Changes in skeletal muscle mitochondria in response to the development of type 2 diabetes or prevention by daily wheel running in hyperphagic OLETF rats

Abstract

The temporal changes in skeletal muscle mitochondrial content and lipid metabolism that precede type 2 diabetes are largely unknown. Here we examined skeletal muscle mitochondrial fatty acid oxidation (MitoFAOX) and markers of mitochondrial gene expression and protein content in sedentary 20- and 40-wk-old hyperphagic, obese Otsuka Long-Evans Tokushima fatty (OLETF-SED) rats. Changes in OLETF-SED rats were compared with two groups of rats who maintained insulin sensitivity: age-matched OLETF rats given access to voluntary running wheels (OLETF-EX) and sedentary, nonobese Long-Evans Tokushima Otsuka (LETO-SED) rats. As expected, glucose tolerance tests revealed insulin resistance at 20 wk that progressed to type 2 diabetes at 40 wk in the OLETF-SED, whereas both the OLETF-EX and LETO-SED maintained whole body insulin sensitivity. At 40 wk, complete MitoFAOX (to CO(2)), beta-hydroxyacyl-CoA dehydrogenase activity, and citrate synthase activity did not differ between OLETF-SED and LETO-SED but were significantly (P < 0.05) higher in OLETF-EX compared with OLETF-SED rats. Genes controlling skeletal muscle MitoFAOX (PGC-1alpha, PPARdelta, mtTFA, cytochrome c) were not different between OLETF-SED and LETO-SED at any age. Compared with the OLETF-SED, the OLETF-EX rats had significantly (P < 0.05) higher skeletal muscle PGC-1alpha, cytochrome c, and mtTFA mRNA levels at 20 and 40 wk and PPARdelta at 40 wk; however, protein content for each of these markers did not differ between groups at 40 wk. Limited changes in skeletal muscle mitochondria were observed during the transition from insulin resistance to type 2 diabetes in the hyperphagic OLETF rat. However, diabetes prevention through increased physical activity appears to be mediated in part through maintenance of skeletal muscle mitochondrial function.

Fig. 1.

Changes in systemic and skeletal muscle glucose homeostasis. Glucose (A) and insulin (B) area under curve (AUC) during an intraperitoneal glucose tolerance test. Values are means ± SE (n = 5–6). Basal (C) and insulin-stimulated (D) glucose transport in the extensor digitorum longus. Values are means ± SE (n = 6–8). #Significant difference between 20- and 40-wk values within respective animal group (P < 0.01). †Significantly different from sedentary Otsuka Long-Evans Tokushima fatty rats (OLETF-SED) at respective age (P < 0.05). *Significantly different from sedentary Long-Evans Tokushima Otsuka rats (LETO-SED) at respective age (P < 0.05). OLETF-EX, rats with access to running wheels.

Fig. 2.

Changes in superoxide dismutase (SOD; A) and catalase (B) activity in the red gastrocnemius (RG) muscle. Values are means ± SE (n = 6–8). #Significant difference between 20- and 40-wk values within respective animal group (P < 0.01). †Significantly different from OLETF-EX at respective age (P < 0.05). *Significantly different from LETO-SED at respective age (P < 0.05).

Fig. 3.

Changes in skeletal muscle β-hydroxyacyl-CoA dehydrogenase (β-HAD; A) and citrate synthase activity (B) and complete (C), incomplete (D), and total (E) palmitate oxidation from isolated RG mitochondria. Values are means ± SE (n = 6–8). #Significant difference between 20- and 40-wk values within respective animal group (P < 0.01). †Significantly different from OLETF-SED at respective age (P < 0.05). *Significantly different from LETO-SED at respective age (P < 0.05). ASMs, acid-soluble metabolites.

Fig. 4.

Changes in silent mating type information regulation 2 homolog 1 (SIRT1; A), peroxisome proliferator-activated receptor (PPAR)γ coactivator-1α (PGC-1α; B), PPARδ (C), mitochondrial transcription factor A (mtTFA; D), and cytochrome c (E) mRNA expression in the RG muscle. Values are means ± SE (n = 6–8). #Significant difference between 20- and 40-wk values within respective animal group (P < 0.01). †Significantly different from OLETF-SED at respective age (P < 0.05). *Significantly different from LETO-SED at respective age (P < 0.05).

Fig. 5.

Changes in silent SIRT1 (A), PGC-1α (B), cytochrome c (C), and cytochrome c oxidase IV (COX-IV)-subunit I protein content (D) in the RG muscle. Values are means ± SE (n = 5–8). #Significant difference between 20- and 40-wk values within respective animal group (P < 0.05). †Significantly different from OLETF-SED at respective age (P < 0.05). *Significantly different from LETO-SED at respective age (P < 0.05). AU, arbitrary units.