High-performance liquid chromatographic assays for free and phosphorylated derivatives of the creatine analogues beta-guanidopropionic acid and 1-carboxy-methyl-2-iminoimidazolidine (cyclocreatine)

Creatine Kinase Equilibration and ΔGATP over an Extended Range of Physiological Conditions: Implications for Cellular Energetics, Signaling, and Muscle Performance

In this report, we establish a straightforward method for estimating the equilibrium constant for the creatine kinase reaction (CK Keq″) over wide but physiologically and experimentally relevant ranges of pH, Mg2+ and temperature. Our empirical formula for CK Keq″ is based on experimental measurements. It can be used to estimate [ADP] when [ADP] is below the resolution of experimental measurements, a typical situation because [ADP] is on the order of micromolar concentrations in living cells and may be much lower in many in vitro experiments. Accurate prediction of [ADP] is essential for in vivo studies of cellular energetics and metabolism and for in vitro studies of ATP-dependent enzyme function under near-physiological conditions. With [ADP], we were able to obtain improved estimates of ΔGATP, necessitating the reinvestigation of previously reported ADP- and ΔGATP-dependent processes. Application to actomyosin force generation in muscle provides support for the hypothesis that, when [Pi] varies and pH is not altered, the maximum Ca2+-activated isometric force depends on ΔGATP in both living and permeabilized muscle preparations. Further analysis of the pH studies introduces a novel hypothesis around the role of submicromolar ADP in force generation.

Phosphocyclocreatine is the dominant form of cyclocreatine in control and creatine transporter deficiency patient fibroblasts

Creatine transporter deficiency (CTD) is a metabolic disorder resulting in cognitive, motor, and behavioral deficits. Cyclocreatine (cCr), a creatine analog, has been explored as a therapeutic strategy for the treatment of CTD. We developed a rapid, selective, and accurate HILIC ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method to simultaneously quantify the intracellular concentrations of cCr, creatine (Cr), creatine-d3 (Cr-d3), phosphocyclocreatine (pcCr), and phosphocreatine (pCr). Using HILIC-UPLC-MS/MS, we measured cCr and Cr-d3 uptake and their conversion to the phosphorylated forms in primary human control and CTD fibroblasts. Altogether, the data demonstrate that cCr enters cells and its dominant intracellular form is pcCr in both control and CTD patient cells. Therefore, cCr may replace creatine as a therapeutic strategy for the treatment of CTD.

Skeletal muscle energetics are compromised only during high-intensity contractions in the Goto-Kakizaki rat model of type 2 diabetes

Type 2 diabetes (T2D) presents with hyperglycemia and insulin resistance, affecting over 30 million people in the United States alone. Previous work has hypothesized that mitochondria are dysfunctional in T2D and results in both reduced ATP production and glucose disposal. However, a direct link between mitochondrial function and T2D has not been determined. In the current study, the Goto-Kakizaki (GK) rat model of T2D was used to quantify mitochondrial function in vitro and in vivo over a broad range of contraction-induced metabolic workloads. During high-frequency sciatic nerve stimulation, hindlimb muscle contractions at 2- and 4-Hz intensities, the GK rat failed to maintain similar bioenergetic steady states to Wistar control (WC) rats measured by phosphorus magnetic resonance spectroscopy, despite similar force production. Differences were not due to changes in mitochondrial content in red (RG) or white gastrocnemius (WG) muscles (cytochrome c oxidase, RG: 22.2 ± 1.6 vs. 23.3 ± 1.7 U/g wet wt; WG: 10.8 ± 1.1 vs. 12.1 ± 0.9 U/g wet wt; GK vs. WC, respectively). Mitochondria isolated from muscles of GK and WC rats also showed no difference in mitochondrial ATP production capacity in vitro, measured by high-resolution respirometry. At lower intensities (0.25-1 Hz) there were no detectable differences between GK and WC rats in sustained energy balance. There were similar phosphocreatine concentrations during steady-state contraction and postcontractile recovery (τ = 72 ± 6 s GK versus 71 ± 2 s WC). Taken together, these results suggest that deficiencies in skeletal muscle energetics seen at higher intensities are not due to mitochondrial dysfunction in the GK rat.

Facile High-Performance Liquid Chromatography Mass Spectrometry Method for Analysis of Cyclocreatine and Phosphocyclocreatine in Complex Mixtures of Amino Acids

Creatine transporter deficiency (CTD) is caused by a defect in the X-linked creatine transporter SLC6A8 gene leading to severe neurologic and physiologic conditions. Cyclocreatine and phosphocyclocreatine supplementation is seen as a potential treatment, but the presence of these compounds within commercially available dietary supplements presents the risk of self-medication. High-performance liquid chromatography-mass spectrometry (HPLC-MS) is an excellent technique to assess composition of complex amino acid mixtures. Herein, we have developed a facile HPLC-MS method using a cyano column in hydrophilic interaction liquid chromatography (HILIC) mode with isocratic elution over 4 min to identify the main components of two commercially available dietary supplements. The relative standard deviation (RSD) for retention time and extracted ion integrated area are <0.3% and 4%, respectively, showing excellent reproducibility. Cyclocreatine and phosphocyclocreatine were not detectable within the dietary supplements, even at ppm levels, demonstrating the power and importance of the developed HPLC-MS method in analyzing complex mixtures.

Effects of altered pyruvate dehydrogenase activity on contracting skeletal muscle bioenergetics

During aerobic exercise (>65% of maximum oxygen consumption), the primary source of acetyl-CoA to fuel oxidative ATP synthesis in muscle is the pyruvate dehydrogenase (PDH) reaction. This study investigated how regulation of PDH activity affects muscle energetics by determining whether activation of PDH with dichloroacetate (DCA) alters the dynamics of the phosphate potential of rat gastrocnemius muscle during contraction. Twitch contractions were induced in vivo over a broad range of intensities to sample submaximal and maximal aerobic workloads. Muscle phosphorus metabolites were measured in vivo before and after DCA treatment by phosphorus nuclear magnetic resonance spectroscopy. At rest, DCA increased PDH activation compared with control (90 ± 12% vs. 23 ± 3%, P < 0.05), with parallel decreases in inorganic phosphate (P_i) of 17% (1.4 ± 0.2 vs. 1.7 ± 0.1 mM, P < 0.05) and an increase in the free energy of ATP hydrolysis (ΔG_ATP) (-66.2 ± 0.3 vs. -65.6 ± 0.2 kJ/mol, P < 0.05). During stimulation DCA increased steady-state phosphocreatine (PCr) and the magnitude of ΔG_ATP, with concomitant reduction in P_i and ADP concentrations. These effects were not due to kinetic alterations in PCr hydrolysis, resynthesis, or glycolytic ATP production and altered the flow-force relationship between mitochondrial ATP synthesis rate and ΔG_ATP. DCA had no significant effect at 1.0- to 2.0-Hz stimulation because physiological mechanisms at these high stimulation levels cause maximal activation of PDH. These data support a role of PDH activation in the regulation of the energetic steady state by altering the phosphate potential (ΔG_ATP) at rest and during contraction.

NAD+ repletion improves muscle function in muscular dystrophy and counters global PARylation

Neuromuscular diseases are often caused by inherited mutations that lead to progressive skeletal muscle weakness and degeneration. In diverse populations of normal healthy mice, we observed correlations between the abundance of mRNA transcripts related to mitochondrial biogenesis, the dystrophin-sarcoglycan complex, and nicotinamide adenine dinucleotide (NAD⁺) synthesis, consistent with a potential role for the essential cofactor NAD⁺ in protecting muscle from metabolic and structural degeneration. Furthermore, the skeletal muscle transcriptomes of patients with Duchene's muscular dystrophy (DMD) and other muscle diseases were enriched for various poly[adenosine 5'-diphosphate (ADP)-ribose] polymerases (PARPs) and for nicotinamide N-methyltransferase (NNMT), enzymes that are major consumers of NAD⁺ and are involved in pleiotropic events, including inflammation. In the mdx mouse model of DMD, we observed significant reductions in muscle NAD⁺ levels, concurrent increases in PARP activity, and reduced expression of nicotinamide phosphoribosyltransferase (NAMPT), the rate-limiting enzyme for NAD⁺ biosynthesis. Replenishing NAD⁺ stores with dietary nicotinamide riboside supplementation improved muscle function and heart pathology in mdx and mdx/Utr^-/- mice and reversed pathology in Caenorhabditis elegans models of DMD. The effects of NAD⁺ repletion in mdx mice relied on the improvement in mitochondrial function and structural protein expression (α-dystrobrevin and δ-sarcoglycan) and on the reductions in general poly(ADP)-ribosylation, inflammation, and fibrosis. In combination, these studies suggest that the replenishment of NAD⁺ may benefit patients with muscular dystrophies or other neuromuscular degenerative conditions characterized by the PARP/NNMT gene expression signatures.

