Ventricular Action Potential Duration Research Articles

KCNQ1 suppression-replacement gene therapy in transgenic rabbits with type 1 long QT syndrome.

Type 1 long QT syndrome (LQT1) is caused by pathogenic variants in the KCNQ1-encoded Kv7.1 potassium channels, which pathologically prolong ventricular action potential duration (APD). Herein, the pathologic phenotype in transgenic LQT1 rabbits is rescued using a novel KCNQ1 suppression-replacement (SupRep) gene therapy. KCNQ1-SupRep gene therapy was developed by combining into a single construct a KCNQ1 shRNA (suppression) and an shRNA-immune KCNQ1 cDNA (replacement), packaged into adeno-associated virus serotype 9, and delivered in vivo via an intra-aortic root injection (1E10 vg/kg). To ascertain the efficacy of SupRep, 12-lead electrocardiograms were assessed in adult LQT1 and wild-type (WT) rabbits and patch-clamp experiments were performed on isolated ventricular cardiomyocytes. KCNQ1-SupRep treatment of LQT1 rabbits resulted in significant shortening of the pathologically prolonged QT index (QTi) towards WT levels. Ventricular cardiomyocytes isolated from treated LQT1 rabbits demonstrated pronounced shortening of APD compared to LQT1 controls, leading to levels similar to WT (LQT1-UT vs. LQT1-SupRep, P < .0001, LQT1-SupRep vs. WT, P = ns). Under β-adrenergic stimulation with isoproterenol, SupRep-treated rabbits demonstrated a WT-like physiological QTi and APD90 behaviour. This study provides the first animal-model, proof-of-concept gene therapy for correction of LQT1. In LQT1 rabbits, treatment with KCNQ1-SupRep gene therapy normalized the clinical QTi and cellular APD90 to near WT levels both at baseline and after isoproterenol. If similar QT/APD correction can be achieved with intravenous administration of KCNQ1-SupRep gene therapy in LQT1 rabbits, these encouraging data should compel continued development of this gene therapy for patients with LQT1.

2-Aminoethoxydiphenyl borate prolongs atrial and shortens ventricular action potential duration through modulation of ion channels in the sarcolemma

Abstract Funding Acknowledgements Type of funding sources: Public grant(s) – National budget only. Main funding source(s): PPP allowance, Health holland ZonmW Background 2-Aminoethoxydiphenyl Borate (2-APB) has been shown to prevent atrial fibrillation in animal models, supposedly through its inhibitory action on the intracellular IP3 receptor and subsequent calcium regulation. However, 2-APB may modulate other ion channels or transporters and the cellular mechanisms associated with the prevention of atrial fibrillation are not well established. We aimed at unravelling the cellular effects of 2-APB on atrial and ventricular action potential and currents. Methods Rabbit (New Zealand White) left atrial and ventricular cardiomyocytes were isolated from Langendorff-perfused hearts by enzymatic dissociation. Adult human atrial cardiomyocytes were obtained by enzymatic dissociation from left atrial appendages (LAA) routinely removed from AF patients undergoing minimally invasive surgical pulmonary vein isolation. Action potentials (APs) and K+ currents were recorded at 36.5 °C with the amphotericin-B-perforated or whole cell patch clamp technique. RNA was isolated from left atrial and ventricular tissue and sequenced on an Illumina HiSeq4000 platform. Results In rabbit atrial cardiomyocytes, extracellular application of 5 uM 2-APB had no effect on action potential duration (APD), but extracellular application of 50 uM 2-APB prolonged the APD by 40% and inhibited an outward K+-current. Inversely, extracellular application of 5 uM 2-APB shortened the APD of rabbit left ventricular cardiomyocytes by 20% while extracellular application 50 uM 2-APB did not prolong the APD. Additionally, 2-APB did not modulate K+ currents in ventricular rabbit cardiomyocytes. Intracellular application (in the patch pipet) of 50 uM 2-APB did not have any effects on atrial APD, suggesting that IP3 receptor inhibition is not involved. However, subsequent application of 50 uM 2-APB in the bath prolonged the APD by 40% indicating that 2-APB prolongs atrial APD by inhibiting one or more K+ channel types in the sarcolemmal membrane from the extracellular site. RNA sequencing analysis revealed potential targets channel including TRP and Kv channels differentially expressed in atria versus ventricle. In human atrial cardiomyocytes from AF patients, application of 50 uM 2-APB prolonged APD by 25%. Conclusions 2-APB prolongs rabbit and human atrial APD and shortens rabbit ventricular APD in a concentration dependent manner, most likely due to modulation of sarcolemmal K+ channels, differentially expressed in the two cell types. Pharmacological studies with 2-APB may reveal novel cellular mechanisms identifying potential new targets for the treatment of atrial fibrillation.

