Effects of epinephrine on lipid metabolism in resting skeletal muscle

β2-Adrenergic Signaling Modulates Mitochondrial Function and Morphology in Skeletal Muscle in Response to Aerobic Exercise

The molecular mechanisms underlying skeletal muscle mitochondrial adaptations induced by aerobic exercise (AE) are not fully understood. We have previously shown that AE induces mitochondrial adaptations in cardiac muscle, mediated by sympathetic stimulation. Since direct sympathetic innervation of neuromuscular junctions influences skeletal muscle homeostasis, we tested the hypothesis that β₂-adrenergic receptor (β₂-AR)-mediated sympathetic activation induces mitochondrial adaptations to AE in skeletal muscle. Male FVB mice were subjected to a single bout of AE on a treadmill (80% Vmax, 60 min) under β₂-AR blockade with ICI 118,551 (ICI) or vehicle, and parameters of mitochondrial function and morphology/dynamics were evaluated. An acute bout of AE significantly increased maximal mitochondrial respiration in tibialis anterior (TA) isolated fiber bundles, which was prevented by β₂-AR blockade. This increased mitochondrial function after AE was accompanied by a change in mitochondrial morphology towards fusion, associated with increased Mfn1 protein expression and activity. β₂-AR blockade fully prevented the increase in Mfn1 activity and reduced mitochondrial elongation. To determine the mechanisms involved in mitochondrial modulation by β₂-AR activation in skeletal muscle during AE, we used C2C12 myotubes, treated with the non-selective β-AR agonist isoproterenol (ISO) in the presence of the specific β₂-AR antagonist ICI or during protein kinase A (PKA) and Gα_i protein blockade. Our in vitro data show that β-AR activation significantly increases mitochondrial respiration in myotubes, and this response was dependent on β₂-AR activation through a Gα_s-PKA signaling cascade. In conclusion, we provide evidence for AE-induced β₂-AR activation as a major mechanism leading to alterations in mitochondria function and morphology/dynamics. β₂-AR signaling is thus a key-signaling pathway that contributes to skeletal muscle plasticity in response to exercise.

The Regulation of Fat Metabolism During Aerobic Exercise

Since the lipid profile is altered by physical activity, the study of lipid metabolism is a remarkable element in understanding if and how physical activity affects the health of both professional athletes and sedentary subjects. Although not fully defined, it has become clear that resistance exercise uses fat as an energy source. The fatty acid oxidation rate is the result of the following processes: (a) triglycerides lipolysis, most abundant in fat adipocytes and intramuscular triacylglycerol (IMTG) stores, (b) fatty acid transport from blood plasma to muscle sarcoplasm, (c) availability and hydrolysis rate of intramuscular triglycerides, and (d) transport of fatty acids through the mitochondrial membrane. In this review, we report some studies concerning the relationship between exercise and the aforementioned processes also in light of hormonal controls and molecular regulations within fat and skeletal muscle cells.

Muscle Lipid Droplets: Cellular Signaling to Exercise Physiology and Beyond

Conventionally viewed as energy storage depots, lipid droplets (LDs) play a central role in muscle lipid metabolism and intracellular signaling, as recognized by recent advances in our biological understanding. Specific subpopulations of muscle LDs, defined by location and associated proteins, are responsible for distinct biological functions. In this review, the traditional view of muscle LDs is examined, and the emerging role of LDs in intracellular signaling is highlighted. The effects of chronic and acute exercise on muscle LD metabolism and signaling is discussed. In conclusion, future directions for muscle LD research are identified. The primary focus will be on human studies, with inclusion of select animal/cellular/non-muscle studies as appropriate, to provide the underlying mechanisms driving the observed findings.

Ghrelin stimulates fatty acid oxidation and inhibits lipolysis in isolated muscle from male rats

Ghrelin is classically known as a central appetite-stimulating hormone but has recently been recognized to have a significant role in peripheral tissue energy metabolism. However, the direct effects of ghrelin on skeletal muscle, a major site for glucose and lipid disposal, remain understudied. We found that the two major ghrelin isoforms, acylated and unacylated ghrelin, were able to significantly increase skeletal muscle fatty acid oxidation (~20%) while incorporation of fatty acids into major lipid pools remained unchanged. The increase in fatty acid oxidation was accompanied by increases in acetyl-CoA carboxylase phosphorylation, a downstream target of AMPK. Ghrelin isoforms had no independent effect on lipolysis under unstimulated conditions, but nearly completely abolished epinephrine-stimulated lipolysis. This effect was generally, but not consistently related to a blunting in the phosphorylation of HSL activation sites, Ser660 and 563. Taken together, these findings suggest that ghrelin isoforms have a direct, acute effect on fatty acid oxidation and lipolysis.

Muscle Lipid Metabolism: Role of Lipid Droplets and Perilipins

Skeletal muscle is one of the main regulators of carbohydrate and lipid metabolism in our organism, and therefore, it is highly susceptible to changes in glucose and fatty acid (FA) availability. Skeletal muscle is an extremely complex tissue: its metabolic capacity depends on the type of fibers it is made up of and the level of stimulation it undergoes, such as acute or chronic contraction. Obesity is often associated with increased FA levels, which leads to the accumulation of toxic lipid intermediates, oxidative stress, and autophagy in skeletal fibers. This lipotoxicity is one of the most common causes of insulin resistance (IR). In this scenario, the "isolation" of certain lipids in specific cell compartments, through the action of the specific lipid droplet, perilipin (PLIN) family of proteins, is conceived as a lifeguard compensatory strategy. In this review, we summarize the cellular mechanism underlying lipid mobilization and metabolism inside skeletal muscle, focusing on the function of lipid droplets, the PLIN family of proteins, and how these entities are modified in exercise, obesity, and IR conditions.

Aerobic Exercise and Pharmacological Therapies for Skeletal Myopathy in Heart Failure: Similarities and Differences

Skeletal myopathy has been identified as a major comorbidity of heart failure (HF) affecting up to 20% of ambulatory patients leading to shortness of breath, early fatigue, and exercise intolerance. Neurohumoral blockade, through the inhibition of renin angiotensin aldosterone system (RAS) and β-adrenergic receptor blockade (β-blockers), is a mandatory pharmacological therapy of HF since it reduces symptoms, mortality, and sudden death. However, the effect of these drugs on skeletal myopathy needs to be clarified, since exercise intolerance remains in HF patients optimized with β-blockers and inhibitors of RAS. Aerobic exercise training (AET) is efficient in counteracting skeletal myopathy and in improving functional capacity and quality of life. Indeed, AET has beneficial effects on failing heart itself despite being of less magnitude compared with neurohumoral blockade. In this way, AET should be implemented in the care standards, together with pharmacological therapies. Since both neurohumoral inhibition and AET have a direct and/or indirect impact on skeletal muscle, this review aims to provide an overview of the isolated effects of these therapeutic approaches in counteracting skeletal myopathy in HF. The similarities and dissimilarities of neurohumoral inhibition and AET therapies are also discussed to identify potential advantageous effects of these combined therapies for treating HF.

