A rise in cytosolic calcium is not necessary for maturation of Xenopus laevis oocytes.

Cytoplasmic free calcium levels during progesterone-induced meiotic maturation in Xenopus laevis oocytes were measured using the photoprotein aequorin. The resting level of [Ca2+]i was 92.6 +/- 30 nM. No significant changes were observed after progesterone addition, although a large pulse of [Ca2+]i was observed upon activation of matured oocytes. These findings are discussed in terms of the role of calcium in maturation and it is concluded that calcium is not the second messenger for progesterone. This conclusion is further supported by the finding that 100 microM TMB-8, a blocker of intracellular calcium release, had no effect on progesterone-induced maturation.