Muscle glycogen content and glucose uptake during exercise in humans: influence of prior exercise and dietary manipulation

A bout of aerobic exercise in the heat increases carbohydrate use but does not enhance the disposal of an oral glucose load, in healthy active individuals

We investigated if a bout of exercise in a hot environment (HEAT) would reduce the postprandial hyperglycemia induced by glucose ingestion. The hypothesis was that HEAT stimulating carbohydrate oxidation and glycogen use would increase the disposal of an ingested glucose load [i.e., oral glucose tolerance test (OGTT); 75 g of glucose]. Separated by at least 1 wk, nine young healthy individuals underwent three trials after an overnight fast in a randomized order. Two trials included 50 min of pedaling at 58 ± 5% V̇o2max either in a thermoneutral (21 ± 1°C; NEUTRAL) or in a hot environment (33 ± 1°C; HEAT) eliciting similar energy expenditure (503 ± 101 kcal). These two trials were compared with a no-exercise trial (NO EXER). Twenty minutes after exercise (or rest), subjects underwent an OGTT, while carbohydrate oxidation (CHOxid, using indirect calorimetry) plasma blood glucose, insulin concentrations (i.e., [glucose], [insulin]), and double tracer glucose kinetics ([U-13C] glucose ingestion and [6,6-2H2] glucose infusion) were monitored for 120 min. At rest, [glucose], [insulin], and rates of appearance/disappearance of glucose in plasma (glucose Ra/Rd) were similar among trials. During exercise, heart rate, tympanic temperature, [glucose], glycogen oxidation, and total CHOxid were higher during HEAT than NEUTRAL (i.e., 149 ± 35 vs. 124 ± 31 µmol·kg-1·min-1, P = 0.010). However, during the following OGTT, glucose Rd was similar in HEAT and NEUTRAL trials (i.e., 25.1 ± 3.6 vs. 25.2 ± 5.3 µmol·kg-1·min-1, P = 0.981). Insulin sensitivity (i.e., ISIndexMATSUDA) only improved in NEUTRAL compared with NO EXER (10.1 ± 4.6 vs. 8.8 ± 3.7 au; P = 0.044). In summary, stimulating carbohydrate use with exercise in a hot environment does not improve postprandial plasma glucose disposal or insulin sensitivity in a subsequent OGTT.NEW & NOTEWORTHY Exercise in the heat increases estimated muscle glycogen use. Reduced muscle glycogen after exercise in the heat could increase insulin-mediated glucose uptake during a subsequent oral glucose tolerance test (OGTT). However, plasma glucose kinetics are not improved during the OGTT in response to a bout of exercise in the heat, and insulin sensitivity worsens. Heat stress activates glucose counterregulatory hormones whose actions may linger during the OGTT, preventing increased glucose uptake.

Effect of an acute long-duration exercise bout on skeletal muscle lipid droplet morphology, GLUT 4 protein, and perilipin protein expression

PURPOSE

Smaller lipid droplet morphology and GLUT 4 protein expression have been associated with greater muscle oxidative capacity and glucose uptake, respectively. The main purpose of this study was to determine the effect of an acute long-duration exercise bout on skeletal muscle lipid droplet morphology, GLUT4, perilipin 3, and perilipin 5 expressions.

METHODS

Twenty healthy men (age 24.0 ± 1.0 years, BMI 23.6 ± 0.4 kg/m2) were recruited for the study. The participants were subjected to an acute bout of exercise on a cycle ergometer at 50% VO2max until they reached a total energy expenditure of 650 kcal. The study was conducted after an overnight fast. Vastus lateralis muscle biopsies were obtained before and immediately after exercise for immunohistochemical analysis to determine lipid, perilipin 3, perilipin 5, and GLUT4 protein contents while GLUT 4 mRNA was quantified using RT-qPCR.

RESULTS

Lipid droplet size decreased whereas total intramyocellular lipid content tended to reduce (p = 0.07) after an acute bout of endurance exercise. The density of smaller lipid droplets in the peripheral sarcoplasmic region significantly increased (0.584 ± 0.04 to 0.638 ± 0.08 AU; p = 0.01) while larger lipid droplets significantly decreased (p < 0.05). GLUT4 mRNA tended to increase (p = 0.05). There were no significant changes in GLUT 4, perilipin 3, and perilipin 5 protein levels.

CONCLUSION

The study demonstrates that exercise may impact metabolism by enhancing the quantity of smaller lipid droplets over larger lipid droplets.

Exacerbated impairments in neuromuscular function when two bouts of team sport match simulations are separated by 48 h

Intermittent team sports, involving high metabolic and mechanical demands, elicit prolonged impairments in neuromuscular function which persist for ∼48-72 h. Whether impairments in neuromuscular function are exacerbated when such exercise is repeated with incomplete recovery is unknown. This study assessed the neuromuscular, heart rate and metabolic responses to two bouts of ∼90 min modified team sport match simulations separated by 48 h in 12 competitive football players. Before and 2 min after both bouts, knee extensor isometric maximal voluntary contraction (MVC), contractile function (Qtw,pot ) and voluntary activation (VA) were measured. Heart rate (HR), sprint time, blood lactate and glucose were measured throughout both bouts. MVC was reduced relative to baseline at post-bout 1 (21 ± 12%; P = 0.003) and pre-bout 2 (14 ± 11%; P = 0.009), and was lower post-bout 2 (33 ± 14%; P < 0.001) relative to post-bout 1 (P = 0.036). Qtw,pot was reduced post-bout 1 (30 ± 11%; P < 0.001) and pre-bout 2 (9 ± 6%; P = 0.004), and was not different post-bout 2 (28 ± 8%; P < 0.001) relative to post-bout 1 (P = 0.872). VA was reduced post-bout 1 (8 ± 7%; P = 0.023), recovered pre-bout 2 (P = 0.133) and was lower post-bout 2 (16 ± 7%; P < 0.001) relative to post-bout 1 (P = 0.029). Total sprint time was longer, and HR, blood lactate and glucose were lower during bout 2 than bout 1 (P ≤ 0.021). Thus, impairments in neuromuscular function are exacerbated when high-intensity intermittent exercise is performed with incomplete recovery concurrent with accentuated reductions in VA. The lower blood lactate and glucose during the second bout might be due, at least in part, to reduced glycogen availability upon commencing exercise and consequently a greater reliance on glucose extraction. NEW FINDINGS: What is the central question of this study? There is limited evidence on whether impairments in neuromuscular function are exacerbated when prolonged high-intensity intermittent exercise is repeated with incomplete recovery: what are the neuromuscular consequences of performing two bouts of a modified team sport match simulations separated by 48 h? What is the main finding and its importance? Impairments in knee extensor force generating capacity are exacerbated concurrent with accentuated reductions in nervous system activation of muscle when prolonged high-intensity intermittent exercise is repeated with 48 h recovery.

