Prostaglandins have been implicated in various aspects of ovarian function including ovulation and luteolysis. In this study, the expression and regulation of inducible prostaglandin G/H synthase (PGHS-2) and PGF2α receptors were investigated in bovine granulosa cells at various stages of differentiation. Firstly, the induction of PGF2α receptor mRNA and PGHS-2 mRNA in preovulatory granulosa cells was evaluated. Granulosa cells were collected from preovulatory follicles and cultured for 1, 4, 7 or 10 days. Cells were treated with hCG (10 iu) or with increasing doses of forskolin (0-10 μmol I-1) for 24 h. Forskolin increased steady-state concentrations of mRNA for PGHS-2 (> 20-fold) and PGF2α receptor (> 1000-fold) in a dose-dependent fashion. Use of selective protein kinase A inhibitor (H89) reduced both hCG- and forskolin-induced expression of PGF2α receptor mRNA and PGHS-2 mRNA. The hypothesis that luteinized granulosa cells would acquire PGF2α responsiveness similar to responses to PGF2α observed in vivo was also evaluated. Treatment with PGF2α (100 nmol I-1) reduced forskolin-induced expression of PGF2α receptor mRNA on days 4, 7 and 10, but not on day 1 of culture (n = 3). Treatment with PGF2α did not change forskolin-induced expression of PGHS-2 mRNA on or before day 4 of culture. In contrast, PGF2α significantly increased PGHS-2 mRNA expression in granulosa cells primed with forskolin for 7 or 10 days. In conclusion, expression of PGHS-2 and PGF2α receptor mRNA is protein kinase A-dependent in preovulatory bovine granulosa cells. Granulosa cells become PGF2α-responsive soon after expression of PGF2α receptor, whereas further differentiation is required before PGF2α induces PGHS-2 mRNA upregulation. These results demonstrate that at least two key transitions are required in PGF2α-induced luteal regression in the mid-cycle corpus luteum.
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