Effects of C-type natriuretic peptide on rat cardiac contractility

C-type natriuretic peptide (CNP): The cardiovascular system and beyond

C-type natriuretic peptide (CNP) represents the 'local' member of the natriuretic peptide family, functioning in an autocrine or paracrine capacity to modulate a hugely diverse portfolio of physiological processes. Whilst the best-characterised of these regulatory roles are in the cardiovascular system, akin to its predominantly endocrine siblings atrial (ANP) and brain (BNP) natriuretic peptides, CNP governs many additional, unrelated mechanisms including bone growth, gamete maturation, auditory processing, and neuronal integrity. Furthermore, there is currently great interest in mimicking the biological activity of CNP for therapeutic gain in many of these disparate organ systems. Herein, we provide an overview of the physiology, pathophysiology and pharmacology of CNP in both cardiovascular and non-cardiovascular settings.

Exploring PKG signaling as a therapeutic avenue for pressure overload, ischemia, and HFpEF

INTRODUCTION

Heart failure (HF) is a complex and heterogeneous syndrome resulting from any diastolic or systolic dysfunction of the cardiac muscle. In addition to comorbid conditions, pressure overload, and myocardial ischemia are associated with cardiac remodeling which manifests as extracellular matrix (ECM) perturbations, impaired cellular responses, and subsequent ventricular dysfunction.

AREAS COVERED

The current review discusses the main aspects of the cyclic guanosine monophosphate (cGMP)-protein kinase G (PKG) pathway (cGMP-PKG) pathway modulators and highlights the promising outcomes of its novel pharmacological boosters.

EXPERT OPINION

Among several signaling pathways involved in the pathogenesis of pressure overload, ischemia and HF with preserved ejection fraction (HFpEF) is cGMP-PKG pathway. This pathway plays a pivotal role in the regulation of cardiac contractility, and modulation of cGMP-PKG signaling, contributing to the development of the diseases. Ventricular cardiomyocytes of HF patients and animal models are known to exhibit reduced cGMP levels and disturbed cGMP signaling including hypophosphorylation of PKG downstream targets. However, restoration of cGMP-PKG signaling improves cardiomyocyte function and promotes cardioprotective effects.

Role of Phosphodiesterase 1 in the Regulation of Real-Time cGMP Levels and Contractility in Adult Mouse Cardiomyocytes

In mouse cardiomyocytes, the expression of two subfamilies of the calcium/calmodulin-regulated cyclic nucleotide phosphodiesterase 1 (PDE1)-PDE1A and PDE1C-has been reported. PDE1C was found to be the major subfamily in the human heart. It is a dual substrate PDE and can hydrolyze both 3',5'-cyclic adenosine monophosphate (cAMP) and 3',5'-cyclic guanosine monophosphate (cGMP). Previously, it has been reported that the PDE1 inhibitor ITI-214 shows positive inotropic effects in heart failure patients which were largely attributed to the cAMP-dependent protein kinase (PKA) signaling. However, the role of PDE1 in the regulation of cardiac cGMP has not been directly addressed. Here, we studied the effect of PDE1 inhibition on cGMP levels in adult mouse ventricular cardiomyocytes using a highly sensitive fluorescent biosensor based on Förster resonance energy transfer (FRET). Live-cell imaging in paced and resting cardiomyocytes showed an increase in cGMP after PDE1 inhibition with ITI-214. Furthermore, PDE1 inhibition and PDE1A knockdown amplified the cGMP-FRET responses to the nitric oxide (NO)-donor sodium nitroprusside (SNP) but not to the C-type natriuretic peptide (CNP), indicating a specific role of PDE1 in the regulation of the NO-sensitive guanylyl cyclase (NO-GC)-regulated cGMP microdomain. ITI-214, in combination with CNP or SNP, showed a positive lusitropic effect, improving the relaxation of isolated myocytes. Immunoblot analysis revealed increased phospholamban (PLN) phosphorylation at Ser-16 in cells treated with a combination of SNP and PDE1 inhibitor but not with SNP alone. Our findings reveal a previously unreported role of PDE1 in the regulation of the NO-GC/cGMP microdomain and mouse ventricular myocyte contractility. Since PDE1 serves as a cGMP degrading PDE in cardiomyocytes and has the highest hydrolytic activities, it can be expected that PDE1 inhibition might be beneficial in combination with cGMP-elevating drugs for the treatment of cardiac diseases.

CNP Promotes Antiarrhythmic Effects via Phosphodiesterase 2

BACKGROUND

Ventricular arrhythmia and sudden cardiac death are the most common lethal complications after myocardial infarction. Antiarrhythmic pharmacotherapy remains a clinical challenge and novel concepts are highly desired. Here, we focus on the cardioprotective CNP (C-type natriuretic peptide) as a novel antiarrhythmic principle. We hypothesize that antiarrhythmic effects of CNP are mediated by PDE2 (phosphodiesterase 2), which has the unique property to be stimulated by cGMP to primarily hydrolyze cAMP. Thus, CNP might promote beneficial effects of PDE2-mediated negative crosstalk between cAMP and cGMP signaling pathways.

METHODS

To determine antiarrhythmic effects of cGMP-mediated PDE2 stimulation by CNP, we analyzed arrhythmic events and intracellular trigger mechanisms in mice in vivo, at organ level and in isolated cardiomyocytes as well as in human-induced pluripotent stem cell-derived cardiomyocytes.

RESULTS

In ex vivo perfused mouse hearts, CNP abrogated arrhythmia after ischemia/reperfusion injury. Upon high-dose catecholamine injections in mice, PDE2 inhibition prevented the antiarrhythmic effect of CNP. In mouse ventricular cardiomyocytes, CNP blunted the catecholamine-mediated increase in arrhythmogenic events as well as in I_CaL, I_NaL, and Ca²⁺ spark frequency. Mechanistically, this was driven by reduced cellular cAMP levels and decreased phosphorylation of Ca²⁺ handling proteins. Key experiments were confirmed in human iPSC-derived cardiomyocytes. Accordingly, the protective CNP effects were reversed by either specific pharmacological PDE2 inhibition or cardiomyocyte-specific PDE2 deletion.

CONCLUSIONS

CNP shows strong PDE2-dependent antiarrhythmic effects. Consequently, the CNP-PDE2 axis represents a novel and attractive target for future antiarrhythmic strategies.

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.

CNP regulates cardiac contractility and increases cGMP near both SERCA and TnI: difference from BNP visualized by targeted cGMP biosensors

AIMS

Guanylyl cyclase-B (GC-B; natriuretic peptide receptor-B, NPR-B) stimulation by C-type natriuretic peptide (CNP) increases cGMP and causes a lusitropic and negative inotropic response in adult myocardium. These effects are not mimicked by NPR-A (GC-A) stimulation by brain natriuretic peptide (BNP), despite similar cGMP increase. More refined methods are needed to better understand the mechanisms of the differential cGMP signalling and compartmentation. The aim of this work was to measure cGMP near proteins involved in regulating contractility to understand compartmentation of cGMP signalling in adult cardiomyocytes.

METHODS AND RESULTS

We constructed several fluorescence resonance energy transfer (FRET)-based biosensors for cGMP subcellularly targeted to phospholamban (PLB) and troponin I (TnI). CNP stimulation of adult rat cardiomyocytes increased cGMP near PLB and TnI, whereas BNP stimulation increased cGMP near PLB, but not TnI. The phosphodiesterases PDE2 and PDE3 constrained cGMP in both compartments. Local receptor stimulation aided by scanning ion conductance microscopy (SICM) combined with FRET revealed that CNP stimulation both in the t-tubules and on the cell crest increases cGMP similarly near both TnI and PLB. In ventricular strips, CNP stimulation, but not BNP, induced a lusitropic response, enhanced by inhibition of either PDE2 or PDE3, and a negative inotropic response. In cardiomyocytes from heart failure rats, CNP increased cGMP near PLB and TnI more pronounced than in cells from sham-operated animals.

