Heart failure enhanced pulmonary vein arrhythmogenesis and dysregulated sodium and calcium homeostasis with increased calcium sparks

Ibrutinib, a Bruton's tyrosine kinase inhibitor, regulates ventricular electromechanical activities and enhances arrhythmogenesis

BACKGROUND

Ibrutinib, a Bruton's tyrosine kinase inhibitor used in cancer therapy, exerts ventricular proarrhythmic effects; however, the underlying mechanisms remain unclear. Excitation-contraction coupling (E-C) disorders are pivotal for the genesis of ventricular arrhythmias (VAs), which arise mainly from the right ventricular outflow tract (RVOT). In this study, we aimed to comprehensively investigate whether ibrutinib regulates the electromechanical activities of the RVOT, leading to enhanced arrhythmogenesis, and explore the underlying mechanisms.

METHODS

We utilized conventional microelectrodes to synchronously record electrical and mechanical responses in rabbit RVOT tissue preparations before and after treatment with ibrutinib (10, 50, and 100 nM) and investigated their electromechanical interactions and arrhythmogenesis during programmed electrical stimulation. The fluorometric ratio technique was used to measure intracellular calcium concentration in isolated RVOT myocytes.

RESULTS

Ibrutinib (10-100 nM) shortened the action potential duration. Ibrutinib at 100 nM significantly increased pacing-induced ventricular tachycardia (VT) (from 0% to 62.5%, n = 8, p = 0.025). Comparisons between pacing-induced VT and non-VT episodes demonstrated that VT episodes had a greater increase in contractility than that of non-VT episodes (402.1 ± 41.4% vs. 232.4 ± 29.2%, p = 0.003). The pretreatment of ranolazine (10 μM, a late sodium current blocker) prevented the occurrence of ibrutinib-induced VAs. Ibrutinib (100 nM) increased late sodium current, reduced intracellular calcium transients, and enhanced calcium leakage in RVOT myocytes.

CONCLUSION

Ibrutinib increased the risk of VAs in the RVOT due to dysregulated electromechanical responses, which can be attenuated by ranolazine or apamin.

Unveiling the Hidden Stroke Threat in Patients With Atrial Fibrillation and Primary Hyperparathyroidism

Recent American College of Cardiology (ACC), American Heart Association (AHA), American College of Clinical Pharmacy (ACCP), and Heart Rhythm Society (HRS) guidelines suggest that patients with atrial fibrillation (AF) at intermediate to low annual risk of ischemic stroke can benefit from consideration of factors that might modify their risk of stroke. The role of nontraditional risk factors, such as primary hyperparathyroidism (PHPT), remains unexplored. In our study, we investigated the potential association between PHPT and the risk of ischemic stroke in patients with AF. Using data from the Nationwide Inpatient Sample Database, a retrospective cohort study focused on the adult population with AF, we stratified the participants based on PHPT presence. Demographic information, co-morbidities, and hospitalization details were extracted using International Classification of Diseases, Tenth revision codes. Propensity score matching was applied, encompassing over 20 confounding variables, including the risk factors outlined in the CHA2DS2-VASc (Congestive heart failure (C), Hypertension (H), Age ≥75 years (A₂), Diabetes Mellitus (D), Stroke/Transient Ischemic Attack (TIA)/Thromboembolism (S₂), Vascular disease (V), Age 65-74 years (A), Sex category [female] (Sc)) score. Multivariate logistic regression analysis was performed after matching to assess the independent impact of PHPT as an ischemic stroke risk factor. A total of 2,051 of the identified 395,249 patients with AF had PHPT. The PHPT group had an average age of 74 years and consisted of more women (66.1% vs 53.0%). After matching, it was observed that the PHPT group had longer hospital stays (5 vs 4 days) and higher hospitalization charges ($45,126 vs $36,644). This group exhibited higher rates of ischemic stroke (6.0% vs 4.4%) and mortality (6.3% vs 4.9%). The adjusted outcomes showed a 1.4-fold increased risk for ischemic stroke and a 1.32-fold increased risk for mortality in the PHPT cohort. The subgroup analysis showed a higher incidence of mortality in men with a high CHA2DS2-VASc score. In conclusion, this study highlights a marked association between PHPT and ischemic stroke in patients with AF, independent of the conventional CHA2DS2-VASc score. The potential mechanisms implicated include vascular changes, cardiac dysfunction, and coagulation cascade alterations. The presence of PHPT should be taken into consideration when deciding the assessment of thromboembolic risk.

The Optimal Management of Patients with Atrial Fibrillation and Acute Heart Failure in the Emergency Department

Atrial fibrillation (AF) and acute heart failure (AHF) are two closely interrelated conditions that frequently coexist in a manifold manner, with AF serving either as the causative factor or as the consequence or even as an innocent bystander. The interplay between these two clinical conditions is complex, given that they share common pathophysiological pathways and they can reciprocally exacerbate each other, thus triggering a vicious cycle that worsens the prognosis and increases the thromboembolic risk. The optimal management of AF in the context of AHF in the emergency department remains a challenge depending on the time onset, as well as the nature and the severity of the associated symptoms. Acute rate control, along with early rhythm control, when indicated, and anticoagulation represent the main pillars of the therapeutic intervention. The purpose of this review is to elucidate the pathophysiological link between AF and AHF and accordingly present a stepwise algorithmic approach for the management of AF in AHF patients in the emergency setting.

Glucagon-like Peptide-1 Receptor Activation Reduces Pulmonary Vein Arrhythmogenesis and Regulates Calcium Homeostasis

Glucagon-like peptide-1 (GLP-1) receptor agonists are associated with reduced atrial fibrillation risk, but the mechanisms underlying this association remain unclear. The GLP-1 receptor agonist directly impacts cardiac Ca2+ homeostasis, which is crucial in pulmonary vein (PV, the initiator of atrial fibrillation) arrhythmogenesis. This study investigated the effects of the GLP-1 receptor agonist on PV electrophysiology and Ca2+ homeostasis and elucidated the potential underlying mechanisms. Conventional microelectrodes and whole-cell patch clamp techniques were employed in rabbit PV tissues and single PV cardiomyocytes before and after GLP-1 (7-36) amide, a GLP-1 receptor agonist. Evaluations were conducted both with and without pretreatment with H89 (10 μM, an inhibitor of protein kinase A, PKA), KN93 (1 μM, an inhibitor of Ca2+/calmodulin-dependent protein kinase II, CaMKII), and KB-R7943 (10 μM, an inhibitor of Na+/Ca2+ exchanger, NCX). Results showed that GLP-1 (7-36) amide (at concentrations of 1, 10, and 100 nM) reduced PV spontaneous activity in a concentration-dependent manner without affecting sinoatrial node electrical activity. In single-cell experiments, GLP-1 (7-36) amide (at 10 nM) reduced L-type Ca2+ current, NCX current, and late Na+ current in PV cardiomyocytes without altering Na+ current. Additionally, GLP-1 (7-36) amide (at 10 nM) increased sarcoplasmic reticulum Ca2+ content in PV cardiomyocytes. Furthermore, the antiarrhythmic effects of GLP-1 (7-36) amide on PV automaticity were diminished when pretreated with H89, KN93, or KB-R7943. This suggests that the GLP-1 receptor agonist may exert its antiarrhythmic potential by regulating PKA, CaMKII, and NCX activity, as well as modulating intracellular Ca2+ homeostasis, thereby reducing PV arrhythmogenesis.

