INTERACTION BETWEEN FAST AND SLOW INACTIVATION IN SKM1 SODIUM-CHANNELS

Rat skeletal muscle (Skm1) sodium channel alpha and beta(1) subunits w ere coexpressed in Xenopus oocytes, and resulting sodium currents were recorded from on-cell macropatches. First, the kinetics and steady-st ate probability of both fast and slow inactivation in Skm1 wild type ( WT) sodium channels were characterized. Next, we confirmed that mutati on of IFM to QQQ (IFM1303QQQ) in the DIII-IV 'inactivation loop' compl etely removed fast inactivation at all voltages. This mutation was the n used to characterize Skm1 slow inactivation without the presence of fast inactivation. The major findings of this paper are as follows: 1) Even with complete removal of fast inactivation by the IFM1303QQQ mut ation, slow inactivation remains intact. 2) In WT channels, similar to 20% of channels fail to slow-inactivate after fast-inactivating, even at very positive potentials. 3) Selective removal of fast inactivatio n by IFM1303QQQ allows stow inactivation to occur more quickly and com pletely than in WT. We conclude that fast inactivation reduces the pro bability of subsequent slow inactivation.