Specific binding sites for gonadotrophin-releasing hormone, LH/chorionic gonadotrophin, low-density lipoprotein, prolactin and FSH in homogenates of human corpus luteum. II: Concentrations throughout the luteal phase of the menstrual cycle and early pregnancy.

Corpora lutea were obtained from 52 women undergoing laparotomy during the luteal phase of the menstrual cycle. In addition, stromal, thecal and granulosa cell preparations were obtained from seven women undergoing ovariectomy during the late follicular-preovulatory phase of the cycle. The specific binding of a 125I-labelled gonadotrophin-releasing hormone (GnRH) agonist [D-Ser(But)6] GnRH(1-9)-ethylamide (buserelin) and of human chorionic gonadotrophin (hCG), human FSH (hFSH), human prolactin (hPRL) and human low-density lipoprotein (hLDL) to tissue homogenates was measured under optimal conditions. Bound LH/hCG was estimated by elution with acid-citrate buffer, followed by radioimmunoassay of released hormone. Binding of GnRH agonist, though variable, was highest in mid-luteal corpus luteum and high binding was also present in three out of four corpora lutea of pregnancy. Binding of LH/hCG increased significantly with luteinization, reaching maximal levels in the mid-luteal phase before falling significantly. Occupancy of LH receptors by bound LH was relatively constant throughout the luteal phase (10.7-35.3%), but occupancy increased to greater than 90% in corpora lutea from early pregnancy. Binding of hFSH was variable, with only five out of 50 corpora lutea having binding greater than 10 pg/micrograms DNA. Similarly, hPRL binding varied markedly with only six out of 44 having binding greater than 50 pg/micrograms DNA. Binding of LDL was highest in the early- to mid-luteal phases of the cycle. In corpora lutea from all stages of the menstrual cycle (excluding corpora albicantia), GnRH agonist binding was highly correlated with the levels of unoccupied and occupied LH receptors (P less than 0.001; n = 49 and n = 48 respectively) and with LDL receptors (P less than 0.002; n = 49). Binding of GnRH agonist was also correlated with PRL binding (P less than 0.05; n = 21) but not with FSH receptors (P greater than 0.4; n = 25). In addition, LDL binding was associated with PRL (P less than 0.005; n = 21) and FSH receptors (P less than 0.05; n = 25) and with endogenously bound LH (P less than 0.03; n = 48), but not with unoccupied LH receptors (P = 0.8; n = 49). Moreover, in corpora lutea from the mid-luteal phase, there was a strong association between GnRH agonist binding and LDL receptors (P less than 0.02; n = 23). The correlations between GnRH agonist binding and a number of important indices of luteal function suggest a physiological role for GnRH-like factors in the human corpus luteum.