Future Directions for Mapping Atrial Fibrillation.

Mapping for AF focuses on the identification of regions of interest that may guide management and - in particular - ablation therapy. Mapping may point to specific mechanisms associated with localised scar or fibrosis, or electrical features, such as localised repetitive, rotational or focal activation. In patients in whom AF is caused by disorganised waves with no spatial predilection, as proposed in the multiwavelet theory for AF, mapping would be of less benefit. The role of AF mapping is controversial at the current time in view of the debate over the underlying mechanisms. However, recent clinical expansions of mapping technologies confirm the importance of understanding the state of the art, including limitations of current approaches and potential areas of future development.

Management of arrhythmias in pulmonary hypertension.

BACKGROUND: Pulmonary hypertension (PH) is a potentially devastating clinical condition with a poor long-term prognosis. Cardiac arrhythmias are frequent in PH, and pulmonary hypertensives are particularly susceptible to the adverse haemodynamic effects of heart rhythm disorders. However, arrhythmia management in PH patients can be more challenging than in the general population due to the particular physiological idiosyncrasies associated with the condition. Here, we summarise and appraise the data pertaining to multimodality treatment of cardiac arrhythmias in PH to help refine the management strategy for this vulnerable patient group. RESULTS: The majority of our understanding of the safety and effectiveness of different arrhythmia treatments in PH is based on observational and retrospective data. Rhythm control is the overall goal, and for atrial and ventricular tachyarrhythmias, referral for catheter ablation, ideally using electroanatomical mapping technology in specialist centres, is the preferable means of achieving this. Contradictory viewpoints are expounded regarding the safety of beta blocker use in PH, though in three small prospective clinical trials and at least six animal models they appear to be well-tolerated. Nevertheless, amiodarone remains the preferred pharmacological treatment. Direct current cardioversion can be carried out effectively to terminate tachyarrhythmias in both the emergency and elective setting, though mechanistic studies demonstrate a higher recurrence rate in PH patients. Individual reports and series suggest that device implantation may be technically challenging and associated with a higher complication rate due to anatomical distortion and chamber enlargement. Modulation of sympathetic input to the heart appears to reduce arrhythmia vulnerability in canine models of PH, and its clinical application in humans is a worthwhile area of further study. CONCLUSION: Prompt restoration of sinus rhythm improves outcomes in PH, and at present, the most reliable and safest strategy for long-term rhythm control is amiodarone and, where possible, ablation. Reinforcement of the evidence base with randomised prospective trials is necessary. This would be particularly beneficial to clarify the role of atrial fibrillation ablation and the safety and efficacy of beta-blockers. In addition, a more comprehensive assessment of the vulnerability of PH patients to potentially fatal brady- and ventricular tachyarrhythmias may help guide recommendations for provision of primary prevention device therapy.

Unmasking Adenosine: The Purinergic Signalling Molecule Critical to Arrhythmia Pathophysiology and Management.

Adenosine was identified in 1929 and immediately recognised as having a potential role in therapy for arrhythmia because of its negative chronotropic and dromotropic effects. Adenosine entered mainstream use in the 1980s as a highly effective agent for the termination of supraventricular tachycardia (SVT) involving the atrioventricular node, as well as for its ability to unmask the underlying rhythm in other SVTs. Adenosine has subsequently been found to have applications in interventional electrophysiology. While considered a safe agent because of its short half-life, adenosine may provoke arrhythmias in the form of AF, bradyarrhythmia and ventricular tachyarrhythmia. Adenosine is also associated with bronchospasm, although this may reflect irritant-induced dyspnoea rather than true obstruction. Adenosine is linked to numerous pathologies relevant to arrhythmia predisposition, including heart failure, obesity, ischaemia and the ageing process itself. This article examines 90 years of experience with adenosine in the light of new European Society of Cardiology guidelines for the management of SVT.

Autonomic modulation of the electrical substrate in mice haploinsufficient for cardiac sodium channels: a model of the Brugada syndrome.

