Luteolysis
Mares
PGF2a
PGFM metabolite

Pregnancy

The concentrations of progesterone (P4) and a metabolite of PGF2a (PGFM) in mares were compared between the interovulatory interval (IOI; n 1⁄4 8) and the corresponding days of pregnancy (n 1⁄4 9). In daily blood samples, P4 increased between the day of ovulation (Day 0) and ~Day 6 and then gradually decreased until the beginning of luteolysis in the IOI group. Before the beginning of luteolysis, there were no significant differences in P4 concentrations between the IOI and early pregnancy. In the IOI, PGFM concentration on the day before the beginning of luteolysis began to increase (P < 0.04) and reached a maximum mean (42.9 ± 11.6 pg/mL) on Day 14. In pregnancy, a novel increase in PGFM occurred from Day 12 to a maximum mean on Day 15 (16.7 ± 3.1 pg/mL). Daily PGFM concentrations were not different between the two groups until the increase just before luteolysis in the IOI. During 8-h sessions of hourly blood sampling, the mean and maximum PGFM concentrations were significantly greater in IOI than in pregnancy for each 8-h session on Days 13, 14, and 15. The minimum was not different between groups on any day. Pulses of PGFM were identified by coefficient of variation during the hourly 8-h sessions on day-sets of Days 4e7, 9e11, and 13e16. Despite the PGFM increase in daily samples between Days 12 and 15 of pregnancy, the amplitude and peaks of CV-identified pulses did not differ in the pregnant mares among the three day-sets. The pulses were similarly small for day-sets 4e7 and 9e11 in the IOI and for all day-sets in pregnancy (eg, amplitude on Days 13e16: 43.4 ± 15.6 pg/mL vs 5.4 ± 1.1 pg/mL for IOI vs pregnancy). Hypothesis 1 was not supported that daily PGFM concentrations in an IOI increase at the intersection between the end of the rapid P4 increase and the gradual P4 decrease. Hypothesis 2 was supported that pregnant mares have low amplitude PGFM pulses during the days of the high amplitude pulses at luteolysis in the IOI.