Creatine uptake in isolated soleus muscle: kinetics and dependence on sodium, but not on insulin

Short-term creatine supplementation changes protein metabolism signaling in hindlimb suspension

The effect of a short-term creatine supplementation on hindlimb suspension (HS)-induced muscle atrophy was investigated. Creatine monohydrate (5 g/kg b.w. per day) or placebo, divided in 2 daily doses, was given by oral gavage for 5 days. Rats were maintained in HS with dietary supplementation concomitantly for 5 days. Body weight, soleus and EDL muscle masses, and cross-sectional areas (CSA) of the muscle fibers were measured. Signaling pathways associated with skeletal muscle mass regulation (FST, MSTN, FAK, IGF-1, MGF, Akt, mTOR, atrogin-1, and MuRF1 expressions, and Akt, S6, GSK3B, and 4EBP1 proteins) were evaluated in the muscles. Soleus muscle exhibited more atrophy than the EDL muscle due to HS. Creatine supplementation attenuated the decrease of wet weight and increased p-4EBP1 protein in the EDL muscle of HS rats. Also, creatine increased mTOR and atrogin-1 expressions in the same muscle and condition. In the absence of HS, creatine supplementation increased FAK and decreased MGF expressions in the EDL muscle. Creatine attenuated the increase in FST expression due to HS in the soleus muscle. MuRF1 expression increased in the soleus muscle due to creatine supplementation in HS animals whereas atrogin-1 expression increased still further in this group compared with untreated HS rats. In conclusion, short-term creatine supplementation changed protein metabolism signaling in soleus and EDL muscles. However, creatine supplementation only slightly attenuated the mass loss of both muscles and did not prevent the CSA reduction and muscle strength decrease induced by HS for 5 days.

Use and Effectiveness of Performance-Enhancing Substances

The use and effects of selected performance-enhancing drugs and nutritional supplements are reviewed. Recent sports medicine studies are mostly double blind and placebo controlled but contain relatively small sample sizes. Their data appear reliable and are reported in reputable journals. Definitions and methods used in sports medicine are provided to enhance the understanding of this literature. The use of performance-enhancing substances is probably under-reported. Anabolic-androgenic steroids are reportedly used in 0% to 1% of women, 0.5% to 3% of high school girls, 1% to 5% of men, 1% to 12% of high school boys, and up to 67% of some groups of elite athletes. The use of combinations of performance-enhancing substances is common. Carbohydrate loading, adequate protein intake, creatine, blood doping, and erythropoietin (epoetin alfa) appear to enhance performance. Anabolic-androgenic steroids enhance performance, but health risks limit their use. Growth hormones and β2 -selective adrenergic agonists may enhance performance, but additional studies are needed. Androstenedione, caffeine, amphetamines, and nonprescription sympathomimetics do not appear to enhance performance. Performance-enhancing drugs have shown some benefit in diseased patients with malnutrition and/or decreases in physical ability. Pharmacists and other health care providers have opportunities to improve the understanding, use, and monitoring of performance-enhancing substances.

Fenugreek increases insulin-stimulated creatine content in L6C11 muscle myotubes

PURPOSE

Creatine uptake by muscle cells is increased in the presence of insulin. Accordingly, compounds with insulin-like actions may also augment creatine uptake. The aim of this study was to investigate whether Trigonella foenum-graecum (fenugreek), an insulin mimetic, increases total intracellular creatine levels in vitro.

METHODS

Total cellular creatine content was measured fluorometrically in L6C11 muscle myotubes treated for 1, 4, and 24 h with 0.5 mM creatine (CR), CR and 20 μg/mL fenugreek seed extract (CR + FEN), CR and 100 nM insulin (CR + INS), and CR + INS + FEN (n = 6 per treatment group). Alterations in the expression of the sodium- and chloride-dependent creatine transporter, SLC6A8, and key signaling proteins in the PI3-K/Akt pathway were determined.

RESULTS

Compared to control (CON), CR + INS + FEN increased total creatine content after 4 h (P < 0.05), whereas all conditions increased SLC6A8 protein expression above CON at this time (P < 0.05). Changes in insulin signaling were demonstrated via increases in Akt^Thr308 phosphorylation, with CR + INS > CON and CR at 1 h (P < 0.05) and with CR + INS + FEN > CON, CR, and CR + INS at 4 h (P < 0.05). In contrast, no changes in PKCζ/λ or GLUT4 phosphorylation were detected.

