A comparison of the LH-releasing activities of LH-RH and its agonistic analogue buserelin in the ovariectomized rat

LH-RH and the potent agonistic analogue (D-Ser(Bu t) 6-des-Gly 10)-LH-RH(1–9)-ethylamide ( (HOE-766 or buserelin) were at several doses either infused or injected intravenously in 5-weeks-ovariectomized rats, which had been treated with either 3 μg estradiol-benzoate (EB) or with oil, 24 h previously. Blood samples for assay of LH were taken during the subsequent 24 h. Potuitary glands were removed at the end of the experiments. Buserelin, when infused, was slightly more effective than LH-RH in releasing LH. When injected, however, buserelin was at the higher dose ranges increasingly more effective as an LH-releasing agent than LH-RH. EB-treatment increased the LH response of the pituitary gland to both peptides in an identical way. It was concluded that buserelin derives its high potency not from its intrinsic LH-releasing activity, which is only slightly greater than that of LH-RH, but from a longer duration of action.