3-Methyl-phenanthrene (3-MP) disrupts the electrical and contractile activity of the heart of the polar fish, navaga cod (Eleginus nawaga)

Alkylated polycyclic aromatic hydrocarbons are abundant in crude oil and are enriched during petroleum refinement but knowledge of their cardiotoxicity remains limited. Polycyclic aromatic hydrocarbons (PAHs) are considered the main hazardous components in crude oil and the tricyclic PAH phenanthrene has been singled out for its direct effects on cardiac tissue in mammals and fish. Here we test the impact of the monomethylated phenanthrene, 3-methylphenanthrene (3-MP), on the contractile and electrical function of the atrium and ventricle of a polar fish, the navaga cod (Eleginus nawaga). Using patch-clamp electrophysiology in atrial and ventricular cardiomyocytes we show that 3-MP is a potent inhibitor of the delayed rectifier current IKr (IC50 = 0.25 μM) and prolongs ventricular action potential duration. Unlike the parent compound phenanthrene, 3-MP did not reduce the amplitude of the L-type Ca2+ current (ICa) but it accelerated current inactivation thus reducing charge transfer across the myocyte membrane and compromising pressure development of the whole heart. 3-MP was a potent inhibitor (IC50 = 4.7 μM) of the sodium current (INa), slowing the upstroke of the action potential in isolated cells, slowing conduction velocity across the atrium measured with optical mapping, and increasing atrio-ventricular delay in a working whole heart preparation. Together, these findings reveal the strong cardiotoxic potential of this phenanthrene derivative on the fish heart. As 3-MP and other alkylated phenanthrenes comprise a large fraction of the PAHs in crude oil mixtures, these findings are worrisome for Arctic species facing increasing incidence of spills and leaks from the petroleum industry. 3-MP is also a major component of polluted air but is not routinely measured. This is also of concern if the hearts of humans and other terrestrial animals respond to this PAH in a similar manner to fish.

The electrical restitution of the non-propagated cardiac ventricular action potential.

Sudden changes in pacing cycle length are frequently associated with repolarization abnormalities initiating cardiac arrhythmias, and physiologists have long been interested in measuring the likelihood of these events before their manifestation. A marker of repolarization stability has been found in the electrical restitution (ER), the response of the ventricular action potential duration to a pre- or post-mature stimulation, graphically represented by the so-called ER curve. According to the restitution hypothesis (ERH), the slope of this curve provides a quantitative discrimination between stable repolarization and proneness to arrhythmias. ER has been studied at the body surface, whole organ, and tissue level, and ERH has soon become a key reference point in theoretical, clinical, and pharmacological studies concerning arrhythmia development, and, despite criticisms, it is still widely adopted. The ionic mechanism of ER and cellular applications of ERH are covered in the present review. The main criticism on ERH concerns its dependence from the way ER is measured. Over the years, in fact, several different experimental protocols have been established to measure ER, which are also described in this article. In reviewing the state-of-the art on cardiac cellular ER, I have introduced a notation specifying protocols and graphical representations, with the aim of unifying a sometime confusing nomenclature, and providing a physiological tool, better defined in its scope and limitations, to meet the growing expectations of clinical and pharmacological research.

