Differential regulation of gonadotropin synthesis and release in ovariectomized ewes after treatment with a luteinizing hormone-releasing hormone antagonist

Our working hypothesis was that synthesis and release of LH, but not FSH, were solely dependent on LHRH. Twenty ovariectomized (OVX) ewes were randomly assigned to one of five treatments (n = 4 per group). Ewes were administered a low (10 micrograms/kg) or high (100 micrograms/kg) dose of LHRH antagonist (LHRH-Ant) at 24-h intervals for 3 or 6 days. Control ewes received vehicle (5% mannitol) at 24-h intervaLs for 6 days. Blood samples were collected every 15 min for 4 h before LHRH-Ant or vehicle and every 2 h during the period of treatment to determine concentrations of LH and FSH. Twenty-four hours after the last treatment with LHRH-Ant or vehicle, anterior pituitaries were collected and divided in half along the midsagittal plane; the number of receptors for LHRH, pituitary content of LH and FSH, and relative amounts of mRNA for aalpha, LHbeta, and FSHbeta subunits were determined. Concentrations of LH in serum decreased (p 0.05) from 25.4 +/- 4.3 ng/ml before LHRH-Ant to less than 0.5 ng/ml within 4 h after the first treatment of LHRH-Ant and remained low ( 0.5 ng/ml) throughout the study. Serum concentrations of FSH declined gradually during the 3- or 6-day period of treatment with LHRH-Ant, from 37.3 +/- 2.4 and 26.5 +/- 4.8 ng/ml to 19.9 +/- 1.8 and 13.7 +/- 2.1 ng/ml, respectively. The magnitude of decline in serum concentrations of LH and FSH did not differ among ewes treated with low or high doses of LHRH-Ant. Pituitary content of LH was not different (p 0.10) from that in controls, whereas pituitary content of FSH was greater (p 0.01) in control ewes compared to ewes treated with LHRH-Ant. Receptors for LHRH were nondetectable ( 0.018 X 10(-16) mol receptor/micrograms protein) in pituitaries after 3 or 6 days of treatment with LHRH-Ant (low or high dose). Relative amounts of mRNA for alpha, LHbeta, and FSHbeta subunits were lower (p 0.01) after 6 days of treatment with LHRH-Ant (low or high dose) than after 3 days of treatment with LHRH-Ant