20α-Hydroxysteroid dehydrogenase (20α-HSD) catabolizes progesterone into 20a-dihydroprogesterone (20α-OHP), which is considered to be a biologically inactive metabolite. In this study, using a Xenopus oocyte translation system, changes in 20α-HSD mRNA content were assessed in the ovary of immature female rats bearing a single generation of corpora lutea, which were induced to form by treatment with pregnant mare serum gonadotropin (PMSG) at the age of 26 days and human chorionic gonadotropin (hCG) at the age of 28 days. The day of the hCG treatment was designed as day 0. Although both enzymatic activities and amount of immunoreactive 20α-HSD continued to increase up to day 9, the ovarian mRNA content remained relatively constant. The content of 20α-HSD mRNA in the ovary abruptly began to increase from day 9, while both enzymatic activity and the amount of immunoreactive 20α-HSD stayed almost constant. Thus, the biphasic change in 20α-HSD mRNA content did not parallel the change in either the enzymatic activities or the degree of immunoreactivity. These results indicate that the mechanism for regulation of 20α-HSD biosynthesis in the ovary during a period of pseudopregnancy involves not only transcriptional but also post-transcriptional processes, probably including stabilization of the mRNA.
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