Mitochondria are sources of metabolic sink and arrhythmias

Physical frailty, genetic predisposition, and incident arrhythmias

BACKGROUND

Cross-sectional evidence suggests a possible link between frailty and atrial fibrillation (AF). It remains unclear whether frailty and incident arrhythmias are longitudinally associated. This study aimed to determine whether the frailty phenotype is longitudinally associated with incident arrhythmias, especially AF.

METHODS

In this prospective cohort of UK Biobank, individuals with arrhythmias at baseline, those without data for frailty phenotype, and no genetic data were excluded. Five domains of physical frailty, including weight loss, exhaustion, low physical activity, low grip strength, and slow gait speed, were assessed. A total of 142 single-nucleotide polymorphisms was used to calculate the polygenic risk score (PRS) for AF. Hospital inpatient records and death records were used to identify incident arrhythmias.

RESULTS

This study included 464 154 middle-aged and older adults (mean age 56.4 ± 8.1 years, 54.7% female) without arrhythmia at baseline. During a median follow-up of 13.4 years (over 5.9 million person-years), 46 454 new-onset arrhythmias cases were recorded. In comparison with non-frailty, the multivariable-adjusted hazard ratios (HRs) of AF were 1.12 (95% CI: 1.09, 1.15, P < 0.0001) and 1.44 (95% CI: 1.36, 1.51, P < 0.0001) for participants with pre-frailty and frailty, respectively. Similar associations were observed for other arrhythmias. We found that slow gait speed presented the strongest risk factor in predicting all arrhythmias, including AF (HR 1.34, 95% CI: 1.30, 1.39), bradyarrhythmias (HR 1.30, 95% CI: 1.22, 1.37), conduction system diseases (HR 1.29, 95% CI: 1.22, 1.36), supraventricular arrhythmias (HR 1.32, 95% CI: 1.19, 1.47), and ventricular arrhythmias (HR 1.37, 95% CI: 1.25, 1.51), with all P values <0.0001. In addition to slow gait speed, weight loss (HR 1.13, 95% CI: 1.09, 1.16, P < 0.0001) and exhaustion (HR 1.11, 95% CI: 1.07, 1.14, P < 0.0001) were significantly associated with incident AF, whereas insignificant associations were observed for physical activity (HR 1.03, 95% CI: 0.996, 1.08, P = 0.099) and low grip strength (HR 1.00, 95% CI: 0.97, 1.03, P = 0.89). We observed a significant interaction between genetic predisposition and frailty on incident AF (P for interaction <0.0001), where those with frailty and the highest tertile of PRS had the highest risk of AF (HR 3.34, 95% CI: 3.08, 3.61, P < 0.0001) compared with those with non-frailty and the lowest tertile of PRS.

CONCLUSIONS

Physical pre-frailty and frailty were significantly and independently associated with incident arrhythmias. Although direct causal inference still needs to be further validated, these results suggested the importance of assessing and managing frailty for arrhythmia prevention.

Mitochondrial network remodeling of the diabetic heart: implications to ischemia related cardiac dysfunction

Mitochondria play a central role in cellular energy metabolism, and their dysfunction is increasingly recognized as a critical factor in the pathogenesis of diabetes-related cardiac pathophysiology, including vulnerability to ischemic events that culminate in myocardial infarction on the one hand and ventricular arrhythmias on the other. In diabetes, hyperglycemia and altered metabolic substrates lead to excessive production of reactive oxygen species (ROS) by mitochondria, initiating a cascade of oxidative stress that damages mitochondrial DNA, proteins, and lipids. This mitochondrial injury compromises the efficiency of oxidative phosphorylation, leading to impaired ATP production. The resulting energy deficit and oxidative damage contribute to functional abnormalities in cardiac cells, placing the heart at an increased risk of electromechanical dysfunction and irreversible cell death in response to ischemic insults. While cardiac mitochondria are often considered to be relatively autonomous entities in their capacity to produce energy and ROS, their highly dynamic nature within an elaborate network of closely-coupled organelles that occupies 30-40% of the cardiomyocyte volume is fundamental to their ability to exert intricate regulation over global cardiac function. In this article, we review evidence linking the dynamic properties of the mitochondrial network to overall cardiac function and its response to injury. We then highlight select studies linking mitochondrial ultrastructural alterations driven by changes in mitochondrial fission, fusion and mitophagy in promoting cardiac ischemic injury to the diabetic heart.

Serum albumin levels and risk of atrial fibrillation: a Mendelian randomization study

Objective

Although several observational studies have linked serum albumin to cardiovascular disease and considered it as an important biomarker, little is known about whether increasing or maintaining serum albumin levels can effectively improve the prognosis of patients with atrial fibrillation. Therefore, this study aims to further explore the causal relationship between serum albumin and atrial fibrillation and its potential mechanism.

Method

Using data from large-scale genome-wide association studies, we conducted a two-sample Mendelian randomization (MR) analysis and a mediation MR analysis, using serum albumin as the exposure variable and atrial fibrillation as the outcome variable. We included 486 serum metabolites as potential mediating factors. To increase the robustness of the analysis, we applied five statistical methods, including inverse variance weighted, weighted median, MR-Egger, simple mode, and weighted mode. Validate the MR results using Bayesian weighted Mendelian randomization method.

Result

The results of the MR analysis indicate a significant inverse association between genetically predicted serum albumin concentration (g/L) and the risk of atrial fibrillation (Beta = -0.172, OR = 0.842, 95% CI: 0.753-0.941, p = 0.002). Further mediation MR analysis revealed that serum albumin may mediate the causal relationship with atrial fibrillation by affecting two serum metabolites, docosatrienoate and oleate/vaccenate, and the mediating effect was significant. In addition, all our instrumental variables showed no heterogeneity and level-multiplicity in the MR analysis. To verify the stability of the results, we also conducted a sensitivity analysis using the leave-one-out method, and the results further confirmed that our findings were robust and reliable. Finally, we conducted a validation using the Bayesian weighted Mendelian randomization method, which demonstrated the reliability of our causal inference results.

Conclusion

This study strongly demonstrates the causal relationship between serum albumin and reduced risk of atrial fibrillation through genetic methods, and reveals the key mediating role of two serum metabolites in this relationship. These findings not only provide a new perspective for our understanding of the role of serum albumin in atrial fibrillation, but also provide new ideas for the prevention and treatment strategies of atrial fibrillation.

Cardiac arrhythmogenesis: roles of ion channels and their functional modification

Cardiac arrhythmias cause significant morbidity and mortality and pose a major public health problem. They arise from disruptions in the normally orderly propagation of cardiac electrophysiological activation and recovery through successive cardiomyocytes in the heart. They reflect abnormalities in automaticity, initiation, conduction, or recovery in cardiomyocyte excitation. The latter properties are dependent on surface membrane electrophysiological mechanisms underlying the cardiac action potential. Their disruption results from spatial or temporal instabilities and heterogeneities in the generation and propagation of cellular excitation. These arise from abnormal function in their underlying surface membrane, ion channels, and transporters, as well as the interactions between them. The latter, in turn, form common regulatory targets for the hierarchical network of diverse signaling mechanisms reviewed here. In addition to direct molecular-level pharmacological or physiological actions on these surface membrane biomolecules, accessory, adhesion, signal transduction, and cytoskeletal anchoring proteins modify both their properties and localization. At the cellular level of excitation-contraction coupling processes, Ca2+ homeostatic and phosphorylation processes affect channel activity and membrane excitability directly or through intermediate signaling. Systems-level autonomic cellular signaling exerts both acute channel and longer-term actions on channel expression. Further upstream intermediaries from metabolic changes modulate the channels both themselves and through modifying Ca2+ homeostasis. Finally, longer-term organ-level inflammatory and structural changes, such as fibrotic and hypertrophic remodeling, similarly can influence all these physiological processes with potential pro-arrhythmic consequences. These normal physiological processes may target either individual or groups of ionic channel species and alter with particular pathological conditions. They are also potentially alterable by direct pharmacological action, or effects on longer-term targets modifying protein or cofactor structure, expression, or localization. Their participating specific biomolecules, often clarified in experimental genetically modified models, thus constitute potential therapeutic targets. The insights clarified by the physiological and pharmacological framework outlined here provide a basis for a recent modernized drug classification. Together, they offer a translational framework for current drug understanding. This would facilitate future mechanistically directed therapeutic advances, for which a number of examples are considered here. The latter are potentially useful for treating cardiac, in particular arrhythmic, disease.

mROS‑calcium feedback loop promotes lethal ventricular arrhythmias and sudden cardiac death in early myocardial ischemia

