The effect of zinc on calcium and hydrogen ion currents in intact snail neurones

The effects of external Zn(2+) on Ca(2+) 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 voltage-dependent H(+) and Ca(2+) 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 concentrations of external Zn(2+) with a K(d) of approximately 16/μmoll(-1). H(+) current activation was also markedly slowed and the block was slow to reverse. Ca(2+) currents, largely free from contamination by outward current, were activated by small depolarizations from a holding potential of -55 mV. Ca(2+) currents were reduced by Zn(2+), but the K(d) for block was more than 80 times greater than for block of H(+) currents. Thus, low concentrations of Zn(2+) provide a method of selectively inhibiting H(+) current in studies of Ca(2+) 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 Zn(2+) blocked the H(+) current and uncovered the underlying Ca(2+) current. The results also suggest that Zn(2+) will be a useful tool in studies of the physiological role of the H(+) pathway.