Loss of insulin signaling may contribute to atrial fibrillation and atrial electrical remodeling in type 1 diabetes.

Atrial fibrillation (AF) is prevalent in diabetes mellitus (DM); however, the basis for this is unknown. This study investigated AF susceptibility and atrial electrophysiology in type 1 diabetic Akita mice using in vivo intracardiac electrophysiology, high-resolution optical mapping in atrial preparations, and patch clamping in isolated atrial myocytes. qPCR and western blotting were used to assess ion channel expression. Akita mice were highly susceptible to AF in association with increased P-wave duration and slowed atrial conduction velocity. In a second model of type 1 DM, mice treated with streptozotocin (STZ) showed a similar increase in susceptibility to AF. Chronic insulin treatment reduced susceptibility and duration of AF and shortened P-wave duration in Akita mice. Atrial action potential (AP) morphology was altered in Akita mice due to a reduction in upstroke velocity and increases in AP duration. In Akita mice, atrial Na+ current (INa) and repolarizing K+ current (IK) carried by voltage gated K+ (Kv1.5) channels were reduced. The reduction in INa occurred in association with reduced expression of SCN5a and voltage gated Na+ (NaV1.5) channels as well as a shift in INa activation kinetics. Insulin potently and selectively increased INa in Akita mice without affecting IK Chronic insulin treatment increased INa in association with increased expression of NaV1.5. Acute insulin also increased INa, although to a smaller extent, due to enhanced insulin signaling via phosphatidylinositol 3,4,5-triphosphate (PIP3). Our study reveals a critical, selective role for insulin in regulating atrial INa, which impacts susceptibility to AF in type 1 DM.

Inhibition of neprilysin with sacubitril without RAS blockage aggravates renal disease in Dahl SS rats.

Salt-sensitive (SS) hypertension is accompanied with severe cardiorenal complications. In this condition, elevated blood pressure (BP) resulting from salt retention is associated with counterintuitively lower levels of atrial natriuretic peptide (ANP). In plasma, ANP is degraded by the neprilysin; therefore, pharmacological inhibition of this metalloprotease (i.e., with sacubitril) can be employed to increase ANP level. We have shown earlier that sacubitril in combination with valsartan (75 µg/day each) had beneficial effects on renal function in Dahl SS rats. The goal of this study was to evaluate the effects of a higher dose of sacubitril on renal damage in this model. To induce hypertension, male Dahl SS rats were fed a 4% NaCl diet (HS) for 21 days, and were administered sacubitril (125 µg/day) or vehicle via s.c. osmotic pumps. At the end of the HS challenge, both groups exhibited similar outcomes for GFR, heart weight, plasma electrolytes, BUN, and creatinine. Sacubitril exacerbated kidney hypertrophy, but did not affect levels of renal fibrosis. We also observed aggravated glomerular lesions and increased formation of protein casts in the sacubitril-treated animals compared to controls. Thus, in Dahl SS rats, administration of sacubitril without renin-angiotensin-system blockage had adverse effects on renal disease progression, particularly in regards to glomerular damage and protein cast formation. We can speculate that while ANP levels are increased because of neprilysin inhibition, there are off-target effects of sacubitril, which are detrimental to renal function in the SS hypertensive state.

Impact of ISK Voltage and Ca2+/Mg2+-Dependent Rectification on Cardiac Repolarization.

