Altered atrial cytosolic calcium handling contributes to the development of postoperative atrial fibrillation

Effects and mechanisms of dexmedetomidine preconditioning on isoproterenol-induced ventricular arrhythmias

Dexmedetomidine (DEX) is commonly used in clinical practice because of its sedative, analgesic, antisympathetic, hemodynamic stabilization and antianxiety effects. Previous clinical studies have demonstrated that DEX plays a role in both the prevention and treatment of perioperative arrhythmias. However, the precise mechanisms underlying the effects of DEX remain unclear. Furthermore, few studies have examined the effect of DEX on cardiac electrophysiology. ECG recording was performed in vivo and ex vivo on C57 mice. Simultaneous recording of membrane voltage (Vm) and [Ca2+]i changes was achieved with dual-dye optical mapping, in which voltage- and Ca2+-sensitive dyes are employed. Simultaneous programmed electrical stimulation was used to pacing and induce arrhythmias. Simulating catecholamine-induced arrhythmias with isoprotereno (ISO) and preconditioning with DEX to investigate the antiarrhythmic effects of DEX. Our findings demonstrated that ISO increased the incidence of ventricular tachycardia or ventricular fibrillation in mice during rapid pacing stimulation. DEX preconditioning reduced the incidence of ISO-induced ventricular arrhythmias. Optical mapping with simultaneous recordings of dual dyes (Vm dye and intracellular Ca2+ dye) revealed that DEX pretreatment attenuated the ISO-induced shortening of action potential duration (APD), calcium transient duration (CaTD), and time-to-peak (TTP) of calcium transients, as well as the ISO-induced increase in repolarization heterogeneity. DEX also slowed the conduction velocity. More importantly, DEX preconditioning significantly reduced the calcium transient alternans ratio at 80-ms, 70-ms, and 60-ms pacing cycles. These findings suggest that DEX preconditioning can reduce the incidence of ventricular arrhythmias induced by acute stress simulated by ISO. Prolongation of action potential duration and calcium transient duration and the maintenance of intracellular calcium homeostasis may be the electrophysiological mechanisms involved.

Calmodulin kinase II inhibition suppresses atrioventricular conduction by regulating intracellular Ca2+ homeostasis

BACKGROUND

Ca2+/calmodulin-dependent protein kinase II (CaMKII) inhibition decelerates atrioventricular node (AVN) conduction, providing a potential treatment of tachycardia. However, the effectiveness of CaMKII inhibition on tachycardia and its underlying mechanism remains unclear.

OBJECTIVE

We aimed to assess the effectiveness of CaMKII inhibition in reducing ventricular rates during atrial fibrillation and to elucidate the underlying mechanism in affecting AVN electrophysiology.

METHODS

Cardiac CaMKII inhibition (AC3-I) mice were used. Transesophageal atrial pacing was performed to evaluate AVN conduction function and to induce atrial fibrillation. Patch-clamp techniques were employed to record action potentials and ionic currents in AVN cells. Intracellular Ca2+ transients and sarcomere length measurements were obtained with the IonOptix system. Masson trichrome stain was used to evaluate fibrosis in the AVN region. Western blotting and immunofluorescence techniques were employed to detect connexin expression and localization.

RESULTS

CaMKII inhibition decreased the ventricular rate during atrial fibrillation and isoproterenol-induced tachycardia. Esophageal electrocardiogram results from AC3-I mice showed longer AVN conduction than in wild-type mice. AN- and N-type AVN cells from AC3-I mice exhibited slower action potential frequencies and diastolic depolarization rates than those of wild-type mice. The study revealed that CaMKII inhibition reduced AVN cell sarcoplasmic reticulum (SR) Ca2+ content, Ca2+ release rate from the SR during diastole, Ca2+ transient amplitude, and SR Ca2+ uptake rate. In addition, CaMKII inhibition prolonged the sarcomere diastole duration and enhanced the sensitivity of sarcomeres to Ca2+.

CONCLUSION

CaMKII inhibition effectively decreases the ventricular rate during atrial fibrillation and tachycardia by slowing down AVN conduction through suppressing Ca2+ overload in AVN cells.

Brain-to-heart cholinergic synapse-calcium signaling mediates ischemic stroke-induced atrial fibrillation

Background: Stroke-related cardiovascular diseases have attracted considerable attention, with atrial fibrillation (AF) being among the most frequent complications. Despite increasing clinical evidence, experimental models of stroke-induced AF are still lacking, hindering mechanistic discoveries and the development of adequate therapeutics targeting this stroke-heart syndrome (SHS). This study aims to create a rat model of ischemic stroke-induced AF (ISIAF) and to explore the efficacy and mechanism of Wenxin Keli (WK), an antiarrhythmic Chinese medicine. Method: The middle cerebral artery occlusion/reperfusion model was adapted to create subacute brain ischemia in rats with normal cardiac function. Invasive electrophysiologic studies and ex vivo optical mapping were performed to evaluate the altered electrophysiological parameters and Ca2+ handling properties. RNA-seq analysis, RT-PCR, and immunohistochemistry (IHC) with immunofluorescence (IF) were employed to assess the SHS model and elucidate the mechanisms of ISIAF and the effects of WK. UPLC/Q-TOF-MS, molecular docking, and whole-cell patch recordings were used to identify the active components of WK for SHS. Results: Ischemic stroke aggravated atrial electrical instability, altered action potential duration (APD), Ca2+ transient duration (CaT), conduction heterogeneity, and spatially discordant alternans in SHS rat hearts. These abnormalities were alleviated by WK. RNA-seq analysis revealed that M3-mediated cholinergic synapse signaling and L-type calcium channel (LTCCs)-mediated Ca2+ signaling play prominent roles in ISIAF development and its reversal by WK. UPLC/Q-TOF-MS analysis identified 19 WK components as the main components in plasma after WK treatment. Molecular docking screening identified Dioscin as the major active component of WK. WK and Dioscin reduced ICa-L in a concentration-dependent manner with a half-maximal inhibitory concentration of 24.254 ± 2.051 mg/mL and 8.666 ± 0.777 µmol/L, respectively. Conclusion: This study established an experimental model of ISIAF capable of characterizing clinically relevant atrial electrophysiological changes post-cerebral ischemia. Molecular mechanistic studies revealed that the cholinergic-calcium signaling pathway is central to this brain-heart syndrome. Ischemic stroke-induced atrial fibrillation is partially reversible by the Chinese medicine Wenxin Keli, which acts via regulation of the cholinergic-calcium signaling pathway, with its active component Dioscin directly binding to IKM3 and inhibiting ICa-L.

Association of epicardial adipose tissue density with postoperative atrial fibrillation after isolated aortic valve replacement

Backgrounds

It is well known that epicardial adipose tissue (EAT) is associated with the development of atrial fibrillation (AF). The aim of this study was to investigate whether EAT density (EAT-d) is associated with the development of new-onset atrial fibrillation (POAF) after aortic valve replacement (AVR).

Methods

We retrospectively studied 143 patients who underwent simple AVR at Department of Cardiovascular Surgery of the General Hospital of Northern Theater Command between June 2020 to August 2023. All patients received cardiac coronary artery computed tomography (CT) before surgery. EAT-d, EAT volume and EAT volume index (EATVI) were quantitatively measured and analysed using EAT analysis software (TIMESlicePro). POAF was detected by 7-day Holter monitoring.

Results

Of 143 patients undergoing AVR, 55 patients (38.46 %) developed POAF after surgery. Male patients and patients who had elder age or smoking history were more likely to develop POAF. On univariable analysis, patients developed POAF had significantly more EAT-d (-79.19(-83.91, -74.69) vs. -81.54(-87.16, -76.76); P = 0.043) and EATVI (4.14(3.32,5.03) vs. 3.90(2.70,4.51); P = 0.043) than patients without POAF. On multivariable analysis, EAT-d and age were independent risk factors for POAF (odds ratio (OR): 1.186, 95 % confidence interval (CI): 1.062-1.324, P = 0.002; OR: 1.119, 95 %CI: 1.055-1.187, P < 0.001). Furthermore, EAT-d was significantly associated with age. Furthermore, EAT-d was associated with cardiac structure changes, such as cardiac left ventricular end-diastolic, left ventricular end-systolic volumes and NT-proBNP before surgery.

Conclusion

EAT-d and age are independent predictors of POAF after simple AVR. EAT-d was related with age.

