Na⁺ transport in the normal and failing heart - remember the balance

Aged garlic extract preserves beta-cell functioning via modulation of nuclear factor kappa-B (NF-κB)/Toll-like receptor (TLR)-4 and sarco endoplasmic reticulum calcium ATPase (SERCA)/Ca2+ in diabetes mellitus

BACKGROUND

Peripheral insulin resistance and compromised insulin secretion from pancreatic β-cells are significant factors and pathogenic hallmarks of diabetes mellitus (DM). NF-κβ/TLR-4 and SERCA/Ca2+ pathways have been identified as potential pathways regulating insulin synthesis by preserving pancreatic β-cell functioning. The current study aimed to evaluate the therapeutic effect of aged garlic extract (AGE) against DM in a streptozotocin (STZ)-induced rat model with particular emphasis on pancreatic β-cell functioning.

METHODS

AGE was characterized by gas chromatography-mass spectrometry (GC-MS), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) to evaluate its physio-chemical characteristics followed by in-vitro anti-diabetic and antioxidant potential. This was followed by the induction of DM in laboratory animals for investigating the therapeutic action of AGE by evaluating the role of NF-κβ/TLR-4 and the SERCA/Ca2+ pathway. The parameters assessed in the present experimental setup encompassed antioxidant parameters, metabolic indicators, insulin concentration, intracellular calcium levels, apoptotic markers (CCK-8 and Caspase Glo-8), and protein expression (P-62 and APACHE-II).

RESULTS

AGE characterization by SEM, GC-MS, and X-ray diffraction (XRD) revealed the presence of phenylalanine, alliin, S-allylmercaptocysteine (SAMC), tryptophan, 1-methyl-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid as major bioactive constituents of AGE. Metabolic studies, including intraperitoneal glucose tolerance test (IPGTT), revealed significantly lower blood glucose levels in the AGE group compared to the disease control group. In contrast, the intraperitoneal insulin tolerance test (ITT) exhibited no significant difference in insulin sensitivity between the AGE supplementation group and the DM control group. Interestingly, AGE was found to have no significant effect on fasting glucose and serum insulin levels. In contrast, AGE supplementation was found to cause significant hypoglycaemia in postprandial blood glucose and insulin levels. Importantly, AGE causes restoration of intracellular Ca2+ levels by modulation of SERCA/Ca2 functioning and inhibition NF-κB/TLR-4 pathway. AGE was found to interact with and inhibit the DR-5/ caspase-8/3 apoptotic complex. Furthermore, microscopic studies revealed degeneration and apoptotic changes in pancreatic β-cells of the DM control group, while supplementation of AGE resulted in inhibition of apoptotic pathway and regeneration of pancreatic β-cells.

CONCLUSION

The current study suggests that AGE enhance glucose homeostasis by exerting their effects on pancreatic β-cells, without ameliorating peripheral sensitivity. Moreover, AGEs promote an increase in β-cell mass by mitigating the apoptosis of pancreatic β-cells. These findings suggest that AGE could aid in developing a viable alternative therapy for diabetes mellitus (DM).

Cardiac function is regulated by the sodium-dependent inhibition of the sodium-calcium exchanger NCX1

The Na+-Ca2+ exchanger (NCX1) is the dominant Ca2+ extrusion mechanism in cardiac myocytes. NCX1 activity is inhibited by intracellular Na+ via a process known as Na+-dependent inactivation. A central question is whether this inactivation plays a physiological role in heart function. Using CRISPR/Cas9, we inserted the K229Q mutation in the gene (Slc8a1) encoding for NCX1. This mutation removes the Na+-dependent inactivation while preserving transport properties and other allosteric regulations. NCX1 mRNA levels, protein expression, and protein localization are unchanged in K229Q male mice. However, they exhibit reduced left ventricular ejection fraction and fractional shortening, while displaying a prolonged QT interval. K229Q ventricular myocytes show enhanced NCX1 activity, resulting in action potential prolongation, higher incidence of aberrant action potentials, a faster decline of Ca2+ transients, and depressed cell shortening. The results demonstrate that NCX1 Na+-dependent inactivation plays an essential role in heart function by affecting both cardiac excitability and contractility.

Dapagliflozin prevents ERK activation and SGLT2-dependent endoglin upregulation in a mechanically provoked cardiac injury model

Sodium-glucose cotransporter 2 inhibitors (SGLT2i) are rapidly gaining ground in the treatment of heart failure (HF) with reduced ejection fraction (HFrEF) and acute myocardial infarction (AMI) by an unknown mechanism. Upregulation of Na+/H+ exchanger 1 (NHE1), SGLT1, and Ca2+/calmodulin-dependent protein kinase II (CaMKII) in the diseased hearts was found to be attenuated by prolonged SGLT2i treatment. Unfortunately, dapagliflozin is not well understood as to how Na+/Ca2+ homeostasis is affected in cardiomyocytes. In this study, we aimed to investigate whether mechanical stretch in cardiomyocytes upregulate SGLT2, resulted to loss of Na+/Ca2+ homeostasis via ERK and eNOS signaling. AMI (+) and AMI (-) serum levels were estimated using ELISA assays of TGFβ-1 or endoglin (CD105). Human cardiomyocyte cell line AC16 was subjected to different stresses: 5% mild and 25% aggressive, at 1 Hz for 24 h. Immunofluorescence assays were used to estimate troponin I, CD105, SGLT1/2, eNOSS633, and ERK1/2T202/Y204 levels was performed for 5% (mild), and 25% elongation for 24 h. AMI (+) serum showed increased TGFβ1 and CD105 compared to AMI (-) patients. In consistent, troponin I, CD105, SGLT1/2, eNOSS633 and ERK1/2T202/Y204 were upregulated after 25% of 24 h cyclic stretch. Dapagliflozin addition caused SGLT2 inhibition, which significantly decreased troponin I, CD105, SGLT1/2, eNOSS633, and ERK1/2T202/Y204 under 25% cyclic stretching. In summary, SGLT2 may have sensed mechanical stretch in a way similar to cardiac overloading as in vivo. By blocking SGLT2 in stretched cardiomyocytes, the AMI biomarkers (CD105, troponin I and P-ERK) were decreased, potentially to rescue eNOS production to maintain normal cellular function. This discovery of CD105 and SGLT2 increase in mechanically stretched cardiomyocytes suggests that SGLT2 may conceive a novel role in direct or indirect sensing of mechanical stretch, prompting the possibility of an in vitro cardiac overloaded cell model, an alternative to animal heart model.

Anti-Arrhythmic Effects of Heart Failure Guideline-Directed Medical Therapy and Their Role in the Prevention of Sudden Cardiac Death: From Beta-Blockers to Sodium-Glucose Cotransporter 2 Inhibitors and Beyond

Sudden cardiac death (SCD) accounts for a substantial proportion of mortality in heart failure with reduced ejection fraction (HFrEF), frequently triggered by ventricular arrhythmias (VA). This review aims to analyze the pathophysiological mechanisms underlying VA and SCD in HFrEF and evaluate the effectiveness of guideline-directed medical therapy (GDMT) in reducing SCD. Beta-blockers, angiotensin receptor-neprilysin inhibitors, and mineralocorticoid receptor antagonists have shown significant efficacy in reducing SCD risk. While angiotensin-converting enzyme inhibitors and angiotensin receptor blockers exert beneficial impacts on the renin-angiotensin-aldosterone system, their direct role in SCD prevention remains less clear. Emerging treatments like sodium-glucose cotransporter 2 inhibitors show promise but necessitate further research for conclusive evidence. The favorable outcomes of those molecules on VA are notably attributable to sympathetic nervous system modulation, structural remodeling attenuation, and ion channel stabilization. A multidimensional pharmacological approach targeting those pathophysiological mechanisms offers a complete and synergy approach to reducing SCD risk, thereby highlighting the importance of optimizing GDMT for HFrEF. The current landscape of HFrEF pharmacotherapy is evolving, with ongoing research needed to clarify the full extent of the anti-arrhythmic benefits offered by both existing and new treatments.

A new approach to characterize cardiac sodium storage by combining fluorescence photometry and magnetic resonance imaging in small animal research

Cardiac myocyte sodium (Na+) homoeostasis is pivotal in cardiac diseases and heart failure. Intracellular Na+ ([Na+]i) is an important regulator of excitation-contraction coupling and mitochondrial energetics. In addition, extracellular Na+ ([Na+]e) and its water-free storage trigger collagen cross-linking, myocardial stiffening and impaired cardiac function. Therefore, understanding the allocation of tissue Na+ to intra- and extracellular compartments is crucial in comprehending the pathophysiological processes in cardiac diseases. We extrapolated [Na+]e using a three-compartment model, with tissue Na+ concentration (TSC) measured by in vivo 23Na-MRI, extracellular volume (ECV) data calculated from T1 maps, and [Na+]i measured by in vitro fluorescence microscopy using Na+ binding benzofuran isophthalate (SBFI). To investigate dynamic changes in Na+ compartments, we induced pressure overload (TAC) or myocardial infarction (MI) via LAD ligation in mice. Compared to SHAM mice, TSC was similar after TAC but increased after MI. Both TAC and MI showed significantly higher [Na+]i compared to SHAM (around 130% compared to SHAM). Calculated [Na+]e increased after MI, but not after TAC. Increased TSC after TAC was primarily driven by increased [Na+]i, but the increase after MI by elevations in both [Na+]i and [Na+]e.

