Effects of metformin on oxidative stress, adenine nucleotides balance, and glucose-induced insulin release impaired by chronic free fatty acids exposure in rat pancreatic islets

Abstract

Background: In rat pancreatic islets, chronic exposure to high free fatty acid (FFA) levels impairs insulin secretion and β cell mass. The mechanisms underlying this defect are not completely understood. Since islets have intrinsically low anti-oxidant enzyme defense, oxidative stress might be responsible for β cell damage.

Aim: In this study, we investigated if FFA could induce oxidative stress in rat pancreatic islets and if metformin might reverse adverse effects.

Material and methods: We cultured rat pancreatic islets in the presence or absence of FFA (oleate/palmitate 2:1, 2 mM) for 72 h. In some experiments, we used metformin (2.5 μg/ml) during the last 24 h.

Results: In our model, glucosestimu lated insulin release was markedly reduced (p<0.005) after chronic FFA exposure, and the ATP/ADP ratio was altered (p<0.05). We observed a significant increase of reactive oxygen species (ROS) (p<0.001), malondialdehyde a lipid peroxidation product (p<0.01) and nitric oxide (NO) levels in the culture media (p<0.001). Inducible NO synthase (iNOS) and heat shock protein-70 (HSP-70) protein expression were also increased (p<0.001 and p<0.01, respectively). When metformin was present during the last 24 h of culture, insulin secretion was restored, and the ATP/ADP ratio was normalized. ROS production, NO production, lipid peroxidation, iNOS and HSP-70 protein expression levels had decreased.

Conclusions: These data indicate that, in rat pancreatic islets, chronic exposure to high FFA induces oxidative stress and that metformin, by reducing this effect, may have a direct beneficial effect on insulin secretion impaired by lipotoxicity.