Effects of beta-guanidinopropionic acid-feeding on the patterns of myosin isoforms in rat fast-twitch muscle

β-guanidinopropionic acid and metformin differentially impact autophagy, mitochondria and cellular morphology in developing C2C12 muscle cells

The serine/threonine kinase AMP-activated protein kinase (AMPK) is a drug target for the treatment of obesity and type 2 diabetes (T2D). Metformin, a widely prescribed anti-hyperglycemic agent, and β-guanidinopropionic acid (β-GPA), a dietary supplement and creatine analog, have been shown to increase activity of AMPK. Macroautophagy is an intracellular degradation pathway for aggregated proteins and dysfunctional organelles, which can be mediated by AMPK. The present study sought to elucidate how metformin and β-GPA affect cell morphology, AMPK activity, autophagy and mitochondrial morphology and function in developing C2C12 myotubes. β-GPA reduced myotube diameter and increased length throughout differentiation, while metformin increased myotube diameter only at the 48 h time point. β-GPA treatment enhanced AMPK signaling and expression of autophagy-related proteins. β-GPA treatment also increased the density of autophagosomes, autolysosomes, and lysosomes. Metformin also increased activation of AMPK after 48 h, but in contrast to β-GPA, led to a dramatic reduction in the density of autophagosomes and lysosomes. Both metformin and β-GPA reduced the mitochondrial oxygen consumption rate, and differentially altered mitochondrial morphology. Obesity and T2D have been shown to increase mitochondrial dysfunction and reduce autophagic flux in skeletal muscle cells. Therefore, β-GPA may help to alleviate the effects of metabolic disease by increasing autophagic flux in skeletal muscle cells. In contrast, the reduction of autophagy by metformin may lead to dysregulation of mitochondrial maintenance, as well as muscle development.

The Contribution of Neuromuscular Stimulation in Elucidating Muscle Plasticity Revisited

Studies carried out during the past 45 years on the effects of chronic low-frequency stimulation on skeletal muscle have revealed a multiplicity of adaptive changes of muscle fibres in response to increased activity. As reflected by induced changes in the metabolic properties, protein profiles of the contractile machinery and elements of the Ca²⁺-regulatory system, all essential components of the muscle fibre undergo pronounced changes in their properties that ultimately lead to their reversible transformation from fast-to-slow phenotype. The chronic low-frequency stimulation experiment thus allows exploring many aspects of the plasticity of mammalian skeletal muscle. Moreover it offers the possibility of elucidating molecular mechanisms that remodel phenotypic properties of a differentiated post-mitotic cell during adaptation to altered functional demands. The understanding of the adaptive potential of muscle can be taken advantage of for repairing muscle damage in various muscle diseases. In addition it can be used to prevent muscle wasting during inactivity and aging. Indeed, pioneering studies are still the sound grounds for the many current applications of Functional Electrical Stimulation and for the related research activities that are still proposed and funded.

Creatine loading elevates the intracellular phosphorylation potential and alters adaptive responses of rat fast-twitch muscle to chronic low-frequency stimulation

This study tested the hypothesis that elevating the intracellular phosphorylation potential (IPP = [ATP]/[ADP]free) within rat fast-twitch tibialis anterior muscles by creatine (Cr) loading would prevent fast-to-slow fibre transitions induced by chronic low-frequency electrical stimulation (CLFS, 10 Hz, 12 h/day). Creatine-control and creatine-CLFS groups drank a solution of 1% Cr + 5% dextrose, ad libitum, for 10 days before and during 10 days of CLFS; dextrose-control and dextrose-CLFS groups drank 5% dextrose. Cr loading increased total Cr (P < 0.025), phosphocreatine (PCr) (P < 0.003), and the IPP (P < 0.0008) by 34%, 45%, and 64%, respectively. PCr and IPP were 46% (P < 0.002) and 76% (P < 0.02) greater in creatine-CLFS than in dextrose-CLFS. Higher IPP was confirmed by a 58% reduction in phospho-AMP-activated protein kinase α (Thr172) (P < 0.006). In dextrose-CLFS, myosin heavy chain (MyHC) I and IIa transcripts increased 32- and 38-fold (P < 0.006), respectively, whereas MyHC-IIb mRNA decreased by 75% (P < 0.03); the corresponding MyHC-I and MyHC-IIa protein contents increased by 2.0- (P < 0.03) and 2.7-fold (P < 0.05), respectively, and MyHC-IIb decreased by 30% (P < 0.03). In contrast, within creatine-CLFS, MyHC-I and MyHC-IIa mRNA were unchanged and MyHC-IIb mRNA decreased by 75% (P < 0.003); the corresponding MyHC isoform contents were not altered. Oxidative reference enzymes were similarly elevated (P < 0.01) in dextrose-CLFS and creatine-CLFS, but reciprocal reductions in glycolytic reference enzymes occurred only in dextrose-CLFS (P < 0.02). Preservation of the glycolytic potential and greater SERCA2 and parvalbumin contents in creatine-CLFS coincided with prolonged time to peak tension and half-rise time (P < 0.01). These results highlight the IPP as an important physiological regulator of muscle fibre plasticity and demonstrate that training-induced changes typically associated with improvements in muscular endurance or increased power output are not mutually exclusive in Cr-loaded muscles.

