Compartmentalized ATP synthesis in skeletal muscle triads

Bioengineered Model of Human LGMD2B Skeletal Muscle Reveals Roles of Intracellular Calcium Overload in Contractile and Metabolic Dysfunction in Dysferlinopathy

Dysferlin is a multi-functional protein that regulates membrane resealing, calcium homeostasis, and lipid metabolism in skeletal muscle. Genetic loss of dysferlin results in limb girdle muscular dystrophy 2B/2R (LGMD2B/2R) and other dysferlinopathies - rare untreatable muscle diseases that lead to permanent loss of ambulation in humans. The mild disease severity in dysferlin-deficient mice and diverse genotype-phenotype relationships in LGMD2B patients have prompted the development of new in vitro models for personalized studies of dysferlinopathy. Here the first 3-D tissue-engineered hiPSC-derived skeletal muscle ("myobundle") model of LGMD2B is described that exhibits compromised contractile function, calcium-handling, and membrane repair, and transcriptomic changes indicative of impaired oxidative metabolism and mitochondrial dysfunction. In response to the fatty acid (FA) challenge, LGMD2B myobundles display mitochondrial deficits and intracellular lipid droplet (LD) accumulation. Treatment with the ryanodine receptor (RyR) inhibitor dantrolene or the dissociative glucocorticoid vamorolone restores LGMD2B contractility, improves membrane repair, and reduces LD accumulation. Lastly, it is demonstrated that chemically induced chronic RyR leak in healthy myobundles phenocopies LGMD2B contractile and metabolic deficit, but not the loss of membrane repair capacity. Together, these results implicate intramyocellular Ca2+ leak as a critical driver of dysferlinopathic phenotype and validate the myobundle system as a platform to study LGMD2B pathogenesis.

Exercise and fatigue: integrating the role of K+, Na+ and Cl- in the regulation of sarcolemmal excitability of skeletal muscle

Perturbations in K+ have long been considered a key factor in skeletal muscle fatigue. However, the exercise-induced changes in K+ intra-to-extracellular gradient is by itself insufficiently large to be a major cause for the force decrease during fatigue unless combined to other ion gradient changes such as for Na+. Whilst several studies described K+-induced force depression at high extracellular [K+] ([K+]e), others reported that small increases in [K+]e induced potentiation during submaximal activation frequencies, a finding that has mostly been ignored. There is evidence for decreased Cl- ClC-1 channel activity at muscle activity onset, which may limit K+-induced force depression, and large increases in ClC-1 channel activity during metabolic stress that may enhance K+ induced force depression. The ATP-sensitive K+ channel (KATP channel) is also activated during metabolic stress to lower sarcolemmal excitability. Taking into account all these findings, we propose a revised concept in which K+ has two physiological roles: (1) K+-induced potentiation and (2) K+-induced force depression. During low-moderate intensity muscle contractions, the K+-induced force depression associated with increased [K+]e is prevented by concomitant decreased ClC-1 channel activity, allowing K+-induced potentiation of sub-maximal tetanic contractions to dominate, thereby optimizing muscle performance. When ATP demand exceeds supply, creating metabolic stress, both KATP and ClC-1 channels are activated. KATP channels contribute to force reductions by lowering sarcolemmal generation of action potentials, whilst ClC-1 channel enhances the force-depressing effects of K+, thereby triggering fatigue. The ultimate function of these changes is to preserve the remaining ATP to prevent damaging ATP depletion.

Pennate myofibrils of the rat temporal muscle

BACKGROUND

The force a muscle exerts is partly determined by anatomical parameters, such as its physiological cross-section. The temporal muscle is structurally heterogeneous. To the authors' knowledge, the ultrastructure of this muscle has been poorly specifically studied.

METHODS

Five adult Wistar rats weighting 350-400g were used as temporal muscle donors. Tissues were specifically processed and studied under transmission electron microscope.

RESULTS

On ultrathin cuts, the general ultrastructural pattern of striated muscles was observed. Moreover, pennate sarcomeres were identified, sharing a one-end insertion on the same Z-disc. Bipennate morphologies resulted when two neighbor sarcomeres, attached on different neighbor Z-discs and separated at that end by a triad, converged to the same Z-disc at the opposite ends, thus building a thicker myofibril distinctively flanked by triads. Tripennate morphologies were identified when sarcomeres from three different Z-discs converged to the same Z-disc at the opposite ends.

CONCLUSIONS

These results support recent evidence of sarcomeres branching gathered in mice. Adequate identification of the sites of excitation-contraction coupling should be on both sides of a myofibril, on bidimensional ultrathin cuts, to avoid false positive results due to putative longitudinal folds of myofibrils.

Fibre type- and localisation-specific muscle glycogen utilisation during repeated high-intensity intermittent exercise

Glycogen particles are situated in key areas of the muscle cell in the vicinity of the main energy-consumption sites and may be utilised heterogeneously dependent on the nature of the metabolic demands. The present study aimed to investigate the time course of fibre type-specific utilisation of muscle glycogen in three distinct subcellular fractions (intermyofibrillar, IMF; intramyofibrillar, Intra; and subsarcolemmal, SS) during repeated high-intensity intermittent exercise. Eighteen moderately to well-trained male participants performed three periods of 10 × 45 s cycling at ∼105% watt max (EX1-EX3) coupled with 5 × 6 s maximal sprints at baseline and after each period. Muscle biopsies were sampled at baseline and after EX1 and EX3. A higher glycogen breakdown rate in type 2 compared to type 1 fibres was found during EX1 for the Intra (-72 vs. -45%) and IMF (-59 vs. -35%) glycogen fractions (P < 0.001) but with no differences for SS glycogen (-52 vs. -40%). In contrast, no fibre type differences were observed during EX2-EX3, where the utilisation of Intra and IMF glycogen in type 2 fibres was reduced, resulting in depletion of all three subcellular fractions to very low levels post-exercise within both fibre types. Importantly, large heterogeneity in single-fibre glycogen utilisation was present with an early depletion of especially Intra glycogen in individual type 2 fibres. In conclusion, there is a clear fibre type- and localisation-specific glycogen utilisation during high-intensity intermittent exercise, which varies with time course of exercise and is characterised by exacerbated pool-specific glycogen depletion at the single-fibre level. KEY POINTS: Muscle glycogen is the major fuel during high-intensity exercise and is stored in distinct subcellular areas of the muscle cell in close vicinity to the main energy consumption sites. In the present study quantitative electron microscopy imaging was used to investigate the utilisation pattern of three distinct subcellular muscle glycogen fractions during repeated high-intensity intermittent exercise. It is shown that the utilisation differs dependent on fibre type, subcellular localisation and time course of exercise and with large single-fibre heterogeneity. These findings expand on our understanding of subcellular muscle glycogen metabolism during exercise and may help us explain how reductions in muscle glycogen can attenuate muscle function even at only moderately lowered whole-muscle glycogen concentrations.

The spatial distribution of glycogen and glycogen consumption in muscle cells

David G. Allen looks at new research from the Nielsen lab.

Specific ATPases drive compartmentalized glycogen utilization in rat skeletal muscle

Glycogen is a key energy substrate in excitable tissue, including in skeletal muscle fibers where it also contributes to local energy production. Transmission electron microscopy imaging has revealed the existence of a heterogenic subcellular distribution of three distinct glycogen pools in skeletal muscle, which are thought to reflect the requirements for local energy stores at the subcellular level. Here, we show that the three main energy-consuming ATPases in skeletal muscles (Ca2+, Na+,K+, and myosin ATPases) utilize different local pools of glycogen. These results clearly demonstrate compartmentalized glycogen metabolism and emphasize that spatially distinct pools of glycogen particles act as energy substrate for separated energy requiring processes, suggesting a new model for understanding glycogen metabolism in working muscles, muscle fatigue, and metabolic disorders. These observations suggest that the distinct glycogen pools can regulate the functional state of mammalian muscle cells and have important implications for the understanding of how the balance between ATP utilization and ATP production is regulated at the cellular level in general and in skeletal muscle fibers in particular.

