Intramyocellular Triacylglycerol Research Articles

Carbohydrate supplementation during prolonged cycling exercise spares muscle glycogen but does not affect intramyocellular lipid use

Using contemporary stable-isotope methodology and fluorescence microscopy, we assessed the impact of carbohydrate supplementation on whole-body and fiber-type-specific intramyocellular triacylglycerol (IMTG) and glycogen use during prolonged endurance exercise. Ten endurance-trained male subjects were studied twice during 3 h of cycling at 63 ± 4% of maximal O2 uptake with either glucose ingestion (CHO trial; 0.7 g CHO kg−1 h−1) or without (CON placebo trial; water only). Continuous infusions with [U-13C] palmitate and [6,6-2H2] glucose were applied to quantify plasma free fatty acids (FFA) and glucose oxidation rates and to estimate intramyocellular lipid and glycogen use. Before and after exercise, muscle biopsy samples were taken to quantify fiber-type-specific IMTG and glycogen content. Plasma glucose rate of appearance (Ra) and carbohydrate oxidation rates were substantially greater in the CHO vs CON trial. Carbohydrate supplementation resulted in a lower muscle glycogen use during the first hour of exercise in the CHO vs CON trial, resulting in a 38 ± 19 and 57 ± 22% decreased utilization in type I and II muscle-fiber glycogen content, respectively. In the CHO trial, both plasma FFA Ra and subsequent plasma FFA concentrations were lower, resulting in a 34 ± 12% reduction in plasma FFA oxidation rates during exercise (P < 0.05). Carbohydrate intake did not augment IMTG utilization, as fluorescence microscopy revealed a 76 ± 21 and 78 ± 22% reduction in type I muscle-fiber lipid content in the CHO and CON trial, respectively. We conclude that carbohydrate supplementation during prolonged cycling exercise does not modulate IMTG use but spares muscle glycogen use during the initial stages of exercise in endurance-trained men.

Significant intramyocellular lipid use during prolonged cycling in endurance-trained males as assessed by three different methodologies

Intramyocellular triacylglycerol (IMTG) has been suggested to represent an important substrate source during exercise. In the present study, IMTG utilization during exercise is assessed through the use of various methodologies. In addition, we identified differences in the use of intramyocellular lipids deposited in the immediate subsarcolemmal (SS) area and those stored in the more central region of the fiber. Contemporary stable isotope technology was applied in combination with muscle tissue sampling before and immediately after 3 h of moderate-intensity cycling exercise (62 +/- 2% Vo(2 max)) in eight well-trained male cyclists. Continuous infusions with [U-13C]palmitate and [6,6-(2)H2]glucose were applied to quantify plasma free fatty acid (FFA) and glucose oxidation rates and to estimate whole body IMTG and glycogen use. Both immunohistochemical analyses of oil red O (ORO)-stained muscle cross sections and biochemical triacylglycerol (TG) extraction were performed to assess muscle lipid content. During exercise, plasma FFA, muscle (and/or lipoprotein)-derived TG, plasma glucose, and muscle glycogen oxidation contributed 24 +/- 2, 22 +/- 3, 11 +/- 1, and 43 +/- 3% to total energy expenditure, respectively. In accordance, a significant net decline in muscle lipid content was observed following exercise as assessed by ORO staining (67 +/- 8%) and biochemical TG extraction (49 +/- 8%), and a positive correlation was observed between methods (r = 0.56; P < 0.05). Lipid depots located in the SS area were utilized to a greater extent than the more centrally located depots. This is the first study to show significant use of IMTG as a substrate source during exercise in healthy males via the concurrent implementation of three major methodologies. In addition, this study shows differences in resting subcellular intramyocellular lipid deposit distribution and in the subsequent net use of these deposits during exercise.

Intrasample variability of intramyocellular triacylglycerol

Intrasample variability of intramyocellular triacylglycerol (imcTG) in the skeletal muscle of rats has been examined. Aliquoting after homogenization of muscle samples reduced imcTG variability considerably compared with aliquoting before homogenization. The results suggested that skeletal muscle samples be homogenized before aliquoting in order to reduce imcTG variability.

