Effects of electroporation on cellular responses to high-intensity electrical shocks

The response of cardiac cells to the external electrical field is a crucial factor contributing to the success and failure of defibrillation. We aimed to determine a possible contribution of electroporation to optically recorded response during high-intensity shocks. We applied 10 ms anodal and cathodal stimuli of different intensities to the rabbit heart epicardium during plateau phase of the action potential with 6-mm-diameter electrode. Transmembrane potential changes were monitored directly under the electrode by an optical technique using voltage-sensitive dye. As stimulus current was increased from 0 to 160 mA/cm², the shock-induced transmembrane potential changes (AV,) also increased while the resting potential after the stimulation remains constant. At higher current densities (> 160 mA/cm²) effects of electroporation were observed: (a) after the initial rapid change at the stimulus onset AV. quickly declined resulting in gradual decreasing of the final ΔVm with increasing of the stimulus strength; (b) post-repolarization elevation of resting potential gradually increased as the stimulus strength increased. These results demonstrate that during strong electrical shocks electroporation changes in transmembrane potential traces are present for hyperpolarized as well as depolarized stimuli of a similar strength. This can explain the multiphasic nature of cellular responses reported for strong electric stimuli.