Glucose uptake is increased in trained vs. untrained muscle during heavy exercise

Exercise performance and health: Role of GLUT4

The glucose transporter GLUT4 is integral for optimal skeletal muscle performance during exercise, as well as for metabolic health. Physiological regulation of GLUT4 translocation during exercise and increased GLUT4 expression following exercise involves multiple, redundant signalling pathways. These include effects of reactive oxygen species (ROS). ROS contribute to GLUT4 translocation that increases skeletal muscle glucose uptake during exercise and stimulate signalling pathways that increase GLUT4 expression. Conversely, ROS can also inhibit GLUT4 translocation and expression in metabolic disease states. The opposing roles of ROS in GLUT4 regulation are ultimately linked to the metabolic state of skeletal muscle and the intricate mechanisms involved give insights into pathways critical for exercise performance and implicated in metabolic health and disease.

GLUT4 localisation with the plasma membrane is unaffected by an increase in plasma free fatty acid availability

BACKGROUND

Insulin-stimulated glucose uptake into skeletal muscle occurs via translocation of GLUT4 from intracellular storage vesicles to the plasma membrane. Elevated free fatty acid (FFA) availability via a lipid infusion reduces glucose disposal, but this occurs in the absence of impaired proximal insulin signalling. Whether GLUT4 localisation to the plasma membrane is subsequently affected by elevated FFA availability is not known.

METHODS

Trained (n = 11) and sedentary (n = 10) individuals, matched for age, sex and body mass index, received either a 6 h lipid or glycerol infusion in the setting of a concurrent hyperinsulinaemic-euglycaemic clamp. Sequential muscle biopsies (0, 2 and 6 h) were analysed for GLUT4 membrane localisation and microvesicle size and distribution using immunofluorescence microscopy.

RESULTS

At baseline, trained individuals had more small GLUT4 spots at the plasma membrane, whereas sedentary individuals had larger GLUT4 spots. GLUT4 localisation with the plasma membrane increased at 2 h (P = 0.04) of the hyperinsulinemic-euglycemic clamp, and remained elevated until 6 h, with no differences between groups or infusion type. The number of GLUT4 spots was unchanged at 2 h of infusion. However, from 2 to 6 h there was a decrease in the number of small GLUT4 spots at the plasma membrane (P = 0.047), with no differences between groups or infusion type.

CONCLUSION

GLUT4 localisation with the plasma membrane increases during a hyperinsulinemic-euglycemic clamp, but this is not altered by elevated FFA availability. GLUT4 appears to disperse from small GLUT4 clusters located at the plasma membrane to support glucose uptake during a hyperinsulinaemic-euglycaemic clamp.

Exercise induces tissue-specific adaptations to enhance cardiometabolic health

The risk associated with multiple cancers, cardiovascular disease, diabetes, and all-cause mortality is decreased in individuals who meet the current recommendations for physical activity. Therefore, regular exercise remains a cornerstone in the prevention and treatment of non-communicable diseases. An acute bout of exercise results in the coordinated interaction between multiple tissues to meet the increased energy demand of exercise. Over time, the associated metabolic stress of each individual exercise bout provides the basis for long-term adaptations across tissues, including the cardiovascular system, skeletal muscle, adipose tissue, liver, pancreas, gut, and brain. Therefore, regular exercise is associated with a plethora of benefits throughout the whole body, including improved cardiorespiratory fitness, physical function, and glycemic control. Overall, we summarize the exercise-induced adaptations that occur within multiple tissues and how they converge to ultimately improve cardiometabolic health.

The physiology of ice hockey performance: An update

Ice hockey is an intense team sport characterized by repeated bursts of fast-paced skating, rapid changes in speed and direction and frequent physical encounters. These are performed in on-ice shifts of ~30-80 s interspersed with longer sequences of passive recovery, resulting in about 15-25 min on-ice time per player. Nearly 50% of the distance is covered at high-intensity skating speeds and with an accentuated intense activity pattern in forwards compared to defensemen. During ice hockey match-play, both aerobic and anaerobic energy systems are significantly challenged, with the heart rate increasing toward maximum levels during each shift, and with great reliance on both glycolytic and phosphagen ATP provision. The high-intensity activity pattern favors muscle glycogen as fuel, leading to pronounced reductions despite the relatively brief playing time, including severe depletion of a substantial proportion of individual fast- and slow-twitch fibers. Player-tracking suggests that the ability to perform high-intensity skating is compromised in the final stages of a game, which is supported by post-game reductions in repeated-sprint ability. Muscle glycogen degradation, in particular in individual fibers, as well as potential dehydration and hyperthermia, may be prime candidates implicated in exacerbated fatigue during the final stages of a game, whereas multiple factors likely interact to impair exercise tolerance during each shift. This includes pronounced PCr degradation, with potential inadequate resynthesis in a proportion of fast-twitch fibers in situations of repeated intense actions. Finally, the recovery pattern is inadequately described, but seems less long-lasting than in other team sports.

Acute physiological outcomes of high-intensity functional training: a scoping review

Background

Systematic reviews and meta-analyses related to high-intensity functional training (HIFT) have been conducted. However, due to a restricted pool of available research, these investigations are often limited in scope. As such, a scoping review investigating the present literature surrounding the acute physiological response to HIFT-based exercise was chosen as a more appropriate structured review.

Methodology

A scoping review was conducted following Arksey and O'Malley's framework. Three large scale databases were searched to reveal any article pertaining to HIFT and related exercise terminology.

Results

A total of 2,241 articles were found during the initial search. Following this, titles, then abstracts, and full-texts were reviewed to determine inclusion eligibility. A total of 60 articles which investigated a combined total of 35 unique HIFT workouts were included within this review.

Conclusions

A variety of physiological parameters and HIFT workouts have been examined. Markers of intensity (e.g., blood lactate concentrations, heart rate) have been most consistently assessed across all studies, and these support the idea that HIFT workouts are typically performed at high-intensity. In contrast, the inclusion of most other measures (e.g., hormonal, markers of inflammation and damage, energy expenditure, performance) has been inconsistent and has thus, limited the possibility for making generalized conclusions. Differences in study methodologies have further impacted conclusions, as different studies have varied in sample population characteristics, workouts assessed, and time points. Though it may be impossible to comprehensively research all possible HIFT workouts, consistent adoption of population definitions and workload quantification may overcome this challenge and assist with future comparisons.

Effects of exercise training on glycaemic control in youths with type 1 diabetes: a systematic review and meta-analysis of randomised controlled trials

AbstractThe aim of the study is to evaluate whether exercise interventions are associated with improved glycaemic control in children and adolescents with type 1 diabetes mellitus (T1DM), and to examine its relationship with the characteristics of the intervention (i.e., type, intensity, length, and duration of the sessions). Eligible criteria were randomised controlled trials of youth aged 6-18 years with T1DM, participating in an exercise-based intervention where glycaemic control is measured (i.e., glycated haemoglobin [HbA1c]). Pooled effect sizes (Hedges'g) were calculated using random-effects inverse-variance analyses. Fourteen studies enrolling 509 patients were analysed. Effect size was expressed as Hedges' g to correct for possible small sample bias. Overall, HbA1c levels in the exercise group (g=-0.38 95% confidence interval [CI], -0.66 to -0.11; mean difference [MD]=-0.62%) were reduced compared with the control group. Concurrent training (g=-0.63 95%CI, -1.05 to -0.21), high-intensity exercise (g=-0.43 95%CI, -0.83 to -0.03), interventions ≥24 weeks (g=-0.92 95%CI, -1.44 to -0.40), and sessions ≥60 minutes (g=-0.71 95%CI, -1.05 to -0.08) showed larger changes (MD=-0.66% to 1.30%). In conclusion, our study suggests that programs longer than 24 weeks with at least 60 min/session of high-intensity concurrent exercise may serve as a supportive therapy to metabolic control in youth with T1DM.

