Characterisation of pulses of 13,14-dihydro-15-keto-PGF2alpha (PGFM) and relationships between PGFM pulses and luteal blood flow before, during, and after luteolysis in mares

Blood collections for characterising 13,14-dihydro-15-keto-PGF2alpha (PGFM) pulses in mares and colour-Doppler examinations for estimating percentage of corpus luteum with blood-flow signals were done hourly for a 24-h session on Day 15 (ovulation = Day 0; n = 13 mares) or during 12-h sessions from Days 12 to 16 (n = 10 mares). Luteolysis was defined as extending from the beginning of a precipitous decrease in progesterone until progesterone was < 2 ng mL(-1). Comparisons were made among preluteolysis, luteolysis, and postluteolysis. Greater prostaglandin F2 alpha activity (mean PGFM concentration per session) occurred during luteolysis than during preluteolysis and postluteolysis. Statistically-detected PGFM pulses were smaller during preluteolysis with a highly variable interval from the last pulse to the beginning of luteolysis. Either two or three pulses were detected in each 24-h session during luteolysis and postluteolysis, after excluding three of eight sessions with no pulses during postluteolysis. Statistically, 17% of pulses during postluteolysis were prominent outliers. The nadir-to-nadir interval during a pulse (5 h), the peak-to-peak interval between pulses (9 h), and the resulting 4-h gap between pulses were similar during and after luteolysis. The decrease in progesterone encompassed the PGFM pulses, without a detectable fluctuation during a pulse. The percentage of corpus luteum with blood-flow signals did not change during the ascending portion of a PGFM pulse and decreased within 2 or 3 h after the peak, even during preluteolysis. Results indicated that a reported increase in luteal blood flow in heifers during the ascending portion of a PGFM pulse does not occur in mares.