Two classes of mammalian skeletal muscle fibers distinguished by metabolite content

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.

MitoRACE: evaluating mitochondrial function in vivo and in single cells with subcellular resolution using multiphoton NADH autofluorescence

KEY POINTS

We developed a novel metabolic imaging approach that provides direct measures of the rate of mitochondrial energy conversion with single-cell and subcellular resolution by evaluating NADH autofluorescence kinetics during the mitochondrial redox after cyanide experiment (mitoRACE). Measures of mitochondrial NADH flux by mitoRACE are sensitive to physiological and pharmacological perturbations in vivo. Metabolic imaging with mitoRACE provides a highly adaptable platform for evaluating mitochondrial function in vivo and in single cells with potential for broad applications in the study of energy metabolism.

ABSTRACT

Mitochondria play a critical role in numerous cell types and diseases, and structure and function of mitochondria can vary greatly among cells or within different regions of the same cell. However, there are currently limited methodologies that provide direct assessments of mitochondrial function in vivo, and contemporary measures of mitochondrial energy conversion lack the spatial resolution necessary to address cellular and subcellular heterogeneity. Here, we describe a novel metabolic imaging approach that provides direct measures of mitochondrial energy conversion with single-cell and subcellular resolution by evaluating NADH autofluorescence kinetics during the mitochondrial redox after cyanide experiment (mitoRACE). MitoRACE measures the rate of NADH flux through the steady-state mitochondrial NADH pool by rapidly inhibiting mitochondrial energetic flux, resulting in an immediate, linear increase in NADH fluorescence proportional to the steady-state NADH flux rate, thereby providing a direct measure of mitochondrial NADH flux. The experiments presented here demonstrate the sensitivity of this technique to detect physiological and pharmacological changes in mitochondrial flux within tissues of living animals and reveal the unique capability of this technique to evaluate mitochondrial function with single-cell and subcellular resolution in different cell types in vivo and in cell culture. Furthermore, we highlight the potential applications of mitoRACE by showing that within single neurons, mitochondria in neurites have higher energetic flux rates than mitochondria in the cell body. Metabolic imaging with mitoRACE provides a highly adaptable platform for evaluating mitochondrial function in vivo and in single cells, with potential for broad applications in the study of energy metabolism.

Skeletal muscle performance in metabolic disease: Microvascular or mitochondrial limitation or both?

One of the clearly established health outcomes associated with chronic metabolic diseases (eg, type II diabetes mellitus) is that the ability of skeletal muscle to maintain contractile performance during periods of elevated metabolic demand is compromised as compared to the fatigue-resistance of muscle under normal, healthy conditions. While there has been extensive effort dedicated to determining the major factors that contribute to the compromised performance of skeletal muscle with chronic metabolic disease, the extent to which this poor outcome reflects a dysfunctional state of the microcirculation, where the delivery and distribution of metabolic substrates can be impaired, versus derangements to normal metabolic processes and mitochondrial function, versus a combination of the two, represents an area of considerable unknown. The purpose of this manuscript is to present some of the current concepts for dysfunction to both the microcirculation of skeletal muscle as well as to mitochondrial metabolism under these conditions, such that these diverse issues can be merged into an integrated framework for future investigation. Based on an interpretation of the current literature, it may be hypothesized that the primary site of dysfunction with earlier stages of metabolic disease may lie at the level of the vasculature, rather than at the level of the mitochondria.

Energetics of contraction

Muscles convert energy from ATP into useful work, which can be used to move limbs and to transport ions across membranes. The energy not converted into work appears as heat. At the start of contraction heat is also produced when Ca(2+) binds to troponin-C and to parvalbumin. Muscles use ATP throughout an isometric contraction at a rate that depends on duration of stimulation, muscle type, temperature and muscle length. Between 30% and 40% of the ATP used during isometric contraction fuels the pumping Ca(2+) and Na(+) out of the myoplasm. When shortening, muscles produce less force than in an isometric contraction but use ATP at a higher rate and when lengthening force output is higher than the isometric force but rate of ATP splitting is lower. Efficiency quantifies the fraction of the energy provided by ATP that is converted into external work. Each ATP molecule provides 100 zJ of energy that can potentially be converted into work. The mechanics of the myosin cross-bridge are such that at most 50 zJ of work can be done in one ATP consuming cycle; that is, the maximum efficiency of a cross-bridge is ∼50%. Cross-bridges in tortoise muscle approach this limit, producing over 90% of the possible work per cycle. Other muscles are less efficient but contract more rapidly and produce more power.

