Class I Anti-Arrhythmics Differentially Regulate Cardiac Sodium Channel Voltage-Sensing Domains

Abstract

Background: Class I anti-arrhythmics target cardiac voltage-gated Na+ channels (NaV1.5). The NaV1.5 α-subunit contains four domains (DI-DIV) with six membrane-spanning segments (S1-S6). S1-S4 form voltage-sensing domains (VSDs), and S5-S6 create the ion-conducting pore. The distinct therapeutic action of subclasses Ia, Ib, and Ic have been traditionally attributed to differences in NaV1.5 access and pore-binding rate. However, others have shown that lidocaine, a local anesthetic and Class Ib anti-arrhythmic interacts with the muscle Na+ channel, NaV1.4, DIII-VSD. Thus, we tested the hypothesis that Class I drug interaction with the NaV1.5 VSDs significantly determines the therapeutic phenotype. Methods: Previously, we created four NaV1.5 DNA constructs with a cysteine introduced to the extracellular S4 of individual VSDs. Channels encoded by these constructs are expressed in Xenopus oocytes and cysteine-labeled with the TAMRA-MTS fluorophore. Ionic current and fluorescence emission corresponding to changes in VSD conformation are then simultaneously recorded using the cut-open oocyte configuration. After control recordings, anti-arrhythmics are administered to the internal solution. When currents are 80% blocked, VSD kinetics are measured. Results: We have not observed significant interaction of Class I drugs with the DI, DII, or DIV VSDs. In contrast, quinidine, lidocaine, and ranolazine all uniquely shift DIII-VSD activation. During control recordings, we observe DIII-VSD activation has V1/2=-108.96±2.72mV. Lidocaine and ranolazine both induced a hyperpolarizing DIII-VSD activation shift (V1/2=-147.28±4.17mV, S.E.M., p=0.003, V1/2=-143.09±1.94mV, S.E.M, p=0.001, respectively) while quinidine caused a large depolarizing shift (V1/2=-80.67±4.39mV, S.E.M, p=0.007). Conclusions: Drug interaction with the DIII-VSD has been tightly linked to use-dependent block of the late Na+ current, a hallmark class Ib drugs. In contrast, class Ia drugs typically reduce peak Na+. Because the DIII-VSD is tightly coupled to NaV channel gating, we propose that the differences in DIII-VSD interaction are determining their unique therapeutic phenotypes.