Gonadotropin-induced changes in the luteinizing hormone receptors of cultured Leydig cells. Evidence of up-regulation in vitro.

Previous in vivo studies have demonstrated that gonadotropic desensitization of luteinizing hormone/human chorionic gonadotropin (hCG) receptors and steroid responses is preceded by an early phase of receptor up-regulation. Hormonal desensitization has been recently reproduced in an in vitro Leydig cell culture, which has now been applied to studies on the early up-regulation of receptors. We performed comparative studies on the binding of 125I-hCG in isolated Leydig cells in plated culture and in suspension. In plated cells the total binding was up to 200% higher than that measured in suspension. This difference was not due to differential internalization. Preincubation with hCG in plated culture for 2 to 6 h increased the number of binding sites measured in suspension. The kinetics of the binding of labeled hCG to plated cells showed a secondary increase which reached its maximum after 3 h of incubation. This increase in hCG binding was not prevented by preincubation with inhibitors of protein synthesis and steroidogenesis or of microtubule or microfilament function. The sensitivities of the testosterone and cAMP responses to hCG in the plated cells were lower than those observed in suspension. These differences were maintained in the presence of a phosphodiesterase inhibitor. These results demonstrated that the cell interaction with a solid substratum is required for the acute up-regulation of the luteinizing hormone receptor and can induce changes in the Leydig cell responsiveness to gonadotropin stimulation.