The Influence of High‐Fat Feeding and Exercise Training on Substrate‐Specific Mitochondrial Respiration in Skeletal Muscle of Female Mice

Abstract

Objective High-fat diet (HFD) induced obesity is associated with excessive lipid supply which drives remodeling of skeletal muscle mitochondria. Exercise stimulates mitochondrial respiration but it is not clear if such adaptations are specific to lipids. Our objective was to determine if HFD-induced obesity would change the sensitivity and capacity for lipid oxidation, and if exercise modifies mitochondrial-specific respiration during HFD. Hypothesis We hypothesized that HFD-induced obesity would increase the sensitivity (intrinsic function) and capacity for lipid respiration while exercise would further augment overall capacity for skeletal muscle mitochondrial substrate oxidation in female mice. Methods We used a 2x2 designof diet and exercise in 10-week old female C57BL/6J mice (n=13-15 per group). Mice consumed diet (either 10% fat or 60% HFD) for 4 weeks then performed 8 weeks of treadmill training for 1 hour per day, 5 days per week or remained sedentary while continuing diet. HFD increased body-weight (P<0.01) by 4 weeks and fat mass measured by dual-energy x-ray absorptiometry at week 12 (P<0.01). We confirmed glucose and insulin intolerance at week 4 of HFD with no change after 8 weeks of treadmill training (P=0.435). Mice rested 24 hours prior to tissue collection. We isolated mitochondria from fresh quadriceps then performed high-resolution respirometry. We measured lipid sensitivity (Km) and capacity (Vmax) using titrations of octanoyl-carnitine (F-linked). Glutamate-succinate were used to determine non-lipid substrate oxidative phosphorylation capacity (N- and S-linked). Primary outcomes were analyzed using 2-way ANOVA (diet x exercise) with Tukey's multiple comparisons (α=0.05). Results The Km for lipid substrates was not different between diet (-7.7%, P=0.66) or exercise (+20.3%, P=0.26) groups. Absolute Vmax was 38% higher in HFD (P=0.018 versus low fat). There was a tendency for 27% higher Vmax after exercise (P=0.074 versus sedentary). For non-lipid substrates, HFD had 23% greater absolute N-linked respiration (P=0.04) and 19% greater S-linked respiration (P=0.02); however, no effects of exercise were detected. Normalizing respiration to mitochondrial protein content eliminated the HFD effects in all substrates. Conclusion We provide evidence that mitochondrial adaptations to HFD, in female mice, are driven primarily by increases in protein abundance rather than intrinsic factors as the normalization of respiration to mitochondrial protein content eliminated HFD effects for increased substrate respiration and there were no differences to lipid sensitivity between diet or exercise. Exercise did not appear to alter the mitochondrial respiratory response to HFD.