CONCLUSION

Fenugreek, when combined with insulin, modulates creatine content via a mechanism which is independent of the activity of SLC6A8, suggesting that an alternative mechanism is responsible for the regulation and facilitation of insulin-mediated creatine uptake in skeletal muscle cells.

Creatine does not promote hypertrophy in skeletal muscle in supplemented compared with nonsupplemented rats subjected to a similar workload

The purpose of this study was to test the hypothesis that creatine (Cr) supplementation may promote an additional hypertrophic effect on skeletal muscle independent of a higher workload on Cr-supplemented trained muscle compared with Cr-nonsupplemented trained muscle. Male Wistar rats (2-3 months old, 250-300 g) were divided randomly into 4 groups (n = 8 per group): nontrained without Cr supplementation (CO), nontrained with Cr supplementation (CR), trained without Cr supplementation (TR), and trained with Cr supplementation (TRCR). Creatine supplementation was given at 0.5 g/kg per day. Trained groups were submitted to a 5-week resistance training program (5 d/wk). The progressive workloads were similar between the Cr-supplemented (TRCR) and Cr-nonsupplemented (TR) trained groups; the only difference between groups was the Cr treatment. After the 5-week experiment, the soleus muscle was dissected to analyze the cross-sectional area (CSA) of the muscle fibers. Resistance training promoted a significant (P < .05) increase in the muscle fibers CSA in the TR group compared with the CO group. However, no additional hypertrophic effect was found when Cr supplementation was added to training (TRCR vs TR comparison, P > .05). In addition, Cr supplementation alone did not promote significant alterations in muscle fiber CSA (CR vs CO comparison, P > .05). We conclude that Cr supplementation does not promote any additional hypertrophic effect on skeletal muscle area when Cr-supplemented trained muscles are submitted to same training regimen than Cr-nonsupplemented trained muscles. Specifically, any benefits of Cr supplementation on hypertrophy gains during resistance training may not be attributed to a direct anabolic effect on the skeletal muscle.

Incubating isolated mouse EDL muscles with creatine improves force production and twitch kinetics in fatigue due to reduction in ionic strength

BACKGROUND

Creatine supplementation can improve performance during high intensity exercise in humans and improve muscle strength in certain myopathies. In this present study, we investigated the direct effects of acute creatine incubation on isolated mouse fast-twitch EDL muscles, and examined how these effects change with fatigue.

METHODS AND RESULTS

The extensor digitorum longus muscle from mice aged 12-14 weeks was isolated and stimulated with field electrodes to measure force characteristics in 3 different states: (i) before fatigue; (ii) immediately after a fatigue protocol; and (iii) after recovery. These served as the control measurements for the muscle. The muscle was then incubated in a creatine solution and washed. The measurement of force characteristics in the 3 different states was then repeated. In un-fatigued muscle, creatine incubation increased the maximal tetanic force. In fatigued muscle, creatine treatment increased the force produced at all frequencies of stimulation. Incubation also increased the rate of twitch relaxation and twitch contraction in fatigued muscle. During repetitive fatiguing stimulation, creatine-treated muscles took 55.1±9.5% longer than control muscles to lose half of their original force. Measurement of weight changes showed that creatine incubation increased EDL muscle mass by 7%.

CONCLUSION

Acute creatine application improves force production in isolated fast-twitch EDL muscle, and these improvements are particularly apparent when the muscle is fatigued. One likely mechanism for this improvement is an increase in Ca(2+) sensitivity of contractile proteins as a result of ionic strength decreases following creatine incubation.