Post-translational Modifications: The Signals at the Intersection of Exercise, Glucose Uptake, and Insulin Sensitivity

Diabetes is a global epidemic, of which type 2 diabetes makes up the majority of cases. Nonetheless, for some individuals, type 2 diabetes is eminently preventable and treatable via lifestyle interventions. Glucose uptake into skeletal muscle increases during and in recovery from exercise, with exercise effective at controlling glucose homeostasis in individuals with type 2 diabetes. Furthermore, acute and chronic exercise sensitizes skeletal muscle to insulin. A complex network of signals converge and interact to regulate glucose metabolism and insulin sensitivity in response to exercise. Numerous forms of post-translational modifications (eg, phosphorylation, ubiquitination, acetylation, ribosylation, and more) are regulated by exercise. Here we review the current state of the art of the role of post-translational modifications in transducing exercise-induced signals to modulate glucose uptake and insulin sensitivity within skeletal muscle. Furthermore, we consider emerging evidence for noncanonical signaling in the control of glucose homeostasis and the potential for regulation by exercise. While exercise is clearly an effective intervention to reduce glycemia and improve insulin sensitivity, the insulin- and exercise-sensitive signaling networks orchestrating this biology are not fully clarified. Elucidation of the complex proteome-wide interactions between post-translational modifications and the associated functional implications will identify mechanisms by which exercise regulates glucose homeostasis and insulin sensitivity. In doing so, this knowledge should illuminate novel therapeutic targets to enhance insulin sensitivity for the clinical management of type 2 diabetes.

Insulin Resistance Does Not Impair Mechanical Overload-Stimulated Glucose Uptake, but Does Alter the Metabolic Fate of Glucose in Mouse Muscle

Skeletal muscle glucose uptake and glucose metabolism are impaired in insulin resistance. Mechanical overload stimulates glucose uptake into insulin-resistant muscle; yet the mechanisms underlying this beneficial effect remain poorly understood. This study examined whether a differential partitioning of glucose metabolism is part of the mechanosensitive mechanism underlying overload-stimulated glucose uptake in insulin-resistant muscle. Mice were fed a high-fat diet to induce insulin resistance. Plantaris muscle overload was induced by unilateral synergist ablation. After 5 days, muscles were excised for the following measurements: (1) [³H]-2-deoxyglucose uptake; (2) glycogen; 3) [5-³H]-glucose flux through glycolysis; (4) lactate secretion; (5) metabolites; and (6) immunoblots. Overload increased glucose uptake ~80% in both insulin-sensitive and insulin-resistant muscles. Overload increased glycogen content ~20% and this was enhanced to ~40% in the insulin-resistant muscle. Overload did not alter glycolytic flux, but did increase muscle lactate secretion 40–50%. In both insulin-sensitive and insulin-resistant muscles, overload increased 6-phosphogluconate levels ~150% and decreased NADP:NADPH ~60%, indicating pentose phosphate pathway activation. Overload increased protein O-GlcNAcylation ~45% and this was enhanced to ~55% in the insulin-resistant muscle, indicating hexosamine pathway activation. In conclusion, insulin resistance does not impair mechanical overload-stimulated glucose uptake but does alter the metabolic fate of glucose in muscle.

Impact of high-intensity interval training and sprint interval training on peripheral markers of glycemic control in metabolic syndrome and type 2 diabetes

Glycemic control is essential to reduce the risk of complications associated with metabolic syndrome (MetS) and type 2 diabetes (T2D). Aerobic and resistance exercise performed alone or in combination improve glycemic control in both conditions. However, perceived lack of time and commitment are considered principal barriers to performing exercise regularly. High intensity interval training (HIIT) and sprint interval training (SIT) can be performed in a fraction of the time required for continuous aerobic exercise. A substantial scientific evidence indicates that HIIT/SIT improve glycemic control to a similar or greater extent than aerobic exercise in populations without MetS or T2D. Likewise, growing evidence suggest that HIIT/SIT improve the glycemic control during MetS and T2D. The aim of this review is to discuss the effects of interval training protocols on peripheral markers of glucose metabolism in patients with MetS and T2D.

Restoration of Muscle Glycogen and Functional Capacity: Role of Post-Exercise Carbohydrate and Protein Co-Ingestion

The importance of post-exercise recovery nutrition has been well described in recent years, leading to its incorporation as an integral part of training regimes in both athletes and active individuals. Muscle glycogen depletion during an initial prolonged exercise bout is a main factor in the onset of fatigue and so the replenishment of glycogen stores may be important for recovery of functional capacity. Nevertheless, nutritional considerations for optimal short-term (3–6 h) recovery remain incompletely elucidated, particularly surrounding the precise amount of specific types of nutrients required. Current nutritional guidelines to maximise muscle glycogen availability within limited recovery are provided under the assumption that similar fatigue mechanisms (i.e., muscle glycogen depletion) are involved during a repeated exercise bout. Indeed, recent data support the notion that muscle glycogen availability is a determinant of subsequent endurance capacity following limited recovery. Thus, carbohydrate ingestion can be utilised to influence the restoration of endurance capacity following exhaustive exercise. One strategy with the potential to accelerate muscle glycogen resynthesis and/or functional capacity beyond merely ingesting adequate carbohydrate is the co-ingestion of added protein. While numerous studies have been instigated, a consensus that is related to the influence of carbohydrate-protein ingestion in maximising muscle glycogen during short-term recovery and repeated exercise capacity has not been established. When considered collectively, carbohydrate intake during limited recovery appears to primarily determine muscle glycogen resynthesis and repeated exercise capacity. Thus, when the goal is to optimise repeated exercise capacity following short-term recovery, ingesting carbohydrate at an amount of ≥1.2 g kg body mass⁻¹·h⁻¹ can maximise muscle glycogen repletion. The addition of protein to carbohydrate during post-exercise recovery may be beneficial under circumstances when carbohydrate ingestion is sub-optimal (≤0.8 g kg body mass⁻¹·h⁻¹) for effective restoration of muscle glycogen and repeated exercise capacity.

Nutritional Ketosis and Mitohormesis: Potential Implications for Mitochondrial Function and Human Health

Impaired mitochondrial function often results in excessive production of reactive oxygen species (ROS) and is involved in the etiology of many chronic diseases, including cardiovascular disease, diabetes, neurodegenerative disorders, and cancer. Moderate levels of mitochondrial ROS, however, can protect against chronic disease by inducing upregulation of mitochondrial capacity and endogenous antioxidant defense. This phenomenon, referred to as mitohormesis, is induced through increased reliance on mitochondrial respiration, which can occur through diet or exercise. Nutritional ketosis is a safe and physiological metabolic state induced through a ketogenic diet low in carbohydrate and moderate in protein. Such a diet increases reliance on mitochondrial respiration and may, therefore, induce mitohormesis. Furthermore, the ketone β-hydroxybutyrate (BHB), which is elevated during nutritional ketosis to levels no greater than those resulting from fasting, acts as a signaling molecule in addition to its traditionally known role as an energy substrate. BHB signaling induces adaptations similar to mitohormesis, thereby expanding the potential benefit of nutritional ketosis beyond carbohydrate restriction. This review describes the evidence supporting enhancement of mitochondrial function and endogenous antioxidant defense in response to nutritional ketosis, as well as the potential mechanisms leading to these adaptations.

