Fatty acids stimulate insulin secretion from human pancreatic islets at fasting glucose concentrations via mitochondria-dependent and -independent mechanisms

BACKGROUND Free fatty acids (FFAs) acutely stimulate insulin secretion from pancreatic islets. Conflicting results have been presented regarding this effect at non-stimulatory glucose concentration, however. The aim of our study was to investigate how long-chain FFAs affect insulin secretion from isolated human pancreatic islets in the presence of physiologically fasting glucose concentrations and to explore the contribution of mitochondria to the effects on secretion. METHODS Insulin secretion from human pancreatic islets was measured from short-term static incubation or perfusion system at fasting glucose concentration (5.5 mM) with or without 4 different FFAs (palmitate, palmitoleate, stearate, and oleate). The contribution of mitochondrial metabolism to the effects of fatty acid-stimulated insulin secretion was explored. RESULTS The average increase in insulin secretion, measured from statically incubated and dynamically perifused human islets, was about 2-fold for saturated free fatty acids (SFAs) (palmitate and stearate) and 3-fold for mono-unsaturated free fatty acids (MUFAs) (palmitoleate and oleate) compared with 5.5 mmol/l glucose alone. Accordingly, MUFAs induced 50 % and SFAs 20 % higher levels of oxygen consumption compared with islets exposed to 5.5 mmol/l glucose alone. The effect was due to increased glycolysis. When glucose was omitted from the medium, addition of the FFAs did not affect oxygen consumption. However, the FFAs still stimulated insulin secretion from the islets although secretion was more than halved. The mitochondria-independent action was via fatty acid metabolism and FFAR1/GPR40 signaling. CONCLUSIONS The findings suggest that long-chain FFAs acutely induce insulin secretion from human islets at physiologically fasting glucose concentrations, with MUFAs being more potent than SFAs, and that this effect is associated with increased glycolytic flux and mitochondrial respiration.