Intramyocellular lipid and glycogen content are reduced following resistance exercise in untrained healthy males

Resistance exercise has recently been shown to improve whole-body insulin sensitivity in healthy males. Whether this is accompanied by an exercise-induced decline in skeletal muscle glycogen and/or lipid content remains to be established. In the present study, we determined fibre-type-specific changes in skeletal muscle substrate content following a single resistance exercise session. After an overnight fast, eight untrained healthy lean males participated in a approximately 45 min resistance exercise session. Muscle biopsies were collected before, following cessation of exercise, and after 30 and 120 min of post-exercise recovery. Subjects remained fasted throughout the test. Conventional light and (immuno)fluorescence microscopy were applied to assess fibre-type-specific changes in intramyocellular triacylglycerol (IMTG) and glycogen content. A significant 27+/-7% net decline in IMTG content was observed in the type I muscle fibres (P<0.05), with no net changes in the type IIa and IIx fibres. Muscle glycogen content decreased with 23+/-6, 40+/-7 and 44+/-7% in the type I, IIa and IIx muscle fibres, respectively (P<0.05). Fibre-type-specific changes in intramyocellular lipid and/or glycogen content correlated well with muscle fibre-type oxidative capacity. During post-exercise recovery, type I muscle fibre lipid content returned to pre-exercise levels within 120 min. No changes in muscle glycogen content were observed during recovery. We conclude that intramyocellular lipid and glycogen stores are readily used during resistance exercise and this is likely associated with the reported increase in whole-body insulin sensitivity following resistance exercise.

Elevated stearoyl-CoA desaturase-1 expression in skeletal muscle contributes to abnormal fatty acid partitioning in obese humans

Obesity and type 2 diabetes are strongly associated with abnormal lipid metabolism and accumulation of intramyocellular triacylglycerol, but the underlying cause of these perturbations are yet unknown. Herein, we show that the lipogenic gene, stearoyl-CoA desaturase 1 (SCD1), is robustly up-regulated in skeletal muscle from extremely obese humans. High expression and activity of SCD1, an enzyme that catalyzes the synthesis of monounsaturated fatty acids, corresponded with low rates of fatty acid oxidation, increased triacylglycerol synthesis and increased monounsaturation of muscle lipids. Elevated SCD1 expression and abnormal lipid partitioning were retained in primary skeletal myocytes derived from obese compared to lean donors, implying that these traits might be driven by epigenetic and/or heritable mechanisms. Overexpression of human SCD1 in myotubes from lean subjects was sufficient to mimic the obese phenotype. These results suggest that elevated expression of SCD1 in skeletal muscle contributes to abnormal lipid metabolism and progression of obesity.

Hormone-Sensitive Lipase as Mediator of Lipolysis in Contracting Skeletal Muscle

The authors propose that the enzyme hormone-sensitive lipase (HSL), which is the rate-limiting enzyme for hydrolysis of triacylglycerol in adipocytes, also regulates the intramyocellular triacylglycerol mobilization and is controlled by mechanisms similar to those regulating glycogen phosphorylase. From an exercise perspective, it is fascinating that the primary enzymatic setting allows simultaneous mobilization of all major extramuscular and intramuscular energy stores.

Dietary &lt;I&gt;trans&lt;/I&gt; fatty acids alter diaphragm phospholipid fatty acid composition, triacylglycerol content and glucose transport in rats

The present study evaluates the effect of dietary trans fatty acids on diaphragm phospholipid fatty acid composition, intramyocellular triacylglycerol content and insulin-stimulated glucose uptake in comparison with dietary saturated fatty acids. Male weanling WNIN rats were divided into three groups and fed for 3 months on one of the following diets containing 10 % oil differing in fatty acid composition: control diet, saturated fatty acid diet and trans fatty acid diet. Dietary trans fatty acids increased the intramyocellular triacylglycerols and decreased the ratio of 20 : 4n-6 to 18 : 2n-6 and long-chain PUFA levels (20 %) in diaphragm phospholipids, indicating inhibition of PUFA biosynthesis. However, saturated fatty acids decreased both 18 : 2n-6 and 20 : 4n-6 without change in the ratio. Trans fatty acid-induced alterations in diaphragm phospholipid fatty acid composition and intramyocellular triacylglycerol content were associated with decreased insulin-stimulated glucose transport in the diaphragm. These observations suggest that dietary trans fatty acids decrease diaphragm insulin sensitivity, possibly due to increased intramyocellular triacylglycerol accumulation and decreased long-chain PUFA in phospholipids.