Protein composition and function of red and white skeletal muscle mitochondria

Red and white muscles are faced with very different energetic demands. However, it is unclear whether relative mitochondrial protein expression is different between muscle types. Mitochondria from red and white porcine skeletal muscle were isolated with a Percoll gradient. Differences in protein composition were determined using blue native (BN)-PAGE, two-dimensional differential in gel electrophoresis (2D DIGE), optical spectroscopy, and isobaric tag for relative and absolute quantitation (iTRAQ). Complex IV and V activities were compared using BN-PAGE in-gel activity assays, and maximal mitochondrial respiration rates were assessed using pyruvate (P) + malate (M), glutamate (G) + M, and palmitoyl-carnitine (PC) + M. Without the Percoll step, major cytosolic protein contamination was noted for white mitochondria. Upon removal of contamination, very few protein differences were observed between red and white mitochondria. BN-PAGE showed no differences in the subunit composition of Complexes I-V or the activities of Complexes IV and V. iTRAQ analysis detected 358 mitochondrial proteins, 69 statistically different. Physiological significance may be lower: at a 25% difference, 48 proteins were detected; at 50%, 14 proteins were detected; and 3 proteins were detected at a 100%. Thus any changes could be argued to be physiologically modest. One area of difference was fat metabolism where four β-oxidation enzymes were ∼25% higher in red mitochondria. This was correlated with a 40% higher rate of PC+M oxidation in red mitochondria compared with white mitochondria with no differences in P+M and G+M oxidation. These data suggest that metabolic demand differences between red and white muscle fibers are primarily matched by the number of mitochondria and not by significant alterations in the mitochondria themselves.

Effects of UCP3 genotype, temperature and muscle type on energy turnover of resting mouse skeletal muscle

Uncoupling protein 3 (UCP3) is a mitochondrial transporter protein which, when over-expressed in mice, is associated with increased metabolic rate, increased feeding and low body weight. This phenotype probably reflects the increased levels of UCP3 partially uncoupling mitochondrial respiration from cellular ATP demands. Consistent with that, mitochondria isolated from muscles of mice that over-express UCP3 are less tightly coupled than those from wild-type mice but the degree of uncoupling is not modulated by likely physiological regulatory factors. To determine whether this also applies to intact muscle fibres, we tested the hypothesis that UCP3 constitutively (i.e. in an unregulated fashion) uncouples mitochondria in muscles from mice that over-expressed human UCP3 (OE mice). The rate of heat production of resting muscles was measured in vitro using bundles of fibres from soleus and extensor digitorum longus muscles of OE, wild-type (WT) and UCP3 knock-out mice. At 20 degrees C, the only significant effect of genotype was that the rate of heat production of OE soleus (3.04+/-0.16 mW g(-1)) was greater than for WT soleus (2.31+/-0.05 mW g(-1)). At physiological temperature (35 degrees C), the rate of heat production was independent of genotype and equal to the expected in vivo rate for skeletal muscles of WT mice. We conclude that at 35 degrees C, the transgenic UCP3 was not constitutively active, but at 20 degrees C in slow-twitch muscle, it was partially activated by unknown factors. The physiological factor(s) that activate mitochondrial uncoupling by UCP3 in vivo was either not present or inactive in resting isolated muscles.

Impaired resting muscle energetics studied by (31)P-NMR in diet-induced obese rats

OBJECTIVE

Mitochondrial activity is altered in skeletal muscle of obese, insulin-resistant or type 2 diabetic patients. We hypothesized that this situation was associated with profound adaptations in resting muscle energetics. For that purpose, we used in vivo (31)P-nuclear magnetic resonance ((31)P-NMR) in male sedentary Wistar rats fed with obesogenic diets known to induce alterations in muscle mitochondrial activity.

METHODS AND PROCEDURES

Two experimental diets (high sucrose and high fat) were provided for 6 weeks at two levels of energy (standard, N and high, H) and compared to control diet. The rates of the adenosine triphosphate (ATP) exchange between phosphocreatine (PCr) and gamma-ATP (k(a)) and beta-adenosine diphosphate (beta-ADP) to beta-ATP (k(b)) were evaluated using (31)P-NMR in resting gastrocnemius muscle. Muscle contents in phosphorylated compounds as well as creatine, were assessed using (31)P-NMR and biochemical assays, respectively.

