Muscle malonyl-CoA decreases during exercise

Acidosis attenuates CPT-I-supported bioenergetics as a potential mechanism limiting lipid oxidation

Fuel interactions in contracting muscle represent a complex interplay between enzymes regulating carbohydrate and fatty acid catabolism, converging in the mitochondrial matrix. While increasing exercise intensity promotes carbohydrate use at the expense of fatty acid oxidation, the mechanisms underlying this effect remain poorly elucidated. As a potential explanation, we investigated whether exercise-induced reductions in intramuscular pH (acidosis) attenuate carnitine palmitoyltransferase-I (CPT-I)-supported bioenergetics, the rate-limiting step for fatty acid oxidation within mitochondria. Specifically, we assessed the effect of a physiologically relevant reduction in pH (pH 7.2 versus 6.8) on single and mixed substrate respiratory responses in murine skeletal muscle isolated mitochondria and permeabilized fibers. While pH did not influence oxidative phosphorylation stoichiometry (ADP/O ratios), coupling efficiency, oxygen affinity, or ADP respiratory responses, acidosis impaired lipid bioenergetics by attenuating respiration with L-carnitine and palmitoyl-CoA, while enhancing the inhibitory effect of malonyl-CoA on CPT-I. These acidotic effects were largely retained following a single bout of intense exercise. At rest, pyruvate and succinate-supported respiration were also impaired by acidosis. However, providing more pyruvate and ADP at pH 6.8 to model increases in glycolytic flux and ATP turnover with intense exercise overcame the acidotic attenuation of carbohydrate-linked oxidative phosphorylation. Importantly, this situation is fundamentally different from lipids where CPT-I substrate sensitivity and availability is impaired at higher power outputs suggesting lipid metabolism may be more susceptible to the effects of acidosis, possibly contributing to fuel shifts with increasing exercise intensity.

Re-Evaluating the Oxidative Phenotype: Can Endurance Exercise Save the Western World?

Mitochondria are popularly called the "powerhouses" of the cell. They promote energy metabolism through the tricarboxylic acid (TCA) cycle and oxidative phosphorylation, which in contrast to cytosolic glycolysis are oxygen-dependent and significantly more substrate efficient. That is, mitochondrial metabolism provides substantially more cellular energy currency (ATP) per macronutrient metabolised. Enhancement of mitochondrial density and metabolism are associated with endurance training, which allows for the attainment of high relative VO2 max values. However, the sedentary lifestyle and diet currently predominant in the Western world lead to mitochondrial dysfunction. Underdeveloped mitochondrial metabolism leads to nutrient-induced reducing pressure caused by energy surplus, as reduced nicotinamide adenine dinucleotide (NADH)-mediated high electron flow at rest leads to "electron leak" and a chronic generation of superoxide radicals (O2-). Chronic overload of these reactive oxygen species (ROS) damages cell components such as DNA, cell membranes, and proteins. Counterintuitively, transiently generated ROS during exercise contributes to adaptive reduction-oxidation (REDOX) signalling through the process of cellular hormesis or "oxidative eustress" defined by Helmut Sies. However, the unaccustomed, chronic oxidative stress is central to the leading causes of mortality in the 21st century-metabolic syndrome and the associated cardiovascular comorbidities. The endurance exercise training that improves mitochondrial capacity and the protective antioxidant cellular system emerges as a universal intervention for mitochondrial dysfunction and resultant comorbidities. Furthermore, exercise might also be a solution to prevent ageing-related degenerative diseases, which are caused by impaired mitochondrial recycling. This review aims to break down the metabolic components of exercise and how they translate to athletic versus metabolically diseased phenotypes. We outline a reciprocal relationship between oxidative metabolism and inflammation, as well as hypoxia. We highlight the importance of oxidative stress for metabolic and antioxidant adaptation. We discuss the relevance of lactate as an indicator of critical exercise intensity, and inferring from its relationship with hypoxia, we suggest the most appropriate mode of exercise for the case of a lost oxidative identity in metabolically inflexible patients. Finally, we propose a reciprocal signalling model that establishes a healthy balance between the glycolytic/proliferative and oxidative/prolonged-ageing phenotypes. This model is malleable to adaptation with oxidative stress in exercise but is also susceptible to maladaptation associated with chronic oxidative stress in disease. Furthermore, mutations of components involved in the transcriptional regulatory mechanisms of mitochondrial metabolism may lead to the development of a cancerous phenotype, which progressively presents as one of the main causes of death, alongside the metabolic syndrome.

New insights into the interaction of carbohydrate and fat metabolism during exercise

Fat and carbohydrate are important fuels for aerobic exercise and there can be reciprocal shifts in the proportions of carbohydrate and fat that are oxidized. The interaction between carbohydrate and fatty acid oxidation is dependent on the intracellular and extracellular metabolic environments. The availability of substrate, both from inside and outside of the muscle, and exercise intensity and duration will affect these environments. The ability of increasing fat provision to downregulate carbohydrate metabolism in the heart, diaphragm and peripheral skeletal muscle has been well studied. However, the regulation of fat metabolism in human skeletal muscle during exercise in the face of increasing carbohydrate availability and exercise intensity has not been well studied until recently. Research in the past 10 years has demonstrated that the regulation of fat metabolism is complex and involves many sites of control, including the transport of fat into the muscle cell, the binding and transport of fat in the cytoplasm, the regulation of intramuscular triacylglycerol synthesis and breakdown, and the transport of fat into the mitochondria. The discovery of proteins that assist in transporting fat across the plasma and mitochondrial membranes, the ability of these proteins to translocate to the membranes during exercise, and the new roles of adipose triglyceride lipase and hormone-sensitive lipase in regulating skeletal muscle lipolysis are examples of recent discoveries. This information has led to the proposal of mechanisms to explain the downregulation of fat metabolism that occurs in the face of increasing carbohydrate availability and when moving from moderate to intense aerobic exercise.

Skeletal Muscle Fuel Selection Occurs at the Mitochondrial Level

Mammals exponentially increase the rate of carbohydrate oxidation as exercise intensity rises, while birds combust lipid almost exclusively while flying at high percentages of aerobic capacity. The fuel oxidized by contracting muscle depends on many factors: whole body fuel storage masses, mobilization, blood transport, cellular uptake, and substrate selection at the level of the mitochondrion. We examined the fuel preferences of mitochondria isolated from mammalian and avian locomotory muscles using two approaches. First, the influence of substrates on the kinetics of respiration (KMADP and Vmax) was evaluated. For all substrates and combinations, KMADP was generally two-fold higher in avian mitochondria. Second, fuel competition between pyruvate, glutamate, and/or palmitoyl-l-carnitine at three levels of ATP free energy was determined using the principle of mass balance and the measured rates of O2 consumption and metabolite accumulation/utilization. Avian mitochondria strongly spared pyruvate from oxidation when another substrate was available and fatty acid was the dominant substrate, regardless of energy state. Mammalian mitochondria exhibited some preference for fatty acid over pyruvate at lower flux (higher energy state), but exhibited much greater tendency to select pyruvate and glutamate when available. Studies in sonicated mitochondria revealed two-fold higher electron transport chain electron conductance in avian mitochondria. We conclude that substantial fuel selection occurs at the level of the mitochondrial matrix and that avian flight muscle mitochondria are particularly biased toward the selection of fatty acid, possibly by facilitating high β-oxidation flux by maintaining a more oxidized matrix.

Metabolic responses to a 48-h ultra-marathon run in middle-aged male amateur runners

PURPOSE

To evaluate ongoing metabolic changes during a 48-h competitive run and a 48-h recovery period, with focus on potential health risks exemplified by heart and skeletal muscle damage biomarkers and oxidative stress-related indices.

METHODS

Blood samples were taken before the race, after 12, 24, and 48 h of running, and after 24 and 48 h of recovery from male amateur runners (N = 7, age 35-59 years, VO2max mean ± SD 57.0 ± 4.0 ml kg(-1) min(-1), total distance covered 183-320 km). The samples were analyzed for morphology, acid-base and electrolyte balance, iron status, lipid profile, interleukin-6, high-sensitivity C-reactive protein, N-terminal pro-brain-type natriuretic peptide, high-sensitivity cardiac troponin T, non-enzymatic antioxidants, activities of selected enzymes including antioxidant enzymes, and total antioxidant status.

