Functional interaction between extracellular sodium, potassium and inactivation gating in HERG channels

We have studied the interaction between extracellular K⁺ (K_o⁺) and extracellular Na⁺ (Na_o⁺) 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 Na_o⁺ 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 Na_o⁺ effect on recovery and Na_o⁺ inhibition of HERG current, and to determine whether inactivation gating plays a critical role in Na_o⁺ inhibition of HERG current. Na_o ⁺ concentration-response relationships for current inhibition and speeding of recovery were different, with Na_o ⁺ less potent at speeding recovery. Na_o ⁺ inhibition of HERG current was relieved by physiological [K⁺]_o, while Na_o ⁺ speeded recovery from inactivation similarly in the absence or presence of physiological [K⁺]_o. To examine the link between Na_o⁺ 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 Na_o⁺, while hyperpolarization-activated currents were augmented by Na_o ⁺. This result reveals a direct link between Na_o⁺ inhibition and a depolarization-induced conformational change, most likely inactivation. We attempted to simulate the disparate concentration-response relationships for the two effects of Na_o⁺ using a kinetic model that included Na_o⁺ site(s) affected by permeation and gating. While a model with only a single dynamic Na_o ⁺ site was inadequate, a model with two distinct Na_o⁺ sites was sufficient to reproduce the data.