Physical activity prevents alterations in mitochondrial ultrastructure and glucometabolic parameters in a high-sugar diet model

Abstract

Endurance exercise is a remarkable intervention for the treatment of many diseases. Mitochondrial changes on skeletal muscle are likely important for many of the benefits provided by exercise. In this study, we aimed to evaluate the effects that a regular physical activity (swimming without workload) has on mitochondrial morphological alterations and glucometabolic parameters induced by a high-sugar diet (HSD). Weaned male Wistar rats fed with a standard diet or a HSD (68% carbohydrate) were subjected to 60 minutes of regular physical activity by swimming (without workload) for four- (20 sessions) or eight-week (40 sessions) periods. After training, animals were euthanized and the sera, adipose tissues, and skeletal muscles were collected for further analysis. The HSD increased body weight after an 8-week period; it also increased the fat pads and the adipose index, resulting in glucose intolerance and insulin resistance (IR). Transmission electron microscopy showed an increase in alterations of mitochondrial ultrastructure in the gastrocnemius muscle, as well as a decrease in superoxide dismutase (SOD) activity, and an increase in protein carbonylation. Regular physical activity partially reverted these alterations in rats fed a HSD, preventing mitochondrial morphological alterations and IR. Moreover, we observed a decrease in Pgc1α expression (qPCR analysis) in STD-EXE group and a less pronounced reduction in HSD-EXE group after an 8-week period. Thus, regular physical activity (swimming without workload) in rats fed a HSD can prevent mitochondrial dysfunction and IR, highlighting the crucial role for physical activity on metabolic homeostasis.

Fig 1. Effects of a high-sugar diet and regular physical activity (swimming without workload) on glucometabolic parameters over 4-week (left panels) and 8-week (right panels) periods.

(A) Fasting glucose in plasma (mmol/L). (B) Area under the curve regarding the oral glucose tolerance test (OGTT). (C) Fasting insulin in serum (pmol/L). (D) Homeostatic model assessment index (HOMA-IR; N = 12). Data are expressed as means ± S.D. Statistically significant differences were determined using a two-way ANOVA to examine the effects of diet (HSD or STD) and regular physical activity (trained or sedentary), followed by Bonferroni post hoc analyses, P<0.05 was considered statistically significant. *Denotes statistical differences compared with the standard diet (STD) group (sedentary or trained), ^#denotes statistically significant differences compared with its untrained control (STD-SED or HSD-SED). STD-SED, sedentary standard chow diet; HSD-SED, sedentary high-sugar diet; STD-EXE, exercised standard chow diet; HSD-EXE, exercised high-sugar diet.

Fig 2. Effects of a high-sugar diet and regular physical activity (swimming without workload) on glycogen and lipids of gastrocnemius muscle fibres over 4-week and 8-week periods.

(A) Quantification of glycogen and (B) lipids densities in gastrocnemius muscle fibres over 4-week (left panels) and 8-week (right panels) periods. N = 75 fields from three animals per group. Data are expressed as means ± S.D. Statistically significant differences were determined using a two-way ANOVA to examine the effects of diet (HSD or STD) and regular physical activity (trained or sedentary), followed by Bonferroni post hoc analyses; P<0.05 was considered statistically significant. *Denotes statistically significant differences compared with the standard diet (STD) group (sedentary or trained), ^#denotes statistically significant differences compared with its untrained control (STD-SED or HSD-SED). STD-SED, sedentary standard chow diet; HSD-SED, sedentary high-sugar diet; STD-EXE, exercised standard chow diet; HSD-EXE, exercised high-sugar diet.

Fig 3. Effects of a high-sugar diet and regular physical activity (swimming without workload) on the mitochondrial profile of gastrocnemius muscle fibres over 4-week and 8-week periods.

(A, C) Quantification of total mitochondria density in gastrocnemius muscle fibres (4-week and 8-week periods, respectively). N = 75 fields from three animals per group. (B, D) Transmission electron micrographs (TEM) of transverse sections of muscle fibres (ultra-structural view), in which mitochondria is highlighted in green (4-week and 8-week periods, respectively). Data are expressed as means ± S.D. Statistically significant differences were determined using a two-way ANOVA to examine the effects of diet (HSD or STD) and regular physical activity (trained or sedentary), followed by Bonferroni post hoc analyses; P<0.05 was considered statistically significant. *Denotes statistically significant differences compared with the standard diet (STD) group (sedentary or trained), ^#denotes statistically significant differences compared with its untrained control (STD-SED or HSD-SED). STD-SED, sedentary standard chow diet; HSD-SED, sedentary high-sugar diet; STD-EXE, exercised standard chow diet; HSD-EXE, exercised high-sugar diet.

Fig 4. Effects of a high-sugar diet and regular physical activity (swimming without workload) on altered mitochondria in gastrocnemius muscle fibres over 4-week and 8-week periods.

(A, B) Quantification of altered mitochondria density in gastrocnemius muscle fibres (4-week and 8-week periods, respectively). N = 75 fields from three animals per group. (C, D) Transmission electron micrographs (TEM) of transverse sections of muscle fibres (ultra-structural view), which represent normal mitochondria (white head arrows) from a STD rat (left panel) and altered mitochondria (white arrows) from a HSD rat (right panel), with swollen appearance, rarefied matrix and damaged cristae. Data are expressed as means ± S.D. Statistically significant differences were determined using a two-way ANOVA to examine the effects of diet (HSD or STD) and regular physical activity (trained or sedentary), followed by Bonferroni post hoc analyses; P<0.05 was considered statistically significant. *Denotes statistically significant differences compared with the standard diet (STD) group (sedentary or trained), ^#denotes statistically significant differences compared with its untrained control (STD-SED or HSD-SED). STD-SED, sedentary standard chow diet; HSD-SED, sedentary high-sugar diet; STD-EXE, exercised standard chow diet; HSD-EXE, exercised high-sugar diet.

Fig 5. Effects of a high-sugar diet and regular physical activity (swimming without workload) on skeletal muscle oxidative stress over 4-week and 8-week periods.

(A) Superoxide dismutase activity assay in gastrocnemius muscle over 4-week (left panels) and 8-week (right panels) periods. (B) Carbonyl groups per mg of protein in gastrocnemius muscle over 4-week (left panels) and 8-week (right panels) periods. N = 6. Data are expressed as means ± S.D. Statistically significant differences were determined using a two-way ANOVA to examine the effects of diet (HSD or STD) and regular physical activity (trained or sedentary), followed by Bonferroni post hoc analyses; P<0.05 was considered statistically significant. *Denotes statistically significant differences compared with the standard diet (STD) group (sedentary or trained), ^#denotes statistically significant differences compared with its untrained control (STD-SED or HSD-SED). STD-SED, sedentary standard chow diet; HSD-SED, sedentary high-sugar diet; STD-EXE, exercised standard chow diet; HSD-EXE, exercised high-sugar diet.

Fig 6. Effects of a high-sugar diet and regular physical activity (swimming without workload) on Pgc1α expression over 4-week and 8-week periods.

(A) Pgc1α levels in the gastrocnemius muscle. Gene expression profiles of the groups were evaluated using the 2^-ΔCq method. rRNA 18S was used as a reference gene. Data are expressed as means ± S.D. Statistically significant differences were determined using a two-way ANOVA to examine the effects of diet (HSD or STD) and regular physical activity (trained or sedentary), followed by Bonferroni post hoc analyses; P<0.05 was considered statistically significant. *Denotes statistically significant differences compared with the standard diet (STD) group (sedentary or trained), ^#denotes statistically significant differences compared with its untrained control (STD-SED or HSD-SED).