Renal denervation reverses ventricular structural and functional remodeling in failing rabbit hearts

Renal denervation (RDN) suppresses the activity of the renin-angiotensin-aldosterone system and inflammatory cytokines, leading to the prevention of cardiac remodeling. Limited studies have reported the effects of renal denervation on ventricular electrophysiology. We aimed to use optical mapping to evaluate the effect of RDN on ventricular structural and electrical remodeling in a tachycardia-induced cardiomyopathy rabbit model. Eighteen rabbits were randomized into 4 groups: sham control group (n = 5), renal denervation group receiving RDN (n = 5), heart failure group receiving rapid ventricular pacing for 1 month (n = 4), and RDN-heart failure group (n = 4). Rabbit hearts were harvested for optical mapping. Different cycle lengths were paced (400, 300, 250, 200, and 150 ms), and the results were analyzed. In optical mapping, the heart failure group had a significantly slower epicardial ventricular conduction velocity than the other three groups. The RDN-heart failure, sham control, and RDN groups had similar velocities. We then analyzed the 80% action potential duration at different pacing cycle lengths, which showed a shorter action potential duration as cycle length decreased (P for trend < 0.01), which was consistent across all groups. The heart failure group had a significantly longer action potential duration than the sham control and RDN groups. Action potential duration was shorter in the RDN-heart failure group than the heart failure group (P < 0.05). Reduction of conduction velocity and prolongation of action potential duration are significant hallmarks of heart failure, and RDN reverses these remodeling processes.

Vascular endothelial growth factor modulates pulmonary vein arrhythmogenesis via vascular endothelial growth factor receptor 1/NOS pathway

Atrial fibrillation (AF) is a common form of arrhythmia with serious public health impacts, but its underlying mechanisms are not yet fully understood. Vascular endothelial growth factor (VEGF) is highly expressed in the atrium of patients with AF, but whether VEGF affects AF pathogenesis remains unclear. Pulmonary veins (PVs) are important sources for the genesis of atrial tachycardia or AF. Therefore, this study assessed the effects of VEGF on PV electrophysiological properties and evaluated its underlying mechanisms. Conventional microelectrodes and whole-cell patch clamps were performed using isolated rabbit PV preparations or single isolated PV cardiomyocytes before and after VEGF or VEGF receptor (VEGFR), Akt, NOS inhibitor administration. We found that VEGF (0.1, 1, and 10 ng/mL) reduced the PV beating rate in a dose-dependent manner. Furthermore, VEGF (10 ng/mL) reduced late diastolic depolarization and diastolic tension. Isoproterenol increased PV beating and burst firing, which was attenuated by VEGF (1 ng/mL). In the presence of VEGFR-1 inhibition (ZM306416 at 10 μM) and L-NAME (100 μM), VEGF (1 ng/mL) did not alter PV spontaneous activity. In isolated PV cardiomyocytes, VEGF (1 ng/mL) decreased L-type calcium, sodium/calcium exchanger, and late sodium currents. In conclusion, we found that VEGF reduces PV arrhythmogenesis by modulating sodium/calcium homeostasis through VEGFR-1/NOS signaling pathway.

Effects of phosphodiesterase-1 inhibitor on pulmonary vein electrophysiology and arrhythmogenesis

INTRODUCTION

Phosphodiesterase (PDE) isoform inhibitors have mechanical and electrical effects on the heart. Inhibition of PDE-1 enzymes is a novel strategy for treating heart failure. However, the electrophysiological effects of PDE-1 inhibition on the heart remain unclear. This study explored the effects of PDE-1 inhibition using ITI-214 on electrical activity in the pulmonary vein (PV), the most common trigger of atrial fibrillation, and investigated the underlying ionic mechanisms.

METHODS

Conventional microelectrodes or whole-cell patch clamps were employed to study the effects of ITI-214 (0.1-10 μM) on PV electrical activity, mechanical responses and ionic currents in isolated rabbit PV tissue specimens and isolated single PV cardiomyocytes.

RESULTS

ITI-214 at 1 μM and 10 μM (but not 0.1 μM) significantly reduced PV spontaneous beating rate (10 ± 2% and 10 ± 3%, respectively) and PV diastolic tension (11 ± 3% and 17 ± 3%, respectively). ITI-24 (1 μM) significantly reduced late sodium current (I_Na-Late ), L-type calcium current (I_Ca-L ) and the reverse mode of the sodium-calcium exchanger (NCX), but it did not affect peak sodium currents.

CONCLUSIONS

ITI-214 reduces PV spontaneous activity and PV diastolic tension by reducing I_Na-Late , I_Ca-L and NCX current. Considering its therapeutic potential in heart failure, targeting PDE-1 inhibition may provide a novel strategy for managing atrial arrhythmogenesis.

Modulated Calcium Homeostasis and Release Events Under Atrial Fibrillation and Its Risk Factors: A Meta-Analysis

Background: Atrial fibrillation (AF) is associated with calcium (Ca²⁺) handling remodeling and increased spontaneous calcium release events (SCaEs). Nevertheless, its exact mechanism remains unclear, resulting in suboptimal primary and secondary preventative strategies.

Methods: We searched the PubMed database for studies that investigated the relationship between SCaEs and AF and/or its risk factors. Meta-analysis was used to examine the Ca²⁺ mechanisms involved in the primary and secondary AF preventative groups.

Results: We included a total of 74 studies, out of the identified 446 publications from inception (1982) until March 31, 2020. Forty-five were primary and 29 were secondary prevention studies for AF. The main Ca²⁺ release events, calcium transient (standardized mean difference (SMD) = 0.49; I² = 35%; confidence interval (CI) = 0.33–0.66; p < 0.0001), and spark amplitude (SMD = 0.48; I² = 0%; CI = −0.98–1.93; p = 0.054) were enhanced in the primary diseased group, while calcium transient frequency was increased in the secondary group. Calcium spark frequency was elevated in both the primary diseased and secondary AF groups. One of the key cardiac currents, the L-type calcium current (I_CaL) was significantly downregulated in primary diseased (SMD = −1.07; I² = 88%; CI = −1.94 to −0.20; p < 0.0001) and secondary AF groups (SMD = −1.28; I² = 91%; CI = −2.04 to −0.52; p < 0.0001). Furthermore, the sodium–calcium exchanger (I_NCX) and NCX1 protein expression were significantly enhanced in the primary diseased group, while only NCX1 protein expression was shown to increase in the secondary AF studies. The phosphorylation of the ryanodine receptor at S2808 (pRyR-S2808) was significantly elevated in both the primary and secondary groups. It was increased in the primary diseased and proarrhythmic subgroups (SMD = 0.95; I² = 64%; CI = 0.12–1.79; p = 0.074) and secondary AF group (SMD = 0.66; I² = 63%; CI = 0.01–1.31; p < 0.0001). Sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) expression was elevated in the primary diseased and proarrhythmic drug subgroups but substantially reduced in the secondary paroxysmal AF subgroup.

