Triglyceride content in skeletal muscle: variability and the source

Combined effects of a ketogenic diet and exercise training alter mitochondrial and peroxisomal substrate oxidative capacity in skeletal muscle

Ketogenic diets (KDs) are reported to improve body weight, fat mass, and exercise performance in humans. Unfortunately, most rodent studies have used a low-protein KD, which does not recapitulate diets used by humans. Since skeletal muscle plays a critical role in responding to macronutrient perturbations induced by diet and exercise, the purpose of this study was to test if a normal-protein KD (NPKD) impacts shifts in skeletal muscle substrate oxidative capacity in response to exercise training (ExTr). A high fat, carbohydrate-deficient NPKD (16.1% protein, 83.9% fat, 0% carbohydrate) was given to C57BL/6J male mice for 6 wk, whereas controls (Con) received a low-fat diet with similar protein (15.9% protein, 11.9% fat, 72.2% carbohydrate). After 3 wk on the diet, mice began treadmill training 5 days/wk, 60 min/day for 3 wks. The NPKD increased body weight and fat mass, whereas ExTr negated a continued rise in adiposity. ExTr increased intramuscular glycogen, whereas the NPKD increased intramuscular triglycerides. Neither the NPKD nor ExTr alone altered mitochondrial content; however, in combination, the NPKD-ExTr group showed increases in PGC-1α and markers of mitochondrial fission/fusion. Pyruvate oxidative capacity was unchanged by either intervention, whereas ExTr increased leucine oxidation in NPKD-fed mice. Lipid metabolism pathways had the most notable changes as the NPKD and ExTr interventions both enhanced mitochondrial and peroxisomal lipid oxidation and many adaptations were additive or synergistic. Overall, these results suggest that a combination of a NPKD and ExTr induces additive and/or synergistic adaptations in skeletal muscle oxidative capacity.NEW & NOTEWORTHY A ketogenic diet with normal protein content (NPKD) increases body weight and fat mass, increases intramuscular triglyceride storage, and upregulates pathways related to protein metabolism. In combination with exercise training, a NPKD induces additive and/or synergistic activation of AMPK, PGC-1α, mitochondrial fission/fusion genes, mitochondrial fatty acid oxidation, and peroxisomal adaptations in skeletal muscle. Collectively, results from this study provide mechanistic insight into adaptations in skeletal muscle relevant to keto-adaptation.

Clearance kinetics of the VGF-derived neuropeptide TLQP-21

TLQP-21 is a multifunctional neuropeptide and a promising new medicinal target for cardiometabolic and neurological diseases. However, to date its clearance kinetics and plasma stability have not been studied. The presence of four arginine residues led us to hypothesize that its half-life is relatively short. Conversely, its biological activities led us to hypothesize that the peptide is still taken up by adipose tissues effectively. [¹²⁵I]TLQP-21 was i.v. administered in rats followed by chasing the plasma radioactivity and assessing tissue uptake. Plasma stability was measured using LC-MS. In vivo lipolysis was assessed by the palmitate rate of appearance.

RESULTS

A small single i.v. dose of [¹²⁵I]TLQP-21 had a terminal half-life of 110 min with a terminal clearance rate constant, k_t, of 0.0063/min, and an initial half-life of 0.97 min with an initial clearance rate constant, k_i, of 0.71/min. The total net uptake by adipose tissue accounts for 4.4% of the entire dose equivalent while the liver, pancreas and adrenal gland showed higher uptake. Uptake by the brain was negligible, suggesting that i.v.-injected peptide does not cross the blood-brain-barrier. TLQP-21 sustained isoproterenol-stimulated lipolysis in vivo. Finally, TLQP-21 was rapidly degraded producing several N-terminal and central sequence fragments after 10 and 60 min in plasma in vitro. This study investigated the clearance and stability of TLQP-21 peptide for the first time. While its pro-lipolytic effect supports and extends previous findings, its short half-life and sequential cleavage in the plasma suggest strategies for chemical modifications in order to enhance its stability and therapeutic efficacy.