Effects of gravitational loading levels on protein expression related to metabolic and/or morphologic properties of mouse neck muscles

The effects of 3 months of spaceflight (SF), hindlimb suspension, or exposure to 2G on the characteristics of neck muscle in mice were studied. Three 8‐week‐old male C57BL/10J wild‐type mice were exposed to microgravity on the International Space Station in mouse drawer system (MDS) project, although only one mouse returned to the Earth alive. Housing of mice in a small MDS cage (11.6 × 9.8‐cm and 8.4‐cm height) and/or in a regular vivarium cage was also performed as the ground controls. Furthermore, ground‐based hindlimb suspension and 2G exposure by using animal centrifuge (n = 5 each group) were performed. SF‐related shift of fiber phenotype from type I to II and atrophy of type I fibers were noted. Shift of fiber phenotype was related to downregulation of mitochondrial proteins and upregulation of glycolytic proteins, suggesting a shift from oxidative to glycolytic metabolism. The responses of proteins related to calcium handling, myofibrillar structure, and heat stress were also closely related to the shift of muscular properties toward fast‐twitch type. Surprisingly, responses of proteins to 2G exposure and hindlimb suspension were similar to SF, although the shift of fiber types and atrophy were not statistically significant. These phenomena may be related to the behavior of mice that the relaxed posture without lifting their head up was maintained after about 2 weeks. It was suggested that inhibition of normal muscular activities associated with gravitational unloading causes significant changes in the protein expression related to metabolic and/or morphological properties in mouse neck muscle.

Inhibition of gravitational loading in space and on the Earth for 3 months caused similar responses of protein expression in mouse neck muscle. Downregulation of mitochondrial proteins and upregulation of glycolytic proteins were induced, suggesting a shift from oxidative to glycolytic metabolism. Furthermore, the responses of proteins, involved in calcium handling, myofibrillar structure, and heat stress, related to the shift of muscular properties toward fast‐twitch type were also noted. It was suggested that inhibition of normal muscular activities associated with gravitational unloading caused significant changes in the protein expression related to metabolic and/or morphological properties in mouse neck muscle.

The therapeutic potential of skeletal muscle plasticity in Duchenne muscular dystrophy: phenotypic modifiers as pharmacologic targets

Duchenne muscular dystrophy (DMD) is a life-limiting, neuromuscular disorder that causes progressive, severe muscle wasting in boys and young men. Although there is no cure, scientists and clinicians can leverage the fact that slower, more oxidative skeletal muscle fibers possess an enhanced degree of resistance to the dystrophic pathology relative to their faster, more glycolytic counterparts, and can thus use this knowledge when investigating novel therapeutic avenues. Several factors have been identified as powerful regulators of muscle plasticity. Some proteins, such as calcineurin, peroxisome proliferator-activated receptor (PPAR) γ coactivator 1α (PGC-1α), PPARβ/δ, and AMP-activated protein kinase (AMPK), when chronically stimulated in animal models, remodel skeletal muscle toward the slow, oxidative myogenic program, whereas others, such as receptor-interacting protein 140 (RIP140) and E2F transcription factor 1 (E2F1), repress this phenotype. Recent studies demonstrating that pharmacologic and physiological activation of targets that shift dystrophic muscle toward the slow, oxidative myogenic program provide appreciable molecular and functional benefits. This review surveys the rationale behind, and evidence for, the study of skeletal muscle plasticity in preclinical models of DMD and highlights the potential therapeutic opportunities in advancing a strategy focused on remodeling skeletal muscle in patients with DMD toward the slow, oxidative phenotype.

