A novel variant of ionotropic glutamate receptor regulates somatostatin secretion from δ-cells of islets of Langerhans

Many metabolic factors affect the secretion of insulin from β-cells and glucagon from α-cells of the islets of Langerhans to regulate blood glucose. Somatostatin from δ-cells, considered a local inhibitor of islet function, reduces insulin and glucagon secretion by activating somatostatin receptors in islet cells. Somatostatin secretion from δ-cells is increased by high glucose via glucose metabolism in a similar way to insulin secretion from β-cells. However, it is unknown how low glucose triggers somatostatin secretion. Because L-glutamate is cosecreted with glucagon from α-cells under low-glucose conditions and acts as a primary intercellular messenger, we hypothesized that glutamate signaling triggers the secretion of somatostatin. In this study, we showed that δ-cells express GluR4c-flip, a newly identified splicing variant of GluR4, an (RS)-α-amino-3-hydroxy-5-methyl- 4-isoxazolepropionic acid (AMPA)-type ionotropic glutamate receptor of rat. After treatment with L-glutamate, AMPA, or kainate, secretion of somatostatin from isolated islets was significantly stimulated under low-glucose conditions. The glutamate-dependent somatostatin secretion was Ca²⁺ dependent and blocked by 6-cyano-7-nitroquinoxaline-2,3-dione. Somatostatin in turn inhibited the secretion of L-glutamate and glucagon from α-cells. These results indicate that L-glutamate triggers somatostatin secretion from δ-cells by way of the GluR4c-flip receptor under low-glucose conditions. The released somatostatin may complete the feedback inhibition of α-cells. Thus, α- and β-cells may conununicate with each other through L-glutamate and somatostatin signaling.