Conclusions: Our study identified that I_CaL is reduced in both the primary and secondary diseased groups. Furthermore, pRyR-S2808 and NCX1 protein expression are enhanced. The remodeling leads to elevated Ca²⁺ functional activities, such as increased frequencies or amplitude of Ca²⁺ spark and Ca²⁺ transient. The main difference identified between the primary and secondary diseased groups is SERCA expression, which is elevated in the primary diseased group and substantially reduced in the secondary paroxysmal AF subgroup. We believe our study will add new evidence to AF mechanisms and treatment targets.

Mechanism of angiotensin receptor-neprilysin inhibitor in suppression of ventricular arrhythmia

BACKGROUND

The mechanisms underlying angiotensin receptor-neprilysin inhibitor (ARNi) suppression of ventricular arrhythmia (VA) are unclear. This study aimed to investigate the mechanism of ARNi-related suppression of VA in a heart failure (HF) model.

METHODS

New Zealand white rabbits (n = 6 per group) were assigned to normal, HF [4 weeks of left ascending artery (LAD) ligation], angiotensin receptor blocker (ARB, valsartan at 27 mg/kg/day for 3 weeks after 1 week of LAD ligation), and ARNi (sacubitril at 34 mg/kg/day and valsartan at 27 mg/kg/day for 3 weeks after 1 week of LAD ligation) groups. Experiments involving echocardiogram, optical mapping, histological of trichrome stain and immunostain, and flow cytometry were performed.

RESULTS

HF group had larger left ventricular (LV) internal dimensions in diastole and systole, and lower LV ejection fraction and fractional shortening than normal, ARB, and ARNi groups. HF group had a prolonged action potential duration (APD) and decreased conduction velocity (CV), which was mitigated in ARB and ARNi groups. HF group had a prolonged QRS duration, QT and QTc intervals, which was reversed in ARB and ARNi groups. HF group had a steeper maximum slope of APD restitutions, which was attenuated in normal, ARB, and ARNi groups. HF group had increased number of phase singularities (PSs) and VA inducibility than normal, ARB, and ARNi groups. A higher content of fibrosis was found in HF group than that in normal, ARB, and ARNi groups. Compared to ARB group, ARNi had a lower context of fibrosis. HF group had more peripheral blood CD4⁺ and CD8⁺ cells count than normal, ARB, and ARNi group.

CONCLUSIONS

In a rabbit model of ischemic HF, ventricular arrhythmogenesis could be suppressed by ARNi treatment. This appears to be mediated by reversing changes in the APD, CV, maximum slope of the APDR, PSs, fibrosis, and inflammation.

Dynamics of Atrial Fibrillation Mechanisms and Comorbidities

Atrial fibrillation (AF) contributes to morbidity and mortality of millions of individuals. Its molecular, cellular, neurohumoral, and hemodynamic pathophysiological mechanisms are complex, and there is increasing awareness that a wide range of comorbidities can contribute to AF-promoting atrial remodeling. Moreover, recent research has highlighted that AF risk is not constant and that the temporal variation in concomitant conditions contributes to the complexity of AF dynamics. In this review, we provide an overview of fundamental AF mechanisms related to established and emerging comorbidities or risk factors and their role in the AF-promoting effects. We focus on the accumulating evidence for the relevance of temporally dynamic changes in these risk factors and the consequence for AF initiation and maintenance. Finally, we highlight the important implications for future research and clinical practice resulting from the dynamic interaction between AF risk factors and mechanisms.

Arginine vasopressin modulates electrical activity and calcium homeostasis in pulmonary vein cardiomyocytes

Background

Atrial fibrillation (AF) frequently coexists with congestive heart failure (HF) and arginine vasopressin (AVP) V1 receptor antagonists are used to treat hyponatremia in HF. However, the role of AVP in HF-induced AF still remains unclear. Pulmonary veins (PVs) are central in the genesis of AF. The purpose of this study was to determine if AVP is directly involved in the regulation of PV electrophysiological properties and calcium (Ca²⁺) homeostasis as well as the identification of the underlying mechanisms.

Methods

Patch clamp, confocal microscopy with Fluo-3 fluorescence, and Western blot analyses were used to evaluate the electrophysiological characteristics, Ca²⁺ homeostasis, and Ca²⁺ regulatory proteins in isolated rabbit single PV cardiomyocytes incubated with and without AVP (1 μM), OPC 21268 (0.1 μM, AVP V1 antagonist), or OPC 41061 (10 nM, AVP V2 antagonist) for 4–6 h.

Results

AVP (0.1 and 1 μM)-treated PV cardiomyocytes had a faster beating rate (108 to 152%) than the control cells. AVP (1 μM) treated PV cardiomyocytes had higher late sodium (Na⁺) and Na⁺/Ca²⁺ exchanger (NCX) currents than control PV cardiomyocytes. AVP (1 μM) treated PV cardiomyocytes had smaller Ca²⁺_i transients, and sarcoplasmic reticulum (SR) Ca²⁺ content as well as higher Ca²⁺ leak. However, combined AVP (1 μM) and OPC 21268 (0.1 μM) treated PV cardiomyocytes had a slower PV beating rate, larger Ca²⁺_i transients and SR Ca²⁺ content, smaller late Na⁺ and NCX currents than AVP (1 μM)-treated PV cardiomyocytes. Western blot experiments showed that AVP (1 μM) treated PV cardiomyocytes had higher expression of NCX and p-CaMKII, and a higher ratio of p-CaMKII/CaMKII.

Conclusions

AVP increases PV arrhythmogenesis with dysregulated Ca²⁺ homeostasis through vasopressin V1 signaling.

Low deceleration capacity is associated with higher stroke risk in patients with paroxysmal atrial fibrillation

Background:

Deceleration capacity (DC) is a non-invasive marker for cardiac autonomic dysfunction; however, few studies have shown that the influence factors of cardiac autonomic dysfunction and the correlations between DC and stroke risk in paroxysmal atrial fibrillation (AF). We aimed to explore the influencing factors of abnormal DC and the relationships between DC and stroke risk in patients with paroxysmal AF.

Methods:

The study included hospitalized paroxysmal AF patients with DC measurements derived from 24-h Holter electrocardiography recordings taken between August 2015 and June 2016. Multivariable regression analysis was performed to evaluate the associations between correlated variables and abnormal DC values. The relationship between DC and ischemic stroke risk scores in patients with paroxysmal AF was analyzed.