AMP-activated protein kinase at the nexus of therapeutic skeletal muscle plasticity in Duchenne muscular dystrophy

Recent studies have highlighted the potential of adenosine monophosphate-activated protein kinase (AMPK) to act as a central therapeutic target in Duchenne muscular dystrophy (DMD). Here, we review the role of AMPK as an important integrator of cell signaling pathways that mediate phenotypic plasticity within the context of dystrophic skeletal muscle. Pharmacological AMPK activation remodels skeletal muscle towards a slower, more oxidative phenotype, which is more pathologically resistant to the lack of dystrophin. Moreover, recent studies suggest that AMPK-activated autophagy may be beneficial for myofiber structure and function in mice with muscular dystrophy. Thus, AMPK may represent an ideal target for intervention because clinically approved pharmacological agonists exist, and because benefits can be derived via two independent yet, complementary biological pathways. The availability of several AMPK activators could therefore lead to the rapid development and implementation of novel and highly effective therapeutics aimed at altering the relentless progression of DMD.

The effect of the creatine analogue beta-guanidinopropionic acid on energy metabolism: a systematic review

Background

Creatine kinase plays a key role in cellular energy transport. The enzyme transfers high-energy phosphoryl groups from mitochondria to subcellular sites of ATP hydrolysis, where it buffers ADP concentration by catalyzing the reversible transfer of the high-energy phosphate moiety (P) between creatine and ADP. Cellular creatine uptake is competitively inhibited by beta-guanidinopropionic acid. This substance is marked as safe for human use, but the effects are unclear. Therefore, we systematically reviewed the effect of beta-guanidinopropionic acid on energy metabolism and function of tissues with high energy demands.

Methods

We performed a systematic review and searched the electronic databases Pubmed, EMBASE, the Cochrane Library, and LILACS from their inception through March 2011. Furthermore, we searched the internet and explored references from textbooks and reviews.

Results

After applying the inclusion criteria, we retrieved 131 publications, mainly considering the effect of chronic oral administration of beta-guanidinopropionic acid (0.5 to 3.5%) on skeletal muscle, the cardiovascular system, and brain tissue in animals. Beta-guanidinopropionic acid decreased intracellular creatine and phosphocreatine in all tissues studied. In skeletal muscle, this effect induced a shift from glycolytic to oxidative metabolism, increased cellular glucose uptake and increased fatigue tolerance. In heart tissue this shift to mitochondrial metabolism was less pronounced. Myocardial contractility was modestly reduced, including a decreased ventricular developed pressure, albeit with unchanged cardiac output. In brain tissue adaptations in energy metabolism resulted in enhanced ATP stability and survival during hypoxia.

Conclusion

Chronic beta-guanidinopropionic acid increases fatigue tolerance of skeletal muscle and survival during ischaemia in animal studies, with modestly reduced myocardial contractility. Because it is marked as safe for human use, there is a need for human data.

Chronic AMPK stimulation attenuates adaptive signaling in dystrophic skeletal muscle

In the present study, we evaluated how a pharmacologically induced phenotype shift in dystrophic skeletal muscle would affect subsequent intracellular signaling in response to a complementary, adaptive physiological stimulus. mdx mice were treated with the AMP-activated protein kinase (AMPK) activator 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR; 500 mg·kg(-1)·day(-1)) for 30 days, and then one-half of the animals were subjected to a bout of treadmill running to induce acute AMPK and p38 MAPK signaling. The mRNA levels of phenotypic modifiers, including peroxisome proliferator-activated receptor-δ (PPARδ), PPARγ coactivator-1α (PGC-1α), receptor interacting protein 140 (RIP 140), and silent information regulator two ortholog 1 (SIRT1) were assessed in skeletal muscle, as well as the expression of the protein arginine methyltransferase genes PRMT1 and CARM1. We found unique AMPK and p38 phosphorylation and expression signatures between dystrophic and healthy muscle. In dystrophic skeletal muscle, treadmill running induced PPARδ, PGC-1α, and SIRT1 mRNAs, three molecules that promote the slow, oxidative myogenic program. In the mdx animals that received the chronic AICAR treatment, running-elicited AMPK and p38 phosphorylation was attenuated compared with vehicle-treated mice. Similarly, acute stress-evoked expression of PPARδ, PGC-1α, and SIRT1 was also blunted by chronic pharmacological AMPK stimulation. Skeletal muscle PRMT1 and CARM1 protein contents were higher in mdx mice compared with wild-type littermates. The acute running-evoked induction of PRMT1 and CARM1 mRNAs was also attenuated by the AICAR treatment. Our data demonstrate that prior pharmacological conditioning is a salient determinant in how dystrophic muscle adapts to subsequent complementary, acute physiological stress stimuli. These results provide insight into possible therapeutic applications of synthetic agonists in neuromuscular diseases, such as during chronic administration to Duchenne muscular dystrophy patients.

