Interrelationships among progesterone, LH, and luteal blood flow during a pulse of a PGF2α metabolite and functional role of LH in the progesterone rebound in heifers

On Day 16 (Day 0 = ovulation) or before the expected transition into the luteolytic period, heifers were not treated (control group, N = 7) or were treated with a single 0.1-mg dose of estradiol (E2) (E2 group, N = 6) or E2 combined with the GnRH antagonist acyline (E2/Ac group, N = 5). Hourly blood samples were collected from hour of treatment (Hour 0) to Hour 20. Estradiol induced a pulse of PGFM, but the peak of the pulse occurred 2 hours later (P < 0.05) and mean PGFM concentrations during the descending portion of the pulse were lower (P < 0.05) in the E2/Ac group than in the E2 group. In the E2 group, concentration of progesterone (P4) decreased (P < 0.05) during the ascending portion of the PGFM pulse, and increased (rebounded; P < 0.05) along with an LH increase during the descending portion. In the E2/Ac group, a rebound in P4 and an increase in LH were not detected during the descending portion of the PGFM pulse. The percentage of CL with color Doppler signals of blood flow increased (P < 0.04) concurrently with the PGFM increase during Hours 0 to 5 and during the ascending portion of the PGFM pulse. Blood flow and PGFM decreased concurrently. The following hypotheses were supported: (1) LH has a positive effect on PGFM pulses; (2) the rebound in P4 and the increase in LH during the descending portion of a PGFM pulse are functionally related; and (3) the increase in luteal blood flow in association with a PGFM pulse represents a direct effect of PGF2α rather than an effect of P4 or LH.