Iron overload decreases CaV1.3-dependent L-type Ca2+ currents leading to bradycardia, altered electrical conduction, and atrial fibrillation

Iron deficiency and supplementation in heart failure

Non-anaemic iron deficiency (NAID) is a strategic target in cardiovascular medicine because of its association with a range of adverse effects in various conditions. Endeavours to tackle NAID in heart failure have yielded mixed results, exposing knowledge gaps in how best to define 'iron deficiency' and the handling of iron therapies by the body. To address these gaps, we harness the latest understanding of the mechanisms of iron homeostasis outside the erythron and integrate clinical and preclinical lines of evidence. The emerging picture is that current definitions of iron deficiency do not assimilate the multiple influences at play in patients with heart failure and, consequently, fail to identify those with a truly unmet need for iron. Additionally, current iron supplementation therapies benefit only certain patients with heart failure, reflecting differences in the nature of the unmet need for iron and the modifying effects of anaemia and inflammation on the handling of iron therapies by the body. Building on these insights, we identify untapped opportunities in the management of NAID, including the refinement of current approaches and the development of novel strategies. Lessons learned from NAID in cardiovascular disease could ultimately translate into benefits for patients with other chronic conditions such as chronic kidney disease, chronic obstructive pulmonary disease and cancer.

The role and mechanism of heme oxygenase-1 in arrhythmias

The global incidence and prevalence of arrhythmias are continuously increasing. However, the precise mechanisms of underlying arrhythmogenesis and the optimal measures for effective treatment remain incompletely understood. The inducible form of heme oxygenase, known as heme oxygenase-1 (HO-1), is recognized as a potent antioxidant molecule capable of exerting anti-inflammatory and anti-apoptotic effects. Recent research indicates that HO-1 plays a role in preventing arrhythmias by mitigating cardiac remodeling, including electrical remodeling, ion remodeling, and structural remodeling. This review aimed to consolidate current knowledge regarding the involvement of HO-1 in arrhythmias and elucidate its underlying mechanisms of action.

The role of iron overload and ferroptosis in arrhythmia pathogenesis

Ferroptosis is a newly discovered form of programmed cell death triggered by intracellular iron overload, which leads to the accumulation of lipid peroxides in various cells. It has been implicated in the pathogenesis and progression of various diseases, including tumors, neurological disorders, and cardiovascular diseases. The intricate mechanism underlying ferroptosis involves an imbalance between the oxidation and antioxidant systems, disturbances in iron metabolism, membrane lipid peroxidation, and dysregulation of amino acid metabolism. We highlight the key molecular mechanisms governing iron overload and ferroptosis, and discuss potential molecular pathways linking ferroptosis with arrhythmias.

Anticoagulation for stroke prevention in atrial fibrillation and treatment of venous thromboembolism and portal vein thrombosis in cirrhosis: guidance from the SSC of the ISTH

While advanced liver disease was previously considered to be an acquired bleeding disorder, there is increasing recognition of an associated prothrombotic state with patients being at higher risk of atrial fibrillation (AF) and stroke and venous thromboembolism (VTE) including portal vein thrombosis (PVT). We review the available literature on epidemiology, pathophysiology, and risk factors and provide guidance on anticoagulant management of these conditions in adults with cirrhosis. In patients with Child-Pugh A or B cirrhosis and AF, we recommend anticoagulation with standard-dose direct oral anticoagulants (DOACs) in accordance with cardiology guideline recommendations for patients without liver disease. In those with Child-Pugh C cirrhosis, there is inadequate evidence with respect to the benefit and risk of anticoagulation for stroke prevention in AF. In patients with cirrhosis and acute deep vein thrombosis or pulmonary embolism, we recommend anticoagulation and suggest use of either a DOAC or low-molecular-weight heparin (LMWH)/vitamin K antagonist (VKA) in Child-Pugh A or B cirrhosis and LMWH alone (or as a bridge to VKA in patients with a normal baseline international normalized ratio) in Child-Pugh C cirrhosis. We recommend anticoagulation for patients with cirrhosis and symptomatic PVT. We suggest anticoagulation for those with asymptomatic, progressing PVT and recommend continuing extended anticoagulation for liver transplant candidates with PVT.

Cardiac Arrest During a Ferric Derisomaltose Infusion Followed by Complete Heart Block: A Case Report

Introduction

Ferric derisomaltose is the newest available parenteral iron formulation. Studies have demonstrated a good safety profile with improved tolerability compared to alternative parenteral iron formulations. To date there have been no reported acute, life-threatening cardiac events associated with ferric derisomaltose.

Case Report

An 86-year-old male who had previously tolerated routine iron infusions received a first dose of ferric derisomaltose at an outpatient infusion clinic. Six minutes into the infusion the patient became unresponsive with no palpable pulse. Return of spontaneous circulation was achieved after two minutes of chest compressions. Electrocardiogram showed complete heart block requiring transcutaneous pacing and vasopressor administration. The patient was transferred to the emergency department for stabilization and then admitted to the cardiac intensive care unit. During admission, the patient received a dual-chamber, permanent pacemaker without complication and was ultimately discharged.

Conclusion

It may be reasonable to consider parenteral iron as a toxicological etiology for patients presenting with complete heart block temporally associated with parenteral iron administration, particularly in patients with underlying conduction abnormalities.

Ferroptosis mechanisms and regulations in cardiovascular diseases in the past, present, and future

Cardiovascular diseases (CVDs) are the main diseases that endanger human health, and their risk factors contribute to high morbidity and a high rate of hospitalization. Cell death is the most important pathophysiology in CVDs. As one of the cell death mechanisms, ferroptosis is a new form of regulated cell death (RCD) that broadly participates in CVDs (such as myocardial infarction, heart transplantation, atherosclerosis, heart failure, ischaemia/reperfusion (I/R) injury, atrial fibrillation, cardiomyopathy (radiation-induced cardiomyopathy, diabetes cardiomyopathy, sepsis-induced cardiac injury, doxorubicin-induced cardiac injury, iron overload cardiomyopathy, and hypertrophic cardiomyopathy), and pulmonary arterial hypertension), involving in iron regulation, metabolic mechanism and lipid peroxidation. This article reviews recent research on the mechanism and regulation of ferroptosis and its relationship with the occurrence and treatment of CVDs, aiming to provide new ideas and treatment targets for the clinical diagnosis and treatment of CVDs by clarifying the latest progress in CVDs research.

Ferroptosis in cardiovascular disease

In the 21st century, cardiovascular disease (CVD) has become one of the leading causes of death worldwide. The prevention and treatment of CVD remain pressing scientific issues. Several recent studies have suggested that ferroptosis may play a key role in CVD. Most studies conducted thus far on ferroptosis and CVD have supported the link. Ferroptosis mediated by different signaling and metabolic pathways can lead to ischemic heart disease, myocarditis, heart failure, ischemia-reperfusion injury, and cardiomyopathy. Still, the specific mechanism of ferroptosis in CVD, the particular organ areas affected, and the stage of disease involved need to be further studied. Therefore, understanding the mechanisms regulating ferroptosis in CVD may improve disease management. Throughout this review, we summarized the mechanism of ferroptosis and its effect on the pathogenesis of CVD. We also predicted and discussed future research directions, aiming to provide new ideas and strategies for preventing and treating CVD.

