Human Ether-à-go-go–related Gene K⁺ Channel Gating Probed with Extracellular Ca²⁺ : Evidence for Two Distinct Voltage Sensors

Human ether-à-go-go–related gene (HERG) encoded K⁺ channels were expressed in Chinese hamster ovary (CHO-K1) cells and studied by whole-cell voltage clamp in the presence of varied extracellular Ca²⁺ concentrations and physiological external K⁺. Elevation of external Ca²⁺ from 1.8 to 10 mM resulted in a reduction of whole-cell K⁺ current amplitude, slowed activation kinetics, and an increased rate of deactivation. The midpoint of the voltage dependence of activation was also shifted +22.3 ± 2.5 mV to more depolarized potentials. In contrast, the kinetics and voltage dependence of channel inactivation were hardly affected by increased extracellular Ca²⁺. Neither Ca²⁺ screening of diffuse membrane surface charges nor open channel block could explain these changes. However, selective changes in the voltage-dependent activation, but not inactivation gating, account for the effects of Ca²⁺ on Human ether-à-go-go–related gene current amplitude and kinetics. The differential effects of extracellular Ca²⁺ on the activation and inactivation gating indicate that these processes have distinct voltage-sensing mechanisms. Thus, Ca²⁺ appears to directly interact with externally accessible channel residues to alter the membrane potential detected by the activation voltage sensor, yet Ca²⁺ binding to this site is ineffective in modifying the inactivation gating machinery.