Intramuscular Lipid Research Articles

A simple and rapid method to characterize lipid fate in skeletal muscle.

BackgroundElevated fatty acids contribute to the development of type 2 diabetes and affect skeletal muscle insulin sensitivity. Since elevated intramuscular lipids and insulin resistance is strongly correlated, aberrant lipid storage or lipid intermediates may be involved in diabetes pathogenesis. The aim of this study was to develop a method to determine the dynamic metabolic fate of lipids in primary human skeletal muscle cells and in intact mouse skeletal muscle. We report a simple and fast method to characterize lipid profiles in skeletal muscle using thin layer chromatography.FindingsThe described method was specifically developed to assess lipid utilization in cultured and intact skeletal muscle. We determined the effect of a pan-diacylglycerol kinase (DGK) class I inhibitor (R59949) on lipid metabolism to validate the method. In human skeletal muscle cells, DGK inhibition impaired diacylglycerol (DAG) conversion to phosphatidic acid and increased triglyceride synthesis. In intact glycolytic mouse skeletal muscle, DGK inhibition triggered the accumulation of DAG species. Conversely, the DGK inhibitor did not affect DAG content in oxidative muscle.ConclusionThis simple assay detects rapid changes in the lipid species composition of skeletal muscle with high sensitivity and specificity. Determination of lipid metabolism in skeletal muscle may further elucidate the mechanisms contributing to the pathogenesis of insulin resistance in type 2 diabetes or obesity.

Activating HSP72 in rodent skeletal muscle increases mitochondrial number and oxidative capacity and decreases insulin resistance.

Induction of heat shock protein (HSP)72 protects against obesity-induced insulin resistance, but the underlying mechanisms are unknown. Here, we show that HSP72 plays a pivotal role in increasing skeletal muscle mitochondrial number and oxidative metabolism. Mice overexpressing HSP72 in skeletal muscle (HSP72Tg) and control wild-type (WT) mice were fed either a chow or high-fat diet (HFD). Despite a similar energy intake when HSP72Tg mice were compared with WT mice, the HFD increased body weight, intramuscular lipid accumulation (triacylglycerol and diacylglycerol but not ceramide), and severe glucose intolerance in WT mice alone. Whole-body VO2, fatty acid oxidation, and endurance running capacity were markedly increased in HSP72Tg mice. Moreover, HSP72Tg mice exhibited an increase in mitochondrial number. In addition, the HSP72 coinducer BGP-15, currently in human clinical trials for type 2 diabetes, also increased mitochondrial number and insulin sensitivity in a rat model of type 2 diabetes. Together, these data identify a novel role for activation of HSP72 in skeletal muscle. Thus, the increased oxidative metabolism associated with activation of HSP72 has potential clinical implications not only for type 2 diabetes but also for other disorders where mitochondrial function is compromised.

Assessment of intramuscular lipid and metabolites of the lower leg using magnetic resonance spectroscopy in boys with Duchenne muscular dystrophy

The purpose of this study was to use proton magnetic resonance spectroscopy to assess intramuscular lipid and metabolites of lower leg muscles in boys with Duchenne muscular dystrophy (DMD) and determine its relationship with strength and functional ability. Spectroscopic measurements were obtained from four muscles of the lower leg in 25 boys with DMD (9.2±3.1years) and 10 healthy boys (10.2±2.6years). Lipid fractions and metabolite concentrations were also determined. Muscle strength, a timed functional test, and the Modified Brooke Lower Extremity Functional Scale were also determined. Lipid fractions were higher (p<0.01) for the DMD group than healthy subjects for all muscles, and lipid fraction was found to be greater in the older DMD boys. The peroneal muscle demonstrated a significant difference in lipid fraction in all DMD age groups. Lipid fractions in all muscles correlated with functional measures (r=0.52–0.70, p<0.001), with smaller inverse correlations with the strength measure (r=−0.36 to −0.56, p<0.05). These findings provide quantifiable information regarding intramuscular lipid and metabolite levels of different muscles across various age groups in boys with DMD and may be used in determining the effect of interventions in future clinical trials.