Na + /K + ATPase-Ca v 1.2 nanodomain differentially regulates intracellular [Na + ], [Ca 2+ ] and local adrenergic signaling in cardiac myocytes

Background

The intracellular Na + concentration ([Na + ] i ) is a crucial but understudied regulator of cardiac myocyte function. The Na + /K + ATPase (NKA) controls the steady-state [Na + ] i and thereby determines the set-point for intracellular Ca 2+ . Here, we investigate the nanoscopic organization and local adrenergic regulation of the NKA macromolecular complex and how it differentially regulates the intracellular Na + and Ca 2+ homeostases in atrial and ventricular myocytes.

Methods

Multicolor STORM super-resolution microscopy, Western Blot analyses, and in vivo examination of adrenergic regulation are employed to examine the organization and function of Na + nanodomains in cardiac myocytes. Quantitative fluorescence microscopy at high spatiotemporal resolution is used in conjunction with cellular electrophysiology to investigate intracellular Na + homeostasis in atrial and ventricular myocytes.

Results

The NKAα1 (NKAα1) and the L-type Ca 2+ -channel (Ca v 1.2) form a nanodomain with a center-to center distance of ∼65 nm in both ventricular and atrial myocytes. NKAα1 protein expression levels are ∼3 fold higher in atria compared to ventricle. 100% higher atrial I NKA , produced by large NKA "superclusters", underlies the substantially lower Na + concentration in atrial myocytes compared to the benchmark values set in ventricular myocytes. The NKA's regulatory protein phospholemman (PLM) has similar expression levels across atria and ventricle resulting in a much lower PLM/NKAα1 ratio for atrial compared to ventricular tissue. In addition, a huge PLM phosphorylation reserve in atrial tissue produces a high ß-adrenergic sensitivity of I NKA in atrial myocytes. ß-adrenergic regulation of I NKA is locally mediated in the NKAα1-Ca v 1.2 nanodomain via A-kinase anchoring proteins.

Conclusions

NKAα1, Ca v 1.2 and their accessory proteins form a structural and regulatory nanodomain at the cardiac dyad. The tissue-specific composition and local adrenergic regulation of this "signaling cloud" is a main regulator of the distinct global intracellular Na + and Ca 2+ concentrations in atrial and ventricular myocytes.

Metabolic Approaches for the Treatment of Dilated Cardiomyopathy

In dilated cardiomyopathy (DCM), where the heart muscle becomes stretched and thin, heart failure (HF) occurs, and the cardiomyocytes suffer from an energetic inefficiency caused by an abnormal cardiac metabolism. Although underappreciated as a potential therapeutic target, the optimal metabolic milieu of a failing heart is still largely unknown and subject to debate. Because glucose naturally has a lower P/O ratio (the ATP yield per oxygen atom), the previous studies using this strategy to increase glucose oxidation have produced some intriguing findings. In reality, the vast majority of small-scale pilot trials using trimetazidine, ranolazine, perhexiline, and etomoxir have demonstrated enhanced left ventricular (LV) function and, in some circumstances, myocardial energetics in chronic ischemic and non-ischemic HF with a reduced ejection fraction (EF). However, for unidentified reasons, none of these drugs has ever been tested in a clinical trial of sufficient size. Other pilot studies came to the conclusion that because the heart in severe dilated cardiomyopathy appears to be metabolically flexible and not limited by oxygen, the current rationale for increasing glucose oxidation as a therapeutic target is contradicted and increasing fatty acid oxidation is supported. As a result, treating metabolic dysfunction in HF may benefit from raising ketone body levels. Interestingly, treatment with sodium-glucose cotransporter-2 inhibitors (SGLT2i) improves cardiac function and outcomes in HF patients with or without type 2 diabetes mellitus (T2DM) through a variety of pleiotropic effects, such as elevated ketone body levels. The improvement in overall cardiac function seen in patients receiving SGLT2i could be explained by this increase, which appears to be a reflection of an adaptive process that optimizes cardiac energy metabolism. This review aims to identify the best metabolic therapeutic approach for DCM patients, to examine the drugs that directly affect cardiac metabolism, and to outline all the potential ancillary metabolic effects of the guideline-directed medical therapy. In addition, a special focus is placed on SGLT2i, which were first studied and prescribed to diabetic patients before being successfully incorporated into the pharmacological arsenal for HF patients.

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.

Antiarrhythmic and Inotropic Effects of Selective Na+/Ca2+ Exchanger Inhibition: What Can We Learn from the Pharmacological Studies?

Life-long stable heart function requires a critical balance of intracellular Ca²⁺. Several ion channels and pumps cooperate in a complex machinery that controls the influx, release, and efflux of Ca²⁺. Probably one of the most interesting and most complex players of this crosstalk is the Na⁺/Ca²⁺ exchanger, which represents the main Ca²⁺ efflux mechanism; however, under some circumstances, it can also bring Ca²⁺ into the cell. Therefore, the inhibition of the Na⁺/Ca²⁺ exchanger has emerged as one of the most promising possible pharmacological targets to increase Ca²⁺ levels, to decrease arrhythmogenic depolarizations, and to reduce excessive Ca²⁺ influx. In line with this, as a response to increasing demand, several more or less selective Na⁺/Ca²⁺ exchanger inhibitor compounds have been developed. In the past 20 years, several results have been published regarding the effect of Na⁺/Ca²⁺ exchanger inhibition under various circumstances, e.g., species, inhibitor compounds, and experimental conditions; however, the results are often controversial. Does selective Na⁺/Ca²⁺ exchanger inhibition have any future in clinical pharmacological practice? In this review, the experimental results of Na⁺/Ca²⁺ exchanger inhibition are summarized focusing on the data obtained by novel highly selective inhibitors.

Benefits of SGLT2 inhibitors in arrhythmias

Some studies have shown that sodium-glucose cotransporter (SGLT) 2 inhibitors can definitively attenuate the occurrence of cardiovascular diseases such as heart failure (HF), dilated cardiomyopathy (DCM), and myocardial infarction. With the development of research, SGLT2 inhibitors can also reduce the risk of arrhythmias. So in this review, how SGLT2 inhibitors play a role in reducing the risk of arrhythmia from the perspective of electrical remodeling and structural remodeling are explored and then the possible mechanisms are discussed. Specifically, we focus on the role of SGLT2 inhibitors in Na+ and Ca2 + homeostasis and the transients of Na+ and Ca2 +, which could affect electrical remodeling and then lead to arrythmia. We also discuss the protective role of SGLT2 inhibitors in structural remodeling from the perspective of fibrosis, inflammation, oxidative stress, and apoptosis. Ultimately, it is clear that SGLT2 inhibitors have significant benefits on cardiovascular diseases such as HF, myocardial hypertrophy and myocardial infarction. It can be expected that SGLT2 inhibitors can reduce the risk of arrhythmia.

Experimental hypothyroidism induces cardiac arrhythmias and ranolazine reverts and prevents the phenotype

AIMS

Hypothyroidism is associated with an increased risk of cardiovascular disease and enhanced susceptibility to arrhythmias. In our investigation, we evaluated the potential involvement of late sodium current (I_Na,late) in cardiac arrhythmias in an experimental murine model of hypothyroidism.

MAIN METHODS

Male Swiss mice were treated with methimazole (0.1 % w/vol, during 21 days) to induce experimental hypothyroidism before ECG, action potential (AP) and intracellular Ca²⁺ dynamics were evaluated. Susceptibility to arrhythmia was measured in vitro and in vivo.

KEY FINDINGS

The results revealed that hypothyroid animals presented ECG alterations (e.g. increased QTc) with the presence of spontaneous sustained ventricular tachycardia. These changes were associated with depolarized resting membrane potential in isolated cardiomyocytes and increased AP duration and dispersion at 90 % of the repolarization. Aberrant AP waveforms were related to increased Ca²⁺ sparks and out-of-pace Ca²⁺ waves. These changes were observed in a scenario of enhanced I_Na,late. Interestingly, ranolazine, a clinically used blocker of I_Na,late, restored the ECG alterations, reduced Ca²⁺ sparks and aberrant waves, decreased the in vitro events and the severity of arrhythmias observed in isolated cardiomyocytes from hypothyroid animals. Using the in vivo dobutamine + caffeine protocol, animals with hypothyroidism developed catecholaminergic bidirectional ventricular tachycardia, but pre-treatment with ranolazine prevented this.

