Luteinizing Hormone Receptors in Human Ovarian Follicles and Corpora Lutea during Menstrual Cycle and Pregnancy*

Abstract

Specific high affinity, low capacity binding of [^l25I]iodo-hCG was demonstrated in homogenates of isolated granulosa cells and corpora lutea of human ovary throughout the menstrual cycle and pregnancy. The binding of radiolabeled hCG to the receptor was time, temperature, ion concentration, and pH dependent. The treatment of ovarian membrane particles with trypsin decreased binding by about 75%, whereas treatment and neuraminidase resulted in a 400% increase in binding. These results suggest a protein nature of the receptor and that a portion of the receptors may be masked in the membrane by sialic acid. The receptor displayed high specificity for hCG and human LH, indicating that they share the same receptor site in the human ovary. The receptor bound hCG with high affinity. The apparent equilibrium association constant for hCG was 2.2 × 10⁹ M^-1 The receptor concentration increased from the early follicular phase to ovulation by about 1.6-fold. After an apparent transient fall associated with the ovulatory period, the receptor concentration increased further by about 4-fold by day 17 of the cycle and thereafter remained virutally unchanged until the end of the cycle. Homogenates of corpora lutea obtained during the follicuar phase aso bound specifically radiolabeled hCG, suggesting that the regression of the corporalutea is not primarily due to the loss of receptors for human LH. The receptor concentration measured in corpora lutea of early pregnancy was clearly lower than that in corpora lutea of the menstrual cycle. This is most likely due to occupation and downregulation of the receptors by high serum hCG. The number of receptors in corpora lutea tended to increase towards the end of pregnancy. Our results indicate that the maturation and subsequent luteinization of human ovarian follicles is associated with a biphasis increase in the number of LH (hCG) receptors in granulosa cells. The changes in the concentration of available receptor may have a determining role in the regulation of follicular and corpus luteum development and function. (J Clin Endocrinol Metab108: 307, 1981)