Calcium Dependence of Steroid and Guanine 3′,5′- Monophosphate Induction of Germinal Vesicle Breakdown in Rana pipiens Oocytes*

Publisher Website
Google Scholar
Abstract
The action of progesterone in the induction of meiotic maturation in amphibian oocytes is thought to be dependent on the presence of exogenous Ca2+ and/or Mg2+ ions. In the present study we report that exogenous Ca2+ is essential only during the first 5–10 min after exposure of Rana pipiens oocytes to steroid and that there is no significant Mg2+ requirement for germinal vesicle (oocyte nucleus) breakdown (GVBD). The order of effectiveness of divalent cations is Ca > Sr > Ba > Mg. GVBD is inhibited by only about 30% if denuded oocytes are transferred from Ca-containing to Ca-free medium containing inducing levels of steroid. If the diffusible Ca2+ pool (t½ < 15 min) is diminished by preincubation in Ca-free, EDTA-containing medium, GVBD is inhibited by 70-80%. Preincubation in Ca-free medium at 0–4 C results in the loss of 55–60% of the denuded oocyte Ca2+ and complete inhibition of steroid-induced GVBD. Progesterone uptake was independent of [Ca2+]o. Progesterone stimulates net 45Ca2+ uptake by denuded oocytes; D-600 inhibits progesterone-induced 45Ca2+ uptake but does not block GVBD. The ionophore A23187 is more effective than progesterone in stimulating 45Ca2+ uptake by denuded oocytes but does not induce GVBD, whereas La3+ blocks 45Ca2+ uptake and induces GVBD. These results suggest that 1) diffusible intracellular Ca2+ is essential but not the sole stimulus for the initial phase in steroid induction of the resumption of meiotic maturation and 2) steroid may both increase Ca2+ influx and release Ca2+ from intracellular reservoirs. We also report 1) a transient increase in [cGMP]i 1–2 h after exposure to steroid and 2) induction of GVBD with low levels (10−10M) of exogenous dibutyryl cGMP, an event requiring elevated exogenous Ca2+ levels. Like steroid, dibutyryl cGMP induction is blocked by cycloheximide, and the cycloheximide-sensitive period (the first 4–5 h) is associated with an increase in the cGMP to cAMP ratio. (Endocrinology106: 1012, 1980)