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

In mammalian eggs, activation by sperm that leads to resumption of mei osis is characterized by an explosive transient increase in intracellu lar calcium ion concentration ([Ca2+](i)), followed by [Ca2+](i) oscil lations. In addition to the spermatozoon, Various treatments can induc e 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 fluores cence ratio imaging) during activation by cooling, using conditions th at ensure a low rate of spontaneous activation. Our present findings d emonstrate 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 perce nt of the responding eggs displayed a single [Ca2+](i) transient, whil e 65% exhibited at least two [Ca2+](i) transients within the time wind ow of the experiment (30-40 min). Twenty-two percent of these eggs dis played high-frequency oscillations (intervals of 3.5-5.9 min). In thes e eggs, the overall pattern of calcium dynamics was similar to that ob served in eggs activated by sperm, as judged by the transient's interv als, duration, and a gradual increase in the amplitude of successive t ransients. The amplitudes of [Ca2+](i) transients, however, were 2-3 t imes lower. We propose that cooling affects [Ca2+](i) homeostasis to p roduce fertilization-like changes in [Ca2+](i), possibly associated wi th parthenogenetic activation. Moreover, great care should be exercise d to prevent temperature changes during egg handling. (C) 1995 Wiiey-L iss, Inc.