SIGNIFICANCE

We concluded that animals with hypothyroidism have increased susceptibility to developing arrhythmias and ranolazine, a clinically used blocker of I_Na,late, is able to correct the arrhythmic phenotype.

Developmental differences in myocardial transmembrane Na+ transport: Implications for excitability and Na+ handling

KEY POINTS

Previous studies showed that myocardial preparations from immature rats are less sensitive to electrical field stimulation than adult preparations. Freshly-isolated ventricular myocytes from neonatal rats showed lower excitability than adult cells, e.g., less negative threshold membrane potential and greater membrane depolarization required for action potential triggering. In addition to differences in mRNA levels for Na⁺ channels isoforms and greater Na⁺ current (I_Na ) density, Na⁺ channel voltage-dependence was shifted to the right in immature myocytes, which seems to be sufficient to decrease excitability, according to computer simulations. Only in neonatal myocytes did cyclic activity promote marked cytosolic Na⁺ accumulation, which was prevented by abolition of systolic Ca²⁺ transients by blockade of Ca²⁺ currents. Developmental changes in I_Na may account for the difference in action potential initiation parameters, but not for cytosolic Na⁺ accumulation, which seems to be due mainly to Na⁺ /Ca²⁺ exchanger-mediated Na⁺ influx.

ABSTRACT

Little is currently known about possible developmental changes in myocardial Na⁺ handling, which may have impact on cell excitability and Ca²⁺ content. Resting intracellular Na⁺ concentration ([Na⁺ ]_i ), measured in freshly-isolated rat ventricular myocytes with CoroNa-green, was not significantly different in neonates (3-5 days old) and adults, but electrical stimulation caused marked [Na⁺ ]_i rise only in neonates. Inhibition of L-type Ca²⁺ current by CdCl₂ abolished not only systolic Ca²⁺ transients, but also activity-dependent intracellular Na⁺ accumulation in immature cells. This indicates that the main Na⁺ influx pathway during activity is the Na⁺ /Ca²⁺ exchanger, rather than voltage-dependent Na⁺ current (I_Na ), which was not affected by CdCl₂ . In immature myocytes, I_Na density was 2-fold greater, inactivation was faster, and the current peak occurred at less negative transmembrane potential (E_m ) than in adults. Na⁺ channel steady-state activation and inactivation curves in neonates showed a rightward shift, which should increase channel availability at diastolic E_m , but also require greater depolarization for excitation, which was observed experimentally and reproduced in computer simulations. Ventricular mRNA levels of Na_v 1.1, Na_v 1.4 and Na_v 1.5 pore-forming isoforms were greater in neonate ventricles, while decrease was seen for the β1 subunit. Both molecular and biophysical changes in the channel profile may contribute to the differences in I_Na density and voltage-dependence, and also to the less negative threshold E_m in neonates, compared to adults. The apparently lower excitability in immature ventricle may confer protection against the development of spontaneous activity in this tissue. Abstract figure legend Little is currently known about possible developmental changes in myocardial Na⁺ transport, which may have impact on cell excitability and other physiological aspects. At the mRNA level, neonatal rat ventricle expresses a greater variety of Na⁺ channel isoforms than in adults. In immature ventricular cardiomyocytes, Na⁺ current (I_Na ) density was greater, but voltage-dependence is shifted to less negative potentials than in adults. This should increase channel availability at diastolic membrane potential, but also require greater depolarization for excitation, which was observed experimentally and reproduced in computer simulation. We also observed that electrical stimulation caused marked intracellular Na⁺ accumulation only in neonates, which was abolished when Ca²⁺ transients and the Na⁺ /Ca²⁺ exchanger (NCX) were inhibited by Cd²⁺ + Ni²⁺ . Thus, it seems that the main Na⁺ influx pathway during activity in neonates is the NCX, rather than voltage-dependent I_Na , which was not affected by these blockers. This article is protected by copyright. All rights reserved.

Beneficial cardiovascular and remodeling effects of SGLT2 inhibitors: pathophysiologic mechanisms

INTRODUCTION

The intent of this paper is to review the data regarding the multipotential effects of the sodium-glucose cotransporter 2 (SGLT 2) inhibitors, their cardiovascular protective effects, and their mechanism of action.

AREAS COVERED

The SGLT2 inhibitors exert their beneficial antidiabetic and cardioprotective effects through increased glucose excretion from the kidneys, blood pressure and weight lowering, vasodilation and other potential beneficial effects. They have been used for the treatment of patients with type 2 diabetes mellitus (T2DM) as well as in patients with cardiovascular disease (CVD), coronary artery disease (CAD),and heart failure with reduced ejection fraction (HFrEF) and heart failure with preserved ejection fraction (HFpEF). In order to get a better understanding of their mechanism of action for their multiple cardiovascular protective effects, a Medline search of the English language literature was conducted between 2015 and February 2022 and 46 pertinent papers were selected.

EXPERT OPINION

The analysis of data clearly demonstrated that the use of the SGLT2 inhibitors besides their antidiabetic effects, provide additional protection against CVD, CAD, and HFrEF and HFpEF, and death, but not stroke, in both diabetic and non-diabetic patients. Therefore, they should be preferably used for the treatment of patients with T2DM with preexisting CVD, CAD, and HFrEF and HFpEF.

Late Sodium Current of the Heart: Where Do We Stand and Where Are We Going?

Late sodium current has long been linked to dysrhythmia and contractile malfunction in the heart. Despite the increasing body of accumulating information on the subject, our understanding of its role in normal or pathologic states is not complete. Even though the role of late sodium current in shaping action potential under physiologic circumstances is debated, it’s unquestioned role in arrhythmogenesis keeps it in the focus of research. Transgenic mouse models and isoform-specific pharmacological tools have proved useful in understanding the mechanism of late sodium current in health and disease. This review will outline the mechanism and function of cardiac late sodium current with special focus on the recent advances of the area.

Sodium-Glucose Cotransporter 2 Inhibitors and Heart Failure: A Bedside-to-Bench Journey

Type 2 diabetes mellitus (T2DM) and heart failure (HF) are multifactorial diseases sharing common risk factors, such as obesity, hyperinsulinemia, and inflammation, with underlying mechanisms including endothelial dysfunction, inflammation, oxidative stress, and metabolic alterations. Cardiovascular benefits of sodium-glucose cotransporter 2 (SGLT2) inhibitors observed in diabetic and non-diabetic patients are also related to their cardiac-specific, SGLT-independent mechanisms, in addition to the metabolic and hemodynamic effects. In search of the possible underlying mechanisms, a research campaign has been launched proposing varied mechanisms of action that include intracellular ion homeostasis, autophagy, cell death, and inflammatory processes. Moreover, the research focus was widened toward cellular targets other than cardiomyocytes. At the moment, intracellular sodium level reduction is the most explored mechanism of direct cardiac effects of SGLT2 inhibitors that mediate the benefits in heart failure in addition to glucose excretion and diuresis. The restoration of cardiac Na⁺ levels with consequent positive effects on Ca²⁺ handling can directly translate into improved contractility and relaxation of cardiomyocytes and have antiarrhythmic effects. In this review, we summarize clinical trials, studies on human cells, and animal models, that provide a vast array of data in support of repurposing this class of antidiabetic drugs.

Calcium Signaling Silencing in Atrial Fibrillation: Implications for Atrial Sodium Homeostasis

Atrial fibrillation (AF) is the most common type of cardiac arrhythmia, affecting more than 33 million people worldwide. Despite important advances in therapy, AF's incidence remains high, and treatment often results in recurrence of the arrhythmia. A better understanding of the cellular and molecular changes that (1) trigger AF and (2) occur after the onset of AF will help to identify novel therapeutic targets. Over the past 20 years, a large body of research has shown that intracellular Ca2+ handling is dramatically altered in AF. While some of these changes are arrhythmogenic, other changes counteract cellular arrhythmogenic mechanisms (Calcium Signaling Silencing). The intracellular Na+ concentration ([Na+])i is a key regulator of intracellular Ca2+ handling in cardiac myocytes. Despite its importance in the regulation of intracellular Ca2+ handling, little is known about [Na+]i, its regulation, and how it might be changed in AF. Previous work suggests that there might be increases in the late component of the atrial Na+ current (INa,L) in AF, suggesting that [Na+]i levels might be high in AF. Indeed, a pharmacological blockade of INa,L has been suggested as a treatment for AF. Here, we review calcium signaling silencing and changes in intracellular Na+ homeostasis during AF. We summarize the proposed arrhythmogenic mechanisms associated with increases in INa,L during AF and discuss the evidence from clinical trials that have tested the pharmacological INa,L blocker ranolazine in the treatment of AF.