Phenotypic flexibility in passerine birds: Seasonal variation in fuel storage, mobilization and transport

Winter acclimatization in small birds living in cold climates produces a winter phenotype characterized by upregulation of metabolic rates to meet enhanced thermoregulatory demands. We measured several key aspects of fuel storage, mobilization and transport in summer and winter to determine whether black-capped chickadees (Poecile atricapillus), white-breasted nuthatches (Sitta carolinensis), and house sparrows (Passer domesticus) seasonally modulate these attributes to meet enhanced winter thermoregulatory demands. In addition, we exposed birds to thermoneutral (control) and severe cold exposure treatments to determine whether acute cold exposure influenced fuel storage, mobilization or transport. Carcass lipid mass and pectoralis intramuscular lipid did not vary significantly between seasons or temperature treatments for any of the study species. Muscle glycogen varied significantly seasonally only for chickadee supracoracoideus and leg muscles, and did not vary among warm or cold treatments for any species. Pectoralis fatty acid binding protein (FABPc) was significantly elevated in winter for chickadees and nuthatches, but not for sparrows. Plasma metabolites showed little consistent variation in response to season or acute cold exposure. Thus, fuel storage and mobilization do not appear to be major targets of adjustment associated with seasonal metabolic flexibility in these species, but modulation of intracellular lipid transport by FABPc may be an important contributor to seasonal phenotypes in some species of small birds.

Birth weight alters the response to postnatal high-fat diet-induced changes in meat quality traits and skeletal muscle proteome of pigs

Low birth weight (LBW) exerts persistent effects on the growth and development of offspring. The present study was conducted to test the hypothesis that LBW alters the response of pigs to high-fat (HF) diet-induced changes in meat quality and skeletal muscle proteome. Normal-birth weight (NBW) and LBW piglets were fed a control diet or a HF diet from weaning to slaughter at 110kg body weight. Most of the meat quality traits were influenced by LBW. Meat quality analysis revealed that LBW piglets had a greater ability to deposit intramuscular lipids than their heavier littermates when fed a HF diet. Increased shear force, lower pH45 min and drip loss were observed in the skeletal muscle of LBW piglets compared with NBW piglets. Proteomic analysis revealed forty-six differentially expressed proteins in the skeletal muscle of LBW and NBW piglets fed the control diet or HF diet. These proteins play a central role in cell structure and motility, glucose and energy metabolism, lipid metabolism, and cellular apoptosis, as well as stress response. Of particular interest is the finding that LBW altered the response to HF diet-induced changes in the expression of proteins related to stress response (heat shock protein) and glucose and energy metabolism (pyruvate kinase, phosphoglycerate mutase, enolase and triosephosphate isomerase). Taken together, our findings revealed that the HF diet-induced changes in the expression of glucose and energy metabolism-related proteins varied between NBW and LBW piglets, which provides a possible mechanism to explain higher intramuscular fat store in LBW pigs when fed a HF diet.

Chronic muscle stimulation improves insulin sensitivity while increasing subcellular lipid droplets and reducing selected diacylglycerol and ceramide species in obese Zucker rats

Although insulin resistance has been associated with accumulations of specific intramuscular fatty acids and altered subcellular localisation of lipid droplets, these concepts remain controversial. Therefore, we aimed to identify specific intramuscular fatty acids and subcellular lipid localisations associated with improved insulin sensitivity following chronic muscle contraction. In lean and insulin-resistant obese Zucker rats the tibialis anterior muscle was stimulated (6 h/day for 6 days). Thereafter, muscles were examined for insulin sensitivity, intramuscular lipid droplet localisation and triacylglycerol (TAG), diacylglycerol (DAG) and ceramide fatty acid composition. In lean and obese animals, regardless of muscle type, chronic muscle contraction improved muscle insulin sensitivity and increased intramuscular levels of total and most C14-C22 TAG fatty acids (p < 0.05). Therefore, accumulation in subcellular lipid droplet compartments reflected the oversupply of lipids within muscle. In contrast, improvements in insulin sensitivity induced by muscle contraction were associated with reductions in specific DAG and ceramide species that were not uniform in red and white muscle of obese rats. However, these reductions were insufficient to fully normalise insulin sensitivity, indicating that other mechanisms are involved. Reductions in 18 C length DAG and ceramide species were the most consistent in red and white muscle and therefore may represent therapeutic targets for improving insulin sensitivity.