Ferroptosis in cardiovascular diseases: role and mechanism

In multicellular organisms, regulatory cell death is a crucial aspect of growth and development. Ferroptosis, which was postulated roughly ten years ago, is a mode of cell death that differs from apoptosis, autophagy, and pyrodeath. This distinct pattern of cell death is triggered by an imbalance between oxidants and antioxidants and strongly associated with the metabolism of iron, lipids, amino acids, and glutathione. A growing body of research has implicated ferroptosis in the incidence and progression of many organ traumas and degenerative diseases. Recently, ferroptosis has gained attention as a crucial regulatory mechanism underlying the initiation and development of a variety of cardiovascular diseases, including myocardial ischemia/reperfusion injury, cardiomyopathy, arrhythmia, chemotherapy, and Corona Virus-2-induced cardiac injury. Pharmacological therapies that inhibit ferroptosis have great potential for the management of cardiovascular disorders. This review discusses the prevalence and regulatory mechanisms of ferroptosis, effect of ferroptosis on the immune system, significance of ferroptosis in cardiovascular diseases, and potential therapeutic value of regulating ferroptosis in a variety of heart diseases.

Ferroptosis: a new strategy for cardiovascular disease

Cardiovascular disease (CVD) is currently one of the prevalent causes of human death. Iron is one of the essential trace elements in the human body and a vital component of living tissues. All organ systems require iron for various metabolic processes, including myocardial and skeletal muscle metabolism, erythropoiesis, mitochondrial function, and oxygen transport. Its deficiency or excess in the human body remains one of the nutritional problems worldwide. The total amount of iron in a normal human body is about 3-5 g. Iron deficiency may cause symptoms such as general fatigue, pica, and nerve deafness, while excessive iron plays a crucial role in the pathophysiological processes of the heart through ferroptosis triggered by the Fenton reaction. It differs from other cell death modes based on its dependence on the accumulation of lipid peroxides and REDOX imbalance, opening a new pathway underlying the pathogenesis and mechanism of CVDs. In this review, we describe the latest research progress on the mechanism of ferroptosis and report its crucial role and association with miRNA in various CVDs. Finally, we summarise the potential therapeutic value of ferroptosis-related drugs or ferroptosis inhibitors in CVDs.

Deficiency of mitochondrial calcium uniporter abrogates iron overload-induced cardiac dysfunction by reducing ferroptosis

Iron overload associated cardiac dysfunction remains a significant clinical challenge whose underlying mechanism(s) have yet to be defined. We aim to evaluate the involvement of the mitochondrial Ca2+ uniporter (MCU) in cardiac dysfunction and determine its role in the occurrence of ferroptosis. Iron overload was established in control (MCUfl/fl) and conditional MCU knockout (MCUfl/fl-MCM) mice. LV function was reduced by chronic iron loading in MCUfl/fl mice, but not in MCUfl/fl-MCM mice. The level of mitochondrial iron and reactive oxygen species were increased and mitochondrial membrane potential and spare respiratory capacity (SRC) were reduced in MCUfl/fl cardiomyocytes, but not in MCUfl/fl-MCM cardiomyocytes. After iron loading, lipid oxidation levels were increased in MCUfl/fl, but not in MCUfl/fl-MCM hearts. Ferrostatin-1, a selective ferroptosis inhibitor, reduced lipid peroxidation and maintained LV function in vivo after chronic iron treatment in MCUfl/fl hearts. Isolated cardiomyocytes from MCUfl/fl mice demonstrated ferroptosis after acute iron treatment. Moreover, Ca2+ transient amplitude and cell contractility were both significantly reduced in isolated cardiomyocytes from chronically Fe treated MCUfl/fl hearts. However, ferroptosis was not induced in cardiomyocytes from MCUfl/fl-MCM hearts nor was there a reduction in Ca2+ transient amplitude or cardiomyocyte contractility. We conclude that mitochondrial iron uptake is dependent on MCU, which plays an essential role in causing mitochondrial dysfunction and ferroptosis under iron overload conditions in the heart. Cardiac-specific deficiency of MCU prevents the development of ferroptosis and iron overload-induced cardiac dysfunction.

Iron and atrial fibrillation: A review

Atrial fibrillation (AF), one of the most common arrhythmias in clinical practice, is classified into paroxysmal, persistent, and permanent AF according to its duration. The development of AF is associated with increased cardiovascular morbidity and mortality. However, the exact etiology of this disease remains poorly understood. Recent studies found disorders of iron metabolism might be involved in the progression of AF. Abnormal iron metabolism in cardiomyocytes provides arrhythmogenic substrates through a variety of mechanisms, including calcium mishandling, ion channel remodeling, and oxidative stress overaction. Interestingly, in AF patients with iron overload, interventions on iron metabolism, such as iron chelators and ferroptosis inhibitors, has been shown to prevent AF via reducing ferroptosis. Herein, we review the possible mechanisms, consequences, and therapeutic implications of altered atrial iron handling for AF pathophysiology.

Review article: diagnosis, pathophysiology and management of atrial fibrillation in cirrhosis and portal hypertension

BACKGROUND

Atrial fibrillation (AF) is the most common arrhythmia and its management in cirrhosis can be challenging due to the altered hepatic metabolism of medications and increased risk of bleeding.

AIMS

To provide a comprehensive overview of the diagnosis, pathophysiology and management of AF in patients with cirrhosis from both a cardiology and a hepatology perspective.

METHODS

An extensive literature search was performed using the terms 'atrial fibrillation' and 'cirrhosis'. Guideline documents and consensus statements were explored.

RESULTS

The prevalence of AF in patients with cirrhosis ranges between 6.6% and 14.2%, while the incidence of new-onset AF in the post-operative period after liver transplant ranged between 6.8% and 10.2%. AF in patients with cirrhosis is associated with adverse outcomes in both pre-transplant and post-transplant settings, including an increased risk of stroke when compared to the general population. We review the pathogenesis of AF in general and in cirrhosis. This review also provides guidance on the management of AF, including the use of anticoagulation and rate versus rhythm control. In the absence of strict contraindications, all patients with cirrhosis and AF should be anticoagulated. The use of DOACs is preferred over vitamin K antagonists. In patients with a high bleeding risk, a DOAC with an approved antidote may be preferred.

CONCLUSIONS

Atrial fibrillation is increased in patients with cirrhosis. AF management requires careful consideration of treatment options. Since patients with cirrhosis were excluded from all major randomised clinical trials, dedicated research on the pathophysiology and management of AF in cirrhosis is needed.

