Evidence for rapid loss of spare hCG receptors in the corpora lutea of the hypophysectomized rat.

In pregnant mare serum gonadotropin treated immature rats hypophysectomized on the day of ovulation (day 1) the corpora lutea (CL) persist as normal morphological structures and produce steroids, especially 20 alpha-dihydroprogesterone (20 alpha-DHP), for at least 40 days (Taya and Greenwald, 1982), although there is a rapid decline in human chorionic gonadotropin (hCG) binding sites (Kim and Greenwald, 1984). In this study the number of occupied and non-occupied hCG receptors in CL from hypophysectomized rats decreased to 14% and 36%, respectively, compared to intact day 5 pseudopregnant animals, but the binding affinity was unchanged. Decreased concentration of occupied hCG receptors paralleled hormonal levels of serum luteinizing hormone (LH) which were measurable in only 9 out of 20 animals and were near the lower limits of the assay (40 pg/ml). Luteal progesterone (P4) production in response to LH was markedly decreased after hypophysectomy, but the maximal P4 response to LH and 20 alpha-DHP production in response to LH and 8-Br-cAMP were the same in hypophysectomized and intact animals. Although hCG receptor concentration in CL decreased significantly after hypophysectomy, LH-stimulated luteal production of cAMP was almost the same in both groups. These results indicate that LH spare receptors which are uncoupled from cAMP exist in the CL of the intact pseudopregnant rat and that after hypophysectomy they quickly disappear within 4 days.