Studies on rat ovarian receptors for lutropin (luteinizing hormone). Interaction with beta-subunit of sheep lutropin.

By using radioimmunoassay, the interaction of sheep lutropin (luteinizing hormone, LH) beta-subunit with rat ovarian receptors was investigated. The binding of beta-subunit was specific, although of much lower order than that of lutropin. Sheep lutropin beta-subunit effectively inhibited the binding of human choriogonadotropin (chorionic gonadotropin, gCG) to the ovary, showing that both occupy the same sites. The binding of sheep lutropin beta-subunit to ovary was not followed by any detectable increase in cyclic AMP. The ovarian response to lutropin in terms of cyclic AMP production was inhibited in the presence of free beta-subunit. The alpha-subunit of lutropin, when used at concentrations where contamination with whole lutropin was negligible, enhanced the degree of binding of beta-subunit; this did not lead to increased cyclic AMP in the tissue. Surprisingly, the binding of beta-subunit in vitro was drastically decreased by the prior removal of all endogenous rat lutropin bound to receptors. The implications of these data are discussed in the light of the reported biological activity of the beta-subunit.