title: Evidence that the peripheral-type benzodiazepine receptor ligand Ro 5-4864 inhibits beta-endorphin release from AtT-20 cells by blockade of voltage-dependent calcium channels.
abstract: The demonstrations that Ro 5-4864, a ligand selective for the peripheral-type benzodiazepine (BZD) binding site, inhibited cellular differentiation and proliferation and that occupancy of the peripheral-type BZD binding site likely mediated the observed BZD effects on diverse endocrine tissues suggested that Ro 5-4864 disrupted a common cellular regulatory event. Using a well-characterized anterior pituitary-derived tumor cell line (AtT-20 cells), which synthesizes and secretes adrenocorticotropic hormone (ACTH), beta-lipotropin hormone (beta-LPH), and beta-endorphin (BE), we have investigated the molecular mechanism of action of Ro 5-4864's capacity to alter BE secretion. Ro 5-4864 inhibits basal and induced BE release from AtT-20 cells, through a cyclic AMP-independent mechanism. Ro 5-4864 completely blocked the corticotropin-releasing hormone and forskolin-induced release of BE without altering the concomitant production of cyclic AMP. The addition to AtT-20 cells of CGP 28392, a dihydropyridine that has been demonstrated in other systems to specifically activate voltage-dependent Ca2+ channels, resulted in a cyclic AMP-independent, dose-related increase in BE secretion. This CGP-induced BE release was blocked by increasing concentrations of Ro 5-4864. In contrast to the capacity of Ro 5-4864 to block CGP-induced BE release, Ro 5-4864 lacked the capacity to block enhanced BE secretion due to the calcium ionophore A23187, which increases intracellular Ca2+ levels independent of the voltage-dependent Ca2+ channels. Our findings suggest that Ro 5-4864 inhibits BE secretion from AtT-20 cells through a blockade of the voltage-dependent membrane Ca2+ channels and this mechanism of action may be responsible for Ro 5-4864's diverse effects observed on other cell types.
mesh_terms: D000818:Animals; D001570:Benzodiazepinones / Q000378:metabolism*; D001665:Binding Sites; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002454:Cell Differentiation; D002455:Cell Division; D002460:Cell Line; D005576:Colforsin / Q000494:pharmacology; D003346:Corticotropin-Releasing Hormone / Q000494:pharmacology; D004723:Endorphins / Q000378:metabolism*; D007473:Ion Channels / Q000378:metabolism*; D051379:Mice; D011725:Pyridines / Q000494:pharmacology; D011963:Receptors, GABA-A / Q000378:metabolism*; D001615:beta-Endorphin
chemical_list: D001570:Benzodiazepinones; D004723:Endorphins; D007473:Ion Channels; D011725:Pyridines; D011963:Receptors, GABA-A; D005576:Colforsin; C028513:4'-chlorodiazepam; D000001:Calcimycin; D001615:beta-Endorphin; C040525:CGP 28392; D003346:Corticotropin-Releasing Hormone; D002118:Calcium