Muscle Energy Balance Research Articles

Deletion Of Muscle Acsl1 Caused Myopathy And Fiber Switch

Skeletal muscle exhibits plasticity and complexity in the choice of fuel substrate. During starvation, muscle initiates protein degradation to provide amino acids for liver gluconeogenesis, during acute exercise, muscle uses glucose as the primary fuel, while during rest or prolonged exercise, muscle switches to use primarily fatty acids. Defective fatty acid metabolism is one of the leading causes of metabolic myopathy. Long chain acyl‐CoA synthetase isoform‐1 (ACSL1) converts long chain fatty acids to long chain acyl‐CoAs, before acyl‐CoAs enter mitochondria for β‐oxidation or are esterified to form complex lipids. Although 5 Acsl isoforms exist, deletion of Acsl1 reduces 90% of ACS activity and fatty acid oxidation (FAO) in gastrocnemius, indicating that ACSL1 is the predominant ACSL isoform in the muscle. The objective of this study was to examine how reduced FAO through deletion of Acsl1 affects muscle physiology. We hypothesized that ACSL1‐directed fatty acid metabolism is essential for energy balance in muscle, and that lack of ACSL1 causes myopathy. To test this hypothesis, we used a mouse model in which ACSL1 is specifically deleted in skeletal muscle (Acsl1M−/−). Compared to control mice (Acsl1flox/flox), Acsl1M−/− mice demonstrated 3.4‐ and 1.5‐fold increases in plasma creatine kinase and aspartate aminotransferase activities, indicating that deletion of Acsl1 led to muscle damage. Gastrocnemius from Acsl1M−/− mice also showed the presence of central nuclei, a marker of muscle regeneration. In addition, we detected expression of caspase 3 protein only in Acsl1M−/− muscle, suggesting that the apoptosis pathway was involved. Different muscle fiber types prefer to metabolize different substrates. To investigate the consequence of Acsl1 deletion on muscle fiber type, succinate dehydrogenase staining was performed and it showed a significantly increased percentage of oxidative fibers in Acsl1M−/− gastrocnemius compared to controls. In addition, gene expression of myosin heavy chain I, an oxidative muscle fiber marker, was 6 fold higher in Acsl1M−/− glycolytic fibers than in controls, suggesting a switch from glycolytic to oxidative fibers. We further hypothesized that the fiber switch in Acsl1M−/− gastrocnemius muscle was caused by increased damage to glycolytic fibers compared to oxidative fibers. In summary, deletion of Acsl1 caused muscle damage and regeneration and promoted a glycolytic to oxidative fiber switch. Ongoing studies are examining protein turnover to determine its contribution to muscle damage.Support or Funding InformationSupported by DK56598.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Putative regulatory factors associated with intramuscular fat content.

Intramuscular fat (IMF) content is related to insulin resistance, which is an important prediction factor for disorders, such as cardiovascular disease, obesity and type 2 diabetes in human. At the same time, it is an economically important trait, which influences the sensorial and nutritional value of meat. The deposition of IMF is influenced by many factors such as sex, age, nutrition, and genetics. In this study Nellore steers (Bos taurus indicus subspecies) were used to better understand the molecular mechanisms involved in IMF content. This was accomplished by identifying differentially expressed genes (DEG), biological pathways and putative regulatory factors. Animals included in this study had extreme genomic estimated breeding value (GEBV) for IMF. RNA-seq analysis, gene set enrichment analysis (GSEA) and co-expression network methods, such as partial correlation coefficient with information theory (PCIT), regulatory impact factor (RIF) and phenotypic impact factor (PIF) were utilized to better understand intramuscular adipogenesis. A total of 16,101 genes were analyzed in both groups (high (H) and low (L) GEBV) and 77 DEG (FDR 10%) were identified between the two groups. Pathway Studio software identified 13 significantly over-represented pathways, functional classes and small molecule signaling pathways within the DEG list. PCIT analyses identified genes with a difference in the number of gene-gene correlations between H and L group and detected putative regulatory factors involved in IMF content. Candidate genes identified by PCIT include: ANKRD26, HOXC5 and PPAPDC2. RIF and PIF analyses identified several candidate genes: GLI2 and IGF2 (RIF1), MPC1 and UBL5 (RIF2) and a host of small RNAs, including miR-1281 (PIF). These findings contribute to a better understanding of the molecular mechanisms that underlie fat content and energy balance in muscle and provide important information for the production of healthier beef for human consumption.