Characterization of lipolytic inhibitor G(0)/G(1) switch gene-2 protein (G0S2) expression in male Sprague-Dawley rat skeletal muscle compared to relative content of adipose triglyceride lipase (ATGL) and comparitive gene identification-58 (CGI-58)

The rate-limiting enzyme in lipolysis, adipose triglyceride lipase (ATGL), is activated by comparative gene identification-58 (CGI-58) and inhibited by the G(0)/G(1) switch gene-2 (G0S2) protein. It is speculated that inhibition of ATGL is through a dose dependent manner of relative G0S2 protein content. There is little work examining G0S2 expression in lipolytic tissues, and the relative expression across oxidative tissues such as skeletal muscle has not yet been described. Three muscles, soleus (SOL), red gastrocnemius (RG), and white gastrocnemius (WG) were excised from 57-day old male Sprague-Dawley rats (n = 9). QRT-PCR was used for mRNA analysis, and western blotting was conducted to determine protein content. ATGL and G0S2 protein content were both greatest in the lipolytic SOL, with the least amount of both ATGL and G0S2 protein content found in the WG. CGI-58 protein content however did not mirror ATGL and G0S2 protein content, since the RG had the greatest CGI-58 protein content when compared to the SOL and WG. When comparing our tissues based on CGI-58-to-ATGL ratio and G0S2-to-ATGL ratio, it was discovered that contrary to oxidative demand, the glycolytic WG had the greatest activator CGI-58-to-ATGL ratio with the oxidative SOL having the least, and no differences in G0S2-to-ATGL across the three muscle types. These data suggest that the content of G0S2 relative to the lipase in skeletal muscle would not predict lipolytic potential.

Metabolomic analysis of simvastatin and fenofibrate intervention in high-lipid diet-induced hyperlipidemia rats

AIM

To investigate the metabolite changes caused by simvastatin or fenofibrate intervention in diet-induced hyperlipidemia rats using a GC-MS-based metabolomic profiling approach.

METHODS

SD rats were fed with high-lipid diet for 4 weeks to induce hyperlipidemia, then the rats were fed with normal diet, and orally administered with simvastatin (10 mg·kg(-1)·d(-1)) or fenofibrate (150 mg·kg(-1)·d(-1)) for 2 weeks. Blood samples were collected once a week, and potential biomarkers were examined using commercial assay kits and a metabolomic approach. The metabolomics data were analyzed using a multivariate statistical technique and a principal component analysis (PCA).

RESULTS

Oral administration of simvastatin or fenofibrate significantly decreased the plasma levels of total cholesterol (TC) and low-density lipoprotein (LDL) cholesterol and increased the plasma level of high-density lipoprotein (HDL) cholesterol in the hyperlipidemia rats. Plasma samples were scattered in the PCA scores plots in response to the diet and to the drugs administered. The main metabolites changed in the hyperlipidemia rats were cholesterol, creatinine, linoleic acid, β-hydroxybutyric acid, tyrosine, isoleucine and ornithine. The plasma level of creatinine was significantly lower in the simvastatin-treated rats than in the fenofibrate-treated rats. The plasma tyrosine concentration was declined following intake of high-lipid diet, which was reversed by fenobrate, but not by simvastatin.

CONCLUSION

A series of potential biomarkers including tyrosine, creatinine, linoleic acid, β-hydroxybutyric acid and ornithine have been identified by metabolomic profiling, which may be used to identify the metabolic changes during hyperlipidemia progression.

PLIN5 deletion remodels intracellular lipid composition and causes insulin resistance in muscle

Defective control of lipid metabolism leading to lipotoxicity causes insulin resistance in skeletal muscle, a major factor leading to diabetes. Here, we demonstrate that perilipin (PLIN) 5 is required to couple intramyocellular triacylglycerol lipolysis with the metabolic demand for fatty acids. PLIN5 ablation depleted triacylglycerol stores but increased sphingolipids including ceramide, hydroxylceramides and sphingomyelin. We generated perilipin 5 (Plin5)(-/-) mice to determine the functional significance of PLIN5 in metabolic control and insulin action. Loss of PLIN5 had no effect on body weight, feeding or adiposity but increased whole-body carbohydrate oxidation. Plin5 (-/-) mice developed skeletal muscle insulin resistance, which was associated with ceramide accumulation. Liver insulin sensitivity was improved in Plin5 (-/-) mice, indicating tissue-specific effects of PLIN5 on insulin action. We conclude that PLIN5 plays a critical role in coordinating skeletal muscle triacylglycerol metabolism, which impacts sphingolipid metabolism, and is requisite for the maintenance of skeletal muscle insulin action.

Enhanced sympathetic activity in mice with brown adipose tissue transplantation (transBATation)

Brown adipose tissue (BAT) burns calories to produce heat, and is thus relevant to energy balance. Interscapular BAT (IBAT) of donor mice was transplanted into recipient mice (transBATation). To test whether transBATation counteracts high-fat diet (HFD)-induced obesity, some sham-operated and recipient mice were fed a HFD (HFD-sham, HFD-trans) while others remained on a standard chow (chow-sham, chow-trans). HFD-trans mice had lower body weight and fat and greater energy expenditure, but similar caloric intake compared with HFD-sham mice. We hypothesized that HFD-trans mice had elevated sympathetic activity compared with HFD-sham mice, contributing to increased energy expenditure and fuel mobilization. This was supported by findings that HFD-trans mice had greater energy expenditure during a norepinephrine challenge test and higher core temperatures after cold exposure than did HFD-sham mice, implicating enhanced whole-body metabolic response and elevated sympathetic activity. Additionally, transBATation selectively increased sympathetic drive to some, but not all, white adipose tissue depots and skeletal muscles, as well as the endogenous IBAT, heart, and liver. Collectively, transBATation confers resistance to HFD-induced obesity via increase in whole-body sympathetic activity, and differential activation of sympathetic drive to some of the tissues involved in energy expenditure and fuel mobilization.

Chemical modulation of glycerolipid signaling and metabolic pathways

Thirty years ago, glycerolipids captured the attention of biochemical researchers as novel cellular signaling entities. We now recognize that these biomolecules occupy signaling nodes critical to a number of physiological and pathological processes. Thus, glycerolipid-metabolizing enzymes present attractive targets for new therapies. A number of fields-ranging from neuroscience and cancer to diabetes and obesity-have elucidated the signaling properties of glycerolipids. The biochemical literature teems with newly emerging small molecule inhibitors capable of manipulating glycerolipid metabolism and signaling. This ever-expanding pool of chemical modulators appears daunting to those interested in exploiting glycerolipid-signaling pathways in their model system of choice. This review distills the current body of literature surrounding glycerolipid metabolism into a more approachable format, facilitating the application of small molecule inhibitors to novel systems. This article is part of a Special Issue entitled Tools to study lipid functions.