CONCLUSION

These targeted biosensors demonstrate that CNP, but not BNP, increases cGMP near TnI in addition to PLB, explaining how CNP, but not BNP, is able to induce lusitropic and negative inotropic responses.

Phosphodiesterases and Compartmentation of cAMP and cGMP Signaling in Regulation of Cardiac Contractility in Normal and Failing Hearts

Cardiac contractility is regulated by several neural, hormonal, paracrine, and autocrine factors. Amongst these, signaling through β-adrenergic and serotonin receptors generates the second messenger cyclic AMP (cAMP), whereas activation of natriuretic peptide receptors and soluble guanylyl cyclases generates cyclic GMP (cGMP). Both cyclic nucleotides regulate cardiac contractility through several mechanisms. Phosphodiesterases (PDEs) are enzymes that degrade cAMP and cGMP and therefore determine the dynamics of their downstream effects. In addition, the intracellular localization of the different PDEs may contribute to regulation of compartmented signaling of cAMP and cGMP. In this review, we will focus on the role of PDEs in regulating contractility and evaluate changes in heart failure.

Cardiovascular Effects of Snake Toxins: Cardiotoxicity and Cardioprotection

Snake venoms, as complex mixtures of peptides and proteins, affect various vital systems of the organism. One of the main targets of the toxic components from snake venoms is the cardiovascular system. Venom proteins and peptides can act in different ways, exhibiting either cardiotoxic or cardioprotective effects. The principal classes of these compounds are cobra cardiotoxins, phospholipases A2, and natriuretic, as well as bradykinin-potentiating peptides. There is another group of proteins capable of enhancing angiogenesis, which include, e.g., vascular endothelial growth factors possessing hypotensive and cardioprotective activities. Venom proteins and peptides exhibiting cardiotropic and vasoactive effects are promising candidates for the design of new drugs capable of preventing or constricting the development of pathological processes in cardiovascular diseases, which are currently the leading cause of death worldwide. For example, a bradykinin-potentiating peptide from Bothrops jararaca snake venom was the first snake venom compound used to create the widely used antihypertensive drugs captopril and enalapril. In this paper, we review the current state of research on snake venom components affecting the cardiovascular system and analyse the mechanisms of physiological action of these toxins and the prospects for their medical application.

Deregulation of Ca2+-Signaling Systems in White Adipocytes, Manifested as the Loss of Rhythmic Activity, Underlies the Development of Multiple Hormonal Resistance at Obesity and Type 2 Diabetes

Various types of cells demonstrate ubiquitous rhythmicity registered as simple and complex Ca²⁺-oscillations, spikes, waves, and triggering phenomena mediated by G-protein and tyrosine kinase coupled receptors. Phospholipase C/IP₃-receptors (PLC/IP₃R) and endothelial NO-synthase/Ryanodine receptors (NOS/RyR)–dependent Ca²⁺ signaling systems, organized as multivariate positive feedback generators (PLC-G and NOS-G), underlie this rhythmicity. Loss of rhythmicity at obesity may indicate deregulation of these signaling systems. To issue the impact of cell size, receptors’ interplay, and obesity on the regulation of PLC-G and NOS-G, we applied fluorescent microscopy, immunochemical staining, and inhibitory analysis using cultured adipocytes of epididumal white adipose tissue of mice. Acetylcholine, norepinephrine, atrial natriuretic peptide, bradykinin, cholecystokinin, angiotensin II, and insulin evoked complex [Ca²⁺]_i responses in adipocytes, implicating NOS-G or PLC-G. At low sub-threshold concentrations, acetylcholine and norepinephrine or acetylcholine and peptide hormones (in paired combinations) recruited NOS-G, based on G proteins subunits interplay and signaling amplification. Rhythmicity was cell size- dependent and disappeared in hypertrophied cells filled with lipids. Contrary to control cells, adipocytes of obese hyperglycemic and hypertensive mice, growing on glucose, did not accumulate lipids and demonstrated hormonal resistance being non responsive to any hormone applied. Preincubation of preadipocytes with palmitoyl-L-carnitine (100 nM) provided accumulation of lipids, increased expression and clustering of IP₃R and RyR proteins, and partially restored hormonal sensitivity and rhythmicity (5–15% vs. 30–80% in control cells), while adipocytes of diabetic mice were not responsive at all. Here, we presented a detailed kinetic model of NOS-G and discussed its control. Collectively, we may suggest that universal mechanisms underlie loss of rhythmicity, Ca²⁺-signaling systems deregulation, and development of general hormonal resistance to obesity.

Multiplicity of Nitric Oxide and Natriuretic Peptide Signaling in Heart Failure

Heart failure (HF) is a common consequence of several cardiovascular diseases and is understood as a vicious cycle of cardiac and hemodynamic decline. The current inventory of treatments either alleviates the pathophysiological features (eg, cardiac dysfunction, neurohumoral activation, and ventricular remodeling) and/or targets any underlying pathologies (eg, hypertension and myocardial infarction). Yet, since these do not provide a cure, the morbidity and mortality associated with HF remains high. Therefore, the disease constitutes an unmet medical need, and novel therapies are desperately needed. Cyclic guanosine-3',5'-monophosphate (cGMP), synthesized by nitric oxide (NO)- and natriuretic peptide (NP)-responsive guanylyl cyclase (GC) enzymes, exerts numerous protective effects on cardiac contractility, hypertrophy, fibrosis, and apoptosis. Impaired cGMP signaling, which can occur after GC deactivation and the upregulation of cyclic nucleotide-hydrolyzing phosphodiesterases (PDEs), promotes cardiac dysfunction. In this study, we review the role that NO/cGMP and NP/cGMP signaling plays in HF. After considering disease etiology, the physiological effects of cGMP in the heart are discussed. We then assess the evidence from preclinical models and patients that compromised cGMP signaling contributes to the HF phenotype. Finally, the potential of pharmacologically harnessing cardioprotective cGMP to rectify the present paucity of effective HF treatments is examined.

C-type natriuretic peptide co-ordinates cardiac structure and function

AIMS

C-type natriuretic peptide (CNP) is an essential endothelium-derived signalling species that governs vascular homoeostasis; CNP is also expressed in the heart but an intrinsic role for the peptide in cardiac function is not established. Herein, we employ unique transgenic strains with cell-specific deletion of CNP to define a central (patho)physiological capacity of CNP in maintaining heart morphology and contractility.

METHODS AND RESULTS

Cardiac structure and function were explored in wild type (WT), cardiomyocyte (cmCNP-/-), endothelium (ecCNP-/-), and fibroblast (fbCNP-/-)-specific CNP knockout mice, and global natriuretic peptide receptor (NPR)-B-/-, and NPR-C-/- animals at baseline and in experimental models of myocardial infarction and heart failure (HF). Endothelium-specific deletion of CNP resulted in impaired coronary responsiveness to endothelium-dependent- and flow-mediated-dilatation; changes mirrored in NPR-C-/- mice. Ex vivo, global ischaemia resulted in larger infarcts and diminished functional recovery in cmCNP-/- and NPR-C-/-, but not ecCNP-/-, vs. WT. The cardiac phenotype of cmCNP-/-, fbCNP-/-, and NPR-C-/- (but not ecCNP-/- or NPR-B-/-) mice was more severe in pressure overload- and sympathetic hyperactivation-induced HF compared with WT; these adverse effects were rescued by pharmacological CNP administration in WT, but not NPR-C-/-, mice. At a molecular level, CNP/NPR-C signalling is impaired in human HF but attenuates activation of well-validated pro-hypertrophic and pro-fibrotic pathways.

