title: Characteristics of somatostatin desensitization in the pituitary tumor cell line AtT-20.
abstract: The molecular mechanisms of somatostatin (SRIF) desensitization were investigated in the anterior pituitary tumor cell line AtT-20. Previous studies have shown that pretreatment of AtT-20 cells with SRIF analogs desensitizes the cells to SRIF inhibition of hormone release, cyclic AMP formation and calcium influx. This desensitization may involve a change in the properties of the SRIF receptors. Pretreatment of AtT-20 cells with Trp8-SRIF reduced the binding of the SRIF analog [125I]CGP 23996 (des-Alal, Gly2-[desamino-Cys3, Tyr11]-3, 14-dicarbasomatostatin) to AtT-20 cell membranes. The loss of [125I]CGP 23996 binding was dependent on the time of Trp8-SRIF treatment and was reversible. The ability of GTP analogs to inhibit [125I]CGP 23996 binding was reduced after Trp8-SRIF treatment, suggesting that the SRIF receptor and the inhibitory G proteins become uncoupled during desensitization. This is indicated further by the decrease in SRIF stimulation of GTPase activity and SRIF inhibition of forskolin-stimulated adenylyl cyclase activity in desensitized membranes. The reduction and recovery of SRIF inhibition of adenylyl cyclase activity after Trp8-SRIF pretreatment has a similar time course as the changes in [125I]CGP 23996 binding. GTP inhibition of forskolin-stimulated adenylyl cyclase activity is also reduced in SRIF-desensitized membranes. The loss of the GTP effect occurs rapidly and does not fully recover after Trp8-SRIF pretreatment. The levels of ADP-ribosylation of inhibitory GTP binding protein, the relative quantity of the alpha subunits of the inhibitory G proteins and their electrophoretic mobility after 2-dimensional gel electrophoretic analysis, are not altered in SRIF-desensitized membranes.(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D000067956:Adenylyl Cyclase Inhibitors; D020558:GTP Phosphohydrolases / Q000032:analysis; D019204:GTP-Binding Proteins / Q000032:analysis; D006160:Guanosine Triphosphate / Q000494:pharmacology; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D008297:Male; D010911:Pituitary Neoplasms / Q000378:metabolism; D017981:Receptors, Neurotransmitter / Q000187:drug effects*; D017481:Receptors, Somatostatin; D013004:Somatostatin / Q000031:analogs & derivatives / Q000378:metabolism / Q000494:pharmacology*; D014407:Tumor Cells, Cultured
chemical_list: D000067956:Adenylyl Cyclase Inhibitors; D017981:Receptors, Neurotransmitter; D017481:Receptors, Somatostatin; D006165:Guanylyl Imidodiphosphate; D013004:Somatostatin; D006160:Guanosine Triphosphate; C038136:CGP 23996; D020558:GTP Phosphohydrolases; D019204:GTP-Binding Proteins