Increase In Action Potential Duration Research Articles

Common human ANK2 variant confers in vivo arrhythmia phenotypes

Human ANK2 (ankyrin-B) loss-of-function variants are directly linked with arrhythmia phenotypes. However, in atypical non-ion channel arrhythmia genes such as ANK2 that lack the same degree of robust structure/function and clinical data, it may be more difficult to assign variant disease risk based simply on variant location, minor allele frequency, and/or predictive structural algorithms. The human ankyrin-B p.L1622I variant found in arrhythmia probands displays significant diversity in minor allele frequency across populations. The objective of this study was to directly test the in vivo impact of ankyrin-B p.L1622I on cardiac electrical phenotypes and arrhythmia risk using a new animal model. We tested arrhythmia phenotypes in a new "knock-in" animal model harboring the human ankyrin-B p.L1622I variant. Ankyrin-B p.L1622I displays reduced posttranslational expression in vivo, resulting in reduced cardiac ankyrin-B expression and reduced association with binding-partner Na/Ca exchanger. Ankyrin-B(L1622I/L1622I) mice display changes in heart rate, atrioventricular and intraventricular conduction, and alterations in repolarization. Furthermore, ankyrin-B(L1622I/L1622I) mice display catecholamine-dependent arrhythmias. At the cellular level, ankyrin-B(L1622I/L1622I) myocytes display increased action potential duration and severe arrhythmogenic afterdepolarizations that provide a mechanistic rationale for the arrhythmias. Our findings support in vivo arrhythmogenic phenotypes of an ANK2 variant with unusual frequency in select populations. On the basis of our findings and current clinical data, we support classification of p.L1622I as a "mild" loss-of-function variant that may confer arrhythmia susceptibility in the context of secondary risk factors including environment, medication, and/or additional genetic variation.

Association Between ACE Gene Polymorphism and QT Dispersion in Patients with Acute Myocardial Infarction.

Background:Angiotensin converting enzyme (ACE) gene polymorphism is associated with high renin-angiotensin system causing myocardial fibrosis and ventricular repolarization abnormality. Based on these findings, this study was designed to determine the association between ACE gene insertion/deletion (I/D) polymorphism and QT dispersion after acute myocardial infarction (MI).Objective and Methods:The study included 108 patients with acute MI. Blood samples were obtained from all the patients for genomic DNA analysis. ECGs were recorded at baseline and at the end of a 6-month follow up. The OT dispersion was manually calculated.Results:The mean age of the patients was 57.5 ±9.9 years (ranging from 36 to 70). The patients with DD genotype showed longer QT dispersion than patients with II or DI genotype at the baseline, while at the end of the six-month follow up the patients with DI genotype showed longer QT dispersion than patients with DD or II genotypes. However, the magnitude of the QT dispersion prolongation was higher in patients carrying the ACE D allele than patients who were not carrying it, at baseline and at the end of six-month follow up (52.5 ±2.6 msn vs. 47.5±2.1 msn at baseline, 57±3.2 msn vs. 53±2.6 msn in months, P: 0.428 and P: 0.613, respectively).Conclusion:Carriers of the D allele of ACE gene I/D polymorphism may be associated with QT dispersion prolongation in patients with MI.An interaction of QT dispersion and ACE gene polymorphism may be associated with an elevation of serum type I-C terminal pro-collagen concentration, possibly leading to myocardial fibrosis, and increased action potential duration.

Arrhythmogenic effect of androgens on the rat heart.

In most species androgens shorten the cardiac action potential and reduce the risk of afterdepolarizations. Despite the central role of the rat model in physiological studies, the effects of androgens on the rat heart are still inconclusive. We therefore performed electrophysiological studies on the perfused rat right ventricular free wall. We found a correlation between androgenic activity and a propensity to generate ventricular ectopic action potentials. We also found that the testosterone treatment increased action potential duration at 90 % of repolarization (APD90), while androgenic inhibition increased the time to peak and decreased APD90. We observed that the voltage-gated potassium channel Kv4.3 and the bi-directional membrane ion transporter NCX in the rat myocardium were regulated by androgenic hormones. One possible explanation for these findings is that due to the expression of specific ion channels in the rat myocardium, the action potential response to its hormonal background is different from those described in other experimental models. Our results indicate that androgenic control of NCX expression plays a key role in determining arrhythmogenicity in the rat heart.