CONCLUSION

C-type natriuretic peptide of cardiomyocyte, endothelial and fibroblast origins co-ordinates and preserves cardiac structure, function, and coronary vasoreactivity via activation of NPR-C. Targeting NPR-C may prove an innovative approach to treating HF and ischaemic cardiovascular disorders.

cGMP signalling in cardiomyocyte microdomains

3',5'-Cyclic guanosine monophosphate (cGMP) is one of the major second messengers critically involved in the regulation of cardiac electrophysiology, hypertrophy, and contractility. Recent molecular and cellular studies have significantly advanced our understanding of the cGMP signalling cascade, its local microdomain-specific regulation and its role in protecting the heart from pathological stress. Here, we summarise recent findings on cardiac cGMP microdomain regulation and discuss their potential clinical significance.

The effect of natriuretic C-type peptide and its change over time on mortality in patients on haemodialysis or haemodiafiltration

Background

C-type natriuretic peptide (CNP) and its co-product N-terminal proCNP (NTproCNP) have been associated with beneficial effects on the cardiovascular system. In prevalent dialysis patients, however, a relation between NTproCNP and mortality has not yet been investigated. Furthermore, as a middle molecular weight substance, its concentration might be influenced by dialysis modality.

Methods

In a cohort of patients treated with haemodialysis (HD) or haemodiafiltration (HDF), levels of NTproCNP were measured at baseline and 6, 12, 24 and 36 months. The relation between serum NTproCNP and mortality and the relation between the 6-month rate of change of NTproCNP and mortality were analysed using Cox regression models. For the longitudinal analyses, linear mixed models were used.

Results

In total, 406 subjects were studied. The median baseline serum NTproCNP was 93 pmol/L and the median follow-up was 2.97 years. No relation between baseline NTproCNP or its rate of change over 6 months and mortality was found. NTproCNP levels remained stable in HD patients, whereas NTproCNP decreased significantly in HDF patients. The relative decline depended on the magnitude of the convection volume.

Conclusions

In our study, levels of NTproCNP appear strongly elevated in prevalent dialysis patients. Second, while NTproCNP remains unaltered in HD patients, its levels decline in individuals treated with HDF, with the decline dependent on the magnitude of the convection volume. Third, NTproCNP is not related to mortality in this population. Thus NTproCNP does not seem to be a useful marker for mortality risk in dialysis patients.

Contrasting signals of cardiovascular health among natriuretic peptides in subjects without heart disease

Natriuretic Peptides (NP) are important in maintaining normal cardiac and metabolic status and have been used to predict cardiovascular events. Whether plasma concentrations of NP products within the normal range reflect cardio-metabolic health is unknown. Plasma NTproANP, NTproBNP and NTproCNP and their bioactive counterparts were measured in a random sample of 348 community dwellers aged 49-51 yr without heart disease and associations sought with established vascular risk factors, echocardiographic indices and a genetic variant previously linked with BNP. Stratified by sex, each of ten vascular risk factors were positively associated with NTproCNP whereas associations with NTproBNP and NTproANP were all negative. In both sexes, higher plasma NTproCNP was associated with higher arterial elastance, lower LV stroke volume and lower LV end diastolic volume. Exactly opposite associations were found with plasma NTproBNP or NTproANP. Sex specific differences were identified: positive association of NTproBNP with LV end systolic volume and the negative association with LV elastance were found only in males. The genetic variant rs198358 was independently associated with NTproBNP but not with NTproANP. In conclusion, higher NTproCNP is likely to be an adaptive response to impaired LV relaxation whereas genetic factors likely contribute to higher NTproBNP and improved cardio-metabolic health at midlife.

Emerging pathways of communication between the heart and non-cardiac organs

The breakthrough discovery of cardiac natriuretic peptides provided the first direct demonstration of the connection between the heart and the kidneys for the maintenance of sodium and volume homeostasis in health and disease. Yet, little is still known about how the heart and other organs cross-talk. Here, we review three physiological mechanisms of communication linking the heart to other organs through: i) cardiac natriuretic peptides, ii) the microRNA-208a/mediator complex subunit-13 axis and iii) the matrix metalloproteinase-2 (MMP-2)/C-C motif chemokine ligand-7/cardiac secreted phospholipase A2 (sPLA2) axis – a pathway which likely applies to the many cytokines, which are cleaved and regulated by MMP-2. We also suggest experimental strategies to answer still open questions on the latter pathway. In short, we review evidence showing how the cardiac secretome influences the metabolic and inflammatory status of non-cardiac organs as well as the heart.

C-type Natriuretic Peptide: A Multifaceted Paracrine Regulator in the Heart and Vasculature

C-type natriuretic peptide (CNP) is an autocrine and paracrine mediator released by endothelial cells, cardiomyocytes and fibroblasts that regulates vital physiological functions in the cardiovascular system. These roles are conveyed via two cognate receptors, natriuretic peptide receptor B (NPR-B) and natriuretic peptide receptor C (NPR-C), which activate different signalling pathways that mediate complementary yet distinct cellular responses. Traditionally, CNP has been deemed the endothelial component of the natriuretic peptide system, while its sibling peptides, atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP), are considered the endocrine guardians of cardiac function and blood volume. However, accumulating evidence indicates that CNP not only modulates vascular tone and blood pressure, but also governs a wide range of cardiovascular effects including the control of inflammation, angiogenesis, smooth muscle and endothelial cell proliferation, atherosclerosis, cardiomyocyte contractility, hypertrophy, fibrosis, and cardiac electrophysiology. This review will focus on the novel physiological functions ascribed to CNP, the receptors/signalling mechanisms involved in mediating its cardioprotective effects, and the development of therapeutics targeting CNP signalling pathways in different disease pathologies.

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.

Distinct submembrane localisation compartmentalises cardiac NPR1 and NPR2 signalling to cGMP

Natriuretic peptides (NPs) are important hormones that regulate multiple cellular functions including cardiovascular physiology. In the heart, two natriuretic peptide receptors NPR1 and NPR2 act as membrane guanylyl cyclases to produce 3′,5′-cyclic guanosine monophosphate (cGMP). Although both receptors protect from cardiac hypertrophy, their effects on contractility are markedly different, from little effect (NPR1) to pronounced negative inotropic and positive lusitropic responses (NPR2) with unclear underlying mechanisms. Here we use a scanning ion conductance microscopy (SICM) approach combined with Förster resonance energy transfer (FRET)-based cGMP biosensors to show that whereas NPR2 is uniformly localised on the cardiomyocyte membrane, functional NPR1 receptors are found exclusively in membrane invaginations called transverse (T)-tubules. This leads to far-reaching CNP/NPR2/cGMP signals, whereas ANP/NPR1/cGMP signals are highly confined to T-tubular microdomains by local pools of phosphodiesterase 2. This provides a previously unrecognised molecular basis for clearly distinct functional effects engaged by different cGMP producing membrane receptors.

Natriuretic peptides (NPs) are important hormones that regulate cardiovascular physiology by increasing cGMP levels in cardiomyocytes. Here the authors use scanning ion conductance microscopy and a cGMP FRET sensor to identify a differential localisation pattern for the natriuretic peptide receptors within the heart.

cGMP Signaling in the Cardiovascular System-The Role of Compartmentation and Its Live Cell Imaging

The ubiquitous second messenger 3′,5′-cyclic guanosine monophosphate (cGMP) regulates multiple physiologic processes in the cardiovascular system. Its intracellular effects are mediated by stringently controlled subcellular microdomains. In this review, we will illustrate the current techniques available for real-time cGMP measurements with a specific focus on live cell imaging methods. We will also discuss currently accepted and emerging mechanisms of cGMP compartmentation in the cardiovascular system.