RESULTS

ATP content increased by 6.7-8.5% in standard-energy high-sucrose (NSU), high-energy high-fat (HF) and high-energy high-sucrose (HSU) groups compared to control (P < 0.05), whereas PCr content decreased by 4.2-6.4% (P < 0.01). Consequently, PCr to ATP ratio decreased in NSU, HF, and HSU groups, compared to control (P < 0.01). Furthermore in high-energy groups (HF and HSU) compared to control, creatine contents were decreased by 14-19% (P < 0.001), whereas k(a) and k(b) fluxes were increased by 89-133% (P < 0.001) and 243-277% (P < 0.01), respectively.

DISCUSSION

Our in vivo data showed adaptations of resting skeletal muscle energetics in response to high-energy diets. Increased activity of enzymes catalyzing ATP production may reflect a compensatory mechanism to face impaired mitochondrial ATP synthesis in order to preserve intracellular energy homeostasis.

S6 kinase deletion suppresses muscle growth adaptations to nutrient availability by activating AMP kinase

S6 kinase (S6K) deletion in metazoans causes small cell size, insulin hypersensitivity, and metabolic adaptations; however, the underlying molecular mechanisms are unclear. Here we show that S6K-deficient skeletal muscle cells have increased AMP and inorganic phosphate levels relative to ATP and phosphocreatine, causing AMP-activated protein kinase (AMPK) upregulation. Energy stress and muscle cell atrophy are specifically triggered by the S6K1 deletion, independent of S6K2 activity. Two known AMPK-dependent functions, mitochondrial biogenesis and fatty acid beta-oxidation, are upregulated in S6K-deficient muscle cells, leading to a sharp depletion of lipid content, while glycogen stores are spared. Strikingly, AMPK inhibition in S6K-deficient cells restores cell growth and sensitivity to nutrient signals. These data indicate that S6K1 controls the energy state of the cell and the AMPK-dependent metabolic program, providing a mechanism for cell mass accumulation under high-calorie diet.

The efficiency of muscle contraction

When a muscle contracts and shortens against a load, it performs work. The performance of work is fuelled by the expenditure of metabolic energy, more properly quantified as enthalpy (i.e., heat plus work). The ratio of work performed to enthalpy produced provides one measure of efficiency. However, if the primary interest is in the efficiency of the actomyosin cross-bridges, then the metabolic overheads associated with basal metabolism and excitation-contraction coupling, together with those of subsequent metabolic recovery process, must be subtracted from the total heat and work observed. By comparing the cross-bridge work component of the remainder to the Gibbs free energy of hydrolysis of ATP, a measure of thermodynamic efficiency is achieved. We describe and quantify this partitioning process, providing estimates of the efficiencies of selected steps, while discussing the errors that can arise in the process of quantification. The dependence of efficiency on animal species, fibre-type, temperature, and contractile velocity is considered. The effect of contractile velocity on energetics is further examined using a two-state, Huxley-style, mathematical model of cross-bridge cycling that incorporates filament compliance. Simulations suggest only a modest effect of filament compliance on peak efficiency, but progressively larger gains (vis-à-vis the rigid filament case) as contractile velocity approaches Vmax. This effect is attributed primarily to a reduction in the component of energy loss arising from detachment of cross-bridge heads at non-zero strain.

Slow skeletal muscles of the mouse have greater initial efficiency than fast muscles but the same net efficiency

The aim of this study was to determine whether the net efficiency of mammalian muscles depends on muscle fibre type. Experiments were performed in vitro (35 degrees C) using bundles of muscle fibres from the slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles of the mouse. The contraction protocol consisted of 10 brief contractions, with a cyclic length change in each contraction cycle. Work output and heat production were measured and enthalpy output (work + heat) was used as the index of energy expenditure. Initial efficiency was defined as the ratio of work output to enthalpy output during the first 1 s of activity. Net efficiency was defined as the ratio of the total work produced in all the contractions to the total, suprabasal enthalpy produced in response to the contraction series, i.e. net efficiency incorporates both initial and recovery metabolism. Initial efficiency was greater in soleus (30 +/- 1%; n=6) than EDL (23 +/- 1%; n=6) but there was no difference in net efficiency between the two muscles (12.6 +/- 0.7% for soleus and 11.7 +/- 0.5% for EDL). Therefore, more recovery heat was produced per unit of initial energy expenditure in soleus than EDL. The calculated efficiency of oxidative phosphorylation was lower in soleus than EDL. The difference in recovery metabolism between soleus and EDL is unlikely to be due to effects of changes in intracellular pH on the enthalpy change associated with PCr hydrolysis. It is suggested that the functionally important specialization of slow-twitch muscle is its low rate of energy use rather than high efficiency.

