ACTIVATION OF ALPHA-2-ADRENERGIC RECEPTORS DECREASES CA-2+ 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, Rb-86+ 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 Bat K 8644. Epinephrine failed to affect the rise in [Ca2+]i induced by carbamylcholine, an agent that mobilizes intracellular Ca2+. Epinephrine also did not change Rb-86+ 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-mu-g/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.