A detailed mathematical model of the human atrial cardiomyocyte: integration of electrophysiology and cardiomechanics

Mechano-electric regulations (MER) play an important role in the maintenance of cardiac performance. Mechano-calcium and mechano-electric feedback (MCF and MEF) pathways adjust the cardiomyocyte contractile force according to mechanical perturbations and affects electro-mechanical coupling. MER integrates all these regulations in one unit resulting in a complex phenomenon. Computational modelling is a useful tool to accelerate the mechanistic understanding of complex experimental phenomena. We have developed a novel model that integrates the MER loop for human atrial cardiomyocytes with proper consideration of feedforward and feedback pathways. The model couples a modified version of the action potential (AP) Koivumäki model with the contraction model by Quarteroni group. The model simulates iso-sarcometric and isometric twitches and the feedback effects on AP and Ca2+-handling. The model showed a biphasic response of Ca2+ transient (CaT) peak to increasing pacing rates and highlights the possible mechanisms involved. The model has shown a shift of the threshold for AP and CaT alternans from 4.6 to 4 Hz under post-operative atrial fibrillation, induced by depressed SERCA activity. The alternans incidence was dependent on a chain of mechanisms including RyRs availability time, MCF coupling, CaMKII phosphorylation, and the stretch levels. As a result, the model predicted a 10% slowdown of conduction velocity for a 20% stretch, suggesting a role of stretch in creation of substrate formation for atrial fibrillation. Overall, we conclude that the developed model provides a physiological CaT followed by a physiological twitch. This model can open pathways for the future studies of human atrial electromechanics. KEY POINTS: With the availability of human atrial cellular data, interest in atrial-specific model integration has been enhanced. We have developed a detailed mathematical model of human atrial cardiomyocytes including the mechano-electric regulatory loop. The model has gone through calibration and evaluation phases against a wide collection of available human in-vitro data. The usefulness of the model for analysing clinical problems has been preliminaryly tested by simulating the increased incidence of Ca2+ transient and action potential alternans at high rates in post-operative atrial fibrillation condition. The model determines the possible role of mechano-electric feedback in alternans incidence, which can increase vulnerability to atrial arrhythmias by varying stretch levels. We found that our physiologically accurate description of Ca2+ handling can reproduce many experimental phenomena and can help to gain insights into the underlying pathophysiological mechanisms.

Study on the mechanisms and Pharmacodynamic substances of Lian-Gui-Ning-Xin-Tang on Arrhythmia Therapy based on Pharmacodynamic-Pharmacokinetic associations

Background

The Chinese herbal compound Lian-Gui-Ning-Xin-Tang (LGNXT), composed of 9 herbs, has a significant antiarrhythmic effect. Previous studies have confirmed that preventing intracellular Ca2+ overload and maintaining intracellular Ca2+ homeostasis may be the important antiarrhythmic mechanisms of LGNXT. Recent studies are focused on elucidating the mechanisms and pharmacodynamic substances of LGNXT.

Purpose

1) To investigate the antiarrhythmic mechanisms of LGNXT; 2) to explore the association of pharmacodynamics (PD) and pharmacokinetics (PK) of the potential pharmacodynamic substances in LGNXT to further verify the mechanisms of action.

Methods

First, pharmacodynamic studies were conducted to determine the effect of LGNXT in arrhythmia at the electrophysiological, molecular, and tissue levels, and the "effect-time" relationship of LGNXT was further proposed. Next, an HPLC-MS/MS method was established to identify the "dose-time" relationship of the 9 potential compounds. Combining the "effect-time" and "dose-time" curves, the active ingredients closely related to the inhibition of inflammation, oxidative stress, and energy metabolism were identified to further verify the mechanisms and pharmacodynamic substances of LGNXT.

Results

Pretreatment with LGNXT could delay the occurrence of arrhythmias and reduce their duration and severity. LGNXT exerted antiarrhythmic effects by inhibiting MDA, LPO, IL-6, and cAMP; restoring Cx43 coupling function; and upregulating SOD, Ca2+-ATPase, and Na+-K+-ATPase levels. PK-PD association showed that nobiletin, methylophiopogonanone A, trigonelline, cinnamic acid, liquiritin, dehydropolisic acid, berberine, and puerarin were the main pharmacodynamic substances responsible for inhibiting the inflammatory response in arrhythmia. Methylophiopogonanone A, dehydropalingic acid, nobiletin, trigonelline, berberine, and puerarin in LGNXT exerted antiarrhythmic effects by inhibiting oxidative stress. Dehydropalingic acid, berberine, cinnamic acid, liquiritin, puerarin, trigonelline, methylophiopogonanone A, nobiletin, and tetrahydropalmatine exerted antiarrhythmic effects by inhibiting the energy-metabolism process.

Conclusions

LGNXT had a positive intervention effect on arrhythmias, especially ventricular tachyarrhythmias, which could inhibit inflammation, oxidative stress, and energy metabolism; positively stabilize the structure, and remodify the function of myocardial cell membranes. Additionally, the PD-PK association study revealed that methylophiopogonanone A, berberine, trigonelline, liquiritin, puerarin, tetrahydropalmatine, nobiletin, dehydropachymic acid, and cinnamic acid directly targeted inflammation, oxidative stress, and energy metabolism, which could be considered the pharmacodynamic substances of LGNXT. Thus, the antiarrhythmic mechanisms of LGNXT were further elucidated.

The association between low serum calcium level and new-onset atrial fibrillation after coronary artery bypass grafting

OBJECTIVES

This study aims to investigate the relationship between serum calcium (SC) levels and the incidence of postoperative atrial fibrillation (POAF) in patients undergoing coronary artery bypass graft surgery.

METHODS

This retrospective, observational cohort study consecutively enrolled patients undergoing isolated coronary artery bypass grafting in Beijing Anzhen Hospital from January 2018 to December 2021. Patients with a previous history of atrial fibrillation or atrial flutter or requiring concomitant cardiac surgery were excluded. A logistic regression model was used to determine predictors of POAF. Multivariable adjustment, inverse probability of treatment weighting and propensity score matching were used to adjust for confounders. Moreover, we conducted univariable and multivariable logistic regression analyses on preoperative and postoperative SC and ionized SC levels.

RESULTS

The analysis encompassed 12 293 patients. The POAF rate was significantly higher in patients with low SC level than those without (1379 [33.9%] vs 2375 [28.9%], P < 0.001). Low SC level was associated with an increased odds ratio of POAF (odds ratio [95% confidence interval]: 1.27 [1.18-1.37], P < 0.001). Inverse probability of treatment weighting and propensity score matching analyses confirmed the results. The increased POAF rate in low SC level group still existed among subgroup analysis based on different age, sex, body mass index, hypertension, hyperlipidaemia, CHA2DS2-VASc and magnesium.

CONCLUSIONS

Low SC level indicates elevated POAF risk in patients undergoing isolated coronary artery bypass graft surgery even after the adjustment for age, sex, cardiovascular risk factors, echocardiographic parameters and laboratory markers.

High-throughput methods for cardiac cellular electrophysiology studies: the road to personalized medicine

Personalized medicine refers to the tailored application of medical treatment at an individual level, considering the specific genotype or phenotype of each patient for targeted therapy. In the context of cardiovascular diseases, implementing personalized medicine is challenging due to the high costs involved and the slow pace of identifying the pathogenicity of genetic variants, deciphering molecular mechanisms of disease, and testing treatment approaches. Scalable cellular models such as human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) serve as useful in vitro tools that reflect individual patient genetics and retain clinical phenotypes. High-throughput functional assessment of these constructs is necessary to rapidly assess cardiac pathogenicity and test new therapeutics if personalized medicine is to become a reality. High-throughput photometry recordings of single cells coupled with potentiometric probes offer cost-effective alternatives to traditional patch-clamp assessments of cardiomyocyte action potential characteristics. Importantly, automated patch-clamp (APC) is rapidly emerging in the pharmaceutical industry and academia as a powerful method to assess individual membrane-bound ionic currents and ion channel biophysics over multiple cells in parallel. Now amenable to primary cell and hiPSC-CM measurement, APC represents an exciting leap forward in the characterization of a multitude of molecular mechanisms that underlie clinical cardiac phenotypes. This review provides a summary of state-of-the-art high-throughput electrophysiological techniques to assess cardiac electrophysiology and an overview of recent works that successfully integrate these methods into basic science research that could potentially facilitate future implementation of personalized medicine at a clinical level.

