Effect of progesterone pre-treatment on steroid secretion rates and follicular fluid insulin-like growth factor-1 concentrations in seasonally anoestrous ewes treated with gonadotrophin releasing hormone

Progesterone priming is known to prevent premature luteal regression in seasonally anoestrous ewes induced to ovulate with small dose multiple injections of gonadotrophin releasing hormone (GnRH), but the precise mechanisms involved are poorly understood. The main objective of the present study was to investigate the effect of progesterone priming, before and after a period of treatment with GnRH, on the biochemical activity of ovarian follicles collected from seasonally anoestrous ewes. Ewes in Groups 1 and 2 (6 ewes/group) received progesterone priming or vehicle alone, respectively, and were slaughtered 72 h later. Ewes in Groups 3 and 4 (6 ewes/group) were similarly primed with progesterone or vehicle only, respectively, and then 72 h later received 2-hourly injections of 250 ng GnRH for 20 h, followed 2 h later by a single bolus injection of 300 μg GnRH. They were then slaughtered 2 h after the bolus injection. All normal growing follicles ≥ 2.0 mm diameter were dissected from the ovaries of all ewes and after measurement of their diameter, were incubated in Eagleʹs Basal Medium for 2 h to measure their steroid secretion rates. The fluid from each follicle was then aspirated and assayed for total insulin-like growth factor-1 (IGF-1) content. The luteinizing hormone receptor content of the whole follicle was estimated using 125I-labelled human chorionic gonadotropin (hCG). The follicles were classified as either non-oestrogenic or oestrogenic if their oestrogen production rate was ≤ 300 or > 300 pg/ml/2h, respectively. The mean diameter of oestrogenic follicles (5.72 ± 0.25 mm) was significantly (P < 0.001) larger than that of non-oestrogenic ones (3.93 ± 0.25 mm). The mean numbers of oestrogenic and non-oestrogenic follicles were not significantly different among the four treatment groups. Oestradiol production rate was significantly (P < 0.001) increased as a result of GnRH treatment, but only in large oestrogenic follicles. Testosterone production rate was, however, significantly (P < 0.05) increased by both GnRH and progesterone in both classes of follicle. While GnRH treatment tended to decrease IGF-1 concentrations in small non-oestrogenic follicles, it significantly (P < 0.05) increased concentrations in large oestrogenic ones. IGF-1 concentrations were, however, significantly (P < 0.05) increased in both classes of follicle collected from progesterone-primed ewes treated with GnRH. Regardless of treatment, follicular fluid IGF-1 concentrations were significantly (P < 0.001) higher in small non-oestrogenic follicles compared to large oestrogenic ones. An increase in 125I-hCG binding to large oestrogenic follicles was observed in response to progesterone (P = 0.07) in GnRH treated ewes. These results indicate differences in the biochemical activity of follicles attributable to progesterone priming which may be casually related to defective luteal function after ovulation.