Suppression of insulin release by galanin and somatostatin is mediated by a G-protein. An effect involving repolarization and reduction in cytoplasmic free Ca²⁺ concentration

The effects of galanin and somatostatin on insulin release, membrane potential, and cytoplasmic free Ca²⁺ concentration ([Ca²⁺](i)) were investigated using β-cells isolated from obese hyperglycemic mice. Whereas insulin release was measured in a column perifusion system, membrane potential and [Ca²⁺](i) were measured with the fluorescent indicators bisoxonol (bis-(1,3-diethylthiobarbiturate)trimethineoxonol) and quin 2, in cell suspensions in a cuvette. Galanin (16 nM) and somatostatin (400 nM) suppressed glucose-stimulated insulin release in parallel to promoting repolarization and a reduction in [Ca²⁺](i). The reduction in [Ca²⁺](i) comprised an initial nadir followed by a slow rise and the establishment of a new steady state level. The slow rise in [Ca²⁺](i) was abolished by 50 μM D-600, a blocker of voltage-activated Ca²⁺ channels. Both peptides suppressed insulin release even when [Ca²⁺](i) was raised by 25 mM K⁺. Under these conditions the inhibition of insulin release was partly reversed by an increase in the glucose concentration. Addition of 5 mM Ca²⁺ to a cell suspension, incubated in the presence of 20 mM glucose and either galanin, somatostatin, or the α₂-adrenergic agonist clonidine (10 nM), induced oscillations in [Ca²⁺](i), this effect disappearing subsequent to the addition of D-600. The effects of galanin, somatostatin, and clonidine on [Ca²⁺](i) were abolished in β-cells treated with pertussis toxin. In accordance with measurements of [Ca²⁺](i) treatment with pertussis toxin reversed the inhibitory effect of galanin on insulin release. The inhibitory action of galanin and somatostatin on insulin release is probably accounted for by not only a repolarization-induced reduction in [Ca²⁺](i) and a decreased sensitivity of the secretory machinery to Ca²⁺, but also by a direct interaction with the exocytotic process. It is proposed that these effects are mediated by a pertussis toxin-sensitive GTP-binding protein.