Cardioprotection by SGLT2 Inhibitors-Does It All Come Down to Na+?

Sodium-glucose co-transporter 2 inhibitors (SGLT2i) are emerging as a new treatment strategy for heart failure with reduced ejection fraction (HFrEF) and—depending on the wistfully awaited results of two clinical trials (DELIVER and EMPEROR-Preserved)—may be the first drug class to improve cardiovascular outcomes in patients suffering from heart failure with preserved ejection fraction (HFpEF). Proposed mechanisms of action of this class of drugs are diverse and include metabolic and hemodynamic effects as well as effects on inflammation, neurohumoral activation, and intracellular ion homeostasis. In this review we focus on the growing body of evidence for SGLT2i-mediated effects on cardiac intracellular Na⁺ as an upstream mechanism. Therefore, we will first give a short overview of physiological cardiomyocyte Na⁺ handling and its deterioration in heart failure. On this basis we discuss the salutary effects of SGLT2i on Na⁺ homeostasis by influencing NHE1 activity, late I_Na as well as CaMKII activity. Finally, we highlight the potential relevance of these effects for systolic and diastolic dysfunction as well as arrhythmogenesis.

Arrhythmogenic and antiarrhythmic actions of late sustained sodium current in the adult human heart

Late sodium current (late INa) inhibition has been proposed to suppress the incidence of arrhythmias generated by pathological states or induced by drugs. However, the role of late INa in the human heart is still poorly understood. We therefore investigated the role of this conductance in arrhythmias using adult primary cardiomyocytes and tissues from donor hearts. Potentiation of late INa with ATX-II (anemonia sulcata toxin II) and E-4031 (selective blocker of the hERG channel) slowed the kinetics of action potential repolarization, impaired Ca2+ homeostasis, increased contractility, and increased the manifestation of arrhythmia markers. These effects could be reversed by late INa inhibitors, ranolazine and GS-967. We also report that atrial tissues from donor hearts affected by atrial fibrillation exhibit arrhythmia markers in the absence of drug treatment and inhibition of late INa with GS-967 leads to a significant reduction in arrhythmic behaviour. These findings reveal a critical role for the late INa in cardiac arrhythmias and suggest that inhibition of this conductance could provide an effective therapeutic strategy. Finally, this study highlights the utility of human ex-vivo heart models for advancing cardiac translational sciences.

Ion current profiles in canine ventricular myocytes obtained by the "onion peeling" technique

The profiles of ion currents during the cardiac action potential can be visualized by the action potential voltage clamp technique. To obtain multiple ion current data from the same cell, the "onion peeling" technique, based on sequential pharmacological dissection of ion currents, has to be applied. Combination of the two methods allows recording of several ion current profiles from the same myocyte under largely physiological conditions. Using this approach, we have studied the densities and integrals of the major cardiac inward (I_Ca, I_NCX, I_Na-late) and outward (I_Kr, I_Ks, I_K1) currents in canine ventricular cells and studied the correlation between them. For this purpose, canine ventricular cardiomyocytes were chosen because their electrophysiological properties are similar to those of human ones. Significant positive correlation was observed between the density and integral of I_Ca and I_Kr, and positive correlation was found also between the integral of I_Ca and I_NCX. No further correlations were detected. The Ca²⁺-sensitivity of K⁺ currents was studied by comparing their parameters in the case of normal calcium homeostasis and following blockade of I_Ca. Out of the three K⁺ currents studied, only I_Ks was Ca²⁺-sensitive. The density and integral of I_Ks was significantly greater, while its time-to-peak value was shorter at normal Ca²⁺ cycling than following I_Ca blockade. No differences were detected for I_Kr or I_K1 in this regard. Present results indicate that the positive correlation between I_Ca and I_Kr prominently contribute to the balance between inward and outward fluxes during the action potential plateau in canine myocytes. The results also suggest that the profiles of cardiac ion currents have to be studied under physiological conditions, since their behavior may strongly be influenced by the intracellular Ca²⁺ homeostasis and the applied membrane potential protocol.

Intracellular Na+ Modulates Pacemaking Activity in Murine Sinoatrial Node Myocytes: An In Silico Analysis

Background: The mechanisms underlying dysfunction in the sinoatrial node (SAN), the heart's primary pacemaker, are incompletely understood. Electrical and Ca2+-handling remodeling have been implicated in SAN dysfunction associated with heart failure, aging, and diabetes. Cardiomyocyte [Na+]i is also elevated in these diseases, where it contributes to arrhythmogenesis. Here, we sought to investigate the largely unexplored role of Na+ homeostasis in SAN pacemaking and test whether [Na+]i dysregulation may contribute to SAN dysfunction. Methods: We developed a dataset-specific computational model of the murine SAN myocyte and simulated alterations in the major processes of Na+ entry (Na+/Ca2+ exchanger, NCX) and removal (Na+/K+ ATPase, NKA). Results: We found that changes in intracellular Na+ homeostatic processes dynamically regulate SAN electrophysiology. Mild reductions in NKA and NCX function increase myocyte firing rate, whereas a stronger reduction causes bursting activity and loss of automaticity. These pathologic phenotypes mimic those observed experimentally in NCX- and ankyrin-B-deficient mice due to altered feedback between the Ca2+ and membrane potential clocks underlying SAN firing. Conclusions: Our study generates new testable predictions and insight linking Na+ homeostasis to Ca2+ handling and membrane potential dynamics in SAN myocytes that may advance our understanding of SAN (dys)function.

Cardiac Energy Metabolism in Heart Failure

Alterations in cardiac energy metabolism contribute to the severity of heart failure. However, the energy metabolic changes that occur in heart failure are complex and are dependent not only on the severity and type of heart failure present but also on the co-existence of common comorbidities such as obesity and type 2 diabetes. The failing heart faces an energy deficit, primarily because of a decrease in mitochondrial oxidative capacity. This is partly compensated for by an increase in ATP production from glycolysis. The relative contribution of the different fuels for mitochondrial ATP production also changes, including a decrease in glucose and amino acid oxidation, and an increase in ketone oxidation. The oxidation of fatty acids by the heart increases or decreases, depending on the type of heart failure. For instance, in heart failure associated with diabetes and obesity, myocardial fatty acid oxidation increases, while in heart failure associated with hypertension or ischemia, myocardial fatty acid oxidation decreases. Combined, these energy metabolic changes result in the failing heart becoming less efficient (ie, a decrease in cardiac work/O₂ consumed). The alterations in both glycolysis and mitochondrial oxidative metabolism in the failing heart are due to both transcriptional changes in key enzymes involved in these metabolic pathways, as well as alterations in NAD redox state (NAD⁺ and nicotinamide adenine dinucleotide levels) and metabolite signaling that contribute to posttranslational epigenetic changes in the control of expression of genes encoding energy metabolic enzymes. Alterations in the fate of glucose, beyond flux through glycolysis or glucose oxidation, also contribute to the pathology of heart failure. Of importance, pharmacological targeting of the energy metabolic pathways has emerged as a novel therapeutic approach to improving cardiac efficiency, decreasing the energy deficit and improving cardiac function in the failing heart.

Decreased GLUT1/NHE1 RNA expression in whole blood predicts disease severity in patients with COVID-19

AIMS

We aimed to assess whether expression of whole-blood RNA of sodium proton exchanger 1 (NHE1) and glucose transporter 1 (GLUT1) is associated with COVID-19 infection and outcome in patients presenting to the emergency department with respiratory infections. Furthermore, we investigated NHE1 and GLUT1 expression in the myocardium of deceased COVID-19 patients.

METHODS AND RESULTS

Whole-blood quantitative assessment of NHE1 and GLUT1 RNA was performed using quantitative PCR in patients with respiratory infection upon first contact in the emergency department and subsequently stratified by SARS-CoV-2 infection status. Assessment of NHE1 and GLUT1 RNA using PCR was also performed in left ventricular myocardium of deceased COVID-19 patients. NHE1 expression is up-regulated in whole blood of patients with COVID-19 compared with other respiratory infections at first medical contact in the emergency department (control: 0.0021 ± 0.0002, COVID-19: 0.0031 ± 0.0003, P = 0.01). The ratio of GLUT1 to NHE1 is significantly decreased in the blood of COVID-19 patients who are subsequently intubated and/or die (severe disease) compared with patients with moderate disease (moderate disease: 0.497 ± 0.083 vs. severe disease: 0.294 ± 0.0336, P = 0.036). This ratio is even further decreased in the myocardium of patients who deceased from COVID-19 in comparison with the myocardium of non-infected donors.

CONCLUSIONS

NHE1 and GLUT1 may be critically involved in the disease progression of SARS-CoV-2 infection. We show here that SARS-CoV-2 infection critically disturbs ion channel expression in the heart. A decreased ratio of GLUT1/NHE1 could potentially serve as a biomarker for disease severity in patients with COVID-19.

