Inhibition of glucose-stimulated insulin release by α₂-adrenoceptor activation is parallelled by both a repolarization and a reduction in cytoplasmic free Ca²⁺ concentration

Effects of the α₂-adrenergic agonist clonidine on insulin release, membrane potential, and cytoplasmic free Ca²⁺ concentration ([Ca²⁺](i)) were investigated using pancreatic β-cells isolated from obese hyperglycemic mice. Addition of 2 μM clonidine promptly inhibited glucose-stimulated insulin release, an effect accompanied by a lowering in both membrane potential and [Ca²⁺](i). Within minutes, the effect on Ca²⁺ was partly reversed, [Ca²⁺](i) attaining a new level, although still significantly lower than in the absence of agonist. This late increase in [Ca²⁺](i) was inhibited by 50 μM D-600, a blocker of voltage-activated Ca²⁺ channels. The inhibitory effects of clonidine on membrane potential, [Ca²⁺](i), and insulin release were abolished by 5 μM of the α₂-adrenergic antagonist yohimbine. Depolarization with high K⁺ increased [Ca²⁺](i) also in the presence of clonidine, conditions accompanied by only a minute release of insulin. Secretion was, however, partly restored by subsequent addition of 20 mM glucose. Addition of 5 mM Ca²⁺ transiently reversed the effects of clonidine on both membrane potential and [Ca²⁺](i). Although the clonidine-induced repolarization should be enough for closing the voltage-activated Ca²⁺ channels with a resulting decrease in [Ca²⁺](i), a direct interaction of the agonist with these channels cannot be excluded. The fact that it was possible to increase [Ca²⁺](i) with only a minor effect on insulin release suggests that the inhibitory effect of clonidine not only is due to a reduction in [Ca²⁺](i), but also involves interference with some more distal step in the insulin secretory machinery.