Lethal ventricular arrhythmia‑sudden cardiac death (LVA‑SCD) occurs frequently during the early stage of myocardial ischemia (MI). However, the mechanism underlying higher LVA‑SCD incidence is still poorly understood. The present study aimed to explore the role of mitochondrial reactive oxygen species (mROS) and Ca2+ crosstalk in promoting LVA‑SCD in early MI. RyR2 S2814A mice and their wild‑type littermates were used. MitoTEMPO was applied to scavenge mitochondrial ROS (mROS). Mice were subjected to severe MI and the occurrence of LVA‑SCD was evaluated. Levels of mitochondrial ROS and calcium (mitoCa2+), cytosolic ROS (cytoROS), and calcium (cytoCa2+), RyR2 Ser‑2814 phosphorylation, CaMKII Met‑282 oxidation, mitochondrial membrane potential (MMP), and glutathione/oxidized glutathione (GSH/GSSG) ratio in the myocardia were detected. Dynamic changes in mROS after hypoxia were investigated using H9c2 cells. Moreover, the myocardial phosphoproteome was analyzed to explore the related mechanisms facilitating mROS‑Ca2+ crosstalk and LVA‑SCD. There was a high incidence (~33.9%) of LVA‑SCD in early MI. Mice who underwent SCD displayed notably elevated levels of myocardial ROS and mROS, and the latter was validated in H9c2 cells. These mice also demonstrated overloads of cytoplasmic and mitochondrial Ca2+, decreased MMP and reduced GSH/GSSG ratio, upregulated RyR2‑S2814 phosphorylation and CaMKII‑M282 oxidation and transient hyperphosphorylation of mitochondrial proteomes in the myocardium. mROS‑specific scavenging by a mitochondria‑targeted antioxidant agent (MitoTEMPO) corrected these SCD‑induced alterations. S2814A mice with a genetically inactivated CaMKII phosphorylation site in RyR2 exhibited decreased overloads in cytoplasmic and mitochondrial Ca2+ and demonstrated similar effects as MitoTEMPO to correct SCD‑induced changes and prevent SCD post‑MI. The data confirmed crosstalk between mROS and Ca2+ in promoting LVA‑SCD. Therefore, we provided evidence that there is a higher incidence of LVA‑SCD in early MI, which may be attributed to a positive feedback loop between mROS and Ca2+ imbalance.

Cardiomyocyte electrophysiology and its modulation: current views and future prospects

Normal and abnormal cardiac rhythms are of key physiological and clinical interest. This introductory article begins from Sylvio Weidmann's key historic 1950s microelectrode measurements of cardiac electrophysiological activity and Singh & Vaughan Williams's classification of cardiotropic targets. It then proceeds to introduce the insights into cardiomyocyte function and its regulation that subsequently emerged and their therapeutic implications. We recapitulate the resulting view that surface membrane electrophysiological events underlying cardiac excitation and its initiation, conduction and recovery constitute the final common path for the cellular mechanisms that impinge upon this normal or abnormal cardiac electrophysiological activity. We then consider progress in the more recently characterized successive regulatory hierarchies involving Ca2+ homeostasis, excitation-contraction coupling and autonomic G-protein signalling and their often reciprocal interactions with the surface membrane events, and their circadian rhythms. Then follow accounts of longer-term upstream modulation processes involving altered channel expression, cardiomyocyte energetics and hypertrophic and fibrotic cardiac remodelling. Consideration of these developments introduces each of the articles in this Phil. Trans. B theme issue. The findings contained in these articles translate naturally into recent classifications of cardiac electrophysiological targets and drug actions, thereby encouraging future iterations of experimental cardiac electrophysiological discovery, and testing directed towards clinical management. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Small-conductance calcium-activated potassium channels in the heart: expression, regulation and pathological implications

Ca2+-activated K+ channels are critical to cellular Ca2+ homeostasis and excitability; they couple intracellular Ca2+ and membrane voltage change. Of these, the small, 4-14 pS, conductance SK channels include three, KCNN1-3 encoded, SK1/KCa2.1, SK2/KCa2.2 and SK3/KCa2.3, channel subtypes with characteristic, EC50 ∼ 10 nM, 40 pM, 1 nM, apamin sensitivities. All SK channels, particularly SK2 channels, are expressed in atrial, ventricular and conducting system cardiomyocytes. Pharmacological and genetic modification results have suggested that SK channel block or knockout prolonged action potential durations (APDs) and effective refractory periods (ERPs) particularly in atrial, but also in ventricular, and sinoatrial, atrioventricular node and Purkinje myocytes, correspondingly affect arrhythmic tendency. Additionally, mitochondrial SK channels may decrease mitochondrial Ca2+ overload and reactive oxygen species generation. SK channels show low voltage but marked Ca2+ dependences (EC50 ∼ 300-500 nM) reflecting their α-subunit calmodulin (CaM) binding domains, through which they may be activated by voltage-gated or ryanodine-receptor Ca2+ channel activity. SK function also depends upon complex trafficking and expression processes and associations with other ion channels or subunits from different SK subtypes. Atrial and ventricular clinical arrhythmogenesis may follow both increased or decreased SK expression through decreased or increased APD correspondingly accelerating and stabilizing re-entrant rotors or increasing incidences of triggered activity. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Mitochondrial Dysfunction in Cardiac Arrhythmias

Electrophysiological and structural disruptions in cardiac arrhythmias are closely related to mitochondrial dysfunction. Mitochondria are an organelle generating ATP, thereby satisfying the energy demand of the incessant electrical activity in the heart. In arrhythmias, the homeostatic supply-demand relationship is impaired, which is often accompanied by progressive mitochondrial dysfunction leading to reduced ATP production and elevated reactive oxidative species generation. Furthermore, ion homeostasis, membrane excitability, and cardiac structure can be disrupted through pathological changes in gap junctions and inflammatory signaling, which results in impaired cardiac electrical homeostasis. Herein, we review the electrical and molecular mechanisms of cardiac arrhythmias, with a particular focus on mitochondrial dysfunction in ionic regulation and gap junction action. We provide an update on inherited and acquired mitochondrial dysfunction to explore the pathophysiology of different types of arrhythmias. In addition, we highlight the role of mitochondria in bradyarrhythmia, including sinus node dysfunction and atrioventricular node dysfunction. Finally, we discuss how confounding factors, such as aging, gut microbiome, cardiac reperfusion injury, and electrical stimulation, modulate mitochondrial function and cause tachyarrhythmia.

Anti-malarial drugs: Mechanisms underlying their proarrhythmic effects

Malaria remains the leading cause of parasitic death in the world. Artemisinin resistance is an emerging threat indicating an imminent need for novel combination therapy. Given the key role of mass drug administration, it is pivotal that the safety of anti-malarial drugs is investigated thoroughly prior to widespread use. Cardiotoxicity, most prominently arrhythmic risk, has been a concern for anti-malarial drugs. We clarify the likely underlying mechanisms by which anti-malarial drugs predispose to arrhythmias. These relate to disruption of (1) action potential upstroke due to effects on the sodium currents, (2) action potential repolarisation due to effects on the potassium currents, (3) cellular calcium homeostasis, (4) mitochondrial function and reactive oxygen species production and (5) cardiac fibrosis. Together, these alterations promote arrhythmic triggers and substrates. Understanding these mechanisms is essential to assess the safety of these drugs, stratify patients based on arrhythmic risk and guide future anti-malarial drug development.

Evidence for the Benefits of Melatonin in Cardiovascular Disease

The pineal gland is a neuroendocrine gland which produces melatonin, a neuroendocrine hormone with critical physiological roles in the circadian rhythm and sleep-wake cycle. Melatonin has been shown to possess anti-oxidant activity and neuroprotective properties. Numerous studies have shown that melatonin has significant functions in cardiovascular disease, and may have anti-aging properties. The ability of melatonin to decrease primary hypertension needs to be more extensively evaluated. Melatonin has shown significant benefits in reducing cardiac pathology, and preventing the death of cardiac muscle in response to ischemia-reperfusion in rodent species. Moreover, melatonin may also prevent the hypertrophy of the heart muscle under some circumstances, which in turn would lessen the development of heart failure. Several currently used conventional drugs show cardiotoxicity as an adverse effect. Recent rodent studies have shown that melatonin acts as an anti-oxidant and is effective in suppressing heart damage mediated by pharmacologic drugs. Therefore, melatonin has been shown to have cardioprotective activity in multiple animal and human studies. Herein, we summarize the most established benefits of melatonin in the cardiovascular system with a focus on the molecular mechanisms of action.

Mechanisms Underlying Antiarrhythmic Properties of Cardioprotective Agents Impacting Inflammation and Oxidative Stress

The prevention of cardiac life-threatening ventricular fibrillation and stroke-provoking atrial fibrillation remains a serious global clinical issue, with ongoing need for novel approaches. Numerous experimental and clinical studies suggest that oxidative stress and inflammation are deleterious to cardiovascular health, and can increase heart susceptibility to arrhythmias. It is quite interesting, however, that various cardio-protective compounds with antiarrhythmic properties are potent anti-oxidative and anti-inflammatory agents. These most likely target the pro-arrhythmia primary mechanisms. This review and literature-based analysis presents a realistic view of antiarrhythmic efficacy and the molecular mechanisms of current pharmaceuticals in clinical use. These include the sodium-glucose cotransporter-2 inhibitors used in diabetes treatment, statins in dyslipidemia and naturally protective omega-3 fatty acids. This approach supports the hypothesis that prevention or attenuation of oxidative and inflammatory stress can abolish pro-arrhythmic factors and the development of an arrhythmia substrate. This could prove a powerful tool of reducing cardiac arrhythmia burden.

