Serum 17,20β-dihydroxy-4-pregnen-3-one Levels in Pregnant and Non-pregnant Female Rockfish, Sebastes schlegeli, Viviparous Teleost, and its Production by Post-ovulatory Follicles

Changes in serum 17α,20β-dihydroxy-4-pregnen-3-one (17,20β-DP) levels around the gestation time of normal pregnant and experimentally non-pregnant females were investigated in the black rockfish, Sebastes schlegeli, a viviparous teleost. The serum 17,20β-DP in both females showed similar changes and levels, increasing from the early to late gestation periods and declining just before parturition in pregnant females and egg-release in non-pregnant females, respectively. These results suggest that the maintenance of high serum levels of 17,20β-DP after oocyte maturation is not correlated with gestation or parturition, but occurs spontaneously in this species. The decline of 17,20β-DP levels prior to egg-release in non-pregnant females tended to occur one week earlier than those prior to parturition in pregnant females, suggesting that both a decline in 17,20β-DP levels in mothers and some response from embryos are needed for a smooth parturition. The post-ovulatory follicles were maintained throughout the gestation period and produced a considerable amount of 17,20β-DP in vitro (3.44–6.96 pg/ml/mg tissue), but little estradiol-17β (0.92–1.66 pg/ml/mg tissue). The production of 17,20β-DP tended to be enhanced by the addition of a precursor steroid, pregnenolone, in the pre-, early and mid-gestation periods. These results strongly suggest that the follicle cells in black rockfish have the ability to synthesize 17,20β-DP during the post-ovulatory period, and high serum 17,20β-DP during gestation is supplied by the post-ovulatory follicles, which in Sebastes are considered to be functionally homologous to the mammalian corpus luteum.