Endocrine secretion of prostaglandin F2α in cyclic gilts is decreased by intrauterine administration of exogenous oxytocin

When administered systemically, oxytocin (OT) stimulates secretion of uterine prostaglandin F2α (PGF2α) in swine, but the role of endometrially-derived OT in control of PGF2α release is not clear. This study determined the effect of exogenous OT, administered into the uterine lumen of intact cyclic gilts, on PGF2α secretion during late diestrus. Intrauterine infusion of 40 USP units OT (in 30 ml 0.9% saline) was performed for 30 min (1 ml/min) into each uterine horn between 7:00 and 9:00 h on days 10, 12, 14 and 16 after estrus. Beginning 20 min before infusion, samples of jugular venous blood were drawn at 5–10-min intervals for 140 min for quantification of 13,14-dihydro-15-keto-PGF2α (PGFM), the major stable metabolite of PGF2α. Progesterone was analyzed in samples collected 0, 60 and 120 min after initiation of OT infusion. Treatment with OT did not alter plasma concentrations of PGFM on days 10 or 12 but decreased (P<0.001) PGFM concentrations for 40 min after onset of infusion on day 16. Concentrations of PGFM also were reduced in the pre-treatment samples on day 14 (P=0.05) and day 16 (P<0.001) in OT-infused gilts. Plasma progesterone declined (P<0.01) between days 10 and 16 in control-infused gilts but did not decline until after day 14 (P<0.001) in gilts infused with OT. These results indicate that when OT is administered into the uterine lumen of pigs during late diestrus, it has an anti-luteolytic effect to reduce endocrine secretion of PGF2α and delay the decline in progesterone that occurs during luteolysis.