Relevance of mitochondrial oxidative stress to arrhythmias: Innovative concepts to target treatments

Cardiac arrhythmia occurs frequently worldwide, and in severe cases can be fatal. Mitochondria are the power plants of cardiomyocytes. In recent studies, mitochondria under certain stimuli produced excessive reactive oxygen species (ROS), which affect the normal function of cardiomyocytes through ion channels and related proteins. Mitochondrial oxidative stress (MOS) plays a key role in diseases with multifactorial etiopathogenesis, such as arrhythmia; MOS can lead to arrhythmias such as atrial fibrillation and ventricular tachycardia. This review discusses the mechanisms of arrhythmias caused by MOS, particularly of ROS produced by mitochondria. MOS can cause arrhythmias by affecting the activities of Ca²⁺-related proteins, the mitochondrial permeability transition pore protein, connexin 43, hyperpolarization-activated cyclic nucleotide-gated potassium channel 4, and ion channels. Based on these mechanisms, we discuss possible new treatments for arrhythmia. Targeted treatments focusing on mitochondria may reduce the progression of arrhythmias, as well as the occurrence of severe arrhythmias, and may be effective for personalized disease prevention.

Emerging Role of Mitophagy in the Heart: Therapeutic Potentials to Modulate Mitophagy in Cardiac Diseases

The normal function of the mitochondria is crucial for most tissues especially for those that demand a high energy supply. Emerging evidence has pointed out that healthy mitochondrial function is closely associated with normal heart function. When these processes fail to repair the damaged mitochondria, cells initiate a removal process referred to as mitophagy to clear away defective mitochondria. In cardiomyocytes, mitophagy is closely associated with metabolic activity, cell differentiation, apoptosis, and other physiological processes involved in major phenotypic alterations. Mitophagy alterations may contribute to detrimental or beneficial effects in a multitude of cardiac diseases, indicating potential clinical insights after a close understanding of the mechanisms. Here, we discuss the current opinions of mitophagy in the progression of cardiac diseases, such as ischemic heart disease, diabetic cardiomyopathy, cardiac hypertrophy, heart failure, and arrhythmia, and focus on the key molecules and related pathways involved in the regulation of mitophagy. We also discuss recently reported approaches targeting mitophagy in the therapy of cardiac diseases.

Mitochondrial Dysfunction Increases Arrhythmic Triggers and Substrates; Potential Anti-arrhythmic Pharmacological Targets

Incidence of cardiac arrhythmias increases significantly with age. In order to effectively stratify arrhythmic risk in the aging population it is crucial to elucidate the relevant underlying molecular mechanisms. The changes underlying age-related electrophysiological disruption appear to be closely associated with mitochondrial dysfunction. Thus, the present review examines the mechanisms by which age-related mitochondrial dysfunction promotes arrhythmic triggers and substrate. Namely, via alterations in plasmalemmal ionic currents (both sodium and potassium), gap junctions, cellular Ca²⁺ homeostasis, and cardiac fibrosis. Stratification of patients' mitochondrial function status permits application of appropriate anti-arrhythmic therapies. Here, we discuss novel potential anti-arrhythmic pharmacological interventions that specifically target upstream mitochondrial function and hence ameliorates the need for therapies targeting downstream changes which have constituted traditional antiarrhythmic therapy.

Cardiac Connexin-43 Hemichannels and Pannexin1 Channels: Provocative Antiarrhythmic Targets

Cardiac connexin-43 (Cx43) creates gap junction channels (GJCs) at intercellular contacts and hemi-channels (HCs) at the peri-junctional plasma membrane and sarcolemmal caveolae/rafts compartments. GJCs are fundamental for the direct cardiac cell-to-cell transmission of electrical and molecular signals which ensures synchronous myocardial contraction. The HCs and structurally similar pannexin1 (Panx1) channels are active in stressful conditions. These channels are essential for paracrine and autocrine communication through the release of ions and signaling molecules to the extracellular environment, or for uptake from it. The HCs and Panx1 channel-opening profoundly affects intracellular ionic homeostasis and redox status and facilitates via purinergic signaling pro-inflammatory and pro-fibrotic processes. These conditions promote cardiac arrhythmogenesis due to the impairment of the GJCs and selective ion channel function. Crosstalk between GJCs and HCs/Panx1 channels could be crucial in the development of arrhythmogenic substrates, including fibrosis. Despite the knowledge gap in the regulation of these channels, current evidence indicates that HCs and Panx1 channel activation can enhance the risk of cardiac arrhythmias. It is extremely challenging to target HCs and Panx1 channels by inhibitory agents to hamper development of cardiac rhythm disorders. Progress in this field may contribute to novel therapeutic approaches for patients prone to develop atrial or ventricular fibrillation.

Metformin preferentially provides neuroprotection following cardiac ischemia/reperfusion in non-diabetic rats

Following acute myocardial infarction, re-establishment of coronary perfusion aggravates further injuries in the heart and remote organs including the brain as a consequence of ischemia/reperfusion (I/R) injury. Since pretreatment with metformin attenuated both cardiac and cerebral I/R injury via AMP-activated protein kinase (AMPK) pathways, we hypothesized that metformin given after ischemia mitigates both cardiac and brain pathologies following cardiac I/R. Male Wistar rats were subjected to either cardiac I/R (30 min-ischemia/120 min-reperfusion; n = 30) or sham operation (n = 5). Metformin 200 mg/kg was given intravenously to the cardiac I/R group (n = 10/group), either during ischemia (D-MET) or at the onset of reperfusion (R-MET). Left ventricular ejection fraction (LVEF) and arrhythmia scores were determined. The heart and brain tissues were collected to determine the extent of injury, mitochondrial function, and apoptosis. Additionally, microglial morphology, Alzheimer's proteins, and dendritic spine density were determined in the brain. Cardiac I/R led to not only reduced LVEF, cardiac mitochondrial dysfunction, and arrhythmias, but also brain mitochondrial dysfunction, apoptosis, Alzheimer's protein aggregation, microglial activation, and dendritic spine loss. A single dose of metformin did not alter p-AMPK/AMPK in both organs. In the heart, impaired LVEF, arrhythmias, infarct size expansion, mitochondrial dysfunction, and apoptosis were not alleviated. On the contrary, metformin attenuated brain mitochondrial dysfunction, apoptosis, and Alzheimer's protein levels. Microglial morphology and dendritic spine density were additionally preserved in D-MET group. In conclusion, metformin given during ischemia preferentially provides neuroprotection against brain mitochondrial dysfunction, apoptosis, microglial activation, and dendritic spine loss in an AMPK-independent manner following cardiac I/R injury.

The debate continues - What is the role of MCU and mitochondrial calcium uptake in the heart?

Since the identification of the mitochondrial calcium uniporter (MCU) in 2011, several studies utilizing genetic models have attempted to decipher the role of mitochondrial calcium uptake in cardiac physiology. Confounding results in various mutant mouse models have led to an ongoing debate regarding the function of MCU in the heart. In this review, we evaluate and discuss the totality of evidence for mitochondrial calcium uptake in the cardiac stress response and highlight recent reports that implicate MCU in the control of homeostatic cardiac metabolism and function. This review concludes with a discussion of current gaps in knowledge and remaining experiments to define how MCU contributes to contractile function, cell death, metabolic regulation, and heart failure progression.

Pro-Arrhythmic Signaling of Thyroid Hormones and Its Relevance in Subclinical Hyperthyroidism

A perennial task is to prevent the occurrence and/or recurrence of most frequent or life-threatening cardiac arrhythmias such as atrial fibrillation (AF) and ventricular fibrillation (VF). VF may be lethal in cases without an implantable cardioverter defibrillator or with failure of this device. Incidences of AF, even the asymptomatic ones, jeopardize the patient's life due to its complication, notably the high risk of embolic stroke. Therefore, there has been a growing interest in subclinical AF screening and searching for novel electrophysiological and molecular markers. Considering the worldwide increase in cases of thyroid dysfunction and diseases, including thyroid carcinoma, we aimed to explore the implication of thyroid hormones in pro-arrhythmic signaling in the pathophysiological setting. The present review provides updated information about the impact of altered thyroid status on both the occurrence and recurrence of cardiac arrhythmias, predominantly AF. Moreover, it emphasizes the importance of both thyroid status monitoring and AF screening in the general population, as well as in patients with thyroid dysfunction and malignancies. Real-world data on early AF identification in relation to thyroid function are scarce. Even though symptomatic AF is rare in patients with thyroid malignancies, who are under thyroid suppressive therapy, clinicians should be aware of potential interaction with asymptomatic AF. It may prevent adverse consequences and improve the quality of life. This issue may be challenging for an updated registry of AF in clinical practice. Thyroid hormones should be considered a biomarker for cardiac arrhythmias screening and their tailored management because of their multifaceted cellular actions.