Cardiovascular Complications in β-Thalassemia: Getting to the Heart of It

Beta thalassemia is an inherited disorder resulting in abnormal or decreased production of hemoglobin, leading to hemolysis and chronic anemia. The long-term complications can affect multiple organ systems, namely the liver, heart, and endocrine. Myocardial iron overload is a common finding in β-thalassemia. As a result, different cardiovascular complications in the form of cardiomyopathy, pulmonary hypertension, arrhythmias, and vasculopathies can occur, and in extreme cases, sudden cardiac death. Each of these complications pertains to underlying etiologies and risk factors, which highlights the importance of early diagnosis and prevention. In this review, we will discuss different types of cardiovascular complications that can manifest in patients with β-thalassemia, in addition to the current diagnostic modalities, preventive and treatment modalities for these complications.

Pathophysiology of Cav1.3 L-type calcium channels in the heart

Ca²⁺ plays a crucial role in excitation-contraction coupling in cardiac myocytes. Dysfunctional Ca²⁺ regulation alters the force of contraction and causes cardiac arrhythmias. Ca²⁺ entry into cardiomyocytes is mediated mainly through L-type Ca²⁺ channels, leading to the subsequent Ca²⁺ release from the sarcoplasmic reticulum. L-type Ca²⁺ channels are composed of the conventional Ca_v1.2, ubiquitously expressed in all heart chambers, and the developmentally regulated Ca_v1.3, exclusively expressed in the atria, sinoatrial node, and atrioventricular node in the adult heart. As such, Ca_v1.3 is implicated in the pathogenesis of sinoatrial and atrioventricular node dysfunction as well as atrial fibrillation. More recently, Ca_v1.3 de novo expression was suggested in heart failure. Here, we review the functional role, expression levels, and regulation of Ca_v1.3 in the heart, including in the context of cardiac diseases. We believe that the elucidation of the functional and molecular pathways regulating Ca_v1.3 in the heart will assist in developing novel targeted therapeutic interventions for the aforementioned arrhythmias.

Gut Microbiota Dysbiosis Induced by a High-Fat Diet Increases Susceptibility to Atrial Fibrillation

BACKGROUND

Obesity is a significant risk factor for atrial fibrillation (AF), and the gut microbiota is closely related to obesity-induced diseases. However, whether the gut microbiota is involved in regulating obesity-induced AF has not been studied. This study investigated whether gut microbiota dysbiosis affects obesity-related AF.

METHODS

Fecal microbes derived from normal diet (ND)-fed and high-fat diet (HD)-fed mice were transplanted into those fed normally. Morphologic, biochemical, functional, histologic, electrophysiological studies, molecular analysis, 16S rRNA gene amplicon sequencing, and RNA-sequencing were performed.

RESULTS

Transplantation of the HD gut microbes in ND-maintained (THD) mice led to a significant increase in the susceptibility to AF. Gut microbiota analysis showed a significant increase in Desulfovibrionaceae, which generated metabolic endotoxemia in THD mice. Transplantation with HD microbes also resulted in significantly increased levels of circulating lipopolysaccharide (LPS), significant disruption in the histologic architecture of the intestinal tissue, and significantly increased proinflammatory cytokines in the left atrium, indicating that atrial inflammation likely contributed to AF susceptibility. RNA-sequencing showed that the THD group had enhanced activation of ferroptosis and TLR4/NF-κB/NLRP3 inflammasome signalling pathway. Inhibiting the ferroptosis or NLRP3 inflammasome signalling pathway significantly improved atrial fibrosis and reduced susceptibility to obesity-related gut dysbiosis-induced AF.

CONCLUSIONS

This study provides evidence showing an original causal role of gut microbiota dysbiosis in the pathogenesis of obesity-related AF, which showed elevated LPS and dysregulation of atrial pathologic remodelling by activating ferroptosis and the TLR4/NF-κB/NLRP3 inflammasome signalling pathway.

Early Detection of Iron Overload Cardiomyopathy in Transfusion Dependent Thalassemia Patients in Sulaimaniyah City, Iraq

Background

Lifelong blood transfusion is recommended for patients with transfusion-dependent thalassemia (TDT) that lead to iron overload and results in cardiomyopathy (CM).

Aim

To assess the accuracy of several electrocardiographic (ECG) data in patients at high risk of arrhythmia, early detection of structural and functional changes in left atrium and ventricle using ECG and echocardiography (two dimensional, M-mode echocardiography along with Doppler studies), and to observe the correlations between plasma B-type atrial natriuretic peptide (BNP)/serum ferritin and ECG with Doppler as well as echocardiographic changes in patients with TDT.

Methods

The current prospective case-control study included 75 TDT patients and 74 control subjects with the mean age of 9.55 and 9.93 years, respectively. Participants were assessed for the socio-demographic, physical examinations, serum ferritin, plasma BNP, ECG, 2D echocardiography, and tissue/pulse wave Doppler.

Results

The mean of serum ferritin and plasma BNP were significantly (p<0.001) higher in the cases (1475.19 ng/mL and 47.63 pg/mL, respectively) than controls (41.3 ng/mL and BNP 23.37 pg/mL, respectively). ECG findings showed significant higher P-wave dispersion, QRS duration, QT duration and dispersion, and JTc dispersion in cases than controls. Echocardiography findings revealed diastolic dysfunction with preserved ejection fraction (EF) in thalassemia cases, as well as significant higher left ventricular (LV) mass, LV internal diameters during systole and diastole (LVIDs and d) and LV posterior wall thickness during diastole (LVPWd) in cases than controls (p<0.05). Also, a significant (p<0.05) correlation between BNP and QT dispersion was found in patients.

Conclusion

These findings suggest the usefulness of ECG, 2D echocardiographic, Doppler studies and plasma BNP, with no significant beneficial effect of serum ferritin level in detecting early cardiac changes in patients with TDT.

Iron in Cardiovascular Disease: Challenges and Potentials

Iron is essential for many biological processes. Inadequate or excess amount of body iron can result in various pathological consequences. The pathological roles of iron in cardiovascular disease (CVD) have been intensively studied for decades. Convincing data demonstrated a detrimental effect of iron deficiency in patients with heart failure and pulmonary arterial hypertension, but it remains unclear for the pathological roles of iron in other cardiovascular diseases. Meanwhile, ferroptosis is an iron-dependent cell death that is distinct from apoptosis, necroptosis, and other types of cell death. Ferroptosis has been reported in several CVDs, namely, cardiomyopathy, atherosclerotic cardiovascular disease, and myocardial ischemia/reperfusion injury. Iron chelation therapy seems to be an available strategy to ameliorate iron overload-related disorders. It is still a challenge to accurately clarify the pathological roles of iron in CVD and search for effective medical intervention. In this review, we aim to summarize the pathological roles of iron in CVD, and especially highlight the potential mechanism of ferroptosis in these diseases.

