Progesterone does not inhibit the increase in pituitary content of luteinizing hormone after removal of estradiol in the ewe

Abstract

Abstract. During late gestation in the ewe, the pituitary content of LH is reduced by about 95%, presumably due to the presence of high concentrations of ovarian steroids. The aim of this study was to determine whether the pituitary content of LH in the ewe can increase after long-term administration of ovarian steroids, when only estradiol (E) is removed or if both E and progesterone (P) must be withdrawn to allow synthesis of LH to occur. Ten ovariectomized ewes were treated with implants containing E and P. After 3 weeks of treatment, the E implants were removed from 5 ewes (–E+P) and both steroid implants were removed from the remaining 5 ewes (−E−P). Five ovariectomized ewes received P implants at the beginning of the experiment and these implants were left in place for the duration of the study; 5 ovariectomized ewes served as controls (C). All animals were injected with 100 μg GnRH iv 3, 6 and 9 weeks after the initiation of treatment. The area under the LH-response curve was used as an indication of the pituitary content of LH. All steroid treatments markedly reduced basal levels of LH. LH levels increased only in −E−P ewes, beginning 6 weeks after initiation of the study. After 3 weeks, −E+P and −E−P ewes released less LH (P < 0.05) in response to GnRH than did C ewes, whereas P animals did not differ from controls. LH release in response to GnRH in −E+P and −E−P groups had increased by 6 and 9 weeks and was not different from that of C ewes. After 9 weeks, LH release in P ewes was reduced (P < 0.05) compared with C ewes. These data suggest that, after the pituitary content of LH has been suppressed by ovarian steroids, the presence of P alone does not inhibit replenishment of the pituitary content of LH.