Atrial natriuretic factor in Purkinje fibers of rabbit heart

Atrial natriuretic peptide signaling co-regulates lipid metabolism and ventricular conduction system gene expression in the embryonic heart

It has been shown that atrial natriuretic peptide (ANP) and its high affinity receptor (NPRA) are involved in the formation of ventricular conduction system (VCS). Inherited genetic variants in fatty acid oxidation (FAO) genes are known to cause conduction abnormalities in newborn children. Although the effect of ANP on energy metabolism in noncardiac cell types is well documented, the role of lipid metabolism in VCS cell differentiation via ANP/NPRA signaling is not known. In this study, histological sections and primary cultures obtained from E11.5 mouse ventricles were analyzed to determine the role of metabolic adaptations in VCS cell fate determination and maturation. Exogenous treatment of E11.5 ventricular cells with ANP revealed a significant increase in lipid droplet accumulation, FAO and higher expression of VCS marker Cx40. Using specific inhibitors, we further identified PPARγ and FAO as critical downstream regulators of ANP-mediated regulation of metabolism and VCS formation.

Characterizing the role of atrial natriuretic peptide signaling in the development of embryonic ventricular conduction system

Patients born with congenital heart defects frequently encounter arrhythmias due to defects in the ventricular conduction system (VCS) development. Although recent studies identified transcriptional networks essential for the heart development, there is scant information on the mechanisms regulating VCS development. Based on the association of atrial natriuretic peptide (ANP) expression with VCS forming regions, it was reasoned that ANP could play a critical role in differentiation of cardiac progenitor cells (CPCs) and cardiomyocytes (CMs) toward a VCS cell lineage. The present study showed that treatment of embryonic ventricular cells with ANP or cell permeable 8-Br-cGMP can induce gene expression of important VCS markers such as hyperpolarization-activated cyclic nucleotide-gated channel-4 (HCN4) and connexin 40 (Cx40). Inhibition of protein kinase G (PKG) via Rp-8-pCPT-cGMPS further confirmed the role of ANP/NPRA/cGMP/PKG pathway in the regulation of HCN4 and Cx40 gene expression. Additional experiments indicated that ANP may regulate VCS marker gene expression by modulating levels of miRNAs that are known to control the stability of transcripts encoding HCN4 and Cx40. Genetic ablation of NPRA revealed significant decreases in VCS marker gene expression and defects in Purkinje fiber arborisation. These results provide mechanistic insights into the role of ANP/NPRA signaling in VCS formation.

Atrial natriuretic peptide inhibits cell cycle activity of embryonic cardiac progenitor cells via its NPRA receptor signaling axis

The biological effects of atrial natriuretic peptide (ANP) are mediated by natriuretic peptide receptors (NPRs), which can either activate guanylyl cyclase (NPRA and NPRB) or inhibit adenylyl cyclase (NPRC) to modulate intracellular cGMP or cAMP, respectively. During cardiac development, ANP serves as an early maker of differentiating atrial and ventricular chamber myocardium. As development proceeds, expression of ANP persists in the atria but declines in the ventricles. Currently, it is not known whether ANP is secreted or the ANP-NPR signaling system plays any active role in the developing ventricles. Thus the primary aims of this study were to 1) examine biological activity of ANP signaling systems in embryonic ventricular myocardium, and 2) determine whether ANP signaling modulates proliferation/differentiation of undifferentiated cardiac progenitor cells (CPCs) and/or cardiomyocytes. Here, we provide evidence that ANP synthesized in embryonic day (E)11.5 ventricular myocytes is actively secreted and processed to its biologically active form. Notably, NPRA and NPRC were detected in E11.5 ventricles and exogenous ANP stimulated production of cGMP in ventricular cell cultures. Furthermore, we showed that exogenous ANP significantly decreased cell number and DNA synthesis of CPCs but not cardiomyocytes and this effect could be reversed by pretreatment with the NPRA receptor-specific inhibitor A71915. ANP treatment also led to a robust increase in nuclear p27 levels in CPCs compared with cardiomyocytes. Collectively, these data provide evidence that in the developing mammalian ventricles ANP plays a local paracrine role in regulating the balance between CPC proliferation and differentiation via NPRA/cGMP-mediated signaling pathways.

