Activation of alpha 2-adrenergic receptors decreases Ca2+ influx to inhibit insulin secretion in a hamster beta-cell line: an action mediated by a guanosine triphosphate-binding protein.
Activation of the sympathetic nervous system inhibits insulin secretion. We tested the hypothesis that activation of alpha 2-adrenergic receptors on the beta-cell by epinephrine or clonidine attenuates insulin release by an effect on the voltage-dependent Ca2+ channel (VDCC) and examined the role of G-proteins in this signal transduction pathway. Using a cultured SV40-transformed hamster beta-cell line (HIT cells) as a model system, we determined the effect of alpha 2-adrenergic agonists on insulin secretion, 86Rb+ efflux (a marker for K+ channel flux), and the free cytosolic Ca2+ level [( Ca2+]i) monitored in fura-2-loaded cells. In a dose-dependent manner, epinephrine and clonidine (10(-8)-10(-5)M) attenuated the increase in [Ca2+]i and insulin secretion induced by either K+ depolarization or stimulation of the VDCC with the agonist Bay K 8644. Epinephrine failed to affect the rise in [Ca2+]i induced by carbamylcholine, an agent that mobilizes intracellular Ca2+. Epinephrine also did not changes 86Rb+ efflux from HIT cells. The inhibitory effects of epinephrine were prevented by the alpha 2-adrenergic antagonist idazoxan, but were unaffected by the alpha 1-adrenergic antagonist phenoxybenzamine. Pretreatment of HIT cells with pertussis toxin (0.1 micrograms/ml) overnight abolished the inhibitory effects of epinephrine and clonidine on both [Ca2+]i and insulin secretion. These data suggest that one mechanism by which alpha 2-adrenergic agonists inhibit insulin secretion is by inhibiting Ca2+ influx through VDCC, an action that is mediated through a pertussis toxin-sensitive G-protein.
Hsu
W H
WH
Department of Medicine, Baylor College of Medicine, Houston, Texas 77030.
Xiang
H D
HD
Rajan
A S
AS
Boyd
A E
AE
D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester / Q000494:pharmacology; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002217:Carbachol / Q000494:pharmacology; D002460:Cell Line; D003000:Clonidine / Q000494:pharmacology; D006224:Cricetinae; D004837:Epinephrine / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000502:physiology*; D007329:Insulin Antagonists / Q000494:pharmacology*; D007515:Islets of Langerhans / Q000378:metabolism*; D037342:Pertussis Toxin; D011188:Potassium / Q000494:pharmacology; D012413:Rubidium / Q000378:metabolism; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D007329:Insulin Antagonists; D010566:Virulence Factors, Bordetella; D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; D002217:Carbachol; D037342:Pertussis Toxin; D019204:GTP-Binding Proteins; D012413:Rubidium; D003000:Clonidine; D011188:Potassium; D002118:Calcium; D004837:Epinephrine