Circulating hormone concentrations within a pulse of a metabolite of prostaglandin F2α during preluteolysis and early luteolysis in heifers

Plasma from hourly blood samples from two previous studies in heifers was assayed for hormones that were not considered in the previous reports. The objective was to determine the intrapulse temporal relationships between a prostaglandin F2α metabolite (PGFM) pulse and various hormones during preluteolysis and luteolysis. Hormone concentrations were centralized to the peak of a PGFM pulse (Hour 0) and evaluated from Hours −3 to 3. Experiment 1 (n = 6) was done during early luteolysis. Progesterone decreased during Hours −3 to 0 and then rebounded, but did not return to prepulse concentrations, and concentration of LH increased between Hours −1 and 2. In Experiment 2 (n = 7), comparisons were made between the last PGFM pulse of preluteolysis and the first pulse of luteolysis. Intrapulse concentrations of LH increased rapidly between Hours 0 and 1 during preluteolysis and gradually between Hours −2 and 2 during luteolysis. Intrapulse differences in cortisol among hours were not significant for preluteolysis and approached significance (P < 0.06) during luteolysis, owing primarily to an apparent increase between Hours −2 and 1. Oxytocin concentrations showed only an hour effect (P < 0.0003) from to an increase between Hours −2 and 0 and a decrease between Hours 0 and 2. Results indicated that oxytocin and PGFM concentrations increased and decreased concomitantly and supported the hypothesis that the reported more prominent rebound in progesterone during the descending portion of a PGFM pulse during preluteolysis than during luteolysis involves a greater transient increase in LH.