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Role of the extracellular Ca²⁺ on the intracellular Ca²⁺ changes in fertilized and activated mouse oocytes

Changes in intracellular calcium concentration ([Ca²⁺]_i) in fertilized mouse oocytes were measured using the calcium-sensitive dye, fura-2. After fertilization, an initial long-lasting [Ca²⁺]_i increase was followed by a periodic [Ca²⁺]_i increase. The periodic increase in [Ca²⁺]_i persisted for 1 to 3 h and all fertilized oocytes extruded the second polar body within 4 h. The mean interval of periodic [Ca²⁺]_i increase was 470 ± 180 s (mean ± SD). The frequency of the periodic [Ca²⁺]_i increase depended on the extracellular calcium concentration. Verapamil and nifedipine inhibited the periodic [Ca²⁺]_i increase. Sixty-five per cent of tested cells extruded the second polar body within 90 min of exposure to 7% ethanol. In these activated oocytes, the long-lasting [Ca²⁺]_i increase was observed. However, no cells showed a repetitive increase in [Ca²⁺]_i Both release of calcium from intracellular stores and influx of extracellular calcium contribute to the increase in [Ca²⁺]_i induced by ethanol. We conclude that the extrusion of the second polar body requires an increase in [Ca²⁺]_i above a certain threshold level and the mobilization of calcium of both the intracellular and extracellular space in mouse oocytes.

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