Free calcium in full grown Xenopus laevis oocyte following treatment with ionophore A 23187 or progesterone.

Free intracellular Ca2+ was monitored in isolated Xenopus laevis oocyte during induced maturation using the Ca2+ -sensitive luminescent protein, aequorin. Internal free Ca2+ was not precisely measured but data suggest it was quite low (in the micromolar range). No change in internal free Ca2+ was detected during maturation induced either by progesterone or by p-chloromercuribenzoate. By contrast, the ionophore A 23187 gave an increase in the free Ca2+ level when there was a raised external Ca2+ (10 mM), conditions which also induce oocyte maturation. About 3 h after progesterone or p-chloromercuribenzoate stimulation, the oocyte membrane potential decreased by about 50 mV while the membrane resistance increased transitorily.