Mitochondrial sensitivity to regulatory signals in muscle energy balance: is it constant during exercise?

The sensitivity of mitochondrial ATP synthesis to regulatory signals elevated by muscle exercise was investigated in a rat model using an integrative approach of physiologic experiments and computer simulation. Time courses of hindlimb muscle phosphocreatine (PCr) and Pi and pH during serial contractions and recovery were recorded using in vivo phosphorus NMR spectroscopy. Muscle contractions were elicited using sciatic nerve pacing over a tenfold range of frequencies (0.25–2 Hz) with concurrent force recording from the Achilles tendon. At each contraction frequency, the mitochondrial ATP synthesis rate was determined from PCr and pH dynamics and correlated with the concentrations of the regulatory metabolites ADP and Pi. Data were analyzed using a computational model of mitochondrial ATP synthesis and accounting for cytosolic fluxes of phosphate buffering and glycolytic pathways. The integrated model was compared to empirical data and used to explore the feasibility of competing hypotheses regarding the control of mitochondrial dehydrogenases and respiratory fluxes.

The role of pH on the thermodynamics and kinetics of muscle biochemistry: An in vivo study by 31P-MRS in patients with myo-phosphorylase deficiency

In this study we assessed ΔG′ ATP hydrolysis, cytosolic [ADP], and the rate of phosphocreatine recovery using Phosphorus Magnetic Resonance Spectroscopy in the calf muscle of a group of patients affected by glycogen myo-phosphorylase deficiency (McArdle disease). The goal was to ascertain whether and to what extent the deficit of the glycogenolytic pathway would affect the muscle energy balance. A typical feature of this pathology is the lack of intracellular acidosis. Therefore we posed the question of whether, in the absence of pH decrease, the rate of phosphocreatine recovery depends on the amount of phosphocreatine consumed during exercise. Results showed that at the end of exercise both [ADP] and ΔG′ ATP of patients were significantly higher than those of matched control groups reaching comparable levels of phosphocreatine concentration. Furthermore, in these patients we found that the rate of phosphocreatine recovery is not influenced by the amount of phosphocreatine consumed during exercise. These outcomes provide experimental evidence that: i) the intracellular acidification occurring in exercising skeletal muscle is a protective factor for the energy consumption; and ii) the influence of pH on the phosphocreatine recovery rate is at least in part related to the kinetic mechanisms of mitochondrial creatine kinase enzyme.

Molecular Mechanism by Which AMP-Activated Protein Kinase Activation Promotes Glycogen Accumulation in Muscle

OBJECTIVEDuring energy stress, AMP-activated protein kinase (AMPK) promotes glucose transport and glycolysis for ATP production, while it is thought to inhibit anabolic glycogen synthesis by suppressing the activity of glycogen synthase (GS) to maintain the energy balance in muscle. Paradoxically, chronic activation of AMPK causes an increase in glycogen accumulation in skeletal and cardiac muscles, which in some cases is associated with cardiac dysfunction. The aim of this study was to elucidate the molecular mechanism by which AMPK activation promotes muscle glycogen accumulation.RESEARCH DESIGN AND METHODSWe recently generated knock-in mice in which wild-type muscle GS was replaced by a mutant (Arg582Ala) that could not be activated by glucose-6-phosphate (G6P), but possessed full catalytic activity and could still be activated normally by dephosphorylation. Muscles from GS knock-in or transgenic mice overexpressing a kinase dead (KD) AMPK were incubated with glucose tracers and the AMPK-activating compound 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) ex vivo. GS activity and glucose uptake and utilization (glycolysis and glycogen synthesis) were assessed.RESULTSEven though AICAR caused a modest inactivation of GS, it stimulated muscle glycogen synthesis that was accompanied by increases in glucose transport and intracellular [G6P]. These effects of AICAR required the catalytic activity of AMPK. Strikingly, AICAR-induced glycogen synthesis was completely abolished in G6P-insensitive GS knock-in mice, although AICAR-stimulated AMPK activation, glucose transport, and total glucose utilization were normal.CONCLUSIONSWe provide genetic evidence that AMPK activation promotes muscle glycogen accumulation by allosteric activation of GS through an increase in glucose uptake and subsequent rise in cellular [G6P].