Chronic AMPK activation evokes the slow, oxidative myogenic program and triggers beneficial adaptations in mdx mouse skeletal muscle

A therapeutic approach for Duchenne muscular dystrophy (DMD) is to up-regulate utrophin in skeletal muscle in an effort to compensate for the lack of dystrophin. We previously hypothesized that promotion of the slow, oxidative myogenic program, which triggers utrophin up-regulation, can attenuate the dystrophic pathology in mdx animals. Since treatment of healthy mice with the AMP-activated protein kinase (AMPK) activator 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) enhances oxidative capacity and elicits a fast-to-slow fiber-type transition, we evaluated the effects of chronic AMPK stimulation on skeletal muscle phenotype and utrophin expression in mdx mice. Daily AICAR administration (500 mg/kg/day, 30 days) of 5-7-week-old mdx animals induced an elevation in mitochondrial cytochrome c oxidase enzyme activity, an increase in myosin heavy-chain type IIa-positive fibers and slower twitch contraction kinetics in the fast, glycolytic extensor digitorum longus muscle. Utrophin expression was significantly enhanced in response to AICAR, which occurred coincident with an elevated β-dystroglycan expression along the sarcolemma. These adaptations were associated with an increase in sarcolemmal structural integrity under basal conditions, as well as during damaging eccentric contractions ex vivo. Notably, peroxisome proliferator-activated receptor γ co-activator-1α (PGC-1α) and silent information regulator two ortholog 1 protein contents were significantly higher in muscle from mdx mice compared with wild-type littermates and AICAR further increased PGC-1α expression. Our data show that AICAR-evoked muscle plasticity results in beneficial phenotypic adaptations in mdx mice and suggest that the contextually novel application of this compound for muscular dystrophy warrants further study.

Effects of creatine supplementation during resistance training on myosin heavy chain (MHC) expression in rat skeletal muscle fibers

The purpose of this study was to utilize a rodent model to test the hypothesis that creatine (Cr) supplementation during resistance training would influence the pattern of slow-twitch muscle myosin heavy chain (MHC) isoforms expression. Male Wistar rats (2-3 months old, 250-300 g) were divided into 4 groups: Nontrained without creatine supplementation (CO), nontrained with creatine supplementation (CR), trained without creatine supplementation (TR), and trained with creatine supplementation (TRCR). TR and TRCR groups were submitted to a resistance training program for 5 weeks (5 days/week) for morphological and biochemical analysis of the soleus muscle. Weightlifting exercise involved jump sessions into water, carrying progressive overload equivalent to percentage of body weight. CR and TRCR groups were given creatine at 0.5 g/kg(-1)/d(-1). Both Cr supplementation and resistance training alone or associated did not result in significant alterations (p > 0.05) in body weight gain, food intake, and muscle weight in the CR, TR and TRCR groups compared to the CO group. Also compared to the CO group, the CR group showed a significant (p < 0.02) increase in MHCI content and a reduction in MHCII; inversely, the TR group increased the MHCII content and reduced MHCI (p < 0.02). When combined, both creatine and resistance training did not promote significant (p > 0.05) changes in MHC content of the TRCR group compared to the CO group. The data show that Cr supplementation provides a potential action to abolish the exercise-induced MHC isoform transitions from slow to fast in slow-twitch muscle. Thus, Cr supplementation might be a suitable strategy to maintaining a slow phenotype in slow muscle during resistance training, which may be favorable to maintenance of muscle oxidative capacity of endurance athletes.

