Rapid reversal of adaptive increases in muscle GLUT-4 and glucose transport capacity after training cessation

Moderate-Intensity Constant and High-Intensity Interval Training Confer Differential Metabolic Benefits in Skeletal Muscle, White Adipose Tissue, and Liver of Candidates to Undergo Bariatric Surgery

Background/Objectives: Bariatric surgery candidates require presurgical physical training, therefore, we compared the metabolic effects of a constant moderate-intensity training program (MICT) vs. a high-intensity interval training (HIIT) in this population. Methods: Seventeen participants performed MICT (n = 9, intensity of 50% of heart rate reserve (HRR) and/or 4-5/10 subjective sensation of effort (SSE)) or HIIT (n = 8, 6 cycles of 2.5 min at 80% of the HRR and/or 7-8/10 of SSE, interspersed by 6 cycles of active rest at 20% of the FCR) for 10 sessions for 4 weeks. After training, tissue samples (skeletal muscle, adipose tissue, and liver) were extracted, and protein levels of adiponectin, GLUT4, PGC1α, phospho-AMPK/AMPK, collagen 1 and TGFβ1 were measured. Results: Participants who performed MICT showed higher protein levels of PGC-1α in skeletal muscle samples (1.1 ± 0.27 vs. 0.7 ± 0.4-fold change, p < 0.05). In the liver samples of the people who performed HIIT, lower protein levels of phospho-AMPK/AMPK (1.0 ± 0.37 vs. 0.52 ± 0.22-fold change), PGC-1α (1.0 ± 0.18 vs. 0.69 ± 0.15-fold change), and collagen 1 (1.0 ± 0.26 vs. 0.59 ± 0.28-fold change) were observed (all p < 0.05). In subcutaneous adipose tissue, higher adiponectin levels were found only after HIIT training (1.1 ± 0.48 vs. 1.9 ± 0.69-fold change, p < 0.05). Conclusions: Our results show that both MICT and HIIT confer metabolic benefits in candidates undergoing bariatric surgery; however, most of these benefits have a program-specific fashion. Future studies should aim to elucidate the mechanisms behind these differences.

A mathematical model of obesity-induced type 2 diabetes and efficacy of anti-diabetic weight reducing drug

The dominant paradigm for modeling the obesity-induced T2DM (type 2 diabetes mellitus) today focuses on glucose and insulin regulatory systems, diabetes pathways, and diagnostic test evaluations. The problem with this approach is that it is not possible to explicitly account for the glucose transport mechanism from the blood to the liver, where the glucose is stored, and from the liver to the blood. This makes it inaccurate, if not incorrect, to properly model the concentration of glucose in the blood in comparison to actual glycated hemoglobin (A1C) test results. In this paper, we develop a mathematical model of glucose dynamics by a system of ODEs. The model includes the mechanism of glucose transport from the blood to the liver, and from the liver to the blood, and explains how obesity is likely to lead to T2DM. We use the model to evaluate the efficacy of an anti-T2DM drug that also reduces weight.

Long- and Short-Term High-Intensity Interval Training on Lipid Profile and Cardiovascular Disorders in Obese Male Adolescents

This study investigated the effects of short-term and long-term periods (8 and 16 weeks) of high-intensity interval training (HIIT) on cardiovascular components, blood lipids, and 6-min walking test performance in obese young boys (age = 16.2 ± 0.7) with >34% body fat. The participants were split into two groups: severe obesity (SOG; n = 17) and moderate obesity (MOG; n = 16). All participants performed on a cycle ergometer for 16 weeks (3 times per week) of HIIT at 100% peak power output at the ventilatory threshold and recovered at 50% of peak power. Except for BMI, both groups improved all body composition measures after 16 weeks, with a higher percentage of change (Δ) in SOG. The 6-min walking test increased in both groups (p < 0.001). Furthermore, cardiovascular variables, blood lactate concentration at rest and after 5-min post-exercise, blood lipids, and insulin concentrations improved significantly in both groups. After 16 weeks, MOG significantly improved in HR_peak, blood glucose concentration, and rating of perceived exertion (RPE), but the percentage of change (Δ) was higher in SOG for all the other variables. SOG showed a higher (Δ) waist-to-hip ratio, maximum heart rate, resting heart rate, systolic blood pressure, blood lactate at 5-min post-exercise, and triglyceride concentrations after 8 and 16 weeks of training. In conclusion, a long-term HIIT program appears to be an appropriate training approach for obese boys with extra body fat. However, considering the RPE values, short-duration training sessions should be planned.

