Changes in PGF2alpha secretion during prolonged luteal phase in mares.

: The aim of this study was to characterize changes in PGF2alpha secretion in mares with persistent corpora lutea that were induced by administering altrenogest during oestrus. In Expt 1, PGF2alpha secretion was compared among mares undergoing normal oestrous cycles (n=7) and mares undergoing prolonged luteal phases (n=6), using the mean 15-ketodihydro-PGF2alpha (PGFM) plasma concentrations, peak PGFM concentrations and number of PGFM surges each day, from day 12 to day 16 of the luteal phase. In Expt 2, oxytocin-induced PGF2alpha secretion was characterized on days 13 and 16 of the luteal phase in mares undergoing normal oestrous cycles (n=6) and in mares undergoing prolonged luteal phases (n=7) by comparing the oxytocin-induced increase in PGFM concentration and total PGF2alpha secretion. In Expt 1, mean PGFM concentrations, peak PGFM concentrations and number of PGFM surges per day were significantly lower in mares undergoing prolonged luteal phases than in mares undergoing normal luteal phases. In Expt 2, the area under the curve for PGFM ng (90 min)(-1) was similar for both groups on day 13 but was significantly lower on day 16 in mares undergoing prolonged luteal phases than in mares undergoing normal luteal phases. No change in total PGF2alpha secretion was observed between day 13 and 16 for mares undergoing normal luteal phases, but a significant decrease was observed from day 13 to day 16 in mares undergoing prolonged luteal phases. On days 13 and 16, the increase in PGFM concentration 5 min after oxytocin administration was significantly higher in mares undergoing normal luteal phases than in mares undergoing prolonged luteal phases. The increase in PGFM concentration 5 min after oxytocin administration was similar on days 13 and 16 for mares undergoing normal luteal phases, but tended to be less on day 16 in mares undergoing prolonged luteal phases. These results indicate that failure of luteolysis in mares undergoing induced prolonged luteal phases is due to decreased uterine sensitivity to oxytocin stimulation or decreased uterine ability to secrete prostaglandin.