Enzyme Trafficking and Co-Clustering Precede and Accurately Predict Human Breast Cancer Recurrences: An Interdisciplinary Review

Although great effort has been expended to understand cancer's origins, less attention has been given to the primary cause of cancer deaths - cancer recurrences and their sequelae. This interdisciplinary review addresses mechanistic features of aggressive cancer by studying metabolic enzyme patterns within ductal carcinoma in situ (DCIS) of the breast lesions. DCIS lesions from patients who did or did not experience a breast cancer recurrence were compared. Several proteins, including phospho-Ser226-glucose transporter type 1, phosphofructokinase type L and phosphofructokinase/fructose 2,6-bisphosphatase type 4 are found in nucleoli of ductal epithelial cells in samples from patients who will not subsequently recur, but traffic to the cell periphery in samples from patients who will experience a cancer recurrence. Large co-clusters of enzymes near plasmalemmata will enhance product formation because enzyme concentrations in clusters are very high while solvent molecules and solutes diffuse through small channels. These structural changes will accelerate aerobic glycolysis. Agglomerations of pentose phosphate pathway and glutathione synthesis enzymes enhance GSH formation. As aggressive cancer lesions are incomplete at early stages, they may be unrecognizable. We have found that machine learning provides superior analyses of tissue images and may be used to identify biomarker patterns associated with recurrent and non-recurrent patients with high accuracy. This suggests a new prognostic test to predict DCIS patients who are likely to recur and those who are at low risk for recurrence. Mechanistic interpretations provide a deeper understanding of anti-cancer drug action and suggest that aggressive metastatic cancer cells are sensitive to reductive chemotherapy.

A century of exercise physiology: key concepts in regulation of glycogen metabolism in skeletal muscle

Glycogen is a branched, glucose polymer and the storage form of glucose in cells. Glycogen has traditionally been viewed as a key substrate for muscle ATP production during conditions of high energy demand and considered to be limiting for work capacity and force generation under defined conditions. Glycogenolysis is catalyzed by phosphorylase, while glycogenesis is catalyzed by glycogen synthase. For many years, it was believed that a primer was required for de novo glycogen synthesis and the protein considered responsible for this process was ultimately discovered and named glycogenin. However, the subsequent observation of glycogen storage in the absence of functional glycogenin raises questions about the true role of the protein. In resting muscle, phosphorylase is generally considered to be present in two forms: non-phosphorylated and inactive (phosphorylase b) and phosphorylated and constitutively active (phosphorylase a). Initially, it was believed that activation of phosphorylase during intense muscle contraction was primarily accounted for by phosphorylation of phosphorylase b (activated by increases in AMP) to a, and that glycogen synthesis during recovery from exercise occurred solely through mechanisms controlled by glucose transport and glycogen synthase. However, it now appears that these views require modifications. Moreover, the traditional roles of glycogen in muscle function have been extended in recent years and in some instances, the original concepts have undergone revision. Thus, despite the extensive amount of knowledge accrued during the past 100 years, several critical questions remain regarding the regulation of glycogen metabolism and its role in living muscle.

Preferent Diaphragmatic Involvement in TK2 Deficiency: An Autopsy Case Study

Our goal was to analyze postmortem tissues of an adult patient with late-onset thymidine kinase 2 (TK2) deficiency who died of respiratory failure. Compared with control tissues, we found a low mtDNA content in the patient’s skeletal muscle, liver, kidney, small intestine, and particularly in the diaphragm, whereas heart and brain tissue showed normal mtDNA levels. mtDNA deletions were present in skeletal muscle and diaphragm. All tissues showed a low content of OXPHOS subunits, and this was especially evident in diaphragm, which also exhibited an abnormal protein profile, expression of non-muscular β-actin and loss of GAPDH and α-actin. MALDI-TOF/TOF mass spectrometry analysis demonstrated the loss of the enzyme fructose-bisphosphate aldolase, and enrichment for serum albumin in the patient’s diaphragm tissue. The TK2-deficient patient’s diaphragm showed a more profound loss of OXPHOS proteins, with lower levels of catalase, peroxiredoxin 6, cytosolic superoxide dismutase, p62 and the catalytic subunits of proteasome than diaphragms of ventilated controls. Strong overexpression of TK1 was observed in all tissues of the patient with diaphragm showing the highest levels. TK2 deficiency induces a more profound dysfunction of the diaphragm than of other tissues, which manifests as loss of OXPHOS and glycolytic proteins, sarcomeric components, antioxidants and overactivation of the TK1 salvage pathway that is not attributed to mechanical ventilation.

Inhibition of glycogenolysis prolongs action potential repriming period and impairs muscle function in rat skeletal muscle

KEY POINTS

Muscle glycogen content is associated with muscle function, but the physiological link between the two is poorly understood. This study investigated the effects of inhibiting glycogenolysis, while maintaining high overall energy status, on different aspects of muscle function. We demonstrate here that Na⁺ ,K⁺ -ATPase activity depends on glycogenolytically derived ATP regardless of high global ATP, with a decrease in activity leading to reduced force production and accelerated fatigue development. The results support the concept of compartmentalized energy transfer with glycogen metabolism playing a crucial role in intramuscular ATP resynthesis and ion regulation. This study gives specific insights into muscular function and may help towards a better understanding of glycogen storage diseases and muscle fatigue.

ABSTRACT

Skeletal muscle glycogen content is associated with muscle function and fatigability. However, little is known about the physiological link between glycogen content and muscle function. Here we aimed to investigate the importance of glycogenolytically derived ATP per se on muscle force and action potential (AP) repriming period, i.e. the time before a second AP can be produced (indicative of Na⁺ ,K⁺ -ATPase activity). Single fibres from rat extensor digitorum longus muscles were isolated and mechanically skinned in order to investigate force production and the AP repriming period while global ATP and PCr concentrations were kept high. The importance of glycogenolytically derived ATP was studied by inhibition of glycogen phosphorylase (1,4-dideoxy-1,4-imino-d-arabinitol (DAB; 2 mm) or CP-316,819 (CP; 10 µm)) or glycogen removal (amyloglucosidase, 20 U ml^-1 ). Tetanic force decreased by (mean (SD)) 21 (15)% (P < 0.001) and 76 (28)% (DAB) or 94 (6)% (CP, P < 0.001) in well-polarized and partially depolarized fibres, respectively. In depolarized fibres, twitch force decreased by 16 (10)% and 55 (26)% with DAB and CP, respectively, with no effect in well-polarized fibres (84 (10)%, P = 0.14). There was no effect of glycogen phosphorylase inhibition on repriming period in well-polarized fibres (median (25th, 75th percentile): 5 (4, 5) vs. 4 (4, 5) ms, P = 0.26), while the repriming period was prolonged from 6 (5, 7) to 8 (7, 10) ms (P = 0.01) in partially depolarized fibres. In line with this, glycogen removal increased repriming period from 5 (5, 6) to 6 (5, 7) ms (P = 0.003) in depolarized fibres. Together, these data strongly indicate that blocking glycogenolysis attenuates Na⁺ ,K⁺ -ATPase activity, which in turn increases the repriming period and reduces force, demonstrating a functional link between glycogenolytically derived ATP and force production.

Sarcoplasmic reticulum and calcium signaling in muscle cells: Homeostasis and disease

The sarco/endoplasmic reticulum is an extensive, dynamic and heterogeneous membranous network that fulfills multiple homeostatic functions. Among them, it compartmentalizes, stores and releases calcium within the intracellular space. In the case of muscle cells, calcium released from the sarco/endoplasmic reticulum in the vicinity of the contractile machinery induces cell contraction. Furthermore, sarco/endoplasmic reticulum-derived calcium also regulates gene transcription in the nucleus, energy metabolism in mitochondria and cytosolic signaling pathways. These diverse and overlapping processes require a highly complex fine-tuning that the sarco/endoplasmic reticulum provides by means of its numerous tubules and cisternae, specialized domains and contacts with other organelles. The sarco/endoplasmic reticulum also possesses a rich calcium-handling machinery, functionally coupled to both contraction-inducing stimuli and the contractile apparatus. Such is the importance of the sarco/endoplasmic reticulum for muscle cell physiology, that alterations in its structure, function or its calcium-handling machinery are intimately associated with the development of cardiometabolic diseases. Cardiac hypertrophy, insulin resistance and arterial hypertension are age-related pathologies with a common mechanism at the muscle cell level: the accumulation of damaged proteins at the sarco/endoplasmic reticulum induces a stress response condition termed endoplasmic reticulum stress, which impairs proper organelle function, ultimately leading to pathogenesis.

