Increasing Concentrations of 17β-Estradiol Has Differential Effects on Secretion of Luteinizing Hormone and Follicle-Stimulating Hormone and Amounts of mRNA for Gonadotropin Subunits during the Follicular Phase of the Bovine Estrous Cycle

Abstract

The hypothesis tested was that 17β-estradiol (E2) would increase amounts of mRNA for α, LHβ, and FSHβ subunits during the follicular phase of the estrous cycle prior to the preovulatory surge of gonadotropins in cows. On Day 16 (Day 0 = estrus) of the estrous cycle, all cows were treated with prostaglandin F2α (PGF2α). Cows served as intact controls (CONT, n = 4) were ovariectomized (OVX, n = 5), or were ovariectomized and administered E2 (OVXE, n = 6) in increasing doses starting at the time of treatment with PGF2α. Cows were bled for 6 h before and for 40 h after PGF2α treatment to characterize pulsatile secretion of LH and FSH. Forty hours after PGF2α treatment, pituitaries were collected for evaluation of amounts of mRNA for α, LHβ, and FSHβ subunits. Amplitude of LH pulses was greater (p \u3c 0.05) in cows from the OVXE than from the CONT group. Concentrations of FSH were greater in cows from both the OVXE and OVX (p \u3c 0.01) groups than from the CONT group. Amounts of mRNA for α and FSHβ subunits were greater (p \u3c 0.01) in pituitaries of cows from the OVX than from the CONT or OVXE groups. Amounts of mRNA for LHP subunit in pituitaries of cows from the OVX group tended to be greater (p \u3c 0.08) than from the CONT group. Cows in the OVXE group tended (p \u3c 0.08) to have greater amounts of mRNA for FSHβ subunit than did CONT cows. Amounts of mRNA for α and LHβ subunits in cows from the OVXE and CONT groups did not differ (p \u3e 0.10). Pituitary weight and content of LH and FSH were not different (p \u3e 0.10) among cows of the different groups. Ovariectomy resulted in enhanced secretion of gonadotropins and increased amounts of mRNA for gonadotropin subunits above values detected in CONT cows. Ovariectomized cows administered E2, in follicular-phase patterns had amounts of mRNA for a and LHβ similar to those in CONT cows even though secretion of LH was enhanced in the OVXE group. We reject our hypothesis and conclude that E2 has a divergent role in regulation of gonadotropins. Release of LH is enhanced by E2, but E2 reduced mRNA for gonadotropin subunits (tended to reduce mRNA for FSHβ subunit) in ovariectomized cows to amounts detected in intact cows during the follicular phase of the bovine estrous cycle