Total peripheral vascular resistance, cardiac output, and plasma C-type natriuretic Peptide level in children with postural tachycardia syndrome

OBJECTIVE

To investigate the total peripheral vascular resistance (TPVR), cardiac output (CO), and plasma C-type natriuretic peptide (CNP) levels in children with postural tachycardia syndrome (POTS) during supine, upright, and return to supine.

STUDY DESIGN

Twenty-nine children with POTS, aged 12 ± 3 years, were recruited, and 32 healthy children, aged 11 ± 2 years, served as controls. Heart rate (HR), blood pressure, TPVR, and CO were continuously monitored with Finapres Medical System, and plasma CNP levels were detected with Sandwich immunoluminescence assay.

RESULTS

In children with POTS, upright TPVR and CO were significantly lower than those in supine position, and they rose again when they returned to supine position. However, in healthy control patients, both TPVR and CO did not change during supine, upright, and supine again positions. Also, in the supine position, there was no significant difference in TPVR and CO between POTS children and control subjects (P > .05). When upright, however, TPVR and CO in children with POTS were significantly lower than those of controls. Plasma CNP levels were significantly greater in children with POTS than that of controls (32.8 ± 9.7 vs 24.2 ± 8.4 [pg/mL], P < .01), and symptom scores and ΔHR positively correlated with plasma CNP levels in children with POTS (symptom scores: r = 0.490, P < .01; ΔHR: r = 0.508, P < .001), but CO negatively correlated with plasma CNP levels (r = -0.446, P < .01).

CONCLUSION

Reduced TPVR and CO associated with the elevated plasma CNP might be involved in the pathogenesis of POTS.

SERCA2 activity is involved in the CNP-mediated functional responses in failing rat myocardium

BACKGROUND AND PURPOSES

Myocardial C-type natriuretic peptide (CNP) levels are increased in heart failure. CNP can induce negative inotropic (NIR) and positive lusitropic responses (LR) in normal hearts, but its effects in failing hearts are not known. We studied the mechanism of CNP-induced NIR and LR in failing hearts and determined whether sarcoplasmatic reticulum Ca(2+) ATPase2 (SERCA2) activity is essential for these responses.

EXPERIMENTAL APPROACH

Contractility, cGMP levels, Ca(2+) transient amplitudes and protein phosphorylation were measured in left ventricular muscle strips or ventricular cardiomyocytes from failing hearts of Wistar rats 6 weeks after myocardial infarction.

KEY RESULTS

CNP increased cGMP levels, evoked a NIR and LR in muscle strips, and caused phospholamban (PLB) Ser(16) and troponin I (TnI) Ser(23/24) phosphorylation in cardiomyocytes. Both the NIR and LR induced by CNP were reduced in the presence of a PKG blocker/cGMP analogue (Rp-8-Br-Pet-cGMPS) and the SERCA inhibitor thapsigargin. CNP increased the amplitude of the Ca(2+) transient and increased SERCA2 activity in cardiomyocytes. The CNP-elicited NIR and LR were not affected by the L-type Ca(2+) channel activator BAY-K8644, but were abolished in the presence of isoprenaline (induces maximal activation of cAMP pathway). This suggests that phosphorylation of PLB and TnI by CNP causes both a NIR and LR. The NIR to CNP in mouse heart was abolished 8 weeks after cardiomyocyte-specific inactivation of the SERCA2 gene.

CONCLUSIONS AND IMPLICATIONS

We conclude that CNP-induced PLB and TnI phosphorylation by PKG in concert mediate both a predictable LR as well as the less expected NIR in failing hearts.

The alteration of NTproCNP plasma levels following anaerobic exercise in physically active young men

Objective:

Amino-terminal propeptide of C-type natriuretic peptide (NTproCNP) is a synthesis product of C-type natriuretic peptide (CNP). In this study, plasma levels of NTproCNP were compared before and after exercise in healthy young subjects who are physically active (PA) or not physically active (NPA).

Methods:

The study was carried on PA group (n=10) who defined the exercise duration more than 2.5 hours per week for at least one year and NPA group (n=10) whose exercise duration was lower than 1.5 hours per week. The level of maximal oxygen consumption was determined. Wingate exercise test was applied on the following day. Plasma NTproCNP levels were measured before the exercise and at the 1^st, 5^th and 30^th minute after the exercise.

Results:

Exercise duration of physically active group was reported as 11.3±5.0 hours per week. Basal NTproCNP levels of the groups were found to be comparable. NTproCNP levels in the 5^th minute (0.93±0.23 pmol/L; p<0.05) and in the 30^th minute (0.77±0.21 pmol/L p<0.05) after exercise were higher than the levels before exercise (0.64±0.29 pmol/L) in PA group. Additionally, the plasma levels of NTproCNP after 5^th minute of exercise were higher in PA group (0.93±0.23 pmol/L) than NPA group (0.74±0.16 pmol/L, p<0.05).

Conclusion:

Being physically active may be a fact affecting the secretion of CNP, which plays a protective role in endothelium, following exercise.

The heart as an endocrine organ

The concept of the heart as an endocrine organ arises from the observation that the atrial cardiomyocytes in the mammalian heart display a phenotype that is partly that of endocrine cells. Investigations carried out between 1971 and 1983 characterised, by virtue of its natriuretic properties, a polypeptide referred to atrial natriuretic factor (ANF). Another polypeptide isolated from brain in 1988, brain natriuretic peptide (BNP), was subsequently characterised as a second hormone produced by the mammalian heart atria. These peptides were associated with the maintenance of extracellular fluid volume and blood pressure. Later work demonstrated a plethora of other properties for ANF and BNP, now designated cardiac natriuretic peptides (cNPs). In addition to the cNPs, other polypeptide hormones are expressed in the heart that likely act upon the myocardium in a paracrine or autocrine fashion. These include the C-type natriuretic peptide, adrenomedullin, proadrenomedullin N-terminal peptide and endothelin-1. Expression and secretion of ANF and BNP are increased in various cardiovascular pathologies and their levels in blood are used in the diagnosis and prognosis of cardiovascular disease. In addition, therapeutic uses for these peptides or related substances have been found. In all, the discovery of the endocrine heart provided a shift from the classical functional paradigm of the heart that regarded this organ solely as a blood pump to one that regards this organ as self-regulating its workload humorally and that also influences the function of several other organs that control cardiovascular function.

Influence of phosphodiesterases and cGMP on cAMP generation and on phosphorylation of phospholamban and troponin I by 5-HT4 receptor activation in porcine left atrium

Our objective was to investigate the role of phosphodiesterase (PDE)3 and PDE4 and cGMP in the control of cAMP metabolism and of phosphorylation of troponin I (TnI) and phospholamban (PLB) when 5-HT4 receptors are activated in pig left atrium. Electrically paced porcine left atrial muscles, mounted in organ baths, received stimulators of particulate guanylyl cyclase (pGC) or soluble guanylyl cyclase (sGC) and/or specific PDE inhibitors followed by 5-HT or the 5-HT4 receptor agonist prucalopride. Muscles were freeze-clamped at different moments of exposure to measure phosphorylation of the cAMP/protein kinase A targets TnI and PLB by immunoblotting and cAMP levels by enzyme immunoassay. Corresponding with the functional results, 5-HT only transiently increased cAMP content, but caused a less quickly declining phosphorylation of PLB and did not significantly change TnI phosphorylation. Under combined PDE3 and PDE4 inhibition, the 5-HT-induced increase in cAMP levels and PLB phosphorylation was enhanced and sustained, and TnI phosphorylation was now also increased. Responses to prucalopride per se and the influence thereupon of PDE3 and PDE4 inhibition were similar except that responses were generally smaller. Stimulation of pGC together with PDE4 inhibition increased 5-HT-induced PLB phosphorylation compared to 5-HT alone, consistent with functional responses. sGC stimulation hastened the fade of inotropic responses to 5-HT, while cAMP levels were not altered. PDE3 and PDE4 control the cAMP response to 5-HT4 receptor activation, causing a dampening of downstream signalling. Stimulation of pGC is able to enhance inotropic responses to 5-HT by increasing cAMP levels, while sGC stimulation decreases contraction to 5-HT cAMP independently.