IL-6 and epinephrine have divergent fiber type effects on intramuscular lipolysis

IL-6 is an exercise-regulated myokine that has been suggested to increase lipolysis in fast-twitch skeletal muscle. However, it is not known if a similar effect is present in slow-twitch muscle. Furthermore, epinephrine increases IL-6 secretion from skeletal muscle, suggesting that IL-6 could play a role in mediating the lipolytic effects of catecholamines. The purpose of this study was to determine whether IL-6 stimulates skeletal muscle lipolysis in a fiber type dependent manner and is required for epinephrine-stimulated lipolysis in murine skeletal muscle. Soleus and extensor digitorum longus (EDL) muscles from male C57BL/6J wild-type and IL-6(-/-) mice were incubated with 1 μM (183 ng/ml) epinephrine or 75 ng/ml recombinant IL-6 (rIL-6) for 60 min. IL-6 treatment increased 5'-AMP-activated protein kinase and signal transducer and activator of transcription 3 phosphorylation and glycerol release in isolated EDL but not soleus muscles from C57BL/6J mice. Conversely, epinephrine increased glycerol release in soleus but not EDL muscles from C57BL/6J mice. Basal lipolysis was elevated in soleus muscle from IL-6(-/-) mice, and this was associated with increases in adipose triglyceride lipase (ATGL) and its coactivator comparative gene identification-58 (CGI-58). The increase in ATGL content does not appear to be due to a loss of IL-6's direct effects, because ex vivo treatment with IL-6 failed to alter the expression of ATGL mRNA in soleus muscle. In summary, IL-6 stimulates lipolysis in glycolytic but not oxidative muscle, whereas the opposite fiber type effect is seen with epinephrine. The absence of IL-6 indirectly upregulates lipolysis, and this is associated with increases in ATGL and its coactivator CGI-58.

Skeletal muscle PLIN3 and PLIN5 are serine phosphorylated at rest and following lipolysis during adrenergic or contractile stimulation

In adipose tissue, access of adipose triglyceride and hormone-sensitive lipases (ATGL and HSL) to the lipid droplet depends on PLIN1 phosphorylation, however, PLIN1 is not expressed in skeletal muscle and the phosphorylation of the expressed PLINs has yet to be investigated. Further, direct interactions between skeletal muscle PLINs and HSL are unknown. We investigated the isolated and combined effects of epinephrine and contraction on PLIN-to-lipase interactions as well as phosphorylation. Isolated rat solei were assigned to one of four 30 min in vitro conditions (25°C): (1) rest; (2) intermittent tetanic stimulation (60 Hz for 150 msec; train rate 20/min); (3) 5 nmol/L epinephrine; (4) intermittent tetanic stimulation and 5 nmol/L epinephrine. Immunoprecipitation of serine phosphorylated proteins followed by Western blotting for PLIN2, PLIN3, PLIN5, revealed that only PLIN2 is not phosphorylated under any of the experimental conditions. This is the first study to show that in whole rat skeletal muscle PLIN3 and PLIN5 are serine phosphorylated. The degree of serine phosphorylation remained unchanged following adrenergic and/or contractile stimulation. Oil red O staining of muscle sections for lipid content shows a significant decrease following each condition, confirming lipolysis occurred (P < 0.05). PLIN2, 3, and 5 all interact with HSL and ATGL, but these interactions were unchanged following treatments. Our results show that in skeletal muscle, PLIN2 is not serine phosphorylated at rest or with lipolytic stimulation and that while PLIN3, PLIN5 are serine phosphorylated at rest, the degree of phosphorylation does not change with lipolytic stimulation.

Contraction-induced lipolysis is not impaired by inhibition of hormone-sensitive lipase in skeletal muscle

In skeletal muscle hormone-sensitive lipase (HSL) has long been accepted to be the principal enzyme responsible for lipolysis of intramyocellular triacylglycerol (IMTG) during contractions. However, this notion is based on in vitro lipase activity data, which may not reflect the in vivo lipolytic activity. We investigated lipolysis of IMTG in soleus muscles electrically stimulated to contract ex vivo during acute pharmacological inhibition of HSL in rat muscles and in muscles from HSL knockout (HSL-KO) mice. Measurements of IMTG are complicated by the presence of adipocytes located between the muscle fibres. To circumvent the problem with this contamination we analysed intramyocellular lipid droplet content histochemically. At maximal inhibition of HSL in rat muscles, contraction-induced breakdown of IMTG was identical to that seen in control muscles (P < 0.001). In response to contractions IMTG staining decreased significantly in both HSL-KO and WT muscles (P < 0.05). In vitro TG hydrolase activity data revealed that adipose triglyceride lipase (ATGL) and HSL collectively account for ∼98% of the TG hydrolase activity in mouse skeletal muscle, other TG lipases accordingly being of negligible importance for lipolysis of IMTG. The present study is the first to demonstrate that contraction-induced lipolysis of IMTG occurs in the absence of HSL activity in rat and mouse skeletal muscle. Furthermore, the results suggest that ATGL is activated and plays a major role in lipolysis of IMTG during muscle contractions.

Effects of fatty acid provision during severe hypoxia on routine and maximal performance of the in situ tilapia heart

The ability to maintain stable cardiac function during environmental hypoxia exposure is crucial for hypoxia tolerance in animals and depends upon the maintenance of cardiac energy balance as well as the state of the heart's extracellular environment (e.g., availability of metabolic fuels). Hypoxic depression of plasma [non-esterified fatty acids] (NEFA), an important cardiac aerobic fuel, is a common response in many species of hypoxia-tolerant fishes, including tilapia. We tested the hypothesis that decreased plasma [NEFA] is important for maintaining stable cardiac function during and following hypoxia exposure, based on the premise that continued reliance upon cardiac fatty acid metabolism under such conditions could impair cardiac function. We examined the effect of severe hypoxia exposure (PO2 < 0.2 kPa) on routine and maximum performance of the in situ perfused tilapia heart under conditions of routine (400 μmol L(-1)) and low (75 μmol L(-1)) [palmitate], which mimicked the in vivo levels of plasma [NEFA] found in normoxic and hypoxic tilapia, respectively. Under both concentrations of palmitate, the in situ tilapia heart showed exceptional hypoxic performance as a result of a high maximum glycolytic potential, confirming our previous results using a perfusate without fatty acids. We additionally provide evidence suggesting that non-contractile ATP demand is depressed in tilapia heart during hypoxia exposure. Cardiac performance during and following severe hypoxia exposure was unaffected by the level of palmitate. Thus, we conclude that hypoxic depression of plasma [NEFA] in fishes does not play a role in cardiac hypoxia tolerance.

Skeletal muscle PLIN proteins, ATGL and CGI-58, interactions at rest and following stimulated contraction

Evidence indicates that skeletal muscle lipid droplet-associated proteins (PLINs) regulate lipolysis through protein-protein interactions on the lipid droplet surface. In adipocytes, PLIN1 is thought to regulate lipolysis by directly interacting with comparative gene identification-58 (CGI-58), an activator of adipose triglyceride lipase (ATGL). Upon lipolytic stimulation, PLIN1 is phosphorylated, releasing CGI-58 to fully activate ATGL and initiate triglyceride breakdown. The absence of PLIN1 in skeletal muscle leads us to believe that other PLIN family members undertake this role. Our purpose was to examine interactions between PLIN2, PLIN3, and PLIN5, with ATGL and its coactivator CGI-58 at rest and following contraction. Isolated rat solei were incubated for 30 min at rest or during 30 min of intermittent tetanic stimulation [150-ms volleys at 60 Hz with a train rate of 20 tetani/min (25°C)] to maximally stimulate intramuscular lipid breakdown. Results show that the interaction between ATGL and CGI-58 increased 128% following contraction (P = 0.041). Further, ATGL interacts with PLIN2, PLIN3, and PLIN5 at rest and following contraction. The PLIN2-ATGL interaction decreased significantly by 21% following stimulation (P = 0.013). Both PLIN3 and PLIN5 coprecipitated with CGI-58 at rest and following contraction, while there was no detectable interaction between PLIN2 and CGI-58 in either condition. Therefore, our findings indicate that in skeletal muscle, during contraction-induced muscle lipolysis, ATGL and CGI-58 strongly associate and that the PLIN proteins work together to regulate lipolysis, in part, by preventing ATGL and CGI-58 interactions at rest.