Inflexibility in intramuscular triglyceride fractional synthesis distinguishes prediabetes from obesity in humans

Whether intramuscular triglyceride (IMTG) concentration or flux is more important in the progression to type 2 diabetes is controversial. Therefore, this study examined IMTG concentration, as well as its fractional synthesis rate (FSR), in obese people with normal glucose tolerance (NGT; n = 20) vs. obese people with prediabetes (PD; n = 19), at rest and during exercise. Insulin action and secretion were assessed using an intravenous glucose tolerance test. [U-(13)C]palmitate was infused for 4 h before and throughout 1.5 h of treadmill walking at 50% VO(2(max)). IMTG concentration was measured by gas chromatograph/mass spectrometer, and FSR by gas chromatography-combustion isotope ratio mass spectrometer, from muscle biopsies taken immediately before and after exercise. Basal IMTG concentration was higher (43 +/- 5.7 vs. 27 +/- 3.9 mg/mg dry weight, P = 0.03) and FSR trended lower (0.23 +/- 0.04 vs. 0.32 +/- 0.05/h, P = 0.075), as did insulin action (S(i); 2.9 +/- 0.43 vs. 3.3 +/- 0.35 x 10(-4)/mU/ml, P = 0.07), in PD vs. NGT. IMTG concentration did not change significantly during exercise, but was no longer different in PD vs. NGT (45 +/- 7.7 vs. 37 +/- 5.8 mg/mg dry weight, P = 0.41). IMTG FSR suppressed during exercise in NGT (-81% to 0.06 +/- 0.13/h, P = 0.02), but not PD (+4% to 0.24 +/- 0.13%/h, P = 0.95). Palmitate oxidation was similar during rest (P = 0.92) and exercise (P = 0.94) between groups, but its source appeared different with more coming from muscle at rest and plasma during exercise in NGT, whereas the converse was true in PD. Altogether, higher basal IMTG concentration that is metabolically inflexible distinguishes obese people with PD from those with NGT.

Cross contamination of intramyocellular lipid signals through loss of bulk magnetic susceptibility effect differences in human muscle using 1H-MRSI at 4 T

Cross contamination of intramyocellular lipid (IMCL) signals through loss of bulk magnetic susceptibility (BMS) differences was detected in human muscles using proton magnetic resonance spectroscopic imaging (1H-MRSI) at 4 T by varying nominal voxel sizes on healthy subjects. In soleus muscle the IMCL content estimated in 1.00-ml-sized voxels was 15% and 30% higher than that in 0.25-ml voxels for nonobese ( P < 0.05) and obese ( P < 0.01) subjects, respectively, whereas no effect was observed on IMCL estimation in tibialis posterior (TP) and tibialis anterior (TA) regions for different voxel sizes. The unbiased 0.25-ml voxel size 1H-MRSI method was applied to measure IMCL content in nonobese sedentary (NOB-Sed), moderately trained (Ath), sedentary obese (OB), and Type 2 diabetic mellitus (DM) subjects. IMCL content in soleus was greatest in OB, with decreasing content in DM, Ath, and NOB-Sed, respectively (12.6 ± 1.6, 9.7 ± 1.8, 7.4 ± 1.0, 4.9 ± 0.5 mmol/kg wet wt; P < 0.05 by ANOVA; P < 0.05 OB vs. NOB-Sed or Ath). In TA, IMCL was equivalently elevated in DM and OB, which was higher than in Ath or NOB-Sed, respectively (4.2 ± 0.4 and 4.2 ± 0.7 vs. 2.7 ± 0.5 and 1.5 ± 0.3 mmol/kg wet wt; ANOVA, P < 0.05; P < 0.05 DM or OB vs. NOB-Sed). We conclude that IMCL content is overestimated when voxel size exceeds 0.25 ml despite measurement by optimized high-resolution 1H-MRSI at high field. When IMCL is measured unbiased by concomitant obesity, we find that it is strongly influenced by muscle type, training status, and the presence of obesity and Type 2 diabetes.

Muscle type-dependent responses to insulin in intramyocellular triglyceride turnover in obese rats

OBJECTIVE

To understand the role of hyperinsulinemia in intramyocellular (imc) triglyceride (TG) accumulation and in regulating imcTG turnover.

RESEARCH METHODS AND PROCEDURES

imcTG was first prelabeled by continuous infusion of [U-(14)C]glycerol (pulse), and then the rate of label loss from the prelabeled imcTG pool (turnover) in gastrocnemius, tibialis anterior, and soleus muscle of awake, high-fat-fed obese rats during the subsequent hyperinsulinemic-euglycemic clamp experiments (chase) was determined.