The electrogenic sodium bicarbonate cotransporter and its roles in the myocardial ischemia-reperfusion induced cardiac diseases

Cardiac tissue ischemia/hypoxia increases glycolysis and lactic acid accumulation in cardiomyocytes, leading to intracellular metabolic acidosis. Sodium bicarbonate cotransporters (NBCs) play a vital role in modulating intracellular pH and maintaining sodium ion concentrations in cardiomyocytes. Cardiomyocytes mainly express electrogenic sodium bicarbonate cotransporter (NBCe1), which has been demonstrated to participate in myocardial ischemia/reperfusion (I/R) injury. This review outlines the structural and functional properties of NBCe1, summarizes the signaling pathways and factors that may regulate the activity of NBCe1, and reviews the roles of NBCe1 in the pathogenesis of I/R-induced cardiac diseases. Further studies revealing the regulatory mechanisms of NBCe1 activity should provide novel therapeutic targets for preventing I/R-induced cardiac diseases.

Proscillaridin A inhibits hepatocellular carcinoma progression through inducing mitochondrial damage and autophagy

Hepatocellular carcinoma (HCC) is one of the leading causes of cancer-related deaths globally. At present, drug options for systemic treatment of HCC are very limited. There is an urgent need to develop additional effective drugs for HCC treatment. In the present study, we found that proscillaridin A (ProA), a cardiac glycoside, exerted a strong anticancer effect on multiple HCC cell lines. ProA significantly inhibited the cell proliferation, migration, and invasion of HCC cells. ProA also had a marked inhibitory effect on the progression of HCC in the MHCC97H xenograft nude mouse model. ProA-mediated suppression of HCC was closely related to cell apoptosis. ProA-treated HCC cells displayed significant mitochondrial damage and elevated reactive oxygen species production, resulting in profound cell apoptosis. Meanwhile, ProA also played a role in autophagy induction in HCC cells. Defects in autophagy partially relieved ProA's anticancer effect in HCC cells. Our findings demonstrate that ProA can effectively inhibit HCC progression and may serve as a potential therapeutic agent for HCC treatment.

The role of calcium homeostasis remodeling in inherited cardiac arrhythmia syndromes

Sudden cardiac death due to malignant ventricular arrhythmias remains the major cause of mortality in the postindustrial world. Defective intracellular Ca²⁺ homeostasis has been well established as a key contributing factor to the enhanced propensity for arrhythmia in acquired cardiac disease, such as heart failure or diabetic cardiomyopathy. More recent advances provide a strong basis to the emerging view that hereditary cardiac arrhythmia syndromes are accompanied by maladaptive remodeling of Ca²⁺ homeostasis which substantially increases arrhythmic risk. This brief review will focus on functional changes in elements of Ca²⁺ handling machinery in cardiomyocytes that occur secondary to genetic mutations associated with catecholaminergic polymorphic ventricular tachycardia, and long QT syndrome.

Late sodium current and calcium homeostasis in arrhythmogenesis

The cardiac late sodium current (I_Na,late) is the small sustained component of the sodium current active during the plateau phase of the action potential. Several studies demonstrated that augmentation of the current can lead to cardiac arrhythmias; therefore, I_Na,late is considered as a promising antiarrhythmic target. Fundamentally, enlarged I_Na,late increases Na⁺ influx into the cell, which, in turn, is converted to elevated intracellular Ca²⁺ concentration through the Na⁺/Ca²⁺ exchanger. The excessive Ca²⁺ load is known to be proarrhythmic. This review describes the behavior of the voltage-gated Na⁺ channels generating I_Na,late in health and disease and aims to discuss the physiology and pathophysiology of Na⁺ and Ca²⁺ homeostasis in context with the enhanced I_Na,late demonstrating also the currently accessible antiarrhythmic choices.

Mitochondrial Ca2+ in heart failure: Not enough or too much?

Ca²⁺ serves as a ubiquitous second messenger mediating a variety of cellular processes including electrical excitation, contraction, gene expression, secretion, cell death and others. The identification of the molecular components of the mitochondrial Ca²⁺ influx and efflux pathways has created a resurgent interest in the regulation of mitochondrial Ca²⁺ balance and its physiological and pathophysiological roles. While the pace of discovery has quickened with the availability of new cellular and animal models, many fundamental questions remain to be answered regarding the regulation and functional impact of mitochondrial Ca²⁺ in health and disease. This review highlights several experimental observations pertaining to key aspects of mitochondrial Ca²⁺ homeostasis that remain enigmatic, particularly whether mitochondrial Ca²⁺ signaling is depressed or excessive in heart failure, which will determine the optimal approach to therapeutic intervention.

SGLT2 Inhibitors: A Novel Player in the Treatment and Prevention of Diabetic Cardiomyopathy

Diabetic cardiomyopathy (DCM) characterized by diastolic and systolic dysfunction independently of hypertension and coronary heart disease, eventually develops into heart failure, which is strongly linked to a high prevalence of mortality in people with diabetes mellitus (DM). Sodium-glucose cotransporter type2 inhibitors (SGLT2Is) are a novel type of hypoglycemic agent in increasing urinary glucose and sodium excretion. Excitingly, the EMPA-REG clinical trial proved that empagliflozin significantly reduced the relative risk of cardiovascular (CV) death and hospitalization for heart failure (HHF) in patients with type 2 DM (T2DM) plus CV disease (CVD). The EMPRISE trial showed that empagliflozin decreased the risk of HHF in T2DM patients with and without a CVD history in routine care. These beneficial effects of SGLT2Is could not be entirely attributed to glucose-lowering or natriuretic action. There could be potential direct mechanisms of SGLT2Is in cardioprotection. Recent studies have shown the effects of SGLT2Is on cardiac iron homeostasis, mitochondrial function, anti-inflammation, anti-fibrosis, antioxidative stress, and renin-angiotensin-aldosterone system activity, as well as GlcNAcylation in the heart. This article reviews the current literature on the effects of SGLT2Is on DCM in preclinical studies. Possible molecular mechanisms regarding potential benefits of SGLT2Is for DCM are highlighted, with the purpose of providing a novel strategy for preventing DCM.

Cardiac foetal reprogramming: a tool to exploit novel treatment targets for the failing heart

As the heart matures during embryogenesis from its foetal stages, several structural and functional modifications take place to form the adult heart. This process of maturation is in large part due to an increased volume and work load of the heart to maintain proper circulation throughout the growing body. In recent years, it has been observed that these changes are reversed to some extent as a result of cardiac disease. The process by which this occurs has been characterized as cardiac foetal reprogramming and is defined as the suppression of adult and re-activation of a foetal genes profile in the diseased myocardium. The reasons as to why this process occurs in the diseased myocardium are unknown; however, it has been suggested to be an adaptive process to counteract deleterious events taking place during cardiac remodelling. Although still in its infancy, several studies have demonstrated that targeting foetal reprogramming in heart failure can lead to substantial improvement in cardiac functionality. This is highlighted by a recent study which found that by modulating the expression of 5-oxoprolinase (OPLAH, a novel cardiac foetal gene), cardiac function can be significantly improved in mice exposed to cardiac injury. Additionally, the utilization of angiotensin receptor neprilysin inhibitors (ARNI) has demonstrated clear benefits, providing important clinical proof that drugs that increase natriuretic peptide levels (part of the foetal gene programme) indeed improve heart failure outcomes. In this review, we will highlight the most important aspects of cardiac foetal reprogramming and will discuss whether this process is a cause or consequence of heart failure. Based on this, we will also explain how a deeper understanding of this process may result in the development of novel therapeutic strategies in heart failure.

Elevated perfusate [Na+] increases contractile dysfunction during ischemia and reperfusion

Recent studies revealed that relatively small changes in perfusate sodium ([Na⁺]_o) composition significantly affect cardiac electrical conduction and stability in contraction arrested ex vivo Langendorff heart preparations before and during simulated ischemia. Additionally, [Na⁺]_o modulates cardiomyocyte contractility via a sodium-calcium exchanger (NCX) mediated pathway. It remains unknown, however, whether modest changes to [Na⁺]_o that promote electrophysiologic stability similarly improve mechanical function during baseline and ischemia-reperfusion conditions. The purpose of this study was to quantify cardiac mechanical function during ischemia-reperfusion with perfusates containing 145 or 155 mM Na⁺ in Langendorff perfused isolated rat heart preparations. Relative to 145 mM Na⁺, perfusion with 155 mM [Na⁺]_o decreased the amplitude of left-ventricular developed pressure (LVDP) at baseline and accelerated the onset of ischemic contracture. Inhibiting NCX with SEA0400 abolished LVDP depression caused by increasing [Na⁺]_o at baseline and reduced the time to peak ischemic contracture. Ischemia-reperfusion decreased LVDP in all hearts with return of intrinsic activity, and reperfusion with 155 mM [Na⁺]_o further depressed mechanical function. In summary, elevating [Na⁺]_o by as little as 10 mM can significantly modulate mechanical function under baseline conditions, as well as during ischemia and reperfusion. Importantly, clinical use of Normal Saline, which contains 155 mM [Na⁺]_o, with cardiac ischemia may require further investigation.