Biomarkers in the clinical management of patients with atrial fibrillation and heart failure

Atrial fibrillation (AF) and heart failure (HF) are two cardiovascular diseases with an increasing prevalence worldwide. These conditions share common pathophysiologiesand frequently co-exit. In fact, the occurrence of either condition can 'cause' the development of the other, creating a new patient group that demands different management strategies to that if they occur in isolation. Regardless of the temproral association of the two conditions, their presence is linked with adverse cardiovascular outcomes, increased rate of hospitalizations, and increased economic burden on healthcare systems. The use of low-cost, easily accessible and applicable biomarkers may hasten the correct diagnosis and the effective treatment of AF and HF. Both AF and HF effect multiple physiological pathways and thus a great number of biomarkers can be measured that potentially give the clinician important diagnostic and prognostic information. These will then guide patient centred therapeutic management. The current biomarkers that offer potential for guiding therapy, focus on the physiological pathways of miRNA, myocardial stretch and injury, oxidative stress, inflammation, fibrosis, coagulation and renal impairment. Each of these has different utility in current clinincal practice.

Iron Overload, Oxidative Stress, and Ferroptosis in the Failing Heart and Liver

Iron accumulation is a key mediator of several cytotoxic mechanisms leading to the impairment of redox homeostasis and cellular death. Iron overload is often associated with haematological diseases which require regular blood transfusion/phlebotomy, and it represents a common complication in thalassaemic patients. Major damages predominantly occur in the liver and the heart, leading to a specific form of cell death recently named ferroptosis. Different from apoptosis, necrosis, and autophagy, ferroptosis is strictly dependent on iron and reactive oxygen species, with a dysregulation of mitochondrial structure/function. Susceptibility to ferroptosis is dependent on intracellular antioxidant capacity and varies according to the different cell types. Chemotherapy-induced cardiotoxicity has been proven to be mediated predominantly by iron accumulation and ferroptosis, whereas there is evidence about the role of ferritin in protecting cardiomyocytes from ferroptosis and consequent heart failure. Another paradigmatic organ for transfusion-associated complication due to iron overload is the liver, in which the role of ferroptosis is yet to be elucidated. Some studies report a role of ferroptosis in the initiation of hepatic inflammation processes while others provide evidence about an involvement in several pathologies including immune-related hepatitis and acute liver failure. In this manuscript, we aim to review the literature to address putative common features between the response to ferroptosis in the heart and liver. A better comprehension of (dys)similarities is pivotal for the development of future therapeutic strategies that can be designed to specifically target this type of cell death in an attempt to minimize iron-overload effects in specific organs.

Ferroportin-mediated ferroptosis involved in new-onset atrial fibrillation with LPS-induced endotoxemia

Sepsis is a known risk factor for new-onset atrial fibrillation (AF), and previous studies have demonstrated that ferroptosis participates in sepsis-induced organ injury development. Nevertheless, the role of ferroptosis in new-onset AF with sepsis remains largely unknown. This study aims to investigate the underlying mechanisms linking ferroptosis and AF caused by sepsis. LPS-induced endotoxemia is often used to model the acute inflammatory response associated with sepsis. Herein, we reported that ferroptosis was significantly activated in LPS-induced endotoxemia rat model. We also observed that ferroportin (Fpn), the only identified mammalian non-heme iron exporter, was downregulated in the atrium of endotoxemia model. Vulnerability to AF was also significantly increased in a endotoxemia rat model. Additionally, Fpn knockdown by shFpn further increased intracellular iron concentration and oxidative stress and exaggerated the AF vulnerability, which was alleviated by ferroptosis inhibition. Mechanistically, silencing Fpn worsened the alterations in calcium handling proteins expression in a endotoxemia rat model. These findings suggest that Fpn-mediated ferroptosis is involved in the new-onset AF with LPS-induced endotoxemia via worsening the calcium handling proteins dysregulation and provides a novel and promising strategy for preventing AF development in sepsis.

Iron Overload, Oxidative Stress and Calcium Mishandling in Cardiomyocytes: Role of the Mitochondrial Permeability Transition Pore

Iron (Fe) plays an essential role in many physiological processes. Hereditary hemochromatosis or frequent blood transfusions often cause iron overload (IO), which can lead to cardiomyopathy and arrhythmias; however, the underlying mechanism is not well defined. In the present study, we assess the hypothesis that IO promotes arrhythmias via reactive oxygen species (ROS) production, mitochondrial membrane potential (∆Ψm) depolarization, and disruption of cytosolic Ca dynamics. In ventricular myocytes isolated from wild type (WT) mice, both cytosolic and mitochondrial Fe levels were elevated following perfusion with the Fe3+/8-hydroxyquinoline (8-HQ) complex. IO promoted mitochondrial superoxide generation (measured using MitoSOX Red) and induced the depolarization of the ΔΨm (measured using tetramethylrhodamine methyl ester, TMRM) in a dose-dependent manner. IO significantly increased the rate of Ca wave (CaW) formation measured in isolated ventricular myocytes using Fluo-4. Furthermore, in ex-vivo Langendorff-perfused hearts, IO increased arrhythmia scores as evaluated by ECG recordings under programmed S1-S2 stimulation protocols. We also carried out similar experiments in cyclophilin D knockout (CypD KO) mice in which the mitochondrial permeability transition pore (mPTP) opening is impaired. While comparable cytosolic and mitochondrial Fe load, mitochondrial ROS production, and depolarization of the ∆Ψm were observed in ventricular myocytes isolated from both WT and CypD KO mice, the rate of CaW formation in isolated cells and the arrhythmia scores in ex-vivo hearts were significantly lower in CypD KO mice compared to those observed in WT mice under conditions of IO. The mPTP inhibitor cyclosporine A (CsA, 1 µM) also exhibited a protective effect. In conclusion, our results suggest that IO induces mitochondrial ROS generation and ∆Ψm depolarization, thus opening the mPTP, thereby promoting CaWs and cardiac arrhythmias. Conversely, the inhibition of mPTP ameliorates the proarrhythmic effects of IO.

Arrhythmias and Sudden Cardiac Death in Beta-Thalassemia Major Patients: Noninvasive Diagnostic Tools and Early Markers

Beta-thalassemias are a group of inherited, autosomal recessive diseases, characterized by reduced or absent synthesis of beta-globin chains of the hemoglobin tetramer, resulting in variable phenotypes, ranging from clinically asymptomatic individuals to severe anemia. Three main forms have been described: heterozygotes, homozygotes β+, and homozygotes β°. Beta-thalassemia major (β-TM), the most serious form, is characterized by an absent synthesis of globin chains that are essential for hemoglobin formation, causing chronic hemolytic anemia. Cardiac complications represent a leading cause of mortality in β-TM patients, although an important and progressive increase of life expectancy has been demonstrated after the introduction of chelating therapies. Iron overload is the primary factor of cardiac damage resulting in thalassemic cardiomyopathy, in which diastolic dysfunction usually happens before systolic impairment and overt heart failure (HF). Although iron-induced cardiomyopathy is slowly progressive and it usually takes several decades for clinical and laboratory features of cardiac dysfunction to manifest, arrhythmias or sudden death may be present without signs of cardiac disease and only if myocardial siderosis is present. Careful analysis of electrocardiograms and other diagnostic tools may help in early identification of high-risk β-TM patients for arrhythmias and sudden cardiac death.