Update on antiarrhythmic drug pharmacology

Cardiac arrhythmias constitute a major public health problem. Pharmacological intervention remains mainstay to their clinical management. This, in turn, depends upon systematic drug classification schemes relating their molecular, cellular, and systems effects to clinical indications and therapeutic actions. This approach was first pioneered in the 1960s Vaughan-Williams classification. Subsequent progress in cardiac electrophysiological understanding led to a lag between the fundamental science and its clinical translation, partly addressed by The working group of the European Society of Cardiology (1991), which, however, did not emerge with formal classifications. We here utilize the recent Revised Oxford Classification Scheme to review antiarrhythmic drug pharmacology. We survey drugs and therapeutic targets offered by the more recently characterized ion channels, transporters, receptors, intracellular Ca²⁺ handling, and cell signaling molecules. These are organized into their strategic roles in cardiac electrophysiological function. Following analysis of the arrhythmic process itself, we consider (a) pharmacological agents directly targeting membrane function, particularly the Na⁺ and K⁺ ion channels underlying depolarizing and repolarizing events in the cardiac action potential. (b) We also consider agents that modify autonomic activity that, in turn, affects both the membrane and (c) the Ca²⁺ homeostatic and excitation-contraction coupling processes linking membrane excitation to contractile activation. Finally, we consider (d) drugs acting on more upstream energetic and structural remodeling processes currently the subject of clinical trials. Such systematic correlations of drug actions and arrhythmic mechanisms at different molecular to systems levels of cardiac function will facilitate current and future antiarrhythmic therapy.

Molecular basis of arrhythmic substrate in ageing murine peroxisome proliferator-activated receptor γ co-activator deficient hearts modelling mitochondrial dysfunction

INTRODUCTION

Ageing and chronic metabolic disorders are associated with mitochondrial dysfunction and cardiac pro-arrhythmic phenotypes which were recently attributed to slowed atrial and ventricular action potential (AP) conduction in peroxisome proliferator-activated receptor γ co-activator deficient (Pgc-1β-/-) mice.

METHODS

We compared expression levels of voltage-gated Na+ channel (NaV1.5) and gap junction channels, Connexins 40 and 43 (Cx40 and Cx43) in the hearts of young and old, and wild-type (WT) and Pgc-1β-/- mice. This employed Western blotting (WB) for NaV1.5, Cx40 and Cx43 in atrial/ventricular tissue lysates, and immunofluorescence (IF) from Cx43 was explored in tissue sections. Results were analysed using two-way analysis of variance (ANOVA) for independent/interacting effects of age and genotype.

RESULTS

In atria, increased age and Pgc-1β-/- genotype each independently decreased both Cx40 and Cx43 expression without interacting effects. In IF experiments, both age and Pgc-1β deletion independently reduced Cx43 expression. In ventricles, age and genotype exerted interacting effects in WB studies of NaV1.5 expression. Young Pgc-1β-/- then showed greater NaV1.5 expression than young WT ventricles. However, neither age nor Pgc-1β deletion affected Cx43 expression, independently or through interacting effects in both WB and IF studies.

CONCLUSION

Similar pro-arrhythmic atrial/ventricular phenotypes arise in aged/Pgc-1β-/- from differing contributions of altered protein expression and functional effects that may arise from multiple acute mechanisms.

Association Between Antiarrhythmic, Electrophysiological, and Antioxidative Effects of Melatonin in Ischemia/Reperfusion

Melatonin is assumed to confer cardioprotective action via antioxidative properties. We evaluated the association between ventricular tachycardia and/or ventricular fibrillation (VT/VF) incidence, oxidative stress, and myocardial electrophysiological parameters in experimental ischemia/reperfusion under melatonin treatment. Melatonin was given to 28 rats (10 mg/kg/day, orally, for 7 days) and 13 animals received placebo. In the anesthetized animals, coronary occlusion was induced for 5 min followed by reperfusion with recording of unipolar electrograms from ventricular epicardium with a 64-lead array. Effects of melatonin on transmembrane potentials were studied in ventricular preparations of 7 rats in normal and “ischemic” conditions. Melatonin treatment was associated with lower VT/VF incidence at reperfusion, shorter baseline activation times (ATs), and activation-repolarization intervals and more complete recovery of repolarization times (RTs) at reperfusion (less baseline-reperfusion difference, ΔRT) (p < 0.05). Superoxide dismutase (SOD) activity was higher in the treated animals and associated with ΔRT (p = 0.001), whereas VT/VF incidence was associated with baseline ATs (p = 0.020). In vitro, melatonin led to a more complete restoration of action potential durations and resting membrane potentials at reoxygenation (p < 0.05). Thus, the antioxidative properties of melatonin were associated with its influence on repolarization duration, whereas the melatonin-related antiarrhythmic effect was associated with its oxidative stress-independent action on ventricular activation.

Atrial Transcriptional Profiles of Molecular Targets Mediating Electrophysiological Function in Aging and Pgc-1β Deficient Murine Hearts

BACKGROUND

Deficiencies in the transcriptional co-activator, peroxisome proliferative activated receptor, gamma, coactivator-1β are implicated in deficient mitochondrial function. The latter accompanies clinical conditions including aging, physical inactivity, obesity, and diabetes. Recent electrophysiological studies reported that Pgc-1β^-/- mice recapitulate clinical age-dependent atrial pro-arrhythmic phenotypes. They implicated impaired chronotropic responses to adrenergic challenge, compromised action potential (AP) generation and conduction despite normal AP recovery timecourses and background resting potentials, altered intracellular Ca²⁺ homeostasis, and fibrotic change in the observed arrhythmogenicity.

OBJECTIVE

We explored the extent to which these age-dependent physiological changes correlated with alterations in gene transcription in murine Pgc-1β^-/- atria.

METHODS AND RESULTS

RNA isolated from murine atrial tissue samples from young (12-16 weeks) and aged (>52 weeks of age), wild type (WT) and Pgc-1β^-/- mice were studied by pre-probed quantitative PCR array cards. We examined genes encoding sixty ion channels and other strategic atrial electrophysiological proteins. Pgc-1β^-/- genotype independently reduced gene transcription underlying Na⁺-K⁺-ATPase, sarcoplasmic reticular Ca²⁺-ATPase, background K⁺ channel and cholinergic receptor function. Age independently decreased Na⁺-K⁺-ATPase and fibrotic markers. Both factors interacted to alter Hcn4 channel activity underlying atrial automaticity. However, neither factor, whether independently or interactively, affected transcription of cardiac Na⁺, voltage-dependent K⁺ channels, surface or intracellular Ca²⁺ channels. Nor were gap junction channels, β-adrenergic receptors or transforming growth factor-β affected.

CONCLUSION

These findings limit the possible roles of gene transcriptional changes in previously reported age-dependent pro-arrhythmic electrophysiologial changes observed in Pgc-1β^-/- atria to an altered Ca²⁺-ATPase (Atp2a2) expression. This directly parallels previously reported arrhythmic mechanism associated with p21-activated kinase type 1 deficiency. This could add to contributions from the direct physiological outcomes of mitochondrial dysfunction, whether through reactive oxygen species (ROS) production or altered Ca²⁺ homeostasis.

Gene and Protein Expression Profile of Selected Molecular Targets Mediating Electrophysiological Function in Pgc-1α Deficient Murine Atria

Increases in the prevalence of obesity, insulin resistance, and metabolic syndrome has led to the increase of atrial fibrillation (AF) cases in the developed world. These AF risk factors are associated with mitochondrial dysfunction, previously modelled using peroxisome proliferator activated receptor-γ (PPARγ) coactivator-1 (Pgc-1)-deficient murine cardiac models. We explored gene and protein expression profiles of selected molecular targets related to electrophysiological function in murine Pgc-1α^−/− atria. qPCR analysis surveyed genes related to Na⁺-K⁺-ATPase, K⁺ conductance, hyperpolarisation-activated cyclic nucleotide-gated (Hcn), Na⁺ channels, Ca²⁺ channels, and indicators for adrenergic and cholinergic receptor modulation. Western blot analysis for molecular targets specific to conduction velocity (Na_v1.5 channel and gap junctions) was performed. Transcription profiles revealed downregulation of molecules related to Na⁺-K⁺-ATPase transport, Hcn-dependent pacemaker function, Na⁺ channel-dependent action potential activation and propagation, Ca²⁺ current generation, calsequestrin-2 dependent Ca²⁺ homeostasis, and adrenergic α_1D dependent protection from hypertrophic change. Na_v1.5 channel protein expression but not gap junction expression was reduced in Pgc-1α^−/− atria compared to WT. Na_v1.5 reduction reflects corresponding reduction in its gene expression profile. These changes, as well as the underlying Pgc-1α^−/− alteration, suggest potential pharmacological targets directed towards either upstream PGC-1 signalling mechanisms or downstream ion channel changes.