Cardiac small conductance Ca2+-activated K+ (SK) channels are activated solely by Ca2+, but the SK current (ISK) is inwardly rectified. However, the impact of inward rectification in shaping action potentials (APs) in ventricular cardiomyocytes under ß-adrenergic stimulation or in disease states remains undefined. Two processes underlie this inward rectification: an intrinsic rectification caused by an electrostatic energy barrier from positively charged amino acids at the inner pore and a voltage-dependent Ca2+/Mg2+ block. Thus, Ca2+ has a biphasic effect on ISK, activating at low [Ca2+] yet inhibiting ISK at high [Ca2+]. We examined the effect of ISK rectification on APs in rat cardiomyocytes by simultaneously recording whole-cell apamin-sensitive currents and Ca2+ transients during an AP waveform and developed a computer model of SK channels with rectification features. The typical profile of ISK during AP clamp included an initial peak (mean 1.6 pA/pF) followed by decay to the point that submembrane [Ca2+] reached ∼10 µM. During the rest of the AP stimulus, ISK either plateaued or gradually increased as the cell repolarized and submembrane [Ca2+] decreased further. We used a six-state gating model combined with intrinsic and Ca2+/Mg2+-dependent rectification to simulate ISK and investigated the relative contributions of each type of rectification to AP shape. This SK channel model replicates key features of ISK recording during AP clamp showing that intrinsic rectification limits ISK at high Vm during the early and plateau phase of APs. Furthermore, the initial rise of Ca2+ transients activates, but higher [Ca2+] blocks SK channels, yielding a transient outward-like ISK trajectory. During the decay phase of Ca2+, the Ca2+-dependent block is released, causing ISK to rise again and contribute to repolarization. Therefore, ISK is an important repolarizing current, and the rectification characteristics of an SK channel determine its impact on early, plateau, and repolarization phases of APs.

PKA phosphorylation underlies functional recruitment of sarcolemmal SK2 channels in ventricular myocytes from hypertrophic hearts.

KEY POINTS: Small-conductance Ca2+ -activated K+ (SK) channels expressed in ventricular myocytes are dormant in health, yet become functional in cardiac disease. SK channels are voltage independent and their gating is controlled by intracellular [Ca2+ ] in a biphasic manner. Submicromolar [Ca2+ ] activates the channel via constitutively-bound calmodulin, whereas higher [Ca2+ ] exerts inhibitory effect during depolarization. Using a rat model of cardiac hypertrophy induced by thoracic aortic banding, we found that functional upregulation of SK2 channels in hypertrophic rat ventricular cardiomyocytes is driven by protein kinase A (PKA) phosphorylation. Using site-directed mutagenesis, we identified serine-465 as the site conferring PKA-dependent effects on SK2 channel function. PKA phosphorylation attenuates ISK rectification by reducing the Ca2+ /voltage-dependent inhibition of SK channels without changing their sensitivity to activating submicromolar [Ca2+ ]i . This mechanism underlies the functional recruitment of SK channels not only in cardiac disease, but also in normal physiology, contributing to repolarization under conditions of enhanced adrenergic drive. ABSTRACT: Small-conductance Ca2+ -activated K+ (SK) channels expressed in ventricular myocytes (VMs) are dormant in health, yet become functional in cardiac disease. We aimed to test the hypothesis that post-translational modification of SK channels under conditions accompanied by enhanced adrenergic drive plays a central role in disease-related activation of the channels. We investigated this phenomenon using a rat model of hypertrophy induced by thoracic aortic banding (TAB). Western blot analysis using anti-pan-serine/threonine antibodies demonstrated enhanced phosphorylation of immunoprecipitated SK2 channels in VMs from TAB rats vs. Shams, which was reversible by incubation of the VMs with PKA inhibitor H89 (1 µmol L-1 ). Patch clamped VMs under basal conditions from TABs but not Shams exhibited outward current sensitive to the specific SK inhibitor apamin (100 nmol L-1 ), which was eliminated by inhibition of PKA (1 µmol L-1 ). Beta-adrenergic stimulation (isoproterenol, 100 nmol L-1 ) evoked ISK in VMs from Shams, resulting in shortening of action potentials in VMs and ex vivo optically mapped Sham hearts. Using adenoviral gene transfer, wild-type and mutant SK2 channels were overexpressed in adult rat VMs, revealing serine-465 as the site that elicits PKA-dependent phosphorylation effects on SK2 channel function. Concurrent confocal Ca2+ imaging experiments established that PKA phosphorylation lessens rectification of ISK via reduction Ca2+ /voltage-dependent inhibition of the channels at high [Ca2+ ] without affecting their sensitivity to activation by Ca2+ in the submicromolar range. In conclusion, upregulation of SK channels in diseased VMs is mediated by hyperadrenergic drive in cardiac hypertrophy, with functional effects on the channel conferred by PKA-dependent phosphorylation at serine-465.

