Abnormal intrastore calcium signaling in chronic heart failure

Functional Impact of Ryanodine Receptor Oxidation on Intracellular Calcium Regulation in the Heart

Type 2 ryanodine receptor (RyR2) serves as the major intracellular Ca²⁺ release channel that drives heart contraction. RyR2 is activated by cytosolic Ca²⁺ via the process of Ca²⁺-induced Ca²⁺ release (CICR). To ensure stability of Ca²⁺ dynamics, the self-reinforcing CICR must be tightly controlled. Defects in this control cause sarcoplasmic reticulum (SR) Ca²⁺ mishandling, which manifests in a variety of cardiac pathologies that include myocardial infarction and heart failure. These pathologies are also associated with oxidative stress. Given that RyR2 contains a large number of cysteine residues, it is no surprise that RyR2 plays a key role in the cellular response to oxidative stress. RyR's many cysteine residues pose an experimental limitation in defining a specific target or mechanism of action for oxidative stress. As a result, the current understanding of redox-mediated RyR2 dysfunction remains incomplete. Several oxidative modifications, including S-glutathionylation and S-nitrosylation, have been suggested playing an important role in the regulation of RyR2 activity. Moreover, oxidative stress can increase RyR2 activity by forming disulfide bonds between two neighboring subunits (intersubunit cross-linking). Since intersubunit interactions within the RyR2 homotetramer complex dictate the channel gating, such posttranslational modification of RyR2 would have a significant impact on RyR2 function and Ca²⁺ regulation. This review summarizes recent findings on oxidative modifications of RyR2 and discusses contributions of these RyR2 modifications to SR Ca²⁺ mishandling during cardiac pathologies.

On the Adjacency Matrix of RyR2 Cluster Structures

In the heart, electrical stimulation of cardiac myocytes increases the open probability of sarcolemmal voltage-sensitive Ca2+ channels and flux of Ca2+ into the cells. This increases Ca2+ binding to ligand-gated channels known as ryanodine receptors (RyR2). Their openings cause cell-wide release of Ca2+, which in turn causes muscle contraction and the generation of the mechanical force required to pump blood. In resting myocytes, RyR2s can also open spontaneously giving rise to spatially-confined Ca2+ release events known as "sparks." RyR2s are organized in a lattice to form clusters in the junctional sarcoplasmic reticulum membrane. Our recent work has shown that the spatial arrangement of RyR2s within clusters strongly influences the frequency of Ca2+ sparks. We showed that the probability of a Ca2+ spark occurring when a single RyR2 in the cluster opens spontaneously can be predicted from the precise spatial arrangements of the RyR2s. Thus, "function" follows from "structure." This probability is related to the maximum eigenvalue (λ1) of the adjacency matrix of the RyR2 cluster lattice. In this work, we develop a theoretical framework for understanding this relationship. We present a stochastic contact network model of the Ca2+ spark initiation process. We show that λ1 determines a stability threshold for the formation of Ca2+ sparks in terms of the RyR2 gating transition rates. We recapitulate these results by applying the model to realistic RyR2 cluster structures informed by super-resolution stimulated emission depletion (STED) microscopy. Eigendecomposition of the linearized mean-field contact network model reveals functional subdomains within RyR2 clusters with distinct sensitivities to Ca2+. This work provides novel perspectives on the cardiac Ca2+ release process and a general method for inferring the functional properties of transmembrane receptor clusters from their structure.

Cytosolic Ca²⁺ buffering determines the intra-SR Ca²⁺ concentration at which cardiac Ca²⁺ sparks terminate

Single ryanodine receptor (RyR) Ca(2+) flux amplitude (i(Ca-RyR)) decreases as intra-sarcoplasmic reticulum (SR) Ca(2+) levels fall during a cardiac Ca(2+) spark. Since i(Ca-RyR) drives the inter-RyR Ca(2+)-induced Ca(2+) release (CICR) that underlies the spark, decreasing i(Ca-RyR) may contribute to spark termination because RyRs that spontaneously close may stay closed. To test this possibility, we simultaneously measured local cytosolic and intra-SR ([Ca(2+)]cyto and [Ca(2+)]SR) during Ca(2+) sparks in permeabilized rabbit ventricular myocytes. Local cytosolic or intra-SR Ca(2+) dynamics were manipulated using Ca(2+) buffers. Buffer manipulations applied in cells had no effect on individual RyR channels reconstituted in planar lipid bilayers. Presence of a fast cytosolic Ca(2+) buffer (BAPTA) significantly suppressed Ca(2+) spark activity and sparks terminated earlier at a higher than usual [Ca(2+)]SR level (∼80% vs. ∼62%). When cytosolic Ca(2+) buffer power was reduced (i.e. cytosolic EGTA level decreased), sparks terminated later and at a lower than usual [Ca(2+)]SR level (∼45% vs. ∼62%). When intra-SR Ca(2+) buffer power was increased, sparks also terminated later and at a lower than usual [Ca(2+)]SR (∼48% vs. ∼62%). These results suggest that cytosolic local control of inter-RyR CICR by i(Ca-RyR) plays a substantial role during the spark termination process. Thus, alterations in local cytosolic Ca(2+) handling dynamics in the dyadic cleft (Ca(2+) buffering, extrusion, etc.) likely influence Ca(2+) spark termination.

NOD1, a new player in cardiac function and calcium handling

AIMS

Inflammation is a significant contributor to cardiovascular disease and its complications; however, whether the myocardial inflammatory response is harmonized after cardiac injury remains to be determined. Some receptors of the innate immune system, including the nucleotide-binding oligomerization domain-like receptors (NLRs), play key roles in the host response after cardiac damage. Nucleotide-binding oligomerization domain containing 1 (NOD1), a member of the NLR family, is expressed in the heart, but its functional role has not been elucidated. We determine whether selective NOD1 activation modulates cardiac function and Ca(2+) signalling.

METHODS AND RESULTS

Mice were treated for 3 days with the selective NOD1 agonist C12-iE-DAP (iE-DAP), and cardiac function and Ca(2+) cycling were assessed. We found that iE-DAP treatment resulted in cardiac dysfunction, measured as a decrease in ejection fraction and fractional shortening. Cardiomyocytes isolated from iE-DAP-treated mice displayed a decrease in the L-type Ca(2+) current, [Ca(2+)]i transients and Ca(2+) load, and decreased expression of phospho-phospholamban, sarcoplasmic reticulum-ATPase, and Na(+)-Ca(2+) exchanger. Furthermore, iE-DAP prompted 'diastolic Ca(2+) leak' in cardiomyocytes, resulting from increased Ca(2+) spark frequency and RyR2 over-phosphorylation. Importantly, these iE-DAP-induced changes in Ca(2+) cycling were lost in NOD1(-/-) mice, indicating that iE-DAP exerts its actions through NOD1. Co-treatment of mice with iE-DAP and a selective inhibitor of NF-κB (BAY11-7082) prevented cardiac dysfunction and Ca(2+) handling impairment induced by iE-DAP.

CONCLUSION

Our data provide the first evidence that NOD1 activation induces cardiac dysfunction associated with excitation-contraction coupling impairment through NF-κB activation and uncover a new pro-inflammatory player in the regulation of cardiovascular function.

Progress in neuregulin/ErbB signaling and chronic heart failure

Heart failure is one of the leading causes of death today. It is a complex clinical syndrome in which the heart has a reduced contraction ability and decreased viable myocytes. Novel approaches to the clinical management of heart failure have been achieved through an understanding of the molecular pathways necessary for normal heart development. Neuregulin-1 (NRG-1) has emerged as a potential therapeutic target based on the fact that mice null for NRG-1 or receptors mediating its activity, ErbB2 and ErbB4, are embryonic lethal and exhibit severe cardiac defects. Preclinical studies performed with animal models of heart failure demonstrate that treatment with NRG-1 significantly improves heart function and survival. Clinical data further support NRG-1 as a promising drug candidate for the treatment of cardiac dysfunction in patients. Recent studies have revealed the mechanism underlying the therapeutic effects of NRG-1/ErbB signaling in the treatment of heart failure. Through activation of upstream signaling molecules such as phosphoinositide 3-kinase, mitogen-activated protein kinase, and focal adhesion kinase, NRG-1/ErbB pathway activation results in increased cMLCK expression and enhanced intracellular calcium cycling. The former is a regulator of the contractile machinery, and the latter triggers cell contraction and relaxation. In addition, NRG-1/ErbB signaling also influences energy metabolism and induces epigenetic modification in cardiac myocytes in a way that more closely resembles healthy heart. These observations reveal potentially new treatment options for heart failure.

