Egg activation events are regulated by the duration of a sustained [Ca ²⁺]_cyt signal in the mouse

Although the dynamics of oscillations of cytosolic Ca²⁺ concentration ([Ca²⁺]_cyt) play important roles in early mammalian development, the impact of the duration when [Ca²⁺] _cyt is elevated is not known. To determine the sensitivity of fertilization-associated responses [i.e., cortical granule exocytosis, resumption of the cell cycle, Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) activity, recruitment of maternal mRNAs] and developmental competence of the parthenotes to the duration of a [Ca²⁺] _cyt transient, unfertilized mouse eggs were subjected to a prolonged [Ca²⁺]_cyt change for 15, 25, or 50 min by means of repetitive Ca²⁺ electropermeabilization at 2-min intervals. The initiation and completion of fertilization-associated responses are correlated with the duration of time in which the [Ca²⁺]_cyt is elevated, with the exception that autonomous CaMKII activity is down-regulated with prolonged elevated [Ca²⁺]_cyt. Activated eggs from 25- or 50-min treatments readily develop to the blastocyst stage with no sign of apoptosis or necrosis and some implant. Ca²⁺ influx into unfertilized eggs causes neither Ca²⁺ release from intracellular stores nor rapid removal of cytosolic Ca²⁺. Thus, the total Ca²⁺ signal input appears to be an important regulatory parameter that ensures completion of fertilization-associated events and oocytes have a surprising degree of tolerance for a prolonged change in [Ca²⁺]_cyt.