Quantification of cyclocreatine in mouse and rat plasma using hydrophilic-interaction ultra-performance liquid chromatography-tandem mass spectrometry

An accurate, rapid and selective method was developed to quantify cyclocreatine in mouse and rat plasma using hydrophilic interaction (HILIC) ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). The plasma samples were prepared by protein precipitation with acetonitrile:methanol (70:30). Chromatographic separation was performed on a HILIC BEH amide column (2.1mm×50mm, 1.7μm) with a 3min gradient elution at a flow rate of 0.5mL/min. For mass spectrometric detection, selected reaction monitoring (SRM) was used; the SRM transitions were m/z 144→98 and m/z 144→56 for cyclocreatine and m/z 148→102 for the internal standard (D4-cyclocreatine) in the positive ionization mode. No endogenous components interfered with the analysis of cyclocreatine and the internal standard in mouse and rat plasma. Plasma calibration curves were constructed in the range of 0.01-25μM. The correlation coefficient of the calibration curves was greater than 0.99. The mean intraday assay accuracy for all quality control (QC) replicates was between 93 and 105%. The mean intraday assay precision (CV%) was 1.9-11% for all QC levels. The HILIC-UPLC-MS/MS method was successfully applied in pharmacokinetic (PK) studies of cyclocreatine in mice and rats for the first time. After a single 30mg/kg oral administration in mice and rats, the AUC_0-∞ (area under the curve) was 84.1μgh/mL and 91.7±18.0μgh/mL, respectively.

Open-Loop Control of Oxidative Phosphorylation in Skeletal and Cardiac Muscle Mitochondria by Ca(2.)

In cardiac muscle, mitochondrial ATP synthesis is driven by demand for ATP through feedback from the products of ATP hydrolysis. However, in skeletal muscle at higher workloads there is an apparent contribution of open-loop stimulation of ATP synthesis. Open-loop control is defined as modulation of flux through a biochemical pathway by a moiety, which is not a reactant or a product of the biochemical reactions in the pathway. The role of calcium, which is known to stimulate the activity of mitochondrial dehydrogenases, as an open-loop controller, was investigated in isolated cardiac and skeletal muscle mitochondria. The kinetics of NADH synthesis and respiration, feedback from ATP hydrolysis products, and stimulation by calcium were characterized in isolated mitochondria to test the hypothesis that calcium has a stimulatory role in skeletal muscle mitochondria not apparent in cardiac mitochondria. A range of respiratory states were obtained in cardiac and skeletal muscle mitochondria utilizing physiologically relevant concentrations of pyruvate and malate, and flux of respiration, NAD(P)H fluorescence, and rhodamine 123 fluorescence were measured over a range of extra mitochondrial calcium concentrations. We found that under these conditions calcium stimulates NADH synthesis in skeletal muscle mitochondria but not in cardiac mitochondria.

The effects of age and muscle contraction on AMPK activity and heterotrimer composition

Sarcopenia is characterized by increased skeletal muscle atrophy due in part to alterations in muscle metabolism. AMP-activated protein kinase (AMPK) is a master regulator of skeletal muscle metabolic pathways which regulate many cellular processes that are disrupted in old-age. Functional AMPK is a heterotrimer composed of α, β and γ subunits, and each subunit can be represented in the heterotrimer by one of two (α1/α2, β1/β2) or three (γ1/γ2/γ3) isoforms. Altered isoform composition affects AMPK localization and function. Previous work has shown that overall AMPK activation with endurance-type exercise is blunted in old vs. young skeletal muscle. However, details regarding the activation of the specific isoforms of AMPK, as well as the heterotrimeric composition of AMPK in old skeletal muscle, are unknown. Our purpose here, therefore, was to determine the effect of old-age on 1) the activation of the α1 and α2 catalytic subunits of AMPK in skeletal muscle by a continuous contraction bout, and 2) the heterotrimeric composition of skeletal muscle AMPK. We studied gastrocnemius (GAST) and tibialis anterior (TA) muscles from young adult (YA; 8months old) and old (O; 30months old) male Fischer344×Brown Norway F1 hybrid rats after an in situ bout of endurance-type contractions produced via electrical stimulation of the sciatic nerve (STIM). AMPKα phosphorylation and AMPKα1 and α2 activities were unaffected by age at rest. However, AMPKα phosphorylation and AMPKα2 protein content and activity were lower in O vs. YA after STIM. Conversely, AMPKα1 content was greater in O vs. YA muscle, and α1 activity increased with STIM in O but not YA muscles. AMPKγ3 overall concentration and its association with AMPKα1 and α2 were lower in O vs. YA GAST. We conclude that activation of AMPKα1 is enhanced, while activation of α2 is suppressed immediately after repeated skeletal muscle contractions in O vs. YA skeletal muscle. These changes are associated with changes in the AMPK heterotrimer composition. Given the known roles of AMPK α1, α2 and γ3, this may contribute to sarcopenia and associated muscle metabolic dysfunction.

Normal cardiac function in mice with supraphysiological cardiac creatine levels

Creatine and phosphocreatine levels are decreased in heart failure, and reductions in myocellular phosphocreatine levels predict the severity of the disease and portend adverse outcomes. Previous studies of transgenic mouse models with increased creatine content higher than two times baseline showed the development of heart failure and shortened lifespan. Given phosphocreatine's role in buffering ATP content, we tested the hypothesis whether elevated cardiac creatine content would alter cardiac function under normal physiological conditions. Here, we report the creation of transgenic mice that overexpress the human creatine transporter (CrT) in cardiac muscle under the control of the α-myosin heavy chain promoter. Cardiac transgene expression was quantified by qRT-PCR, and human CrT protein expression was documented on Western blots and immunohistochemistry using a specific anti-CrT antibody. High-energy phosphate metabolites and cardiac function were measured in transgenic animals and compared with age-matched, wild-type controls. Adult transgenic animals showed increases of 5.7- and 4.7-fold in the content of creatine and free ADP, respectively. Phosphocreatine and ATP levels were two times as high in young transgenic animals but declined to control levels by the time the animals reached 8 wk of age. Transgenic mice appeared to be healthy and had normal life spans. Cardiac morphometry, conscious echocardiography, and pressure-volume loop studies demonstrated mild hypertrophy but normal function. Based on our characterization of the human CrT protein expression, creatine and phosphocreatine content, and cardiac morphometry and function, these transgenic mice provide an in vivo model for examining the therapeutic value of elevated creatine content for cardiac pathologies.

Iron deficiency causes a shift in AMP-activated protein kinase (AMPK) subunit composition in rat skeletal muscle

Background

As a cellular energy sensor, the 5’AMP-activated protein kinase (AMPK) is activated in response to energy stresses such as hypoxia and muscle contraction. To determine effects of iron deficiency on AMPK activation and signaling, as well as the AMPK subunit composition in skeletal muscle, rats were fed a control (C=50-58 mg/kg Fe) or iron deficient (ID=2-6 mg/kg Fe) diet for 6–8 wks.

Results

Their respective hematocrits were 47.5% ± 1.0 and 16.5% ± 0.6. Iron deficiency resulted in 28.3% greater muscle fatigue (p<0.01) in response to 10 min of stimulation (1 twitch/sec) and was associated with a greater reduction in phosphocreatine (C: Resting 24.1 ± 0.9 μmol/g, Stim 13.1 ± 1.5 μmol/g; ID: Resting 22.7 ± 1.0 μmol/g, Stim 3.2 ± 0.7 μmol/g; p<0.01) and ATP levels (C: Resting 5.89 ± 0.48 μmol/g, Stim 6.03 ± 0.35 μmol/g; ID: Resting 5.51 ± 0.20 μmol/g, Stim 4.19 ± 0.47 μmol/g; p<0.05). AMPK activation increased with stimulation in muscles of C and ID animals. A reduction in Cytochrome c and other iron-dependent mitochondrial proteins was observed in ID animals (p<0.01). The AMPK catalytic subunit (α) was examined because both isoforms are known to play different roles in responding to energy challenges. In ID animals, AMPKα2 subunit protein content was reduced to 71.6% of C (p<0.05), however this did not result in a significant difference in resting AMPKα2 activity. AMPKα1 protein was unchanged, however an overall increase in AMPKα1 activity was observed (C: 0.91 pmol/mg/min; ID: 1.63 pmol/mg/min; p<0.05). Resting phospho Acetyl CoA Carboxylase (pACC) was unchanged. In addition, we observed significant reductions in the β2 and γ3 subunits of AMPK in response to iron deficiency.

Conclusions

This study indicates that chronic iron deficiency causes a shift in the expression of AMPKα, β, and γ subunit composition. Iron deficiency also causes chronic activation of AMPK as well as an increase in AMPKα1 activity in exercised skeletal muscle.

Impaired adaptability of in vivo mitochondrial energetics to acute oxidative insult in aged skeletal muscle

Periods of elevated reactive oxygen species (ROS) production are a normal part of mitochondrial physiology. However, little is known about age-related changes in the mitochondrial response to elevated ROS in vivo. Significantly, ROS-induced uncoupling of oxidative phosphorylation has received attention as a negative feedback mechanism to reduce mitochondrial superoxide production. Here we use a novel in vivo spectroscopy system to test the hypothesis that ROS-induced uncoupling is diminished in aged mitochondria. This system simultaneously acquires (31)P magnetic resonance and near-infrared optical spectra to non-invasively measure phosphometabolite and O(2) concentrations in mouse skeletal muscle. Using low dose paraquat to elevate intracellular ROS we assess in vivo mitochondrial function in young, middle aged, and old mice. Oxidative phosphorylation was uncoupled to the same degree in response to ROS at each age, but this uncoupling was associated with loss of phosphorylation capacity and total ATP in old mice only. Using mice lacking UCP3 we demonstrate that this in vivo uncoupling is independent of this putative uncoupler of skeletal muscle mitochondria. These data indicate that ROS-induced uncoupling persists throughout life, but that oxidative stress leads to mitochondrial deficits and loss of ATP in aged organisms that may contribute to impaired function and degeneration.