RESULTS

Post-absorptive basal fractional imcTG turnover rate in soleus was 0.010 +/- 0.001/min, significantly lower than that in gastrocnemius (0.026 +/- 0.002/min, p < 0.001) or tibialis anterior (0.030 +/- 0.002/min, p < 0.0001), a pattern reciprocal to their imcTG pool size. Insulin infusion at 25 pmol/kg per minute resulted in pathophysiological hyperinsulinemia (5-fold increase over the baseline value). This caused an increase in imcTG turnover by 3-fold in soleus (0.029 +/- 0.006/min, p = 0.002) but a decrease in gastrocnemius (0.012 +/- 0.003/min, p = 0.001) and in tibialis anterior (0.0064 +/- 0.001/min, p < 0.0001). Pathophysiological hyperinsulinemia suppressed hormone-sensitive lipase activity in heart (p = 0.01) and mesenteric fat (p = 0.05) but not in skeletal muscle (p > 0.05). The pool size of imcTG was not affected by hyperinsulinemia.

DISCUSSION

The results demonstrated muscle-type dependence in the response of imcTG turnover to hyperinsulinemia in the obesity model. The reciprocal insulin effects on imcTG turnover in oxidative vs. oxidative-glycolytic muscle indicated a possibility that oxidative muscle contributes more to insulin resistance under hyperinsulinemia if imcTG-fatty acid oxidation is a function of turnover. imcTG turnover does not seem to regulate imcTG pool size acutely.

Growth hormone-induced insulin resistance is associated with increased intramyocellular triglyceride content but unaltered VLDL-triglyceride kinetics

The ability of growth hormone (GH) to stimulate lipolysis and cause insulin resistance in skeletal muscle may be causally linked, but the mechanisms remain obscure. We investigated the impact of GH on the turnover of FFA and VLDL-TG, intramuscular triglyceride content (IMTG), and insulin sensitivity (euglycemic clamp) in nine healthy men in a randomized double-blind placebo-controlled crossover study after 8 days treatment with (A) Placebo+Placebo, (B) GH (2 mg daily)+Placebo, and (C) GH (2 mg daily)+Acipimox (250 mgx3 daily). In the basal state, GH (B) increased FFA levels (P<0.05), palmitate turnover (P<0.05), and lipid oxidation (P=0.05), but VLDL-TG kinetics were unaffected. Administration of acipimox (C) suppressed basal lipolysis but did not influence VLDL-TG kinetics. In the basal state, IMTG content increased after GH (B; P=0.03). Insulin resistance was induced by GH irrespective of concomitant acipimox (P<0.001). The turnover of FFA and VLDL-TG was suppressed by hyperinsulinemia during placebo and GH, whereas coadministration of acipimox induced a rebound increase FFA turnover and VLDL-TG clearance. We conclude that these results show that GH-induced insulin resistance is associated with increased IMTG and unaltered VLDL-TG kinetics; we hypothesize that fat oxidation in muscle tissue is an important primary effect of GH and that circulating FFA rather than VLDL-TG constitute the major source for this process; and the role of IMTG in the development of GH-induced insulin resistance merits future research.

Desaturation of skeletal muscle structural and depot lipids in obese individuals during a very-low-calorie diet intervention

OBJECTIVE

This study investigated whether a very-lowcalorie dietary intervention (VLCD) may influence composition of skeletal muscle cell membrane phospholipid and composition and concentration of intramyocellular triglyceride (IMTG) in obese subjects. The working hypothesis proposed that a VLCD would decrease saturated fatty acids (FAs) and increase long-chain polyunsaturated FAs (LCPUFAs) in muscular structural lipids, as such changes have been associated with improved insulin sensitivity.

RESEARCH METHODS AND PROCEDURES

Skeletal muscle biopsies (vastus lateralis) were obtained from 13 obese subjects (nine women) before and after 8 weeks on VLCD (approximately 600 to 800 kcal/d). FA composition in muscle cell membrane phospholipid and concentration and FA composition of IMTG were determined by gas-liquid chromatography.