Insights into the development of insulin resistance: Unraveling the interaction of physical inactivity, lipid metabolism and mitochondrial biology

While impairments in peripheral tissue insulin signalling have a well-characterized role in the development of insulin resistance and type 2 diabetes (T2D), the specific mechanisms that contribute to these impairments remain debatable. Nonetheless, a prominent hypothesis implicates the presence of a high-lipid environment, resulting in both reactive lipid accumulation and increased mitochondrial reactive oxygen species (ROS) production in the induction of peripheral tissue insulin resistance. While the etiology of insulin resistance in a high lipid environment is rapid and well documented, physical inactivity promotes insulin resistance in the absence of redox stress/lipid-mediated mechanisms, suggesting alternative mechanisms-of-action. One possible mechanism is a reduction in protein synthesis and the resultant decrease in key metabolic proteins, including canonical insulin signaling and mitochondrial proteins. While reductions in mitochondrial content associated with physical inactivity are not required for the induction of insulin resistance, this could predispose individuals to the detrimental effects of a high-lipid environment. Conversely, exercise-training induced mitochondrial biogenesis has been implicated in the protective effects of exercise. Given mitochondrial biology may represent a point of convergence linking impaired insulin sensitivity in both scenarios of chronic overfeeding and physical inactivity, this review aims to describe the interaction between mitochondrial biology, physical (in)activity and lipid metabolism within the context of insulin signalling.

Preclinical rodent models of physical inactivity-induced muscle insulin resistance: challenges and solutions

Physical inactivity influences the development of muscle insulin resistance yet is far less understood than diet-induced muscle insulin resistance. Progress in understanding the mechanisms of physical inactivity-induced insulin resistance is limited by a lack of an appropriate preclinical model of muscle insulin resistance. Here, we discuss differences between diet and physical inactivity-induced insulin resistance, the advantages and disadvantages of the available rodent inactivity models to study insulin resistance, and our current understanding of the mechanisms of muscle insulin resistance derived from such preclinical inactivity designs. The burgeoning rise of health complications emanating from metabolic disease presents an alarming issue with mounting costs for health care and a reduced quality of life. There exists a pressing need for more complete understanding of mechanisms behind the development and progression of metabolic dysfunction. Since lifestyle modifications such as poor diet and lack of physical activity are primary catalysts of metabolic dysfunction, rodent models have been formed to explore mechanisms behind these issues. Particularly, the use of a high-fat diet has been pervasive and has been an instrumental model to gain insight into mechanisms underlying diet-induced insulin resistance (IR). However, physical inactivity (and to some extent muscle disuse) is an often overlooked and much less frequently studied lifestyle modification, which some have contended is the primary contributor in the initial development of muscle IR. In this mini-review we highlight some of the key differences between diet- and physical inactivity-induced development of muscle IR and propose reasons for the sparse volume of academic research into physical inactivity-induced IR including infrequent use of clearly translatable rodent physical inactivity models.