Lipid metabolism in skeletal muscle: generation of adaptive and maladaptive intracellular signals for cellular function

Fatty acids derived from adipose tissue lipolysis, intramyocellular triacylglycerol lipolysis, or de novo lipogenesis serve a variety of functions in skeletal muscle. The two major fates of fatty acids are mitochondrial oxidation to provide energy for the myocyte and storage within a variety of lipids, where they are stored primarily in discrete lipid droplets or serve as important structural components of membranes. In this review, we provide a brief overview of skeletal muscle fatty acid metabolism and highlight recent notable advances in the field. We then 1) discuss how lipids are stored in and mobilized from various subcellular locations to provide adaptive or maladaptive signals in the myocyte and 2) outline how lipid metabolites or metabolic byproducts derived from the actions of triacylglycerol metabolism or β-oxidation act as positive and negative regulators of insulin action. We have placed an emphasis on recent developments in the lipid biology field with respect to understanding skeletal muscle physiology and discuss unanswered questions and technical limitations for assessing lipid signaling in skeletal muscle.

A single prior bout of exercise protects against palmitate-induced insulin resistance despite an increase in total ceramide content

Ceramide accumulation has been implicated in the impairment of insulin-stimulated glucose transport in skeletal muscle following saturated fatty acid (FA) exposure. Importantly, a single bout of exercise can protect against acute lipid-induced insulin resistance. The mechanism by which exercise protects against lipid-induced insulin resistance is not completely known but may occur through a redirection of FA toward triacylglycerol (TAG) and away from ceramide and diacylglycerol (DAG). Therefore, in the current study, an in vitro preparation was used to examine whether a prior bout of exercise could confer protection against palmitate-induced insulin resistance and whether the pharmacological [50 μM fumonisin B(1) (FB1)] inhibition of ceramide synthesis in the presence of palmitate could mimic the protective effect of exercise. Soleus muscle of sedentary (SED), exercised (EX), and SED in the presence of FB1 (SED+FB1) were incubated with or without 2 mM palmitate for 4 h. This 2-mM palmitate exposure impaired insulin-stimulated glucose transport (-28%, P < 0.01) and significantly increased ceramide, DAG, and TAG accumulation in the SED group (P < 0.05). A single prior bout of exercise prevented the detrimental effects of palmitate on insulin signaling and caused a partial redistribution of FA toward TAG (P < 0.05). However, the net increase in ceramide content in response to palmitate exposure in the EX group was not different compared with SED, despite the maintenance of insulin sensitivity. The incubation of soleus from SED rats with FB1 (SED+FB1) prevented the detrimental effects of palmitate and caused a redirection of FA toward TAG accumulation (P < 0.05). Therefore, this research suggests that although inhibiting ceramide accumulation can prevent the detrimental effects of palmitate, a single prior bout of exercise appears to protect against palmitate-induced insulin resistance, which may be independent of changes in ceramide content.

Metabolic challenges reveal impaired fatty acid metabolism and translocation of FAT/CD36 but not FABPpm in obese Zucker rat muscle

We examined, in muscle of lean and obese Zucker rats, basal, insulin-induced, and contraction-induced fatty acid transporter translocation and fatty acid uptake, esterification, and oxidation. In lean rats, insulin and contraction induced the translocation of the fatty acid transporter FAT/CD36 (43 and 41%, respectively) and plasma membrane-associated fatty acid binding protein (FABPpm; 19 and 60%) and increased fatty acid uptake (63 and 40%, respectively). Insulin and contraction increased lean muscle palmitate esterification and oxidation 72 and 61%, respectively. In obese rat muscle, basal levels of sarcolemmal FAT/CD36 (+33%) and FABPpm (+14%) and fatty acid uptake (+30%) and esterification (+32%) were increased, whereas fatty acid oxidation was reduced (-28%). Insulin stimulation of obese rat muscle increased plasmalemmal FABPpm (+15%) but not plasmalemmal FAT/CD36, blunted fatty acid uptake and esterification, and failed to reduce fatty acid oxidation. In contracting obese rat muscle, the increases in fatty acid uptake and esterification and FABPpm translocation were normal, but FAT/CD36 translocation was impaired and fatty acid oxidation was blunted. There was no relationship between plasmalemmal fatty acid transporters and palmitate partitioning. In conclusion, fatty acid metabolism is impaired at several levels in muscles of obese Zucker rats; specifically, they are 1) insulin resistant with respect to FAT/CD36 translocation and fatty acid uptake, esterification, and oxidation and 2) contraction resistant with respect to fatty acid oxidation and FAT/CD36 translocation, but, conversely, 3) obese muscles are neither insulin nor contraction resistant at the level of FABPpm. Finally, 4) there is no evidence that plasmalemmal fatty acid transporters contribute to the channeling of fatty acids to specific metabolic destinations within the muscle.

Effect of intermittent fasting and refeeding on insulin action in healthy men

Insulin resistance is currently a major health problem. This may be because of a marked decrease in daily physical activity during recent decades combined with constant food abundance. This lifestyle collides with our genome, which was most likely selected in the late Paleolithic era (50,000-10,000 BC) by criteria that favored survival in an environment characterized by fluctuations between periods of feast and famine. The theory of thrifty genes states that these fluctuations are required for optimal metabolic function. We mimicked the fluctuations in eight healthy young men [25.0 +/- 0.1 yr (mean +/- SE); body mass index: 25.7 +/- 0.4 kg/m(2)] by subjecting them to intermittent fasting every second day for 20 h for 15 days. Euglycemic hyperinsulinemic (40 mU.min(-1).m(-2)) clamps were performed before and after the intervention period. Subjects maintained body weight (86.4 +/- 2.3 kg; coefficient of variation: 0.8 +/- 0.1%). Plasma free fatty acid and beta-hydroxybutyrate concentrations were 347 +/- 18 and 0.06 +/- 0.02 mM, respectively, after overnight fast but increased (P < 0.05) to 423 +/- 86 and 0.10 +/- 0.04 mM after 20-h fasting, confirming that the subjects were fasting. Insulin-mediated whole body glucose uptake rates increased from 6.3 +/- 0.6 to 7.3 +/- 0.3 mg.kg(-1).min(-1) (P = 0.03), and insulin-induced inhibition of adipose tissue lipolysis was more prominent after than before the intervention (P = 0.05). After the 20-h fasting periods, plasma adiponectin was increased compared with the basal levels before and after the intervention (5,922 +/- 991 vs. 3,860 +/- 784 ng/ml, P = 0.02). This experiment is the first in humans to show that intermittent fasting increases insulin-mediated glucose uptake rates, and the findings are compatible with the thrifty gene concept.