Creatine Supplementation Reduces Muscle Inosine Monophosphate during Endurance Exercise in Humans

Creatine (Cr) supplementation has been shown to attenuate increases in plasma ammonia and hypoxanthine during intense endurance exercise lasting 1 h, suggesting that Cr supplementation may improve muscle energy balance (matching of ATP resynthesis to ATP demand) during such exercise. We hypothesized that Cr supplementation would improve muscle energy balance (as assessed by muscle inosine monophosphate (IMP) accumulation) during intense endurance exercise. Seven well-trained men completed two experimental trials involving approximately 1 h of intense endurance exercise (cycling 45 min at 78+/-1% & OV0312;O2 peak followed by completion of 251+/-6 kJ as quickly as possible (performance ride)). Subjects ingested approximately 42 g.d dextrose for 5 d before the first experimental trial (CON), then approximately 21 g Cr monohydrate plus approximately 21 g.d dextrose for 5 d before the second experimental trial (CREAT). Trials were ordered because of the long washout time for Cr. Subjects were blinded to the order of the trials. Creatine supplementation significantly (P< 0.05) increased muscle total Cr (resting values: CREAT: 138.1+/-7.9; CON: 117.7+/- 6.5 mmol.kg dm). No difference was seen between treatments in any measured muscle or blood metabolite after the first 45 min of exercise. Despite the performance ride completion time being similar in the two treatments ( approximately 13.5 min, approximately 86% & OV0312;O2 peak), IMP at the end of the performance ride was significantly (P<0.05) lower in CREAT than in CON (CREAT: 1.2+/- 0.6; CON: 2.0+/- 0.7 mmol.kg dm). Raising muscle total Cr content before exercise appears to improve the ability of the muscle to maintain energy balance during intense aerobic exercise, but not during more moderate exercise intensities.

Temperature and pH dependence of energy balance by 31P- and 1H-MRS in anaerobic frog muscle

The temperature ( T)-dependence of energy consumption of resting anaerobic frog gastrocnemii exposed to different, changing electrochemical gradients was assessed. To this aim, the rate of ATP resynthesis (Δ∼P/Δ t) was determined by 31P- and 1H-MRS as the sum of the rates of PCr hydrolysis (Δ[PCr]/Δ t) and of anaerobic glycolysis (Δ[La]/ Δ t, based on a ∼P/La ratio of 1.5). The investigated T levels were 15, 20 and 25 °C, whereas initial extracellular pH (pHe) values were 7.9, 7.3 and 7.0, i.e. higher, equal or lower, respectively, than intracellular pH (pHi). The latter was changing with T according to the neutrality point (dpH/d T=−0.0165 pH units/°C). Both rates of PCr hydrolysis and of lactate accumulation and that of their sum, expressed as Δ∼P/Δ t, were highly T-dependent. By contrast, the pHe-dependence of the muscle energy balance was nil or extremely limited at 15 and 20 °C, respectively, but remarkable at 25 °C (with a depression of the ATP resynthesis rate up to 25% with a decrease of pHe from 7.9 to 7.0). The pHe-dependent reduction of metabolic rate was associated with a down-regulation of anaerobic glycolysis due to reduced activity of ion-transporters controlling acid–base balance and/or to a shift from Na +/H + to a more efficient Na +-dependent Cl −/HCO 3 − exchanger. Uncoupling of glycogenolysis from P-metabolite concentrations, both as function of T (≥20 °C) and of pHe (≤7.3), was also shown, attributable to a T-dependence of glycolytic enzyme activity and/or H + ion transport systems. The described metabolic slowdown observed in isolated muscle preparations subjected to the combined regimes of anoxia/acidosis implies that the mechanism determining survival time at the cellular level is mediated by exchange transport systems. A similar mechanism might affect muscle metabolism of homeotherms during chronic hypoxia and/or ischemia.

Psychobiological Aspects of Somatoform Disorders: Contributions of Monoaminergic Transmitter Systems

Objective: To evaluate the possible biological aspects of ‘unexplained physical symptoms’, this study examined serotonergic and noradrenergic monoamino acids in somatoform disorders and/or depression. Methods: Blood samples of 150 subjects from 4 groups (somatization syndrome; depression; depression and somatization; controls) were analyzed for amino acids contributing to the serotonergic and noradrenergic system and peripheral muscle energy balance (tryptophan, valine, leucine, isoleucine, phenylalanine and tyrosine). Results: Tryptophan, branched chain amino acids and other serotonergic amino acids were decreased in patients with somatoform symptoms, even when no depression was present. Conclusions: We conclude that serotonergic amino acids are biological correlates of multiple unexplained symptoms. Ways of action do not only involve brain mechanisms, but also energy metabolism in peripheral muscles.

