We have studied the interaction between extracellular K+ (Ko+) and extracellular Na+ (Nao+) in human ether-à-go-go related gene (HERG)-encoded K+ channels expressed in Chinese hamster ovary (CHO-K1) cells, using the whole-cell voltage clamp technique. Prior studies indicate that Nao+ potently inhibits HERG current (IC50 3mM) by binding to an outer pore site, and also speeds recovery from inactivation. In this study, we sought to explore the relationship between the Nao+ effect on recovery and Nao+ inhibition of HERG current, and to determine whether inactivation gating plays a critical role in Nao+ inhibition of HERG current. Nao + concentration-response relationships for current inhibition and speeding of recovery were different, with Nao + less potent at speeding recovery. Nao + inhibition of HERG current was relieved by physiological [K+]o, while Nao + speeded recovery from inactivation similarly in the absence or presence of physiological [K+]o. To examine the link between Nao+ block and inactivation using an independent approach, we studied hyperpolarization-activated currents uncoupled from inactivation in the S4-S5 linker mutant D540K. Depolarization-activated D540K currents were inhibited by Nao+, while hyperpolarization-activated currents were augmented by Nao +. This result reveals a direct link between Nao+ inhibition and a depolarization-induced conformational change, most likely inactivation. We attempted to simulate the disparate concentration-response relationships for the two effects of Nao+ using a kinetic model that included Nao+ site(s) affected by permeation and gating. While a model with only a single dynamic Nao + site was inadequate, a model with two distinct Nao+ sites was sufficient to reproduce the data.
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