Exercise and health: historical perspectives and new insights

Since ancient times, the health benefits of regular physical activity/exercise have been recognized and the classic studies of Morris and Paffenbarger provided the epidemiological evidence in support of such an association. Cardiorespiratory fitness, often measured by maximal oxygen uptake, and habitual physical activity levels are inversely related to mortality. Thus, studies exploring the biological bases of the health benefits of exercise have largely focused on the cardiovascular system and skeletal muscle (mass and metabolism), although there is increasing evidence that multiple tissues and organ systems are influenced by regular exercise. Communication between contracting skeletal muscle and multiple organs has been implicated in exercise benefits, as indeed has other interorgan "cross-talk." The application of molecular biology techniques and "omics" approaches to questions in exercise biology has opened new lines of investigation to better understand the beneficial effects of exercise and, in so doing, inform the optimization of exercise regimens and the identification of novel therapeutic strategies to enhance health and well-being.

Exogenous carbohydrate and regulation of muscle carbohydrate utilisation during exercise

PURPOSE

Carbohydrates (CHO) are one of the fundamental energy sources during prolonged steady state and intermittent exercise. The consumption of exogenous CHO during exercise is common place, with the aim to enhance sporting performance. Despite the popularity around exogenous CHO use, the process by which CHO is regulated from intake to its use in the working muscle is still not fully appreciated. Recent studies utilizing the hyperglycaemic glucose clamp technique have shed light on some of the potential barriers to CHO utilisation during exercise. The present review addresses the role of exogenous CHO utilisation during exercise, with a focus on potential mechanisms involved, from glucose uptake to glucose delivery and oxidation at the different stages of regulation.

METHODS

Narrative review.

RESULTS

A number of potential barriers were identified, including gastric emptying, intestinal absorption, blood flow (splanchnic and muscle), muscle uptake and oxidation. The relocation of glucose transporters plays a key role in the regulation of CHO, particularly in epithelial cells and subsequent transport into the blood. Limitations are also apparent when CHO is infused, particularly with regards to blood flow and uptake within the muscle.

CONCLUSION

We highlight a number of potential barriers involved with the regulation of both ingested and infused CHO during exercise. Future work on the influence of longitudinal training within the regulation processes (such as the gut) is warranted to further understand the optimal type, dose and method of CHO delivery to enhance sporting performance.

Is GLUT4 translocation the answer to exercise-stimulated muscle glucose uptake?

Exercise in humans increases muscle glucose uptake up to 100-fold compared with rest. The magnitude of increase depends on exercise intensity and duration. Although knockout of glucose transporter type 4 (GLUT4) convincingly has shown that GLUT4 is necessary for exercise to increase muscle glucose uptake, studies only show an approximate twofold increase in GLUT4 translocation to the muscle cell membrane when transitioning from rest to exercise. Therefore, there is a big discrepancy between the increase in glucose uptake and GLUT4 translocation. It is suggested that either the methods for measurements of GLUT4 translocation in muscle grossly underestimate the real translocation of GLUT4 or, alternatively, GLUT4 intrinsic activity increases in muscle during exercise, perhaps due to increased muscle temperature and/or mechanical effects during contraction/relaxation cycles.

Tuning fatty acid oxidation in skeletal muscle with dietary fat and exercise

Both the consumption of a diet rich in fatty acids and exercise training result in similar adaptations in several skeletal muscle proteins. These adaptations are involved in fatty acid uptake and activation within the myocyte, the mitochondrial import of fatty acids and further metabolism of fatty acids by β-oxidation. Fatty acid availability is repeatedly increased postprandially during the day, particularly during high dietary fat intake and also increases during, and after, aerobic exercise. As such, fatty acids are possible signalling candidates that regulate transcription of target genes encoding proteins involved in muscle lipid metabolism. The mechanism of signalling might be direct or indirect targeting of peroxisome proliferator-activated receptors by fatty acid ligands, by fatty acid-induced NAD⁺-stimulated activation of sirtuin 1 and/or fatty acid-mediated activation of AMP-activated protein kinase. Lactate might also have a role in lipid metabolic adaptations. Obesity is characterized by impairments in fatty acid oxidation capacity, and individuals with obesity show some rigidity in increasing fatty acid oxidation in response to high fat intake. However, individuals with obesity retain improvements in fatty acid oxidation capacity in response to exercise training, thereby highlighting exercise training as a potential method to improve lipid metabolic flexibility in obesity.

Exercise and GLUT4

The glucose transporter GLUT4 is critical for skeletal muscle glucose uptake in response to insulin and muscle contraction/exercise. Exercise increases GLUT4 translocation to the sarcolemma and t-tubule and, over the longer term, total GLUT4 protein content. Here, we review key aspects of GLUT4 biology in relation to exercise, with a focus on exercise-induced GLUT4 translocation, postexercise metabolism and muscle insulin sensitivity, and exercise effects on GLUT4 expression.

Benefits of physical activity in children and adolescents with type 1 diabetes: A systematic review

AIMS

We performed a systematic review of clinical trials investigating the health benefits of physical activity in pediatric patients with type 1 diabetes.

METHODS

To perform this systematic review, search strategies were created and adapted to four databases. Only randomized controlled trials written in English before 1998 and that answered to the PICOS criteria were included. The PRISMA guidelines were followed to ensure highest scientific rigor within studies.

RESULTS

Seven studies out of 2655 were included in this systematic review according to the inclusion criteria. These studies showed positive gains on global health: blood lipid profile, physical fitness, quality of life and body size and body composition but only one demonstrated a positive effect on glycemic control.

CONCLUSION

Globally, physical activity exerts a positive impact on metabolic (i.e., decrease in total cholesterol, improvement of physical fitness, etc.) and psychological health in children with type 1 diabetes. Yet variations in study protocols or sample size restrict statistical power to reach the outcome of improving glycemic control in most studies. Here, we address the measured outcomes in individual trials and discuss potential key elements to consider for future clinical trials.

