EFFECT OF PROGESTERONE PRETREATMENT ON STEROID-SECRETION RATES AND FOLLICULAR-FLUID INSULIN-LIKE GROWTH-FACTOR-I CONCENTRATIONS IN SEASONALLY ANESTROUS EWES TREATED WITH GONADOTROPIN-RELEASING-HORMONE

Progesterone priming is known to prevent premature luteal regression i n seasonally anoestrous ewes induced to ovulate with small dose multip le injections of gonadotrophin releasing hormone (GnRH), but the preci se mechanisms involved are poorly understood. The main objective of th e 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 ewe s. 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 progester one or vehicle only, respectively, and then 72 h later received 2-hour ly injections of 250 ng GnRH for 20 h, followed 2 h later by a single bolus injection of 300 mu GnRH. They were then slaughtered 2 h after t he bolus injection, All normal growing follicles greater than or equal to 2.0 mm diameter were dissected from the ovaries of all ewes and af ter measurement of their diameter, were incubated in Eagle's Basal Med ium for 2 h to measure their steroid secretion rates. The fluid from e ach follicle was then aspirated and assayed for total insulin-like gro wth factor-1 (IGF-1) content. The luteinizing hormone receptor content of the whole follicle was estimated using I-125-labelled human chorio nic gonadotropin (hCG). The follicles were classified as either non-oe strogenic or oestrogenic if their oestrogen production rate was less t han or equal to 300 or >300 pg/ml/2h, respectively. The mean diameter of oestrogenic follicles (5.72 +/- 0.25 mm) was significantly (P < 0.0 01) larger than that of non-oestrogenic ones (3.93 +/- 0.25 mm). The m ean numbers of oestrogenic and non-oestrogenic follicles were not sign ificantly different among the four treatment groups. Oestradiol produc tion rate was significantly(P < 0.001) increased as a result of GnRH t reatment, but only in large oestrogenic follicles. Testosterone produc tion rate was, however, significantly (P < 0.05) increased by both GnR H and progesterone in both classes of follicle. While GnRH treatment t ended to decrease IGF-I concentrations in small non-oestrogenic follic les, it significantly (P < 0.05) increased concentrations in large oes trogenic ones. IGF-I concentrations were, however, significantly (P < 0.05) increased in both classes of follicle collected from progesteron e-primed ewes treated with GnRH. Regardless of treatment, follicular f luid IGF-I concentrations were significantly (P < 0.001) higher in sma ll non-oestrogenic follicles compared to large oestrogenic ones. An in crease in I-125-hCG binding to large oestrogenic follicles was observe d in response to progesterone (P = 0.07) in GnRH treated ewes. These r esults indicate differences in the biochemical activity of follicles a ttributable to progesterone priming which may be casually related to d efective luteal function after ovulation.