Differential effects of low-dose sacubitril and/or valsartan on renal disease in salt-sensitive hypertension.

Diuretics and renin-angiotensin system blockers are often insufficient to control the blood pressure (BP) in salt-sensitive (SS) subjects. Abundant data support the proposal that the level of atrial natriuretic peptide may correlate with the pathogenesis of SS hypertension. We hypothesized here that increasing atrial natriuretic peptide levels with sacubitril, combined with renin-angiotensin system blockage by valsartan, can be beneficial for alleviation of renal damage in a model of SS hypertension, the Dahl SS rat. To induce a BP increase, rats were challenged with a high-salt 4% NaCl diet for 21 days, and chronic administration of vehicle or low-dose sacubitril and/or valsartan (75 µg/day each) was performed. Urine flow, Na+ excretion, and water consumption were increased on the high-salt diet compared with the starting point (0.4% NaCl) in all groups but remained similar among the groups at the end of the protocol. Upon salt challenge, we observed a mild decrease in systolic BP and urinary neutrophil gelatinase-associated lipocalin levels (indicative of alleviated tubular damage) in the valsartan-treated groups. Sacubitril, as well as sacubitril/valsartan, attenuated the glomerular filtration rate decline induced by salt. Alleviation of protein cast formation and lower renal medullary fibrosis were observed in the sacubitril/valsartan- and valsartan-treated groups, but not when sacubitril alone was administered. Interestingly, proteinuria was mildly mitigated only in rats that received sacubitril/valsartan. Further studies of the effects of sacubitril/valsartan in the setting of SS hypertension, perhaps involving a higher dose of the drug, are warranted to determine if it can interfere with the progression of the disease.

Dysfunction in the ßII spectrin-dependent cytoskeleton underlies human arrhythmia.

BACKGROUND: The cardiac cytoskeleton plays key roles in maintaining myocyte structural integrity in health and disease. In fact, human mutations in cardiac cytoskeletal elements are tightly linked to cardiac pathologies, including myopathies, aortopathies, and dystrophies. Conversely, the link between cytoskeletal protein dysfunction and cardiac electric activity is not well understood and often overlooked in the cardiac arrhythmia field. METHODS AND RESULTS: Here, we uncover a new mechanism for the regulation of cardiac membrane excitability. We report that ßII spectrin, an actin-associated molecule, is essential for the posttranslational targeting and localization of critical membrane proteins in heart. ßII spectrin recruits ankyrin-B to the cardiac dyad, and a novel human mutation in the ankyrin-B gene disrupts the ankyrin-B/ßII spectrin interaction, leading to severe human arrhythmia phenotypes. Mice lacking cardiac ßII spectrin display lethal arrhythmias, aberrant electric and calcium handling phenotypes, and abnormal expression/localization of cardiac membrane proteins. Mechanistically, ßII spectrin regulates the localization of cytoskeletal and plasma membrane/sarcoplasmic reticulum protein complexes, including the Na/Ca exchanger, ryanodine receptor 2, ankyrin-B, actin, and αII spectrin. Finally, we observe accelerated heart failure phenotypes in ßII spectrin-deficient mice. CONCLUSIONS: Our findings identify ßII spectrin as critical for normal myocyte electric activity, link this molecule to human disease, and provide new insight into the mechanisms underlying cardiac myocyte biology.

Ankyrin-G coordinates intercalated disc signaling platform to regulate cardiac excitability in vivo.