Abstract P3145: Enhancing Cardiac Cav1.2 Ion Channel Function By Targeted Deubiquitination

Ventricular excitation-contraction coupling relies on precise spatial arrangement of voltage-gated calcium channels CaV1.2 within T-tubular dyadic junctions proximal to ryanodine receptors (RyR2) that gate Ca2+ release from the sarcoplasmic reticulum. Impaired CaV1.2 trafficking is linked to devastating cardiovascular diseases such as arrythmias and cardiac hypertrophy. Ubiquitination is an important regulatory mechanism that controls ion channel surface trafficking and turnover. Emerging evidence hint at a prominent role for CaV1.2 ubiquitination to underlie heart failure disease progression, that suppresses CaV1.2 currents (ICa,L), resulting in harmful compensatory mechanisms. Targeted protein stabilization (TPS) is a burgeoning therapeutic approach for undruggable targets. We hypothesized that harnessing deubiquitinases using TPS can counteract pathological ubiquitination of CaV1.2, overcoming the need for hormonal remodeling and lead to novel treatment. To this end, we sought to develop engineered deubiquitinases (enDUBs) that can increase functional expression of CaV1.2 channels in the heart. The catalytic domain of ubiquitin specific protease 21 (USP21) fused to nb.F3 (F3-USP21) robustly increased CaV1.2 currents reconstituted in HEK293 cells as well as endogenous ICa,L in mammalian ventricular cardiomyocytes. This increase was within the physiological range of the impact of β-adrenergic stimulation of CaV1.2 and did not alter ventricular action potential duration. The mechanism of nbF3-USP21-induced increase in ICa,L is augmented CaV1.2 surface density. Reassuringly, this approach did not adversely interfere with the correct placement of CaV1.2 channels at dyadic junctions as cardiomyocytes expressing nb.F3-USP21 displayed normal colocalization of CaV1.2 with RyR2 at T-tubules. In conclusion, our study provides evidence that targeting deubiquitinases using TPS could be a viable therapeutic approach for rescuing CaV1.2 functional expression and these findings may pave the way for future development of TPS-based therapies for cardiovascular diseases such as arrhythmias and cardiac hypertrophy.

Levosimendan attenuates electrical alternans and prevents ventricular arrhythmia during therapeutic hypothermia in isolated rabbit hearts

Therapeutic hypothermia (TH) increases the susceptibility to ventricular arrhythmias (VAs) by prolonging action potential duration (APD) and facilitating arrhythmogenic spatially discordant alternans (SDA). Levosimendan, a calcium sensitizer, has been reported to shorten APD by enhancing the adenosine triphosphate (ATP)-sensitive K current. The purpose of this study was to test the hypothesis that, during TH, levosimendan shortens the already prolonged APD, attenuates SDA, and prevents VA. Langendorff-perfused isolated rabbit hearts were subjected to TH (30°C) for 15 minutes, followed by treatment with either levosimendan 0.5 μM (n = 9) or vehicle (n = 8) for an additional 30 minutes under TH. Using an optical mapping system, epicardial APD was evaluated by S1 pacing. SDA threshold was defined as the longest pacing cycle length (PCL) that induces the phenomenon of SDA. Ventricular fibrillation (VF) inducibility was evaluated by burst pacing for 30 seconds at the shortest PCL that achieved 1:1 ventricular capture. During TH, levosimendan shortened ventricular APD (PCL 400 ms; from 259 ± 8 ms to 241 ± 18 ms; P = .036) and decreased SDA threshold (from 327 ± 88 ms to 311 ± 68 ms; P = .011). VF inducibility was lowered from 39% ± 30% to 14% ± 12% with levosimendan (P = .018), whereas APD at PCL 400 ms (P = .161), SDA threshold (P = 1), and VF inducibility (P = .173) were not changed by vehicle. During TH, levosimendan could protect hearts against VA by shortening APD and decreasing SDA threshold. Enhancing ATP-sensitive K current with levosimendan might be a novel approach to preventing VA during TH.

