Regulation of Pituitary Gonadotrophic Secretion: Inhibition by Estrogen or Inactivation by the Ovaries?

Abstract

The control of pituitary gonadotrophic secretion could be either by inhibition by estrogens or inactivation by the ovaries. The traditional explanation of the control of pituitary gonadotrophin secretion is inhibition by estrogen; however, in postmenopausal women, castrated women and [female] castrated rats substitution of physiologic amts. of estrogen fails to suppress gonadotrophins to normal levels. Since inhibition by estrogen is an inadequate explanation, the possibility of ovarian inactivation of gonadotrophins has been investigated. Ovaries of adult [female] rats were transplanted into the spleen. By this operation, the ovaries continue to be stimulated by pituitary gonadotrophins and could inactivate the gonadotrophins if this occurs, whereas the pituitaries are removed from the action of estrogens since the ovaries in their position in the spleen secrete the estrogen into the portal circulation and the estrogens are inactivated by the liver before they reach the general systemic circulation. This is attested to by the general castrate appearance of the vaginal smear, uterus, and thymus of the exptl. animals. The pituitaries of these rats were assayed for their gonadotrophic content and were found to be more similar to normal pituitaries than to castrate pituitaries. Since the removal of estrogen from the systemic circulation did not result in castrate gonadotrophin levels of the pituitaries of the exptl. animals, it is concluded that the ovaries inactivate pituitary gonadotrophins and in this manner serve to control pituitary gonadotrophic content and secretion.