Effect of creatine supplementation on creatine and glycogen content in rat skeletal muscle

The effects of high dose creatine feeding (5 g kg(-1) BW day(-1), 5 days) on creatine content, glucose transport, and glycogen accumulation in white gastrocnemius, red gastrocnemius and soleus muscles of the rat was investigated. Isolated rat hindquarters of creatine fed and control rats were perfused with a standard medium containing either insulin alone (0, 100 or 20 000 microU mL(-1)) or in combination with creatine (2 or 10 mmol L(-1)). Furthermore, plasma insulin concentration was measured in normal rats during creatine feeding, as well as in anaesthetized rats during intravenous creatine infusion. Five days of creatine feeding increased (P < 0.05) total creatine content in soleus (+ 20%) but not in red gastrocnemius (+15%, n.s.) and white gastrocnemius (+ 10%, n.s.). In parallel, glycogen content was markedly elevated (P < 0.05) in soleus (+ 40%), less (P < 0.05) in red gastrocnemius (+ 15%), and not in white gastrocnemius (+ 10%, n.s.). Glucose transport rate, muscle GLUT-4 content, glycogen synthase activity in perfused muscles and glycogen synthesis rate were not significantly altered by creatine feeding in either muscle type. Furthermore, high dose creatine feeding raised (P < 0.05) plasma creatine concentration fivefold but did not alter circulating insulin level. It is concluded that short-term high dose creatine feeding enhances creatine disposal and glycogen storage in rat skeletal muscle. However, the creatine and glycogen response to creatine supplementation is markedly greater in oxidative than in glycolytic muscles.

Therapeutic options in other metabolic myopathies

Adult patients with metabolic myopathies typically present with exercise-induced pain, cramps, fatigue, and myoglobinuria. The current therapeutic options of glycogen and lipid storage myopathies include dietary treatments, excersise training, and pharmacological supplementations. Herein is a review of evidence from randomized controlled trials in McArdle disease (glycogen storage disease type V, muscle phosphorylase deficiency) and carnitine palmitoyltransferase (CPT) 2 deficiency. A brief overview on current treatment options in rhabdomyolysis is also included because patients with McArdle disease and CPT 2 often experience such potentially life-threatening complications.

The neuroprotective role of creatine

Significant progress has been made in identifying neuroprotective agents and their translation to patients with neurological disorders. While the direct causative pathways of neurodegeneration remain unclear, they are under great clinical and experimental investigation. There are a number of interrelated pathogenic mechanisms triggering molecular events that lead to neuronal death. One putative mechanism reported to play a prominent role in the pathogenesis of neurological diseases is impaired energy metabolism. If reduced energy stores play a role in neuronal loss, then therapeutic strategies that buffer intracellular energy levels may prevent or impede the neurodegenerative process. Recent studies suggest that impaired energy production promotes neurological disease onset and progression. Sustained ATP levels are critical to cellular homeostasis and may have both direct and indirect influence on pathogenic mechanisms associated with neurological disorders. Creatine is a critical component in maintaining cellular energy homeostasis, and its administration has been reported to be neuroprotective in a wide number of both acute and chronic experimental models of neurological disease. In the context of this chapter, we will review the experimental evidence for creatine supplementation as a neurotherapeutic strategy in patients with neurological disorders, including Huntington's disease, Parkinson's disease, amyotrophic lateral sclerosis, and Alzheimer's disease, as well as in ischemic stroke, brain and spinal cord trauma, and epilepsy.

The effect of creatine supplementation on mass and performance of rat skeletal muscle

This study investigated the effect of dietary creatine supplementation on hypertrophy and performance of rat skeletal muscle. Male Sprague-Dawley rats underwent either tibialis anterior ablation or partial ablation of the plantaris/gastrocnemius to induce compensatory hypertrophy of the extensor digitorum longus (EDL) or soleus respectively, or sham surgery. Creatine (300 mg/kg) was administered to one half of each group for 5 weeks, after which force production was measured. With the leg fixed at the knee and ankle, the distal tendon of the EDL or soleus was attached to a force transducer and the muscle was electrically stimulated via the sciatic nerve. Synergist ablation resulted in a significant increase in EDL mass and in soleus mass relative to control muscles. However, no effect of creatine supplementation on muscle mass or performance was found between control and either group of creatine-treated rats. Despite an apparent increase in muscle creatine content, creatine supplementation did not augment muscle hypertrophy or force production in rat EDL or soleus muscle, providing evidence that the potential benefits of creatine supplementation are not due to a direct effect on muscle but rather to an enhanced ability to train.