AMP kinase activation with AICAR simultaneously increases fatty acid and glucose oxidation in resting rat soleus muscle

5-Amino-4-imidazolecarboxamide riboside (AICAR), a pharmacological activator of AMP-activated protein kinase (AMPK), acutely stimulates glucose uptake and fatty acid (FA) oxidation in skeletal muscle. However, it is not fully understood whether AICAR-induced changes in glucose oxidation are secondary to changes in FA oxidation (i.e. glucose fatty acid cycle), or what role AMPK may be playing in the regulation of intramuscular triacylglycerol (TAG) esterification and hydrolysis. We examined the acute (60 min) effects of AICAR (2 mm) on FA metabolism, glucose oxidation and pyruvate dehydrogenase (PDH) activation in isolated resting rat soleus muscle strips exposed to two different FA concentrations (low fatty acid, LFA, 0.2 mm; high fatty acid, HFA, 1 mm). AICAR significantly increased AMPK alpha2 activity (+192%; P<0.05) over 60 min, and simultaneously increased both FA (LFA: +33%, P<0.05; HFA: +36%, P<0.05) and glucose (LFA: +105%, P<0.05; HFA: +170, P<0.001) oxidation regardless of FA availability. While there were no changes in TAG esterification, AICAR did increase the ratio of FA partitioned to oxidation relative to TAG esterification (LFA: +15%, P<0.05; HFA: +49%, P<0.05). AICAR had no effect on endogenous TAG hydrolysis and oxidation in resting soleus. The stimulation of glucose oxidation with AICAR was associated with an increase in PDH activation (+126%; P<0.05) but was without effect on pyruvate, an allosteric activator of the PDH complex, suggesting that AMPK may stimulate PDH directly. In conclusion, AMPK appears to be an important regulator of both FA metabolism and glucose oxidation in resting skeletal muscle.

Brief Food Restriction in Old Animals Decreases Triglyceride Content and Insulin-Stimulated Triglyceride Synthesis

To determine the effects of brief food restriction on fatty acid (FA) metabolism in old muscle, hind limbs of 24-month F344/BN rats fed either ad libitum (AL) or 60% food restricted (FR) for 28 days were perfused under hyperglycemic-hyperinsulinemic conditions. Basal glucose and insulin levels were significantly lower (p<.05) in FR rats. Although palmitate uptake was not affected by food restriction, palmitate oxidation was 49% lower (2.2+/-0.3 vs 4.3+/-0.7 nmol . g-1 . min-1, p<.05) in FR versus AL animals, respectively. Compared to AL animals, FR animals had 25%-43% (p<.05) lower muscle triglyceride (TG) levels and hyperinsulinemic TG synthesis rates. Higher glucose uptake rates occurred in FR rats (p<.05). In conclusion, our results indicate that brief food restriction in old animals improves insulin sensitivity as it pertains to both glucose uptake and FA oxidation. Together with the decrease in nonoxidative FA disposal, the decreased FA oxidation under hyperinsulinemic conditions may significantly contribute to food restriction-induced reduction in muscle TG.

Hormone-sensitive lipase activity and triacylglycerol hydrolysis are decreased in rat soleus muscle by cyclopiazonic acid

Cyclopiazonic acid (CPA) is a sarcoplasmic reticulum Ca2+-ATPase inhibitor that increases intracellular calcium. The role of CPA in regulating the oxidation and esterification of palmitate, the hydrolysis of intramuscular lipids, and the activation of hormone-sensitive lipase (HSL) was examined in isolated rat soleus muscles at rest. CPA (40 micro M) was added to the incubation medium to levels that resulted in subcontraction increases in muscle tension, and lipid metabolism was monitored using the previously described pulse-chase procedure. CPA did not alter the cellular energy state, as reflected by similar muscle contents of ATP, phosphocreatine, free AMP, and free ADP. CPA increased total palmitate uptake into soleus muscle (11%, P < 0.05) and was without effect on palmitate oxidation. This resulted in greater esterification of exogenous palmitate into the triacylglycerol (18%, P < 0.05) and phospholipid (89%, P < 0.05) pools. CPA decreased (P < 0.05) intramuscular lipid hydrolysis, and this occurred as a result of reduced HSL activity (20%, P < 0.05). Incubation of muscles with 3 mM caffeine, which is also known to increase Ca2+ without affecting the cellular energy state, reduced HSL activity (24%, P < 0.05). KN-93, a calcium/calmodulin-dependent kinase inhibitor (CaMKII), blocked the effects of CPA and caffeine, and HSL activity returned to preincubation values. The results of the present study demonstrate that CPA simultaneously decreases intramuscular triacylglycerol (IMTG) hydrolysis and promotes lipid storage in isolated, intact soleus muscle. The decreased IMTG hydrolysis is likely mediated by reduced HSL activity, possibly via the CaMKII pathway. These responses are not consistent with the increased hydrolysis and decreased esterification observed in contracting muscle when substrate availability and the hormonal milieu are tightly controlled. It is possible that more powerful signals or a higher [Ca2+] may override the lipid-storage effect of the CPA-mediated effects during muscular contractions.

Fate of fatty acids at rest and during exercise: regulatory mechanisms

Fatty acids are a major fuel source for humans both at rest and during exercise. Plasma free fatty acids (FFA), although present only in micromolar concentrations, are the major circulating lipid fuel. FFA availability can increase two- to four-fold with moderate intensity exercise. Other potential sources of fatty acids include circulating very low-density lipoprotein (VLDL) triglycerides (TGs) ( approximately 1/5 the fuel availability of FFA) and intramyocellular TGs ( approximately 2 mmol kg-1 muscle). At rest approximately 40% of systemic FFA uptake occurs in the splanchnic bed and uptake in legs is approximately 15-20%. During leg exercise the uptake of FFA in leg tissue increases to 30-60% of systemic uptake and splanchnic uptake decreases to 15%. The fate of VLDL TG fatty acids has not been adequately studied. Intramyocellular TG hydrolysis increases during exercise, but the factors that regulate this response are not clear. The fact that contraction of isolated muscles can stimulate the hydrolysis and oxidation of intramyocellular TGs (in the absence of hormonal or neural input) suggests an intracellular regulation of this process. Additional regulation from changes in catecholamines and insulin may also occur. During moderate intensity exercise circulating FFA and intramyocellular TG provide roughly equal portions of fatty acids for oxidation. In addition to endurance training, dietary factors have been shown to modulate the fatty acid oxidation response to exercise. Much remains to be learned about fatty acid trafficking during exercise. What role do VLDL TG play? How is the oxidation of intramyocellular TGs regulated? Techniques to address these questions in humans are only now becoming available.