Role of AMP-Activated Protein Kinase for Regulating Post-exercise Insulin Sensitivity

Skeletal muscle insulin resistance precedes development of type 2 diabetes (T2D). As skeletal muscle is a major sink for glucose disposal, understanding the molecular mechanisms involved in maintaining insulin sensitivity of this tissue could potentially benefit millions of people that are diagnosed with insulin resistance. Regular physical activity in both healthy and insulin-resistant individuals is recognized as the single most effective intervention to increase whole-body insulin sensitivity and thereby positively affect glucose homeostasis. A single bout of exercise has long been known to increase glucose disposal in skeletal muscle in response to physiological insulin concentrations. While this effect is identified to be restricted to the previously exercised muscle, the molecular basis for an apparent convergence between exercise- and insulin-induced signaling pathways is incompletely known. In recent years, we and others have identified the Rab GTPase-activating protein, TBC1 domain family member 4 (TBC1D4) as a target of key protein kinases in the insulin- and exercise-activated signaling pathways. Our working hypothesis is that the AMP-activated protein kinase (AMPK) is important for the ability of exercise to insulin sensitize skeletal muscle through TBC1D4. Here, we aim to provide an overview of the current available evidence linking AMPK to post-exercise insulin sensitivity.

An improved glucose transport assay system for isolated mouse skeletal muscle tissues

There is a growing demand for a system in the field of sarcopenia and diabetes research that could be used to evaluate the effects of functional food ingredients that enhance muscle mass/contractile force or muscle glucose uptake. In this study, we developed a new type of in vitro muscle incubation system that systemizes an apparatus for muscle incubation, using an electrode, a transducer, an incubator, and a pulse generator in a compact design. The new system enables us to analyze the muscle force stimulated by the electric pulses and glucose uptake during contraction and it may thus be a useful tool for analyzing the metabolic changes that occur during muscle contraction. The system may also contribute to the assessments of new food ingredients that act directly on skeletal muscle in the treatment of sarcopenia and diabetes.

The Accumulative Effect of Concentric-Biased and Eccentric-Biased Exercise on Cardiorespiratory and Metabolic Responses to Subsequent Low-Intensity Exercise: A Preliminary Study

The study investigated the accumulative effect of concentric-biased and eccentric-biased exercise on cardiorespiratory, metabolic and neuromuscular responses to low-intensity exercise performed hours later. Fourteen young men cycled at low-intensity (~60 rpm at 50% maximal oxygen uptake) for 10 min before, and 12 h after: concentric-biased, single-leg cycling exercise (CON) (performed ~19:30 h) and eccentric-biased, double-leg knee extension exercise (ECC) (~06:30 h the following morning). Respiratory measures were sampled breath-by-breath, with oxidation values derived from stoichiometry equations. Knee extensor neuromuscular function was assessed before and after CON and ECC. Cardiorespiratory responses during low-intensity cycling were unchanged by accumulative CON and ECC. The RER was lower during low-intensity exercise 12 h after CON and ECC (0.88 ± 0.08), when compared to baseline (0.92 ± 0.09; p = 0.02). Fat oxidation increased from baseline (0.24 ± 0.2 g·min(-1)) to 12 h after CON and ECC (0.39 ± 0.2 g·min(-1); p = 0.01). Carbohydrate oxidation decreased from baseline (1.59 ± 0.4 g·min(-1)) to 12 h after CON and ECC (1.36 ± 0.4 g·min(-1); p = 0.03). These were accompanied by knee extensor force loss (right leg: -11.6%, p < 0.001; left leg: -10.6%, p = 0.02) and muscle soreness (right leg: 2.5 ± 0.9, p < 0.0001; left leg: 2.3 ± 1.2, p < 0.01). Subsequent concentric-biased and eccentric-biased exercise led to increased fat oxidation and decreased carbohydrate oxidation, without impairing cardiorespiration, during low-intensity cycling. An accumulation of fatiguing and damaging exercise increases fat utilisation during low intensity exercise performed as little as 12 h later.

Exercise and Gene Expression

Acute and transient changes in gene transcription following a single exercise bout, if reinforced by repeated exercise stimuli, result in the longer lasting effects on protein expression and function that form the basis of skeletal muscle training adaptations. Changes in skeletal muscle gene expression occur in response to multiple stimuli associated with skeletal muscle contraction, various signaling kinases that respond to these stimuli, and numerous downstream pathways and targets of these kinases. In addition, DNA methylation, histone acetylation and phosphorylation, and micro-RNAs can alter gene expression via epigenetic mechanisms. Contemporary studies rely upon "big omics data," in combination with computational and systems biology, to interrogate, and make sense of, the complex interactions underpinning exercise adaptations. The exciting potential is a greater understanding of the integrative biology of exercise.

Nutrition and the adaptation to endurance training

Maximizing metabolic stress at a given level of mechanical stress can improve the adaptive response to endurance training, decrease injury, and potentially improve performance. Calcium and metabolic stress, in the form of heat, decreases in the adenosine triphosphate/adenosine diphosphate ratio, glycogen depletion, caloric restriction, and oxidative stress, are the primary determinants of the adaptation to training. These stressors increase the activity and amount of peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC-1α), a protein that can directly induce the primary adaptive responses to endurance exercise: mitochondrial biogenesis, angiogenesis, and increases in fat oxidation. The activity of PGC-1α is regulated by its charge (phosphorylation and acetylation), whereas its transcription is regulated by proteins that bind to myocyte enhancing factor 2, enhancer box, and cyclic adenosine monophosphate response element sites within the PGC-1α promoter. This brief review will describe what is known about the control of PGC-1α by these metabolic stressors. As the duration of calcium release and the amount of metabolic stress, and therefore the activation of PGC-1α, can be directly modulated by training and nutrition, a simple strategy can be generated to maximize the adaptive response to endurance training.

Neuromuscular responses to mild-muscle damaging eccentric exercise in a low glycogen state

The aim of this study was to examine the effect of low muscle glycogen on the neuromuscular responses to maximal eccentric contractions. Fourteen healthy men (22 ± 3 years) performed single-leg cycling (20 min at ~75% maximal oxygen uptake (V̇O2 max); eight 90 s sprints at a 1:1 work-to-rest ratio (5% decrements from 90% to 55% V̇O2 max until exhaustion) the evening before 100 eccentric (1.57 rads(-1)) with reduced (RED) and normal glycogen (NORM). Neuromuscular responses were measured during and up to 48 h after with maximal voluntary and involuntary (twitch, 20 Hz and 50 Hz) isometric contractions. During eccentric contractions, peak torque decreased (RED: -16.1 ± 2.5%; NORM: -6.2 ± 5.1%) and EMG frequency increased according to muscle length. EMG activity decreased for RED only. After eccentric contractions, maximal isometric force was reduced up to 24h for NORM (-13.5 ± 5.8%) and 48 h for RED (-7.4 ± 10.9%). Twelve hours after eccentric contractions, twitch force and the 20:50 Hz ratio were decreased for RED but not for NORM. Immediate involuntary with prolonged voluntary force loss suggests that reduced glycogen is associated with increased susceptibility to mild muscle-damaging eccentric exercise with contributions of peripheral and central mechanisms to be different during recovery.

