Effects of a Na+/Ca2+ exchanger inhibitor on pulmonary vein electrical activity and ouabain-induced arrhythmogenicity

Ziprasidone triggers inflammasome signaling via PI3K-Akt-mTOR pathway to promote atrial fibrillation

BACKGROUND

Second-generation antipsychotics increase the risk of atrial fibrillation. This study explores whether the atypical antipsychotic ziprasidone triggers inflammasome signaling, leading to atrial arrhythmia.

METHODS

Electromechanical and pharmacological assessments were conducted on the rabbit left atria (LA). The patch-clamp technique was used to measure ionic channel currents in single cardiomyocytes. Detection of cytosolic reactive oxygen species production was performed in atrial cardiomyocytes.

RESULTS

The duration of action potentials at 50 % and 90 % repolarization was dose-dependently shortened in ziprasidone-treated LA. Diastolic tension in LA increased after ziprasidone treatment. Ziprasidone-treated LA showed rapid atrial pacing (RAP) triggered activity. PI3K inhibitor, Akt inhibitor and mTOR inhibitor abolished the triggered activity elicited by ziprasidone in LA. The NLRP3 inhibitor MCC950 suppressed the ziprasidone-induced post-RAP-triggered activity. MCC950 treatment reduced the reverse-mode Na+/Ca2+ exchanger current in ziprasidone-treated myocytes. Cytosolic reactive oxygen species production decreased in ziprasidone-treated atrial myocytes after MCC950 treatment. Protein levels of inflammasomes and proinflammatory cytokines, including NLRP3, caspase-1, IL-1β, IL-18, and IL-6 were observed to be upregulated in myocytes treated with ziprasidone.

CONCLUSIONS

Our findings suggest ziprasidone induces atrial arrhythmia, potentially through upregulation of the NLRP3 inflammasome and enhancement of reactive oxygen species production via the PI3K/Akt/mTOR pathway.

Adenosine monophosphate-regulated protein kinase inhibition modulates electrophysiological characteristics and calcium homeostasis of rabbit right ventricular outflow tract

BACKGROUND

Metabolic stress predisposes to ventricular arrhythmias and sudden cardiac death. Right ventricular outflow tract (RVOT) is the common origin of ventricular arrhythmias. Adenosine monophosphate-regulated protein kinase (AMPK) activation is an important compensatory mechanism for cardiac remodeling during metabolic stress.

OBJECTIVES

The purpose of this study was to access whether AMPK inhibition would modulate RVOT electrophysiology, calcium (Ca2+ ) regulation, and RVOT arrhythmogenesis or not.

METHODS

Conventional microelectrodes were used to record electrical activity before and after compound C (10 µM, an AMPK inhibitor) in isoproterenol (1 µM)-treated rabbit RVOT tissue preparations under electrical pacing. Whole-cell patch-clamp and confocal microscopic examinations were performed in baseline and compound C-treated rabbit RVOT cardiomyocytes to investigate ionic currents and intracellular Ca2+ transients in isolated rabbit RVOT cardiomyocytes.

RESULTS

Compound C decreased RVOT contractility, and reversed isoproterenol increased RVOT contractility. Compound C decreased the incidence, rate, and duration of isoproterenol-induced RVOT burst firing under rapid pacing. Compared to baseline, compound C-treated RVOT cardiomyocytes had a longer action potential duration, smaller intracellular Ca2+ transients, late sodium (Na+ ), peak L-type Ca2+ current density, Na+ -Ca2+ exchanger, transient outward potassium (K+ ) current, and rapid and slow delayed rectifier K+ currents.

CONCLUSION

AMPK inhibition modulates RVOT electrophysiological characteristics and Ca2+ homeostasis, contributing to lower RVOT arrhythmogenic activity. Accordingly, AMPK inhibition might potentially reduce ventricular tachyarrhythmias.

OpiCa1-PEG-PLGA nanomicelles antagonize acute heart failure induced by the cocktail of epinephrine and caffeine

Background

Reducing Ca2+ content in the sarcoplasmic reticulum (SR) through ryanodine receptors (RyRs) by calcin is a potential intervention strategy for the SR Ca2+ overload triggered by β-adrenergic stress in acute heart diseases.

Methods

OpiCal-PEG-PLGA nanomicelles were prepared by thin film dispersion, of which the antagonistic effects were observed using an acute heart failure model induced by epinephrine and caffeine in mice. In addition, cardiac targeting, self-stability as well as biotoxicity were determined.

Results

The synthesized OpiCa1-PEG-PLGA nanomicelles were elliptical with a particle size of 72.26 nm, a PDI value of 0.3, and a molecular weight of 10.39 kDa. The nanomicelles showed a significant antagonistic effect with 100 % survival rate to the death induced by epinephrine and caffeine, which was supported by echocardiography with significantly recovered heart rate, ejection fraction and left ventricular fractional shortening rate. The FITC labeled nanomicelles had a strong membrance penetrating capacity within 2 h and cardiac targeting within 12 h that was further confirmed by immunohistochemistry with a self-prepared OpiCa1 polyclonal antibody. Meanwhile, the nanomicelles can keep better stability and dispersibility in vitro at 4 °C rather than 20 °C or 37 °C, while maintain a low but stable plasma OpiCa1 concentration in vivo within 72 h. Finally, no obvious biotoxicities were observed by CCK-8, flow cytometry, H&E staining and blood biochemical examinations.

Conclusion

Our study also provide a novel nanodelivery pathway for targeting RyRs and antagonizing the SR Ca2+ disordered heart diseases by actively releasing SR Ca2+ through RyRs with calcin.

Ziprasidone Induces Rabbit Atrium Arrhythmogenesis via Modification of Oxidative Stress and Sodium/Calcium Homeostasis

Background: Atypical antipsychotics increase the risk of atrial arrhythmias and sudden cardiac death. This study investigated whether ziprasidone, a second-generation antipsychotic, affected intracellular Ca²⁺ and Na⁺ regulation and oxidative stress, providing proarrhythmogenic substrates in atriums. Methods: Electromechanical analyses of rabbit atrial tissues were conducted. Intracellular Ca²⁺ monitoring using Fluo-3, the patch-clamp method for ionic current recordings, and a fluorescence study for the detection of reactive oxygen species and intracellular Na⁺ levels were conducted in enzymatically dissociated atrial myocytes. Results: Ziprasidone-treated atriums showed sustained triggered activities after rapid pacing, which were inhibited by KN-93 and ranolazine. A reduced peak L-type Ca²⁺ channel current and enhanced late Na⁺ current were observed in ziprasidone-treated atrial myocytes, together with an increased cytosolic Na⁺ level. KN-93 suppressed the enhanced late Na⁺ current in ziprasidone-treated atrial myocytes. Atrial myocytes treated with ziprasidone showed reduced Ca²⁺ transient amplitudes and sarcoplasmic reticulum (SR) Ca²⁺ stores, and increased SR Ca²⁺ leakage. Cytosolic and mitochondrial reactive oxygen species production was increased in atrial myocytes treated with ziprasidone. TNF-α and NLRP3 were upregulated in ziprasidone-treated myocytes, and the level of phosphorylated calcium/calmodulin-dependent protein kinase II protein was increased. Conclusions: Our results suggest that ziprasidone increases the occurrence of atrial triggered activity and causes intracellular Ca²⁺ and Na⁺ dysregulation, which may result from enhanced oxidative stress and activation of the TNF-α/NLRP3 inflammasome pathway in ziprasidone-treated myocytes.