Acute bout of exercise downregulates thioredoxin-interacting protein expression in rat contracting skeletal muscles

We previously reported that in rat skeletal muscle, disuse (i.e., decreased muscle contractile activity) rapidly increases thioredoxin-interacting protein (TXNIP), which is implicated in the reduced glucose uptake. Accordingly, we sought herein to (a) determine the effect of exercise (i.e., increased muscle contractile activity) on muscle TXNIP protein expression, and (b) elucidate the mechanisms underlying the changes of TXNIP protein expression in response to exercise. Rat epitrochlearis and soleus muscles were dissected out after an acute bout of 3-hr swimming (without weight loading) or 3-hr treadmill running (15% grade at 9m/min). In a separate protocol, the isolated epitrochlearis and soleus muscles were incubated for 3 hr with AMP-dependent protein kinase activator AICAR. Immediately after the cessation of the 3-hr swimming, the TXNIP protein was decreased in epitrochlearis but not in soleus muscle. Conversely, 3-hr treadmill running decreased the TXNIP protein in soleus but not in epitrochlearis muscle. TXNIP protein was decreased concomitantly with reduced postexercise muscle glycogen, showing that a decrease in TXNIP protein expression occurs in muscles that are recruited during exercise. In addition, 3-hr incubation with AICAR decreased TXNIP protein in both isolated epitrochlearis and soleus muscles. Our results suggest that (a) an acute bout of exercise downregulates TXNIP protein expression in rat contracting skeletal muscles, and (b) the reduction in TXNIP protein expression in contracting muscles is probably mediated by AMPK activation, at least in part.

Adaptations in GLUT4 Expression in Response to Exercise Detraining Linked to Downregulation of Insulin-Dependent Pathways in Cardiac but not in Skeletal Muscle Tissue

Insulin resistance is associated with cardiometabolic risk factors, and exercise training can improve insulin-mediated glucose uptake. However, few studies have demonstrated the reversibility of exercise-induced benefits. Thus, the authors examine the time-response effects of exercise training and detraining on glucose transporter 4 (GLUT4) content, insulin-dependent and insulin-independent pathways in cardiac and gastrocnemius muscle tissues of spontaneously hypertensive rats. Thirty-two male spontaneously hypertensive rats, 4 months old, were assigned to (n = 8/group): T (exercise training: 10-week treadmill exercise, 50-70% maximum effort capacity, 1 hr/day, 5 days/week); D2 (exercise training + 2-day detraining), D4 (exercise training + 4-day detraining); and S (no exercise). The authors evaluated insulin resistance, maximum effort capacity, GLUT4 content, p-IRS-1Tyr1179, p-AS160Ser588, p-AMPKα1Thr172, and p-CaMKIIThr286 in cardiac and gastrocnemius muscle tissues (Western blot). In response to exercise training, there were improvements in insulin resistance (15.4%; p = .010), increased GLUT4 content (microsomal, 29.4%; p = .012; plasma membrane, 27.1%; p < .001), p-IRS-1 (42.2%; p < .001), p-AS160 (60.0%; p < .001) in cardiac tissue, and increased GLUT4 content (microsomal, 29.4%; p = .009; plasma membrane, 55.5%; p < .001), p-IRS-1 (28.1%; p = .018), p-AS160 (76.0%; p < .001), p-AMPK-α1 (37.5%; p = .026), and p-CaMKII (30.0%; p = .040) in the gastrocnemius tissue. In D4 group, the exercise-induced increase in GLUT4 was reversed (plasma membrane, -21.3%; p = .027), p-IRS1 (-37.1%; p = .008), and p-AS160 (-82.6%; p < .001) in the cardiac tissue; p-AS160 expression (-35.7%; p = .034) was reduced in the gastrocnemius. In conclusion, the cardiac tissue is more susceptible to exercise adaptations in the GLUT4 content and signaling pathways than the gastrocnemius muscle. This finding may be explained by particular characteristics of insulin-dependent and insulin-independent pathways in the muscle tissues studied.