RESULTS

The sustained ultra-endurance run caused hypocapnic alkalosis with slight hyperkalemia and hypocalcemia, but no hyponatremia. Blood biochemistry showed severe muscle but not liver damage, and an acute inflammatory response. These effects were evidenced by leukocytosis, several fold rises in interleukin-6 and high sensitivity C-reactive protein, extreme elevations in serum levels of muscle enzymes, and marked increases in cardiac biomarker levels. Most of the changes dissolved during the 48 h post-race recovery. Neither the iron pool, nor erythropoiesis, nor pro-oxidant/antioxidant balance were substantially affected.

CONCLUSIONS

The changes consequent on the ultra-endurance run do not pose a serious health risk in men who begin their endeavor with ultra-endurance running in mid-life. There is some circumstantial evidence that hyperventilatory hypocapnia may modulate inflammatory response by stimulating the release of interleukin-6 from working skeletal muscles.

AMPK regulation of fatty acid metabolism and mitochondrial biogenesis: implications for obesity

Skeletal muscle plays an important role in regulating whole-body energy expenditure given it is a major site for glucose and lipid oxidation. Obesity and type 2 diabetes are causally linked through their association with skeletal muscle insulin resistance, while conversely exercise is known to improve whole body glucose homeostasis simultaneously with muscle insulin sensitivity. Exercise activates skeletal muscle AMP-activated protein kinase (AMPK). AMPK plays a role in regulating exercise capacity, skeletal muscle mitochondrial content and contraction-stimulated glucose uptake. Skeletal muscle AMPK is also thought to be important for regulating fatty acid metabolism; however, direct genetic evidence in this area is currently lacking. This review will discuss the current paradigms regarding the influence of AMPK in regulating skeletal muscle fatty acid metabolism and mitochondrial biogenesis at rest and during exercise, and highlight the potential implications in the development of insulin resistance.

Identification of a novel malonyl-CoA IC(50) for CPT-I: implications for predicting in vivo fatty acid oxidation rates

Published values regarding the sensitivity (IC(50)) of CPT-I (carnitine palmitoyltransferase I) to M-CoA (malonyl-CoA) inhibition in isolated mitochondria are inconsistent with predicted in vivo rates of fatty acid oxidation. Therefore we have re-examined M-CoA inhibition kinetics under various P-CoA (palmitoyl-CoA) concentrations in both isolated mitochondria and PMFs (permeabilized muscle fibres). PMFs have an 18-fold higher IC(50) (0.61 compared with 0.034 μM) in the presence of 25 μM P-CoA and a 13-fold higher IC(50) (6.3 compared with 0.49 μM) in the presence of 150 μM P-CoA compared with isolated mitochondria. M-CoA inhibition kinetics determined in PMFs predicts that CPT-I activity is inhibited by 33% in resting muscle compared with >95% in isolated mitochondria. Additionally, the ability of M-CoA to inhibit CPT-I appears to be dependent on P-CoA concentration, as the relative inhibitory capacity of M-CoA is decreased with increasing P-CoA concentrations. Altogether, the use of PMFs appears to provide an M-CoA IC(50) that better reflects the predicted in vivo rates of fatty acid oxidation. These findings also demonstrate that the ratio of [P-CoA]/[M-CoA] is critical for regulating CPT-I activity and may partially rectify the in vivo disconnect between M-CoA content and CPT-I flux within the context of exercise and Type 2 diabetes.

Unsaturation of mitochondrial membrane lipids is related to palmitate oxidation in subsarcolemmal and intermyofibrillar mitochondria

Membrane lipid composition is thought to influence the function of integral membrane proteins; however, the potential for lipid composition to influence overall mitochondrial long-chain fatty acids (LCFA) oxidation is currently unknown. Therefore, the naturally occurring variability of LCFA oxidation rates within subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria in muscles with varying oxidative potentials (heart → red → white) was utilized to examine this relationship. To this end, SS and IMF mitochondria were isolated and palmitate oxidation rates were compared to membrane phospholipid composition. Among tissues, rates of palmitate oxidation in mitochondria displayed a 2.5-fold range, creating the required range to determine potential relationships with membrane lipid composition. In general, the percent mole fraction of phospholipid head groups and major fatty acid subclasses were similar in all mitochondria studied. However, rates of palmitate oxidation were positively correlated with both the unsaturation index and relative abundance of cardiolipin within mitochondria (r = 0.57 and 0.49, respectively; p < 0.05). Thus, these results suggest that mitochondrial LCFA oxidation may be significantly influenced by the total unsaturation and percent mole fraction of cardiolipin of the mitochondrial membrane, whereas other indices of membrane structure (e.g., percent mole fraction of other predominant membrane phospholipids, chain length, and ratio of phosphatidylcholine to phosphatidylethanolamine) were not significantly correlated.

Skeletal muscle fatty acid oxidation is not directly associated with AMPK or ACC2 phosphorylation

Rescue of palmitate-induced insulin resistance has been linked with improvements in fatty acid oxidation, but importantly, not always with concurrently altered AMPK or ACC2 phosphorylation. Therefore, we examined the interrelationships among AMPK, ACC2, and fatty acid oxidation under 12 controlled conditions in isolated muscle. Incubation of soleus muscle (0–12 h) did not alter fatty acid oxidation, but did increase AMPK and ACC2 phosphorylation (24%–30%). Muscle incubation with palmitate (2 mmol·L–1) inhibited palmitate oxidation (∼55%), but paradoxically, this was associated with increased AMPK and ACC2 phosphorylation (∼50%). Addition of an AMPK activator (thujone) to control (no palmitate) muscle increased AMPK and ACC2 phosphorylation (∼25%) but did not alter palmitate oxidation. Addition of AMPK inhibitors, compound C (50 µmol·L–1) or adenine 9-β-d-arabinofuranoside (Ara; 2.5 mmol·L–1), to thujone-treated muscles (no palmitate) did not alter palmiate oxidation but reduced AMPK phosphorylation (32%–42%), while ACC2 phosphorylation remained above basal level (+14%–18%). Finally, in palmitate-treated muscle, thujone increased AMPK (+100%) and ACC2 phosphorylation (+52%) and restored palmitate oxidation. Compound C or Ara, administered along with thujone in palmitate-treated muscle, only partly blunted palmitate oxidation recovery despite inhibiting AMPK phosphorylation (–22%), although ACC2 phosphorylation remained upregulated (+33%). Among these experiments, AMPK phosphorylation and ACC2 phosphorylation were positively correlated. However, AMPK phosphorylation was not correlated with palmitate oxidation, and unexpectedly, palmitate oxidation was negatively correlated with ACC2 phosphorylation. Our study, in accordance with a growing body of evidence, indicates that neither AMPK phosphorylation nor ACC2 phosphorylation is by itself an appropriate marker of fatty acid oxidation, and further serves to question their regulatory role.

Metabolic fuels: regulating fluxes to select mix

Animals must regulate the fluxes of multiple fuels to support changing metabolic rates that result from variation in physiological circumstances. The aim of fuel selection strategies is to exploit the advantages of individual substrates while minimizing the impact of disadvantages. All exercising mammals share a general pattern of fuel selection: at the same %V(O(2,max)) they oxidize the same ratio of lipids to carbohydrates. However, highly aerobic species rely more on intramuscular fuels because energy supply from the circulation is constrained by trans-sarcolemmal transfer. Fuel selection is performed by recruiting different muscles, different fibers within the same muscles or different pathways within the same fibers. Electromyographic analyses show that shivering humans can modulate carbohydrate oxidation either through the selective recruitment of type II fibers within the same muscles or by regulating pathway recruitment within type I fibers. The selection patterns of shivering and exercise are different: at the same %V(O(2,max)), a muscle producing only heat (shivering) or significant movement (exercise) strikes a different balance between lipid and carbohydrate oxidation. Long-distance migrants provide an excellent model to characterize how to increase maximal substrate fluxes. High lipid fluxes are achieved through the coordinated upregulation of mobilization, transport and oxidation by activating enzymes, lipid-solubilizing proteins and membrane transporters. These endurance athletes support record lipolytic rates in adipocytes, use lipoprotein shuttles to accelerate transport and show increased capacity for lipid oxidation in muscle mitochondria. Some migrant birds use dietary omega-3 fatty acids as performance-enhancing agents to boost their ability to process lipids. These dietary fatty acids become incorporated in membrane phospholipids and bind to peroxisome proliferator-activated receptors to activate membrane proteins and modify gene expression.