cGMP-dependent protein kinases (cGK)

cGMP-dependent protein kinases (cGK) are serine/threonine kinases that are widely distributed in eukaryotes. Two genes-prkg1 and prkg2-code for cGKs, namely, cGKI and cGKII. In mammals, two isozymes, cGKIα and cGKIβ, are generated from the prkg1 gene. The cGKI isozymes are prominent in all types of smooth muscle, platelets, and specific neuronal areas such as cerebellar Purkinje cells, hippocampal neurons, and the lateral amygdala. The cGKII prevails in the secretory epithelium of the small intestine, the juxtaglomerular cells, the adrenal cortex, the chondrocytes, and in the nucleus suprachiasmaticus. Both cGKs are major downstream effectors of many, but not all, signalling events of the NO/cGMP and the ANP/cGMP pathways. cGKI relaxes smooth muscle tone and prevents platelet aggregation, whereas cGKII inhibits renin secretion, chloride/water secretion in the small intestine, the resetting of the clock during early night, and endochondral bone growth. This chapter focuses on the involvement of cGKs in cardiovascular and non-cardiovascular processes including cell growth and metabolism.

Study of the regulation of the inotropic response to 5-HT4 receptor activation via phosphodiesterases and its cross-talk with C-type natriuretic peptide in porcine left atrium

We studied how 5-HT(4) receptor-mediated inotropic responses are regulated at the level of cAMP in porcine left atrium. We used selective phosphodiesterase (PDE) inhibitors to assess which PDE subtypes are responsible for the fade with time of inotropic responses to 5-HT(4) receptor activation with 5-HT and the 5-HT(4) receptor agonist prucalopride. A possible cross-talk via PDEs between cGMP and 5-HT(4) receptor-induced cAMP signalling was evaluated. Electrically paced left atrial pectinate muscles from young male pigs (15-25 kg) were studied in vitro. Simultaneous inhibition of PDE3 plus PDE4 subtypes was necessary to increase the amplitude and completely prevent the fade of the inotropic response to 5-HT and prucalopride. When responses to 5-HT or prucalopride had faded 1 h after addition, the nonspecific PDE-inhibitor IBMX still fully recovered inotropic responses. Stimulation of particulate guanylyl cyclase, together with PDE2 and PDE4 inhibition, delayed the fade of the response to 5-HT, while stimulation of soluble guanylyl cyclase independently of PDEs accelerated the fade of the response to 5-HT. In conclusion, both PDE3 and PDE4 subtypes are responsible for the suppression and the fade of the inotropic response to 5-HT and prucalopride. Signalling through the 5-HT(4) receptor remains fully active for at least 90 min with PDEs continuously regulating the response. cGMP levels, elevated by activation of particulate guanylyl cyclase under PDE2 inhibition, can indirectly enhance 5-HT(4) receptor-mediated signalling, at least when also PDE4 is inhibited, presumably through inhibition of PDE3. Elevation of cGMP generated by soluble guanylyl cyclase attenuates responses to 5-HT independently of PDEs.

Acute effects of intravenous nesiritide on cardiac contractility in heart failure

BACKGROUND

Although nesiritide is a potent vasodilator, studies using myocytes and isolated muscle strips have shown that recombinant B-type natriuretic peptide (BNP; nesiritide) decreases contractility. We sought to determine whether nesiritide decreases contractility in heart failure patients.

METHODS AND RESULTS

Twenty-five heart failure patients underwent left heart catheterization (using a pressure-volume conductance catheter) and echocardiography at baseline and after a 2 mcg/kg bolus and 30-minute nesiritide infusion (0.01 mcg.kg.min). From invasive and noninvasive measurements, left ventricular (LV) systolic function indices were calculated, including ejection fraction, end-systolic elastance (E(es); single-beat invasive and noninvasive methods) and preload-recruitable stroke work (PRSW; noninvasive, single-beat method). The mean age was 60 +/- 11 years, 48% were male, 56% had coronary disease, and 64% had hypertension. Although nesiritide did not change LV ejection fraction, it did decrease contractility on pressure-volume analysis. Noninvasive E(es) decreased from 2.6 +/- 1.6 to 2.0 +/- 1.4 mm Hg/mL (P = .02). For those with reduced ejection fraction, E(es) decreased by invasive (P = .006) and noninvasive (P = .02) methods. PRSW decreased from 76 +/- 37 to 62 +/- 28 g/cm(2) (P = .003). On tissue Doppler imaging, nesiritide reduced the systolic annular tissue velocity of the mitral annulus from 8.0 +/- 1.9 to 6.9 +/- 1.3 cm/s (P = .04).

CONCLUSIONS

Nesiritide infusion acutely decreases derived measures of contractility and systolic function in patients with chronic heart failure.

Neutral endopeptidase inhibitor suppresses the early phase of atrial electrical remodeling in a canine rapid atrial pacing model

INTRODUCTION

We examined the acute effects of neutral endopeptidase inhibitor on the hemodynamics and electrical properties of dogs subjected to rapid atrial pacing.

METHODS

Ten beagle dogs were used and divided into two groups with and without candoxatril, a neutral endopeptidase inhibitor preadministration. Before and after the 6 hours rapid atrial pacing from the right atrial appendage, the hemodynamics, atrial effective refractory period, and monophasic action potential duration of the right atrial appendage were measured and blood samples were collected. Atrial tissue was also excised after the experiment.

RESULTS

Candoxatril significantly increased plasma ANP levels (Control: 88.4 +/- 50.25 vs. Candoxatril: 197.1 +/- 32.09 pg/ml, p = 0.004) and prevented reductions in atrial effective refractory period and monophasic action potential duration. We further demonstrated that the treated animals exhibited significantly higher levels of atrial tissue cyclic GMP (Control: 28.1 +/- 1.60 fmol/mg vs. Candoxatril: 44.5 +/- 12.28 fmol/mg, p = 0.034) as well as that of plasma cyclic GMP (Control: 32 +/- 5.5 vs. Candoxatril: 42 +/- 7.1 pg/ml, p = 0.028).

CONCLUSION

Candoxatril suppressed the shortening of atrial effective refractory period and monophasic action potential duration in the rapid atrial pacing model. As plasma ANP and the atrial tissue levels of cyclic GMP were higher in the Candoxatril group than the control, this effect was considered to appear through the reduction of calcium overload caused by ANP and cyclic GMP.