Exposing cardiomyocytes to subclinical concentrations of doxorubicin rapidly reduces their creatine transport

Doxorubicin is commonly used to treat leukemia, lymphomas, and solid tumors, such as soft tissue sarcomas or breast cancer. A major side effect of doxorubicin therapy is dose-dependent cardiotoxicity. Doxorubicin's effects on cardiac energy metabolism are emerging as key elements mediating its toxicity. We evaluated the effect of doxorubicin on [(14)C]creatine uptake in rat neonatal cardiac myocytes and HL-1 murine cardiac cells expressing the human creatine transporter protein. A significant and irreversible decrease in creatine transport was detected after an incubation with 50-100 nmol/l doxorubicin. These concentrations are well below peak plasma levels (5 μmol/l) and within the ranges (25-250 nmol/l) for steady-state plasma concentrations reported after the administration of 15-90 mg/m(2) doxorubicin for chemotherapy. The decrease in creatine transport was not solely because of increased cell death due to doxorubicin's cytotoxic effects. Kinetic analysis showed that doxorubicin decreased V(max), K(m), and creatine transporter protein content. Cell surface biotinylation experiments confirmed that the amount of creatine transporter protein present at the cell surface was reduced. Cardiomyocytes rely on uptake by a dedicated creatine transporter to meet their intracellular creatine needs. Our findings show that the cardiomyocellular transport capacity for creatine is substantially decreased by doxorubicin administration and suggest that this effect may be an important early event in the pathogenesis of doxorubicin-mediated cardiotoxicity.

Reduced coupling of oxidative phosphorylation in vivo precedes electron transport chain defects due to mild oxidative stress in mice

Oxidative stress and mitochondrial function are at the core of many degenerative conditions. However, the interaction between oxidative stress and in vivo mitochondrial function is unclear. We used both pharmacological (2 week paraquat (PQ) treatment of wild type mice) and transgenic (mice lacking Cu, Zn-superoxide dismutase (SOD1(-/-))) models to test the effect of oxidative stress on in vivo mitochondrial function in skeletal muscle. Magnetic resonance and optical spectroscopy were used to measure mitochondrial ATP and oxygen fluxes and cell energetic state. In both models of oxidative stress, coupling of oxidative phosphorylation was significantly lower (lower P/O) at rest in vivo in skeletal muscle and was dose-dependent in the PQ model. Despite this reduction in efficiency, in vivo mitochondrial phosphorylation capacity (ATPmax) was maintained in both models, and ex vivo mitochondrial respiration in permeabilized muscle fibers was unchanged following PQ treatment. In association with the reduced P/O, PQ treatment led to a dose-dependent reduction in PCr/ATP ratio and increased phosphorylation of AMPK. These results indicate that oxidative stress uncouples oxidative phosphorylation in vivo and results in energetic stress in the absence of defects in the mitochondrial electron transport chain.

Skeletal muscle dysfunction in muscle-specific LKB1 knockout mice

Liver kinase B1 (LKB1) is a tumor-suppressing protein that is involved in the regulation of muscle metabolism and growth by phosphorylating and activating AMP-activated protein kinase (AMPK) family members. Here we report the development of a myopathic phenotype in skeletal and cardiac muscle-specific LKB1 knockout (mLKB1-KO) mice. The myopathic phenotype becomes overtly apparent at 30-50 wk of age and is characterized by decreased body weight and a proportional reduction in fast-twitch skeletal muscle weight. The ability to ambulate is compromised with an often complete loss of hindlimb function. Skeletal muscle atrophy is associated with a 50-75% reduction in mammalian target of rapamycin pathway phosphorylation, as well as lower peroxisome proliferator-activated receptor-alpha coactivator-1 content and cAMP response element binding protein phosphorylation (43 and 40% lower in mLKB1-KO mice, respectively). Maximum in situ specific force production is not affected, but fatigue is exaggerated, and relaxation kinetics are slowed in the myopathic mice. The increased fatigue is associated with a 30-78% decrease in mitochondrial protein content, a shift away from type IIA/D toward type IIB muscle fibers, and a tendency (P=0.07) for decreased capillarity in mLKB1-KO muscles. Hearts from myopathic mLKB1-KO mice exhibit grossly dilated atria, suggesting cardiac insufficiency and heart failure, which likely contributes to the phenotype. These findings indicate that LKB1 plays a critical role in the maintenance of both skeletal and cardiac function.

Simultaneous determination of inositol and inositol phosphates in complex biological matrices: quantitative ion-exchange chromatography/tandem mass spectrometry

myo-Inositol (Ins) and myo-inositol phosphates (InsPs) are widely distributed in plants and animals. The evaluation of the distribution of Ins and InsPs in cells and plant sources can impact the understanding of their role in nutrition, cellular processes and diseases, and how they may be modulated by diet. We developed an anion-exchange chromatography/tandem mass spectrometry (HPLC/ESI-MS/MS) method for the separation and simultaneous quantitation of Ins and different naturally occurring phosphorylated inositol compounds. Chromatographic separation was achieved in 30 min on a commercial anion-exchange column (0.5 x 150 mm) using a gradient of 200 mM ammonium carbonate buffer (pH 9.0) and 5% methanol in H(2)O. Analytes were identified by selective reaction monitoring using a triple quadrupole mass spectrometer in negative ion electrospray ionization mode. Adenosine 5'-monophosphate was used as a general internal standard for quantitation. Detection is linear in the range of 0.25-400 pmol for Ins, InsP(1), InsP(4), and InsP(5), 40-400 pmol for InsP(2) and InsP(3), and 60-400 pmol for InsP(6), with a minimum r(2) > 0.994. The limit of detection is 0.25 pmol with a signal-to-noise ratio of 10:1 for all analytes. The intra-day and inter-day variations were within 17% at three concentration levels. Recovery values for the seven analytes spiked into extraction solution or different matrices were between 63 and 121%. Using this approach, Ins and InsPs were measured in three different plant samples and in cultured cells, illustrating significant differences in the distribution of inositol compounds in food samples compared to cells and between cell types.

Phosphocreatine as an energy source for actin cytoskeletal rearrangements during myoblast fusion

Myoblast fusion is essential for muscle development, postnatal growth and muscle repair after injury. Recent studies have demonstrated roles for actin polymerization during myoblast fusion. Dynamic cytoskeletal assemblies directing cell-cell contact, membrane coalescence and ultimately fusion require substantial cellular energy demands. Various energy generating systems exist in cells but the partitioning of energy sources during myoblast fusion is unknown. Here, we demonstrate a novel role for phosphocreatine (PCr) as a spatiotemporal energy buffer during primary mouse myoblast fusion with nascent myotubes. Creatine treatment enhanced cell fusion in a creatine kinase (CK)-dependent manner suggesting that ATP-consuming reactions are replenished through the PCr/CK system. Furthermore, selective inhibition of actin polymerization prevented myonuclear addition following creatine treatment. As myotube formation is dependent on cytoskeletal reorganization, our findings suggest that PCr hydrolysis is coupled to actin dynamics during myoblast fusion. We conclude that myoblast fusion is a high-energy process, and can be enhanced by PCr buffering of energy demands during actin cytoskeletal rearrangements in myoblast fusion. These findings implicate roles for PCr as a high-energy phosphate buffer in the fusion of multiple cell types including sperm/oocyte, trophoblasts and macrophages. Furthermore, our results suggest the observed beneficial effects of oral creatine supplementation in humans may result in part from enhanced myoblast fusion.

Reduced mitochondrial coupling in vivo alters cellular energetics in aged mouse skeletal muscle

The mitochondrial theory of ageing proposes that the accumulation of oxidative damage to mitochondria leads to mitochondrial dysfunction and tissue degeneration with age. However, no consensus has emerged regarding the effects of ageing on mitochondrial function, particularly for mitochondrial coupling (P/O). One of the main barriers to a better understanding of the effects of ageing on coupling has been the lack of in vivo approaches to measure P/O. We use optical and magnetic resonance spectroscopy to independently quantify mitochondrial ATP synthesis and O2 uptake to determine in vivo P/O. Resting ATP demand (equal to ATP synthesis) was lower in the skeletal muscle of 30-month-old C57Bl/6 mice compared to 7-month-old controls (21.9 +/- 1.5 versus 13.6 +/- 1.7 nmol ATP (g tissue)(-1) s(-1), P = 0.01). In contrast, there was no difference in the resting rates of O2 uptake between the groups (5.4 +/- 0.6 versus 8.4 +/- 1.6 nmol O2 (g tissue)(-1) s(-1)). These results indicate a nearly 50% reduction in the mitochondrial P/O in the aged animals (2.05 +/- 0.07 versus 1.05 +/- 0.36, P = 0.02). The higher resting ADP (30.8 +/- 6.8 versus 58.0 +/- 9.5 micromol g(-1), P = 0.05) and decreased energy charge (ATP/ADP) (274 +/- 70 versus 84 +/- 16, P = 0.03) in the aged mice is consistent with an impairment of oxidative ATP synthesis. Despite the reduced P/O, uncoupling protein 3 protein levels were not different in the muscles of the two groups. These results demonstrate reduced mitochondrial coupling in aged skeletal muscle that alters cellular metabolism and energetics.