A murine line haploinsufficient in the cardiac sodium channel has been used to model human Brugada syndrome: a disease causing sudden cardiac death due to lethal ventricular arrhythmias. We explored the effects of cholinergic tone on electrophysiological parameters in wild-type and genetically modified, heterozygous, Scn5a+/- knockout mice. Scn5a+/- ventricular slices showed longer refractory periods than wild-type both at baseline and during isoprenaline challenge. Scn5a+/- hearts also showed lower conduction velocities and increased mean increase in delay than did littermate controls at baseline and blunted responses to isoprenaline challenge. Carbachol exerted limited effects but reversed the effects of isoprenaline with coapplication. Scn5a+/- mice showed a reduction in conduction reserve in that isoprenaline no longer increased conduction velocity, and this was not antagonized by muscarinic agonists.

Ageing in Pgc-1ß-/- mice modelling mitochondrial dysfunction induces differential expression of a range of genes regulating ventricular electrophysiology.

Mice deficient in mitochondrial promoter peroxisome proliferator activated receptor-γ co-activator-1ß (Pgc-1ß-/- ) is a valuable model for metabolic diseases and has been found to present with several pathologies including ventricular arrhythmia. In the present study, our aim was to shed light on the molecular mechanisms behind the observed arrhythmic substrate by studying how the expression of selected genes critical for cardiac function differs in wild-type (WT) compared with Pgc-1ß knockout mice and young compared with aged mice. We found that a clear majority of genes are down-regulated in the Pgc-1ß-/- ventricular tissue compared with the WT. Although most individual genes are not significantly differentially expressed, a pattern is apparent when the genes are grouped according to their functional properties. Genes encoding proteins relating to ATPase activity, potassium ion channels relating to repolarisation and resting membrane potential, and genes encoding proteins in the cAMP pathway are found to be significantly down-regulated in the Pgc-1ß deficient mice. On the contrary, the pacemaker channel genes Hcn3 and Hcn4 are up-regulated in subsets of the Pgc-1ß deficient tissue. Furthermore, we found that with age, especially in the Pgc-1ß-/- genotype, most genes are up-regulated including genes relating to the resting membrane potential, calcium homeostasis, the cAMP pathway, and most of the tested adrenoceptors. In conclusion, we here demonstrate how a complex pattern of many modest changes at gene level may explain major functional differences of the action potential related to ageing and mitochondrial dysfunction.

Pharmacological Modulation of Right Ventricular Endocardial-Epicardial Gradients in Brugada Syndrome.

Background We explored the hypothesis that increased cholinergic tone exerts its proarrhythmic effects in Brugada syndrome (BrS) through increasing dispersion of transmural repolarization in patients with spontaneous and drug-induced BrS. Methods BrS and supraventricular tachycardia patients were studied after deploying an Ensite Array in the right ventricular outflow tract and a Cardima catheter in the great cardiac vein to record endo and epicardial signals, respectively. S1-S2 restitution curves from the right ventricular apex were conducted at baseline and after edrophonium challenge to promote increased cholinergic tone. The local unipolar electrograms were then analyzed to study transmural conduction and repolarization dynamics. Results The study included 8 BrS patients (5 men:3 women; mean age, 56 years) and 8 controls patients with supraventricular tachycardia (5 men:3 women; mean age, 48 years). Electrophysiological studies in controls demonstrated shorter endocardial than epicardial right ventricular activation times (mean difference: 26 ms; P<0.001). In contrast, patients with BrS showed longer endocardial than epicardial activation time (mean difference: -15 ms; P=0.001). BrS hearts, compared with controls, showed significantly larger transmural gradients in their activation recovery intervals (mean intervals, 20.5 versus 3.5 ms; P<0.01), with longer endocardial than epicardial activation recovery intervals. Edrophonium challenge increased such gradients in both controls (to a mean of 16 ms [ P<0.001]) and BrS (to 29.7 ms; P<0.001). However, these were attributable to epicardial and endocardial activation recovery interval prolongations in control and BrS hearts, respectively. Dynamic changes in repolarization gradients were also observed across the BrS right ventricular wall in BrS. Conclusions Differential contributions of conduction and repolarization were identified in BrS which critically modulated transmural dispersion of repolarization with significant cholinergic effects only identified in the patients with BrS. This has important implications for explaining the proarrhythmic effects of increased vagal tone in BrS, as well as evaluating autonomic modulation and epicardial ablation as therapeutic strategies.

