Temporal relationship of the prolactin‐dependent LH‐induced LH receptor to the LH stimulus

AbstractThe time course for LH induction of luteinizing hormone (LH) receptors as reflected in binding of 125l‐labeled hCG was investigated in hypophysecto‐mized adult male rats. A low dose of oLH (10 μg) was administered to hypophysectomized adult male rats following pretreatments with prolactin, follicle‐stimulating hormone (FSH), growth hormone (GH), or saline. Testicular binding of hCG was determined at different times following the LH injection using Leydig cell membrane preparations from a testicular homogenate. Seven days after hypophysectomy, hCG binding was at a nadir of 19 ± 7% (mean ± SD) of control values. Pretreatment with prolactin (100 μg/day) for 7 days was associated with a nonsignificantly different hCG binding that was 30 ± 5% of control values. Prolactin pretreatment plus a single 10 μg LH i.p. injection increased 125l hCG binding up to 56 ± 10% of control values within 30 minutes of the LH injection. Luteinizing hormone‐induced hCG binding persisted at a high level (51 ± 4% of control values) for 2 hours but returned to hypophysectomized control levels 6 hours after the i.p. LH injection.Seven days pretreatment with FSH or GH at 100 μg/day plus 10‐μg LH injections was also tested. Neither FSH nor GH had a statistically significant effect on hCG binding nor could they mimic the ability of prolactin to allow for LH induction of hCG binding in the hypophysectomized adult male rats.These studies suggest that the induction or “up‐regulation” of Leydig cell hCG binding by ovine LH is rapid and specifically dependent upon pre‐exposure to prolactin.