IK1-enhanced human-induced pluripotent stem cell-derived cardiomyocytes: an improved cardiomyocyte model to investigate inherited arrhythmia syndromes.

Currently available induced pluripotent stem cell-derived cardiomyocytes (iPS-CMs) do not ideally model cellular mechanisms of human arrhythmic disease due to lack of a mature action potential (AP) phenotype. In this study, we create and characterize iPS-CMs with an electrically mature AP induced by potassium inward rectifier (IK1) enhancement. The advantages of IK1-enhanced iPS-CMs include the absence of spontaneous beating, stable resting membrane potentials at approximately -80 mV and capability for electrical pacing. Compared with unenhanced, IK1-enhanced iPS-CMs calcium transient amplitudes were larger (P < 0.05) with a typical staircase pattern. IK1-enhanced iPS-CMs demonstrated a twofold increase in cell size and membrane capacitance and increased DNA synthesis compared with control iPS-CMs (P < 0.05). Furthermore, IK1-enhanced iPS-CMs expressing the F97C-CAV3 long QT9 mutation compared with wild-type CAV3 demonstrated an increase in AP duration and late sodium current. IK1-enhanced iPS-CMs represent a more mature cardiomyocyte model to study arrhythmia mechanisms.

Divergent electrophysiologic profile of fluconazole and voriconazole in an experimental whole-heart model of proarrhythmia

In several case reports a prolongation of the QT-interval and even proarrhythmic effects of fluconazole and voriconazole were reported. The aim of the present study was to investigate if application of fluconazole or voriconazole has the potential to provoke polymorphic ventricular tachycardia in a sensitive model of proarrhythmia. In female rabbits, fluconazole (10, 30 and 50µM, n=6) and voriconazole (10, 30 and 50µM, n=6) were infused after obtaining baseline data. Eight endocardial and epicardial monophasic action potentials and a simultaneously recorded 12-lead ECG showed a significant QT prolongation after application of fluconazole as compared with baseline (10µM:+12ms, 30µM:+22ms, 50µM:+37ms; P<0.05) accompanied by an increase of action potential duration (APD90). Administration of voriconazole also induced QT prolongation (30µM:+10ms, 50µM:+20ms, P<0.05). Spatial dispersion of repolarization remained stable in voriconazole-treated hearts while fluconazole induced a significant increase (30µM:+15ms, 50µM:+16ms; P<0.05). Lowering of potassium concentration in bradycardic AV-blocked hearts did not provoke any early afterdepolarizations (EADs) or polymorphic ventricular tachycardia in voriconazole-treated hearts. Application of fluconazole led to the reproducible induction of EADs in 4 of 6 hearts and polymorphic ventricular tachycardia in 3 of 6 hearts (36 episodes). In the present study, voriconazole demonstrated a safe electrophysiologic profile despite significant QT prolongation. In contrast, fluconazole led to a more marked prolongation of myocardial repolarization combined with a more marked increase of dispersion of repolarization. These results imply that application of fluconazole might be torsadogenic and the QT-interval should be closely monitored.

The β1-Adrenergic Receptor Blocker, Metoprolol, Improves Survival and Electrical Remodeling in Rats with Pulmonary Artery Hypertension

Right ventricular (RV) failure is the leading cause of death in patients with pulmonary arterial hypertension (PAH). Pre-clinical studies have shown that β-blockers can improve survival in PAH and RV failure, however effects on electrical remodeling are unknown.Male Wistar rats were injected with saline (controls, CON) or 60mg/kg of monocrotaline (MCT) to induce PAH and RV failure. Metoprolol (10mg/kg) or placebo was administered daily from day 15 post MCT injection. For the acquisition of in vivo ECG measurements telemetric devices were implanted in CON, placebo treated MCT (FAIL) and β-blocker treated MCT (BB). Rats were removed when heart failure symptoms developed. Recordings occurred daily during light and dark cycles.Survival of the BB rats was significantly improved with a median time to failure of 32 days post injection compared to the FAIL rats median of 23 days (P BB > CON with all groups significantly different to each other (P< 0.05-0.001). The time from T-wave peak to end (Tpe) followed the same pattern but FAIL and BB were not significantly different from each other (P< 0.05-0.01), N = 9-10 animals in each group.We show that β1-blockade delays the onset of HF in PAH rats. In addition it partially reverses the increase in action potential duration (QT interval) possibly by reversing effects on K+ channel expression. There also was a trend for reduced dispersion of repolarisation (Tpe). These effects could help reduce pro-arrhythmic substrates of PAH rat hearts.Supported by the British Heart Foundation

