Ca2+ oscillations and sensitization of Ca2+ release in unfertilized mouse eggs injected with a sperm factor

The mechanisms by which a sperm factor caused Ca2+ oscillations in unfertilized mouse eggs was investigated by monitoring the fluorescence of intracellular Fluo-3. Injecting sperm extracts triggered Ca2+ oscillations that varied in number and frequency rather than amplitude. All the sperm factor-induced Ca2+ transients showed a characteristic rapid increase and decline. The oscillations persisted for variable times in eggs bathed in Ca2+ free media and were distinct in character from those caused by inserting pipettes containing inositol 1,4,5-trisphosphate (InsP3). InsP3-induced Ca2+ oscillations were always of high frequency but varied considerably in form and amplitude depending upon InsP3 concentration. The oscillations produced by injecting Ca2+ into unfertilized eggs were similar to those produced with low doses of InsP3 but different from those caused by sperm extract injection. After injection of sperm extracts further injection of InsP3, or Ca2+, produced high frequency large amplitude Ca2+ transients. It is concluded that the sperm factor mediates its effects by sensitizing Ca2+ release mechanisms.