RATIONALE: Nav1.5 (SCN5A) is the primary cardiac voltage-gated Nav channel. Nav1.5 is critical for cardiac excitability and conduction, and human SCN5A mutations cause sinus node dysfunction, atrial fibrillation, conductional abnormalities, and ventricular arrhythmias. Further, defects in Nav1.5 regulation are linked with malignant arrhythmias associated with human heart failure. Consequently, therapies to target select Nav1.5 properties have remained at the forefront of cardiovascular medicine. However, despite years of investigation, the fundamental pathways governing Nav1.5 membrane targeting, assembly, and regulation are still largely undefined. OBJECTIVE: Define the in vivo mechanisms underlying Nav1.5 membrane regulation. METHODS AND RESULTS: Here, we define the molecular basis of an Nav channel regulatory platform in heart. Using new cardiac-selective ankyrin-G(-/-) mice (conditional knock-out mouse), we report that ankyrin-G targets Nav1.5 and its regulatory protein calcium/calmodulin-dependent kinase II to the intercalated disc. Mechanistically, ßIV-spectrin is requisite for ankyrin-dependent targeting of calcium/calmodulin-dependent kinase II-δ; however, ßIV-spectrin is not essential for ankyrin-G expression. Ankyrin-G conditional knock-out mouse myocytes display decreased Nav1.5 expression/membrane localization and reduced INa associated with pronounced bradycardia, conduction abnormalities, and ventricular arrhythmia in response to Nav channel antagonists. Moreover, we report that ankyrin-G links Nav channels with broader intercalated disc signaling/structural nodes, as ankyrin-G loss results in reorganization of plakophilin-2 and lethal arrhythmias in response to ß-adrenergic stimulation. CONCLUSIONS: Our findings provide the first in vivo data for the molecular pathway required for intercalated disc Nav1.5 targeting/regulation in heart. Further, these new data identify the basis of an in vivo cellular platform critical for membrane recruitment and regulation of Nav1.5.

EHD3-dependent endosome pathway regulates cardiac membrane excitability and physiology.

RATIONALE: Cardiac function is dependent on the coordinate activities of membrane ion channels, transporters, pumps, and hormone receptors to tune the membrane electrochemical gradient dynamically in response to acute and chronic stress. Although our knowledge of membrane proteins has rapidly advanced during the past decade, our understanding of the subcellular pathways governing the trafficking and localization of integral membrane proteins is limited and essentially unstudied in vivo. In the heart, to our knowledge, there are no in vivo mechanistic studies that directly link endosome-based machinery with cardiac physiology. OBJECTIVE: To define the in vivo roles of endosome-based cellular machinery for cardiac membrane protein trafficking, myocyte excitability, and cardiac physiology. METHODS AND RESULTS: We identify the endosome-based Eps15 homology domain 3 (EHD3) pathway as essential for cardiac physiology. EHD3-deficient hearts display structural and functional defects including bradycardia and rate variability, conduction block, and blunted response to adrenergic stimulation. Mechanistically, EHD3 is critical for membrane protein trafficking, because EHD3-deficient myocytes display reduced expression/localization of Na/Ca exchanger and L-type Ca channel type 1.2 with a parallel reduction in Na/Ca exchanger-mediated membrane current and Cav1.2-mediated membrane current. Functionally, EHD3-deficient myocytes show increased sarcoplasmic reticulum [Ca], increased spark frequency, and reduced expression/localization of ankyrin-B, a binding partner for EHD3 and Na/Ca exchanger. Finally, we show that in vivo EHD3-deficient defects are attributable to cardiac-specific roles of EHD3 because mice with cardiac-selective EHD3 deficiency demonstrate both structural and electric phenotypes. CONCLUSIONS: These data provide new insight into the critical role of endosome-based pathways in membrane protein targeting and cardiac physiology. EHD3 is a critical component of protein trafficking in heart and is essential for the proper membrane targeting of select cellular proteins that maintain excitability.

