Muscle-Specific Adaptations, Impaired Oxidative Capacity and Maintenance of Contractile Function Characterize Diet-Induced Obese Mouse Skeletal Muscle

BACKGROUND:The effects of diet-induced obesity on skeletal muscle function are largely unknown, particularly as it relates to changes in oxidative metabolism and morphology. PRINCIPAL FINDINGS:Compared to control fed mice, mice fed a high fat diet (HFD; 60% kcal: fat) for 8 weeks displayed increased body mass and insulin resistance without overt fasting hyperglycemia (i.e. pre-diabetic). Histological analysis revealed a greater oxidative potential in the HFD gastrocnemius/plantaris (increased IIA, reduced IIB fiber-type percentages) and soleus (increased I, IIA cross-sectional areas) muscles, but no change in fiber type percentages in tibialis anterior muscles compared to controls. Intramyocellular lipid levels were significantly increased relative to control in HFD gastrocnemius/plantaris, but were similar to control values in the HFD soleus. Using a novel, single muscle fiber approach, impairments in complete palmitate and glucose oxidation (72.8+/-6.6% and 61.8+/-9.1% of control, respectively; p<0.05) with HFD were detected. These reductions were consistent with measures made using intact extensor digitorum longus and soleus muscles. Compared to controls, no difference in succinate dehydrogenase or citrate synthase enzyme activities were observed between groups in any muscle studied, however, short-chain fatty acyl CoA dehydrogenase (SCHAD) activity was elevated in the HFD soleus, but not tibialis anterior muscles. Despite these morphological and metabolic alterations, no significant difference in peak tetanic force or low-frequency fatigue rates were observed between groups. CONCLUSIONS:These findings indicate that HFD induces early adaptive responses that occur in a muscle-specific pattern, but are insufficient to prevent impairments in oxidative metabolism with continued high-fat feeding. Moreover, the morphological and metabolic changes which occur with 8 weeks of HFD do not significantly impact muscle contractile properties

Morphometric changes are muscle-specific.

<p>Serial cross-sections from control (CON, white bars in all graphs) and high fat diet (HFD, black bars in all graphs) fed mouse muscles [TA (top left only), gastrocnemius/plantaris complex (GP, all middle graphs), and soleus (all right graphs)] were examined for (A) fiber type composition, (B) area, (C) SDH stain intensity, and (D) IMCL stain intensity. Representative images of all stains used, performed on GP muscle cross-sections are shown to the left of graphs B–D with fiber type (type I, and types II- A, D, B) labeled with CON on the left and HFD on the right. (B) Metachromatic fiber type stain was used to assess fiber type. (C) SDH and (D) Oil-Red-O stains are graphically represented by arbitrary units (A.U.) of optical intensity measurements, with greater values for more intense stains and normalized to a percentage of all the control value means for each graph (% CON). All measurements were taken on an average of 51–331 total fibers/animal with a Nikon Eclipse 90i microscope (N = 3–4). Significance is represented by * vs. CON, p<0.005.</p

Eight weeks of a high fat diet (HFD) elicits pre-diabetes.

<p>(A) Fasted body mass was assessed before experimental diet began and after 4 and 8 weeks (N = 19 CON, 20 HFD). (B) Epididymal fat mass after 8 weeks of diet intervention (N = 19 CON, N = 20 HFD). (C) Intraperitoneal glucose tolerance test (IPGTT) performed after an overnight fast (16 hrs) 1 week before harvest (N = 19 CON, N = 18 HFD). (D) Plasma insulin levels assessed 4 weeks into diet intervention (8 hr fast, N = 10) and at IPGTT 45 minute time-point (16 hr fast, N = 4). Significance is represented by * vs. CON at same time point (A–D), a or b vs. 0 weeks within diet group, and c vs. 4 weeks within diet group, p<0.005.</p

Impaired palmitate and glucose oxidation in HFD single fibers.

<p>(A) Palmitate (N = 19, average of 17 fibers/dish) and (B) glucose (N = 12 CON, N = 10 HFD, average of 23 fibers/dish) oxidation in single fibers derived from EDL and peroneus muscles was similarly impaired in mice fed a high-fat diet (HFD) compared to control (CON). Values were normalized to control values for each experiment and significance is represented by * vs. CON, p<0.005.</p

<i>In situ</i> contractile analysis reveals trend towards force decrements, yet unaltered peak force and fatigue.

<p>Relative tetanic force production [in Newtons (N) per gram (g) of wet muscle mass] in the gastrocnemius/plantaris muscle group of high-fat diet (HFD) mice compared to control (CON) was (A) not different before (Pre) or after (Post) the fatigue protocol. (B) There was no difference between diets over all frequencies used to test force production pre-fatigue, however there was a significant main effect of diet post-fatigue. (C) Contractile force, relative to initial (% initial), throughout a 2 minute low-frequency fatigue protocol was not different between diet groups. Significance is represented by * vs. CON, p<0.005.</p