconstrictive agent, has inotropic and mitogenic actions, modulates salt and water homeostasis and plays an important role in the maintenance of vascular tone and blood pressure in healthy subjects. Endothelin-1, as well as ET-A and ECE-1, also has an important role in cardiovascular development, as observed by the variety of abnormalities related to neural crest-derived tissues in mouse embryos deficient of a member of the ET-1/ECE-1/ET-A pathway. Various evidence indicates that endogenous endothelin-1 may contribute to the pathophysiology of conditions associated with sustained vasoconstriction, such as heart failure. In heart failure, elevated circulating levels of both endothelin-1 and big-endothelin-1 are observed; in failing hearts an activation of the endothelin system is found: tissue level of ET-1 is increased with respect to non-failing hearts as well as receptor density, due mainly to an upregulation of the ET-A subtype, the prevalent receptor subclass in cardiac tissue. Finally, studies in both humans and animal models of cardiovascular disease show that inhibition of the endothelin function (anti-endothelin strategy) is associated with an improvement of haemodynamic conditions; these observations indicate that endothelin receptor antagonists or endothelin-converting enzyme inhibitors may constitute a novel and potentially important class of agents for the treatment of this disease.

Preservation of the right atrial appendage improves reduced plasma atrial natriuretic peptide levels after the maze procedure

OBJECTIVES

The present study was conducted to determine whether preservation of the right atrial appendage lessens the decrease of plasma atrial natriuretic peptide levels after the maze procedure and whether the increase of plasma atrial natriuretic peptides improves the ability of the kidneys to excrete the fluid load after the operation.

METHODS

We evaluated 42 patients who underwent the maze procedure. The right atrial appendage was preserved in 22 patients but not in 20. Blood samples were obtained before and after the operation for measurement of atrial natriuretic peptides. To evaluate the influence of atrial natriuretic peptides on the ability of the kidneys, we also measured body weight, fluid balance, and the doses of furosemide and dopamine administered after the operation.

RESULTS

The restoration to sinus rhythm at 1 month after was comparable in the two groups. Plasma atrial natriuretic peptide levels significantly increased after the operation in patients in whom the right atrial appendage was preserved (1 day after: 23.4 +/- 17.8 vs 3 days after: 42.7 +/- 23.6 and 7 days after: 36.3 +/- 23.7 pg/mL, P <.05) but not in patients in whom the right atrial appendage was not preserved (1 day after: 20.0 +/- 19.6, 3 days after: 28.5 +/- 19.3, and 7 days after: 23.0 +/- 16.1 pg/mL). Furthermore, plasma atrial natriuretic peptide levels were significantly lower in patients in whom the right atrial appendage was not preserved than in patients in whom the right atrial appendage was preserved at 3 and 7 days after the operation. The fluid balance during the first 7 days of the postoperative period was comparable in the two groups, although the total dose of dopamine used in the same period was significantly smaller in patients in whom the right atrial appendage was preserved than in patients in whom the right atrial appendage was not preserved (155.3 +/- 119.0 vs 244.9 +/- 129.0 microg/kg, P <.05).

CONCLUSIONS

The present study showed that preservation of the right atrial appendage lessens the decrease of plasma atrial natriuretic peptide levels after the maze procedure and that increased plasma atrial natriuretic peptides may improve the ability of the kidneys to excrete the fluid load after the operation.

Expression and distribution of brain natriuretic peptide in human right atria

OBJECTIVES

We investigated expression of brain natriuretic peptide (BNP) as well as atrial natriuretic peptide (ANP) and their genes in human right atria. Their relations with atrial pressure were also examined.

BACKGROUND

The BNP plays a roll in electrolyte-fluid homeostasis such as ANP. The tissue level is reported to be elevated in the failing ventricles. However, expression and transmural distribution of BNP in the atria remain unclear.

METHODS

Expression of ANP and BNP was immunohistochemically investigated in the right atrial (RA) specimens from 21 patients who had undergone cardiac surgery. The mRNA of specimens were quantitatively measured by Northern blot analysis and also evaluated by in situ hybridization. In addition, plasma levels of ANP and BNP were measured in the patients.

RESULTS

The BNP immunoreactivity was diffusely seen in RA tissue of patients with mean RA pressure (mRAP) of 5 mm Hg or more, but it was noted only in the subendocardial half of the atria of those with mRAP less than 5 mm Hg. There was a significant correlation between the incidence of BNP-positive myocytes and mRAP (r = 0.850, p < 0.0001). Conversely, ANP-positive myocytes were found diffusely in all cases. In Northern blot analysis, the mRNAs levels of ANP and BNP in the atrial tissue were positively correlated with the mRAP (ANP, p = 0.775, p < 0.005 and BNP, p = 0.771, p < 0.005). In situ hybridization confirmed these findings. The mRNA levels were significantly correlated to each other (r = 0.845, p < 0.0002). Plasma ANP and BNP levels were elevated in the patients compared with that in controls; however, none were significantly correlated with the mRAP.

CONCLUSIONS

Expression of BNP and BNP mRNA is augmented in the atria with increased pressure, and distributed predominantly in the subendocardial side. The level of BNP mRNA was well correlated with that of ANP mRNA. Thus, these two genes might be commonly regulated in response to atrial pressure.

Atrial natriuretic peptide secretion and body fluid balance after bilateral atrial appendectomy by the maze procedure

OBJECTIVES

One of the earliest recognized postoperative complications of the maze procedure was the fluid retention in the immediate postoperative period. Routine postoperative administration of diuretics markedly reduces the frequency and severity of the fluid retention. However, the cause of the abnormal fluid balance is still uncertain.

