Channelrhodopsin ion selectivity determines mechanisms and efficacy of optogenetic defibrillation in human atria and ventricles

Abstract Cardiac re‐entrant arrhythmias are a major cause of sudden cardiac death. Optogenetic defibrillation has emerged as a promising anti‐arrhythmic strategy based on photoactivation of channelrhodopsins, yet the efficacy and potential adverse consequences of activating different classes of light‐gated ion channels have not been systematically evaluated. Previous studies assessed either cation (e.g. ChR2) or anion (e.g. GtACR1) non‐selective channels, both of which depolarize resting cardiomyocytes. Recently identified light‐gated K + channels (e.g. WiChR) suppress cardiomyocyte activity while maintaining the membrane potential near its resting state, but their defibrillation potential remains unexplored. We used computational simulations to compare ChR2, GtACR1 and WiChR. Single‐cell simulations showed that ChR2 and GtACR1 markedly increased diastolic intracellular Ca 2+ (by 42.6% and 52.6%, respectively), whereas WiChR induced only minimal changes (by 4.0%), suggesting lower pro‐arrhythmogenic risk. WiChR, however, slightly increased intracellular Na + (by 15.1% vs . 0.1% and 3.4% for ChR2 and GtACR1), consistent with the residual Na + permeability of currently available K + ‐conducting channelrhodopsins. Simulations in human ventricles and atria showed that GtACR1 most effectively terminated re‐entrant arrhythmias at low light intensities, whereas WiChR achieved comparable efficacy at ≥5 mW/mm 2 . Complementary tissue‐scale simulations revealed that defibrillation occurs through depolarization within the excitable gap followed by fast Na + channel inactivation (ChR2 and partly GtACR1), or through reduced membrane resistance at sufficiently high light intensities (large‐conductance channels GtACR1 and WiChR). Overall, we demonstrate that channelrhodopsin ion selectivity is a key determinant of both optogenetic defibrillation efficacy and its underlying mechanisms. image Translational perspective Re‐entrant arrhythmias are a leading cause of sudden cardiac death. Standard therapies, including the activation of implantable cardioverter‐defibrillators, carry burdens such as pain and myocardial injury. Optogenetic defibrillation has been suggested as a potentially gentler alternative, replacing high‐voltage electrical shocks by photoactivation of light‐gated ion channels expressed in cardiac cells. Determining which light‐gated ion channel optimally balances efficacy and adverse effects on cardiac electrophysiology is critical for clinical translation. We addressed this question using computational simulations and validation experiments in zebrafish hearts. We identify ion selectivity as a key determinant of both the success rate for arrhythmia termination and the underlying mechanisms: non‐selective cation‐ and anion‐conducting channels terminate re‐entry efficiently while promoting intracellular Ca 2+ accumulation, a plausible substrate for pro‐arrhythmic side effects that could undermine therapeutic safety. K + ‐conducting channels largely avoid this pro‐arrhythmic risk by holding cardiomyocytes near their resting potential, albeit at the cost of requiring transmural illumination, while also causing mild Na + accumulation attributable to residual Na + permeability. Together, these trade‐offs define clear clinical implications: K + ‐conducting channels may provide the safest option for optogenetic defibrillation, particularly once variants with further enhanced K + selectivity and red‐shifted activation spectra for improved operation within blood‐perfused myocardium become available. Key points We use computational simulations to compare cation non‐selective (ChR2), anion non‐selective (GtACR1) and K + ‐selective (WiChR) channelrhodopsins for optogenetic termination of re‐entrant arrhythmia. Single‐cardiomyocyte simulations suggest that ChR2 and GtACR1 activation can cause progressive accumulation of intracellular Ca 2+ , which is minimized when using WiChR. Intracellular microelectrode recordings from hearts of transgenic adult zebrafish validate the simulated voltage dynamics seen with ChR2, GtACR1 or WiChR activation. Simulations of human left ventricles and atria indicate that GtACR1 is most effective in terminating re‐entrant arrhythmia at low light intensities, whereas WiChR becomes similarly effective at higher intensities. Tissue‐scale simulations indicate distinct defibrillation mechanisms: excitable gap extinction by de novo action potential initiation followed by inactivation of fast Na + channels for depolarizing channelrhodopsins (ChR2, GtACR1), and reduction in membrane resistance for the large‐conductance channels (GtACR1, WiChR), effectively clamping the membrane potential at each channel's reversal potential at high light levels.