Inhibition of the P2X7 receptor prevents atrial proarrhythmic remodeling in experimental post-operative atrial fibrillation

BACKGROUND

Post-operative atrial fibrillation (POAF) is a common complication in patients undergoing cardiac surgery. The purinergic receptor P2X7 (P2X7R) is involved in some cardiovascular diseases, whereas its effects on atrial fibrillation (AF) are unclear.

OBJECTIVE

This study was to assess the effect of P2X7R on atrial arrhythmogenic remodeling in the rat model of sterile pericarditis (SP).

METHODS

Male Sprague-Dawley (SD) rats were used to induce the SP model. Electrocardiogram, atrial electrophysiological protocol, histology, mRNA sequencing, real-time quantitative PCR, western blot, and Elisa assay were performed.

RESULTS

SP significantly up-regulated P2X7R expression; increased AF susceptibility; reduced the protein expression of ion channels including Nav1.5, Cav1.2, Kv4.2, Kv4.3, and Kv1.5; caused atrial fibrosis; increased norepinephrine (NE) level in plasma; promoted the production of inflammatory cytokines such as TNF-α, IL-1β, and IL-6; increased the accumulation of immune cells (CD68- and MPO- positive cells); and activated NLRP3 inflammasome signaling pathway. P2X7R antagonist Brilliant Blue G (BBG) mitigated SP-induced alterations. The mRNA sequencing demonstrated that BBG prevented POAF mainly by regulating the immune system. In addition, another selective P2X7R antagonist A740003, and IL-1R antagonist anakinra also reduced AF inducibility in the SP model.

CONCLUSIONS

P2X7R inhibition prevents SP-induced atrial proarrhythmic remodeling, which is closely associated with the improvement of inflammatory changes, ion channel expression, atrial fibrosis, and sympathetic activation. The findings point to P2X7R inhibition as a promising target for AF (particularly POAF) and perhaps other conditions.

Implications for neutrophils in cardiac arrhythmias

Cardiac arrhythmias commonly occur as a result of aberrant electrical impulse formation or conduction in the myocardium. Frequently discussed triggers include underlying heart diseases such as myocardial ischemia, electrolyte imbalances, or genetic anomalies of ion channels involved in the tightly regulated cardiac action potential. Recently, the role of innate immune cells in the onset of arrhythmic events has been highlighted in numerous studies, correlating leukocyte expansion in the myocardium to increased arrhythmic burden. Here, we aim to call attention to the role of neutrophils in the pathogenesis of cardiac arrhythmias and their expansion during myocardial ischemia and infectious disease manifestation. In addition, we will elucidate molecular mechanisms associated with neutrophil activation and discuss their involvement as direct mediators of arrhythmogenicity.

Atrial fibrillation-associated electrical remodelling in human induced pluripotent stem cell-derived atrial cardiomyocytes: a novel pathway for antiarrhythmic therapy development

AIMS

Atrial fibrillation (AF) is associated with tachycardia-induced cellular electrophysiology alterations which promote AF chronification and treatment resistance. Development of novel antiarrhythmic therapies is hampered by the absence of scalable experimental human models that reflect AF-associated electrical remodelling. Therefore, we aimed to assess if AF-associated remodelling of cellular electrophysiology can be simulated in human atrial-like cardiomyocytes derived from induced pluripotent stem cells in the presence of retinoic acid (iPSC-aCM), and atrial-engineered human myocardium (aEHM) under short term (24 h) and chronic (7 days) tachypacing (TP).

METHODS AND RESULTS

First, 24-h electrical pacing at 3 Hz was used to investigate whether AF-associated remodelling in iPSC-aCM and aEHM would ensue. Compared to controls (24 h, 1 Hz pacing) TP-stimulated iPSC-aCM presented classical hallmarks of AF-associated remodelling: (i) decreased L-type Ca2+ current (ICa,L) and (ii) impaired activation of acetylcholine-activated inward-rectifier K+ current (IK,ACh). This resulted in action potential shortening and an absent response to the M-receptor agonist carbachol in both iPSC-aCM and aEHM subjected to TP. Accordingly, mRNA expression of the channel-subunit Kir3.4 was reduced. Selective IK,ACh blockade with tertiapin reduced basal inward-rectifier K+ current only in iPSC-aCM subjected to TP, thereby unmasking an agonist-independent constitutively active IK,ACh. To allow for long-term TP, we developed iPSC-aCM and aEHM expressing the light-gated ion-channel f-Chrimson. The same hallmarks of AF-associated remodelling were observed after optical-TP. In addition, continuous TP (7 days) led to (i) increased amplitude of inward-rectifier K+ current (IK1), (ii) hyperpolarization of the resting membrane potential, (iii) increased action potential-amplitude and upstroke velocity as well as (iv) reversibly impaired contractile function in aEHM.

CONCLUSIONS

Classical hallmarks of AF-associated remodelling were mimicked through TP of iPSC-aCM and aEHM. The use of the ultrafast f-Chrimson depolarizing ion channel allowed us to model the time-dependence of AF-associated remodelling in vitro for the first time. The observation of electrical remodelling with associated reversible contractile dysfunction offers a novel platform for human-centric discovery of antiarrhythmic therapies.

Research on atrial fibrillation mechanisms and prediction of therapeutic prospects: focus on the autonomic nervous system upstream pathways

Atrial fibrillation (AF) is the most common clinical arrhythmia disorder. It can easily lead to complications such as thromboembolism, palpitations, dizziness, angina, heart failure, and stroke. The disability and mortality rates associated with AF are extremely high, significantly affecting the quality of life and work of patients. With the deepening of research into the brain-heart connection, the link between AF and stroke has become increasingly evident. AF is now categorized as either Known Atrial Fibrillation (KAF) or Atrial Fibrillation Detected After Stroke (AFDAS), with stroke as the baseline. This article, through a literature review, briefly summarizes the current pathogenesis of KAF and AFDAS, as well as the status of their clinical pharmacological and non-pharmacological treatments. It has been found that the existing treatments for KAF and AFDAS have limited efficacy and are often associated with significant adverse reactions and a risk of recurrence. Moreover, most drugs and treatment methods tend to focus on a single mechanism pathway. For example, drugs targeting ion channels primarily modulate ion channels and have relatively limited impact on other pathways. This limitation underscores the need to break away from the "one disease, one target, one drug/measurement" dogma for the development of innovative treatments, promoting both drug and non-drug therapies and significantly improving the quality of clinical treatment. With the increasing refinement of the overall mechanisms of KAF and AFDAS, a deeper exploration of physiological pathology, and comprehensive research on the brain-heart relationship, it is imperative to shift from long-term symptom management to more precise and optimized treatment methods that are effective for almost all patients. We anticipate that drugs or non-drug therapies targeting the central nervous system and upstream pathways can guide the simultaneous treatment of multiple downstream pathways in AF, thereby becoming a new breakthrough in AF treatment research.

Does the small conductance Ca2+-activated K+ current ISK flow under physiological conditions in rabbit and human atrial isolated cardiomyocytes?

BACKGROUND

The small conductance Ca2+-activated K+ current (ISK) is a potential therapeutic target for treating atrial fibrillation.

AIM

To clarify, in rabbit and human atrial cardiomyocytes, the intracellular [Ca2+]-sensitivity of ISK, and its contribution to action potential (AP) repolarisation, under physiological conditions.

METHODS

Whole-cell-patch clamp, fluorescence microscopy: to record ion currents, APs and [Ca2+]i; 35-37°C.

RESULTS

In rabbit atrial myocytes, 0.5 mM Ba2+ (positive control) significantly decreased whole-cell current, from -12.8 to -4.9 pA/pF (P < 0.05, n = 17 cells, 8 rabbits). By contrast, the ISK blocker apamin (100 nM) had no effect on whole-cell current, at any set [Ca2+]i (∼100-450 nM). The ISK blocker ICAGEN (1 μM: ≥2 x IC50) also had no effect on current over this [Ca2+]i range. In human atrial myocytes, neither 1 μM ICAGEN (at [Ca2+]i ∼ 100-450 nM), nor 100 nM apamin ([Ca2+]i ∼ 250 nM) affected whole-cell current (5-10 cells, 3-5 patients/group). APs were significantly prolonged (at APD30 and APD70) by 2 mM 4-aminopyridine (positive control) in rabbit atrial myocytes, but 1 μM ICAGEN had no effect on APDs, versus either pre-ICAGEN or time-matched controls. High concentration (10 μM) ICAGEN (potentially ISK-non-selective) moderately increased APD70 and APD90, by 5 and 26 ms, respectively. In human atrial myocytes, 1 μM ICAGEN had no effect on APD30-90, whether stimulated at 1, 2 or 3 Hz (6-9 cells, 2-4 patients/rate).