Expression of brain natriuretic peptide in the rat heart studies during heart growth and in relation to sympathectomy

Brain natriuretic peptide (BNP) might be of importance during heart development and is described to be increasingly expressed in congestive heart failure and to affect the progress of this condition. However, details in the normal expression of BNP are still unclear in various parts of the adult and growing heart, including the conduction system. In this study, we investigated the expression of BNP in relation to that of atrial natriuretic peptide (ANP) in the growing as well as in the adult rat heart. The effects of chemical sympathectomy in adult rats were also examined. Contrary to previous BNP immunohistochemical studies, the BNP antiserum was preabsorbed with an excess of ANP before staining to abolish the crossreactivity with ANP. There was a pronounced BNP immunoreaction in the auricles, the trabeculated ventricular walls, and the peripheral parts of the conduction system at 0-1 days postnatally. The degree of immunoreaction gradually decreased with increasing age. A similar developmental pattern was seen concerning ANP expression, but the magnitude of the latter clearly exceeded that for BNP. Immunoreaction for BNP was never detected in the atrioventricular (AV) node and AV bundle at any stage. In contrast to the situation for ANP previously observed, no obvious changes in BNP immunoreaction patterns were observed in response to sympathectomy. This is the first study to thoroughly demonstrate the expression of BNP in the various regions of the rat heart during growth and in the normal and sympathectomized adult stage. The observations are related to possible functions of natriuretic peptides in the growing and adult heart.

Natriuretic peptides in relation to the cardiac innervation and conduction system

During the past two decades, the heart has been known to undergo endocrine action, harbouring peptides with hormonal activities. These, termed "atrial natriuretic peptide (ANP)," "brain natriuretic peptide (BNP)," and "C-type natriuretic peptide (CNP)," are polypeptides mainly produced in the cardiac myocardium, where they are released into the circulation, producing profound hypotensive effects due to their diuretic, natriuretic, and vascular dilatory properties. It is, furthermore, well established that cardiac disorders such as congestive heart failure and different forms of cardiomyopathy are combined with increased expression of ANP and BNP, leading to elevated levels of these peptides in the plasma. Besides the occurrence of natriuretic peptides (NPs) in the ordinary myocardium, the presence of ANP in the cardiac conduction system has been described. There is also evidence of ANP gene expression in nervous tissue such as the nodose ganglion and the superior cervical ganglion of the rat, ganglia known to be involved in the neuronal regulation of the heart. Furthermore, in the mammalian heart, ANP appears to affect the cardiac autonomic nervous system by sympathoinhibitory and vagoexcitatory actions. This article provides an overview of the relationship between the cardiac conduction system, the cardiac innervation and NPs in the mammalian heart and provides data for the concept that ANP is also involved in neuronal cardiac regulation.

Lack of evidence for functional natriuretic peptide receptors in the heart of the cane toad, Bufo marinus

Several studies have shown that the heart of species from each vertebrate class contains natriuretic peptide binding sites which suggests that ANP released from the heart may act in a paracrine/autocrine fashion. The present study used a set of techniques to study cell surface receptors in order to investigate the presence and nature of NPRs in the heart of the cane toad, Bufo marinus. Autoradiographical studies of both atria and ventricle showed no variation between total and non-specific binding, indicating a lack of NP binding sites in these tissues. This was confirmed with in vitro binding studies in which increasing concentrations of ANP did not compete for any specific binding. Increasing concentrations of ANP did not increase cGMP generation and physiological experiments showed that both ANP and CNP had no effect on the force or rate of contraction of a sino-atrial preparation. Molecular expression studies, however, showed that mRNA for NPRs was expressed in the heart, in spite of the lack of evidence for NPR on the cell surface. Overall, this study showed that no functional NPRs are present in the heart, and provided evidence that the heart is not a target organ for NP action in B. marinus.

Immunohistochemical expression of atrial natriuretic peptide (ANP) in the conducting system and internodal atrial myocardium of human hearts

Expression and distribution of atrial natriuretic peptide (ANP) were studied immunohistochemically in the conducting system and internodal atrial myocardium of 5 adult human hearts. Myocytes from the sinus node and compact atrioventricular node were usually ANP-negative; only a very few cells exhibited ANP immunoreactivity. These ANP-positive myocytes were small and did not appear to be trapped working atrial myocytes which are larger than nodal cells. The transitional cell zones of the sinus node and the atrioventricular node were composed of bundles of ANP-positive myocytes, intermingled with non-reactive myocytes. The internodal atrial myocardium exhibited a comparable intensity of myocyte staining in each case examined. Thus, morphologically distinct connecting pathways between the sinus node and the atrioventricular node with regard to myocyte ANP immunoreactivity could not be demonstrated, reinforcing the notion that they actually do not exist. The penetrating bundle, branching bundle and bundle branches were usually composed of ANP-negative myocytes although some ANP-positive myocytes were observed in the branching bundle and bundle branches in 4 cases. Myocytes from the ventricular conducting tissue presenting ANP immunoreactivity have been designated Purkinje fibers and have been found in several mammalian species.

