Low temperature and fertilization‐induced Ca2+ changes in rat eggs

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Abstract
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.