Chronic nitrates blunt the effects of not only nitric oxide but also natriuretic peptides in cardiac myocytes

Exposure to nitrates causes tachyphylaxis to nitric oxide (NO), which reduces the effects of the second messenger cyclic guanosine-3',-5'-monophosphate (cyclic GMP). We tested the hypothesis that prolonged exposure to NO would also blunt the effects of natriuretic peptides. Cardiac myocytes were isolated from control (N=7) and chronic nitroglycerin (patched, N=7) rabbits. Patched animals received a transdermal nitroglycerin patch (0.3mg/h for 5 days). Myocyte function was determined at baseline, after C-type natriuretic peptide (CNP, 10(-8) and 10(-7)M) or brain natriuretic peptide (BNP, 10(-8) and 10(-7)M) or S-nitroso-N-acetyl-penicilliamine (SNAP, a NO donor, 10(-6) and 10(-5)M) followed by KT5823 (a cyclic GMP protein kinase inhibitor, 10(-6)M). Soluble and particulate guanylyl cyclase activities were measured in vitro and phosphoprotein analysis was performed. In control animals, CNP 10(-8)M (5.14+/-0.5%) and 10(-7)M (4.4+/-0.7%) significantly reduced percentage shortening from baseline (6.1+/-1.6%). KT5823 restored percentage shortening to 4.9+/-0.8%. Similar data were obtained with BNP and SNAP. In patched animals, CNP, BNP, SNAP had no significant effects on percentage shortening. The data on maximal rate of shortening and relaxation were consistent with these results. Guanylyl cyclase activities were not different in the control and patched animals. The myocytes from control and patched animals had similar protein phosphorylation patterns. Our data suggested that in addition to NO, the responses to both natriuretic peptides were downregulated after chronic exposure to nitroglycerin, but these effects were not due to changes in either guanylyl cyclase or cyclic GMP protein kinase, suggesting an altered downstream pathway.

Natriuretic peptide receptor B signaling in the cardiovascular system: protection from cardiac hypertrophy

Natriuretic peptides (NP) represent a family of structurally homologous but genetically distinct peptide hormones involved in regulation of fluid and electrolyte balance, blood pressure, fat metabolism, cell proliferation, and long bone growth. Recent work suggests a role for natriuretic peptide receptor B (NPR-B) signaling in regulation of cardiac growth by either a direct effect on cardiomyocytes or by modulation of other signaling pathways including the autonomic nervous system. The research links NPR-B for the first time to a cardiac phenotype in vivo and underlines the importance of the NP in the cardiovascular system. This manuscript will focus on the role of NPR-B and its ligand C-type natriuretic peptide in cardiovascular physiology and disease and will evaluate these new findings in the context of the known function of this receptor, with a perspective on how future research might further elucidate NPR-B function.

Altered role of C-type natriuretic peptide-activated pGC-cGMP-PDE3-cAMP signaling in hyperthyroid beating rabbit atria

The role of C-type natriuretic peptide (CNP) in the pathophysiology of atrial function in hyperthyroidism has not been defined. This study was to define the role of CNP-activated particulate (p) guanylyl cyclase (GC)-cGMP-phosphodiesterase (PDE)3 signaling in the regulation of cAMP levels and contractile and secretory functions in the atria from hyperthyroid rabbits. Experiments were performed in perfused beating rabbit atria. CNP was used to activate pGC. In euthyroid atria from sham-treated rabbits, CNP (100 nM) increased cGMP and cAMP efflux by 176.7+/-17.7 and 55.3+/-10.0%, respectively. CNP decreased stroke volume and pulse pressure and ANP release by 51+/-7 and 41+/-2 and 60.4+/-3.2%, respectively. Pretreatment with milrinone blocked the CNP-induced increase of cAMP but without significant changes in decrease of atrial dynamics and ANP release. In hyperthyroid atria, CNP-induced increase of cGMP levels was accentuated, while CNP-induced increase of cAMP was attenuated. The gain of cAMP, i.e., change in cAMP efflux concentration in terms of cGMP was attenuated in the hyperthyroid compared to euthyroid atria. CNP rather increased atrial dynamics in hyperthyroid atria instead of decrease. CNP-induced decrease in atrial ANP release was attenuated. Pretreatment with milrinone blocked the CNP-induced increase of cAMP levels concomitantly with a decrease of atrial dynamics. The present study demonstrates that altered role of CNP-activated pGC-cGMP-PDE3-cAMP signaling is involved in the pathophysiology of hyperthyroid heart.

Negative inotropic effects of C-type natriuretic peptide are attenuated in hypertrophied ventricular myocytes associated with reduced cyclic GMP production

BACKGROUND

We tested the hypothesis that the negative inotropic effects of C-type natriuretic peptide (CNP) would be diminished in renal hypertensive (one-kidney-one-clip, 1K1C) hypertrophic rabbit hearts and that this attenuated effect would be due either to decreased cyclic GMP production or to reduced signaling.

MATERIAL AND METHODS

Using isolated control and 1K1C ventricular myocytes, cell shortening data (video edge detection) were collected: (1) at baseline and after CNP 10(-8,-7) M, followed by KT5823 (KT), a cyclic GMP-dependent protein kinase inhibitor; or (2) at baseline, following KT pre-treatment and subsequent CNP 10(-8,-7) M. In addition, cyclic GMP levels were determined by radioimmunoassay at baseline and CNP 10(-7) M.

RESULTS

In control myocytes, CNP decreased percent shortening (5.7 +/- 0.4 versus 4.0 +/- 0.4% at 10(-7) M), maximal rate of shortening (58.7 +/- 5.1 versus 45.2 +/- 3.6 microm/sec) and maximal rate of relaxation (57.1 +/- 4.9 versus 44.1 +/- 3.4 microm/sec) in a concentration-dependent manner. These effects were attenuated by subsequent KT administration. CNP failed to produce these negative functional effects in 1K1C myocytes. When pre-treated with KT, CNP had no negative functional effect in either normal and 1K1C myocytes. Basal levels of cyclic GMP were similar in control versus 1K1C myocytes; however, CNP produced a significant rise in cyclic GMP level in control (63.6 +/- 7.8 versus 83.5 +/- 11.3 pmol/10(5) myocytes) but not in 1K1C (49.2 +/- 2.6 versus 52.7 +/- 5.6) myocytes.

CONCLUSIONS

Thus, CNP acted through the cyclic GMP protein kinase in control myocytes. We conclude that in hypertrophic cardiac myocytes, the decreased effect of CNP was because of decreased production of cyclic GMP.

Cyclic GMP signaling and regulation of SERCA activity during cardiac myocyte contraction

We tested the hypothesis that cGMP-induced reductions in cardiac myocyte function were related to activation of the sarcoplasmic reticulum Ca2+-ATPase (SERCA) and cGMP-dependent phosphorylation of phospholamban. Ventricular myocyte function was measured using a video edge detector (n = 11 rabbits). Thapsigargin (TG) or cyclopiazonic acid (CPA) were used to inhibit SERCA. 8-Bromo-cGMP was added at 10(-6), 10(-5) M followed by TG 10(-8) M or KT5823 (cGMP-protein kinase inhibitor, 10(-6) M) prior to TG or CPA. Cyclic GMP-dependent protein phosphorylation and immunoblotting with anti-phospholamban antibody were examined. TG 10(-8) M significantly increased percent shortening (from 6.6+/-0.7 to 9.1+/-1.3%). Cyclic GMP 10(-5) M significantly decreased cell shortening from 9.3+/-0.9 to 5.1+/-0.6%. This was partially reversed by KT5823 (5.1+/-0.6 to 8.2+/-1.4%) suggesting that negative functional effects of cGMP were partially through the cGMP-dependent protein kinase. Addition of TG after cGMP also reduced the negative effects of cGMP on myocyte shortening suggesting involvement of SERCA in cGMP signaling. TG after cGMP and KT5823 treatment did not alter myocyte contractility (8.2+/-1.4 to 7.2+/-1.3%). CPA had similar effects as those of TG. Protein phosphorylation and immunoblotting showed that phospholamban was a target of the cGMP protein kinase. These results indicated that the cyclic GMP-induced reductions in myocyte function were partially mediated through the action of SERCA. It further suggested that cGMP signaling affects myocyte function through phosphorylation of phospholamban which regulates SERCA activity.