Fibroblast Growth Factor 1 Reduces Pulmonary Vein and Atrium Arrhythmogenesis via Modification of Oxidative Stress and Sodium/Calcium Homeostasis

Rationale

Atrial fibrillation is a critical health burden. Targeting calcium (Ca²⁺) dysregulation and oxidative stress are potential upstream therapeutic strategies. Fibroblast growth factor (FGF) 1 can modulate Ca²⁺ homeostasis and has antioxidant activity. The aim of this study was to investigate whether FGF1 has anti-arrhythmic potential through modulating Ca²⁺ homeostasis and antioxidant activity of pulmonary vein (PV) and left atrium (LA) myocytes.

Methods

Patch clamp, western blotting, confocal microscopy, cellular and mitochondrial oxidative stress studies were performed in isolated rabbit PV and LA myocytes treated with or without FGF1 (1 and 10 ng/mL). Conventional microelectrodes were used to record electrical activity in isolated rabbit PV and LA tissue preparations with and without FGF1 (3 μg/kg, i.v.).

Results

FGF1-treated rabbits had a slower heart rate than that observed in controls. PV and LA tissues in FGF1-treated rabbits had slower beating rates and longer action potential duration than those observed in controls. Isoproterenol (1 μM)-treated PV and LA tissues in the FGF1-treated rabbits showed less changes in the increased beating rate and a lower incidence of tachypacing (20 Hz)-induced burst firing than those observed in controls. FGF1 (10 ng/mL)-treated PV and LA myocytes had less oxidative stress and Ca²⁺ transient than those observed in controls. Compared to controls, FGF1 (10 ng/mL) decreased I_Na−L in PV myocytes and lowered I_to, I_Kr−tail in LA myocytes. Protein kinase C (PKC)ε inhibition abolished the effects of FGF1 on the ionic currents of LA and PV myocytes.

Conclusion

FGF1 changes PV and LA electrophysiological characteristics possibly via modulating oxidative stress, Na⁺/Ca²⁺ homeostasis, and the PKCε pathway.

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.

Fast voltage-dependent sodium (NaV ) currents are functionally expressed in mouse corpus cavernosum smooth muscle cells

BACKGROUND AND PURPOSE

Corpus cavernosum smooth muscle (CCSM) exhibits phasic contractions that are coordinated by ion channels. Mouse models are commonly used to study erectile dysfunction, but there are few published electrophysiological studies of mouse CCSM. We describe, for the first time, voltage-dependent sodium (Na_V ) currents in mouse CCSM and investigate their function.

EXPERIMENTAL APPROACH

Electrophysiological, pharmacological, and immunocytochemical studies on isolated CCSM cells. Tension measurements in whole tissue.

KEY RESULTS

A fast, voltage-dependent sodium current was induced by depolarising steps. Steady-state activation and inactivation curves revealed a window current between -60 and -30 mV. Two populations of Na_V currents, ('TTX-sensitive') and ('TTX-insensitive'), were distinguished. TTX-sensitive current showed 48% block with the Na_V -subtype-specific blockers ICA-121431 (Na_V 1.1-1.3), PF-05089771 (Na_V 1.7), and 4,9-anhydro-TTX (Na_V 1.6). TTX-insensitive current was insensitive to A803467, a Na_V 1.8 blocker. Immunocytochemistry confirmed the expression of Na_V 1.5 and Na_V 1.4 in freshly dispersed CCSM cells. Veratridine, a Na_V activator, reduced time-dependent inactivation of the current and increased the duration of evoked action potentials. Veratridine induced phasic contractions in CCSM strips. This effect was reversible with TTX and nifedipine but not by KB-R7943.

CONCLUSION AND IMPLICATIONS

We report, for the first time, a fast voltage-dependent sodium current in mouse CCSM. Stimulation of this current increases the contractility of corpus cavernosum in vitro, suggesting that it may contribute to the mechanisms of detumescence, and potentially serve as a clinically relevant target for pharmaceutical intervention in erectile dysfunction. Further work will be necessary to define its role.

Phosphodiesterase-1 inhibitor modulates Ca2+ regulation in sirtuin 1-deficient mouse cardiomyocytes

BACKGROUND

Phosphodiesterase inhibitors can be used to enhance second messenger signaling to regulate intracellular Ca²⁺ cycling. This study investigated whether ITI-214, a selective phosphodiesterase-1 inhibitor, modulates intracellular Ca²⁺ regulation, resulting in a positive inotropic effect in sirtuin 1 (Sirt1)-deficient cardiomyocytes.

METHODS

Mice with cardiac-specific Sirt1 knockout (Sirt1^-/-) were used, with Sirt1^flox/flox mice serving as controls. Electromechanical analyses of ventricular tissues were conducted, and we monitored intracellular Ca²⁺ using Fluo-3 as well as reactive oxygen species production in isolated cardiomyocytes.

RESULTS

Sirt1^-/- ventricles showed prolonged action potential duration at 90% repolarization and increased contractile force after treatment with ITI-214. The rates and sustained durations of burst firing in ventricles were higher and longer, respectively, in Sirt1^-/- ventricles than in controls. ITI-214 treatment decreased the rates and shortened the durations of burst firing in Sirt1^-/- mice. Sirt1^-/- cardiomyocytes showed reduced Ca²⁺ transient amplitudes and sarcoplasmic reticulum (SR) Ca²⁺ stores compared to those in control cardiac myocytes, which was reversed after ITI-214 treatment. SR Ca²⁺ leakage was larger in Sirt1^-/- cardiac myocytes than in control myocytes. ITI-214 reduced SR Ca²⁺ leakage in Sirt1^-/- cardiac myocytes. Increased levels of reactive oxygen species in Sirt1^-/- cardiomyocytes compared to those in controls were reduced after ITI-214 treatment. Levels of Ca²⁺ regulatory proteins, including ryanodine receptor 2, phospholamban, and sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPase 2a were not affected by ITI-214 administration.

CONCLUSIONS

Our results suggest that ITI-214 improves intracellular Ca²⁺ regulation, which in turn exerts inotropic effects and suppresses arrhythmic events in Sirt1-deficient ventricular myocytes.

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.

Concurrent increases in post-pacing action potential duration and contractility predict occurrence of ventricular arrhythmia

Excitation-contraction coupling from the integration of action potential duration (APD) and muscle contractility plays an important role in arrhythmogenesis. We aimed to determine whether distinctive excitation-contraction coupling contributes to the genesis of ventricular tachycardias (VTs). Action potential (AP) and mechanical activity were simultaneously recorded under electrical pacing (cycle lengths from 1000 to 100 ms) in the tissue model created from isolated rabbit right ventricular outflow tracts treated with NS 5806 (10 μM, transient outward potassium current enhancer), pinacidil (2 μM, ATP-sensitive potassium channel opener), and pilsicainide (5 μM, sodium channel blocker). There were 15 (9.9%) inducible VT episodes (group 1) and 136 (90.1%) non-inducible VT episodes (group 2) in our tissue model. Group 1 had greater post-pacing increases of the first occurrence of AP at 90% repolarization (ΔAPD₉₀, p < 0.001) and contractility (ΔContractility, p = 0.003) compared with group 2. Triggered VT episodes were common (72.7%) in cases with a ΔAPD₉₀ > 15% and a ΔContractility > 270%, but were undetectable in those with a ΔAPD₉₀ < 15% and a ΔContractility < 270%. In those with pacing-induced VTs, KB-R7943 (10 μM, a Na⁺-Ca²⁺ exchanger inhibitor, NCX inhibitor) significantly reduced the occurrence of VTs from 100.0 to 20.0% (15/15 to 3/15 episodes, p < 0.001). Concurrent increases in both post-pacing APD and contractility resulted in the occurrence of ventricular arrhythmias. NCX inhibition may be a potential therapeutic strategy for ventricular arrhythmias.

