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

Follicular-phase concentrations of progesterone, estradiol-17β, LH, FSH, and a PGF2α metabolite and daily clustering of prolactin pulses, based on hourly blood sampling and hourly detection of ovulation in heifers

Circulating concentrations of hormones were determined each hour in 13 heifers from the end of the luteolytic period to ovulation (follicular phase, 3.5 days). Diameter of the preovulatory follicle was determined every 8 hours, and the time of ovulation was determined hourly. The diameter of the preovulatory follicle decreased 0.8 ± 0.1 mm/h in heifers when there was 1 to 3 hours between the last two diameter measurements before ovulation. The concentration of progesterone (P4) after the end of the luteolytic period (P4 < 1 ng/mL) changed (P < 0.0001), as shown by a continued decrease until Hour -57 (Hour 0 = ovulation), then was maintained at approximately 0.2 ng/mL until 2 hours before the peak of the LH surge at Hour -26, and then a decrease to 0.1 ng/mL along with a decrease in estradiol-17β. Concentrations of LH gradually increased (P < 0.007) and concentrations of FSH gradually decreased (P < 0.0001) after the end of luteolysis until the beginning nadirs of the respective preovulatory surges. A cluster of prolactin (PRL) pulses occurred (P < 0.0001) each day with approximately 24 hours between the maximum value of successive clusters. Hourly concentrations of a PGF2α metabolite decreased (P < 0.007) until Hour -40, but did not differ among hours thereafter. Novel observations included the gradual increase in LH and decrease in FSH until the beginning of the preovulatory surges and follicle diameter decrease a few hours before ovulation. Results supported the following hypotheses: (1) change in the low circulating P4 concentrations during the follicular phase are temporally associated with change in LH concentrations; and (2) PRL pulses occur in a cluster each day during the follicular phase of the estrous cycle.

Circadian influence on the preovulatory LH surge, ovulation, and prolactin concentrations in heifers

A novel circadian study of the effect of clock hours on the preovulatory LH surge, ovulation, and maximal PRL concentration was done in 13 nontreated Holstein heifers. Hourly blood sampling and hourly ultrasound examinations to detect the hour of ovulation began at 8 and 48 hours, respectively, after CL area (cm(2)) had decreased 15% from the area at 15 days postovulation. The resulting experimental period began at the beginning of postluteolysis (progesterone, <1 ng/mL) and encompassed a mean of 3.5 days until ovulation. The frequency of the peak of the preovulatory LH surge for the three 8-hour periods of a 24-hour day was different (P < 0.02) between 2:00 AM to 9:00 AM (N = 9), 10:00 AM to 5:00 PM (N = 3), and 6:00 PM to 1:00 AM (N = 1). The median was 6:00 AM. The frequency of ovulations for 8-hour periods was different (P < 0.02) between 3:00 AM to 10:00 AM (N = 9), 11:00 AM to 6:00 PM (N = 3), and 7:00 PM to 2:00 AM (N = 1). The median was 7:30 AM. Two or three clusters of PRL pulses occurred during the 3.5 days. Based on all available PRL pulse clusters (N = 36), the clock hours of the maximal concentration/cluster was greater (P < 0.0001) for 9:00 AM to 2:00 PM (N = 33 clusters) than for each of the three other 6-hour periods (N = 0, 1, or 2 per period). The median was 11:30 AM. The hypothesis was supported that the peak of the preovulatory LH surge, ovulation, and maximal PRL concentration during pulse clusters occur with greater frequency during certain clock hours in heifers.

Inhibition of prostaglandin biosynthesis during postluteolysis and effects on CL regression, prolactin, and ovulation in heifers

The beginning of postluteolysis (progesterone, <1 ng mL(-1)) in heifers was targeted by using 8 h after ultrasonic detection of a 25% decrease in CL area (cm2) and was designated Hour 0. Flunixin meglumine (FM; n=10) to inhibit PGF2α secretion or vehicle (n=9) were given intramuscularly at Hours 0, 4, 8, 16, 24, 32, and 40. The dose of FM was 2.5 mg/kg at each treatment. Blood sampling and measurement of the CL and dominant follicle were done every 8 h beginning 14 days postovulation in each group. Blood samples for detection of pulses of PRL and pulses of a metabolite of PGF2α (PGFM) were obtained every hour for 24 h beginning at Hour 0. Pulse concentrations of both PGFM and PRL were lower in the FM group than in the vehicle group. Concentration of PRL was greatest at the peak of a PGFM pulse. Neither CL area (cm2) nor progesterone concentration differed between groups during Hours 0 to 48 (postluteolysis). Ovulation occurred in nine of nine heifers in the vehicle group and in three of 10 heifers in the FM group. The anovulatory follicles in the FM group grew to 36.2±2.9 mm, and the wall became thickened from apparent luteinization. The hypothesis that PGF2α was involved in the continued P4 decrease and structural CL regression during postluteolysis was not supported. However, the hypotheses that pulses of PGFM and PRL were temporally related and that systemic FM treatment induced an anovulatory follicle were supported.