AMP-activated protein kinase control of fat metabolism in skeletal muscle

AMP-activated protein kinase (AMPK) has emerged as a key regulator of skeletal muscle fat metabolism. Because abnormalities in skeletal muscle metabolism contribute to a variety of clinical diseases and disorders, understanding AMPK's role in the muscle is important. It was originally shown to stimulate fatty acid (FA) oxidation decades ago, and since then much research has been accomplished describing this role. In this brief review, we summarize much of these data, particularly in relation to changes in FA oxidation that occur during skeletal muscle exercise. Potential roles for AMPK exist in regulating FA transport into the mitochondria via interactions with acetyl-CoA carboxylase, malonyl-CoA decarboxylase, and perhaps FA transporter/CD36 (FAT/CD36). Likewise, AMPK may regulate transport of FAs into the cell through FAT/CD36. AMPK may also regulate capacity for FA oxidation by phosphorylation of transcription factors such as CREB or coactivators such as PGC-1alpha.

AMPK and the biochemistry of exercise: implications for human health and disease

AMPK (AMP-activated protein kinase) is a phylogenetically conserved fuel-sensing enzyme that is present in all mammalian cells. During exercise, it is activated in skeletal muscle in humans, and at least in rodents, also in adipose tissue, liver and perhaps other organs by events that increase the AMP/ATP ratio. When activated, AMPK stimulates energy-generating processes such as glucose uptake and fatty acid oxidation and decreases energy-consuming processes such as protein and lipid synthesis. Exercise is perhaps the most powerful physiological activator of AMPK and a unique model for studying its many physiological roles. In addition, it improves the metabolic status of rodents with a metabolic syndrome phenotype, as does treatment with AMPK-activating agents; it is therefore tempting to attribute the therapeutic benefits of regular physical activity to activation of AMPK. Here we review the acute and chronic effects of exercise on AMPK activity in skeletal muscle and other tissues. We also discuss the potential role of AMPK activation in mediating the prevention and treatment by exercise of specific disorders associated with the metabolic syndrome, including Type 2 diabetes and Alzheimer's disease.

Adenosine 5'-monophosphate-activated protein kinase regulation of fatty acid oxidation in skeletal muscle

AMP-activated protein kinase (AMPK) is master regulator of energy balance through suppression of ATP-consuming anabolic pathways and enhancement of ATP-producing catabolic pathways. AMPK is activated by external metabolic stresses and subsequently orchestrates a complex downstream signaling cascade that mobilizes the cell for efficient energy production. AMPK has emerged as a key kinase driving lipid oxidation in skeletal muscle, and this function has important implications for exercise adaptations as well as metabolic defects associated with obesity.

LKB1 and the regulation of malonyl-CoA and fatty acid oxidation in muscle

5'-AMP-activated protein kinase (AMPK), by way of its inhibition of acetyl-CoA carboxylase (ACC), plays an important role in regulating malonyl-CoA levels and the rate of fatty acid oxidation in skeletal and cardiac muscle. In these tissues, LKB1 is the major AMPK kinase and is therefore critical for AMPK activation. The purpose of this study was to determine how the lack of muscle LKB1 would affect malonyl-CoA levels and/or fatty-acid oxidation. Comparing wild-type (WT) and skeletal/cardiac muscle-specific LKB1 knockout (KO) mice, we found that the 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR)-stimulated decrease in malonyl-CoA levels in WT heart and quadriceps muscles was entirely dependent on the presence of LKB1, as was the AICAR-induced increase in fatty-acid oxidation in EDL muscles in vitro, since these responses were not observed in KO mice. Likewise, the decrease in malonyl-CoA levels after muscle contraction was attenuated in KO gastrocnemius muscles, suggesting that LKB1 plays an important role in promoting the inhibition of ACC, likely by activation of AMPK. However, since ACC phosphorylation still increased and malonyl-CoA levels decreased in KO muscles (albeit not to the levels observed in WT mice), whereas AMPK phosphorylation was entirely unresponsive, LKB1/AMPK signaling cannot be considered the sole mechanism for inhibiting ACC during and after muscle activity. Regardless, our results suggest that LKB1 is an important regulator of malonyl-CoA levels and fatty acid oxidation in skeletal muscle.

A malonyl-CoA fuel sensing mechanism in muscle: effects of insulin, glucose and denervation

Increases in the concentration of malonyl-CoA in skeletal muscle have been observed in the KKA(Y) mouse, an obese rodent with high plasma insulin and glucose levels. To assess whether insulin and glucose directly regulate malonyl-CoA in muscle, soleus muscles from young rats were incubated with insulin and glucose at various concentrations, and their content of malonyl-CoA was determined. In addition, the effect on malonyl-CoA of denervation and electrically-induced muscle contractions was assessed. The concentration of malonyl-CoA in the soleus, taken directly from a rat fed ad libitum, was 2.0 +/- 0.2 nmol/g. In muscles incubated for 20 min in a medium devoid of added insulin and glucose, the concentration was decreased to 0.8 +/- 0.2 nmol/g. When the medium contained 0.5, 7.5, or 30 mM glucose, malonyl-CoA concentrations were 1.3 +/- 0.1, 1.8 +/- 0.1, or 2.4 +/- 0.2 nmol/g, respectively, in the absence of insulin and 1.7 +/- 0.1, 4.6 +/- 0.3 and 5.5 +/- 0.6 nmol/g. in its presence (10 mU/ml). Compared with its level in a control muscle, the concentration of malonyl-CoA increased 3-fold in the soleus 6-8 h after denervation and remained 2-fold higher for > or = 48 h. In contrast, muscle contractions induced by sciatic nerve stimulation, in vivo, acutely decreased the concentration of malonyl-CoA by 30-35%. The results indicate that insulin and glucose, and probably contractile activity, regulate the concentration of malonyl-CoA in muscle. They suggest that malonyl-CoA is a component of a fuel-sensing and signaling mechanism that responds to changes in the fuel milieu and possibly the energy expenditure of the muscle cell.

Metabolic flexibility is conserved in diabetic myotubes

The purpose of this study was to test the hypothesis that metabolic inflexibility is an intrinsic defect. Glucose and lipid oxidation were studied in human myotubes established from healthy lean and obese subjects and patients with type 2 diabetes (T2D). In lean myotubes, glucose oxidation is raised by increasing glucose concentrations (0-20 mmol/l) and acute insulin stimulation (P < 0.05), whereas it is inhibited by palmitate (PA). PA oxidation is raised by increasing PA concentrations (0-0.6 mmol/l), whereas 1.0 mmol/l PA inhibits its own oxidation (P < 0.05). Furthermore, PA oxidation is increased by acute insulin stimulation (P < 0.05) and inhibited by glucose. Even 0.05 mM PA and 2.5 mM glucose significantly reduce glucose and PA oxidation (P < 0.05), respectively. Glucose and PA oxidation are insulin-sensitive in myotubes established from lean (46% and 17% glucose and PA oxidation, respectively; P < 0.05 vs. basal), obese (31% and 14%; P < 0.05), and T2D (17% and 8%; P < 0.05) subjects. PA supplementation reduces both basal and insulin-stimulated glucose oxidation by 33-44% (P < 0.05), and myotubes are still insulin-sensitive in all three groups (P < 0.05). Therefore, the metabolic inflexibility described in obese and diabetic patients is not an intrinsic defect; rather, it is based on an extramuscular mechanism (i.e., the inability to vary extracellular fatty acid concentrations during insulin stimulation). Thus, skeletal muscles are metabolic-flexible per se.

