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Ligand specificity of uterine oestrogen and progesterone receptors in the subadult African elephant, Loxodonta africana

The ligand specificity of progesterone and oestrogen receptors in the uteri of four nonpregnant, nonlactating African elephants, killed during routine culling in the Kruger National Park, were determined. The mean (± SEM) K_d values of the oestrogen (0.18 ± 0.019 x 10^–9 mol l^–1, n = 12) and progesterone (0.22 ± 0.025 x 10^–9 mol l^–1, n = 12) receptors were essentially similar when [³H]promegestone was used as radioligand in the progesterone receptor assays. However, when [³H]progesterone was used as radioligand, the progesterone receptor exhibited a significantly higher K_d value (1.03 ± 0.132 x 10^–9 mol l^–1, n = 12) than that of the oestrogen receptor. The use of the different radioligands did not significantly affect the quantitative values obtained for the progesterone receptor. Both the oestrogen and the progesterone receptors displayed a high ligand specificity. The 5-reduced metabolites of progesterone exhibited a high relative binding affinity for the progesterone receptor 5-pregnane-3,20-dione: relative binding affinity = 43%; 5-pregnane-3-ol-20-one: relative binding affinity = 20%) but the synthetic antiprogestin RU 486 did not compete successfully with progesterone in competitive binding studies. However, norethindrone (relative binding affinity = 293%) competed successfully for binding to the progesterone receptor, and may have some potential in the future development of a technique to control reproductive output in the African elephant.