Adaptive responses to creatine loading and exercise in fast-twitch rat skeletal muscle

We investigated the effects of chronic creatine loading and voluntary running (Run) on muscle fiber types, proteins that regulate intracellular Ca2+, and the metabolic profile in rat plantaris muscle to ascertain the bases for our previous observations that creatine loading results in a higher proportion of myosin heavy chain (MHC) IIb, without corresponding changes in contractile properties. Forty Sprague-Dawley rats were assigned to one of four groups: creatine-fed sedentary, creatine-fed run-trained, control-fed sedentary, and control-fed run-trained animals. Proportion and cross-sectional area increased 10% and 15% in type IIb fibers and the proportion of type IIa fibers decreased 11% in the creatine-fed run-trained compared with the control-fed run-trained group (P < 0.03). No differences were observed in fast Ca2+-ATPase isoform SERCA1 content (P > 0.49). Creatine feeding alone induced a 41% increase (P < 0.03) in slow Ca2+-ATPase (SERCA2) content, which was further elevated by 33% with running (P < 0.02). Run training alone reduced parvalbumin content by 50% (P < 0.05). By comparison, parvalbumin content was dramatically decreased by 75% (P < 0.01) by creatine feeding alone but was not further reduced by run training. These adaptive changes indicate that elevating the capacity for high-energy phosphate shuttling, through creatine loading, alleviates the need for intracellular Ca2+ buffering by parvalbumin and increases the efficiency of Ca2+ uptake by SERCAs. Citrate synthase and 3-hydroxyacyl-CoA dehydrogenase activities were elevated by run training (P < 0.003) but not by run training + creatine feeding. This indicates that creatine loading during run training supports a faster muscle phenotype that is adequately supported by the existing glycolytic potential, without changes in the capacity for terminal substrate oxidation.

Effects of long-term creatine feeding and running on isometric functional measures and myosin heavy chain content of rat skeletal muscles

The purpose of this study was to investigate whether creatine (Cr) supplementation during 12 weeks of phasic high-frequency voluntary wheel running would result in a faster myosin heavy chain (MHC) isoform profile in the rat mixed fast-twitch plantaris and alter its corresponding isometric contractile properties. The fast-twitch extensor digitorum longus and medial gastrocnemius and slow-twitch soleus were also studied. Forty weanling Sprague-Dawley male rats were assigned to one of four groups: creatine-sedentary (Cre-Sed); creatine-voluntary running (Cre-Run); control-sedentary (Con-Sed); control-voluntary running (Con-Run). Daily running distance was similar between Cre-Run and Con-Run. Average daily Cr ingestion was also similar being 2.4+/-0.17 and 3.0+/-0.14 g/kg in Cre-Sed and Cre-Run, respectively. Total creatine (TCr) content was elevated (P<0.03) in the plantaris of Cre-Run [211.4+/-16.9 mmol/kg dry weight (dw)], compared with Con-Run (175.1+/-5.69). In the plantaris, MHCIIb was 13% greater (P<0.00001) in Cre-Run compared with Con-Run, while MHCIId/x and MHCIIa were lower in Cre-Run by 7 and 6% (P<0.0002), respectively. No differences were observed in twitch force, time-to-peak tension, half-rise time or half-fall time. Greater tetanic force production (P<0.05) in Cre-Sed compared with Con-Sed corresponded to a 12% increase in MHCIId/x (P<0.0001) and a 12% decrease in MHCIIb (P<0.0006). The fatigue index of the plantaris at 10 s (FI(10s)) was reduced only after running (Cre-Run vs Con-Run), while in all other muscles the FI(10s) was lower only in the Cre-Sed group. In conclusion, Cr supplementation had differential effects on MHC isoform content and fatigability that depended on the level of contractile activity. Cr feeding combined with running exercise resulted in a faster MHC-based phenotype in the rat plantaris but the impact on associated isometric contractile properties was minimal.

Increased resistance to fatigue in creatine kinase deficient muscle is not due to improved contractile economy

There has been speculation on the origin of the increased endurance of skeletal muscles in creatine kinase (CK)-deficient mice. Important factors that have been raised include the documented increased mitochondrial capacity and alterations in myosin heavy chain (MyHC) isoform composition in CK-deficient muscle. More recently, the absence of inorganic phosphate release from phosphocreatine hydrolysis in exercising CK-deficient muscle has been postulated to contribute to the lower fatigueability in skeletal muscle. In this study, we tested the hypothesis that the reported shift in MyHC composition to slower isoforms in CK-deficient muscle leads to a decrease in oxygen cost of twitch performance. To that aim, extensor digitorum longus (EDL) and soleus (SOL) muscles were isolated from wild-type (WT) and knock-out mice deficient in the cytoplasmic muscle-type and sarcomeric mitochondrial isoenzymes of CK, and oxygen consumption per twitch time-tension-integral (TTI) was measured. The results show that the adaptive response to loss of CK function does not involve any major change to contractile economy of skeletal muscle.

