Reduction in voltage-gated K⁺ currents in primary cultured rat pancreatic β-cells by linoleic acids

Free fatty acids (FFAs), in addition to glucose, have been shown to stimulate insulin release through the G protein-coupled receptor (GPCR)40 receptor in pancreatic β-cells. Intracellular free calcium concentration ([Ca²⁺]_i) in β-cells is elevated by FFAs, although the mechanism underlying the [Ca²⁺]_i increase is still unknown. In this study, we investigated the action of linoleic acid on voltage-gated K⁺ currents. Nystatin-perforated recordings were performed on identified rat β-cells. In the presence of nifedipine, tetrodotoxin, and tolbutamide, voltage-gated K⁺ currents were observed. The transient current represents less than 5%, whereas the delayed rectifier current comprises more than 95%, of the total K⁺ currents. A long-chain unsaturated FFA, linoleic acid (10 μM), reversibly decreased the amplitude of K⁺ currents (to less than 10%). This reduction was abolished by the cAMP/protein kinase A system inhibitors H89 (1 μM) and Rp-cAMP (10 μM) but was not affected by protein kinase C inhibitor. In addition, forskolin and 8′-bromo-cAMP induced a similar reduction in the K⁺ current as that evoked by linoleic acid. Insulin secretion and cAMP accumulation in β-cells were also increased by linoleic acid. Methyl linoleate, which has a similar structure to linoleic acid but no binding affinity to GPR40, did not change K⁺ currents. Treatment of cultured cells with GPR40-specific small interfering RNA significantly reduced the decrease in K⁺ current induced by linoleic acid, whereas the cAMP-induced reduction of K⁺ current was not affected. We conclude that linoleic acid reduces the voltage-gated K⁺ current in rat β-cells through GPR40 and the cAMP-protein kinase A system, leading to an increase in [Ca²⁺]_i and insulin secretion.