The effect of Sirt1 deficiency on Ca2+ and Na+ regulation in mouse ventricular myocytes

This study addressed the hypothesis that cardiac Sirtuin 1 (Sirt1) deficiency alters cardiomyocyte Ca²⁺ and Na⁺ regulation, leading to cardiac dysfunction and arrhythmogenesis. We used mice with cardiac‐specific Sirt1 knockout (Sirt1^−/−). Sirt1^flox/flox mice were served as control. Sirt1^−/− mice showed impaired cardiac ejection fraction with increased ventricular spontaneous activity and burst firing compared with those in control mice. The arrhythmic events were suppressed by KN93 and ranolazine. Reduction in Ca²⁺ transient amplitudes and sarcoplasmic reticulum (SR) Ca²⁺ stores, and increased SR Ca²⁺ leak were shown in the Sirt1^−/− mice. Electrophysiological measurements were performed using patch‐clamp method. While L‐type Ca²⁺ current (I_{Ca, L}) was smaller in Sirt1^−/− myocytes, reverse‐mode Na⁺/Ca²⁺ exchanger (NCX) current was larger compared with those in control myocytes. Late Na⁺ current (I_{Na, L}) was enhanced in the Sirt1^−/− mice, alongside with elevated cytosolic Na⁺ level. Increased cytosolic and mitochondrial reactive oxygen species (ROS) were shown in Sirt1^−/− mice. Sirt1^−/− cardiomyocytes showed down‐regulation of L‐type Ca²⁺ channel α1c subunit (Cav1.2) and sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPase 2a (SERCA2a), but up‐regulation of Ca²⁺/calmodulin‐dependent protein kinase II and NCX. In conclusions, these findings suggest that deficiency of Sirt1 impairs the regulation of intracellular Ca²⁺ and Na⁺ in cardiomyocytes, thereby provoking cardiac dysfunction and arrhythmogenesis.

Effects of ANP on pulmonary vein electrophysiology, Ca2+ homeostasis and adrenergic arrhythmogenesis via PKA

Atrial fibrillation (AF) is the most common form of arrhythmia and increases the risk of stroke and heart failure (HF). Pulmonary veins (PVs) are important sources of triggers that generate AF, and calcium (Ca²⁺ ) overload participates in PV arrhythmogenesis. Neurohormonal activation is an important cause of AF. Higher atrial natriuretic peptide (ANP) level predicts paroxysmal AF occurrence in HF patients. However, it is not clear if ANP directly modulates electrophysiological characteristics and Ca²⁺ homeostasis in the PVs. Conventional microelectrodes, whole-cell patch-clamp, and the Fluo-3 fluorimetric ratio technique were performed using isolated rabbit PV preparations or single isolated PV cardiomyocytes before and after ANP administration. We found that ANP (1, 10, and 100 nmol/L) concentration-dependently decreased spontaneous activity in PV preparations. ANP (100 nmol/L) decreased isoproterenol (1 μmol/L)-induced PV spontaneous activity and burst firing. AP811 (100 nmol/L, NPR-C agonist), H89 (1μmol/L, PKA inhibitor) decreased isoproterenol-induced PV spontaneous activity or burst firing, but successive administration of ANP had no further effect on PV activity. KT5823 (1 μmol/L, PKG inhibitor) decreased isoproterenol-induced PV spontaneous activity but did not change isoproterenol-induced PV burst firing, whereas successive administration of ANP did not change isoproterenol-induced PV burst firing. ANP decreased intracellular Ca²⁺ transient and sarcoplasmic reticulum Ca²⁺ content in single PV cardiomyocytes. ANP decreased the late sodium current, L-type Ca²⁺ current, but did not change nickel-sensitive Na⁺ -Ca²⁺ exchanger current in single PV cardiomyocytes. In conclusion, ANP directly regulates PV electrophysiological characteristics and Ca²⁺ homeostasis and attenuates isoproterenol-induced arrhythmogenesis through NPR-C/cAMP/PKA signal pathway.

Sarco/endoplasmic reticulum Ca2+-ATPase is a more effective calcium remover than sodium-calcium exchanger in human embryonic stem cell-derived cardiomyocytes

Human pluripotent stem cell (hPSCs)-derived ventricular (V) cardiomyocytes (CMs) display immature Ca2+–handing properties with smaller transient amplitudes and slower kinetics due to such differences in crucial Ca2+-handling proteins as the poor sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) pump but robust Na+-Ca2+ exchanger (NCX) activities in human embryonic stem cell (ESC)-derived VCMs compared with adult. Despite their fundamental importance in excitation-contraction coupling, the relative contribution of SERCA and NCX to Ca2+-handling of hPSC-VCMs remains unexplored. We systematically altered the activities of SERCA and NCX in human embryonic stem cell-derived ventricular cardiomyocytes (hESC-VCMs) and their engineered microtissues, followed by examining the resultant phenotypic consequences. SERCA overexpression in hESC-VCMs shortened the decay of Ca2+ transient at low frequencies (0.5 Hz) without affecting the amplitude, SR Ca2+ content and Ca2+ baseline. Interestingly, short hairpin RNA-based NCX suppression did not prolong the transient decay, indicating a compensatory response for Ca2+ removal. Although hESC-VCMs and their derived microtissues exhibited negative frequency-transient/force responses, SERCA overexpression rendered them less negative at high frequencies (>2 Hz) by accelerating Ca2+ sequestration. We conclude that for hESC-VCMs and their microtissues, SERCA, rather than NCX, is the main Ca2+ remover during diastole; poor SERCA expression is the leading cause for immature negative-frequency/force responses, which can be partially reverted by forced expression. Combinatorial approach to mature calcium handling in hESC-VCMs may help shed further mechanistic insights.

NEW & NOTEWORTHY In this study of human pluripotent stem cell-derived cardiomyocytes, we studied the role of sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) and Na+-Ca2+ exchanger (NCX) in Ca2+ handling. Our data support the notion that SERCA is more effective in cytosolic calcium removal than the NCX.

Lemnalol Modulates the Electrophysiological Characteristics and Calcium Homeostasis of Atrial Myocytes

Sepsis, an inflammatory response to infection provoked by lipopolysaccharide (LPS), is associated with high mortality, as well as ischemic stroke and new-onset atrial arrhythmia. Severe bacterial infections causing sepsis always result in profound physiological changes, including fever, hypotension, arrhythmia, necrosis of tissue, systemic multi-organ dysfunction and finally death. LPS challenge-induced inflammatory responses during sepsis may increase the likelihood of the arrhythmogenesis. Lemnalol is known to possess potent anti-inflammatory effects. This study examined whether Lemnalol (0.1 μM) could modulate the electrophysiological characteristics and calcium homeostasis of atrial myocytes under the influence of LPS (1μg/mL). Under challenge with LPS, Lemnalol-treated LA myocytes, had a longer AP duration at 20%, 50% and 90% repolarization of the amplitude, compared to the LPS-treated cells. LPS-challenged LA myocytes showed increased late sodium current, Na⁺-Ca²⁺ exchanger current, transient outward current, rapid component of delayed rectifier potassium current, tumor necrosis factor-α, NF-κB and increased phosphorylation of ryanodine receptor (RyR), but a lower L-type Ca²⁺ current than the control LA myocytes. Exposure to Lemnalol reversed the LPS-induced effects. The LPS-treated and control groups of LA myocytes, with or without the existence of Lemnalol. showed no apparent alterations in the sodium current amplitude or Cav1.2 expression. The expression of sarcoendoplasmic reticulum calcium transport ATPase (SERCA2) was reduced by LPS treatment, while Lemnalol ameliorated the LPS-induced alterations. The phosphorylation of RyR was enhanced by LPS treatment, while Lemnalol attenuated the LPS-induced alterations. In conclusion, Lemnalol modulates LPS-induced alterations of LA calcium homeostasis and blocks the NF-κB pathways, which may contribute to the attenuation of LPS-induced arrhythmogenesis.