The role of HIF-1 in hypoxic response in the skeletal muscle

During endurance training, exercising skeletal muscle experiences severe and repetitive oxygen stress, and the muscle's ability to cope with and improve its function through that stress is central to its role in the body. The primary transcriptional response factor for hypoxic adaptation is hypoxia inducible factor-1alpha (HIF-1alpha), which upregulates glycolysis and angiogenesis in response to low levels of tissue oxygenation. To examine the role of HIF-1alpha in endurance training, we have created mice specifically lacking skeletal muscle HIF-1alpha and subjected them to an endurance training protocol. We found that only wild type mice improve their oxidative capacity, as measured by the respiratory exchange ratio; surprisingly, we found that HIF-1alpha null mice have already upregulated this parameter without training. Furthermore, untrained HIF-1alpha null mice have an increased capillary to fiber ratio, and elevated oxidative enzyme activities. These changes correlate with constitutively activated AMP-activated protein kinase in the HIF-1alpha null muscles. Additionally, HIF-1alpha null muscles have decreased expression of pyruvate dehydrogenase kinase I, a HIF-1alpha target that inhibits oxidative metabolism. This data demonstrates that removal of HIF-1alpha causes an adaptive response in skeletal muscle akin to endurance training, and provides evidence for the suppression of mitochondrial biogenesis by HIF-1alpha in normal tissue.

Muscle type-specific fatty acid metabolism in insulin resistance: an integrated in vivo study in Zucker diabetic fatty rats

Intramyocellular lipid content (IMCL) serves as a good biomarker of skeletal muscle insulin resistance (IR). However, intracellular fatty acid metabolites [malonyl-CoA, long-chain acyl-CoA (LCACoA)] rather than IMCL are considered to be responsible for IR. This study aimed to investigate dynamics of IMCL and fatty acid metabolites during fed-to-starved-to-refed transition in lean and obese (IR) Zucker diabetic fatty rats in the following different muscle types: soleus (oxidative), extensor digitorum longus (EDL, intermediary), and white tibialis anterior (wTA, glycolytic). In the fed state, IMCL was significantly elevated in obese compared with lean rats in all three muscle types (soleus: 304%, EDL: 333%, wTA: 394%) in the presence of elevated serum triglycerides but similar levels of free fatty acids (FFA), malonyl-CoA, and total LCACoAs. During starvation, IMCL in soleus remained relatively constant, whereas in both rat groups IMCL increased significantly in wTA and EDL after comparable dynamics of starvation-induced FFA availability. The decreases of malonyl-CoA in wTA and EDL during starvation were more pronounced in lean than in obese rats, although there were no changes in soleus muscles for both groups. The concomitant increase in IMCL with the fall of malonyl-CoA support the concept that, as a reaction to starvation-induced FFA availability, muscle will activate lipid oxidation more the lower its oxidative capacity and then store the rest as IMCL.

AMPK: a key sensor of fuel and energy status in skeletal muscle

Contraction induces marked metabolic changes in muscle, and the AMP-activated protein kinase (AMPK) is a good candidate to explain these effects. Recent work using a muscle-specific knockout of the upstream kinase, LKB1, has confirmed that the LKB1-->AMPK cascade is the signaling pathway responsible for many of these effects.

Quantification of malonyl-coenzyme A in tissue specimens by high-performance liquid chromatography/mass spectrometry

We present a validated high-performance liquid chromatography/mass spectrometry (HPLC/MS) method for the quantification of malonyl-coenzyme A (CoA) in tissues. The assay consists of extraction of malonyl-CoA from tissue using 10% trichloroacetic acid, isolation using a reversed-phase solid-phase extraction column, HPLC separation, and detection using electrospray MS. Quantification was performed using an internal standard ([(13)C(3)]malonyl-CoA) and multiple-point standard curves from 50 to 1000pmol. The procedure was validated by performing recovery, accuracy, and precision studies. Recoveries of malonyl-CoA were determined to be 28.8+/-0.9, 48.5+/-1.8, and 44.7+/-4.4% (averages+/-SD, n=5) for liver, heart, and skeletal muscle, respectively. Accuracy was demonstrated by the addition of known amounts of malonyl-CoA to tissue samples. The malonyl-CoA detected was compared with the malonyl-CoA added, and the resulting relationships were linear with slopes and regression coefficients equal to 1. Precision was demonstrated by repetitive analysis of identical samples. These showed a within-run variation between 5 and 11%, and the interbatch repeatability was essentially the same. This procedure was then applied to rat liver, heart, and skeletal muscle, where the malonyl-CoA contents were found to be 1.9+/-0.6, 1.3+/-0.4, and 0.7+/-0.2nmol/g wet weight, respectively, for these tissues. This analytical approach can be extended to the quantification of other acyl-CoA species with no significant modification.

Identification of fatty acid translocase on human skeletal muscle mitochondrial membranes: essential role in fatty acid oxidation

Fatty acid translocase (FAT/CD36) is a transport protein with a high affinity for long-chain fatty acids (LCFA). It was recently identified on rat skeletal muscle mitochondrial membranes and found to be required for palmitate uptake and oxidation. Our aim was to identify the presence and elucidate the role of FAT/CD36 on human skeletal muscle mitochondrial membranes. We demonstrate that FAT/CD36 is present in highly purified human skeletal mitochondria. Blocking of human muscle mitochondrial FAT/CD36 with the specific inhibitor sulfo-N-succimidyl-oleate (SSO) decreased palmitate oxidation in a dose-dependent manner. At maximal SSO concentrations (200 muM) palmitate oxidation was decreased by 95% (P<0.01), suggesting an important role for FAT/CD36 in LCFA transport across the mitochondrial membranes. SSO treatment of mitochondria did not affect mitochondrial octanoate oxidation and had no effect on maximal and submaximal carnitine palmitoyltransferase I (CPT I) activity. However, SSO treatment did inhibit palmitoylcarnitine oxidation by 92% (P<0.001), suggesting that FAT/CD36 may be playing a role downstream of CPT I activity, possibly in the transfer of palmitoylcarnitine from CPT I to carnitine-acylcarnitine translocase. These data provide new insight regarding human skeletal muscle mitochondrial fatty acid (FA) transport, and suggest that FAT/CD36 could be involved in the cellular and mitochondrial adaptations resulting in improved and/or impaired states of FA oxidation.

Green tea extract improves running endurance in mice by stimulating lipid utilization during exercise

A series of polyphenols known as catechins are abundant in green tea, which is consumed mainly in Asian countries. The effects of catechin-rich green tea extract (GTE) on running endurance and energy metabolism during exercise in BALB/c mice were investigated. Mice were divided into four groups: nonexercise control, exercise control (Ex-cont), exercise+0.2% GTE, and exercise+0.5% GTE groups. Treadmill running time to exhaustion, plasma biochemical parameters, skeletal muscle glycogen content, beta-oxidation activity, and malonyl-CoA content immediately after exercise were measured at 8-10 wk after the initiation of the experiment. Oxygen consumption and respiratory exchange ratio were measured using indirect calorimetry. Running times to exhaustion in mice fed 0.5% GTE were 30% higher than in Ex-cont mice and were accompanied by a lower respiratory exchange ratio, higher muscle beta-oxidation activity, and lower malonyl-CoA content. In addition, muscle glycogen content was high in the GTE group compared with the Ex-cont group. Plasma lactate concentrations in mice fed GTE were significantly lower after exercise, concomitant with an increase in free fatty acid concentrations. Catechins, which are the main constituents of GTE, did not show significant effects on peroxisome proliferator-activated receptor-alpha or delta-dependent luciferase activities. These results suggest that the endurance-improving effects of GTE were mediated, at least partly, by increased metabolic capacity and utilization of fatty acid as a source of energy in skeletal muscle during exercise.

