Effects of LH and PGF2.ALPHA. in Equine Dominant Follicles Observed by MDS.

Abstract

ABSTRACT. Microdialysis System (MDS) is a novel technique used for investigation of molecule secretion between different cell populations. Local hormonal secretion at follicular wall has been still unclear. This MDS study was used to determine progesterone (P 4 ), androstenedione (A 4 ), estradiol-17β (E 2 ) and Prostaglandin F 2α (PGF 2α ) release in mare pre-ovulatory follicles. Follicles larger than 30 mm were isolated from the ovary and follicular fluid aspirated for hormone assay. Follicular fluid collected from small, middle and large follicles were analyzed by EIA. The concentrations of P 4 and PGF 2α were similar among the different sizes of follicles. The release of A 4 was observed in middle and large follicles. E 2 concentration was observed in middle follicles and was higher in large follicles compared with middle follicles. Follicular wall was cut and incubated for MDS and when LH was infused, there was an increase in P4 and A 4 release. PGF 2α release was considerably high after LH infusion compared to the control group. Infusion of PGF 2α increased P 4 and A 4 release but there was no change in E 2 release. This results suggest that in pre-ovulatory follicles, LH stimulates theca interna cells and also PGF 2α seemed to have a mediator role to induce steroid hormone production and luteinization of follicular cells. The nature of the mechanisms involved in selection of large follicles is still a perplexing research problem in reproduction. KEY WORDS: follicle, mare, MDS, ovulation, steroid-hormone.J. Vet. Med. Sci. 64(2): 119-122, 2002 Mares are seasonal polyestrous animals and the estrous cycle have several unique characteristics, including a long period of estrus, a considerable larger size of preovulatory follicles and the absence of a typical LH surge compared with other species [2,7,9]. However, endocrinological changes that occur in the different follicular compartments (theca, granulosa cells and follicular fluid) during follicular development, maturation, and ovulation still remain unclear.Follicles isolated from mare ovaries maintained the ability to secrete steroid hormones and to respond to hormonal stimulation in vitro [18]. The same study demonstrated a variable effect of LH on progesterone (P 4 ), androstenedione (A 4 ), and estradiol-17β (E 2 ) production depending on the stage of the estrous cycle. A 4 is one of the main secretory products of the theca layer that serve as substrate for the synthesis of estradiol in the granulosa cells [4,19]. These secretory products (steroids and/or prostaglandins) are accumulated in the follicular fluid and may be used as indicators of the follicular development, atretic stage or the proximity of ovulation [21,22]. Moreover, other report suggests that progesterone can modulate ovarian activity by altering FSH and LH receptor in the follicular wall [11]. In addition, prostaglandins have been considered as an essential mediator for the LH action on the ovulatory processes in several species, including the mares [13].Prominent structural and secre...