Iron overload and arrhythmias: Influence of confounding factors

Arrhythmias as a cardiac complication of iron overload (IO) have been well described for decades in the clinical literature. They are assumed to be directly associated with the myocardial accumulation of iron. However, the influence of heart failure and elevated oxidative stress, which are major arrhythmogenic confounding factors associated with IO on arrhythmias, has not been critically reviewed in the published literature. A comprehensive narrative review of published articles in PubMed was conducted to address the influence of confounding factors of IO on arrhythmias. The previous data may have been largely confounded by the other cardiac complications of IO, particularly heart failure. The previous studies on IO-related arrhythmias lack proper age-gender-matched control subjects and/or comparison groups with properly controlled confounding factors to assess accurately their etiology and clinical significance. Given the above considerations, further mechanistic investigations to clarify the etiology and clinical relevance of IO-induced arrhythmias are needed. In addition, investigations to develop arrhythmia management strategy specific to IO, are warranted.

Involvement of cytosolic and mitochondrial iron in iron overload cardiomyopathy: an update

Iron overload cardiomyopathy (IOC) is a major cause of death in patients with diseases associated with chronic anemia such as thalassemia or sickle cell disease after chronic blood transfusions. Associated with iron overload conditions, there is excess free iron that enters cardiomyocytes through both L- and T-type calcium channels thereby resulting in increased reactive oxygen species being generated via Haber-Weiss and Fenton reactions. It is thought that an increase in reactive oxygen species contributes to high morbidity and mortality rates. Recent studies have, however, suggested that it is iron overload in mitochondria that contributes to cellular oxidative stress, mitochondrial damage, cardiac arrhythmias, as well as the development of cardiomyopathy. Iron chelators, antioxidants, and/or calcium channel blockers have been demonstrated to prevent and ameliorate cardiac dysfunction in animal models as well as in patients suffering from cardiac iron overload. Hence, either a mono-therapy or combination therapies with any of the aforementioned agents may serve as a novel treatment in iron-overload patients in the near future. In the present article, we review the mechanisms of cytosolic and/or mitochondrial iron load in the heart which may contribute synergistically or independently to the development of iron-associated cardiomyopathy. We also review available as well as potential future novel treatments.

Increased Ferritin Concentration and Risk of Atrial Fibrillation and Heart Failure in Men and Women: Three Studies of the Danish General Population Including 35799 Individuals

BACKGROUND

Moderately increased plasma ferritin, as a biomarker of iron overload, has been associated with higher rates of cardiovascular death and heart failure. However, the association of moderately increased plasma ferritin with risk of atrial fibrillation in the general population is unknown.

METHODS

We examined the association of plasma ferritin concentrations with risk of atrial fibrillation and heart failure in metaanalyses of 35799 men and women from 3 studies of the Danish general population: the Copenhagen City Heart Study, the Danish General Suburban Population Study, and the Copenhagen General Population Study.

RESULTS

Multivariable adjusted fixed effects odds ratios for atrial fibrillation were 1.23 (95% CI, 1.05-1.44; P = 0.005) in men for ferritin concentration ≥300 μg/L vs <300 μg/L, 1.13 (95% CI, 0.93-1.38; P = 0.22) in women for ≥200 μg/L vs <200 μg/L, and 1.19 (95% CI, 1.06-1.35; P = 0.005) in both sexes combined (P _{sex interaction} = 0.52). Corresponding fixed effects odds ratios for heart failure were 1.16 (95% CI, 0.98-1.37; P = 0.08) in men, 0.86 (95% CI, 0.67-1.10; P = 0.23) in women, and 1.05 (95% CI, 0.91-1.21; P = 0.45) in both sexes combined (P _{sex interaction} = 0.05). Multivariable adjusted fixed effects odds ratio for atrial fibrillation per step increase in ferritin concentrations was 1.13 (95% CI, 1.06-1.21; P _trend = 0.0005) in both sexes combined (P _{sex interaction} = 0.59); the corresponding value for heart failure was 1.03 (95% CI, 0.95-1.11; P _trend = 0.47) (P _{sex interaction} = 0.08). In sensitivity analyses, there was no evidence of U-shaped relationships between plasma ferritin concentrations and risk of atrial fibrillation or heart failure in men or women.

CONCLUSIONS

Increased ferritin concentration is associated with increased risk of atrial fibrillation in the general population.

Cellular Electrophysiology of Iron-Overloaded Cardiomyocytes

Iron, the most abundant transition metal element in the human body, plays an essential role in many physiological processes. However, without a physiologically active excretory pathway, iron is subject to strict homeostatic processes acting upon its absorption, storage, mobilization, and utilization. These intricate controls are perturbed in primary and secondary hemochromatoses, leading to a deposition of excess iron in multiple vital organs including the heart. Iron overload cardiomyopathy is the leading cause of mortality in patients with iron overload conditions. Apart from mechanical deterioration of the siderotic myocardium, arrhythmias reportedly contribute to a substantial portion of cardiac death associated with iron overload. Despite this significant impact, the cellular mechanisms of electrical disturbances in an iron-overloaded heart are still incompletely characterized. This review article focuses on cellular electrophysiological studies that directly investigate the effects of iron overload on the function of cardiac ion channels, including trans-sarcolemmal and sarcoplasmic reticulum Ca²⁺ fluxes, as well as cardiac action potential morphology. Our ultimate aim is to provide a comprehensive summary of the currently available information that will encourage and facilitate further mechanistic elucidation of iron-induced pathoelectrophysiological changes in the heart.

Association of GSTT1/GSTM1 and ApoE variants with left ventricular diastolic dysfunction in thalassaemia major patients

BACKGROUND

Cardiomyocytes are particularly susceptible to complications from iron loading. The blood transfusions in thalassaemia major create loading of iron that cannot be naturally excreted. Apolipoprotein E and Glutathione S-transferase act as the scavenger of free radicals, which are generated due to excess iron. The variants of Apolipoprotein E (ApoE) and Glutathione S-transferase (GST) may play a role in oxidative damage-induced cardiomyopathy, so we aimed to study the association of genetic variants of these genes on diastolic dysfunction in our patients.

MATERIALS AND METHODS

One hundred and five β-thalassaemia patients older than 10 years were enrolled for the study. Two-dimensional and M-mode echocardiography analysis was done in all patients. Genotyping of the genetic variants of aforementioned genes was done using the PCR-RFLP method. Serum Glutathione S-transferase levels were estimated by ELISA.

RESULTS

Diastolic dysfunction was observed in 24 (22.8%) patients, whereas left ventricular hypertrophy was present in 37(35.2%) patients. There was a significant association of GSTM1 null allele with diastolic dysfunction only. Serum GST levels were also positively correlated with e/a and e/e' ratio. Positive association of ApoE E2 allele with the diastolic dysfunction was also seen.

