Temporal interrelationships at 15-min intervals among oxytocin, LH, and progesterone during a pulse of a prostaglandin F2α metabolite in heifers

A pulse of a PGF2α metabolite (PGFM) was induced by treatment with 0.1 mg of estradiol-17β on Day 15 (Day 0 = ovulation; n = 9 heifers). Blood samples were taken every 15 min for 9 h beginning at treatment (Hour 0). For PGFM and LH, an intraassay-CV method was used to detect fluctuations in the 15-min samples and pulses in the hourly samples. A mean of 6.9 ± 0.4 PGFM fluctuations/9 h were superimposed on the hourly PGFM concentrations, compared to 2.1 ± 0.5 LH fluctuations/9 h (P < 0.02). An increase (P < 0.02) in oxytocin began 15 min before the beginning nadir of the PGFM pulse. A transient increase in progesterone did not occur at the beginning nadir of the PGFM pulse. Progesterone decreased (P < 0.02) during the ascending portion and increased (P < 0.03) as a rebound during the descending portion of the PGFM pulse. The peak of an LH pulse occurred 1.5 ± 0.4 h (range, 0.25–2.75 h) after the peak of the PGFM pulse. The wide range in the interval from a PGFM peak to an LH peak obscured the contribution of increasing LH to the rebound. The results did not support the hypothesis that oxytocin and PGFM increase concurrently. Results supported the hypothesis that the immediate transient progesterone increase that has been demonstrated with exogenous PGF2α does not occur during the ascending portion of an endogenous PGFM pulse. The hypothesis that the progesterone rebound after the peak of a PGFM pulse is temporally related to an LH pulse was supported.