Contraction-mediated phosphorylation of AMPK is lower in skeletal muscle of adenylate kinase-deficient mice

The activity of AMP-activated protein kinase (AMPK) increases during muscle contractions as a result of elevated AMP concentration. We tested whether activation of AMPK would be altered during contractions in adenylate kinase (AK) 1-deficient (AK1-/-) mice, because they have a reduced capacity to form AMP. The right gastrocnemius-soleus-plantaris muscle group was stimulated via the sciatic nerve at 2 Hz for 30 min in both wild-type (WT) and AK1-/- animals. Initial force production was not different between the two groups (129.2 +/- 3.3 g vs. 140.9 +/- 8.5 g for WT and AK1-/-, respectively); however, force production by AK1-/- mice was significantly greater over the 30-min stimulation period, and final tension was 85 +/- 4.5% of initial in WT and 102 +/- 3.2% of initial in AK1-/- mice. Western blot analysis showed that AMPK phosphorylation with contractions was clearly increased in WT muscles (4.0 +/- 1.1 above resting values), but did not change noticeably with AK deficiency (1.6 +/- 0.4 above WT resting values). However, increases in phosphorylation of acetyl CoA carboxylase were robust in both WT and AK1-/- muscles and not different between the two groups. These results suggest that reduced formation of AMP during contractions in skeletal muscle of AK1-/- mice results in reduced phosphorylation of AMPK. However, altered AMPK signaling was not apparent in the phosphorylation status of acetyl CoA carboxylase, a typical marker of AMPK activity.

Skeletal muscle contractile performance and ADP accumulation in adenylate kinase-deficient mice

The production of AMP by adenylate kinase (AK) and subsequent deamination by AMP deaminase limits ADP accumulation during conditions of high-energy demand in skeletal muscle. The goal of this study was to investigate the consequences of AK deficiency (-/-) on adenine nucleotide management and whole muscle function at high-energy demands. To do this, we examined isometric tetanic contractile performance of the gastrocnemius-plantaris-soleus (GPS) muscle group in situ in AK1(-/-) mice and wild-type (WT) controls over a range of contraction frequencies (30-120 tetani/min). We found that AK1(-/-) muscle exhibited a diminished inosine 5'-monophosphate formation rate (14% of WT) and an inordinate accumulation of ADP ( approximately 1.5 mM) at the highest energy demands, compared with WT controls. AK-deficient muscle exhibited similar initial contractile performance (521 +/- 9 and 521 +/- 10 g tension in WT and AK1(-/-) muscle, respectively), followed by a significant slowing of relaxation kinetics at the highest energy demands relative to WT controls. This is consistent with a depressed capacity to sequester calcium in the presence of high ADP. However, the overall pattern of fatigue in AK1(-/-) mice was similar to WT control muscle. Our findings directly demonstrate the importance of AMP formation and subsequent deamination in limiting ADP accumulation. Whole muscle contractile performance was, however, remarkably tolerant of ADP accumulation markedly in excess of what normally occurs in skeletal muscle.

Mechanical stimuli regulate rapamycin-sensitive signalling by a phosphoinositide 3-kinase-, protein kinase B- and growth factor-independent mechanism

In response to growth factors, mTOR (mammalian target of rapamycin) has been identified as a central component of the signalling pathways that control the translational machinery and cell growth. Signalling through mTOR has also been shown to be necessary for the mechanical load-induced growth of cardiac and skeletal muscles. Although the mechanisms involved for mechanically induced activation of mTOR are not known, it has been suggested that activation of PI3K (phosphoinositide 3-kinase) and protein kinase B (Akt), via the release of locally acting growth factors, underlies this process. In the present study, we show that mechanically stimulating (passive stretch) the skeletal muscle ex vivo results in the activation of mTOR-dependent signalling events. The activation of mTOR-dependent signalling events was necessary for an increase in translational efficiency, demonstrating the physiological significance of this pathway. Using pharmacological inhibitors, we show that activation of mTOR-dependent signalling occurs through a PI3K-independent pathway. Consistent with these results, mechanically induced signalling through mTOR was not disrupted in muscles from Akt1-/- mice. In addition, ex vivo co-incubation experiments, along with in vitro conditioned-media experiments, demonstrate that a mechanically induced release of locally acting autocrine/paracrine growth factors was not sufficient for the activation of the mTOR pathway. Taken together, our results demonstrate that mechanical stimuli can activate the mTOR pathway independent of PI3K/Akt1 and locally acting growth factors. Thus mechanical stimuli and growth factors provide distinct inputs through which mTOR co-ordinates an increase in the translational efficiency.

Phosphate uptake in rat skeletal muscle is reduced during isometric contractions

During contractions, there is a net efflux of phosphate from skeletal muscle, likely because of an elevated intracellular inorganic phosphate (P(i)) concentration. Over time, contracting muscle could incur a substantial phosphate deficit unless P(i) uptake rates were increased during contractions. We used the perfused rat hindquarter preparation to assess [(32)P]P(i) uptake rates in muscles at rest or over a range of energy expenditures during contractions at 0.5, 3, or 5 Hz for 30 min. P(i) uptake rates were reduced during contractions in a pattern that was dependent on contraction frequency and fiber type. In soleus and red gastrocnemius, [(32)P]P(i) uptake rates declined by approximately 25% at 0.5 Hz and 50-60% at 3 and 5 Hz. Uptake rates in white gastrocnemius decreased by 65-75% at all three stimulation frequencies. These reductions in P(i) uptake are not likely confounded by changes in precursor [(32)P]P(i) specific activity in the interstitium. In soleus and red gastrocnemius, declines in P(i) uptake rates were related to energy expenditure over the contraction duration. These data imply that P(i) uptake in skeletal muscle is acutely modulated during contractions and that decreases in P(i) uptake rates, in combination with expected increases in P(i) efflux, exacerbate the net loss of phosphate from the cell. Enhanced uptake of P(i) must subsequently occur because skeletal muscle typically maintains a relatively constant total phosphate pool.

Phosphate uptake and PiT-1 protein expression in rat skeletal muscle

Skeletal muscle fiber types differ in their contents of total phosphate, which includes inorganic phosphate (P(i)) and high-energy organic pools of ATP and phosphocreatine (PCr). At steady state, uptake of P(i) into the cell must equal the rate of efflux, which is expected to be a function of intracellular P(i) concentration. We measured (32)P-labeled P(i) uptake rates in different muscle fiber types to determine whether they are proportional to cellular P(i) content. P(i) uptake rates in isolated, perfused rat hindlimb muscles were linear over time and highest in soleus (2.42 +/- 0.17 micromol x g(-1) x h(-1)), lower in red gastrocnemius (1.31 +/- 0.11 micromol x g(-1) x h(-1)), and lowest in white gastrocnemius (0.49 +/- 0.06 micromol x g(-1) x h(-1)). Reasonably similar rates were obtained in vivo. P(i) uptake rates at plasma P(i) concentrations of 0.3-1.7 mM confirm that the P(i) uptake process is nearly saturated at normal plasma P(i) levels. P(i) uptake rate correlated with cellular P(i) content (r = 0.99) but varied inversely with total phosphate content. Sodium-phosphate cotransporter (PiT-1) protein expression in soleus and red gastrocnemius were similar to each other and seven- to eightfold greater than PiT-1 expression in white gastrocnemius. That the PiT-1 expression pattern did not match the pattern of P(i) uptake across fiber types implies that other factors are involved in regulating P(i) uptake in skeletal muscle. Furthermore, fractional turnover of the cellular P(i) pool (0.67, 0.57, and 0.33 h(-1) in soleus, red gastrocnemius, and white gastrocnemius, respectively) varies among fiber types, indicating differential management of intracellular P(i), likely due to differences in resistance to P(i) efflux from the fiber.

Mitochondrial coupling in vivo in mouse skeletal muscle

The coupling of mitochondrial ATP synthesis and oxygen consumption (ratio of ATP and oxygen fluxes, P/O) plays a central role in cellular bioenergetics. Reduced P/O values are associated with mitochondrial pathologies that can lead to reduced capacity for ATP synthesis and tissue degeneration. Previous work found a wide range of values for P/O in normal mitochondria. To measure mitochondrial coupling under physiological conditions, we have developed a procedure for determining the P/O of skeletal muscle in vivo. This technique measures ATPase and oxygen consumption rates during ischemia with31P magnetic resonance and optical spectroscopy, respectively. This novel approach allows the independent quantitative measurement of ATPase and oxygen flux rates in intact tissue. The quantitative measurement of oxygen consumption is made possible by our ability to independently measure the saturations of hemoglobin (Hb) and myoglobin (Mb) from optical spectra. Our results indicate that the P/O in skeletal muscle of the mouse hindlimb measured in vivo is 2.16 ± 0.24. The theoretical P/O for resting muscle is 2.33. Systemic treatment with 2,4-dinitrophenol to partially uncouple mitochondria does not affect the ATPase rate in the mouse hindlimb but nearly doubles the rate of oxygen consumption, reducing in vivo P/O to 1.37 ± 0.22. These results indicate that only a small fraction of the oxygen consumption in resting mouse skeletal muscle is nonphosphorylating under physiological conditions, suggesting that mitochondria are more tightly coupled than previously thought.