Novel heterozygous mutations in the PGAM2 gene with negative exercise testing

Pathogenic variants in the PGAM2 gene are associated with glycogen storage disease type X (GSDX) and is characterized by exercise induced muscle cramping, weakness, myoglobinuria, and often tubular aggregates in skeletal muscle. We report here a patient diagnosed with GSDX at 52 years of age with a normal increase in post-exercise lactate with both anaerobic and aerobic exercise. Genetic testing found two novel PGAM2 variants (c.426C > A, p.Tyr142Ter and c.533delG, p.Gly178Alafs*31).

No Muscle Is an Island: Integrative Perspectives on Muscle Fatigue

Muscle fatigue has been studied with a variety approaches, tools and technologies. The foci of these studies have ranged tremendously, from molecules to the entire organism. Single cell and animal models have been used to gain mechanistic insight into the fatigue process. The theme of this review is the concept that the mechanisms of muscle fatigue do not occur in isolation in vivo: muscular work is supported by many complex physiological systems, any of which could fail during exercise and thus contribute to fatigue. To advance our overall understanding of fatigue, a combination of models and approaches is necessary. In this review, we examine the roles that neuromuscular properties, intracellular glycogen, oxygen metabolism, and blood flow play in the fatigue process during exercise and pathological conditions.

Analysis of mitochondrial organization and function in the Drosophila blastoderm embryo

Mitochondria are inherited maternally as globular and immature organelles in metazoan embryos. We have used the Drosophila blastoderm embryo to characterize their morphology, distribution and functions in embryogenesis. We find that mitochondria are relatively small, dispersed and distinctly distributed along the apico-basal axis in proximity to microtubules by motor protein transport. Live imaging, photobleaching and photoactivation analyses of mitochondrially targeted GFP show that they are mobile in the apico-basal axis along microtubules and are immobile in the lateral plane thereby associating with one syncytial cell. Photoactivated mitochondria distribute equally to daughter cells across the division cycles. ATP depletion by pharmacological and genetic inhibition of the mitochondrial electron transport chain (ETC) activates AMPK and decreases syncytial metaphase furrow extension. In summary, we show that small and dispersed mitochondria of the Drosophila blastoderm embryo localize by microtubule transport and provide ATP locally for the fast syncytial division cycles. Our study opens the possibility of use of Drosophila embryogenesis as a model system to study the impact of maternal mutations in mitochondrial morphology and metabolism on embryo patterning and differentiation.

Power-duration relationship: Physiology, fatigue, and the limits of human performance

The duration that exercise can be maintained decreases as the power requirements increase. In this review, we describe the power-duration (PD) relationship across the full range of attainable power outputs in humans. We show that a remarkably small range of power outputs is sustainable (power outputs below the critical power, CP). We also show that the origin of neuromuscular fatigue differs considerably depending on the exercise intensity domain in which exercise is performed. In the moderate domain (below the lactate threshold, LT), fatigue develops slowly and is predominantly of central origin (residing in the central nervous system). In the heavy domain (above LT but below CP), both central and peripheral (muscle) fatigue are observed. In this domain, fatigue is frequently correlated with the depletion of muscle glycogen. Severe-intensity exercise (above the CP) is associated with progressive derangements of muscle metabolic homeostasis and consequent peripheral fatigue. To counter these effects, muscle activity increases progressively, as does pulmonary oxygen uptake ([Formula: see text]), with task failure being associated with the attainment of [Formula: see text] max. Although the loss of homeostasis and thus fatigue develop more rapidly the higher the power output is above CP, the metabolic disturbance and the degree of peripheral fatigue reach similar values at task failure. We provide evidence that the failure to continue severe-intensity exercise is a physiological phenomenon involving multiple interacting mechanisms which indicate a mismatch between neuromuscular power demand and instantaneous power supply. Valid integrative models of fatigue must account for the PD relationship and its physiological basis.

Skeletal muscle fatigue

Skeletal muscle fatigue is defined as the fall of force or power in response to contractile activity. Both the mechanisms of fatigue and the modes used to elicit it vary tremendously. Conceptual and technological advances allow the examination of fatigue from the level of the single molecule to the intact organism. Evaluation of muscle fatigue in a wide range of disease states builds on our understanding of basic function by revealing the sources of dysfunction in response to disease.

Muscle glycogen stores and fatigue

  Studies performed at the beginning of the last century revealed the importance of carbohydrate as a fuel during exercise, and the importance of muscle glycogen on performance has subsequently been confirmed in numerous studies. However, the link between glycogen depletion and impaired muscle function during fatigue is not well understood and a direct cause-and-effect relationship between glycogen and muscle function remains to be established. The use of electron microscopy has revealed that glycogen is not homogeneously distributed in skeletal muscle fibres, but rather localized in distinct pools. Furthermore, each glycogen granule has its own metabolic machinery with glycolytic enzymes and regulating proteins. One pool of such glycogenolytic complexes is localized within the myofibrils in close contact with key proteins involved in the excitation-contraction coupling and Ca2+ release from the sarcoplasmic reticulum (SR). We and others have provided experimental evidence in favour of a direct role of decreased glycogen, localized within the myofibrils, for the reduction in SR Ca2+ release during fatigue. This is consistent with compartmentalized energy turnover and distinctly localized glycogen pools being of key importance for SR Ca2+ release and thereby affecting muscle contractility and fatigability.

Physiological aspects of the subcellular localization of glycogen in skeletal muscle

Glucose is stored in skeletal muscle fibers as glycogen, a branched-chain polymer observed in electron microscopy images as roughly spherical particles (known as β-particles of 10–45 nm in diameter), which are distributed in distinct localizations within the myofibers and are physically associated with metabolic and scaffolding proteins. Although the subcellular localization of glycogen has been recognized for more than 40 years, the physiological role of the distinct localizations has received sparse attention. Recently, however, studies involving stereological, unbiased, quantitative methods have investigated the role and regulation of these distinct deposits of glycogen. In this report, we review the available literature regarding the subcellular localization of glycogen in skeletal muscle as investigated by electron microscopy studies and put this into perspective in terms of the architectural, topological, and dynamic organization of skeletal muscle fibers. In summary, the distribution of glycogen within skeletal muscle fibers has been shown to depend on the fiber phenotype, individual training status, short-term immobilization, and exercise and to influence both muscle contractility and fatigability. Based on all these data, the available literature strongly indicates that the subcellular localization of glycogen has to be taken into consideration to fully understand and appreciate the role and regulation of glycogen metabolism and signaling in skeletal muscle. A full understanding of these phenomena may prove vital in elucidating the mechanisms that integrate basic cellular events with changing glycogen content.

Compartmentation of GAPDH

The concept of the cytosol as a space that contains discrete zones of metabolites is discussed relative to the contribution of GAPDH. GAPDH is directed to very specific cell compartments. This chapter describes the utilization of GAPDH's enzymatic function for focal demands (i.e. ATP/ADP and NAD(+)/NADH), and offers a speculative role for GAPDH as perhaps moderating local concentrations of inorganic phosphate and hydrogen ions (i.e. co-substrate and co-product of the glycolytic reaction, respectively). Where known, the structural features of the binding between GAPDH and the compartment components are discussed. The nuances, which are associated with the intracellular distribution of GAPDH, appear to be specific to the cell-type, particularly with regards to the various plasma membrane proteins to which GAPDH binds. The chapter includes discussion on the curious observation of GAPDH being localized to the external surface of the plasma membrane in a human cell type. The default perspective has been that GAPDH localization is synonymous with compartmentation of glycolytic energy. The chapter discusses GAPDH translocation to the nucleus and to non-nuclear cellular structures, emphasizing its glycolytic function. Nevertheless, it is becoming clear that alternate functions of GAPDH play a role in compartmentation, particularly in the translocation to the nucleus.