The use of stoned olive cake and rolled linseed in the diet of intensively reared lambs: effect on the intramuscular fatty-acid composition

The aim of the present study was to evaluate the effect of the inclusion of stoned olive cake and rolled linseed in a concentrate-based diet for lambs on the fatty-acid composition of polar and non-polar intramuscular lipids of the longissimus dorsi muscle. To achieve this objective, 32 Appenninica lambs were randomly distributed into four groups of eight lambs each and were fed conventional cereal-based concentrates (diet C); concentrates containing 20% on a dry matter (DM) basis of rolled linseed (diet L); concentrates containing 35% DM of stoned olive cake (diet OC); and concentrates containing both rolled linseed (10% DM) and stoned olive cake (17% DM; diet OCL). The concentrates were administered together with grass hay at a 20:80 forage:concentrate ratio. Growing performances and carcass traits were evaluated. The fatty-acid composition was analysed in the total intramuscular lipids, as well as in the polar and neutral lipids. The average feed intake and the growth performance of lambs were not affected by the dietary treatments, as a consequence of similar nutritional characteristics of the diets. The inclusion of rolled linseed in the L and OCL diets increased the content of C18:3 n-3 in intramuscular total lipids, which was threefold higher in meat from the L lambs and more than twofold higher in meat from the OCL lambs compared with the C and OC treatments. The n-6:n-3 ratio significantly decreased in the meat from lambs in the L and OCL groups, reaching values below 3. The L treatment resulted in the highest level of trans-18:1 fatty acids in the muscle. Regardless of the dietary treatment, the t10-18:1 was the major isomer, representing 55%, 45%, 49% and 45% of total trans-18:1 for C, L, OC and OCL treatments, respectively. Neutral lipids from the OC-fed lambs contained the highest amount of c9-18:1 (more than 36% of total fatty acids); however, the content of c9-18:1 did not differ between the OC and C lambs, suggesting an intensive biohydrogenation of dietary c9-18:1 in the case of OC treatment. The highest content of c9,t11-18:2 was detected in the intramuscular fat from the L-fed lambs, followed by the OCL treatment. A similar trend was observed in the neutral lipid fraction and, to a lower extent, in the polar lipids.

Cholesterol and Fatty Acids Oxidation in Meat from Three Muscles of Massese Suckling Lambs Slaughtered at Different Weights

Eighteen Massese male lambs fed mainly with maternal milk were slaughtered at 11, 14 and 17 kg. Samples of Longissimus dorsi (LD), Triceps brachii (TB) and Semimembranosus (Sm) muscles were collected. Total intramuscular lipids were extracted by means of a mixture of chloroform methanol 2/1. Cholesterol content and its oxidation product (COP) were determined by a gas chromatography apparatus equipped with an apolar 30 m column. Fatty acid oxidation was evaluated by means of thio-barbituric acid reactive substances (TBARS) extracting the sample with aqueous acidic solution. The effect of slaughter weight on oxidation of intramuscular lipids was found only in TB muscles. In this muscle the cholesterol content showed a decreasing trend, while the content of COPs significantly increased with the age of animals. Among the COPs, the 7-ketocholesterol and 7β-hydroxycholesterol were the most abundant, followed by α- and β-epoxy-cholesterol and cholestan-triol. The content of TBARS did not vary owing to a similar fatty acid composition of intramuscular fat across weight of slaughter. In any case, the values of TBARS did not reach the threshold of the detection of off-flavour in meat.

Ectopic fat assessment focusing on cardiometabolic and renal risk.

It is well known that people with high levels of body fat are at higher risk for developing diabetes mellitus, kidney disease, and cardiovascular disorders. Since individuals who are slightly overweight, or even individuals of normal weight, can vary in body fat distribution, their metabolic profiles and the degree of association of these profiles with cardiometabolic risk factors may differ. Fat distribution might be more of a predictive factor for cardiorenometabolic risk than obesity itself, which has led researchers to investigate whether ectopic fat accumulation may partially account for the development of cardiorenometabolic disorders. In addition to visceral obesity, fat can accumulate in the liver and muscle, and these intrahepatic and intramuscular lipid stores are associated with insulin resistance and adverse metabolic phenotypes. More recently, pericardial fat, perivascular fat, and perirenal fat were found to be associated with coronary atherosclerosis, cardiovascular diseases, and kidney damage, respectively. Thus, regional fat distribution may play a key role in understanding the development of cardiorenometabolic diseases in nonobese people.

