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 (d)s of 2 and 14.3 mM.