GAPDH in anesthesia

Thus far, two independent laboratories have shown that inhaled anesthetics directly affect GAPDH structure and function. Additionally, it has been demonstrated that GAPDH normally regulates the function of GABA (type A) receptor. In light of these literature observations and some less direct findings, there is a discussion on the putative role of GAPDH in anesthesia. The binding site of inhaled anesthetics is described from literature reports on model proteins, such as human serum albumin and apoferritin. In addition to the expected hydrophobic residues that occupy the binding cavity, there are hydrophilic residues at or in very close proximity to the site of anesthetic binding. A putative binding site in the bacterial analog of the human GABA (type A) receptor is also described. Additionally, GAPDH may also play a role in anesthetic preconditioning, a phenomenon that confers protection of cells and tissues to future challenges by noxious stimuli. The central thesis regarding this paradigm is that inhaled anesthetics evoke an intra-molecular protein dehydration that is recognized by the cell, eliciting a very specific burst of chaperone gene expression. The chaperones that are implicated are associated with conferring protection against dehydration-induced protein aggregation.

Role of glycogen availability in sarcoplasmic reticulum Ca2+ kinetics in human skeletal muscle

Glucose is stored as glycogen in skeletal muscle. The importance of glycogen as a fuel during exercise has been recognized since the 1960s; however, little is known about the precise mechanism that relates skeletal muscle glycogen to muscle fatigue. We show that low muscle glycogen is associated with an impairment of muscle ability to release Ca(2+), which is an important signal in the muscle activation. Thus, depletion of glycogen during prolonged, exhausting exercise may contribute to muscle fatigue by causing decreased Ca(2+) release inside the muscle. These data provide indications of a signal that links energy utilization, i.e. muscle contraction, with the energy content in the muscle, thereby inhibiting a detrimental depletion of the muscle energy store.

Subcellular localization-dependent decrements in skeletal muscle glycogen and mitochondria content following short-term disuse in young and old men

Previous studies have shown that skeletal muscle glycogen and mitochondria are distributed in distinct subcellular localizations, but the role and regulation of these subcellular localizations are unclear. In the present study, we used transmission electron microscopy to investigate the effect of disuse and aging on human skeletal muscle glycogen and mitochondria content in subsarcolemmal (SS), intermyofibrillar (IMF), and intramyofibrillar (intra) localizations. Five young (∼23 yr) and five old (∼66 yr) recreationally active men had their quadriceps muscle immobilized for 2 wk by whole leg casting. Biopsies were obtained from m. vastus lateralis before and after the immobilization period. Immobilization induced a decrement of intra glycogen content by 54% (P < 0.001) in both age groups and in two ultrastructurally distinct fiber types, whereas the content of IMF and SS glycogen remained unchanged. A localization-dependent decrease (P = 0.03) in mitochondria content following immobilization was found in both age groups, where SS mitochondria decreased by 33% (P = 0.02), superficial IMF mitochondria decreased by 20% (P = 0.05), and central IMF mitochondria remained unchanged. In conclusion, our findings demonstrate a localization-dependent adaptation to immobilization in glycogen and mitochondria content of skeletal muscles of both young and old individuals. Specifically, this suggests that short-term disuse preferentially affects glycogen particles located inside the myofibrils and that mitochondria volume plasticity can be dependent on the distance to the fiber border.

Peripheral fatigue: high-energy phosphates and hydrogen ions

Peripheral fatigue results from an overactivity-induced decline in muscle function that originates from non-central nervous system mechanisms. A common symptom of fatigue is a feeling of tiredness or weariness because of overexertion, such as that associated with intense or prolonged physical exercise. Fatigue is worsened by low physical fitness and chronic illnesses. These conditions may intensify fatigue to levels that limit physical and social functioning and severely diminish health-related quality of life. Although etiologic aspects of peripheral fatigue are often associated with regulatory system (neurologic, endocrine, immunologic, muscular) and support system (cardiovascular, pulmonary, metabolic, renal, digestive, skeletal) limitations, final mediation occurs in muscle cells as a result of altered crossbridge functioning. Specifically, the final product and ionic metabolite accumulation that result from adenosine triphosphate hydrolysis appear to inhibit crossbridge formation and activation. Thus, clinical manifestations of peripheral fatigue often can be observed as limitations placed upon muscle or cardiorespiratory endurance, here defined as fatigue resistance. An overview of the common pathways by which peripheral fatigue can be mediated is provided. Product inhibition of contractile chemistry is brought into focus as a common pathway through which the mechanisms of peripheral fatigue often act.

Sarcoplasmic reticulum function in smooth muscle

The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a "one model fits all" approach to this subject, we have tried to synthesize conclusions wherever possible.

Novel role of calpain-3 in the triad-associated protein complex regulating calcium release in skeletal muscle

Calpain-3 (CAPN3) is a non-lysosomal cysteine protease that is necessary for normal muscle function, as mutations in CAPN3 result in an autosomal recessive form of limb girdle muscular dystrophy type 2A. To elucidate the biological roles of CAPN3 in skeletal muscle, we performed a search for potential substrates and interacting partners. By yeast-two-hybrid analysis we identified the glycolytic enzyme aldolase A (AldoA) as a binding partner of CAPN3. In co-expression studies CAPN3 degraded AldoA; however, no accumulation of AldoA was observed in total extracts from CAPN3-deficient muscles suggesting that AldoA is not an in vivo substrate of CAPN3. Instead, we found CAPN3 to be necessary for recruitment of AldoA to one specific location, namely the triads, which are structural components of muscle responsible for calcium transport and excitation-contraction coupling. Both aldolase and CAPN3 are present in the triad-enriched fraction and are able to interact with ryanodine receptors (RyR) that form major calcium release channels. Levels of triad-associated AldoA and RyR were decreased in CAPN3-deficient muscles compared with wild-type. Consistent with these observations we found calcium release to be significantly reduced in fibers from CAPN3-deficient muscles. Together, these data suggest that CAPN3 is necessary for the structural integrity of the triad-associated protein complex and that impairment of calcium transport is a phenotypic feature of CAPN3-deficient muscle.

The regulatory beta subunit of phosphorylase kinase interacts with glyceraldehyde-3-phosphate dehydrogenase

Skeletal muscle phosphorylase kinase (PhK) is an (alphabetagammadelta) 4 hetero-oligomeric enzyme complex that phosphorylates and activates glycogen phosphorylase b (GP b) in a Ca (2+)-dependent reaction that couples muscle contraction with glycogen breakdown. GP b is PhK's only known in vivo substrate; however, given the great size and multiple subunits of the PhK complex, we screened muscle extracts for other potential targets. Extracts of P/J (control) and I/lnJ (PhK deficient) mice were incubated with [gamma- (32)P]ATP with or without Ca (2+) and compared to identify potential substrates. Candidate targets were resolved by two-dimensional polyacrylamide gel electrophoresis, and phosphorylated glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was identified by matrix-assisted laser desorption ionization mass spectroscopy. In vitro studies showed GAPDH to be a Ca (2+)-dependent substrate of PhK, although the rate of phosphorylation is very slow. GAPDH does, however, bind tightly to PhK, inhibiting at low concentrations (IC 50 approximately 0.45 microM) PhK's conversion of GP b. When a short synthetic peptide substrate was substituted for GP b, the inhibition was negligible, suggesting that GAPDH may inhibit predominantly by binding to the PhK complex at a locus distinct from its active site on the gamma subunit. To test this notion, the PhK-GAPDH complex was incubated with a chemical cross-linker, and a dimer between the regulatory beta subunit of PhK and GAPDH was formed. This interaction was confirmed by the fact that a subcomplex of PhK missing the beta subunit, specifically an alphagammadelta subcomplex, was unable to phosphorylate GAPDH, even though it is catalytically active toward GP b. Moreover, GAPDH had no effect on the conversion of GP b by the alphagammadelta subcomplex. The interactions described herein between the beta subunit of PhK and GAPDH provide a possible mechanism for the direct linkage of glycogenolysis and glycolysis in skeletal muscle.

Skeletal Muscle Fatigue: Cellular Mechanisms

Repeated, intense use of muscles leads to a decline in performance known as muscle fatigue. Many muscle properties change during fatigue including the action potential, extracellular and intracellular ions, and many intracellular metabolites. A range of mechanisms have been identified that contribute to the decline of performance. The traditional explanation, accumulation of intracellular lactate and hydrogen ions causing impaired function of the contractile proteins, is probably of limited importance in mammals. Alternative explanations that will be considered are the effects of ionic changes on the action potential, failure of SR Ca2+release by various mechanisms, and the effects of reactive oxygen species. Many different activities lead to fatigue, and an important challenge is to identify the various mechanisms that contribute under different circumstances. Most of the mechanistic studies of fatigue are on isolated animal tissues, and another major challenge is to use the knowledge generated in these studies to identify the mechanisms of fatigue in intact animals and particularly in human diseases.