Exercise, GLUT4, and skeletal muscle glucose uptake

Glucose is an important fuel for contracting muscle, and normal glucose metabolism is vital for health. Glucose enters the muscle cell via facilitated diffusion through the GLUT4 glucose transporter which translocates from intracellular storage depots to the plasma membrane and T-tubules upon muscle contraction. Here we discuss the current understanding of how exercise-induced muscle glucose uptake is regulated. We briefly discuss the role of glucose supply and metabolism and concentrate on GLUT4 translocation and the molecular signaling that sets this in motion during muscle contractions. Contraction-induced molecular signaling is complex and involves a variety of signaling molecules including AMPK, Ca(2+), and NOS in the proximal part of the signaling cascade as well as GTPases, Rab, and SNARE proteins and cytoskeletal components in the distal part. While acute regulation of muscle glucose uptake relies on GLUT4 translocation, glucose uptake also depends on muscle GLUT4 expression which is increased following exercise. AMPK and CaMKII are key signaling kinases that appear to regulate GLUT4 expression via the HDAC4/5-MEF2 axis and MEF2-GEF interactions resulting in nuclear export of HDAC4/5 in turn leading to histone hyperacetylation on the GLUT4 promoter and increased GLUT4 transcription. Exercise training is the most potent stimulus to increase skeletal muscle GLUT4 expression, an effect that may partly contribute to improved insulin action and glucose disposal and enhanced muscle glycogen storage following exercise training in health and disease.

More than a store: regulatory roles for glycogen in skeletal muscle adaptation to exercise

The glycogen content of muscle determines not only our capacity for exercise but also the signaling events that occur in response to exercise. The result of the shift in signaling is that frequent training in a low-glycogen state results in improved fat oxidation during steady-state submaximal exercise. This review will discuss how the amount or localization of glycogen particles can directly or indirectly result in this differential response to training. The key direct effect discussed is carbohydrate binding, whereas the indirect effects include the metabolic shift toward fat oxidation, the increase in catecholamines, and osmotic stress. Although our understanding of the role of glycogen in response to training has expanded exponentially over the past 5 years, there are still many questions remaining as to how stored carbohydrate affects the muscular adaptation to exercise.

Training with low muscle glycogen enhances fat metabolism in well-trained cyclists

PURPOSE

To determine the effects of training with low muscle glycogen on exercise performance, substrate metabolism, and skeletal muscle adaptation.

METHODS

Fourteen well-trained cyclists were pair-matched and randomly assigned to HIGH- or LOW-glycogen training groups. Subjects performed nine aerobic training (AT; 90 min at 70% VO2max) and nine high-intensity interval training sessions (HIT; 8 × 5-min efforts, 1-min recovery) during a 3-wk period. HIGH trained once daily, alternating between AT on day 1 and HIT the following day, whereas LOW trained twice every second day, first performing AT and then, 1 h later, performing HIT. Pretraining and posttraining measures were a resting muscle biopsy, metabolic measures during steady-state cycling, and a time trial.

RESULTS

Power output during HIT was 297 ± 8 W in LOW compared with 323 ± 9 W in HIGH (P < 0.05); however, time trial performance improved by ∼10% in both groups (P < 0.05). Fat oxidation during steady-state cycling increased after training in LOW (from 26 ± 2 to 34 ± 2 μmol·kg−¹·min−¹, P < 0.01). Plasma free fatty acid oxidation was similar before and after training in both groups, but muscle-derived triacylglycerol oxidation increased after training in LOW (from 16 ± 1 to 23 ± 1 μmol·kg−¹·min−¹, P < 0.05). Training with low muscle glycogen also increased β-hydroxyacyl-CoA-dehydrogenase protein content (P < 0.01).

CONCLUSIONS

Training with low muscle glycogen reduced training intensity and, in performance, was no more effective than training with high muscle glycogen. However, fat oxidation was increased after training with low muscle glycogen, which may have been due to the enhanced metabolic adaptations in skeletal muscle.

Effect of glycogen availability on human skeletal muscle protein turnover during exercise and recovery

We examined the effect of carbohydrate (CHO) availability on whole body and skeletal muscle protein utilization at rest, during exercise, and during recovery in humans. Six men cycled at ∼75% peak O2 uptake (V̇o2peak) to exhaustion to reduce body CHO stores and then consumed either a high-CHO (H-CHO; 71 ± 3% CHO) or low-CHO (L-CHO; 11 ± 1% CHO) diet for 2 days before the trial in random order. After each dietary intervention, subjects received a primed constant infusion of [1-13C]leucine and l-[ring-2H5]phenylalanine for measurements of the whole body net protein balance and skeletal muscle protein turnover. Muscle, breath, and arterial and venous blood samples were obtained at rest, during 2 h of two-legged kicking exercise at ∼45% of kicking V̇o2peak, and during 1 h of recovery. Biopsy samples confirmed that the muscle glycogen concentration was lower in the L-CHO group versus the H-CHO group at rest, after exercise, and after recovery. The net leg protein balance was decreased in the L-CHO group compared with at rest and compared with the H-CHO condition, which was primarily due to an increase in protein degradation (area under the curve of the phenylalanine rate of appearance: 1,331 ± 162 μmol in the L-CHO group vs. 786 ± 51 μmol in the H-CHO group, P < 0.05) but also due to a decrease in protein synthesis late in exercise. There were no changes during exercise in the rate of appearance compared with rest in the H-CHO group. Whole body leucine oxidation increased above rest in the L-CHO group only and was higher than in the H-CHO group. The whole body net protein balance was reduced in the L-CHO group, largely due to a decrease in whole body protein synthesis. These data extend previous findings by others and demonstrate, using contemporary stable isotope methodology, that CHO availability influences the rates of skeletal muscle and whole body protein synthesis, degradation, and net balance during prolonged exercise in humans.

Scoparia dulcis (SDF7) endowed with glucose uptake properties on L6 myotubes compared insulin

AIM OF THE STUDY

Insulin stimulates glucose uptake and promotes the translocation of glucose transporter 4 (Glut 4) to the plasma membrane on L6 myotubes. The aim of this study is to investigate affect of Scoparia dulcis Linn water extracts on glucose uptake activity and the Glut 4 translocation components (i.e., IRS-1, PI 3-kinase, PKB/Akt2, PKC and TC 10) in L6 myotubes compared to insulin.

