Abstract:
The present study is directed at obtaining information on the interaction between uterine steroid receptors and circulating steroids. Furthermore the study aimed at quantifying the affinity of these receptors for possible contraceptive agents that could be applied to constrain individual reproductive output. Scatchard analysis showed that both progesterone and oestrogen receptors had high binding affinities for their appropriate ligands. The binding affinity of the oestrogen receptor decreased with progressing pregnancy while that of the progesterone receptor showed no definite pattern. The concentrations of the two types of receptors were positively correlated and decreased with advancing pregnancy. The down-regulation of the progesterone receptor could possibly be attributed to an increase in the circulating levels of both 5α-pregnane-3,20-dione and progesterone. In the pregnant animals the uterus horn containing the foetus had a lower receptor concentration than the corresponding uterus horn that was not in contact with the developing foetus. Both the oestrogen and progesterone receptors displayed high levels of specificity for their natural ligands. The relatively high binding affinity of 5α-pregnane-3 ,20-dione for its appropriate receptor, indicates that this hormone may be of biological importance to the maintenance of pregnancy in this species and in this regard needs :further investigation. Competitive binding assays showed that mifepristone (RU 486) could not be used to terminate pregnancy in the African elephant as the progesterone receptor had a very low affinity for this steroid. Norethindrone and levonorgestrel may, however, be of potential use when considering their relatively high affinity to the uterine progesterone receptor.