Congenital long QT syndrome in children

Long QT syndrome (LQTS) is a cardiac repolarization disorder characterized by prolonged QT interval on the electrocardiogram (ECG) and increased propensity to ventricular tachyarrhythmias and cardiac events. LQTS might be acquired or congenital, which presents a group of channelopathies that occur due to mutation in one of 15 so far identified genes. The most frequent types of congenital LTQS are LQT1, LQT2 and LQT3. Prolonged or delayed repolarization leads to the increase of action potential duration which predisposes early afterdepolarization, as well as the amplification of transmural dispersion of repolarization, both contributing to the development of Torsades de Pointes ventricular tachycardia. Clinical manifestations of LQTS are palpitations, syncope, aborted cardiac arrest or sudden cardiac death, but it can also be asymptomatic. Trigger factors for symptoms are specific for certain genotype. LQTS examination includes thorough clinical and family history focused on distinctive data (repeated syncopes, cases of sudden cardiac death in the family, hereditary arrhythmias), resting ECG, exercise stress testing and genetic analysis, with additional methods (serial ECG records, 24h ECG Holter, epinephrine test). Clinical LQTS diagnosis is based on Schwartz's scoring system, while the criteria for final diagnosis of LQTS depend on Schwartz's score, QT interval duration, presence of pathogenic mutation and clinical symptoms. Treatment approach begins with lifestyle modifications and β-blockers therapy, while other options include implantable cardioverter- defibrillator, permanent pacemaker or surgical sympathectomy. Sudden cardiac death is the reason of 90% of sudden deaths in young athletes, while LQTS is one of its causes. Recommendations for physical activities in children with congenital LQTS arise from the ones for adults and they presume very strict limitations. Further researches are expected to advance the understanding of genotype-phenotype correlation of congenital LQTS and enable eventual genetically-guided personalized treatment, novel insight into rare LQTS types, as well as more precise recommendations for physical activity of LQTS children.

Abstract 19749: Cardiac-specific Deletion of Caveolin-3 Delays Repolarization and Increases Susceptibility to Ventricular Arrhythmia

Caveolin-3 (Cav-3) is an essential scaffolding protein for formation of caveolae in muscle cells. Cav-3 is part of a macromolecular complex including several ion channels. Mutations in Cav-3 have been associated with the inherited long QT syndrome as well as a variety of skeletal myopathies. To investigate the role of Cav-3 in heart and whether loss of function of Cav-3 explains the long QT phenotype, we generated cardiac-specific, inducible Cre-lox Cav-3 knockout mice. 8 week old mice were treated with tamoxifen in the chow to induce cardiac-specific recombination. Western blot analysis and transmitted electron microscopy demonstrated a graded loss of Cav-3 and caveolae in Cav-3 KO heterozygous mice (Cav-3-/+), Cav-3 KO homozygous mice (Cav-3-/-) relative to the littermate controls mice (WT). Echocardiography revealed no significant difference in %EF, %FS, LV chamber dimensions, and LV wall thickness between the different genotypes. Histopathological examination demonstrated no significant difference in HW/BW ratio, cardiac structure or fibrosis comparing Cav-3-/- and WT mice. Telemetry ECG recordings revealed a significant increase in QTc interval Cav-3-/- (68.5±7 ms) compared to WT (54.83±6 ms). Whole cell patch clamp analysis from isolated ventricular myocytes indicated a progressive increase in action potential duration (APD) with loss of Cav-3: WT (APD50: 4.7 ± 1ms; APD90: 28.0±3 ms; n=9); Cav-3-/+(APD50: 10.3±2 ms; APD90: 42.4±3 ms; n=13), Cav-3-/- (APD50: 32.4±6ms; APD90: 97.4±7ms; n=12). Whole cell voltage clamp measurements from Cav-3-/- revealed increased late INa, decrease in ICa,L, Ito,Iss current density without altering peak INa compared to WT cells, and these current changes were adequate to explain the increased APD based on computational representation using the Morotti et al. mouse ventricular cell model. Intracardiac programmed electrical stimulation (ventricular burst pacing) induced VT/Vfib in 8 out of 9 Cav3-/- but none of WT mice (0/5). Our results demonstrate that loss of Cav-3 and caveolae in adult mice does not alter cardiac structure or contractile function but leads to prolonged APD, an increased in QTc, and increased susceptibility to ventricular arrhythmias.

