LOW-TEMPERATURE AND FERTILIZATION-INDUCED CA2+ CHANGES IN RAT EGGS

In mammalian eggs, activation by sperm that leads to resumption of meiosis is characterized by an explosive transient increase in intracellular calcium ion concentration ([Ca2+](i)), followed by [Ca2+](i) oscillations. In addition to the spermatozoon, Various treatments can induce parthenogenetic activation, accompanied by an elevation of [Ca2+](i). It has been reported that cooling can induce egg activation, yet the mechanism of this phenomenon has not been elucidated. in the present study we followed changes in egg [Ca2(+)]i (measured by Fura-2 fluorescence ratio imaging) during activation by cooling, using conditions that ensure a low rate of spontaneous activation. Our present findings demonstrate that cooling induces egg activation as manifested by [Ca2+](i) transient(s) and second polar body extrusion. Seventy-eight of 104 eggs responded to cooling with increased [Ca2+](i). Thirty-five percent of the responding eggs displayed a single [Ca2+](i) transient, while 65% exhibited at least two [Ca2+](i) transients within the time window of the experiment (30-40 min). Twenty-two percent of these eggs displayed high-frequency oscillations (intervals of 3.5-5.9 min). In these eggs, the overall pattern of calcium dynamics was similar to that observed in eggs activated by sperm, as judged by the transient's intervals, duration, and a gradual increase in the amplitude of successive transients. The amplitudes of [Ca2+](i) transients, however, were 2-3 times lower. We propose that cooling affects [Ca2+](i) homeostasis to produce fertilization-like changes in [Ca2+](i), possibly associated with parthenogenetic activation. Moreover, great care should be exercised to prevent temperature changes during egg handling. (C) 1995 Wiiey-Liss, Inc.