Impaired sinoatrial node function and increased susceptibility to atrial fibrillation in mice lacking natriuretic peptide receptor C.

Natriuretic peptides (NPs) are critical regulators of the cardiovascular system that are currently viewed as possible therapeutic targets for the treatment of heart disease. Recent work demonstrates potent NP effects on cardiac electrophysiology, including in the sinoatrial node (SAN) and atria. NPs elicit their effects via three NP receptors (NPR-A, NPR-B and NPR-C). Among these receptors, NPR-C is poorly understood. Accordingly, the goal of this study was to determine the effects of NPR-C ablation on cardiac structure and arrhythmogenesis. Cardiac structure and function were assessed in wild-type (NPR-C(+/+)) and NPR-C knockout (NPR-C(-/-)) mice using echocardiography, intracardiac programmed stimulation, patch clamping, high-resolution optical mapping, quantitative polymerase chain reaction and histology. These studies demonstrate that NPR-C(-/-) mice display SAN dysfunction, as indicated by a prolongation (30%) of corrected SAN recovery time, as well as an increased susceptibility to atrial fibrillation (6% in NPR-C(+/+) vs. 47% in NPR-C(-/-)). There were no differences in SAN or atrial action potential morphology in NPR-C(-/-) mice; however, increased atrial arrhythmogenesis in NPR-C(-/-) mice was associated with reductions in SAN (20%) and atrial (15%) conduction velocity, as well as increases in expression and deposition of collagen in the atrial myocardium. No differences were seen in ventricular arrhythmogenesis or fibrosis in NPR-C(-/-) mice. This study demonstrates that loss of NPR-C results in SAN dysfunction and increased susceptibility to atrial arrhythmias in association with structural remodelling and fibrosis in the atrial myocardium. These findings indicate a critical protective role for NPR-C in the heart.

Mitochondrial Ca 2+ uniporter (MCU) variants form plasma-membrane channels.

MCU is widely recognized as a responsible gene for encoding a pore-forming subunit of highly mitochondrial-specific and Ca 2+ -selective channel, mitochondrial Ca 2+ uniporter complex (mtCUC). Here, we report a novel short variant derived from the MCU gene (termed MCU-S) which lacks mitochondria-targeted sequence and forms a Ca 2+ - permeable channel outside of mitochondria. MCU-S was ubiquitously expressed in all cell-types/tissues, with particularly high expression in human platelets. MCU-S formed Ca 2+ channels at the plasma membrane, which exhibited similar channel properties to those observed in mtCUC. MCU-S channels at the plasma membrane served as an additional Ca 2+ influx pathway for platelet activation. Our finding is completely distinct from the originally reported MCU gene function and provides novel insights into the molecular basis of MCU variant-dependent cellular Ca 2+ handling.

Effects of elevation of ANP and its deficiency on cardiorenal function.

Atrial natriuretic peptide (ANP), encoded by Nppa, is a vasodilatory hormone that promotes salt excretion. Genome-wide association studies identified Nppa as a causative factor of blood pressure development, and in humans, ANP levels were suggested as an indicator of salt sensitivity. This study aimed to provide insights into the effects of ANP on cardiorenal function in salt-sensitive hypertension. To address this question, hypertension was induced in SSNPPA-/- (KO of Nppa in the Dahl salt-sensitive [SS] rat background) or SSWT (WT Dahl SS) rats by a high-salt (HS) diet challenge (4% NaCl for 21 days). Chronic infusion of ANP in SSWT rats attenuated the increase in blood pressure and cardiorenal damage. Overall, the SSNPPA-/- strain demonstrated higher blood pressure and intensified cardiac fibrosis (with no changes in ejection fraction) compared with SSWT rats. Furthermore, SSNPPA-/- rats exhibited kidney hypertrophy and higher glomerular injury scores, reduced diuresis, and lower sodium and chloride excretion than SSWT when fed a HS diet. Additionally, the activity of epithelial Na+ channel (ENaC) was found to be increased in the collecting ducts of the SSNPPA-/- rats. Taken together, these data show promise for the therapeutic benefits of ANP and ANP-increasing drugs for treating salt-sensitive hypertension.

