Na⁺ Interaction with the Pore of Shaker B K⁺ Channels: Zero and Low K⁺ Conditions

The Shaker B K⁺ conductance (G_K) collapses (in a reversible manner) if the membrane is depolarized and then repolarized in, 0 K⁺, Na⁺-containing solutions (Gómez-Lagunas, F. 1997. J. Physiol. 499:3–15; Gómez-Lagunas, F. 1999. Biophys. J. 77:2988–2998). In this work, the role of Na⁺ ions in the collapse of G_K in 0-K⁺ solutions, and in the behavior of the channels in low K⁺_, was studied. The main findings are as follows. First, in 0-K⁺ solutions, the presence of Na⁺ ions is an important factor that speeds the collapse of G_K. Second, external Na⁺ fosters the drop of G_K by binding to a site with a K_d = 3.3 mM. External K⁺ competes, in a mutually exclusive manner, with Na_o⁺ for binding to this site, with an estimated K_d = 80 μM. Third, NMG and choline are relatively inert regarding the stability of G_K; fourth, with [K_o⁺] = 0, the energy required to relieve Na_i⁺ block of Shaker (French, R.J., and J.B. Wells. 1977. J. Gen. Physiol. 70:707–724; Starkus, J.G., L. Kuschel, M. Rayner, and S. Heinemann. 2000. J. Gen. Physiol. 110:539–550) decreases with the molar fraction of Na_i⁺ (X_Na,i), in an extent not accounted for by the change in Δμ_Na. Finally, when X_Na,i = 1, G_K collapses by the binding of Na_i⁺ to two sites, with apparent K_ds of 2 and 14.3 mM.