Induction of PGFM pulses and luteolysis by sequential estradiol-17β treatments in heifers

The effects of sequential induction of PGFM pulses by estradiol-17β (E2) on prominence of PGFM pulses and progesterone (P4) concentration were studied in heifers. Three treatments of vehicle (n = 12) or E2 (n = 12) at doses of 0.05 or 0.1 mg were given at 12-h intervals beginning on Day 15 postovulation. Blood samples were collected every 12 h from Days 13–24 and hourly for 12 h after the first and third treatments. On Day 15, all heifers were in preluteolysis and on Day 16 were in preluteolysis in the vehicle-treated heifers (n = 11) and either preluteolysis (n = 4) or luteolysis (n = 8) in the E2-treated heifers. Peak concentration of induced PGFM pulses during preluteolysis on Day 15 was greater (P < 0.04) than for pulses during preluteolysis on Day 16. The interval from ovulation to the beginning of luteolysis was shorter (P < 0.04) in the E2-treated heifers than in the vehicle-treated heifers. An E2-induced PGFM pulse was less prominent (P < 0.008) in heifers in temporal association with a transient resurgence in P4 than in heifers with a progressive P4 decrease. The hypothesis that repeated E2 exposure stimulates increasing prominence of PGFM pulses was not supported. Instead, repeated exposure reduced the prominence of PGFM pulses, in contrast to the stimulation from the first E2 treatment. Reduced prominence of a PGF2α pulse during luteolysis can lead to a transient resurgence in P4 concentration.