α7 Nicotinic acetylcholine receptor mediates right ventricular fibrosis and diastolic dysfunction in pulmonary hypertension.

Right ventricular (RV) fibrosis is a key feature of maladaptive RV hypertrophy and dysfunction and is associated with poor outcomes in pulmonary hypertension (PH). However, mechanisms and therapeutic strategies to mitigate RV fibrosis remain unrealized. Previously, we identified that cardiac fibroblast α7 nicotinic acetylcholine receptor (α7 nAChR) drives smoking-induced RV fibrosis. Here, we sought to define the role of α7 nAChR in RV dysfunction and fibrosis in the settings of RV pressure overload as seen in PH. We show that RV tissue from PH patients has increased collagen content and ACh expression. Using an experimental rat model of PH, we demonstrate that RV fibrosis and dysfunction are associated with increases in ACh and α7 nAChR expression in the RV but not in the left ventricle (LV). In vitro studies show that α7 nAChR activation leads to an increase in adult ventricular fibroblast proliferation and collagen content mediated by a Ca2+/epidermal growth factor receptor (EGFR) signaling mechanism. Pharmacological antagonism of nAChR decreases RV collagen content and improves RV function in the PH model. Furthermore, mice lacking α7 nAChR exhibit improved RV diastolic function and have lower RV collagen content in response to persistently increased RV afterload, compared with WT controls. These finding indicate that enhanced α7 nAChR signaling is an important mechanism underlying RV fibrosis and dysfunction, and targeted inhibition of α7 nAChR is a potentially novel therapeutic strategy in the setting of increased RV afterload.

Cell free DNA as a diagnostic and prognostic marker for cardiovascular diseases.

Release of cell free DNA (cfDNA) from damaged or dead cells routinely occurs in normal physiology. Recently, cfDNA has emerged as an essential biomarker in cardiovascular disease (CVD) of potential prognostic and diagnostic significance. Within the last decade, significant research efforts have been devoted to uncovering the mechanisms mediating cfDNA release and its outcome-predicting ability. The current review focuses on the pathways for cfDNA release in myocardial infarction, heart failure and hypertension, and discusses implementation of cfDNA monitoring to assess the overall development of these disease states and predict future complications.

Renal Glomerular Mitochondria Function in Salt-Sensitive Hypertension.

Salt-sensitive (SS) hypertension is accompanied with an early onset of proteinuria, which results from the loss of glomerular podocytes. Here, we hypothesized that glomerular damage in the SS hypertension occurs in part due to mitochondria dysfunction, and we used a unique model of freshly isolated glomeruli to test this hypothesis. In order to mimic SS hypertension, we used Dahl SS rats, an established animal model. Animals were fed a 0.4% NaCl (normal salt, NS) diet or challenged with a high salt (HS) 4% NaCl diet for 21 days to induce an increase in blood pressure (BP). Similar to previous studies, we found that HS diet caused renal hypertrophy, increased BP, glomerulosclerosis, and renal lesions such as fibrosis and protein casts. We did not observe changes in mitochondrial biogenesis in the renal cortex or isolated glomeruli fractions. However, Seahorse assay performed on freshly isolated glomeruli revealed that basal mitochondrial respiration, maximal respiration, and spare respiratory capacity were lower in the HS compared to the NS group. Using confocal imaging and staining for mitochondrial H2O2 using mitoPY1, we detected an intensified response to an acute H2O2 application in the podocytes of the glomeruli isolated from the HS diet fed group. TEM analysis showed that glomerular mitochondria from the HS diet fed group have structural abnormalities (swelling, enlargement, less defined cristae). Therefore, we report that glomerular mitochondria in SS hypertension are functionally and structurally defective, and this impairment could eventually lead to loss of podocytes and proteinuria. Thus, the glomerular-mitochondria axis can be targeted in novel treatment strategies for hypertensive glomerulosclerosis.

