Cardiac sodium transport and excitation-contraction coupling

Cellular-level analyses of SCN5A mutations in left ventricular noncompaction cardiomyopathy suggest electrophysiological mechanisms for ventricular tachycardia

Left ventricular noncompaction cardiomyopathy (LVNC) is a cardiovascular disease characterized by arrhythmia and heart failure. In this study, LVNC myocardial samples were collected from patients who underwent heart transplantation and were analyzed using exome sequencing. Approximately half of the LVNC patients carried SCN5A variants, which are associated with clinical symptoms of ventricular tachycardia. To investigate the electrophysiological functions of these SCN5A variants and the underlying mechanism by which they increase arrhythmia susceptibility in LVNC patients, functional evaluations were conducted in CHO-K1 cells and human embryonic stem cell-derived cardiomyocytes (hESC-CMs) using patch-clamp or microelectrode array (MEA) techniques. These findings demonstrated that these SCN5A mutants exhibited gain-of-function properties, leading to increased channel activation and enhanced fast inactivation in CHO-K1 cells. Additionally, these mutants enhanced the excitability and contractility of the cardiomyocyte population in hESC-CMs models. All SCN5A variants induced fibrillation-like arrhythmia and increased the heart rate in cardiomyocytes. However, the administration of Lidocaine, an antiarrhythmic drug that acts on sodium ion channels, was able to rescue or alleviate fibrillation-like arrhythmias and secondary beat phenomenon. Based on these findings, it is speculated that SCN5A variants may contribute to susceptibility to arrhythmia in LVNC patients. Furthermore, the construction of cardiomyocyte models with SCN5A variants and their application in drug screening may facilitate the development of precise therapies for arrhythmia in the future.

Genetic risk factors for drug-induced long QT syndrome: findings from a large real-world case-control study

Aim: Drug-induced long QT syndrome (diLQTS), an adverse effect of many drugs, can lead to sudden cardiac death. Candidate genetic variants in cardiac ion channels have been associated with diLQTS, but several limitations of previous studies hamper clinical utility. Materials & methods: Thus, the purpose of this study was to assess the associations of KCNE1-D85N, KCNE2-I57T and SCN5A-G615E with diLQTS in a large observational case-control study (6,083 self-reported white patients treated with 27 different high-risk QT-prolonging medications; 12.0% with diLQTS). Results: KCNE1-D85N significantly associated with diLQTS (adjusted odds ratio: 2.24 [95% CI: 1.35-3.58]; p = 0.001). Given low minor allele frequencies, the study had insufficient power to analyze KCNE2-I57T and SCN5A-G615E. Conclusion: KCNE1-D85N is a risk factor for diLQTS that should be considered in future clinical practice guidelines.

Exposure to echimidine impairs the heart development and function of zebrafish larvae

Pyrrolizidine alkaloids (PAs) are a class of phytotoxins that are widely distributed and can be consumed by humans through their daily diets. Echimidine is one of the most abundant PAs, but its safety, particularly its effects on development, is not fully understood. In this study, we used a zebrafish model to assess the developmental toxicity of echimidine. Zebrafish embryos were exposed to echimidine at concentrations of 0.02, 0.2, and 2 mg/L for 96 h. Our study revealed that embryonic exposure to echimidine led to developmental toxicity, characterized by delayed hatching and reduced body length. Additionally, echimidine exposure had a notable impact on heart development in larvae, causing tachycardia and reducing stroke volume (SV)and cardiac output (CO). Upon exposing the transgenic zebrafish strain Tg(cmlc2:EGFP) to echimidine, we observed atrial dilation and thinning of the atrial wall in developing embryos. Moreover, our findings indicated abnormal expression of genes associated with cardiac development (including gata4, tbx5, nkx2.5 and myh6) and genes involved in calcium signaling pathways (such as cacna1aa, cacna1sa, ryr2a, ryr2b, atp2a2a, atp2a2b, slc8a1, slc8a3 and slc8a4a). In summary, our findings demonstrate that echimidine may impair cardiac development and function in zebrafish larvae by disrupting calcium transport, leading to developmental toxicity. These findings provide insights regarding the safety of products containing PAs in food and medicine.

Hypoxia and HIF-1α Regulate the Activity and Expression of Na,K-ATPase Subunits in H9c2 Cardiomyoblasts

The optimal function of the Na,K-ATPase (NKA) pump is essential for the heart. In ischemic heart disease, NKA activity decreases due to the decreased expression of the pump subunits. Here, we tested whether the hypoxia-inducible transcription factor (HIF-1α), the key signaling molecule regulating the adaptation of cells to hypoxia, is involved in controlling the expression and cellular dynamics of α1- and β1-NKA isoforms and of NKA activity in in-vitro hypoxic H9c2 cardiomyoblasts. HIF-1α was silenced through adenoviral infection, and cells were kept in normoxia (19% O2) or hypoxia (1% O2) for 24 h. We investigated the mRNA and protein expression of α1-, β1-NKA using RT-qPCR and Western blot in whole-cell lysates, cell membranes, and cytoplasmic fractions after labeling the cell surface with NHS-SS-biotin and immunoprecipitation. NKA activity and intracellular ATP levels were also measured. We found that in hypoxia, silencing HIF-1α prevented the decreased mRNA expression of α1-NKA but not of β1-NKA. Hypoxia decreased the plasma membrane expression of α1-NKA and β1- NKA compared to normoxic cells. In hypoxic cells, HIF-1α silencing prevented this effect by inhibiting the internalization of α1-NKA. Total protein expression was not affected. The decreased activity of NKA in hypoxic cells was fully prevented by silencing HIF-1α independent of cellular ATP levels. This study is the first to show that in hypoxic H9c2 cardiomyoblasts, HIF-1α controls the internalization and membrane insertion of α1-NKA subunit and of NKA activity. The mechanism behind this regulation needs further investigation.

Hypoxic Stress-Dependent Regulation of Na,K-ATPase in Ischemic Heart Disease

In cardiomyocytes, regular activity of the Na,K-ATPase (NKA) and its Na/K pump activity is essential for maintaining ion gradients, excitability, propagation of action potentials, electro-mechanical coupling, trans-membrane Na⁺ and Ca²⁺ gradients and, thus, contractility. The activity of NKA is impaired in ischemic heart disease and heart failure, which has been attributed to decreased expression of the NKA subunits. Decreased NKA activity leads to intracellular Na⁺ and Ca²⁺ overload, diastolic dysfunction and arrhythmias. One signal likely related to these events is hypoxia, where hypoxia-inducible factors (HIF) play a critical role in the adaptation of cells to low oxygen tension. HIF activity increases in ischemic heart, hypertension, heart failure and cardiac fibrosis; thus, it might contribute to the impaired function of NKA. This review will mainly focus on the regulation of NKA in ischemic heart disease in the context of stressed myocardium and the hypoxia-HIF axis and argue on possible consequences of treatment.

Hypercontractile Cardiac Phenotype in Mice with Migraine-Associated Mutation in the Na+,K+-ATPase α2-Isoform

Two α-isoforms of the Na⁺,K⁺-ATPase (α₁ and α₂) are expressed in the cardiovascular system, and it is unclear which isoform is the preferential regulator of contractility. Mice heterozygous for the familial hemiplegic migraine type 2 (FHM2) associated mutation in the α₂-isoform (G301R; α₂^+/G301R mice) have decreased expression of cardiac α₂-isoform but elevated expression of the α₁-isoform. We aimed to investigate the contribution of the α₂-isoform function to the cardiac phenotype of α₂^+/G301R hearts. We hypothesized that α₂^+/G301R hearts exhibit greater contractility due to reduced expression of cardiac α₂-isoform. Variables for contractility and relaxation of isolated hearts were assessed in the Langendorff system without and in the presence of ouabain (1 µM). Atrial pacing was performed to investigate rate-dependent changes. The α₂^+/G301R hearts displayed greater contractility than WT hearts during sinus rhythm, which was rate-dependent. The inotropic effect of ouabain was more augmented in α₂^+/G301R hearts than in WT hearts during sinus rhythm and atrial pacing. In conclusion, cardiac contractility was greater in α₂^+/G301R hearts than in WT hearts under resting conditions. The inotropic effect of ouabain was rate-independent and enhanced in α₂^+/G301R hearts, which was associated with increased systolic work.

