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 F SH, were solely dependent on LHRH. Twenty ovariectomized (OVX) ewes we re randomly assigned to one of five treatments (n = 4 per group). Ewes were administered a low (10 mu g/kg) or high (100 mu g/kg) dose of LH RH antagonist (LHRH-Ant) at 24-h intervals for 3 or 6 days. Control ew es 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 concentrati ons of LH and FSH. Twenty-four hours after the last treatment with LHR H-Ant or vehicle, anterior pituitaries were collected and divided in h alf along the midsagittal plane; the number of receptors for LHRH, pit uitary content of LH and FSH, and relative amounts of mRNA for (alpha, LH beta, and FSH beta 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-An t and remained low (< 0.5 ng/ml) throughout the study. Serum concentra tions of FSH declined gradually during the 3- or 6-day period of treat ment 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 treate d 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(-1 6) mol receptor/mu g protein) in pituitaries after 3 or 6 days of trea tment with LHRH-Ant (low or high dose). Relative amounts of mRNA for a lpha, LH beta, and FSH beta subunits were lower (P < 0.01) after 6 day s of treatment with LHRH-Ant (low or high dose) than after 3 days of t reatment with LHRH-Ant (low or high dose). The LHRH was, therefore, re quired to maintain steady state amounts of mRNA for FSH and LH and to maintain pituitary stores of FSH but not LH. Our data support the hypo thesis that differential regulation of LH and FSH release occurs in ew es. While synthesis and release of LH are dependent on LHRH, synthesis but not release of FSH appears to be dependent on LHRH.