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.

Overexpression of the Na+ /Ca2+ exchanger influences ouabain-mediated spontaneous Ca2+ activity but not positive inotropy

Administration of digitalis in heart failure (HF) increases quality of life but does not carry a prognostic benefit. Digitalis is an indirect inhibitor of the Na⁺ /Ca²⁺ exchanger (NCX), which is overexpressed in HF. We therefore used the cardiac glycoside ouabain in Ca²⁺ imaging experiments and patch-clamp experiments in isolated ventricular myocytes from nonfailing transgenic NCX overexpressor mice (OE). In field-stimulated myocytes, ouabain (1-100 μm) increased the amplitude of the Ca²⁺ transient in OE and wild-type (WT) similarly. Ouabain-mediated spontaneous Ca²⁺ -activity was significantly more pronounced in OE compared to WT myocytes at higher concentrations (100 μm). Also, at very high concentrations (1000 μm) of ouabain, the number of cells with hypercontraction leading to cell death was higher in OE. Ouabain (10 μm) shortened the action potential duration in both genotypes. Our findings suggest that the proarrhythmic but not the inotropic effects of cardiac glycosides are enhanced by increased NCX expression. This may offer an explanation for the observed lack of prognostic benefit but increased quality of life in HF, which is accompanied by NCX upregulation.

Hydrogen sulphide increases pulmonary veins and atrial arrhythmogenesis with activation of protein kinase C

Hydrogen sulphide (H₂ S), one of the most common toxic air pollutants, is an important aetiology of atrial fibrillation (AF). Pulmonary veins (PVs) and left atrium (LA) are the most important AF trigger and substrate. We investigated whether H₂ S may modulate the arrhythmogenesis of PVs and atria. Conventional microelectrodes and whole-cell patch clamp were performed in rabbit PV, sinoatrial node (SAN) or atrial cardiomyocytes before and after the perfusion of NaHS with or without chelerythrine (a selective PKC inhibitor), rottlerin (a specific PKC δ inhibitor) or KB-R7943 (a NCX inhibitor). NaHS reduced spontaneous beating rates, but increased the occurrences of delayed afterdepolarizations and burst firing in PVs and SANs. NaHS (100 μmol/L) increased I_KATP and I_NCX in PV and LA cardiomyocytes, which were attenuated by chelerythrine (3 μmol/L). Chelerythrine, rottlerin (10 μmol/L) or KB-R7943 (10 μmol/L) attenuated the arrhythmogenic effects of NaHS on PVs or SANs. NaHS shortened the action potential duration in LA, but not in right atrium or in the presence of chelerythrine. NaHS increased PKC activity, but did not translocate PKC isoforms α, ε to membrane in LA. In conclusion, through protein kinase C signalling, H₂ S increases PV and atrial arrhythmogenesis, which may contribute to air pollution-induced AF.

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.

Excavatolide B Modulates the Electrophysiological Characteristics and Calcium Homeostasis of Atrial Myocytes

Severe bacterial infections caused by sepsis always result in profound physiological changes, including fever, hypotension, arrhythmia, necrosis of tissue, systemic multi-organ dysfunction, and finally death. The lipopolysaccharide (LPS) provokes an inflammatory response under sepsis, which may increase propensity to arrhythmogenesis. Excavatolide B (EXCB) possesses potent anti-inflammatory effects. However, it is not clear whether EXCB could modulate the electrophysiological characteristics and calcium homeostasis of atrial myocytes. This study investigated the effects of EXCB on the atrial myocytes exposed to lipopolysaccharide. A whole-cell patch clamp and indo-1 fluorimetric ratio technique was employed to record the action potential (AP), ionic currents, and intracellular calcium ([Ca²⁺]_i) in single, isolated rabbit left atrial (LA) cardiomyocytes, with and without LPS (1 μg/mL) and LPS + EXCB administration (10 μM) for 6 ± 1 h, in order to investigate the role of EXCB on atrial electrophysiology. In the presence of LPS, EXCB-treated LA myocytes (n = 13) had a longer AP duration at 20% (29 ± 2 vs. 20 ± 2 ms, p < 0.05), 50% (52 ± 4 vs. 40 ± 3 ms, p < 0.05), and 90% (85 ± 5 vs. 68 ± 3 ms, p < 0.05), compared to the LPS-treated cells (n = 12). LPS-treated LA myocytes showed a higher late sodium current, Na⁺/Ca²⁺ exchanger current, transient outward current, and delayed rectifier potassium current, but a lower l-type Ca²⁺ current, than the control LA myocytes. Treatment with EXCB reversed the LPS-induced alterations of the ionic currents. LPS-treated, EXCB-treated, and control LA myocytes exhibited similar Na⁺ currents. In addition, the LPS-treated LA myocytes exhibited a lower [Ca²⁺]_i content and higher sarcoplasmic reticulum calcium content, than the controls. EXCB reversed the LPS-induced calcium alterations. In conclusion, EXCB modulates LPS-induced LA electrophysiological characteristics and calcium homeostasis, which may contribute to attenuating LPS-induced arrhythmogenesis.

Epigallocatechin-3-gallate modulates arrhythmogenic activity and calcium homeostasis of left atrium

BACKGROUND

Atrial fibrillation (AF) is the commonest sustained arrhythmia, and increases the risk of stroke, heart failure, and mortality. Calcium (Ca²⁺) overload and oxidative stress are thought to participate in the pathogenesis of AF. Epigallocatechin-3-gallate (EGCG) has an antioxidative effect and been shown to be beneficial in promoting cardiovascular health. However, it is not clear if EGCG directly modulates the electrophysiological characteristics and Ca²⁺ homeostasis of the left atrium (LA).

METHODS AND RESULTS

Conventional microelectrodes, whole-cell patch-clamp, and Fluo-3 fluorometric ratio technique were performed using the isolated rabbit LA preparations or isolated single LA cardiomyocytes before and after EGCG treatment. EGCG (0.01, 0.1, 1, and 10μM) which concentration-dependently decreased the APD₂₀ by 13±8%, 25±5%, 31±6%, and 37±5%, APD₅₀ by 9±8%, 22±6%, 32±7%, and 40±4%, and APD₉₀ by 2±12%, 9±8%, 24±10%, and 34±5% in LA preparations. EGCG (0.1μM) decreased the late sodium (Na⁺) current, L-type Ca²⁺ current, nickel-sensitive Na⁺-Ca²⁺ exchanger current, and transient outward current, but did not change the Na⁺ current and ultra-rapid delayed rectifier potassium current in LA cardiomyocytes. EGCG decreased intracellular Ca²⁺ transient and sarcoplasmic reticulum Ca²⁺ content in LA cardiomyocytes. Furthermore, EGCG decreased isoproterenol (ISO, 1μM)-induced burst firing. KT5823 (1μM) or KN93 (1μM) decreased the incidences of ISO-induced LA burst firing, which became lower with EGCG treatment. H89 (10μM) and KN92 (1μM) did not suppress the incidence of ISO-induced LA burst firing. However, EGCG decreased the incidences of ISO-induced LA burst firing in the presence of H89 or KN92.

CONCLUSION

EGCG directly regulates LA electrophysiological characteristics and Ca²⁺ homeostasis, and suppresses ISO-induced atrial arrhythmogenesis through inhibiting Ca²⁺/calmodulin or cGMP-dependent protein kinases.