Analysis of the effect of cortisone on the QT interval

BackgroundCardiac death caused by malignant arrhythmias is very prevalent. Prolongation of the QT interval is a relevant aspect in arrhythmia mechanisms. Prior studies have revealed that the QTc interval could be shortened by cortisone. Moreover, in an animal model of long QT syndrome, cortisone treatment shortens the ventricular action potential duration. The present study investigated the effect of methylprednisolone (MPS) on the QTc interval in cardiovascularly healthy humans. MethodsPatients who had just been diagnosed with multiple sclerosis receiving MPS therapy were analysed prospectively. Demographic data, laboratory values, anti-arrhythmic medication and baseline and follow-up ECGs were extracted from the patients' medical records. ResultsSeventy-eight patients were included. The mean ± standard deviation age was 47 ± 15 years. The values of the electrolytes were normal. All patients were treated with MPS for 3 or 5 days. The heart rate increased at the beginning of MPS therapy and decreased during the subsequent period. ECG measurements showed that the QTc interval was prolonged at the beginning of MPS therapy and shortened over the course of treatment. The longest QTc intervals were obtained by calculation with Bazett‘s formula. ConclusionsIn humans, cortisone shortens the QTc interval over time. The analysis indicates a cumulative effect of cortisone that lasts longer. The results of our pilot study reveal that cortisone might be added to therapeutic strategies in patients with long QT syndromes. Further clinical studies have to be carried out to analyze potential clinical options.

Data-driven, patient-specific simulations of long-term heart rate variability and cardiac alternans formation.

Individuals suffering from chronic heart failure (CHF) have a greater risk for death mediated by ventricular arrhythmias, which may be related to changes in heart rate. Heart rate constantly fluctuates from beat-to-beat on a timescale ranging from seconds to hours under physiological conditions, termed heart rate variability (HRV). Individuals with CHF have reduced HRV and faster heart rates, both of which influence the formation of pro-arrhythmic alternans, a beat-to-beat alternation in the action potential duration (APD) and intracellular calcium (Ca) levels. It is unclear how long-term changes in heart rate influence alternans and arrhythmia formation. We obtained sequences of RR intervals (time between successive R waves in an electrocardiogram) from individuals with normal sinus rhythm (NSR) and CHF from a Physionet database. These RR-sequences are used to define the pacing period for a discrete-time coupled map model of APD and intracellular Ca cycling for a single cardiac myocyte, modified to account for human heart rates and pathological Ca-remodeling in CHF. These modifications include fitting human ventricular myocyte APD restitution parameters, including a new term for Ca leak from the sarcoplasmic reticulum (SR), and scaling SR Ca-related parameters. Importantly, this minimal model enables simulation of long-term trends, on the order of 24 hours, in large populations. We find that the alternations in APD and Ca vary over time, such that there are periods with both small and large beat-to-beat differences in both populations. However, the CHF population tends to have a larger alternations magnitude, demonstrating that CHF-associated heart rates promotes more pro-arrhythmic phenomena. Future work will extend this methodology to determine what statistical properties govern the CHF RR-sequences, and further to test to what extent pro-arrhythmic alternans depends on CHF-related changes in heart rate or remodeling.

Scenarios for Increasing, Decreasing and Stability of Tpe/QT Ratio (Simulation Study)

The objective of this simulation was to study the physiological meaning of the novel index for arrhythmic risk stratification – Tpe/QT ratio (the interval from the peak to the end of the T-wave (Tpe) on electrocardiogram divided by QT interval). Methods and Results: The role of two factors determining Tpe/QT ratio – action potential duration (APD) and dispersion of repolarization (DOR) – was studied in silico at two levels: ventricular wall segment and the whole heart ventricles. The simulations performed in the framework of both the segment and the entire ventricles’ models showed that Tpe/QT magnitude reflects the dynamic relationship between the longest and the shortest APD rather than the magnitude of DOR, as interval Tpe does. Tpe/QT ratio remained unchanged even at large DOR, provided that the longer the ventricular APD, the greater the gap between the longest and the shortest of them. The imbalance between the longest and the shortest APD values led to the increased or decreased Tpe/QT. Conclusion: Simulations showed that Tpe/QT is not just a corrected Tpe interval, but it reflects the balance between the longest and the shortest APD in the heart ventricles.