RESULTS

Baseline BMI was 36.0 +/- 3.4 kg/m2. Weight loss was 9.3 +/- 1.1 kg (8.8 +/- 1.1%; p < 0.0001); loss of adipose tissue was 5.9 +/- 0.9 kg (p < 0.0001). Insulin resistance (by homeostasis model assessment) decreased (-44 +/- 7%; p < 0.001). Muscle cell membrane phospholipid saturated FAs decreased (-3.2 +/- 1.3%; p < 0.05), whereas monounsaturated FAs (4.3 +/- 1.7%; p < 0.05), LCPUFAs (11 +/- 6%; p < 0.05), and the ratio of LCPUFAs to saturated FAs (12 +/- 5%; p < 0.05) increased. IMTG decreased, but not significantly (-5%). IMTG-saturated FAs decreased (-3.3 +/- 1.5%; p < 0.05), whereas LCPUFAn-3 (29 +/- 9%; p < 0.01), LCPUFAn-6 (33 +/- 9%; p < 0.01), and the ratio of LCPUFAs to saturated FAs (34 +/- 8%; p < 0.001) increased. Plasma total cholesterol (-15 +/- 6%; p < 0.05), low-density lipoprotein-cholesterol (-16 +/- 5%; p < 0.01), high-density lipoprotein-cholesterol (-8 +/- 2%; p < 0.01), and plasma triglyceride (-19 +/- 12%; p = 0.10) all decreased during the VLCD.

DISCUSSION

Desaturation of both muscle cell membrane phospholipid and IMTG was significant but modest during a VLCD in obese subjects. Further research must delineate whether such changes in skeletal muscle structural and depot lipid composition themselves are enough to promote the observed improvements in insulin action.

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.

Assessment of intramuscular triglycerides: contribution to metabolic abnormalities

PURPOSE OF REVIEW

The purpose of this review is to highlight recent methodological advances that have helped us to understand the role of intramuscular triglycerides in human health as well as prevention and treatment of disease. In addition to both invasive (muscle biopsy) and noninvasive methods, techniques to assess other lipids within muscle will be reviewed.

RECENT FINDINGS

In addition to intramuscular triglycerides, other lipids contained within skeletal muscle such as diacylglycerol and ceramides may have an important role in human metabolic disease. Both ex-vivo and in-vivo methods to obtain serial measurements of these muscle lipids as a result of experimental perturbation have recently provided unique insight into their respective roles in human metabolism.

SUMMARY

Triglyceride accumulation within skeletal muscle has received considerable interest due to its potential role in insulin resistance and fatty acid metabolism of obesity, aging and type 2 diabetes. Observations that triglycerides themselves may be used as a fuel source for exercising muscle, and thus may not be detrimental in all circumstances, has in part prompted the development and implementation of analytical methods to quantify the fatty acid composition of muscle triglycerides as well as other lipid species within muscle, e.g. diacylglycerol and ceramides. In addition, noninvasive computed tomography, magnetic resonance spectroscopy and magnetic resonance imaging methods have been applied to examine the serial effects of intervention on muscle triglycerides.

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.

Rosiglitazone enhances glucose tolerance by mechanisms other than reduction of fatty acid accumulation within skeletal muscle

We hypothesized that improved glucose tolerance with rosiglitazone treatment would coincide with decreased levels of i.m. triacylglycerol (IMTG), diacylglycerol, and ceramide. Obese Zucker rats were randomly divided into two experimental groups: control (n = 9) and rosiglitazone (n = 9), with lean Zucker rats (n = 9) acting as a control group for obese controls. Rats received either vehicle or 3 mg/kg rosiglitazone for 6 wk. Glucose tolerance was impaired (P < 0.01) in obese compared with lean rats, but was normalized after rosiglitazone treatment. IMTG content was higher in obese compared with lean rats (70.5 +/- 5.1 vs. 27.5 +/- 2.0 micromol/g dry mass; P < 0.05) and increased an additional 30% (P < 0.05) with rosiglitazone treatment. Intramuscular fatty acid composition shifted toward a higher proportion of monounsaturates (P < 0.05) in obese rosiglitazone-treated rats due to an increase in palmitoleate (16:1; P < 0.05). Rosiglitazone treatment increased (P < 0.05) skeletal muscle diacylglycerol and ceramide levels by 65% and 100%, respectively, compared with obese rats, but elevated muscle diacylglycerol was not associated with changes in the total or membrane contents of the diacylglycerol-sensitive protein kinase C isoforms theta;, delta, alpha, and beta. In summary, we observed a disassociation among skeletal muscle IMTG, diacylglycerol and ceramide content, and glucose tolerance with rosiglitazone treatment in obese Zucker rats. Our data suggest, therefore, that rosiglitazone enhances glucose tolerance by mechanisms other than reduction of fatty acid accumulation within skeletal muscle.

