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

Abstract

Changes in intracellular calcium concentration ([Ca(2+)](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.