Results:

We studied 259 hospitalized patients with paroxysmal AF (143 [55.2%] male, mean age 66.4 ± 12.0 years); 38 patients of them showed abnormal DC values. In the univariate analysis, age, hypertension, heart failure, and previous stroke/transient ischemic attack (TIA) were significantly associated with abnormal DC values. Among these factors, a history of previous stroke/TIA (odds ratio = 2.861, 95% confidence interval: 1.356–6.039) were independently associated with abnormal DC values in patients with paroxysmal AF. The abnormal DC group showed a higher stroke risk with the score of congestive heart failure, hypertension, age >75 years, diabetes mellitus, previous stroke and TIA (CHADS₂) (2.25 ± 1.48 vs. 1.40 ± 1.34, t = −4.907, P = 0.001) and CHA₂DS₂-vascular disease, age 65–74 years and female category (VASc) (3.76 ± 1.95 vs. 2.71 ± 1.87, t = −4.847, P = 0.001) scores. Correlation analysis showed that DC was negatively correlated with CHADS₂ scores (r = −0.290, P < 0.001) and CHA₂DS₂-VASc scores (r = −0.263, P < 0.001).

Conclusions:

Lower DC is closely associated with previous stroke/TIA, and is also correlated negatively with higher stroke risk scores in patients with paroxysmal AF. It could be a potential indicator of stroke risk in paroxysmal AF patients.

Heart Failure Differentially Modulates Natural (Sinoatrial Node) and Ectopic (Pulmonary Veins) Pacemakers: Mechanism and Therapeutic Implication for Atrial Fibrillation

Heart failure (HF) frequently coexists with atrial fibrillation (AF) and dysfunction of the sinoatrial node (SAN), the natural pacemaker. HF is associated with chronic adrenergic stimulation, neurohormonal activation, abnormal intracellular calcium handling, elevated cardiac filling pressure and atrial stretch, and fibrosis. Pulmonary veins (PVs), which are the points of onset of ectopic electrical activity, are the most crucial AF triggers. A crosstalk between the SAN and PVs determines PV arrhythmogenesis. HF has different effects on SAN and PV electrophysiological characteristics, which critically modulate the development of AF and sick sinus syndrome. This review provides updates to improve our current understanding of the effects of HF in the electrical activity of the SAN and PVs as well as therapeutic implications for AF.

Angiotensin II Induces Automatic Activity of the Isolated Guinea Pig Pulmonary Vein Myocardium through Activation of the IP₃ Receptor and the Na⁺-Ca2+ Exchanger

The automaticity of the pulmonary vein myocardium is known to be the major cause of atrial fibrillation. We examined the involvement of angiotensin II in the automatic activity of isolated guinea pig pulmonary vein preparations. In tissue preparations, application of angiotensin II induced an automatic contractile activity; this effect was mimicked by angiotensin I and blocked by losartan, but not by PD123,319 or carvedilol. In cardiomyocytes, application of angiotensin II induced an increase in the frequency of spontaneous Ca²⁺ sparks and the generation of Ca²⁺ transients; these effects were inhibited by losartan or xestospongin C. In tissue preparations, angiotensin II caused membrane potential oscillations, which lead to repetitive generation of action potentials. Angiotensin II increased the diastolic depolarization slope of the spontaneous or evoked action potentials. These effects of angiotensin II were inhibited by SEA0400. In tissue preparations showing spontaneous firing of action potentials, losartan, xestospongin C or SEA0400 decreased the slope of the diastolic depolarization and inhibited the firing of action potentials. In conclusion, in the guinea pig pulmonary vein myocardium, angiotensin II induces the generation of automatic activity through activation of the IP₃ receptor and the Na⁺-Ca²⁺ exchanger.

Possibilities and perspectives of using cardiac contractility modulation in patients with chronic heart failure and atrial fibrillation

Heart failure is one of the main health care problems all over the world. Although, there are many drugs with proven effectiveness and hi-tech devices, there is a continuous process of searching new possibilities in heart failure prophylaxis going on because of huge economic burden and impact on life quality. Developing of atrial fibrillation in heart failure patients increases the risks of hospitalization and all-cause mortality. Appearance of new Optimizer Smart® system of cardiac contractility modulation is a perspective way of treatment in patients with heart failure and atrial fibrillation, who are not a candidate or have not got a good result from cardiac resynchronization therapy (CRT).

Age as a Critical Determinant of Atrial Fibrillation: A Two-sided Relationship

The incidence of atrial fibrillation (AF), the most common sustained arrhythmia and a major public health burden, increases exponentially with age. However, mechanisms underlying this long-recognized association remain incompletely understood. Experimental and human studies have demonstrated the involvement of aging in several arrhythmogenic processes, including atrial electrical and structural remodelling, disturbed calcium homeostasis, and enhanced atrial ectopic activity/increased vulnerability to re-entry induction. Given this wide range of putative mechanisms, the task of delineating the specific effects of aging responsible for AF promotion is not simple, as aging is itself associated with increasing prevalence of a host of AF-predisposing conditions, including heart failure, coronary artery disease, and hypertension. Although we usually think of old age promoting AF, there is also evidence that young age may actually have a protective effect against AF occurrence. For example, the low AF incidence among populations of young patients with significant structural congenital heart disease and substantial atrial enlargement/remodelling suggests that younger age might protect against fibrillation in the diseased atrium; efforts at understating how younger age may prevent AF might be helpful in elucidating missing mechanistic links between AF and age. The goal of this paper is to review the epidemiologic and pathophysiologic evidence regarding mechanisms underlying age-related AF. Although the therapeutic options for AF have recently improved, major gaps still remain and a better understanding of the special relationship between age and AF may be important for the identification of new targets for therapeutic innovation.

Atrial fibrillation in aging population

With aging, the pathogenesis processes of atrial fibrillation (AF) are heightened. In this article, we review the mechanisms that predispose elderly patients to AF. We also highlight the unique features in diagnosis, stroke prevention, and treatment strategies for the elderly patient with AF.

Fibroblast growth factor 23 dysregulates late sodium current and calcium homeostasis with enhanced arrhythmogenesis in pulmonary vein cardiomyocytes

Fibroblast growth factor 23 (FGF23), elevated in chronic renal failure, increases atrial arrhythmogenesis and dysregulates calcium homeostasis. Late sodium currents (I_Na-Late) critically induces ectopic activity of pulmoanry vein (the most important atrial fibrillation trigger). This study was to investigate whether FGF23 activates the I_Na-Late leading to calcium dysregulation and increases PV arrhythmogenesis. Patch clamp, western blot, and confocal microscopy were used to evaluate the electrical activities, calcium homeostasis, and mitochondrial reactive oxygen species (ROS) in PV cardiomyocytes with or without FGF23 (0.1 or 1 ng/mL) incubation for 4~6 h. Compared to the control, FGF23 (1 ng/mL, but not 0.1 ng/mL)-treated PV cardiomyocytes had a faster beating rate. FGF23 (1 ng/mL)-treated PV cardiomyocytes had larger I_Na-Late, calcium transients, and mitochondrial ROS than controls. However, ranolazine (an inhibitor of I_Na-Late) attenuated FGF23 (1 ng/mL)-increased beating rates, calcium transients and mitochondrial ROS. FGF23 (1 ng/mL)-treated PV cardiomyocytes exhibited larger phosphorylation of calcium/calmodulin-dependent protein kinase II (CaMKII). Chelerythrine chloride (an inhibitor of protein kinase C) decreased I_Na-Late in FGF23 (1 ng/mL)-treated PV cardiomyocytes. However, KN93 (a selective CaMKII blocker) decreased I_Na-Late in control and FGF23 (1 ng/mL)-treated PV cardiomyocytes to a similar extent. In conclusion, FGF23 increased PV arrhythmogenesis through sodium and calcium dysregulation by acting protein kinase C signaling.

