Ionic requirements for induction of maturation (meiosis) in full-grown and medium-sized Xenopus laevis oocytes

There is already good evidence that calcium ions are involved in the induction of maturation in full-grown amphibian oocytes; we show here that other cations (K+, Mg2+) also play a role in this process. Full-grown (1.3 mm in diameter) and medium-sized (0.8-1.0 mm in diameter) oocytes were compared in the present study. It was found that, provided the medium is K+-free, valinomycin induces maturation in full-grown, but not in medium-sized, oocytes. Increasing the CaCl2 (20 mM) or the MgSO4 (40 mM) content of the medium induces maturation in full-grown, but not in medium-sized, oocytes; however, the latter undergo germinal vesicle breakdown after treatment with either progesterone or ionophore A23187 if there is an excess of Ca2+ or Mg2+ in the medium. Maturation is possible in a Na+-free medium, but amiloride inhibits germinal vesicle breakdown when NaCl is present in the medium. It is concluded that maturation is controlled by changes in the balance between the various ions rather than by Ca2+ alone.