FAST INACTIVATION IN SHAKER K- PROPERTIES OF IONIC AND GATING CURRENTS( CHANNELS )

Fast inactivating Shaker H4 potassium channels and nonconducting pore mutant Shaker H4 W434F channels have been used to correlate the instal lation and recovery of the fast inactivation of ionic current with cha nges in the kinetics of gating current known as ''charge immobilizatio n'' (Armstrong, C.M., and F. Bezanilla. 1977. J. Gen. Physiol. 70:567- 590.). Shaker H4 W434F gating currents are very similar to those of th e conducting clone recorded in potassium-free solutions. This mutant c hannel allows the recording of the total gating charge return, even wh en returning from potentials that would largely inactivate conducting channels. As the depolarizing potential increased, the OFF gating curr ents decay phase at -90 mV return potential changed from a single fast component to at least two components, the slower requiring similar to 200 ms for a full charge return. The charge immobilization onset and the ionic current decay have an identical time course. The recoveries of gating current (Shaker H4 W434F) and ionic current (Shaker H4) in 2 mM external potassium have at least two components. Both recoveries a re similar at -120 and -90 mV. In contrast, at higher potentials (-70 and -50 mV), the gating charge recovers significantly more slowly than the ionic current. A model with a single inactivated state cannot acc ount for all our data, which strongly support the existence of ''paral lel'' inactivated states. In this model, a fraction of the charge can be recovered upon repolarization while the channel pore is occupied by the NH2-terminus region.