Na+ interaction with the pore of Shaker BK+ channels:: Zero and low K+ conditions

The ShakerB K+ conductance (GK) collapses (in a reversible manner) if the membrane is depolarized and then repolarized in, 0 K+, Na+-containing solutions (Gomez-Lagunas, F. 1997. J. Physiol. 499:3-15; Gomez-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 muM. 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,X-i), in an extent not accounted for by the change in Delta mu (Na). Finally, when X-Na,X-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.