Endurance training-induced changes in alkali light chain patterns in type IIB fibers of the rat

The effects of endurance training on the expression of myosin were electrophoretically analyzed in the deep portion of vastus lateralis muscle from the rat. A 10-wk running program led to increases (P < 0.01) in myosin heavy chain (MHC) 2a and 2d with a decrease (P < 0.01) in MHC(2b). Training also evoked a rearrangement of the isomyosin pattern with decreases in fast isomyosin (FM) 1 (P < 0.01) and FM2 (P < 0.05) and a rise in intermediate isomyosin (P < 0.01). These changes were accompanied by a 61% decrease (P < 0.01) in myosin light chain (MLC) 3F (11.8 +/- 2.7 vs. 4.6 +/- 4.2%). Two-dimensional electrophoresis made it possible to separate the triplet of isomyosins (FMb) consisting of MHC(2b). Training elicited a 26% decrease (P < 0.05) in the FM1b fraction within FMb, i.e., FM1b/(FM1b + FM2b + FM3b) (24.2 +/- 5.5 vs. 18.0 +/- 4.3%). These changes resulted in a 10% decrease (P < 0.05) in the MLC(3F) fraction, i.e., MLC(3F)/(MLC(1F) + MLC(3F)), in FMb (44.9 +/- 4.5 vs. 40.3 +/- 3.2%). These results suggest that endurance training may exert the depressive effect on the contractile velocity of type IIB fibers and that a training-induced decrease in the contractile velocity of whole muscle may be caused by alterations in fast alkali MLC complements within a given fiber type as well as by transitions in MHC-based fiber populations.

Effect of creatine manipulation on fast-twitch skeletal muscle of the mouse

1. The effect of short-term, reversible alteration of muscle total creatine content (Crtot) on force development was sought in fast-twitch extensor digitorum longus (EDL) muscles of female mice. 2. Three in vivo interventions were investigated: 1% creatine-supplementation, treatment with the creatine-uptake inhibitor beta-guanidino propionic acid (beta-GPA; 1%) or beta-GPA treatment followed by creatine supplementation for 5 days. 3. The Crtot of isolated muscles, determined using reverse-phase high-performance liquid chromatography, was 133 +/- 38 mmol/kg dry in 11 EDL control muscles and was not significantly affected by dietary creatine-supplementation (152 +/- 25 mmol/kg dry; n = 8). Significant creatine depletion was observed in the beta-GPA-fed group (65 +/- 6 mmol/kg dry; n = 9) and this was reversed by 5 days of creatine supplementation (133 +/- 21 mmol/kg dry; n = 10). 4. Creatine depletion did not affect maximum tetanic stress. However, when muscle creatine was restored by creatine supplementation, a substantial increase in tetanic stress was observed. Significant correlations were observed between Crtot and maximum tetanic stress (r = 0.56) and relaxation rate (r = 0.43). The enhancement of force was not due to effects of creatine on muscle fibre type because neither mechanical tests of fibre characteristics nor the fibre types of the muscles were affected. 5. We conclude that, in muscles that contain large numbers of fast-twitch fibres, maximum tetanic stress is determined, in part, by muscle creatine stores.

High level of uncoupling protein 1 expression in muscle of transgenic mice selectively affects muscles at rest and decreases their IIb fiber content

The mitochondrial uncoupling protein of brown adipose tissue (UCP1) was expressed in skeletal muscle and heart of transgenic mice at levels comparable with the amount found in brown adipose tissue mitochondria. These transgenic mice have a lower body weight, and when related to body weight, food intake and energy expenditure are increased. A specific reduction of muscle mass was observed but varied according to the contractile activity of muscles. Heart and soleus muscle are unaffected, indicating that muscles undergoing regular contractions, and therefore with a continuous mitochondrial ATP production, are protected. In contrast, the gastrocnemius and plantaris muscles showed a severely reduced mass and a fast to slow shift in fiber types promoting mainly IIa and IIx fibers at the expense of fastest and glycolytic type IIb fibers. These observations are interpreted as a consequence of the strong potential dependence of the UCP1 protonophoric activity, which ensures a negligible proton leak at the membrane potential observed when mitochondrial ATP production is intense. Therefore UCP1 is not deleterious for an intense mitochondrial ATP production and this explains the tolerance of the heart to a high expression level of UCP1. In muscles at rest, where ATP production is low, the rise in membrane potential enhances UCP1 activity. The proton return through UCP1 mimics the effect of a sustained ATP production, permanently lowering mitochondrial membrane potential. This very likely constitutes the origin of the signal leading to the transition in fiber types at rest.

