Progesterone-induced maturational events in oocytes of Xenopus laevis: I. Continuous necessity for diffusible calcium and magnesium

Oocytes of Xenopus laevis were induced to undergo germinal vesicle breakdown in vitro by progesterone, administered in buffered saline media. The effect of calcium and magnesium on this complex event was studied by adding or withdrawing these ions from the medium and observing the effect on the kinetics of germinal vesicle breakdown. It was found that the initial entry or binding of progesterone is not sensitive to the presence of these ions in the medium, but subsequent events within the oocyte leading to germinal vesicle breakdown requires their continual presence. Either ion alone in the medium can support a slightly reduced and morphologically atypical response to the hormone but magnesium is more efficient and both ions together give the best result. Since graded additions of divalent cations to the medium or injected directly into oocytes do not mimic the effect that progesterone induces, it is concluded that progesterone does not act by simply raising the intracellular levels of either ion.