Metabolic landscape in cardiac aging: insights into molecular biology and therapeutic implications

Cardiac aging is evident by a reduction in function which subsequently contributes to heart failure. The metabolic microenvironment has been identified as a hallmark of malignancy, but recent studies have shed light on its role in cardiovascular diseases (CVDs). Various metabolic pathways in cardiomyocytes and noncardiomyocytes determine cellular senescence in the aging heart. Metabolic alteration is a common process throughout cardiac degeneration. Importantly, the involvement of cellular senescence in cardiac injuries, including heart failure and myocardial ischemia and infarction, has been reported. However, metabolic complexity among human aging hearts hinders the development of strategies that targets metabolic susceptibility. Advances over the past decade have linked cellular senescence and function with their metabolic reprogramming pathway in cardiac aging, including autophagy, oxidative stress, epigenetic modifications, chronic inflammation, and myocyte systolic phenotype regulation. In addition, metabolic status is involved in crucial aspects of myocardial biology, from fibrosis to hypertrophy and chronic inflammation. However, further elucidation of the metabolism involvement in cardiac degeneration is still needed. Thus, deciphering the mechanisms underlying how metabolic reprogramming impacts cardiac aging is thought to contribute to the novel interventions to protect or even restore cardiac function in aging hearts. Here, we summarize emerging concepts about metabolic landscapes of cardiac aging, with specific focuses on why metabolic profile alters during cardiac degeneration and how we could utilize the current knowledge to improve the management of cardiac aging.

Gene mutations in comorbidity of epilepsy and arrhythmia

Epilepsy is one of the most common neurological disorders, and sudden unexpected death in epilepsy (SUDEP) is the most severe outcome of refractory epilepsy. Arrhythmia is one of the heterogeneous factors in the pathophysiological mechanism of SUDEP with a high incidence in patients with refractory epilepsy, increasing the risk of premature death. The gene co-expressed in the brain and heart is supposed to be the genetic basis between epilepsy and arrhythmia, among which the gene encoding ion channel contributes to the prevalence of "cardiocerebral channelopathy" theory. Nevertheless, this theory could only explain the molecular mechanism of comorbid arrhythmia in part of patients with epilepsy (PWE). Therefore, we summarized the mutant genes that can induce comorbidity of epilepsy and arrhythmia and the possible corresponding treatments. These variants involved the genes encoding sodium, potassium, calcium and HCN channels, as well as some non-ion channel coding genes such as CHD4, PKP2, FHF1, GNB5, and mitochondrial genes. The relationship between genotype and clinical phenotype was not simple linear. Indeed, genes co-expressed in the brain and heart could independently induce epilepsy and/or arrhythmia. Mutant genes in brain could affect cardiac rhythm through central or peripheral regulation, while in the heart it could also affect cerebral electrical activity by changing the hemodynamics or internal environment. Analysis of mutations in comorbidity of epilepsy and arrhythmia could refine and expand the theory of "cardiocerebral channelopathy" and provide new insights for risk stratification of premature death and corresponding precision therapy in PWE.

Cardiovascular Disease in Obstructive Sleep Apnea: Putative Contributions of Mineralocorticoid Receptors

Obstructive sleep apnea (OSA) is a chronic and highly prevalent condition that is associated with oxidative stress, inflammation, and fibrosis, leading to endothelial dysfunction, arterial stiffness, and vascular insulin resistance, resulting in increased cardiovascular disease and overall mortality rates. To date, OSA remains vastly underdiagnosed and undertreated, with conventional treatments yielding relatively discouraging results for improving cardiovascular outcomes in OSA patients. As such, a better mechanistic understanding of OSA-associated cardiovascular disease (CVD) and the development of novel adjuvant therapeutic targets are critically needed. It is well-established that inappropriate mineralocorticoid receptor (MR) activation in cardiovascular tissues plays a causal role in a multitude of CVD states. Clinical studies and experimental models of OSA lead to increased secretion of the MR ligand aldosterone and excessive MR activation. Furthermore, MR activation has been associated with worsened OSA prognosis. Despite these documented relationships, there have been no studies exploring the causal involvement of MR signaling in OSA-associated CVD. Further, scarce clinical studies have exclusively assessed the beneficial role of MR antagonists for the treatment of systemic hypertension commonly associated with OSA. Here, we provide a comprehensive overview of overlapping mechanistic pathways recruited in the context of MR activation- and OSA-induced CVD and propose MR-targeted therapy as a potential avenue to abrogate the deleterious cardiovascular consequences of OSA.

Purinoreceptors and ectonucleotidases control ATP-induced calcium waveforms and calcium-dependent responses in microglia: Roles of P2 receptors and CD39 in ATP-stimulated microglia

Adenosine triphosphate (ATP) and its metabolites drive microglia migration and cytokine production by activating P2X- and P2Y- class purinergic receptors. Purinergic receptor activation gives rise to diverse intracellular calcium (Ca2+ signals, or waveforms, that differ in amplitude, duration, and frequency. Whether and how these characteristics of diverse waveforms influence microglia function is not well-established. We developed a computational model trained with data from published primary murine microglia studies. We simulate how purinoreceptors influence Ca2+ signaling and migration, as well as, how purinoreceptor expression modifies these processes. Our simulation confirmed that P2 receptors encode the amplitude and duration of the ATP-induced Ca2+ waveforms. Our simulations also implicate CD39, an ectonucleotidase that rapidly degrades ATP, as a regulator of purinergic receptor-induced Ca2+ responses. Namely, it was necessary to account for CD39 metabolism of ATP to align the model's predicted purinoreceptor responses with published experimental data. In addition, our modeling results indicate that small Ca2+ transients accompany migration, while large and sustained transients are needed for cytokine responses. Lastly, as a proof-of-principal, we predict Ca2+ transients and cell membrane displacements in a BV2 microglia cell line using published P2 receptor mRNA data to illustrate how our computer model may be extrapolated to other microglia subtypes. These findings provide important insights into how differences in purinergic receptor expression influence microglial responses to ATP.

Human-gained heart enhancers are associated with species-specific cardiac attributes

The heart, a vital organ which is first to develop, has adapted its size, structure and function in order to accommodate the circulatory demands for a broad range of animals. Although heart development is controlled by a relatively conserved network of transcriptional/chromatin regulators, how the human heart has evolved species-specific features to maintain adequate cardiac output and function remains to be defined. Here, we show through comparative epigenomic analysis the identification of enhancers and promoters that have gained activity in humans during cardiogenesis. These cis-regulatory elements (CREs) are associated with genes involved in heart development and function, and may account for species-specific differences between human and mouse hearts. Supporting these findings, genetic variants that are associated with human cardiac phenotypic/disease traits, particularly those differing between human and mouse, are enriched in human-gained CREs. During early stages of human cardiogenesis, these CREs are also gained within genomic loci of transcriptional regulators, potentially expanding their role in human heart development. In particular, we discovered that gained enhancers in the locus of the early human developmental regulator ZIC3 are selectively accessible within a subpopulation of mesoderm cells which exhibits cardiogenic potential, thus possibly extending the function of ZIC3 beyond its conserved left-right asymmetry role. Genetic deletion of these enhancers identified a human gained enhancer that was required for not only ZIC3 and early cardiac gene expression at the mesoderm stage but also cardiomyocyte differentiation. Overall, our results illuminate how human gained CREs may contribute to human-specific cardiac attributes, and provide insight into how transcriptional regulators may gain cardiac developmental roles through the evolutionary acquisition of enhancers.