Impaired calcium-calmodulin-dependent inactivation of Cav1.2 contributes to loss of sarcoplasmic reticulum calcium release refractoriness in mice lacking calsequestrin 2

AIMS

In cardiac muscle, Ca(2+) release from sarcoplasmic reticulum (SR) is reduced with successively shorter coupling intervals of premature stimuli, a phenomenon known as SR Ca(2+) release refractoriness. We recently reported that the SR luminal Ca(2+) binding protein calsequestrin 2 (Casq2) contributes to release refractoriness in intact mouse hearts, but the underlying mechanisms remain unclear. Here, we further investigate the mechanisms responsible for physiological release refractoriness.

METHODS AND RESULTS

Gene-targeted ablation of Casq2 (Casq2 KO) abolished SR Ca(2+) release refractoriness in isolated mouse ventricular myocytes. Surprisingly, impaired Ca(2+)-dependent inactivation of L-type Ca(2+) current (ICa), which is responsible for triggering SR Ca(2+) release, significantly contributed to loss of Ca(2+) release refractoriness in Casq2 KO myocytes. Recovery from Ca(2+)-dependent inactivation of ICa was significantly accelerated in Casq2 KO compared to wild-type (WT) myocytes. In contrast, voltage-dependent inactivation measured by using Ba(2+) as charge carrier was not significantly different between WT and Casq2 KO myocytes. Ca(2+)-dependent inactivation of ICa was normalized by intracellular dialysis of excess apo-CaM (20 μM), which also partially restored physiological Ca(2+) release refractoriness in Casq2 KO myocytes.

CONCLUSIONS

Our findings reveal that the intra-SR protein Casq2 is largely responsible for the phenomenon of SR Ca(2+) release refractoriness in murine ventricular myocytes. We also report a novel mechanism of impaired Ca(2+)-CaM-dependent inactivation of Cav1.2, which contributes to the loss of SR Ca(2+) release refractoriness in the Casq2 KO mouse model and, therefore, may further increase risk for ventricular arrhythmia in vivo.

Mechanisms of SR calcium release in healthy and failing human hearts

Normal heart contraction and rhythm relies on the proper flow of calcium ions (Ca2+) into cardiac cells and between their intracellular organelles, and any disruption can lead to arrhythmia and sudden cardiac death. Electrical excitation of the surface membrane activates voltage-dependent L-type Ca2+ channels to open and allow Ca2+ to enter the cytoplasm. The subsequent increase in cytoplasmic Ca2+ concentration activates calcium release channels (RyR2) located at specialised Ca2+ release sites in the sarcoplasmic reticulum (SR), which serves as an intracellular Ca2+ store. Animal models have provided valuable insights into how intracellular Ca2+ transport mechanisms are altered in human heart failure. The aim of this review is to examine how Ca2+ release sites are remodelled in heart failure and how this affects intracellular Ca2+ transport with an emphasis on Ca2+ release mechanisms in the SR. Current knowledge on how heart failure alters the regulation of RyR2 by Ca2+ and Mg2+ and how these mechanisms control the activity of RyR2 in the confines of the Ca2+ release sites is reviewed.

Heart failure duration progressively modulates the arrhythmia substrate through structural and electrical remodeling

AIMS

Ventricular arrhythmias are a common cause of death in patients with heart failure (HF). Structural and electrical abnormalities in the heart provide a substrate for such arrhythmias. Canine tachypacing-induced HF models of 4-6 weeks duration are often used to study pathophysiology and therapies for HF. We hypothesized that a chronic canine model of HF would result in greater electrical and structural remodeling than a short term model, leading to a more arrhythmogenic substrate.

MAIN METHODS

HF was induced by ventricular tachypacing for one (short-term) or four (chronic) months to study remodeling.

KEY FINDINGS

Left ventricular contractility was progressively reduced, while ventricular hypertrophy and interstitial fibrosis were evident at 4 month but not 1 month of HF. Left ventricular myocyte action potentials were prolonged after 4 (p<0.05) but not 1 month of HF. Repolarization instability and early afterdepolarizations were evident only after 4 months of HF (p<0.05), coinciding with a prolonged QTc interval (p<0.05). The transient outward potassium current was reduced in both HF groups (p<0.05). The outward component of the inward rectifier potassium current was reduced only in the 4 month HF group (p<0.05). The delayed rectifier potassium currents were reduced in 4 (p<0.05) but not 1 month of HF. Reactive oxygen species were increased at both 1 and 4 months of HF (p<0.05).

SIGNIFICANCE

Reduced Ito, outward IK1, IKs, and IKr in HF contribute to EAD formation. Chronic, but not short term canine HF, results in the altered electrophysiology and repolarization instability characteristic of end-stage human HF.

Regulation of RYR2 by sarcoplasmic reticulum Ca(2+)

Ca(2+) is arguably the most important ion involved in the contraction of the heart. The cardiac ryanodine receptor (RyR2), the major Ca(2+) release channel located in the sarcoplasmic reticulum (SR) membrane, is responsible for releasing the bulk of Ca(2+) required for contraction. Moreover, RyR2 is also crucial for maintaining SR Ca(2+) homeostasis by releasing Ca(2+) from the SR when it becomes overloaded with Ca(2+) . During normal contraction, RyR2 is activated by cytosolic Ca(2+) , whereas during store overload conditions, the opening of RyR2 is governed by SR Ca(2+) . Although the process of the cytosolic control of RyR2 is well established, the molecular mechanism by which SR luminal Ca(2+) regulates RyR2 has only recently been elucidated and remains controversial. In addition to the activation of RyR2, SR luminal Ca(2+) also determines when the RyR2 channel closes. RyR2-mediated Ca(2+) release from the SR does not continue until the SR is completely depleted. Rather, it ceases when SR luminal Ca(2+) falls below a certain level. Given the importance of SR Ca(2+) , it is not surprising that the SR luminal Ca(2+) level is tightly controlled by SR Ca(2+) -buffering proteins. Consequently, the opening and closing of RyR2 is heavily influenced by the presence of such proteins, particularly those associated with RyR2, such as calsequestrin and the histidine-rich Ca(2+) -binding protein. These proteins appear to indirectly alter RyR2 activity by modifying the microdomain SR Ca(2+) level surrounding RyR2.

Glycoproteins identified from heart failure and treatment models

Conduction abnormalities can lead to dyssynchronous contraction, which significantly worsens morbidity and mortality of heart failure. Cardiac resynchronization therapy (CRT) can reverse ventricular remodeling and improve cardiac function. Although the underlying molecular changes are unknown, the use of a canine model of dyssynchronous heart failure (DHF) and CRT has shown that there are global changes across the cardiac proteome. This study determines changes in serum glycoprotein concentration from DHF and CRT compared to normal. We hypothesize that CRT invokes protective or advantageous pathways that can be reflected in the circulating proteome. Two prong discovery approaches were carried out on pooled normal, DHF, and CRT samples composed of individual canine serum to determine the overall protein concentration and the N-linked glycosites of circulating glycoproteins. The level of the glycoproteins was altered in DHF and CRT compared to control sera, with 63 glycopeptides substantially increased in DHF and/or CRT. Among the 32 elevated glycosite-containing peptides in DHF, 13 glycopeptides were reverted to normal level after CRT therapy. We further verify the changes of glycopeptides using label-free LC-MS from individual canine serum. Circulating glycoproteins such as alpha-fetoprotein, alpha-2-macroglobulin, galectin-3-binding protein, and collectin-10 show association to failing heart and CRT treatment model.

