Background Little is known about the mechanisms underlying the transition from paroxysmal to persistent atrial fibrillation (AF). In an ovine model of long-standing persistent AF (LS-PAF) we tested the hypothesis that the rate of electrical and/or structural remodeling, assessed by dominant frequency (DF) changes, determines the time at which AF becomes persistent. Methods and Results Self-sustained AF was induced by atrial tachypacing. Seven sheep were sacrificed 11.5±2.3 days after the transition to persistent AF and without reversal to sinus rhythm (SR); 7 sheep were sacrificed after 341.3±16.7 days of LS-PAF. Seven sham-operated animals were in SR for 1 year. DF was monitored continuously in each group. RT-PCR, western blotting, patch-clamping and histological analyses were used to determine changes in functional ion channel expression and structural remodeling. Atrial dilatation, mitral valve regurgitation, myocyte hypertrophy, and atrial fibrosis occurred progressively and became statistically significant after the transition to persistent AF, with no evidence for left ventricular dysfunction. DF increased progressively during the paroxysmal-to-persistent AF transition and stabilized when AF became persistent. Importantly, the rate of DF increase (dDF/dt) correlated strongly with the time to persistent AF. Significant action potential duration (APD) abbreviation, secondary to functional ion channel protein expression changes (CaV1.2, NaV1.5 and KV4.2 decrease; Kir2.3 increase), was already present at the transition and persisted for one-year follow up. Conclusions In the sheep model of LS-PAF, the rate of DF increase predicts the time at which AF stabilizes and becomes persistent, reflecting changes in APD and densities of sodium, L-type calcium and inward rectifier currents.

Objectives To determine whether Gal-3 mediates sustained atrial fibrillation (AF)-induced atrial structural and electrical remodeling and contributes to AF perpetuation. Background Galectin-3 (Gal-3) mediates extracellular matrix remodeling in heart failure, but its role in AF progression remains unexplored. Methods We examined intracardiac blood samples from patients with AF (N=55) to identify potential biomarkers of AF recurrence. In a sheep model of tachypacing-induced AF (N=20), we tested the effects of Gal-3 inhibition during AF progression. Results In patients, intracardiac serum Gal-3 levels were greater in persistent than paroxysmal AF and independently predicted atrial tachyarrhythmia recurrences after a single ablation procedure. In the sheep model, both Gal-3 and TGF-β1 were elevated in the atria of persistent AF animals. The Gal-3 inhibitor GM-CT-01 (GMCT) reduced both Gal-3 and TGF-β1-induced sheep atrial fibroblast migration and proliferation in vitro. GMCT (12 mg/kg twice/week) prevented the increase in serum procollagen type III N-terminal peptide seen during progression to persistent AF, and also mitigated atrial dilatation, myocyte hypertrophy, fibrosis, and the expected increase in dominant frequency of excitation. Atria of GMCT-treated animals had significantly less TGF-β1-Smad2/3 signaling pathway activation and expression of α-smooth muscle actin and collagen than saline-treated animals. Ex-vivo hearts from GMCT-treated animals had significantly longer action potential durations and fewer rotors and wavebreaks during AF, and myocytes had lower functional expression of inward rectifier K+ channel (Kir2.3) than saline-treated animals. Importantly, GMCT increased the probability of spontaneous AF termination, decreased AF inducibility and reduced overall AF burden. Conclusions Inhibiting Gal-3 during AF progression might be useful as an adjuvant treatment to improve outcomes of catheter ablation for persistent AF. Gal-3 inhibition may be a potential new upstream therapy for prevention of AF progression.

Heart disease, a leading cause of death in the developed world, is overwhelmingly correlated with arrhythmias, where heart muscle cells, myocytes, beat abnormally. Cardiac arrhythmias are usually managed by electric shock intervention, antiarrhythmic drugs, surgery, and/or catheter ablation. Despite recent improvements in techniques, ablation procedures are still limited by the risk of complications from unwanted cellular damage, caused by the nonspecific delivery of ablative energy to all heart cell types. We describe an engineered nanoparticle containing a cardiac-targeting peptide (CTP) and a photosensitizer, chlorin e6 (Ce6), for specific delivery to myocytes. Specificity was confirmed in vitro using adult rat heart cell and human stem cell-derived cardiomyocyte and fibroblast cocultures. In vivo, the CTP-Ce6 nanoparticles were injected intravenously into rats and, upon laser illumination of the heart, induced localized, myocyte-specific ablation with 85% efficiency, restoring sinus rhythm without collateral damage to other cell types in the heart, such as fibroblasts. In both sheep and rat hearts ex vivo, upon perfusion of CTP-Ce6 particles, laser illumination led to the formation of a complete electrical block at the ablated region and restored the physiological rhythm of the heart. This nano-based, cell-targeted approach could improve ablative technologies for patients with arrhythmias by reducing currently encountered complications.