The Role of the Na+/Ca2+ Exchanger in Aberrant Intracellular Ca2+ in Cardiomyocytes of Chagas-Infected Rodents

Chagas disease is produced by the parasite Trypanosoma cruzi (T. cruzi), which is the leading cause of death and morbidity in Latin America. We have shown that in patients with Chagas cardiomyopathy, there is a chronic elevation of diastolic Ca²⁺ concentration ([Ca²⁺]_d), associated with deterioration to further address this issue, we explored the role Na⁺/Ca²⁺ exchanger (NCX). Experiments were carried out in noninfected C57BL/6 mice and infected with blood-derived trypomastigotes of the T. cruzi Y strain. Anesthetized mice were sacrificed and the cardiomyocytes were enzymatically dissociated. Diastolic [Ca²⁺] ([Ca²⁺]_d) was measured using Ca²⁺ selective microelectrodes in cardiomyocytes from control mice (CONT) and cardiomyocytes from T. cruzi infected mice in the early acute phase (EAP) at 20 dpi, in the acute phase (AP) at 40 dpi, and in the chronic phase (CP) at 120 dpi. [Ca²⁺]_d was 1.5-times higher in EAP, 2.6-times in AP, and 3.4-times in CP compared to CONT. Exploring the reverse mode activity of NCX, we replaced extracellular Na⁺ in equivalent amounts with N-methyl-D-glucamine. Reduction of [Na⁺]_e to 65 mM caused an increase in [Ca²⁺]_d of 1.7 times in cardiomyocytes from CONT mice, 2 times in EAP infected mice, 2.4 times in AP infected mice and 2.8 in CP infected mice. The Na⁺ free solution caused a further elevation of [Ca²⁺]_d of 2.5 times in cardiomyocytes from CONT, 2.8 times in EAP infected mice, 3.1 times in AP infected mice, and 3.3 times in CP infected mice. Extracellular Ca²⁺ withdrawal reduced [Ca²⁺]_d in both CONT and cardiomyocytes from Chagas-infected mice and prevented the increase in [Ca²⁺]_d induced by Na⁺ depletion. Preincubation with 10µM KB-R7943 or in 1µM YM-244769 reduced [Ca²⁺]_d in cardiomyocytes from infected mice, but not control mice. Furthermore, both NCX blockers prevented the increase in [Ca²⁺]_d associated with exposure to a solution without Na⁺. These results suggest that Ca²⁺ entry through the reverse NCX mode plays a significant role in the observed [Ca²⁺]_d dyshomeostasis in Chagas infected cardiomyocytes. Additionally, NCX inhibitors may be a viable therapeutic approach for treating patients with Chagas cardiomyopathy.

Regulation of Cardiac Contractility by the Alpha 2 Subunit of the Na+/K+-ATPase

Cytosolic Na + concentrations regulate cardiac excitation-contraction coupling and contractility. Inhibition of the Na⁺/K⁺-ATPase (NKA) activity increases cardiac contractility by increasing cytosolic Ca²⁺ levels, as increased cytosolic Na⁺ levels are coupled to less Ca²⁺ extrusion and/or increased Ca²⁺ influx from the Na⁺/Ca²⁺-exchanger. NKA consists of one α subunit and one β subunit, with α1 and α2 being the main α isoforms in cardiomyocytes. Substantial evidence suggests that NKAα2 is the primary regulator of cardiac contractility despite being outnumbered by NKAα1 in cardiomyocytes. This review will mainly focus on differential regulation and subcellular localization of the NKAα1 and NKAα2 isoforms, and their relation to the proposed concept of subcellular gradients of Na⁺ in cardiomyocytes. We will also discuss the potential roles of NKAα2 in mediating cardiac hypertrophy and ventricular arrhythmias.

The genetics of drug-induced QT prolongation: evaluating the evidence for pharmacodynamic variants

Drug-induced long QT syndrome (diLQTS) is an adverse effect of many commonly prescribed drugs, and it can increase the risk for lethal ventricular arrhythmias. Genetic variants in pharmacodynamic genes have been associated with diLQTS, but the strength of the evidence for each of those variants has not yet been evaluated. Therefore, the purpose of this review was to evaluate the strength of the evidence for pharmacodynamic genetic variants associated with diLQTS using a novel, semiquantitative scoring system modified from the approach used for congenital LQTS. KCNE1-D85N and KCNE2-T8A had definitive and strong evidence for diLQTS, respectively. The high level of evidence for these variants supports current consideration as risk factors for patients that will be prescribed a QT-prolonging drug.

Myogenin suppresses apoptosis induced by angiotensin II in human induced pluripotent stem cell-derived cardiomyocytes

BACKGROUND

Angiotensin II (Ang II), an important component of the renin-angiotensin system (RAS), plays a critical role in the pathogenesis of cardiovascular disorders. In addition, human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) have been considered as a promising platform for studying personalized medicine for heart diseases. However, whether Ang II can induce the apoptosis of hiPSC-CMs is not known.

METHODS

In this study, we treated hiPSC-CMs with different concentrations of Ang II [0 nM (vehicle as a control), 1 nM, 10 nM, 100 nM, 1 μM, 10 μM, 100 μM, and 1 mM] for various time periods (24 h, 48 h, 6 days, and 10 days) and analyzed the viability and apoptosis of hiPSC-CMs.

RESULTS

We found that treatment with 1 mM Ang II for 10 days reduced the viability of hiPSC-CMs by 41% (p = 2.073E-08) and increased apoptosis by 2.74-fold, compared to the control group (p = 6.248E-12). MYOG, which encodes the muscle-specific transcription factor myogenin, was also identified as an apoptosis-suppressor gene in Ang II-treated hiPSC-CMs. Ectopic MYOG expression decreased the apoptosis and increased the viability of Ang II-treated hiPSC-CMs. Further analysis of the RNA sequencing (RNA-seq) data illustrated that myogenin ameliorated Ang II-induced apoptosis of hiPSC-CMs by downregulating the expression of proinflammatory genes.

CONCLUSION

Our findings suggest that Ang II induces the apoptosis of hiPSC-CMs and that myogenin attenuates Ang II-induced apoptosis.

Coupling of the Na+/K+-ATPase to Ankyrin B controls Na+/Ca2+ exchanger activity in cardiomyocytes

AIMS

Ankyrin B (AnkB) is an adaptor protein that assembles Na+/K+-ATPase (NKA) and Na+/Ca2+ exchanger (NCX) in the AnkB macromolecular complex. Loss-of-function mutations in AnkB cause the AnkB syndrome in humans, characterized by ventricular arrhythmias and sudden cardiac death. It is unclear to what extent NKA binding to AnkB allows regulation of local Na+ and Ca2+ domains and hence NCX activity.

METHODS AND RESULTS

To investigate the role of NKA binding to AnkB in cardiomyocytes, we synthesized a disruptor peptide (MAB peptide) and its AnkB binding ability was verified by pulldown experiments. As opposed to control, the correlation between NKA and NCX currents was abolished in adult rat ventricular myocytes dialyzed with MAB peptide, as well as in cardiomyocytes from AnkB+/- mice. Disruption of NKA from AnkB (with MAB peptide) increased NCX-sensed cytosolic Na+ concentration, reduced Ca2+ extrusion through NCX, and increased frequency of Ca2+ sparks and Ca2+ waves without concomitant increase in Ca2+ transient amplitude or SR Ca2+ load, suggesting an effect in local Ca2+ domains. Selective inhibition of the NKAα2 isoform abolished both the correlation between NKA and NCX currents and the increased rate of Ca2+ sparks and waves following NKA/AnkB disruption, suggesting that an AnkB/NKAα2/NCX domain controls Ca2+ fluxes in cardiomyocytes.

CONCLUSION

NKA binding to AnkB allows ion regulation in a local domain, and acute disruption of the NKA/AnkB interaction using disruptor peptides lead to increased rate of Ca2+ sparks and waves. The functional effects were mediated through the NKAα2 isoform. Disruption of the AnkB/NKA/NCX domain could be an important pathophysiological mechanism in the AnkB syndrome.

Increased Burden of Ion Channel Gene Variants Is Related to Distinct Phenotypes in Pediatric Patients With Left Ventricular Noncompaction

BACKGROUND

Left ventricular noncompaction (LVNC) is a hereditary type of cardiomyopathy. Although it is associated with high morbidity and mortality, the related ion channel gene variants in children have not been fully investigated. This study aimed to elucidate the ion channel genetic landscape of LVNC and identify genotype-phenotype correlations in a large Japanese cohort.

METHODS

We enrolled 206 children with LVNC from 2002 to 2017 in Japan. LVNC was classified as follows: LVNC with congenital heart defects, arrhythmia, dilated phenotype, or normal function. In the enrolled patients, 182 genes associated with cardiomyopathy were screened using next-generation sequencing.

RESULTS

We identified 99 pathogenic variants in 40 genes in 87 patients. Of the pathogenic variants, 8.8% were in genes associated with channelopathies, 27% were in sarcomere genes, and 11.5% were in mitochondrial genes. Ion channel gene variants were mostly associated with the arrhythmia classification, whereas sarcomere and mitochondrial gene variants were associated with the dilated phenotype. Echocardiography revealed that the group with ion channel gene variants had almost normal LV ejection fraction and LV diastolic diameter Z scores. Fragmented QRS, old age, and an arrhythmia phenotype were the most significant risk factors for ventricular tachycardia (P=0.165, 0.0428, and 0.0074, respectively). Moreover, the group with ion channel variants exhibited a greater risk of a higher prevalence of arrhythmias such as ventricular tachycardia, rather than congestive heart failure.