MATERIALS AND METHODS

Extract from TLC fraction-7 (SDF7) was used in this study. The L6 myotubes were treated by various concentrations of SDF7 (1 to 50 microg/ml) and insulin (1 to 100 nM). The glucose uptake activities of L6 myotubes were evaluated using 2-Deoxy-D-glucose uptake assay in with or without fatty acid-induced medium. The Glut 4 translocation components in SDF7-treated L6 myotubes were detected using immunoblotting and quantified by densitometry compared to insulin. Plasma membrane lawn assay and glycogen colorimetry assay were carried out in SDF7- and insulin-treated L6 myotubes in this study.

RESULTS

Here, our data clearly shows that SDF7 possesses glucose uptake properties on L6 myotubes that are dose-dependent, time-dependent and plasma membrane Glut 4 expression-dependent. SDF7 successfully stimulates glucose uptake activity as potent as insulin at a maximum concentration of 50 microg/ml at 480 min on L6 myotubes. Furthermore, SDF7 stimulates increased Glut 4 expression and translocation to plasma membranes at equivalent times. Even in the insulin resistance stage (free fatty acids-induced), SDF7-treated L6 myotubes were found to be more capable at glucose transport than insulin treatment.

CONCLUSIONS

Thus, we suggested that Scoparia dulcis has the potential to be categorized as a hypoglycemic medicinal plant based on its good glucose transport properties.

The role of interleukin-6 in insulin resistance, body fat distribution and energy balance

Interleukin-6 (IL-6) is a central player in the regulation of inflammation, haematopoiesis, immune response and host defense mechanisms. During the last decade, an accumulating amount of data suggested a pivotal role for IL-6 in metabolic processes, thus fortifying the picture of IL-6 as a multifaceted, pleiotropic cytokine. Because of its secretion by adipose tissue and contracting skeletal muscle and its broad action on central and peripheral organs, IL-6 has been termed an adipokine and a myokine. Its quantitative release from adipose tissue results in a subclinical, systemic elevation of IL-6 plasma levels with increasing body fat content, which may be implicated in the proinflammatory state leading to insulin resistance. On the other hand, IL-6 produced in the working muscle during physical activity could act as an energy sensor by activating AMP-activated kinase and enhancing glucose disposal, lipolysis and fat oxidation. In addition, both impaired IL-6 secretion and action are risk factors for weight gain. Thus, IL-6 clearly has lipolytic effects and anti-obesity potential. However, the application of IL-6 itself is at least limited by a narrow therapeutic range and its important function for a balanced inflammatory response. Further studies on the molecular basis of the metabolic effects of IL-6 could elucidate novel therapeutic strategies for custom-designed, IL-6-related substances.

Efeitos da ingestão prévia de carboidrato de alto índice glicêmico sobre a resposta glicêmica e desempenho durante um treino de força

O objetivo deste estudo foi examinar os efeitos da ingestão prévia de carboidrato no desempenho físico e comportamento glicêmico durante o treino de força. Oito voluntários realizaram 2 sessões de exercício de força (7 exercícios com 3 séries na intensidade de 70% de 1 repetição máxima), nas quais ingeriram bebida composta de carboidrato (maltodextrina) ou placebo. A bebida foi ingerida 15 minutos antes do início da sessão, a ordem das sessões foi randomizada, e essas foram separadas por 7 dias de intervalo. A glicemia foi mensurada em 4 momentos: antes da ingestão da bebida, 15 minutos após a ingestão da bebida, na metade do treino, e ao final do mesmo. O desempenho físico nos dois dias de treino foi influenciado somente pela variação no número das repetições executadas, as quais foram inseridas no cálculo da tonelagem total de treino executada nas respectivas sessões (repetições · séries · carga). A freqüência cardíaca foi continuamente monitorada e a concentração de lactato foi mensurada ao término da sessão. A glicemia esteve aumentada somente aos 15 minutos após a ingestão da bebida com carboidrato (de 98,25 ± 17,77mg/dL para 133,12 ± 22,76mg/dL, p = 0,015), enquanto que no dia da bebida placebo não foram observadas alterações significativas nestes momentos (de 98,25 ± 13,69mg/dL para 94,38 ± 12,21mg/dL, p = 1,000). A tonelagem total de treino, freqüência cardíaca e concentração final de lactato foram semelhantes nos dois treinos de força. Mesmo com o aumento da glicemia pré-exercício após a ingestão da bebida com carboidrato, os resultados do estudo não indicam que a ingestão prévia de carboidrato à sessão de exercício de força pode ser uma suplementação eficaz para aumentar o desempenho físico.

Physiological roles of muscle-derived interleukin-6 in response to exercise

PURPOSE OF REVIEW

To discuss recent findings with regard to the regulation of muscle-derived interleukin-6 as well as the possible physiological and metabolic roles of interleukin-6 in response to exercise.

RECENT FINDINGS

Contraction-induced transcription and release of interleukin-6 is primarily regulated by an altered intramuscular milieu in response to exercise. Accordingly, changes in calcium homeostasis, impaired glucose availability and increased formation of reactive oxygen species are all associated with exercise and capable of activating transcription factors known to regulate interleukin-6 synthesis. Acute interleukin-6 administration to humans increases lipolysis, fat oxidation and insulin-mediated glucose disposal. Adenosine monophosphate-activated protein kinase activation by interleukin-6 appears to play an important role in modulating some of these metabolic effects. Interleukin-6 facilitates an antiinflammatory milieu and may exert some of its biological effects via inhibition of the proinflammatory cytokine tumor necrosis factor-alpha.

SUMMARY

The discovery of contracting muscle as a cytokine-producing organ opens a new paradigm: skeletal muscle is an endocrine organ that in response to contractions produces and releases 'myokines', which subsequently can modulate the metabolic and immunological response to exercise in several tissues. In our view, interleukin-6 may be one of several myokines.

Physical activity and modulation of systemic low-level inflammation

It has been recognized for some time that cardiovascular disease and type 2 diabetes are, to a major extent, inflammatory disorders associated with an environment characterized by a sedentary lifestyle together with abundant intakes of calories. Systemic low-level inflammation is suggested to be a cause as well as consequence of pathological processes with local tumor necrosis factor alpha production as an important biological driver. It is hypothesized that physical inactivity contributes to an enhanced proinflammatory burden independently of obesity, as regular muscle contractions mediate signals with myokines/cytokines as important messengers, which suppress proinflammatory activity at distant sites as well as within skeletal muscle. Muscle-derived interleukin (IL)-6 is considered to possess a central role in anti-inflammatory activities and health beneficial effects in relation to physical exercise. It is discussed how this fits the consistent observation that enhanced plasma levels of IL-6 represent a strong risk marker in chronic disorders associated with systemic low-level inflammation and all-cause mortality.

An immune origin of type 2 diabetes?