Marked heterogeneity of human skeletal muscle lipolysis at rest

In this study, variations in lipolysis among different muscle groups were examined by measuring local net glycerol release in vivo in healthy, normal-weight subjects (n = 11) during rested, postabsorptive conditions. Microdialysis of the gastrocnemius, deltoid, and vastus lateralis muscle regions revealed that extracellular glycerol concentrations of these three muscle regions were 84.7 +/- 6.7, 59.7 + 7.3, and 56.4 +/- 7.5 micro mol/l, respectively, and the arterial plasma glycerol concentration was 44.8 +/- 2.3 micro mol/l (P = 0.0003-0.006, gastrocnemius vs. others). Local tissue blood flow, as measured by Xe clearance, did not differ among the regions. Net glycerol release was significantly higher in gastrocnemius muscle than in the two other regions. There were no regional differences in glycerol uptake when studied during glycerol infusion. Gastrocnemius muscle showed a dominance of type 1 fibers (70%), whereas the vastus lateralis muscle had equal distribution of fiber types (P = 0.02). No differences in intramuscular triaclyceride content, perimuscular fat, or the adipocyte-specific protein perilipin were observed among the muscle regions. Triglyceride turnover in the gastrocnemius muscle was 3.3 + 1.4% over 24 h, which is about 10 times more rapid than the turnover rate in subcutaneous adipose tissue (P < 0.01). Thus there were marked differences in lipolytic activity among skeletal muscle groups at rest, possibly reflecting variations in fiber type.

Intramuscular triacylglycerol utilization in human skeletal muscle during exercise: is there a controversy?

Intramuscular triacylglyerols (IMTGs) represent a potentially important energy source for contracting human skeletal muscle. Although the majority of evidence from isotope tracer and (1)H-magnetic resonance spectroscopy (MRS) studies demonstrate IMTG utilization during exercise, controversy regarding the importance of IMTG as a metabolic substrate persists. The controversy stems from studies that measure IMTG in skeletal muscle biopsy samples and report no significant net IMTG degradation during prolonged moderate-intensity (55-70% maximal O(2) consumption) exercise lasting 90-120 min. Although postexercise decrements in IMTG levels are often reported from direct muscle measurements, the marked between-biopsy variability (approximately 23%) that has been reported with this technique in untrained subjects is larger than the expected decrease in IMTG content, effectively precluding significant findings. In contrast, recent data obtained in endurance-trained subjects demonstrated reduced variability between duplicate biopsies (approximately 12%), and significant changes in IMTG were detected after 120 min of moderate-intensity exercise. Therefore, it is our contention that the muscle biopsy, isotope tracer, and (1)H-MRS techniques report significant and energetically important oxidation of free fatty acids derived from IMTGs during prolonged moderate exercise.

Hormone-sensitive lipase: control of intracellular tri-(di-)acylglycerol and cholesteryl ester hydrolysis

Hormone-sensitive lipase (HSL) is an intracellular neutral lipase that is capable of hydrolyzing triacylglycerols, diacylglycerols, monoacylglycerols, and cholesteryl esters, as well as other lipid and water soluble substrates. HSL activity is regulated post-translationally by phosphorylation and also by pretranslational mechanisms. The enzyme is highly expressed in adipose tissue and steroidogenic tissues, with lower amounts expressed in cardiac and skeletal muscle, macrophages, and islets. Studies of the structure of HSL have identified several amino acids and regions of the molecule that are critical for enzymatic activity and regulation of HSL. This has led to important insights into its function, including the interaction of HSL with other intracellular proteins, such as adipocyte lipid binding protein. Accumulating evidence has defined important functions for HSL in normal physiology, affecting adipocyte lipolysis, steroidogenesis, spermatogenesis, and perhaps insulin secretion and insulin action; however, direct links between abnormal expression or genetic variations of HSL and human disorders, such as obesity, insulin resistance, type 2 diabetes, and hyperlipidemia, await further clarification. The published reports examining the regulation, and function of HSL in normal physiology and disease are reviewed in this paper.

Leptin increases FA oxidation in lean but not obese human skeletal muscle: evidence of peripheral leptin resistance

The adipocyte-derived hormone leptin has been shown to acutely increase fatty acid (FA) oxidation and decrease esterification in resting rodent skeletal muscle. However, the effects of leptin on human skeletal muscle FA metabolism are completely unknown. In these studies, we have utilized an isolated human skeletal muscle preparation combined with the pulse-chase technique to measure FA metabolism with and without leptin in lean and obese human skeletal muscle. Under basal conditions (in the absence of leptin), muscle from the obese demonstrated significantly elevated levels of total FA uptake (+72%, P = 0.038) and enhanced rates of FA esterification into triacylglycerol (+102%, P = 0.042) compared with lean subjects. In the presence of leptin, lean muscle had elevated rates of endogenous (+103%, P = 0.01) and exogenous (+150%, P = 0.03) palmitate oxidation. When the ratio of esterification to exogenous oxidation was examined, leptin reduced this ratio (-47%, P = 0.032), demonstrating the increased partitioning of FA toward oxidation and away from storage. Contrary to these findings in lean muscle, leptin had no effect on FA metabolism in skeletal muscle of the obese. This study provides the first evidence that leptin increases FA oxidation in skeletal muscle of lean, but not obese humans, thus demonstrating the development of leptin resistance in obese human skeletal muscle.

Impaired fatty acid oxidation in muscle of aging rats perfused under basal conditions

The purpose of the present study was to examine the utilization of fatty acids (FA) and muscle substrates by skeletal muscle in young, middle-aged, and old adult rats under conditions of euglycemia with low insulin levels. Male Fischer 344 x Brown Norway rats aged 5, 15, or 24 mo underwent hindlimb perfusion with a medium of 8 mM glucose, 1 mM palmitate, 25 microU/ml insulin, [1-(14)C]palmitate, and [3-(3)H]glucose. Glucose and palmitate uptake were similar among age groups. The percent and total palmitate oxidized (nmol.min(-1).g(-1)) were 30-36 and 41-49% lower (P < 0.05) in 15-mo- and 24-mo-old than in 5-mo-old animals. Compared with 5-mo- and 15-mo-old animals, pre- and postperfusion muscle triglyceride (TG) levels were significantly (P < 0.05) elevated 91-305% in red and 118-219% in white muscles of 24-mo-old animals. Fatty acid-binding protein content was 40-64% higher (P < 0.05) in 24-mo- than in 5-mo- or 15-mo-old animals. In red muscle, hormone-sensitive lipase (HSL) content was 28% lower (P < 0.05) in 24-mo- than in 5-mo-old animals. These results indicate that, under euglycemic conditions in the presence of low insulin levels, the reduction in FA disposal to oxidation and the decrease in HSL content may contribute to the accumulation of TG in muscle of old animals.