CONCLUSION

ISK does not flow in human or rabbit atrial cardiomyocytes with [Ca2+]i set within the global average diastolic-systolic range, nor during APs stimulated at physiological or supra-physiological (≤3 Hz) rates.

Physiology of intracellular calcium buffering

Calcium signaling underlies much of physiology. Almost all the Ca2+ in the cytoplasm is bound to buffers, with typically only ∼1% being freely ionized at resting levels in most cells. Physiological Ca2+ buffers include small molecules and proteins, and experimentally Ca2+ indicators will also buffer calcium. The chemistry of interactions between Ca2+ and buffers determines the extent and speed of Ca2+ binding. The physiological effects of Ca2+ buffers are determined by the kinetics with which they bind Ca2+ and their mobility within the cell. The degree of buffering depends on factors such as the affinity for Ca2+, the Ca2+ concentration, and whether Ca2+ ions bind cooperatively. Buffering affects both the amplitude and time course of cytoplasmic Ca2+ signals as well as changes of Ca2+ concentration in organelles. It can also facilitate Ca2+ diffusion inside the cell. Ca2+ buffering affects synaptic transmission, muscle contraction, Ca2+ transport across epithelia, and the killing of bacteria. Saturation of buffers leads to synaptic facilitation and tetanic contraction in skeletal muscle and may play a role in inotropy in the heart. This review focuses on the link between buffer chemistry and function and how Ca2+ buffering affects normal physiology and the consequences of changes in disease. As well as summarizing what is known, we point out the many areas where further work is required.

Non-invasive electromechanical assessment during atrial fibrillation identifies underlying atrial myopathy alterations with early prognostic value

Electromechanical characterization during atrial fibrillation (AF) remains a significant gap in the understanding of AF-related atrial myopathy. This study reports mechanistic insights into the electromechanical remodeling process associated with AF progression and further demonstrates its prognostic value in the clinic. In pigs, sequential electromechanical assessment during AF progression shows a progressive decrease in mechanical activity and early dissociation from its electrical counterpart. Atrial tissue samples from animals with AF reveal an abnormal increase in cardiomyocytes death and alterations in calcium handling proteins. High-throughput quantitative proteomics and immunoblotting analyses at different stages of AF progression identify downregulation of contractile proteins and progressive increase in atrial fibrosis. Moreover, advanced optical mapping techniques, applied to whole heart preparations during AF, demonstrate that AF-related remodeling decreases the frequency threshold for dissociation between transmembrane voltage signals and intracellular calcium transients compared to healthy controls. Single cell simulations of human atrial cardiomyocytes also confirm the experimental results. In patients, non-invasive assessment of the atrial electromechanical relationship further demonstrate that atrial electromechanical dissociation is an early prognostic indicator for acute and long-term rhythm control.

Pericardial Inflammatory Mediators That Can Drive Postoperative Atrial Fibrillation in Cardiac Surgery Patients

Postoperative atrial fibrillation (POAF) is a common dysrhythmia that affects a significant number of patients undergoing cardiac surgery. Many studies aim to better understand this complex postsurgical complication by analysing circulating biomarkers in patients who develop POAF. More recently, the pericardial space was shown to contain inflammatory mediators that could trigger POAF. In this review we summarise recent studies that examine the immune mediators present in the pericardial space and their potential implications for the pathophysiology of POAF in cardiac surgery patients. Ongoing research in this area should better delineate the multifactorial etiology of POAF, where specific markers may be targeted to reduce the incidence of POAF and improve outcomes for this patient population.

Lysine crotonylation of SERCA2a correlates to cardiac dysfunction and arrhythmia in Sirt1 cardiac-specific knockout mice

Protein post-translational modifications (PTMs) are important regulators of protein functions and produce proteome complexity. SIRT1 has NAD+-dependent deacylation of acyl-lysine residues. The present study aimed to explore the correlation between lysine crotonylation (Kcr) on cardiac function and rhythm in Sirt1 cardiac-specific knockout (ScKO) mice and related mechanism. Quantitative proteomics and bioinformatics analysis of Kcr were performed in the heart tissue of ScKO mice established with a tamoxifen-inducible Cre-loxP system. The expression and enzyme activity of crotonylated protein were assessed by western blot, co-immunoprecipitation, and cell biology experiment. Echocardiography and electrophysiology were performed to investigate the influence of decrotonylation on cardiac function and rhythm in ScKO mice. The Kcr of SERCA2a was significantly increased on Lys120 (1.973 folds). The activity of SERCA2a decreased due to lower binding energy of crotonylated SERCA2a and ATP. Changes in expression of PPAR-related proteins suggest abnormal energy metabolism in the heart. ScKO mice had cardiac hypertrophy, impaired cardiac function, and abnormal ultrastructure and electrophysiological activities. We conclude that knockout of SIRT1 alters the ultrastructure of cardiac myocytes, induces cardiac hypertrophy and dysfunction, causes arrhythmia, and changes energy metabolism by regulating Kcr of SERCA2a. These findings provide new insight into the role of PTMs in heart diseases.

Electrophysiological and calcium-handling development during long-term culture of human-induced pluripotent stem cell-derived cardiomyocytes

Human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) are increasingly used for personalised medicine and preclinical cardiotoxicity testing. Reports on hiPSC-CM commonly describe heterogenous functional readouts and underdeveloped or immature phenotypical properties. Cost-effective, fully defined monolayer culture is approaching mainstream adoption; however, the optimal age at which to utilise hiPSC-CM is unknown. In this study, we identify, track and model the dynamic developmental behaviour of key ionic currents and Ca2+-handling properties in hiPSC-CM over long-term culture (30-80 days). hiPSC-CMs > 50 days post differentiation show significantly larger ICa,L density along with an increased ICa,L-triggered Ca2+-transient. INa and IK1 densities significantly increase in late-stage cells, contributing to increased upstroke velocity and reduced action potential duration, respectively. Importantly, our in silico model of hiPSC-CM electrophysiological age dependence confirmed IK1 as the key ionic determinant of action potential shortening in older cells. We have made this model available through an open source software interface that easily allows users to simulate hiPSC-CM electrophysiology and Ca2+-handling and select the appropriate age range for their parameter of interest. This tool, together with the insights from our comprehensive experimental characterisation, could be useful in future optimisation of the culture-to-characterisation pipeline in the field of hiPSC-CM research.

Association of postoperative atrial fibrillation with higher dosing ratios of protamine-to-heparin

Background: Postoperative atrial fibrillation (POAF) is defined as new-onset AF in the immediate postoperative period. The relatively high incidence of POAF after cardiac surgery is well described, but pathophysiological mechanisms underlying the initiation, maintenance, and progression of POAF may be multifactorial and have not yet been comprehensively characterized. One of the mechanisms includes altered Ca2+ kinetics. Accumulating evidence has suggested that altered atrial cytosolic calcium handling contributes to the development of POAF, protamine reversibly modulates the calcium release channel/ryanodine receptor 2 (RyR2) and voltage-dependent cardiac RyR2. However, it is currently unknown whether such abnormalities contribute to the arrhythmogenic substrate predisposing patients to the development of POAF. Methods: We have retrospectively analyzed 147 patients who underwent cardiac surgery with cardiopulmonary bypass support. Of these, 40 patients were excluded from the analysis because of pre-existing AF. All patients received heparin followed by protamine at different dosing ratios of protamine-to-heparin, depending on the periods studied. Results: The dosing ratio of protamine-to-heparin = 1.0 was compared with higher dosing ratios of protamine-to-heparin >1.0 up to 1.7. POAF developed in 15 patients (15/107 = 14%), of these, 5 out of 57 patients (33.3%) in the dosing ratio of protamine-to-heparin = 1.0 and 10 out of 35 patients (66.7%) in the higher dosing ratios of protamine-to-heparin. Statistical significance was observed in patients with higher dosing ratios of protamine-to-heparin, compared with the dosing ratio of protamine-to-heparin = 1.0 (odds ratio = 3.890, 95% CI = 1.130–13.300, p-value = 0.031). When types of diseases were analyzed in terms of higher dosing ratios of protamine-to-heparin, only valvular disorders were significantly associated with POAF (p = 0.04). Conclusions: Protamine is clinically utilized to reverse heparin overdose and has been shown to display immunological and inflammatory alterations. However, its association with POAF has not been reported. Our results provide evidence that higher dosing ratios of protamine-to-heparin may increase the incidence of POAF.

