REPEATED ADMINISTRATION OF PROSTAGLANDIN-F(2-ALPHA) DURING THE EARLY LUTEAL-PHASE CAUSES PREMATURE LUTEOLYSIS IN THE PIG

Previous investigators considered pig corpora lutea refractory to the luteolytic effects of prostaglandin (PG) F2alpha before Day 12 of the estrous cycle. This study was designed to determine whether multiple i njections of PGR2alpha would result in a sustained reduction of serum progesterone and luteolysis, leading to significant shortening of the estrous cycle and interestrous interval. On Days 5-10 of an estrous cy cle, gilts (n = 4) received injections of 12.5 mg PGF2alpha (dinoprost tromethamine) i.m. every 12 h, or vehicle (PBS; n = 4) according to t he same schedule. Mean interestrous interval in PGF2alpha-treated gilt s was reduced (p < 0.001) to 13.3 +/- 0.5 days compared with 19.8 +/- 0.6 days for control gilts. Serum progesterone declined below 1 ng/ml by Day 10.5 in PGF2alpha-treated gilts compared to Day 17.5 in control animals. Serum concentrations of estradiol-17beta (E2) reached maxima l levels in PGF2alpha-treated gilts earlier (Day 12.5) in the cycle th an in control gilts (Day 19.5). Peak E2 and LH concentrations coincide d with the periestrous period, suggesting that PGF2alpha-induced estru s is accompanied by normal follicular development and ovulation. These results demonstrate that the pig is susceptible to the luteolytic eff ects of PGF2alpha before Day 12 if repeated injections are given from Day 5 through Day 10.