Subclinical, low-grade systemic inflammation has been observed in patients with type 2 diabetes and in those at increased risk of the disease. This may be more than an epiphenomenon. Alleles of genes encoding immune/inflammatory mediators are associated with the disease, and the two major environmental factors the contribute to the risk of type 2 diabetes-diet and physical activity-have a direct impact on levels of systemic immune mediators. In animal models, targeting of immune genes enhanced or suppressed the development of obesity or diabetes. Obesity is associated with the infiltration and proinflammatory activity of macrophages in adipose tissue, and immune mediators may be important regulators of insulin resistance, mitochondrial function, ectopic lipid storage and beta cell dysfunction or death. Intervention studies targeting these pathways would help to determine the contribution of an activated innate immune system to the development of type 2 diabetes.

Muscle glycogen and metabolic regulation

Muscle glycogen is an important fuel for contracting skeletal muscle during prolonged strenuous exercise, and glycogen depletion has been implicated in muscle fatigue. It is also apparent that glycogen availability can exert important effects on a range of metabolic and cellular processes. These processes include carbohydrate, fat and protein metabolism during exercise, post-exercise glycogen resynthesis, excitation-contraction coupling, insulin action and gene transcription. For example, low muscle glycogen is associated with reduced muscle glycogenolysis, increased glucose and NEFA uptake and protein degradation, accelerated glycogen resynthesis, impaired excitation-contraction coupling, enhanced insulin action and potentiation of the exercise-induced increases in transcription of metabolic genes. Future studies should identify the mechanisms underlying, and the functional importance of, the association between glycogen availability and these processes.

Complex systems model of fatigue: integrative homoeostatic control of peripheral physiological systems during exercise in humans

Fatigue is hypothesised as being the result of the complex interaction of multiple peripheral physiological systems and the brain. In this new model, all changes in peripheral physiological systems such as substrate depletion or metabolite accumulation act as afferent signallers which modulate control processes in the brain in a dynamic, non-linear, integrative manner.

Regulation of fuel metabolism by preexercise muscle glycogen content and exercise intensity

To date, the results of studies that have examined the effects of altering preexercise muscle glycogen content and exercise intensity on endogenous carbohydrate oxidation are equivocal. Differences in the training status of subjects between investigations may, in part, explain these inconsistent findings. Accordingly, we determined the relative effects of exercise intensity and carbohydrate availability on patterns of fuel utilization in the same subjects who performed a random order of four 60-min rides, two at 45% and two at 70% of peak O2 uptake (V̇o2 peak), after exercise-diet intervention to manipulate muscle glycogen content. Preexercise muscle glycogen content was 596 ± 43 and 202 ± 21 mmol/kg dry mass ( P < 0.001) for high-glycogen (HG) and low-glycogen (LG) conditions, respectively. Respiratory exchange ratio was higher for HG than LG during exercise at both 45% (0.85 ± 0.01 vs. 0.74 ± 0.01; P < 0.001) and 70% (0.90 ± 0.01 vs. 0.79 ± 0.01; P < 0.001) of V̇o2 peak. The contribution of whole body muscle glycogen oxidation to energy expenditure differed between LG and HG for exercise at both 45% (5 ± 2 vs. 45 ± 5%; P < 0.001) and 70% (25 ± 3 vs. 60 ± 3%; P < 0.001) of V̇o2 peak. Yet, despite marked differences in preexercise muscle glycogen content and its subsequent utilization, rates of plasma glucose disappearance were similar under all conditions. We conclude that, in moderately trained individuals, muscle glycogen availability (low vs. high) does not influence rates of plasma glucose disposal during either low- or moderate-intensity exercise.

Statistical analyses of repeated measures in physiological research: a tutorial

Experimental designs involving repeated measurements on experimental units are widely used in physiological research. Often, relatively many consecutive observations on each experimental unit are involved and the data may be quite nonlinear. Yet evidently, one of the most commonly used statistical methods for dealing with such data sets in physiological research is the repeated-measurements ANOVA model. The problem herewith is that it is not well suited for data sets with many consecutive measurements; it does not deal with nonlinear features of the data, and the interpretability of the model may be low. The use of inappropriate statistical models increases the likelihood of drawing wrong conclusions. The aim of this article is to illustrate, for a reasonably typical repeated-measurements data set, how fundamental assumptions of the repeated-measurements ANOVA model are inappropriate and how researchers may benefit from adopting different modeling approaches using a variety of different kinds of models. We emphasize intuitive ideas rather than mathematical rigor. We illustrate how such models represent alternatives that 1) can have much higher interpretability, 2) are more likely to meet underlying assumptions, 3) provide better fitted models, and 4) are readily implemented in widely distributed software products.

Leg glucose and protein metabolism during an acute bout of resistance exercise in humans

The present study investigated the responses of leg glucose and protein metabolism during an acute bout of resistance exercise. Seven subjects (5 men, 2 women) were studied at rest and during a strenuous lower body resistance exercise regimen consisting of ∼8 sets of 10 repetitions of leg press at ∼75% 1 repetition maximum and 8 sets of 8 repetitions of knee extensions at ∼80% 1 repetition maximum. l-[ ring-2H5]phenylalanine was infused throughout the study for measurement of phenylalanine rates of appearance, disappearance, protein synthesis, and protein breakdown across the leg. Femoral arterial and venous blood samples were collected at rest and during exercise for determination of leg blood flow, concentrations of glucose, lactate, alanine, glutamine, glutamate, leucine, and phenylalanine, and phenylalanine enrichments. Muscle biopsies were obtained at rest and immediately after exercise. Leg blood flow was nearly three times ( P < 0.009) higher and glucose uptake more than five times higher ( P = 0.009) during exercise than at rest. Leg lactate release was 86 times higher than rest during the exercise bout. Although whole body phenylalanine rate of appearance, an indicator of whole body protein breakdown, was reduced during exercise; leg phenylalanine rate of appearance, rate of disappearance, protein synthesis, and protein breakdown did not change. Arterial and venous alanine concentrations and glutamate uptake were significantly higher during exercise than at rest. We conclude that lower body resistance exercise potently stimulates leg glucose uptake and lactate release. In addition, muscle protein synthesis is not elevated during a bout of resistance exercise.

Skeletal muscle adaptation: training twice every second day vs. training once daily

Low muscle glycogen content has been demonstrated to enhance transcription of a number of genes involved in training adaptation. These results made us speculate that training at a low muscle glycogen content would enhance training adaptation. We therefore performed a study in which seven healthy untrained men performed knee extensor exercise with one leg trained in a low-glycogen (Low) protocol and the other leg trained at a high-glycogen (High) protocol. Both legs were trained equally regarding workload and training amount. On day 1, both legs (Low and High) were trained for 1 h followed by 2 h of rest at a fasting state, after which one leg (Low) was trained for an additional 1 h. On day 2, only one leg (High) trained for 1 h. Days 1 and 2 were repeated for 10 wk. As an effect of training, the increase in maximal workload was identical for the two legs. However, time until exhaustion at 90% was markedly more increased in the Low leg compared with the High leg. Resting muscle glycogen and the activity of the mitochondrial enzyme 3-hydroxyacyl-CoA dehydrogenase increased with training, but only significantly so in Low, whereas citrate synthase activity increased in both Low and High. There was a more pronounced increase in citrate synthase activity when Low was compared with High. In conclusion, the present study suggests that training twice every second day may be superior to daily training.