Ablation of Complex Fractionated Electrograms Improves Outcome in Persistent Atrial Fibrillation of Over 2 Year's Duration.

AIM: There is controversy and sparse data on whether substrate based techniques in addition to pulmonary vein isolation (PVI) confer benefit in the catheter ablation of persistent atrial fibrillation (AF), especially if long standing. We performed an observational study to assess whether substrate based ablation improved freedom from atrial arrhythmia. METHODS: A total of 286 patients undergoing first ablation procedures for persistent AF with PVI only(n = 79), PVI plus linear ablation(n = 85), or PVI plus complex fractionated electrogram (CFAE) and linear ablation(n = 107) were followed. Primary end point was freedom from atrial arrhythmia at one year. RESULTS: Mean duration of pre-procedure time in AF was 28+/-27 months.There were no differences in baseline characteristics between groups except a higher proportion of patients with a severely dilated LA in those receiving PVI+CFAEs+lines. Freedom from atrial arrhythmia was higher with a PVI+CFAE+lines strategy then for PVI alone (HR 1.56, 95% CI: 1.04-2.34, p=0.032) but was not higher with PVI+lines. Benefit of substrate modification was conferred for preprocedure times in AF of over 30 months. The occurrence of atrial tachycardia was higher when lines were added to the ablation strategy (HR 0.08, 95% CI: 0.01-0.59, p=0.014). Freedom from atrial arrhythmia at 1 year was higher with lower patient age, use of general anaesthetic (GA), normal or mildly dilated left atrium and decreasing time in AF. CONCLUSIONS: In patients with long standing persistent AF of over 30 months duration,CFAE ablation resulted in improved freedom from atrial arrhythmia. Increased freedom from atrial arrhythmia occurs in patients who are younger and have smaller atria, and with GA procedures. Linear ablation did not improve outcome and resulted in a higher incidence of atrial tachycardia.

Interplay of cell-cell contacts and RhoA/MRTF-A signaling regulates cardiomyocyte identity.

Cell-cell and cell-matrix interactions guide organ development and homeostasis by controlling lineage specification and maintenance, but the underlying molecular principles are largely unknown. Here, we show that in human developing cardiomyocytes cell-cell contacts at the intercalated disk connect to remodeling of the actin cytoskeleton by regulating the RhoA-ROCK signaling to maintain an active MRTF/SRF transcriptional program essential for cardiomyocyte identity. Genetic perturbation of this mechanosensory pathway activates an ectopic fat gene program during cardiomyocyte differentiation, which ultimately primes the cells to switch to the brown/beige adipocyte lineage in response to adipogenesis-inducing signals. We also demonstrate by in vivo fate mapping and clonal analysis of cardiac progenitors that cardiac fat and a subset of cardiac muscle arise from a common precursor expressing Isl1 and Wt1 during heart development, suggesting related mechanisms of determination between the two lineages.

Continuous and minimally-interrupted direct oral anticoagulant are both safe compared with vitamin K antagonist for atrial fibrillation ablation: An updated meta-analysis.