The 'Reverse FDUF' Mechanism of Atrial Excitation-Contraction Coupling Sustains Calcium Alternans-A Hypothesis

Cardiac calcium alternans is defined as beat-to-beat alternations of Ca transient (CaT) amplitude and has been linked to cardiac arrhythmia, including atrial fibrillation. We investigated the mechanism of atrial alternans in isolated rabbit atrial myocytes using high-resolution line scan confocal Ca imaging. Alternans was induced by increasing the pacing frequency until stable alternans was observed (1.6-2.5 Hz at room temperature). In atrial myocytes, action potential-induced Ca release is initiated in the cell periphery and subsequently propagates towards the cell center by Ca-induced Ca release (CICR) in a Ca wave-like fashion, driven by the newly identified 'fire-diffuse-uptake-fire' (FDUF) mechanism. The development of CaT alternans was accompanied by characteristic changes of the spatio-temporal organization of the CaT. During the later phase of the CaT, central [Ca]_i exceeded peripheral [Ca]_i that was indicative of a reversal of the subcellular [Ca]_i gradient from centripetal to centrifugal. This gradient reversal resulted in a reversal of CICR propagation, causing a secondary Ca release during the large-amplitude alternans CaT, thereby prolonging the CaT, enhancing Ca-release refractoriness and reducing Ca release on the subsequent beat, thus enhancing the degree of CaT alternans. Here, we propose the 'reverse FDUF' mechanism as a novel cellular mechanism of atrial CaT alternans, which explains how the uncoupling of central from peripheral Ca release leads to the reversal of propagating CICR and to alternans.

A modern automated patch-clamp approach for high throughput electrophysiology recordings in native cardiomyocytes

Crucial conventional patch-clamp approaches to investigate cellular electrophysiology suffer from low-throughput and require considerable experimenter expertise. Automated patch-clamp (APC) approaches are more experimenter independent and offer high-throughput, but by design are predominantly limited to assays containing small, homogenous cells. In order to enable high-throughput APC assays on larger cells such as native cardiomyocytes isolated from mammalian hearts, we employed a fixed-well APC plate format. A broad range of detailed electrophysiological parameters including action potential, L-type calcium current and basal inward rectifier current were reliably acquired from isolated swine atrial and ventricular cardiomyocytes using APC. Effective pharmacological modulation also indicated that this technique is applicable for drug screening using native cardiomyocyte material. Furthermore, sequential acquisition of multiple parameters from a single cell was successful in a high throughput format, substantially increasing data richness and quantity per experimental run. When appropriately expanded, these protocols will provide a foundation for effective mechanistic and phenotyping studies of human cardiac electrophysiology. Utilizing scarce biopsy samples, regular high throughput characterization of primary cardiomyocytes using APC will facilitate drug development initiatives and personalized treatment strategies for a multitude of cardiac diseases.

An altered automated patch-clamp (APC) approach enables high-throughput recordings from native pig cardiomyocytes and human iPSC-derived cardiomyocytes.

Neprilysins regulate muscle contraction and heart function via cleavage of SERCA-inhibitory micropeptides

Muscle contraction depends on strictly controlled Ca²⁺ transients within myocytes. A major player maintaining these transients is the sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPase, SERCA. Activity of SERCA is regulated by binding of micropeptides and impaired expression or function of these peptides results in cardiomyopathy. To date, it is not known how homeostasis or turnover of the micropeptides is regulated. Herein, we find that the Drosophila endopeptidase Neprilysin 4 hydrolyzes SERCA-inhibitory Sarcolamban peptides in membranes of the sarcoplasmic reticulum, thereby ensuring proper regulation of SERCA. Cleavage is necessary and sufficient to maintain homeostasis and function of the micropeptides. Analyses on human Neprilysin, sarcolipin, and ventricular cardiomyocytes indicates that the regulatory mechanism is evolutionarily conserved. By identifying a neprilysin as essential regulator of SERCA activity and Ca²⁺ homeostasis in cardiomyocytes, these data contribute to a more comprehensive understanding of the complex mechanisms that control muscle contraction and heart function in health and disease.

Restrictive Atrial Dysfunction in Cardiac Amyloidosis: Differences between Immunoglobulin Light Chain and Transthyretin Cardiac Amyloidosis Patients

Background: In cardiac amyloidosis, the prevalence of thromboembolic events and atrial fibrillation is higher in transthyretin amyloidosis compared to immunoglobulin light chain amyloidosis. Therefore, we hypothesize that transthyretin cardiac amyloidosis patients have worse atrial function. Purpose: To explore the left atrial function by conventional ultrasound and strain analysis in immunoglobulin light chain- and transthyretin cardiac amyloidosis patients. Methods: In cardiac amyloidosis patients in our Amyloidosis Expert Center, echocardiographic strain analysis was performed using speckle tracking. Results: The data of 53 cardiac amyloidosis patients (83% male, mean age 70 years) were analyzed. Transthyretin cardiac amyloidosis patients (n = 24, 45%) were older (75 ± 5.6 vs. 65 ± 7.2 years, p < 0.001) and had more left ventricular (LV) hypertrophy than immunoglobulin light chain cardiac amyloidosis patients (n = 29, 55%). However, LV systolic and diastolic function did not differ, nor did left atrial dimensions (LAVI 56(24) vs. 50(31) mL/m2). Left atrial reservoir strain was markedly lower in transthyretin cardiac amyloidosis (7.4(6.2) vs. 13.6(14.7), p = 0.017). This association was independent of other measurements of the left atrial and ventricular function. Conclusions: Transthyretin cardiac amyloidosis patients had lower left atrial reservoir function compared to immunoglobulin light chain cardiac amyloidosis patients although the left atrial geometry was similar. Interestingly, this association was independent of left atrial- and LV ejection fraction and global longitudinal strain. Further research is warranted to assess the impact of impaired left atrial dysfunction in transthyretin cardiac amyloidosis on atrial fibrillation burden and prognosis.

Characterization of pre-existing arrhythmogenic substrate associated with de novo early and late postoperative atrial fibrillation

BACKGROUND

PoAF is the most common complication after cardiac surgery and may occur in patients with pre-existing arrhythmogenic substrate. Characterization of this substrate could aid in identifying patients at risk for PoAF. We therefore compared intra-atrial conduction parameters and electrogram morphology between patients without and with early- (≤5 days after surgery) and late- (up to 5 years) postoperative atrial fibrillation (PoAF).

METHODS AND RESULTS

Epicardial mapping of the right and left atrium and Bachmann's Bundle (BB) was performed during sinus rhythm (SR) in 263 patients (207male, 67 ± 11 years). Unipolar potentials were classified as single, short or long double and fractionated potentials. Unipolar voltage, fractionation delay (time difference between the first and last deflection), conduction velocity (CV) and conduction block (CB) prevalence were measured. Comparing patients without (N = 166) and with PoAF (N = 97), PoAF was associated with lower CV and more CB at BB. Unipolar voltages were lower and more low-voltage areas were found at the left and right atrium and BB in PoAF patients. These differences were more pronounced in patients with late-PoAF (6%), which could even occur up to 5 years after surgery. Although several electrophysiological parameters were related to PoAF, age was the only independent predictor.

CONCLUSIONS

Patients with de novo PoAF have more extensive arrhythmogenic substrate prior to cardiac surgery compared to those who remained in SR, which is even more pronounced in late-PoAF patients. Future studies should evaluate whether intra-operative electrophysiological examination enables identification of patients at risk for developing PoAF and hence (preventive) therapy.

The Impact of Left Atrial Mechanics on Adverse Events and Clinical Outcome After Cardiac Surgery

OBJECTIVES

Post-operative atrial fibrillation (POAF) represents a common complication after cardiac surgery that is associated with unfavorable clinical outcome. Identifying patients at risk for POAF is crucial but challenging. This study aimed to investigate the prognostic potential of speckle tracking echocardiography (STE) on POAF and fatal adverse events from a long-term perspective.

METHODS

A total of 124 patients undergoing elective cardiac surgery were prospectively enrolled and underwent pre-operative STE. Patients were followed prospectively for the occurrence of POAF within the entire hospitalization and reaching the secondary end-points cardiovascular (CV) and all-cause mortality.

