High-Lard and High-Fish Oil Diets Differ in Their Effects on Insulin Resistance Development, Mitochondrial Morphology and Dynamic Behaviour in Rat Skeletal Muscle

Fish oil (mainly omega 3 polyunsaturated fatty acids), differently from lard (mainly saturated fatty acids) has been sug- gested to have anti-inflammatory effects associated with amelioration of insulin sensibility. An important role in skele- tal muscle insulin resistance development has been recently attributed to mitochondrial dynamic behavior. Mitochon- dria are dynamic organelles that frequently undergo fission/fusion processes and a shift toward fission process has been associated with skeletal muscle mitochondrial dysfunction and insulin resistance development. The present work aimed to evaluate if the replacement of lard with fish oil in high-fat diet positively affect skeletal muscle mitochondrial dy- namic behavior in association with the improvement of insulin-resistance. Body weight gain, systemic insulin-resistance (glucose/insulin ratio), serum TNFα levels and skeletal muscle lipid content were assessed in rats fed a high-lard or high-fish-oil diet for 6 weeks. In skeletal muscle sections, immunohistochemical analysis were performed to detect the presence of insulin receptor substrate 1 (IRS1) and tyrosine phosphorylated IRS1 (key factor in insulin signalling path- way) as well as to detect the main proteins involved in mitochondrial fusion (MFN2 and OPA1) and fission (DRP1 and Fis1) processes. Skeletal muscle mitochondrial ultrastructural features were assessed by electron microscopy. High-fish oil feeding induced lower body weight gain, systemic inflammation and insulin-resistance development as well as skeletal muscle lipid accumulation compared to high-lard feeding. Skeletal muscle sections from high-fish oil fed rats exhibited a greater number of immunoreactive fibers for MFN2 and OPA1 proteins as well as weaker immunostaining for DRP1 and Fis1 compared to sections from high-lard fed rats. Electron microscopy observations suggested a promi- nent presence of fission events in L rats and fusion events in F rats. The positive effect of the replacement of lard with fish oil in high-fat diet on systemic and skeletal muscle insulin sensibility was associated to changes in mitochondrial dynamic behavior.