Project title Cellular respiration in muscle energy balance

Project title Cellular respiration in muscle energy balance

Cellular energetics analysis by a mathematical model of energy balance: estimation of parameters in human skeletal muscle.

Cellular energy balance requires that the physiological demands by ATP-utilizing functions be matched by ATP synthesis to sustain muscle activity. We devised a new method of analysis of these processes in data from single individuals. Our approach is based on the logic of current information on the major mechanisms involved in this energy balance and can quantify not directly measurable parameters that govern those mechanisms. We use a mathematical model that simulates by ordinary, nonlinear differential equations three components of cellular bioenergetics (cellular ATP flux, mitochondrial oxidative phosphorylation, and creatine kinase kinetics). We incorporate data under resting conditions, during the transition toward a steady state of stimulation and during the transition during recovery back to the original resting state. Making use of prior information about the kinetic parameters, we fitted the model to previously published dynamic phosphocreatine (PCr) and inorganic phosphate (P(i)) data obtained in normal subjects with an activity-recovery protocol using (31)P nuclear magnetic resonance spectroscopy. The experiment consisted of a baseline phase, an ischemic phase (during which muscle stimulation and PCr utilization occurred), and an aerobic recovery phase. The model described satisfactorily the kinetics of the changes in PCr and P(i) and allowed estimation of the maximal velocity of oxidative phosphorylation and of the net ATP flux in individuals both at rest and during stimulation. This work lays the foundation for a quantitative, model-based approach to the study of in vivo muscle energy balance in intact muscle systems, including human muscle.

Energy metabolism and interstitial fluid displacement in human gastrocnemius during short ischemic cycles

Energy metabolism and interstitial fluid displacement were studied in the human gastrocnemius during three subsequent 5-min ischemia-reperfusion periods [ischemic preconditioning (IP)]. The muscle energy balance was assessed by combining near-infrared spectroscopy (NIRS) and 31P-nuclear magnetic resonance spectroscopy (31P-NMRS). The interstitial fluid displacement was determined by combining NIRS and 23Na-NMRS. No changes in total energy consumption or in the fractional contribution of the underlying energy sources (aerobic glycolysis, anaerobic glycolysis, and Lohmann reaction) were observed in the muscle during the tested IP protocol. Oxygen consumption in the muscle region of interest, as estimated by NIRS, was approximately 8 micromol . 100 g-1 . min-1 and did not change during IP. Phosphocreatine and ATP concentrations did not change over the whole experimental period. A slight but significant (P < 0.05) increase in intracellular pH was observed. Compared with the control, a 10% greater interstitial fluid content per muscle unit volume was observed at the end of the IP protocol. It is concluded that, at variance with cardiac muscle, repeated 5-min ischemia-reperfusion cycles do not induce metabolic changes in human gastrocnemius but alter the interstitial fluid readjustment. The techniques developed in the present study may be useful in identifying protocols suitable for skeletal muscle preconditioning and to explain the functional basis of this procedure.

Exercise performance of fish.

How is the muscle fiber designed to accomplish the diversity of tasks performed by striated muscle? Basically, a common contractile mechanism and a similar organization of metabolism in striated muscles are used to generate a wide spectrum of speeds and durations of contraction. The speed of contraction ranges from manyfold within an animal to over a hundred-fold between animals, owing to variation in the intrinsic velocities of the myosin isoforms. Recruiting fibers that contain the myosin isoform that contracts at the appropriate velocity varies the speed of locomotion at minimal cost. The magnitude and duration of the energy supply required to meet this contractile demand depends on the size of the cellular energy buffer and the capacities of the metabolic pathways. The faster the contractile speed, the larger the PCr pool and the greater the glycolytic capacity to meet a high rate of ATP use. Slower-contracting fibers have a smaller buffer for the short term, but an increased oxidative capacity for continuous energy supply to maintain energy balance over the long term. In general, fibers trade contractile speed for duration of performance, but a number of exceptions exist where rapid contractions are maintained for extended periods. In the face of this heterogeneity of properties, common features are found that assure an energy balance. The PCr/ATP buffer system offers a simple mechanism of feedback control of energy supply despite the wide range of high-energy phosphate concentrations and oxidative capacities found in skeletal muscle. An oxygen balance system also appears to be present in the terminal structures of the respiratory system, the capillaries, and mitochondria. Despite the diversity of these structures, a rather constant ratio of oxygen delivery capacity to mitochondrial oxidative capacity is found in vertebrate striated muscles. Finally, a critical feature of muscle energy balance that remains unresolved is (are) the mechanism(s) controlling mitochondrial respiration in heart. Feedback control appears to account for linking ATP supply to demand in skeletal muscle, but the mechanisms governing respiratory control in heart are still under vigorous investigation. Thus, the links between contractile demand and oxidative phosphorylation are still unresolved in this tissue, which may indicate that a key element is missing in our understanding of the cellular energetics of exercise.