Resveratrol ameliorates cardiac dysfunction induced by pressure overload in rats via structural protection and modulation of Ca(2+) cycling proteins

Background

Cardiac hypertrophy is a compensatory stage of the heart in response to stress such as pressure overload (PO), which can develop into heart failure (HF) if left untreated. Resveratrol has been reported to prevent the development of hypertrophy and contractile dysfunction induced by PO. However, other studies found that resveratrol treatment for a longer period of time failed to regress cardiac hypertrophy. The aim of this study is to determine the timing of resveratrol treatment to achieve antihypertrophic effect and investigate whether resveratrol prevents the development of HF through preservation of myocardium structure and modulation of Ca²⁺ handling proteins.

Methods

To generate rats with cardiac hypertrophy, male Sprague–Dawley rats were subjected to PO (aortic banding procedure) for 4 weeks. Sham-operated animals served as controls. Rats with cardiac hypertrophy were given resveratrol (4 mg/kg/day) for 4, 6, and 8 weeks, respectively. Histological and echocardiographic analysis and transmission electron microscopy were performed to assess cardiac structure and function. The levels of Ca²⁺ handling proteins were measured by western blot analysis.

Results

Histological analysis showed that resveratrol treatment regressed developed cardiac hypertrophy at 8 and 10 weeks postsurgery, but not at 12 weeks. However, resveratrol strongly and continuously prevented the development of cardiac dysfunction and dilation of cardiac chamber as evaluated by echocardiography and H&E staining of heart cross-sections. In addition, PO-induced cardiac fibrosis was completely inhibited by resveratrol treatment. Resveratrol markedly prevented the disrupted myocardium but partially rescued mitochondrial abnormality in banded rats. Moreover, resveratrol prevented the alteration of Ca²⁺ handling proteins induced by aortic banding, including downregulation of sarcoplasmic reticulum Ca²⁺ ATPase ₂ (SERCA₂) and ryanodine receptor ₂ (RyR₂), hypophosphorylated phospholamban (PLB), upregulation of Na⁺/Ca²⁺-exchangers (NCX₁) and increased expression and phosphorylation of Ca²⁺/calmodulin -dependent protein kinase II (CaMKII). Moreover, resveratrol alleviated the decreased SERCA activity induced by aortic banding.

Conclusions

Resveratrol effectively prevented the transition from compensatory to decompensatory stage of cardiac hypertrophy induced by PO, but this effect is dependent on the timing of treatment. We suggest that resveratrol may exert beneficial effects on cardiac hypertrophy through protection of cardiac structure and modulation of Ca²⁺ handling proteins.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-014-0323-x) contains supplementary material, which is available to authorized users.

Calcium-activated potassium current modulates ventricular repolarization in chronic heart failure

The role of I_KCa in cardiac repolarization remains controversial and varies across species. The relevance of the current as a therapeutic target is therefore undefined. We examined the cellular electrophysiologic effects of I_KCa blockade in controls, chronic heart failure (HF) and HF with sustained atrial fibrillation. We used perforated patch action potential recordings to maintain intrinsic calcium cycling. The I_KCa blocker (apamin 100 nM) was used to examine the role of the current in atrial and ventricular myocytes. A canine tachypacing induced model of HF (1 and 4 months, n = 5 per group) was used, and compared to a group of 4 month HF with 6 weeks of superimposed atrial fibrillation (n = 7). A group of age-matched canine controls were used (n = 8). Human atrial and ventricular myocytes were isolated from explanted end-stage failing hearts which were obtained from transplant recipients, and studied in parallel. Atrial myocyte action potentials were unchanged by I_KCa blockade in all of the groups studied. I_KCa blockade did not affect ventricular myocyte repolarization in controls. HF caused prolongation of ventricular myocyte action potential repolarization. I_KCa blockade caused further prolongation of ventricular repolarization in HF and also caused repolarization instability and early afterdepolarizations. SK2 and SK3 expression in the atria and SK3 in the ventricle were increased in canine heart failure. We conclude that during HF, I_KCa blockade in ventricular myocytes results in cellular arrhythmias. Furthermore, our data suggest an important role for I_KCa in the maintenance of ventricular repolarization stability during chronic heart failure. Our findings suggest that novel antiarrhythmic therapies should have safety and efficacy evaluated in both atria and ventricles.

[Ryanodine receptor, calcium leak and arrhythmias]

The participation of the ionic Ca(2+) release channel/ryanodine receptor in cardiac excitation-contraction coupling is well known since the late '80s, when various seminal papers communicated its purification for the first time and its identity with the "foot" structures located at the terminal cisternae of the sarcoplasmic reticulum. In addition to its main role as the Ca(2+) channel responsible for the transient Ca(2+) increase that activates the contractile machinery of the cardiomyocytes, the ryanodine receptor releases Ca(2+) during the relaxation phase of the cardiac cycle, giving rise to a diastolic Ca(2+) leak. In normal physiological conditions, diastolic Ca(2+) leak regulates the proper level of luminal Ca(2+), but in pathological conditions it participates in the generation of both, acquired and hereditary arrhythmias. Very recently, several groups have focused their efforts into the development of pharmacological tools to control the altered diastolic Ca(2+) leak via ryanodine receptors. In this review, we focus our interest on describing the participation of cardiac ryanodine receptor in the diastolic Ca(2+) leak under physiological or pathological conditions and also on the therapeutic approaches to control its undesired exacerbated activity during diastole.

Calcium in health and disease

Evolution has exploited the chemical properties of Ca(2+), which facilitate its reversible binding to the sites of irregular geometry offered by biological macromolecules, to select it as a carrier of cellular signals. A number of proteins bind Ca(2+) to specific sites: those intrinsic to membranes play the most important role in the spatial and temporal regulation of the concentration and movements of Ca(2+) inside cells. Those which are soluble, or organized in non-membranous structures, also decode the Ca(2+) message to be then transmitted to the targets of its regulation. Since Ca(2+) controls the most important processes in the life of cells, it must be very carefully controlled within the cytoplasm, where most of the targets of its signaling function reside. Membrane channels (in the plasma membrane and in the organelles) mediate the entrance of Ca(2+) into the cytoplasm, ATPases, exchangers, and the mitochondrial Ca(2+) uptake system remove Ca(2+) from it. The concentration of Ca(2+) in the external spaces, which is controlled essentially by its dynamic exchanges in the bone system, is much higher than inside cells, and can, under conditions of pathology, generate a situation of dangerous internal Ca(2+) overload. When massive and persistent, the Ca(2+) overload culminates in the death of the cell. Subtle conditions of cellular Ca(2+) dyshomeostasis that affect individual systems that control Ca(2+), generate cell disease phenotypes that are particularly severe in tissues in which the signaling function of Ca(2+) has special importance, e.g., the nervous system.

Oxidation- and CaMKII-mediated sarcoplasmic reticulum Ca(2+) leak triggers atrial fibrillation in aging

BACKGROUND

Advanced age is a well-recognized predisposition to atrial fibrillation (AF). However, the cellular electrophysiological changes that underlie the heightened susceptibility to AF in aged individuals remain poorly understood. Sarcoplasmic reticulum (SR) Ca(2+) leak that results from posttranslational modification of type 2 ryanodine receptor channels (RyR2) has been implicated in arrhythmogenesis. We hypothesize that aging alters atrial myocytes Ca(2+) homeostasis and RyR2 function, which create a substrate for AF initiation.

METHODS AND RESULTS

We examined the susceptibility to AF in aged (24 months) and young adult (4-5 months) mice using an intraesophageal atrial electrical stimulation protocol. Aged mice showed significant higher AF induction rate (43.3%, n = 30) than young adults (8.8%, n = 34, P < 0.01). In accordance with these in vivo findings, significantly increased diastolic SR Ca(2+) leak and arrhythmogenic Ca(2+) activities with reduced SR Ca(2+) content were observed in aged atrial myocytes. Western blot showed RyR2 oxidation and phosphorylation at Ser2814 (Ca(2+) /calmodulin-dependent protein kinase II [CaMKII] site), but not phosphorylation at Ser2808 (protein kinase A [PKA] and CaMKII site), were increased in aged atrial myocytes. The selective CaMKII inhibitor (KN-93), as well as the antioxidant reagent (DTT) reversed the diastolic Ca(2+) leak and the frequency of spontaneous Ca(2+) transients in aged atrial myocytes, whereas PKA inhibition with H-89 was ineffective.