Constant-Moderate and High-Intensity Interval Training Have Differential Benefits on Insulin Sensitive Tissues in High-Fat Fed Mice

In a mouse model of diet-induced obesity, this study determined if two exercise prescriptions with equivalent time and distance covered, [constant-moderate endurance (END) and high intensity interval training (HIIT)], exert differential metabolic benefits on insulin sensitive tissues. Male 10 week old C57BL/6 mice were fed a high fat diet (HFD; 45% kcal fat) ad libitum for 10 weeks and for a further 10 weeks they underwent END or HIIT training (3 × 40 min sessions/wk). Untrained HFD and chow-fed mice acted as controls. At 30 weeks of age, mice were sacrificed and quadriceps muscle, subcutaneous adipose tissue (SAT) and liver were excised. Neither END nor HIIT altered body weight or composition in HFD mice. In quadriceps, HFD decreased high-molecular weight adiponectin protein, which was normalized by END and HIIT. In contrast, HIIT but not END reversed the HFD-driven decrease in the adiponectin receptor 1 (AdipoR1). In SAT, both programs tended to decrease collagen VI protein (p = 0.07–0.08) in HFD, whereas only HIIT induced an increase in the mRNA (3-fold vs. HFD untrained) and protein (2-fold vs. HFD untrained) of UCP1. In liver, only END reversed collagen I accumulation seen in HFD untrained mice. Our results suggest that HIIT may promote better systemic metabolic changes, compared to END, which may be the result of the normalization of muscle AdipoR1 and increased UCP1 seen in SAT. However, END was more effective in normalizing liver changes, suggesting differential metabolic effects of END and HIIT in different tissues during obesity.

Inactivity induces resistance to the metabolic benefits following acute exercise

Acute exercise improves postprandial lipemia, glucose tolerance, and insulin sensitivity, all of which are risk factors for cardiovascular disease. However, recent research suggests that prolonged sedentary behavior might abolish these healthy metabolic benefits. Accordingly, this study aimed to elucidate the impact of an acute bout of exercise on postprandial plasma triglyceride, glucose, and insulin concentrations after 4 days of prolonged sitting (~13.5 h/day). Ten untrained to recreationally active men ( n = 5) and women ( n = 5) completed a counterbalanced, crossover study. Four days of prolonged sitting without exercise (SIT) were compared with 4 days of prolonged sitting with a 1-h bout of treadmill exercise (SIT + EX; 63.1 ± 5.2% V̇o2max) on the evening of the fourth day. The following morning, participants completed a high-fat/glucose tolerance test (HFGTT), during which plasma was collected over a 6-h period and analyzed for triglycerides, glucose, and insulin. No differences between trials ( P > 0.05) were found in the overall plasma triglyceride, glucose, or insulin responses during the HFGTT. This lack of difference between trials comes with similarly low physical activity (~3,500–4,000 steps/day) on each day except for the 1-h bout of exercise during SIT + EX the day before the HFGTT. These data indicate that physical inactivity (e.g., sitting ~13.5 h/day and <4,000 steps/day) creates a condition whereby people become “resistant” to the metabolic improvements that are typically derived from an acute bout of aerobic exercise (i.e., exercise resistance).

NEW & NOTEWORTHY In people who are physically inactive and sitting for a majority of the day, a 1-h bout of vigorous exercise failed to improve lipid, glucose, and insulin metabolism measured the next day. It seems that something inherent to inactivity and/or prolonged sitting makes the body resistant to the 1 h of exercise preventing the normally derived metabolic improvements following exercise.

Exercise Increases Glucose Transporter-4 Levels on Peripheral Blood Mononuclear Cells

PURPOSE

Glucose transporter 4 (GLUT4) plays a key role in the pathophysiology of type 2 diabetes. Glucose transporter 4 is upregulated in response to exercise, enhancing cellular glucose transport in skeletal muscle tissue. This mechanism appears to remain intact in individuals with insulin resistance. Details of the mechanism are poorly understood and are challenging to study due to the invasive nature of muscle biopsy. Peripheral blood mononuclear cells (PBMC) have documented insulin-sensitive GLUT4 activity and may serve as a proxy tissue for studying skeletal muscle GLUT4. The purpose of this study was to investigate whether GLUT4 in PBMC is affected by conditioning.

