LH response diminishes following increasing dosage of estradiol

ABSTRACT

Four - day cyclic female rats were ovariectomized in the morning of day 1 of diestrus and injected on this and the following two days at 09.00 h either with constant or with increasing doses of estradiol benzoate [EB]. The serum concentrations of LH and estradiol - 17 beta were determined 4 h after the last injection. Significant inhibition of LH secretion resulting from three injections of 0. 25 micro g EB/100g b. w. was completely prevented when 0.75 micro g EB/100g b. w. were administered at increasing dosage. Markedly higher serum concentrations of estradiol were found after the administration of 0.05, 0.10 and 0.25 than after three injections of 0.13 micro micro g EB/100 g b. w; but the circulating LH level did not differ between both groups. Even the total amount of 4.0 micro g EB/100 g b. w. did not inhibit LH secretion when increasing dosage was applied. Although preliminary, the data indicate a hitherto unknown action of estrogens in the control of LH secretion which may be of importance for the therapeutic use of these hormones