The occurrence of atrial fibrillation (AF), one of the most socially significant arrhythmias, is associated with the presence of areas of fibrosis. Fibrosis introduces conduction heterogeneity into the cardiac tissue and, thus, may be a substrate for spiral wave reentry, which provokes the onset of AF and is often associated with its persistence. Despite results from computer and animal models of cardiac tissues, data on the conditions under which microreentries occur in human tissues are limited. In this work, we conducted a study of the new approach to modeling the fibrous atrial tissue, which takes into account the cellular structure and conduction in fibrosis areas. Using the Potts model, we created a realistic texture of atrial tissues remodeled by fibroblasts and showed the presence of pathways in such a system with a low proportion of fibroblasts. Our study revealed the relationship between the shape of the cells’ action potential, their location in the tissue, and the direction of the wave propagation. The wavefront obtained in the model creates a dynamic heterogeneity of the tissue, which affects the migration and pinning of spiral waves, and explains the formation of microreentries in the cardiac tissue. In the future, such a model can become a potential tool for predictive modeling of AF and the search for ablation target identification.

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Background/Objectives: Patient-derived mesenchymal stem cells (MSCs) represent a promising avenue for myocardial regeneration, yet therapeutic application remains limited by inconsistent differentiation capacity and the absence of standardized cardiogenic induction protocols. This study demonstrates a proof-of-concept for guiding patient-specific bone marrow MSCs toward a functional cardiomyocyte phenotype using paracrine signals from differentiating iPSC-derived cardiomyocytes (iPSC-CMs). Materials and Methods: MSCs were maintained in conditioned medium from a concurrent, validated iPSC-CM differentiation protocol, with evaluation via immunocytochemistry, optical mapping, and whole-cell patch-clamp recordings. Results: Differentiated MSCs acquired organized sarcomeric architecture with cross-striations and displayed spontaneous calcium oscillations with decay kinetics matching source iPSC-CMs (CaT50 ≈ 283 ms vs. 301 ms). In co-culture, MSC-derived cells exhibited synchronized calcium dynamics with iPSC-CMs, confirming functional coupling, while patch-clamp detected hallmark cardiac ion currents (INa, ICa,L, and IKv). Morphologically, MSC-CMs displayed more mature, elongated rod-like shapes. Conclusions: Although current densities indicate partial immaturity, their reproducible detection validates successful cardiomyogenic commitment. This “parallel differentiation” platform eliminates donor-specific protocol tuning, providing a streamlined, paracrine-mediated approach to generate autologous cardiomyocyte-like cells for disease modeling, pharmacological testing, and future regenerative applications.

BACKGROUND: Human cardiomyocytes obtained by directed differentiation from induced pluripotent stem cells are a promising model for evaluating arrhythmogenicity and the electrophysiological response to beta-adrenergic stimulation. Isoprenaline is a non-selective beta-adrenergic agonist that is widely used in cardiac electrophysiology studies and clinical practice to treat bradycardia, heart block, and cardiac arrest. However, there is limited quantitative data on its effects on human cardiac ion channels because most studies are conducted on animal models under non-physiological conditions. AIM: The study aimed to evaluate the effects of isoprenaline on voltage-gated ion channels in human ventricular cardiomyocytes obtained by directed differentiation of human induced pluripotent stem cells from a healthy donor. METHODS: The patch clamp technique was used to record currents of ion channels, including fast sodium channels, L-type calcium channels, and delayed rectifier potassium channels. Human ventricular cardiomyocytes were obtained by directed differentiation of human induced pluripotent stem cells from a healthy donor. RESULTS: At a concentration of 1 μM, isoprenaline was found to significantly increase ion channel activity, including an increase in the amplitude of fast sodium currents (73%), L-type calcium currents (120%), and delayed rectifier potassium currents (98%). Additional parameters of electrophysiological activity were also obtained, providing a deeper understanding of a beta-adrenergic response in human cardiomyocytes. CONCLUSION: Human cardiomyocytes obtained by directed differentiation were assessed as a reliable model to evaluate arrhythmogenicity and responses to beta-adrenergic stimulation. These quantitative data can be used to create mathematical models of cardiac function and predict the behavior of cardiac tissue under sympathetic stimulation.

Voltage-gated sodium channels are crucial to action potential propagation in excitable tissues. Voltage-clamp measurements of sodium current are very challenging and are usually performed at room temperature due to the high amplitude and fast activation of the current. In this study, we measured sodium current’s voltage dependence in stem-cell-derived cardiomyocytes at physiological temperature. Although apparent activation and inactivation curves measured as the sodium current amplitude dependence on voltage step is within the range reported in previous studies, we demonstrate a systematic error in our measurements that is due to deviation of membrane potential from the command potential of the amplifier. We show how this artifact can be accounted for by the computer simulation of the patch-clamp experiment. This patch-clamp model optimization technique yields a surprising result: −11.5 mV half-activation and −87 mV half-inactivation of the sodium current. Although the half-activation is strikingly different from what was previously believed to be typical for the cardiac sodium current, we show that this estimate explains conduction velocity dependence on extracellular potassium in hyperkalemic conditions. Key points: Voltage gated sodium currents play a crucial role in excitable tissues including neurons, cardiac and skeletal muscles. Measurement of sodium current is challenging because of its high amplitude and rapid kinetics, especially at physiological temperature. We have used the patch-clamp technique to measure human sodium current voltage-dependence in human induced pluripotent stem cell-derived cardiomyocytes. The patch-clamp data was processed by optimization of the model accounting for voltage-clamp experiment artifacts, revealing a large difference between apparent parameters of sodium current and the results of the optimization. We conclude that actual sodium current activation is extremely depolarized in comparison to previous studies. The new sodium current model provides a better understanding of action potential propagation, we demonstrate that it explains propagation in hyperkalemic conditions.