Abstract 19945: Acute Alcohol Intoxication and Hemorrhagic Shock Lead to Microvascular Hyperpermeability and Cardiovascular Arrhythmias

Background: Binge drinking combined with trauma is a major cause of mortality in US. It is known that acute alcohol intoxication (AAI) causes deleterious electrical changes in the heart, including involving QTc dispersion. We also recently demonstrated that acute alcohol intoxication (AAI) increases microvascular permeability. In the current study we investigated the consequences of combined hemorrhagic shock and resuscitation (HSR) and AAI on both microvascular permeability and cardiac electrical activity. Hypothesis and Methods: We hypothesized that AAI combined with HSR causes microvascular leakage and electrical dysfunction in the heart. We used an established rat model of AAI/HSR to test the microvascular and cardiac effects. Results: We identified that combined AAI/HSR significantly (p&lt;0.0007) prolongs the QTc interval (ms) compared to control water fed, sham rats (190.8±7.1 vs. 138.4±4.4, p&lt;0.05). Ex vivo acquired left ventricular monophasic action potential (AP) data revealed increased AP duration in the AAI/HSR group at 20 (9.7±0.6 vs. 5.5±0.2; p&lt;0.0004 ), 50 (16±1.1 vs. 8.8±0.6; p&lt;0.001), and 70% (24.7±2.4 vs. 19.5±0.5; p&lt;0.03) repolarization level relative to water/sham controls. Microvascular hyperpermeability was apparent in the mesenteric microcirculation in the AAI/HSR group compared to AAI/sham, water/HSR, and water/sham controls (IOI of 1061.7±208 HSR-alcohol vs. 698.4±311 HSR-water, p&lt;0.05; vs. 209.5±42 sham-alcohol, p&lt;0.001; and vs. 62.3±9 sham-water, p&lt;0.001). Conclusions: We report for the first time the cardiovascular hallmarks of AAI/HSR associated injury, which include prolonged QTc and AP durations and elevated microvascular permeability.

A delayed chronic pain like condition with decreased Kv channel activity in a rat model of Gulf War Illness pain syndrome

Following their return from deployment, Gulf War (GW) veterans reported widespread joint and muscle pain at rates that far exceeded those of soldiers returning from other conflicts. It is widely believed that exposure to insecticides, repellants and nerve gas prophylactics contributed to the symptoms of Gulf War Illness (GWI), but an animal model of GW pain has been elusive. In our previous work, we observed that 4–8 weeks exposure to pyridostigmine bromide (PB), permethrin and chlorpyrifos could produce persistent alterations in the physiology of Nav1.9 and Kv7 expressed in deep tissue nociceptors of the dorsal root ganglion. However, behavioral assessments from these same rats were not consistent with a delayed pain syndrome similar to that of GWI pain. In the present studies, we intensified the exposure to anticholinesterases PB and chlorpyrifos while retaining the same dosages. Animals receiving the intensified protocol for 30 days exhibited significant increases in resting for about 8 weeks after exposure. Thereafter, all measures were comparable to controls. Animals treated with intensified anticholinesterases for 60 days exhibited increased resting and reduced movement 12 weeks post-exposure. In whole cell patch studies, muscle and vascular nociceptor KDR and Kv7 ion channels exhibited increased amplitude relative to controls (e.g., normalized current and/or peak conductance) at 8 weeks post-exposures; however, at 12 weeks post-exposure, the amplitude of these currents was significantly decreased in muscle nociceptors. In current clamp studies, muscle nociceptors also manifested increased action potential duration, afterhyperpolarization and increased discharge to muscarinic agonists 12 weeks post-exposure. The decline in activity of muscle nociceptor KDR and Kv7 channel proteins was consistent with increased nociceptor excitability and a delayed myalgia in rats exposed to GW chemicals.

Cardiac tissue slices: preparation, handling, and successful optical mapping.

