Changes in Cyclic AMP Levels in the Amphibian Ovarian Follicle Following Progesterone Induction of Meiotic Maturation: Effect of Phosphodiesterase Inhibitors and Exogenous Calcium on Germinal Vesicle Breakdown

Abstract

In several cell systems, factors that decrease cyclic AMP levels stimulate cell division, while factors that increase cyclic AMP levels cause a decrease in mitotic activity. A similar pattern is seen for the progesterone initiation of the meiotic divisions in the Rana pipiens oocyte. Following exposure of ovarian follicles in vitro to inducing levels of steroid, [cAMP] i decreased from 7.5–0.9 pmol/mg dry wt to 2.0–0.2 pmol/mg dry wt by 4–5 h and remained at about 35% of the unstimulated (control) level during germinal vesicle breakdown (GVBD) and the completion of the first meiotic division. By 24 h [cAMP] i rose to about 10 pmol/mg dry wt and remained elevated until second metaphase arrest. After fertilization, [cAMP] i remained relatively unchanged but decreased three to four fold by late blastula stage. Exogenous DBcAMP reversibly blocked GVBD if added at any time during the first 4–5 h after exposure to steroid. [DBcAMP] 0 which blocked GVBD had no inhibitory effect on 14 C-leucine uptake and incorporation. Phosphodiesterase inhibitors (SQ 20,006, theophylline, caffeine) blocked both GVBD and the decrease in [cAMP] i , but did not significantly inhibit 14 C-leucine uptake and incorporation. Steroid-induced GVBD occurs without the continued presence of Ca 2+ in the medium, but exogenous Ca 2+ is essential for inhibition of GVBD by DBcAMP or phosphodiesterase inhibitors, but not by cycloheximide. The results suggest that a decrease in [cAMP] i may be specific and obligatory for nuclear membrane dissolution during the first meiotic division and that a primary action of progesterone may be to increase free Ca 2+ in the oocyte cytoplasm, which in turn may activate a phosphodiesterase and lower intracellular cyclic AMP levels.