SK channel enhancers attenuate Ca2+-dependent arrhythmia in hypertrophic hearts by regulating mito-ROS-dependent oxidation and activity of RyR.

Aims: Plasmamembrane small conductance Ca2+-activated K+ (SK) channels were implicated in ventricular arrhythmias in infarcted and failing hearts. Recently, SK channels were detected in the inner mitochondria membrane (IMM) (mSK), and their activation protected from acute ischaemia-reperfusion injury by reducing intracellular levels of reactive oxygen species (ROS). We hypothesized that mSK play an important role in regulating mitochondrial function in chronic cardiac diseases. We investigated the role of mSK channels in Ca2+-dependent ventricular arrhythmia using rat model of cardiac hypertrophy induced by banding of the ascending aorta thoracic aortic banding (TAB). Methods and results: Dual Ca2+ and membrane potential optical mapping of whole hearts derived from TAB rats revealed that membrane-permeable SK enhancer NS309 (2 µM) improved aberrant Ca2+ homeostasis and abolished VT/VF induced by ß-adrenergic stimulation. Using whole cell patch-clamp and confocal Ca2+ imaging of cardiomyocytes derived from TAB hearts (TCMs) we found that membrane-permeable SK enhancers NS309 and CyPPA (10 µM) attenuated frequency of spontaneous Ca2+ waves and delayed afterdepolarizations. Furthermore, mSK inhibition enhanced (UCL-1684, 1 µM); while activation reduced mitochondrial ROS production in TCMs measured with MitoSOX. Protein oxidation assays demonstrated that increased oxidation of ryanodine receptors (RyRs) in TCMs was reversed by SK enhancers. Experiments in permeabilized TCMs showed that SK enhancers restored SR Ca2+ content, suggestive of substantial improvement in RyR function. Conclusion: These data suggest that enhancement of mSK channels in hypertrophic rat hearts protects from Ca2+-dependent arrhythmia and suggest that the protection is mediated via decreased mitochondrial ROS and subsequent decreased oxidation of reactive cysteines in RyR, which ultimately leads to stabilization of RyR-mediated Ca2+ release.

Electrophysiological effects of natriuretic peptides in the heart are mediated by multiple receptor subtypes.

Natriuretic peptides (NPs) are a family of cardioprotective hormones with numerous beneficial effects in cardiovascular system. The NP family includes several peptides including atrial NP (ANP), B-type NP (BNP), C-type NP (CNP) and Dendroaspis NP (DNP). These peptides elicit their effects by binding to three distinct cell surface receptors called natriuretic peptide receptors A, B and C (NPR-A, NPR-B and NPR-C). NPR-A (which binds ANP, BNP and DNP) and NPR-B (which is selective for CNP) are particulate guanylyl cyclase (GC)-linked receptors that mediate increases in cGMP upon activation. cGMP can then target several downstream signaling molecules including protein kinase G (PKG), phosphodiesterase 2 (PDE2) and phosphodiesterase 3 (PDE3). NPR-C, which is able to bind all NPs with comparable affinity, is coupled to the activation of inhibitory G-proteins (Gi) that inhibit adenylyl cyclase (AC) activity and reduce cAMP levels. NPs are best known for their ability to regulate blood volume and fluid homeostasis. More recently, however, it has become apparent that NPs are essential regulators of cardiac electrophysiology and arrhythmogenesis. Evidence for this comes from numerous studies of the effects of NPs on cardiac electrophysiology and ion channel function in different regions and cell types within the heart, as well as the identification of mutations in the NP system that cause atrial fibrillation in humans. Despite the strong evidence that NPs regulate cardiac electrophysiology different studies have reported varying effects of NPs. The reasons for disparate observations are not fully understood, but likely occur as a result of several factors, including the fact that NP signaling can be highly complex and involve multiple receptors and/or downstream signaling molecules which may be differentially activated in different conditions. The goal of this review is to provide a comprehensive summary of the different effects of NPs on cardiac electrophysiology that have been described and to provide rationale and explanation for why different results may be obtained in different studies.