CONCLUSIONS

Patients having Glutathione S-transferase M1 allele and Apolipoprotein E E2 allele are predisposed to oxidative stress-induced cardiac injury.

Management of cardiac hemochromatosis

Iron-overload syndromes may be hereditary or acquired. Patients may be asymptomatic early in the disease. Once heart failure develops, there is rapid deterioration. Cardiac hemochromatosis is characterized by a dilated cardiomyopathy with dilated ventricles, reduced ejection fraction, and reduced fractional shortening. Deposition of iron may occur in the entire cardiac conduction system, especially the atrioventricular node. Cardiac hemochromatosis should be considered in any patient with unexplained heart failure. Screening for systemic iron overload with serum ferritin and transferin saturation should be performed. If these tests are consistent with iron overload, further noninvasive and histologic confirmation is indicated to confirm organ involvement with iron overload. Cardiac magnetic resonance imaging is superior to other diagnostic tests since it can quantitatively assess myocardial iron load. Therapeutic phlebotomy is the therapy of choice in nonanemic patients with cardiac hemochromatosis. Therapeutic phlebotomy should be started in men with serum ferritin levels of 300 μg/l or more and in women with serum ferritin levels of 200 μg/l or more. Therapeutic phlebotomy consists of removing 1 unit of blood (450 to 500 ml) weekly until the serum ferritin level is 10 to 20 μg/l and maintenance of the serum ferritin level at 50 μg/l or lower thereafter by periodic removal of blood. Phlebotomy is not a treatment option in patients with anemia (secondary iron-overload disorders) nor in patients with severe congestive heart failure. In these patients, the treatment of choice is iron chelation therapy.

Application of Whole Exome Sequencing in the Clinical Diagnosis and Management of Inherited Cardiovascular Diseases in Adults

BACKGROUND

With the advent of high throughput sequencing, the identification of genetic causes of cardiovascular disease (CVD) has become an integral part of medical diagnosis and management and at the forefront of personalized medicine in this field. The use of whole exome sequencing for clinical diagnosis, risk stratification, and management of inherited CVD has not been previously evaluated.

METHODS AND RESULTS

We analyzed the results of whole exome sequencing in first 200 adult patients with inherited CVD, who underwent genetic testing at the Yale Program for Cardiovascular Genetics. Genetic diagnosis was reached and reported with a success rate of 26.5% (53 of 200 patients). This compares to 18% (36 of 200) that would have been diagnosed using commercially available genetic panels (P=0.04). Whole exome sequencing was particularly useful for clinical diagnosis in patients with aborted sudden cardiac death, in whom the primary insult for the presence of both depressed cardiac function and prolonged QT had remained unknown. The analysis of the remaining cases using genome annotation and disease segregation led to the discovery of novel candidate genes in another 14% of the cases.

CONCLUSIONS

Whole exome sequencing is an exceptionally valuable screening tool for its capability to establish the clinical diagnosis of inherited CVDs, particularly for poorly defined cases of sudden cardiac death. By presenting novel candidate genes and their potential disease associations, we also provide evidence for the use of this genetic tool for the identification of novel CVD genes. Creation and sharing of exome databases across centers of care should facilitate the discovery of unknown CVD genes.

Heart Rate Recovery as a Novel Test for Predicting Cardiac Involvement in Beta-Thalassemia Major

BACKGROUND

Abnormal heart rate recovery (HRR) is predictive of cardiac mortality. Autonomic abnormalities in beta-thalassemia major (TM) patients have been reported in previous studies. However, the importance of low HRR in exercise stress test in TM patients has not yet been ascertained. Therefore, this study will be the first of its kind in the literature.

METHODS

Exercise stress test was performed on 56 TM patients who were being treated at the Thalassemia Center of our hospital, along with 46 non-TM iron deficiency anemia (IDA) patients as a control group. Values for HHR were recorded at 1, 2, 3, 4 and 5 min, and HRR was calculated by the difference of heart rate at peak exercise and at a specific time interval following the onset of recovery.

RESULTS

All HRR values were found to be lower in TM patients compared to those in the IDA group. Exercise capacity [metabolic equivalents (METs)] was also found to be low in these patients (p < 0.001) as well. Total exercise time was significantly lower in the TM group compared to the IDA group (8.40 ± 1.7 min vs. 11.17 ± 1.51 min, p < 0.001). Exercise capacity (METs) was also lower in the TM group compared to the IDA group. Mean T2* value was 28.3 ± 13.7 ms in TM patients on magnetic resonance imaging (MRI). In addition, there are 18 TM patients with T2* value was < 20 ms.

CONCLUSIONS

This study found that TM was independently associated with low HRR. Such a condition is an indicator of autonomic dysfunction in TM patients, since abnormal HRR is related to impaired autonomic response. In addition, impaired HRR may be a marker of early cardiac involvement in patients, whose T2* value is high on MRI. Modifying HRR with a cardiac rehabilitation program in TM patients with impaired HRR is a field open for further investigation.

Physical and functional interaction of Snapin with Cav1.3 calcium channel impacts channel protein trafficking in atrial myocytes

The L-type Ca²⁺ channel (LTCC) Cav1.3 plays a critical role in generating electrical activity in atrial myocytes and cardiac pacemaker cells. However, the molecular and functional basis of Cav1.3 modulation in atrial myocytes has not yet been fully understood. By using the yeast two-hybrid system (Y2H), a Cav1.3-associated protein was screened, which was identified as Snapin. Physical interaction and co-localization between Snapin and Cav1.3 were then confirmed in both the heterologous expression system and mouse atrial myocytes. Direct interaction between them was additionally addressed in a GST pull down assay. Furthermore, both total and membrane expressions of Cav1.3 were significantly impaired by Snapin overexpression, resulting in the ubiquitin-proteasomal degradation of Cav1.3 and a consequent reduction of the densities of whole-cell I_Ca-L. Snapin-induced down-regulation of Cav1.3 was reversed by SNAP-23 competitively. What is more important is that the depressed-expression of Cav1.3 paralleled with enhanced-expression of Snapin was documented in atrial samples from atrial fibrillation (AF) patients. Our results provide the evidence of a direct regulatory role of Snapin on Cav1.3 channels in atrial myocytes, and highlight a potential role of Snapin in the regulation of Cav1.3 in atrial arrhythmogenesis.

