title: GTP-dependent inhibition of insulin secretion by epinephrine in permeabilized RINm5F cells. Lack of correlation between insulin secretion and cyclic AMP levels.
abstract: The mechanism by which alpha 2-adrenergic agonists inhibit exocytosis was investigated in electrically permeabilized insulin secreting RINm5F cells. In this preparation alpha 2-adrenoceptors remain coupled to adenylate cyclase, since basal- and forskolin-stimulated cyclic AMP production was lowered by epinephrine and clonidine by 30-50%. Cyclic AMP levels did not correlate with the rate of insulin secretion. Thus, at low Ca2+, forskolin enhanced cyclic AMP levels 5-fold without eliciting secretion, and Ca2+-stimulated secretion was associated with decreased cyclic AMP accumulation. Epinephrine (plus propranolol) inhibited Ca2+-induced insulin secretion in a GTP-dependent manner. The maximal inhibition (43%) occurred at 500 microM GTP. Clonidine also inhibited Ca2+-stimulated secretion. Replacement of GTP by GDP or by the nonhydrolyzable GTP analog guanosine 5'-(3-O-thio)triphosphate as well as treatment of the cells with pertussis toxin prior to permeabilization abolished epinephrine inhibition of insulin secretion. Pertussis toxin did not affect Ca2+-stimulated secretion. Insulin release stimulated by 1,2-didecanoyl glycerol was also lowered by epinephrine suggesting an effect distal to the activation of protein kinase C (Ca2+/phospholipid-dependent enzyme). These results taken together with the ability of epinephrine to inhibit ionomycin-induced insulin secretion in intact cells suggest that alpha 2-adrenergic inhibition is distal to the generation of second messengers. A model is proposed for alpha 2-adrenoceptor coupling to two effector systems, namely the adenylate cyclase and the exocytotic site in insulin-secreting cells.
authors.affiliation: Institut de Biochimie Clinique, Centre Médical Universitaire, Genève, Switzerland.
authors.forename: S
authors.initials: S
authors.lastname: Ullrich
authors.forename: C B
authors.initials: CB
authors.lastname: Wollheim
mesh_terms: D037361:Adenylate Cyclase Toxin; D002118:Calcium / Q000494:pharmacology; D002460:Cell Line; D002463:Cell Membrane Permeability; D003000:Clonidine / Q000494:pharmacology; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism*; D004837:Epinephrine / Q000494:pharmacology*; D004987:Ethers / Q000494:pharmacology; D006160:Guanosine Triphosphate / Q000494:pharmacology*; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D015759:Ionomycin; D007700:Kinetics; D008954:Models, Biological; D037342:Pertussis Toxin; D011433:Propranolol / Q000494:pharmacology; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
chemical_list: D037361:Adenylate Cyclase Toxin; D004987:Ethers; D007328:Insulin; D010566:Virulence Factors, Bordetella; D005576:Colforsin; D015759:Ionomycin; D006160:Guanosine Triphosphate; D011433:Propranolol; D000242:Cyclic AMP; D037342:Pertussis Toxin; D003000:Clonidine; D002118:Calcium; D004837:Epinephrine