Hormonal regulation of testicular luteinizing hormone receptors. Effects on cyclic AMP and testosterone responses in isolated Leydig cells.

Hormone-induced regulation of luteinizing hormone (LH) receptors and desensitization of target cell responses were studied in purified Leydig cells prepared from testicular interstitial tissue of rats treated with 0.2, 1, and 10 pg of human chorionic gonadotropin (hCG). The extent and rate of LH receptor loss after hCG were dose-related, with maximum decreases of 50, 80, and 95% at 4, 2, and 1 days after treatment with 0.2, 1, and 10 pg of hCG, respectively. After 10 pg of hCG, testicular LH receptors were almost completely abolished for 4 days, rose slightly at 6 days, and returned to normal at 10 days. Cyclic AMP responses to stimulation by hCG in vitro were generally reduced in proportion to receptor loss, with no change in sensitivity to gonadotropin. However, cyclic AMP responses to cholera toxin were retained by interstitial cells from hCG-treated rats. The testosterone responses to hCG, cholera toxin, and dibutyryl cyclic AMP were enhanced in interstitial cells from rats treated with 0.2 pg of hCG and were markedly depressed in cells from rats given 1 and 10 pg of hCG. The reduced maximum testosterone responses of partially desensitized interstitial cells could not be raised to the control levels by high concentrations of hCG in vitro at 4 days, but were restored after 6 days, with a 2-fold decrease in sensitivity to hCG. The enhanced testosterone responses to hCG, choleragen, and dibutyryl cyclic AMP observed after the low dose of hCG (0.2 pg) reflect physiological stimulation of Leydig cell function, whereas reduced testosterone responses after higher doses are manifestations of receptor loss and desensitization of steps in the steroidogenic pathway. Measurement of occupied receptors showed that minor degrees of occupancy were followed by major loss of receptor sites. The decrease in LH’ receptors was less marked in detergent-solubilized preparations, suggesting that the extensive receptor loss observed in isolated cells was partly due to sequestration or internalization of binding sites. The loss of LH receptors in isolated Leydig cells persisted for several days after land lo-pg doses of hCG, indicating