ANP enhances bradycardic reflexes in normotensive but not spontaneously hypertensive rats

Baroreflex control of heart rate in spontaneously hypertensive rats (SHR) is defective, largely because of a poor vagal contribution to the reflex. We have demonstrated previously that atrial natriuretic peptide (ANP) enhances reflex bradycardia in normotensive rats through an action on nonarterial vagal afferent pathways. In the present study, we investigated whether ANP could reverse the baroreflex abnormality in SHR. Heart rate reflexes were activated by three different methods in conscious, instrumented SHR and Wistar-Kyoto rats (WKY) in the presence of intravenous infusions of vehicle (saline) or rat ANP (150 ng/kg per minute). Heart rate responses were measured by (1) the steady-state changes in blood pressure after alternating slow infusions (over approximately 15 to 30 seconds) of a pressor (methoxamine) and depressor (nitroprusside) drug (stimulating predominantly arterial baroreceptors), (2) the ramp method of rapid infusion of methoxamine (over < 10 seconds; stimulating arterial and cardiopulmonary baroreceptors), and (3) the von Bezold-Jarisch method of activating chemically sensitive cardiac receptors through serotonin injections. ANP enhanced the heart rate range of the arterial baroreflex (steady-state method) by 13 +/- 3% in WKY but had no significant effect on the sensitivity or any other parameter of the steady-state baroreflex. When a very rapid rise in blood pressure was elicited by the ramp method in WKY, ANP significantly enhanced baroreflex bradycardia (sensitivity increased by 29 +/- 9%, P < .05). ANP also enhanced the bradycardia of the von Bezold-Jarisch reflex (by 33 +/- 16%, P < .05) in WKY. By contrast, ANP did not influence baroreceptor or chemoreceptor heart rate reflex responses in SHR. We conclude that in normotensive rats, ANP facilitates cardiopulmonary bradycardic reflexes. The lack of effect of ANP in SHR may be related to an underlying structural or genetic alteration in their cardiac sensors, perhaps associated with cardiac hypertrophy, that prevents the ANP-induced activation of cardiac sensory afferents, resulting in cardioinhibition.

Natriuretic peptide immunoreactivity in nerve structures and Purkinje fibres of human, pig and sheep hearts

Atrial natriuretic peptide is a well-described peptide in cardiac Purkinje fibres and has been shown to interfere with the autonomic regulation in the heart of various species, including man. Recently, we detected immunoreactivity for the peptide in intracardial ganglionic cells and nerve fibre varicosities of bovine hearts, by the use of a modified immunostaining technique that induced an improved detection of natriuretic peptides. These findings raised the question as to whether natriuretic peptides are detectable in these tissues in man and other species. The conduction system from human, pig and sheep hearts was dissected processed with antisera against atrial natriuretic peptide and the closely related brain natriuretic peptide. Immunostaining for the brain natriuretic peptide was detected in some Purkinje fibres in all of these species. Interestingly, in pig, sheep and human hearts, some ganglionic cells and nerve fibres showed atrial natriuretic peptide immunoreactivity, particularly in the soma of human ganglionic cells. This is the first study showing immunoreactivity for the atrial natriuretic peptide in nerve structures and for the brain natriuretic peptide in Purkinje fibres of the human heart. The results give a morphological correlate for the documented effects of atrial natriuretic peptide on the heart autonomic nervous system and for the presumable effects of brain natriuretic peptide in the conduction system of man.

Atrial natriuretic peptide and cardiac electrophysiology: autonomic and direct effects

Atrial natriuretic peptide (ANP) has varied effects on cardiac electrophysiologic parameters including heart rate, intraatrial conduction time, and refractory period. ANP's vagoexcitatory and sympathoinhibitory actions as well as its direct actions on cardiac ion currents may be responsible for some of these effects. This review discusses the role of ANP in cardiac electrophysiology, its interactions with the autonomic nervous system and baroreceptor reflex, and its effects on cardiac ion currents.

cGMP and atrial natriuretic factor regulate cell volume of rabbit atrial myocytes