Skeletal Muscle Lipid Metabolism in Exercise and Insulin Resistance

Lipids as fuel for energy provision originate from different sources: albumin-bound long-chain fatty acids (LCFA) in the blood plasma, circulating very-low-density lipoproteins-triacylglycerols (VLDL-TG), fatty acids from triacylglycerol located in the muscle cell (IMTG), and possibly fatty acids liberated from adipose tissue adhering to the muscle cells. The regulation of utilization of the different lipid sources in skeletal muscle during exercise is reviewed, and the influence of diet, training, and gender is discussed. Major points deliberated are the methods utilized to measure uptake and oxidation of LCFA during exercise in humans. The role of the various lipid-binding proteins in transmembrane and cytosolic transport of lipids is considered as well as regulation of lipid entry into the mitochondria, focusing on the putative role of AMP-activated protein kinase (AMPK), acetyl CoA carboxylase (ACC), and carnitine during exercise. The possible contribution to fuel provision during exercise of circulating VLDL-TG as well as the role of IMTG is discussed from a methodological point of view. The contribution of IMTG for energy provision may not be large, covering ∼10% of total energy provision during fasting exercise in male subjects, whereas in females, IMTG may cover a larger proportion of energy delivery. Molecular mechanisms involved in breakdown of IMTG during exercise are also considered focusing on hormone-sensitive lipase (HSL). Finally, the role of lipids in development of insulin resistance in skeletal muscle, including possible molecular mechanisms involved, is discussed.

Interactions of metabolic hormones, adipose tissue and exercise

Physiological and psychological systems work together to determine energy intake and output, and thus maintain adipose tissue. In addition, adipose tissue secretes leptin and cytokines, which induces satiety and has been linked to catecholamines, cortisol, insulin, human growth hormone, thyroid hormones, gonadotropin and lipolysis. Thus, adipose tissue is acted upon by a number of physiological stimuli, including hormones, and simultaneously, is an active component in the regulation of its own lipid content. All of the hormones mentioned above are associated with each other and respond to exercise and exercise training. Thus, exercise is one of the major links between the hormonal modulators of energy intake and output. It appears that the sympathetic nervous system and the catecholamines are key components facilitating the lipolytic activity during exercise. These two neuroendocrine factors directly affect adipose metabolism and metabolic hormones that influence adipose metabolism. Acute low- and moderate-intensity exercise causes hormonal changes that facilitate lipolytic activity. Exercise training reduces these hormonal responses, but the sensitivity to these hormones increases so that lipolysis may be facilitated. Large amounts of adipose tissue blunt the metabolic hormonal responses to exercise, but the sensitivity of these hormones is increased; thus maintaining normal lipolytic activity. Although the physiological role of the endocrine system during exercise and training is significant, other training effects may have as great, or greater influence on lipolytic activity in adipose tissue.

Dynamic changes in fat oxidation in human primary myocytes mirror metabolic characteristics of the donor

Metabolic flexibility of skeletal muscle, that is, the preference for fat oxidation (FOx) during fasting and for carbohydrate oxidation in response to insulin, is decreased during insulin resistance. The aim of this study was to test the hypothesis that the capacity of myotubes to oxidize fat in vitro reflects the donor's metabolic characteristics. Insulin sensitivity (IS) and metabolic flexibility of 16 healthy, young male subjects was determined by euglycemic hyperinsulinemic clamp. Muscle samples were obtained from vastus lateralis, cultured, and differentiated into myotubes. In human myotubes in vitro, we measured suppressibility (glucose suppression of FOx) and adaptability (an increase in FOx in the presence of high palmitate concentration). We termed these dynamic changes in FOx metabolic switching. In vivo, metabolic flexibility was positively correlated with IS and maximal oxygen uptake and inversely correlated with percent body fat. In vitro suppressibility was inversely correlated with IS and metabolic flexibility and positively correlated with body fat and fasting FFA levels. Adaptability was negatively associated with percent body fat and fasting insulin and positively correlated with IS and metabolic flexibility. The interindividual variability in metabolic phenotypes was preserved in human myotubes separated from their neuroendocrine environment, which supports the hypothesis that metabolic switching is an intrinsic property of skeletal muscle.

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.

Regulation of CPT I activity in intermyofibrillar and subsarcolemmal mitochondria from human and rat skeletal muscle

Carnitine palmitoyltransferase I (CPT I) is considered the rate-limiting enzyme in the transfer of long-chain fatty acids (LCFA) into the mitochondria and is reversibly inhibited by malonyl-CoA (M-CoA) in vitro. In rat skeletal muscle, M-CoA levels decrease during exercise, releasing the inhibition of CPT I and increasing LCFA oxidation. However, in human skeletal muscle, M-CoA levels do not change during moderate-intensity exercise despite large increases in fat oxidation, suggesting that M-CoA is not the sole regulator of increased CPT I activity during exercise. In the present study, we measured CPT I activity in intermyofibrillar (IMF) and subsarcolemmal (SS) mitochondria isolated from human vastus lateralis (VL), rat soleus (Sol), and red gastrocnemius (RG) muscles. We tested whether exercise-related levels ( approximately 65% maximal O2 uptake) of calcium and adenylate charge metabolites (free AMP, ADP, and Pi) could override the M-CoA-induced inhibition of CPT I activity and explain the increased CPT I flux during exercise. Protein content was approximately 25-40% higher in IMF than in SS mitochondria in all muscles. Maximal CPT I activity was similar in IMF and SS mitochondria in all muscles (VL: 282 +/- 46 vs. 280 +/- 51; Sol: 390 +/- 81 vs. 368 +/- 82; RG: 252 +/- 71 vs. 278 +/- 44 nmol.min-1.mg protein-1). Sensitivity to M-CoA did not differ between IMF and SS mitochondria in all muscles (25-31% inhibition in VL, 52-70% in Sol and RG). Calcium and adenylate charge metabolites did not override the M-CoA-induced inhibition of CPT I activity in mitochondria isolated from VL, Sol, and RG muscles. Decreasing pH from 7.1 to 6.8 reduced CPT I activity by approximately 34-40% in both VL mitochondrial fractions. In summary, this study reports no differences in CPT I activity or sensitivity to M-CoA between IMF and SS mitochondria isolated from human and rat skeletal muscles. Exercise-induced increases in calcium and adenylate charge metabolites do not appear responsible for upregulating CPT I activity in human or rat skeletal muscle during moderate aerobic exercise.

Regulatory mechanisms in the interaction between carbohydrate and lipid oxidation during exercise

At the onset of exercise, signals from inside and outside the muscle cell increase the availability of carbohydrate (CHO) and fat to provide the fuel required for ATP production. CHO and fat oxidation are the dominant sources of aerobic ATP production and both pathways must be heavily upregulated during exercise to meet the increased energy demand. Within this paradigm, there is room for shifts between the proportion of energy that is provided from CHO and fat. It has long been known that increasing the availability of endogenous or exogenous CHO can increase the oxidation of CHO and decrease the oxidation of fat. The opposite is also true. While descriptive studies documenting these changes are numerous, the mechanisms regulating these shifts in fuel use in the face of constant energy demand have not been thoroughly elucidated. It would be expected, for example, that any fat-induced shift in CHO metabolism would target the enzymes that play key roles in regulating CHO metabolism and oxidation. Inside the muscle these could include glucose uptake (GLUT4) and phosphorylation (hexokinase), glycogenolysis (glycogen phosphorylase), glycolysis (phosphofructokinase) and conversion to acetyl CoA (pyruvate dehydrogenase). The same would be expected for a CHO-induced down regulation of fat metabolism and oxidation and might target transport of long chain fatty acids into the cell (fatty acid translocase CD36), release of fatty acids from intramuscular triacylglycerol (hormone sensitive lipase) and transport into the mitochondria (carnitine palmitoyl transferase complex). This review summarizes the work describing the interaction between CHO and fat metabolism in human skeletal muscle during exercise and presents the theories that may account for CHO/fat interaction during exercise.