The Mitochondrial Translocator Protein and the Emerging Link Between Oxidative Stress and Arrhythmias in the Diabetic Heart

The mitochondrial translocator protein (TSPO) is a key outer mitochondrial membrane protein that regulates the activity of energy-dissipating mitochondrial channels in response to oxidative stress. In this article, we provide an overview of the role of TSPO in the systematic amplification of reactive oxygen species (ROS) through an autocatalytic process known as ROS-induced ROS-release (RIRR). We describe how this TSPO-driven process destabilizes the mitochondrial membrane potential leading to electrical instability at the cellular and whole heart levels. Finally, we provide our perspective on the role of TSPO in the pathophysiology of diabetes, in general and diabetes-related arrhythmias, in particular.

Age-dependent electrocardiographic changes in Pgc-1β deficient murine hearts

Increasing evidence implicates chronic energetic dysfunction in human cardiac arrhythmias. Mitochondrial impairment through Pgc-1β knockout is known to produce a murine arrhythmic phenotype. However, the cumulative effect of this with advancing age and its electrocardiographic basis have not been previously studied. Young (12-16 weeks) and aged (>52 weeks), wild type (WT) (n = 5 and 8) and Pgc-1β^-/- (n = 9 and 6), mice were anaesthetised and used for electrocardiographic (ECG) recordings. Time intervals separating successive ECG deflections were analysed for differences between groups before and after β1-adrenergic (intraperitoneal dobutamine 3 mg/kg) challenge. Heart rates before dobutamine challenge were indistinguishable between groups. The Pgc-1β^-/- genotype however displayed compromised nodal function in response to adrenergic challenge. This manifested as an impaired heart rate response suggesting a functional defect at the level of the sino-atrial node, and a negative dromotropic response suggesting an atrioventricular conduction defect. Incidences of the latter were most pronounced in the aged Pgc-1β^-/- mice. Moreover, Pgc-1β^-/- mice displayed electrocardiographic features consistent with the existence of a pro-arrhythmic substrate. Firstly, ventricular activation was prolonged in these mice consistent with slowed action potential conduction and is reported here for the first time. Additionally, Pgc-1β^-/- mice had shorter repolarisation intervals. These were likely attributable to altered K⁺ conductance properties, ultimately resulting in a shortened QT_c interval, which is also known to be associated with increased arrhythmic risk. ECG analysis thus yielded electrophysiological findings bearing on potential arrhythmogenicity in intact Pgc-1β^-/- systems in widespread cardiac regions.

Omega-3 Index and Anti-Arrhythmic Potential of Omega-3 PUFAs

Omega-3 polyunsaturated fatty acids (PUFAs), namely eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are permanent subjects of interest in relation to the protection of cardiovascular health and the prevention of the incidence of both ventricular and atrial arrhythmias. The purpose of this updated review is to focus on the novel cellular and molecular effects of omega-3 PUFAs, in the context of the mechanisms and factors involved in the development of cardiac arrhythmias; to provide results of the most recent studies on the omega-3 PUFA anti-arrhythmic efficacy and to discuss the lack of the benefit in relation to omega-3 PUFA status. The evidence is in the favor of omega-3 PUFA acute and long-term treatment, perhaps with mitochondria-targeted antioxidants. However, for a more objective evaluation of the anti-arrhythmic potential of omega-3 PUFAs in clinical trials, it is necessary to monitor the basal pre-interventional omega-3 status of individuals, i.e., red blood cell content, omega-3 index and free plasma levels. In the view of evidence-based medicine, it seems to be crucial to aim to establish new approaches in the prevention of cardiac arrhythmias and associated morbidity and mortality that comes with these conditions.

Pro-arrhythmic atrial phenotypes in incrementally paced murine Pgc1β-/- hearts: effects of age

New Findings

What is the central question of this study?

Can we experimentally replicate atrial pro‐arrhythmic phenotypes associated with important chronic clinical conditions, including physical inactivity, obesity, diabetes mellitus and metabolic syndrome, compromising mitochondrial function, and clarify their electrophysiological basis?

What is the main finding and its importance?

Electrocardiographic and intracellular cardiomyocyte recording at progressively incremented pacing rates demonstrated age‐dependent atrial arrhythmic phenotypes in Langendorff‐perfused murine Pgc1β^−/− hearts for the first time. We attributed these to compromised action potential conduction and excitation wavefronts, whilst excluding alterations in recovery properties or temporal electrophysiological instabilities, clarifying these pro‐arrhythmic changes in chronic metabolic disease.

Atrial arrhythmias, most commonly manifesting as atrial fibrillation, represent a major clinical problem. The incidence of atrial fibrillation increases with both age and conditions associated with energetic dysfunction. Atrial arrhythmic phenotypes were compared in young (12–16 week) and aged (>52 week) wild‐type (WT) and peroxisome proliferative activated receptor, gamma, coactivator 1 beta (Ppargc1b)‐deficient (Pgc1β^−/−) Langendorff‐perfused hearts, previously used to model mitochondrial energetic disorder. Electrophysiological explorations were performed using simultaneous whole‐heart ECG and intracellular atrial action potential (AP) recordings. Two stimulation protocols were used: an S1S2 protocol, which imposed extrasystolic stimuli at successively decremented intervals following regular pulse trains; and a regular pacing protocol at successively incremented frequencies. Aged Pgc1β^−/− hearts showed greater atrial arrhythmogenicity, presenting as atrial tachycardia and ectopic activity. Maximal rates of AP depolarization (dV/dt_max) were reduced in Pgc1β^−/− hearts. Action potential latencies were increased by the Pgc1β^−/− genotype, with an added interactive effect of age. In contrast, AP durations to 90% recovery (APD₉₀) were shorter in Pgc1β^−/− hearts despite similar atrial effective recovery periods amongst the different groups. These findings accompanied paradoxical decreases in the incidence and duration of alternans in the aged and Pgc1β^−/− hearts. Limiting slopes of restitution curves of APD₉₀ against diastolic interval were correspondingly reduced interactively by Pgc1β^−/− genotype and age. In contrast, reduced AP wavelengths were associated with Pgc1β^−/− genotype, both independently and interacting with age, through the basic cycle lengths explored, with the aged Pgc1β^−/− hearts showing the shortest wavelengths. These findings thus implicate AP wavelength in possible mechanisms for the atrial arrhythmic changes reported here.

Effects of ageing on pro-arrhythmic ventricular phenotypes in incrementally paced murine Pgc-1β -/- hearts

A range of chronic clinical conditions accompany cardiomyocyte energetic dysfunction and constitute independent risk factors for cardiac arrhythmia. We investigated pro-arrhythmic and arrhythmic phenotypes in energetically deficient C57BL mice with genetic ablation of the mitochondrial promoter peroxisome proliferator-activated receptor-γ coactivator-1β (Pgc-1β), a known model of ventricular arrhythmia. Pro-arrhythmic and cellular action potential (AP) characteristics were compared in intact Langendorff-perfused hearts from young (12–16 week) and aged (> 52 week), wild-type (WT) and Pgc-1β^−/− mice. Simultaneous electrocardiographic and intracellular microelectrode recordings were made through successive trains of 100 regular stimuli at progressively incremented heart rates. Aged Pgc-1β^−/− hearts displayed an increased incidence of arrhythmia compared to other groups. Young and aged Pgc-1β^−/− hearts showed higher incidences of alternans in both AP activation (maximum AP upshoot velocity (dV/dt)_max and latency), recovery (action potential duration (APD₉₀) and resting membrane potential (RMP) characteristics compared to WT hearts. This was particularly apparent at lower pacing frequencies. These findings accompanied reduced (dV/dt)_max and increased AP latency values in the Pgc-1β^−/− hearts. APs observed prior to termination of the protocol showed lower (dV/dt)_max and longer AP latencies, but indistinguishable APD₉₀ and RMPs in arrhythmic compared to those in non-arrhythmic hearts. APD restitution analysis showed that Pgc-1β^−/− and WT hearts showed similar limiting gradients. However, Pgc-1β^−/− hearts had shortened plateau AP wavelengths, particularly in aged Pgc-1β^−/− hearts. Pgc-1β^−/− hearts therefore show pro-arrhythmic instabilities attributable to altered AP conduction and activation rather than recovery characteristics.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

Xanthohumol Modulates Calcium Signaling in Rat Ventricular Myocytes: Possible Antiarrhythmic Properties