BACKGROUND: The appropriate and safe peri-procedural anticoagulation schedule for patients on a direct oral anticoagulant (DOAC) undergoing AF ablation is not known. We aimed to evaluate the safety and efficacy of both continuous and minimally-interrupted novel oral anticoagulant (DOAC) strategies compared with uninterrupted vitamin K antagonist (VKA) for atrial fibrillation (AF) ablation. METHODS: We searched electronic databases for randomized or prospective controlled observational studies comparing DOAC (continuous or interrupted) versus uninterrupted VKA. The primary endpoint was major bleeding. Secondary endpoints were total bleeding (composite of major and minor bleeding) and symptomatic thromboembolism. Data were analyzed by random-effects modeling and sensitivity analyses performed according to study design and peri-procedural DOAC schedule. RESULTS: Thirteen studies (4 randomized, 9 observational) with 5463 patients were included in final analysis (45% on DOAC). DOAC was associated with less major bleeding compared with VKA in pooled randomized studies (odds ratio [OR] 0.27, 95% confidence interval [CI] 0.09-0.80, p = 0.03, I2 = 0%), however there was no difference on overall analyses (OR 0.70, 95% CI 0.39-1.24, p = 0.22, I2 = 27%). When stratified by DOAC dose schedule, there was no difference in major bleeding for continuous DOAC (OR 0.48, 95% CI 0.21-1.11, p = 0.09, I2 = 6%) or minimally-interrupted DOAC (OR 0.81, 95% CI 0.37-1.76, p = 0.60, I2 = 43%) compared with VKA. There was no difference between DOAC and VKA for risk of total bleeding (p = 0.20) or symptomatic thromboembolism (p = 0.78). CONCLUSION: Continuous and minimally-interrupted DOAC are both safe and non-inferior peri-procedural anticoagulation strategies compared with uninterrupted VKA for AF ablation. DOAC in general is associated with reduced major bleeding as demonstrated in pooled randomized studies.

Improved outcome and cost effectiveness in ablation of persistent atrial fibrillation under general anaesthetic.

Aims: Outcome of persistent atrial fibrillation (AF) ablation remains suboptimal. Techniques employed to reduce arrhythmia recurrence rate are more likely to be embraced if cost-effectiveness can be demonstrated. A single-centre observational study assessed whether use of general anaesthesia (GA) in persistent AF ablation improved outcome and was cost-effective. Methods and results: Two hundred and ninety two patients undergoing first ablation procedures for persistent AF under conscious sedation or GA were followed. End points were freedom from listing for repeat ablation at 18 months and freedom from recurrence of atrial arrhythmia at 1 year. Freedom from atrial arrhythmia was higher in patients who underwent ablation under GA rather than sedation (63.9% vs. 42.3%, hazard ratio (HR) 1.87, 95% confidence interval (CI): 1.23-2.86, P = 0.002). Significantly fewer GA patients were listed for repeat procedures (29.2% vs. 42.7%, HR 1.62, 95% CI: 1.01-2.60, P = 0.044). Despite GA procedures costing slightly more, a saving of £177 can be made per patient in our centre for a maximum of two procedures if all persistent AF ablations are performed under GA. Conclusions: In patients with persistent AF, it is both clinical and economically more effective to perform ablation under GA rather than sedation.

Age-dependent electrocardiographic changes in Pgc-1ß deficient murine hearts.

Increasing evidence implicates chronic energetic dysfunction in human cardiac arrhythmias. Mitochondrial impairment through Pgc-1ß knockout is known to produce a murine arrhythmic phenotype. However, the cumulative effect of this with advancing age and its electrocardiographic basis have not been previously studied. Young (12-16 weeks) and aged (>52 weeks), wild type (WT) (n = 5 and 8) and Pgc-1ß-/- (n = 9 and 6), mice were anaesthetised and used for electrocardiographic (ECG) recordings. Time intervals separating successive ECG deflections were analysed for differences between groups before and after ß1-adrenergic (intraperitoneal dobutamine 3 mg/kg) challenge. Heart rates before dobutamine challenge were indistinguishable between groups. The Pgc-1ß-/- genotype however displayed compromised nodal function in response to adrenergic challenge. This manifested as an impaired heart rate response suggesting a functional defect at the level of the sino-atrial node, and a negative dromotropic response suggesting an atrioventricular conduction defect. Incidences of the latter were most pronounced in the aged Pgc-1ß-/- mice. Moreover, Pgc-1ß-/- mice displayed electrocardiographic features consistent with the existence of a pro-arrhythmic substrate. Firstly, ventricular activation was prolonged in these mice consistent with slowed action potential conduction and is reported here for the first time. Additionally, Pgc-1ß-/- mice had shorter repolarisation intervals. These were likely attributable to altered K+ conductance properties, ultimately resulting in a shortened QTc interval, which is also known to be associated with increased arrhythmic risk. ECG analysis thus yielded electrophysiological findings bearing on potential arrhythmogenicity in intact Pgc-1ß-/- systems in widespread cardiac regions.