Malonyl coenzyme A and the regulation of functional carnitine palmitoyltransferase-1 activity and fat oxidation in human skeletal muscle

Physiological hyperglycemia with hyperinsulinemia reduces fat oxidation in skeletal muscle. The mechanism responsible for this decrease in fat oxidation in human muscle is not known and may contribute to the development of insulin resistance. We hypothesized that the transfer of long-chain fatty acids (LCFAs) into the mitochondria via carnitine palmitoyltransferase-1 (CPT-1) is inhibited by increased malonyl coenzyme A (malonyl-CoA) (a known potent inhibitor of CPT-1) in human muscle during hyperglycemia with hyperinsulinemia. We studied six healthy subjects after an overnight fast and during an induced 5-hour period of hyperglycemia with hyperinsulinemia. Muscle fatty acid oxidation was calculated using stable isotope methodology combined with blood sampling from the femoral artery and vein of one leg. Muscle functional CPT-1 activity was assessed by concurrently infusing an LCFA tracer and a CPT-independent medium-chain fatty acid tracer. Muscle biopsies were obtained from the vastus lateralis after the periods of fasting and hyperglycemia with hyperinsulinemia. Hyperglycemia with hyperinsulinemia decreased LCFA oxidation, but had no effect on LCFA uptake or medium-chain fatty acid oxidation across the leg. Malonyl-CoA concentration significantly increased from 0.13 +/- 0.01 to 0.35 +/- 0.07 nmol/g during hyperglycemia with hyperinsulinemia. We conclude that hyperglycemia with hyperinsulinemia increases malonyl-CoA, inhibits functional CPT-1 activity, and shunts LCFA away from oxidation and toward storage in human muscle.

Coordinate regulation of malonyl-CoA decarboxylase, sn-glycerol-3-phosphate acyltransferase, and acetyl-CoA carboxylase by AMP-activated protein kinase in rat tissues in response to exercise

Changes in the concentration of malonyl-CoA in many tissues have been related to alterations in the activity of acetyl-CoA carboxylase (ACC), the rate-limiting enzyme in its formation. In contrast, little is known about the physiological role of malonyl-CoA decarboxylase (MCD), an enzyme responsible for malonyl-CoA catabolism. In this study, we examined the effects of voluntary exercise on MCD activity in rat liver, skeletal muscle, and adipose tissue. In addition, the activity of sn-glycerol-3-phosphate acyltransferase (GPAT), which like MCD and ACC can be regulated by AMP-activated protein kinase (AMPK), was assayed. Thirty min after the completion of a treadmill run, MCD activity was increased approximately 2-fold, malonyl-CoA levels were reduced, and ACC and GPAT activities were diminished by 50% in muscle and liver. These events appeared to be mediated via activation of AMPK since: 1) AMPK activity was concurrently increased by exercise in both tissues; 2) similar findings were observed after the injection of 5-amino 4 imidazole carboxamide, an AMPK activator; 3) changes in the activity of GPAT and ACC paralleled that of MCD; and 4) the increase in MCD activity in muscle was reversed in vitro by incubating immunoprecipitated enzyme from the exercised muscle with protein phosphatase 2A, and it was reproduced by incubating immunopurified MCD from resting muscle with purified AMPK. An unexpected finding was that exercise caused similar changes in the activities of ACC, MCD, GPAT, and AMPK and the concentration of malonyl-CoA in adipose tissue.

IN CONCLUSION

MCD, GPAT, and ACC are coordinately regulated by AMPK in liver and adipose tissue in response to exercise, and except for GPAT, also in muscle. The results suggest that AMPK activation plays a major role in regulating lipid metabolism in many cells following exercise. They also suggest that in each of them, it acts to increase fatty acid oxidation and decrease its esterification.

Regulation of skeletal muscle fat oxidation during exercise in humans

Fat and carbohydrate are the major energy substrates during aerobic exercise in well-fed humans. The regulation of fat metabolism during exercise has not been as thoroughly studied as carbohydrate metabolism, especially in human skeletal muscle. Traditionally, it was believed that the regulation of skeletal muscle fat metabolism was mainly at the level of the delivery of free fatty acids to the muscle (adipose tissue lipolysis) and transport of the long chain fatty acids into the mitochondria. It is now known that the transport of fatty acids into the muscle cell and the regulation of muscle triacylglycerol lipase activity are also important sites of regulation. New lines of research are currently underway examining the regulation of fat metabolism in skeletal muscle at the level of fat transport across the sarcolemmal and mitochondrial membranes and regulation of TG lipase activity in both rodent and human models. A major goal of this research is to determine the regulatory signals that control the up-regulation of fat metabolism during the transition from rest to low and moderate aerobic exercise (30-65% (.)VO(2max)) and the down-regulation that occurs when exercising at intense aerobic exercise (approximately 85% (.)VO(2max)). Although it is expected that the signals that activate carbohydrate metabolism during exercise (Ca and free ADP, AMP, and P(i)) would also play a role in fat metabolism, this has not been demonstrated to date.

Regulation of fat metabolism in skeletal muscle

Regulation of carbohydrate and fat utilization by skeletal muscle at rest and during exercise has been the subject of investigation since the early 1960s when Randle et al. proposed the so-called glucose-fatty acid cycle to explain the reciprocal relationship between carbohydrate and fat metabolism. The suggested mechanisms were based on the premise that an increase in fatty acid (FA) availability would result in increased fat metabolism and inhibition of carbohydrate metabolism. Briefly, accumulation of acetyl-CoA would result in inhibition of pyruvate dehydrogenase (PDH), accumulation of citrate would inhibit phosphofructokinase (PFK), and accumulation of glucose-6-phosphate (G6P) would reduce hexokinase (HK) activity. Ultimately, this would inhibit carbohydrate metabolism with increasing availability and oxidation of FA. Although there is some evidence for the existence of the glucose-FA cycle at rest and during low-intensity exercise, it cannot explain substrate use at moderate to high exercise intensities. More recently, evidence has accumulated that increases in glycolytic flux may decrease fat metabolism. Potential sites of regulation are the transport of FA into the sarcoplasma, lipolysis of intramuscular triacylglycerol (IMTG) by hormone-sensitive lipase (HSL), and transport of FA across the mitochondrial membrane. There are several potential regulators of fat oxidation: first, malonyl-CoA concentration, which is formed from acetyl-CoA, catalyzed by the enzyme acetyl-CoA carboxylase (ACC), which in turn will inhibit carnitine palmitoyl transferase I (CPT I). Another possible mechanism is accumulation of acetyl-CoA that will result in acetylation of the carnitine pool, reducing the free carnitine concentration. This could theoretically reduce FA transport into the mitochondria. There is also some recent evidence that CPT I is inhibited by small reductions in pH that might be observed during exercise at high intensities. It is also possible that FA entry into the sarcolemma is regulated by translocation of FAT/CD36 in a similar manner to glucose transport by GLUT-4. Studies suggest that the regulatory mechanisms may be different at rest and during exercise and may change as the exercise intensity increases. Regulation of skeletal muscle fat metabolism is clearly multifactorial, and different mechanisms may dominate in different conditions.

Lipid oxidation fuels recovery from exhaustive exercise in white muscle of rainbow trout

The oxidative utilization of lipid and carbohydrate was examined in white muscle of rainbow trout (Oncorhynchus mykiss) at rest, immediately after exhaustive exercise, and for 32-h recovery. In addition to creatine phosphate and glycolysis fueling exhaustive exercise, near maximal activation of pyruvate dehydrogenase (PDH) at the end of exercise points to oxidative phosphorylation of carbohydrate as an additional source of ATP during exercise. Within 15 min postexercise, PDH activation returned to resting values, thus sparing accumulated lactate from oxidation. Glycogen synthase activity matched the rate of glycogen resynthesis and represented near maximal activation. Decreases in white muscle free carnitine, increases in long-chain fatty acyl carnitine, and sustained elevations of acetyl-CoA and acetyl carnitine indicate a rapid utilization of lipid to supply ATP for recovery. Increases in malonyl-CoA during recovery suggest that malonyl-CoA may not regulate carnitine palmitoyltransferase-1 in trout muscle during recovery, but instead it may act to elongate short-chain fatty acids for mitochondrial oxidation. In addition, decreases in intramuscular triacylglycerol and in plasma nonesterified fatty acids indicate that both endogenous and exogenous lipid fuels may be oxidized during recovery.

