Pulses of prolactin before, during, and after luteolysis and synchrony with pulses of a metabolite of prostaglandin F2α in heifers

Pulses of prolactin (PRL) and a metabolite of prostaglandin F2α (PGFM) were determined from hourly blood samples collected before, during, and after luteolysis (n = 7 heifers). Progesterone concentrations were used to partition the results into six 12-h sets from 12 h before to 36 h after luteolysis. Pulses of PRL with a nadir-to-nadir interval of 4.4 ± 0.2 h were detected in each 12-h set. Pulses were rhythmic (P < 0.05) in six heifers, beginning 12 h before the end of luteolysis. The peak of a PRL pulse was greater (P < 0.05) for the 12 h after the end of luteolysis than for other 12-h sets, except for the last set of luteolysis. Area under the curve of a pulse was greater (P < 0.05) for the 24 h that encompassed the end of luteolysis than for two previous 12-h sets. Synchrony between the peaks of PRL and PGFM pulses was greater (P < 0.03) during and after luteolysis (same hour for 29/39 pairs) than before luteolysis (0/12). Concentration of PRL centralized to the peak (Hour 0) of PGFM pulses was greater (P < 0.05) at Hours 0 and 1 than at Hours −2, −1, and 3. Results supported the hypothesis that PRL is secreted in pulses in heifers. The pulses were most prominent and rhythmic during the last 12 h of luteolysis and thereafter. The pulse peaks of PRL and PGFM were synchronized for most PRL pulses during and after luteolysis.