Ovarian progesterone levels during in vitro oocyte maturation and ovulation in Xenopus laevis.

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Endogenous progesterone was measured in ovaries of the frog Xenopus laevis following the administration of gonadotropins that induce meiotic maturation and ovulation of large oocytes. Ovarian pieces were incubated for 0, 1, 3, 5, or 10 h in Gurdon’s solution in the presence or absence of human chorionic gonadotropin (HCG; 20 lU/mI) or a frog pituitary homogenate (FPH; 0.04 pituitary/mI). Each incubation sample was scored for ovulation and maturation at the end of its incubation period, homogenized in the medium, and extracted for progesterone with petroleum ether. Following purification on Sephadex LH-20, extracts were assayed for progesterone by a radioimmunoassay that was validated for use with frog ovarian tissue. In 13 experiments, mean (! SEM) progesterone concentration in untreated ovarian tissue was 3.7 ± 0.5 ng/gm at time zero and progesterone levels did not change significantly during 10 h of incubation. HCG-treated tissue (n = 6) exhibited a linear, two-fold increase in progesterone for the first 3 h of incubation and maintained that level for the remainder of the incubation. FPH treatment (n = 10) produced a linear six-fold increase in progesterone over the course of 10 h. Mean progesterone content across time was significantly greater (P<0.005) in FPH-treated tissue than in HCG-treated tissue. Both differed significantly (P<0.05) from untreated controls. In tissue treated with HCG the percent oocyte maturation was correlated (P<0.10) with both mean progesterone concentration across time and progesterone concentration at 10 h of incubation; ovulation did not occur. In FPH-treated tissue progesterone concentration was not correlated with percent maturation but was correlated (P<0.05) with the number of ovulations/gm. Treatment of ovarian pieces with the gonadotropins following a 10 h incubation in Gurdon’s solution produced effects similar to those following treatment at time zero. In vivo treatment with HCG prior to in vitro incubation of ovarian tissue with HCG or FPH resulted in an eight- or eleven-fold increase, respectively, in peak progesterone concentration as compared to tissue that was not pretreated in vivo. In two experiments large, preovulatory follicles contained sufficiently more (P<0.05) progesterone following FPH treatment than did smaller follicles. Both large and small FPH-treated follicles had significantly higher levels of progesterone than untreated control tissue.