High triacylglycerol turnover rate in human skeletal muscle

In the present study we investigated the relationship between plasma fatty acids (FA) and intramuscular triacylglycerol (IMTAG) kinetics of healthy volunteers. With this aim [U-(13)C]-palmitate was infused for 10 h and FA kinetics determined across the leg. In addition, the rate of FA incorporation into IMTAG in vastus lateralis muscle was determined during two consecutive 4-h periods (2-6 h and 6-10 h). Fifty to sixty per cent of the FA taken up from the circulation were esterified into IMTAG, whereas 32 and 42% were oxidized between 2-6 and 6-10 h, respectively. IMTAG fractional synthesis rate was 3.4 +/- 0.8% h(-1) and did not change between the two 4- h periods, despite an increase in arterial FA concentration (34%, P < 0.01). IMTAG concentration was also unchanged, implying that the IMTAG fractional synthesis rate was balanced by an equal rate of breakdown. FA oxidation increased over time, which could be due to the observed decline in plasma insulin concentration (-74%, P < 0.01). In conclusion, a substantial fraction of the fatty acids entering skeletal muscle in post-absorptive healthy individuals is esterified into IMTAG, due to its high turnover rate (29 h pool(-1)). An increase in FA level, as a consequence of short-term fasting, does not seem to increase IMTAG synthesis rate and pool size.

Níveis de triglicerídeos intra e extracelulares em músculos humanos mediante ¹H-ERM: um estudo de caso

O objetivo deste estudo foi analisar o consumo de triglicerídeos intra (IT) e extracelulares (ET) nos músculos sóleo, tibial anterior e vasto medial após uma prova de quatro horas de ciclismo de estrada. Esta pesquisa caracterizou-se por ser um estudo de caso de um ciclista que participa de competições internacionais. Os estudos de ressonância magnética utilizaram os seguintes parâmetros espectrais impostos para o ajuste no domínio do tempo, como a distância das freqüências entre os sinais de IT e ET. Os valores de amplitudes dos triglicerídeos intra e extracelulares foram divididos pela ressonância de água. Concluímos que o músculo vasto medial do ciclista apresentou maior consumo de triglicerídeos depois de quatro horas de ciclismo em estrada. Portanto, constatou-se que um trabalho com intensidade de 80% da freqüência cardíaca máxima permitiu consumo de triglicerídeos intramusculares durante o exercício.

Evidence for increased and insulin-resistant lipolysis in skeletal muscle of high-fat-fed rats

The metabolic and isotopic profiles of glycerol in skeletal muscle were examined using awake, fasted lean and high-fat-induced obese rats, and hyperinsulinemic-euglycemic clamp was performed to assess the effect of insulin. During the clamp, Intralipid (no heparin; Fresnius Kabi Clayton, Clayton, NC), free fatty acids, glycerol, and glucose were coinfused to maintain their respective basal plasma levels in both groups. At steady-state, [U-(14)C]glycerol was infused intravenously for 120 minutes followed by muscle biopsy. The classical phenotypic characteristics of obesity, namely, reduced insulin-stimulated glucose uptake, a failure to suppress systemic lipolysis by insulin, and elevated plasma fatty acid concentration, were observed in the obese rats. Novel observations showed that in the basal state, the isotopic specific activity (S.A.) of glycerol (dpm/nmol) in gastrocnemius (0.03 v 0.12), soleus (0.05 v 0.12), and tibialis anterior (0.03 v 0.12) was significantly lower (all P <.003) in obese than in lean rats despite similar concentrations, indicating an active basal intramyocellular lipolysis. In addition, the lipolysis appeared resistant to insulin because the suppression of muscle glycerol during the clamp was 8%, 12%, and 8% in obese compared to 67%, 71%, and 63% in the lean control for gastrocnemius (P =.001), soleus (P =.007), and tibialis anterior (P =.004), respectively. The active intracellular lipolysis likely disturbs metabolic functions that may contribute to insulin resistance.