Left ventricular dysfunction in heart failure with preserved ejection fraction-molecular mechanisms and impact on right ventricular function

The current classification of heart failure (HF) based on left ventricular (LV) ejection fraction (EF) identifies a large group of patients with preserved ejection fraction (HFpEF) with significant morbidity and mortality but without prognostic benefit from current HF therapy. Co-morbidities and conditions such as arterial hypertension, diabetes mellitus, chronic kidney disease, adiposity and aging shape the clinical phenotype and contribute to mortality. LV diastolic dysfunction and LV structural remodeling are hallmarks of HFpEF, and are linked to remodeling of the cardiomyocyte and extracellular matrix. Pulmonary hypertension (PH) and right ventricular dysfunction (RVD) are particularly common in HFpEF, and mortality is up to 10-fold higher in HFpEF patients with vs. without RV dysfunction. Here, we review alterations in cardiomyocyte function (i.e., ion homeostasis, sarcomere function and cellular metabolism) associated with diastolic dysfunction and summarize the main underlying cellular pathways. The contribution and interaction of systemic and regional upstream signaling such as chronic inflammation, neurohumoral activation, and NO-cGMP-related pathways are outlined in detail, and their diagnostic and therapeutic potential is discussed in the context of preclinical and clinical studies. In addition, we summarize prevalence and pathomechanisms of RV dysfunction in the context of HFpEF and discuss mechanisms connecting LV and RV dysfunction in HFpEF. Dissecting the molecular mechanisms of LV and RV dysfunction in HFpEF may provide a basis for an improved classification of HFpEF and for therapeutic approaches tailored to the molecular phenotype.

Intracellular calcium leak in heart failure and atrial fibrillation: a unifying mechanism and therapeutic target

Ca²⁺ is a fundamental second messenger in all cell types and is required for numerous essential cellular functions, including cardiac and skeletal muscle contraction. The intracellular concentration of free Ca²⁺ ([Ca²⁺]) is regulated primarily by ion channels, pumps (ATPases), exchangers and Ca²⁺-binding proteins. Defective regulation of [Ca²⁺] is found in a diverse spectrum of pathological states that affect all the major organs. In the heart, abnormalities in the regulation of cytosolic and mitochondrial [Ca²⁺] occur in heart failure (HF) and atrial fibrillation (AF), two common forms of heart disease and leading contributors to morbidity and mortality. In this Review, we focus on the mechanisms that regulate ryanodine receptor 2 (RYR2), the major sarcoplasmic reticulum (SR) Ca²⁺-release channel in the heart, how RYR2 becomes dysfunctional in HF and AF, and its potential as a therapeutic target. Inherited RYR2 mutations and/or stress-induced phosphorylation and oxidation of the protein destabilize the closed state of the channel, resulting in a pathological diastolic Ca²⁺ leak from the SR that both triggers arrhythmias and impairs contractility. On the basis of our increased understanding of SR Ca²⁺ leak as a shared Ca²⁺-dependent pathological mechanism in HF and AF, a new class of drugs developed in our laboratory, known as rycals, which stabilize RYR2 channels and prevent Ca²⁺ leak from the SR, are undergoing investigation in clinical trials.

Membrane current evoked by mitochondrial Na+-Ca2+ exchange in mouse heart

The electrogenicity of mitochondrial Na+-Ca2+ exchange (NCXm) had been controversial and no membrane current through it had been reported. We succeeded for the first time in recording NCXm-mediated currents using mitoplasts derived from mouse ventricle. Under conditions that K+, Cl-, and Ca2+ uniporter currents were inhibited, extra-mitochondrial Na+ induced inward currents with 1 μM Ca2+ in the pipette. The half-maximum concentration of Na+ was 35.6 mM. The inward current was diminished without Ca2+ in the pipette, and was augmented with 10 μM Ca2+. The Na+-induced inward currents were largely inhibited by CGP-37157, an NCXm blocker. However, the reverse mode of NCXm, which should be detected as an outward current, was hardly induced by extra-mitochondrial application of Ca2+ with Na+ in the pipette. It was concluded that NCXm is electrogenic. This property may be advantageous for facilitating Ca2+ extrusion from mitochondria, which has large negative membrane potential.

Late Sodium Current Inhibitors as Potential Antiarrhythmic Agents

Based on recent findings, an increased late sodium current (INa,late) plays an important pathophysiological role in cardiac diseases, including rhythm disorders. The article first describes what is INa,late and how it functions under physiological circumstances. Next, it shows the wide range of cellular mechanisms that can contribute to an increased INa,late in heart diseases, and also discusses how the upregulated INa,late can play a role in the generation of cardiac arrhythmias. The last part of the article is about INa,late inhibiting drugs as potential antiarrhythmic agents, based on experimental and preclinical data as well as in the light of clinical trials.

Calcium Signaling in Cardiomyocyte Function

Rhythmic increases in intracellular Ca²⁺ concentration underlie the contractile function of the heart. These heart muscle-wide changes in intracellular Ca²⁺ are induced and coordinated by electrical depolarization of the cardiomyocyte sarcolemma by the action potential. Originating at the sinoatrial node, conduction of this electrical signal throughout the heart ensures synchronization of individual myocytes into an effective cardiac pump. Ca²⁺ signaling pathways also regulate gene expression and cardiomyocyte growth during development and in pathology. These fundamental roles of Ca²⁺ in the heart are illustrated by the prevalence of altered Ca²⁺ homeostasis in cardiovascular diseases. Indeed, heart failure (an inability of the heart to support hemodynamic needs), rhythmic disturbances, and inappropriate cardiac growth all share an involvement of altered Ca²⁺ handling. The prevalence of these pathologies, contributing to a third of all deaths in the developed world as well as to substantial morbidity makes understanding the mechanisms of Ca²⁺ handling and dysregulation in cardiomyocytes of great importance.

The Control of Diastolic Calcium in the Heart: Basic Mechanisms and Functional Implications

Normal cardiac function requires that intracellular Ca²⁺ concentration be reduced to low levels in diastole so that the ventricle can relax and refill with blood. Heart failure is often associated with impaired cardiac relaxation. Little, however, is known about how diastolic intracellular Ca²⁺ concentration is regulated. This article first discusses the reasons for this ignorance before reviewing the basic mechanisms that control diastolic intracellular Ca²⁺ concentration. It then considers how the control of systolic and diastolic intracellular Ca²⁺ concentration is intimately connected. Finally, it discusses the changes that occur in heart failure and how these may result in heart failure with preserved versus reduced ejection fraction.

Modulation of the cardiac Na+-Ca2+ exchanger by cytoplasmic protons: Molecular mechanisms and physiological implications

A precise temporal and spatial control of intracellular Ca²⁺ concentration is essential for a coordinated contraction of the heart. Following contraction, cardiac cells need to rapidly remove intracellular Ca²⁺ to allow for relaxation. This task is performed by two transporters: the plasma membrane Na⁺-Ca²⁺ exchanger (NCX) and the sarcoplasmic reticulum (SR) Ca²⁺-ATPase (SERCA). NCX extrudes Ca²⁺ from the cell, balancing the Ca²⁺entering the cytoplasm during systole through L-type Ca²⁺ channels. In parallel, following SR Ca²⁺ release, SERCA activity replenishes the SR, reuptaking Ca²⁺ from the cytoplasm. The activity of the mammalian exchanger is fine-tuned by numerous ionic allosteric regulatory mechanisms. Micromolar concentrations of cytoplasmic Ca²⁺ potentiate NCX activity, while an increase in intracellular Na⁺ levels inhibits NCX via a mechanism known as Na⁺-dependent inactivation. Protons are also powerful inhibitors of NCX activity. By regulating NCX activity, Ca²⁺, Na⁺ and H⁺ couple cell metabolism to Ca²⁺ homeostasis and therefore cardiac contractility. This review summarizes the recent progress towards the understanding of the molecular mechanisms underlying the ionic regulation of the cardiac NCX with special emphasis on pH modulation and its physiological impact on the heart.

Electrophysiological and Contractile Effects of Disopyramide in Patients With Obstructive Hypertrophic Cardiomyopathy: A Translational Study

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In patients with HCM and symptomatic LVOT-obstruction, first treatment with disopyramide leads to a marked reduction of LVOT gradients, with a slight decrease of resting ejection fraction and a modest increase of corrected QT interval, highlighting high efficacy and safety.