Oxygen regulation and limitation to cellular respiration in mouse skeletal muscle in vivo

In skeletal muscle, intracellular Po2 can fall to as low as 2-3 mmHg. This study tested whether oxygen regulates cellular respiration in this range of oxygen tensions through direct coupling between phosphorylation potential and intracellular Po2. Oxygen may also behave as a simple substrate in cellular respiration that is near saturating levels over most of the physiological range. A novel optical spectroscopic method was used to measure tissue oxygen consumption (Mo2) and intracellular Po2 using the decline in hemoglobin and myoglobin saturation in the ischemic hindlimb muscle of Swiss-Webster mice. 31P magnetic resonance spectroscopic determinations yielded phosphocreatine concentration ([PCr]) and pH in the same muscle volume. Intracellular Po2 fell to <2 mmHg during the ischemic period without a change in the muscle [PCr] or pH. The constant phosphorylation state despite the decline in intracellular Po2 rejects the hypothesis that direct coupling between these two variables results in a regulatory role for oxygen in cellular respiration. A second set of experiments tested the relationship between intracellular Po2 and Mo2. In vivo Mo2 in mouse skeletal muscle was increased by systemic treatment with 2 and 4 mg/kg body wt 2,4-dinitrophenol to partially uncouple mitochondria. Mo2 was not dependent on intracellular Po2 above 3 mmHg in the three groups despite a threefold increase in Mo2. These results indicate that Mo2 and the phosphorylation state of the cell are independent of intracellular Po2 throughout the physiological range of oxygen tensions. Therefore, we reject a regulatory role for oxygen in cellular respiration and conclude that oxygen acts as a simple substrate for respiration under physiological conditions.

Validation of a simple liquid chromatography assay for creatine suitable for pharmacokinetic applications, determination of plasma protein binding and verification of percent labeled claim of various creatine products

Creatine has been quantified in various tissues by a range of methodologies. This paper reports on the development and validation of a simplified HPLC assay to determine plasma creatine, plasma protein binding of creatine, creatine in microdialysate and creatine in over-the-counter products. An isocratic, reversed-phase (C(18)) HPLC assay, using potassium phosphate monobasic (pH 4) as a mobile phase, was validated in human plasma and microdialysis perfusion fluid (normal saline). The lower limit of quantification for the assay was 1 mg l(-1) in saline and 5 mg l(-1) in plasma. The RSD was below 6% and accuracy was below 12% in both matrices. Protein binding in human plasma was found to be negligible (<10%). Over-the-counter creatine monohydrate products tested contained 100% creatine monohydrate. This assay was found to be suitable for pharmacokinetic studies and the assessment of plasma creatine and skeletal muscle microdialysate.

Muscle creatine uptake and creatine transporter expression in response to creatine supplementation and depletion

The total creatine pool size [Cr(total); creatine (Cr) + phosphocreatine (PCr)] is crucial for optimal energy utilization in skeletal muscle, especially at the onset of exercise and during intense contractions. The Cr(total) likely is controlled by long-term modulation of Cr uptake via the sodium-dependent Cr transporter (CrT). To test this hypothesis, adult male Sprague-Dawley rats were fed 1% Cr, their muscle Cr(total) was reduced by approximately 85% [1% beta-guanidinoproprionic acid (beta-GPA)], or their muscle Cr(total) was repleted (1% Cr after beta-GPA depletion). Cr uptake was assessed by skeletal muscle (14)C-Cr accumulation to Cr and PCr by using hindlimb perfusion, and CrT protein content was assessed by Western blot. Cr uptake rate decreased with dietary Cr supplementation in the white gastrocnemius (WG; 45%) only. Depletion of muscle Cr(total) to approximately 15% of normal increased Cr uptake in the soleus (21%) and red gastrocnemius (22%), corresponding to 70-150% increases in muscle CrT content. In contrast, the inherently lower Cr uptake rate in the WG was unchanged with depletion of muscle Cr(total) even though CrT band density was increased by 230%. Thus there was no direct relationship between apparent muscle CrT abundance and Cr uptake rates. However, Cr uptake rates scaled inversely with decreases in muscle Cr(total) in the high-oxidative muscle types but not in the WG. This implies that factors controlling Cr uptake are different among fiber types. These observations may help explain the influence of initial muscle Cr(total), time dependency, and variations in muscle Cr(total) accumulation during Cr supplementation.

N-acetylaspartate to total creatine ratio in the hippocampal CA1 sector after transient cerebral ischemia in gerbils: influence of neuronal elements, reactive gliosis, and tissue atrophy

The authors compared temporal profiles of N-acetylaspartate (NAA) and the NAA/total creatine ratio with neuronal and astrocytic densities and with tissue atrophy in the hippocampal CA1 sector of gerbils after 5-minute bilateral forebrain ischemia and subsequent reperfusion for up to 6 months. The CA1 sector was dissected from 20- micro m lyophilized sections (n = 5) for NAA, phosphocreatine, and creatine assays using high-performance liquid chromatography. Adjacent 10- micro m sections were used for immunohistochemical analysis to follow neuronal and astrocytic responses. The NAA concentration was significantly (P<0.01) decreased after 7 days but leveled off thereafter. The NAA/total creatine (phosphocreatine + creatine) ratio was significantly decreased after 7 days and further decreased (P<0.05) after 6 months. Extensive neuronal damage developed beyond 7 days, while reactive astrogliosis progressed throughout the observation period. There was a good linear correlation (P<0.01) between astroglial density and the NAA/total creatine ratio beyond 7 days. The thickness of the CA1 sector was significantly reduced after 1 month and further reduced after 6 months. Although both NAA level and the NAA/total creatine ratio seemed to be indicators of neuronal damage, the latter could be influenced by reactive astrogliosis with progression of tissue atrophy.

Creatine uptake and creatine transporter expression among rat skeletal muscle fiber types

Total creatine (Cr(total) = phosphocreatine + creatine) concentrations differ substantially among mammalian skeletal muscle. Because the primary means to add Cr(total) to muscle is uptake of creatine through the sodium-dependent creatine transporter (CrT), differences in creatine uptake and CrT expression could account for the variations in [Cr(total)] among muscle fiber types. To test this hypothesis, hindlimbs of adult rats were perfused with 0.05-1 mM [(14)C]creatine for up to 90 min. Creatine uptake rates at 1 mM creatine were greatest in the soleus (140 +/- 8.8 nmol x h(-1) x g(-1)), less in the red gastrocnemius (117 +/- 8.3), and least in the white gastrocnemius (97 +/- 10.7). These rates were unaltered by time, insulin concentration, or increased perfusate sodium concentration. Conversely, creatine uptake rates were correspondingly decreased among fiber types by lower creatine and sodium concentrations. The CrT protein content by Western blot analysis was similarly greatest in the soleus, less in the red gastrocnemius, and least in the white gastrocnemius, whereas CrT mRNA was not different. Creatine uptake rates differ among skeletal muscle fiber sections in a manner reasonably assigned to the 58-kDa band of the CrT. Furthermore, creatine uptake rates scale inversely with creatine content, with the lowest uptake rate in the fiber type with the highest Cr(total) and vice versa. This suggests that the creatine pool fractional turnover rate is not common across muscle phenotypes and, therefore, is differentially regulated.

Phosphocreatine content of freeze-clamped muscle: influence of creatine kinase inhibition

The study of cellular energetics is critically dependent on accurate measurement of high-energy phosphates. Muscle values of phosphocreatine (PCr) vary greatly between in vivo measurements (i.e., by nuclear magnetic resonance) and chemical measurements determined from muscles isolated and quick-frozen. The source of this difference has not been experimentally identified. A likely cause is activation of ATPases and phosphotransfer from PCr to ADP. Therefore, rat hindlimb skeletal muscle was perfused either with or without 2 mM iodoacetamide, a creatine kinase inhibitor, and muscle was freeze-clamped either at rest or after contraction. Creatine kinase inhibition resulted in approximately 6 micromol/g higher PCr and lower creatine in the freeze-clamped soleus, red gastrocnemius, and white gastrocnemius. This PCr content difference was reduced when the initial PCr content was decreased with prior contractions. Therefore, the amount of PCr artifact appears to scale with initial PCr content within a fiber-type section. This artifact directly affects the measurement and, thus, the calculations of muscle energetic parameters from studies using isolated and frozen muscle.

Complement regulatory protein CD59 involves c-SRC related tyrosine phosphorylation of the creatine transporter in skeletal muscle during sepsis

BACKGROUND

Myocellular creatine (Cr) uptake is predominantly governed by the creatine transporter (CreaT) and plays a pivotal role in skeletal muscle energy metabolism. The CreaT belongs to a neurotransmitter transporter family that is functionally regulated by protein tyrosine kinase induced tyrosine phosphorylation. Recently, complement regulatory protein CD59 has been found not only to protect host tissue from C5b-9 complex attack that occurs in sepsis but also to initiate the activation of Src family kinase and tyrosine phosphorylation of its downstream proteins. The purpose of this study was to determine the association between myocellular free Cr, c-Src related tyrosine phosphorylation of the CreaT, and CD59 during sepsis.

METHODS

Male Sprague-Dawley rats (250 to 300 g) were randomized to undergo cecal ligation and puncture (CLP) or sham operation. Fast-twitch gastrocnemius muscles were harvested 24 hours after operation. Myocellular free Cr levels were measured by high-performance liquid chromatography. Combination of protein immunoprecipitation with Western blotting was used to assess tyrosine phosphorylation status of the CreaT and the association between CD59, c-Src, and CreaT.