Brief food restriction increases FA oxidation and glycogen synthesis under insulin-stimulated conditions

To determine the effects of brief food restriction on fatty acid (FA) metabolism, hindlimbs of F344/BN rats fed either ad libitum (AL) or food restricted (FR) to 60% of baseline food intake for 28 days were perfused under hyperglycemic-hyperinsulinemic conditions (20 mM glucose, 1 mM palmitate, 1,000 microU/ml insulin, [3-(3)H]glucose, and [1-(14)C]palmitate). Basal glucose and insulin levels were significantly lower (P < 0.05) in FR vs. AL rats. Palmitate uptake (34.3 +/- 2.7 vs. 24.5 +/- 3.1 nmol/g/min) and oxidation (3.8 +/- 0.2 vs. 2.7 +/- 0.3 nmol.g(-1).min(-1)) were significantly higher (P < 0.05) in FR vs. AL rats, respectively. Glucose uptake was increased in FR rats and was accompanied by significant increases in red and white gastrocnemius glycogen synthesis, indicating an improvement in insulin sensitivity. Although muscle triglyceride (TG) levels were not significantly different between groups, glucose uptake and total preperfusion TG concentration were negatively correlated (r(2) = 0.27, P < 0.05). In conclusion, our results show that under hyperglycemic-hyperinsulinemic conditions, brief FR resulted in an increase in FA oxidative disposal that may contribute to the improvement in insulin sensitivity.

Fatty acid oxidation and triacylglycerol hydrolysis are enhanced after chronic leptin treatment in rats

Leptin acutely increases fatty acid (FA) oxidation and triacylglycerol (TG) hydrolysis and decreases TG esterification in oxidative rodent muscle. However, the effects of chronic leptin administration on FA metabolism in skeletal muscle have not been examined. We hypothesized that chronic leptin treatment would enhance TG hydrolysis as well as the capacity to oxidize FA in soleus (SOL) muscle. Female Sprague-Dawley rats were infused for 2 wk with leptin (LEPT; 0.5 mg x kg(-1) x day(-1)) by use of subcutaneously implanted miniosmotic pumps. Control (AD-S) and pair-fed (PF-S) animals received saline-filled implants. Subsequently, FA metabolism was monitored for 45 min in isolated, resting, and contracting (20 tetani/min) SOL muscles by means of pulse-chase procedures. Food intake (-33 +/- 2%, P < 0.01) and body mass (-12.5 +/- 4%, P = 0.01) were reduced in both LEPT and PF-S animals. Leptin levels were elevated (+418 +/- 7%, P < 0.001) in treated animals but reduced in PF-S animals (-73 +/- 8%, P < 0.05) relative to controls. At rest, TG hydrolysis was increased in leptin-treated rats (1.8 +/- 2.2, AD-S vs. 23.5 +/- 8.1 nmol/g wet wt, LEPT; P < 0.001). In contracting SOL muscles, TG hydrolysis (1.5 +/- 0.6, AD-S vs. 3.6 +/- 1.0 micromol/g wet wt, LEPT; P = 0.02) and palmitate oxidation (18.3 +/- 6.7, AD-S vs. 45.7 +/- 9.9 nmol/g wet wt, LEPT; P < 0.05) were both significantly increased by leptin treatment. Chronic leptin treatment had no effect on TG esterification either at rest or during contraction. Markers of overall (citrate synthase) and FA (hydroxyacyl-CoA dehydrogenase) oxidative capacity were unchanged with leptin treatment. Protein expression of hormone-sensitive lipase (HSL) was also unaltered following leptin treatment. Thus leptin-induced increases in lipolysis are likely due to HSL activation (i.e., phosphorylation). Increased FA oxidation secondary to chronic leptin treatment is not due to an enhanced oxidative capacity and may be a result of enhanced flux into the mitochondrion (i.e., carnitine palmitoyltransferase I regulation) or electron transport uncoupling (i.e., uncoupling protein-3 expression).

Gender differences in substrate utilization during submaximal exercise in endurance-trained subjects

Substrate utilization across the leg during 90 min of bicycle exercise at 58% of peak oxygen uptake (VO(2 peak)) was studied in seven endurance-trained males and seven endurance-trained, eumenorrheic females by applying arteriovenous catheterization, stable isotopes, and muscle biopsies. The female and male groups were matched according to VO(2 peak) per kilogram of lean body mass, physical activity level, and training history of the subjects. All subjects consumed the same diet, well controlled in terms of nutrient composition as well as energy content, for 8 days preceding the experiment, and all females were tested in the midfollicular phase of the menstrual cycle. During exercise, respiratory exchange ratio (RER) and leg respiratory quotient (RQ) were similar in females and males. Myocellular triacylglycerol (TG) degradation was negligible in males but amounted to 12.4 +/- 3.2 mmol/kg dry wt in females and corresponded to 25.0 +/- 6.0 and 5.0 +/- 7.3% of total oxygen uptake in females and males, respectively (P < 0.05). Utilization of plasma fatty acids (12.0 +/- 2.5 and 9.6 +/- 1.5%), blood glucose (13.6 +/- 1.5 and 14.3 +/- 1.5%), and glycogen (48.5 +/- 4.9 and 42.8 +/- 2.1%) were similar in females and males. Thus, in females, measured substrate oxidation accounted for 99% of the leg oxygen uptake, whereas in males 28% of leg oxygen uptake was unaccounted for in terms of measured oxidized lipid substrates. These findings may indicate that males utilized additional lipid sources, presumably very low density lipoprotein-TG or TG located between muscle fibers. On the basis of RER and leg RQ, it is concluded that no gender difference existed in the relative contribution from carbohydrate and lipids to the oxidative metabolism across the leg during submaximal exercise at the same relative workload. However, an effect of gender appears to occur in the utilization of the different lipid sources.

Insulin increases FA uptake and esterification but reduces lipid utilization in isolated contracting muscle

We examined the effect of insulin on the synthesis and degradation of muscle lipid pools [phospholipid (PL), diacylglycerol (DG), triacylglycerol (TG)] and palmitate oxidation in isolated resting and contracting (20 tetani/min) soleus muscles. Lipid metabolism was monitored using the previously defined pulse-chase procedure. At rest, insulin significantly increased total palmitate uptake into soleus muscle (+49%, P < 0.05), corresponding to enhanced DG (+60%, P < 0.05) and TG (+61%, P < 0.05) esterification, but blunted palmitate oxidation (-38%, P < 0.05) and TG hydrolysis (-34%, P < 0.05). During muscle contraction, when total palmitate uptake was increased, insulin further enhanced uptake (+21%, P < 0.05) and esterification of fatty acids (FA) to PL (+73%, P < 0.05), DG (+19%, P < 0.05), and TG (+161%, P < 0.01). Despite a profound shift in the relative partitioning of FA away from esterification and toward oxidation during contraction, the increase in palmitate oxidation and TG hydrolysis was significantly blunted by insulin [oxidation, -24% (P = 0.05); hydrolysis, -83% (P < 0.01)]. The effects of insulin on FA esterification (stimulation) and oxidation (inhibition) during contraction were reduced in the presence of the phosphatidylinositol 3-kinase inhibitor LY-294002. In summary, the effects of insulin and contraction on palmitate uptake and esterification are additive, while insulin opposes the stimulatory effect of contraction on FA oxidation and TG hydrolysis. Insulin's modulatory effects on muscle FA metabolism during contraction are mediated at least in part through phosphatidylinositol 3-kinase.