HSL and ATGL: the movers and shakers of muscle lipolysis

Intramuscular triglyceride (IMTG) accounts for only a fraction of the total lipid store in the human body. However, given its significant contribution as a substrate for ATP synthesis during exercise, and its proposed role in the development of skeletal muscle insulin resistance, IMTG metabolism has proved to be an area of great scientific interest over the last 20 years. Studies investigating IMTG utilisation during exercise remained equivocal, until methods using immunofluorescence microscopy enabled the muscle fibre type-specific assessment of IMTG content without the interference of extramyocellular adipocytes. Studies using this method convincingly demonstrated that in trained individuals 60–70% of IMTG can be depleted in type I muscle fibres during prolonged moderate-intensity exercise, and can account for up to 50% of total lipid oxidation (van Loon, 2004). Of more clinical relevance were studies highlighting the potential role of intramuscular lipid accumulation in the development of skeletal muscle insulin resistance. This hypothesis has been refined over the last decade, as highly insulin-sensitive, endurance-trained athletes exhibit higher levels of total IMTG and diacylglycerol (DAG) (Amati et al. 2011). It is now hypothesised that the accumulation of ceramides and certain species of membrane-bound DAG are responsible for the inhibition of insulin-mediated glucose uptake.

Enteric Neuropathy Can Be Induced by High Fat Diet In Vivo and Palmitic Acid Exposure In Vitro

ObjectiveObese and/or diabetic patients have elevated levels of free fatty acids and increased susceptibility to gastrointestinal symptoms. Since the enteric nervous system is pivotal in regulating gastrointestinal functions alterations or neuropathy in the enteric neurons are suspected to occur in these conditions. Lipid induced intestinal changes, in particular on enteric neurons, were investigated in vitro and in vivo using primary cell culture and a high fat diet (HFD) mouse model.DesignMice were fed normal or HFD for 6 months. Intestines were analyzed for neuronal numbers, remodeling and lipid accumulation. Co-cultures of myenteric neurons, glia and muscle cells from rat small intestine, were treated with palmitic acid (PA) (0 – 10−3 M) and / or oleic acid (OA) (0 – 10−3 M), with or without modulators of intracellular lipid metabolism. Analyses were by immunocyto- and histochemistry.ResultsHFD caused substantial loss of myenteric neurons, leaving submucous neurons unaffected, and intramuscular lipid accumulation in ileum and colon. PA exposure in vitro resulted in neuronal shrinkage, chromatin condensation and a significant and concentration-dependent decrease in neuronal survival; OA exposure was neuroprotective. Carnitine palmitoyltransferase 1 inhibition, L-carnitine- or alpha lipoic acid supplementation all counteracted PA-induced neuronal loss. PA or OA alone both caused a significant and concentration-dependent loss of muscle cells in vitro. Simultaneous exposure of PA and OA promoted survival of muscle cells and increased intramuscular lipid droplet accumulation. PA exposure transformed glia from a stellate to a rounded phenotype but had no effect on their survival.ConclusionsHFD and PA exposure are detrimental to myenteric neurons. Present results indicate excessive palmitoylcarnitine formation and exhausted L-carnitine stores leading to energy depletion, attenuated acetylcholine synthesis and oxidative stress to be main mechanisms behind PA-induced neuronal loss.High PA exposure is suggested to be a factor in causing diabetic neuropathy and gastrointestinal dysregulation.

Changes of Intramuscular Fat Composition, Lipid Oxidation and Lipase Activity in Biceps femoris and Semimembranosus of Xuanwei Ham During Controlled Salting Stages

