ALLOSTERIC EFFECTS OF PERMEATING CATIONS ON GATING CURRENTS DURING K+CHANNEL DEACTIVATION

K+ channel gating currents are usually measured ill the absence of per meating ions, when a common feature of channel closing is a rising pha se of off-gating current and slow subsequent decay. Current models of gating invoke a concerted rearrangement of subunits just before die op en state to explain this very slow charge return from opening potentia ls. We have measured gating currents from the voltage-gated K+ channel , Kv1.5, highly overexpressed in human embryonic kidney cells. In tile presence of permeating K+ or Cs+, we show, by comparison with data ob tained in the absence of permeant ions, that there is a rapid return o f charge after depolarizations, Measurement of off-gating currents on repolarization before and after K+ dialysis from cells allowed a compa rison of off-gating current amplitudes and time course in the same cel ls. Parallel experiments utilizing the low permeability of Cs+ through Kv1.5 revealed similar rapid charge return during measurements of off -gating currents at E-Cs. Such effects could not be reproduced in a no nconducting mutant (W472F) of Kv1.5, in which, by definition, ion perm eation was macroscopically absent. This preservation of a fast kinetic structure of off-gating currents on return from potentials at which c hannels open suggests an allosteric modulation by permeant cations. Th is may arise fi om a direct action on a slow step late in the activati on pathway, or via a retardation in the rate of C-type inactivation. T he activation energy barrier for K+ channel closing is reduced, which may be important during repetitive action potential spiking where ion channels characteristically undergo continuous cyclical activation and deactivation.