CONCLUSIONS

This is the first study that focused on genotype-phenotype correlations in a large pediatric LVNC patient cohort with ion channel gene variants that were determined using next-generation sequencing. Ion channel gene variants were strongly correlated with arrhythmia phenotypes. Genetic testing and phenotype specification allow for appropriate medical management of specific LVNC targets.

Co-localization and confinement of ecto-nucleotidases modulate extracellular adenosine nucleotide distributions

Nucleotides comprise small molecules that perform critical signaling roles in biological systems. Adenosine-based nucleotides, including adenosine tri-, di-, and mono-phosphate, are controlled through their rapid degradation by diphosphohydrolases and ecto-nucleotidases (NDAs). The interplay between nucleotide signaling and degradation is especially important in synapses formed between cells, which create signaling 'nanodomains'. Within these 'nanodomains', charged nucleotides interact with densely-packed membranes and biomolecules. While the contributions of electrostatic and steric interactions within such nanodomains are known to shape diffusion-limited reaction rates, less is understood about how these factors control the kinetics of nucleotidase activity. To quantify these factors, we utilized reaction-diffusion numerical simulations of 1) adenosine triphosphate (ATP) hydrolysis into adenosine monophosphate (AMP) and 2) AMP into adenosine (Ado) via two representative nucleotidases, CD39 and CD73. We evaluate these sequentially-coupled reactions in nanodomain geometries representative of extracellular synapses, within which we localize the nucleotidases. With this model, we find that 1) nucleotidase confinement reduces reaction rates relative to an open (bulk) system, 2) the rates of AMP and ADO formation are accelerated by restricting the diffusion of substrates away from the enzymes, and 3) nucleotidase co-localization and the presence of complementary (positive) charges to ATP enhance reaction rates, though the impact of these contributions on nucleotide pools depends on the degree to which the membrane competes for substrates. As a result, these contributions integratively control the relative concentrations and distributions of ATP and its metabolites within the junctional space. Altogether, our studies suggest that CD39 and CD73 nucleotidase activity within junctional spaces can exploit their confinement and favorable electrostatic interactions to finely control nucleotide signaling.

ArrhythmoGenoPharmacoTherapy

This review is focusing on the understanding of various factors and components governing and controlling the occurrence of ventricular arrhythmias including (i) the role of various ion channel-related changes in the action potential (AP), (ii) electrocardiograms (ECGs), (iii) some important arrhythmogenic mediators of reperfusion, and pharmacological approaches to their attenuation. The transmembrane potential in myocardial cells is depending on the cellular concentrations of several ions including sodium, calcium, and potassium on both sides of the cell membrane and active or inactive stages of ion channels. The movements of Na+, K+, and Ca2+ via cell membranes produce various currents that provoke AP, determining the cardiac cycle and heart function. A specific channel has its own type of gate, and it is opening and closing under specific transmembrane voltage, ionic, or metabolic conditions. APs of sinoatrial (SA) node, atrioventricular (AV) node, and Purkinje cells determine the pacemaker activity (depolarization phase 4) of the heart, leading to the surface manifestation, registration, and evaluation of ECG waves in both animal models and humans. AP and ECG changes are key factors in arrhythmogenesis, and the analysis of these changes serve for the clarification of the mechanisms of antiarrhythmic drugs. The classification of antiarrhythmic drugs may be based on their electrophysiological properties emphasizing the connection between basic electrophysiological activities and antiarrhythmic properties. The review also summarizes some important mechanisms of ventricular arrhythmias in the ischemic/reperfused myocardium and permits an assessment of antiarrhythmic potential of drugs used for pharmacotherapy under experimental and clinical conditions.

Genetic Variants on SCN5A, KCNQ1, and KCNH2 in Patients with Ventricular Arrhythmias during Acute Myocardial Infarction in a Chinese Population

OBJECTIVE

Acute myocardial infarction (AMI) remains a leading cause of morbidity and mortality worldwide. About half of sudden deaths from AMI are mainly because of malignant ventricular arrhythmias (VA) after AMI. The sodium channel gene SCN5A and potassium channel genes KCNQ1 and KCNH2 have been widely reported to be genetic risk factors for arrhythmia including Brugada syndrome and long QT syndrome (LQTS). A few studies reported the association of SCN5A variant with ventricular tachycardia (VT)/ventricular fibrillation (VF) complicating AMI. However, little is known about the role of KCNQ1 and KCNH2 in AMI with VA (AMI_VA). This study focuses on investigating the potential variants on SCN5A, KCNQ1, and KCNH2 contributing to AMI with VA in a Chinese population.

MATERIALS AND METHODS

In total, 139 patients with AMI_VA, and 337 patients with AMI only, were included. Thirty exonic sites were selected to be screened. Sanger sequencing was used to detect variants. A subsequent association study was also performed between AMI_VA and AMI.

RESULTS

Twelve variants [5 on KCNH2(NM_000238.3), 3 on KCNQ1(NM_000218.2), and 4 on SCN5A(NM_198056.2)] were identified in AMI_VA patients. Only 5 (KCNH2: c.2690A>C; KCNQ1: c.1927G>A, c.1343delC; SCN5A: c.1673A>G, c.3578G>A) of them are missense variants. Two (KCNQ1: c.1343delC and SCN5A: c.3578G>A) of the missense variants were predicted to be clinically pathogenic. All these variants were further genotyped in an AMI without VA group. The association study identified a statistically significant difference in genotype frequency of KCNH2: c.1539C>T and KCNH2: c.1467C>T between the AMI and AMI_VA groups. Moreover, 2 rare variants (KCNQ1: c.1944C>T and SCN5A: c.3621C>T) showed an elevated allelic frequency (more than 1.5-fold) in the AMI_VA group when compared to the AMI group.

CONCLUSION

Twelve variants (predicting from benign/VUS to pathogenic) were identified on KCNH2, KCNQ1, and SCN5A in patients with AMI_VA. Genotype frequency comparison between AMI_VA and AMI identified 2 significant common variants on KCNH2. Meanwhile, the allelic frequency of 2 rare variants on KCNQ1 and SCN5A, respectively, were identified to be enriched in AMI_VA, although there was no statistical significance. The present study suggests that the ion-channel genes KCNH2, KCNQ1, and SCN5A may contribute to the pathogenesis of VA during AMI.

Quantitative determination of cellular [Na+] by fluorescence lifetime imaging with CoroNaGreen

Meyer et al. establish the suitability of the sodium-sensitive indicator dye CoroNaGreen for fluorescence lifetime imaging inside cells. This approach represents a valuable tool for quantitative and dynamic determination of intracellular sodium concentrations independent of dye concentration.

Fluorescence lifetime imaging microscopy (FLIM) with fluorescent ion sensors enables the measurement of ion concentrations based on the detection of photon emission events after brief excitation with a pulsed laser source. In contrast to intensity-based imaging, it is independent of dye concentration, photobleaching, or focus drift and has thus been successfully employed for quantitative analysis of, e.g., calcium levels in different cell types and cellular microdomains. Here, we tested the suitability of CoroNaGreen for FLIM-based determination of sodium concentration ([Na⁺]) inside cells. In vitro measurements confirmed that fluorescence lifetimes of CoroNaGreen (CoroNaFL) increased with increasing [Na⁺]. Moreover, CoroNaFL was largely independent of changes in potassium concentration or viscosity. Changes in pH slightly affected FL in the acidic range (pH ≤ 5.5). For intracellular determination of [Na⁺], HEK293T cells were loaded with the membrane-permeable form of CoroNaGreen. Fluorescence decay curves of CoroNaGreen, derived from time-correlated single-photon counting, were approximated by a bi-exponential decay. In situ calibrations revealed a sigmoidal dependence of CoroNaFL on [Na⁺] between 0 and 150 mM, exhibiting an apparent K_d of ∼80 mM. Based on these calibrations, a [Na⁺] of 17.6 mM was determined in the cytosol. Cellular nuclei showed a significantly lower [Na⁺] of 13.0 mM, whereas [Na⁺] in perinuclear regions was significantly higher (26.5 mM). Metabolic inhibition or blocking the Na⁺/K⁺-ATPase by removal of extracellular K⁺ caused significant [Na⁺] increases in all cellular subcompartments. Using an alternative approach for data analysis (“Ratio FLIM”) increased the temporal resolution and revealed a sequential response to K⁺ removal, with cytosolic [Na⁺] increasing first, followed by the nucleus and finally the perinuclear regions. Taken together, our results show that CoroNaGreen is suitable for dynamic, FLIM-based determination of intracellular [Na⁺]. This approach thus represents a valuable tool for quantitative determination of [Na⁺] and changes thereof in different subcellular compartments.

