Expression of messenger ribonucleic acid encoding for steroidogenic acute regulatory protein and enzymes, and luteinizing hormone receptor during the spring transitional season in equine follicles.

The period of spring transition, from the anovulatory to the ovulatory season, is characterized in many mares by cyclical growth and regression of large dominant follicles. These follicles produce only low concentrations of estradiol and it is thought that acquisition of steroidogenic competence by large follicles during spring transition is prerequisite in stimulating LH prior to first ovulation. In situ hybridization was used to localize and quantify expression of factors that play a key role in follicular steroidogenesis: StAR, P450scc (CYP11A1), P450c17 (CYP17), P450arom (CYP19), and LH receptor (LHr). One ovary was obtained from mares on the day after detection of an actively growing 30 mm transitional anovulatory follicle (defined as the transitional follicle), and the remaining ovary was removed at the third estrus of the breeding season on the day after the preovulatory follicle reached 30 mm in diameter (defined as the preovulatory follicle). Messenger RNAs encoding StAR, CYP11A1, and CYP17 were detected only in theca cells and CYP19 mRNA was confined to the granulosa layer. There was significantly lower expression of mRNAs for the steroidogenic enzymes, StAR (P<0.001) and LHr (P<0.05) in transitional follicles than in preovulatory follicles. In conclusion, large equine follicles during spring transition have low levels of mRNA encoding steroidogenic enzymes, StAR and LHr which will contribute to the steroidogenic incompetence of dominant follicles during spring transition and their subsequent regression.

Non-genomic progesterone receptors in the mammalian ovary: some unresolved issues.

In addition to their well-documented genomic effects, steroid hormones may also exert actions that are: (i) rapid, (ii) insensitive to inhibitors of transcription, (iii) mimicked by steroids coupled to cell membrane-impermeant molecules, and (iv) demonstrable in cells that do not express the classic genomic progesterone receptor (gPR). Such 'non-genomic' effects have been described for all the major classes of steroids (progesterone, oestrogens, androgens and corticoids), as well as for thyroid hormones, retinoids and vitamin D(3). Rapid, membrane-mediated effects of progesterone have been studied most intensively in human spermatozoa and in the Xenopus oocyte. However, similar non-genomic actions of progesterone and other steroids have now been described in a wide variety of different tissues in many species. The first putative membrane steroid receptor to be cloned was that for the pig membrane progesterone receptor (mPR). Subsequently, similar genes were cloned from rats and cattle, and two related mPRs have been described in humans. Despite accumulating evidence for cell-surface membrane actions of steroids, a number of uncertainties remain as to the properties and identity of such 'receptors' and their cellular actions. Furthermore, some rapid steroid effects may be mediated through membrane-associated 'classical' steroid receptors, and steroid receptors may be capable of activating other signalling pathways non-classically. This review focuses on some of these unresolved issues, taking as its model the actions of progesterone in the mammalian ovary.