Prostaglandin-induced shortening of the luteal phase in cattle affects progesterone receptors, uterine secretory function and pregnancy rate

In ruminants, progesterone exposure before ovulation is required for normal luteal function in the subsequent cycle. Inadequate luteal function, i.e. a short luteal phase or low progesterone concentrations, causes pregnancy losses. Dairy cow fertility programmes often involve prostaglandin treatment to regress corpora lutea and allow timed insemination. We have therefore studied uterine function in cows in which oestrus has been induced at different times of the luteal phase. In a preliminary experiment, oestrus was induced with PGF2alpha on days 6, 9 or 12 of the cycle or cows were left untreated (n=8 per group) and were inseminated during oestrus. Pregnancy rate was reduced when oestrus had been induced early. In the main experiment, cows were treated at different days of the cycle with PGF2alpha. (day 6, day 12, control; n=8 per group). During following oestrus and dioestrus, endometrial biopsies were taken and analysed for progesterone and oestrogen receptors and for lectin binding. The percentage of progesterone receptor positive nuclei during the subsequent luteal phase was significantly lower in cows in which oestrus had been induced on day 6 than in both other groups. Oestrogen receptor positive cells did not differ between groups. Out of the five lectins tested, three showed in at least one location differences between groups, indicating changes in uterine secretions after induction of oestrus during the early luteal phase. In conclusion, the time at which the luteal phase is terminated affects uterine progesterone receptor expression. Induction of luteolysis early in the luteal phase thus may negatively affect fertility.