Combined effects of calcium and dibutyryl cyclic AMP on germinal vesicle breakdown in the mouse oocyte

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Abstract
Mouse oocytes were cultured in the presence of dibutyryl cAMP (dbcAMP) and various agents that affected cytoplasmic Ca concentrations. Treatments that inhibited Ca uptake potentiated the inhibitory effect of dbcAMP and treatments which stimulated cellular Ca uptake overcame the effect of dbcAMP. Elevated extracellular Ca (> 10 mM) significantly decreased the inhibitory effect of concentrations of dbcAMP up to 150 .mu.M when compared to control levels of Ca (1.7 mM). In addition, the Ca ionophore A23187 [calcimycin] (> 1 .mu.M) significantly overcame the effect of dbcAMP in media that contained 1.7 or 20 mM Ca. In the presence of 41 .mu.M dbcAMP the Ca antagonist verapamil increased (in a dose-dependent fashion) the percentage of oocytes blocked at the germinal vesicle stage from 21% with 10 .mu.M verapamil to 99% with 200 .mu.M. A similar dose-dependent, reversible potentiation of the effect of dbcAMP was found with tetracaine, which also lowers cytoplasmic Ca concentrations. Apparently, a minimum level of cytoplasm Ca is required for the initiation of germinal vesicle breakdown and the action of dbcAMP is mediated by its effect upon this Ca.
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