Role of GPR40 in fatty acid action on the beta cell line INS-1E.

GPR40 is a G protein-coupled receptor expressed preferentially in beta cells, that has been implicated in mediating free fatty acid-stimulated insulin release. GPR40 RNAi impaired the ability of palmitic acid (PA) to increase both insulin secretion and intracellular calcium ([Ca2+]i). The PA-dependent [Ca2+]i increase was attenuated by inhibitors of Galphaq, PLC, and SERCA. Thus GPR40 activates the Galphaq pathway, leading to release of Ca2+ from the ER. Yet the GPR40-dependent [Ca2+]i rise was dependent on extracellular Ca2+ and elevated glucose, and was blocked by inhibition of L-type calcium channels (LTCC) or opening of the K(ATP) channel; this suggests that GPR40 promotes Ca2+ influx through up-regulation of LTCC pre-activated by glucose and membrane depolarization. Taken together, the data indicate that GPR40 mediates the increase in [Ca2+]i and insulin secretion through the Galphaq-PLC pathway, resulting in release of Ca2+ from the ER and leading to up-regulation of Ca2+ influx via LTCC.