Cardiac tissue slices are becoming increasingly popular as a model system for cardiac electrophysiology and pharmacology research and development. Here, we describe in detail the preparation, handling, and optical mapping of transmembrane potential and intracellular free calcium concentration transients (CaT) in ventricular tissue slices from guinea pigs and rabbits. Slices cut in the epicardium-tangential plane contained well-aligned in-slice myocardial cell strands (“fibers”) in subepicardial and midmyocardial sections. Cut with a high-precision slow-advancing microtome at a thickness of 350 to 400 μm, tissue slices preserved essential action potential (AP) properties of the precutting Langendorff-perfused heart. We identified the need for a postcutting recovery period of 36 min (guinea pig) and 63 min (rabbit) to reach 97.5% of final steady-state values for AP duration (APD) (identified by exponential fitting). There was no significant difference between the postcutting recovery dynamics in slices obtained using 2,3-butanedione 2-monoxime or blebistatin as electromechanical uncouplers during the cutting process. A rapid increase in APD, seen after cutting, was caused by exposure to ice-cold solution during the slicing procedure, not by tissue injury, differences in uncouplers, or pH-buffers (bicarbonate; HEPES). To characterize intrinsic patterns of CaT, AP, and conduction, a combination of multipoint and field stimulation should be used to avoid misinterpretation based on source-sink effects. In summary, we describe in detail the preparation, mapping, and data analysis approaches for reproducible cardiac tissue slice-based investigations into AP and CaT dynamics.

Abstract 13190: Integrin Linked Kinase is Required for Integrity of Cardiac Ion Current and Electrophysiologic Function

Introduction: The cardiac adaptor protein integrin-linked kinase (ILK) is a regulator of cardiac structure/function. This study investigated the ventricular cardiomyocyte ion channel/electrophysiological remodeling caused by cardiomyocyte-directed ILK deletion (ILK-KO) in mice. Methods: Mice expressing the mckCRE transgene and 2 loxP1-flanked ILK-alleles (ILK-KO) were compared to littermate controls (WT). Perforated and tight-seal patch clamp were used respectively to record action potentials (APs) and ion currents in isolate ventricular cardiomyocytes. Cardiac function was assessed by echocardiography and rhythms were recorded by 24 hr telemetry in conscious mice. Results: ILK-KO mice died suddenly (50% mortality at 8 wks and 100% mortality at 18 wks) with progressive dilated cardiomyopathy. Spontaneous ventricular tachyarrhythmias (VTs) were recorded in 60% (*P&lt;0.001 vs WT) at 10 wks. At 5 wks, prior to cardiac dysfunction, ILK-KO increased action potential duration (APD) by 68%*. I CaL and I K1 were unchanged, but fast inactivating transient outward (I to,f ) and slowly inactivating delayed rectifier (I K,slow ) K currents were reduced by 40%* and 34%* respectively. At 10 wks, LV systolic, diastolic dimensions increased by 72%*, 31%* respectively and LVEF decreased by 42%*; I CaL , I K1 , I to,f and I K,slow were all significantly decreased (by 23%, 32%, 38%, 53% respectively). APD was increased by 114%* and EADs/triggered activity were recorded. To assess mechanisms of specific K + current interactions, ILK was immunoprecitated (IPed) from cardiomyocyte membranes and K + channel subunits detected by immunoblot (Figure). Kv4.2 co-IPed with ILK, indicating direct physical interaction; whereas Kv1.4, Kv4.3 and KChIP2 did not. Conclusions: Cardiomyocyte ILK-deletion causes dilated cardiomyopathy, extensive ionic remodeling, VT and sudden death. Early K + current changes and co-IP point to a specific interaction with Kv4.2 subunits in pathogenesis.

NS5806 partially restores action potential duration but fails to ameliorate calcium transient dysfunction in a computational model of canine heart failure

