Estriol- and estradiol-17 beta-induced luteinizing hormone release in ovariectomized cows and ewes

Two studies were conducted with ovariectomized animals to compare luteinizing hormone (LH) responses during the 24-hr period following an IM injection of either estriol or estradiol-17 beta. In Exp. 1, six long-term ovariectomized (6 months) beef cows were randomly assigned to be given 1 mg of estriol or estradiol 17 beta injected in 4 ml of corn oil. Two months later, the experiment was replicated in a switch back design. In Exp. 2, eight ewes that had been ovariectomized for 1 to 4 months were randomly assigned to be given 150 micrograms of estriol or estradiol-17 beta injected in 1.5 ml of corn oil. The initial increase in serum LH concentrations occurred earlier in both cows (P less than .01) and ewes (P less than .05) in response to estriol (8 to 9 hr) than in response to estradiol 17 beta (12 to 18 hr). In addition, duration of the decrease in LH release that occurred 1 hr after estrogen injection in ewes was 3 hr shorter (P less than .05) in response to estriol than in response to estradiol-17 beta. The total area under the LH response curve was greater for both cows (P less than .05) and ewes (P less than .01) given estradiol-17 beta than for those given estriol. Administration of estradiol-17 beta induced a 10-fold increase in serum estradiol-17 beta concentrations and a two to fourfold increase in serum estrone concentrations. Both estrogens returned to basal concentrations by 24 hr after administration. Peak serum estriol exceeded 300 pg/ml within 1 hr of estriol injection, but decreased to nondetectable levels within 12 hours. We concluded that estriol can stimulate LH release from the pituitary. Furthermore, the differential LH response to the two estrogens suggests that the positive effects of estriol are mediated earlier but that estradiol-17 beta is a more potent stimulus when administered as a single IM injection.