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In single cardiomyocytes and intact trabeculae from surgical samples of patients with obstructive HCM, in vitro treatment with 5 μmol/l disopyramide lowered force and Ca2⁺ transients while reducing action potential duration and the rate of arrhythmic afterdepolarizations.

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These effects are mediated by the combined inhibition of peak and late Na⁺ currents, L-type Ca²⁺ current, delayed-rectifier K⁺ current, and ryanodine receptors.

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In addition to the negative inotropic effect of disopyramide, in vitro results suggest additional antiarrhythmic actions.

Disopyramide is effective and safe in patients with obstructive hypertrophic cardiomyopathy. However, its cellular and molecular mechanisms of action are unknown. We tested disopyramide in cardiomyocytes from the septum of surgical myectomy patients: disopyramide inhibits multiple ion channels, leading to lower Ca transients and force, and shortens action potentials, thus reducing cellular arrhythmias. The electrophysiological profile of disopyramide explains the efficient reduction of outflow gradients but also the limited prolongation of the QT interval and the absence of arrhythmic side effects observed in 39 disopyramide-treated patients. In conclusion, our results support the idea that disopyramide is safe for outpatient use in obstructive patients.

The Ionotropic P2X4 Receptor has Unique Properties in the Heart by Mediating the Negative Chronotropic Effect of ATP While Increasing the Ventricular Inotropy

Background: Mounting evidence indicate that reducing the sinoatrial node (SAN) activity may be a useful therapeutic strategy to control of heart failure. Purines, like ATP and its metabolite adenosine, consistently reduce the SAN spontaneous activity leading to negative cardiac chronotropy, with variable effects on the force of myocardial contraction (inotropy). Apart from adenosine A₁ receptors, the human SAN expresses high levels of ATP-sensitive ionotropic P2X4 receptors (P2X4R), yet their cardiac role is unexplored. Methods: Here, we investigated the activity of P2 purinoceptors on isolated spontaneously beating atria (chronotropy) and on 2 Hz-paced right ventricular (RV, inotropy) strips from Wistar rats. Results: ATP (pEC ₅₀ = 4.05) and its stable analogue ATPγS (pEC ₅₀ = 4.69) concentration-dependently reduced atrial chronotropy. Inhibition of ATP breakdown into adenosine by NTPDases with POM-1 failed to modify ATP-induced negative chronotropy. The effect of ATP on atrial rate was attenuated by a broad-spectrum P2 antagonist, PPADS, as well as by 5-BDBD, which selectively blocks the P2X4R subtype; however, no effect was observed upon blocking the A₁ receptor with DPCPX. The P2X4R positive allosteric modulator, ivermectin, increased the negative chronotropic response of ATP. Likewise, CTP, a P2X agonist that does not generate adenosine, replicated the P2X4R-mediated negative chronotropism of ATP. Inhibition of the Na⁺/Ca²⁺ exchanger (NCX) with KB-R7943 and ORM-10103, but not blockage of the HCN channel with ZD7288, mimicked the effect of the P2X4R blocker, 5-BDBD. In paced RV strips, ATP caused a mild negative inotropic effect, which magnitude was 2 to 3-fold increased by 5-BDBD and KB-R7943. Immunofluorescence confocal microscopy studies confirm that cardiomyocytes of the rat SAN and RV co-express P2X4R and NCX1 proteins. Conclusions: Data suggest that activation of ATP-sensitive P2X4R slows down heart rate by reducing the SAN activity while increasing the magnitude of ventricular contractions. The mechanism underlying the dual effect of ATP in the heart may involve inhibition of intracellular Ca²⁺-extrusion by bolstering NCX function in the reverse mode. Thus, targeting the P2X4R activation may create novel well-tolerated heart-rate lowering drugs with potential benefits in patients with deteriorated ventricular function.

Calcium Signaling and Reactive Oxygen Species in Mitochondria

In heart failure, alterations of Na⁺ and Ca²⁺ handling, energetic deficit, and oxidative stress in cardiac myocytes are important pathophysiological hallmarks. Mitochondria are central to these processes because they are the main source for ATP, but also reactive oxygen species (ROS), and their function is critically controlled by Ca²⁺ During physiological variations of workload, mitochondrial Ca²⁺ uptake is required to match energy supply to demand but also to keep the antioxidative capacity in a reduced state to prevent excessive emission of ROS. Mitochondria take up Ca²⁺ via the mitochondrial Ca²⁺ uniporter, which exists in a multiprotein complex whose molecular components were identified only recently. In heart failure, deterioration of cytosolic Ca²⁺ and Na⁺ handling hampers mitochondrial Ca²⁺ uptake and the ensuing Krebs cycle-induced regeneration of the reduced forms of NADH (nicotinamide adenine dinucleotide) and NADPH (nicotinamide adenine dinucleotide phosphate), giving rise to energetic deficit and oxidative stress. ROS emission from mitochondria can trigger further ROS release from neighboring mitochondria termed ROS-induced ROS release, and cross talk between different ROS sources provides a spatially confined cellular network of redox signaling. Although low levels of ROS may serve physiological roles, higher levels interfere with excitation-contraction coupling, induce maladaptive cardiac remodeling through redox-sensitive kinases, and cell death through mitochondrial permeability transition. Targeting the dysregulated interplay between excitation-contraction coupling and mitochondrial energetics may ameliorate the progression of heart failure.

Altered Intracellular Calcium Homeostasis and Arrhythmogenesis in the Aged Heart

Aging of the heart is associated with a blunted response to sympathetic stimulation, reduced contractility, and increased propensity for arrhythmias, with the risk of sudden cardiac death significantly increased in the elderly population. The altered cardiac structural and functional phenotype, as well as age-associated prevalent comorbidities including hypertension and atherosclerosis, predispose the heart to atrial fibrillation, heart failure, and ventricular tachyarrhythmias. At the cellular level, perturbations in mitochondrial function, excitation-contraction coupling, and calcium homeostasis contribute to this electrical and contractile dysfunction. Major determinants of cardiac contractility are the intracellular release of Ca²⁺ from the sarcoplasmic reticulum by the ryanodine receptors (RyR2), and the following sequestration of Ca²⁺ by the sarco/endoplasmic Ca²⁺-ATPase (SERCa2a). Activity of RyR2 and SERCa2a in myocytes is not only dependent on expression levels and interacting accessory proteins, but on fine-tuned regulation via post-translational modifications. In this paper, we review how aberrant changes in intracellular Ca²⁺ cycling via these proteins contributes to arrhythmogenesis in the aged heart.

Effects of the Inhibition of Late Sodium Current by GS967 on Stretch-Induced Changes in Cardiac Electrophysiology

PURPOSE

Mechanical stretch increases sodium and calcium entry into myocytes and activates the late sodium current. GS967, a triazolopyridine derivative, is a sodium channel blocker with preferential effects on the late sodium current. The present study evaluates whether GS967 inhibits or modulates the arrhythmogenic electrophysiological effects of myocardial stretch.

METHODS

Atrial and ventricular refractoriness and ventricular fibrillation modifications induced by acute stretch were studied in Langendorff-perfused rabbit hearts (n = 28) using epicardial multiple electrodes and high-resolution mapping techniques under control conditions and during the perfusion of GS967 at different concentrations (0.03, 0.1, and 0.3 μM).

RESULTS

On comparing ventricular refractoriness, conduction velocity and wavelength obtained before stretch had no significant changes under each GS967 concentration while atrial refractoriness increased under GS967 0.3 μM. Under GS967, the stretch-induced changes were attenuated, and no significant differences were observed between before and during stretch. GS967 0.3 μM diminished the normal stretch-induced changes resulting in longer (less shortened) atrial refractoriness (138 ± 26 ms vs 95 ± 9 ms; p < 0.01), ventricular refractoriness (155 ± 18 ms vs 124 ± 16 ms; p < 0.01) and increments in spectral concentration (23 ± 5% vs 17 ± 2%; p < 0.01), the fifth percentile of ventricular activation intervals (46 ± 8 ms vs 31 ± 3 ms; p < 0.05), and wavelength of ventricular fibrillation (2.5 ±0.5 cm vs 1.7 ± 0.3 cm; p < 0.05) during stretch. The stretch-induced increments in dominant frequency during ventricular fibrillation (control = 38%, 0.03 μM = 33%, 0.1 μM = 33%, 0.3 μM = 14%; p < 0.01) and the stretch-induced increments in arrhythmia complexity index (control = 62%, 0.03μM = 41%, 0.1 μM = 32%, 0.3 μM = 16%; p < 0.05) progressively decreased on increasing the GS967 concentration.

CONCLUSIONS

GS967 attenuates stretch-induced changes in cardiac electrophysiology.

Changes in cellular Ca2+ and Na+ regulation during the progression towards heart failure in the guinea pig

Key points

During compensated hypertrophy in vivo fractional shortening (FS) remains constant until heart failure (HF) develops, when FS decreases from 70% to 39%.