Cardioprotective effect of abscisic acid in a rat model of type 3 cardio-renal syndrome: Role of NOX-4, P-53, and HSP-70

Cardiorenal syndrome (CRS) is a complex heart and kidney pathophysiologic disorder that leads to a bidirectional interrelationship between them. Abscisic acid (ABA) is a phytohormone that is present in plants, and is known to regulate fundamental physiological functions. This study aimed to explore the efficacy of ABA in surgically induced-CRS type 3 rats. Rats were randomly and equally divided into four groups. Rats in Group 1 received saline (Sham group), Group 2 included control induced-CRS rats, Group 3 rats (CRS+ABA) included CRS rats treated with ABA and Group 4 (CRS + ABA + Verapamil + propofol) were CRS rats treated with Verapamil, propofol and ABA. The rats were treated with the drugs daily for four weeks. At the end of the study, relative heart weight corrected QT interval (QTc), mean blood pressure (MBP), kidney functions, oxidative stress, NADPH oxidase 4 (NOX4), protein 53 (P53), and heat shock proteins-70 (HSP-70) expression was assessed and recorded.ABA led to a significant shortening of the ventricular action potential duration indicated by QTc. Furthermore, it significantly lowered heart weight, MBP, serum creatinine, NOX-4, and P-53 expression and augmented HSP-70 expression. In contrast, adding calcium channel blockers (CCBs) to ABA mitigated this effect. The results suggested that ABA has a potential protective role in CRS-induced cardiac hypertrophy and arrhythmia that could be mediated through inhibition of P-53, NOX-4, and an increase in HSP-70 expression.

Testosterone does not shorten action potential duration in Langendorff-perfused rabbit ventricles.

Women have longer baseline QT intervals than men. Because previous studies showed that testosterone and 5α-dihydrotestosterone shorten the ventricular action potential duration (APD) in animal models, differential testosterone concentrations may account for the sex differences in QT interval. The purpose of this study was to test the hypothesis that testosterone shortens the APD in Langendorff-perfused rabbit ventricles. We performed optical mapping studies in hearts with or without testosterone administration. Acute studies included 26 hearts using 2 different protocols, including 17 without and 9 with atrioventricular (AV) block. For chronic studies, we implanted testosterone pellets subcutaneously in 7 female rabbits for 2-3 weeks before optical mapping studies during complete AV block. Six rabbits without pellet implantation served as controls. The hearts in the acute studies were paced with a pacing cycle length (PCL) of 200-300 ms and mapped at baseline and after administration of 1 nM, 10 nM, 100 nM, and 3 μM of testosterone. There was no shortening of APD80 at any PCL. Instead, a lengthening of APD80 was noted at higher concentrations. There were no sex differences in testosterone responses. In chronic studies, heart rates were 136 ± 5 bpm before and 148 ± 9 bpm after (P = .10) while QTc intervals were 314 ± 9 ms before and 317 ± 99 ms after (P= .69) testosterone pellet implantation, respectively. Overall, ventricular APD80 in the pellet group was longer than in the control group at 300- to 700-ms PCL. Testosterone does not shorten ventricular repolarization in rabbit hearts.

Incorrectly corrected? QT interval analysis in rats and mice.

QT interval, a surrogate measure for ventricular action potential duration (APD) in the surface ECG, is widely used to identify cardiac abnormalities and drug safety. In humans, cardiac APD and QT interval are prominently affected by heart rate (HR), leading to widely accepted formulas to correct the QT interval for HR changes (QT corrected - QTc). While QTc is widely used in the clinic, the proper way to correct the QT interval in small mammals such as rats and mice is not clear. Over the years, empiric correction formulas were developed for rats and mice, which are widely used in the literature. Recent experimental findings obtained from pharmacological and direct pacing experiments in unanesthetized rodents show that the rate-adaptation properties are markedly different from those in humans and the use of existing QTc formulae can lead to major errors in data interpretation. In the present review, these experimental findings are summarized and discussed.

Protein Kinase D1 Regulates Cardiac Hypertrophy, Potassium Channel Remodeling, and Arrhythmias in Heart Failure.