METHODS

We recruited 16 student athletes from the cross-country running and skiing teams and fifteen sedentary students matched for age and sex from the University of Alaska Fairbanks. Peripheral blood mononuclear cells were collected with mononuclear cell separation tubes. The GLUT4 concentrations were measured using a commercially available enzyme linked immunosorbent assay. Additionally, correlations between PBMC GLUT4 and common indicators of insulin resistance were examined.

RESULTS

Results indicate significantly higher PBMC GLUT4 levels in conditioned athletes than in their sedentary counterparts, similar to what has been documented in myocytes. Females were observed to have higher PBMC GLUT4 levels than males. Correlations were not detected between PBMC GLUT4 and hemoglobin A1c, glucose, insulin, homeostatic model assessment of insulin resistance, body mass index, or body fat.

CONCLUSIONS

This study provides evidence to support exploration of PBMC as a proxy tissue for studying GLUT4 response to exercise or other noninsulin factors.

Exercise training, Glut-4 protein abundance and glutamine in skeletal muscle of mature and very old horses

Two groups of unfit Standardbred mares (adult: 9-14 years, 540 kg, n=7) and old (20-25 years, 530 kg, n=5) were used to test two hypotheses, first, that aging and training would alter plasma and muscle glutamine [Gln] and glutamate [Glu] and second, that aging and training would alter Glut-4 expression in skeletal muscle. All animals were housed on pasture with free access to grass and all received hay and supplementation with a commercially prepared supplement (15% crude protein and 3.00 Mcal/kg dry matter) in individual stalls. Mares were fed to meet or exceed NRC (2007) nutrient recommendations for moderate to heavy exercise. The mares were exercise trained in a free-stall motorised circular exercise machine for 30 min/d, 5 d/week, for 8 weeks. Work intensity during training was set at a relative intensity of ~60% of the maximum heart rate, previously determined during an incremental exercise test (GXT). Blood samples and muscle biopsies (gluteus) obtained before and after 8 weeks of training were used for measurement of [Gln], [Glu] and Glut-4 abundance. Samples were collected before the initiation of training and at 24 h after cessation of last bout of exercise in the training period. All samples were immediately frozen in liquid nitrogen and stored at -80 °C until enzymatic analysis for [Gln], [Glu] and Western Blot analysis for Glut-4 protein abundance. Data were analysed by one-way or two-way ANOVA for repeated measures and the Pearson correlation method. Post-hoc differences were identified with the Tukey test. Significance was set at P<0.05. There were no differences (P>0.05) in muscle [Glu] due to aging. Training decreased (P<0.05) muscle [Glu] from 7,561±701 nmol/g of tissue (mean ± standard error) in pre-training samples to 4,491±701 nmol/g of tissue post-training. Plasma [Gln] decreased (P<0.05) with training (368±14 nmol/ml vs 317±14 nmol/ml). There was a trend (P=0.063) towards an effect of aging. There were significant interactions between age and training for plasma [Gln]. Old mares had lower (P<0.05) post-exercise plasma [Gln] (224±21 nmol/ml) when compared with pre-exercise plasma [Gln] (372±21 nmol/ml). Post-training, plasma [Gln] was lower (P<0.05) in the old mares compared to adult mares [Gln] (224±21 nmol/ml vs 410±18 nmol/ml). There was an effect (P<0.05) of age on muscle [Gln] (old = 6,126 ±870 nmol/g of tissue; adult = 3,176±735 nmol/g of tissue); however, there were no changes (P>0.05) due to training. Glut-4 abundance analysis did not differ (P>0.05) between the young adult and old horses; however, there was a trend (P=0.063) towards an effect of training when samples from both groups were pooled. It was concluded that training and aging produce changes in plasma and muscle [Gln], which may affect immune function in athletic horses, but not in Glut-4.