RESULTS

Within the study population 43.5% (n = 53) of enrolled individuals developed POAF. After a median follow-up of 3.9 years, 25 (20.2%) patients died. We observed that patients presenting with POAF had lower global peak atrial longitudinal strain (PALS) values compared to the non-POAF arm (POAF: 14.8% [95% CI : 10.9-17.8] vs non-POAF: 19.4% [95% CI : 14.8-23.5], p < 0.001). Moreover, global PALS was a strong and independent predictor for POAF (adjusted Odds Ratio per 1-SD: 0.37 [95% CI : 0.22-0.65], p < 0.001), and independently associated with mortality (adjusted Hazard Ratio per 1-SD: 0.63 [95% CI : 0.40-0.99], p = 0.048). CART analysis revealed a cut-off value of < 17% global PALS as high-risk for both POAF and mortality.

CONCLUSION

Global PALS is associated with the development of POAF following surgery in an unselected patient population undergoing CABG and/or valve surgery. Since patients with global PALS <17% face a poor long-term prognosis, routine assessment of global PALS needs to be considered in terms of proper secondary prevention in the era of personalized medicine.

Atrial fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia despite substantial efforts to understand the pathophysiology of the condition and develop improved treatments. Identifying the underlying causative mechanisms of AF in individual patients is difficult and the efficacy of current therapies is suboptimal. Consequently, the incidence of AF is steadily rising and there is a pressing need for novel therapies. Research has revealed that defects in specific molecular pathways underlie AF pathogenesis, resulting in electrical conduction disorders that drive AF. The severity of this so-called electropathology correlates with the stage of AF disease progression and determines the response to AF treatment. Therefore, unravelling the molecular mechanisms underlying electropathology is expected to fuel the development of innovative personalized diagnostic tools and mechanism-based therapies. Moreover, the co-creation of AF studies with patients to implement novel diagnostic tools and therapies is a prerequisite for successful personalized AF management. Currently, various treatment modalities targeting AF-related electropathology, including lifestyle changes, pharmaceutical and nutraceutical therapy, substrate-based ablative therapy, and neuromodulation, are available to maintain sinus rhythm and might offer a novel holistic strategy to treat AF.

A Mathematical Model for Electrical Activity in Pig Atrial Tissue

State of the art mathematical models are currently used to bridge the gap between basic research conducted in the laboratory and preclinical research conducted on large animals, which ultimately paves the way for clinical translation. In this regard, there is a great need for models that can be used alongside experiments for in-depth investigation and validation. One such experimental model is the porcine atrium, which is commonly used to study the mechanisms of onset and control of atrial fibrillation in the context of its surgical management. However, a mathematical model of pig atria is lacking. In this paper, we present the first ionically detailed mathematical model of porcine atrial electrophysiology, at body temperature. The model includes 12 ionic currents, 4 of which were designed based on experimental patch-clamp data directly obtained from literature. The formulations for the other currents are adopted from the human atrial model, and modified for porcine specificity based on our measured restitution data for different action potential characteristics: resting membrane potential, action potential amplitude, maximum upstroke velocity and action potential duration and different levels of membrane voltage repolarization. The intracellular Ca²⁺ dynamics follows the Luo-Rudy formulation for guinea pig ventricular cardiomyocytes. The resulting model represents “normal” cells which are formulated as a system of ordinary differential equations. We extend our model to two dimensions to obtain plane wave propagation in tissue with a velocity of 0.58 m/s and a wavelength of 8 cm. The wavelength reduces to 5 cm when the tissue is paced at 200 ms. Using S1-S2 cross-field protocol, we demonstrate in an 11.26 cm square simulation domain, the ability to initiate single spiral waves (rotation period ≃ 180 ms) that remain stable for more than 40 s. The spiral tip exhibits hypermeander. In agreement with previous experimental results using pig atria, our model shows that early repolarization is primarily driven by a calcium-mediated chloride current, I_ClCa, which is completely inactivated at high pacing frequencies. This is a condition that occurs only in porcine atria. Furthermore, the model shows spatiotemporal chaos with reduced repolarization.

Strain Echocardiography to Predict Postoperative Atrial Fibrillation

Postoperative atrial fibrillation (POAF) complicates 15% to 40% of cardiovascular surgeries. Its incidence progressively increases with aging, reaching 50% in octogenarians. This arrhythmia is usually transient but it increases the risk of embolic stroke, prolonged hospital stay, and cardiovascular mortality. Though many pathophysiological mechanisms are known, POAF prediction is still a hot topic of discussion. Doppler echocardiogram and, lately, strain echocardiography have shown significant capacity to predict POAF. Alterations in oxidative stress, calcium handling, mitochondrial dysfunction, inflammation, fibrosis, and tissue aging are among the mechanisms that predispose patients to the perfect "atrial storm". Manifestations of these mechanisms have been related to enlarged atria and impaired function, which can be detected prior to surgery. Specific alterations in the atrial reservoir and pump function, as well as atrial dyssynchrony determined by echocardiographic atrial strain, can predict POAF and help to shed light on which patients could benefit from preventive therapy.

Interleukin-6-Mediated-Ca2+ Handling Abnormalities Contributes to Atrial Fibrillation in Sterile Pericarditis Rats

Pre-existing Ca²⁺ handling abnormalities constitute the arrhythmogenic substrate in patients developing postoperative atrial fibrillation (POAF), a common complication after cardiac surgery. Postoperative interleukin (IL)-6 levels are associated with atrial fibrosis in several animal models of POAF, contributing to atrial arrhythmias. Here, we hypothesize that IL-6-mediated-Ca²⁺ handling abnormalities contribute to atrial fibrillation (AF) in sterile pericarditis (SP) rats, an animal model of POAF. SP was induced in rats by dusting atria with sterile talcum powder. Anti-rat-IL-6 antibody (16.7 μg/kg) was administered intraperitoneally at 30 min after the recovery of anesthesia. In vivo electrophysiology, ex vivo optical mapping, western blots, and immunohistochemistry were performed to elucidate mechanisms of AF susceptibility. IL-6 neutralization ameliorated atrial inflammation and fibrosis, as well as AF susceptibility in vivo and the frequency of atrial ectopy and AF with a reentrant pattern in SP rats ex vivo. IL-6 neutralization reversed the prolongation and regional heterogeneity of Ca²⁺ transient duration, relieved alternans, reduced the incidence of discordant alternans, and prevented the reduction and regional heterogeneity of the recovery ratio of Ca²⁺ transient. In agreement, western blots showed that IL-6 neutralization reversed the reduction in the expression of ryanodine receptor 2 (RyR2) and phosphorylated phospholamban. Acute IL-6 administration to isolated rat hearts recapitulated partial Ca²⁺ handling phenotype in SP rats. In addition, intraperitoneal IL-6 administration to rats increased AF susceptibility, independent of fibrosis. Our results reveal that IL-6-mediated-Ca²⁺ handling abnormalities in SP rats, especially RyR2-dysfunction, independent of IL-6-induced-fibrosis, early contribute to the development of POAF by increasing propensity for arrhythmogenic alternans.

Increased cytosolic calcium buffering contributes to a cellular arrhythmogenic substrate in iPSC-cardiomyocytes from patients with dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is a major risk factor for heart failure and is associated with the development of life-threatening cardiac arrhythmias. Using a patient-specific induced pluripotent stem cell-derived cardiomyocyte (iPSC-CM) model harbouring a mutation in cardiac troponin T (R173W), we aim to examine the cellular basis of arrhythmogenesis in DCM patients with this mutation. iPSC from control (Ctrl) and DCM-TnT-R173W donors from the same family were differentiated into iPSC-CM and analysed through optical action potential (AP) recordings, simultaneous measurement of cytosolic calcium concentration ([Ca²⁺]_i) and membrane currents and separately assayed using field stimulation to detect the threshold for AP- and [Ca²⁺]_i-alternans development. AP duration was unaltered in TnT-R173W iPSC-CM. Nevertheless, TnT-R173W iPSC-CM showed a strikingly low stimulation threshold for AP- and [Ca²⁺]_i-alternans. Myofilaments are known to play a role as intracellular Ca²⁺ buffers and here we show increased Ca²⁺ affinity of intracellular buffers in TnT-R173W cells, indicating increased myofilament sensitivity to Ca²⁺. Similarly, EMD57033, a myofilament Ca²⁺ sensitiser, replicated the abnormal [Ca²⁺]_i dynamics observed in TnT-R173W samples and lowered the threshold for alternans development. In contrast, application of a Ca²⁺ desensitiser (blebbistatin) to TnT-R173W iPSC-CM was able to phenotypically rescue Ca²⁺ dynamics, normalising Ca²⁺ transient profile and minimising the occurrence of Ca²⁺ alternans at physiological frequencies. This finding suggests that increased Ca²⁺ buffering likely plays a major arrhythmogenic role in patients with DCM, specifically in those with mutations in cardiac troponin T. In addition, we propose that modulation of myofilament Ca²⁺ sensitivity could be an effective anti-arrhythmic target for pharmacological management of this disease.