Brief intense exercise followed by passive recovery modifies the pattern of fuel use in humans during subsequent sustained intermittent exercise

The role of work period duration as the principal factor influencing carbohydrate metabolism during intermittent exercise has been investigated. Fuel oxidation rates and muscle glycogen and free carnitine content were compared between two protocols of sustained intermittent intense exercise with identical treadmill speed and total work duration. In the first experiment subjects (n=6) completed 40 min of intermittent treadmill running involving a work : recovery cycle of 6 : 9 s or 24 : 36 s on separate days. With 24 : 36 s exercise a higher rate of carbohydrate oxidation approached significance (P=0.057), whilst fat oxidation rate was lower (P < or = 0.01) and plasma lactate concentration higher (P < or = 0.01). Muscle glycogen was lower post-exercise with 24 : 36 s (P < or = 0.05). Muscle free carnitine decreased (P < or = 0.05), but there was no difference between protocols. In the second experiment a separate group of subjects (n=5) repeated the intermittent exercise protocols with the addition of a 10-min bout of intense exercise, followed by 43 +/- 5 min passive recovery, prior to sustained (40 min) intermittent exercise. For this experiment the difference in fuel use observed previously between 6 : 9 s and 24 : 36 s was abolished. Carbohydrate and fat oxidation, plasma lactate and muscle glycogen levels were similar in 6 : 9 s and 24 : 36 s. When compared with the first experiment, this result was because of reduced carbohydrate oxidation in 24 : 36 s (P < or = 0.05). There was no difference, and no change, in muscle free carnitine between protocols. A 10-min bout of intense exercise, followed by 43 +/- 5 min of passive recovery, substantially modifies fuel use during subsequent intermittent intense exercise.

Insulin resistance and associated metabolic abnormalities in muscle: effects of exercise

Skeletal muscle is a major site of insulin resistance. In addition to glucose transport, oxidative disposal and storage defects, insulin resistant muscle exhibit many other metabolic abnormalities. After a brief review of insulin resistance determinants, we will focus on muscular abnormalities in obesity and type 2 diabetes. Glucose and lipid metabolism defects will be analysed and their interactions discussed. Exercise can improve many of these muscular abnormalities and the mechanisms underlying exercise-induced benefits have been clarified during the past decades. Therefore, exercise training has proved to be useful in the management of insulin resistant states, i.e. mainly obesity, especially in its truncal distribution, and type 2 diabetes. However, exercise prescription remains poorly codified, and results on glycaemic control are sometimes conflicting. In the last part of this review, we will emphasize the pathophysiological basis for an individualized exercise prescription in insulin resistant subjects.

Effects of acute (-)-hydroxycitrate supplementation on substrate metabolism at rest and during exercise in humans

BACKGROUND

(-)-Hydroxycitrate (HCA), a competitive inhibitor of ATP-citrate lyase, should reduce the extramitochondrial acetyl-CoA pool. It has been hypothesized that HCA ingestion can reduce malonyl-CoA concentrations and consequently increase fatty acid oxidation in vivo.

OBJECTIVE

This study investigated the acute effects of HCA supplementation on substrate utilization at rest and during exercise in endurance-trained humans.

DESIGN

Ten cyclists [x+/- SD) age: 24 +/- 2 y, weight: 73 +/- 2 kg, maximal oxygen uptake: 4.95 +/- 0.11 L/min, maximal work output (W:max): 408 +/- 8 W] were studied at rest and during 2 h of exercise at 50% W:max on 2 occasions. Both 45 and 15 min before exercise and 30 and 60 min after the start of exercise, 3.1 mL/kg body wt of an HCA solution (19 g/L) or placebo was ingested. Total fat and carbohydrate oxidation rates were assessed. Blood samples were collected at 15-min intervals at rest and every 30 min during exercise.

RESULTS

Plasma HCA concentrations increased after HCA ingestion up to 0.39 +/- 0.02 mmol/L (82.0 +/- 4.8 mg/L). However, no significant differences in total fat and carbohydrate oxidation rates were observed between trials. Accordingly, plasma glucose, glycerol, and fatty acid concentrations did not differ between trials. Plasma lactate concentrations were significantly lower in the HCA than in the placebo trial after 30 min of exercise but at the end of the exercise period they did not differ between trials.

CONCLUSION

HCA, even when provided in large quantities, does not increase total fat oxidation in vivo in endurance-trained humans.

Effects of high fat provision on muscle PDH activation and malonyl-CoA content in moderate exercise

This study examined the effects of elevated free fatty acid (FFA) provision on the regulation of pyruvate dehydrogenase (PDH) activity and malonyl-CoA (M-CoA) content in human skeletal muscle during moderate-intensity exercise. Seven men rested for 30 min and cycled for 10 min at 40% and 10 min at 65% of maximal O(2) uptake while being infused with either Intralipid and heparin (Int) or saline (control). Muscle biopsies were taken at 0, 1 (rest-to-exercise transition), 10, and 20 min. Exercise plasma FFA were elevated (0.99 +/- 0.11 vs. 0.33 +/- 0.03 mM), and the respiratory exchange ratio was reduced during Int (0.87 +/- 0.02) vs. control (0.91 +/- 0.01). PDH activation was lower during Int at 1 min (1.33 +/- 0.19 vs. 2.07 +/- 0.14 mmol. min(-1). kg(-1) wet muscle) and throughout exercise. Muscle pyruvate was reduced during Int at rest [0.17 +/- 0.03 vs. 0.25 +/- 0.03 mmol/kg dry muscle (dm)] but increased above control during exercise. NADH was higher during Int vs. control at rest and 1 min of exercise (0.122 +/- 0.016 vs. 0.102 +/- 0.005 and 0.182 +/- 0.016 vs. 0.150 +/- 0.016 mmol/kg dm), but not at 10 and 20 min. M-CoA was lower during Int vs. control at rest and 20 min of exercise (1.12 +/- 0.22 vs. 1.43 +/- 0.17 and 1.33 +/- 0.16 vs. 1.84 +/- 0.17 micromol/kg dm). The reduced PDH activation with elevated FFA during the rest-to-exercise transition was related to higher mitochondrial NADH at rest and 1 min of exercise and lower muscle pyruvate at rest. The decreased M-CoA may have increased fat oxidation during exercise with elevated FFA by reducing carnitine palmitoyltransferase I inhibition and increasing mitochondrial FFA transport.

Physical activity as a metabolic stressor

Both physical activity and diet stimulate processes that, over time, alter the morphologic composition and biochemical function of the body. Physical activity provides stimuli that promote very specific and varied adaptations according to the type, intensity, and duration of exercise performed. There is further interest in the extent to which diet or supplementation can enhance the positive stimuli. Prolonged walking at low intensity presents little metabolic, hormonal, or cardiovascular stress, and the greatest perturbation from rest appears to be from increased fat oxidation and plasma free fatty acid mobilization resulting from a combination of increased lipolysis and decreased reesterification. More intense jogging or running largely stimulates increased oxidation of glycogen and triacylglycerol, both of which are stored directly within the muscle fibers. Furthermore, these intramuscular stores of carbohydrate and fat appear to be the primary substrates for the enhanced oxidative and performance ability derived from endurance training-induced increases in muscle mitochondrial density. Weightlifting that produces fatigue in brief periods (ie, in 15-90 s and after 15 repetitive contractions) elicits a high degree of motor unit recruitment and muscle fiber stimulation. This is a remarkably potent stimulus for altering protein synthesis in muscle and increasing neuromuscular function. The metabolic stress of physical activity can be measured by substrate turnover and depletion, cardiovascular response, hormonal perturbation, accumulation of metabolites, or even the extent to which the synthesis and degradation of specific proteins are altered, either acutely or by chronic exercise training.

Physiology of a microgravity environment invited review: microgravity and skeletal muscle

Spaceflight (SF) has been shown to cause skeletal muscle atrophy; a loss in force and power; and, in the first few weeks, a preferential atrophy of extensors over flexors. The atrophy primarily results from a reduced protein synthesis that is likely triggered by the removal of the antigravity load. Contractile proteins are lost out of proportion to other cellular proteins, and the actin thin filament is lost disproportionately to the myosin thick filament. The decline in contractile protein explains the decrease in force per cross-sectional area, whereas the thin-filament loss may explain the observed postflight increase in the maximal velocity of shortening in the type I and IIa fiber types. Importantly, the microgravity-induced decline in peak power is partially offset by the increased fiber velocity. Muscle velocity is further increased by the microgravity-induced expression of fast-type myosin isozymes in slow fibers (hybrid I/II fibers) and by the increased expression of fast type II fiber types. SF increases the susceptibility of skeletal muscle to damage, with the actual damage elicited during postflight reloading. Evidence in rats indicates that SF increases fatigability and reduces the capacity for fat oxidation in skeletal muscles. Future studies will be required to establish the cellular and molecular mechanisms of the SF-induced muscle atrophy and functional loss and to develop effective exercise countermeasures.

