Effects of Ovariectomy and Steroid Treatment on Hypophyseal Sensitivity to Purified LH-Releasing Factor (LRF)

Abstract

The sensitivity of the pituitary gland to partially purified LRF was investigated both in vivo and in vitro. LRF produced an increasein, plasma LH but not FSH in vivo, but it increased the release of both hormones from anteriorpituitaries incubated in vitro. Ovariectomy appeared to lower sensitivity below that encountered in proestrouS animals, but treatment of ovariectomized animals with 50 μg of estradiol benzoate subcutaneously 3 days before greatly enhanced the sensitivity to LRF both in vivo and in vitro. Treatment of ovariectomized animalswith 25 mg of progesterone in the presence or absence of estrogen failed to modify the responsiveness of the glands. Sensitization of the glands by estrogen was associated with a decline inplasma levels, but no alteration in pituitary contentof LH. Hypothalamic gonadotropin-releasingactivity was reduced by estrogen and the ratio of FSH to LH-releasing activity was reduced suggestingthat estrogen preferentially reduces synthesisand/or release of FSH-releasing factor. It is concluded that the estrogen-primed rat is ahighly sensitive assay animal for LRF. Since LHrelease of the ovariectomized rats was inhibited byestrogen in the face of heightened pituitary sensitivity to LRF, it is concluded that estrogen feedsback at least in part at the hypothalamic level toblock LH release in ovariectomized females. (Endocrinology94: 518, 1974)