Background Structural and electrophysiological remodeling characterize heart failure (HF) enhancing arrhythmias. PKD1 (protein kinase D1) is upregulated in HF and mediates pathological hypertrophic signaling, but its role in K+ channel remodeling and arrhythmogenesis in HF is unknown. Methods and Results We performed echocardiography, electrophysiology, and expression analysis in wild-type and PKD1 cardiomyocyte-specific knockout (cKO) mice following transverse aortic constriction (TAC). PKD1-cKO mice exhibited significantly less cardiac hypertrophy post-TAC and were protected from early decline in cardiac contractile function (3 weeks post-TAC) but not the progression to HF at 7 weeks post-TAC. Wild-type mice exhibited ventricular action potential duration prolongation at 8 weeks post-TAC, which was attenuated in PKD1-cKO, consistent with larger K+ currents via the transient outward current, sustained current, inward rectifier K+ current, and rapid delayed rectifier K+ current and increased expression of corresponding K+ channels. Conversely, reduction of slowly inactivating K+ current was independent of PKD1 in HF. Acute PKD inhibition slightly increased transient outward current in TAC and sham wild-type myocytes but did not alter other K+ currents. Sham PKD1-cKO versus wild-type also exhibited larger transient outward current and faster early action potential repolarization. Tachypacing-induced action potential duration alternans in TAC animals was increased and independent of PKD1, but diastolic arrhythmogenic activities were reduced in PKD1-cKO. Conclusions Our data indicate an important role for PKD1 in the HF-related hypertrophic response and K+ channel downregulation. Therefore, PKD1 inhibition may represent a therapeutic strategy to reduce hypertrophy and arrhythmias; however, PKD1 inhibition may not prevent disease progression and reduced contractility in HF.

Abstract P2096: Long Qt Syndrome Type 3 Phenotype Dependence On Extracellular Sodium And The Perinexus In A Genetic Mouse Model

Background: Long QT Syndrome Type 3 (LQT3) is caused by cardiac voltage-gated sodium channel (Nav1.5) gain of function and characterized by action potential duration (APD) prolongation. A guinea pig model of drug-induced LQT3 demonstrated that elevating sodium and widening the perinexus, an intercalated disc cleft adjacent to gap junctions, individually and synergistically exacerbate APD prolongation. The synergistic effect suggests that high sodium with cardiac edema is a risk factor for LQT3 patients. However, it remains unclear if this synergistic effect occurs in a genetic model of LQT3. Purpose: To investigate individual and combined effects of sodium concentration and perinexal widening in a genetic mouse model of LQT3. Methods: Ventricular action potential duration at 30 (APD30) and 90 (APD90) percent of repolarization was measured from optically mapped, Langendorff-perfused wild type (WT) and ΔKPQ mouse hearts. Hearts were perfused with sodium concentrations of 145 or 160 mM, in the absence or presence of an intercalated disc adhesion inhibitor (βadp1) inducing perinexal widening. Results: At baseline with 145 mM sodium, APD30 and APD90 were significantly greater in ΔKPQ than WT hearts. Surprisingly, increased sodium (160 mM), significantly decreased APD90 in WT but not ΔKPQ hearts, which is inconsistent with results from the drug-induced LQT3 guinea pig model. In contrast, 160mM sodium significantly decreased APD30 in both genotypes suggesting elevated sodium increases the transient outward potassium current (Ito). Interestingly, with 145 mM sodium, βadp1 treatment did not significantly change APD30 or APD90 in either genotype. However, increasing sodium from 145 to 160 mM with βadp1 significantly increased APD90 in ΔKPQ hearts but not WT. Conclusions: Synergistic APD prolongation was modest in a genetic LQT3 mouse when elevating sodium and widening the perinexus relative to drug-induced LQT3 in guinea pig. A significant difference in action potential morphology between mice and guinea pig is the over-expression of Ito in mice and its absence in guinea pig. Future studies will investigate whether transient outward potassium channels contribute to the effect of high sodium on APD shortening during early repolarization.

Abstract P2091: Age-dependent Remodeling Of The Perinexus Modulates The Phenotype And Arrhythmic Risk Of Long Qt Syndrome Type 3