Muscle metabolic, SR Ca2+-cycling responses to prolonged cycling, with and without glucose supplementation

This study investigated the effects of prolonged exercise, with and without glucose supplementation, on metabolism and sarcoplasmic reticulum (SR) Ca2+-handling properties in working vastus lateralis muscle. Fifteen untrained volunteers [peak O2consumption (V̇o2peak) = 3.45 ± 0.17 l/min; mean ± SE] cycled at ∼60% V̇o2peakon two occasions, during which they were provided with either an artificially sweetened placebo beverage (NG) or a 6% glucose (G) beverage (∼1.00 g carbohydrate/kg body mass). Beverage supplementation started at 30 min of exercise and continued every 15 min thereafter. SR Ca2+handling, metabolic, and substrate responses were assessed in tissue extracted from the vastus lateralis at rest, after 30 min and 90 min of exercise, and at fatigue in both conditions. Plasma glucose during G was 15–23% higher ( P < 0.05) than those observed during NG following 60 min of exercise until fatigue. Cycle time to fatigue was increased ( P < 0.05) by ∼19% during G (137 ± 7 min) compared with NG (115 ± 6 min). Prolonged exercise reduced ( P < 0.05) maximal Ca2+-ATPase activity (−18.4%), SR Ca2+uptake (−27%), and both Phase 1 (−22.2%) and Phase 2 (−34.2%) Ca2+-release rates during NG. The exercise-induced reductions in SR Ca2+-cycling properties were not altered during G. The metabolic responses to exercise were all unaltered by glucose supplementation, since no differences in respiratory exchange ratios, carbohydrate and lipid oxidation rates, and muscle metabolite and glycogen contents were observed between NG and G. These results indicate that the maintenance of blood glucose homeostasis by glucose supplementation is without effect in modifying the muscle metabolic, endogenous glycogen, or SR Ca2+-handling responses.

Measurement of sarcoplasmic reticulum Ca2+ ATPase activity using high-performance liquid chromatography

The goal of this investigation was to develop an assay whereby we could measure changes in ATP, ADP, and phosphocreatine (PCr) during stimulation of the sarcoplasmic reticulum (SR) Ca2+ ATPase. After stopping the enzyme reaction, compounds were extracted by perchloric acid and separated by reversed-phase high-performance liquid chromatography (HPLC). Absorbance of ATP and ADP was monitored at 260 nm, and detection of PCr was done at 205 nm. Chromatograms show that peaks associated with each compound are clearly separated and easily detected. The SR Ca2+ ATPase assay was run for various time periods and using varying free [Ca2+]. The changes in ATP and ADP contents were linear with increasing time and varied as expected with increasing free [Ca2+]. The ATPase activities determined using changes in ATP and ADP were nearly identical to those determined using previously established assays. When PCr was added to the assay, we were able to confirm that the Ca2+ ATPase uses ATP that is synthesized locally from PCr via creatine kinase (CK). The results indicate that this is a valid and reliable method for examining SR Ca2+ ATPase activity and for investigating its interaction with CK.

Na+-K+ pumps in the transverse tubular system of skeletal muscle fibers preferentially use ATP from glycolysis

The Na(+)-K(+) pumps in the transverse tubular (T) system of a muscle fiber play a vital role keeping K(+) concentration in the T-system sufficiently low during activity to prevent chronic depolarization and consequent loss of excitability. These Na(+)-K(+) pumps are located in the triad junction, the key transduction zone controlling excitation-contraction (EC) coupling, a region rich in glycolytic enzymes and likely having high localized ATP usage and limited substrate diffusion. This study examined whether Na(+)-K(+) pump function is dependent on ATP derived via the glycolytic pathway locally within the triad region. Single fibers from rat fast-twitch muscle were mechanically skinned, sealing off the T-system but retaining normal EC coupling. Intracellular composition was set by the bathing solution and action potentials (APs) triggered in the T-system, eliciting intracellular Ca(2+) release and twitch and tetanic force responses. Conditions were selected such that increased Na(+)-K(+) pump function could be detected from the consequent increase in T-system polarization and resultant faster rate of AP repriming. Na(+)-K(+) pump function was not adequately supported by maintaining cytoplasmic ATP concentration at its normal resting level ( approximately 8 mM), even with 10 or 40 mM creatine phosphate present. Addition of as little as 1 mM phospho(enol)pyruvate resulted in a marked increase in Na(+)-K(+) pump function, supported by endogenous pyruvate kinase bound within the triad. These results demonstrate that the triad junction is a highly restricted microenvironment, where glycolytic resynthesis of ATP is critical to meet the high demand of the Na(+)-K(+) pump and maintain muscle excitability.

An enquiry into metabolite domains

It is currently assumed that two or more pools of the same metabolite can coexist in the cytosolic compartment of mammalian cells. These pools are thought to be generated by the differential subcellular location of enzymes and transporters, much in the way calcium microdomains arise by the combined workings of channels, buffers, and pumps. With the aim of estimating the amplitude and spatial dimensions of these metabolite pools, we developed an analytical tool based on Brownian diffusion and the turnover numbers of the proteins involved. The outcome of the analysis is that ATP, glucose, pyruvate, lactate, and glutamate cannot be concentrated at their sources to an extent that would affect their downstream targets. For these metabolites, and others produced by slow enzymes or transporters and present at micromolar concentrations or higher, the cytosol behaves as a well-mixed, homogenous compartment. In contrast, the analysis showed microdomains known to be generated by calcium channels and revealed that calcium and pH nanodomains are to be found in the vicinity of slow enzymes and transporters in the steady state. The analysis can be readily applied to any other molecule, provided knowledge is available about rate of production, average concentration, and diffusion coefficient. Our main conclusion is that the notion of cytosolic compartmentation of metabolites needs reevaluation, as it seems to be in conflict with the underlying physical chemistry.

Influence of activation frequency on cellular signalling pathways during fatiguing contractions in rat skeletal muscle

Activation frequency as a regulator of physiological responses in skeletal muscle, independent of contractile force, has received little attention. Here, the length-tension and force-frequency relationships were employed to keep active contractile force equal, despite a twofold difference in stimulation frequency (15 versus 30 Hz). Rat tibialis anterior muscles were tested in situ using 15 Hz stimulation at optimal length (15 Hz) and 30 Hz stimulation at shortened and lengthened positions (30 Hz(sub) and 30 Hz(supra)). Muscles were subjected to 1, 15, 30 and 80 Hz stimulation trains before and after 2 min of fatiguing stimulation. The principal findings were that the two 30 Hz protocols produced greater 38 kDa MAPK (p38) phosphorylation than the 15 Hz protocol (1.4- to 1.5-fold versus 1.1-fold), as well as greater fatigue (65-78 versus 43% decline in contraction force). In contrast, c-jun amino terminal kinase (JNK) phosphorylation appeared most responsive to total (active plus passive) tension such that the changes followed the pattern: 30 Hz(supra) > 15 Hz > 30 Hz(sub), while 44 and 42 kDa extracellular regulated kinase (ERK1/2) phosphorylation was not significantly increased in response to any of the protocols studied. Neither glycogen depletion nor myofibre damage accounted for any of the findings, but a decline in muscle excitation (m-wave) may have contributed to the greater fatigue seen at higher frequencies. These data suggest that neuromuscular activation frequency can influence certain signalling pathways in skeletal muscle, independent of force production.

