Human ether-a-go-go-related gene K+ channel gating probed with extracellular Ca2+ - Evidence for two distinct voltage sensors

Human ether-a-go-go-related gene (HERG) encoded K+ channels were expressed in Chinese hamster ovary (CHO-K1) cells and studied by whole-cell voltage c lamp in the presence of varied extracellular Ca2+ concentrations and physio logical external K+. Elevation of external Ca2+ from 1.8 to 10 mM resulted in a reduction of whole-cell K+ current amplitude, slowed activation kineti cs, and an increased rate of deactivation. The midpoint of the voltage depe ndence of activation was also shifted +22.3 +/- 2.5 mV to more depolarized potentials. In contrast, the kinetics and voltage dependence of channel ina ctivation were hardly affected by increased extracellular Ca2+. Neither Ca2 + screening of diffuse membrane surface charges nor open channel block coul d explain these changes. However, selective changes in the voltage-dependen t activation, but not inactivation gating, account for the effects of Ca2on Human ether-a-go-go-related gene current amplitude and kinetics. The dif ferential effects of extracellular Ca2+ on the activation and inactivation gating indicate that these processes have distinct voltage-sensing mechanis ms. Thus, Ca2+ appears to directly interact with externally accessible chan nel residues to alter the membrane potential detected by the activation vol tage sensor, yet Ca2+ binding to this site is ineffective in modifying the inactivation gating machinery.