Atrial natriuretic peptide stimulates Na+-dependent Ca2+ efflux from freshly isolated adult rat cardiomyocytes

Physiological and Pathophysiological Effects of C-Type Natriuretic Peptide on the Heart

Simple Summary

C-type natriuretic peptide (CNP) is the third member of the natriuretic peptide family. Unlike atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP), CNP was not previously regarded as an important cardiac modulator. However, recent studies have revealed the physiological and pathophysiological importance of CNP in the heart; in concert with its cognate natriuretic peptide receptor-B (NPR-B), CNP has come to be regarded as the major heart-protective natriuretic peptide in the failed heart. In this review, I introduce the history of research on CNP in the cardiac field.

Abstract

C-type natriuretic peptide (CNP) is the third member of the natriuretic peptide family. Unlike other members, i.e., atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP), which are cardiac hormones secreted from the atrium and ventricle of the heart, respectively, CNP is regarded as an autocrine/paracrine regulator with broad expression in the body. Because of its low expression levels compared to ANP and BNP, early studies failed to show its existence and role in the heart. However, recent studies have revealed the physiological and pathophysiological importance of CNP in the heart; in concert with the distribution of its specific natriuretic peptide receptor-B (NPR-B), CNP has come to be regarded as the major heart-protective natriuretic peptide in the failed heart. NPR-B generates intracellular cyclic guanosine 3′,5′-monophosphate (cGMP) upon CNP binding, followed by various molecular effects including the activation of cGMP-dependent protein kinases, which generates diverse cytoprotective actions in cardiomyocytes, as well as in cardiac fibroblasts. CNP exerts negative inotropic and positive lusitropic responses in both normal and failing heart models. Furthermore, osteocrin, the intrinsic and specific ligand for the clearance receptor for natriuretic peptides, can augment the effects of CNP and may supply a novel therapeutic strategy for cardiac protection.

Hypertrophic phenotype in cardiac cell assemblies solely by structural cues and ensuing self-organization

In vitro models of cardiac hypertrophy focus exclusively on applying "external" dynamic signals (electrical, mechanical, and chemical) to achieve a hypertrophic state. In contrast, here we set out to demonstrate the role of "self-organized" cellular architecture and activity in reprogramming cardiac cell/tissue function toward a hypertrophic phenotype. We report that in neonatal rat cardiomyocyte culture, subtle out-of-plane microtopographic cues alter cell attachment, increase biomechanical stresses, and induce not only structural remodeling, but also yield essential molecular and electrophysiological signatures of hypertrophy. Increased cell size and cell binucleation, molecular up-regulation of released atrial natriuretic peptide, altered expression of classic hypertrophy markers, ion channel remodeling, and corresponding changes in electrophysiological function indicate a state of hypertrophy on par with other in vitro and in vivo models. Clinically used antihypertrophic pharmacological treatments partially reversed hypertrophic behavior in this in vitro model. Partial least-squares regression analysis, combining gene expression and functional data, yielded clear separation of phenotypes (control: cells grown on flat surfaces; hypertrophic: cells grown on quasi-3-dimensional surfaces and treated). In summary, structural surface features can guide cardiac cell attachment, and the subsequent syncytial behavior can facilitate trophic signals, unexpectedly on par with externally applied mechanical, electrical, and chemical stimulation.

Atrial natriuretic peptide production and natriuretic peptide receptors in the human uterus and their effect on myometrial relaxation

OBJECTIVE

The objective of the study was to identify the effect of atrial natriuretic peptide (ANP) on uterine contractility, production of ANP, and natriuretic peptide receptor (NPR) expression in human myometrial tissue.

STUDY DESIGN

In an institutional review board-approved study, gravid human myometrium was obtained from patients undergoing cesarean section. Uterine contractility was examined using isometric force tension studies. After regular uterine contractions were obtained with oxytocin, ANP was added in increasing concentrations. ANP concentration was measured from myometrial tissue using radioimmunoassay (RIA). Primary myometrial cell culture was performed and treated with nifedipine vs oxytocin. RIA was performed on these cells and the cell culture media. Western blot analysis was performed on uterine tissue samples for natriuretic peptide receptors.

RESULTS

With increasing concentration of ANP (starting at 3 pM), myometrial contraction frequency decreased. ANP was identified in primary cultured myometrial cells and cell culture media. Myometrial ANP concentration increased with advancing gestational age. The concentration of ANP decreased within myometrial cells treated with oxytocin. The amount of ANP in the cell culture media increased from cells treated with nifedipine. Western blot identified NPR-A, -B, and -C in myometrial tissue. NPR-A expression was significantly increased in preterm samples.