Greater effects of high- compared with moderate-intensity interval training on cardio-metabolic variables, blood leptin concentration and ratings of perceived exertion in obese adolescent females

This study examined the effects of high- vs. moderate-intensity interval training on cardiovascular fitness, leptin levels and ratings of perceived exertion (RPE) in obese female adolescents. Forty-seven participants were randomly assigned to one of three groups receiving either a 1:1 ratio of 15 s of effort comprising moderate-intensity interval training (MIIT at 80% maximal aerobic speed: MAS) or high-intensity interval training (HIIT at 100% MAS), with matched 15 s recovery at 50% MAS, thrice weekly, or a no-training control group. The HIIT and MIIT groups showed improved (p < 0.05) body mass (BM), BMI Z-score, and percentage of body fat (%BF). Only the HIIT group showed decreased waist circumference (WC) (p = 0.017). The effect of exercise on maximal oxygen uptake (VO2max) was significant (p = 0.019, ES = 0.48 and p = 0.010, ES = 0.57, HIIT and MIIT, respectively). The decrease of rate-pressure product (RPP) (p < 0.05, ES = 0.53 and ES = 0.46, HIIT and MIIT, respectively) followed the positive changes in resting heart rate and blood pressures. Blood glucose, insulin level and the homeostasis model assessment index for insulin decreased (p < 0.05) in both training groups. Significant decreases occurred in blood leptin (p = 0.021, ES = 0.67 and p = 0.011, ES = 0.73) and in RPE (p = 0.001, ES = 0.76 and p = 0.017, ES = 0.57) in HIIT and MIIT, respectively. In the post-intervention period, blood leptin was strongly associated with %BF (p < 0.001) and VO2max (p < 0.01) in the HIIT and MIIT groups, respectively, while RPE was strongly associated with BM (p < 0.01) in the HIIT group. The results suggest that high-intensity interval training may produce more positive effects on health determinants in comparison with the same training mode at a moderate intensity.

The emergence of sedentary behaviour physiology and its effects on the cardiometabolic profile in young and older adults

It has recently emerged that sedentary behaviour is independent of a lack of physical activity as individuals can be sufficiently active, based on the recommended physical activity guidelines, but also spend the majority of their waking hours engaging in sedentary behaviour. Individuals who follow this pattern of physical activity and sedentary behaviour are known as 'active couch potatoes'. Sedentary behaviour has been found to have detrimental effects on cardiometabolic markers associated with cardiovascular disease. Since the positive effects of moderate-to-vigorous intensity physical activity do not necessarily negate the deleterious effects of sedentary behaviour on cardiometabolic markers, it is postulated that engaging in light physical activity is an intervention that will successfully reduce levels of sedentary behaviour and may hence improve health markers of quality of life. We propose that such lifestyle changes may be particularly relevant to older populations as these engage in sedentary behaviour for the majority of their waking hours, thereby adding to the negative aging effect on cardiometabolic markers.

The effect of acute exercise on GLUT4 levels in peripheral blood mononuclear cells of sled dogs

Using sled dogs as exercise model, our objectives of this study were to (1) assess the effects of one acute bout of high-intensity exercise on surface GLUT4 concentrations on easily accessible peripheral blood mononuclear cells (PBMC) and (2) compare our findings with published research on exercise induced GLUT4 in skeletal muscle. During the exercise bout, dogs ran 5 miles at approximately 90% of VO₂ max. PMBC were collected before exercise (baseline), immediately after exercise and after 24 h recovery.GLUT4 was measured via ELISA. Acute exercise resulted in a significant increase on surface GLUT4 content on PBMC. GLUT4 was increased significantly immediately after exercise (~50%; p<0.05) and reduced slightly by 24 h post-exercise as compared to baseline (~22%; p>0.05). An effect of acute exercise on GLUT4 levels translocated to the cell membrane was observed, with GLUT4 levels not yet returned to baseline after 24 h post-exercise. In conclusion, the present investigation demonstrated that acute high-intensity exercise increased GLUT4 content at the surface of PBMC of sled dogs as it has been reported in skeletal muscle in other species. Our findings underline the potential use of peripheral blood mononuclear cell GLUT4 protein content as minimally invasive proxy to investigate relationships between insulin sensitivity, insulin resistance, GLUT4 expression and glucose metabolism.