Cardiac arrhythmia is a major cause of mortality in cardiovascular pathologies. A host of drugs targeted to sarcolemmal Na⁺, Ca²⁺, and K⁺ channels has had limited success clinically. Recently, Ca²⁺ signaling has been target of pharmacotherapy based on finding that leaky ryanodine receptors elevate local Ca²⁺ concentrations causing membrane depolarizations that trigger arrhythmias. In this study, we report that xanthohumol, an antioxidant extracted from hops showing therapeutic effects in other pathologies, suppresses aberrant ryanodine receptor Ca²⁺ release. The effects of xanthohumol (5-1000 nM) on Ca²⁺ signaling pathways were probed in isolated rat ventricular myocytes incubated with Fluo-4 AM using the perforated patch-clamp technique. We found that 5-50 nM xanthohumol reduced the frequency of spontaneously occurring Ca²⁺ sparks (>threefold) and Ca²⁺ waves in control myocytes and in cells subjected to Ca²⁺ overload caused by the following: 1) exposure to low K⁺ solutions, 2) periods of high frequency electrical stimulation, 3) exposures to isoproterenol, or 4) caffeine. At room temperatures, 50-100 nM xanthohumol reduced the rate of relaxation of electrically- or caffeine-triggered Ca²⁺transients, without suppressing I_Ca, but this effect was small and reversed by isoproterenol at physiologic temperatures. Xanthohumol also suppressed the Ca²⁺ content of the SR and its rate of recirculation. The stabilizing effects of xanthohumol on the frequency of spontaneously triggered Ca²⁺ sparks and waves combined with its antioxidant properties, and lack of significant effects on Na⁺ and Ca²⁺ channels, may provide this compound with clinically desirable antiarrhythmic properties.

A technical review of optical mapping of intracellular calcium within myocardial tissue

Optical mapping of Ca(2+)-sensitive fluorescence probes has become an extremely useful approach and adopted by many cardiovascular research laboratories to study a spectrum of myocardial physiology and disease conditions. Optical mapping data are often displayed as detailed pseudocolor images, providing unique insight for interpreting mechanisms of ectopic activity, action potential and Ca(2+) transient alternans, tachycardia, and fibrillation. Ca(2+)-sensitive fluorescent probes and optical mapping systems continue to evolve in the ongoing effort to improve therapies that ease the growing worldwide burden of cardiovascular disease. In this technical review we provide an updated overview of conventional approaches for optical mapping of Cai (2+) within intact myocardium. In doing so, a brief history of Cai (2+) probes is provided, and nonratiometric and ratiometric Ca(2+) probes are discussed, including probes for imaging sarcoplasmic reticulum Ca(2+) and probes compatible with potentiometric dyes for dual optical mapping. Typical measurements derived from optical Cai (2+) signals are explained, and the analytics used to compute them are presented. Last, recent studies using Cai (2+) optical mapping to study arrhythmias, heart failure, and metabolic perturbations are summarized.

Effects of salvianolic acid B on liver mitochondria of rats with nonalcoholic steatohepatitis

AIM: To investigate the effects of salvianolic acid B (Sal B) on the morphological characteristics and functions of liver mitochondria of rats with nonalcoholic steatohepatitis (NASH).

METHODS: A total of 60 male Sprague-Dawley rats were randomly divided into three groups: (1) a normal group fed a normal diet; (2) an NASH model group; and (3) a Sal B-treated group fed a high-fat diet. Two rats from each group were executed at the end of the 12^th week to detect pathological changes. The rats in the Sal B-treated group were gavaged with 20 mL/kg Sal B (1 mg/mL) daily. The model group received an equal volume of distilled water as a control. At the end of the 24^th weekend, the remaining rats were executed. Serum biochemical parameters and liver histological characteristics were observed. Malondialdehyde (MDA) and superoxide dismutase (SOD) in the liver were determined. Protein expression of CytC and caspase-3 was determined by immunohistochemistry. The mRNA transcripts of mitofusin-2 (Mfn2) and NF-κB in the liver tissue were detected by real-time PCR. Mitochondrial membrane potential was detected using a fluorescence spectrophotometer. Mitochondrial respiratory function was detected using a Clark oxygen electrode.

RESULTS: The model group showed significantly higher ALT, AST, TG, TC and MDA but significantly lower SOD than the normal group. In the model group, the histological characteristics of inflammation and steatosis were also evident; mitochondrial swelling and crest were shortened or even disappeared. CytC (18.46 ± 1.21 vs 60.01 ± 3.43, P < 0.01) and caspase-3 protein expression (30.26 ± 2.56 vs 83.31 ± 5.12, P < 0.01) increased significantly. The mRNA expression of NF-κB increased (0.81 ± 0.02 vs 0.91 ± 0.03, P < 0.05), whereas the mRNA expression of Mfn2 decreased (1.65 ± 0.31 vs 0.83 ± 0.16, P < 0.05). Mitochondrial membrane potential also decreased and breathing of rats was weakened. Steatosis and inflammation degrees in the treatment group were significantly alleviated compared with those of the model group. In the treatment group, mitochondrial swelling was alleviated. CytC (60.01 ± 3.43 vs 30.52 ± 2.01, P < 0.01) and caspase-3 protein expression (83.31 ± 5.12 vs 40.15 ± 3.26, P < 0.01) significantly decreased. The mRNA expression of NF-κB also decreased (0.91 ± 0.03 vs 0.74 ± 0.02, P < 0.01), whereas the mRNA expression of Mfn2 increased (0.83 ± 0.16 vs 1.35 ± 0.23, P < 0.01). Mitochondrial membrane potential increased and respiratory function was enhanced.

CONCLUSION: Sal B can treat NASH by protecting the morphological characteristics and functions of liver mitochondria, regulating lipid metabolism, controlling oxidative stress and lipid peroxidation and inhibiting apoptosis.

β3-Adrenoceptor Impairs Mitochondrial Biogenesis and Energy Metabolism During Rapid Atrial Pacing-Induced Atrial Fibrillation

BACKGROUND

The β3-adrenoceptor (β3-AR) is implicated in cardiac remodeling. Since metabolic dysfunction due to loss of mitochondria plays an important role in heart diseases, we examined the effects of β3-AR on mitochondrial biogenesis and energy metabolism in atrial fibrillation (AF).

METHODS

Atrial fibrillation was created by rapid atrial pacing in adult rabbits. Rabbits were randomly divided into 4 groups: control, pacing (P7), β3-AR antagonist (L748337), and β3-AR agonist (BRL37344) groups. Atrial effective refractory period (AERP) and AF induction rate were measured. Atrial concentrations of adenine nucleotides and phosphocreatine were quantified through high-performance liquid chromatography. Mitochondrial DNA content was determined. Real-time polymerase chain reaction and Western blot were used to examine the expression levels of signaling intermediates related to mitochondrial biogenesis.

RESULTS

After pacing for 7 days, β3-AR was significantly upregulated, AERP was reduced, and the AF induction rate was increased. The total adenine nucleotides pool was significantly reduced due to the decrease in adenosine triphosphate (ATP). The P7 group showed decreased activity of F0F1-ATPase. Mitochondrial DNA content was decreased and mitochondrial respiratory chain subunits were downregulated after pacing. Furthermore, expression of transcription factors involved in mitochondrial biogenesis, including peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α), nuclear respiratory factor 1 (NRF-1), and mitochondrial transcription factor A (Tfam), was lower in the P7 group in response to β3-AR activation. Further stimulation of β3-AR with BRL37344 exacerbated these effects, together with a significant decrease in the levels of phosphocreatine. In contrast, inhibition of β3-AR with L748337 partially restored mitochondrial biogenesis and energy metabolism of atria in the paced rabbits.

CONCLUSION

The activation of β3-AR contributes to atrial metabolic remodeling via transcriptional downregulation of PGC-1α/NRF-1/Tfam pathway that are involved in mitochondrial biogenesis, which ultimately perturbs mitochondrial function in rapid pacing-induced AF. The β3-AR is therefore a potential novel therapeutic target for the treatment or prevention of AF.

Effect of sodium nitrite on ischaemia and reperfusion-induced arrhythmias in anaesthetized dogs: is protein S-nitrosylation involved?

Background and Purpose

To provide evidence for the protective role of inorganic nitrite against acute ischaemia and reperfusion-induced ventricular arrhythmias in a large animal model.

Experimental Approach

Dogs, anaesthetized with chloralose and urethane, were administered intravenously with sodium nitrite (0.2 µmolkg^-1min^-1) in two protocols. In protocol 1 nitrite was infused 10 min prior to and during a 25 min occlusion of the left anterior descending (LAD) coronary artery (NaNO₂-PO; n = 14), whereas in protocol 2 the infusion was started 10 min prior to reperfusion of the occluded vessel (NaNO₂-PR; n = 12). Control dogs (n = 15) were infused with saline and subjected to the same period of ischaemia and reperfusion. Severities of ischaemia and ventricular arrhythmias, as well as changes in plasma nitrate/nitrite (NOx) levels in the coronary sinus blood, were assessed throughout the experiment. Myocardial superoxide and nitrotyrosine (NT) levels were determined during reperfusion. Changes in protein S-nitrosylation (SNO) and S-glutathionylation were also examined.