Gap junction modifiers regulate electrical activities of the sinoatrial node and pulmonary vein: Therapeutic implications in atrial arrhythmogenesis

BACKGROUND

Gap junction (GJ) dysfunctions predispose cardiac tissues to various arrhythmias. Sinoatrial node (SAN) and pulmonary veins (PVs) are closely related atrial dysrhythmia. This study evaluated whether GJ modifications modulate SAN and PVs electrical activities.

METHODS

Conventional microelectrodes were used to record action potentials in isolated rabbit SAN, PVs, and connected PV-SAN tissue preparations before and after heptanol (GJ inhibitor) and PQ1 (GJ enhancer) administration with and without isoproterenol. A whole-cell patch clamp was used to record the electrical activities before and after heptanol in single SAN and PV cardiomyocytes.

RESULTS

Heptanol (1, 3, and 10μM) reduced the spontaneous beating rates of isolated SAN preparations but not PVs. Heptanol (10μM) decelerated the SAN leading rhythm in the PV-SAN preparations and induced PV burst firings without (3 of 6, 50%) and with (6 of 6, 100%) isoproterenol (1μM). Heptanol (10μM) also reduced the spontaneous beating rates in single SAN cardiomyocyte, but not PV cardiomyocyte, with a decreased pacemaker current. PQ1 (50 and 500nM) treatment did not change the spontaneous beating rates in isolated SAN and PV preparations. In the connected PV-SAN preparations, PQ1 (500nM) did not induce any PV firing even having additional isoproterenol treatment (1μM). Moreover, PQ1 (500nM) prevented heptanol-induced electrical changes in SAN and PVs preparations.

CONCLUSION

GJ dysfunction modulates SAN and PV electrical activity, which may contribute to atrial arrhythmogenesis. GJ enhancer has a therapeutic potential in SAN dysfunction and atrial arrhythmogenesis.

Role of the dysfunctional ryanodine receptor - Na(+)-Ca(2+)exchanger axis in progression of cardiovascular diseases: What we can learn from pharmacological studies?

Abnormal Ca(2+)homeostasis is often associated with chronic cardiovascular diseases, such as hypertension, heart failure or cardiac arrhythmias, and typically contributes to the basic ethiology of the disease. Pharmacological targeting of cardiac Ca(2+)handling has great therapeutic potential offering invaluable options for the prevention, slowing down the progression or suppression of the harmful outcomes like life threatening cardiac arrhythmias. In this review we outline the existing knowledge on the involvement of malfunction of the ryanodine receptor and the Na(+)-Ca(2+)exchanger in disturbances of Ca(2+)homeostasis and discuss important proof of concept pharmacological studies targeting these mechanisms in context of hypertension, heart failure, atrial fibrillation and ventricular arrhythmias. We emphasize the promising results of preclinical studies underpinning the potential benefits of the therapeutic strategies based on ryanodine receptor or Na(+)-Ca(2+)exchanger inhibition.

Latrunculin B modulates electrophysiological characteristics and arrhythmogenesis in pulmonary vein cardiomyocytes

AF (atrial fibrillation) is the most common sustained arrhythmia, and the PVs (pulmonary veins) play a critical role in triggering AF. Stretch causes structural remodelling, including cytoskeleton rearrangement, which may play a role in the genesis of AF. Lat-B (latrunculin B), an inhibitor of actin polymerization, is involved in Ca(2+) regulation. However, it is unclear whether Lat-B directly modulates the electrophysiological characteristics and Ca(2+) homoeostasis of the PVs. Conventional microelectrodes, whole-cell patch-clamp, and the fluo-3 fluorimetric ratio technique were used to record ionic currents and intracellular Ca(2+) within isolated rabbit PV preparations, or within isolated single PV cardiomyocytes, before and after administration of Lat-B (100 nM). Langendorff-perfused rabbit hearts were exposed to acute and continuous atrial stretch, and we studied PV electrical activity. Lat-B (100 nM) decreased the spontaneous electrical activity by 16±4% in PV preparations. Lat-B (100 nM) decreased the late Na(+) current, L-type Ca(2+) current, Na(+)/Ca(2+) exchanger current, and stretch-activated BKCa current, but did not affect the Na(+) current in PV cardiomyocytes. Lat-B reduced the transient outward K(+) current and ultra-rapid delayed rectifier K(+) current, but increased the delayed rectifier K(+) current in isolated PV cardiomyocytes. In addition, Lat-B (100 nM) decreased intracellular Ca(2+) transient and sarcoplasmic reticulum Ca(2+) content in PV cardiomyocytes. Moreover, Lat-B attenuated stretch-induced increased spontaneous electrical activity and trigger activity. The effects of Lat-B on the PV spontaneous electrical activity were attenuated in the presence of Y-27632 [10 μM, a ROCK (Rho-associated kinase) inhibitor] and cytochalasin D (10 μM, an actin polymerization inhibitor). In conclusion, Lat-B regulates PV electrophysiological characteristics and attenuates stretch-induced arrhythmogenesis.

Renal failure induces atrial arrhythmogenesis from discrepant electrophysiological remodeling and calcium regulation in pulmonary veins, sinoatrial node, and atria

BACKGROUND

Renal failure (RF) increases the risk of atrial fibrillation (AF), but arrhythmogenic mechanism is unclear. The present study investigated the electrophysiological effects of RF on AF trigger (pulmonary veins, PVs) and substrate (atria) and evaluated potential underlying mechanisms.

METHODS

Electrocardiographic, echocardiographic, and biochemical studies were conducted in rabbits with and without antibiotic-induced mild (creatinine=1.5-6.0 mg/dl) and advanced (creatinine>6.0 mg/dl) RF. Conventional microelectrode techniques, western blotting, and histological examinations were performed using the isolated rabbit PV, left atrium (LA), right atrium (RA) and sinoatrial node (SAN).

RESULTS

Advanced RF rabbits (n=18) had a higher incidence (33.3% vs. 11.1% and 0%, p<0.05) of atrial arrhythmia than mild RF (n=18) and control (n=18) rabbits. Advanced RF rabbits exhibited faster PV spontaneous activities, longer action potential duration (APD) in the LA, higher fibrosis in the LA, and slower SAN beating rates than control rabbits, but had a similar APD and fibrosis in the RA. Caffeine (1 mM) increased advanced RF PV arrhythmogenesis, which is blocked by flecainide (10 μM), or KB-R7943 (10 μM). Moreover, advanced RF rabbits had a higher expression of the Na+/Ca2+ exchanger, protein kinase A, phosphorylated ryanodine receptor (Serine 2808), and phosphorylated phospholamban (Serine 16) in PVs, and a higher expression of Cav 1.2 in the LA, and a lower expression of hyperpolarization-activated cyclic nucleotide-gated potassium channel 4 in the SAN.

CONCLUSIONS

Advanced RF increases atrial arrhythmia by modulating the distinctive electrophysiological characteristics of the PV, LA, and SAN.

Selective Na(+) /Ca(2+) exchanger inhibition prevents Ca(2+) overload-induced triggered arrhythmias

BACKGROUND AND PURPOSE

Augmented Na(+) /Ca(2+) exchanger (NCX) activity may play a crucial role in cardiac arrhythmogenesis; however, data regarding the anti-arrhythmic efficacy of NCX inhibition are debatable. Feasible explanations could be the unsatisfactory selectivity of NCX inhibitors and/or the dependence of the experimental model on the degree of Ca(2+) i overload. Hence, we used NCX inhibitors SEA0400 and the more selective ORM10103 to evaluate the efficacy of NCX inhibition against arrhythmogenic Ca(2+) i rise in conditions when [Ca(2+) ]i was augmented via activation of the late sodium current (INaL ) or inhibition of the Na(+) /K(+) pump.

