A conformational change of the cytoplasmic vestibule of the voltage-gated Na channel gives rise to a very stable inactivated state, ultra-slow inactivation (Ius). Mutations in the selectivity filter region which increase the permeability to K+ produce Ius. Therefore, an increase in the diameter of the selectivity filter may result in an interaction with the adjacent S6 segment, giving rise to Ius. Hence, Ius should be enhanced if large cations pass through the channel. The mutation K1237E in the rNav 1.4 allows permeation of both K+ and large organic cations. Following a 300s depolarizing prepulse,80% of the channels recovered from Ius ([tau]rec=120 s) when Na+ was the permeating ion. During permeation with choline 100 % of the channels recovered from Ius, indicating that residency of these large cations in the channel increased the likelihood of entry into Ius.

I1575 in the S6 segment of domain IV is predicted to be adjacent to the selectivity filter. The mutation I1575E/K1237E abolished the modulation of Ius by choline. Hence, Ius may result from a widening of the selectivity filter giving rise to an interaction with I1575 in the adjacent S6 segment. The selectivity filter forms the roof of the inner vestibule at the level of I1575 of S6.

Support: Austrian Science Fund (FWF) P17509-B11