Individual variation in contractile cost and recovery in a human skeletal muscle.

This study determined the variation among individuals in ATP use during contraction and ATP synthesis after stimulation in a human limb muscle. Muscle energetics were evaluated using a metabolic stress test that separates ATP utilization from synthesis by 31P NMR spectroscopy. Epicutaneous supramaximal twitch stimulation (1 Hz) of the median and ulnar nerves was applied in combination with ischemia of the finger and wrist flexors in eight normal subjects. The linear creatine phosphate (PCr) breakdown during ischemic stimulation defined ATP use (delta PCr per twitch or approximately P/twitch) and was highly reproducible as shown by the relative standard deviation [(standard deviation/mean) x 100] of 11% in three repeated measures. The time constant of the monoexponential PCr change during aerobic recovery represented ATP synthesis rate and also showed a low relative standard deviation (9%). Individuals were found to differ significantly in both mean approximately P/twitch (PCr breakdown rates, 0.29-0.45% PCr per sec or % PCr per twitch; ANOVA, p < 0.001) and in mean recovery time constants (41-74 sec; ANOVA, P < 0.001). This range of approximately P/twitch corresponded with the range of fiber types reported for a flexor muscle. In addition, approximately P/twitch was negatively correlated with a metabolite marker of slow-twitch fiber composition (Pi/ATP). The nearly 2-fold range of recovery time constants agreed with the range of mitochondrial volume densities found in human muscle biopsies. These results indicate that both components involved in the muscle energy balance--oxidative capacity and contractile costs--vary among individuals in human muscle and can be measured noninvasively by 31 P NMR.

Reappraisal of diffusion, solubility, and consumption of oxygen in frog skeletal muscle, with applications to muscle energy balance.

Previously we tested the validity of the one-dimensional diffusion equation for O2 in the excised frog sartorius muscle and used it to measure the diffusion coefficient (D) for O2 in this muscle and the time course of its rate of O2 consumption (Qo2) after a tetanus (Mahler, 1978, 1979, J. Gen. Physiol., 71:533-557, 559-580, 73:159-174). A transverse section of the frog sartorius is in fact well fit by a hemi-ellipse with width divided by maximum thickness averaging 5.1 +/- 0.2. Using the previous techniques with the two-dimensional diffusion equation and this hemi-elliptical boundary yields a value for D that is 30% smaller than reported previously; the revised values at 0, 10, and 22.8 degrees C are 6.2, 7.9, and 10.8 X 10(-6) cm2/s, respectively. After a tetanus at 20 degrees C, Qo2 rose quickly to a peak and then declined exponentially, with a time constant (tau) approximately 15% faster than that reported previously; tau averaged 2.1 min in Rana temporaria and 2.6 min in Rana pipiens. A technique was devised to measure the solubility (alpha) of O2 in intact, respiring muscles, and yielded alpha (muscle)/alpha (H2O) = 1.26 +/- 0.04. With these modifications, the values for O2 consumption obtained with the diffusion method were in agreement with those measured by the direct method of Kushmerick and Paul (1976, J. Physiol. [Lond.]., 254:693-709). Using results from both methods, at 20 degrees C the ratio of phosphorylcreatine split during a tetanus to O2 consumption during recovery ranged from 5.2 to 6.2 mumol/mumol, and postcontractile ATP hydrolysis was estimated to be 13.6 +/- 4.1 (n = 3) nmol/mumol total creatine.