EXPERIMENTAL APPROACH

Action potentials (APs) were recorded from canine papillary muscles and Purkinje fibres by microelectrodes. NCX current (INCX ) was determined in ventricular cardiomyocytes utilizing the whole-cell patch clamp technique. Ca(2+) i transients (CaTs) were monitored with a Ca(2+) -sensitive fluorescent dye, Fluo-4.

KEY RESULTS

Enhanced INaL increased the Ca(2+) load and AP duration (APD). SEA0400 and ORM10103 suppressed INCX and prevented/reversed the anemone toxin II (ATX-II)-induced [Ca(2+) ]i rise without influencing APD, CaT or cell shortening, or affecting the ATX-II-induced increased APD. ORM10103 significantly decreased the number of strophanthidin-induced spontaneous diastolic Ca(2+) release events; however, SEA0400 failed to restrict the veratridine-induced augmentation in Purkinje-ventricle APD dispersion.

CONCLUSIONS AND IMPLICATIONS

Selective NCX inhibition - presumably by blocking rev INCX (reverse mode NCX current) - is effective against arrhythmogenesis caused by [Na(+) ]i -induced [Ca(2+) ]i elevation, without influencing the AP waveform. Therefore, selective INCX inhibition, by significantly reducing the arrhythmogenic trigger activity caused by the perturbed Ca(2+) i handling, should be considered as a promising anti-arrhythmic therapeutic strategy.

Selective and non-selective non-steroidal anti-inflammatory drugs differentially regulate pulmonary vein and atrial arrhythmogenesis

BACKGROUND

Non-steroidal anti-inflammatory drugs (NSAIDs) increase the risk of atrial fibrillation (AF). This study investigated whether selective and non-selective NSAIDs differentially regulate the arrhythmogenesis of pulmonary veins and atria.

METHODS

Conventional microelectrodes were used to record action potentials (APs) in isolated rabbit PVs, sinoatrial node (SAN), left atrium (LA), and right atrium (RA) preparations before and after celecoxib or indomethacin administration. A whole-cell patch clamp was used to record the sodium-calcium exchanger (NCX) current, L-type calcium current (ICa-L), and late sodium current (INa-late) before and after celecoxib administration in isolated PV cardiomyocytes.

RESULTS

Celecoxib (0.3, 1, and 3 μM) reduced PV spontaneous beating rates, and induced delayed afterdepolarizations and burst firings in four of eight PV preparations (50%, p<0.05). Celecoxib also reduced SAN beating rates and decreased AP durations (APDs) in RA and LA, but did not change the resting membrane potential. Indomethacin (0.3, 1, 3, and 10 μM) changed neither the PV or SAN beating rates nor RA APDs, but it reduced LA APDs. Celecoxib (3 μM) significantly increased the NCX current and decreased the ICa-L, but did not change the INa-late. Ranolazine (10 μM) suppressed celecoxib (3 μM)-induced PV burst firings in 6 (86%, p<0.05) of 7 PVs. KB-R7943 (10 μM) suppressed celecoxib (3 μM)-induced PV burst firings in 5 (71%, p<0.05) of 7 PVs.

CONCLUSIONS

Selective and non-selective NSAIDs differentially modulate PV and atrial electrophysiological characteristics. Celecoxib increased PV triggered activity through enhancement of the NCX current, which contributed to its arrhythmogenesis.

Paradoxical effects of KB-R7943 on arrhythmogenicity in a chronic myocardial infarction rabbit model

BACKGROUND

Na(+)/Ca(2+) exchanger blockade has been reported to be anti-arrhythmic in different models. The effects of KB-R7943, a Na(+)/Ca(2+) exchanger blocker, on arrhythmogenesis in hearts with chronic myocardial infarction (MI) remain unclear.

METHODS

Dual voltage and intracellular Ca(2+) (Cai) optical mapping was performed in nine rabbit hearts with chronic MI and four control hearts. Electrophysiology studies including inducibility of ventricular tachyarrhythmias, ventricular fibrillation dominant frequency, action potential, Cai alternans, Cai decay, and conduction velocity were performed. The same protocol was repeated in the presence of KB-R7943 (0.5, 1, and 5μM) after the baseline studies.

RESULTS

KB-R7943 was effective in suppressing afterdepolarizations and spontaneous ventricular tachyarrhythmias in hearts with chronic MI. Surprisingly, KB-R7943 increased the inducibility of ventricular tachyarrhythmias in a dose-dependent manner (11%, 11%, 22%, and 56% at baseline and with 0.5, 1, and 5μM KB-R7943, respectively, p=0.02). Optical mapping analysis revealed that the underlying mechanisms of the induced ventricular tachyarrhythmias were probably spatially discordant alternans with wave breaks and rotors. Further analysis showed that KB-R7943 significantly enhanced both action potential (p=0.033) and Cai (p=0.001) alternans, prolonged Cai decay (tau value) in a dose-dependent manner (p=0.004), and caused heterogeneous conduction delay especially at peri-infarct zones during rapid burst pacing. In contrast, KB-R7943 had insignificant effects in control hearts.

CONCLUSIONS

In this chronic MI rabbit model, KB-R7943 has contrasting effects on arrhythmogenesis, suppressing afterdepolarizations and spontaneous ventricular tachyarrhythmias, but enhancing the inducibility of tachyarrhythmias. The mechanism is probably the enhanced spatially discordant alternans because of prolonged Cai decay and heterogeneous conduction delay.

Combined vildagliptin and metformin exert better cardioprotection than monotherapy against ischemia-reperfusion injury in obese-insulin resistant rats

BACKGROUND

Obese-insulin resistance caused by long-term high-fat diet (HFD) consumption is associated with left ventricular (LV) dysfunction and increased risk of myocardial infarction. Metformin and vildagliptin have been shown to exert cardioprotective effects. However, the effect of these drugs on the hearts under obese-insulin resistance with ischemia-reperfusion (I/R) injury is unclear. We hypothesized that combined vildagliptin and metformin provide better protective effects against I/R injury than monotherapy in obese-insulin resistant rats.

METHODOLOGY

Male Wistar rats were fed either HFD or normal diet. Rats in each diet group were divided into 4 subgroups to receive vildagliptin, metformin, combined vildagliptin and metformin, or saline for 21 days. Ischemia due to left anterior descending artery ligation was allowed for 30-min, followed by 120-min reperfusion. Metabolic parameters, heart rate variability (HRV), LV function, infarct size, mitochondrial function, calcium transient, Bax and Bcl-2, and Connexin 43 (Cx43) were determined. Rats developed insulin resistance after 12 weeks of HFD consumption. Vildagliptin, metformin, and combined drugs improved metabolic parameters, HRV, and LV function. During I/R, all treatments improved LV function, reduced infarct size and Bax, increased Bcl-2, and improved mitochondrial function in HFD rats. However, only combined drugs delayed the time to the first VT/VF onset, reduced arrhythmia score and mortality rate, and increased p-Cx43 in HFD rats.

CONCLUSION

Although both vildagliptin and metformin improved insulin resistance and attenuate myocardial injury caused by I/R, combined drugs provided better outcomes than single therapy by reducing arrhythmia score and mortality rate.

A monounsaturated fatty acid (oleic acid) modulates electrical activity in atrial myocytes with calcium and sodium dysregulation

BACKGROUND

Obesity and metabolic syndrome are important risk factors for atrial fibrillation. High plasma concentrations of monounsaturated fatty acids, including oleic acid (OLA), are frequently noted in obese individuals and patients with metabolic syndrome. However, it is not clear whether monounsaturated fatty acids (MUFAs) can directly modulate the electrophysiological characteristics of atrial myocytes.

