The effects of external Zn2+ on Ca2+ and H+ currents in the soma of intact Helix neurones were investigated using standard two-electrode voltage-clamp procedures. Cells were exposed to a 0Na+, tetraethylammonium (TEA+) saline and clamped with Cs+-filled electrodes, which allows separation of voltagedependent H+ and Ca2+ currents using different holding potentials. Outward H+ currents, activated by depolarizations from holding potentials in the range −15 to −10 mV, were rapidly blocked by low

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... centrations of external Zn2+ with a Kd of approximately 16/zmoll-1. H+ current activation was also markedly slowed and the block was slow to reverse. Ca2+ currents, largely free from contamination by outward current, were activated by small depolarizations from a holding potential of −55 mV. Ca2+ currents were reduced by Zn2+, but the Kd for block was more than 80 times greater than for block of H+ currents. Thus, low concentrations of Zn2+ provide a method of selectively inhibiting H+ current in studies of Ca2+ current. This was demonstrated in cells which slowly acidified following exposure to 0Na+, TEA+ saline, leading to an increased outward H+ current. Washing with low concentrations of Zn2+ blocked the H+ current and uncovered the underlying Ca2+ current. The results also suggest that Zn2+ will be a useful tool in studies of the physiological role of the H+ pathway.