Interaction between the pore and a fast gate of the cardiac sodium channel

Permeant ions affect a fast gating process observed in human cardiac sodium channels (Townsend, C., H.A. Hartmann, and R. Horn. 1997. J. Gen. Physiol. 110:11-21). Removal of extracellular pet-meant ions causes a reduction of open probability at positive membrane potentials. These results suggest an intimate relationship between the ion-conducting pore and the gates of the channel. We tested this hypothesis by three sets of manipulations designed to affect the binding of cations within the pore: application of intracellu lar pore blockers, mutagenesis of residues known to contribute to permeatio n, and chemical modification of a native cysteine residue (C373) near the e xtracellular mouth of the pore The coupling between extracellular permeant ions and this fast gating process is abolished both by pore blockers and by a mutation that severely affects selectivity. A more superficial pore muta tion or chemical modification of C373 reduces single channel conductance wh ile preserving both selectivity of the pore and the modulatory effects of e xtracellular cations. Our results demonstrate a modulatory gating role for a region deep within the pore and suggest that the structure of the permeat ion pathway is largely preserved when a channel is closed.