Hub microRNAs and genes in the development of atrial fibrillation identified by weighted gene co-expression network analysis

Co-expression network may contribute to better understanding molecular interaction patterns underlying cellular processes. To explore microRNAs (miRNAs) expression patterns correlated with AF, we performed weighted gene co-expression network analysis (WGCNA) based on the dataset GSE28954. Thereafter, we predicted target genes using experimentally verified databases (ENOCRI, miRTarBase, and Tarbase), and overlapped genes with differentially expressed genes (DEGs) from GSE79768 were identified as key genes. Integrated analysis of association between hub miRNAs and key genes was conducted to screen hub genes. In general, we identified 3 differentially expressed miRNAs (DEMs) and 320 DEGs, predominantly enriched in inflammation-related functional items. Two significant modules (red and blue) and hub miRNAs (hsa-miR-146b-5p and hsa-miR-378a-5p), which highly correlated with AF-related phenotype, were detected by WGCNA. By overlapping the DEGs and predicted target genes, 38 genes were screened out. Finally, 9 genes (i.e. ATP13A3, BMP2, CXCL1, GABPA, LIF, MAP3K8, NPY1R, S100A12, SLC16A2) located at the core region in the miRNA-gene interaction network were identified as hub genes. In conclusion, our study identified 2 hub miRNAs and 9 hub genes, which may improve the understanding of molecular mechanisms and help to reveal potential therapeutic targets against AF.

The Effect of Gender and Sex Hormones on Cardiovascular Disease, Heart Failure, Diabetes, and Atrial Fibrillation in Sleep Apnea

Sleep apnea is a highly prevalent disorder with increasing impact on healthcare systems worldwide. Previous studies have been conducted primarily with male subjects, and prevalence and severity of sleep apnea in women are underestimated. Recent clinical and basic science evidence increasingly points to different mechanisms in men and women with sleep-disordered breathing (SDB). SDB is associated with a variety of comorbidities, including cardiovascular disease, heart failure, diabetes, and atrial fibrillation. In this review, we discuss sex-dependent mechanisms of SDB in select associated conditions to sharpen our clinical understanding of these sex-dependent inherent differences.

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.

Sleep-disordered breathing is independently associated with reduced atrial connexin 43 expression

BACKGROUND

Patients with atrial fibrillation (AF) exhibit decreased atrial expression of connexin (Cx), which has been causally linked to a proarrhythmogenic substrate. Interestingly, patients with sleep-disordered breathing (SDB) are at increased risk of AF, but the mechanisms remain unclear.

OBJECTIVE

We tested the hypothesis that patients with SDB have reduced atrial Cx expression independent of important comorbidities.

METHODS

We analyzed right atrial appendage biopsies from 77 patients undergoing coronary artery bypass grafting. Patients were tested for SDB by polygraphy before surgery. Expression of Cx40 and Cx43 messenger RNA was quantified using real-time quantitative polymerase chain reaction and Western blot (Cx43). Structural atrial remodeling was investigated histologically and by quantitative polymerase chain reaction. Postoperative AF was assessed by 12-lead electrocardiography.

RESULTS

Patients were stratified according to apnea-hypopnea index (SDB if apnea-hypopnea index ≥15 per hour, n = 32 vs n = 45). Patients with SDB had significantly lower atrial Cx43 expression, which was negatively correlated with apnea-hypopnea index and oxygen desaturation index. No significant increase in atrial fibrosis or expression of hypertrophy and inflammatory markers was observed. Interestingly, SDB remained the strongest independent predictor of decreased atrial Cx43 expression in a multivariate logistic regression model including age, sex, diabetes, and heart failure with reduced ejection fraction (odds ratio 7.58; 95% confidence interval 1.891-30.375; P = .004). Moreover, reduced atrial Cx43 expression was strongly associated with the occurrence of postoperative AF (odds ratio 15.749; 95% confidence interval 1.072-231.472; P = .044).

CONCLUSION

Patients with SDB exhibited decreased atrial Cx43 expression, which correlated with the severity of SDB. This correlation was independent of several concomitant diseases and may be linked to an increased risk of AF after cardiac surgery.

Optimising risk factors for atrial fibrillation post-cardiac surgery

Postoperative atrial fibrillation (POAF) is an ongoing complication following cardiac surgery, with an incidence of 15%-60%. It is associated with substantial mortality and morbidity, as well increased hospital stays and healthcare costs. The pathogenesis is not fully understood, but the literature suggests that POAF occurs when transient, postoperative triggers act on vulnerable atrial tissue produced by preoperative, procedure-induced and postoperative processes such as inflammation, oxidative stress, autonomic dysfunction and electrophysiological remodelling of the atrial tissues. This sets the stage for arrhythmogenic mechanisms, such as ectopic firing secondary to triggered activity and re-entry mechanisms generating POAF. Preoperative factors include advanced age, sex, ethnicity, cardiovascular risk factors, preoperative drugs, electrocardiogram and echocardiogram abnormalities. Procedural factors include: the use of cardiopulmonary bypass and aortic cross clamp, type of cardiac surgery, use of hypothermia, left ventricular venting, bicaval cannulation and exclusion of the left atrial appendage. Postoperative factors include postoperative drugs, electrolyte and fluid balance and infection. This review explores the pathogenesis of POAF and the contribution of these perioperative factors in the development of POAF. Patients can be risk stratified for targeted treatment and prophylaxis, and how these factors can be attenuated to improve POAF outcomes following cardiac surgery.

Current Drug Treatment Strategies for Atrial Fibrillation and TASK-1 Inhibition as an Emerging Novel Therapy Option

Atrial fibrillation (AF) is the most common sustained arrhythmia with a prevalence of up to 4% and an upwards trend due to demographic changes. It is associated with an increase in mortality and stroke incidences. While stroke risk can be significantly reduced through anticoagulant therapy, adequate treatment of other AF related symptoms remains an unmet medical need in many cases. Two main treatment strategies are available: rate control that modulates ventricular heart rate and prevents tachymyopathy as well as rhythm control that aims to restore and sustain sinus rhythm. Rate control can be achieved through drugs or ablation of the atrioventricular node, rendering the patient pacemaker-dependent. For rhythm control electrical cardioversion and pharmacological cardioversion can be used. While electrical cardioversion requires fasting and sedation of the patient, antiarrhythmic drugs have other limitations. Most antiarrhythmic drugs carry a risk for pro-arrhythmic effects and are contraindicated in patients with structural heart diseases. Furthermore, catheter ablation of pulmonary veins can be performed with its risk of intraprocedural complications and varying success. In recent years TASK-1 has been introduced as a new target for AF therapy. Upregulation of TASK-1 in AF patients contributes to prolongation of the action potential duration. In a porcine model of AF, TASK-1 inhibition by gene therapy or pharmacological compounds induced cardioversion to sinus rhythm. The DOxapram Conversion TO Sinus rhythm (DOCTOS)-Trial will reveal whether doxapram, a potent TASK-1 inhibitor, can be used for acute cardioversion of persistent and paroxysmal AF in patients, potentially leading to a new treatment option for AF.

Human Atrial Fibrillation Is Not Associated With Remodeling of Ryanodine Receptor Clusters

The release of Ca²⁺ by ryanodine receptor (RyR2) channels is critical for cardiac function. However, abnormal RyR2 activity has been linked to the development of arrhythmias, including increased spontaneous Ca²⁺ release in human atrial fibrillation (AF). Clustering properties of RyR2 have been suggested to alter the activity of the channel, with remodeling of RyR2 clusters identified in pre-clinical models of AF and heart failure. Whether such remodeling occurs in human cardiac disease remains unclear. This study aimed to investigate the nanoscale organization of RyR2 clusters in AF patients – the first known study to examine this potential remodeling in diseased human cardiomyocytes. Right atrial appendage from cardiac surgery patients with paroxysmal or persistent AF, or without AF (non-AF) were examined using super-resolution (dSTORM) imaging. Significant atrial dilation and cardiomyocyte hypertrophy was observed in persistent AF patients compared to non-AF, with these two parameters significantly correlated. Interestingly, the clustering properties of RyR2 were remarkably unaltered in the AF patients. No significant differences were identified in cluster size (mean ∼18 RyR2 channels), density or channel packing within clusters between patient groups. The spatial organization of clusters throughout the cardiomyocyte was also unchanged across the groups. RyR2 clustering properties did not significantly correlate with patient characteristics. In this first study to examine nanoscale RyR2 organization in human cardiac disease, these findings indicate that RyR2 cluster remodeling is not an underlying mechanism contributing to altered channel function and subsequent arrhythmogenesis in human AF.

