Intracellular CA2+ response of rabbit oocytes to electrical stimulation

Abstract

Electrical stimulation is known to cause activation in mammalian oocytes, possibly by eliciting an elevation in intracellular calcium (Ca²⁺). This study reports intracellular Ca²⁺ concentrations in mature rabbit oocytes using the Ca²⁺ 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 Ca²⁺ levels ranged from 30 to 90 nM. The intracellular Ca²⁺ 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 Ca²⁺ transients. The size of the cytoplasmic Ca²⁺ rise was influenced by the duration of the pulse, the field strength and the concentrations of external Ca²⁺ (P < 0.05). Oocytes electrically stimulated in the presence of 100 μM CaCl₂, which evoked Ca²⁺ 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 Ca²⁺. 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 Ca²⁺ response to the pulse was similar (P < 0.05). These results indicate that electrical pulse parameters and extracellular Ca²⁺ concentrations can be used to modulate intracellular Ca²⁺ levels and optimize oocyte activation rates. Furthermore, the data suggest that as the oocyte ages it becomes more responsive to a given intracellular Ca²⁺ elevation.