In ovariectomized heifers, oestradiol decreases concentrations of LH in serum for approximately 12 h after which LH is released in a surge comparable in size and duration to the preovulatory surge. Using this model, we measured LH release induced by LH releasing hormone (LH-RH) from pituitary explants taken from ovariectomized heifers before or after an oestradiol-induced LH surge. These changes were related to changes in LH concentrations in serum and pituitary glands and hypothalamic LH-RH content.
Twenty Holstein heifers were randomly assigned to one of four treatment groups to be killed 0, 6, 12, or 24 h after the injection of 500 μg oestradiol-17β. Jugular blood was collected at −2, −1 and 0 h then at intervals of 2 h until slaughter. Pituitary glands were collected and ≃2 mm3 explants were exposed to 4 ng LH-RH/ml medium for 30 min (superfusion) or 4 ng LH-RH/ml medium for 2 h in Erlenmeyer flasks. Levels of LH were measured in the medium. Hypothalami, collected at autopsy, were assayed for LH-RH content. To determine pituitary LH content, an additional 15 ovariectomized heifers were killed, five each at 0, 12 and 24 h after the injection of 500 μg oestradiol. In both groups of heifers, oestradiol reduced serum LH concentrations to ≃ 1 ng/ml, a level which persisted for 12 h, when LH was released in a surge. Pituitary sensitivity to LH-RH was increased at 6 and 12 h after the injection of oestradiol, but was markedly decreased at 24 h, i.e. after the LH surge. Despite this twofold increase in capacity of the pituitary gland to release LH in response to LH-RH, pituitary LH content did not change during 12 h after oestradiol treatment. However, LH content decreased after the LH surge and this decrease was associated with a decrease in pituitary responsiveness to LH-RH. Hypothalamic LH-RH content was not altered by these treatments.
We have interpreted our results as evidence that oestradiol exerts a positive feedback effect on the pituitary gland of ovariectomized heifers such that pituitary sensitivity to LH-RH is increased twofold by the time the LH surge is initiated. In addition, oestradiol causes a transitory inhibition of LH-RH release as shown by the fact that serum LH concentrations remained low during the interval from injection of oestradiol until the beginning of the LH surge despite the fact that pituitary sensitivity to LH-RH is increased at this time. Depletion of a readily releasable pool of pituitary LH may be the mechanism by which the LH surge is terminated.
Studies on the Measurements of LH-releasing Hormone (LH-RH) by Radioimmunoassay (RIA)