The therapeutic role of creatine in Huntington's disease

Huntington's disease (HD) is an autosomal dominant and fatal neurological disorder characterized by a clinical triad of progressive choreiform movements, psychiatric symptoms, and cognitive decline. HD is caused by an expanded trinucleotide CAG repeat in the gene coding for the protein huntingtin. No proven treatment to prevent the onset or to delay the progression of HD currently exists. While a direct causative pathway from the gene mutation to the selective neostriatal neurodegeneration remains unclear, it has been hypothesized that interactions of the mutant huntingtin protein or its fragments may result in a number of interrelated pathogenic mechanisms triggering a cascade of molecular events that lead to the untimely neuronal death observed in HD. One putative pathological mechanism reported to play a prominent role in the pathogenesis of HD is mitochondrial dysfunction and the subsequent reduction of cellular energy. Indeed, if mitochondrial impairment and reduced energy stores play roles in the neuronal loss in HD, then a therapeutic strategy that buffers intracellular energy levels may ameliorate the neurodegenerative process. Sustained ATP levels may have both direct and indirect importance in ameliorating the severity of many of the pathogenic mechanisms associated with HD. Creatine, a guanidino compound produced endogenously and acquired exogenously through diet, is a critical component in maintaining much needed cellular energy. As such, creatine is one of a number of ergogens that may provide a relatively safe and immediately available therapeutic strategy to HD patients that may be the cornerstone of a combined treatment necessary to delay the relentless progression of HD.

Targeting cellular energy production in neurological disorders

The concepts of energy dysregulation and oxidative stress and their complicated interdependence have rapidly evolved to assume primary importance in understanding the pathophysiology of numerous neurological disorders. Therefore, neuroprotective strategies addressing specific bioenergetic defects hold particular promise in the treatment of these conditions (i.e., amyotrophic lateral sclerosis, Huntington's disease, Parkinson's disease, Friedreich's ataxia, mitochondrial cytopathies and other neuromuscular diseases), all of which, to some extent, share 'the final common pathway' leading to cell death through either necrosis or apoptosis. Compounds such as creatine monohydrate and coenzyme Q(10) offer substantial neuroprotection against ischaemia, trauma, oxidative damage and neurotoxins. Miscellaneous agents, including alpha-lipoic acid, beta-OH-beta-methylbutyrate, riboflavin and nicotinamide, have also been shown to improve various metabolic parameters in brain and/or muscle. This review will highlight the biological function of each of the above mentioned compounds followed by a discussion of their utility in animal models and human neurological disease. The balance of this work will be comprised of discussions on the therapeutic applications of creatine and coenzyme Q(10).

Pharmacokinetics of the dietary supplement creatine

Creatine is a nonessential dietary component that, when supplemented in the diet, has shown physiological benefits in athletes, in animal-based models of disease and in patients with various muscle, neurological and neuromuscular disease. The clinical relevance of creatine supplementation is based primarily on its role in ATP generation, and cells may be able to better handle rapidly changing energy demands with supplementation. Although the pharmacological outcome measures of creatine have been investigated, the behaviour of creatine in the blood and muscle is still not fully understood. Creatine is most probably actively absorbed from the gastrointestinal tract in a similar way to amino acids and peptides. The distribution of creatine throughout the body is largely determined by the presence of creatine transporters. These transporters not only serve to distribute creatine but serve as a clearance mechanism because of creatine 'trapping' by skeletal muscle. Besides the pseudo-irreversible uptake by skeletal muscle, creatine clearance also depends on renal elimination and degradation to creatinine. Evidence suggests that creatine pharmacokinetics are nonlinear with respect to dose size and frequency. Skeletal muscle, the largest depot of creatine, has a finite capacity to store creatine. As such, when these stores are saturated, both volume of distribution and clearance can decrease, thus leading to complex pharmacokinetic situations. Additionally, other dietary components such as caffeine and carbohydrate can potentially affect pharmacokinetics by their influence on the creatine transporter. Disease and age may also affect the pharmacokinetics, but more information is needed. Overall, there are very limited pharmacokinetic data available for creatine, and further studies are needed to define absorption characteristics, clearance kinetics and the effect of multiple doses. Additionally, the relationship between plasma creatine and muscle creatine needs to be elucidated to optimise administration regimens.

