Background Alternans of intracellular Ca2+ (Cai) underlies T-wave alternans, a predictor of cardiac arrhythmias. A related phenomenon, T-Wave Lability (TWL), precedes Torsade de Pointes (TdP) in patients and animal models with impaired repolarization. However, the role of Cai in TWL remains unexplored. Methods Action potentials (APs) and Cai transients, (CaTs) were mapped optically from paced Langendorff female rabbit hearts (n=8) at 1.2s cycle length, after AV node ablation. Hearts were perfused with normal Tyrode's solution then with dofetilide (0.5 μM) and reduced [K+] (2 mM) and [Mg2+] (0.5 mM) to elicit long QT type 2 (LQT2). Lability of EKG, voltage and Cai signals were evaluated during regular paced rhythm, before and after dofetilide perfusion. Results In LQT2, lability of Cai, voltage and EKG signals increased during paced rhythm, before the appearance of early afterdepolarizations (EADs). LQT2 resulted in AP prolongation and multiple (1-3) additional Cai upstrokes, while APs remained monophasic. When EADs appeared, Cai rose before voltage upstrokes at the origins of propagating EADs. Interventions (i.e. ryanodine and thapsigargin, n=3 or low [Ca]o and nifedipine, n=4) that suppressed Cai oscillations also abolished EADs. Conclusions In LQT2, Cai oscillations (CaiO) precede EADs by minutes, indicating that they result from spontaneous sarcoplasmic reticulum Ca2+ release rather than spontaneous ICaL reactivation. CaiO likely produce oscillations of Na/Ca exchange current, INCX. Depolarizing INCX during the AP plateau contributes to the generation of EADs by re-activating Ca2+-channels that have recovered from inactivation. TWL reflects CaTs and APs lability that occur before EADs and TdP.

Background Atrial fibrillation (AF) contributes significantly to morbidity and mortality in elderly patients and has been correlated with enhanced age-dependent atrial fibrosis. Reversal of atrial fibrosis has been proposed as therapeutic strategy to suppress AF. Objective To test the ability of relaxin to reverse aged-dependent atrial fibrosis and suppress AF. Methods Aged F-344 rats (24-months old) were treated with subcutaneous infusion of vehicle or relaxin (0.4 mg/kg/day) for 2-weeks. Rat hearts were excised, perfused on a Langendorff apparatus and stained with voltage and Ca2+ indicator dyes. Optical mapping and programmed electrical stimulation was used to test arrhythmia vulnerability and changes in electrophysiological characteristics. Changes in protein expression and Na+-current density (INa) were measured by tissue immunofluorescence and whole-cell patch clamp technique. Results In aged rats, sustained AF was readily induced with a premature pulse (n=7/8) and relaxin treatment suppressed sustained AF by a premature impulse or burst pacing (n=1/6) (p<0.01). Relaxin significantly increased atrial action potential conduction velocity and decreased atrial fibrosis. Relaxin-treatment increased Nav1.5 expression (n=6; 36±10%) and decreased total collagen and collagen I (n=5–6; 55–66±15%) in aged atria (p<0.05) and decreased collagen I&III and TGF-β1 mRNA (p<0.05). Voltage-clamp experiments demonstrated that relaxin-treatment (100nM for 2 days) increased atrial INa by 46±4% (n=12–13/group, p<0.02). Conclusion Relaxin suppresses AF through an increase in atrial conduction velocity by decreasing atrial fibrosis and increasing INa. This data provides compelling evidence that relaxin may serve as an effective therapy to manage AF in geriatric patients by reversing fibrosis and modulating cardiac ionic currents.

Ventricular tachycardia (VT) is the most common and potentially lethal complication following myocardial infarction (MI). Biological correction of the conduction inhomogeneity that underlies re-entry could be a major advance in infarction therapy. As minimal increases in conduction of infarcted tissue markedly influence VT susceptibility, we reasoned that enhanced propagation of the electrical signal between non-excitable cells within a resolving infarct might comprise a simple means to decrease post-infarction arrhythmia risk. We therefore tested lentivirus-mediated delivery of the gap-junction protein Connexin 43 (Cx43) into acute myocardial lesions. Cx43 was expressed in (myo)fibroblasts and CD45+ cells within the scar and provided prominent and long lasting arrhythmia protection in vivo. Optical mapping of Cx43 injected hearts revealed enhanced conduction velocity within the scar, indicating Cx43-mediated electrical coupling between myocytes and (myo)fibroblasts. Thus, Cx43 gene therapy, by direct in vivo transduction of non-cardiomyocytes, comprises a simple and clinically applicable biological therapy that markedly reduces post-infarction VT.

Healthy aging results in cardiac structural and electrical remodeling that increases susceptibility to cardiovascular diseases. Relaxin, an insulin-like hormone, suppresses atrial fibrillation, inflammation and fibrosis in aged rats but the mechanisms-of-action are unknown. Here we show that relaxin treatment of aged rats reverses pathological electrical remodeling (increasing Nav1.5 expression and localization of Connexin43 to intercalated disks) by activating canonical Wnt signaling. In isolated adult ventricular myocytes, relaxin upregulated Nav1.5 (EC50 = 1.3 nM) by a mechanism inhibited by the addition of Dickkopf-1. Furthermore, relaxin increased the levels of connexin43, Wnt1, and cytosolic and nuclear β-catenin. Treatment with Wnt1 or CHIR-99021 (a GSK3β inhibitor) mimicked the relaxin effects. In isolated fibroblasts, relaxin blocked TGFβ-induced collagen elevation in a Wnt dependent manner. These findings demonstrate a close interplay between relaxin and Wnt-signaling resulting in myocardial remodeling and reveals a fundamental mechanism of great therapeutic potential.