Ovary

Effects of a 6-Day Treatment with Medroxyprogesterone Acetate after Prostaglandin F₂-Induced Luteolysis at Mid-Cycle on Antral Follicular Development and Ovulation Rate in Non-Prolific Western White-Faced Ewes

Abstract

Medroxyprogesterone acetate (MAP) from intravaginal sponges prolongs the lifespan of large ovarian follicles when administered after prostaglandin F₂(PgF₂)-induced luteolysis early in the luteal phase of ewes. The present study was designed to see if a PgF₂/MAP treatment applied at mid-cycle would alter the pattern of antral follicle growth and increase ovulation rate in non-prolific ewes. A single injection of PgF₂ (15 mg, i.m.) was given and an intravaginal MAP (60 mg) sponge was inserted for 6 d, on ~day 8 after ovulation, in 7 (Experiment 1), 8 (Experiment 2) or 11 (Experiment 3) ultrasonographically monitored, cycling Western white-faced ewes; seven ewes (Experiment 1) served as untreated controls. Blood samples were collected each day and also every 12 min for 6 h, half-way through the period of treatment with MAP (Experiment 1), or every 4 h, from 1 d before to 1 d after sponging (Experiment 2). Seventeen out of 26 treated ewes (Experiment 1: n=6; Experiment 2: n=5; and Experiment 3: n=6) ovulated 1 to 6 d after PgF₂, but this did not affect the emergence of ensuing follicular waves (Experiments 1 and 2). These ovulations, confirmed by laparotomy and histological examinations of the ovaries (Experiment 3), were not preceded by an increase in LH/FSH secretion and did not result in corpora lutea (CL), as evidenced by transrectal ultrasonography and radioimmunoassay of serum progesterone (Experiments 1 and 2). Following the removal of MAP sponges, the mean ovulation rate was 3.1±0.4 in treated and 2.0±0.3 in control ewes (Experiment 1; P<0.05). In Experiments 1 and 2, the ovulation rate after treatment (3.1±0.4 and 2.8±0.4) was also greater than the pre-treatment ovulation rate (1.9±0.3 and 1.9±0.1, respectively). Ovulations of follicles from 2 consecutive waves before ovulation were seen in 5 treated but only in 2 control ewes (Experiment 1), and in 7 ewes in Experiment 2. There were no significant differences between the MAP-treated and control ewes in mean daily serum concentrations of FSH and estradiol, and no differences in the parameters of LH/FSH secretion, based on frequent blood sampling. Treatment of non-prolific Western white-faced ewes with PgF₂ and MAP at mid-cycle changed follicular dynamics and increased ovulation rate by approximately 50%. These effects of MAP, in the absence of luteal progesterone, may not be mediated by changes in gonadotropin secretion.