Angiotensin 1-7 modulates electrophysiological characteristics and calcium homoeostasis in pulmonary veins cardiomyocytes via MAS/PI3K/eNOS signalling pathway

BACKGROUND

Atrial fibrillation (AF) is the most common sustained arrhythmia, and pulmonary veins (PVs) play a critical role in triggering AF. Angiotensin (Ang)-(1-7) regulates calcium (Ca²⁺ ) homoeostasis and also plays a critical role in cardiovascular pathophysiology. However, the role of Ang-(1-7) in PV arrhythmogenesis remains unclear.

MATERIALS AND METHODS

Conventional microelectrodes, whole-cell patch-clamp and the fluo-3 fluorimetric ratio technique were used to record ionic currents and intracellular Ca²⁺ in isolated rabbit PV preparations and in single isolated PV cardiomyocytes, before and after administration of Ang-(1-7).

RESULTS

Ang (1-7) concentration dependently (0.1, 1, 10 and 100 nmol/L) decreased PV spontaneous electrical activity. Ang-(1-7) (100 nmol/L) decreased the late sodium (Na⁺ ), L-type Ca²⁺ and Na⁺ -Ca²⁺ exchanger currents, but did not affect the voltage-dependent Na⁺ current in PV cardiomyocytes. In addition, Ang-(1-7) decreased intracellular Ca²⁺ transient and sarcoplasmic reticulum Ca²⁺ content in PV cardiomyocytes. A779 (a Mas receptor blocker, 3 μmol/L), L-NAME (a NO synthesis inhibitor, 100 μmol/L) or wortmannin (a specific PI3K inhibitor, 10 nmol/L) attenuated the effects of Ang-(1-7) (100 nmol/L) on PV spontaneous electric activity.

CONCLUSION

Ang-(1-7) regulates PV electrophysiological characteristics and Ca²⁺ homoeostasis via Mas/PI3K/eNOS signalling pathway.

Aging Modulates the Substrate and Triggers Remodeling in Atrial Fibrillation

Aging plays a critical role in the genesis of atrial fibrillation (AF) and also increases the risks of cardiac dysfunction and stroke in AF patients. AF is caused by increased AF triggering from abnormalities of the thoracic vein and/or modulated substrate (atrial) with enhancement of AF maintenance. Clinical and laboratory evidence indicates that aging is significant in the creation of atrial electrical and structural remodeling that leads to increased susceptibility to AF occurrence. Aging is commonly associated with cardiovascular comorbidities, oxidative stress, calcium dysregulation, atrial myopathy with apoptosis, and fibrosis, which all contribute to the genesis of AF. This review updates the current understanding of the effects of aging on the pathophysiology of AF.

Redox and Activation of Protein Kinase A Dysregulates Calcium Homeostasis in Pulmonary Vein Cardiomyocytes of Chronic Kidney Disease

Background

Chronic kidney disease (CKD) increases the occurrence of atrial fibrillation and pulmonary vein (PV) arrhythmogenesis. Calcium dysregulation and reactive oxygen species (ROS) enhance PV arrhythmogenic activity. The purposes of this study were to investigate whether CKD modulates PV electrical activity through dysregulation of calcium homeostasis and ROS.

Methods and Results

Biochemical and electrocardiographic studies were conducted in rabbits with and without CKD (induced by 150 mg/kg per day neomycin sulfate and 500 mg/kg per day cefazolin). Confocal microscopy with fluorescence and a whole‐cell patch clamp were applied to study calcium homeostasis and electrical activities in control and CKD isolated single PV cardiomyocytes with or without treatment with H89 (1 μmol/L, a protein kinase A inhibitor) and MPG (N‐[2‐mercaptopropionyl]glycine; 100 μmol/L, a ROS scavenger). The ROS in mitochondria and cytosol were evaluated via intracellular dye fluorescence and lipid peroxidation. CKD rabbits had excessive atrial premature captures over those of control rabbits. Compared with the control, CKD PV cardiomyocytes had a faster beating rate and larger calcium transient amplitudes, sarcoplasmic reticulum calcium contents, sodium/calcium exchanger currents, and late sodium currents but smaller L‐type calcium current densities. CKD PV cardiomyocytes had a higher frequency and longer duration of calcium sparks and more ROS in the mitochondria and cytosol than did controls. Moreover, H89 suppressed all calcium sparks in CKD PV cardiomyocytes, and H89‐ and MPG‐treated CKD PV cardiomyocytes had similar calcium transients compared with control PV cardiomyocytes.

Conclusions

CKD increases PV arrhythmogenesis with enhanced calcium‐handling abnormalities through activation of protein kinase A and ROS.

Atrial Fibrillation: Epidemiology, Pathophysiology, and Clinical Outcomes

The past 3 decades have been characterized by an exponential growth in knowledge and advances in the clinical treatment of atrial fibrillation (AF). It is now known that AF genesis requires a vulnerable atrial substrate and that the formation and composition of this substrate may vary depending on comorbid conditions, genetics, sex, and other factors. Population-based studies have identified numerous factors that modify the atrial substrate and increase AF susceptibility. To date, genetic studies have reported 17 independent signals for AF at 14 genomic regions. Studies have established that advanced age, male sex, and European ancestry are prominent AF risk factors. Other modifiable risk factors include sedentary lifestyle, smoking, obesity, diabetes mellitus, obstructive sleep apnea, and elevated blood pressure predispose to AF, and each factor has been shown to induce structural and electric remodeling of the atria. Both heart failure and myocardial infarction increase risk of AF and vice versa creating a feed-forward loop that increases mortality. Other cardiovascular outcomes attributed to AF, including stroke and thromboembolism, are well established, and epidemiology studies have championed therapeutics that mitigate these adverse outcomes. However, the role of anticoagulation for preventing dementia attributed to AF is less established. Our review is a comprehensive examination of the epidemiological data associating unmodifiable and modifiable risk factors for AF and of the pathophysiological evidence supporting the mechanistic link between each risk factor and AF genesis. Our review also critically examines the epidemiological data on clinical outcomes attributed to AF and summarizes current evidence linking each outcome with AF.