Compensated hypertrophy is accompanied by an increase in I_Na,late and a decrease in Na⁺,K⁺‐ATPase current. These changes persist as HF develops.

SR Ca²⁺ content increases during compensated hypertrophy then decreases in HF. In healthy cells, increases in SR Ca²⁺ content and Ca²⁺ transients can be achieved by the same amount of inhibition of the Na⁺,K⁺‐ATPase as measured in the diseased cells.

SERCA function remains constant during compensated hypertrophy then decreases in HF, when there is also an increase in spark frequency and spark‐mediated Ca²⁺ leak.

We suggest an increase in I_Na,late and a decrease in Na⁺,K⁺‐ATPase current and function alters the balance of Ca²⁺ flux mediated by the Na⁺/Ca²⁺ exchange that limits early contractile impairment.

Abstract

We followed changes in cardiac myocyte Ca²⁺ and Na⁺ regulation from the formation of compensated hypertrophy (CH) until signs of heart failure (HF) are apparent using a trans‐aortic pressure overload (TAC) model. In this model, in vivo fractional shortening (FS) remained constant despite HW:BW ratio increasing by 39% (CH) until HF developed 150 days post‐TAC when FS decreased from 70% to 39%. Using live and fixed fluorescence imaging and electrophysiological techniques, we found an increase in I_Na,late from –0.34 to –0.59 A F⁻¹ and a decrease in Na⁺,K⁺‐ATPase current from 1.09 A F⁻¹ to 0.54 A F⁻¹ during CH. These changes persisted as HF developed (I_Na,late increased to –0.82 A F⁻¹ and Na⁺,K⁺‐ATPase current decreased to 0.51 A F⁻¹). Sarcoplasmic reticulum (SR) Ca²⁺ content increased during CH then decreased in HF (from 32 to 15 μm l⁻¹) potentially supporting the maintenance of FS in the whole heart and Ca²⁺ transients in single myocytes during the former stage. We showed using glycoside blockade in healthy myocytes that increases in SR Ca²⁺ content and Ca²⁺ transients can be driven by the same amount of inhibition of the Na⁺,K⁺‐ATPase as measured in the diseased cells. SERCA function remains constant in CH but decreases (τ for SERCA‐mediated Ca²⁺ removal changed from 6.3 to 3.0 s⁻¹) in HF. In HF there was an increase in spark frequency and spark‐mediated Ca²⁺ leak. We suggest an increase in I_Na,late and a decrease in Na⁺,K⁺‐ATPase current and function alters the balance of Ca²⁺ flux mediated by the Na⁺/Ca²⁺ exchange that limits early contractile impairment.

During compensated hypertrophy in vivo fractional shortening (FS) remains constant until heart failure (HF) develops, when FS decreases from 70% to 39%.

Compensated hypertrophy is accompanied by an increase in I_Na,late and a decrease in Na⁺,K⁺‐ATPase current. These changes persist as HF develops.

SR Ca²⁺ content increases during compensated hypertrophy then decreases in HF. In healthy cells, increases in SR Ca²⁺ content and Ca²⁺ transients can be achieved by the same amount of inhibition of the Na⁺,K⁺‐ATPase as measured in the diseased cells.

SERCA function remains constant during compensated hypertrophy then decreases in HF, when there is also an increase in spark frequency and spark‐mediated Ca²⁺ leak.

We suggest an increase in I_Na,late and a decrease in Na⁺,K⁺‐ATPase current and function alters the balance of Ca²⁺ flux mediated by the Na⁺/Ca²⁺ exchange that limits early contractile impairment.

Molecular Mechanisms Responsible for Diastolic Dysfunction in Diabetes Mellitus Patients

In diabetic patients, cardiomyopathy is an important cause of heart failure, but its pathophysiology has not been completely understood thus far. Myocardial hypertrophy and diastolic dysfunction have been considered the hallmarks of diabetic cardiomyopathy (DCM), while systolic function is affected in the latter stages of the disease. In this article we propose the potential pathophysiological mechanisms responsible for myocardial hypertrophy and increased myocardial stiffness leading to diastolic dysfunction in this specific entity. According to our model, increased myocardial stiffness results from both cellular and extracellular matrix stiffness as well as cell–matrix interactions. Increased intrinsic cardiomyocyte stiffness is probably the most important contributor to myocardial stiffness. It results from the impairment in cardiomyocyte cytoskeleton. Several other mechanisms, specifically affected by diabetes, seem to also be significantly involved in myocardial stiffening, i.e., impairment in the myocardial nitric oxide (NO) pathway, coronary microvascular dysfunction, increased inflammation and oxidative stress, and myocardial sodium glucose cotransporter-2 (SGLT-2)-mediated effects. Better understanding of the complex pathophysiology of DCM suggests the possible value of drugs targeting the listed mechanisms. Antidiabetic drugs, NO-stimulating agents, anti-inflammatory agents, and SGLT-2 inhibitors are emerging as potential treatment options for DCM.

Direct cardiovascular impact of SGLT2 inhibitors: mechanisms and effects

Diabetes is a global epidemic and a leading cause of death with more than 422 million patients worldwide out of whom around 392 million alone suffer from type 2 diabetes (T2D). Sodium-glucose cotransporter 2 inhibitors (SGLT2i) are novel and effective drugs in managing glycemia of T2D patients. These inhibitors gained recent clinical and basic research attention due to their clinically observed cardiovascular protective effects. Although interest in the study of various SGLT isoforms and the effect of their inhibition on cardiovascular function extends over the past 20 years, an explanation of the effects observed clinically based on available experimental data is not forthcoming. The remarkable reduction in cardiovascular (CV) mortality (38%), major CV events (14%), hospitalization for heart failure (35%), and death from any cause (32%) observed over a period of 2.6 years in patients with T2D and high CV risk in the EMPA-REG OUTCOME trial involving the SGLT2 inhibitor empagliflozin (Empa) have raised the possibility that potential novel, more specific mechanisms of SGLT2 inhibition synergize with the known modest systemic improvements, such as glycemic, body weight, diuresis, and blood pressure control. Multiple studies investigated the direct impact of SGLT2i on the cardiovascular system with limited findings and the pathophysiological role of SGLTs in the heart. The direct impact of SGLT2i on cardiac homeostasis remains controversial, especially that SGLT1 isoform is the only form expressed in the capillaries and myocardium of human and rodent hearts. The direct impact of SGLT2i on the cardiovascular system along with potential lines of future research is summarized in this review.

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.

Proarrhythmic Remodeling of Calcium Homeostasis in Cardiac Disease; Implications for Diabetes and Obesity

A rapid growth in the incidence of diabetes and obesity has transpired to a major heath issue and economic burden in the postindustrial world, with more than 29 million patients affected in the United States alone. Cardiovascular defects have been established as the leading cause of mortality and morbidity of diabetic patients. Over the last decade, significant progress has been made in delineating mechanisms responsible for the diminished cardiac contractile function and enhanced propensity for malignant cardiac arrhythmias characteristic of diabetic disease. Rhythmic cardiac contractility relies upon the precise interplay between several cellular Ca²⁺ transport protein complexes including plasmalemmal L-type Ca²⁺ channels (LTCC), Na⁺-Ca²⁺ exchanger (NCX1), Sarco/endoplasmic Reticulum (SR) Ca²⁺-ATPase (SERCa2a) and ryanodine receptors (RyR2s), the SR Ca²⁺ release channels. Here we provide an overview of changes in Ca²⁺ homeostasis in diabetic ventricular myocytes and discuss the therapeutic potential of targeting Ca²⁺ handling proteins in the prevention of diabetes-associated cardiomyopathy and arrhythmogenesis.

Proarrhythmic Remodeling of Calcium Homeostasis in Cardiac Disease; Implications for Diabetes and Obesity

A rapid growth in the incidence of diabetes and obesity has transpired to a major heath issue and economic burden in the postindustrial world, with more than 29 million patients affected in the United States alone. Cardiovascular defects have been established as the leading cause of mortality and morbidity of diabetic patients. Over the last decade, significant progress has been made in delineating mechanisms responsible for the diminished cardiac contractile function and enhanced propensity for malignant cardiac arrhythmias characteristic of diabetic disease. Rhythmic cardiac contractility relies upon the precise interplay between several cellular Ca²⁺ transport protein complexes including plasmalemmal L-type Ca²⁺ channels (LTCC), Na⁺-Ca²⁺ exchanger (NCX1), Sarco/endoplasmic Reticulum (SR) Ca²⁺-ATPase (SERCa2a) and ryanodine receptors (RyR2s), the SR Ca²⁺ release channels. Here we provide an overview of changes in Ca²⁺ homeostasis in diabetic ventricular myocytes and discuss the therapeutic potential of targeting Ca²⁺ handling proteins in the prevention of diabetes-associated cardiomyopathy and arrhythmogenesis.