Inhibition Of Adipose Tissue Lipolysis Increases Intramuscular Lipid Use In Type 2 Diabetes Patients

Numerous studies have reported the association between elevated plasma free fatty acid (FFA) concentrations, intramyocellular triacylglycerol (IMTG) accumulation and the development of insulin resistance. Excess IMTG accretion is the result of an imbalance between FFA uptake, IMTG deposition and its subsequent mobilisation and/or oxidation. Recent data indicate that this imbalance, and not the actual size or distribution of the IMTG pool, is responsible for the development of insulin resistance. Therefore, the turnover of the IMTG pool should form a major therapeutic target in the prevention and treatment of skeletal muscle insulin resistance. PURPOSE To investigate the consequences of adipose tissue lipolytic inhibition on skeletal muscle substrate use. METHODS Ten type 2 diabetes patients were studied at rest, during exercise and subsequent recovery under normal, fasting conditions (CON) and following administration of a nicotinic acid analogue (LFA). Continuous [U-13C]palmitate and [6,6-2H2]glucose infusions were applied to quantify plasma FFA and glucose oxidation rates, and to estimate intramyocellular lipid and glycogen use. Muscle biopsies were collected before and after exercise to determine net changes in fibre type specific lipid and glycogen content. RESULTS In the LFA trial, plasma FFA rate of appearance was effectively reduced, resulting in 2–5 fold lower plasma FFA concentrations (P<0.001). Plasma FFA oxidation rates were substantially reduced at rest, during exercise and recovery in the LFA trial, which was compensated by an increase in both IMTG and endogenous carbohydrate use. Plasma glucose disposal rates did not differ between trials. During exercise, IMTG use averaged 0.18 ± 0.05 vs 0.08 ± 0.03 g/min in the LFA and CON trial, respectively (P<0.01). In accordance to these tracer data, a greater net decline in type I and type II muscle fiber lipid content was observed following exercise in the LFA vs the CON trial (P<0.05). CONCLUSION This study shows that plasma FFA availability regulates IMTG use and introduces adipose tissue lipolytic inhibition as a means to augment IMTG use in type 2 diabetes patients. The latter seems to represent an effective strategy to improve whole-body insulin sensitivity.

Inhibition Of Adipose Tissue Lipolysis Increases Intramuscular Lipid Use In Type 2 Diabetes Patients

Numerous studies have reported the association between elevated plasma free fatty acid (FFA) concentrations, intramyocellular triacylglycerol (IMTG) accumulation and the development of insulin resistance. Excess IMTG accretion is the result of an imbalance between FFA uptake, IMTG deposition and its subsequent mobilisation and/or oxidation. Recent data indicate that this imbalance, and not the actual size or distribution of the IMTG pool, is responsible for the development of insulin resistance. Therefore, the turnover of the IMTG pool should form a major therapeutic target in the prevention and treatment of skeletal muscle insulin resistance. PURPOSE To investigate the consequences of adipose tissue lipolytic inhibition on skeletal muscle substrate use. METHODS Ten type 2 diabetes patients were studied at rest, during exercise and subsequent recovery under normal, fasting conditions (CON) and following administration of a nicotinic acid analogue (LFA). Continuous [U-13C]palmitate and [6,6-2H2]glucose infusions were applied to quantify plasma FFA and glucose oxidation rates, and to estimate intramyocellular lipid and glycogen use. Muscle biopsies were collected before and after exercise to determine net changes in fibre type specific lipid and glycogen content. RESULTS In the LFA trial, plasma FFA rate of appearance was effectively reduced, resulting in 2–5 fold lower plasma FFA concentrations (P<0.001). Plasma FFA oxidation rates were substantially reduced at rest, during exercise and recovery in the LFA trial, which was compensated by an increase in both IMTG and endogenous carbohydrate use. Plasma glucose disposal rates did not differ between trials. During exercise, IMTG use averaged 0.18 ± 0.05 vs 0.08 ± 0.03 g/min in the LFA and CON trial, respectively (P<0.01). In accordance to these tracer data, a greater net decline in type I and type II muscle fiber lipid content was observed following exercise in the LFA vs the CON trial (P<0.05). CONCLUSION This study shows that plasma FFA availability regulates IMTG use and introduces adipose tissue lipolytic inhibition as a means to augment IMTG use in type 2 diabetes patients. The latter seems to represent an effective strategy to improve whole-body insulin sensitivity.

Use of intramuscular triacylglycerol as a substrate source during exercise in humans.

Fat and carbohydrate are the principal substrates that fuel aerobic ATP synthesis in skeletal muscle. Most endogenous fat is stored as triacylglycerol in subcutaneous and deep visceral adipose tissue. Smaller quantities of triacylglycerol are deposited as lipid droplets inside skeletal muscle fibers. The potential role of intramyocellular triacylglycerol (IMTG) as a substrate source during exercise in humans has recently regained much of its interest because of the proposed functional relationship between IMTG accumulation and the development of skeletal muscle insulin resistance. Exercise likely represents an effective means to prevent excess IMTG accretion by stimulating its rate of oxidation. However, there is much controversy on the actual contribution of the IMTG pool as a substrate source during exercise. The apparent discrepancy in the literature likely stems from methodological difficulties that have been associated with the methods used to estimate IMTG oxidation during exercise. However, recent studies using stable isotope methodology, 1H-magnetic resonance spectroscopy, and electron and/or immunofluorescence microscopy all support the contention that the IMTG pool can function as an important substrate source during exercise. Although more research is warranted, IMTG mobilization and/or oxidation during exercise seem to be largely determined by exercise intensity, exercise duration, macronutrient composition of the diet, training status, gender, and/or age. In addition, indirect evidence suggests that the capacity to mobilize and/or oxidize IMTG is substantially impaired in an obese and/or Type 2 diabetic state. As we now become aware that skeletal muscle has an enormous capacity to oxidize IMTG stores during exercise, more research is warranted to develop combined exercise, nutritional, and/or pharmacological interventions to effectively stimulate IMTG oxidation in sedentary, obese, and/or Type 2 diabetes patients.