The study investigates how increased Ito, as mediated by the activator NS5806, affects excitation-contraction coupling in chronic heart failure (HF). We hypothesized that restoring spike-and-dome morphology of the action potential (AP) to a healthy phenotype would be insufficient to restore the intracellular Ca(2) (+) transient (CaT), due to HF-induced remodelling of Ca(2+) handling. An existing mathematical model of the canine ventricular myocyte was modified to incorporate recent experimental data from healthy and failing myocytes, resulting in models of both healthy and HF epicardial, midmyocardial, and endocardial cell variants. Affects of NS5806 were also included in HF models through its direct interaction with Kv4.3 and Kv1.4. Single-cell simulations performed in all models (control, HF, and HF + drug) and variants (epi, mid, and endo) assessed AP morphology and underlying ionic processes with a focus on calcium transients (CaT), how these were altered in HF across the ventricular wall, and the subsequent effects of varying compound concentration in HF. Heart failure model variants recapitulated a characteristic increase in AP duration (APD) in the disease. The qualitative effects of application of half-maximal effective concentration (EC50) of NS5806 on APs and CaT are heterogeneous and non-linear. Deepening in the AP notch with drug is a direct effect of the activation of Ito; both Ito and consequent alteration of IK1 kinetics cause decrease in AP plateau potential. Decreased APD50 and APD90 are both due to altered IK1. Analysis revealed that drug effects depend on transmurality. Ca(2+) transient morphology changes-increased amplitude and shorter time to peak-are due to direct increase in ICa,L and indirect larger SR Ca(2+) release subsequent to Ito activation. Downstream effects of a compound acting exclusively on sarcolemmal ion channels are difficult to predict. Remediation of APD to pre-failing state does not ameliorate dysfunction in CaT; however, restoration of notch depth appears to impart modest benefit and a likelihood of therapeutic value in modulating early repolarization.

Human stem cell-derived cardiomyocytes detect drug-mediated changes in action potentials and ion currents

IntroductionIt has been proposed that proarrhythmia assessment for safety pharmacology testing includes the use of human pluripotent stem cell-derived cardiomyocytes (hiPSC-CM) to detect drug-induced changes in cardiac electrophysiology. This study measured the actions of diverse agents on action potentials (AP) and ion currents recorded from hiPSC-CM. MethodsDuring AP experiments, the hiPSC-CM were paced at 1Hz during a baseline period, and when increasing concentrations of test compound were administered at 4-minute intervals. AP parameters, including duration (APD60 and APD90), resting membrane potential, rate of rise, and amplitude, were measured throughout the entire experiment. Voltage clamp experiments with E-4031 and nifedipine were similarly conducted. ResultsE-4031 produced a dose-dependent prolongation of cardiac action potential and blocked the hERG/IKr current with an IC50 of 17nM. At 3nM, dofetilide significantly increased APD90. Astemizole significantly increased APD60 and APD90 at 30nM. Terfenadine significantly increased APD90 at concentrations greater than 10nM. Fexofenadine, a metabolite of terfenadine, did not produce any electrophysiologic changes in cardiac action potentials. Flecainide produced a dose-dependent prolongation of the cardiac action potential at 1 and 3μM. Acute exposure to nifedipine significantly decreased APD60 and APD90 and produced a dose-dependent block of calcium current with an IC50 of 0.039μM. Verapamil first shortened APD60 and APD90 in a dose-dependent manner, until a compensating increase in APD90, presumably via hERG blockade, was observed at 1 and 3μM. Following a chronic exposure (20–24h) to clinically relevant levels of pentamidine, a significant increase in action potential duration was accompanied by early afterdepolarizations (EADs). DiscussionThese experiments show the ability of AP measured from hiPSC-CM to record the interactions of various ion channels via AP recording and avoid the limitations of using several single ion channel assays in a noncardiac tissue.

Application of a novel curcumin analog in the management of diabetic cardiomyopathy.

Diabetes mellitus is becoming an epidemic health threat and represents one of the most prevalent chronic noncommunicable disorders. Cardiovascular complications are considered the leading cause of death for diabetic patients. Diabetes leads to undesired changes in cardiac structure and function, a condition commonly known as diabetic cardiomyopathy, which occurs independent of macro- and microvascular comorbidities in diabetes (1,2). Both systolic and diastolic dysfunctions have been demonstrated in diabetic hearts including prolonged duration of contraction and relaxation, reduced velocity of contraction and relaxation, and depressed myocardial contractility (1,3). In particular, clinical evaluation using electrocardiogram and echocardiography has revealed substantial functional changes in diabetic hearts, including shorter left ventricular ejection time, prolonged pre-ejection duration, increased wall stiffness, decreased fractional shortening, decreased rate of left ventricular filling, and increased action potential duration (4,5). To date, a plethora of cellular and molecular mechanisms have been postulated for the onset and development of diabetic cardiomyopathy, including reduced energy production due to decreases in mitochondrial respiration and pyruvate dehydrogenase activity, accumulation of reactive oxygen species, oxidative stress, apoptosis, impaired autophagy, and malfunction of cardiac contractile and intracellular Ca2+ regulatory proteins such as myosin, sarco/endoplasmic reticulum Ca2+-ATPase (SERCA), and Na+-Ca2+ exchanger (1–3,5,6). The high morbidity and mortality for diabetic cardiomyopathy warrant aggressive clinical management involving lifestyle modification, control of glucose and lipid abnormalities, and treatment of hypertension and …

Effects of proarrhythmic drugs on relaxation time and beating pattern in rat engineered heart tissue.

