Zn²⁺ blocks the voltage activated calcium current of Aplysia neurons

We have investigated the effect of Zn²⁺ on voltage-activated calcium currents of Aplysia neurons, using conventional two-electrode voltage-clamp techniques. The peak of these currents was reversibly reduced by Zn²⁺ (50% reduction at 3.75 mM; total block at 20 mM), while the current-voltage relation and the activation and inactivation curves were shifted to depolarized voltages. The effects of Zn²⁺ were concentration-dependent. The Hill coefficient was 1.62. The high concentrations required, the shift of the current-voltage relation and the effects on activation and inactivation are best explained by a charge-screening effect combined with a specific binding site for Zn²⁺ near the entrance of the channel.