Epac-induced ryanodine receptor type 2 activation inhibits sodium currents in atrial and ventricular murine cardiomyocytes.

Acute RyR2 activation by exchange protein directly activated by cAMP (Epac) reversibly perturbs myocyte Ca2+ homeostasis, slows myocardial action potential conduction, and exerts pro-arrhythmic effects. Loose patch-clamp studies, preserving in vivo extracellular and intracellular conditions, investigated Na+ current in intact cardiomyocytes in murine atrial and ventricular preparations following Epac activation. Depolarising steps to varying test voltages activated typical voltage-dependent Na+ currents. Plots of peak current against depolarisation from resting potential gave pretreatment maximum atrial and ventricular currents of -20.23 ± 1.48 (17) and -29.8 ± 2.4 (10) pA/µm2 (mean ± SEM [n]). Challenge by 8-CPT (1 µmol/L) reduced these currents to -11.21 ± 0.91 (12) (P < .004) and -19.3 ± 1.6 (11) pA/µm2 (P < .04) respectively. Currents following further addition of the RyR2 inhibitor dantrolene (10 µmol/L) (-19.91 ± 2.84 (13) and -26.6 ± 1.7 (17)), and dantrolene whether alone (-19.53 ± 1.97 (8) and -27.6 ± 1.9 (14)) or combined with 8-CPT (-19.93 ± 2.59 (12) and -29.9 ± 2.5(11)), were indistinguishable from pretreatment values (all P >> .05). Assessment of the inactivation that followed by applying subsequent steps to a fixed voltage 100 mV positive to resting potential gave concordant results. Half-maximal inactivation voltages and steepness factors, and time constants for Na+ current recovery from inactivation in double-pulse experiments, were similar through all the pharmacological conditions. Intracellular sharp microelectrode membrane potential recordings in intact Langendorff-perfused preparations demonstrated concordant variations in maximum rates of atrial and ventricular action potential upstroke, (dV/dt)max . We thus demonstrate an acute, reversible, Na+ channel inhibition offering a possible mechanism for previously reported pro-arrhythmic slowing of AP propagation following modifications of Ca2+ homeostasis, complementing earlier findings from chronic alterations in Ca2+ homeostasis in genetically-modified RyR2-P2328S hearts.

Age-dependent atrial arrhythmic phenotype secondary to mitochondrial dysfunction in Pgc-1ß deficient murine hearts.

INTRODUCTION: Ageing and several age-related chronic conditions including obesity, insulin resistance and hypertension are associated with mitochondrial dysfunction and represent independent risk factors for atrial fibrillation (AF). MATERIALS AND METHODS: Atrial arrhythmogenesis was investigated in Langendorff-perfused young (3-4 month) and aged (>12 month), wild type (WT) and peroxisome proliferator activated receptor-γ coactivator-1ß deficient (Pgc-1ß-/-) murine hearts modeling age-dependent chronic mitochondrial dysfunction during regular pacing and programmed electrical stimulation (PES). RESULTS AND DISCUSSION: The Pgc-1ß-/- genotype was associated with a pro-arrhythmic phenotype progressing with age. Young and aged Pgc-1ß-/- hearts showed compromised maximum action potential (AP) depolarization rates, (dV/dt)max, prolonged AP latencies reflecting slowed action potential (AP) conduction, similar effective refractory periods and baseline action potential durations (APD90) but shortened APD90 in APs in response to extrasystolic stimuli at short stimulation intervals. Electrical properties of APs triggering arrhythmia were similar in WT and Pgc-1ß-/- hearts. Pgc-1ß-/- hearts showed accelerated age-dependent fibrotic change relative to WT, with young Pgc-1ß-/- hearts displaying similar fibrotic change as aged WT, and aged Pgc-1ß-/- hearts the greatest fibrotic change. Mitochondrial deficits thus result in an arrhythmic substrate, through slowed AP conduction and altered repolarisation characteristics, arising from alterations in electrophysiological properties and accelerated structural change.