Fatty acid composition of neutral lipids (NLs), phospholipids (PLs) and free fatty acids (FFAs) from intramuscular fat (IMF), lipid oxidation and lipase activity in muscle Semimembranosus (SM) and msucle Biceps femoris (BF) of dry-cured Xuanwei ham during the 90-d salting stages were analysed. The salt content increased from 0.34 to 3.52% in BF and from 0.10 to 5.42% in SM during the 90 d salting stage, respectively. PLs of IMF in both BF and SM decreased 54.70% (P<0.001) and 34.64% (P<0.05), furthermore, the saturated fatty acids (SFA), monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA) of PLs in both muscles were hydrolysed almost isochronously. FFAs were increased from 0.46 g 100 g−1 lipids to 2.92 g 100 g−1 lipids in BF at the end of salting, which was lower than SM (from 1.29 g 100 g−1 lipids to 9.70 g 100 g−1 lipids). The activities of acid lipase, neutral lipase and acid phospholipase all remained active in the 90 d. The thiobarbituric acid reactive substances (TBARS) was slowly increased to 1.34 mg kg−1 muscle in BF and to 2.44 mg kg−1 muscle in SM during the salting stage. In conclusion, the controlled salting process prompted the hydrolysis of PLs of IMF notably and increased the lipid oxidation of muscles within some limits.

Metformin limits ceramide-induced senescence in C2C12 myoblasts

High lipid and ceramide concentrations are hallmarks of obese and/or insulin resistant skeletal muscle, yet little is known about its role on cell cycle and senescence. The purpose of this study was to examine the role of ceramide on muscle senescence, and whether metformin limited this response. MethodsLow passage, proliferating C2C12 myoblasts were treated with a control, 50μM C2-ceramide (8h), and/or 2mM metformin, then examined for insulin sensitivity, cell senescence, cell proliferation, cell cycle, protein expression of cell cycle regulators. ResultsCeramide treatment caused a dephosphorylation (p<0.05) of Akt and 4E-BP1, regardless of the presence of insulin. The ceramide treated myoblasts displayed higher β-galactosidase staining (p<0.05), reduced BrDu incorporation and total number of cells (p<0.05), and an increased proportion of cells in G2-phase (p<0.05) versus control cultures. Ceramide treatment also upregulated (p<0.05) p53 and p21 protein expression, that was reversed by either pifithrin-α or shRNA for p53. Metformin limited (p<0.05) ceramide's effects on insulin signaling, senescence, and cell cycle regulation. ConclusionsHigh ceramide concentrations reduced myoblast proliferation that was associated with aberrant cell cycle regulation and a senescent phenotype, which could provide an understanding of skeletal muscle cell adaptation during conditions of high intramuscular lipid deposition and/or obesity.

An Optimized Histochemical Method to Assess Skeletal Muscle Glycogen and Lipid Stores Reveals Two Metabolically Distinct Populations of Type I Muscle Fibers

Skeletal muscle energy metabolism has been a research focus of physiologists for more than a century. Yet, how the use of intramuscular carbohydrate and lipid energy stores are coordinated during different types of exercise remains a subject of debate. Controversy arises from contradicting data from numerous studies, which used different methodological approaches. Here we review the “pros and cons” of previously used histochemical methods and describe an optimized method to ensure the preservation and specificity of detection of both intramyocellular carbohydrate and lipid stores. For optimal preservation of muscle energy stores, air drying cryosections or cycles of freezing-thawing need to be avoided. Furthermore, optimization of the imaging settings in order to specifically image intracellular lipid droplets stained with oil red O or Bodipy-493/503 is shown. When co-staining lipid droplets with associated proteins, Bodipy-493/503 should be the dye of choice, since oil red O creates precipitates on the lipid droplets blocking the light. In order to increase the specificity of glycogen stain, an antibody against glycogen is used. The resulting method reveals the existence of two metabolically distinct myosin heavy chain I expressing fibers: I-1 fibers have a smaller crossectional area, a higher density of lipid droplets, and a tendency to lower glycogen content compared to I-2 fibers. Type I-2 fibers have similar lipid content than IIA. Exhaustive exercise lead to glycogen depletion in type IIA and IIX fibers, a reduction in lipid droplets density in both type I-1 and I-2 fibers, and a decrease in the size of lipid droplets exclusively in type I-1 fibers.