Rescuing cardiac automaticity in L-type Cav1.3 channelopathies and beyond

Pacemaker activity of the sino-atrial node generates the heart rate. Disease of the sinus node and impairment of atrioventricular conduction induce an excessively low ventricular rate (bradycardia), which cannot meet the needs of the organism. Bradycardia accounts for about half of the total workload of clinical cardiologists. The 'sick sinus' syndrome (SSS) is characterized by sinus bradycardia and periods of intermittent atrial fibrillation. Several genetic or acquired risk factors or pathologies can lead to SSS. Implantation of an electronic pacemaker constitutes the only available therapy for SSS. The incidence of SSS is forecast to double over the next 50 years, with ageing of the general population thus urging the development of complementary or alternative therapeutic strategies. In recent years an increasing number of mutations affecting ion channels involved in sino-atrial automaticity have been reported to underlie inheritable SSS. L-type Ca_v 1.3 channels play a major role in the generation and regulation of sino-atrial pacemaker activity and atrioventricular conduction. Mutation in the CACNA1D gene encoding Ca_v 1.3 channels induces loss-of-function in channel activity and underlies the sino-atrial node dysfunction and deafness syndrome (SANDD). Mice lacking Ca_v 1.3 channels (Ca_v 1.3^-/- ) fairly recapitulate SSS and constitute a precious model to test new therapeutic approaches to handle this disease. Work in our laboratory shows that targeting G protein-gated K⁺ (I_KACh ) channels effectively rescues SSS of Ca_v 1.3^-/- mice. This new concept of 'compensatory' ion channel targeting shines new light on the principles underlying the pacemaker mechanism and may open the way to new therapies for SSS.

Electrocardiographic Presentation, Cardiac Arrhythmias, and Their Management in β-Thalassemia Major Patients

Beta-thalassemia major (β-TM) is a genetic hemoglobin disorder characterized by an absent synthesis of globin chains that are essential for hemoglobin formation, causing chronic hemolytic anemia. Clinical management of thalassemia major consists in regular long-life red blood cell transfusions and iron chelation therapy to remove iron introduced in excess with transfusions. Iron deposition in combination with inflammatory and immunogenic factors is involved in the pathophysiology of cardiac dysfunction in these patients. Heart failure and arrhythmias, caused by myocardial siderosis, are the most important life-limiting complications of iron overload in beta-thalassemia patients. Cardiac complications are responsible for 71% of global death in the beta-thalassemia major patients. The aim of this review was to describe the most frequent electrocardiographic abnormalities and arrhythmias observed in β-TM patients, analyzing their prognostic impact and current treatment strategies.

Impact of Iron and Homocysteine Levels on T Peak-to-End Interval and Tp-e/QT Ratio in Elite Athletes

BACKGROUND

Electrocardiography (ECG) is frequently used in preparticipation evaluation of competitive athletes. Repolarization heterogeneities on ECG is a well-known indicator for malignant ventricular arrhythmias and sudden cardiac death. We aimed to investigate the effect of iron and homocysteine levels on arrhythmogenic indicators, T peak-to-end (Tp-e) interval, and Tp-e/QT ratio in elite athletes.

METHODS

A total of 72 players (48 football and 24 basketball) with a mean age of 25.4 ± 5.0 years were included to the analysis. Blood biochemistry, homocysteine level, and iron parameters (transferrin saturation and serum iron) were obtained by standard methods. Duration of QRS, QT interval, and Tp-e interval were measured manually on the precordial leads and Tp-e/QT ratio was calculated.

RESULTS

Baseline demographic and clinical characteristics of the study population were compared in two groups according to the median value of Tp-e/QT = 0.219. Both iron and transferrin saturations were higher in the above median group (P = 0.001 and P = 0.002, respectively), however, homocysteine levels were not statistically different among two groups (P = 0.405). In correlation analysis, both serum iron and transferrin saturation were significantly correlated with Tp-e interval (r = 0.368; P = 0.001 and r = 0.394; P = 0.00, respectively) and Tp-e/QT ratio (r = 0.357; P = 0.002 and r = 0.372; P = 0.001, respectively). Multiple stepwise regression analysis revealed that transferrin saturation was an independent predictor of Tp-e interval and Tp-e/QT ratio (β = 0.325; P = 0.002 and β = 0.372; P = 0.001, respectively).

CONCLUSION

This study showed an independent relationship between iron status and Tp-e interval and Tp-e/QT ratios of elite sport players which were also not correlated with serum homocysteine levels.

G protein-gated IKACh channels as therapeutic targets for treatment of sick sinus syndrome and heart block

Dysfunction of pacemaker activity in the sinoatrial node (SAN) underlies "sick sinus" syndrome (SSS), a common clinical condition characterized by abnormally low heart rate (bradycardia). If untreated, SSS carries potentially life-threatening symptoms, such as syncope and end-stage organ hypoperfusion. The only currently available therapy for SSS consists of electronic pacemaker implantation. Mice lacking L-type Cav1.3 Ca(2+) channels (Cav1.3(-/-)) recapitulate several symptoms of SSS in humans, including bradycardia and atrioventricular (AV) dysfunction (heart block). Here, we tested whether genetic ablation or pharmacological inhibition of the muscarinic-gated K(+) channel (IKACh) could rescue SSS and heart block in Cav1.3(-/-) mice. We found that genetic inactivation of IKACh abolished SSS symptoms in Cav1.3(-/-) mice without reducing the relative degree of heart rate regulation. Rescuing of SAN and AV dysfunction could be obtained also by pharmacological inhibition of IKACh either in Cav1.3(-/-) mice or following selective inhibition of Cav1.3-mediated L-type Ca(2+) (ICa,L) current in vivo. Ablation of IKACh prevented dysfunction of SAN pacemaker activity by allowing net inward current to flow during the diastolic depolarization phase under cholinergic activation. Our data suggest that patients affected by SSS and heart block may benefit from IKACh suppression achieved by gene therapy or selective pharmacological inhibition.

Inherited Structural Heart Diseases With Potential Atrial Fibrillation Occurrence

Inherited cardiac diseases inducing structural remodeling of the myocardium sometimes develop arrhythmias of various kinds. Among these rhythm disturbances, atrial fibrillation is well known to frequently worsen the prognosis of the primary disorder by increasing morbidity and mortality, especially because of a higher rate of heart failure. In this manuscript, we have reviewed the literature on the most important inherited structural cardiac diseases in whose clinical history atrial fibrillation may occur fairly often.

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

Background—

Atrial natriuretic peptide (ANP) is a hormone with numerous beneficial cardiovascular effects. Recently, a mutation in the ANP gene, which results in the generation of a mutant form of ANP (mANP), was identified and shown to cause atrial fibrillation in people. The mechanism(s) through which mANP causes atrial fibrillation is unknown. Our objective was to compare the effects of wild-type ANP and mANP on atrial electrophysiology in mice and humans.

Methods and Results—

Action potentials (APs), L-type Ca 2+ currents ( I Ca,L ), and Na + current were recorded in atrial myocytes from wild-type or natriuretic peptide receptor C knockout (NPR-C −/− ) mice. In mice, ANP and mANP (10–100 nmol/L) had opposing effects on atrial myocyte AP morphology and I Ca,L . ANP increased AP upstroke velocity ( V max ), AP duration, and I Ca,L similarly in wild-type and NPR-C −/− myocytes. In contrast, mANP decreased V max , AP duration, and I Ca,L , and these effects were completely absent in NPR-C −/− myocytes. ANP and mANP also had opposing effects on I Ca,L in human atrial myocytes. In contrast, neither ANP nor mANP had any effect on Na + current in mouse atrial myocytes. Optical mapping studies in mice demonstrate that ANP sped electric conduction in the atria, whereas mANP did the opposite and slowed atrial conduction. Atrial pacing in the presence of mANP induced arrhythmias in 62.5% of hearts, whereas treatment with ANP completely prevented the occurrence of arrhythmias.

