Facilitation of recovery from inactivation by external Na+ and location of the activation gate in neuronal Na+ channels.

Fast inactivation of the Na(+) channel presumably is produced by binding of the inactivating peptide (the "hinged lid") to the internal pore mouth of the activated channel. It has been shown that recovery from inactivation in Na(+) channels begins with a delay, which corresponds to deactivation of the channel, and is then followed by an exponential phase, which corresponds to unbinding of the inactivating peptide. We found that the exponential phase is approximately 1.6-fold faster in 150 mm than in 0 mm external Na(+), but the initial delays are the same. External Na(+) also increases the late steady-state Na(+) current during a step depolarization and shifts the inactivation curve accordingly but has no effect on the activation and deactivation kinetics of the current. Quantitative analysis of the data reveals that external Na(+) has the same facilitation effect on the unbinding of the bound inactivating peptide whether the channel is activated or deactivated but has no effect on the other gating processes of the channel. These findings suggest that permeating Na(+) ions directly knock off the bound inactivating peptide and that channel activation or deactivation does not affect the accessibility of the bound inactivation peptide to external Na(+). The activation gate (the key gating change transforming a Na(+)-nonconducting pore into a Na(+)-conducting one) therefore should not be located external to the inactivation gate, which presumably is already located close to the internal end of the pore.