Key Results

Compared with controls, sodium nitrite administered either pre-occlusion or pre-reperfusion markedly suppressed the number and severity of ventricular arrhythmias during occlusion and increased survival (0% vs. 50 and 92%) upon reperfusion. There were also significant decreases in superoxide and NT levels in the nitrite treated dogs. Compared with controls, increased SNO was found only in NaNO₂-PR dogs, whereas S-glutathionylation occurred primarily in NaNO₂-PO dogs.

Conclusions

Intravenous infusion of nitrite profoundly reduced the severity of ventricular arrhythmias resulting from acute ischaemia and reperfusion in anaesthetized dogs. This effect, among several others, may result from an NO-mediated reduction in oxidative stress, perhaps through protein SNO and/or S-glutathionylation.

Mitochondrial depolarization and electrophysiological changes during ischemia in the rabbit and human heart

Instability of the inner mitochondrial membrane potential (ΔΨm) has been implicated in electrical dysfunction, including arrhythmogenesis during ischemia-reperfusion. Monitoring ΔΨm has led to conflicting results, where depolarization has been reported as sporadic and as a propagating wave. The present study was designed to resolve the aforementioned difference and determine the unknown relationship between ΔΨm and electrophysiology. We developed a novel imaging modality for simultaneous optical mapping of ΔΨm and transmembrane potential (Vm). Optical mapping was performed using potentiometric dyes on preparations from 4 mouse hearts, 14 rabbit hearts, and 7 human hearts. Our data showed that during ischemia, ΔΨm depolarization is sporadic and changes asynchronously with electrophysiological changes. Spatially, ΔΨm depolarization was associated with action potential duration shortening but not conduction slowing. Analysis of focal activity indicated that ΔΨm is not different within the myocardium where the focus originates compared with normal ventricular tissue. Overall, our data suggest that during ischemia, mitochondria maintain their function at the expense of sarcolemmal electrophysiology, but ΔΨm depolarization does not have a direct association to ischemia-induced arrhythmias.

Mitochondrial ion channels/transporters as sensors and regulators of cellular redox signaling

SIGNIFICANCE

Mitochondrial ion channels/transporters and the electron transport chain (ETC) serve as key sensors and regulators for cellular redox signaling, the production of reactive oxygen species (ROS) and nitrogen species (RNS) in mitochondria, and balancing cell survival and death. Although the functional and pharmacological characteristics of mitochondrial ion transport mechanisms have been extensively studied for several decades, the majority of the molecular identities that are responsible for these channels/transporters have remained a mystery until very recently.

RECENT ADVANCES

Recent breakthrough studies uncovered the molecular identities of the diverse array of major mitochondrial ion channels/transporters, including the mitochondrial Ca2+ uniporter pore, mitochondrial permeability transition pore, and mitochondrial ATP-sensitive K+ channel. This new information enables us to form detailed molecular and functional characterizations of mitochondrial ion channels/transporters and their roles in mitochondrial redox signaling.

CRITICAL ISSUES

Redox-mediated post-translational modifications of mitochondrial ion channels/transporters and ETC serve as key mechanisms for the spatiotemporal control of mitochondrial ROS/RNS generation.

FUTURE DIRECTIONS

Identification of detailed molecular mechanisms for redox-mediated regulation of mitochondrial ion channels will enable us to find novel therapeutic targets for many diseases that are associated with cellular redox signaling and mitochondrial ion channels/transporters.

Functional crosstalk between the mitochondrial PTP and KATP channels determine arrhythmic vulnerability to oxidative stress

Background: Mitochondrial permeability transition pore (mPTP) opening is a terminal event leading to mitochondrial dysfunction and cell death under conditions of oxidative stress (OS). However, mPTP blockade with cyclosporine A (CsA) has shown variable efficacy in limiting post-ischemic dysfunction and arrhythmias. We hypothesized that strong feedback between energy dissipating (mPTP) and cardioprotective (mK_ATP) channels determine vulnerability to OS.

Methods and Results: Guinea pig hearts (N = 61) were challenged with H₂O₂ (200 μM) to elicit mitochondrial membrane potential (ΔΨ_m) depolarization. High-resolution optical mapping was used to measure ΔΨ_m or action potentials (AP) across the intact heart. Hearts were treated with CsA (0.1 μM) under conditions that altered the activity of mK_ATP channels either directly or indirectly via its regulation by protein kinase C. mPTP blockade with CsA markedly blunted (P < 0.01) OS-induced ΔΨ_m depolarization and delayed loss of LV pressure (LVP), but did not affect arrhythmia propensity. Surprisingly, prevention of mK_ATP activation with the chemical phosphatase BDM reversed the protective effect of CsA, paradoxically exacerbating OS-induced ΔΨ_m depolarization and accelerating arrhythmia onset in CsA treated compared to untreated hearts (P < 0.05). To elucidate the putative molecular mechanisms, mPTP inhibition by CsA was tested during conditions of selective PKC inhibition or direct mK_ATP channel activation or blockade. Similar to BDM, the specific PKC inhibitor, CHE (10 μM) did not alter OS-induced ΔΨ_m depolarization directly. However, it completely abrogated CsA-mediated protection against OS. Direct pharmacological blockade of mK_ATP, a mitochondrial target of PKC signaling, equally abolished the protective effect of CsA on ΔΨ_m depolarization, whereas channel activation with 30 μM Diazoxide protected against ΔΨ_m depolarization (P < 0.0001). Conditions that prevented mK_ATP activation either directly or indirectly via PKC inhibition led to accelerated ΔΨ_m depolarization and early onset of VF in response to OS. Investigation of the electrophysiological substrate revealed accelerated APD shortening in response to OS in arrhythmia-prone hearts.

Conclusions: Cardioprotection by CsA requires mK_ATP channel activation through a PKC-dependent pathway. Increasing mK_ATP activity during CsA administration is required for limiting OS-induced electrical dysfunction.

Mechano-electrical coupling as framework for understanding functional remodeling during LBBB and CRT

It is not understood why, after onset of left bundle-branch block (LBBB), acute worsening of cardiac function is followed by a further gradual deterioration of function, whereas most adverse cardiac events lead to compensatory adaptations. We investigated whether mechano-electrical coupling (MEC) can explain long-term remodeling with LBBB and cardiac resynchronization therapy (CRT). To this purpose, we used an integrative modeling approach relating local ventricular electrophysiology, calcium handling, and excitation-contraction coupling to global cardiovascular mechanics and hemodynamics. Each ventricular wall was composed of multiple mechanically and electrically coupled myocardial segments. MEC was incorporated by allowing adaptation of L-type Ca2+ current aiming at minimal dispersion of local external work, an approach that we previously applied to replicate T-wave memory in a synchronous heart after a period of asynchronous activation. LBBB instantaneously decreased left-ventricular stroke work and increased end-diastolic volume. During sustained LBBB, MEC reduced intraventricular dispersion of mechanical workload and repolarization. However, MEC-induced reduction in contractility in late-activated regions was larger than the contractility increase in early-activated regions, resulting in further decrease of stroke work and increase of end-diastolic volume. Upon the start of CRT, stroke work increased despite a wider dispersion of mechanical workload. During sustained CRT, MEC-induced reduction in dispersion of workload and repolarization coincided with a further reduction in end-diastolic volume. In conclusion, MEC may represent a useful framework for better understanding the long-term changes in cardiac electrophysiology and contraction following LBBB as well as CRT.

Diversity of mitochondrial Ca²⁺ signaling in rat neonatal cardiomyocytes: evidence from a genetically directed Ca²⁺ probe, mitycam-E31Q

I(Ca)-gated Ca(2+) release (CICR) from the cardiac SR is the main mechanism mediating the rise of cytosolic Ca(2+), but the extent to which mitochondria contribute to the overall Ca(2+) signaling remains controversial. To examine the possible role of mitochondria in Ca(2+) signaling, we developed a low affinity mitochondrial Ca(2+) probe, mitycam-E31Q (300-500 MOI, 48-72h) and used it in conjunction with Fura-2AM to obtain simultaneous TIRF images of mitochondrial and cytosolic Ca(2+) in cultured neonatal rat cardiomyocytes. Mitycam-E31Q staining of adult feline cardiomyocytes showed the typical mitochondrial longitudinal fluorescent bandings similar to that of TMRE staining, while neonatal rat cardiomyocytes had a disorganized tubular or punctuate appearance. Caffeine puffs produced rapid increases in cytosolic Ca(2+) while simultaneously measured global mitycam-E31Q signals decreased more slowly (increased mitochondrial Ca(2+)) before decaying to baseline levels. Similar, but oscillating mitycam-E31Q signals were seen in spontaneously pacing cells. Withdrawal of Na(+) increased global cytosolic and mitochondrial Ca(2+) signals in one population of mitochondria, but unexpectedly decreased it (release of Ca(2+)) in another mitochondrial population. Such mitochondrial Ca(2+) release signals were seen not only during long lasting Na(+) withdrawal, but also when Ca(2+) loaded cells were exposed to caffeine-puffs, and during spontaneous rhythmic beating. Thus, mitochondrial Ca(2+) transients appear to activate with a delay following the cytosolic rise of Ca(2+) and show diversity in subpopulations of mitochondria that could contribute to the plasticity of mitochondrial Ca(2+) signaling.

