title: Desensitization of the cAMP system in mouse Leydig cells by hCG, cholera toxin, dibutyryl cAMP and cAMP: localization of the 'lesion' to the guanine nucleotide regulatory protein-adenylate cyclase complex.
abstract: The mechanism of hCG-induced desensitization of the cAMP system was studied in Percoll-purified mouse Leydig cells. Pretreatment of Leydig cells with hCG resulted in a time- and dose-dependent decrease in the capacity of hCG-induced cAMP formation. Maximal desensitization (approximately 90%) was induced by only partial prior stimulation. Desensitization, however, was not observed without a prior increase in cAMP or testosterone production. Pretreatment of the cells with N6,O2'-dibutyryl cAMP (DBcAMP) also induced a dose- and time-dependent densensitization. cAMP was only effective in the presence of the phosphodiesterase inhibitor 1-methyl-3-isobutylxanthine (MIX). Cholera toxin desensitized the hormone-induced cAMP response as drastically as hCG. Cholera toxin was unable to reverse the refractory state induced by one of the agonists. hCG-induced desensitization was not associated with a loss in [125I]hCG binding or an increase in maximal phosphodiesterase activity, and appeared not to be dependent on protein synthesis. Membranes from hCG, cholera toxin of DBcAMP-desensitized cells showed an impaired adenylate cyclase activity in response to hCG, hCG plus beta-gamma-imidoguanosine 5'-triphosphate (GPPNP) and NaF. In conclusion, hCG-induced desensitization of the adenylate cyclase system in mouse Leydig cells can be mimicked by cholera toxin, DBcAMP and cAMP, indicating a cAMP-mediated process. The site of the 'lesion' has to be localized to the guanine nucleotide regulatory protein-adenylate cyclase complex rather than to its uncoupling from the hormone receptor.
mesh_terms: D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D000262:Adenylyl Cyclases / Q000378:metabolism; D000818:Animals; D003994:Bucladesine / Q000494:pharmacology; D002772:Cholera Toxin / Q000494:pharmacology*; D006063:Chorionic Gonadotropin / Q000494:pharmacology*; D000242:Cyclic AMP / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D019204:GTP-Binding Proteins; D007700:Kinetics; D007985:Leydig Cells / Q000187:drug effects* / Q000378:metabolism; D008297:Male; D051379:Mice; D011956:Receptors, Cell Surface / Q000378:metabolism
chemical_list: D006063:Chorionic Gonadotropin; D011956:Receptors, Cell Surface; D003994:Bucladesine; D002772:Cholera Toxin; D000242:Cyclic AMP; D019204:GTP-Binding Proteins; D000262:Adenylyl Cyclases; D015056:1-Methyl-3-isobutylxanthine