Effect of Pregnancy on Oxytocin-induced Release of Prostaglandin F2α in Heifers

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Abstract
The effects of pregnancy on the release of prostaglandin F2.alpha. (PGF2.alpha.) in response to oxytocin (OT) has been examined. Fourteen cyclic heifers received one intravenous injection of 1 IU OT (n=6) or 100 IU OT (n=8) 17, 18, or 19 days (Day 17-19) after the onset of estrus (Day 0). Five of these animals also received 100 IU OT at Days 6 and 13 to determine the effect of OT at different times of the cycle. Frequent blood samples were taken for 60 min before and for 90 min after OT injection for the measurement of 15-keto-13,14-dihydro-PGF2.alpha. (PGFM) by radioimmunoassay. The experiment was then repeated using the same animals at Day 17-19 of pregnancy (confirmed by the recovery of an embryo the day after OT injection). Following the injection of 1 IU OT, plasma PGFM reached its peak within 30 min with the increase significantly lower (P < 0.05) in pregnant (1.13 .+-. 0.10-fold) than in nonpregnant animals (2.07 .+-. 0.27-fold). However, because only 3 of the 6 cyclic animals showed a response to 1 IU OT, the dose was increased to 100 IU in subsequent experiments. The animals that received 100 IU at Days 6 and 13 had no significant increase in PGFM concentrations (1.18 .+-. 0.05-fold and 1.01 .+-. 0.04-fold, respectively). At Day 17-19 the increase in plasma PGFM reached its peak 5-15 min after 100 IU OT and the increase was significantly greater in nonpregnant (3.23 .+-. 0.17-fold) than in pregnant (1.21 .+-. 0.02-fold; P = 0.003) heifers. Six of 11 animals injected at Day 17-19 of the cycle showed a decrease in progesterone (P4) the day after OT administration. These data show that the release of PGF2.alpha. in response to OT is suppressed in pregnant animals in vivo, suggesting an antiluteolytic role for the embryo in luteostasis.