Effects of Wild-Type and Mutant Forms of Atrial Natriuretic Peptide on Atrial Electrophysiology and Arrhythmogenesis.

BACKGROUND: Atrial natriuretic peptide (ANP) is a hormone with numerous beneficial cardiovascular effects. Recently, a mutation in the ANP gene, which results in the generation of a mutant form of ANP (mANP), was identified and shown to cause atrial fibrillation in people. The mechanism(s) through which mANP causes atrial fibrillation is unknown. Our objective was to compare the effects of wild-type ANP and mANP on atrial electrophysiology in mice and humans. METHODS AND RESULTS: Action potentials (APs), L-type Ca(2+) currents (ICa,L), and Na(+) current were recorded in atrial myocytes from wild-type or natriuretic peptide receptor C knockout (NPR-C(-/-)) mice. In mice, ANP and mANP (10-100 nmol/L) had opposing effects on atrial myocyte AP morphology and ICa,L. ANP increased AP upstroke velocity (Vmax), AP duration, and ICa,L similarly in wild-type and NPR-C(-/-) myocytes. In contrast, mANP decreased Vmax, AP duration, and ICa,L, and these effects were completely absent in NPR-C(-/-) myocytes. ANP and mANP also had opposing effects on ICa,L in human atrial myocytes. In contrast, neither ANP nor mANP had any effect on Na(+) current in mouse atrial myocytes. Optical mapping studies in mice demonstrate that ANP sped electric conduction in the atria, whereas mANP did the opposite and slowed atrial conduction. Atrial pacing in the presence of mANP induced arrhythmias in 62.5% of hearts, whereas treatment with ANP completely prevented the occurrence of arrhythmias. CONCLUSIONS: These findings provide mechanistic insight into how mANP causes atrial fibrillation and demonstrate that wild-type ANP is antiarrhythmic.

Protein phosphatase 2A regulatory subunit B56α limits phosphatase activity in the heart.

Protein phosphatase 2A (PP2A) is a serine/threonine-selective holoenzyme composed of a catalytic, scaffolding, and regulatory subunit. In the heart, PP2A activity is requisite for cardiac excitation-contraction coupling and central in adrenergic signaling. We found that mice deficient in the PP2A regulatory subunit B56α (1 of 13 regulatory subunits) had altered PP2A signaling in the heart that was associated with changes in cardiac physiology, suggesting that the B56α regulatory subunit had an autoinhibitory role that suppressed excess PP2A activity. The increase in PP2A activity in the mice with reduced B56α expression resulted in slower heart rates and increased heart rate variability, conduction defects, and increased sensitivity of heart rate to parasympathetic agonists. Increased PP2A activity in B56α(+/-) myocytes resulted in reduced Ca(2+) waves and sparks, which was associated with decreased phosphorylation (and thus decreased activation) of the ryanodine receptor RyR2, an ion channel on intracellular membranes that is involved in Ca(2+) regulation in cardiomyocytes. In line with an autoinhibitory role for B56α, in vivo expression of B56α in the absence of altered abundance of other PP2A subunits decreased basal phosphatase activity. Consequently, in vivo expression of B56α suppressed parasympathetic regulation of heart rate and increased RyR2 phosphorylation in cardiomyocytes. These data show that an integral component of the PP2A holoenzyme has an important inhibitory role in controlling PP2A enzyme activity in the heart.