Atrial natriuretic factor (ANF) reduces the volume of atrial myocytes by inhibiting Na+/K+/2Cl- cotransport. We determined the role of cGMP and cAMP in ANF-induced shrinkage by using digital video microscopy to measure cell volume; volumes are reported relative to control. ANF (1 mumol/L) reversibly reduced atrial cell volume from 1.0 to 0.915 +/- 0.005 (mean +/- SEM). This effect was mimicked by 10 mumol/L 8-bromo-cGMP (8-Br-cGMP), which decreased myocyte volume to 0.894 +/- 0.007 with an ED50 of 0.99 +/- 0.05 mumol/L. In contrast, 100 mumol/L 8-bromo-cAMP (8-Br-cAMP) did not affect volume, and activating the cAMP pathway with 100 mumol/L 8-Br-cAMP did not alter the volume decrease caused by 8-Br-cGMP or ANF. Inhibition of Na+/K+/2Cl- cotransport with bumetanide (1 mumol/L) also reduced cell volume and prevented further shrinkage on subsequent exposure to 8-Br-cGMP. Similarly, 8-Br-cGMP (10 mumol/L) prevented further shrinkage by ANF. Block of Na(+)-H+ exchange, a participant in volume regulation in other cells, did not alter the response to 8-Br-cGMP. More evidence implicating cGMP was obtained by altering its metabolism. LY83583 (10 mumol/L), a guanylate cyclase inhibitor, blocked ANF-induced cell shrinkage. Zaprinast (100 mumol/L), a cGMP-specific phosphodiesterase inhibitor, markedly potentiated the effect of a threshold concentration of ANF (0.01 mumol/L). The actions of ANF, LY83583, and zaprinast on cGMP levels were verified by radioimmunoassay. These data strongly support the idea that the cGMP cascade is the intracellular signaling pathway responsible for ANF-induced atrial cell shrinkage.(ABSTRACT TRUNCATED AT 250 WORDS)

Gene expression of natriuretic peptide receptors in myocardial cells

Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are cardiac hormones that serve to unload the heart through their effects on the kidney and vasculature. Whether the heart itself represents a site of action for these peptides is currently the subject of debate. Although functional studies indicate that ANP has some effects on isolated myocytes, several studies have been unable to detect binding of the hormone to these cells. The present study demonstrates that the genes for all three natriuretic peptide receptor (NPR) subtypes, NPR-A, NPR-B, and NPR-C, are expressed in the rat heart. For microlocalization of the receptor mRNAs in myocytes and nonmyocytic cells, a combination of cell isolation and reverse transcription-polymerase chain reaction (RT-PCR) was used. Cardiac myocytes were isolated by enzymatic dissociation of rat ventricular tissue, purified by density gradient centrifugation, and collected as single cells under microscopic control. Analysis by RT-PCR revealed the presence of transcripts for NPR-A as well as NPR-B and NPR-C. However, cGMP generation in purified myocytes was stimulated only by ANP and BNP, which specifically bind to NPR-A, whereas C-type natriuretic peptide (CNP, an NPR-B agonist) was ineffective. Therefore, rat ventricular myocytes appear to produce predominantly NPR-A. The expression of NPR-B may be low or even absent. The mRNAs for all three NPRs were also found in cultures of fibroblasts from the rat heart. In contrast to the myocytes, large increases in cGMP were observed in response not only to ANP but also to CNP.

Natriuretic peptide receptor mRNAs in the rat and human heart

Functional studies indicate that atrial natriuretic peptide (ANP), a member of the natriuretic peptide family, has direct effects on cardiac muscle cells. However, conventional ligand-binding studies designed to establish the presence of natriuretic peptide-binding sites in the heart have yielded conflicting results. There are discrepancies also between the latter and the receptor distribution predicted from the pattern of the mRNA transcripts localized by in situ hybridization. Here we have employed the technique of cDNA amplification with the polymerase chain reaction to confirm the presence of natriuretic peptide A, B, and C receptor mRNAs in rat and human cardiac tissue. In the rat heart, the distribution of the A and B receptor transcripts appears to be relatively homogeneous; in contrast, the C type mRNA is concentrated principally in the atria, with no difference between the left and right sides of the heart. A and B receptor DNA products were obtained after amplification of left, but not right, ventricular cDNA from the heart of a 16-yr-old male with cystic fibrosis; the yield of C receptor DNA was similar for both ventricles. If these mRNA transcripts are translated into functional receptors in the rat and human heart, ANP and the other natriuretic peptides may have direct effects on cardiac function, including regulation of natriuretic peptide release via a short feedback loop, modulation of contractility of the heart, or activation of cardiac reflexes.

