Evidence that the preovulatory rise in intrafollicular progesterone may not be required for ovulation in cattle

Abstract

Despite ample evidence pointing to an obligatory involvement of progesterone in ovulation, the mechanisms responsible for the ovulation promoting effects of intrafollicular progesterone are unclear. The objectives of this study were to determine if ovulation, luteinization and the gonadotropin surge-induced regulation of select extracellular matrix-degrading enzymes and their inhibitors, and mRNAs for prostaglandin (PG) biosynthesis and metabolizing enzymes are blocked following suppression of the intrafollicular increase in progesterone. Bovine preovulatory follicles were injected with the 3 beta-hydroxysteroid dehydrogenase inhibitor trilostane or diluent and collected at 0, 12, and 24 h after GnRH induction of the preovulatory LH surge. Intrafollicular trilostane administration blocked the preovulatory increase in follicular fluid progesterone resulting in concentrations similar to those observed at time 0 post-GnRH injection. The preovulatory increase in follicular fluid PGE(2) and PGF(2alpha) was reduced in trilostane-treated follicles and accompanied by upregulation of prostaglandin dehydrogenase mRNA in the granulosal and thecal cells. However, follicle rupture was not blocked by inhibition of the preovulatory rise in intrafollicular progesterone, and normal serum progesterone concentrations were observed during subsequent luteal development. Effects of trilostane administration on preovulatory changes in mRNA abundance and protein/activity in preovulatory follicles for most regulators of extracellular matrix remodeling examined were distinct from changes previously observed following the inhibition of intrafollicular prostaglandin synthesis. Results suggest that the preovulatory increase in intrafollicular progesterone may not be obligatory for bovine follicle rupture, luteinization, or regulation of prominent matrix-degrading proteinases and their inhibitors associated with ovulation.