Dynamic progesterone responses to simulation of a natural pulse of a metabolite of prostaglandin F2α in heifers

The effects of prostaglandin F2α (PGF) on circulating progesterone concentration were studied in four groups (n = 4) of Holstein heifers 9 d after ovulation. The progesterone response to simulation of a pulse of 13,14-dihydro-15-keto-PGF (PGFM) by a 2-h intrauterine (IU) infusion of 0.5 mg of PGF was compared with the response to a PGF-bolus IU injection of 4 mg. The beginning of infusion and time of injection were designated Minute 0. Progesterone concentration did not change significantly between Minute 0 and Hour 48 in control or IU vehicle-treated groups. In the PGF-bolus group, progesterone concentration increased (P < 0.05) between Minutes 0 and 10 and then decreased. In the PGF-infusion group, simulation of a PGFM pulse was not associated with an initial transient increase in progesterone. The first significant decrease (P < 0.05) in progesterone began at Minute 20 and continued until Hour 1. The progesterone concentration then began to rebound (P < 0.05) at Hour 1 and peaked at Hour 3 at almost the same concentration as at the start of PGF infusion. The progesterone again decreased after Hour 3 and increased again between Hours 24 and 48. In summary: (1) an initial transient increase in progesterone was not detected in association with an individual simulated pulse of PGF, indicating that the frequently reported pronounced transient increase after a bolus luteolytic dose of PGF is a nonphysiological response and (2) simulation of a PGFM pulse resulted in a distinct transient rebound in progesterone beginning at Hour 1 of the PGF infusion.