Effect of oleic acid on arginine-induced glucagon secretion by the isolated perfused rat pancreas

Robust increase in glucagon secretion after oral protein intake, but not after glucose or lipid intake in Japanese people without diabetes

Few studies in Asian populations have analyzed how glucagon secretion is affected by ingested glucose, proteins or lipids, individually. To investigate the fluctuations of glucagon secretion after the intake of each of these nutrients, 10 healthy volunteers underwent oral loading tests using each of glucose, proteins and lipids, and blood levels of glucose, insulin and glucagon were measured every 30 min for 120 min. Whereas glucagon secretion was suppressed and minimally affected by oral glucose intake and lipid intake, respectively, oral protein intake robustly increased glucagon secretion, as well as insulin secretion. Further studies are needed to elucidate the mechanism by which protein loading increases glucagon secretion.

Acute stimulation of glucagon secretion by linoleic acid results from GPR40 activation and [Ca2+]i increase in pancreatic islet {alpha}-cells

The role of free fatty acids (FFAs) in glucagon secretion has not been well established, and the involvement of FFA receptor GPR40 and its downstream signaling pathways in regulating glucagon secretion are rarely demonstrated. In this study, it was found that linoleic acid (LA) acutely stimulated glucagon secretion from primary cultured rat pancreatic islets. LA at 20 and 40 μmol/l dose-dependently increased glucagon secretion both at 3 mmol/l glucose and at 15 mmol/l glucose, although 15 mmol/l glucose reduced basal glucagon levels. LA induced an increase in cytoplasmic free calcium concentrations ([Ca(2)(+)](i)) in identified rat α-cells, which is reflected by increased Fluo-3 intensity under confocal microscopy recording. The increase in [Ca(2)(+)](i) was partly inhibited by removal of extracellular Ca(2)(+) and eliminated overall by further exhaustion of intracellular Ca(2)(+) stores using thapsigargin treatment, suggesting that both Ca(2)(+) release and Ca(2)(+) influx contributed to the LA-stimulated increase in [Ca(2)(+)](i) in α-cells. Double immunocytochemical stainings showed that GPR40 was expressed in glucagon-positive α-cells. LA-stimulated increase in [Ca(2)(+)](i) was blocked by inhibition of GPR40 expression in α-cells after GPR40-specific antisense treatment. The inhibition of phospholipase C activity by U73122 also blocked the increase in [Ca(2)(+)](i) by LA. It is concluded that LA activates GPR40 and phospholipase C (and downstream signaling pathways) to increase Ca(2)(+) release and associated Ca(2)(+) influx through Ca(2)(+) channels, resulting in increase in [Ca(2)(+)](i) and glucagon secretion.

GPR40 is expressed in glucagon producing cells and affects glucagon secretion

The free fatty acid receptor, GPR40, has been coupled with insulin secretion via its expression in pancreatic beta-cells. However, the role of GPR40 in the release of glucagon has not been studied and previous attempts to identify the receptor in alpha-cells have been unfruitful. Using double-staining for glucagon and GPR40 expression, we demonstrate that the two are expressed in the same cells in the periphery of mouse islets. In-R1-G9 hamster glucagonoma cells respond dose-dependently to linoleic acid stimulation by elevated phosphatidyl inositol hydrolysis and glucagon release and the cells become increasingly responsive to fatty acid stimulation when overexpressing GPR40. Isolated mouse islets also secrete glucagon in response to linoleic acid, a response that was abolished by antisense treatment against GPR40. This study demonstrates that GPR40 is present and active in pancreatic alpha-cells and puts further emphasis on the importance of this nutrient sensing receptor in islet function.

The short-term effect of fatty acids on glucagon secretion is influenced by their chain length, spatial configuration, and degree of unsaturation: studies in vitro

The influence of fatty acids on beta cell function has been well established whereas little is known about the role of fatty acids on alpha cell function. The aim of our study was to investigate the short-term effects of chain length, spatial configuration, and degree of unsaturation of fatty acids on glucagon secretion from isolated mouse islets and alpha tumor cell 1 clone 6 cells (alpha TC1-6 cells). Glucagon release was measured with different saturated and unsaturated fatty acids as well as cis and trans isomers of fatty acids at low and high glucose. Palmitate (0.1-0.5 mmol/L) immediately stimulated glucagon release in a dose-dependent manner from both isolated islets and alpha TC 1-6 cells. The longer chain length of saturated fatty acids, the higher glucagon responses were obtained. The average fold increase in glucagon to saturated fatty acids (0.3 mmol/L) compared to control was octanoate 1.5, laurate 2.0, myristate 2.9, palmitate 5.4, and stearate 6.2, respectively. Saturated fatty acids were more effective than unsaturated fatty acids in stimulating glucagon secretion. At an equimolar concentration, trans-fatty acids were more potent than their cis isomers. Fatty acids immediately stimulate glucagon secretion from isolated mouse islets pancreatic alpha cells. The chain length, spatial configuration, and degree of unsaturation of fatty acids influence the glucagonotropic effect.