Concentrations of circulating hormones during the interval between pulses of a PGF2α metabolite in mares and heifers

The temporal relationship of several hormones to a metabolite of prostaglandin F2α (PGFM) was studied in mares and heifers from the beginning of the first PGFM pulse during luteolysis to the end of the second pulse. Mares (n = 7) were selected with a 9-h interval between the peaks of the two pulses. In mares, estradiol-17β (estradiol) increased (P < 0.05) within each PGFM pulse and plateaued for a mean of 6 h between the pulses, resulting in a stepwise estradiol increase. Progesterone decreased linearly (P < 0.0001) throughout the intra-pulse and inter-pulse intervals of PGFM. In heifers (n = 6), inter-pulse intervals were variable, and therefore Hours 1–4 of the first pulse (Hour 0 = PGFM peak) and Hours −4 to −1 of the second pulse were used to represent the mean 8-h interval between peaks of the two pulses. Estradiol increased (P < 0.05) during the ascending portion of each PGFM pulse and then decreased (P < 0.05) beginning at Hour −1 of the first PGFM pulse and Hour 0 of the second pulse. The 1-h delay during the second pulse was accompanied by an apparent increase in PRL. A transient decrease in estradiol occurred in individuals between PGFM pulses at a mean of 5 h after the first PGFM peak, concomitant with a transient LH increase (P < 0.05). Results indicated that estradiol plateaued in mares and fluctuated in heifers during the interval between PGFM pulses. Heifers also showed temporal relationships between estradiol and LH and apparently between estradiol and PRL.