Differential regional expression of three natriuretic peptide receptor genes within primate tissues

The natriuretic peptide receptors are three homologous cell surface proteins, each with a single transmembrane domain. The atrial natriuretic peptide receptor type A (ANPRA) and the homologous receptor type B (ANPRB) are both membrane guanylyl cyclases that synthesize cyclic GMP as an intracellular second messenger. The third receptor in this family, the atrial natriuretic peptide receptor type C (ANPRC), is not coupled to cyclic GMP production. We report on the distribution of the ANPRA, ANPRB, and ANPRC mRNAs in rhesus monkey tissues assayed by in situ hybridization. ANPRA mRNA is most abundantly expressed in the kidney glomerulus, adrenal zona glomerulosa, pituitary, cerebellum, and endocardial endothelial cells of the right and left atrium and right ventricle. In contrast, abundant ANPRB expression appears to be confined to the adrenal medulla, pituitary, and cerebellum. ANPRC mRNA appeared to be expressed very differently than ANPRA and ANPRB. In the heart, ANPRC mRNA is expressed most prominently in endocardial endothelial cells of all four chambers but is also found throughout the myocardium only in the right atrium. These data identify major sites of natriuretic peptide receptor mRNA expression and suggest that there may be prominent cell type-specific differential distribution of these receptors in central and peripheral targets for the natriuretic peptides.

Differential regional expression of three natriuretic peptide receptor genes within primate tissues

The natriuretic peptide receptors are three homologous cell surface proteins, each with a single transmembrane domain. The atrial natriuretic peptide receptor type A (ANPRA) and the homologous receptor type B (ANPRB) are both membrane guanylyl cyclases that synthesize cyclic GMP as an intracellular second messenger. The third receptor in this family, the atrial natriuretic peptide receptor type C (ANPRC), is not coupled to cyclic GMP production. We report on the distribution of the ANPRA, ANPRB, and ANPRC mRNAs in rhesus monkey tissues assayed by in situ hybridization. ANPRA mRNA is most abundantly expressed in the kidney glomerulus, adrenal zona glomerulosa, pituitary, cerebellum, and endocardial endothelial cells of the right and left atrium and right ventricle. In contrast, abundant ANPRB expression appears to be confined to the adrenal medulla, pituitary, and cerebellum. ANPRC mRNA appeared to be expressed very differently than ANPRA and ANPRB. In the heart, ANPRC mRNA is expressed most prominently in endocardial endothelial cells of all four chambers but is also found throughout the myocardium only in the right atrium. These data identify major sites of natriuretic peptide receptor mRNA expression and suggest that there may be prominent cell type-specific differential distribution of these receptors in central and peripheral targets for the natriuretic peptides.

Atrial natriuretic factor decreases cell volume of rabbit atrial and ventricular myocytes

The effect of atrial natriuretic factor (ANF) on cell volume was studied using video microscopy of rabbit atrial and ventricular myocytes. Each cell served as its own control, and relative cell volumes were determined. ANF (1 microM) significantly decreased relative cell volume to 0.929 +/- 0.006 (n = 7) in atrial and 0.930 +/- 0.013 (n = 5) in ventricular myocytes (normalized to volume without ANF). Reduction of volume was detectable at greater than or equal to 0.01 microM ANF, and the ED50 was 0.072 +/- 0.007 microM (n = 15). The effect of ANF also was examined under hypotonic (0.55T, 168 mosmol/l) and hypertonic (1.82T, 560 mosmol/l) conditions; osmolarity was adjusted using mannitol with NaCl fixed at 65 mM. In 0.55T, 1 microM ANF decreased cell volume to 0.941 +/- 0.014 (n = 5) in atrial and 0.942 +/- 0.017 (n = 7) in ventricular cells (normalized to 0.55T without ANF). In contrast, 1 microM ANF had no effect on atrial (n = 13) or ventricular (n = 11) cell volume in 1.82T. The hypothesis that ANF decreases cell volume by inhibiting Na(+)-K(+)-2Cl- cotransport was tested by blocking the cotransporter with bumetanide (10 microM). After inhibition of Na(+)-K(+)-2Cl- cotransport, 1 microM ANF failed to reduce cell volume in either atrial (n = 6) or ventricular myocytes (n = 6). Block of ANF-induced cell shrinkage by bumetanide was not due to changes in cell volume, since similar results were obtained using atrial (n = 7) and ventricular (n = 7) cells swollen in hypotonic (0.80T, 244 mosmol/l) solution. Replacement of Na+ with N-methyl-D-glucamine or Cl- with methanesulfonate abolished the ability of both ANF and bumetanide to decrease volume of atrial and ventricular cells in 1T and 0.8T solution. These data suggest that ANF can decrease the volume of atrial and ventricular cells under isotonic and hypotonic conditions by a mechanism that may involve Na(+)-K(+)-2Cl- cotransport. An ANF-induced decrease in cell volume may act as negative feedback and inhibit the stretch-induced release of ANF from atrial and ventricular cells. Furthermore, it may contribute to cell volume maintenance in myocytes in the setting of congestive heart failure or myocardial hypoxia when ANF release is elevated.