Substrate concentration and metabolism in left and right muscles of rats

Skeletal muscle fiber heterogeneity among muscle groups is well known; however, laterality of muscle metabolism has not been addressed. In the present studies, metabolite concentrations in left and right gastrocnemius, tibialis anterior, quadriceps, and soleus muscles and their response to exogenous insulin have been compared in fasted awake rats. The results indicated that the concentrations of muscle free glycerol (P >.4), glycerol 3-phosphate (P >.1) nonesterified fatty acids (NEFA) (P >.6) and intramyocellular triglycerides (imcTG) (P >.08) are comparable between left and right of the same muscle, and are similar among mixed glycolytic-oxidative muscles. The concentration of free glycerol in soleus responded to exogenous insulin in a pattern distinct to that seen for the mixed muscles. The results support interchangeable use of left and right side of same muscles, and probably among different muscles of similar fiber type, but not muscles of different fiber types.

Intramyocellular lipids form an important substrate source during moderate intensity exercise in endurance-trained males in a fasted state

Both stable isotope methodology and fluorescence microscopy were applied to define the use of intramuscular triglyceride (IMTG) stores as a substrate source during exercise on a whole-body as well as on a fibre type-specific intramyocellular level in trained male cyclists. Following an overnight fast, eight subjects were studied at rest, during 120 min of moderate intensity exercise (60 % maximal oxygen uptake capacity (VO2,max)) and 120 min of post-exercise recovery. Continuous infusions of [U-13C]palmitate and [6,6-2H2]glucose were administered at rest and during subsequent exercise to quantify whole-body plasma free fatty acid (FFA) and glucose oxidation rates and the contribution of other fat sources (sum of muscle- plus lipoprotein-derived TG) and muscle glycogen to total energy expenditure. Fibre type-specific intramyocellular lipid content was determined in muscle biopsy samples collected before, immediately after and 2 h after exercise. At rest, fat oxidation provided 66 +/- 5 % of total energy expenditure, with FFA and other fat sources contributing 48 +/- 6 and 17 +/- 3 %, respectively. FFA oxidation rates increased during exercise, and correlated well with the change in plasma FFA concentrations. Both the use of other fat sources and muscle glycogen declined with the duration of exercise, whereas plasma glucose production and utilisation increased (P < 0.001). On average, FFA, other fat sources, plasma glucose and muscle glycogen contributed 28 +/- 3, 15 +/- 2, 12 +/- 1 and 45 +/- 4 % to total energy expenditure during exercise, respectively. Fluorescence microscopy revealed a 62 +/- 7 % net decline in muscle lipid content following exercise in the type I fibres only, with no subsequent change during recovery. We conclude that IMTG stores form an important substrate source during moderate intensity exercise in endurance-trained male athletes following an overnight fast, with the oxidation rate of muscle- plus lipoprotein-derived TG being decreased with the duration of exercise.

Accelerated intramyocellular triglyceride synthesis in skeletal muscle of high-fat-induced obese rats

OBJECTIVE

To test the hypothesis that the synthesis of intramyocellular triglycerides (imcTG) in skeletal muscle is increased in obese rats in which the content of imcTG is known to be abnormally high.

ANIMALS

Sprague-Dawley male lean and high-fat-induced obese rats were studied at the age of 4, 8 and 12 months after an overnight fast, awake.

MEASUREMENTS

[U-(14)C]glycerol was continuously infused intravenously for 2 h followed by muscle biopsies, and intracellular glycerol incorporation into imcTG was determined. imcTG content, intramyocellular free glycerol concentration and specific activity, systemic glycerol flux and plasma glycerol, free fatty acid (FFA) and glucose concentrations were also determined.

RESULTS

The rates of incorporation of intramyocellular glycerol into imcTG (nmol/g wet muscle/h) were markedly accelerated in obese rats compared to their lean littermates at all ages: 66+/-12 vs 12+/-2 (P=0.02) for gastrocnemius and 74+/-29 vs 31+/-7 (P=0.09) for soleus when 4 months old; 223+/-29 vs 58+/-27 (P=0.001) for gastrocnemius, 224+/-28 vs 70+/-21 (P=0.001) for soleus and 294+/-78 vs 49+/-22 (P=0.02) for tibialis anterior when 8 months old; and 25+/-4 vs 11+/-2 (P=0.01) for gastrocnemius and 22+/-8 vs 8.4+/-3 (P=0.04) for soleus when 12 months old. As expected, this was accompanied by a higher imcTG content in virtually all muscles at all ages tested.

