LETHALITY OF SKELETAL MYOCYTES EXPOSED TO HIGH-INTENSITY ELECTRIC FIELDS: COMPARISON WITH CARDIOMYOCYTES

Introduction: Cardiac diseases are responsible for the highest mortality rate worldwide. Severe cardiac arrythmias (e.g., ventricular fibrillation) compromise blood pumping by the heart, thus representing a life-threatening condition. Such arrythmias, may be reverted by electric defibrillation, in which high-intensity electric fields (HIEF) are applied to the thorax, and cell injury and even death are likely to occur. During external defibrillation, the skeletal muscles of the thoracic wall are also exposed to HIEF, however, little information is available on the vulnerability of skeletal myocytes to the HIEF deleterious effects. Objective: To evaluate quantitatively and compare the vulnerability of skeletal and cardiac myocytes to the lethal HIEF effect. Methods: Protocols of animal care and use were approved by CEUA/UNICAMP (No. 4429-1 e 6042-1/2022). Skeletal and cardiac myocytes were isolated from murine flexor digitorum brevis muscle, and rat left ventricles, respectively, by enzymatic digestion. After determination of the stimulation threshold field intensity (ET) for each cell, the preparation was stimulated at 0.5 Hz with biphasic, rectangular voltage pulses with 10 ms duration and circa-threshold intensity for ~10 min, after which a single high-amplitude pulse (15x the average ET) was delivered to generate HIEF. Cells were rested for 15-30 min for recovery, and the protocol was repeated, increasing the HIEF-stimulus amplitude until cell death ensued, verified by irreversible hypercontracture and electrical refractoriness, and Trypan blue inclusion in some cases. The relationship between probability of cell death x E intensity was obtained by survival analysis, and the intensity corresponding to the probability of lethality = 0.5 (E0.5) was determined by nonlinear fitting. For both cell type, the mean angle between E application and the cell major axis was ~20⁰, and N= 7. Results: Although skeletal cells were much longer than cardiomyocytes (mean ± standard deviation: 431 ± 52 vs. 139 ± 11 µm; p< 0.001), ET was comparable (3.98 ± 1.72 vs. 3.79 ± 0.45 µm; p> 0.79). Lethality curves in these cell types were significantly different (p< 0.001, Mantel-Cox test). In skeletal myocytes, E0.5 was estimated as 112.2 V/cm (~38x ET), which was over 55% higher than that in cardiomyocytes (72 V/cm, ~19x ET). Conclusion: The present results, which represent the first report of a quantitative evaluation of skeletal myocyte sensitivity to HIEF, indicate that these cells are considerably more resistant to strong shocks than cardiomyocytes, which is good news, because skeletal myocytes are closer to the surface electrodes, thus exposed to greater HIEF. Nevertheless, the greater vulnerability of the myocardium requires that strategies be devised to minimize the deleterious effects of defibrillation-like HIEF.