Blood flow-restricted training enhances thigh glucose uptake during exercise and muscle antioxidant function in humans

This study examined the effects of blood-flow-restricted (BFR)-training on thigh glucose uptake at rest and during exercise in humans and the muscular mechanisms involved. Ten active men (~25 y; VO_2max ~50 mL/kg/min) completed six weeks of training, where one leg trained with BFR (cuff pressure: ~180 mmHg) and the other leg without BFR. Before and after training, thigh glucose uptake was determined at rest and during exercise at 25% and 90% of leg incremental peak power output by sampling of femoral arterial and venous blood and measurement of femoral arterial blood flow. Furthermore, resting muscle samples were collected. After training, thigh glucose uptake during exercise was higher than before training only in the BFR-trained leg (p < 0.05) due to increased glucose extraction (p < 0.05). Further, BFR-training substantially improved time to exhaustion during exhaustive exercise (11 ± 5% vs. CON-leg; p = 0.001). After but not before training, NAC infusion attenuated (~50-100%) leg net glucose uptake and extraction during exercise only in the BFR-trained leg, which coincided with an increased muscle abundance of Cu/Zn-SOD (39%), GPX-1 (29%), GLUT4 (28%), and nNOS (18%) (p < 0.05). Training did not affect Mn-SOD, catalase, and VEGF abundance in either leg (p > 0.05), although Mn-SOD was higher in BFR-leg vs. CON-leg after training (p < 0.05). The ratios of p-AMPK-Thr¹⁷²/AMPK and p-ACC-Ser⁷⁹/ACC, and p-ACC-Ser⁷⁹, remained unchanged in both legs (p > 0.05), despite a higher p-AMPK-Thr¹⁷² in BFR-leg after training (38%; p < 0.05). In conclusion, BFR-training enhances glucose uptake by exercising muscles in humans probably due to an increase in antioxidant function, GLUT4 abundance, and/or NO availability.

Bilateral Asymmetries in Ultrasound Assessments of the Rectus Femoris throughout an NCAA Division I Volleyball Preseason

The purpose of the study was to assess glycogen content of the rectus femoris (RF) muscles utilizing high-frequency ultrasound throughout an intensive, nine-day preseason training period in NCAA division I volleyball athletes. In the morning prior to the beginning of practice, athletes (n = 13) left and right RF muscles were assessed via ultrasound to quantify muscle fuel ratings (0–100 score range). The recommended location of the RF ultrasound scans were based on manufacturer guidelines, and the same technician recorded the daily measurements. To assess daily training load, session ratings of perceived exertion (s-RPE) were utilized. A paired t-test revealed a large significant difference between left (51.7 ± 17.9) and right (32.8 ± 17.4) RF muscle fuel ratings (p < 0.001). There was also a major effect of time on s-RPE (p < 0.001) and left (dominant) RF fuel rating (p = 0.001). s-RPE decreased from the beginning to the end of the training camp. However, left RF fuel ratings increased from the first to the second day, then remained elevated all throughout the preseason. In conclusion, all athletes were left-leg dominant and had a 57.6% bilateral asymmetry between their left and right RF muscle fuel ratings despite changes in training load. High-frequency ultrasounds are a noninvasive assessment tool that can determine glycogen replenishment asymmetries in the RF.

Exercise Metabolism: Fuels for the Fire

During exercise, the supply of adenosine triphosphate (ATP) is essential for the energy-dependent processes that underpin ongoing contractile activity. These pathways involve both substrate-level phosphorylation, without any need for oxygen, and oxidative phosphorylation that is critically dependent on oxygen delivery to contracting skeletal muscle by the respiratory and cardiovascular systems and on the supply of reducing equivalents from the degradation of carbohydrate, fat, and, to a limited extent, protein fuel stores. The relative contribution of these pathways is primarily determined by exercise intensity, but also modulated by training status, preceding diet, age, gender, and environmental conditions. Optimal substrate availability and utilization before, during, and after exercise is critical for maintaining exercise performance. This review provides a brief overview of exercise metabolism, with expanded discussion of the regulation of muscle glucose uptake and fatty acid uptake and oxidation.

Actovegin, a non-prohibited drug increases oxidative capacity in human skeletal muscle

Actovegin, a deproteinized haemodialysate of calf blood, is suggested to have ergogenic properties, but this potential effect has never been investigated in human skeletal muscle. To investigate this purported ergogenic effect, we measured the mitochondrial respiratory capacity in permeabilized human skeletal muscle fibres acutely exposed to Actovegin in a low and in a high dose. We found that Actovegin, in the presence of complex I-linked substrates increased the oxidative phosphorylation (OXPHOS) capacity significantly in a concentration-dependent manner (19 ± 3, 31 ± 4 and 45 ± 4 pmol/mg/s). Maximal OXPHOS capacity with complex I and II-linked substrate was increased when the fibres were exposed to the high dose of Actovegin (62 ± 6 and 77 ± 6 pmol/mg/s) (p < .05). The respiratory capacity of the electron transfer system as well as Vmax and Km were also increased in a concentration-dependent manner after Actovegin exposure (70 ± 6, 79 ± 6 and 88 ± 7 pmol/mg/s; 13 ± 2, 25 ± 3 and 37 ± 4 pmol/mg/s; 0.08 ± 0.02, 0.21 ± 0.03 and 0.36 ± 0.03 mM, respectively) (p < .05). In summary, we report for the first time that Actovegin has a marked effect on mitochondrial oxidative function in human skeletal muscle. Mitochondrial adaptations like this are also seen after a training program in human subjects. Whether this improvement translates into an ergogenic effect in athletes and thus reiterates the need to include Actovegin on the World Anti-Doping Agency's active list remains to be investigated.

Quantitative immunofluorescence microscopy of subcellular GLUT4 distribution in human skeletal muscle: effects of endurance and sprint interval training

Increases in insulin‐mediated glucose uptake following endurance training (ET) and sprint interval training (SIT) have in part been attributed to concomitant increases in glucose transporter 4 (GLUT4) protein content in skeletal muscle. This study used an immunofluorescence microscopy method to investigate changes in subcellular GLUT4 distribution and content following ET and SIT. Percutaneous muscle biopsy samples were taken from the m. vastus lateralis of 16 sedentary males in the overnight fasted state before and after 6 weeks of ET and SIT. An antibody was fully validated and used to show large (> 1 μm) and smaller (<1 μm) GLUT4‐containing clusters. The large clusters likely represent trans‐Golgi network stores and the smaller clusters endosomal stores and GLUT4 storage vesicles (GSVs). Density of GLUT4 clusters was higher at the fibre periphery especially in perinuclear regions. A less dense punctate distribution was seen in the rest of the muscle fibre. Total GLUT4 fluorescence intensity increased in type I and type II fibres following both ET and SIT. Large GLUT4 clusters increased in number and size in both type I and type II fibres, while the smaller clusters increased in size. The greatest increases in GLUT4 fluorescence intensity occurred within the 1 μm layer immediately adjacent to the PM. The increase in peripheral localisation and protein content of GLUT4 following ET and SIT is likely to contribute to the improvements in glucose homeostasis observed after both training modes.

e12085

This paper first describes the development of a novel confocal immunofluorescence microscopy method that allows quantitation of GLUT4 content in the plasma membrane and 1 µm layers below it in a muscle fibre‐type specific manner. Skeletal muscle biopsies obtained from sedentary young men before and after 6 weeks of traditional endurance training (ET) and sprint interval training (SIT) were then analysed to show for the first time increases in large and small GLUT4 clusters with greater increases in the layer within 1 µm of the plasma membrane, which is the layer from which most GLUT4 fusion events emanate. This training‐induced redistribution is likely to contribute to the increase in insulin sensitivity seen following both ET and SIT.

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.

