Calcium influx mediates the voltage-dependence of sperm entry into sea urchin eggs.

Sperm entry was monitored in voltage-clamped sea urchin eggs following insemination in a variety of artificial seawaters. In regular seawater, maintaining the membrane potential at increasingly negative values progressively inhibits sperm entry. Reducing Ca(2+) relieves the inhibition, shifting the sperm entry vs voltage relationship toward more negative potentials. Raising Ca(2+) shifts the relationship in the other direction. Large changes in Na(+) or Mg(2+) do not affect sperm entry although changing Na(+) dramatically changes the currents following sperm attachment. Applying one of seven different calcium channel blockers or replacing Ca(2+) with Ba(2+) or Sr(2+) or microinjecting calcium chelators into the cytoplasm relieves the block to sperm entry at negative potentials. We conclude that the block to sperm entry at negative potentials is mediated by calcium which crosses the membrane and acts at an intracellular site.