Heart failure modulates electropharmacological characteristics of sinoatrial nodes

The impact of heart failure (HF) on sinoatrial node (SAN) channel regulation and electropharmacological responses has remained elusive. The present study aimed to investigate the effects of HF on the electrical activity of SANs with and without pharmacological interventions. Action potentials (APs) were recorded in isolated SANs from normal rabbits (control) and those with HF (rapid ventricular pacing for 4 weeks) prior to and after administration of a funny current blocker (ivabradine; 0.1, 0.3, 3 or 10 µM), a calmodulin kinase II inhibitor (KN-93; 0.3 or 3 µM), a sarcoplasmic reticulum Ca2+ release inhibitor (ryanodine; 0.3 or 3 µM), a sodium current inhibitor (tetrodotoxin; 1, 3 or 10 µM) and a late sodium current inhibitor (ranolazine; 10 µM). Western blot analysis was used to investigate the protein expression in SANs from normal rabbits and those with HF. Control SANs had a higher beating rate than SANs from rabbits with HF (2.3±0.1 vs. 1.5±0.1 Hz; P<0.001). Similarly, ivabradine (10 µM), KN-93 (3 µM), ranolazine (10 µM) and ryanodine (3 µM) decreased the beating rates of SANs in the control (n=6) and HF (n=6) groups. Ivabradine treatment resulted in a higher incidence of AP block in HF vs. control SANs (66.7 vs. 0%; P<0.05). Tetrodotoxin (1, 3 or 10 µM) decreased the beating rate to a higher extent in SANs from rabbits with HF than in those from control rabbits and induced a higher incidence of AP block (66.7 vs. 0%; P<0.05). Furthermore, SANs from rabbits with HF had higher protein levels of phospholamban (PLB) and lower levels of hyperpolarization-activated cyclic nucleotide-gated potassium channel 4, ryanodine receptor and phosphorylated PLB than control SANs. In conclusion, HF modulates electropharmacological responses in the SAN by channel regulation, which may result in SAN dysfunction.

Pleiotropic Effects of Myocardial MMP-9 Inhibition to Prevent Ventricular Arrhythmia

Observational studies have established a strong association between matrix metalloproteinase-9 (MMP-9) and ventricular arrhythmia. However, whether MMP-9 has a causal link to ventricular arrhythmia, as well as the underlying mechanism, remains unclear. Here, we investigated the mechanistic involvement of myocardial MMP-9 in the pathophysiology of ventricular arrhythmia. Increased levels of myocardial MMP-9 are linked to ventricular arrhythmia attacks after angiotensin II (Ang II) treatment. MMP-9-deficient mice were protected from ventricular arrhythmia. Increased expressions of protein kinase A (PKA) and ryanodine receptor phosphorylation at serine 2808 (pS2808) were correlated with inducible ventricular arrhythmia. MMP-9 deficiency consistently prevented PKA and pS2808 increases after Ang II treatment and reduced ventricular arrhythmia. Calcium dynamics were examined via confocal imaging in isolated murine cardiomyocytes. MMP-9 inhibition prevents calcium leakage from the sarcoplasmic reticulum and reduces arrhythmia-like irregular calcium transients via protein kinase A and ryanodine receptor phosphorylation. Human induced pluripotent stem cell-derived cardiomyocytes similarly show that MMP-9 inhibition prevents abnormal calcium leakage. Myocardial MMP-9 inhibition prevents ventricular arrhythmia through pleiotropic effects, including the modulation of calcium homeostasis and reduced calcium leakage.

B-Type Natriuretic Peptide Modulates Pulmonary Vein Arrhythmogenesis: A Novel Potential Contributor to the Genesis of Atrial Tachyarrhythmia in Heart Failure

BACKGROUND

Heart failure (HF) plays a critical role in the genesis of atrial fibrillation (AF). A high B-type natriuretic peptide (BNP) level occurs in patients with HF and in patients with AF. However, the role of BNP in the pathophysiology of AF is not clear. The purposes of this study were to evaluate the effects of BNP on pulmonary vein (PV) arrhythmogenesis.

METHODS AND RESULTS

Whole-cell patch clamp and fluorescence were used to study the action potential, ionic currents, and calcium homeostasis in isolated single rabbit PV cardiomyocytes before and after a BNP infusion, with or without ODQ (10 μM), milrinone (50 μM), or ouabain (1 μM). BNP increased PV spontaneous activity by 28.2 ± 7.5% at 100 nM and by 23.8 ± 9.1% at 300 nM. Similar to those with BNP, milrinone 50 μM increased the PV beating rate from 3.0 ± 0.2 to 3.6 ± 0.3 Hz (P < 0.0005, n = 7). In the presence of ODQ application, BNP didn't change PV spontaneous activity. BNP (100 nM) increased calcium transients (F/F₀ from 1.6 ± 0.1 to 1.9 ± 0.2, n = 20, P < 0.05) and increased the pacemaker current (0.4 ± 0.1 to 1.0 ± 0.2 pA/pF, n = 17, P < 0.0005) in PV cardiomyocytes. Moreover, BNP (100 nM) increased the transient inward current, sodium currents, sodium-calcium exchanger currents, and L-type calcium current; but reduced late sodium currents and the Na-K pump in PV cardiomyocytes.

CONCLUSION

BNP increases PV arrhythmogenesis, which may contribute to the genesis of atrial tachyarrhythmogenesis in HF. Cyclic GMP activation, phosphodiesterase 3 inhibition and Na⁺ /K⁺ -ATPase inhibition might participate in the BNP modulation of PV electrophysiology.

Colchicine modulates calcium homeostasis and electrical property of HL-1 cells

Colchicine is a microtubule disruptor that reduces the occurrence of atrial fibrillation (AF) after an operation or ablation. However, knowledge of the effects of colchicine on atrial myocytes is limited. The aim of this study was to determine if colchicine can regulate calcium (Ca²⁺) homeostasis and attenuate the electrical effects of the extracellular matrix on atrial myocytes. Whole‐cell clamp, confocal microscopy with fluorescence, and western blotting were used to evaluate the action potential and ionic currents of HL‐1 cells treated with and without (control) colchicine (3 nM) for 24 hrs. Compared with control cells, colchicine‐treated HL‐1 cells had a longer action potential duration with smaller intracellular Ca²⁺ transients and sarcoplasmic reticulum (SR) Ca²⁺ content by 10% and 47%, respectively. Colchicine‐treated HL‐1 cells showed a smaller L‐type Ca²⁺ current, reverse mode sodium–calcium exchanger (NCX) current and transient outward potassium current than control cells, but had a similar ultra‐rapid activating outward potassium current and apamin‐sensitive small‐conductance Ca²⁺‐activated potassium current compared with control cells. Colchicine‐treated HL‐1 cells expressed less SERCA2a, total, Thr17‐phosphorylated phospholamban, Cav1.2, CaMKII, NCX, Kv1.4 and Kv1.5, but they expressed similar levels of the ryanodine receptor, Ser16‐phosphorylated phospholamban and Kv4.2. Colchicine attenuated the shortening of the collagen‐induced action potential duration in HL‐1 cells. These findings suggest that colchicine modulates the atrial electrical activity and Ca²⁺ regulation and attenuates the electrical effects of collagen, which may contribute to its anti‐AF activity.

