Selective activation of tetrodotoxin-sensitive sodium channels by AaH-II drives arrhythmogenic late Na+ current in the heart.

Increased late sodium current (INaL) is a central mechanism underlying both inherited and acquired cardiac arrhythmias. Although Nav1.5 is the dominant cardiac sodium channel, multiple tetrodotoxin-sensitive (TTX-S) Nav isoforms are also expressed in cardiomyocytes, particularly within transverse tubules, where they influence excitation-contraction coupling. Whether these channels contribute directly to pathological INaL and arrhythmogenesis remains unresolved, in part because available pharmacological tools lack isoform selectivity. Herein, our objective was to determine the contribution of TTX-S Nav channels to pathological cardiac INaL and arrhythmia and to establish selective pharmacological tools to dissect their role. Using automated patch-clamp and human Nav isoform profiling, we show that the reference INaL inducer ATX-II predominantly activates TTX-resistant Nav1.5 Nav channels. However, ATX-II lacks Nav isoform selectivity when inappropriately used, questioning the conclusions reached by numerous cardiac INaL studies. In contrast, AaH-II, a peptide from Androctonus australis hector scorpion venom, more selectively and potently enhances INaL through TTX-S Nav isoforms. In human iPS-derived cardiomyocytes, TTX-S INaL leads to action potential prolongation. In adult ventricular cardiomyocytes, AaH-II induces abnormal Ca2+ handling and spontaneous Ca2+ release events. In isolated hearts and in vivo, selective TTX-S INaL activation produces conduction abnormalities, QT prolongation, and ventricular proarrhythmic events, which are prevented by nanomolar tetrodotoxin concentrations that spare Nav1.5. These findings demonstrate that TTX-S sodium channels are sufficient to generate arrhythmogenic late Na+ current in the heart, independently of Nav1.5. AaH-II provides a powerful new tool to selectively probe TTX-S INaL and reveals these channels as previously underappreciated contributors to cardiac electrical instability. Targeting TTX-S Nav channels may therefore represent a novel and potentially safer strategy for antiarrhythmic therapy.