Human corpus luteum: luteinizing hormone and chorionic gonadotropin receptors during the menstrual cycle

To characterize and determine the concentration of LH/hCG receptors in human corpora lutea of the menstrual cycle, we measured occupied and unoccupied receptors and determined the association (Ka) and dissociation (Kd) constants individually in 23 corpora lutea (CL) and 4 corpora albicantia obtained at the time of tubal ligation from 25 normal cycling women. We found no [125I]hCG binding in any of the corpora albicantia. Scatchard plot analysis for each CL revealed a linear binding plot indicative of a single set of LH/hCG receptors. The mean concentration of unoccupied receptors was 36 +/- 10 (+/- SE) fmol/mg protein in the early luteal phase (days 15-19; n = 5), 64 +/- 11 fmol/mg protein in the midluteal phase (days 20-25; n = 13), and 42 +/- 19 fmol/mg protein in the late luteal phase (days 26-30; n = 5). The concentrations of occupied receptors were 56 +/- 8, 46 +/- 6, and 54 +/- 12 fmol/mg protein in the early, mid-, and late luteal phases, respectively. Total (occupied plus unoccupied) receptor concentrations reached maximum levels of 110 +/- 11 fmol/mg protein in the midluteal phase. Ka increased progressively from 12 +/- 4 X 10(9) mol/L-1 in the early luteal phase to 19 +/- 7 X 10(9) and 21 +/- 8 X 10(9) mol/L-1 in the mid- and late luteal phases. We conclude that in normal CL, 1) total and unoccupied LH/hCG receptor levels parallel progesterone secretion; 2) changes in the binding affinity may be important in sustaining and/or rescuing the CL; and 3) loss of LH/hCG receptors is probably related to luteolysis.