METHODS

Whole-cell patch clamp, indo-1 fluorescence, and Western blot analyses were used to record the action potentials (APs), ionic currents, and protein expressions of HL-1 myocytes incubated with and without (control) OLA (0.5mM) for 24h.

RESULTS

Compared to control myocytes (n=14), OLA-treated myocytes (n=16) had shorter APD90 (65 ± 6 vs. 85 ± 6 ms, p<0.05) and APD50 (24 ± 6 vs. 38 ± 4 ms, p<0.05) with a higher incidence of delayed afterdepolarizations (35.7% vs. 7%, p<0.05), which were suppressed by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS, a blocker of the calcium-activated chloride current). In addition, OLA-treated myocytes (n=19) exhibited larger calcium transients (0.54 ± 0.06 vs. 0.38 ± 0.05 R410/485, p<0.05), and sarcoplasmic reticular calcium contents (0.91 ± 0.05 vs. 0.64 ± 0.08 R410/485, p<0.05) than control myocytes (n=15). OLA-treated myocytes had larger late sodium currents, smaller sodium-calcium exchanger currents, and smaller sodium-potassium pump currents. Moreover OLA-treated myocytes had higher expressions of sarcoplasmic reticular Ca(2+)-ATPase and calmodulin kinase II, but lower expression of the sodium-potassium ATPase protein than control myocytes.

CONCLUSIONS

MUFAs can regulate atrial electrophysiological characteristics with calcium and sodium dysregulation, which may contribute to atrial arrhythmogenesis.

Leptin modulates electrophysiological characteristics and isoproterenol-induced arrhythmogenesis in atrial myocytes

Background

Obesity is an important risk factor for atrial fibrillation (AF). Leptin is an important adipokine. However, it is not clear whether leptin directly modulates the electrophysiological characteristics of atrial myocytes.

Results

Whole cell patch clamp and indo-1 fluorescence were used to record the action potentials (APs) and ionic currents in isolated rabbit left atrial (LA) myocytes incubated with and without (control) leptin (100 nM) for 1 h to investigate the role of leptin on atrial electrophysiology. Leptin-treated LA myocytes (n = 19) had longer 20% of AP duration (28 ± 3 vs. 21 ± 2 ms, p < 0.05), but similar 50% of AP duration (51 ± 4 vs. 50 ± 3 ms, p > 0.05), and 90% of AP duration (89 ± 5 vs. 94 ± 4 ms, p > 0.05), as compared to the control (n = 22). In the presence of isoproterenol (10 nM), leptin-treated LA myocytes (n = 21) showed a lower incidence (19% vs. 54.2%, p < 0.05) of delayed afterdepolarization (DAD) than the control (n = 24). Leptin-treated LA myocytes showed a larger sodium current, but a smaller ultra-rapid delayed rectifier potassium current, and sodium-calcium exchanger current than the control. Leptin-treated and control LA myocytes exhibited a similar late sodium current, inward rectifier potassium current, transient outward current and L-type calcium current. In addition, the leptin-treated LA myocytes (n = 38) exhibited a smaller intracellular Ca²⁺ transient (0.21 ± 0.01 vs. 0.26 ± 0.01 R410/485, p < 0.05) and sarcoplasmic reticulum Ca²⁺ content (0.35 ± 0.02 vs. 0.43 ± 0.03 R410/485, p < 0.05) than the control LA myocytes (n = 42).

Conclusions

Leptin regulates the LA electrophysiological characteristics and attenuates isoproterenol-induced arrhythmogenesis.

The cardioprotective effect of hypertonic saline is associated with inhibitory effect on macrophage migration inhibitory factor in sepsis

Sepsis can cause myocardial dysfunction, which contributes to the high mortality of sepsis. Hypertonic saline (HS) has been reported to increase myocardial contractility in sepsis. In the present study, mechanisms of action of HS resuscitation (4 mL of 7.5% NaCl per kilogram) on cardiac function have been evaluated in septic rats. HS was administered 1 h after LPS (10 mg/kg, i.v.) challenge. The mean arterial blood pressure significantly decreased 4 h after LPS challenge, and septic shock was observed at the end of experiment (6 h). Posttreatment with HS prevented hypotension caused by LPS and significantly improved cardiac function, evidenced by increases in left ventricular developed pressure, mean +dP/dt and -dP/dt. The amplitude of electrical-stimulated intracellular Ca(2+) transient in isolated single cardiomyocytes was significantly reduced after 6 h LPS insult, which was recovered by HS. In addition, LPS resulted in significant increases in neutrophil myeloperoxidase activity, macrophage migration inhibitory factor (MIF), and NF-κB phospho-p65 protein levels in myocardium at 6 h, which were significantly attenuated by HS. In conclusion, HS improved myocardial contractility and prevented circulatory failure induced by endotoxemia, which may attribute to improvement of intracellular calcium handling process and inhibitory effects on neutrophil infiltration and MIF production in hearts.

The dipeptidyl peptidase-4 inhibitor-sitagliptin modulates calcium dysregulation, inflammation, and PPARs in hypertensive cardiomyocytes

BACKGROUND

Hypertension induces cardiac dysfunction, calcium (Ca(2+)) dysregulation, and arrhythmogenesis. Dipeptidyl peptidase (DPP)-4 inhibitors, an antidiabetic agent with anti-inflammation and anti-hypertension potential, may regulate peroxisome proliferator-activated receptors (PPARs)-α, -γ, and -δ and Ca(2+) homeostasis.

OBJECTIVE

The purpose of this study was to investigate whether DPP-4 inhibitor, sitagliptin, can modulate PPARs and Ca(2+) handling proteins in hypertensive hearts.

METHODS

A Western blot analysis was used to evaluate protein expressions of myocardial PPAR isoforms, tumor necrosis factor (TNF)-α, interleukin (IL)-6, sarcoplasmic reticulum ATPase (SERCA2a), Na(+)-Ca(2+) exchanger (NCX), ryanodine receptor (RyR), voltage-dependent Ca(2+) (CaV1.2), slow-voltage potassium currents (Kvs), angiotensin II type 1 receptor (AT1R), and receptor of advanced glycated end-products (RAGE) from Wistar-Kyoto (WKY) rats, spontaneously hypertensive rats (SHR), and SHR treated with sitagliptin (10mg/kg for 4weeks). Conventional microelectrodes were used to record action potentials (APs) in the ventricular myocytes from each group.

RESULTS

Compared to the control group, SHR had lower cardiac PPAR-α and PPAR-δ protein expressions, but had greater cardiac PPAR-γ levels, and TNF-α, IL-6, RAGE, and AT1R protein expressions, which were ameliorated in the sitagliptin-treated SHR. SHR had prolonged QT interval and AP duration with less SERCA2a and RyR, and greater CaV1.2 expressions, which were also attenuated in sitagliptin-treated SHR.

CONCLUSIONS

Sitagliptin significantly changed the cardiac electrophysiological characteristics and Ca(2+) regulation, which may have been caused by its effects on cardiac PPARs, proinflammatory cytokines, and AT1R.

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.