Effect of androgen deprivation therapy on muscle attenuation in men with prostate cancer

Aging skeletal muscle is associated with not only a reduction in muscle size and strength but also in muscle quality which reflects an increase in fatty infiltration of muscle. In men with prostate cancer, androgen deprivation therapy (ADT) accelerates this loss of muscle size and strength, but it is unknown if muscle quality is also adversely affected. Therefore, we examined the effects of ADT on muscle attenuation, an indirect measure of intramuscular lipid content, as well as the muscle cross-sectional area (CSA) in men with prostate cancer. Pre- and post-CT scans of the pelvis in 39 men aged 49-78 years receiving leuprorelin were examined. The time between baseline and follow-up scans was 14.6-20 weeks after the commencement of ADT. Changes in skeletal muscle attenuation in Hounsfield units of the rectus femoris and the CSA of the rectus femoris, sartorius and quadricep muscles were assessed. Muscle attenuation of the rectus femoris muscle was significantly reduced following the initiation of ADT by 18.9% (P < 0.001). In addition, there was a significant decrease (P < 0.001) in the CSA for the sartorius, quadriceps and rectus femoris muscles. There was no effect of Zometa on muscle attenuation or muscle CSA. Our results indicate that not only muscle size but also muscle quality may be adversely affected by the undertaking of ADT in men with prostate cancer. Consequently, interventions to counteract deteriorations to both muscle mass and possibly muscle quality should be considered in men receiving ADT.

Effects of cannabinoid receptor 1 (brain) on lipid accumulation by transcriptional control of CPT1A and CPT1B

CB1 (also known as CNR1), a main receptor for cannabinoids acting at PPARs, can enhance fat deposition. Carnitine palmitoyltransferase-1 (CPT1), an enzyme responsible for the transport of long-chain fatty acids for β-oxidation, is closely related to fat deposition. Whether CB1 can regulate intramuscular adipocytes lipid accumulation through regulation of CPT1 is unclear. Based on the investigation of tissue- and breed-specific CPT1A and CPT1B mRNA expression levels in Jinhua and Landrace pigs, we studied the effects of CB1 on lipid accumulation and CPT1B expression by treating porcine intramuscular adipocytes with CB1 antagonist Δ9-THC and antagonist SR141716. Results showed that muscle CPT1 mRNA was expressed at higher levels in the longissimus dorsi and subcutaneous fat. Liver CPT1A mRNA expression levels were higher in the pancreas, duodenum and liver. Compared with Landrace pigs, CPT1A and CPT1B in the longissimus dorsi of Jinhua pigs were significantly higher and positively correlated with intramuscular fat content. However, for subcutaneous fat, CPT1 levels were significantly lower and negatively correlated with body fat percentage. Δ9-THC significantly increased CB1 mRNA levels and lipid accumulation but decreased CPT1A and CPT1B mRNA levels. Conversely, SR141716 reduced CB1 mRNA levels but increased CPT1A and CPT1B mRNA levels, resulting in decreased lipid accumulation. The CPT1 antagonist etomoxir did not affect CB1 expression, suggesting that CB1 is likely upstream of CPT1A and CPT1B. Meanwhile, PPARA expression was greatly decreased when CPT1A and CPT1B were inhibited and enhanced when CPT1A and CPT1B were activated. Taken together, these data indicate that CB1 can affect intramuscular fat deposition by regulating both CPT1A and CPT1B mRNA expression, with the PPARA signal pathway likely playing a major role in this process.

Structural and biochemical characteristics of bovine intramuscular connective tissue and beef quality

The aim of the study was to evaluate the impact of structural and biochemical characteristics of muscle intramuscular connective tissue on beef quality. The experimental design was based on three muscles of three breeds sampled as fresh material and cooked at 55°C (Longissimus thoracis and Semimembranosus) or at 70°C (Semimembranosus and Biceps femoris) for quality assessment. The results showed that muscle characteristics influence beef quality differently from one muscle to another. In grilled LT, proteoglycan content contributed negatively to juiciness, and intramuscular lipids were linked positively to tenderness, flavour, residues and overall liking scores. In grilled SM, cross-link and lipid contents were involved in beef quality. In BF cooked to 70°C, perimysial branch points were negatively linked to tenderness. In SM cooked to 70°C, perimysial area was involved in beef quality. These results should allow a better understanding of the factors involved in background toughness, in juiciness and flavour of meat.