Background: The Long-QT Syndrome Type 3 (LQT3) is characterized by action potential duration (APD) prolongation due to cardiac voltage-gated sodium channel (Nav1.5) gain of function. Arrhythmia risk increases with age in LQT3 patients, but it is unknown whether this is due to progressive APD prolongation. Recent studies have shown that the LQT3 phenotype can be exacerbated by widening the perinexus, an intercalated disc nanodomain adjacent to gap junctions. However, it remains unclear if the perinexus mediates the LQT3 phenotype during aging. Objective: To investigate the role of the perinexus during aging in LQT3. Methods: Ventricular epicardial APD was obtained by optically mapping Langendorff-perfused adult (3-month) and aged (15-month) guinea pig hearts. LQT3 phenotype was induced with anemone toxin (ATXII), and the perinexus was widened with an intercalated disc adhesion inhibitor (βadp1). Perinexal width (Wp) was quantified by transmission electron microscopy. Susceptibility to arrhythmia was assessed with burst pacing in the absence and presence of βadp1 and evaluated from electrocardiograms (ECG). Results: APD was not different between adult and aged hearts at baseline, with ATXII, or ATXII+βadp1. Wp was significantly wider in adult LQT3 hearts either without or with βadp1 relative to aged hearts (without βadp1: 29.6±2.5 vs 22.9±3.8 nm; with βadp1: 38.3±1.7 vs 29.9±1.2 nm). These data suggest the perinexus narrows with aging and attenuates previously reported cellular APD prolongation such that there is no difference in whole-heart, epicardial APD between adult and aged LQT3 hearts. However, ECG Tpeak-Tend interval was not prolonged in adult, but significantly prolonged in aged hearts after treatment with ATXII+βadp1 (18.7±5.6 vs 35.5±11.5 ms), suggesting that βadp1 increases transmural dispersion of repolarization during aging in LQT3. Furthermore, ATXII+βadp1 did not induce arrhythmias in adult (0/6) but was arrhythmogenic in aged hearts (3/5). Conclusions: Perinexal narrowing associated with aging attenuates APD prolongation and reduces arrhythmic risk in aged LQT3 hearts. Loss of perinexal adhesion in aged LQT3 hearts results in significant and heterogeneous APD prolongation and increased risk of arrhythmia.

In vivo Overexpression of Electrogenic Sodium/Bicarbonate Cotransporter (NBCe1) by AAV9 Modifies the Cardiac Action Potential and the QT Interval in Mice.

Cardiac cells depend on specific sarcolemmal ion transporters to assure the correct intracellular pH regulation. The sodium/bicarbonate cotransporter (NBC) is one of the major alkalinizing mechanisms. In the heart two different NBC isoforms have been described: the electroneutral NBCn1 (1Na+:1) and the electrogenic NBCe1 (1Na+:2). NBCe1 generates an anionic repolarizing current that modulates the action potential duration (APD). In addition to regulating the pH, the NBC is a source of sodium influx. It has been postulated that NBC could play a role in the development of hypertrophy. The aim of this research was to study the contribution of NBCe1 in heart electrophysiology and in the development of heart hypertrophy in an in vivo mouse model with overexpression of NBCe1. Heart NBCe1 overexpression was achieved by a recombinant cardiotropic adeno-associated virus (AAV9) and was evidenced by western-blot and qPCR. AAV9-mCherry was used as a transduction control. NBCe1 overexpression fails to increase heart growth. Patch clamp and electrocardiogram were performed. We observed a reduction on both, ventricular myocytes APD and electrocardiogram QT interval corrected by cardiac rate, emphasizing for the first time NBCe1 relevance for the electrical activity of the heart.

Suppressor TRNA-mediated rescue of hERG LQTS2 nonsense mutations

Congenital long QT syndrome (LQTS), characterized by prolongation of the ventricular action potential duration and an extended QT interval on the electrocardiogram, increases the propensity for lethal ventricular arrythmias and sudden cardiac death. In particular, LQTS type 2 (LQT2) results from deleterious loss-of-function mutations in the human ether-à-go-go related gene (hERG), which encodes for the pore-forming subunit of a voltage-dependent potassium channel in cardiac cells. LQTS2-associated nonsense mutations in hERG are characterized by single nucleotide changes which convert an amino acid-encoding codon into one of the three protein translation stop codons.