The effects of ageing and adrenergic challenge on electrocardiographic phenotypes in a murine model of long QT syndrome type 3.

Long QT Syndrome 3 (LQTS3) arises from gain-of-function Nav1.5 mutations, prolonging action potential repolarisation and electrocardiographic (ECG) QT interval, associated with increased age-dependent risk for major arrhythmic events, and paradoxical responses to ß-adrenergic agents. We investigated for independent and interacting effects of age and Scn5a+/ΔKPQ genotype in anaesthetised mice modelling LQTS3 on ECG phenotypes before and following ß-agonist challenge, and upon fibrotic change. Prolonged ventricular recovery was independently associated with Scn5a+/ΔKPQ and age. Ventricular activation was prolonged in old Scn5a+/ΔKPQ mice (p = 0.03). We associated Scn5a+/ΔKPQ with increased atrial and ventricular fibrosis (both: p < 0.001). Ventricles also showed increased fibrosis with age (p < 0.001). Age and Scn5a+/ΔKPQ interacted in increasing incidences of repolarisation alternans (p = 0.02). Dobutamine increased ventricular rate (p < 0.001) and reduced both atrioventricular conduction (PR segment-p = 0.02; PR interval-p = 0.02) and incidences of repolarisation alternans (p < 0.001) in all mice. However, in Scn5a+/ΔKPQ mice, dobutamine delayed the changes in ventricular repolarisation following corresponding increases in ventricular rate. The present findings implicate interactions between age and Scn5a+/ΔKPQ in prolonging ventricular activation, correlating them with fibrotic change for the first time, adding activation abnormalities to established recovery abnormalities in LQTS3. These findings, together with dynamic electrophysiological responses to ß-adrenergic challenge, have therapeutic implications for ageing LQTS patients.

Effects of ageing on pro-arrhythmic ventricular phenotypes in incrementally paced murine Pgc-1ß -/- hearts.

A range of chronic clinical conditions accompany cardiomyocyte energetic dysfunction and constitute independent risk factors for cardiac arrhythmia. We investigated pro-arrhythmic and arrhythmic phenotypes in energetically deficient C57BL mice with genetic ablation of the mitochondrial promoter peroxisome proliferator-activated receptor-γ coactivator-1ß (Pgc-1ß), a known model of ventricular arrhythmia. Pro-arrhythmic and cellular action potential (AP) characteristics were compared in intact Langendorff-perfused hearts from young (12-16 week) and aged (> 52 week), wild-type (WT) and Pgc-1ß -/- mice. Simultaneous electrocardiographic and intracellular microelectrode recordings were made through successive trains of 100 regular stimuli at progressively incremented heart rates. Aged Pgc-1ß -/- hearts displayed an increased incidence of arrhythmia compared to other groups. Young and aged Pgc-1ß -/- hearts showed higher incidences of alternans in both AP activation (maximum AP upshoot velocity (dV/dt)max and latency), recovery (action potential duration (APD90) and resting membrane potential (RMP) characteristics compared to WT hearts. This was particularly apparent at lower pacing frequencies. These findings accompanied reduced (dV/dt)max and increased AP latency values in the Pgc-1ß -/- hearts. APs observed prior to termination of the protocol showed lower (dV/dt)max and longer AP latencies, but indistinguishable APD90 and RMPs in arrhythmic compared to those in non-arrhythmic hearts. APD restitution analysis showed that Pgc-1ß -/- and WT hearts showed similar limiting gradients. However, Pgc-1ß -/- hearts had shortened plateau AP wavelengths, particularly in aged Pgc-1ß -/- hearts. Pgc-1ß -/- hearts therefore show pro-arrhythmic instabilities attributable to altered AP conduction and activation rather than recovery characteristics.

