VOLTAGE-INDEPENDENT GATING TRANSITIONS IN SQUID AXON POTASSIUM CHANNELS

We have investigated the actions of internal and external Zn2+ on squi d axon K channel ionic and gating currents. As has been noted previous ly, application of Zn2+ to either membrane surface substantially slowe d the activation of these channels with little or no change in deactiv ation. internal Zn2+ (near 200-300 nM) slowed channel activation by up to sixfold over the range of membrane voltages from -30 to +50 mV. Ex ternal Zn2+ (10 mM) produced an approximate twofold slowing of activat ion from -40 to +40 mV. We found that the changes in ionic current act ivation kinetics were accompanied by less than a twofold slowing of ch annel-gating currents in a narrow range of potentials near -30 mV. The re was, at most, only a few percent reduction of charge movement assoc iated with Zn2+ application. We conclude that these ions interact with channel components involved in weakly voltage-dependent conformationa l changes. Although there are some differences in detail, the general similarity of the actions of both internal and external Zn2+ on channe l function suggests that the modified channel-gating step involves ami no acids accessible to both the internal and external membrane surface .