Objectives: Somatostatin (SST) inhibits insulin secretion from pancreatic B-cells through a reduction of intracellular free calcium ([Ca2+]i). The influx of Ca2+ is mediated by voltage-operated Ca2+ channels (VOCCs). The role of VOCCs of the R-type (CaV2.3) in SST-mediated processes is unknown. Therefore, we designed a study to identify SST-receptor subtypes (SSTR) in insulinoma cells (INS-1) and characterize the role of the CaV2.3 in mediating the effects of SST in these cells.
Methods: The expression of SSTRs in INS-1 cells was detected by RT-PCR. The effects of highly SSTR-selective agonists (SSTR-Ag) on cyclic AMP, insulin secretion and [Ca2+]i were measured by ELISA, RIA and cell fluorescence imaging. VOCCs were characterized by patch-clamp technique.
Results: INS-1 cells express SSTR2 and SSTR3. SSTR2-selective agonist (SSTR2-Ag) more potently reduced cyclic AMP production than SSTR3-Ag. SSTR2-Ag transiently increased [Ca2+]i which then rapidly decreased below the basal. Blockade of L- and R-type channels modulated [Ca2+]i changes in response to SSTR2-Ag treatment. In contrast, SSTR3-Ag lowered [Ca2+]i after 30 min, only. Blockade of R-type channels of cells treated with SSTR3-Ag less potently influenced [Ca2+]i than SST or SSTR2-Ag. SST (EC50: 0.04 nM) and SSTR2-Ag (EC50: 0.06 nM) more potently inhibited 20 mM glucose/10 nM exendin-4-stimulated insulin secretion than SSTR3-Ag. The specific R-type channel blocker SNX-482 more potently reduced the inhibition of insulin secretion by SST and SSTR2-Ag as compared to SSTR3-Ag.
Conclusions: INS-1 cells express SSTR2 and SSTR3. SSTR2-Ag more effectively reduces intracellular cyclic AMP-accumulation and insulin secretion than SSTR2-Ag. Blockade of R-type Ca2+ channels prevents SSTR2- and SSTR3-induced inhibition of insulin secretion, suggesting that these agonists inhibit insulin secretion through modulation of R-type channel activity.