Clinical characterisation of a novel SCN5A variant associated with progressive malignant arrhythmia and dilated cardiomyopathy

INTRODUCTION

The SCN5A gene is implicated in many arrhythmogenic and cardiomyopathic processes. We identified a novel SCN5A variant in a family with significant segregation in individuals affected with progressive sinus and atrioventricular nodal disease, atrial arrhythmia, dilated cardiomyopathy, and early sudden cardiac arrest.

METHODS

A patient pedigree was created following the clinical evaluation of three affected individuals, two monozygotic twins and a paternal half-brother, which lead to the evaluation of a paternal half-sister (four siblings with the same father and three mothers) all of whom experienced varying degrees of atrial arrhythmias, conduction disease, and dilated cardiomyopathy in addition to a paternal history of unexplained death in his 50s with similar autopsy findings. The index male underwent sequencing of 58 genes associated with cardiomyopathies. Sanger sequencing was used to provide data for bases with insufficient coverage and for bases in some known regions of genomic segmental duplications. All clinically significant and novel variants were confirmed by independent Sanger sequencing.

RESULTS

All relatives tested were shown to have the same SCN5A variant of unknown significance (p. Asp197His) and the monozygotic twins shared a co-occurring NEXN (p. Glu575*). Segregation analysis demonstrates likely pathogenic trait for the SCN5A variant with an additional possible role for the NEXN variant in combination.

CONCLUSIONS

There is compelling clinical evidence suggesting that the SCN5A variant p. Asp197His may be re-classified as likely pathogenic based on the segregation analysis of our family of interest. Molecular mechanism studies are pending.

2019 HRS expert consensus statement on evaluation, risk stratification, and management of arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) is an arrhythmogenic disorder of the myocardium not secondary to ischemic, hypertensive, or valvular heart disease. ACM incorporates a broad spectrum of genetic, systemic, infectious, and inflammatory disorders. This designation includes, but is not limited to, arrhythmogenic right/left ventricular cardiomyopathy, cardiac amyloidosis, sarcoidosis, Chagas disease, and left ventricular noncompaction. The ACM phenotype overlaps with other cardiomyopathies, particularly dilated cardiomyopathy with arrhythmia presentation that may be associated with ventricular dilatation and/or impaired systolic function. This expert consensus statement provides the clinician with guidance on evaluation and management of ACM and includes clinically relevant information on genetics and disease mechanisms. PICO questions were utilized to evaluate contemporary evidence and provide clinical guidance related to exercise in arrhythmogenic right ventricular cardiomyopathy. Recommendations were developed and approved by an expert writing group, after a systematic literature search with evidence tables, and discussion of their own clinical experience, to present the current knowledge in the field. Each recommendation is presented using the Class of Recommendation and Level of Evidence system formulated by the American College of Cardiology and the American Heart Association and is accompanied by references and explanatory text to provide essential context. The ongoing recognition of the genetic basis of ACM provides the opportunity to examine the diverse triggers and potential common pathway for the development of disease and arrhythmia.

Assessment of PEEP-Ventilation and the Time Point of Parallel-Conductance Determination for Pressure-Volume Analysis Under β-Adrenergic Stimulation in Mice

Aim: Cardiac pressure-volume (PV loop) analysis under β-adrenergic stimulation is a powerful method to simultaneously determine intrinsic cardiac function and β-adrenergic reserve in mouse models. Despite its wide use, several key approaches of this method, which can affect murine cardiac function tremendously, have not been experimentally investigated until now. In this study, we investigate the impact of three lines of action during the complex procedure of PV loop analysis: (i) the ventilation with positive end-expiratory pressure, (ii) the time point of injecting hypertonic saline to estimate parallel-conductance, and (iii) the implications of end-systolic pressure-spikes that may arise under β-adrenergic stimulation.

Methods and Results: We performed pressure-volume analysis during β-adrenergic stimulation in an open-chest protocol under Isoflurane/Buprenorphine anesthesia. Our analysis showed that (i) ventilation with 2 cmH₂O positive end-expiratory pressure prevented exacerbation of peak inspiratory pressures subsequently protecting mice from macroscopic pulmonary bleedings. (ii) Estimations of parallel-conductance by injecting hypertonic saline prior to pressure-volume recordings induced dilated chamber dimensions as depicted by elevation of end-systolic volume (+113%), end-diastolic volume (+40%), and end-diastolic pressure (+46%). Further, using this experimental approach, the preload-independent contractility (PRSW) was significantly impaired under basal conditions (−17%) and under catecholaminergic stimulation (−14% at 8.25 ng/min Isoprenaline), the β-adrenergic reserve was alleviated, and the incidence of ectopic beats was increased >5-fold. (iii) End-systolic pressure-spikes were observed in 26% of pressure-volume recordings under stimulation with 2.475 and 8.25 ng/min Isoprenaline, which affected the analysis of maximum pressure (+11.5%), end-diastolic volume (−8%), stroke volume (−10%), and cardiac output (−11%).

Conclusions: Our results (i) demonstrate the advantages of positive end-expiratory pressure ventilation in open-chest instrumented mice, (ii) underline the perils of injecting hypertonic saline prior to pressure-volume recordings to calibrate for parallel-conductance and (iii) emphasize the necessity to be aware of the consequences of end-systolic pressure-spikes during β-adrenergic stimulation.

Evidence for heterogeneous subsarcolemmal Na+ levels in rat ventricular myocytes

The intracellular Na⁺ concentration ([Na⁺]) regulates cardiac contractility. Previous studies have suggested that subsarcolemmal [Na⁺] is higher than cytosolic [Na⁺] in cardiac myocytes, but this concept remains controversial. Here, we used electrophysiological experiments and mathematical modeling to test whether there are subsarcolemmal pools with different [Na⁺] and dynamics compared with the bulk cytosol in rat ventricular myocytes. A Na⁺ dependency curve for Na⁺-K⁺-ATPase (NKA) current was recorded with symmetrical Na⁺ solutions, i.e., the same [Na⁺] in the superfusate and internal solution. This curve was used to estimate [Na⁺] sensed by NKA in other experiments. Three experimental observations suggested that [Na⁺] is higher near NKA than in the bulk cytosol: 1) when extracellular [Na⁺] was high, [Na⁺] sensed by NKA was ~6 mM higher than the internal solution in quiescent cells; 2) long trains of Na⁺ channel activation almost doubled this gradient; compared with an even intracellular distribution of Na⁺, the increase of [Na⁺] sensed by NKA was 10 times higher than expected, suggesting a local Na⁺ domain; and 3) accumulation of Na⁺ near NKA after trains of Na⁺ channel activation dissipated very slowly. Finally, mathematical models assuming heterogeneity of [Na⁺] between NKA and the Na⁺ channel better reproduced experimental data than the homogeneous model. In conclusion, our data suggest that NKA-sensed [Na⁺] is higher than [Na⁺] in the bulk cytosol and that there are differential Na⁺ pools in the subsarcolemmal space, which could be important for cardiac contractility and arrhythmogenesis. NEW & NOTEWORTHY Our data suggest that the Na⁺-K⁺-ATPase-sensed Na⁺ concentration is higher than the Na⁺ concentration in the bulk cytosol and that there are differential Na⁺ pools in the subsarcolemmal space, which could be important for cardiac contractility and arrhythmogenesis. Listen to this article's corresponding podcast at https://ajpheart.podbean.com/e/heterogeneous-sodium-in-ventricular-myocytes/ .

Hypokalemia-Induced Arrhythmias and Heart Failure: New Insights and Implications for Therapy

Routine use of diuretics and neurohumoral activation make hypokalemia (serum K⁺ < 3. 5 mM) a prevalent electrolyte disorder among heart failure patients, contributing to the increased risk of ventricular arrhythmias and sudden cardiac death in heart failure. Recent experimental studies have suggested that hypokalemia-induced arrhythmias are initiated by the reduced activity of the Na⁺/K⁺-ATPase (NKA), subsequently leading to Ca²⁺ overload, Ca²⁺/Calmodulin-dependent kinase II (CaMKII) activation, and development of afterdepolarizations. In this article, we review the current mechanistic evidence of hypokalemia-induced triggered arrhythmias and discuss how molecular changes in heart failure might lower the threshold for these arrhythmias. Finally, we discuss how recent insights into hypokalemia-induced arrhythmias could have potential implications for future antiarrhythmic treatment strategies.