Effect of exhaustive ultra-endurance exercise in muscular glycogen and both Alpha1 and Alpha2 Ampk protein expression in trained rats

Glycogen is the main store of readily energy in skeletal muscle and plays a key role in muscle function, demonstrated by the inability to sustain prolonged high-intensity exercise upon depletion of these glycogen stores. With prolonged exercise, glycogen depletion occurs and 5'-AMP-activated protein kinase (AMPK), a potent regulator of muscle metabolism and gene expression, is activated promoting molecular signalling that increases glucose uptake by muscular skeletal cells. The aim of this study was primarily to determine the effect of ultra-endurance exercise on muscle glycogen reserves and secondly to verify the influence of this type of exercise on AMPK protein expression. Twenty-four male Wistar rats, 60 days old, were divided into four experimental groups: sedentary, sedentary exhausted (SE), endurance trained (T) and endurance trained exhausted (TE). The animals ran for 10 to 90 min/day, 5 days/week, for 12 weeks to attain trained status. Rats were killed immediately after the exhaustion protocol, which consisted of running on a treadmill (at approximately 60% Vmax until exhaustion). Optical density of periodic acid-Schiff was detected and glycogen depletion observed predominantly in type I muscle fibres of the TE group and in both type I and II muscle fibres in the SE group. Plasma glucose decreased only in the TE group. Hepatic glycogen was increased in T group and significantly depleted in TE group. AMPK protein expression was significantly elevated in TE and T groups. In conclusion, acute exhaustive ultra-endurance exercise promoted muscle glycogen depletion. It seems that total AMPK protein and gene expression is more influenced by status training.

Is exercise a therapeutic tool for improvement of cardiovascular risk factors in adolescents with type 1 diabetes mellitus? A randomised controlled trial

BACKGROUND

Type 1 diabetes mellitus (T1DM) is associated with a high risk for early atherosclerotic complications especially risk of coronary heart disease.

OBJECTIVE

To evaluate the impact of six months exercise prgram on glycemic control, plasma lipids values, blood pressure, severity and frequency of hypoglycemia, anthropometric measurements and insulin dose in a sample of adolescents with T1DM.

RESEARCH DESIGN AND METHODS

A total of 196 type 1 diabetic patients participated in the study. They were classified into three groups: Group (A) did not join the exercise program(n = 48), group (B) attended the exercise sessions once/week (n = 75), group (C) attended the exercise sessions three times/week (n = 73). Studied parameters were evaluated before and six months after exercise programe.

RESULTS

Exercise improved glycemic control by reducing HbA1c values in exercise groups (P = 0.03, P = 0.01 respectively) and no change in those who were not physically active (P = 0.2). Higher levels of HbA1c were associated with higher levels of cholesterol, LDL-c, and triglycerides (P = 0.000 each). In both groups, B and C, frequent exercise improved dyslipidemia and reduced insulin requirements significantly (P = 0.00 both), as well as a reduction in BMI (P = 0.05, P = 0.00 respectively) and waist circumference(P = 0.02, P = 0.00 respectively). The frequency of hypoglycemic attacks were not statistically different between the control group and both intervention groups (4.7 +/- 3.56 and 4.82 +/- 4.23, P = 0.888 respectively). Reduction of blood pressure was statistically insignificant apart from the diastolic blood presure in group C (P = 0.04).

CONCLUSION

Exercise is an indispensable component in the medical treatment of patients with T1DM as it improves glycemic control and decreases cardiovascular risk factors among them.

Effects of contraction on localization of GLUT4 and v-SNARE isoforms in rat skeletal muscle

In skeletal muscle, contractions increase glucose uptake due to a translocation of GLUT4 glucose transporters from intracellular storage sites to the surface membrane. Vesicle-associated membrane proteins (VAMPs) are believed to play an important role in docking and fusion of the GLUT4 transporters at the surface membrane. However, knowledge about which VAMP isoforms colocalize with GLUT4 vesicles in mature skeletal muscle and whether they translocate during muscle contractions is incomplete. The aim of the present study was to further identify VAMP isoforms, which are associated with GLUT4 vesicles and examine which VAMP isoforms translocate to surface membranes in skeletal muscles undergoing contractions. VAMP2, VAMP3, VAMP5, and VAMP7 were enriched in immunoprecipitated GLUT4 vesicles. In response to 20 min of in situ contractions, there was a redistribution of GLUT4 (+64 +/- 13%), transferrin receptor (TfR; +75 +/- 22%), and insulin-regulated aminopeptidase (IRAP; +70 +/- 13%) to fractions enriched in heavy membranes away from low-density membranes (-32 +/- 7%; -18 +/- 12%; -33 +/- 9%; respectively), when compared with the resting contralateral muscle. Similarly, there was a redistribution of VAMP2 (+240 +/- 40%), VAMP5 (+79 +/- 9%), and VAMP7 (+79 +/- 29%), but not VAMP3, to fractions enriched in heavy membranes away from low-density membranes (-49 +/- 10%, -54 +/- 9%, -14 +/- 11%, respectively) in contracted vs. resting muscle. In summary, VAMP2, VAMP3, VAMP5, and VAMP7 coimmunoprecipitate with intracellular GLUT4 vesicles in muscle, and VAMP2, VAMP5, VAMP7, but not VAMP3, translocate to the cell surface membranes similar to GLUT4, TfR, and IRAP in response to muscle contractions. These findings suggest that VAMP2, VAMP5, and VAMP7 may be involved in translocation of GLUT4 during muscle contractions.

Glucose ingestion during endurance training does not alter adaptation

Glucose ingestion during exercise attenuates activation of metabolic enzymes and expression of important transport proteins. In light of this, we hypothesized that glucose ingestion during training would result in 1) an attenuation of the increase in fatty acid uptake and oxidation during exercise, 2) lower citrate synthase (CS) and β-hydroxyacyl-CoA dehydrogenase (β-HAD) activity and glycogen content in skeletal muscle, and 3) attenuated endurance performance enhancement in the trained state. To investigate this we studied nine male subjects who performed 10 wk of one-legged knee extensor training. They trained one leg while ingesting a 6% glucose solution (Glc) and ingested a sweetened placebo while training the other leg (Plc). The subjects trained their respective legs 2 h at a time on alternate days 5 days a week. Endurance training increased peak power (Pmax) and time to fatigue at 70% of Pmax ∼14% and ∼30%, respectively. CS and β-HAD activity increased and glycogen content was greater after training, but there were no differences between Glc and Plc. After training the rate of oxidation of palmitate (Rox) and the % of rate of disappearance that was oxidized (%Rdox) changed. %Rdox was on average 16.4% greater during exercise after training whereas, after exercise %Rdox was 30.4% lower. Rox followed the same pattern. However, none of these parameters were different between Glc and Plc. We conclude that glucose ingestion during training does not alter training adaptation related to substrate metabolism, mitochondrial enzyme activity, glycogen content, or performance.