Effect of muscle creatine content manipulation on contractile properties in mouse muscles

The effects of muscle creatine manipulation on contractile properties in oxidative and glycolytic muscles were evaluated. Whereas control mice (NMRi; n = 12) received normal chow (5 g daily), three experimental groups were created by adding creatine monohydrate (CR group; 5%, 1 week; n = 13); beta-guanidinoproprionic acid, an inhibitor of cellular creatine uptake (beta-GPA group; 1%, 2 weeks; n = 12); or CR following beta-GPA (beta-GPA+CR group; n = 11). Total creatine (TCr) and the contractile properties of incubated soleus and extensor digitorum longus (EDL) muscles were determined. For the soleus, compared with control, TCr increased in the CR group (+25%), decreased in beta-GPA group (-50%), and remained stable in the beta-GPA+CR group, whereas, for the EDL, TCr was similar in the CR, and lower in the beta-GPA (-40%) and beta-GPA+CR (-15%) groups. None of the experimental groups (CR, beta-GPA, or beta-GPA+CR) showed changes in peak tension (P(peak)), time to peak tension, or relaxation in soleus or EDL during twitch or tetanic stimulation. For the soleus, fatigue reduced P(peak) to approximately 60% of initial P(peak); 5 min of recovery restored P(peak) to values approximately 15% higher in CR than in controls. P(peak) recovery was not affected by beta-GPA or beta-GPA+CR in the soleus or any treatment in the EDL. Thus, peak tension recovery is enhanced by creatine intake in oxidative but not glycolytic muscles. This may be implicated in the beneficial action of creatine loading.

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

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

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

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

Evaluation of the stability of creatine in solution prepared from effervescent creatine formulations

The objectives of this study were to determine the cause of the crystallization in a large volume creatine supplement solution made from effervescent powders containing di-creatine citrate, and to characterize these crystals using thermal analyses and x-ray diffractometry. Creatine effervescent powders were dissolved in deionized water (pH 6.2) and stored both at room temperature (RT) (25 degrees C) and refrigerated condition (4 degrees C) over a period of 45 days. Creatine concentration was determined using high-performance liquid chromatography (HPLC). Intrinsic dissolution and saturated solubility of creatine, creatine monohydrate, and di-creatine citrate in water were determined and compared. Crystal growth was detected only in the refrigerated samples on the seventh day of storage. Differential Scanning Calorimetry (DSC) and x-ray diffraction (XRD) studies revealed that the crystals formed were of creatine monohydrate. Ninety percent creatine degradation was observed within 45 days for RT samples. However, at refrigerated condition this degradation was 80% within the same time period. The pH of the RT samples also increased from 3.6 to 4.5 during storage. No such increase was observed in the case of refrigerated samples. The intrinsic dissolution rate constants of the compounds decreased in the following order: di-creatine citrate > creatine > creatine monohydrate. In conclusion, di-creatine citrate used in effervescent formulation dissociates to creatine in aqueous solution and eventually crystallizes out as creatine monohydrate. Significant decrease in solubility and effect of pH contribute to this crystallization process.

Oral creatine supplementation and skeletal muscle metabolism in physical exercise

Creatine is the object of growing interest in the scientific literature. This is because of the widespread use of creatine by athletes, on the one hand, and to some promising results regarding its therapeutic potential in neuromuscular disease on the other. In fact, since the late 1900s, many studies have examined the effects of creatine supplementation on exercise performance. This article reviews the literature on creatine supplementation as an ergogenic aid, including some basic aspects relating to its metabolism, pharmacokinetics and side effects. The use of creatine supplements to increase muscle creatine content above approximately 20 mmol/kg dry muscle mass leads to improvements in high-intensity, intermittent high-intensity and even endurance exercise (mainly in nonweightbearing endurance activities). An effective supplementation scheme is a dosage of 20 g/day for 4-6 days, and 5 g/day thereafter. Based on recent pharmacokinetic data, new regimens of creatine supplementation could be used. Although there are opinion statements suggesting that creatine supplementation may be implicated in carcinogenesis, data to prove this effect are lacking, and indeed, several studies showing anticarcinogenic effects of creatine and its analogues have been published. There is a shortage of scientific evidence concerning the adverse effects following creatine supplementation in healthy individuals even with long-term dosage. Therefore, creatine may be considered as a widespread, effective and safe ergogenic aid.