Application of Spectral Phasor analysis to sodium microenvironments in myoblast progenitor cells

Sodium ions (Na+) are key regulators of molecular events in many cellular processes, yet the dynamics of this ion remain poorly defined. Developing approaches to identify and characterise Na+ microenvironments will enable more detailed elucidation of the mechanisms of signal transduction. Here we report the application of Spectral Phasor analysis to the Na+ fluorophore, CoroNa Green, to identify and spatially map spectral emissions that report Na+ microenvironments. We use differentiating stem cells where Na+ fluxes were reported as an antecedent. Myoblast stem cells were induced to differentiate by serum starvation and then fixed at intervals between 0 and 40-minutes of differentiation prior to addition of CoroNa Green. The fluorescent intensity was insufficient to identify discrete Na+ microenvironments. However, using Spectral Phasor analysis we identified spectral shifts in CoroNa Green fluorescence which is related to the Na+ microenvironment. Further, spectral-heterogeneity appears to be contingent on the distance of Na+ from the nucleus in the early stages of differentiation. Spectral Phasor analysis of CoroNa Green in fixed stem cells demonstrates for the first time that CoroNa Green has unique spectral emissions depending on the nature of the Na+ environment in differentiating stem cells. Applying Spectral Phasor analysis to CoroNa Green in live stem cells is likely to further elucidate the role of Na+ microenvironments in the differentiation process.

SCN5A Variants: Association With Cardiac Disorders

The SCN5A gene encodes the alpha subunit of the main cardiac sodium channel Na_v1.5. This channel predominates inward sodium current (INa) and plays a critical role in regulation of cardiac electrophysiological function. Since 1995, SCN5A variants have been found to be causatively associated with Brugada syndrome, long QT syndrome, cardiac conduction system dysfunction, dilated cardiomyopathy, etc. Previous genetic, electrophysiological, and molecular studies have identified the arrhythmic and cardiac structural characteristics induced by SCN5A variants. However, due to the variation of disease manifestations and genetic background, impact of environmental factors, as well as the presence of mixed phenotypes, the detailed and individualized physiological mechanisms in various SCN5A-related syndromes are not fully elucidated. This review summarizes the current knowledge of SCN5A genetic variations in different SCN5A-related cardiac disorders and the newly developed therapy strategies potentially useful to prevent and treat these disorders in clinical setting.

Beyond Intracellular Signaling: The Ins and Outs of Second Messengers Microdomains

A typical characteristic of eukaryotic cells compared to prokaryotes is represented by the spatial heterogeneity of the different structural and functional components: for example, most of the genetic material is surrounded by a highly specific membrane structure (the nuclear membrane), continuous with, yet largely different from, the endoplasmic reticulum (ER); oxidative phosphorylation is carried out by organelles enclosed by a double membrane, the mitochondria; in addition, distinct domains, enriched in specific proteins, are present in the plasma membrane (PM) of most cells. Less obvious, but now generally accepted, is the notion that even the concentration of small molecules such as second messengers (Ca²⁺ and cAMP in particular) can be highly heterogeneous within cells. In the case of most organelles, the differences in the luminal levels of second messengers depend either on the existence on their membrane of proteins that allow the accumulation/release of the second messenger (e.g., in the case of Ca²⁺, pumps, exchangers or channels), or on the synthesis and degradation of the specific molecule within the lumen (the autonomous intramitochondrial cAMP system). It needs stressing that the existence of a surrounding membrane does not necessarily imply the existence of a gradient between the cytosol and the organelle lumen. For example, the nuclear membrane is highly permeable to both Ca²⁺ and cAMP (nuclear pores are permeable to solutes up to 50 kDa) and differences in [Ca²⁺] or [cAMP] between cytoplasm and nucleoplasm are not seen in steady state and only very transiently during cell activation. A similar situation has been observed, as far as Ca²⁺ is concerned, in peroxisomes.

A phytochemical study of the Cuphea glutinosa from Southern Brazil: Na+,K+-ATPase activity inhibition and antioxidant properties

This study investigated the antioxidant activity of Cuphea glutinosa (CG) and its effect on Na⁺, K⁺-ATPase from cardiac muscle. The ethanolic extract showed higher antioxidant capacity compared to aqueous and ethyl acetate fraction. Ethyl acetate fraction showed β-sitosterol-3-O-β-glucoside, kaempferol, quercetin, isoquercetin, gallic acid methyl ester, and gallic acid. The ethanolic extract also reduced the Na⁺,K⁺-ATPase activity. CG presented a promising antioxidant activity and inhibitory effect on the Na⁺, K⁺-ATPase activity, supporting biochemical evidences the popular use of this plant in the treatment of heart failure.

No fuzzy space for intracellular Na+ in healthy ventricular myocytes

Sachse et al. highlight work that reveals a Na⁺-dependent inactivation mechanism in the Na⁺/K⁺ pump.

Mung bean (Phaseolus radiatus L.) polyphenol extract attenuates aluminum-induced cardiotoxicity through an ROS-triggered Ca2+/JNK/NF-κB signaling pathway in rats

Aluminum (Al) has been linked to the development of some cardiovascular diseases and mung bean is a functional food with the ability to detoxify. We aimed to evaluate the preventive effect and possible underlying mechanisms of the mung bean polyphenol extract (MPE) on Al-induced cardiotoxicity. Control, AlCl₃ (171.8 mg Al per kg body weight), MPE + AlCl₃ (Al-treatment plus 200 mg MPE per kg body weight), and a group of MPE per se were used. Al intake induced a significant increase of serum CK and LDH activity and the level of Na⁺, Ca²⁺, malondialdehyde and advanced oxidation protein products in the AlCl₃-treated rats' heart tissue. Administration of MPE significantly improved the integrity and normal ion levels of heart tissue, and attenuated oxidative damage and the accumulation of Al in Al-treated rats. MPE significantly inhibited Al-induced increase of myocardial p-JNK, cytoplasmic NF-κB, cytochrome C, and caspase-9 protein expressions. Therefore, these results showed that MPE has a cardiac protective effect against Al-induced biotoxicity through ROS-JNK and NF-κB-mediated caspase pathways. Furthermore, the stability constant for the vitexin-Al complex was analyzed (log K = log K₁ + log K₂ = 4.91 + 4.85 = 9.76). We found that MPE-mediated protection against Al-cardiotoxicity is connected both with MPE antioxidant and chelation properties.

Mineralocorticoid regulation of cell function: the role of rapid signalling and gene transcription pathways

The mineralocorticoid receptor (MR) and mineralocorticoids regulate epithelial handling of electrolytes, and induces diverse effects on other tissues. Traditionally, the effects of MR were ascribed to ligand-receptor binding and activation of gene transcription. However, the MR also utilises a number of intracellular signalling cascades, often by transactivating unrelated receptors, to change cell function more rapidly. Although aldosterone is the physiological mineralocorticoid, it is not the sole ligand for MR. Tissue-selective and mineralocorticoid-specific effects are conferred through the enzyme 11β-hydroxysteroid dehydrogenase 2, cellular redox status and properties of the MR itself. Furthermore, not all aldosterone effects are mediated via MR, with implication of the involvement of other membrane-bound receptors such as GPER. This review will describe the ligands, receptors and intracellular mechanisms available for mineralocorticoid hormone and receptor signalling and illustrate their complex interactions in physiology and disease.

The water extract of Veratrilla baillonii could attenuate the subacute toxicity induced by Aconitum brachypodum

BACKGROUND

Aconitum brachypodum Diels (Family Ranunculaceae) is a Chinese ethnodrug and is well known for both its therapeutic application and high toxicity. However, no detoxication strategy is available for the complete elimination of the toxicity of Aconitum plants. Veratrilla baillonii Franch is believed to possess antitoxic effects on the toxicity induced by Aconitum plants and has been clinically used for hundreds of time by Naxi and Lisu nationalities in Yunnan Province of China. To further address the mechanism of the detoxication of Veratrilla baillonii, the effect of water decoction of Veratrilla baillonii (WVBF) on subacute toxicology of SD rats induced by Aconitum brachypodum (CFA), a genus Aconitum, was determined and studied in the present work.

METHODS

The clinical behavior and number of survivors for different dosage of WVBF (25, 50, 100mg/kg) on CFA (4mg/kg) induced rats were observed until day 28. Histological changes and haematological parameters were evaluated. Moreover, Na⁺-K⁺-ATPase pathway in heart as well as key enzymes in liver were determined to further discuss the mechanism.