Metabolic stress-like condition can be induced by prolonged strenuous exercise in athletes

Few studies have examined energy metabolism during prolonged, strenuous exercise. We wanted therefore to investigate energy metabolic consequences of a prolonged period of continuous strenuous work with very high energy expenditure. Twelve endurance-trained athletes (6 males and 6 females) were recruited. They performed a 7-h bike race on high work-load intensity. Physiological, biochemical, endocrinological, and anthropometric muscular compartment variables were monitored before, during, and after the race. The energy expenditure was high, being 5557 kcal. Work-load intensity (% of VO(2) peak) was higher in females (77.7%) than in men (69.9%). Muscular glycogen utilization was pronounced, especially in type I fibres (>90%). Additionally, muscular triglyceride lipolysis was considerably accelerated. Plasma glucose levels were increased concomitantly with an unchanged serum insulin concentration which might reflect an insulin resistance state in addition to proteolytic glyconeogenesis. Increased reactive oxygen species (malondialdehyde (MDA)) were additional signs of metabolic stress. MDA levels correlated with glycogen utilization rate. A relative deficiency of energy substrate on a cellular level was indicated by increased intracellular water of the leg muscle concomitantly with increased extracellular levels of the osmoregulatory amino acid taurine. A kindred nature of a presumed insulin-resistant state with less intracellular availability of glucose for erythrocytes was also indicated by the findings of decreased MCV together with increased MCHC (haemoconcentration) after the race. This strenuous energy-demanding work created a metabolic stress-like condition including signs of insulin resistance and deteriorated intracellular glucose availability leading to compromised fuelling of ion pumps, culminating in a disturbed cellular osmoregulation indicated by taurine efflux and cellular swelling.

The anti-diabetic AMPK activator AICAR reduces IL-6 and IL-8 in human adipose tissue and skeletal muscle cells

Increased production of inflammatory cytokines is suggested to be of importance for initiation and progression of insulin resistance. Chronic treatment with the AMP analogue 5-aminoimidazole-4-carboxamide-1-beta-d-ribonucleoside (AICAR) has been shown to improve insulin sensitivity and prevent the development of diabetes in rodents. We investigated the effects of AICAR on the expression and production of inflammatory cytokines from human adipose tissue and skeletal muscle cells as well as intact rat skeletal muscles in vitro. AICAR dose-dependently decreased interleukin-6 (IL-6) gene expression and secretion in human adipose tissue and in human skeletal muscle cells. In parallel, AICAR inhibited interleukin-8 (IL-8) secretion in human adipose tissue and skeletal muscle cells, as well as reduced IL-8 gene expression in skeletal muscle cells. In intact rat extensor digitorum longus (EDL) muscle fibres AICAR markedly decreased IL-6 and IL-8 gene expression. In conclusion, AICAR inhibits the production of IL-6 and IL-8 human adipose tissue and in skeletal muscle cells. We suggest that decreased cytokine production might play a role for the AICAR-induced increase in insulin sensitivity.

Fat and carbohydrate metabolism during submaximal exercise in children

During exercise, the contribution of fat and carbohydrate to energy expenditure is largely modulated by the intensity of exercise. Age, a short- or long-term diet enriched in carbohydrate or fat substrate stores, training and gender are other factors that have also been found to affect this balance. These factors have been extensively studied in adults from the perspective of improving performance in athletes, or from a health perspective in people with diseases. During the last decade, lifestyle changes associated with high-energy diets rich in lipid and reduced physical activity have contributed to the increase in childhood obesity. This lifestyle change has emerged as a serious health problem favouring the early development of cardiovascular diseases, insulin resistance or type 2 diabetes mellitus. Increasing physical activity levels in young people is important to increase energy expenditure and promote muscle oxidative capacity. Therefore, it is surprising that the regulation of balance between carbohydrate and lipid use during exercise has received much less attention in children than in adults. In this review, we have focused on the factors that affect carbohydrate and lipid metabolism during exercise and have identified areas that may be relevant in explaining the higher contribution of lipid to energy expenditure in children when compared with adults. Low muscle glycogen content is possibly associated with a low activity of glycolytic enzymes and high oxidative capacity, while lower levels of sympathoadrenal hormones are likely to favour lipid metabolism in children. Changes in energetic metabolism occurring during adolescence are also dependent on pubertal events with an increase in testosterone in boys and estrogen and progesterone in girls. The profound effects of ovarian hormones on carbohydrate and fat metabolism along with their effects on oxidative enzymes could explain that differences in substrate metabolism have not always been observed between girls and women. Finally, although the regulatory mechanisms of fat and carbohydrate balance during exercise are quite well identified, there are a lack of data specific to children and most of the evidences reported in this review were drawn from studies in adults. Isotope tracer techniques and nuclear magnetic resonance will allow non-invasive investigation of the metabolism of the different substrate sources in skeletal muscle.

Training at high exercise intensity promotes qualitative adaptations of mitochondrial function in human skeletal muscle

This study explored mitochondrial capacities to oxidize carbohydrate and fatty acids and functional optimization of mitochondrial respiratory chain complexes in athletes who regularly train at high exercise intensity (ATH, n = 7) compared with sedentary (SED, n = 7). Peak O(2) uptake (Vo(2max)) was measured, and muscle biopsies of vastus lateralis were collected. Maximal O(2) uptake of saponin-skinned myofibers was evaluated with several metabolic substrates [glutamate-malate (V(GM)), pyruvate (V(Pyr)), palmitoyl carnitine (V(PC))], and the activity of the mitochondrial respiratory complexes II and IV were assessed using succinate (V(s)) and N,N,N',N'-tetramethyl-p-phenylenediamine dihydrochloride (V(TMPD)), respectively. Vo(2max) was higher in ATH than in SED (57.8 +/- 2.2 vs. 31.4 +/- 1.3 ml.min(-1).kg(-1), P < 0.001). V(GM) was higher in ATH than in SED (8.6 +/- 0.5 vs. 3.3 +/- 0.3 micromol O(2).min(-1).g dry wt(-1), P < 0.001). V(Pyr) was higher in ATH than in SED (8.7 +/- 1.0 vs. 5.5 +/- 0.2 micromol O(2).min(-1).g dry wt(-1), P < 0.05), whereas V(PC) was not significantly different (5.3 +/- 0.9 vs. 4.4 +/- 0.5 micromol O(2).min(-1).g dry wt(-1)). V(S) was higher in ATH than in SED (11.0 +/- 0.6 vs. 6.0 +/- 0.3 micromol O(2).min(-1).g dry wt(-1), P < 0.001), as well as V(TMPD) (20.1 +/- 1.0 vs. 16.2 +/- 3.4 micromol O(2).min(-1).g dry wt(-1), P < 0.05). The ratios V(S)/V(GM) (1.3 +/- 0.1 vs. 2.0 +/- 0.1, P < 0.001) and V(TMPD)/V(GM) (2.4 +/- 1.0 vs. 5.2 +/- 1.8, P < 0.01) were lower in ATH than in SED. In conclusion, comparison of ATH vs. SED subjects suggests that regular endurance training at high intensity promotes the enhancement of maximal mitochondrial capacities to oxidize carbohydrate rather than fatty acid and induce specific adaptations of the mitochondrial respiratory chain at the level of complex I.