METHODS

We evaluated 24 patients: 15 patients underwent the maze procedure (maze group) and 9 patients did not (nonmaze group). Blood samples were obtained before and in the time course after operation for atrial natriuretic peptide measurement. To evaluate the influence of atrial natriuretic peptide on the body fluid balance, we also measured the amount of body fluid balance and the total doses of furosemide and dopamine administered after operation. To examine the effect of the maze procedure on atrial natriuretic peptide secretion in chronic phase, we measured plasma atrial natriuretic peptide levels during dynamic exercise in 21 patients who had undergone cardiac operations 2 years before.

RESULTS

Plasma atrial natriuretic peptide levels in the nonmaze group significantly increased after operation. In contrast, plasma atrial natriuretic peptide levels in the maze group did not increase, and these levels were significantly lower than in the nonmaze group. Although significantly greater doses of furosemide and dopamine were administered to the maze group than to the nonmaze group, the body fluid balance in the maze group was comparable with that in the nonmaze group in the early postoperative period. The response of atrial natriuretic peptide secretion by exercise was significantly attenuated in the maze group (n = 12) compared with the nonmaze group (n = 9) even 2 years after surgery, although there were no significant differences in heart rate or blood pressure during exercise between two groups.

CONCLUSIONS

These results suggest that the maze procedure attenuates atrial natriuretic peptide secretion in the early postoperative period and persists in chronic phase. This attenuated atrial natriuretic peptide secretion may reduce the ability of the kidneys to handle fluid load early after surgery.

Plasma and cardiac tissue atrial and brain natriuretic peptides in experimental heart failure

OBJECTIVES

This study evaluated the role of changes in heart rate, cardiac filling pressures and cardiac tissue atrial and brain natriuretic peptides in the modulation of their plasma levels in a model of heart failure.

BACKGROUND

Atrial and brain natriuretic peptides constitute a dual natriuretic peptide system that regulates circulatory homeostasis.

METHODS

The effects of 1) acute ventricular pacing, 2) acute volume expansion, and 3) volume expansion after 1 week of continuous pacing on plasma atrial and brain natriuretic peptide levels were compared in eight dogs. Atrial and ventricular tissue levels of the peptides were examined in 5 normal dogs (control group), 21 dogs paced for 1 week (group 1) and 10 dogs paced for 3 weeks (group 2).

RESULTS

Both acute pacing and volume expansion increased plasma atrial natriuretic peptide levels (from 53 +/- 41 to 263 +/- 143 pg/ml [mean +/- SD], p < 0.01, and from 38 +/- 23 to 405 +/- 221 pg/ml, p < 0.001, respectively). After 1 week, there was a marked increase in plasma levels of atrial natriuretic peptide, but the level did not increase further with volume expansion (from 535 +/- 144 to 448 +/- 140 pg/ml, p = 0.72). By contrast, plasma brain natriuretic peptide levels increased only modestly with acute pacing (from 12 +/- 4 to 20 +/- 8 pg/ml, p < 0.05) and after pacing for 1 week (from 13 +/- 4 to 48 +/- 20 pg/ml, p < 0.05) but did not change with acute or repeat volume expansion. In groups 1 and 2, atrial tissue levels of atrial natriuretic peptide (1.9 +/- 1.3 and 2.0 +/- 0.9 ng/mg, respectively) were lower than those in the control group (11.7 +/- 6.8 ng/mg, both p < 0.001), whereas ventricular levels were similar to those in the control group. Atrial tissue brain natriuretic peptide levels in groups 1 and 2 were similar to those in the control group. However, ventricular levels in group 2 (0.018 +/- 0.006 ng/mg) were increased compared with those in the control group (0.013 +/- 0.006 ng/mg, p < 0.05) and in group 1 (0.011 +/- 0.006 ng/mg, p < 0.05).

CONCLUSIONS

Atrial and brain natriuretic peptides respond differently to changes in heart rate and atrial pressures. Reduced atrial tissue atrial natriuretic peptide levels in heart failure may indicate reduced storage after enhanced cardiac release. However, the relatively modest change in cardiac tissue brain natriuretic peptide levels suggests that the elevated plasma levels may be mediated by mechanisms other than increased atrial pressures.

Atrial natriuretic factor as a volume regulator

Atrial natriuretic factor, originally isolated from the atrium of the heart, has been found to consist of three major groups: atrial natriuretic peptide (ANP), B-form natriuretic peptide (BNP), and C-form natriuretic peptide (CNP). In addition, ANP exists in its precursor form, pro-ANP, an active ANP with a longer peptide chain (urodilatin) and an antiparallel dimer of active ANP. Sites and production of these diverse forms of the peptides are also diverse, depending on pathologic states. Three major subtypes of ANP receptors exist; these include a clearance receptor and two types of a transmembrane receptor with guanylyl cyclase structures in their intracellular domain. The latter exists at least in two forms, one of which is found mainly in the brain. All the actions of ANP mediated by the transmembrane form of ANP receptors are mediated by cGMP generated by the guanylyl cyclase in the cytosolic domain of the receptor. Among the numerous effects of ANP, its major effects are stimulation of natriuresis and diuresis by the kidney through its hemodynamic and tubular effects. In addition, ANP causes vasodilatation and fluid volume reduction by direct actions on vascular smooth muscle cells, and inhibition of secretion of hormones, such as aldosterone, from adrenal cortex and norepinephrine from peripheral adrenergic neurons. Centrally mediated effects on the regulation of the fluid volume may also be important.