CONCLUSION

ANP has a dose dependent effect on uterine relaxation. ANP is present in human myometrial cells and appears to be secreted by myometrial cells. The concentration of ANP may vary with gestational age and modulators of uterine contractility. NPR-A, -B, and -C receptor proteins are present in myometrial tissue. NPR-A levels may correlate with gestational age.

Human atrial natriuretic peptide suppresses torsades de pointes in rabbits

BACKGROUND

The increase in inward current, primarily L-type Ca2+ current, facilitates torsades de pointes (TdP). Because human atrial natriuretic peptide (ANP) moderates the L-type Ca2+ current, in our study it was hypothesized that ANP counteracts TdP.

METHODS AND RESULTS

We tested the effect of ANP, guanosine 3', 5'-cyclic monophosphate analogue (8-bromo cGMP) and hydralazine on the occurrence of TdP in a rabbit model. In control rabbits, administration of methoxamine and nifekalant almost invariably caused TdP (14/15). In contrast, ANP (10 microg . kg(-1) . min(-1)) markedly abolished TdP (2/15), whereas hydralazine failed to show a comparable anti-arrhythmic action (10/15). TdP occurred only in 1 of 15 rabbits treated with 8-bromo cGMP. Presence of early afterdepolarization-like hump in the ventricular monophasic action potential was associated with the occurrence of TdP.

CONCLUSION

Results suggest that ANP affects TdP in the rabbit model, and that this anti-arrhythmic effect of ANP is not necessarily shared by other vasodilating agents.

Natriuretic peptides modify Pseudomonas fluorescens cytotoxicity by regulating cyclic nucleotides and modifying LPS structure

Background

Nervous tissues express various communication molecules including natriuretic peptides, i.e. Brain Natriuretic Peptide (BNP) and C-type Natriuretic Peptide (CNP). These molecules share structural similarities with cyclic antibacterial peptides. CNP and to a lesser extent BNP can modify the cytotoxicity of the opportunistic pathogen Pseudomonas aeruginosa. The psychrotrophic environmental species Pseudomonas fluorescens also binds to and kills neurons and glial cells, cell types that both produce natriuretic peptides. In the present study, we investigated the sensitivity of Pseudomonas fluorescens to natriuretic peptides and evaluated the distribution and variability of putative natriuretic peptide-dependent sensor systems in the Pseudomonas genus.

Results

Neither BNP nor CNP modified P. fluorescens MF37 growth or cultivability. However, pre-treatment of P. fluorescens MF37 with BNP or CNP provoked a decrease of the apoptotic effect of the bacterium on glial cells and an increase of its necrotic activity. By homology with eukaryotes, where natriuretic peptides act through receptors coupled to cyclases, we observed that cell-permeable stable analogues of cyclic AMP (dbcAMP) and cyclic GMP (8BcGMP) mimicked the effect of BNP and CNP on bacteria. Intra-bacterial concentrations of cAMP and cGMP were measured to study the involvement of bacterial cyclases in the regulation of P. fluorescens cytotoxicity by BNP or CNP. BNP provoked an increase (+49%) of the cAMP concentration in P. fluorescens, and CNP increased the intra-bacterial concentrations of cGMP (+136%). The effect of BNP and CNP on the virulence of P. fluorescens was independent of the potential of the bacteria to bind to glial cells. Conversely, LPS extracted from MF37 pre-treated with dbcAMP showed a higher necrotic activity than the LPS from untreated or 8BcGMP-pre-treated bacteria. Capillary electrophoresis analysis suggests that these different effects of the LPS may be due, at least in part, to variations in the structure of the macromolecule.

Conclusion

These observations support the hypothesis that P. fluorescens responds to natriuretic peptides through a putative sensor system coupled to a cyclase that could interfere with LPS synthesis and thereby modify the overall virulence of the micro-organism.

