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 beta-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 microM 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 alpha 2-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 beta-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.
Nilsson
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Department of Medical Cell Biology, Uppsala University, Sweden.
Arkhammar
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Rorsman
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Berggren
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PO
D000818:Animals; D002118:Calcium / Q000502:physiology*; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology; D005260:Female; D019204:GTP-Binding Proteins / Q000502:physiology*; D019004:Galanin; D005711:Gallopamil / Q000494:pharmacology; D005947:Glucose / Q000494:pharmacology; D006943:Hyperglycemia / Q000378:metabolism; D066298:In Vitro Techniques; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000187:drug effects / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D051379:Mice; D008820:Mice, Obese; D009479:Neuropeptides / Q000494:pharmacology*; D010455:Peptides / Q000494:pharmacology*; D011188:Potassium / Q000494:pharmacology; D013004:Somatostatin / Q000494:pharmacology*
D007328:Insulin; D009479:Neuropeptides; D010455:Peptides; D005711:Gallopamil; D013004:Somatostatin; D019004:Galanin; D019204:GTP-Binding Proteins; D005947:Glucose; D011188:Potassium; D002118:Calcium