Temporal changes in serum prostaglandin F2α and oxytocin in dairy cows with short luteal phases after the first postpartum ovulation

Luteolysis in the cow depends upon an interaction between prostaglandin F 2α (PGF 2α ) and oxytocin. The objectives of our study were 1) to determine oxytocin concentrations in postpartum dairy cows and 2) to identify the temporal relationship between oxytocin and PGF 2α release patterns during luteolysis in normal and abbreviated estrous cycles in the postpartum period. Serum oxytocin and PGF 2α metabolite (PGFM) concentrations from nine cows which had short estrous cycles (< 17 d) were compared with those of six cows which had normal estrous cycles. Serum basal oxytocin concentrations in short estrous cycle cows (23.7 to 31.1 pg/ml) were higher (P<0.05) than those of normal estrous cycle cows (14.6 to 19.8 pg/ml). Oxytocin concentrations increased to peak values in both short and normal cycle cows, during luteolysis. Basal PGFM concentrations (112.2 to 137.4 pg/ml) were higher in cows with short cycle (P<0.05) than in cows with normal cycles (62.9 to 87.5 pg/ml). The increase in PGFM concentrations during luteolysis was significant in both normal cycle and short cycle cows (P<0.05). Increases in serum PGFM concentrations were always associated with increases in serum oxytocin concentrations in normal cycle and short cycle cows and the levels decreased simultaneously before the subsequent estrus. Results support the idea of a positive relationship between PGF 2α and oxytocin concentration during the estrous cycle as well as a possible synergistic action of these hormones in the induction of luteolysis in dairy cattle.