Adaptations in skeletal muscle disuse or decreased-use atrophy

Those factors that seem to play some role in inducing adaptations of skeletal muscle in vivo are discussed. The role of myogenesis in maintaining and repairing muscle during atrophic and hypertrophic states is discussed, including pointing out that the modulation of myonuclear number is one means of adapting to varying chronic levels of neuromuscular activity. Finally, we point out the potential consequences of muscle atrophy on the control of movement and the susceptibility to fatigue.

Activity-independent neural influences on cat soleus motor unit phenotypes

The physiological and phenotypic properties of motor units in the cat soleus muscle were studied after 4 months of inactivity induced by spinal cord isolation (SI). The soleus of some SI cats were stimulated for 30 min/day during an isometric (SI-I), shortening (SI-S), or lengthening (SI-L) phase of a simulated step cycle. Mean maximum tetanic tensions were approximately 15, 26, 32, and 51% of the control in the SI, SI-S, SI-L, and SI-I groups. Mean time-to-peak tension was approximately 50% shorter than the control in all SI groups. One motor unit was glycogen-depleted in each muscle via repetitive stimulation. Eighteen physiologically slow and 9 fast motor units from the spinal cord-isolated groups consisted of fibers that contained only slow myosin heavy chain (MHC) and sarco(endo)plasmic reticulum calcium-adenotriphosphatase (SERCA) isoforms. Two motor units (physiologically fast) consisted primarily of fibers that contained both fast and slow MHC and SERCA. These data reflect a dissociation between isometric speed-related properties and MHC and SERCA isoforms following inactivity. The predominance of fibers containing both fast and slow MHC and SERCA isoforms in 2 motor units demonstrates a strong motoneuronal influence on the muscle-fiber phenotype even when the motoneurons are silent.

Creatine and creatinine metabolism

The goal of this review is to present a comprehensive survey of the many intriguing facets of creatine (Cr) and creatinine metabolism, encompassing the pathways and regulation of Cr biosynthesis and degradation, species and tissue distribution of the enzymes and metabolites involved, and of the inherent implications for physiology and human pathology. Very recently, a series of new discoveries have been made that are bound to have distinguished implications for bioenergetics, physiology, human pathology, and clinical diagnosis and that suggest that deregulation of the creatine kinase (CK) system is associated with a variety of diseases. Disturbances of the CK system have been observed in muscle, brain, cardiac, and renal diseases as well as in cancer. On the other hand, Cr and Cr analogs such as cyclocreatine were found to have antitumor, antiviral, and antidiabetic effects and to protect tissues from hypoxic, ischemic, neurodegenerative, or muscle damage. Oral Cr ingestion is used in sports as an ergogenic aid, and some data suggest that Cr and creatinine may be precursors of food mutagens and uremic toxins. These findings are discussed in depth, the interrelationships are outlined, and all is put into a broader context to provide a more detailed understanding of the biological functions of Cr and of the CK system.

Fibre type-specific gene expression activated by chronic electrical stimulation of adult mouse skeletal muscle fibres in culture

1. Fast-twitch skeletal muscle fibres were enzymatically dissociated from adult mouse flexor digitorum brevis (FDB) muscles and maintained in culture without or with chronic low frequency stimulation (one 5 s train of 5 Hz pulses per minute) for up to 6 days. Single fibre reverse transcriptase-polymerase chain reaction was conducted to coamplify beta-myosin heavy chain (beta-MHC) and alpha-skeletal actin mRNA from the same fibre. 2. Chronic low frequency electrical stimulation of FDB fibres in culture increased the level of mRNA for beta-MHC. In unstimulated fibres there was a slight decline in the beta-MHC mRNA level. As an internal control there was no increase in the level of mRNA for alpha-actin in the identical individual stimulated or unstimulated fibres. 3. Neither the percentage of fibres exhibiting beta-MHC protein nor the Ca2+ transients recorded from individual fibres subjected to the same pattern of stimulation showed any difference between stimulated and unstimulated fibres over the period in culture. 4. This system provides a convenient in vitro model system for studying activity-dependent control of fibre type-specific gene expression in adult skeletal muscle fibres in culture.

