BackgroundThe effects of diet-induced obesity on skeletal muscle function are largely unknown, particularly as it relates to changes in oxidative metabolism and morphology.Principal FindingsCompared 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.ConclusionsThese 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.

Mammalian skeletal muscle cells have the ability to regulate volume in response to increases or decreases in extracellular osmolarity. In the present study we measured the time course of change in single fibre intracellular calcein fluorescence (volume indicastor) and width in response to varied 200 mosmol/L increase in extracellular osmolarity using NaCl or sucrose. Adult mouse EDL single fibres were isolated using collagenase and incubated in DMEM prior to and during experimentation. Fibres were loaded with calcein‐AM for 30 min, and triple‐rinsed with calcein‐free DMEM. After obtaining baseline images NaCl or sucrose solution was added. Fibre images were obtained at 3–6 s intervals for up to 60 min. Fibre images were analyzed for intensity and width at 2–3 sites. Increased osmolarity resulted in a rapid increase in fibre fluorescence and decrease in fibre width. Both variables gradually recovered to baseline values within ~45 min. Bumetanide, an inhibitor of the sodium‐potassium‐2 chloride cotransporter (NKCC) impaired recovery. There was a linear relationship between increases in fibre fluorescent intensity and decreases in fibre width. It is concluded that the NKCC is involved in regulatory volume increase in skeletal muscle, and that changes in fluorescence intensity can be used as an indicator of changes in cell volume.Supported by NSERC of Canada.

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Diet‐induced obesity (DIO) is increasing in prevalence among youth; a time where robust muscle growth occurs. We hypothesized that consumption of a high fat diet (HFD) in early adulthood would negatively impact mouse muscle metabolism and contractile function. HFD (~60% kcal fat for 8 weeks) induced an obese and insulin resistant phenotype with reduced relative muscle mass (72 ± 1.9%) compared to control.Using isolated single muscle fibers for analysis, HFD induced a significant impairment in palmitate and glucose oxidation (72.8 ± 6.6% and 61.8 ± 9.1% of control respectively). This novel technique was validated in intact muscle. Impaired insulin responses were noted for glucose metabolism and glycogen synthesis in HFD muscle vs. control, while glucose and palmitate uptake was not altered between groups. Preliminary RT‐PCR results demonstrate no difference in CPT‐1, PGC‐1α, PPARα or UCP‐2 expression. Further gene and protein analysis into the mechanisms underlying changes in fat uptake and oxidation are currently underway. Despite metabolic impairments, HFD muscle exhibited no loss of maximal contractile force or alteration in fatigue rates.Our data suggest that HFD skeletal muscle is highly adaptive in the face of impaired substrate oxidation, as demonstrated by the preservation of contractile properties. These findings contribute to our understanding of the impact of DIO on developing skeletal muscle.