Effect of endurance exercise training on Ca2+ calmodulin-dependent protein kinase II expression and signalling in skeletal muscle of humans

Here the hypothesis that skeletal muscle Ca(2+)-calmodulin-dependent kinase II (CaMKII) expression and signalling would be modified by endurance training was tested. Eight healthy, young men completed 3 weeks of one-legged endurance exercise training with muscle samples taken from both legs before training and 15 h after the last exercise bout. Along with an approximately 40% increase in mitochondrial F(1)-ATP synthase expression, there was an approximately 1-fold increase in maximal CaMKII activity and CaMKII kinase isoform expression after training in the active leg only. Autonomous CaMKII activity and CaMKII autophosphorylation were increased to a similar extent. However, there was no change in alpha-CaMKII anchoring protein expression with training. Nor was there any change in expression or Thr(17) phosphorylation of the CaMKII substrate phospholamban with training. However, another CaMKII substrate, serum response factor (SRF), had an approximately 60% higher phosphorylation at Ser(103) after training, with no change in SRF expression. There were positive correlations between the increases in CaMKII expression and SRF phosphorylation as well as F(1)ATPase expression with training. After training, there was an increase in cyclic-AMP response element binding protein phosphorylation at Ser(133), but not expression, in muscle of both legs. Taken together, skeletal muscle CaMKII kinase isoform expression and SRF phosphorylation is higher with endurance-type exercise training, adaptations that are restricted to active muscle. This may contribute to greater Ca(2+) mediated regulation during exercise and the altered muscle phenotype with training.

Whole-body fat oxidation determined by graded exercise and indirect calorimetry: a role for muscle oxidative capacity?

During whole-body exercise, peak fat oxidation occurs at a moderate intensity. This study investigated whole-body peak fat oxidation in untrained and trained subjects, and the presence of a relation between skeletal muscle oxidative enzyme activity and whole-body peak fat oxidation. Healthy male subjects were recruited and categorized into an untrained (N=8, VO(2max) 3.5+/-0.1 L/min) and a trained (N=8, VO(2max) 4.6+/-0.2 L/min) group. Subjects performed a graded exercise test commencing at 60 W for 8 min followed by 35 W increments every 3 min. On a separate day, muscle biopsies were obtained from vastus lateralis and a 3 h bicycle exercise test was performed at 58% of VO(2max). Whole-body fat oxidation was calculated during prolonged and graded exercise from the respiratory exchange ratio using standard indirect calorimetry equations. Based on the graded exercise test, whole-body peak fat oxidation was determined. The body composition was determined by DEXA. Whole-body peak fat oxidation (250+/-25 and 462+/-33 mg/min) was higher (P<0.05) and occurred at a higher (P<0.05) relative workload (43.5+/-1.8% and 49.9+/-1.2% VO(2max)) in trained compared with untrained subjects, respectively. Muscle citrate synthase activity and beta-hydroxy-acyl-CoA-dehydrogenase activity were higher (49% and 35%, respectively, P<0.05) in trained compared with untrained subjects. Both lean body mass and maximal oxygen uptake were significantly correlated to whole-body peak fat oxidation (r(2)=0.57, P<0.001), but leg muscle oxidative capacity was not correlated to whole-body peak fat oxidation. In conclusion, whole-body peak fat oxidation occurred at a higher relative exercise load in trained compared with untrained subjects. Whole-body peak fat oxidation was not significantly related to leg muscle oxidative capacity, but was related to lean body mass and maximal oxygen uptake. This may suggest that leg muscle oxidative activity is not the main determinant of whole-body peak fat oxidation.

Training-induced changes in membrane transport proteins of human skeletal muscle

Training improves human physical performance by inducing structural and cardiovascular changes, metabolic changes, and changes in the density of membrane transport proteins. This review focuses on the training-induced changes in proteins involved in sarcolemmal membrane transport. It is concluded that the same type of training affects many transport proteins, suggesting that all transport proteins increase with training, and that both sprint and endurance training in humans increase the density of most membrane transport proteins. There seems to be an upper limit for these changes: intense training for 6-8 weeks substantially increases the density of membrane proteins, whereas years of training (as performed by athletes) have no further effect. Studies suggest that training-induced changes at the protein level are important functionally. The underlying factors responsible for these changes in transport proteins might include changes in substrate concentration, but the existence of "exercise factors" mediating these responses is more likely. Exercise factors might include Ca(2+), mitogen-activated protein kinases, adenosine monophosphate kinases, other kinases, or interleukin-6. Although the magnitudes of training-induced changes have been investigated at the protein level, the underlying signal mechanisms have not been fully described.

Skeletal muscle glucose uptake during exercise: how is it regulated?

The increase in skeletal muscle glucose uptake during exercise results from a coordinated increase in rates of glucose delivery (higher capillary perfusion), surface membrane glucose transport, and intracellular substrate flux through glycolysis. The mechanism behind the movement of GLUT4 to surface membranes and the subsequent increase in transport by muscle contractions is largely unresolved, but it is likely to occur through intracellular signaling involving Ca(2+)-calmodulin-dependent protein kinase, 5'-AMP-activated protein kinase, and possibly protein kinase C.

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.

Effect of training status on regional disposal of circulating free fatty acids in the liver and skeletal muscle during physiological hyperinsulinemia

OBJECTIVE

Fat metabolism is increasingly implicated in the pathogenesis of type 2 diabetes. Endurance training has been shown to prevent hepatic steatosis and to alter skeletal muscle fat metabolism, and regional free fatty acid (FFA) uptake adaptations were suggested as a mechanism. Thus, we tested whether endurance training modifies the uptake of plasma FFAs occurring in the liver and in skeletal muscle during anabolic, i.e., hyperinsulinemic, conditions.

RESEARCH DESIGN AND METHODS

Trained and untrained healthy male subjects underwent positron emission tomography scanning of the liver and thigh regions, with the FFA analog 14(R,S)-[(18)F]fluoro-6-thia-heptadecanoic acid, during euglycemic hyperinsulinemia. Tracer influx rate constants in skeletal muscle (MK(i)) and liver (LK(i)) were multiplied by plasma FFA levels to obtain FFA uptake for skeletal muscle (MFU) and liver (LFU), respectively.

RESULTS

Athletes showed increased Vo(2max) (P < 0.0001), insulin-mediated glucose disposal (M value, 61 +/- 4 vs. 46 +/- 3 micromol. min(-1). kg(-1), P = 0.01), and plasma lactate levels during the clamp and lower percentage of body fat mass (P = 0.002). MK(i) was 25% higher in athletes than in sedentary men (P = 0.03). In all subjects, MK(i) and MFU were positively correlated with the M value (r = 0.56, P = 0.02, and r = 0.51, P = 0.03, respectively) and with plasma lactate levels (r = 0.63, P = 0.006, and r = 0.63, P = 0.005, respectively). LK(i) was significantly reduced by 20% in the athletes (P = 0.04). By multiple regression, LFU was inversely correlated with the two fitness categories (P = 0.008), and it was lower in athletes. Linear fitting of liver data showed time consistency, indicating no release of FFAs as a mechanism for the reduced liver retention in athletes.

CONCLUSIONS

We conclude that endurance training promotes insulin-mediated glucose and FFA disposal in skeletal muscle, while lowering hepatic FFA uptake. Such changes may result in a divergent pattern of fat accumulation in the two organs.

