Figure 4. External Ca²⁺ and Mg²⁺ had similar effects on the surface potential.

To study the effect of changes in [Ca²⁺]_o on membrane current, avoiding the voltage shift due to the non-specific screening effect of Ca²⁺ ions, the solutions were balanced with Mg²⁺. This figure shows that the voltage at the peak of the Na⁺ current evoked by a voltage ramp (0.66 mV ms⁻¹; inset) was not affected by changing the external solution from 4.5 mm Ca²⁺ and 0.5 mm Mg²⁺ (trace a) to 0.5 mm Ca²⁺ and 4.5 mm Mg²⁺ (trace b; Int 1 and Ext 13 in Table 1). The voltage at the peak was -43 ± 0.5 and -45 ± 3.4 mV with 4.5 mm Ca²⁺ and 4.5 mm Mg²⁺, respectively (n = 15). The difference was not significant (P > 0.05). This indicates that Ca²⁺ and Mg²⁺ ions exert the same screening effect on the negative fixed charges at the mouth of the Na⁺ channels.