Stimulatory effects of leptin and muscle contraction on fatty acid metabolism are not additive

Leptin has been shown to acutely stimulate fatty acid oxidation and triacylglycerol hydrolysis in skeletal muscle. These effects are similar to those induced by muscle contraction alone. Several studies have demonstrated that, during aerobic exercise, plasma leptin concentrations are well maintained; however, none has examined whether the stimulatory effects of leptin and contraction on muscle lipid metabolism are additive. This is the first study to examine the direct effect of leptin on lipid and carbohydrate (CHO) metabolism in isolated oxidative muscle over a range of contraction intensities. We examined the effect of leptin (10 microg/ml) on the synthesis and degradation of muscle lipid pools [phospholipid (PL), diacylglycerol (DG), triacylglycerol (TG)] and palmitate oxidation in isolated resting and contracting (2, 8, and 20 tetani/min) soleus muscles. At rest, leptin increased fatty acid oxidation (+ 40%, P < 0.05) and TG hydrolysis (+ 47%, P < 0.05), while blunting TG esterification (-20%, P < 0.05). Glucose oxidation was unaffected at rest in the presence of leptin. During tetanic contraction, fatty acid oxidation (+20-114%, P < 0.05) and TG esterification (+ 19-33%, P < 0.05) as well as net TG utilization (+ 23%, P < 0.05) were all significantly increased. However, leptin was without further effect on any of these parameters during contraction. Net utilization of intramuscular glycogen, as well as glucose oxidation, was unaffected during contraction by leptin. The findings of the present study indicate that leptin has an important influence on lipid metabolism in resting muscle, but not during contraction.

Proteolytic cleavage product of 30-kDa adipocyte complement-related protein increases fatty acid oxidation in muscle and causes weight loss in mice

Adipocyte complement-related protein (30 kDa) (Acrp30), a secreted protein of unknown function, is exclusively expressed in differentiated adipocytes; its mRNA is decreased in obese humans and mice. Here we describe novel pharmacological properties of the protease-generated globular head domain of Acrp30 (gAcrp30). Acute treatment of mice with gAcrp30 significantly decreased the elevated levels of plasma free fatty acids caused either by administration of a high fat test meal or by i.v. injection of Intralipid. This effect of gAcrp30 was caused, at least in part, by an acute increase in fatty acid oxidation by muscle. As a result, daily administration of a very low dose of gAcrp30 to mice consuming a high-fat/sucrose diet caused profound and sustainable weight reduction without affecting food intake. Thus, gAcrp30 is a novel pharmacological compound that controls energy homeostasis and exerts its effect primarily at the peripheral level.

Proteolytic cleavage product of 30-kDa adipocyte complement-related protein increases fatty acid oxidation in muscle and causes weight loss in mice

Adipocyte complement-related protein (30 kDa) (Acrp30), a secreted protein of unknown function, is exclusively expressed in differentiated adipocytes; its mRNA is decreased in obese humans and mice. Here we describe novel pharmacological properties of the protease-generated globular head domain of Acrp30 (gAcrp30). Acute treatment of mice with gAcrp30 significantly decreased the elevated levels of plasma free fatty acids caused either by administration of a high fat test meal or by i.v. injection of Intralipid. This effect of gAcrp30 was caused, at least in part, by an acute increase in fatty acid oxidation by muscle. As a result, daily administration of a very low dose of gAcrp30 to mice consuming a high-fat/sucrose diet caused profound and sustainable weight reduction without affecting food intake. Thus, gAcrp30 is a novel pharmacological compound that controls energy homeostasis and exerts its effect primarily at the peripheral level.

Stimulation of hormone-sensitive lipase activity by contractions in rat skeletal muscle

Because the enzymic regulation of muscle triglyceride breakdown is poorly understood we studied whether neutral lipase in skeletal muscle is activated by contractions. Incubated soleus muscles from 70 g rats were electrically stimulated for 60 min. Neutral lipase activity against triacylglycerol increased after 1 and 5 min of contractions [0.36 +/- 0.02 (basal) versus 0.49 +/- 0.05 (1 min) and 0.54 +/- 0.05 (5 min) m-unit.mg of protein(-1), means +/- S.E.M., P < 0.05]. After 10 min the neutral lipase activity (0.40 +/- 0.05 m-unit.mg of protein(-1)) had decreased to basal values (P > 0.05). The contraction-mediated increase in lipase activity was increased by approximately 110% when muscle was stimulated in the presence of okadaic acid. Conversely, treatment of muscle homogenate with alkaline phosphatase completely reversed the contraction-mediated lipase activation. Lipase activity did not change during contractions when analysed in the presence of anti-hormone-sensitive-lipase (HSL) antibody [0.17 +/- 0.02 (basal) versus 0.21 +/- 0.02 (5 min) m-unit.mg of protein(-1), P > 0.05]. Furthermore, immunoprecipitation with affinity-purified anti-HSL antibody reduced muscle-HSL protein concentration by 81+/-4% and caused similar reductions in lipase activity against triacylglycerol and in the contraction-induced increase in this activity. Neither prior sympathectomy [0.33+/- 0.02 (basal) versus 0.53 +/- 0.06 (5 min) m-unit.mg of protein(-1), P < 0.05] nor propranolol impaired the lipase response to contractions. Glycogen phosphorylase activity in the absence of AMP increased after 1 min [27.3 +/- 3.1 versus 8.9 +/- 1.8% (activity without AMP/total activity with AMP), P < 0.05] and returned to basal levels after 5 min. In conclusion, skeletal-muscle-immunoreactive HSL is transiently stimulated by contractions and the mechanism probably involves phosphorylation. The time course of HSL activation is similar to that of glycogen phosphorylase. Apparently, the two enzymes are regulated in parallel by contraction-induced as well as hormonal mechanisms, allowing simultaneous recruitment of all major extra- and intra-muscular energy stores.

Endurance training increases FFA oxidation and reduces triacylglycerol utilization in contracting rat soleus

We examined the effects of 8 wk of intense endurance training on free fatty acid (FFA) transporters and metabolism in resting and contracting soleus muscle using pulse-chase procedures. Endurance training increased maximal citrate synthase activity in red muscles (+54 to +91%; P </= 0.05) but failed to increase cytosolic fatty acid binding protein content, mRNA for fatty acyl-CoA synthase, and the putative FFA transporters or transport of palmitic acid into giant sarcolemmal vesicles. At rest, only triacylglycerol (TG) synthesis was significantly increased by training (+100.9 +/- 8.7 vs. +66.6 +/- 6.7 nmol/g wet wt; P </= 0.05). Muscle contraction increased TG synthesis (+46%; P </= 0.05) and palmitate oxidation (+115%; P </= 0.05) in untrained rats. Endurance training further enhanced synthesis of monoacylglycerol (MG), diacylglycerol (DG) and TG during contraction (+36, +69 and +71%, respectively; P </= 0.05), as well as exogenous palmitate oxidation (+41%; P </= 0.05) relative to untrained rats. Compared with those in untrained rats, TG breakdown and oxidation during contraction were reduced after training by 49 and 30%, respectively (P </= 0.05). In conclusion, endurance training 1) increases FFA oxidation and incorporation into endogenous lipid pools during contraction and 2) reduces the rate of intramuscular TG utilization during contraction when exogenous FFA availability is adequate. The enhanced FFA uptake subsequent to training appears to be independent of altered maximal transport rates of FFA into the muscle cell.