Atrial natriuretic peptide attenuates elevations in Ca2+ and protects hepatocytes by stimulating net plasma membrane Ca2+ efflux

Elevations in intracellular Ca(2+) concentration and calpain activity are common early events in cellular injury, including that of hepatocytes. Atrial natriuretic peptide is a circulating hormone that has been shown to be hepatoprotective. The aim of this study was to examine the effects of atrial natriuretic peptide on potentially harmful elevations in cytosolic free Ca(2+) and calpain activity induced by extracellular ATP in rat hepatocytes. We show that atrial natriuretic peptide, through protein kinase G, attenuated both the amplitude and duration of ATP-induced cytosolic Ca(2+) rises in single hepatocytes. Atrial natriuretic peptide also prevented stimulation of calpain activity by ATP, taurolithocholate, or Ca(2+) mobilization by thapsigargin and ionomycin. We therefore investigated the cellular Ca(2+) handling mechanisms through which ANP attenuates this sustained elevation in cytosolic Ca(2+). We show that atrial natriuretic peptide does not modulate the release from or re-uptake of Ca(2+) into intracellular stores but, through protein kinase G, both stimulates plasma membrane Ca(2+) efflux from and inhibits ATP-stimulated Ca(2+) influx into hepatocytes. These findings suggest that stimulation of net plasma membrane Ca(2+) efflux (to which both Ca(2+) efflux stimulation and Ca(2+) influx inhibition contribute) is the key process through which atrial natriuretic peptide attenuates elevations in cytosolic Ca(2+) and calpain activity. Moreover we propose that plasma membrane Ca(2+) efflux is a valuable, previously undiscovered, mechanism through which atrial natriuretic peptide protects rat hepatocytes, and perhaps other cell types, against Ca(2+)-dependent injury.

Gene expression of the natriuretic peptide system in atrial tissue of patients with paroxysmal and persistent atrial fibrillation

INTRODUCTION

Circulating cardiac natriuretic peptides play an important role in maintaining volume homeostasis, especially during conditions affecting hemodynamics. During atrial fibrillation (AF), levels of plasma atrial natriuretic peptide (ANP) becomes elevated. The aim of this study was to gather information about gene expression of the natriuretic peptide system on the atrial level in patients with AF.

METHODS AND RESULTS

Right atrial appendages of 36 patients with either paroxysmal or persistent AF were compared with 36 case matched controls in sinus rhythm for mRNA expression of pro- atrial natriuretic peptide (pro-ANP), pro-brain natriuretic peptide (pro-BNP), and their natriuretic peptide receptor type-A (NPR-A). We investigated patients without (n = 36) and with (n = 36) valvular disease. Persistent AF was associated with higher mRNA expression of pro-BNP (+66%, P = 0.04, in patients without valvular disease, and +69%, P < 0.01, in patients with valvular disease) and lower mRNA expression of NPR-A (-58%, P = 0.02, in patients without valvular disease, and -62 %, P < 0.01, in patients with valvular disease). The mRNA content of pro-ANP was only increased in patients with valvular disease (+12%, P = 0.03). No changes were observed in patients with paroxysmal AF.

CONCLUSION

This study demonstrates that persistent, but not paroxysmal, AF induces alterations in gene expression of pro-BNP and NPR-A on the atrial level. Although AF generally is associated with an increase of plasma ANP level, a change in mRNA content of pro-ANP is only observed in the presence of concomitant valvular disease and is of minor magnitude.

Role of natriuretic peptides in ion transport mechanisms

Natriuretic peptides (NP) act as ligands on the guanylyl cyclase family of receptors. The NP binding site on these receptors is extracellular and the guanylyl cyclase and protein kinase domains are intracellular. The guanylyl cyclase receptor catalyzes the synthesis of the second messenger molecule, cGMP, which activates protein kinase. This in turn is involved in the phosphorylation of various ion transport proteins. Ion transport proteins, which are modulated by NP and are thought to underlie the natriuretic and diuretic actions of NP, include: (a) calcium-activated K+ channels; (b) ATP-sensitive K+ channels; (c) inwardly-rectifying K+ channels; (d) outwardly-rectifying K+ channels; (e) L-type Ca2+ channels; (f) Cl- channels including cystic fibrosis transmembrane conductance regulator Cl- channels; (g) Na+- K+ 2Cl- co-transporter; (h) Na+- K+ ATPase; (i) Na+ channels; (j) stretch-activated channels; and (k) water channels. It appears that NP modulate the kinetics, rather than the conductance, of ion channels. Some of these channels, like the Ca2+, ATP-sensitive K+ and stretch-activated channels, are also involved in NP secretion. In addition, the structural properties of the NP, e.g., ovCNP-22 and ovCNP-39, appear to confer on them the ability to form ion channels. These CNP-formed ion channels can modify the trans-membrane signal transduction and second messenger systems underlying NP-induced pathological effects.