CONCLUSIONS

Aging increases both the oxidation and CaMKII-phosphorylation of RyR2, which result in diastolic SR Ca(2+) leak and facilitate AF initiation. These results contribute to the electrophysiological remodeling of aged atria and suggest a therapeutic strategy for AF treatment in aging.

Ca handling during excitation-contraction coupling in heart failure

In the heart, coupling between excitation of the surface membrane and activation of contractile apparatus is mediated by Ca released from the sarcoplasmic reticulum (SR). Several components of Ca machinery are perfectly arranged within the SR network and the T-tubular system to generate a regular Ca cycling and thereby rhythmic beating activity of the heart. Among these components, ryanodine receptor (RyR) and SR Ca ATPase (SERCA) complexes play a particularly important role and their dysfunction largely underlies abnormal Ca homeostasis in diseased hearts such as in heart failure. The abnormalities in Ca regulation occur at practically all main steps of Ca cycling in the failing heart, including activation and termination of SR Ca release, diastolic SR Ca leak, and SR Ca uptake. The contributions of these different mechanisms to depressed contractile function and enhanced arrhythmogenesis may vary in different HF models. This brief review will therefore focus on modifications in RyR and SERCA structure that occur in the failing heart and how these molecular modifications affect SR Ca regulation and excitation-contraction coupling.

Automated detection and analysis of Ca(2+) sparks in x-y image stacks using a thresholding algorithm implemented within the open-source image analysis platform ImageJ

Previous studies have used analysis of Ca(2+) sparks extensively to investigate both normal and pathological Ca(2+) regulation in cardiac myocytes. The great majority of these studies used line-scan confocal imaging. In part, this is because the development of open-source software for automatic detection of Ca(2+) sparks in line-scan images has greatly simplified data analysis. A disadvantage of line-scan imaging is that data are collected from a single row of pixels, representing only a small fraction of the cell, and in many instances x-y confocal imaging is preferable. However, the limited availability of software for Ca(2+) spark analysis in two-dimensional x-y image stacks presents an obstacle to its wider application. This study describes the development and characterization of software to enable automatic detection and analysis of Ca(2+) sparks within x-y image stacks, implemented as a plugin within the open-source image analysis platform ImageJ. The program includes methods to enable precise identification of cells within confocal fluorescence images, compensation for changes in background fluorescence, and options that allow exclusion of events based on spatial characteristics.

Genetic ablation of ryanodine receptor 2 phosphorylation at Ser-2808 aggravates Ca(2+)-dependent cardiomyopathy by exacerbating diastolic Ca2+ release

Phosphorylation of the cardiac ryanodine receptor (RyR2) by protein kinase A (PKA) at Ser-2808 is suggested to mediate the physiological 'fight or flight' response and contribute to heart failure by rendering the sarcoplasmic reticulum (SR) leaky for Ca(2+). In the present study, we examined the potential role of RyR2 phosphorylation at Ser-2808 in the progression of Ca(2+)-dependent cardiomyopathy (CCM) by using mice genetically modified to feature elevated SR Ca(2+) leak while expressing RyR2s that cannot be phosphorylated at this site (S2808A). Surprisingly, rather than alleviating the disease phenotype, constitutive dephosphorylation of Ser-2808 aggravated CCM as manifested by shortened survival, deteriorated in vivo cardiac function, exacerbated SR Ca(2+) leak and mitochondrial injury. Notably, the deteriorations of cardiac function, myocyte Ca(2+) handling, and mitochondria integrity were consistently worse in mice with heterozygous ablation of Ser-2808 than in mice with complete ablation. Wild-type (WT) and CCM myocytes expressing unmutated RyR2s exhibited a high level of baseline phosphorylation at Ser-2808. Exposure of these CCM cells to protein phosphatase 1 caused a transitory increase in Ca(2+) leak attributable to partial dephosphorylation of RyR2 tetramers at Ser-2808 from more fully phosphorylated state. Thus, exacerbated Ca(2+) leak through partially dephosphorylated RyR2s accounts for the prevalence of the disease phenotype in the heterozygous S2808A CCM mice. These results do not support the importance of RyR2 hyperphosphorylation in Ca(2+)-dependent heart disease, and rather suggest roles for the opposite process, the RyR2 dephosphorylation at this residue in physiological and pathophysiological Ca(2+) signalling.

Differential effects of the peroxynitrite donor, SIN-1, on atrial and ventricular myocyte electrophysiology

Oxidative stress has been implicated in the pathogenesis of heart failure and atrial fibrillation and can result in increased peroxynitrite production in the myocardium. Atrial and ventricular canine cardiac myocytes were superfused with 3-morpholinosydnonimine-N-ethylcarbamide (SIN-1), a peroxynitrite donor, to evaluate the acute electrophysiologic effects of peroxynitrite. Perforated whole-cell patch clamp techniques were used to record action potentials. SIN-1 (200 µM) increased the action potential duration (APD) in atrial and ventricular myocytes; however, in the atria, APD prolongation was rate independent, whereas in the ventricle APD, prolongation was rate dependent. In addition to prolongation of the action potential, beat-to-beat variability of repolarization was significantly increased in ventricular but not in atrial myocytes. We examined the contribution of intracellular calcium cycling to the effects of SIN-1 by treating myocytes with the SERCA blocker, thapsigargin (5-10 µM). Inhibition of calcium cycling prevented APD prolongation in the atrial and ventricular myocytes, and prevented the SIN-1-induced increase in ventricular beat-to-beat APD variability. Collectively, these data demonstrate that peroxynitrite affects atrial and ventricular electrophysiology differentially. A detailed understanding of oxidative modulation of electrophysiology in specific chambers is critical to optimize therapeutic approaches for cardiac diseases.

'Ryanopathy': causes and manifestations of RyR2 dysfunction in heart failure

The cardiac ryanodine receptor (RyR2), a Ca(2+) release channel on the membrane of the sarcoplasmic reticulum (SR), plays a key role in determining the strength of the heartbeat by supplying Ca(2+) required for contractile activation. Abnormal RyR2 function is recognized as an important part of the pathophysiology of heart failure (HF). While in the normal heart, the balance between the cytosolic and intra-SR Ca(2+) regulation of RyR2 function maintains the contraction-relaxation cycle, in HF, this behaviour is compromised by excessive post-translational modifications of the RyR2. Such modification of the Ca(2+) release channel impairs the ability of the RyR2 to properly deactivate leading to a spectrum of Ca(2+)-dependent pathologies that include cardiac systolic and diastolic dysfunction, arrhythmias, and structural remodelling. In this article, we present an overview of recent advances in our understanding of the underlying causes and pathological consequences of abnormal RyR2 function in the failing heart. We also discuss the implications of these findings for HF therapy.

JTV519 (K201) reduces sarcoplasmic reticulum Ca²⁺ leak and improves diastolic function in vitro in murine and human non-failing myocardium

BACKGROUND AND PURPOSE

Ca²⁺ leak from the sarcoplasmic reticulum (SR) via ryanodine receptors (RyR2s) contributes to cardiomyocyte dysfunction. RyR2 Ca²⁺ leak has been related to RyR2 phosphorylation. In these conditions, JTV519 (K201), a 1,4-benzothiazepine derivative and multi-channel blocker, stabilizes RyR2s and decrease SR Ca²⁺ leak. We investigated whether JTV519 stabilizes RyR2s without increasing RyR2 phosphorylation in mice and in non-failing human myocardium and explored underlying mechanisms.

EXPERIMENTAL APPROACH

SR Ca²⁺ leak was induced by ouabain in murine cardiomyocytes. [Ca²⁺]-transients, SR Ca²⁺ load and RyR2-mediated Ca²⁺ leak (sparks/waves) were quantified, with or without JTV519 (1 µmol·L⁻¹). Contribution of Ca²⁺ -/calmodulin-dependent kinase II (CaMKII) was assessed by KN-93 and Western blot (RyR2-Ser(2814) phosphorylation). Effects of JTV519 on contractile force were investigated in non-failing human ventricular trabeculae.

