Sperm and its soluble extract cause transient increases in intracellular calcium concentration and in membrane potential of sea urchin zygotes.

Fertilization is known to initiate a transient increase of intracellular calcium concentration and an accompanying change of membrane potential in sea urchin eggs. If the fertilization membrane and hyaline layer are removed, sperm can again enter fertilized eggs (refertilization). We have found that Ca2+ and voltage transients were repeatedly induced in fertilized eggs during refertilization. Similar changes were also obtained by external application of a soluble extract of sperm to fertilized eggs. This sperm extract caused no changes in unfertilized eggs. The active factor in the sperm extract survives heating (100 degrees C, 10 min) and incubation with pronase. Its molecular weight is less than 1300.