Studies on the Relative Roles of Pituitary and Progesterone in the Induction of Meiotic Maturation in the Amphibian Oocyte

In the amphibian, gonadotropins act on the epithelial cells surrounding the oocyte to produce and/or release progesterone which in turn acts at the oocyte surface to initiate the resumption of meiotic maturation. Since maturation is reported to require continuous exposure to gonadotropins but only brief (5- 15 min) exposure to progesterone, it was of interest to reexamine the interrelationships between the two hormonal stimuli as well as the kinetics of progesterone production, metabolism, and biological activity. Germinal vesicle breakdown (GVBD) required continuous exposure to 0.005 pituitaries/ml for 6.0 h to produce 50% GVBD that occurred at 10.1 h. Actinomycin D (10 μg/ml) completely inhibited pituitary induced GVBD when added during the first 5-6 h; 5096 inhibition occurred when added at 7.3 h. Thus, actinomycin D continues to inhibit 1-2 h after the requirement for pituitary stimulation. Pituitary stimulation produced a 4-fold increase in ³H-acetate incorporation into progesterone and acetate conversion to progesterone was relatively constant during successive 2 h pulses throughout the 10 h period leading to GVBD. There was no significant metabolism of ³H-acetate derived progesterone when follicles were treated with pituitary extract, although the same follicles rapidly concentrated and metabolized exogenous ³H- or ¹⁴C-progesterone. The length of continuous progesterone exposure required for 50% GVBD varied from 11 h at 0.158 μM to less than 0.08 h at 15.8 μM. The time to 5096 GVBD was only delayed by about 10% (1.5 h) when maximal and micimal progesterone levels were compared. A comparison of ³H-progesterone uptake and response (GVBD) as a function of [progesterone]₀ indicated that uptake of 2-4 μmol 1^-1 cell water will induce 50% GVBD. These results indicate that a threshold uptake must be reached to initiate resumption of meiosis and that this level must be maintained throughout the period leading to nuclear breakdown. Under physiologic conditions, gonadotropins stimulate progesterone production and this progesterone is protected from, or inaccessible to, steroid metabolizing enzymes.