KEY RESULTS

Ouabain increased systolic and diastolic cytosolic [Ca²⁺](i) , SR [Ca²⁺], and SR Ca²⁺ leak (Ca²⁺ spark (SparkF) and Ca²⁺ wave frequency), independently of CaMKII and RyR-Ser(2814) phosphorylation. JTV519 decreased SparkF but also SR Ca²⁺ load. At matched SR [Ca²⁺], Ca²⁺ leak was significantly reduced by JTV519, but it had no effect on fractional Ca²⁺ release or Ca²⁺ wave propagation velocity. In human muscle, JTV519 was negatively inotropic at baseline but significantly enhanced ouabain-induced force and reduced its deleterious effects on diastolic function.

CONCLUSIONS AND IMPLICATIONS

JTV519 was effective in reducing SR Ca²⁺ leak by specifically regulating RyR2 opening at diastolic [Ca²⁺](i) in the absence of increased RyR2 phosphorylation at Ser(2814) , extending the potential use of JTV519 to conditions of acute cellular Ca²⁺ overload.

[Molecular and genetic background of sudden cardiac death]

Despite recent findings on the functional, structural and genetic background of sudden cardiac death, the incidence is still relatively high in the entire population. A thorough knowledge on susceptibility, as well as pathophysiology behind the development of malignant arrhythmias will help us to identify individuals at risk and prevent sudden cardiac death. This article presents a review of the current literature on the role of altered intracellular Ca2+ handling, acute myocardial ischaemia, cardiac autonomic innervation, renin-angiotensin-aldosterone system, monogenic and complex heritability in the pathogenesis of sudden cardiac death.

Up-regulation of sarcoplasmic reticulum Ca(2+) uptake leads to cardiac hypertrophy, contractile dysfunction and early mortality in mice deficient in CASQ2

AIMS

Although aberrant Ca(2+) release (i.e. Ca(2+) 'leak') from the sarcoplasmic reticulum (SR) through cardiac ryanodine receptors (RyR2) is linked to heart failure (HF), it remains unknown whether and under what conditions SR-derived Ca(2+) can actually cause HF. We tested the hypothesis that combining dysregulated RyR2 function with facilitated Ca(2+) uptake into SR will exacerbate abnormal SR Ca(2+) release and induce HF. We also examined the mechanisms for these alterations.

METHODS AND RESULTS

We crossbred mice deficient in expression of cardiac calsequestrin (CASQ2) with mice overexpressing the skeletal muscle isoform of SR Ca(2+)ATPase (SERCA1a). The new double-mutant strains displayed early mortality, congestive HF with left ventricular dilated hypertrophy, and decreased ejection fraction. Intact right ventricular muscle preparations from double-mutant mice preserved normal systolic contractile force but were susceptible to spontaneous contractions. Double-mutant cardiomyocytes while preserving normal amplitude of systolic Ca(2+) transients displayed marked disturbances in diastolic Ca(2+) handling in the form of multiple, periodic Ca(2+) waves and wavelets. Dysregulated myocyte Ca(2+) handling and structural and functional cardiac pathology in double-mutant mice were associated with increased rate of apoptotic cell death. Qualitatively similar results were obtained in a hybrid strain created by crossing CASQ2 knockout mice with mice deficient in phospholamban.

CONCLUSION

We demonstrate that enhanced SR Ca(2+) uptake combined with dysregulated RyR2s results in sustained diastolic Ca(2+) release causing apoptosis, dilated cardiomyopathy, and early mortality. Our data also suggest that up-regulation of SERCA activity must be advocated with caution as a therapy for HF in the context of abnormal RyR2 function.

Store-dependent deactivation: cooling the chain-reaction of myocardial calcium signaling

In heart cells, Ca(2+) released from the internal storage unit, the sarcoplasmic reticulum (SR) through ryanodine receptor (RyR2) channels is the predominant determinant of cardiac contractility. Evidence obtained in recent years suggests that SR Ca(2+) release is tightly regulated not only by cytosolic Ca(2+) but also by intra-store Ca(2+) concentration. Specifically, Ca(2+)-induced Ca(2+) release (CICR) that relies on auto-catalytic action of Ca(2+) at the cytosolic side of RyR2s is precisely balanced and counteracted by RyR2 deactivation dependent on a reciprocal decrease of Ca(2+) at the luminal side of RyR2s. Dysregulation of this inherently unstable Ca(2+) signaling is considered to be an underlying cause of triggered arrhythmias, and is associated with genetic and acquired forms of sudden cardiac death. In this article, we present an overview of recent advances in our understanding of the regulatory role luminal Ca(2+) plays in Ca(2+) handling, with a particular emphasis on the role of Ca(2+)release refractoriness in aberrant Ca(2+) release.

Simulation of arrhythmogenic effect of rogue RyRs in failing heart by using a coupled model

Cardiac cells with heart failure are usually characterized by impairment of Ca²⁺ handling with smaller SR Ca²⁺ store and high risk of triggered activities. In this study, we developed a coupled model by integrating the spatiotemporal Ca²⁺ reaction-diffusion system into the cellular electrophysiological model. With the coupled model, the subcellular Ca²⁺ dynamics and global cellular electrophysiology could be simultaneously traced. The proposed coupled model was then applied to study the effects of rogue RyRs on Ca²⁺ cycling and membrane potential in failing heart. The simulation results suggested that, in the presence of rogue RyRs, Ca²⁺ dynamics is unstable and Ca²⁺ waves are prone to be initiated spontaneously. These release events would elevate the membrane potential substantially which might induce delayed afterdepolarizations or triggered action potentials. Moreover, the variation of membrane potential depolarization is indicated to be dependent on the distribution density of rogue RyR channels. This study provides a new possible arrhythmogenic mechanism for heart failure from subcellular to cellular level.

Influence of cyclovirobuxine D on intracellular [Ca(2+)] regulation and the expression of the calcium cycling proteins in rat myocytes

Our previous studies have shown that cyclovirobuxine D (CVB-D) ameliorated the cardiac function in heart failure rats. Considering the relationship between cardiac function and [Ca(2+)]i, and the role of calcium cycling on regulating [Ca(2+)]i, the present study was designed to evaluate the influence of CVB-D on the calcium transient of myocytes from neonatal rats and adult heart failure (HF) rats. The expression of calcium cycling proteins, including L-type calcium channel (LTCC), ryanodine receptor 2 (RYR2), sarcoplasmic reticulum calcium ATPase 2a (SERCA2a) and sodium-calcium exchanger (NCX), were investigated to explore the underlying mechanism. CVB-D increased the intensity of calcium transient, accelerated the process of calcium transient and attenuation in the neonatal and adult myocytes. Furthermore, CVB-D shortened T(peak) and T(attenuation) in the adult myocytes and slowed down the heart rate of neonatal myocytes. Besides, CVB-D increased the expression of RYR2 and SERCA2a, decreased the expression of NCX, but showed no significant effect on LTCC. Thus, it was concluded that CVB-D increased the release and uptake of Ca(2+) in systolic and diastolic period, respectively. CVB-D might not only facilitate the utilization of intracellular Ca(2+), but also prevent the loss of Ca(2+).

Advanced glycation end product cross-link breaker attenuates diabetes-induced cardiac dysfunction by improving sarcoplasmic reticulum calcium handling

Diabetic heart disease is a distinct clinical entity that can progress to heart failure and sudden death. However, the mechanisms responsible for the alterations in excitation-contraction coupling leading to cardiac dysfunction during diabetes are not well known. Hyperglycemia, the landmark of diabetes, leads to the formation of advanced glycation end products (AGEs) on long-lived proteins, including sarcoplasmic reticulum (SR) Ca²⁺ regulatory proteins. However, their pathogenic role on SR Ca²⁺ handling in cardiac myocytes is unknown. Therefore, we investigated whether an AGE cross-link breaker could prevent the alterations in SR Ca²⁺ cycling that lead to in vivo cardiac dysfunction during diabetes. Streptozotocin-induced diabetic rats were treated with alagebrium chloride (ALT-711) for 8 weeks and compared to age-matched placebo-treated diabetic rats and healthy rats. Cardiac function was assessed by echocardiographic examination. Ventricular myocytes were isolated to assess SR Ca²⁺ cycling by confocal imaging and quantitative Western blots. Diabetes resulted in in vivo cardiac dysfunction and ALT-711 therapy partially alleviated diastolic dysfunction by decreasing isovolumetric relaxation time and myocardial performance index (MPI) (by 27 and 41% vs. untreated diabetic rats, respectively, P < 0.05). In cardiac myocytes, diabetes-induced prolongation of cytosolic Ca²⁺ transient clearance by 43% and decreased SR Ca²⁺ load by 25% (P < 0.05); these parameters were partially improved after ALT-711 therapy. SERCA2a and RyR2 protein expression was significantly decreased in the myocardium of untreated diabetic rats (by 64 and 36% vs. controls, respectively, P < 0.05), but preserved in the treated diabetic group compared to controls. Collectively, our results suggest that, in a model of type 1 diabetes, AGE accumulation primarily impairs SR Ca²⁺ reuptake in cardiac myocytes and that long-term treatment with an AGE cross-link breaker partially normalized SR Ca²⁺ handling and improved diabetic cardiomyopathy.