Dipolar coupling and ordering effects observed in magnetic resonance spectra of skeletal muscle

Skeletal muscle is a biological structure with a high degree of organization at different spatial levels. This order influences magnetic resonance (MR) in vivo-in particular 1H-spectra-by a series of effects that have very distinct physical sources and biomedical applications: (a) bulk fat (extramyocellular lipids, EMCL) along fasciae forms macroscopic plates, changing the susceptibility within these structures compared to the spherical droplets that contain intra-myocellular lipids (IMCL); this effect leads to a separation of the signals from EMCL and IMCL; (b) dipolar coupling effects due to anisotropic motional averaging have been shown for 1H-resonances of creatine, taurine, and lactate; (c) aromatic protons of carnosine show orientation-dependent effects that can be explained by dipolar coupling, chemical shift anisotropy or by relaxation anisotropy; (d) limited rotational freedom and/or compartmentation may explain differences of 1H-MR-visibility of the creatine/phosphocreatine resonances; (e) lactate 1H-MR resonances are reported to reveal information on tissue compartmentation; (f) transverse relaxation of water and metabolites show multiple components, indicative of intra-, extracellular and/or macromolecular-bound pools, and in addition dipolar or J-coupling lead to a modulation of the signal decay, hindering straightforward interpretation; (g) diffusion weighted 31P-MRS has shown restricted diffusion of phosphocreatine; (h) magnetization transfer (MT) indicates that there is a motionally restricted proton pool in spin-exchange with free creatine; reduced availability or restricted motion of creatine is particularly important for an estimation of ADP from 31P-MR spectra, and in addition MT effects may alter the signal intensity of creatine 1H-resonances following water-suppression pulses; (i) transcytolemmal water-exchange can be studied in 1H-MRS by contrast-agents applied to the extracellular space; (k) transport of glucose across the cell membrane has been studied in diabetes patients using a combination of 13C- and 31P-MRS; and l residual quadrupolar interaction in 23Na MR spectra from human skeletal muscle suggest that sodium ions are bound to ordered muscular structures.

Contributions of dynamic phosphorus-31 magnetic resonance spectroscopy to the analysis of muscle fiber distribution

RATIONALE AND OBJECTIVES

In high-performance athletes, conclusions regarding the muscle fiber distribution were to be drawn from dynamic 31phosphorus magnetic resonance spectroscopy (31P MRS).

METHODS

Eleven volleyball players (V), eight bodybuilders (B), and 22 nonathletic volunteers (N) were examined by dynamic 31P MRS. During rest, exhaustive exercise, and recovery, respectively, up to 60 consecutive phosphorus spectra of the quadriceps muscle were acquired by "time series" in 36 s each. Two main spectroscopic approaches to the spectroscopic analysis of muscle fiber distribution were applied: evaluation of the ratio Pi/PCr at rest and the computer-assisted analysis of the Pi-peak at its exercise-induced line width maximum.

RESULTS

At rest, the bodybuilders showed a significant lower Pi/PCr (0.07 +/- 0.03), in comparison with the volleyball players (0.11 +/- 0.03) and the nonathletic volunteers (0.11 +/- 0.02). The computer-assisted analysis of the Pi-peak at its line width maximum revealed a significantly lower pH of both of the subpeaks in the bodybuilders [6.30 versus 6.37 (V) and 6.38 (N); 6.89 versus 6.92 (V, N)], whereas the volleyball players provided the largest proportion of oxidative muscle fibers (68%), compared to bodybuilders (64%) and nonathletic volunteers (59%). A correlation between the ratio Pi/PCr and the area of the subpeak with the high pH (representing oxidative fibers) could not be demonstrated.

CONCLUSIONS

Spectroscopic results during rest and exercise may be influenced by the muscle fiber distribution of the respective volunteer. The applied spectroscopic approaches to the analysis of muscle fiber composition are not compatible with each other; depending on the applied method, the classification of a muscle fiber as type I or type II fiber may change. The influence of physiologic factors like muscle fiber distribution on spectroscopic results has to be considered in the interpretation of pathological conditions.