ecent clinical studies have demonstrated that longterm right ventricular apical (RVA) pacing imposes a risk of heart failure, ventricular arrhythmias, and cardiac death. 1-5 RVA pacing causes left ventricular (LV) mechanical dyssynchrony because of altered ventricular excitation that bypasses the His-Purkinje system. 6-9 Longterm RVA pacing results in LV dilatation associated with asymmetric LV hypertrophy, 10,11 regional myocardial perfusion defects 12-14 and a decrease in the LV ejection fraction (LVEF). 12,15,16 Pacing on the right ventricular (RV) septum, RV outflow tract and His or para-His bundle has been introduced as a potentially favorable alternative to RVA pacing to preserve a more physiologic ventricular activation. 8 However, previous investigations of alternative pacing sites have yielded inconsistent results, 17-23 which may be attributable, in part, to the fact that the pacing site was determined on a topological rather than functional basis. 24 Indeed, acute hemodynamic studies have demonstrated that individual optimization of the RV pacing sites could preserve LV performance in patients without LV dysfunction, and that there are substantial individual variations in the optimal pacing sites. 25,26 The paced QRS duration seems to be a practical indicator for determining the optimal RV pacing site. 14,19,22,23 However, information on the most appropriate pacing site to preserve long-term LV function is still limited.To address this issue, we investigated the effect of RV septal (RVS) pacing guided by QRS morphology on longterm LV mechanical synchronicity and function in patients with normal QRS duration and preserved LV function at baseline. Methods PatientsWe retrospectively studied 55 patients (22 men, 32 women; 70±10 years) undergoing dual-chamber pacemaker implantation for advanced atrioventricular block (AVB; n=33) or sinus node dysfunction (SND, n=22). In 40 patients (n=24 for AVB, n=16 for SND), pace mapping was carried out at the junction between the upper and middle segments of the RV septum using a hand-shaped stylet under fluoroscopy

Background The aldosterone inhibitor eplerenone (EPL) has been shown to reduce the incidence of AF in patients with systolic heart failure, but the mechanism is unknown. Objectives We hypothesized that by reducing atrial dilatation and fibrosis in the absence of heart failure EPL also reduces AF burden and prevents AF perpetuation. Methods We conducted a randomized-controlled study in 34 sheep that were atrially tachypaced (13±1 weeks). We compared daily oral EPL (N=19) versus sugar-pill (SP) treatment (N=15) from the start of tachypacing. The endpoint was a continuous 7-day stretch of persistent AF (N=29) or completion of 23-weeks tachypacing (N=5). Results EPL significantly reduced the rate of left atrial dilatation increase during AF progression. Atria from EPL-treated sheep had less smooth muscle actin protein, collagen-III expression, interstitial atrial fibrosis and cell hypertrophy than SP-treated sheep atria. However, EPL did not modify the AF-induced increase in the rate of dominant frequency and ion channel densities seen under SP treatment, but prolonged the time to persistent AF in 26% of animals. It also reduced the degree of fibrillatory conduction, AF inducibility and AF burden. Conclusions In the sheep model, EPL mitigates fibrosis and atrial dilatation, modifies AF inducibility and AF complexity, and prolongs the transition to persistent AF in 26% of animals, but it does not prevent AF-induced electrical remodeling or AF persistence. The results highlight structural remodeling as a central upstream target to reduce AF burden, and the need to prevent electrical remodeling to avert AF perpetuation.

Both acute and chronic CaM/CaMKII inhibition improves conduction characteristics and enhances localization of Cx43 in the intercalated disc. In the absence of fibrosis, this reduced the susceptibility for arrhythmias.

Abstract.Bepridil is effective for conversion of atrial fibrillation to sinus rhythm and in the treatment of drug-refractory ventricular tachyarrhythmias. We investigated the effects of bepridil on electrophysiological properties and spiral-wave (SW) reentry in a 2-dimensional ventricular muscle layer of isolated rabbit hearts by optical mapping. Ventricular tachycardia (VT) induced in the presence of bepridil (1 μM) terminated earlier than in the control. Bepridil increased action potential duration (APD) by 5% -8% under constant pacing and significantly increased the space constant. There was a linear relationship between the wavefront curvature (κ) and local conduction . Dye transfer assay in cultured rat cardiomyocytes confirmed that bepridil increased intercellular coupling. SW reentry in the presence of bepridil was characterized by decremental conduction near the rotation center, prominent drift, and self-termination by collision with boundaries. These results indicate that bepridil causes an increase of intercellular coupling and a moderate APD prolongation, and this combination compromises wavefront propagation near the rotation center of SW reentry, leading to its drift and early termination.

The acetylcholine-activated inward rectifier potassium current ( I) is constitutively active in persistent atrial fibrillation (AF). We tested the hypothesis that the blocking of I with the small molecule chloroquine terminates persistent AF. We used a sheep model of tachypacing-induced, persistent AF, molecular modeling, electrophysiology, and structural biology approaches. The 50% inhibition/inhibitory concentration of I block with chloroquine, measured by patch clamp, was 1 μM. In optical mapping of sheep hearts with persistent AF, 1 μM chloroquine restored sinus rhythm. Molecular modeling suggested that chloroquine blocked the passage of a hydrated potassium ion through the intracellular domain of Kir3.1 (a molecular correlate of I) by interacting with residues D260 and F255, in proximity to I228, Q227, and L299. HN heteronuclear single-quantum correlation of purified Kir3.1 intracellular domain confirmed the modeling results. F255, I228, Q227, and L299 underwent significant chemical-shift perturbations upon drug binding. We then crystallized and solved a 2.5 Å X-ray structure of Kir3.1 with F255A mutation. Modeling of chloroquine binding to the mutant channel suggested that the drug's binding to the pore becomes off centered, reducing its ability to block a hydrated potassium ion. Patch clamp validated the structural and modeling data, where the F255A and D260A mutations significantly reduced I block by chloroquine. With the use of numerical and structural biology approaches, we elucidated the details of how a small molecule could block an ion channel and exert antiarrhythmic effects. Chloroquine binds the I channel at a site formed by specific amino acids in the ion-permeation pathway, leading to decreased I and the subsequent termination of AF.-Takemoto, Y., Slough, D. P., Meinke, G., Katnik, C., Graziano, Z. A., Chidipi, B., Reiser, M., Alhadidy, M. M., Ramirez, R., Salvador-Montañés, O., Ennis, S., Guerrero-Serna, G., Haburcak, M., Diehl, C., Cuevas, J., Jalife, J., Bohm, A., Lin,Y.-S., Noujaim, S. F. Structural basis for the antiarrhythmic blockade of a potassium channel with a small molecule.