Abnormal termination of Ca2+ release is a common defect of RyR2 mutations associated with cardiomyopathies

RATIONALE

Naturally occurring mutations in the cardiac ryanodine receptor (RyR2) have been associated with both cardiac arrhythmias and cardiomyopathies. It is clear that delayed afterdepolarization resulting from abnormal activation of sarcoplasmic reticulum Ca2+ release is the primary cause of RyR2-associated cardiac arrhythmias. However, the mechanism underlying RyR2-associated cardiomyopathies is completely unknown.

OBJECTIVE

In the present study, we investigate the role of the NH2-terminal region of RyR2 in and the impact of a number of cardiomyopathy-associated RyR2 mutations on the termination of Ca2+ release.

METHODS AND RESULTS

The 35-residue exon-3 region of RyR2 is associated with dilated cardiomyopathy. Single-cell luminal Ca2+ imaging revealed that the deletion of the first 305 NH2-terminal residues encompassing exon-3 or the deletion of exon-3 itself markedly reduced the luminal Ca2+ threshold at which Ca2+ release terminates and increased the fractional Ca2+ release. Single-cell cytosolic Ca2+ imaging also showed that both RyR2 deletions enhanced the amplitude of store overload-induced Ca2+ transients in HEK293 cells or HL-1 cardiac cells. Furthermore, the RyR2 NH2-terminal mutations, A77V, R176Q/T2504M, R420W, and L433P, which are associated with arrhythmogenic right ventricular displasia type 2, also reduced the threshold for Ca2+ release termination and increased fractional release. The RyR2 A1107M mutation associated with hypertrophic cardiomyopathy had the opposite action (i.e., increased the threshold for Ca2+ release termination and reduced fractional release).

CONCLUSIONS

These results provide the first evidence that the NH2-terminal region of RyR2 is an important determinant of Ca2+ release termination, and that abnormal fractional Ca2+ release attributable to aberrant termination of Ca2+ release is a common defect in RyR2-associated cardiomyopathies.

Remodeling of calcium handling in human heart failure

Heart failure (HF) is an increasing public health problem accelerated by a rapidly aging global population. Despite considerable progress in managing the disease, the development of new therapies for effective treatment of HF remains a challenge. To identify targets for early diagnosis and therapeutic intervention, it is essential to understand the molecular and cellular basis of calcium handling and the signaling pathways governing the functional remodeling associated with HF in humans. Calcium (Ca(2+)) cycling is an essential mediator of cardiac contractile function, and remodeling of calcium handling is thought to be one of the major factors contributing to the mechanical and electrical dysfunction observed in HF. Active research in this field aims to bridge the gap between basic research and effective clinical treatments of HF. This chapter reviews the most relevant studies of calcium remodeling in failing human hearts and discusses their connections to current and emerging clinical therapies for HF patients.

Multiscale modeling of calcium cycling in cardiac ventricular myocyte: macroscopic consequences of microscopic dyadic function

In cardiac ventricular myocytes, calcium (Ca) release occurs at distinct structures (dyads) along t-tubules, where L-type Ca channels (LCCs) appose sarcoplasmic reticulum (SR) Ca release channels (RyR2s). We developed a model of the cardiac ventricular myocyte that simulates local stochastic Ca release processes. At the local Ca release level, the model reproduces Ca spark properties. At the whole-cell level, the model reproduces the action potential, Ca currents, and Ca transients. Changes in microscopic dyadic properties (e.g., during detubulation in heart failure) affect whole-cell behavior in complex ways, which we investigated by simulating changes in the dyadic volume and number of LCCs/RyR2s in the dyad, and effects of calsequestrin (CSQN) as a Ca buffer (CSQN buffer) or a luminal Ca sensor (CSQN regulator). We obtained the following results: 1), Increased dyadic volume and reduced LCCs/RyR2s decrease excitation-contraction coupling gain and cause asynchrony of SR Ca release, and interdyad coupling partially compensates for the reduced synchrony. 2), Impaired CSQN buffer depresses Ca transients without affecting the synchrony of SR Ca release. 3), When CSQN regulator function is impaired, interdyad coupling augments diastolic Ca release activity to form Ca waves and long-lasting Ca release events.

Adrenergic signaling controls RGK-dependent trafficking of cardiac voltage-gated L-type Ca2+ channels through PKD1

RATIONALE

The Rad-Gem/Kir-related family (RGKs) consists of small GTP-binding proteins that strongly inhibit the activity of voltage-gated calcium channels. Among RGKs, Rem1 is strongly and specifically expressed in cardiac tissue. However, the physiological role and regulation of RGKs, and Rem1 in particular, are largely unknown.

OBJECTIVE

To determine if Rem1 function is physiologically regulated by adrenergic signaling and thus impacts voltage-gated L-type calcium channel (VLCC) activity in the heart.

METHODS AND RESULTS

We found that activation of protein kinase D1, a protein kinase downstream of α(1)-adrenergic signaling, leads to direct phosphorylation of Rem1 at Ser18. This results in an increase of the channel activity and plasma membrane expression observed by using a combination of electrophysiology, live cell confocal microscopy, and immunohistochemistry in heterologous expression system and neonatal cardiomyocytes. In addition, we show that stimulation of α(1)-adrenergic receptor-protein kinase D1-Rem1 signaling increases transverse-tubule VLCC expression that results in increased L-type Ca(2+) current density in adult ventricular myocytes.

CONCLUSION

The α(1)-adrenergic stimulation releases Rem1 inhibition of VLCCs through direct phosphorylation of Rem1 at Ser18 by protein kinase D1, resulting in an increase of the channel activity and transverse-tubule expression. Our results uncover a novel molecular regulatory mechanism of VLCC trafficking and function in the heart and provide the first demonstration of physiological regulation of RGK function.

Calcium signaling in cardiac myocytes

Calcium (Ca(2+)) is a critical regulator of cardiac myocyte function. Principally, Ca(2+) is the link between the electrical signals that pervade the heart and contraction of the myocytes to propel blood. In addition, Ca(2+) controls numerous other myocyte activities, including gene transcription. Cardiac Ca(2+) signaling essentially relies on a few critical molecular players--ryanodine receptors, voltage-operated Ca(2+) channels, and Ca(2+) pumps/transporters. These moieties are responsible for generating Ca(2+) signals upon cellular depolarization, recovery of Ca(2+) signals following cellular contraction, and setting basal conditions. Whereas these are the central players underlying cardiac Ca(2+) fluxes, networks of signaling mechanisms and accessory proteins impart complex regulation on cardiac Ca(2+) signals. Subtle changes in components of the cardiac Ca(2+) signaling machinery, albeit through mutation, disease, or chronic alteration of hemodynamic demand, can have profound consequences for the function and phenotype of myocytes. Here, we discuss mechanisms underlying Ca(2+) signaling in ventricular and atrial myocytes. In particular, we describe the roles and regulation of key participants involved in Ca(2+) signal generation and reversal.

