Oleic acid interacts with GPR40 to induce Ca2+ signaling in rat islet β-cells:: mediation by PLC and L-type Ca2+ channel and link to insulin release

It has long been thought that long-chain free fatty acids (FFAs) stimulate insulin secretion via mechanisms involving their metabolism in pancreatic beta-cells. Recently, it was reported that FFAs function as endogenous ligands for GPR40, a G protein-coupled receptor, to amplify glucose-stimulated insulin secretion in an insulinoma cell line and rat islets. However, signal transduction mechanisms for GPR40 in beta-cells are little known. The present study was aimed at elucidating GPR40-linked Ca2+ signaling mechanisms in rat pancreatic beta-cells. We employed oleic acid (OA), an FFA that has a high affinity for the rat GPR40, and examined its effect on cytosolic Ca2+ concentration ([Ca2+](i)) in single beta-cells by fura 2 fluorescence imaging. OA at 1 - 10 mu M concentration-dependently increased [Ca2+](i) in the presence of 5.6, 8.3, and 11.2 mM, but not 2.8 mM, glucose. OA-induced [Ca2+](i) increases at 11.2 mM glucose were inhibited in beta-cells transfected with small interfering RNA targeted to rat GPR40 mRNA. OA-induced [Ca2+](i) increases were also inhibited by phospholipase C (PLC) inhibitors, U73122 and neomycin, Ca2+-free conditions, and an L-type Ca2+ channel blocker, nitrendipine. Furthermore, OA increased insulin release from isolated islets at 8.3 mM glucose, and it was markedly attenuated by PLC and L-type Ca2+ channel inhibitors. These results demonstrate that OA interacts with GPR40 to increase [Ca2+](i) via PLC- and L-type Ca2+ channel-mediated pathway in rat islet beta-cells, which may be link to insulin release.