A weakly coupled version of the Huxley crossbridge model can simulate energetics of amphibian and mammalian skeletal muscle

This study aimed to establish whether quantitatively accurate predictions of the rate of crossbridge-dependent energy output from shortening muscle could be made on the basis of a 2-state model of crossbridge kinetics incorporating weak coupling between mechanical cycles and ATP hydrolysis. The model was based on Huxley's (1957) model but included rapid detachment, without ATP hydrolysis, of crossbridges when their strain energy increased sufficiently that crossbridge free energy exceeded that of the unbound state (Cooke et al., 1994). An expression was derived relating force to steady-state velocity in terms of the model's rate constants. The values of the rate constants that both provided the best fit through force-velocity data and correctly predicted crossbridge-dependent rate of energy output during an isometric contraction were found and used to predict the variation in rate of energy liberation with shortening velocity. The model predictions closely matched the estimated crossbridge energetics of frog sartorius muscle, including the decline in rate of enthalpy output at high shortening velocities. Data from fast- and slow-twitch muscles of the mouse were also simulated. The velocity-dependence of rate of energy liberation from fast-twitch EDL muscle was well described by the model. The model overestimated crossbridge-dependent energy output from slow-twitch soleus at low shortening velocities but provided accurate predictions of energy output at high velocities. In terms of this model, the distinctive energetics of fast and slow muscles cannot be explained exclusively by differences in cross-bridge detachment rate; differences in the relative rates of crossbridge attachment must also be considered to explain the different relations between energy output and shortening velocity.

Phosphorus metabolites in different muscles of the rat leg by 31P image-selected in vivo spectroscopy

The difference in concentration of phosphorylated metabolites in muscles with different fiber composition was studied in vivo by localized 31P nuclear magnetic resonance spectroscopy in the rat hindlimb 120-160 microliters volumes were selected in regions containing the soleus and gastrocnemius muscles. Concentrations of phosphocreatine (PCr), adenosine triphosphate and inorganic phosphate (Pi) were determined and intracellular pH was calculated in the respective muscle groups. The highest level of PCr was found in the gastrocnemius muscle, containing 30.7 mmoles/dm3 tissue compared to 22.3 mmoles/dm3 in the soleus muscle. Pi was significantly lower in gastrocnemius (1.9 mmoles/dm3) than in soleus (3.2 mmoles/dm3). The ATP concentration was 6.7 and 6.4 mmoles/dm3 and pH was determined to 7.11 and 7.09 in the gastrocnemius and soleus muscle, respectively. Our NMR data show that it is possible to measure high-energy phosphates with precision in small localized volumes with the ISIS method using a Helmholtz coil. Earlier biochemical data are confirmed by these in vivo NMR results. Localized in vivo 31P NMR spectroscopy can contribute to the understanding of the underlying mechanisms of several metabolic events in different regions of the tissue. The method can be used for future studies of varying ischemia tolerance, varying degrees of adaptation to exercise with regard to oxidative capacity, and pH compartmentation in muscles with different fiber composition.

Skeletal muscle: a paradigm for testing principles of bioenergetics

Muscular activity converts chemical energy into useful work and metabolism restores muscle to its original state. This essay explores the organization and interactions of the regulatory system(s) which allow this energy balance to occur. The term "energy balance" is used in a biochemical rather than a thermodynamic sense--concerned not with deductions from the physical principles of thermodynamics, but rather with those enzymatic processes which nature evolved and which operate at remarkably fixed stoichiometry. Energy balance is a statement of conservation of energy put into biochemical observables. 31P NMR spectroscopy is one of the most useful techniques for investigating these questions quantitatively under physiological conditions in vivo. The author (1) describes the rules or principles of biochemical energy balance; (2) discusses sample results from human muscle to demonstrate its use in studying this class of questions; (3) presents a simple model of integrated cellular respiration to demonstrate its sufficiency to account for the main observations.

Bioenergetics and muscle cell types

Potential complexities in biochemical and bioenergetic interpretation due fiber type heterogeneity are not significant for human muscle. Paradigms for understanding muscle bioenergetics then can be understood from a set of basic premises of biochemical energy balance 1) ATP provides the energy for all forms of muscle work; 2) chemical energy is stored in cells as phosphocreatine, a biochemical capacitor; 3) the sum of the coupled ATPases sets the demand side of the balance and defines energetic states; and 4) this demand is supplied by aerobic metabolism and the products of the coupled ATPases provide control signals for regulation of energy balance. We speculate that cytoplasmic signals at work in energy balance may also control muscle plasticity.