Medroxyprogesterone Acetate: INTRODUCTION
Cyclic ewes have regular, emerging waves of antral follicles that grow to ovulatory diameters (>5 mm) before regression or ovulation. The interovulatory interval typically consists of three or four follicular waves. In nonprolific breeds such as the Western white-face, Suffolk, Texel, and Ile-de-France, ovulatory follicles originate mostly in the last wave of the cycle, but in prolific Finn ewes, ovulatory follicles are recruited also from the penultimate wave of the estrous cycle, which emerges during the midluteal phase.
It was shown that lower than normal luteal phase concentrations of progesterone prolonged follicular lifespan in ewes. Similarly, the lifespan of large antral follicles was prolonged in ewes injected with PGF2a on Day 6 and treated with vaginal sponges soaked with medroxyprogesterone acetate (MAP), from Days 5 to 19 after ovulation. Therefore, the MAP treatment appeared to mimic the effects of a low progesterone regimen in ewes. Interestingly, in the breeding season, mean serum concentrations of progesterone are higher in nonprolific Western white-faced ewes compared to prolific Finn sheep. Lower circulating concentrations of progesterone in Finn sheep may facilitate the prolongation of the lifespan of follicles in the penultimate wave so that they can ovulate with follicles from the final wave of the cycle.
The aim of this study was to determine whether a short treatment with MAP would increase the ovulation rate by causing ovulations from two consecutive antral follicle waves. Luteal and endocrine function after treatment (i.e., corpus luteum [CL] formation, progesterone secretion, and changes in circulating concentrations of FSH and estradiol) were also examined. The study was replicated to confirm the findings.