SEGREGATION OF THE PATHWAYS LEADING TO CORTICAL REACTION AND CELL-CYCLE ACTIVATION IN THE RAT EGG

At fertilization of the mammalian egg, resumption of the cell cycle an d the cortical reaction are two events of Egg activation, correlated w ith an increase in intracellular Ca2+ concentration and activation of protein kinase C. To evaluate the pathways leading to both events, rat eggs were parthenogenetically activated by the calcium ionophore iono mycin, or by the protein kinase C activators 12-O-tetradecanoyl phorbo l-13-acetate (TPA) or 1-oleoyl-2-acetylglycerol (OAG). Cortical granul e exudate was visualized by the lectin Lens culinaris and Texas Red st reptavidin, using a confocal microscope. Resumption of meiosis was det ected by Hoechst dye, and intracellular Ca2+ concentration by fura-2. Ionomycin triggered bath a cortical reaction and resumption of meiosis , while chelation of intracellular Ca2+ rise by BAPTA-AM -bis-(O-amino pbenoxy)-ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester) reveal ed a segregation between these two events. A low Ca2+ transient (simil ar to 150 nM) induced a partial cortical reaction in half of the eggs, but the meiotic status was not affected. TPA triggered a cortical rea ction with neither resumption of meiosis nor intracellular Ca2+ rise, while OAG induced both aspects of activation, as well as a significant intracellular Ca2+ rise. We conclude that in the cascade of events le ading to egg activation, the initial Ca2+ rise is followed by a segreg ation in the pathway. A relatively low Ca2+ rise is sufficient to indu ce a partial cortical reaction. However, a higher level of Ca2+ is req uired to complete the cortical reaction and resumption of meiosis. The activation of the cell cycle is Ca2+-dependent, but protein kinase C- independent.