Creatine transporter activity and content in the rat heart supplemented by and depleted of creatine

The intracellular creatine concentration is an important bioenergetic parameter in cardiac muscle. Although creatine uptake is known to be via a NaCl-dependent creatine transporter (CrT), its localization and regulation are poorly understood. We investigated CrT kinetics in isolated perfused hearts and, by using cardiomyocytes, measured CrT content at the plasma membrane or in total lysates. Rats were fed control diet or diet supplemented with creatine or the creatine analog beta-guanidinopropionic acid (beta-GPA). Creatine transport in control hearts followed saturation kinetics with a K(m) of 70 +/- 13 mM and a V(max) of 3.7 +/- 0.07 nmol x min(-1) x g wet wt(-1). Creatine supplementation significantly decreased the V(max) of the CrT (2.7 +/- 0.17 nmol x min(-1) x g wet wt(-1)). This was matched by an approximately 35% decrease in the plasma membrane CrT; the total CrT pool was unchanged. Rats fed beta-GPA exhibited a >80% decrease in tissue creatine and increase in beta-GPA(total). The V(max) of the CrT was increased (6.0 +/- 0.25 nmol x min(-1) x g wet wt(-1)) and the K(m) decreased (39.8 +/- 3.0 mM). The plasma membrane CrT increased about fivefold, whereas the total CrT pool remained unchanged. We conclude that, in heart, creatine transport is determined by the content of a plasma membrane isoform of the CrT but not by the total cellular CrT pool.

Dietary creatine supplementation and exercise performance: why inconsistent results?

Over the past few years there has been considerable interest in both the use of creatine (Cr) supplementation by athletes and the documentation of its effects by scientists. Some believe that this nitrogen-containing compound found in meat and fish has a performance-enhancing capability as important for brief intense exercise efforts as dietary carbohydrate is for activities where glycogen supplies limit performance. The mechanisms thought to be responsible for any ergogenic effect of acute (few d) Cr supplementation include: increased stores of muscle phosphocreatine (PCr), faster regeneration of PCr during exercise recovery, enhanced adenosine triphosphate (ATP) production from glycolysis secondary to increased hydrogen ion buffering, and/or possible shortened post contraction muscle relaxation time. With chronic (wk mo) supplementation when combined with strength training, Cr may alter muscle protein metabolism directly (via decreasing protein breakdown or increasing synthesis) and/or indirectly as a result of a greater training load made possible by its acute ergogenic effects on strength and power. Cr supplementation is not banned by the International Olympic Committee and, with the exception of a small increase in body mass (approximately 1 kg) over the initial 36 d, does not appear to have any adverse side effects, at least with short-term use. Few scientific data are available for more prolonged use (mo or y) but considering the large numbers of athletes using Cr over the past 6+ y and the absence of reported problems, it may be that the often discussed somewhat nebulous long term adverse effects are presently being overestimated. Intakes of 285-300 mg Cr/kg body mass 1 over 36 d or 3050 mg/kg body mass 1 over approximately 4 wk are sufficient to produce benefits (muscle mass and high intensity power gains); however, not all study results are consistent. The focus of this review is to outline some possible explanations for the inconsistent observations reported in the literature. Clearly, if proven to be consistent the benefits of Cr supplementation could extend far beyond the athletic arena to include individuals who experience muscle weakness for a variety of other reasons (e.g., age/disuse, muscle disease, exposure to microgravity, etc).

Effects of creatine loading and depletion on rat skeletal muscle contraction

1. In humans, the effects of dietary creatine supplementation are controversial, with some studies showing increased muscle force and fatigue resistance and others reporting no effect on exercise performance. Little is known about the effects of creatine on muscle contractile properties. 2. Rats were fed a standard diet, creatine for 10 days or beta-guanidinopropionate, which depletes muscle creatine, for 7 days. Contractile properties were measured in isolated extensor digitorum longus and sternohyoid muscle as representative limb and upper airway dilator muscles, respectively. 3. Creatine had no effect on specific twitch and tetanic tension, contractile kinetics, twitch/tetanus tension ratio, the tension-frequency relationship or fatigue in both muscles. beta-Guanidinopropionate had no effect on the twitch and tetanic tension, contractile kinetics, twitch/tetanus tension ratio or tension-frequency relationship, but significantly increased (P < 0.05, anova) fatigue in both muscles. 4. Therefore, although creatine depletion increases fatigue, creatine loading has no effects on extensor digitorum longus and sternohyoid muscle contractile properties.