CONCLUSION

The synthesis of imcTG in skeletal muscle is grossly increased in obese rats, which likely contributes to abnormal imcTG accumulation.

Human skeletal muscle fatty acid and glycerol metabolism during rest, exercise and recovery

This study was conducted to investigate skeletal muscle fatty acid (FA) and glycerol kinetics and to determine the contribution of skeletal muscle to whole body FA and glycerol turnover during rest, 2 h of one-leg knee-extensor exercise at 65 % of maximal leg power output, and 3 h of recovery. To this aim, the leg femoral arterial-venous difference technique was used in combination with a continuous infusion of [U-(13)C]palmitate and [(2)H(5)]glycerol in five post-absorptive healthy volunteers (22 +/- 3 years). The influence of contamination from non-skeletal muscle tissues, skin and subcutaneous adipose tissue, on FA and glycerol kinetics was studied by catheterization of the femoral vein in antegrade and retrograde directions. Substantially higher net leg FA and glycerol uptakes were observed with a retrograde compared to an antegrade catheter position, as a result of a much lower tracer-calculated leg FA and glycerol release. The whole body FA rate of appearance (R(a)) increased with exercise and decreased rapidly in recovery but stayed higher compared to pre-exercise. The leg net FA uptake decreased immediately on cessation of exercise to near pre-exercise level, but the tracer FA uptake and release decreased slowly and reached constant values after approximately 1.5 h of recovery similar to pre-exercise. Whole body FA reesterification (FA R(d) - FA oxidation; R(d), rate of disappearance) was approximately 400 micromol min(-1) at rest and during exercise, and increased during recovery to 495 micromol min(-1). Leg FA reesterification was 17 micromol min(-1) at rest and decreased to 9 micromol min(-1) during recovery, due to a larger fraction of leg FA uptake being directed to oxidation. A net glycerol exchange across the leg could not be detected under all conditions, but a substantial leg glycerol uptake was observed, which was substantially higher during exercise. Total body skeletal muscle FA and glycerol uptake/release was estimated to account for 18-25 % of whole body R(d) or R(a).

IN CONCLUSION

(1) skeletal muscle FA and glycerol metabolism, using the leg arterial-venous difference method, can only be studied if contamination from skin and subcutaneous adipose tissue is prevented; (2) whole body FA reesterification is unchanged when going from rest to exercise, but is increased during recovery; (3) in post-absorptive man total body skeletal muscle contributes 17-24 % to whole body FA and glycerol turnover and FA reesterification at rest; (4) glycerol is taken up by skeletal muscle and the uptake increases many fold during exercise.

Positional distribution of CLA in TAG of lamb tissues

The content and positional distribution of CLA in TAG fractions of lamb tissues was examined with either preformed CLA or the linoleic acid precursor of CLA in the diet as experimental treatments. The CLA content of phospholipid (PL) from these tissues was also examined. Thirteen lambs were randomized to the following dietary treatments: (i) control diet (no supplement); (ii) CLA supplementation (0.33 g d(-1) for 21 d prior to weaning) to milk-replacer of pre-ruminating lambs, or (iii) feeding linoleic acid-rich oil (6% safflower oil on a dry matter basis) to weaned ruminating lambs. At slaughter, tissue samples were procured from diaphragm, rib muscle, and subcutaneous (SC) adipose tissue. Safflower oil supplementation in the diet resulted in an increase in CLA content of the TAG from diaphragm, rib muscle, and SC adipose tissue by about threefold (P< 0.05) on a mol% basis. CLA was localized to the sn-1/3 positions of TAG. Animals that received pre-formed CLA, however, had increased proportions of CLA at the sn-2 position of TAG from SC adipose tissue, suggesting that there were tissue-specific dietary effects and possible age-related effects on the mode of FA incorporation into TAG. Safflower oil supplementation in the diet had no effect on the CLA content of PL from diaphragm, rib muscle, and SC adipose tissue, suggesting that CLA was preferentially incorporated into the TAG of these tissues.