Mitochondrial modulation of Ca2+ -induced Ca2+ -release in rat sensory neurons

Ca2+ -induced Ca2+ -release (CICR) from ryanodine-sensitive Ca2+ stores provides a mechanism to amplify and propagate a transient increase in intracellular calcium concentration ([Ca2+]i). A subset of rat dorsal root ganglion neurons in culture exhibited regenerative CICR when sensitized by caffeine. [Ca2+]i oscillated in the maintained presence of 5 mM caffeine and 25 mM K+. Here, CICR oscillations were used to study the complex interplay between Ca2+ regulatory mechanisms at the cellular level. Oscillations depended on Ca2+ uptake and release from the endoplasmic reticulum (ER) and Ca2+ influx across the plasma membrane because cyclopiazonic acid, ryanodine, and removal of extracellular Ca2+ terminated oscillations. Increasing caffeine concentration decreased the threshold for action potential-evoked CICR and increased oscillation frequency. Mitochondria regulated CICR by providing ATP and buffering [Ca2+]i. Treatment with the ATP synthase inhibitor, oligomycin B, decreased oscillation frequency. When ATP concentration was held constant by recording in the whole cell patch-clamp configuration, oligomycin no longer affected oscillation frequency. Aerobically derived ATP modulated CICR by regulating the rate of Ca2+ sequestration by the ER Ca2+ pump. Neither CICR threshold nor Ca2+ clearance by the plasma membrane Ca2+ pump were affected by inhibition of aerobic metabolism. Uncoupling electron transport with carbonyl cyanide p-trifluoromethoxy-phenyl-hydrazone or inhibiting mitochondrial Na+/Ca2+ exchange with CGP37157 revealed that mitochondrial buffering of [Ca2+]i slowed oscillation frequency, decreased spike amplitude, and increased spike width. These findings illustrate the interdependence of energy metabolism and Ca2+ signaling that results from the complex interaction between the mitochondrion and the ER in sensory neurons.

The voltage-dependent anion channel in endoplasmic/sarcoplasmic reticulum: characterization, modulation and possible function

In recent years, it has been recognized that there is a metabolic coupling between the cytosol, ER/SR and mitochondria. In this cross-talk, mitochondrial Ca(2+) homeostasis and ATP production and supply play a major role. The primary transporter of adenine nucleotides, Ca(2+)and other metabolites into and out of mitochondria is the voltage-dependent anion channel (VDAC) located at the outer mitochondrial membrane, at a crucial position in the cell. VDAC has been established as a key player in mitochondrial metabolite and ion signaling and it has also been proposed that VDAC is present in extramitochondrial membranes. Thus, regulation of VDAC, as the main interface between mitochondrial and cellular metabolism, by other molecules is of utmost importance. This article reviews localization and function of VDAC, and focuses on VDAC as a skeletal muscle sarcoplasmic reticulum channel. The regulation of VDAC activity by associated proteins and by inhibitors is also presented. Several aspects of the physiological relevance of VDAC to Ca(2+) homeostasis and mitochondria-mediated apoptosis will be discussed.

Molecular aspects of rapid, reversible, Ca2+-dependent de-phosphorylation of pp63/parafusin during stimulated exo-endocytosis in Paramecium cells

Ca2+ signalling governs stimulated exocytosis and exocytosis-coupled endocytosis also in Paramecium cells. Upon stimulation, the < or =10(3) dense-core exocytotic organelles (trichocysts) can be synchronously (80 ms) released, followed by endocytotic membrane resealing (350 ms) and retrieval. Paramecium is the most synchronous dense-core exocytotic system known, allowing to dissect rapidly reversible Ca2+-dependent phenomena. This holds for the reversible de-/re-phosphorylation cycle of a 63 kD phosphoprotein, pp63/parafusin (pf), which we have cloned, immuno-localised, and characterised as phosphoglucomutase, the enzyme funneling glucose into the glycolytic pathway. It was isolated ex vivo, followed by MALDI analysis, while X-ray structure analysis was performed after heterologous expression. We found multiple phosphorylation of superficial Ser/Thr residues. Although present also in exo(-) mutants, pp63/pf is selectively de-phosphorylated only in exo(+) strains during synchronous exocytosis (80 ms) and re-phosphorylated within approximately 20 s, i.e., the time required to re-establish [Ca2+] homeostasis. We have isolated relevant protein phosphatases and kinases and probed their activity on pp63/pf in vitro. We consider Ca2+/calmodulin-activated PP2B (calcineurin, whose subunits have been cloned) relevant for de-phosphorylation. Re-phosphorylation can be achieved by two protein kinases that also have been cloned. One is activated by cGMP (PKG) which in turn is formed by Ca2+-activated guanylate cyclase. Another kinase, casein kinase 2, is inhibited by Ca2+ and, hence, activated with some delay in parallel to decreasing [Ca2+] after exocytosis. In total, several Ca2+-sensitive cycles cooperate whose protein components have been localised to the cell cortex. Regulation of the phosphorylation degree of pp63/pf may affect structure binding on a microscale and/or its enzymatic activity. All this may serve fueling substrate into glycolysis with increased ATP re-formation (compromised in exo(-) mutants) and NADH formation, with effects on Ca2+ signalling including mobilisation from cortical stores (alveolar sacs) and overall effects on ATP and Ca2+ dynamics during synchronous exo- and endocytosis.

Glyceraldehyde-3-phosphate dehydrogenase is a GABAA receptor kinase linking glycolysis to neuronal inhibition

Protein phosphorylation is crucial for regulating synaptic transmission. We describe a novel mechanism for the phosphorylation of the GABA(A) receptor, which mediates fast inhibition in the brain. A protein copurified and coimmunoprecipitated with the phosphorylated receptor alpha1 subunit; this receptor-associated protein was identified by purification and microsequencing as the key glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Molecular constructs demonstrated that GAPDH directly phosphorylates the long intracellular loop of GABA(A) receptor alpha1 subunit at identified serine and threonine residues. GAPDH and the alpha1 subunit were found to be colocalized at the neuronal plasma membrane. In keeping with the GAPDH/GABA(A) receptor molecular association, glycolytic ATP produced locally at plasma membranes was consumed for this alpha1 subunit phosphorylation, possibly within a single macrocomplex. The membrane-attached GAPDH is thus a dual-purpose enzyme, a glycolytic dehydrogenase, and a receptor-associated kinase. In acutely dissociated cortical neurons, the rundown of the GABA(A) responses was essentially attributable to a Mg(2+)-dependent phosphatase activity, which was sensitive to vanadate but insensitive to okadaic acid or fluoride. Rundown was significantly reduced by the addition of GAPDH or its reduced cofactor NADH and nearly abolished by the addition of its substrate glyceraldehyde-3-phosphate (G3P). The prevention of rundown by G3P was abolished by iodoacetamide, an inhibitor of the dehydrogenase activity of GAPDH, indicating that the GABA(A) responses are maintained by a glycolysis-dependent phosphorylation. Our results provide a molecular mechanism for the direct involvement of glycolysis in neurotransmission.

Mechanisms and management of fatigue in health and disease: symposium introduction

Exercise intolerance is a condition commonly experienced by both the healthy and those with disease. Yet we have only a limited understanding of the underlying mechanisms and, consequently, the management of this condition. In this Symposium, a major objective was to address the role of the muscle cell in weakness and fatigue. We have focused on addressing the advances made in characterizing the basis of muscle cell contractility with particular respect to the processes and proteins involved in excitation and contraction, and how these processes can be modified during repetitive activity. Three reviews are provided on this subject. Each addresses a specific link in the cascade of events from neural activation of the muscle to the generation of force. In the first review the processes involved in signal transduction in the sarcolemma and T-tubule, and which regulate membrane excitability, are examined. The second review analyzes the sarcoplasmic reticulum regulation of the intracellular messenger that controls the myofibrillar complex, namely free calcium. The final review in this series deals with the events regulating actin-myosin behaviour and the mechanical response. All reviews place special emphasis on how different sites can be modified by repetitive activity and, as a consequence, how they can represent a potential source of fatigue. Since it is important to understand the nature, manifestations, and measurement of weakness and fatigue, a comprehensive review on these topics is also provided.

Membrane excitability, weakness, and fatigue

A failure in membrane excitability, defined as an inability of the sarcolemma and T-tubule to translate the neural discharge command into repetitive action potentials, represents an inviting cause of mechanical disfunction in both health and disease. A failure at this level would precipitate a disturbance in signal transmission between the T-tubule and the calcium release channels of the sarcoplasmic reticulum, resulting in reduced release of Ca2+, lower cytosolic free Ca2+ levels, and depressed myofibrillar activation and force generation. The ability of the sarcolemma and T-tubules to conduct repetitive action potentials is intimately dependent on active transport of Na+ and K+ following an action potential. The active transport of these cations is mediated by the Na+-K+-ATPase, an integral membrane protein that uses the energy from the hydrolysis of 1 ATP to transport 3 Na+ out of the cell and 2 K+ into the cell. A failure to recruit sufficient Na+-K+-ATPase activity during contractile activity could result in a rundown of the transmembrane gradients for Na+ and K+, leading to a loss of membrane excitability. The Na+-K+-ATPase activity depends on the amount and isoform composition of the protein, substrate availability, and acute regulatory factors. Each of these factors is examined as a potential cause of altered activation of the Na+-K+-ATPase activity and loss of membrane excitability in fatigue. Regular exercise represents a potent stimulus for upregulating Na+-K+-ATPase levels and for increasing the ability for cation transport across the sarcolemma and T-tubule membrane. As such, training may be a valuable tool in the management of fatigue in health and disease.