Ablation of the androgen receptor gene modulates atrial electrophysiology and arrhythmogenesis with calcium protein dysregulation

Androgen deficiency is important in the pathophysiology of atrial fibrillation. Androgen regulates cardiac electrophysiology and calcium (Ca(2+)) homeostasis. The purpose of this study is to evaluate whether androgen receptor knockout (ARKO) can modulate atrial electrophysiology and arrhythmogenesis with modulation of Ca(2+) homeostasis proteins. We used conventional microelectrodes to study the action potential (AP) in left atrium (LA) tissues prepared from wild-type (WT) and ARKO mice (aged 6-10 months) before and after the administration of isoproterenol, hypocalcemic/hypercalcemic solutions, and ouabain. Echocardiography and Western blots were used to evaluate the cardiac function and expression levels of ionic channel proteins in WT and ARKO LAs. ARKO LAs had larger LA diameter with decreased LA fractional shortening than did WT LAs. In the current study, we found that ARKO LAs had a lower negative resting membrane potential and a greater 90% AP duration (APD) than did WT LAs. Isoproterenol increased the incidence and amplitude of delayed afterdepolarizations (DADs) in ARKO LAs but not in WT LAs. Hypocalcemic solutions prolonged APD in WT and ARKO LAs but increased DAD amplitude only in ARKO LAs. Hypercalcemic solutions shortened APD in ARKO LAs but not in WT LAs. Ouabain increased DAD amplitude in ARKO LAs but not in WT LAs. ARKO LAs expressed higher amounts of Ca(2+)/calmodulin-dependent protein kinase II, Na(+)/Ca(2+) exchanger, and phosphorylated phospholamban (Ser-16/Thr-17 site) and less Cav1.2, Kir2.1, Kir3.1, and Kv7.1 than WT LAs. These observations indicate that ARKO alters atrial electrophysiology with increased atrial arrhythmogenesis.

Structure, function and clinical relevance of the cardiac conduction system, including the atrioventricular ring and outflow tract tissues

It is now over 100years since the discovery of the cardiac conduction system, consisting of three main parts, the sinus node, the atrioventricular node and the His-Purkinje system. The system is vital for the initiation and coordination of the heartbeat. Over the last decade, immense strides have been made in our understanding of the cardiac conduction system and these recent developments are reviewed here. It has been shown that the system has a unique embryological origin, distinct from that of the working myocardium, and is more extensive than originally thought with additional structures: atrioventricular rings, a third node (so called retroaortic node) and pulmonary and aortic sleeves. It has been shown that the expression of ion channels, intracellular Ca(2+)-handling proteins and gap junction channels in the system is specialised (different from that in the ordinary working myocardium), but appropriate to explain the functioning of the system, although there is continued debate concerning the ionic basis of pacemaking. We are beginning to understand the mechanisms (fibrosis and remodelling of ion channels and related proteins) responsible for dysfunction of the system (bradycardia, heart block and bundle branch block) associated with atrial fibrillation and heart failure and even athletic training. Equally, we are beginning to appreciate how naturally occurring mutations in ion channels cause congenital cardiac conduction system dysfunction. Finally, current therapies, the status of a new therapeutic strategy (use of a specific heart rate lowering drug) and a potential new therapeutic strategy (biopacemaking) are reviewed.

Reverse mode of the sodium/calcium exchanger subtype 3 in interstitial cells of Cajal from rat bladder

OBJECTIVE

To investigate how the sodium/calcium exchanger subtype 3 (NCX3) and its reverse mode contribute to the function of interstitial cells of Cajal (ICCs) from the rat bladder.

METHODS

The study used 20 female Wistar rats. We observed the expression of the NCX3 expression in the bladder using reverse transcriptase-polymerase chain reaction and Western blotting. The NCX3 in ICCs was also confirmed by double-labeled fluorescence. NCX3 functions in reverse mode of ICCs were observed using confocal microscopy with preload fluo-3AM, and its currents were evaluated using the whole-cell patch clamp technique, with or without the NCX3 inhibitor KB-R7943 (5 and 30μM), with an afterward identification of ICCs using single-cell polymerase chain reaction.

RESULTS

NCX3 was confirmed in rat bladder ICCs. The time required for the intracellular calcium concentration [Ca(2+)]i of NCX3 was enhanced by KB-R7943 (5μM, P ≤.01). Moreover, KB-R7943 (5 and 30μM) significantly decreased the currents generated by the reverse mode of NCX3 from the ICCs (P <.05).

CONCLUSION

NCX3 is expressed in rat bladder ICCs. The reverse mode of NCX3 can generate [Ca(2+)]i of the bladder ICCs.

Extracellular matrix of collagen modulates arrhythmogenic activity of pulmonary veins through p38 MAPK activation

Atrial fibrillation (AF) is the most common sustained arrhythmia. Cardiac fibrosis with enhanced extracellular collagen plays a critical role in the pathophysiology of AF through structural and electrical remodeling. Pulmonary veins (PVs) are important foci for AF genesis. The purpose of this study was to evaluate whether collagen can directly modulate PV arrhythmogenesis. Action potentials and ionic currents were investigated in isolated male New Zealand rabbit PV cardiomyocytes with and without collagen incubation (10μg/ml, 5-7h) using the whole-cell patch-clamp technique. Compared to control PV cardiomyocytes (n=25), collagen-treated PV cardiomyocytes (n=22) had a faster beating rate (3.2±04 vs. 1.9±0.2Hz, p<0.005) and a larger amplitude of delayed afterdepolarization (16±2 vs. 10±1mV, p<0.01). Moreover, collagen-treated PV cardiomyocytes showed a larger transient outward potassium current, small-conductance Ca(2+)-activated K(+) current, inward rectifier potassium current, pacemaker current, and late sodium current than control PV cardiomyocytes, but amplitudes of the sodium current, sustained outward potassium current, and L-type calcium current were similar. Collagen increased the p38 MAPK phosphorylation in PV cardiomyocytes as compared to control. The change of the spontaneous activity and action potential morphology were ameliorated by SB203580 (the p38 MAPK catalytic activity inhibitor), indicating that collagen can directly increase PV cardiomyocyte arrhythmogenesis through p38 MAPK activation, which may contribute to the pathogenesis of AF.

Atrial fibrillation in the elderly: the potential contribution of reactive oxygen species

Atrial fibrillation (AF) is the most commonly encountered cardiac arrhythmia, and is a significant source of healthcare expenditures throughout the world. It is an arrhythmia with a very clearly defined predisposition for individuals of advanced age, and this fact has led to intense study of the mechanistic links between aging and AF. By promoting oxidative damage to multiple subcellular and cellular structures, reactive oxygen species (ROS) have been shown to induce the intra- and extra-cellular changes necessary to promote the pathogenesis of AF. In addition, the generation and accumulation of ROS have been intimately linked to the cellular processes which underlie aging. This review begins with an overview of AF pathophysiology, and introduces the critical structures which, when damaged, predispose an otherwise healthy atrium to AF. The available evidence that ROS can lead to damage of these critical structures is then reviewed. Finally, the evidence linking the process of aging to the pathogenesis of AF is discussed.

Omega-3 Polyunsaturated Fatty Acid Supplementation: Mechanism and Current Evidence in Atrial Fibrillation

Atrial fibrillation (AF) is the most prevalent arrhythmia and is associated with considerable morbidity and mortality. Available pharmacologic antiarrhythmic therapies are often ineffective in preventing the recurrence of AF, possibly because these drugs target a single pathophysiological mechanism. Given their beneficial effects on ventricular arrhythmias, omega-3 polyunsaturated fatty acids (n-3 PUFAs) have recently been investigated as possible candidates in the treatment of supraventricular arrhythmias. In this review, we explore the current understanding of the antiarrhythmic effects attributed to n-3 PUFAs including direct modulation of ionic channels, improvement of membrane fluidity, anti-inflammatory and antifibrotic effects, and modulation of sympatho-vagal balance. We will then focus on the results of epidemiologic studies exploring the associations between nutritional intake of n3 PUFAs and the incidence of AF, and will review the findings of the clinical trials investigating the effects of n-3 PUFAs supplementation in the prophylaxis of AF and in the prevention of its recurrences.