Transforming growth factor ß receptor inhibition prevents ventricular fibrosis in a mouse model of progressive cardiac conduction disease.

AIMS: Loss-of-function mutations in SCN5A, the gene encoding NaV1.5 channel, have been associated with inherited progressive cardiac conduction disease (PCCD). We have proposed that Scn5a heterozygous knock-out (Scn5a+/-) mice, which are characterized by ventricular fibrotic remodelling with ageing, represent a model for PCCD. Our objectives were to identify the molecular pathway involved in fibrosis development and prevent its activation. METHODS AND RESULTS: Our study shows that myocardial interstitial fibrosis occurred in Scn5a+/- mice only after 45 weeks of age. Fibrosis was triggered by transforming growth factor ß (TGF-ß) pathway activation. Younger Scn5a+/- mice were characterized by a higher connexin 43 expression than wild-type (WT) mice. After the age of 45 weeks, connexin 43 expression decreased in both WT and Scn5a+/- mice, although the decrease was larger in Scn5a+/- mice. Chronic inhibition of cardiac sodium current with flecainide (50 mg/kg/day p.o) in WT mice from the age of 6 weeks to the age of 60 weeks did not lead to TGF-ß pathway activation and fibrosis. Chronic inhibition of TGF-ß receptors with GW788388 (5 mg/kg/day p.o.) in Scn5a+/- mice from the age of 45 weeks to the age of 60 weeks prevented the occurrence of fibrosis. However, current data could not detect reduction in QRS duration with GW788388. CONCLUSION: Myocardial fibrosis secondary to a loss of NaV1.5 is triggered by TGF-ß signalling pathway. Those events are more likely secondary to the decreased NaV1.5 sarcolemmal expression rather than the decreased Na+ current per se. TGF-ß receptor inhibition prevents age-dependent development of ventricular fibrosis in Scn5a+/- mouse.

Arrhythmic effects of Epac-mediated ryanodine receptor activation in Langendorff-perfused murine hearts are associated with reduced conduction velocity.

Recent papers have attributed arrhythmic substrate in murine RyR2-P2328S hearts to reduced action potential (AP) conduction velocities (CV), reflecting acute functional inhibition and/or reduced expression of sodium channels. We explored for acute effects of direct exchange protein directly activated by cAMP (Epac)-mediated ryanodine receptor-2 (RyR2) activation on arrhythmic substrate and CV. Monophasic action potential (MAP) recordings demonstrated that initial steady (8 Hz) extrinsic pacing elicited ventricular tachycardia (VT) in 0 of 18 Langendorff-perfused wild-type mouse ventricles before pharmacological intervention. The Epac activator 8-CPT (8-(4-chlorophenylthio)-2'-O-methyladenosine-3',5'-cyclic monophosphate) (VT in 1 of 7 hearts), and the RyR2 blocker dantrolene, either alone (0 of 11) or with 8-CPT (0 of 9) did not then increase VT incidence (P>.05). Both progressively increased pacing rates and programmed extrasystolic (S2) stimuli similarly produced no VT in untreated hearts (n=20 and n=9 respectively). 8-CPT challenge then increased VT incidences (5 of 7 and 4 of 8 hearts respectively; P<.05). However, dantrolene, whether alone (0 of 10 and 1 of 13) or combined with 8-CPT (0 of 10 and 0 of 13) did not increase VT incidence relative to those observed in untreated hearts (P>.05). 8-CPT but not dantrolene, whether alone or combined with 8-CPT, correspondingly increased AP latencies (1.14±0.04 (n=7), 1.04±0.03 (n=10), 1.09±0.05 (n=8) relative to respective control values). In contrast, AP durations, conditions for 2:1 conduction block and ventricular effective refractory periods remained unchanged throughout. We thus demonstrate for the first time that acute RyR2 activation reversibly induces VT in specific association with reduced CV.