The assessment of proarrhythmic risks of drugs remains challenging. To evaluate the suitability of rat engineered heart tissue (EHT) for detecting proarrhythmic effects. We monitored drug effects on spontaneous contractile activity and, in selected cases, on action potentials (sharp microelectrode) and Ca2+ transients (Fura-2) and contraction under electrical pacing. The Ito-blocker inhibitor 4-aminopyridine increased action potential duration and T2 and caused aftercontractions, which were abolished by inhibitors of ryanodine receptors (RyR2; JTV-519) or sodium calcium exchanger (NCX; SEA0400). 77 Drugs were then tested at 1-10-100× free therapeutic plasma concentrations (FTPC): Inhibitors of IKr, IKs, Ito, antiarrhythmics (8), drugs withdrawn from market for torsades des pointes arrhythmias (TdP, 5), drugs with measurable (7) or isolated TdP incidence (13), drugs considered safe (14), 28 new chemical entities (NCE). Inhibitors of IKr or IKs had no effect alone, but substantially prolonged relaxation time (T2) when combined at high concentration. 15/33 drugs associated with TdP and 6/14 drugs considered non-torsadogenic (cibenzoline, diltiazem, ebastine, ketoconazole, moxifloxacin, and phenytoin) induced concentration-dependent T2 prolongations (10-100× FTPC). Bepridil, desipramine, imipramine, thioridazine, and erythromycin induced irregular beating. Three NCE prolonged T2, one reduced force. Drugs inhibiting repolarization prolong relaxation in rat EHTs and cause aftercontractions involving RyR2 and NCX. Insensitivity to IKr inhibitors makes rat EHTs unsuitable as general proarrhythmia screen, but favors detection of effects on Ito, IKs + Ito or IKs + IKr. Screening a large panel of drugs suggests that effects on these currents, in addition to IKr, are more common than anticipated.Electronic supplementary materialThe online version of this article (doi:10.1007/s00395-014-0436-7) contains supplementary material, which is available to authorized users.

Vernakalant in an Experimental Model of Pacing-Induced Heart Failure: Lack of Proarrhythmia Despite Prolongation of Repolarization

BackgroundThe present ESC guidelines on atrial fibrillation have introduced vernakalant (VER) for pharmacologic cardioversion of atrial fibrillation. The aim of the present study was to investigate possible proarrhythmic effects of vernakalant in an experimental model of heart failure (HF). Methods and ResultsIn 12 female rabbits, HF was induced with the use of 4 weeks of rapid ventricular pacing. Twelve rabbits were sham operated. Isolated hearts demonstrated a significant prolongation of myocardial repolarization after induction of HF. Vernakalant caused a concentration-dependent (10 μmol/L and 30 μmol/L) increase of action potential duration (APD90) and QT interval without affecting spatial and temporal dispersion of repolarization. The increase in APD90 was accompanied by a greater increase in refractory period resulting in a significant increase in post-repolarization refractoriness. In control conditions, programmed ventricular stimulation and burst pacing led to ventricular fibrillation (VF) in 2 of the 12 sham (4 episodes) and in 3 of the 12 HF (24 episodes) subjects. In the presence of 30 μmol/L vernakalant, VF was no longer inducible in both groups (0 episodes). In the presence of low K+ concentration, neither sham nor HF vernakalant-treated subjects developed early after-depolarizations or ventricular tachyarrhythmias. ConclusionIn the present study, application of vernakalant led to a significant prolongation of myocardial repolarization and increased post-repolarization refractoriness but did not induce early after-depolarization and therefore did not cause proarrhythmia in failing hearts.

Direct measurements of SR free Ca reveal the mechanism underlying the transient effects of RyR potentiation under physiological conditions.

