Differential Action of Small Doses of Estradiol on Gonadotrophins in the Rat1

Abstract

The effect of small doses of estradiol on the changes in pituitary and serum follicle-stimulating hormone (FSH) and luteinizing hormone (LH, ICSH) following ovariectomy in young rats was evaluated. Quantitative and specific bio-assay methods (the HCG-augmentation assay of Steelman and Pohley for FSH and the ovarian ascorbic acid depletion assay of Parlow for LH), as well as a “total gonadotrophin” bio-assay (ovarian weight method) were used. In daily doses of 0.4 or 0.1μg, or less, administered during a period of 16 days immediately after ovariectomy, estradiol inhibited the postovariectomy increase in pituitary and serum LH concentration. However, at these same doses estradiol failed to inhibit the postovariectomy elevation in pituitary FSH. At a higher daily dose of 2.0 μg, estradiol exhibited a significant inhibitory effect on the elevation in both pituitary and serum FSH. These data indicate that LH is far more responsive than FSH to the inhibitory action of physiological doses of estradiol. The “total gonadotrophin” bio-assay detected the inhibitory effect of estradiol only at a daily dose of 0.4 μg of this steroid. Therefore, this method was less discriminating quantitatively than the more specific bio-assay methods. These results may contribute to an explanation of some conclusions of others on estrogengonadotrophin interrelationships, obtained by means of nonspecific or “total gonadotrophin” end-points. (Endocrinology75: 1, 1964)