C-type natriuretic peptide and guanylyl cyclase B receptor

Guanylyl cyclases (GC) are widely distributed enzymes that signal via the production of the second messenger cGMP. The particulate guanylyl cyclases share a similar topology: an extracellular ligand binding domain and intracellular regulatory kinase-homology and cyclase catalytic domains. The natriuretic peptide receptors GC-A and -B mediate the effects of a family of peptides, atrial, B- and C-type natriuretic peptide (ANP, BNP and CNP, respectively), with natriuretic, diuretic and vasorelaxant properties. ANP and BNP, through the activation of GC-A, act as endocrine hormones to regulate blood pressure and volume, and inhibit cardiac hypertrophy. CNP, on the other hand, acts in an autocrine/paracrine fashion to induce vasorelaxation and vascular remodeling, and to regulate bone growth through its cognate receptor GC-B. GC-B, like GC-A, is phosphorylated in the basal state, and undergoes both homologous and heterologous desensitization, reflected by dephosphorylation of specific sites in the kinase-homology domain. This review will examine the structure and function of GC-B, and summarize the physiological processes in which this receptor is thought to participate.

Differential effects of cGMP produced by soluble and particulate guanylyl cyclase on mouse ventricular myocytes

Particulate guanylyl cyclase (pGC) and soluble guanylyl cyclase (sGC) are cGMP-generation systems distributed in different intracellular locations. Our aim was to test the hypothesis that the functional effects of cGMP produced by pGC and sGC on contraction and Ca2+ transients would differ in ventricular myocytes. We measured myocyte shortening from adult mice using a video edge-detector and investigated the functional changes after stimulating pGC with C-type natriuretic peptide (CNP; 10(-8) M and 10(-7) M) or sGC with S-nitroso-N-acetyl-penicillamine (SNAP; nitric oxide donor; 10(-6) M and 10(-5) M). Significant concentration-dependent decreases in percentage shortening (PCS), maximal rate of shortening (RSmax), and relaxation (RRmax) were produced by CNP. To a similar degree, SNAP concentration-dependently reduced PCS, RSmax, and RRmax. The addition of Rp-8-[(4-chlorophenyl)thio]-cGMPS triethylamine (cGMP-dependent protein kinase inhibitor; 5 x 10(-6) M) or erythro-9-(2-hydroxy-3-nonyl) adenine (cGMP-stimulated cAMP phosphodiesterase inhibitor; 10(-5) M) reduced the responses induced by CNP or SNAP, suggesting that their actions were through cGMP-mediated pathways. While SNAP significantly increased intracellular cGMP concentration by 57%, CNP had little effect on cGMP production. We also found that CNP markedly decreased the amplitude of Ca2+ transients while SNAP had little effect, suggesting the cGMP generated by sGC may decrease myofilament Ca2+ sensitivity. The small amount of cGMP generated by pGC had a major effect in reducing Ca2+ level. This study suggested the existence of compartmentalization for cGMP in ventricular myocytes.

Autocrine and paracrine actions of natriuretic peptides in the heart

The natriuretic peptides, atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP), are a family of polypeptide mediators exerting numerous actions in cardiovascular homeostasis. ANP and BNP are cardiac derived, being secreted and up-regulated in myocardium in response to many pathophysiological stimuli. CNP is an endothelium-derived mediator. The classical endocrine effects of ANP and BNP on fluid homeostasis and blood pressure, especially in conditions characterised by left ventricular dysfunction, are well recognised and extensively researched. However, there is accumulating evidence that, in addition to endocrine actions, ANP and BNP exhibit important autocrine and paracrine functions within the heart and coronary circulation. These include regulation of myocyte growth, inhibition of fibroblast proliferation and extracellular matrix deposition, a cytoprotective anti-ischaemic (preconditioning-like) function, and influences on coronary endothelium and vascular smooth muscle proliferation and contractility. Most if not all of these actions can be ascribed to particulate guanylyl cyclase activation because the ANP/BNP receptor, natriuretic peptide receptor (NPR)-A, has an intracellular guanylyl cyclase domain. Subsequent elevation of the intracellular second messenger cGMP may exert diverse physiological effects through activation of cGMP-dependent protein kinases (cGK), predominantly cGK-I. However, there appear to be other contributory mechanisms in several of these actions, including the augmentation of nitric oxide synthesis. These diverse actions may represent counterregulatory mechanisms in the pathophysiology of many cardiovascular diseases, not just those typified by left ventricular dysfunction. Ultimately, insights from the autocrine/paracrine actions of natriuretic peptides may provide routes to therapeutic application in cardiac diseases of natriuretic peptides and drugs that modify their availability.

CNP-induced changes in pHi, cGMP/cAMP and mRNA expression of natriuretic peptide receptors in human trabecular meshwork cells

It has been demonstrated that natriuretic peptides lower intraocular pressure, however, the underlying cellular mechanism(s) mediating this response remain(s) to be determined. The purpose of this study was to investigate the effects of C-type natriuretic peptide (CNP) on pH(i), cGMP/cAMP and expression of atrial natriuretic peptide receptor (NPR-A), brain natriuretic peptide receptor (NPR-B) and C-type natriuretic peptide receptor (NPR-C), in HTM cells. At concentrations of 10(-7) M, CNP caused an acidification of pH(i). In addition, CNP caused a dose-dependent increase in cGMP formation and inhibition of forskolin-stimulated cAMP accumulation. These changes were not significantly altered in the absence of 10(-3) M isobutylmethylxanthine (IBMX). Treatment with the NPR-A antagonist, anantin, produced no influence on basal cGMP/cAMP levels, the CNP-stimulated cGMP accumulation and CNP-induced inhibition of forskolin-stimulated cAMP accumulation. However, CNP-induced reduction of forskolin-stimulated cAMP accumulation was inhibited by pretreatment with pertussis toxin (PTX). Furthermore, NPRB receptors were predominantly expressed and pretreatment with CNP (10(-7) M, 24hr) enhanced all NPR mRNAs expression which was not altered by higher concentrations or longer incubation. Results demonstrate that NPR-A, NPR-B and NPR-C receptors' expression can be up-regulated by CNP treatment. CNP activates NPR-B receptors preferentially to increase cGMP accumulation and acts through the PTX-sensitive cAMP-signaling pathway leading to a decrease in pH(i).

High and low gain switches for regulation of cAMP efflux concentration: distinct roles for particulate GC- and soluble GC-cGMP-PDE3 signaling in rabbit atria

This study tests the hypothesis that particulate (p) guanylyl cyclase (GC) and soluble (s) GC are involved in the distinct roles for the regulation of cGMP-PDE-cAMP signaling and of mechanical and secretory functions in the heart. Experiments were performed in perfused beating rabbit atria. C-type natriuretic peptide (CNP) and SIN-1, an NO donor, or BAY 41-2272 (BAY), a direct activator for sGC, were used to activate pGC and sGC, respectively. CNP and SIN-1 increased cGMP and cAMP efflux in a concentration-dependent manner. Increase in cAMP was a function of cGMP. The changes in cAMP efflux concentration in terms of cGMP were much more prominent in the atria treated with CNP than in the atria treated with SIN-1. Increase in cAMP efflux concentration was blocked by milrinone but not changed by EHNA. BAY increased cGMP but not cAMP in a concentration-dependent manner. CNP and SIN-1 decreased atrial stroke volume and myocytic ANP release. The decreases in terms of cGMP efflux concentration were much more prominent in the atria treated with CNP than in the atria treated with SIN-1 or BAY. Milrinone accentuated GC agonist-induced decreases in atrial stroke volume and ANP release. In the presence of ODQ, SIN-1 or BAY induced effects were not observed. These data suggest that pGC and sGC activations have distinct roles via cGMP-PDE3-cAMP signaling in the cardiac atrium: high and low gain switches, respectively, for the regulation of cAMP levels and contractile and secretory functions.