Effect of insulin and contraction up on glucose transport in skeletal muscle

The major glucose transporter protein expressed in skeletal muscle is GLUT4. Both muscle contraction and insulin induce translocation of GLUT4 from the intracellular pool to the plasma membrane. The intracellular pathways that lead to contraction- and insulin-stimulated GLUT4 translocation seem to be different, allowing the attainment of a maximal effect when acting together. Insulin utilizes a phosphatidylinositol 3-kinase-dependent mechanism, whereas the exercise signal may be initiated by calcium release from the sarcoplasmic reticulum or from autocrine- or paracrine-mediated activation of glucose transport. During exercise skeletal muscle utilizes more glucose than when at rest. However, endurance training leads to decreased glucose utilization during sub-maximal exercise, in spite of a large increase in the total GLUT4 content associated with training. The mechanisms involved in this reduction have not been totally elucidated, but appear to cause the decrease of the amount of GLUT4 translocated to the plasma membrane by altering the exercise-induced enhancement of glucose transport capacity. On the other hand, the effect of resistance training is controversial. Recent studies, however, demonstrated the improvement in insulin sensitivity correlated with increasing muscle mass. New studies should be designed to define the molecular basis for these important adaptations to skeletal muscle. Since during exercise the muscle may utilize insulin-independent mechanisms to increase glucose uptake, the mechanisms involved should provide important knowledge to the understanding and managing peripheral insulin resistance.

Resistance training with vascular occlusion: metabolic adaptations in human muscle

Two recent studies have reported increases in strength and whole muscle cross-sectional area after low-intensity resistance training (LIT) with vascular occlusion (OCC) that are greater than LIT alone (e.g., 22, 25). The OCC stress might be expected to induce metabolic alterations that are consistent with compromised oxygen delivery rather than an increase in strength per se, but this has not been studied.

PURPOSE

We examined the effect of LIT and LIT+OCC on resting metabolites in m. biceps brachii and elbow flexor strength.

METHODS

Eight men (19.5 +/- 0.4 yr) performed 8 wk of LIT at approximately 50% of one-repetition maximum (2 sessions per week; 3-6 sets, 8-10 repetitions, final set to failure); one arm trained with OCC and the other without (CON). :Biopsies obtained before and 72 h after the final training bout revealed that resting [glycogen] was higher (P <or= 0.05) in both arms after LIT (CON: 452 +/- 20 vs 325 +/- 28, OCC: 501 +/- 12 vs 332 +/- 28 mmol.kg-1 dry weight) and the increase was larger in the OCC arm (P <or= 0.05). Resting [ATP] was lower (P <or= 0.05) after LIT in both arms (CON: 20.5 +/- 0.5 vs 22.8 +/- 0.7, OCC: 18.2 +/- 0.6 vs 23.1 +/- 0.5 mmol.kg-1 dry weight), and the decrease was larger in the OCC arm (P <or= 0.05). Maximal isotonic and isokinetic elbow flexor strength increased (P <or= 0.05) after training to a similar extent in both arms.

CONCLUSION

We conclude that [glycogen] was increased and [ATP] was decreased in resting human muscle, 72 h after an 8-wk LIT protocol. OCC potentiated the metabolic changes, perhaps by inducing an ischemic stimulus that enhanced muscle glucose transport and adenine nucleotide catabolism after LIT, but did not augment the increases in strength.

Skeletal muscle glucose uptake response to exercise in trained and untrained men

PURPOSE

Endurance training enhances skeletal muscle glucose uptake at rest, but the responses to different exercise intensities are unknown. In the present study, we tested whether glucose uptake is enhanced in trained men during low-, moderate-, and high-intensity exercise as compared with untrained men.

METHODS

Seven trained and untrained men were studied without any dietary manipulation during bicycle exercise at relative intensities of 30%, 55%, and 75% of maximal oxygen consumption ([OV0312]O(2max)) on three separate days. Glucose uptake in the quadriceps femoris muscle was directly measured using positron emission tomography (PET) and 18F-fluoro-deoxy-glucose ([18F]FDG). [18F]FDG was injected 10 min after the start of the exercise. Thereafter exercise was continued for another 25 min. PET scanning was conducted immediately after completion of the exercise. The measured glucose uptake values reflect the situation during exercise due to chemical characteristics of the [18F]FDG.

RESULTS

Muscle glucose uptake increased from 30% to 55% [OV0312]O(2max) intensity exercise similarly in both groups (P < 0.05). However, from 55% to 75% [OV0312]O(2max) intensity exercise, only athletes were able to further enhance glucose uptake. Furthermore, at highest intensity, glucose uptake was significantly higher in trained than in untrained men (236.6 +/- 29.6 vs 176.3 +/- 22.4 micromol.kg-1.min-1, P < 0.05). There were no differences in plasma glucose, insulin, or lactate in any time point at 75% [OV0312]O(2max) intensity between groups.

CONCLUSIONS

These results show that skeletal muscle glucose uptake is higher in trained than in untrained men at high relative exercise intensity, although at lower relative exercise intensities no differences are observed. Thus, endurance training improves the capacity of contraction-induced glucose uptake in skeletal muscle.

The effect of graded exercise on IL-6 release and glucose uptake in human skeletal muscle

In this study, the hypothesis that the release of interleukin (IL)-6 from human muscle is linked to exercise intensity and muscle glucose uptake was investigated. In the overnight fasted state, seven healthy males performed knee extension exercise, kicking with both legs, each at 25 % of maximal power (W(max)) for 45 min (eliciting 23 +/- 1 % of pulmonary maximal oxygen uptake, V(O2,max)) and then simultaneously with one leg at 65 % and the other leg at 85 % W(max) for 35 min (40 +/- 1 % of pulmonary V(O2,max)). Blood was sampled from a femoral artery and both femoral veins, and blood flow was determined by thermodilution. Thigh plasma flow (0.15 +/- 0.01, 1.4 +/- 0.2, 2.0 +/- 0.1 and 2.3 +/- 0.2 l min(-1) thigh(-1) at rest and 25 %, 65 % and 85 % W(max), respectively) and thigh oxygen uptake (0.02 +/- 0.01, 0.27 +/- 0.03, 0.48 +/- 0.04 and 0.55 +/- 0.05 l min(-1) thigh(-1) at rest and 25 %, 65 % and 85 % W(max), respectively) increased with increasing exercise intensity (P < 0.05). Also, thigh IL-6 release (0.4 +/- 0.1, 1.3 +/- 0.5, 1.5 +/- 0.6 and 2.5 +/- 0.7 ng min(-1) thigh(-1) at rest and 25 %, 65 % and 85 % W(max), respectively) and thigh glucose uptake (0.05 +/- 0.01, 0.3 +/- 0.05, 0.75 +/- 0.16, 1.07 +/- 0.15 mmol min(-1) thigh(-1) at rest and 25 %, 65 % and 85 % W(max), respectively) increased with increasing exercise intensity (P < 0.05). During the last 35 min of exercise, arterial catecholamine concentrations were higher (P < 0.05) than at rest and during low-intensity exercise. During exercise, thigh IL-6 release was positively related to both thigh glucose uptake (P < 0.001) and thigh glucose delivery (P < 0.005), but not to thigh glucose extraction. Thigh IL-6 release was also positively related to arterial plasma adrenaline concentration. The pre-exercise muscle glycogen concentration tended to correlate with the arteriovenous IL-6 concentration difference at rest, and the postexercise glycogen concentration was inversely correlated with IL-6 release during the final 35 min of exercise. In conclusion, the study indicates that IL-6 release from human muscle is positively related to exercise intensity, arterial adrenaline concentration and muscle glucose uptake. This supports the hypothesis that IL-6 may be linked to the regulation of glucose homeostasis during exercise. The observation of a relationship between IL-6 release and muscle glycogen store both at rest and after exercise suggests that IL-6 may act as a carbohydrate sensor.

