Intracellular CA 2+ response of rabbit oocytes to electrical stimulation

Electrical stimulation is known to cause activation in mammalian oocytes, possibly by eliciting an elevation in intracellular calcium (Ca 2+ ). This study reports intracellular Ca 2+ concentrations in mature rabbit oocytes using the Ca 2+ indicator fura‐2. Calcium levels were determined prior to, during, and after the administration of an electrical pulse (3.6 kV/cm for 60 μsec). Baseline Ca 2+ levels ranged from 30 to 90 nM. The intracellular Ca 2+ transient evoked by a pulse, peaked at 11 sec, was highly variable in amplitude (40–300 nM) and returned to prepulse levels within 300 sec. Electrically stimulated oocytes did not exhibit repetitive Ca 2+ transients. The size of the cytoplasmic Ca 2+ rise was influenced by the duration of the pulse, the field strength and the concentrations of external Ca 2+ ( P < 0.05). Oocytes electrically stimulated in the presence of 100 μM CaCl 2 , which evoked Ca 2+ transients with a mean magnitude of 120 nM, activated at a higher rate ( P < 0.05) than oocytes stimulated in the presence of either higher or lower levels of external Ca 2+ . Although oocytes electrically shocked at 16–18 hr after administration of human chorionic gonadotropin (hphCG) activated at a lower rate than oocytes stimulated at 22–24 hphCG ( P < 0.05), their intracellular Ca 2+ response to the pulse was similar ( P < 0.05). These results indicate that electrical pulse parameters and extracellular Ca 2+ concentrations can be used to modulate intracellular Ca 2+ levels and optimize oocyte activation rates. Furthermore, the data suggest that as the oocyte ages it becomes more responsive to a given intracellular Ca 2+ elevation. © 1992 Wiley‐Liss, Inc.