Atrial Dyssynchrony Measured by Strain Echocardiography as a Marker of Proarrhythmic Remodeling and Oxidative Stress in Cardiac Surgery Patients

Aging leads to structural and electrophysiological changes that increase the risk of postoperative atrial arrhythmias; however, noninvasive preoperative markers of atrial proarrhythmic conditions are still needed. This study is aimed at assessing whether interatrial dyssynchrony determined using two-dimensional speckle tracking echocardiography relates to proarrhythmic structural and functional remodeling. A cohort of 45 patients in sinus rhythm referred for cardiac surgery was evaluated by echocardiography and surface electrocardiogram the day before the intervention. Transmembrane potential, connexin, and potassium channel distribution, inflammatory, and nitrooxidative markers were measured from right atrial tissue obtained from patients. A difference greater than 40 milliseconds between right and left atrial free wall contraction confirmed the presence of interatrial dyssynchrony in 21 patients. No difference in relation with age, previous diseases, and 2-dimensional echocardiographic findings as well as average values of global longitudinal right and left atrial strain were found between synchronic and dyssynchronic patients. Postoperative atrial fibrillation incidence increased from 8.3% in the synchronic group to 33.3% in the dyssynchronic ones. P wave duration showed no difference between groups. Action potentials from dyssynchronous patients decreased in amplitude, maximal rate of depolarization, and hyperpolarized. Duration at 30% of repolarization increased, being markedly shorter at 90% of repolarization. Only the dyssynchronous group showed early and delayed afterdepolarizations. Atrial tissue of dyssynchronous patients displayed lateralization of connexin 40 and increased connexin 43 expression and accumulation of tumor necrosis factor-α in the intercalated disc. Tumor necrosis factor-α did not colocalize, however, with lateralized connexin 40. Nitroxidative marks and KATP channels increased perivascularly and in myocytes. Our results demonstrate that, as compared to a traditional surface electrocardiogram, the novel noninvasive echocardiographic evaluation of interatrial dyssynchrony provides a better identification of nonaged-related proarrhythmic atrial remodeling with increased susceptibility to postoperative atrial fibrillation.

Cellular and mitochondrial mechanisms of atrial fibrillation

The molecular mechanisms underlying atrial fibrillation (AF), the most common form of arrhythmia, are poorly understood and therefore target-specific treatment options remain an unmet clinical need. Excitation–contraction coupling in cardiac myocytes requires high amounts of adenosine triphosphate (ATP), which is replenished by oxidative phosphorylation in mitochondria. Calcium (Ca²⁺) is a key regulator of mitochondrial function by stimulating the Krebs cycle, which produces nicotinamide adenine dinucleotide for ATP production at the electron transport chain and nicotinamide adenine dinucleotide phosphate for the elimination of reactive oxygen species (ROS). While it is now well established that mitochondrial dysfunction plays an important role in the pathophysiology of heart failure, this has been less investigated in atrial myocytes in AF. Considering the high prevalence of AF, investigating the role of mitochondria in this disease may guide the path towards new therapeutic targets. In this review, we discuss the importance of mitochondrial Ca²⁺ handling in regulating ATP production and mitochondrial ROS emission and how alterations, particularly in these aspects of mitochondrial activity, may play a role in AF. In addition to describing research advances, we highlight areas in which further studies are required to elucidate the role of mitochondria in AF.

Systematic review of pre-clinical therapies for post-operative atrial fibrillation

BACKGROUND

Post-operative atrial fibrillation (POAF) is a frequent cardiothoracic surgery complication that increases hospital stay, mortality and costs. Despite decades of research, there has been no systematic overview and meta-analysis of preclinical therapies for POAF in animal models.

METHODS

We performed a systematic search of MEDLINE and EMBASE from their inception through September 2020 to determine the effect of preclinical POAF therapies on primary efficacy outcomes using a prospectively registered protocol (CRD42019155649). Bias was assessed using the SYRCLE tool and CAMARADES checklist.

RESULTS

Within the 26 studies that fulfilled our inclusion criteria, we identified 4 prevention strategies including biological (n = 5), dietary (n = 2), substrate modification (n = 2), and pharmacological (n = 17) interventions targeting atrial substrate, cellular electrophysiology or inflammation. Only one study altered more than 1 pathophysiological mechanism. 73% comprised multiple doses of systemic therapies. Large animal models were used in 81% of the studies. Preclinical therapies altogether attenuated atrial fibrosis (SMD -2.09; 95% confidence interval [CI] -2.95 to -1.22; p < 0.00001; I2 = 47%), AF inducibility (RR 0.40; 95% CI 0.21 to 0.79; p = 0.008; I2 = 39%), and AF duration (SMD -2.19; 95% CI -3.05 to -1.32; p < 0.00001; I2 = 50%). However, all the criteria needed to evaluate the risk of bias was unclear for many outcomes and only few interventions were independently validated by more than 1 research group.

CONCLUSION

Treatments with therapies targeting atrial substrate, cellular electrophysiology or inflammation reduced POAF in preclinical animal models compared to controls. Improving the quality of outcome reporting, independently validating promising approaches and targeting complimentary drivers of POAF are promising means to improve the clinical translation of novel therapies for this highly prevalent and clinically meaningful disease.

Q. M. Muskens A, Boersma E, de Groot NMS, Brundel BJJM. Evaluating Serum Heat Shock Protein Levels as Novel Biomarkers for Atrial Fibrillation

Background: Staging of atrial fibrillation (AF) is essential to understanding disease progression and the accompanied increase in therapy failure. Blood-based heat shock protein (HSP) levels may enable staging of AF and the identification of patients with higher risk for AF recurrence after treatment. Objective: This study evaluates the relationship between serum HSP levels, presence of AF, AF stage and AF recurrence following electrocardioversion (ECV) or pulmonary vein isolation (PVI). Methods: To determine HSP27, HSP70, cardiovascular (cv)HSP and HSP60 levels, serum samples were collected from control patients without AF and patients with paroxysmal atrial fibrillation (PAF), persistent (PeAF) and longstanding persistent (LSPeAF) AF, presenting for ECV or PVI, prior to intervention and at 3-, 6- and 12-months post-PVI. Results: The study population (n = 297) consisted of 98 control and 199 AF patients admitted for ECV (n = 98) or PVI (n = 101). HSP27, HSP70, cvHSP and HSP60 serum levels did not differ between patients without or with PAF, PeAF or LSPeAF. Additionally, baseline HSP levels did not correlate with AF recurrence after ECV or PVI. However, in AF patients with AF recurrence, HSP27 levels were significantly elevated post-PVI relative to baseline, compared to patients without recurrence. Conclusions: No association was observed between baseline HSP levels and the presence of AF, AF stage or AF recurrence. However, HSP27 levels were increased in serum samples of patients with AF recurrence within one year after PVI, suggesting that HSP27 levels may predict recurrence of AF after ablative therapy.

Atrial fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia despite substantial efforts to understand the pathophysiology of the condition and develop improved treatments. Identifying the underlying causative mechanisms of AF in individual patients is difficult and the efficacy of current therapies is suboptimal. Consequently, the incidence of AF is steadily rising and there is a pressing need for novel therapies. Research has revealed that defects in specific molecular pathways underlie AF pathogenesis, resulting in electrical conduction disorders that drive AF. The severity of this so-called electropathology correlates with the stage of AF disease progression and determines the response to AF treatment. Therefore, unravelling the molecular mechanisms underlying electropathology is expected to fuel the development of innovative personalized diagnostic tools and mechanism-based therapies. Moreover, the co-creation of AF studies with patients to implement novel diagnostic tools and therapies is a prerequisite for successful personalized AF management. Currently, various treatment modalities targeting AF-related electropathology, including lifestyle changes, pharmaceutical and nutraceutical therapy, substrate-based ablative therapy, and neuromodulation, are available to maintain sinus rhythm and might offer a novel holistic strategy to treat AF.