Somatostatin potently inhibits insulin secretion from pancreatic β-cells. It does so via activation of ATP-sensitive K+-channels (KATP) and G protein-regulated inwardly rectifying K+-channels, which act to decrease voltage-gated Ca2+-influx, a process central to exocytosis. Because KATP channels, and indeed insulin secretion, is controlled by glucose oxidation, we investigated whether somatostatin inhibits insulin secretion by direct effects on glucose metabolism. Oxidative metabolism in β-cells was monitored by measuring changes in the O2 consumption (ΔO2) of isolated mouse islets and MIN6 cells, a murine-derived β-cell line. In both models, glucose-stimulated ΔO2, an effect closely associated with inhibition of KATP channel activity and induction of electrical activity (r > 0.98). At 100 nm, somatostatin abolished glucose-stimulated ΔO2 in mouse islets (n = 5, P < 0.05) and inhibited it by 80 ± 28% (n = 17, P < 0.01) in MIN6 cells. Removal of extracellular Ca2+, 5 mm Co2+, or 20 μm nifedipine, conditions that inhibit voltage-gated Ca2+ influx, did not mimic but either blocked or reduced the effect of the peptide on ΔO2. The nutrient secretagogues, methylpyruvate (10 mm) and α-ketoisocaproate (20 mm), also stimulated ΔO2, but this was unaffected by somatostatin. Somatostatin also reversed glucose-induced hyperpolarization of the mitochondrial membrane potential monitored using rhodamine-123. Application of somatostatin receptor selective agonists demonstrated that the peptide worked through activation of the type 5 somatostatin receptor. In conclusion, somatostatin inhibits glucose metabolism in murine β-cells by an unidentified Ca2+-dependent mechanism. This represents a new signaling pathway by which somatostatin can inhibit cellular functions regulated by glucose metabolism.
Regulation of voltage-gated K+channels by glucose metabolism in pancreatic β-cells