RESULTS

The results showed that the exposure of CFA led to some subacute toxicity to rats, especially male ones, accompanied with abnormality of serum biochemical index in rats' serum. The toxicological target organs of CFA may be the heart, liver, kidney and brain. It is demonstrated that WVBF could attenuate the toxicity induced by Aconitum brachypodum via promoting the metabolic enzymes CYP3A1 and CYP3A2 in liver, downregulating the expression of Sodium/Calcium exchanger 1 (NCX1) and SCN5A sodium channal mRNA, and inducing Na⁺/K⁺-ATPase activity in heart. This study provides insights into detoxifying measures of Aconitum plants.

CONCLUSIONS

Aconitum brachypodum may lead to subacute toxicity of rats after long term of administration, and the toxicity could be attenuated by Veratrilla baillonii via promoting the metabolic enzymes in liver, downregulating the expression of NCX1 and SCN5A mRNA, and inducing Na⁺/K⁺-ATPase activity in heart.

Changes in Intracellular Na+ following Enhancement of Late Na+ Current in Virtual Human Ventricular Myocytes

The slowly inactivating or late Na+ current, INa-L, can contribute to the initiation of both atrial and ventricular rhythm disturbances in the human heart. However, the cellular and molecular mechanisms that underlie these pro-arrhythmic influences are not fully understood. At present, the major working hypothesis is that the Na+ influx corresponding to INa-L significantly increases intracellular Na+, [Na+]i; and the resulting reduction in the electrochemical driving force for Na+ reduces and (may reverse) Na+/Ca2+ exchange. These changes increase intracellular Ca2+, [Ca2+]i; which may further enhance INa-L due to calmodulin-dependent phosphorylation of the Na+ channels. This paper is based on mathematical simulations using the O'Hara et al (2011) model of baseline or healthy human ventricular action potential waveforms(s) and its [Ca2+]i homeostasis mechanisms. Somewhat surprisingly, our results reveal only very small changes (≤ 1.5 mM) in [Na+]i even when INa-L is increased 5-fold and steady-state stimulation rate is approximately 2 times the normal human heart rate (i.e. 2 Hz). Previous work done using well-established models of the rabbit and human ventricular action potential in heart failure settings also reported little or no change in [Na+]i when INa-L was increased. Based on our simulations, the major short-term effect of markedly augmenting INa-L is a significant prolongation of the action potential and an associated increase in the likelihood of reactivation of the L-type Ca2+ current, ICa-L. Furthermore, this action potential prolongation does not contribute to [Na+]i increase.

Current-Voltage Relationship for Late Na(+) Current in Adult Rat Ventricular Myocytes

It is now well established that the slowly inactivating component of the Na(+) current (INa-L) in the mammalian heart is a significant regulator of the action potential waveform. This insight has led to detailed studies of the role of INa-L in a number of important and challenging pathophysiological settings. These include genetically based ventricular arrhythmias (LQT 1, 2, and 3), ventricular arrhythmias arising from progressive cardiomyopathies (including diabetic), and proarrhythmic abnormalities that develop during local or global ventricular ischemia. Inhibition of INa-L may also be a useful strategy for management of atrial flutter and fibrillation. Many important biophysical parameters that characterize INa-L have been identified; and INa-L as an antiarrhythmia drug target has been studied extensively. However, relatively little information is available regarding (1) the ion transfer or current-voltage relationship for INa-L or (2) the time course of its reactivation at membrane potentials similar to the resting or diastolic membrane potential in mammalian ventricle. This chapter is based on our preliminary findings concerning these two very important physiological/biophysical descriptors for INa-L. Our results were obtained using whole-cell voltage clamp methods applied to enzymatically isolated rat ventricular myocytes. A chemical agent, BDF 9148, which was once considered to be a drug candidate in the Na(+)-dependent inotropic agent category has been used to markedly enhance INa-L current. BDF acts in a potent, selective, and reversible fashion. These BDF 9148 effects are compared and contrasted with the prototypical activator of INa-L, a sea anemone toxin, ATX II.

Pivotal role of α2 Na+ pumps and their high affinity ouabain binding site in cardiovascular health and disease

Reduced smooth muscle (SM)-specific α2 Na⁺ pump expression elevates basal blood pressure (BP) and increases BP sensitivity to angiotensin II (Ang II) and dietary NaCl, whilst SM-α2 overexpression lowers basal BP and decreases Ang II/salt sensitivity. Prolonged ouabain infusion induces hypertension in rodents, and ouabain-resistant mutation of the α2 ouabain binding site (α2^R/R mice) confers resistance to several forms of hypertension. Pressure overload-induced heart hypertrophy and failure are attenuated in cardio-specific α2 knockout, cardio-specific α2 overexpression and α2^R/R mice. We propose a unifying hypothesis that reconciles these apparently disparate findings: brain mechanisms, activated by Ang II and high NaCl, regulate sympathetic drive and a novel neurohumoral pathway mediated by both brain and circulating endogenous ouabain (EO). Circulating EO modulates ouabain-sensitive α2 Na⁺ pump activity and Ca²⁺ transporter expression and, via Na⁺ /Ca²⁺ exchange, Ca²⁺ homeostasis. This regulates sensitivity to sympathetic activity, Ca²⁺ signalling and arterial and cardiac contraction.

Quantifying the Influence of the Crowded Cytoplasm on Small Molecule Diffusion

Cytosolic crowding can influence the thermodynamics and kinetics of in vivo chemical reactions. Most significantly, proteins and nucleic acid crowders reduce the accessible volume fraction, ϕ, available to a diffusing substrate, thereby reducing its effective diffusion rate, Deff, relative to its rate in bulk solution. However, Deff can be further hindered or even enhanced, when long-range crowder/diffuser interactions are significant. To probe these effects, we numerically estimated Deff values for small, charged molecules in representative, cytosolic protein lattices up to 0.1 × 0.1 × 0.1 μm(3) in volume via the homogenized Smoluchowski electro-diffusion equation. We further validated our predictions against Deff estimates from ϕ-dependent analytical relationships, such as the Maxwell-Garnett (MG) bound, as well as explicit solutions of the time-dependent electro-diffusion equation. We find that in typical, moderately crowded cell cytoplasm (ϕ ≈ 0.8), Deff is primarily determined by ϕ; in other words, diverse protein shapes and heterogeneous distributions only modestly impact Deff. However, electrostatic interactions between diffusers and crowders, particularly at low electrolyte ionic strengths, can substantially modulate Deff. These findings help delineate the extent that cytoplasmic crowders influence small molecule diffusion, which ultimately may shape the efficiency and timing of intracellular signaling pathways. More generally, the quantitative agreement between computationally expensive solutions of the time-dependent electro-diffusion equation and its comparatively cheaper homogenized form suggest that the latter is a broadly effective model for diffusion in wide-ranging, crowded biological media.

Enzyme localization, crowding, and buffers collectively modulate diffusion-influenced signal transduction: Insights from continuum diffusion modeling

Biochemical reaction networks consisting of coupled enzymes connect substrate signaling events with biological function. Substrates involved in these reactions can be strongly influenced by diffusion "barriers" arising from impenetrable cellular structures and macromolecules, as well as interactions with biomolecules, especially within crowded environments. For diffusion-influenced reactions, the spatial organization of diffusion barriers arising from intracellular structures, non-specific crowders, and specific-binders (buffers) strongly controls the temporal and spatial reaction kinetics. In this study, we use two prototypical biochemical reactions, a Goodwin oscillator, and a reaction with a periodic source/sink term to examine how a diffusion barrier that partitions substrates controls reaction behavior. Namely, we examine how conditions representative of a densely packed cytosol, including reduced accessible volume fraction, non-specific interactions, and buffers, impede diffusion over nanometer length-scales. We find that diffusion barriers can modulate the frequencies and amplitudes of coupled diffusion-influenced reaction networks, as well as give rise to "compartments" of decoupled reactant populations. These effects appear to be intensified in the presence of buffers localized to the diffusion barrier. These findings have strong implications for the role of the cellular environment in tuning the dynamics of signaling pathways.

Regulation of cardiomyocyte autophagy by calcium

Calcium signaling plays a crucial role in a multitude of events within the cardiomyocyte, including cell cycle control, growth, apoptosis, and autophagy. With respect to calcium-dependent regulation of autophagy, ion channels and exchangers, receptors, and intracellular mediators play fundamental roles. In this review, we discuss calcium-dependent regulation of cardiomyocyte autophagy, a lysosomal mechanism that is often cytoprotective, serving to defend against disease-related stress and nutrient insufficiency. We also highlight the importance of the subcellular distribution of calcium and related proteins, interorganelle communication, and other key signaling events that govern cardiomyocyte autophagy.