Eating, exercise, and "thrifty" genotypes: connecting the dots toward an evolutionary understanding of modern chronic diseases

Survival of Homo sapiens during evolution was dependent on the procurement of food, which in turn was dependent on physical activity. However, food supply was never consistent. Thus it is contended that the ancient hunter-gatherer had cycles of feast and famine, punctuated with obligate periods of physical activity and rest. Hence, gene selection in the Late-Paleolithic era was probably influenced by physical activity and rest. To ensure survival during periods of famine, certain genes evolved to regulate efficient intake and utilization of fuel stores. Such genes were termed "thrifty genes" in 1962. Furthermore, convincing evidence shows that this ancient genome has remained essentially unchanged over the past 10,000 years and certainly not changed in the past 40-100 years. Although the absolute caloric intake of modern-day humans is likely lower compared with our hunter-gatherer ancestors, it is nevertheless in positive caloric balance in the majority of the US adult population mainly due to the increased sedentary lifestyle in present society. We contend that the combination of continuous food abundance and physical inactivity eliminates the evolutionarily programmed biochemical cycles emanating from feast-famine and physical activity-rest cycles, which in turn abrogates the cycling of certain metabolic processes, ultimately resulting in metabolic derangements such as obesity and Type 2 diabetes. In this context, we postulate that perhaps a crucial mechanism to break the stall of the metabolic processes would be via exercise through the regulation of "physical activity genes," some of which may also be potential candidates for the "thrifty genes" of our hunter-gatherer ancestors. Therefore, the identification of such "thrifty gene" candidates would help provide insight into the pathogenetic processes of the numerous physical inactivity-mediated disorders.

Cerebral carbohydrate cost of physical exertion in humans

Above a certain level of cerebral activation the brain increases its uptake of glucose more than that of O(2), i.e., the cerebral metabolic ratio of O(2)/(glucose + 12 lactate) decreases. This study quantified such surplus brain uptake of carbohydrate relative to O(2) in eight healthy males who performed exhaustive exercise. The arterial-venous differences over the brain for O(2), glucose, and lactate were integrated to calculate the surplus cerebral uptake of glucose equivalents. To evaluate whether the amount of glucose equivalents depends on the time to exhaustion, exercise was also performed with beta(1)-adrenergic blockade by metoprolol. Exhaustive exercise (24.8 +/- 6.1 min; mean +/- SE) decreased the cerebral metabolic ratio from a resting value of 5.6 +/- 0.2 to 3.0 +/- 0.4 (P < 0.05) and led to a surplus uptake of glucose equivalents of 9 +/- 2 mmol. beta(1)-blockade reduced the time to exhaustion (15.8 +/- 1.7 min; P < 0.05), whereas the cerebral metabolic ratio decreased to an equally low level (3.2 +/- 0.3) and the surplus uptake of glucose equivalents was not significantly different (7 +/- 1 mmol; P = 0.08). A time-dependent cerebral surplus uptake of carbohydrate was not substantiated and, consequently, exhaustive exercise involves a brain surplus carbohydrate uptake of a magnitude comparable with its glycogen content.

Influence of resistance exercise training on glucose control in women with type 2 diabetes

The objective of the study was to evaluate the effects of acute and chronic resistance training on glucose and insulin responses to a glucose load in women with type 2 diabetes. Subjects consisted of type 2 diabetic women (n = 7) and age-matched controls (n = 8) with normal glucose tolerance. All subjects participated in 3 oral glucose tolerance tests: pretraining, 12 to 24 hours after the first exercise session (acute) and 60 to 72 hours after the final training session (chronic). Exercise training consisted of a whole body resistance exercise program using weight-lifting machines 3 days per week for 6 weeks. Resistance training was effective in increasing strength of all muscle groups in all subjects. Integrated glucose concentration expressed as area under the curve (AUC) was 3,355.0 +/- 324.6 mmol/L. min pretraining, improved significantly (P <.01) after the acute bout of exercise (2,868 +/- 324.0 mmol/L. min), but was not improved with chronic training (3,206.0 +/- 337.0 mmol/L. min) in diabetic subjects. A similar pattern of significance was observed with peak glucose concentration (pre: 20.2 +/-1.4 mmol/L; acute: 17.2 +/- 1.7 mmol/L; chronic: 19.9 +/- 1.7 mmol/L). There were no significant changes in insulin concentrations after any exercise bout in the diabetic subjects. There were no changes in glucose or insulin levels in control subjects. An acute bout of resistance exercise was effective in improving integrated glucose concentration, including reducing peak glucose concentrations in women with type 2 diabetes, but not age-matched controls. There were no significant changes in insulin concentrations for either group. Resistance exercise offers an alternative to aerobic exercise for improving glucose control in diabetic patients. To realize optimal glucose control benefits, individuals must follow a regular schedule that includes daily exercise.

Glucose ingestion blunts hormone-sensitive lipase activity in contracting human skeletal muscle

To examine the effect of attenuated epinephrine and elevated insulin on intramuscular hormone sensitivity lipase activity (HSLa) during exercise, seven men performed 120 min of semirecumbent cycling (60% peak pulmonary oxygen uptake) on two occasions while ingesting either 250 ml of a 6.4% carbohydrate (GLU) or sweet placebo (CON) beverage at the onset of, and at 15 min intervals throughout, exercise. Muscle biopsies obtained before and immediately after exercise were analyzed for HSLa. Blood samples were simultaneously obtained from a brachial artery and a femoral vein before and during exercise, and leg blood flow was measured by thermodilution in the femoral vein. Net leg glycerol and lactate release and net leg glucose and free fatty acid (FFA) uptake were calculated from these measures. Insulin and epinephrine were also measured in arterial blood before and throughout exercise. During GLU, insulin was elevated (120 min: CON, 11.4 +/- 2.4, GLU, 35.3 +/- 6.9 pM, P < 0.05) and epinephrine suppressed (120 min: CON, 6.1 +/- 2.5, GLU, 2.1 +/- 0.9 nM; P < 0.05) compared with CON. Carbohydrate feeding also resulted in suppressed (P < 0.05) HSLa relative to CON (120 min: CON, 1.71 +/- 0.18, GLU, 1.27 +/- 0.16 mmol.min-1.kg dry mass-1). There were no differences in leg lactate or glycerol release when trials were compared, but leg FFA uptake was lower (120 min: CON, 0.29 +/- 0.06, GLU, 0.82 +/- 0.09 mmol/min) and leg glucose uptake higher (120 min: CON, 3.16 +/- 0.59, GLU, 1.37 +/- 0.37 mmol/min) in GLU compared with CON. These results demonstrate that circulating insulin and epinephrine play a role in HSLa in contracting skeletal muscle.