Intramyocellular triacylglycerol as a substrate source during exercise.

The role of intramyocellular triacylglycerol (IMTG) as a substrate source during exercise has recently regained much attention as a result of the proposed functional relationship between IMTG accumulation and the development of insulin resistance. It has been speculated that elevated NEFA delivery and/or impaired fatty acid (FA) oxidation result in intramyocellular accumulation of triacylglycerol and FA metabolites, which are likely to induce defects in the insulin signalling cascade, causing insulin resistance. The progressive accumulation of IMTG in sedentary patients and patients who are obese and/or have type 2 diabetes should therefore form a major therapeutic target, and efforts should be made to develop interventions that prevent excess IMTG accretion by stimulating their rate of oxidation. Although regular exercise is likely to represent such an effective means, there is much controversy about the actual contribution of the IMTG pool as a substrate source during exercise. The apparent discrepancy in the published literature might be explained by differences in the applied research protocol and the selected subject population, but most of all by the techniques that have been employed to estimate IMTG use during exercise. Data obtained in trained-endurance athletes indicate that athletes can substantially reduce their IMTG pool following a single exercise session. With the growing awareness that skeletal muscle has a tremendous potential to oxidise IMTG during prolonged moderate-intensity exercise, more research is warranted to develop combined exercise, nutritional and/or pharmacological interventions that can stimulate IMTG oxidation in sedentary patients and patients who are obese and/or have type 2 diabetes.

Regulation and role of hormone-sensitive lipase in rat skeletal muscle.

Intramyocellular triacylglycerol (TG) is an important energy store, and the energy content of this depot is higher than the energy content of the muscle glycogen depot. It has recently been shown that the mobilization of fatty acids from this TG pool may be regulated by the neutral lipase hormone-sensitive lipase (HSL). This enzyme is known to be rate limiting for intracellular TG hydrolysis in adipose tissue. The presence of HSL has been demonstrated in all muscle fibre types by Western blotting of muscle fibres isolated by collagenase treatment or after freeze-drying. The content of HSL varies between fibre types, being higher in oxidative fibres than in glycolytic fibres. When analysed under conditions optimal for HSL, neutral lipase activity in muscle can be stimulated by adrenaline as well as by contractions. These increases are abolished by the presence of anti-HSL antibody during analysis. Moreover, immunoprecipitation with affinity-purified anti-HSL antibody causes similar reductions in muscle HSL protein concentration and in measured neutral lipase responses to contractions. The immunoreactive HSL in muscle is stimulated by adrenaline via beta-adrenergic activation of cAMP-dependent protein kinase (PKA). From findings in adipocytes it is likely that PKA phosphorylates HSL at residues Ser(563), Ser(659) and Ser(660). Contraction probably also enhances muscle HSL activity by phosphorylation, because the contraction-induced increase in HSL activity is elevated by the protein phosphatase inhibitor okadaic acid and reversed by alkaline phosphatase. A novel signalling pathway in muscle by which HSL activity may be stimulated by protein kinase C (PKC) via extracellular signal-regulated kinase (ERK) has been demonstrated. In contrast to previous findings in adipocytes, in muscle the activation of ERK is not necessary for stimulation of HSL by adrenaline. However, contraction-induced HSL activation is mediated by PKC, at least partly via the ERK pathway. In fat cells ERK is known to phosphorylate HSL at Ser(600). Hence, phosphorylation of different sites may explain the finding that in muscle the effects of contractions and adrenaline on HSL activity are partially additive. In line with the view that the two stimuli act by different mechanisms, training increases contraction-mediated HSL activation but diminishes adrenaline-mediated HSL activation in muscle. In conclusion, HSL is present in skeletal muscle and can be activated by phosphorylation in response to both adrenaline and muscle contractions. Training increases contraction-mediated HSL activation, but decreases adrenaline-mediated HSL activation in muscle.