Single Ion Occupancy and Steady-state Gating of Na Channels in Squid Giant Axon

The properties of the small fraction of tetrodotoxin (TTX)-sensitive Na channels that remain open in the steady state were studied in internally dialyzed voltage clamped squid giant axons. The observed Ussing flux ratio exponent (n′) of 0.97 ± 0.03 (calculated from simultaneous measurements of TTX-sensitive current and ²²Na efflux) and nonindependent behavior of Na current at high internal [Na] are explained by a one-site (“1s”) permeation model characterized by a single effective binding site within the channel pore in equilibrium with internal Na ions (apparent equilibrium dissociation constant K_Nai(0) = 0.61 ± 0.08 M). Steady-state open probability of the TTX-sensitive channels can be modeled by the product p_ap_∞, where p_a represents voltage-dependent activation described by a Boltzmann distribution with midpoint V_a = −7 mV and effective valence z_a = 3.2 (Vandenberg, C.A., and F. Bezanilla. 1991. Biophys. J. 60:1499–1510) coupled to voltage-independent inactivation by an equilibrium constant (Bezanilla, F., and C.M. Armstrong. 1977. J. Gen. Physiol. 70:549–566) K_eq = 770. The factor p_∞ represents voltage-dependent inactivation with empirical midpoint V_∞= −83 ± 5 mV and effective valence z_∞ = 0.55 ± 0.03. The composite p_ap_∞1s model describes the steady-state voltage dependence of the persistent TTX-sensitive current well.