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

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

Creatine supplementation: exploring the role of the creatine kinase/phosphocreatine system in human muscle

The effect of oral creatine supplementation on high-intensity exercise performance has been extensively studied over the past ten years and its ergogenic potential in young healthy subjects is now well documented. Recently, research has shifted from performance evaluation towards elucidating the mechanisms underlying enhanced muscle functional capacity after creatine supplementation. In this review, we attempt to summarise recent advances in the understanding of potential mechanisms of action of creatine supplementation at the level of skeletal muscle cells. By increasing intracellular creatine content, oral creatine ingestion conceivably stimulates operation of the creatine kinase (CK)/phosphocreatine (PCr) system, which in turn facilitates muscle relaxation. Furthermore, evidence is accumulating to suggest that creatine supplementation can beneficially impact on muscle protein and glycogen synthesis. Thus, muscle hypertrophy and glycogen supercompensation are candidate factors to explain the ergogenic potential of creatine ingestion. Additional issues discussed in this review are the fibre-type specificity of muscle creatine metabolism, the identification of responders versus non-responders to creatine intake, and the scientific background concerning potential side effects of creatine supplementation.

Creatine and the creatine transporter: a review

The cellular role of creatine (Cr) and Cr phosphate (CrP) has been studied extensively in neural, cardiac and skeletal muscle. Several studies have demonstrated that alterations in the cellular total Cr (Cr + CrP) concentration in these tissues can produce marked functional and/or structural change. The primary aim of this review was to critically evaluate the literature that has examined the regulation of cellular total Cr content. In particular, the review focuses on the regulation of the activity and gene expression of the Cr transporter (CreaT), which is primarily responsible for cellular Cr uptake. Two CreaT genes (CreaT1 and CreaT2) have been identified and their chromosomal location and DNA sequencing have been completed. From these data, putative structures of the CreaT proteins have been formulated. Transcription products of the CreaT2 gene are expressed exclusively in the testes, whereas CreaT1 transcripts are found in a variety of tissues. Recent research has measured the expression of the CreaT1 protein in several tissues including neural, cardiac and skeletal muscle. There is very little information available about the factors regulating CreaT gene expression. There is some evidence that suggests the intracellular Cr concentration may be involved in the regulatory process but there is much more to learn before this process is understood. The activity of the CreaT protein is controlled by many factors. These include substrate concentration, transmembrane Na+ gradients, cellular location, and various hormones. It is also likely that transporter activity is influenced by its phosphorylation state and by its interaction with other plasma membrane proteins. The extent of CreaT protein glycosylation may vary within cells, the functional significance of which remains unclear.

The effects of creatine dietary supplementation on anterior compartment pressure in the lower leg during rest and following exercise

OBJECTIVE

To examine the effects of creatine supplementation on anterior compartment pressure of the lower leg at rest and following exercise.

DESIGN

14 college-age males received creatine or placebo supplementation for 34 days. At baseline, anterior compartment pressure was measured preexercise, immediately postexercise, and 1, 5, and 15 minutes postexercise after a level treadmill run for 20 minutes at 80% of maximal aerobic power.

INTERVENTION

Following baseline testing, subjects began a 6-day creatine or placebo loading phase at a dosage of 0.3 g x kg body mass(-1) x d(-1). This was followed by a 28-day maintenance phase at a dosage of 0.03 g x kg body mass(-1) x d(-1). Subjects and investigators were blinded as to treatment administration. Subjects continued to exercise during the supplementation period. After 6 days and 34 days of supplementation. anterior compartment pressure was measured at rest and following exercise.

RESULTS

Creatine supplementation for 6 days significantly increased compartment pressure compared with the placebo group at rest (76%), immediately post- (150%), 1 minute post- (125%), 5 minute post- (106%), and 15 minute postexercise (77%). Anterior compartment pressures continued to remain significantly higher for the creatine group compared with the placebo group at rest (72%), immediately post- (125%) and 1 minute postexercise (180%) after 34 days of creatine supplementation.