Fatty acid import into mitochondria

The mitochondrial carnitine system plays an obligatory role in beta-oxidation of long-chain fatty acids by catalyzing their transport into the mitochondrial matrix. This transport system consists of the malonyl-CoA sensitive carnitine palmitoyltransferase I (CPT-I) localized in the mitochondrial outer membrane, the carnitine:acylcarnitine translocase, an integral inner membrane protein, and carnitine palmitoyltransferase II localized on the matrix side of the inner membrane. Carnitine palmitoyltransferase I is subject to regulation at the transcriptional level and to acute control by malonyl-CoA. The N-terminal domain of CPT-I is essential for malonyl-CoA inhibition. In liver CPT-I activity is also regulated by changes in the enzyme's sensitivity to malonyl-CoA. As fluctuations in tissue malonyl-CoA content are parallel with changes in acetyl-CoA carboxylase activity, which in turn is under the control of 5'-AMP-activated protein kinase, the CPT-I/malonyl-CoA system is part of a fuel sensing gauge, turning off and on fatty acid oxidation depending on the tissue's energy demand. Additional mechanism(s) of short-term control of CPT-I activity are emerging. One proposed mechanism involves phosphorylation/dephosphorylation dependent direct interaction of cytoskeletal components with the mitochondrial outer membrane or CPT-I. We have proposed that contact sites between the outer and inner mitochondrial membranes form a microenvironment which facilitates the carnitine transport system. In addition, this system includes the long-chain acyl-CoA synthetase and porin as components.

Sensitivity of CPT I to malonyl-CoA in trained and untrained human skeletal muscle

The present study examined the sensitivity of carnitine palmitoyltransferase I (CPT I) activity to its inhibitor malonyl-CoA (M-CoA), and simulated metabolic conditions of rest and exercise, in aerobically trained and untrained humans. Maximal CPT I activity was measured in mitochondria isolated from resting human skeletal muscle. Mean CPT I activity was 492.8 +/- 72.8 and 260.8 +/- 33.6 micromol. min(-1). kg wet muscle(-1) in trained and untrained subjects, respectively (pH 7.0, 37 degrees C). The sensitivity to M-CoA was greater in trained muscle; the IC(50) for M-CoA was 0.17 +/- 0.04 and 0.49 +/- 0.17 microM in trained and untrained muscle, respectively. The presence of acetyl-CoA, free coenzyme A (CoASH), and acetylcarnitine, in concentrations simulating rest and exercise conditions did not release the M-CoA-induced inhibition of CPT I activity. However, CPT I activity was reduced at pH 6.8 vs. pH 7.0 in both trained and untrained muscle in the presence of physiological concentrations of M-CoA. The results of this study indicate that aerobic training is associated with an increase in the sensitivity of CPT I to M-CoA. Accumulations of acetyl-CoA, CoASH, and acetylcarnitine do not counteract the M-CoA-induced inhibition of CPT I activity. However, small decreases in pH produce large reductions in the activity of CPT I and may contribute to the decrease in fat metabolism that occurs during moderate and intense aerobic exercise intensities.

Substrate profile in rat soleus muscle fibers after hindlimb unloading and fatigue

Limb muscles from rats flown in space and after hindlimb unloading (HU) show an increased fatigability, and spaceflight has been shown to result in a reduced ability to oxidize fatty acids. The purpose of this investigation was to determine the effects of HU on the substrate content in fast- and slow-twitch fibers and to assess the substrate utilization patterns in single slow type I fibers isolated from control and HU animals. A second objective was to assess whether HU altered the ability of the heart or limb muscle to oxidize pyruvate or palmitate. After 2 wk of HU, single fibers were isolated from the freeze-dried soleus and gastrocnemius muscles. HU increased the glycogen content in all fiber types, and it increased lactate, ATP, and phosphocreatine in the slow type I fiber. After HU, the type I fiber substrate profile was shifted toward that observed in fast fibers. For example, fiber glycogen increased from 179 +/- 16 to 285 +/- 25 mmol/kg dry wt, which approached the 308 +/- 23 mmol/kg dry wt content observed in the post-HU type IIa fiber. With contractile activity, the type I fiber from the HU animal showed a greater utilization of glycogen and accumulation of lactate compared with the control type I fiber. HU had no effect on the ability of crude homogenate or mitochondria fractions from the soleus or gastrocnemius to oxidize pyruvate or palmitate. The increased fatigability after HU may have resulted from an elevated glycolysis producing an increased cell lactate and a decreased pH.

Regulation of fatty acid oxidation in skeletal muscle

Researchers using animals are beginning to elucidate the control of fatty acid metabolism in muscle at the molecular and enzymatic level. This review examines the physiological data that has been collected from human subjects in the context of the proposed control mechanisms. A number of factors, including the availability of free fatty acids and the abundance of fatty acid transporters, may influence the rate of muscle fatty acid oxidation. However, the predominant point of control appears to be the rate at which fatty acyl-coenzyme A is transported into the mitochondria by the carnitine palmitoyl transferase system. In turn, evidence suggests that the intracellular concentration of malonyl-coenzyme A in muscle is an important regulator of carnitine palmitoyl transferase-I activity. Malonyl-coenzyme A is increased by glucose, which is likely the mechanism whereby glucose intake suppresses the transfer of fatty acids into the mitochondria for subsequent oxidation. In contrast, malonyl-coenzyme A levels decrease during exercise, which enables increased fatty acid oxidation. However, for any given carnitine palmitoyl transferase-I activity, there may be an effect of free fatty acid availability on fatty acid oxidation, particularly at low levels of free fatty acids. Nonetheless, the rate of glucose or glycogen metabolism is probably the primary regulator of the balance between glucose and fatty acid oxidation in muscle.

Regulation of fat/carbohydrate interaction in human skeletal muscle during exercise

There has been continued interest in the regulation of fat and carbohydrate utilization in muscle tissue, as they are the main substrates for energy production during exercise in well fed humans. Many investigations have demonstrated that increasing fat availability increases fat oxidation and decreases carbohydrate use in the whole body and skeletal muscle. However, little work has been performed in skeletal muscle to identify the mechanisms, and specific biochemical signals which mediate these shifts in fuel selection. The classic work in this area, using contracting heart and resting diaphragm muscle, led to a theory that could explain reciprocal changes in fat and carbohydrate use from a biochemical perspective (glucose-fatty acid cycle). Using this information, we embarked on a number of studies demonstrating that the biochemical regulation of fat/carbohydrate interaction in human skeletal muscle during exercise is different than exists in the continually active heart and diaphragm muscles. By increasing the availability of free fatty acids to the working muscles, it was demonstrated that carbohydrate downregulation during moderate and intense aerobic exercise occurred mainly at glycogen phosphorylase, the enzyme that regulates the degradation of muscle glycogen. There was also coordinate downregulation of pyruvate dehydrogenase activity at low and moderate exercise intensities. We have also investigated the roles that carnitine palmitoyl-transferase I and its regulator malonyl-CoA play in governing the transport of long-chain fatty acids into the mitochondria for oxidation, and therefore the impact on carbohydrate use in human skeletal muscle. The regulation of Carnitine Palmitoyltransferase I (CPTI) activity appears to involve more than changes in the concentration of malonyl-CoA. Several additional mechanisms may exist that interact with or override the effects of malonyl-CoA during exercise, although this work is just beginning. The present work has identified several regulatory sites in the pathways of carbohydrate and fat metabolism that help explain the regulation of fat/carbohydrate interaction in human skeletal muscle during exercise.