The vascular Ca2+-sensing receptor regulates blood vessel tone and blood pressure

The extracellular calcium-sensing receptor CaSR is expressed in blood vessels where its role is not completely understood. In this study, we tested the hypothesis that the CaSR expressed in vascular smooth muscle cells (VSMC) is directly involved in regulation of blood pressure and blood vessel tone. Mice with targeted CaSR gene ablation from vascular smooth muscle cells (VSMC) were generated by breeding exon 7 LoxP-CaSR mice with animals in which Cre recombinase is driven by a SM22α promoter (SM22α-Cre). Wire myography performed on Cre-negative [wild-type (WT)] and Cre-positive (SM22α)CaSR(Δflox/Δflox) [knockout (KO)] mice showed an endothelium-independent reduction in aorta and mesenteric artery contractility of KO compared with WT mice in response to KCl and to phenylephrine. Increasing extracellular calcium ion (Ca(2+)) concentrations (1-5 mM) evoked contraction in WT but only relaxation in KO aortas. Accordingly, diastolic and mean arterial blood pressures of KO animals were significantly reduced compared with WT, as measured by both tail cuff and radiotelemetry. This hypotension was mostly pronounced during the animals' active phase and was not rescued by either nitric oxide-synthase inhibition with nitro-l-arginine methyl ester or by a high-salt-supplemented diet. KO animals also exhibited cardiac remodeling, bradycardia, and reduced spontaneous activity in isolated hearts and cardiomyocyte-like cells. Our findings demonstrate a role for CaSR in the cardiovascular system and suggest that physiologically relevant changes in extracellular Ca(2+) concentrations could contribute to setting blood vessel tone levels and heart rate by directly acting on the cardiovascular CaSR.

Resveratrol, a red wine antioxidant, reduces atrial fibrillation susceptibility in the failing heart by PI3K/AKT/eNOS signaling pathway activation

BACKGROUND

Resveratrol has shown benefits in reducing ventricular remodeling and arrhythmias.

OBJECTIVE

This study aimed to assess the therapeutic efficacy of resveratrol in reducing atrial fibrillation (AF) in a heart failure (HF) model and to explore the underlying mechanisms.

METHODS

HF rabbits were created 4 weeks after undergoing coronary ligation. Group 1 (n = 6) was divided into subgroups of (a) normal rabbits, (b) HF sham rabbits, and (c) HF rabbits treated for 1 week with intraperitoneal injections of resveratrol, (d) resveratrol plus wortmannin, or (e) resveratrol plus diphenyleneiodonium chloride (DPI). All rabbits underwent epicardial catheter stimulation. Collagen content, messenger RNA and protein expression in ion channels, and phosphoinositide 3-kinase (PI3K)/AKT/endothelial nitric oxide synthase (eNOS) signaling pathways were studied in left atrial appendage (LAA) preparations. To investigate acute drug effects on left atrial electrophysiology, groups 2 a through 2e (n = 6 per group) were subjected to Langendorff perfusion.

RESULTS

Higher AF inducibility was found in the HF group and groups that were given PI3K and eNOS inhibitors than in the normal and resveratrol-treated groups (P < .001). Histologic analysis of the LAA revealed a decrease in fibrosis in resveratrol-treated groups compared with the HF group (8.95% ± 1.53% vs 26.62% ± 2.19%, P < .001). In real-time polymerase chain reaction analysis, ion channels including Kv1.4, Kv1.5, KvLQT1, Kir2.1, Nav1.5, Cav1.2, NCX, SERCA2a, and phospholamban were upregulated by resveratrol. PI3K, AKT, and eNOS messenger RNA and protein expression were upregulated by resveratrol but were inhibited by the coadministration of wortmannin and DPI.

CONCLUSION

Resveratrol decreases left atrial fibrosis and regulates variation in ion channels to reduce AF through the PI3K/AKT/eNOS signaling pathway.

Histone deacetylase inhibition reduces pulmonary vein arrhythmogenesis through calcium regulation

Pulmonary veins (PVs) play a critical role in the pathophysiology of atrial fibrillation (AF). Histone deacetylases (HDACs) are vital to calcium homeostasis and AF genesis. However, the electrophysiological effects of HDAC inhibition were unclear. This study evaluated whether HDAC inhibition can regulate PV electrical activity through calcium modulation. Whole-cell patch-clamp, confocal microscopic with fluorescence, and Western blot were used to evaluate electrophysiological characteristics and Ca(2+) dynamics in isolated rabbit PV cardiomyocytes with and without MPT0E014 (a pan HDAC inhibitor), MS-275 (HDAC1 and 3 inhibitor), and MC-1568 (HDAC4 and 6 inhibitor) for 5~8h. Atrial electrical activity and induced-AF (rapid atrial pacing and acetylcholine infusion) were measured in rabbits with and without MPT0E014 (10mg/kg treated for 5 hours) in vivo. MPT0E014 (1 μM)-treated PV cardiomyocytes (n=12) had slower beating rates (2.1 ± 0.2 vs. 2.8 ± 0.1 Hz, p < 0.05) than control PV cardiomyocytes. However, control (n=11) and MPT0E014 (1 μM)-treated (n = 12) SAN cardiomyocytes had similar beating rates (3.2 ± 0.2 vs. 2.9 ± 0.3 Hz). MS-275-treated PV cardiomyocytes (n = 12, 2.3 ± 0.2 Hz), but not MC-1568-treated PV cardiomyocytes (n=14, 3.1 ± 0.3 Hz) had slower beating rates than control PV cardiomocytes. MPT0E014-treated PV cardiomyocytes (n=14) had a lower frequency (2.4 ± 0.6 vs. 0.3 ± 0.1 spark/mm/s, p < 0.05) of Ca(2+) sparks than control PV (n=17) cardiomyocytes. As compared to control, MPT0E014-treated PV cardiomyocytes had reduced Ca(2+) transient amplitudes, sodium-calcium exchanger currents, and ryanodine receptor expressions. Moreover, MPT0E014-treated rabbits had less AF and shorter AF duration than control rabbits. In conclusions, HDAC inhibition reduced PV arrhythmogenesis and AF inducibility with modulation on calcium homeostasis.

