An increase of intracellular free Ca2+ is essential for spontaneous meiotic resumption by mouse oocytes

The involvement of calcium ions in the mechanism of meiotic resumption has been studied in mouse oocytes made resistant to the lethal effects of calcium-free medium (CFM) by zona pellucida removal (De Felici et al., '89). We show here that such oocytes undergo meiotic resumption in CFM (as evaluated by germinal vesicle breakdown, GVBD) at a rate comparable to that shown by oocytes cultured in medium containing 1.7 mM Ca2+. The addition to CFM of 50 u M Quin2/AM (a membrane permeable, high affinity Ca2+ chelator) totally prevents GVBD, while purported antagonists of Ca2+ release from intracellular stores, such as 150 uM 8-(N,N-diethylamino)octyl-3-4-5 trimethoxybenzoate (TMB-8) or 300 uM chlortetracycline, only cause a slight meiotic delay. On the other hand, if the oocytes are pre-incubated for 30 min in CFM supplemented with 100 uM TBM-8 plus 0.2 mM dibutyryl-cyclic AMP (dbcAMP, a reversible inhibitor of GVBD), and then cultured in the same medium, without dbcAMP, a sustained inhibition of meiotic maturation is obtained. Our observations suggest that an increase in intracellular free Ca2+ is essential for meiotic resumption by mouse oocytes; in the experimental absence of external Ca2+, release of the cation from internal stores is sufficient to allow meiotic resumption.