Cardiac-specific miRNA in cardiogenesis, heart function, and cardiac pathology (with focus on myocardial infarction)

Cardiac miRNAs (miR-1, miR133a, miR-208a/b, and miR-499) are abundantly expressed in the myocardium. They play a central role in cardiogenesis, heart function and pathology. While miR-1 and miR-133a predominantly control early stages of cardiogenesis supporting commitment of cardiac-specific muscle lineage from embryonic stem cells and mesodermal precursors, miR-208 and miR-499 are involved in the late cardiogenic stages mediating differentiation of cardioblasts to cardiomyocytes and fast/slow muscle fiber specification. In the heart, miR-1/133a control cardiac conductance and automaticity by regulating all phases of the cardiac action potential. miR-208/499 located in introns of the heavy chain myosin genes regulate expression of sarcomeric contractile proteins. In cardiac pathology including myocardial infarction (MI), expression of cardiac miRNAs is markedly altered that leads to deleterious effects associated with heart wounding, arrhythmia, increased apoptosis, fibrosis, hypertrophy, and tissue remodeling. In acute MI, circulating levels of cardiac miRNAs are significantly elevated making them to be a promising diagnostic marker for early diagnosis of acute MI. Great cardiospecific capacity of these miRNAs is very helpful for enhancing regenerative properties and survival of stem cell and cardiac progenitor transplants and for reprogramming of mature non-cardiac cells to cardiomyocytes.

Inhibitory efficacy of bufadienolides on Na+,K+-pump activity versus cell proliferation

Bufadienolides are cytotoxic drugs that may form the basis for anticancer agents. Due to structural and functional similarity to cardiotonic glycosides, application is restricted. We, therefore, investigated correlation of their putative anticancer effects with inhibition of Na⁺,K⁺pumps. The natural bufalin and three derivatives were tested. The anticancer effects of the drugs were checked by observing their inhibitory effects on proliferation of rat liver cancer cells using MTT assay. Inhibition of Na⁺,K⁺-pump was determined by measuring pump-mediated current of rat α1/β1 and α2/β1 Na⁺,K⁺pumps expressed in Xenopus oocytes.

All tested bufadienolides inhibited cell proliferation and Na⁺,K⁺pump activity. An activity coefficient A=100xIC₅₀^{Na,K pump}/IC₅₀^{proliferation} was used to describe drug effectivity as anticancer drug. Natural bufalin exhibited lowest effectivity on cell proliferation, and also the A value for rat α1 isoform was the lowest (0.08), the α2 isoform was much less sensitive (A=1.00). The highest A values were obtained for the BF238 derivative with A=0.88 and 2.64 for the α1 and α2 isoforms, respectively. Therefore, we suggest that search for bufalin derivatives with high anticancer effect and low affinity for both Na⁺,K⁺pump isoforms may be a promising strategy for development of anticancer drugs.

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Effects of bufadienolides on Na pump are not correlated with their cytotoxicity.

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Bufadienolides with high anticancer effect but low side effect may exist.

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BF238 may form a basis for further anticancer drug research and development.

Glutathionylation-Dependence of Na(+)-K(+)-Pump Currents Can Mimic Reduced Subsarcolemmal Na(+) Diffusion

The existence of a subsarcolemmal space with restricted diffusion for Na(+) in cardiac myocytes has been inferred from a transient peak electrogenic Na(+)-K(+) pump current beyond steady state on reexposure of myocytes to K(+) after a period of exposure to K(+)-free extracellular solution. The transient peak current is attributed to enhanced electrogenic pumping of Na(+) that accumulated in the diffusion-restricted space during pump inhibition in K(+)-free extracellular solution. However, there are no known physical barriers that account for such restricted Na(+) diffusion, and we examined if changes of activity of the Na(+)-K(+) pump itself cause the transient peak current. Reexposure to K(+) reproduced a transient current beyond steady state in voltage-clamped ventricular myocytes as reported by others. Persistence of it when the Na(+) concentration in patch pipette solutions perfusing the intracellular compartment was high and elimination of it with K(+)-free pipette solution could not be reconciled with restricted subsarcolemmal Na(+) diffusion. The pattern of the transient current early after pump activation was dependent on transmembrane Na(+)- and K(+) concentration gradients suggesting the currents were related to the conformational poise imposed on the pump. We examined if the currents might be accounted for by changes in glutathionylation of the β1 Na(+)-K(+) pump subunit, a reversible oxidative modification that inhibits the pump. Susceptibility of the β1 subunit to glutathionylation depends on the conformational poise of the Na(+)-K(+) pump, and glutathionylation with the pump stabilized in conformations equivalent to those expected to be imposed on voltage-clamped myocytes supported this hypothesis. So did elimination of the transient K(+)-induced peak Na(+)-K(+) pump current when we included glutaredoxin 1 in patch pipette solutions to reverse glutathionylation. We conclude that transient K(+)-induced peak Na(+)-K(+) pump current reflects the effect of conformation-dependent β1 pump subunit glutathionylation, not restricted subsarcolemmal diffusion of Na(+).

The role of mutations in the SCN5A gene in cardiomyopathies

The SCN5A gene encodes the alpha-subunit of the Nav1.5 ion channel protein, which is responsible for the sodium inward current (INa). Since 1995 several hundred mutations in this gene have been found to be causative for inherited arrhythmias including Long QT syndrome, Brugada syndrome, cardiac conduction disease, sudden infant death syndrome, etc. As expected these syndromes are primarily electrical heart diseases leading to life-threatening arrhythmias with an "apparently normal heart". In 2003 a new form of dilated cardiomyopathy was identified associated with mutations in the SCN5A gene. Recently mutations have been also found in patients with arrhythmogenic right ventricular cardiomyopathy and atrial standstill. The purpose of this review is to outline and analyze the following four topics: 1) SCN5A genetic variants linked to different cardiomyopathies; 2) clinical manifestations of the known mutations; 3) possible molecular mechanisms of myocardial remodeling; and 4) the potential implications of gene-specific treatment for those disorders. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

Inhibitory effect of Scutia buxifolia extracts, fractions, and ursolic acid on Na(+), K(+)-ATPase activity in vitro in membranes purified from rat hearts

ETHNOPHARMACOLOGICAL RELEVANCE

Scutia buxifolia is a tree native to South America and is used as a cardiotonic agent; however, this property has not been associated with a clear mechanism or a specific compound.

AIM OF THE STUDY

Given the importance of Na(+),K(+)-ATPase as a drug target in the treatment of heart failure, this study aimed to investigate the possible inhibitory effect of S. buxifolia crude extract and fractions (dichloromethane, ethyl acetate, and butanolic fractions), and identified compounds with effects on the activity of Na(+),K(+)-ATPase in vitro.

MATERIALS AND METHODS

First, we characterized the crude extract and fractions by high-performance liquid chromatography, and then monitored their effects on the activity of Na(+),K(+)-ATPase obtained from heart muscle and brain membranes of adult male Wistar rats.

RESULTS

We identified gallic acid, chlorogenic acid, caffeic acid, rutin, quercitrin, quercetin, and ursolic acid in S. buxifolia stem bark and leaves; quercitrin and ursolic acid were the main compounds in the ethyl acetate and dichloromethane fractions from leaves and stem bark. The crude extract (3 and 30mg/ml), and the ethyl acetate and dichloromethane fractions (0.1 and 1mg/ml) of both the stem bark and leaves inhibited Na(+),K(+)-ATPase activity in heart and brain samples. We found that, of the identified compounds, only ursolic acid (0.1mg/ml) was able to diminish Na(+), K(+)-ATPase activity in heart and brain samples.

CONCLUSIONS

These data indicated that the cardiotonic effects of S. buxifolia may be due to the inhibition of Na(+),K(+)-ATPase activity in heart muscle, supporting the popular use of this plant as a treatment for heart failure.

The current status of heart failure diagnostic biomarkers

Heart failure (HF) affects approximately 23 million individuals worldwide and this number is increasing, due to an aging and growing population. Early detection of HF is crucial in the management of this debilitating disease. Current diagnostic methods for HF rely heavily on clinical imaging techniques and blood analysis, which makes them less than ideal for population-based screening purposes. Studies focusing on developing novel biomarkers for HF have utilized various techniques and biological fluids, including urine and saliva. Promising results from these studies imply that these body fluids can be used in evaluating the clinical manifestation of HF and will one day be integrated into a clinical workflow and facilitate HF management.