C-type natriuretic peptide induces human colonic myofibroblast relaxation

Intestinal response to injury requires coordinated regulation of the tension exerted by subepithelial myofibroblasts (SEM). However, the signals governing relaxation of intestinal SEM have not been investigated. Our aim was to test the hypothesis that signal transduction pathways initiated by C-type natriuretic peptide (CNP) induce intestinal SEM relaxation. We directly quantified the effects of CNP on isometric tension exerted by cultured human colonic SEM. We also measured the effects of CNP on cGMP content, myosin regulatory light chain (MLC) phosphorylation, and cytosolic Ca2+ concentration. CNP induced relaxation of SEM within 10 s. By 10 min, relaxation reached a plateau that was sustained for 2 h. CNP-induced relaxation was saturable, with a maximal decrease in tension (51.7 +/- 3.8 dyn) observed at 250 nM. SEM relaxation in response to CNP constituted approximately 23% of total basal tension. CNP increased intracellular cGMP content and reduced MLC phosphorylation. Effects of CNP on cGMP and MLC exhibited the same dose dependence as CNP-induced relaxation. MLC phosphorylation decreased within 2 min of CNP exposure and was sustained for at least 45 min. CNP also stimulated a large transient increase in cytosolic Ca2+ concentration that occurred within 30 s and was nearly complete by 1 min. We also observed that calyculin-A, a potent inhibitor of MLC phosphatase, completely abolished the reduction in MLC phosphorylation induced by CNP. These results suggest that CNP induces intestinal SEM relaxation through cGMP-associated reductions in MLC phosphorylation. Moreover, these findings raise the possibility that CNP plays a role in intestinal wound healing.

Increased effects of C-type natriuretic peptide on contractility and calcium regulation in murine hearts overexpressing cyclic GMP-dependent protein kinase I

1. C-type natriuretic peptide (CNP) and its receptor guanylyl cyclase (GC-B) are expressed in the heart and modulate cardiac contractility in a cGMP-dependent manner. Since the distal cellular signalling pathways remain unclear, we evaluated the peptide effects on cardiac function and calcium regulation in wild-type (WT) and transgenic mice with cardiac overexpression of cGMP-dependent protein kinase I (PKG ITG). 2. In isolated, perfused working WT hearts, CNP (10 nm) provoked an immediate increase in the maximal rates of contraction and relaxation, a small increase in the left ventricular systolic pressure and a decrease in the time of relaxation. These changes in cardiac function were accompanied by a marked increase in the levels of Ser16-phosphorylated phospholamban (PLB). 3. In PKG ITG hearts, the effects of CNP on cardiac contractility and relaxation as well as on PLB phosphorylation were markedly enhanced. 4. CNP increased cell shortening and systolic Cai2+ levels, and accelerated Cai2+ decay in isolated, Indo-1/AM-loaded WT cardiomyocytes, and these effects were enhanced in PKG I-overexpressing cardiomyocytes. 5. 8-pCPT-cGMP, a membrane-permeable PKG activator, mimicked the contractile and molecular actions of CNP, the effects again being more pronounced in PKG ITG hearts. In contrast, the cardiac responses to beta-adrenergic stimulation were not different between genotypes. 6. Taken together, our data indicate that PKG I is a downstream target activated by the CNP/GC-B/cGMP-signalling pathway in cardiac myocytes. cGMP/PKG I-stimulated phosphorylation of PLB and subsequent activation of the sarcoplasmic reticulum Ca2+ pump appear to mediate the positive inotropic and lusitropic responses to CNP.

Myocardial production of C-type natriuretic peptide in chronic heart failure

BACKGROUND

C-type natriuretic peptide (CNP) is a vasodilator produced by the vascular endothelium. It shares structural and physiological properties with the cardiac hormones atrial natriuretic peptide and brain natriuretic peptide (BNP), but little is known about its pathophysiological role in chronic heart failure (CHF). We assessed the hypothesis that CNP is produced by the heart in patients with CHF.

METHODS AND RESULTS

Myocardial CNP production was determined (difference in plasma levels between the aortic root and coronary sinus [CS]) in 9 patients undergoing right and left heart catheterization as part of their CHF assessment (all male, age 59+/-9 years; New York Heart Association class 2.2+/-0.1; left ventricular ejection fraction 29+/-5%; creatinine 105+/-8 micro mol/L [all values mean+/-SEM]). BNP, established as originating from myocardium, was assessed from the same samples as a positive control. Analyses were performed by a blinded operator using a standard competitive radioimmunoassay kit (Peninsula Laboratories, Bachem Ltd UK). A step-up (29%) in plasma CNP concentration was found from the aorta to the CS (3.55+/-1.53 versus 4.59+/-1.54 pg/mL, respectively; P=0.035). The mean increase in CNP was 0.90+/-0.35 pg/mL (range 0.05 to 2.80 pg/mL). BNP levels increased by 57% from aorta to CS (86.0+/-20.5 versus 135.0+/-42.2 pg/mL; P=0.01). CS CNP levels correlated with mean pulmonary capillary wedge pressure (r=0.82, P=0.007).

CONCLUSIONS

We have shown that CNP is produced by the heart in patients with CHF. Although further evaluation is required to define its full pathophysiological role in this condition, CNP may represent an important new local mediator in the heart.

Natriuretic peptide receptor-B in adult rat ventricle is predominantly confined to the nonmyocyte population

We assessed the cellular localization and relative concentration of the C-type natriuretic peptide (CNP) guanylate cyclase-B (GC-B) receptor in the adult rat heart ventricle by several techniques. In frozen sections of the ventricle, anti-receptor antibody stained the vasculature and cells interstitial to myocytes, but not the myocytes themselves. The same antibody detected GC-B in immunoblots of protein extracts of nonmyocytes, but not myocytes and recognized an equivalent protein in extracts of cultured cardiac fibroblasts, but not A7r5 rat smooth muscle cells. In functional assays, CNP-induced cGMP accumulation per milligram cell protein was an order of magnitude greater in cultured cardiac fibroblasts than in A7r5 smooth muscle cells and two orders of magnitude greater than in freshly isolated cardiac myocytes. Modulation of cGMP accumulation by phosphodiesterases (PDEs) was cell specific as determined by antagonist pharmacological profiles, PDE1 in fibroblasts, PDE2 in A7r5 cells, and PDE3 in myocytes, suggesting that significant but low-level cGMP response to CNP measured in heart myocytes is not due to nonmyocyte contamination. Fibroblasts of cardiac origin do not show an interactive relationship between receptor responsiveness to CNP, cGMP levels, and proliferation-related mitogen-activated signal transduction pathways. Whereas previous reports suggest CNP exerts significant effects in neonatal rat cardiomyocytes, our results suggest that fibroblasts are likely the most responsive cell type (cGMP production) in the adult rat heart.

C-type natriuretic peptide has a negative inotropic effect on cardiac myocytes

C-type natriuretic peptide (CNP) has vasodilatory and antimitogenic actions, but its role in the control of cardiac function is unclear. We studied the effect of CNP on cultured, beating neonatal rat cardiac myocytes. CNP caused a significant reduction in the amplitude of contraction and a significant accumulation of intracellular cyclic GMP. The effect of a membrane permeable cyclic GMP on cell contraction was similar to that of CNP. CNP caused no change in Ca2+ transients. Blockade of natriuretic peptide receptors abolished the effects of CNP on contraction and accumulation of intracellular cyclic GMP. Blockade of cyclic GMP-dependent protein kinase abolished the effect of CNP on myocyte contraction. We conclude that CNP has a negative inotropic effect on neonatal rat cardiac myocytes. The effect of CNP is mediated via natriuretic peptide receptor(s) causing elevation of intracellular cyclic GMP which possibly activates protein kinase and causes attenuation of myofilament sensitivity to Ca2+.