Modulation of sarcoplasmic reticulum Ca2+ release by glycolysis in cat atrial myocytes

In cardiac myocytes, glycolysis and excitation-contraction (E-C) coupling are functionally coupled. We studied the effects of inhibitors (2-deoxy-D-glucose (2-DG), iodoacetate (IAA)), intermediates (glucose-6-phosphate (G6P), fructose-6-phosphate (F6P), fructose-1,6-bisphosphate (FBP), phosphoenolpyruvate (PEP)) and products (pyruvate, L-lactate) of glycolysis on sarcoplasmic reticulum (SR) Ca(2+) release and uptake in intact and permeabilized cat atrial myocytes. In field-stimulated (0.5-0.7 Hz) intact myocytes, 2-DG (10 mm) and IAA (1 mm) caused elevation of diastolic [Ca(2+)](i) and [Ca(2+)](i) transient alternans (Ca(2+) alternans) followed by a decrease of the amplitude of the [Ca(2+)](i) transient. Focal application of 2-DG resulted in local Ca(2+) alternans that was confined to the region of exposure. 2-DG and IAA slowed the decay kinetics of the [Ca(2+)](i) transient and delayed its recovery (positive staircase) after complete SR depletion, suggesting impaired activity of the SR Ca(2+)-ATPase (SERCA). 2-DG and IAA reduced the rate of reuptake of Ca(2+) into the SR which was accompanied by a 15-20% decrease of SR Ca(2+) load. Major changes of mitochondrial redox state (measured as FAD autofluorescence) were not observed after inhibition of glycolysis. Pyruvate (10 mm) and L-lactate (10 mm) elicited similar changes of the [Ca(2+)](i) transient. Pyruvate, L-lactate and IAA - but not 2-DG - induced intracellular acidosis. Recording of single channel activity of ryanodine receptors (RyRs) incorporated into lipid bilayers revealed complex modulation by glycolytic intermediates and products (1 mm each): some were without effect (G6P, PEP, L-lactate) while others either increased (F6P, +40%; FBP, +265%) or decreased (pyruvate, -58%) the open probability of the RyR. Consistent with these findings, spontaneous SR Ca(2+) release (Ca(2+) sparks) in permeabilized myocytes was facilitated by FBP and inhibited by pyruvate. The results indicate that in atrial myocytes glycolysis regulates Ca(2+) release from the SR by multiple mechanisms including direct modulation of RyR activity by intermediates and products of glycolysis and modulation of SERCA activity through local changes of glycolytically derived ATP.

Glycogen debranching enzyme is associated with rat skeletal muscle sarcoplasmic reticulum

AIMS

Gel electrophoresis revealed a band of molecular weight approximately 160 000 Da associated with the skeletal muscle sarcoplasmic reticulum (SR) vesicle preparations. This investigation sought to examine glycogen debranching enzyme associated with skeletal muscle SR.

METHODS

Sarcoplasmic reticulum samples were also taken from muscle whose glycogen content had been reduced either via stimulation of the sciatic nerve or alpha-amylase treatment of muscle homogenates.

RESULTS

The stimulation protocol reduced whole muscle glycogen by 86% (7.4 +/- 0.4 vs. 1.0 +/- 0.3 microg mg(-1) wet mass, P < or = 0.05). Glycogen associated with the SR was reduced by 82% in the stimulation protocol (533 +/- 82 vs. 96 +/- 7 microg mg(-1) protein) and by 94% in alpha-amylase treatment (493 +/- 11 vs. 29 +/- 2 microg mg(-1) protein), respectively. Gel electrophoresis and Western blots revealed that the content of glycogen debranching enzyme was reduced by approximately 53% as a result of muscle stimulation and by approximately 46% in alpha-amylase treatment (P < or = 0.05). In addition, glycogen debranching enzyme activity was reduced by 61% in stimulated samples compared with control (20.3 +/- 1.0 vs. 8.0 +/- 1.2 nmol mg(-1) min(-1), respectively), a value consistent with reductions observed from gel electrophoresis and Western blots.

CONCLUSION

These results confirm that similar to glycogen phosphorylase, glycogen debranching enzyme is associated with the skeletal muscle SR and is dissociated under exercise conditions.

Mitochondrial function in intact skeletal muscle fibres of creatine kinase deficient mice

Creatine kinase (CK) has a central role in skeletal muscle, acting as a fast energy buffer and shuttle between sites of energy production (mitochondria) and consumption (cross-bridges and ion pumps). Unexpectedly, isolated fast-twitch skeletal muscle cells of mice deficient in both cytosolic and mitochondrial CK (CK-/-) are highly fatigue resistant during stimulation protocols that stress aerobic metabolism. We have now studied different aspects of mitochondrial function in CK-/- skeletal muscle. Intact, single fibres of flexor digitorum brevis (FDB) muscles were fatigued by repeated tetanic stimulation (70 Hz, 350 ms duration, duty cycle 0.14). Under control conditions, CK-/- FDB fibres were more fatigue resistant than wild-type fibres. However, after mitochondrial inhibition with cyanide, force declined markedly faster in CK-/- fibres than in wild-type fibres. The rapid force decline in CK-/- fibres was not due to decreased myoplasmic [Ca2+] during tetani (measured with indo-1), which in these fibres remained virtually constant during fatigue in the presence of cyanide. Intact, single fibres of highly oxidative soleus muscles were fatigued by repeated tetani (50 Hz, 500 ms duration, duty cycle 0.5). All CK-/- soleus fibres tested (n = 9) produced > 40 % force at the end of the fatiguing stimulation period (500 tetani), whereas force fell to < 40 % before 500 tetani in two of three wild-type fibres. Mitochondrial [Ca2+] (measured with rhod-2 and confocal microscopy) increased during repeated tetanic stimulation in CK-/- but not in wild-type FDB fibres. In conclusion, mitochondria and energy shuttling operate effectively in CK-/- fibres and this is associated with an increase in mitochondrial [Ca2+].

Imbalance of plasma membrane ion leak and pump relationship as a new aetiological basis of certain disease states

The basis for life is the ability of the cell to maintain ion gradients across biological membranes. Such gradients are created by specific membrane-bound ion pumps [adenosine triphosphatases (ATPases)]. According to physicochemical rules passive forces equilibrate (dissipate) ion gradients. The cholesterol/phospholipid ratio of the membrane and the degree of saturation of phospholipid fatty acids are important factors for membrane molecular order and herewith a determinant of the degree of non-specific membrane leakiness. Other operative principles, i.e. specific ion channels can be opened and closed according to mechanisms that are specific to the cell. Certain compounds called ionophores can be integrated in the plasma membrane and permit specific inorganic ions to pass. Irrespective of which mechanism ions leak across the plasma membrane the homeostasis may be kept by increasing ion pumping (ATPase activity) in an attempt to restore the physiological ion gradient. The energy source for this work seems to be glycolytically derived ATP formation. Thus an increase in ion pumping is reflected by increased ATP hydrolysis and rate of glycolysis. This can be measured as an accumulation of breakdown products of ATP and end-products of anaerobic glycolysis (lactate). In certain disease entities, the balance between ATP formation and ion pumping may be disordered resulting in a decrease in inter alia (i.a.) cellular energy charge, and an increase in lactate formation and catabolites of adenylates. Cardiac syndrome X is proposed to be due to an excessive leakage of potassium ions, leading to electrocardiographic (ECG) changes, abnormal Tl-scintigraphy of the heart and anginal pain (induced by adenosine). Cocksackie B3 infections, a common agent in myocarditis might also induce an ionophore-like effect. Moreover, Alzheimer's disease is characterized by the formation of extracellular amyloid deposits in the brain of patients. Perturbation of cellular membranes by the amyloid peptide during the development of Alzheimer's disease is one of several mechanisms proposed to account for the toxicity of this peptide on neuronal membranes. We have studied the effects of the peptide and fragments thereof on 45Ca2+-uptake in human erythrocytes and the energetic consequences. Treatment of erythrocytes with the beta 1-40 peptide, results in qualitatively similar nucleotide pattern and decrease of energy charge as the treatment with Ca2+-ionophore A23187. Finally, in recent studies we have revealed and published in this journal that a rare condition, Tarui's disease or glycogenosis type VII, primarily associated with a defect M-subunit of phosphofructokinase, demonstrates as a cophenomenon an increased leak of Ca2+ into erythrocytes.

