Properties of a Sodium Channel (Nax) Activated by Strong Depolarization of Xenopus Oocytes

Short (<1 sec) duration depolarization of Xenopus laevis oocytes to voltages greater than +40 mV activates a sodium-selective channel (Na(x)) with sodium permeability five to six times greater than the permeability of other monovalent cations examined, including K+, Rb+, Cs+, TMA+, and Choline+. The permeability to Li+ is about equal to that of Na+. This channel was present in all oocytes examined. The kinetics, voltage dependence and pharmacology of Na(x)distinguish it from TTX-sensitive or epithelial sodium channels. It is also different from the sodium channel of Xenopus oocytes activated by prolonged depolarization, which is more highly selective for Na+, requires prolonged depolarization to be activated, and is blocked by Li+. Intracellular Mg2+ reversibly inhibits Na(x), whereas extracellular Mg2+ does not have an inhibitory effect. Intracellular Mg2+ inhibition of Na(x), is voltage dependent, suggesting that Mg2+ binding occurs within the membrane field. Eosin is also a reversible voltage-dependent intracellular inhibitor of Na(x), suggesting that a P-type ATPase may mediate the current. An additional cytoplasmic factor is involved in maintaining Na(x) since the current runs down in internally perfused oocytes and excised membrane patches. The rundown is reversible by reintroduction of the membrane patch into oocyte cytoplasm. The cytoplasmic factor is not ATP, because ATP has no effect on Na(x) current magnitude in either cut-open or inside-out patch preparations. Extracellular Gd3+ is also an inhibitor of Na(x). Na(x) activation follows a sigmoid time course. Its half-maximal activation potential is +100 mV and the effective valence estimated from the steepness of conductance activation is 1.0. Na(x) deactivates monoexponentially upon return to the holding potential (-40 mV). The deactivation rate is voltage dependent, increasing at more negative membrane potentials.