Real-time detection of somatostatin release from single islets reveals hypersecretion in type 2 diabetes

Aim

Somatostatin from pancreatic δ-cells is a paracrine regulator of insulin and glucagon secretion, but the release kinetics and whether secretion is altered in diabetes is unclear. This study aimed to improve understanding of somatostatin secretion by developing a tool for real-time detection of somatostatin release from individual pancreatic islets.

Methods

Reporter cells responding to somatostatin with cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i) changes were generated by co-expressing somatostatin receptor SSTR2, the G-protein Gα15 and a fluorescent Ca²⁺ sensor in HeLa cells.

Results

Somatostatin induced dose-dependent [Ca²⁺]_i increases in reporter cells with half-maximal and maximal effects at 1.6 ± 0.4 and ~30 nM, respectively. Mouse and human islets induced reporter cell [Ca²⁺]_i elevations that were inhibited by the SSTR2 antagonist CYN154806. Depolarization of islets by high K⁺, K_ATP channel blockade or increasing the glucose concentration from 3 to 11 mM evoked concomitant elevations of [Ca²⁺]_i in islets and reporter cells. Exposure of islets to glucagon, GLP-1 and ghrelin also triggered reporter cell [Ca²⁺]_i responses, whereas little effect was obtained by islet exposure to insulin, glutamate, GABA and urocortin-3. Islets from type 2 diabetic human donors induced higher reporter cell [Ca²⁺]_i responses at 11 mM and after K⁺ depolarization compared with non-diabetic islets, although fewer δ-cells were identified by immunostaining.

Conclusion

Type 2 diabetes is associated with hypersecretion of somatostatin, which has implications for paracrine regulation of insulin and glucagon secretion. The new reporter cell assay for real-time detection of single-islet somatostatin release holds promise for further studies of somatostatin secretion in islet physiology and pathophysiology.