Effects of oral contraceptives on glucose flux and substrate oxidation rates during rest and exercise

We examined the effects of oral contraceptives (OC) on glucose flux and whole body substrate oxidation rates during rest (90 min) and two exercise intensities [60-min leg ergometer cycling at 45 and 65% peak O(2) uptake (Vo(2 peak))]. Eight healthy, eumenorrheic women were studied during the follicular and luteal phases before OC and the inactive and high-dose phases after 4 mo of a low-dose, triphasic OC. Subjects were studied in the morning 3 h after a standardized (308 kcal) breakfast. There were significant reductions in glucose rates of appearance and disappearance during exercise of both intensities with OC but not rest. There were no phase effects on substrate oxidation during rest or exercise. These results are interpreted to mean that, in women fed several hours before study, 1) OC decreases glucose flux, but not overall carbohydrate and lipid oxidation rates during moderate-intensity exercise; and 2) synthetic ovarian hormone analogs in the doses contained in OC have greater metabolic effects on glucose metabolism during exercise than do endogenous ovarian hormones.

Changes in skeletal muscle GLUT4 content and muscle membrane glucose transport following 6 weeks of exercise training

This study examined changes in skeletal muscle GLUT4 content and glucose transport in isolated muscle membranes (GT) from horses before and 2 min after standardised submaximal exercise tests (SET) prior to and after completion of 6 weeks of training. Seven horses, age 3-9 years, body mass mean +/- s.e. 530 +/- 19 kg, and sedentary for at least 4 months, completed 6 weeks of training on a treadmill. An initial SET (UT) was performed on a 4 degree incline at a speed equivalent to 55% of pretraining VO2max and was repeated post-training at the same absolute workload (ABS). A third SET (REL) was performed at 55% of post-training VO2max. There was no significant pre- to postexercise change in GLUT4 content before or after training. Following training, total GLUT4 content was increased 2- or 3-fold in pre-exercise biopsies (pre UT: 0.30 +/- 0.05; pre ABS: 1.05 +/- 0.32; pre REL: 1.34 +/- 0.28 arbitrary units) (P<0.05) with similar increases in postexercise GLUT4 content (P<0.05) (post UT: 033 +/- 0.06; post ABS: 1.19 +/- 0.44; post REL: 1.43 +/- 0.31). GT increased 2.5- to 6-fold in postexercise muscle membrane vesicles in UT over a range of glucose concentrations. After training (ABS and REL), there was a 25-50% attenuation (P<0.05) in membrane GT in response to exercise in ABS and REL. These finding indicate that moderate intensity exercise training increased middle gluteal muscle GLUT4 content, but this change was not reflected in an increase in muscle membrane glucose transport activity in postexercise muscle samples.

Plasma glucose kinetics during prolonged exercise in trained humans when fed carbohydrate

Nine endurance-trained men exercised on a cycle ergometer at approximately 68% peak O2 uptake to the point of volitional fatigue [232 +/- 14 (SE) min] while ingesting an 8% carbohydrate solution to determine how high glucose disposal could increase under physiological conditions. Plasma glucose kinetics were measured using a primed, continuous infusion of [6,6-2H]glucose and the appearance of ingested glucose, assessed from [3-3H]glucose that had been added to the carbohydrate drink. Plasma glucose was increased (P < 0.05) after 30 min of exercise but thereafter remained at the preexercise level. Glucose appearance rate (R(a)) increased throughout exercise, reaching its peak value of 118 +/- 7 micromol. kg(-1). min(-1) at fatigue, whereas gut R(a) increased continuously during exercise, peaking at 105 +/- 10 micromol. kg(-1). min(-1) at the point of fatigue. In contrast, liver glucose output never rose above resting levels at any time during exercise. Glucose disposal (R(d)) increased throughout exercise, reaching a peak value of 118 +/- 7 micromol. kg(-1). min(-1) at fatigue. If we assume 95% oxidation of glucose R(d), estimated exogenous glucose oxidation at fatigue was 1.36 +/- 0.08 g/min. The results of this study demonstrate that glucose uptake increases continuously during prolonged, strenuous exercise when carbohydrate is ingested and does not appear to limit exercise performance.

Myocardial and skeletal muscle glucose uptake during exercise in humans

The purpose of this study was to investigate the effects of exercise on myocardial glucose uptake and whether the pattern of glucose uptake is the same as in skeletal muscle. Glucose uptake was measured using positron emission tomography (PET) and 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG). Twelve healthy men were studied during rest, while 14 subjects were studied after 35 min of bicycle exercise corresponding to 30, 55 and 75 % of maximal oxygen consumption (*VO2,max)) on three separate days. [(18)F]FDG was injected 10 min after the start of exercise and exercise continued for a further 25 min. Myocardial and skeletal muscle PET scanning was commenced directly after the completion of the exercise bout. As compared to the resting state, exercise doubled myocardial glucose uptake at the 30 % (P = 0.056) and 55 % intensity levels (P < 0.05), while at the 75 % intensity level glucose uptake was reduced significantly compared to the lower exercise intensities. There was no significant difference between the highest intensity level and the resting state (P = 0.18). At rest and during low-intensity exercise, myocardial glucose uptake was inversely associated with circulating levels of free fatty acids. However, during higher exercise intensities when plasma lactate concentrations increased significantly, this association disappeared. In contrast to myocardial responses, skeletal muscle glucose uptake rose in parallel with exercise intensity from rest to 30 % and then 55 % *VO2,max) (P < 0.001) and tended to increase further at the intensity of 75 % *VO2,max) (P = 0.065). In conclusion, these results demonstrate that myocardial glucose uptake is increased during mild- and moderate-intensity exercise, but is decreased during high-intensity exercise. This finding suggests that the increased myocardial energy that is needed during high-intensity exercise is supplied by substrates other than glucose.

Adaptations to training in endurance cyclists: implications for performance

Our present scientific knowledge of the effects of specific training interventions undertaken by professional cyclists on selected adaptive responses in skeletal muscle and their consequences for improving endurance performance is limited: sport scientists have found it difficult to persuade elite cyclists to experiment with their training regimens and access to muscle and blood samples from these athletes is sparse. Owing to the lack of scientific study we present a theoretical model of some of the major training-induced adaptations in skeletal muscle that are likely to determine performance capacity in elite cyclists. The model includes, but is not limited to, skeletal muscle morphology, acid-base status and fuel supply. A working premise is that the training-induced changes in skeletal muscle resulting from the high-volume, high-intensity training undertaken by elite cyclists is at least partially responsible for the observed improvements in performance. Using experimental data we provide evidence to support the model.