Conclusions—

These findings provide mechanistic insight into how mANP causes atrial fibrillation and demonstrate that wild-type ANP is antiarrhythmic.

Iron-induced damage in cardiomyopathy: oxidative-dependent and independent mechanisms

The high incidence of cardiomyopathy in patients with hemosiderosis, particularly in transfusional iron overload, strongly indicates that iron accumulation in the heart plays a major role in the process leading to heart failure. In this context, iron-mediated generation of noxious reactive oxygen species is believed to be the most important pathogenetic mechanism determining cardiomyocyte damage, the initiating event of a pathologic progression involving apoptosis, fibrosis, and ultimately cardiac dysfunction. However, recent findings suggest that additional mechanisms involving subcellular organelles and inflammatory mediators are important factors in the development of this disease. Moreover, excess iron can amplify the cardiotoxic effect of other agents or events. Finally, subcellular misdistribution of iron within cardiomyocytes may represent an additional pathway leading to cardiac injury. Recent advances in imaging techniques and chelators development remarkably improved cardiac iron overload detection and treatment, respectively. However, increased understanding of the pathogenic mechanisms of iron overload cardiomyopathy is needed to pave the way for the development of improved therapeutic strategies.

Functional role of voltage gated Ca(2+) channels in heart automaticity

Pacemaker activity of automatic cardiac myocytes controls the heartbeat in everyday life. Cardiac automaticity is under the control of several neurotransmitters and hormones and is constantly regulated by the autonomic nervous system to match the physiological needs of the organism. Several classes of ion channels and proteins involved in intracellular Ca(2+) dynamics contribute to pacemaker activity. The functional role of voltage-gated calcium channels (VGCCs) in heart automaticity and impulse conduction has been matter of debate for 30 years. However, growing evidence shows that VGCCs are important regulators of the pacemaker mechanisms and play also a major role in atrio-ventricular impulse conduction. Incidentally, studies performed in genetically modified mice lacking L-type Cav1.3 (Cav1.3(-/-)) or T-type Cav3.1 (Cav3.1(-/-)) channels show that genetic inactivation of these channels strongly impacts pacemaking. In cardiac pacemaker cells, VGCCs activate at negative voltages at the beginning of the diastolic depolarization and importantly contribute to this phase by supplying inward current. Loss-of-function of these channels also impairs atrio-ventricular conduction. Furthermore, inactivation of Cav1.3 channels promotes also atrial fibrillation and flutter in knockout mice suggesting that these channels can play a role in stabilizing atrial rhythm. Genomic analysis demonstrated that Cav1.3 and Cav3.1 channels are widely expressed in pacemaker tissue of mice, rabbits and humans. Importantly, human diseases of pacemaker activity such as congenital bradycardia and heart block have been attributed to loss-of-function of Cav1.3 and Cav3.1 channels. In this article, we will review the current knowledge on the role of VGCCs in the generation and regulation of heart rate and rhythm. We will discuss also how loss of Ca(2+) entry through VGCCs could influence intracellular Ca(2+) handling and promote atrial arrhythmias.

Increased atrial arrhythmia susceptibility induced by intense endurance exercise in mice requires TNFα

Atrial fibrillation (AF) is the most common supraventricular arrhythmia that, for unknown reasons, is linked to intense endurance exercise. Our studies reveal that 6 weeks of swimming or treadmill exercise improves heart pump function and reduces heart-rates. Exercise also increases vulnerability to AF in association with inflammation, fibrosis, increased vagal tone, slowed conduction velocity, prolonged cardiomyocyte action potentials and RyR2 phosphorylation (CamKII-dependent S2814) in the atria, without corresponding alterations in the ventricles. Microarray results suggest the involvement of the inflammatory cytokine, TNFα, in exercised-induced atrial remodelling. Accordingly, exercise induces TNFα-dependent activation of both NFκB and p38MAPK, while TNFα inhibition (with etanercept), TNFα gene ablation, or p38 inhibition, prevents atrial structural remodelling and AF vulnerability in response to exercise, without affecting the beneficial physiological changes. Our results identify TNFα as a key factor in the pathology of intense exercise-induced AF.

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

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

Effects of natriuretic peptides on electrical conduction in the sinoatrial node and atrial myocardium of the heart

Natriuretic peptides, including B-type and C-type natriuretic peptide (BNP and CNP), are powerful regulators of the cardiovascular system; however, their electrophysiological effects in the heart, particularly in the sinoatrial node (SAN), are incompletely understood. We have used high-resolution optical mapping to measure the effects of BNP and CNP, and the roles of natriuretic peptide receptors (NPR-A, NPR-B and NPR-C), on electrical conduction within the SAN and atrial myocardium. In basal conditions BNP and CNP (50-500 nm) increased conduction velocity (CV) within the SAN by ∼30% at the high dose and shifted the initial exit site superiorly. These effects sped conduction from the SAN to the surrounding atrial myocardium and were mediated by the NPR-A and NPR-B receptors. In the presence of isoproterenol (1 μm) the NPR-C receptor made a major contribution to the effects of BNP and CNP in the heart. In these conditions BNP, CNP and the NPR-C agonist cANF each decreased SAN CV and shifted the initial exit site inferiorly. The effects of cANF (30% reduction) were larger than BNP or CNP (∼15% reduction), indicating that BNP and CNP activate multiple natriuretic peptide receptors. In support of this, the inhibitory effects of BNP were absent in NPR-C knockout mice, where BNP instead elicited a further increase (∼25%) in CV. Measurements in externally paced atrial preparations demonstrate that the effects of natriuretic peptides on CV are partially independent of changes in cycle length. These data provide detailed novel insight into the complex effects of natriuretic peptides and their receptors on electrical conduction in the heart.

Prooxidant mechanisms in iron overload cardiomyopathy

Iron overload cardiomyopathy (IOC), defined as the presence of systolic or diastolic cardiac dysfunction secondary to increased deposition of iron, is emerging as an important cause of heart failure due to the increased incidence of this disorder seen in thalassemic patients and in patients of primary hemochromatosis. At present, although palliative treatment by regular iron chelation was recommended; whereas IOC is still the major cause for mortality in patient with chronic heart failure induced by iron-overloading. Because iron is a prooxidant and the associated mechanism seen in iron-overload heart is still unclear; therefore, we intend to delineate the multiple signaling pathways involved in IOC. These pathways may include organelles such as calcium channels, mitochondria; paracrine effects from both macrophages and fibroblast, and novel mediators such as thromboxane A2 and adiponectin; with increased oxidative stress and inflammation found commonly in these signaling pathways. With further understanding on these complex and inter-related molecular mechanisms, we can propose potential therapeutic strategies to ameliorate the cardiac toxicity induced by iron-overloading.