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(765) EFFECT OF DEPLETING LUTEAL OXYTOCIN (OT) ON PREGNANCY RATES IN SHEEP. Bacon, Charlotte1, 2, Keator, Christopher1, Milvae, Robert1, McCracken, John1, 1 University of Connecticut, Storrs, CT2 University of Hartford, West Hartford, CT ABSTRACT- Previous studies in ruminants indicated that depletion of luteal OT, either by the infusion of noradrenaline in cows (Kotwicka and Skarzynski, J Reprod Fert 1993;97:411) or by subluteolytic infusions of PGF2 KEY WORDS: oxytocin, pregnancy, PGF2 |
in sheep (McCracken et al, Biol Reprod 2000;62: Suppl 1, 125) did not alter estrous cycle length, suggesting that luteal OT may not be a necessary requirement for luteolysis in ruminants. In the latter study, sheep depleted of luteal OT at midcycle and untreated controls all showed a decline in plasma progesterone (P) on cycle day 14. However, the rate of decline of P in OT-depleted sheep was steeper than in control sheep. This indicated that untreated control sheep had a longer period of P influence than OT-depleted sheep, which in the event of pregnancy, might provide a longer window for the blastocyst to establish itself via interferon-tau synthesis. To test the hypothesis that luteal OT might be advantageous for the establishment of pregnancy, estrus was synchronized with Lutalyse in 10 sheep which were mated with fertile rams. The mated sheep were then randomly assigned to either an untreated control group (n=5) or to a treatment group (n=5) which received six 30 min systemic infusions of PGF2
g/min) at 8hr intervals on cycle days 10 and 11, thus depleting luteal OT by about 95%. From days 12 to 19 after mating, blood was sampled twice daily to monitor peripheral plasma P concentration and a vasectomized ram determined whether any mated animals returned to estrus. Results: As shown by P conc. of less than 1.0ng/ml and confirmed by the marker ram, 2/5 control and 1/5 treated animals were not pregnant by day 19. Chi-square analysis indicated no significant difference (p = 0.49) in pregnancy status between the groups. However, when P conc. was compared between the animals maintaining pregnancy in each group, there was a significant treatment by day interaction (p = 0.048). Moreover, from days 18 to 19, mean P conc. in control sheep increased from 3.84 to 7.79 ng/ml whereas P conc. in treated sheep decreased from 4.05 to 2.98 ng/ml. These results suggest that luteal OT may play some as yet unexplained role in maintaining early pregnancy in sheep. However, larger groups of animals and longer sampling periods will be required to confirm these findings. (Supported by USDA grant # 98-35203-6635).