Proarrhythmic effects of carbon monoxide in human ventricular tissue: insights from computational modeling

Epidemiological studies have demonstrated that ambient air pollution has been closely associated with cardiovascular diseases. Carbon monoxide (CO) is a common ambient air pollutant that can cause adverse effects on the heart. CO is known to cause tissue ischemia, resulting in ventricular arrhythmias. However, accumulating biological studies showed that CO could exert effects on multiple cardiac ionic channels under normoxic conditions, which might indicate new proarrhythmic mechanisms other than ischemia-mediated electrophysiology changes. In this work, we evaluated the functional impacts of CO on human ventricles using a multi-scale model of human ventricular tissue. Experimental data regarding the effects of CO on different ion channels were incorporated into the cell model to explore the alterations of ventricular electrophysiology. Simulation results suggested that CO significantly prolonged the duration of ventricular action potentials, enhanced the transmural dispersion of repolarization, and reduced the adaptability of ventricular tissue to fast heart rates. In addition, simulated pseudo-ECGs showed consistent manifestations with the clinical observation that CO caused an apparent QT interval prolongation and T-wave widening, indicating that CO affected the heart's abnormal ventricular repolarization.

B-PO04-011 INHIBITION OF KV11.1 CHANNELS IN ISOLATED HYPOKALEMIC RABBIT HEARTS WAS PROARRHYTHMIC, WHEREAS KCA2 CHANNEL INHIBITION WAS SAFE

Hypokalemia increases the risk of ventricular arrhythmia by reducing the cardiac repolarization reserve and prolonging the QT-interval. The proarrhythmic risk of hypokalemia is further exacerbated by classical Class 3 antiarrhythmic drugs that inhibit Kv11.1-channels such as dofetilide especially at slow heart rates. To examine the proarrhythmic potential of selective Kv11.1 inhibitor (dofetilide), a selective KCa2 inhibitor (AP14145), and a mixed Kv11.1/ KCa2 inhibitor (AP30663) in a setting of normo- and hypokalemia. Isolated rabbit hearts were AV-ablated and perfused with normokalemic (4 mM K+) solution at baseline. This was followed by a 20-minute period perfusion with drug or vehicle control. Finally, a 20-minute period of perfusion followed where drug was still present, but the solution was hypokalemic (2.5 mM K+). Animals were randomly assigned to 4 groups: Time matched controls, dofetilide (10nM), AP14145 (3μM), or AP30663 (1.5 μM). Ventricular action potential duration (APD90) and triangulation (APD30-90) were measured during ventricular pacing at heart rates of 92, 120, and 200 BPM. During unpaced periods, proarrhythmia was quantified by short term variability of repolarisation and a proarrhythmia score. Dofetilide increased APD90, APD30-90, and proarrhythmia markers, compared to time matched controls especially at slow heart rates and during hypokalemia. Neither AP14145 nor AP30663 affected any of these parameters regardless of heart rate or K+ concentration. These results support that KCa2 inhibition may be associated with a better cardiac safety profile than Kv11.1 inhibition.

Adaptation of ventricular repolarization dispersion during heart rate increase in humans: A roller coaster process

BackgroundRegional differences in ventricular activation sequence and action potential duration and morphology result in dispersion in ventricular repolarization (VR). VR dispersion is a key factor in arrhythmogenesis. We studied the adaptation of global VR dispersion in humans during normal and abnormal ventricular activation, and the relation to the QT adaptation (hysteresis). MethodsWe measured global VR dispersion as T amplitude, T area, and ventricular gradient (VG), using continuous Frank vectorcardiography, in response to abrupt and sustained atrial (AP) or ventricular pacing (VP) aiming at 120 bpm, in 21 subjects with permanent pacemakers. ResultsFollowing pacing start, VR adaptation showed an initially rapid and complex tri-phasic pattern, most pronounced for T amplitude. There were major differences in the patterns of VR dispersion adaptation following abrupt AP vs VP, confirming that the adaptation pattern is activation dependent. In response to AP, an instantaneous decrease in VR dispersion occurred, followed by an increase and then a slow decrease, all at a lower level than baseline. In contrast, following VP there was an immediate increase to ~4× baseline in T amplitude and T area (but not in VG), with a subsequent biphasic adaptation lasting longer during VP than AP. The initial rapid changes occurred within the time for QT adaptation to reach steady-state. ConclusionsOur results corroborate and expand data from animal and invasive human studies, showing similarities of the adaptation pattern on different scales. The initial rapidly changing VR adaptation phase presumably reflects a window of increased vulnerability to arrhythmias.