Pulses of the prostaglandin F2alpha (PGF) metabolite 13,14-dihydro-15-keto-PGF (PGFM) were compared among heifers that were in the preluteolytic, luteolytic, and postluteolytic periods (n = 7 or 8 heifers/period). Hourly blood sampling was done in 18-h sessions 15, 16, or 17 days after ovulation. Hourly sampling and statistical identification of a PGFM pulse allowed novel comparisons of PGFM pulses among the three periods. Each period had a similar number of PGFM pulses (2.3 ± 0.2). The pulses were more prominent during the luteolytic period than during the other periods, as indicated by significantly greater concentration for the peak and amplitude between nadir and peak. Significantly more fluctuations that did not meet the definition of a pulse occurred at the beginning of the preluteolytic period and end of the postluteolytic period than during the luteolytic period. The same nadir ended a pulse and began the next pulse in 85% of adjacent pulses. Seven heifers were selected objectively, based on a progesterone concentration >5 ng/ml at Hour −3 (Hour 0 = peak of PGFM pulse) and a progressive decrease in progesterone from Hours −3 to 0. Progesterone increased (P < 0.03) between Hours 0 and 1, remained at a mean plateau at Hours 1 and 2, and then decreased. Results support the hypothesis of a transient intrapulse rebound in progesterone during an individual PGFM pulse, but only during the first portion of luteolysis. These findings should be considered in future proposals on the mechanisms involved in the effects of PGF on progesterone concentrations.
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Vol. 82 • No. 6