CORPUS LUTEAL FUNCTION IN NONPREGNANT MARES FOLLOWING INTRAUTERINE ADMINISTRATION OF PROSTAGLANDIN E(2) OR ESTRADIOL-17-BETA

The objective of this study was to test the hypothesis that intrauteri ne administration of prostaglandin E(2) (PGE(2)) or estradiol-17 beta (E-17 beta) would prolong CL function in nonpregnant mares. Nonpregnan t mares were continuously infused with 240 mu g/d of PGE(2), 6 mu g/d of E-17 beta, or vehicle (sham-treated) on Days 10 to 16 post ovulatio n (ovulation = Day 0), using osmotic minipumps surgically placed into the uterine lumen on Day 10 (n = 11 per group). Nonpregnant and pregna nt mares served as negative and positive controls, respectively (n = 1 1 per group). Mares were defined as having prolonged CL function if pl asma progesterone remained >2.5 ng/ml and if ovulation did not occur o n Days 9 to 30. Corpus luteal function was prolonged until Day 30 in 1 /11 nonpregnant mares, 4/11 sham-treated mares, 6/11 E-17 beta-treated mares, 8/11 PGE(2)-treated mares, and 11/11 pregnant mares. The incid ence of prolonged CL function was similar (P=0.16) in the sham-treated and nonpregnant mares. The hypothesis that PGE(2) would prolong CL fu nction in nonpregnant mares was supported, since the incidence of prol onged CL function was higher (P=0.003) in PGE(2)-treated versus nonpre gnant mares, tended to be higher (P=0.09) in PGE(2)-versus sham-treate d mares, and was not lower (P=0.11) in PGE(2)-treated versus pregnant mares. The hypothesis that E-17 beta would prolong CL function in nonp regnant mares was not supported, since the incidence of prolonged CL f unction was not higher (P=0.34) in E-17 beta-versus sham-treated mares , and was lower (P=0.02) in E-17 beta-treated versus pregnant mares. T hese results demonstrate that intrauterine administration of a pharmac ologic dose of PGE(2) initiated prolonged CL function in nonpregnant m ares. Further experiments are needed to confirm the role of conceptus secretion of PGE(2) in CL maintenance, and to determine the mechanism of action of PGE(2) within the equine reproductive tract.