Inhibition of alpha-adrenergic vasoconstriction in exercising human thigh muscles

The mechanisms underlying metabolic inhibition of sympathetic responses within exercising skeletal muscle remain incompletely understood. The aim of the present study was to test whether alpha(2)-adrenoreceptor-mediated vasoconstriction was more sensitive to metabolic inhibition than alpha(1)-vasoconstriction during dynamic knee-extensor exercise. We studied healthy volunteers using two protocols: (1) wide dose ranges of the alpha-adrenoreceptor agonists phenylephrine (PE, alpha(1) selective) and BHT-933 (BHT, alpha(2) selective) were administered intra-arterially at rest and during 27 W knee-extensor exercise (n= 13); (2) flow-adjusted doses of PE (0.3 microg kg(-1) l(-1)) and BHT (15 microg kg(-1) l(-1)) were administered at rest and during ramped exercise (7 W to 37 W; n= 10). Ultrasound Doppler and thermodilution techniques provided direct measurements of femoral blood flow (FBF). PE (0.8 microg kg(-1)) and BHT (40 microg kg(-1)) produced comparable maximal reductions in FBF at rest (-58 +/- 6 versus-64 +/- 4%). Despite increasing the doses, PE (1.6 microg kg(-1) min(-1)) and BHT (80 microg kg(-1) min(-1)) caused significantly smaller changes in FBF during 27 W exercise (-13 +/- 4 versus-3 +/- 5%). During ramped exercise, significant vasoconstriction at lower intensities (7 and 17 W) was seen following PE (-16 +/- 5 and -16 +/- 4%), but not BHT (-2 +/- 4 and -4 +/- 5%). At the highest intensity (37 W), FBF was not significantly changed by either drug. Collectively, these data demonstrate metabolic inhibition of alpha-adrenergic vasoconstriction in large postural muscles of healthy humans. Both alpha(1)- and alpha(2)-adrenoreceptor agonists produce comparable vasoconstriction in the resting leg, and dynamic thigh exercise attenuates alpha(1)- and alpha(2)-mediated vasoconstriction similarly. However, alpha(2)-mediated vasoconstriction appears more sensitive to metabolic inhibition, because alpha(2) is completely inhibited even at low workloads, whereas alpha(1) becomes progressively inhibited with increasing workloads.

Signalling to glucose transport in skeletal muscle during exercise

Exercise-induced glucose uptake in skeletal muscle is mediated by an insulin-independent mechanism. Although the signalling events that increase glucose transport in response to muscle contraction are not fully elucidated, the aim of the present review is to briefly present the current understanding of the molecular signalling mechanisms involved. Glucose uptake may be regulated by Ca++-sensitive contraction-related mechanisms possibly involving protein kinase C, and by mechanisms that reflect the metabolic status of the muscle and may involve the AMP-activated protein kinase. Furthermore the p38 mitogen activated protein kinase may be involved. Still, the picture is incomplete and a substantial part of the exercise/contraction-induced signalling mechanism to glucose transport remains unknown.

Glucose ingestion attenuates interleukin-6 release from contracting skeletal muscle in humans

To examine whether glucose ingestion during exercise affects the release of interleukin-6 (IL-6) from the contracting limb, seven men performed 120 min of semi-recumbent cycling on two occasions while ingesting either 250 ml of a 6.4 % carbohydrate (GLU trial) or sweet placebo (CON trial) beverage at the onset of, and at 15 min intervals throughout, exercise. Muscle biopsies obtained before and immediately after exercise were analysed for glycogen and IL-6 mRNA expression. Blood samples were simultaneously obtained from a brachial artery and a femoral vein prior to and during exercise and leg blood flow was measured by thermodilution in the femoral vein. Net leg IL-6 release, and net leg glucose and free fatty acid (FFA) uptake, were calculated from these measurements. The arterial IL-6 concentration was lower (P < 0.05) after 120 min of exercise in GLU, but neither intramuscular glycogen nor IL-6 mRNA were different when comparing GLU with CON. However, net leg IL-6 release was attenuated (P < 0.05) in GLU compared with CON. This corresponded with an enhanced (P < 0.05) glucose uptake and a reduced (P < 0.05) FFA uptake in GLU. These results demonstrate that glucose ingestion during exercise attenuates leg IL-6 release but does not decrease intramuscular expression of IL-6 mRNA.

Acute interleukin-6 administration does not impair muscle glucose uptake or whole-body glucose disposal in healthy humans

The cytokine interleukin (IL)-6 has recently been linked with type 2 diabetes mellitus and has been suggested to affect glucose metabolism. To determine whether acute IL-6 administration affects whole-body glucose kinetics or muscle glucose uptake, 18 healthy young men were assigned to one of three groups receiving a high dose of recombinant human IL-6 (HiIL-6; n = 6), a low dose of IL-6 (LoIL-6; n = 6) or saline (Con; n = 6) infused into one femoral artery for 3 h. The stable isotope [6,6-2H2] glucose was infused into a forearm vein throughout the 3 h infusion period and for a further 3 h after the cessation of infusion (recovery) to determine endogenous glucose production and whole-body glucose disposal. Infusion with HiIL-6 and LoIL-6 resulted in a marked (P < 0.05) increase in systemic IL-6 concentration throughout the 3 h of infusion (mean arterial plasma [IL-6]s of 319 and 143 pg ml-1 for HiIL-6 and LoIL-6, respectively), followed by a rapid decline (P < 0.05) during the recovery period. Subjects experienced clinical symptoms such as shivering and discomfort during HiIL-6 administration, but were asymptomatic during LoIL-6 administration. In addition, only HiIL-6 elevated (P < 0.05) plasma adrenaline (epinephrine). IL-6 infusion, irrespective of dose, did not result in any changes to endogenous glucose production, whole-body glucose disposal or leg- glucose uptake. These data demonstrate that acute IL-6 administration does not impair whole-body glucose disposal, net leg-glucose uptake, or increase endogenous glucose production at rest in healthy young humans.

Myogenin induces higher oxidative capacity in pre-existing mouse muscle fibres after somatic DNA transfer

Muscle is a permanent tissue, and in the adult pronounced changes can occur in pre-existing fibres without the formation of new fibres. Thus, the mechanisms responsible for phenotype transformation in the adult might be distinct from mechanisms regulating muscle differentiation during muscle formation and growth. Myogenin is a muscle-specific, basic helix-loop-helix transcription factor that is important during early muscle differentiation. It is also expressed in the adult, where its role is unknown. In this study we have overexpressed myogenin in glycolytic fibres of normal adult mice by electroporation and single-cell intracellular injection of expression vectors. Myogenin had no effects on myosin heavy chain fibre type, but induced a considerable increase in succinate dehydrogenase and NADH dehydrogenase activity, with some type IIb fibres reaching the levels observed histochemically in normal type IIx and IIa fibres. mRNA levels for malate dehydrogenase were similarly altered. The size of the fibres overexpressing myogenin was reduced by 30-50 %. Thus, the transfected fibres acquired a phenotype reminiscent of the phenotype obtained by endurance training in man and other animals, with a higher oxidative capacity and smaller size. We conclude that myogenin can alter pre-existing glycolytic fibres in the intact adult animal.