Development of biophysically detailed electrophysiological models for pacemaking and non-pacemaking human pulmonary vein cardiomyocytes

Ectopic foci originating from the pulmonary veins (PVs) have been suggested as the underlying cause for generating atrial arrhythmias that include atrial fibrillation (AF). Recent experimental findings indicate two types of PV cells: pacemaking and non-pacemaking. In this study, we have developed two mathematical models for human PV cardiomyocytes with and without pacemaking activities. The models were reconstructed by modifying an existing model of the human right atrium to incorporate extant experimental data on the electrical differences between the two cell types. Differences in their action potential (AP) profiles and automaticity were reproduced by the models, which can be attributed to the observed differences in the current densities of I(NCX), I(to), I(Na) and I(Ca-L), as well as the difference in the channel kinetics of I(Ca-L) and inclusion of the I(f) and I(Ca-T) currents in the pacemaking cells. The developed models provide a useful tool suitable for studying the substrates for generating AF.

Heat shock protein inducer modifies arrhythmogenic substrate and inhibits atrial fibrillation in the failing heart

BACKGROUND

Geranylgeranylacetone (GGA) has been reported up-regulating heat shock protein (HSP) expression, and protecting against atrial remodeling. This study aimed to investigate the effects of GGA on atrial electrophysiology and inducibility of atrial fibrillation (AF) in heart failure (HF) model.

METHODS AND RESULTS

HF rabbits were created 4 weeks after coronary artery ligation. Monophasic action potential recordings and multielectrode array were used to record the electrophysiological characteristics of left atrium (LA) in normal, or HF rabbits with (HF-GGA) and without (HF-control) oral administration of GGA (200 mg/kg, 24 h before experiments). The mRNA and protein expressions of ionic channels were measured by Western blot and PCR. HF-GGA LA (n = 10), similar to normal LA (n = 10) had a shorter action potential duration (APD) and effective refractory period than HF-control LA (n = 10). HF-GGA LA had less triggered activity and APD alternans (20% vs. 100%, P = 0.001), lower maxima slope of restitution curve of APD (0.94 ± 0.04 vs.1.69 ± 0.04, P < 0.001), and less inducibility of AF (50% vs. 100%, P = 0.033) than HF-control LA. HF-GGA LA had a shorter activation time and higher conduction velocity than HF-control LA. HF-GGA LA had a higher mRNA expression of Cav1.2, Nav1.5, Kir2.1, Kv1.4, Kv7.1, Kv11.1, sarcoplasmic reticulum Ca(2+)-ATPase, and higher phosphorylation of phospholamban than HF-control LA.

CONCLUSIONS

GGA decreases triggered activity, dispersion of APD and inducibility of AF in failing heart through induction of HSP, and modulation of ionic channels and calcium homeostasis.

The arrhythmogenic consequences of increasing late INa in the cardiomyocyte

This review presents the roles of cardiac sodium channel NaV1.5 late current (late INa) in generation of arrhythmic activity. The assumption of the authors is that proper Na(+) channel function is necessary to the maintenance of the transmembrane electrochemical gradient of Na(+) and regulation of cardiac electrical activity. Myocyte Na(+) channels' openings during the brief action potential upstroke contribute to peak INa and initiate excitation-contraction coupling. Openings of Na(+) channels outside the upstroke contribute to late INa, a depolarizing current that persists throughout the action potential plateau. The small, physiological late INa does not appear to be critical for normal electrical or contractile function in the heart. Late INa does, however, reduce the net repolarizing current, prolongs action potential duration, and increases cellular Na(+) loading. An increase of late INa, due to acquired conditions (e.g. heart failure) or inherited Na(+) channelopathies, facilitates the formation of early and delayed afterpolarizations and triggered arrhythmias, spontaneous diastolic depolarization, and cellular Ca(2+) loading. These in turn increase the spatial and temporal dispersion of repolarization time and may lead to reentrant arrhythmias.

Calcium signalling microdomains and the t-tubular system in atrial mycoytes: potential roles in cardiac disease and arrhythmias

The atria contribute 25% to ventricular stroke volume and are the site of the commonest cardiac arrhythmia, atrial fibrillation (AF). The initiation of contraction in the atria is similar to that in the ventricle involving a systolic rise of intracellular Ca(2+) concentration ([Ca(2+)](i)). There are, however, substantial inter-species differences in the way systolic Ca(2+) is regulated in atrial cells. These differences are a consequence of a well-developed and functionally relevant transverse (t)-tubule network in the atria of large mammals, including humans, and its virtual absence in smaller laboratory species such as the rat. Where T-tubules are absent, the systolic Ca(2+) transient results from a 'fire-diffuse-fire' sequential recruitment of Ca(2+) release sites from the cell edge to the centre and hence marked spatiotemporal heterogeneity of systolic Ca(2+). Conversely, the well-developed T-tubule network in large mammals ensures a near synchronous rise of [Ca(2+)](i). In addition to synchronizing the systolic rise of [Ca(2+)](i), the presence of T-tubules in the atria of large mammals, by virtue of localization of the L-type Ca(2+) channels and Na(+)-Ca(2+) exchanger antiporters on the T-tubules, may serve to, respectively, accelerate changes in the amplitude of the systolic Ca(2+) transient during inotropic manoeuvres and lower diastolic [Ca(2+)](i). On the other hand, the presence of T-tubules and hence wider cellular distribution of the Na(+)-Ca(2+) exchanger may predispose the atria of large mammals to Ca(2+)-dependent delayed afterdepolarizations (DADs); this may be a determining factor in why the atria of large mammals spontaneously develop and maintain AF.

Heart failure epicardial fat increases atrial arrhythmogenesis

BACKGROUND

Obesity is an important risk factor for atrial fibrillation (AF) and heart failure (HF). The effects of epicardial fat on atrial electrophysiology were not clear. This study was to evaluate whether HF may modulate the effects of epicardial fat on atrial electrophysiology.

METHODS

Conventional microelectrodes recording was used to record the action potential in left (LA) and right (RA) atria of healthy (control) rabbits before and after application of epicardial fat from control or HF (ventricular pacing of 360-400 bpm for 4 weeks) rabbits. Adipokine profiles were checked in epicardial fat of control and HF rabbits.

RESULTS

The LA 90% of AP duration was prolonged by control epicardial fat (from 77 ± 6 to 87 ± 7 ms, p<0.05, n=7), and by HF epicardial fat (from 78 ± 3 to 98 ± 4 ms, p<0.001, n=9). However, control or HF epicardial fat did not change the AP morphology in RA. HF epicardial fat increased the contractility in LA (61 ± 11 vs. 35 ± 6 mg, p=0.001), but not in RA. Control fat did not change the LA or RA contractility. Moreover, control and HF epicardial fat induced early and delayed afterdepolarizations in LA and RA, but only HF epicardial fat provoked spontaneous activity and burst firing in LA (n=3/9, 33.3% vs. n=0/7, 0%, n=0/9, 0%, p<0.05). Compared to control fat, HF epicardial fat, had lower resistin, C-reactive protein and serum amyloid A, but similar interleukin-6, leptin, monocyte chemotactic protein-1, adiponectin and adipsin.

CONCLUSIONS

HF epicardial fat increases atrial arrhythmogenesis, which may contribute to the higher atrial arrhythmia in obesity.