Abstract
The main objective of the experiment was to investigate the mechanism by which progesterone priming eliminates defective luteal function in anoestrous ewes induced to ovulate with GnRH. Animals in group 1 (no. = 10) were primed with a single i.m. injection of progesterone in corn oil 3 days before the start of GnRH treatment, while ewes in group 2 (no. = 10) received corn oil alone and served as untreated controls. Ewes in group 3 (no. = 10), which served as positive controls, were treated with an intra-vaginal progestagen sponge for 7 days, and this was removed just before the start of GnRH treatment. Ewes in all three groups were induced to ovulate by administration of 2-h injections of GnRH (250 ng per injection) for 54 h. Frequent blood samples for LH, FSH and progesterone analysis were taken around the time of both progesterone injection and GnRH treatment, as well as daily thereafter to monitor luteal function, and laparoscopy was performed 3 and 7 days after GnRH treatment. The incidence of ovulation was similar for all the three groups (8/10, 7/10 and 9/10 for groups 1, 2 and 3 respectively). Hoioever, both laparoscopic examination and plasma progesterone concentrations revealed that the incidence of normal luteal function was significantly higher in progesterone-primed animals group 1:7/8; group 3:9/9) compared ivith controls (group 2:0/7), P < 0-05) with no difference between groups 1 and 3. Injection of progesterone on day −3 significantly suppressed mean LH concentrations (P < 0·05), but mean FSH concentrations were not altered. However, there were no significant differences between groups in LH and FSH concentrations over the period of GnRH treatment, nor in the timing, duration and height of pre-ovulatory LH and FSH surges. These results suggest that progesterone priming may eliminate defective luteal function either by changing LH concentrations at the time of progesterone administration or through mechanisms not involving gonadotropin secretion.