Muscle glycogenolysis is not activated by changes in cytosolic P-metabolites: a 31P and 1H MRS demonstration

Skeletal muscle contraction and glycogenolysis are closely coupled. The standard explanation for this coupling, as taught in modern biochemistry textbooks, is that the metabolic products of contraction (ADP, AMP, P(i)) feed back to activate glycogenolytic enzymes, thus providing for resynthesis of ATP. However, both in vivo (31)P MRS analyses and chemical analyses of muscle extracts have provided results that are contrary to this theory, at least in its simplest form. The MRS studies suffer from ambiguous assumptions. More importantly, in (31)P MRS studies the dependent and independent variables are often confounded because the glycogenolytic rate is calculated from the same data which are used to calculate the other metabolic variables. The analysis of biopsies has been necessarily quite limited, and suffers from a different set of experimental artifacts. Thus, the problem of contraction-glycogenolysis-coupling was reassessed using a quantitatively accurate (1)H MRS method. It is confirmed that glycogenolysis and contractions are closely coupled during repetitive exercise, while glycogenolysis and P-metabolite concentrations are not. A simple metabolic feedback system cannot explain contraction-glycogenolysis-coupling.

Local calcium gradients during excitation-contraction coupling and alternans in atrial myocytes

Subcellular Ca(2+) signalling during normal excitation-contraction (E-C) coupling and during Ca(2+) alternans was studied in atrial myocytes using fast confocal microscopy and measurement of Ca(2+) currents (I(Ca)). Ca(2+) alternans, a beat-to-beat alternation in the amplitude of the [Ca(2+)](i) transient, causes electromechanical alternans, which has been implicated in the generation of cardiac fibrillation and sudden cardiac death. Cat atrial myocytes lack transverse tubules and contain sarcoplasmic reticulum (SR) of the junctional (j-SR) and non-junctional (nj-SR) types, both of which have ryanodine-receptor calcium release channels. During E-C coupling, Ca(2+) entering through voltage-gated membrane Ca(2+) channels (I(Ca)) triggers Ca(2+) release at discrete peripheral j-SR release sites. The discrete Ca(2+) spark-like increases of [Ca(2+)](i) then fuse into a peripheral 'ring' of elevated [Ca(2+)](i), followed by propagation (via calcium-induced Ca(2+) release, CICR) to the cell centre, resulting in contraction. Interrupting I(Ca) instantaneously terminates j-SR Ca(2+) release, whereas nj-SR Ca(2+) release continues. Increasing the stimulation frequency or inhibition of glycolysis elicits Ca(2+) alternans. The spatiotemporal [Ca(2+)](i) pattern during alternans shows marked subcellular heterogeneities including longitudinal and transverse gradients of [Ca(2+)](i) and neighbouring subcellular regions alternating out of phase. Moreover, focal inhibition of glycolysis causes spatially restricted Ca(2+) alternans, further emphasising the local character of this phenomenon. When two adjacent regions within a myocyte alternate out of phase, delayed propagating Ca(2+) waves develop at their border. In conclusion, the results demonstrate that (1) during normal E-C coupling the atrial [Ca(2+)](i) transient is the result of the spatiotemporal summation of Ca(2+) release from individual release sites of the peripheral j-SR and the central nj-SR, activated in a centripetal fashion by CICR via I(Ca) and Ca(2+) release from j-SR, respectively, (2) Ca(2+) alternans is caused by subcellular alterations of SR Ca(2+) release mediated, at least in part, by local inhibition of energy metabolism, and (3) the generation of arrhythmogenic Ca(2+) waves resulting from heterogeneities in subcellular Ca(2+) alternans may constitute a novel mechanism for the development of cardiac dysrhythmias.

Subcellular Ca2+ alternans represents a novel mechanism for the generation of arrhythmogenic Ca2+ waves in cat atrial myocytes

Ca(2+) alternans is a potentially arrhythmogenic beat-to-beat alternation of the amplitude of the action potential-induced [Ca(2+)](i) transient in cardiac myocytes. Despite its pathophysiological significance the cellular mechanisms underlying Ca(2+) alternans are poorly understood. Recent evidence, however, points to the modulation of Ca(2+)-induced Ca(2+) release (CICR) from the sarcoplasmic reticulum (SR) by localized alterations in energy metabolism as an important determinant of Ca(2+) alternans. We therefore studied the subcellular properties of Ca(2+) alternans in field-stimulated cat atrial myocytes employing fast two-dimensional fluorescence confocal microscopy. Ca(2+) alternans was elicited by an increase in stimulation frequency or by metabolic interventions targeting glycolysis. Marked subcellular variations in the time of onset, the magnitude, and the phase of alternans were observed. Longitudinal and transverse gradients of Ca(2+) alternans were found as well as neighbouring subcellular regions alternating out-of-phase. Moreover, focal inhibition of glycolysis resulted in spatially restricted Ca(2+) alternans. When two adjacent regions within a myocyte alternated out-of-phase, steep [Ca(2+)](i) gradients developed at their border giving rise to delayed propagating Ca(2+) waves. The results demonstrate that Ca(2+) alternans is a subcellular phenomenon caused by modulation of SR Ca(2+) release, which is mediated, at least in part, by local inhibition of energy metabolism. The generation of arrhythmogenic Ca(2+) waves by subcellular variations in the phase of Ca(2+) alternans represents a novel mechanism for the development of atrial disrhythmias.

Brownian dynamics of interactions between aldolase mutants and F-actin

Previous Brownian dynamics (BD) simulations (Ouporov IG, Knull HR and Thomasson KA 1999. Biophys. J. 76: 17-27) of complex formation between rabbit aldolase and F-actin have identified three lysine residues (K288, K293 and K341) on aldolase and acidic residues (DEDE) at the N-terminus of actin as important to binding. BD simulations of computer models of aldolase mutants with any of these lysine residues replaced by alanine show reduced binding energy; the greatest effect of a single substitution is for K341A, and replacement of all three lysines greatly reduces binding. BD simulations of wild-type rabbit aldolase vs altered F-actin show that binding is decreased if any one of the four N-terminal acidic residues is replaced by alanine and binding is greatly reduced if three or more of the N-terminal acidic residues are replaced; none of the four actin residues appear more critical for binding than the others.

Effects of glucose on contractile function, [Ca2+]i, and glycogen in isolated mouse skeletal muscle

Extensor digitorum longus muscles were stimulated to contract to fatigue and allowed to recover for 2 h in the absence or presence of 5.5 or 11 mM extracellular glucose. This was followed by a second fatigue run, which ended when the absolute force was the same as at the end of the first run. During the first fatigue run, the fluorescence ratio for indo 1 increased [reflecting an increase in myoplasmic free Ca2+ concentration ([Ca2+]i)] during the initial tetani, peaking at approximately 115% of the first tetanic value, followed by a continuous decrease to approximately 90% at fatigue. During the first fatigue run, myofibrillar Ca2+ sensitivity was significantly decreased. During the second run, the number of tetani was 57 +/- 6% of initial force in muscles that recovered in the absence of glucose and 110 +/- 6 and 119 +/- 2% of initial force in muscles that recovered in 5.5 and 11 mM glucose, respectively. Fluorescence ratios during the first, peak, and last tetani did not differ significantly between the first and second fatigue runs during any of the three conditions. Glycogen decreased by almost 50% during the first fatigue run and did not change further after recovery in the absence of glucose. After recovery in the presence of 5.5 and 11 mM glucose, glycogen increased 32 and 42% above the nonstimulated control value (P < 0.01). These data demonstrate that extracellular glucose delays the decrease of tetanic force and [Ca2+]i during fatiguing stimulation and that glycogen supercompensation following contraction can occur in the absence of insulin.