Temporality of ovarian steroids and LH/FSH pulse profiles encompassing selection of the dominant follicle in heifers†.

The tested hypotheses were (1) LH/FSH pulses and F2 diameter are diminished by P4 and, (2) E2 increases during the transition to deviation and alters LH/FSH pulses. On Day 5 (Day 0 = ovulation), heifers were randomized into an untreated group (HiP4, n = 11), and a prostaglandin analog treated group (NoP4, n = 10). On Day 6, a follicular wave was induced by follicle ablation. Ultrasound and blood collections were performed every 12 h from Days 7 to 11. Blood was collected every 15 min for 10 h on Day 9 (largest follicle expected to be ~7.5 mm). Estradiol was ~75% greater (0.36 ± 0.14 vs 0.63 ± 0.19 pg/mL) in heifers with F1 ≥ 7.2 mm than in heifers with F1 < 7.2 mm. The HiP4 had smaller second largest follicle (F2) diameter, lower estradiol (P = 0.06), LH pulse baseline and peak concentrations (P < 0.007), in addition to half the frequency of LH/FSH pulses (4.1 ± 0.3 vs 9.6 ± 0.7 in 10 h) than the NoP4. Within HiP4, heifers with F1 ≥ 7.2 mm had ~25% fewer (P = 0.03) LH pulses compared to heifers with F1 < 7.2 mm. In contrast, within the NoP4, heifers with F1 ≥ 7.2 mm had ~75% greater LH (P = 0.05) and FSH (P = 0.08) pulse amplitude. We propose that greater F2 diameter at deviation in low P4 is related to greater LH baseline and peak concentrations, and greater frequency of LH/FSH pulses. A greater increase in E2 after F1 reaches ~7.2 mm results in further stimulation of LH/FSH pulse amplitude. Elevated P4 not only diminished frequency of LH/FSH pulses but also converted an E2 increase into a negative feedback effect on LH/FSH pulse frequency leading to smaller F2 at deviation.