Crystalline mitochondrial inclusion bodies isolated from creatine depleted rat soleus muscle

Rats were fed a 2% guanidino propionic acid diet for up to 18 weeks to induce cellular creatine depletion by inhibition of creatine uptake by this creatine analogue. Ultrastructural analysis of creatine depleted tissues showed that mitochondrial intermembrane inclusion bodies appeared in all skeletal muscles analysed, after 11 weeks of feeding. Heart had relatively few even after 18 weeks of analogue feeding and none were evident in kidney, brain or liver. These structures were strongly immuno-positive for sarcomeric mitochondrial creatine kinase and upon removal from mitochondria, the inclusion bodies were shown to diffract to a resolution of 2.5 nm. Two-dimensional image analysis and three-dimensional reconstruction revealed arrays of creatine kinase octamers with additional components between the octameric structures. The same mitochondria had a 3-fold higher extractable specific creatine kinase activity than controls. Molecular mass gel filtration of inclusion body containing mitochondrial extracts from analogue fed rat solei revealed mitochondrial creatine kinase eluting as an aggregate of an apparent molecular mass &gt; or = 2,000 kDa. Mitochondrial creatine kinase of control soleus mitochondrial extract eluted as an octamer, with a molecular mass of 340 kDa. Respiration measurements of control solei mitochondria displayed creatine mediated stimulation of oxidative phosphorylation that was absent in analogue-fed rat solei mitochondria. The latter also had 19% and 14% slower rates of state 4 and maximal state 3 respiration, respectively, than control mitochondria. These results indicate that mitochondrial creatine kinase co-crystallises with another component within the inter membrane space of select mitochondria in creatine depleted skeletal muscle, and is inactive in situ.

Improved fatigue resistance not associated with maximum oxygen consumption in creatine-depleted rats

Effects of feeding of either creatine or its analog beta-guanidinopropionic acid (beta-GPA) on endurance work capacity and oxygen consumption were studied in rats. Resting high-energy phosphate contents in hindlimb muscles were lower in the beta-GPA group and higher in the creatine group than in controls. The glycogen contents in resting hindlimb muscles of rats fed beta-GPA were significantly higher than those in controls. The endurance run and swimming times to exhaustion were significantly greater (32-70%) in the beta-GPA group than in the control and creatine groups. However, there were no beneficial effects on the maximum oxygen consumption (VO2max) and oxygen transport capacity of blood by the feeding of beta-GPA. None of these parameters were significantly influenced by creatine supply. Both maximum exercise time and VO2max in the beta-GPA group were not changed by normalization of glycogen levels. The activities of mitochondrial enzymes in skeletal muscles were higher in the beta-GPA group than in the controls. Thus endurance capacity is improved if the respiratory capacity of muscles is increased, even when the contents of high-energy phosphates in muscles are lower. Increased endurance capacity was not directly associated with the elevated levels of muscle glycogen, oxygen transport capacity of blood, or VO2max.

Mammalian skeletal muscle fiber type transitions

Mammalian skeletal muscle is an extremely heterogeneous tissue, composed of a large variety of fiber types. These fibers, however, are not fixed units but represent highly versatile entities capable of responding to altered functional demands and a variety of signals by changing their phenotypic profiles. This adaptive responsiveness is the basis of fiber type transitions. The fiber population of a given muscle is in a dynamic state, constantly adjusting to the current conditions. The full range of adaptive ability spans fast to slow characteristics. However, it is now clear that fiber type transitions do not proceed in immediate jumps from one extreme to the other, but occur in a graded and orderly sequential manner. At the molecular level, the best examples of these stepwise transitions are myofibrillar protein isoform exchanges. For the myosin heavy chain, this entails a sequence going from the fastest (MHCIIb) to the slowest (MHCI) isoform, and vice-versa. Depending on the basal protein isoform profile and hence the position within the fast-slow spectrum, the adaptive ranges of different fibers vary. A simple transition scheme has emerged from the multitude of data collected on fiber type conversions under a variety of conditions.