Several cloned sodium channels were expressed in oocytes and compared with respect to their sensitivity to internal Mg2+ concerning the open-channel block and to external Ca2+ concerning open-channel block and shifts in steady-state activation. 2. A quantitative comparison between wild-type II channels and a mutant with a positive charge in the S4 segment of repeat I neutralized (K226Q) revealed no significant differences in the Mg2+ block. 3. The blocking effect of extracellular Ca2+ ions on single-channel inward currents was studied for type II, mutant K226Q and type III. A quantitative comparison showed that all three channel types differ significantly in their Ca2+ sensitivity. 4. The influence of extracellular Ca2+ on the voltage dependence of steady-state activation of macroscopic currents was compared for type II and K226Q channels. Extracellular Ca2+ increases the voltage of half-maximal activation, V1/2, more for K226Q than for wild-type II channels; a plot of V1/2 against [Ca]o is twice as steep for the mutant K226Q as for the wild-type on a logarithmic concentration scale. 5. The differential effects of extracellular Ca2+ and intracellular Mg2+ on wild-type II and K226Q channels are discussed in terms of structural models of the Na+ channel protein.
