Rationale: Myofibroblasts typically appear in the myocardium after insults to the heart like mechanical overload and infarction. Apart from contributing to fibrotic remodeling, myofibroblasts induce arrhythmogenic slow conduction and ectopic activity in cardiomyocytes after establishment of heterocellular electrotonic coupling in vitro. So far, it is not known whether ␣-smooth muscle actin (␣-SMA) containing stress fibers, the cytoskeletal components that set myofibroblasts apart from resident fibroblasts, are essential for myofibroblasts to develop arrhythmogenic interactions with cardiomyocytes.Objective: We investigated whether pharmacological ablation of ␣-SMA containing stress fibers by actin-targeting drugs affects arrhythmogenic myofibroblast-cardiomyocyte cross-talk. Methods and Results:Experiments were performed with patterned growth cell cultures of neonatal rat ventricular cardiomyocytes coated with cardiac myofibroblasts. The preparations exhibited slow conduction and ectopic activity under control conditions. Exposure to actin-targeting drugs (Cytochalasin D, Latrunculin B, Jasplakinolide) for 24 hours led to disruption of ␣-SMA containing stress fibers. In parallel, conduction velocities increased dose-dependently to values indistinguishable from cardiomyocyte-only preparations and ectopic activity measured continuously over 24 hours was completely suppressed. Mechanistically, antiarrhythmic effects were due to myofibroblast hyperpolarization (Cytochalasin D, Latrunculin B) and disruption of heterocellular gap junctional coupling (Jasplakinolide), which caused normalization of membrane polarization of adjacent cardiomyocytes. Conclusions:The results suggest that ␣-SMA containing stress fibers importantly contribute to myofibro- Key Words: arrhythmia Ⅲ conduction Ⅲ fibroblast Ⅲ gap junction Ⅲ myocardial structure F ibrotic remodeling of the working myocardium is a common consequence of various insults to the heart like pressure/volume overload, infarction, genetic predisposition, and old age. 1 Apart from compromising mechanical function, fibrotic remodeling is a major cause of cardiac arrhythmias. Arrhythmogenesis in fibrotically remodeled hearts is thought to be due to the disruption of the normally dense and orderly three-dimensional cytoarchitecture of parenchymal cells by excessive amounts of extracellular matrix (ECM) produced by stromal cells. The resulting disorganization of the electrically excitable substrate causes slow conduction (discontinuous and/or "zigzag" conduction) and conduction blocks that contribute to the precipitation of arrhythmias. 2,3 Stromal cells responsible for fibrotic remodeling include so-called myofibroblasts (MFBs), which contribute to fibrosis by hypersecretion of ECM proteins. 4 MFBs, which are not normally present in healthy myocardia, typically appear in the context of the pathologies mentioned and tend to locally persist for extended periods of time.Apart from contributing to fibrotic remodeling, MFBs recently came into focus as a cell type that might direct...

Conflicts of interest are often declared in the S1 guidelines of the AWMF, but they are only rarely assessed by external evaluators. Clear rules should be issued for how experts' declared conflicts of interest should be acted upon, whether they are of a financial nature or not.

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