RESULTS

Myocellular free Cr levels were 70% greater after CLP. Tyrosine phosphorylation of the CreaT was significantly increased after CLP as compared to sham operation. Tyrosine phosphorylated c-Src (Tyr-416) in the CreaT-c-Src immune complex was 24% higher after CLP. Sepsis also increased protein expression of tyrosine phosphorylated c-Src (Tyr-416) or CreaT in the CD59-c-Src or CD59-CreaT complex by 20% or 30%, respectively.

CONCLUSIONS

During sepsis, an increase in myocellular free Cr levels is associated with enhanced tyrosine phosphorylation of the CreaT, which is likely induced by active c-Src. CD59 is physically associated with both c-Src and CreaT, which suggests that CD59 may participate in the regulation of myocellular Cr metabolism via the CreaT during sepsis.

Cr supplementation decreases tyrosine phosphorylation of the CreaT in skeletal muscle during sepsis

Myocellular creatine (Cr) uptake is predominantly governed by a sodium-dependent Cr transporter (CreaT) and plays a pivotal role in skeletal muscle energy metabolism. The CreaT belongs to a neurotransmitter transporter family that can be functionally regulated by protein tyrosine kinase-induced tyrosine phosphorylation. The association between myocellular Cr and c-Src-related tyrosine phosphorylation of the CreaT and the influence of oral Cr supplementation on this association were investigated during sepsis. Animals were randomized to receive standard rat chow or standard rat chow with oral Cr supplementation for 4 days followed by cecal ligation and puncture (CLP) or sham operation. Fast-twitch gastrocnemius muscles were harvested 24 h after operation. Myocellular free Cr levels were 70% higher after CLP. Western blotting of the immunoprecipitated CreaT with an anti-phosphotyrosine or anti-phospho-c-Src (Y-416) antibody revealed that tyrosine phosphorylation of the CreaT and tyrosine-phosphorylated c-Src (Tyr(416)) expression in the CreaT-c-Src complex were significantly increased after CLP compared with sham operation. These changes were observed in homogenates and plasma membrane fractions of gastrocnemius muscles. Although oral Cr supplementation increased myocellular free Cr levels equivalently in CLP and sham-operated animals, c-Src-related tyrosine phosphorylation of the CreaT in homogenates and plasma membrane fractions of gastrocnemius muscles was, however, downregulated in Cr-supplemented CLP animals compared with Cr-supplemented sham-operated rats. During sepsis, increased myocellular free Cr levels are associated with enhanced tyrosine phosphorylation of the CreaT, which is likely induced by active c-Src. Oral Cr supplementation downregulates c-Src-related tyrosine phosphorylation of the CreaT. The data suggest that myocellular Cr homeostasis and CreaT activity are tightly regulated and closely related during sepsis.

Role of skeletal muscle Na+-K+ ATPase activity in increased lactate production in sub-acute sepsis

Bacterial sepsis is frequently accompanied by increased blood concentration of lactic acid, which traditionally is attributed to poor tissue perfusion, hypoxia and anaerobic glycolysis. Therapy aimed at improving oxygen delivery to tissues often does not correct the hyperlactatemia, suggesting that high blood lactate in sepsis is not due to hypoxia. Various tissues, including skeletal muscle, demonstrate increased lactate production under well-oxygenated conditions when the activity of the Na+-K+ ATPase is stimulated. Although both muscle Na+-K+ ATPase activity and muscle plasma membrane content of Na+, K+-ATPase subunits are increased in sepsis, no studies in vivo have demonstrated correlation between lactate production and changes in intracellular Na+ and K+ resulting from increased Na+-K+ pump activity in sepsis. Plasma concentrations of lactate and epinephrine, a known stimulator of the Na+-K+ pump, were increased in rats made septic by E. coli injection. Muscle lactate content was significantly increased in septic rats, although muscle ATP and phosphocreatine remained normal, suggesting oxygen delivery remained adequate for mitochondrial energy metabolism. In septic rats, muscle intracellular ratio of Na+:K+ was significantly reduced, indicating increased Na+-K+ pump activity. These data thus demonstrate that increased muscle lactate during sepsis correlates with evidence of elevated muscle Na+-K+ ATPase activity, but not with evidence of impaired oxidative metabolism. This study also further supports a role for epinephrine in this process.

Influence of ribose on adenine salvage after intense muscle contractions

The influence of ribose supplementation on skeletal muscle adenine salvage rates during recovery from intense contractions and subsequent muscle performance was evaluated using an adult rat perfused hindquarter preparation. Three minutes of tetanic contractions (60 tetani/min) decreased ATP content in the calf muscles by approximately 50% and produced an equimolar increase in IMP. Effective recovery of muscle ATP 1 h after contractions was due to reamination of IMP via the purine nucleotide cycle and was complete in the red gastrocnemius but incomplete in the white gastrocnemius muscle section. Adenine salvage rates in recovering muscle averaged 45 +/- 4, 49 +/- 5, and 30 +/- 3 nmol. h(-1). g(-1) for plantaris, red gastrocnemius, and white gastrocnemius muscle, respectively, which were not different from values in corresponding nonstimulated muscle sections. Adenine salvage rates increased five- to sevenfold by perfusion with approximately 4 mM ribose (212 +/- 17, 192 +/- 9, and 215 +/- 14 nmol. h(-1). g(-1) in resting muscle sections, respectively). These high rates were sustained in recovering muscle, except for a small (approximately 20%) but significant (P < 0.001) decrease in the white gastrocnemius muscle. Ribose supplementation did not affect subsequent muscle force production after 60 min of recovery. These data indicate that adenine salvage rates were essentially unaltered during recovery from intense contractions.

ATP from glycolysis is required for normal sodium homeostasis in resting fast-twitch rodent skeletal muscle

Myocellular sodium homeostasis is commonly disrupted during critical illness for unknown reasons. Recent data suggest that changes in intracellular sodium content and the amount of ATP provided by glycolysis are closely related. The role of glycolysis and oxidative phosphorylation in providing fuel to the Na(+)-K(+) pump was investigated in resting rat extensor digitorum longus muscles incubated at 30 degrees C for 1 h. Oxidative inhibition with carbonyl cyanide m-chlorophenylhydrazone, known as CCCP (0.2 microM), or by hypooxygenation did not alter myocellular sodium or potassium content ([Na(+)](i), [K(+)](i), respectively), whereas treatment with iodoacetic acid (0.3 mM), which effectively blocked glycolysis, dramatically increased [Na(+)](i) and the [Na(+)](i)/[K(+)](i) ratio. Experiments using ouabain and measurements of myocellular high-energy phosphates indicate that Na(+)-K(+)-ATPase activity is only impaired when glycolysis is inhibited. The data suggest that normal glycolysis is required to regulate intracellular sodium in fast-twitch skeletal muscles, because it is the predominant source of the fuel for the Na(+)-K(+)-ATPase.

Adrenergic blockade reduces skeletal muscle glycolysis and Na(+), K(+)-ATPase activity during hemorrhage

BACKGROUND

Recent evidence suggests that hyperlactatemia in shock may reflect accelerated aerobic glycolysis linked to activity of the Na(+), K(+)-ATPase rather than hypoxia. Epinephrine stimulates glycolysis in resting muscle largely by stimulating Na(+), K(+)-ATPase activity. This study evaluates the effects of hemorrhagic shock, with and without combined alpha- and beta-adrenergic receptor blockade, on lactate production, glycogenolysis, Na(+)-K(+) pump activity, and high-energy phosphates in rat skeletal muscle.

METHODS

Male Sprague-Dawley rats in four treatment groups were studied: unhemorrhaged control not receiving blockers (CN), controls receiving blockers (CB), shocked animals not receiving blockers (SN), and shocked rats receiving blockers (SB). Shocked rats (SN and SB) were bled to a MAP of 40 mm Hg, maintained for 60 min. Blocker groups (CB and SB) received propranolol and phenoxybenzamine. Arterial blood was drawn for plasma lactate, epinephrine, norepinephrine, and gas analysis. Lactate, glycogen, glucose 6-phosphate, ATP, phosphocreatine, and intracellular Na(+) and K(+) were determined in extensor digitorum longus and soleus muscles. For comparison, muscles were exposed to epinephrine and/or ouabain in vitro.

RESULTS

With the exception of P(a)CO(2), HCO(3), and base excess in the SN group, no significant differences in arterial blood gas parameters were noted. Adrenergic blockade significantly reduced plasma lactate concentration. In shocked rats, adrenergic blockade significantly reduced muscle lactate and glucose 6-phosphate accumulation. Intracellular Na(+):K(+) ratio was decreased in SN rats, implying increased Na(+)-K(+) pump activity. Adrenergic blockade raised the intracellular Na(+):K(+) ratio in shocked animals, implying decreased pump activity. Epinephrine exposure in vitro stimulated muscle lactate production, raised glucose 6-phosphate content, and significantly reduced soleus phosphocreatine stores.

CONCLUSIONS

Neither hypoxia nor defective oxidative metabolism appeared responsible for increased glycolysis during hemorrhagic shock. Adrenergic blockade concurrently reduced plasma lactate, muscle levels of lactate and glucose 6-phosphate, and muscle Na(+)-K(+) pump activity during shock. Rapid skeletal muscle aerobic glycolysis in response to increased plasma epinephrine levels may be an important contributor to increased glycolysis in muscle and increased plasma lactate during hemorrhagic shock.

