Rapid and slow voltage-dependent conformational changes in segment IVS6 of voltage-gated Na+ channels

Mutations in segment IVS6 of voltage-gated Nai channels affect fast-inactivation, slow-inactivation, local anesthetic action, and batrachotoxin (BTX) action. To detect conformational changes associated with these processes, we substituted a cysteine for a valine at position 1583 in the rat adult skeletal muscle sodium channel alpha-subunit, and examined the accessibility of the substituted cysteine to modification by 2-aminoethyl methanethiosulfonate (MTS-EA) in excised macropatches. MTS-EA causes an irreversible reduction in the peak current when applied both internally and externally, with a reaction rate that is strongly voltage-dependent. The rate increased when exposures to MTS-EA occurred during brief conditioning pulses to progressively more depolarized voltages, but decreased when exposures occurred at the end of prolonged depolarizations, revealing two conformational changes near site 1583, one coupled to fast inactivation, and one tightly associated with slow inactivation. Tetraethylammonium, a pore blocker, did not affect the reaction rate from either direction, while BTX, a lipophilic activator of sodium channels, completely prevented the modification reaction from occurring from either direction. We conclude that there are two inactivation-associated conformational changes in the vicinity of site 1583, that the reactive site most likely faces away from the pore, and that site 1583 comprises part of the BTX receptor.