This study shows that spiral waves in atrial cardiomyocyte monolayers can be terminated effectively by a light-induced depolarizing current, produced by the arrhythmogenic substrate itself, upon optogenetic engineering. These results provide proof-of-concept for shockless defibrillation.

Mesenchymal stem cells (MSCs) show unexplained differences in differentiation potential. In this study, differentiation of human (h) MSCs derived from embryonic, fetal and adult sources toward cardiomyocytes, endothelial and smooth muscle cells was investigated. Labeled hMSCs derived from embryonic stem cells (hESC-MSCs), fetal umbilical cord, bone marrow, amniotic membrane and adult bone marrow and adipose tissue were co-cultured with neonatal rat cardiomyocytes (nrCMCs) or cardiac fibroblasts (nrCFBs) for 10 days, and also cultured under angiogenic conditions. Cardiomyogenesis was assessed by human-specific immunocytological analysis, whole-cell current-clamp recordings, human-specific qRT-PCR and optical mapping. After co-culture with nrCMCs, significantly more hESC-MSCs than fetal hMSCs stained positive for α-actinin, whereas adult hMSCs stained negative. Furthermore, functional cardiomyogenic differentiation, based on action potential recordings, was shown to occur, but not in adult hMSCs. Of all sources, hESC-MSCs expressed most cardiac-specific genes. hESC-MSCs and fetal hMSCs contained significantly higher basal levels of connexin43 than adult hMSCs and co-culture with nrCMCs increased expression. After co-culture with nrCFBs, hESC-MSCs and fetal hMSCs did not express α-actinin and connexin43 expression was decreased. Conduction velocity (CV) in co-cultures of nrCMCs and hESC-MSCs was significantly higher than in co-cultures with fetal or adult hMSCs. In angiogenesis bioassays, only hESC-MSCs and fetal hMSCs were able to form capillary-like structures, which stained for smooth muscle and endothelial cell markers.Human embryonic and fetal MSCs differentiate toward three different cardiac lineages, in contrast to adult MSCs. Cardiomyogenesis is determined by stimuli from the cellular microenvironment, where connexin43 may play an important role.

Background-After intramyocardial injection, mesenchymal stem cells (MSCs) may engraft and influence host myocardium.However, engraftment rate and pattern of distribution are difficult to control in vivo, hampering assessment of potential adverse effects. In this study, the role of the engraftment patterns of MSCs on arrhythmicity in controllable in vitro models is investigated. Methods and Results-Cocultures of 4×105 neonatal rat cardiomyocytes and 7% or 28% adult human MSCs (hMSCs) in diffuse or clustered distribution patterns were prepared. Electrophysiological effects were studied by optical mapping and patch-clamping. In diffuse cocultures, hMSCs dose-dependently decreased neonatal rat cardiomyocyte excitability, slowed conduction, and prolonged action potential duration until 90% repolarization (APD 90 ). Triggered activity (14% versus 0% in controls) and increased inducibility of re-entry (53% versus 6% in controls) were observed in 28% hMSC cocultures. MSC clusters increased APD 90 , slowed conduction locally, and increased re-entry inducibility (23%), without increasing triggered activity. Pharmacological heterocellular electric uncoupling increased excitability and conduction velocity to 133% in 28% hMSC cocultures, but did not alter APD 90 . Transwell experiments showed that hMSCs dosedependently increased APD 90 , APD dispersion, inducibility of re-entry and affected specific ion channel protein levels, whereas excitability was unaltered. Incubation with hMSC-derived exosomes did not increase APD in neonatal rat cardiomyocyte cultures. Conclusions-Adult hMSCs affect arrhythmicity of neonatal rat cardiomyocyte cultures by heterocellular coupling leading to depolarization-induced conduction slowing and by direct release of paracrine factors that negatively affect repolarization rate. The extent of these detrimental effects depends on the number and distribution pattern of hMSCs.These results suggest that caution should be urged against potential adverse effects of myocardial hMSC engraftment.(Circ Arrhythm Electrophysiol. 2013;6:380-391.)

Arrhythmogeneity of fibrotic myocardial cultures is mediated by Cx43 expression in MFBs. Reduced expression of Cx43 causes a more negative MDP of CMCs. This preserves CMC excitability, limits prolongation of repolarization and thereby strongly reduces the incidence of spontaneous re-entrant tachyarrhythmias.

Both BV ≤ 1.81 mV and UV ≤ 7.95 mV are useful for detection of scar during epicardial EAM, in the absence of ≥ 2.8 mm fat. However, EC can be used to detect scar covered with fat. A newly developed algorithm combining UV and EC can differentiate between scar and viable myocardium. Unipolar voltage but not BV could detect intramural scar.