Cellular mechanism underlying burn serum-generated bidirectional regulation of excitation-contraction coupling in isolated rat cardiomyocytes

Myocardial depressant factors have long been recognized to be present in burn serum (BS) and contribute to burn-generated cardiac contractile dysfunction. However, much of the cellular and molecular mechanism for its role in the development of the cardiac deficiency remains unknown. In this study, we investigated the effect of BS on myocardial contractility and Ca handling in single rat cardiomyocytes. The results revealed that BS (5% by volume) bidirectionally regulated cardiac excitation-contraction (EC) coupling. The action potential-elicited Ca transient and cell shortening were increased by 28.0% ± 9.7% and 34.7% ± 12.5% within 20 min after BS stimulation (the upregulation phase), but decreased by 20.5% ± 6.8% and 32.3% ± 5.1% at 60 min after BS stimulation (the downregulation phase). There was a 32.0% ± 5.8% reduction in sarcoplasmic reticulum (SR) Ca content at the downregulation phase, whereas no alteration was detected at the upregulation phase. The incidences of spontaneous Ca sparks and Ca waves were significantly increased after BS stimulation, no matter at the upregulation or downregulation phase. The hyperactive Ca sparks and Ca waves could be completely abolished by antioxidative treatment (vitamin A, 0.2 mM; and vitamin E, 1 mM) and partially reversed by NOS inhibitor L-NAME (100 μM), but not by blocking Ca influx with nifedipine (1 μM). With the normalization of Ca sparks, BS-induced alterations of action potential-elicited Ca transient and contractility were prevented by antioxidative therapy. Taken together, we propose that BS-associated bidirectional regulation of EC coupling is attributed largely to oxidative stress-induced hyperactivity of ryanodine receptors, increasing EC coupling through enhancing intracellular Ca release initially, but subsequently decreasing EC coupling by partially depleting SR Ca content through enhancement of Ca spark-mediated SR leak.

Tetrahydrobiopterin depletion and NOS2 uncoupling contribute to heart failure-induced alterations in atrial electrophysiology

AIMS

Heart failure is a common antecedent to atrial fibrillation; both heart failure and atrial fibrillation are associated with increased myocardial oxidative stress. Chronic canine heart failure reduces atrial action potential duration and atrial refractoriness. We hypothesized that inducible nitric oxide synthase 2 (NOS2) contributes to atrial oxidative stress and electrophysiologic alterations.

METHODS AND RESULTS

A 16-week canine tachypacing model of heart failure was used (n= 21). At 10 weeks, dogs were randomized to either placebo (n = 12) or active treatment (n = 9) with NOS cofactor, tetrahydrobiopterin (BH(4), 50 mg), and NOS substrate (L-arginine, 3 g) twice daily for 6 weeks. A group of matched controls (n = 7) was used for comparison. Heart failure increased atrial NOS2 and reduced atrial BH(4), while L-arginine was unchanged. Treatment reduced inducible atrial fibrillation and normalized the heart failure-induced shortening of the left atrial myocyte action potential duration. Treatment increased atrial [BH(4)] while [L-arginine] was unchanged. Treatment did not improve left ventricular function or dimensions. Heart failure-induced reductions in atrial [BH(4)] resulted in NOS uncoupling, as measured by NO and superoxide anion (O(2)(·-)) production, while BH(4) and L-arginine treatment normalized NO and O(2)(·-). Heart failure resulted in left atrial oxidative stress, which was attenuated by BH(4) and L-arginine treatment.

CONCLUSION

Chronic non-ischaemic heart failure results in atrial oxidative stress and electrophysiologic abnormalities by depletion of BH(4) and uncoupling of NOS2. Modulation of NOS2 activity by repletion of BH(4) may be a safe and effective approach to reduce the frequency of atrial arrhythmias during heart failure.

Redox signaling in cardiac myocytes

The heart has complex mechanisms that facilitate the maintenance of an oxygen supply-demand balance necessary for its contractile function in response to physiological fluctuations in workload as well as in response to chronic stresses such as hypoxia, ischemia, and overload. Redox-sensitive signaling pathways are centrally involved in many of these homeostatic and stress-response mechanisms. Here, we review the main redox-regulated pathways that are involved in cardiac myocyte excitation-contraction coupling, differentiation, hypertrophy, and stress responses. We discuss specific sources of endogenously generated reactive oxygen species (e.g., mitochondria and NADPH oxidases of the Nox family), the particular pathways and processes that they affect, the role of modulators such as thioredoxin, and the specific molecular mechanisms that are involved-where this knowledge is available. A better understanding of this complex regulatory system may allow the development of more specific therapeutic strategies for heart diseases.

Pathophysiological mechanisms of atrial fibrillation: a translational appraisal

Atrial fibrillation (AF) is an arrhythmia that can occur as the result of numerous different pathophysiological processes in the atria. Some aspects of the morphological and electrophysiological alterations promoting AF have been studied extensively in animal models. Atrial tachycardia or AF itself shortens atrial refractoriness and causes loss of atrial contractility. Aging, neurohumoral activation, and chronic atrial stretch due to structural heart disease activate a variety of signaling pathways leading to histological changes in the atria including myocyte hypertrophy, fibroblast proliferation, and complex alterations of the extracellular matrix including tissue fibrosis. These changes in electrical, contractile, and structural properties of the atria have been called "atrial remodeling." The resulting electrophysiological substrate is characterized by shortening of atrial refractoriness and reentrant wavelength or by local conduction heterogeneities caused by disruption of electrical interconnections between muscle bundles. Under these conditions, ectopic activity originating from the pulmonary veins or other sites is more likely to occur and to trigger longer episodes of AF. Many of these alterations also occur in patients with or at risk for AF, although the direct demonstration of these mechanisms is sometimes challenging. The diversity of etiological factors and electrophysiological mechanisms promoting AF in humans hampers the development of more effective therapy of AF. This review aims to give a translational overview on the biological basis of atrial remodeling and the proarrhythmic mechanisms involved in the fibrillation process. We pay attention to translation of pathophysiological insights gained from in vitro experiments and animal models to patients. Also, suggestions for future research objectives and therapeutical implications are discussed.

Dynamic calcium movement inside cardiac sarcoplasmic reticulum during release

RATIONALE

Intra-sarcoplasmic reticulum (SR) free [Ca] ([Ca](SR)) provides the driving force for SR Ca release and is a key regulator of SR Ca release channel gating during normal SR Ca release or arrhythmogenic spontaneous Ca release events. However, little is known about [Ca](SR) spatiotemporal dynamics.

OBJECTIVE

To directly measure local [Ca](SR) with subsarcomeric spatiotemporal resolution during both normal global SR Ca release and spontaneous Ca sparks and to evaluate the quantitative implications of spatial [Ca](SR) gradients.

METHODS AND RESULTS

Intact and permeabilized rabbit ventricular myocytes were subjected to direct simultaneous measurement of cytosolic [Ca] and [Ca](SR) and FRAP (fluorescence recovery after photobleach). We found no detectable [Ca](SR) gradients between SR release sites (junctional SR) and Ca uptake sites (free SR) during normal global Ca release, clear spatiotemporal [Ca](SR) gradients during isolated Ca blinks, faster intra-SR diffusion in the longitudinal versus transverse direction, 3- to 4-fold slower diffusion of fluorophores in the SR than in cytosol, and that intra-SR Ca diffusion varies locally, dependent on local SR connectivity. A computational model clarified why spatiotemporal gradients are more detectable in isolated local releases versus global releases and provides a quantitative framework for understanding intra-SR Ca diffusion.

CONCLUSIONS

Intra-SR Ca diffusion is rapid, limiting spatial [Ca](SR) gradients during excitation-contraction coupling. Spatiotemporal [Ca](SR) gradients are apparent during Ca sparks, and these observations constrain models of dynamic Ca movement inside the SR. This has important implications for myocyte SR Ca handling, synchrony, and potentially arrhythmogenic spontaneous contraction.

The relationship between arrhythmogenesis and impaired contractility in heart failure: role of altered ryanodine receptor function

AIMS

In heart failure (HF), abnormal myocyte Ca(2+) handling has been implicated in cardiac arrhythmias and contractile dysfunction. In the present study, we investigated the relationships between Ca(2+) handling, reduced myocyte contractility, and enhanced arrhythmogenesis during HF progression in a canine model of non-ischaemic HF.

