MACROSCOPIC NA+ CURRENTS IN THE NONCONDUCTING SHAKER POTASSIUM CHANNEL MUTANT W434F

C-type inactivation in Shaker potassium channels inhibits K+ permeatio n. The associated structural changes appear to involve the outer regio n of the pore. Recently, we have shown that C-type inactivation involv es a change in the selectivity of the Shaker channel, such that C-type inactivated channels show maintained voltage-sensitive activation and deactivation of Na+ and Li+ currents in K+-free solutions, although t hey show no measurable ionic currents in physiological solutions. In a ddition, it appears that the effective block of ion conduction produce d by the mutation W434F in the pore region may be associated with perm anent C-type inactivation of W434F channels. These conclusions predict that permanently C-type inactivated W434F channels would also show Na + and Li+ currents (in K+-free solutions) with kinetics similar to tho se seen in C-type-inactivated Shaker channels. This paper confirms tha t prediction and demonstrates that activation and deactivation paramet ers for this mutant can be obtained from macroscopic ionic current mea surements. We also show that the prolonged Na+ tail currents typical o f C-type inactivated channels involve an equivalent prolongation of th e return of gating charge, thus demonstrating that the kinetics of gat ing charge return in W434F channels can be markedly altered by changes in ionic conditions.