HORMONE PRODUCTION INVIVO AND INVITRO FROM FOLLICLES AT DIFFERENT STAGES OF THE ESTROUS-CYCLE IN THE SHEEP

This experiment was undertaken in order to investigate the production of inhibin, oestradiol and androstenedione by ovarian follicles at different stages of the oestrous cycle in sheep. Twenty-four Scottish Blackface ewes were allocated to four groups of six ewes, i.e. those operated on during the luteal phase (day 10), and those operated on during the follicular phase 24-30, 36 and 60 h after the induction of luteal regression by an injection of 125-mu-g cloprostenol on day 10 of the luteal phase. Samples of jugular and ovarian venous blood were collected under anaesthesia and ovaries were then removed and all follicles larger than 3 mm diameter dissected out and incubated in medium for 2 h. After injection of cloprostenol, luteal regression occurred as indicated by a fall in the secretion rate of progesterone. The ovarian secretion rate of inhibin was similar at all stages of the follicular phase and during the luteal phase while, in contrast, the secretion rate of oestradiol was significantly (P < 0.05) elevated in the group 24 h after injection of cloprostenol. There was good correlation between the in-vivo ovarian secretion rate and production rate during incubation in vitro for both inhibin (r = 0.57) and oestradiol (r = 0.60). When follicle diameter was compared with in-vitro hormone production there was good correlation for inhibin (r = 0.72) with larger follicles producing more inhibin, while the value for oestradiol was somewhat lower (r = 0.57) owing to the presence of large atretic follicles with low oestradiol production. Androstenedione production showed a lower correlation with follicle diameter (r = 0.39). When the four time periods were compared separately, there were significantly (P < 0.05) more follicles with high in-vitro oestradiol production (> 90 fmol/min) in the group at 36 h than in the other three groups, while inhibin release in relation to follicle size was similar in the four groups. Large oestrogenic follicles were responsible for 90% of the total oestradiol production during culture while only providing 55% of the total inhibin production, with large non-oestrogenic and small follicles contributing 33% and 12% of inhibin production respectively. From the results of this study we conclude that while oestradiol is mainly produced by the large oestrogenic follicles, a considerable amount of inhibin is also produced by large non-oestrogenic and small follicles. We also found that a lack of variation in inhibin secretion rate in the intact animal was paralleled by a lack of variation in the pattern of inhibin produced from individual follicles.