CONCLUSIONS

These data indicate that creatine supplementation abnormally increases anterior compartment pressure in the lower leg at rest and following 20 minutes of level running at 80% of maximal aerobic power.

Creatine transporter protein content, localization, and gene expression in rat skeletal muscle

The present study examined the gene expression and cellular localization of the creatine transporter (CreaT) protein in rat skeletal muscle. Soleus (SOL) and red (RG) and white gastrocnemius (WG) muscles were analyzed for CreaT mRNA, CreaT protein, and total creatine (TCr) content. Cellular location of the CreaT protein was visualized with immunohistochemical analysis of muscle cross sections. TCr was higher (P < or = 0.05) in WG than in both RG and SOL, and was higher in RG than in SOL. Total CreaT protein content was greater (P < or = 0.05) in SOL and RG than in WG. Two bands (55 and 70 kDa) of the CreaT protein were found in all muscle types. Both the 55-kDa (CreaT-55) and the 70-kDa (CreaT-70) bands were present in greater (P < or = 0.05) amounts in SOL and RG than in WG. SOL and RG had a greater amount (P < or = 0.05) of CreaT-55 than CreaT-70. Immunohistochemical analysis revealed that the CreaT was mainly associated with the sarcolemmal membrane in all muscle types. CreaT mRNA expression per microgram of total RNA was similar across the three muscle types. These data indicate that rat SOL and RG have an enhanced potential to transport Cr compared with WG, despite a higher TCr in the latter.

Adverse effects of creatine supplementation: fact or fiction?

The consumption of oral creatine monohydrate has become increasingly common among professional and amateur athletes. Despite numerous publications on the ergogenic effects of this naturally occurring substance, there is little information on the possible adverse effects of this supplement. The objectives of this review are to identify the scientific facts and contrast them with reports in the news media, which have repeatedly emphasised the health risks of creatine supplementation and do not hesitate to draw broad conclusions from individual case reports. Exogenous creatine supplements are often consumed by athletes in amounts of up to 20 g/day for a few days, followed by 1 to 10 g/day for weeks, months and even years. Usually, consumers do not report any adverse effects, but body mass increases. There are few reports that creatine supplementation has protective effects in heart, muscle and neurological diseases. Gastrointestinal disturbances and muscle cramps have been reported occasionally in healthy individuals, but the effects are anecdotal. Liver and kidney dysfunction have also been suggested on the basis of small changes in markers of organ function and of occasional case reports, but well controlled studies on the adverse effects of exogenous creatine supplementation are almost nonexistent. We have investigated liver changes during medium term (4 weeks) creatine supplementation in young athletes. None showed any evidence of dysfunction on the basis of serum enzymes and urea production. Short term (5 days), medium term (9 weeks) and long term (up to 5 years) oral creatine supplementation has been studied in small cohorts of athletes whose kidney function was monitored by clearance methods and urine protein excretion rate. We did not find any adverse effects on renal function. The present review is not intended to reach conclusions on the effect of creatine supplementation on sport performance, but we believe that there is no evidence for deleterious effects in healthy individuals. Nevertheless, idiosyncratic effects may occur when large amounts of an exogenous substance containing an amino group are consumed, with the consequent increased load on the liver and kidneys. Regular monitoring is compulsory to avoid any abnormal reactions during oral creatine supplementation.

Role of plasma membrane transporters in muscle metabolism

Muscle plays a major role in metabolism. Thus it is a major glucose-utilizing tissue in the absorptive state, and changes in muscle insulin-stimulated glucose uptake alter whole-body glucose disposal. In some conditions, muscle preferentially uses lipid substrates, such as fatty acids or ketone bodies. Furthermore, muscle is the main reservoir of amino acids and protein. The activity of many different plasma membrane transporters, such as glucose carriers and transporters of carnitine, creatine and amino acids, play a crucial role in muscle metabolism by catalysing the influx or the efflux of substrates across the cell surface. In some cases, the membrane transport process is subjected to intense regulatory control and may become a potential pharmacological target, as is the case with the glucose transporter GLUT4. The goal of this review is the molecular characterization of muscle membrane transporter proteins, as well as the analysis of their possible regulatory role.