Purine salvage to adenine nucleotides in different skeletal muscle fiber types

Rates of purine salvage of adenine and hypoxanthine into the adenine nucleotide (AdN) pool of the different skeletal muscle phenotype sections of the rat were measured using an isolated perfused hindlimb preparation. Tissue adenine and hypoxanthine concentrations and specific activities were controlled over a broad range of purine concentrations, ranging from 3 to 100 times normal, by employing an isolated rat hindlimb preparation perfused at a high flow rate. Incorporation of [(3)H]adenine or [(3)H]hypoxanthine into the AdN pool was not meaningfully influenced by tissue purine concentration over the range evaluated (approximately 0.10-1.6 micromol/g). Purine salvage rates were greater (P < 0.05) for adenine than for hypoxanthine (35-55 and 20-30 nmol x h(-1) x g(-1), respectively) and moderately different (P < 0.05) among fiber types. The low-oxidative fast-twitch white muscle section exhibited relatively low rates of purine salvage that were approximately 65% of rates in the high-oxidative fast-twitch red section of the gastrocnemius. The soleus muscle, characterized by slow-twitch red fibers, exhibited a high rate of adenine salvage but a low rate of hypoxanthine salvage. Addition of ribose to the perfusion medium increased salvage of adenine (up to 3- to 6-fold, P < 0.001) and hypoxanthine (up to 6- to 8-fold, P < 0.001), depending on fiber type, over a range of concentrations up to 10 mM. This is consistent with tissue 5-phosphoribosyl-1-pyrophosphate being rate limiting for purine salvage. Purine salvage is favored over de novo synthesis, inasmuch as delivery of adenine to the muscle decreased (P < 0.005) de novo synthesis of AdN. Providing ribose did not alter this preference of purine salvage pathway over de novo synthesis of AdN. In the absence of ribose supplementation, purine salvage rates are relatively low, especially compared with the AdN pool size in skeletal muscle.

Large energetic adaptations of elderly muscle to resistance and endurance training

This study determined the cellular energetic and structural adaptations of elderly muscle to exercise training. Forty male and female subjects (69.2 +/- 0.6 yr) were assigned to a control group or 6 mo of endurance (ET) or resistance training (RT). We used magnetic resonance spectroscopy and imaging to characterize energetic properties and size of the quadriceps femoris muscle. The phosphocreatine and pH changes during exercise yielded the muscle oxidative properties, glycolytic ATP synthesis, and contractile ATP demand. Muscle biopsies taken from the same site as the magnetic resonance measurements were used to determine myosin heavy chain isoforms, metabolite concentrations, and mitochondrial volume densities. The ET group showed changes in all energetic pathways: oxidative capacity (+31%), contractile ATP demand (-21%), and glycolytic ATP supply (-56%). The RT group had a large increase in oxidative capacity (57%). Only the RT group exhibited change in structural properties: a rise in mitochondrial volume density (31%) and muscle size (10%). These results demonstrate large energetic, but smaller structural, adaptations by elderly muscle with exercise training. The rise in oxidative properties with both ET and RT suggests that the aerobic pathway is particularly sensitive to exercise training in elderly muscle. Thus elderly muscle remains adaptable to chronic exercise, with large energetic changes accompanying both ET and RT.

Oxidative capacity and ageing in human muscle

This study determined the decline in oxidative capacity per volume of human vastus lateralis muscle between nine adult (mean age 38.8 years) and 40 elderly (mean age 68.8 years) human subjects (age range 25-80 years). We based our oxidative capacity estimates on the kinetics of changes in creatine phosphate content ([PCr]) during recovery from exercise as measured by (31)P magnetic resonance (MR) spectroscopy. A matched muscle biopsy sample permitted determination of mitochondrial volume density and the contribution of the loss of mitochondrial content to the decline in oxidative capacity with age. The maximal oxidative phosphorylation rate or oxidative capacity was estimated from the PCr recovery rate constant (k(PCr)) and the [PCr] in accordance with a simple electrical circuit model of mitochondrial respiratory control. Oxidative capacity was 50 % lower in the elderly vs. the adult group (0.61 +/- 0.04 vs. 1.16 +/- 0.147 mM ATP s(-1)). Mitochondrial volume density was significantly lower in elderly compared with adult muscle (2.9 +/- 0.15 vs. 3.6 +/- 0.11 %). In addition, the oxidative capacity per mitochondrial volume (0.22 +/- 0.042 vs. 0.32 +/- 0.015 mM ATP (s %)(-1)) was reduced in elderly vs. adult subjects. This study showed that elderly subjects had nearly 50 % lower oxidative capacity per volume of muscle than adult subjects. The cellular basis of this drop was a reduction in mitochondrial content, as well as a lower oxidative capacity of the mitochondria with age.

Sublytic complement attack increases intracellular sodium in rat skeletal muscle

BACKGROUND

Although excessive complement activation and deranged sodium homeostasis in skeletal muscle are characteristic in sepsis, their relationship has not been examined. This study was designed to determine if sublytic complement activation can directly mediate changes in myocellular sodium content.

MATERIALS AND METHODS

Fast-twitch extensor digitorum longus muscles were freshly isolated from infant rats. Unsensitized muscles were incubated at 30 degrees C for 60 min in the media containing 10% human or rat serum under conditions of no complement activation, activation by zymosan, inactivation by heat, C7 or C9 deficiency, selective inhibition of complement pathway, and inhibition of Na(+)-K(+) ATPase by ouabain. Intracellular sodium ([Na(+)](i)) and potassium ([K(+)](i)) contents of the muscles, myocellular ATP, and LDH release from the muscles were then determined.

RESULTS

Normal human serum significantly increased [Na(+)](i) and the [Na(+)](i)/[K(+)](i) ratio in the muscles as well as zymosan-activated serum. Heat inactivation, C7 deficiency, and inhibition of the alternative pathway completely abolished the cationic changes. Average LDH release was identical in all groups and less than 6%. Complement activation did not impair ouabain-sensitive Na(+)-K(+) ATPase activity in the muscles or alter myocellular ATP. Thus, the observed alterations are not likely due to dysfunction of Na(+)-K(+) pump or depletion of myocellular energy. Instead, alterations in [Na(+)](i) were dependent upon the amount of C9 added to C9-deficient serum, which suggests that the alterations are likely dependent on transmembrane pores created by membrane attack complexes (MAC).

CONCLUSIONS

Sublytic amounts of MAC formed as a result of complement activation can directly alter [Na(+)](i) in ex vivo skeletal muscle.

Noninvasive measurement of gene expression in skeletal muscle

We have developed a noninvasive detection method for expression of viral-mediated gene transfer. A recombinant adenovirus was constructed by using the gene for arginine kinase (AK), which is the invertebrate correlate to the vertebrate ATP-buffering enzyme, creatine kinase. Gene expression was noninvasively monitored using (31)P-magnetic resonance spectroscopy ((31)P-MRS). The product of the AK enzyme, phosphoarginine (PArg), served as an MRS-visible reporter of AK expression. The recombinant adenovirus coding for arginine kinase (rAdCMVAK) was injected into the right hindlimbs of neonatal mice. Two weeks after injection of rAdCMVAK, a unique (31)P-MRS resonance was observed. It was observable in all rAdCMVAK injected hindlimbs and was not present in the contralateral control or the vehicle injected limb. PArg and phosphocreatine (PCr) concentrations were calculated to be 11.6 +/- 0.90 and 13.6 +/- 1.1 mM respectively in rAdCMVAK injected limbs. AK activity was demonstrated in vivo by monitoring the decreases in PArg and ATP resonances during prolonged ischemia. After 1 h of ischemia intracellular pH was 6.73 +/- 0.06, PCr/ATP was decreased by 77 +/- 8%, whereas PArg/ATP was decreased by 50 +/- 15% of basal levels. PArg and PCr returned to basal levels within 5 min of the restoration of blood flow. AK activity persisted for at least 8 mo after injection, indicating that adenoviral-mediated gene transfer can produce stable expression for long periods of time. Therefore, the cDNA encoding AK provides a useful reporter gene that allows noninvasive and repeated monitoring of gene expression after viral mediated gene transfer to muscle.

Anisotropic orientation of lactate in skeletal muscle observed by dipolar coupling in (1)H NMR spectroscopy

Double quantum (DQ), J-resolved (1)H NMR spectra from rat and bovine skeletal muscle showed a splitting frequency ( approximately 24 Hz) for the lactate methyl protons that varied with the orientation of the muscle fibers relative to the magnetic field. In contrast, spectra of lactate in solution consist of a J-coupled methyl doublet and a J-coupled methine quartet (J(HH) = 7 Hz) with no sensitivity to sample orientation. Spectra acquired in magnetic fields of 4.7, 7, and 11 T showed that the splitting was not due to inhomogeneities in magnetic susceptibility within the muscle, because the magnitude of the splitting did not scale with the strength of B(0) fields. Triple quantum coherence (TQC) spectra revealed two distinct transition frequencies on the methyl resonance. These frequencies resulted from intra-methyl and methine-methyl couplings in this four spin system (A(3)X). Decoupling experiments on the triple quantum coherence showed that the observed frequency splitting was due mainly to the dipolar interactions between the methine and methyl protons of the lactate molecule. Thus, all the proton resonances of the lactate molecules in muscle behave anisotropically in the magnetic field. Adequate design and interpretation of spectroscopic experiments to measure lactate in muscle, and possibly in any cell and organ which contain asymmetric structures, require that both the dipolar coupling described here and the well-known scalar coupling be taken into account.