METHODS AND RESULTS

Key Ca(2+) handling parameters were determined by measuring cytosolic and intra-sarcoplasmic reticulum (SR) [Ca(2+)] in isolated ventricular myocytes at different stages of HF. The progression of HF was associated with an early and continuous increase in ryanodine receptor (RyR2)-mediated SR Ca(2+) leak. The increase in RyR2 activity was paralleled by an increase in the frequency of diastolic spontaneous Ca(2+) waves (SCWs) in HF myocytes under conditions of β-adrenergic stimulation. In addition to causing arrhythmogenic-delayed afterdepolarizations, SCWs decreased the amplitude of subsequent electrically evoked Ca(2+) transients by depleting SR Ca(2+). At late stages of HF, Ca(2+) release oscillated essentially independent of electrical pacing. The increased propensity for the generation of SCWs in HF myocytes was attributable to reduced ability of the RyR2 channels to become refractory following Ca(2+) release. The progressive alterations in RyR2 function and Ca(2+) cycling in HF myocytes were associated with sequential modifications of RyR2 by CaMKII-dependent phosphorylation and thiol oxidation.

CONCLUSION

These findings suggest that destabilized RyR2 activity due to excessive CaMKII phopshorylation and oxidation resulting in impaired post-release refractoriness is a common mechanism involved in arrhythmogenesis and contractile dysfunction in the failing heart.

L-type calcium current (ICa,L) and inward rectifier potassium current (IK1) are involved in QT prolongation induced by arsenic trioxide in rat

The present study was designed to study the effects of As(2)O(3) on QT interval prolongation and to explore the potential ionic mechanisms in isolated rat ventricular cardiomyocytes. The rats of As(2)O(3) group were treated with 0.8 mg·kg(-1)·d(-1) As(2)O(3) intravenously for 7 days consecutively and the control group with saline. The ECG was recorded to calculate heart rate-corrected QT interval (QTc). Single cardiomyocytes were isolated by using collagenase II, and the action potential duration (APD) and ion currents were recorded by whole-cell patch clamp. [Ca(2+)](i) was examined by confocal laser scanning microscopy. Our data showed that both QTc and APD were prolonged significantly after As(2)O(3)treatment. Meanwhile, As(2)O(3) suppressed I(K1) and shifted the reversal potential to more positive direction. Moreover, the density of I(Ca,L) was augmented significantly, and the steady-state activation curve became more negative, whereas, the inactivation and reactivation of I(Ca,L) were not changed notably after As(2)O(3) administration. Furthermore, the maximal [Ca(2+)](i) was enhanced obviously by either KCl or caffeine stimulation in As(2)O(3)-treated cardiomyocytes. Our results show that the potential mechanism of As(2)O(3)-induced QT interval prolongation in rat might be relative to disturbing the fine balance of transmembrane currents (increasing I(Ca,L) and decreasing I(K1)) and causing APD prolongation.

Ca²+ spark-dependent and -independent sarcoplasmic reticulum Ca²+ leak in normal and failing rabbit ventricular myocytes

Sarcoplasmic reticulum (SR) Ca²(+) leak is an important component of cardiac Ca²(+) signalling. Together with the SR Ca²(+)-ATPase (SERCA)-mediated Ca²(+) uptake, diastolic Ca²(+) leak determines SR Ca²(+) load and, therefore, the amplitude of Ca²(+) transients that initiate contraction. Spontaneous Ca²(+) sparks are thought to play a major role in SR Ca²(+) leak. In this study, we determined the quantitative contribution of sparks to SR Ca²(+) leak and tested the hypothesis that non-spark mediated Ca²(+) release also contributes to SR Ca²(+) leak. We simultaneously measured spark properties and intra-SR free Ca²(+) ([Ca²(+)](SR)) after complete inhibition of SERCA with thapsigargin in permeabilized rabbit ventricular myocytes. When [Ca²(+)](SR) declined to 279 ± 10 μm, spark activity ceased completely; however SR Ca²(+) leak continued, albeit at a slower rate. Analysis of sparks and [Ca²(+)](SR) revealed, that SR Ca²(+) leak increased as a function of [Ca²(+)](SR), with a particularly steep increase at higher [Ca²(+)](SR) ( >600 μm) where sparks become a major pathway of SR Ca²(+) leak. At low [Ca²(+)](SR) (< 300 μm), however, Ca²(+) leak occurred mostly as non-spark-mediated leak. Sensitization of ryanodine receptors (RyRs) with low doses of caffeine increased spark frequency and SR Ca²(+) leak. Complete inhibition of RyR abolished sparks and significantly decreased SR Ca²(+) leak, but did not prevent it entirely, suggesting the existence of RyR-independent Ca²(+) leak. Finally, we found that RyR-mediated Ca²(+) leak was enhanced in myocytes from failing rabbit hearts. These results show that RyRs are the main, but not sole contributor to SR Ca²(+) leak. RyR-mediated leak occurs in part as Ca²(+) sparks, but there is clearly RyR-mediated but Ca²(+) sparks independent leak.

The ryanodine receptor channel as a molecular motif in atrial fibrillation: pathophysiological and therapeutic implications

Atrial fibrillation (AF) is the most common cardiac arrhythmia and is associated with substantial morbidity and mortality. It causes profound changes in sarcoplasmic reticulum (SR) Ca(2+) homeostasis, including ryanodine receptor channel dysfunction and diastolic SR Ca(2+) leak, which might contribute to both decreased contractile function and increased propensity to atrial arrhythmias. In this review, we will focus on the molecular basis of ryanodine receptor channel dysfunction and enhanced diastolic SR Ca(2+) leak in AF. The potential relevance of increased incidence of spontaneous SR Ca(2+) release for both AF induction and/or maintenance and the development of novel mechanism-based therapeutic approaches will be discussed.

Neuregulin signaling and heart failure

Neuregulin-1 (NRG-1), a ligand of receptor tyrosine kinases of the ErbB family, plays a critical role in cardiovascular development and maintenance of adult heart function. Results from cellular, animal, and clinical experiments have shown NRG-1 to be a promising drug candidate for restoring cardiac function after cardiac injury. Various mechanisms have been suggested to be involved in this process, such as improving sarcomeric structure or cell-cell adhesion, promoting proliferation and survival of cardiac myocytes, balancing Ca(2+) homeostasis, modulating inotropic effects, promoting angiogenesis, and preventing atherosclerosis. However, the contribution of these effects to the restoration of cardiac function remains to be estimated, and it may depend on the specific events that led to heart failure. Meanwhile, distinct and crossed signaling pathways downstream of NRG-1 may play a role in these underlying mechanisms, resulting in a complicated network of signaling mediating the function of NRG-1.

Frequency and release flux of calcium sparks in rat cardiac myocytes: a relation to RYR gating

Cytosolic calcium concentration in resting cardiac myocytes locally fluctuates as a result of spontaneous microscopic Ca²⁺ releases or abruptly rises as a result of an external trigger. These processes, observed as calcium sparks, are fundamental for proper function of cardiac muscle. In this study, we analyze how the characteristics of spontaneous and triggered calcium sparks are related to cardiac ryanodine receptor (RYR) gating. We show that the frequency of spontaneous sparks and the probability distribution of calcium release flux quanta of triggered sparks correspond quantitatively to predictions of an allosteric homotetrameric model of RYR gating. This model includes competitive binding of Ca²⁺ and Mg²⁺ ions to the RYR activation sites and allosteric interaction between divalent ion binding and channel opening. It turns out that at rest, RYRs are almost fully occupied by Mg²⁺. Therefore, spontaneous sparks are most frequently evoked by random openings of the highly populated but rarely opening Mg₄RYR and CaMg₃RYR forms, whereas triggered sparks are most frequently evoked by random openings of the less populated but much more readily opening Ca₂Mg₂RYR and Ca₃MgRYR forms. In both the spontaneous and the triggered sparks, only a small fraction of RYRs in the calcium release unit manages to open during the spark because of the limited rate of Mg²⁺ unbinding. This mechanism clarifies the unexpectedly low calcium release flux during elementary release events and unifies the theory of calcium signaling in resting and contracting cardiac myocytes.