AimsMost of the calcium that activates contraction is released from the sarcoplasmic reticulum (SR) through the ryanodine receptor (RyR). It is controversial whether activators of the RyR produce a maintained increase in the amplitude of the systolic Ca transient. We therefore aimed to examine the effects of activation of the RyR in large animals under conditions designed to be as physiological as possible while simultaneously measuring SR and cytoplasmic Ca.Methods and resultsExperiments were performed on ventricular myocytes from canine and ovine hearts. Cytoplasmic Ca was measured with fluo-3 and SR Ca with mag-fura-2. Application of caffeine resulted in a brief increase in the amplitude of the systolic Ca transient accompanied by an increase of action potential duration. These effects disappeared with a rate constant of ∼3 s−1. Similar effects were seen in cells taken from sheep in which heart failure had been induced by rapid pacing. The decrease of Ca transient amplitude was accompanied by a decrease of SR Ca content. During this phase, the maximum (end-diastolic) SR Ca content fell while the minimum systolic increased.ConclusionsThis study shows that, under conditions designed to be as physiological as possible, potentiation of RyR opening has no maintained effect on the systolic Ca transient. This result makes it unlikely that potentiation of the RyR has a maintained role in positive inotropy.

Calmodulin mutations associated with long QT syndrome prevent inactivation of cardiac L-type Ca2 + currents and promote proarrhythmic behavior in ventricular myocytes

Recent work has identified missense mutations in calmodulin (CaM) that are associated with severe early-onset long-QT syndrome (LQTS), leading to the proposition that altered CaM function may contribute to the molecular etiology of this subset of LQTS. To date, however, no experimental evidence has established these mutations as directly causative of LQTS substrates, nor have the molecular targets of CaM mutants been identified. Here, therefore, we test whether expression of CaM mutants in adult guinea-pig ventricular myocytes (aGPVM) induces action-potential prolongation, and whether affiliated alterations in the Ca2+ regulation of L-type Ca2+ channels (LTCC) might contribute to such prolongation. In particular, we first overexpressed CaM mutants in aGPVMs, and observed both increased action potential duration (APD) and heightened Ca2+ transients. Next, we demonstrated that all LQTS CaM mutants have the potential to strongly suppress Ca2+/CaM-dependent inactivation (CDI) of LTCCs, whether channels were heterologously expressed in HEK293 cells, or present in native form within myocytes. This attenuation of CDI is predicted to promote action-potential prolongation and boost Ca2+ influx. Finally, we demonstrated how a small fraction of LQTS CaM mutants (as in heterozygous patients) would nonetheless suffice to substantially diminish CDI, and derange electrical and Ca2+ profiles. In all, these results highlight LTCCs as a molecular locus for understanding and treating CaM-related LQTS in this group of patients.

An electrocardiographic sign of ischemic preconditioning.

Ischemic preconditioning is a form of intrinsic cardioprotection where an episode of sublethal ischemia protects against subsequent episodes of ischemia. Identifying a clinical biomarker of preconditioning could have important clinical implications, and prior work has focused on the electrocardiographic ST segment. However, the electrophysiology biomarker of preconditioning is increased action potential duration (APD) shortening with subsequent ischemic episodes, and APD shortening should primarily alter the T wave, not the ST segment. We translated findings from simulations to canine to patient models of preconditioning to test the hypothesis that the combination of increased [delta (Δ)] T wave amplitude with decreased ST segment elevation characterizes preconditioning. In simulations, decreased APD caused increased T wave amplitude with minimal ST segment elevation. In contrast, decreased action potential amplitude increased ST segment elevation significantly. In a canine model of preconditioning (9 mongrel dogs undergoing 4 ischemia-reperfusion episodes), ST segment amplitude increased more than T wave amplitude during the first ischemic episode [ΔT/ΔST slope = 0.81, 95% confidence interval (CI) 0.46-1.15]; however, during subsequent ischemic episodes the T wave increased significantly more than the ST segment (ΔT/ΔST slope = 2.43, CI 2.07-2.80) (P < 0.001 for interaction of occlusions 2 vs. 1). A similar result was observed in patients (9 patients undergoing 2 consecutive prolonged occlusions during elective percutaneous coronary intervention), with an increase in slope of ΔT/ΔST of 0.13 (CI -0.15 to 0.42) in the first occlusion to 1.02 (CI 0.31-1.73) in the second occlusion (P = 0.02). This integrated analysis of the T wave and ST segment goes beyond the standard approach to only analyze ST elevation, and detects cellular electrophysiology changes of preconditioning.