Fatty acid transport proteins chronically relocate to the transverse-tubules in muscle from obese Zucker rats but are resistant to further insulin-induced translocation

ObjectivesRecently, we have demonstrated that FA transport proteins are located within the t-tubule fraction of rodent muscle, and that insulin stimulation causes their translocation to this membrane fraction. Chronic relocation of the FA transport protein FAT/CD36 to the sarcolemma is observed in obese rodents and humans, and correlates with intramuscular lipid accumulation and insulin resistance. It is not known whether in an obese, insulin resistant state FA transporters also chronically relocate to the t-tubules. Furthermore, it is not known whether the insulin-stimulated translocation of the various FA transport proteins to the t-tubules is impaired in insulin resistance. MethodsSarcolemmal and t-tubule membrane fractions were isolated via differential centrifugation from muscles of lean and obese female Zucker rats during basal or insulin stimulated conditions. FA transport proteins were measured via western blot on both membrane fractions. ResultsOur results demonstrate that in muscle from insulin resistant Zucker rats, FAT/CD36, FABPpm and FATP1 are all increased on the t-tubules in the basal state (+72%, +120%, and +69%, respectively), potentially contributing to the accumulation of intramuscular lipids. Insulin failed to increase the content of the FA transport proteins on either the t-tubule or sarcolemma above the elevated basal levels, analogous to the well characterized impairment of insulin-stimulated GLUT4 translocation to both membrane domains in obesity. ConclusionFA transport proteins chronically relocate to the t-tubule domain in insulin resistant muscle, potentially contributing to lipid accumulation. Further translocation of the FA transport proteins to this domain during insulin stimulation, however, is impaired.

Colour of fat, and colour, fatty acid composition and sensory characteristics of muscle from heifers offered alternative forages to grass silage in a finishing ration

The effect of type of silage offered to beef heifers during the finishing period on aspects of beef quality was determined. In two experiments, a diet based on grass silage (GS) was compared with a diet based on maize silage (MS) or whole-crop wheat silage (WCW). Compared to the GS-based diet, increasing the amount of MS linearly increased fat whiteness while the increase in fat whiteness due to WCW was dependent on the stage of crop maturity at harvesting. There was no effect of diet on muscle colour or on muscle pH measured at 48h post-mortem, drip loss, taste panel traits after 14days ageing or shear force values at 2, 7 or 14days ageing. The alternative silages decreased the n−3 polyunsaturated fatty acid proportion and increased the linoleic:linolenic acid ratio in intramuscular lipid. It is concluded that type of silage affects fat colour and fatty acid composition of muscle but not the other muscle characteristics examined.

The Effects of Long-Term Valsartan Treatment on Skeletal Muscle Fatty Acid Handling in Humans With Impaired Glucose Metabolism

Blocking the renin-angiotensin system reduces the incidence of type 2 diabetes mellitus in humans with impaired glucose metabolism (IGM). Nevertheless, underlying mechanisms remain to be established. The purpose of this study was to investigate the effects of the angiotensin II type 1 receptor blocker valsartan (VAL) on skeletal muscle fatty acid (FA) handling in subjects with IGM. This was a randomized, double-blind placebo-controlled trial at Maastricht University Medical Center. INTERVENTION/MAIN OUTCOMES/PARTICIPANTS: Fasting and postprandial skeletal muscle FA handling were assessed at baseline and after 26 weeks of treatment with VAL or placebo in 26 subjects with IGM. Fasting and postprandial skeletal muscle FA handling were determined by combining the forearm balance technique with stable isotopes of palmitate. [²H₂]-Palmitate was infused iv to label endogenous triacylglycerol (TAG) and free fatty acid (FFA) in the circulation, and [U-¹³C]-palmitate was incorporated in a high-fat mixed meal (2.6 MJ, 61% energy from fat) to label chylomicron TAG. Muscle biopsy samples were taken to determine im TAG, diacylglycerol (DAG), FFA, and phospholipid contents, their fractional synthetic rates and degree of saturation, and mRNA expression of oxidative genes. VAL decreased saturation of im TAG and DAG fractions but did not affect net muscle uptake of [²H₂]-palmitate, very low-density lipoprotein ([²H₂])-TAG and chylomicron ([U-¹³C])-TAG, and muscle mRNA expression. VAL decreased FA spillover, as estimated by circulating [U-¹³C]-palmitate, and FFA rate of appearance and tended to decrease chylomicron TAG concentrations. VAL treatment for 26 weeks decreased saturation of skeletal muscle TAG and DAG stores, suggesting altered intramuscular lipid partitioning of FA. The VAL-induced reduction in postprandial FA spillover, endogenous lipolysis, and chylomicron TAG concentrations indicate improved adipose tissue lipid buffering capacity.