Stimulation of both aerobic glycolysis and Na(+)-K(+)-ATPase activity in skeletal muscle by epinephrine or amylin

Epinephrine and amylin stimulate glycogenolysis, glycolysis, and Na(+)-K(+)-ATPase activity in skeletal muscle. However, it is not known whether these hormones stimulate glycolytic ATP production that is specifically coupled to ATP consumption by the Na(+)-K(+) pump. These studies correlated glycolysis with Na(+)-K(+)-ATPase activity in resting rat extensor digitorum longus and soleus muscles incubated at 30 degrees C in well-oxygenated medium. Lactate production rose three- to fourfold, and the intracellular Na(+)-to-K(+) ratio (Na(+)/K(+)) fell with increasing concentrations of epinephrine or amylin. In muscles exposed to epinephrine at high concentrations (5 x 10(-7) and 5 x 10(-6) M), ouabain significantly inhibited glycolysis by approximately 70% in either muscle and inhibited glycogenolysis by approximately 40 and approximately 75% in extensor digitorum longus and soleus, respectively. In the absence of ouabain, but not in its presence, statistically significant inverse correlations were observed between lactate production and intracellular Na(+)/K(+) for each hormone. Epinephrine had no significant effect on oxygen consumption or ATP content in either muscle. These results suggest for the first time that stimulation of glycolysis and glycogenolysis in resting skeletal muscle by epinephrine or amylin is closely linked to stimulation of active Na(+)-K(+) transport.

Increased skeletal muscle Na+, K+-ATPase activity as a cause of increased lactate production after hemorrhagic shock

BACKGROUND

Lactate production after hemorrhagic shock may be produced by aerobic glycolysis, which has been linked to activity of the Na+/K+ pump in smooth muscle and other tissues. We tested whether increased muscle Na+/K+ pump activity after shock was linked to increased lactate production.

METHODS

Male Sprague-Dawley rats were subjected to 1 or 2 hours of hemorrhagic shock and then resuscitated with shed blood and normal saline. After 24 hours, pairs of extensor digitorum longus muscles were preincubated for 30 minutes in Krebs buffer (95:5, O2:CO2) with 10 mmol/L glucose. One muscle served as a control and was incubated in buffer alone; the other was incubated in buffer with 1 mmol/L ouabain, an inhibitor of the Na+, K+-ATPase. Lactate, ADP, ATP, glycogen, and creatinine-phosphate were determined.

RESULTS

Under these well-oxygenated conditions, muscles from shocked rats produced about twice as much lactate as sham muscles. Inhibition of the Na+/K+ pump by ouabain significantly reduced lactate production.

CONCLUSIONS

Hypoxia is unlikely to account for increased muscle lactate production after resuscitated hemorrhagic shock, because high lactate production persists under well-oxygenated incubation conditions. Inhibition of shock-induced lactate production by ouabain indicates energetic coupling of glycolysis to the Na+, K+-ATPase.

Responses of mouse fast and slow skeletal muscle to streptozotocin diabetes: myosin isoenzymes and phosphorous metabolites

A condition similar to insulin-dependent diabetes mellitus (IDDM) was induced in male CD-1 mice by injection of streptozotocin (STZ). Five weeks after treatment, the fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (SOL) muscles were isolated for analysis. Phosphorous metabolites were quantified by 31P-NMR and HPLC, native myosin was characterized electrophoretically, and activities of metabolic enzymes were measured spectrophotometrically. Relative to control animals, STZ-diabetes resulted in a significant 32% decrease in the FM1 isoform of myosin in EDL and a 24% decrease in IM myosin of SOL. Mass-specific activities of phosphofructokinase, citrate synthase, and cytochrome oxidase were significantly lower in SOL from STZ-diabetic mice than in controls by 23, 18, and 36%, respectively. Intracellular ATP was significantly lower in SOL from STZ-diabetic mice than in controls (3.44 +/- 0.20 mumol g-1 wet weight vs. 4.61 +/- 0.20 mumol g-1, respectively), as was creatine phosphate (11.98 +/- 0.80 mumol g-1 wet weight vs. 14.22 +/- 0.44 mumol g-1). In contrast to results from SOL, there were no significant changes in phosphorus metabolites or enzyme activity in EDL. These results show that the effects of IDDM on levels of phosphorus containing metabolites and maximal activities of key regulatory enzymes in muscle are markedly fiber-type specific. It is suggested that the muscle type-specific effects of STZ-diabetes may be a consequence of differential accumulation of intracellular fatty acids.

Creatine kinase equilibration follows solution thermodynamics in skeletal muscle. 31P NMR studies using creatine analogs

The hypothesis tested was whether creatine kinase (CK) equilibrates with its substrates and products in the cytosol as if in solution. We used the creatine analogs cyclocreatine (cCr) or beta-guanidopropionate (beta GPA) to test if mass action ratios (gamma) for CK in muscle could be predicted from combined equilibrium constants (Kcomb) measured in solutions mimicking the intracellular environment. Mice were fed cCr or beta GPA and their muscles assayed for substrates and products of the CK reaction by 31P NMR spectroscopy and high performance liquid chromatography. After three weeks of feeding, gamma was indistinguishable from Kcomb in cCr-treated muscles demonstrating both PCr/Cr and phospho-analog/analog must have equilibrated with a constant and uniform cellular ATP/ADP ratio. In beta GPA-treated muscles, gamma was smaller than Kcomb due to a higher content of muscle beta GPA. Feeding beta GPA for 9-12 weeks resulted in a closer agreement between Kcomb and gamma, suggesting ATP/ADP ratios are not uniform within the muscle perhaps due to transient metabolic stress in some cells. From this analysis it follows that calculation of free ADP from the CK equilibrium for a heterogeneous population of cells with respect to total Cr and ATP content is correct only if chemical potentials of these cells are uniform.

Effect of physiological ADP concentrations on contraction of single skinned fibers from rabbit fast and slow muscles

To directly assess the possible role of ADP in muscle fatigue, we have studied the effect of physiological MgADP levels on maximum Ca(2+)-activated isometric force and unloaded shortening velocity (Vus) of single skinned fiber segments from rabbit fast-twitch (psoas) and slow-twitch (soleus) muscles. MgADP concentration was changed in a controlled and well-buffered manner by varying creatine (Cr) in solutions, which also contained MgATP, phosphocreatine (PCr), and creatine kinase (CK). To quantify ADP as a function of Cr added, we determined the apparent equilibrium constant (K') of CK for the conditions of our experiments (pH 7.1, 3 mM Mg2+, 12 degrees C): K' = (sigma [Cr]. sigma [ATP])/(sigma [PCr]. sigma [ADP]) = 260 +/- 3 (SE). In this manner, ADP was altered essentially as occurs during stimulation in vivo but without the concomitant changes in pH and P(i), which affect force and Vus. As ADP (and Cr) was increased, force and Vus decreased in both fiber types; at the highest ADP level used, 200 microM, normalized force was 96.6 +/- 1.7% for psoas (n = 6) and 93.7 +/- 2.8% for soleus (n = 6), and Vus was 80.4 +/- 2.4% for psoas and 91.3 +/- 7.7% for soleus. Diffusion-reaction calculations indicated that radial gradients of metabolite concentrations within fibers could not explain the small effects of ADP on fiber mechanics, and experiments verified that metabolite levels were well buffered within fibers by the CK reaction. Exogenous CK was added to bathing solutions at 290 U/ml, threefold above that necessary to maintain Vus independent of CK concentration; in the absence of PCr and exogenous CK, at least a fourfold increased MgATP was necessary to maintain Vus at the control level. Adenylate kinase activity was not detectable; thus myofibrillar adenosine-triphosphatase and exogenous CK activities were the major determinants of nucleotide levels within activated cells. Cr alone (in absence of PCr and exogenous CK) also decreased force and Vus, presumably by a nonspecific mechanism. Over the physiological range, altered ADP had little or no effect on force or Vus in well-buffered conditions. It is therefore likely that other factors decrease force and Vus during muscular fatigue.

Contractile economy and aerobic recovery metabolism in skeletal muscle adapted to creatine depletion

Mice were treated for 7-12 wk with the creatine analogue beta-guanidinopropionic acid (beta-GPA). Treatment reduced total creatine to approximately 5% of control values in soleus (SOL) and extensor digitorum longus (EDL) muscles. In both muscles from treated mice, phosphorylated beta-GPA accumulated and resting [ATP] decreased by approximately 50%. Relative to controls, cytochrome oxidase and citrate synthase activities increased significantly in EDL from treated mice, but not in SOL; creatine kinase activity decreased significantly in SOL, but not in EDL. Measurements of poststimulation energy metabolism show that the energy cost to maintain tension in SOL and EDL from treated mice was approximately 50% of that in control muscle. Relative to controls, first-order rate constants of poststimulation O2 demand were 2- and 3.6-fold greater in SOL and EDL, respectively, from treated mice. Increased economy of SOL and EDL from treated mice is consistent with previously reported changes in myosin isoenzymes. Increases in rate constants of O2 utilization in creatine-depleted muscle are inconsistent with the hypothesis that cytoplasmic or mitochondrial creatine kinase is rate limiting for cellular respiration.