Evidence that the subunit structure of gonadotropin receptor is preserved during regression of rat corpus luteum.

The level of hCG/LH receptor has been shown to undergo marked changes during the life span of rat corpus luteum. To evaluate whether these fluctuations are due to changes in the receptor subunit structure or receptor protein content, the 125I-hCG binding activity and the receptor subunit structure were determined during different time periods of pseudopregnancy. The maximum 125I-hCG binding activity was observed on day 7, after which it decreased by 20 and 45% on day 11 and day 14, respectively. The Scatchard analysis of 125I-hCG binding data showed that the decrease in binding activity was caused by a change in the number of binding sites rather than a change in the binding affinity. The LH/hCG receptor in ovarian membranes obtained on days 7, 11 and 14 were then characterized by the method of affinity cross-linking. All four subunits of the LH/hCG receptor were detected in the ovarian membranes at all stages while the intensity decreased parallel to a decrease in hCG binding from day 7 to day 14. These results suggest that the decrease in 125I-hCG binding activity in rat ovarian membranes from day 7 to day 14 of pseudopregnancy is due to a decrease in receptor concentration rather than a change in the receptor subunit structure.