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Progesterone inhibition of the development of the luteolytic signal in cows

Mated cows that subsequently undergo luteolysis have significantly lower plasma concentrations of progesterone than do cows in which pregnancy is maintained. Two groups of ovariectomized cows were treated with progesterone and oestradiol to recreate luteal phases exhibiting the extremes of plasma concentrations of progesterone (6.0 ± 0.4 and 12.4 ± 0.8 ng ml^–1) normally seen in intact cows. The effect of this difference in progesterone on the development of the luteolytic signal was determined by monitoring the basal and oxytocin-stimulated plasma concentrations of the principal metabolite of prostaglandin F₂, PGFM on days 12–16 of the simulated luteal phases. Basal PGFM concentrations were similar in the low and high progesterone groups on all days. The mean plasma concentration of PGFM showed a larger increase following oxytocin in the low progesterone group on days 13 and 14 (P < 0.05) and on days 15 and 16 (P < 0.1). These results demonstrate that a low plasma concentration of progesterone results in the development of a stronger luteolytic signal, and provide an explanation for the fact that cows with lower plasma concentrations of progesterone are more prone to embryo loss.