High-fat, low-carbohydrate diet promotes arrhythmic death and increases myocardial ischemia-reperfusion injury in rats

High-fat, low-carbohydrate diets (HFLCD) are often eaten by humans for a variety of reasons, but the effects of such diets on the heart are incompletely understood. We evaluated the impact of HFLCD on myocardial ischemia/reperfusion (I/R) using an in vivo model of left anterior descending coronary artery ligation. Sprague-Dawley rats (300 g) were fed HFLCD (60% calories fat, 30% protein, 10% carbohydrate) or control (CONT; 16% fat, 19% protein, 65% carbohydrate) diet for 2 wk and then underwent open chest I/R. At baseline (preischemia), diet did not affect left ventricular (LV) systolic and diastolic function. Oil red O staining revealed presence of lipid in the heart with HFLCD but not in CONT. Following I/R, recovery of LV function was decreased in HFLCD. HFLCD hearts exhibited decreased ATP synthase and increased uncoupling protein-3 gene and protein expression. HFLCD downregulated mitochondrial fusion proteins and upregulated fission proteins and store-operated Ca(2+) channel proteins. HFLCD led to increased death during I/R; 6 of 22 CONT rats and 16 of 26 HFLCD rats died due to ventricular arrhythmias and hemodynamic shock. In surviving rats, HFLCD led to larger infarct size. We concluded that in vivo HFLCD does not affect nonischemic LV function but leads to greater myocardial injury during I/R, with increased risk of death by pump failure and ventricular arrhythmias, which might be associated with altered cardiac energetics, mitochondrial fission/fusion dynamics, and store-operated Ca(2+) channel expression.

Mitochondrial channels: ion fluxes and more

The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.

Dipeptidyl peptidase-4 inhibitor reduces infarct size and preserves cardiac function via mitochondrial protection in ischaemia-reperfusion rat heart

AIM

We hypothesized that dipeptidyl peptidase (DPP)-4 inhibitor (vildagliptin) reduces fatal arrhythmias, cardiac dysfunction and infarct size caused by ischaemia-reperfusion (I/R) injury via its attenuation of cardiac mitochondrial dysfunction.

METHODS

In total, 26 rats were randomized to receive either 1 mL normal saline solution or 2.0 mg/kg vildagliptin intravenously (n = 13/group) 30 min prior to a 30-min left anterior descending coronary artery occlusion, followed by a 120-min reperfusion. Arrhythmia scores, cardiac functions, infarct size and mitochondrial function were evaluated.

RESULTS

Vildagliptin reduced the infarct size by 44% and mitigated cardiac dysfunction by preserving cardiac function without altering the incidence of cardiac arrhythmias. Vildagliptin increased expression of Bcl-2 and pro-caspase3 in the ischaemic area, whereas Bax and phosphorylated-connexin43/total-connexin43 were not altered. Vildagliptin attenuated cardiac mitochondrial dysfunction by reducing the reactive oxygen species level and mitochondrial swelling.

CONCLUSIONS

DPP-4 inhibitor provides cardioprotection by reducing the infarct size and ameliorating cardiac dysfunction in I/R hearts by attenuating cardiac mitochondrial dysfunction and cardiomyocyte apoptosis.

Blockade of mitochondrial calcium uniporter prevents cardiac mitochondrial dysfunction caused by iron overload

AIM

Iron overload in the heart can lead to iron-overload cardiomyopathy and cardiac arrhythmia. In the past decades, growing evidence has suggested that cardiac mitochondrial dysfunction is associated with the development of cardiac dysfunction and lethal arrhythmias. Despite these facts, the effect of iron overload on cardiac mitochondrial function is still unclear. In this study, we determined the effects of iron overload on the cardiac mitochondrial function and the routes of cardiac mitochondrial iron uptake. We tested the hypothesis that iron overload can lead to cardiac mitochondrial dysfunction and that mitochondrial calcium uniporter (MCU) plays a major role for cardiac mitochondrial iron uptake under iron-overload condition. Cardiac mitochondrial function was assessed via the determination of mitochondrial swelling, mitochondrial reactive oxygen species (ROS) production and mitochondrial membrane potential changes.

METHODS

Isolated cardiac mitochondria from male Wistar rats were used in this study. To determine the routes for cardiac mitochondrial iron uptake, isolated mitochondria were exposed to MCU blocker (Ru360), mitochondrial permeability transition pore (mPTP) blocker (cyclosporin A) and an iron chelator (deferoxamine).

RESULTS

We found that (i) iron overload caused cardiac mitochondrial dysfunction, indicated by increased ROS production, mitochondrial membrane depolarization and mitochondrial swelling; and (ii) only MCU blocker completely protected cardiac mitochondrial dysfunction caused by iron overload.

CONCLUSIONS

These findings strongly suggest that MCU could be the major route for iron uptake into cardiac mitochondria. The inhibition of MCU could be the novel pharmacological intervention for preventing iron-overload cardiomyopathy.

Mitochondrial targets for arrhythmia suppression: is there a role for pharmacological intervention?

Mitochondrial dysfunction is a hallmark of common cardiovascular disorders, including ischemia-reperfusion injury, hypertrophy, heart failure, and diabetes mellitus. While the role of the mitochondrial network in regulating energy production and cell death pathways is well established, its active control of other critical cellular functions, including excitation-contraction coupling and excitability, is less understood. The purpose of this focused review article is to highlight the growing mechanistic link between mitochondrial dysfunction and arrhythmogenesis. The goal is not to provide a comprehensive listing of all factors by which mitochondrial bioenergetics and altered cellular redox status affect ion channel function but rather to focus on one central mechanism of arrhythmogenesis which arises from a mitochondrial origin. In doing so, we discuss the role of mitochondrial targets for suppressing arrhythmias through this mechanism.

Glutathione oxidation unmasks proarrhythmic vulnerability of chronically hyperglycemic guinea pigs

Chronic hyperglycemia in type-1 diabetes mellitus is associated with oxidative stress (OS) and sudden death. Mechanistic links remain unclear. We investigated changes in electrophysiological (EP) properties in a model of chronic hyperglycemia before and after challenge with OS by GSH oxidation and tested reversibility of EP remodeling by insulin. Guinea pigs survived for 1 mo following streptozotocin (STZ) or saline (sham) injection. A treatment group received daily insulin for 2 wk to reverse STZ-induced hyperglycemia (STZ + Ins). EP properties were measured using high-resolution optical action potential mapping before and after challenge of hearts with diamide. Despite elevation of glucose levels in STZ compared with sham-operated (P = 0.004) and STZ + Ins (P = 0.002) animals, average action potential duration (APD) and arrhythmia propensity were not altered at baseline. Diamide promoted early (<10 min) formation of arrhythmic triggers reflected by a higher arrhythmia scoring index in STZ (P = 0.045) and STZ + Ins (P = 0.033) hearts compared with sham-operated hearts. APD heterogeneity underwent a more pronounced increase in response to diamide in STZ and STZ + Ins hearts compared with sham-operated hearts. Within 30 min, diamide resulted in spontaneous incidence of ventricular tachycardia and ventricular fibrillation (VT/VF) in 3/6, 2/5, 1/5, and 0/4 STZ, STZ + Ins, sham-operated, and normal hearts, respectively. Hearts prone to VT/VF exhibited greater APD heterogeneity (P = 0.010) compared with their VT/VF-free counterparts. Finally, altered EP properties in STZ were not rescued by insulin. In conclusion, GSH oxidation enhances APD heterogeneity and increases arrhythmia scoring index in a guinea pig model of chronic hyperglycemia. Despite normalization of glycemic levels by insulin, these proarrhythmic properties are not reversed, suggesting the importance of targeting antioxidant defenses for arrhythmia suppression.

Calcium signaling in cardiac mitochondria

Mitochondrial Ca signaling contributes to the regulation of cellular energy metabolism, and mitochondria participate in cardiac excitation-contraction coupling (ECC) through their ability to store Ca, shape the cytosolic Ca signals and generate ATP required for contraction. The mitochondrial inner membrane is equipped with an elaborate system of channels and transporters for Ca uptake and extrusion that allows for the decoding of cytosolic Ca signals, and the storage of Ca in the mitochondrial matrix compartment. Controversy, however remains whether the fast cytosolic Ca transients underlying ECC in the beating heart are transmitted rapidly into the matrix compartment or slowly integrated by the mitochondrial Ca transport machinery. This review summarizes established and novel findings on cardiac mitochondrial Ca transport and buffering, and discusses the evidence either supporting or arguing against the idea that Ca can be taken up rapidly by mitochondria during ECC.