•

Acute exercise increased GLUT4 translocation in peripheral mononuclear cells

•

GLUT4 translocation in peripheral mononuclear cells was insulin independent

•

Mononuclear cells are a potential proxy tissue to study GLUT4 translocation

Conditioning causes an increase in glucose transporter-4 levels in mononuclear cells in sled dogs

This study was designed to investigate the effects of physical conditioning on the expression of the insulin sensitive glucose transporter-4 protein (GLUT4) on mononuclear cells and HOMA-IR levels in dogs and compared to results reported in human skeletal muscle and the skeletal muscle of rodent models. Blood was sampled from conditioned dogs (n = 8) and sedentary dogs (n = 8). The conditioned dogs were exercised four months prior the experiment and were following a uniform training protocol, whereas the sedentary dogs were not. GLUT4 expression in mononuclear cells and plasma insulin levels were measured using commercially available enzyme-linked immunosorbent assay (ELISA). Blood glucose levels were determined using blood plasma. HOMA-IR was calculated using plasma insulin and blood glucose levels using the linear approximation formula. Our results indicate that the state of conditioning had a significant effect on the GLUT4 expression at the surface of mononuclear cells. HOMA-IR was also affected by conditioning in dogs. GLUT4 levels in mononuclear cells of sled dogs were inversely correlated with the homeostasis model assessment of insulin sensitivity. This study demonstrates that conditioning increases GLUT4 levels in mononuclear cells of sled dogs as it has been previously reported in skeletal muscle. Our results support the potential of white blood cells as a proxy tissue for studying insulin signaling and may lead to development of a minimally invasive and direct marker of insulin resistance. This may be the first report of GLUT4 in mononuclear cells in response to exercise and measured with ELISA.

Adiponectin is not required for exercise training-induced improvements in glucose and insulin tolerance in mice

Adiponectin (Ad) is a potent insulin‐sensitizing adipokine that has been found to activate pathways involved in the adaptation to exercise. Therefore, we examined whether Ad is required for the increased insulin response observed following exercise training in Ad knockout mice (AdKO). Eight weeks of exercise training significantly increased glucose and insulin tolerance in both wild type (WT) and AdKO mice. There were no differences in glucose tolerance between genotypes but insulin tolerance was improved to a greater extent in AdKO compared to WT mice following exercise training (+26%, P < 0.05). There were no genotype differences in the insulin‐stimulated phosphorylation of AKT or AS160 in red or white gastrocnemius muscle (RG, WG). Exercise training increased total AKT and AS160 protein content in RG and total AS160 protein content in WG. There were no genotype differences in total AKT or AS160. However, exercise training induced a more robust increase in total AS160 in RG from AdKO (+44 ± 8%, P < 0.05) compared to WT mice (+28 ± 7%, P = 0.06). There were no differences in total GLUT4 or FAT/CD36 in RG or WG in WT or AdKO, with or without exercise training. Similarly, there were no differences in RER, VO₂, or activity between any groups. Our results indicate the presence of Ad is not required for exercise‐induced increases in insulin response. Furthermore, it appears that exercise may improve insulin sensitivity to a greater extent in the absence of Ad, suggesting the presence of an unknown compensatory mechanism.

Collectively, our results demonstrate that the absence of Ad does not impair the capacity of endurance exercise training to increase glucose and insulin tolerance in AdKO mice. In addition, there were no impairments in insulin signaling or in the protein content of AKT or AS160. Taken together with previous findings, our data indicate that AdKO mice may have sufficient compensations to override the absence of Ad.

Mechanisms regulating GLUT4 transcription in skeletal muscle cells are highly conserved across vertebrates

The glucose transporter 4 (GLUT4) plays a key role in glucose uptake in insulin target tissues. This transporter has been extensively studied in many species in terms of its function, expression and cellular traffic and complex mechanisms are involved in its regulation at many different levels. However, studies investigating the transcription of the GLUT4 gene and its regulation are scarce. In this study, we have identified the GLUT4 gene in a teleost fish, the Fugu (Takifugu rubripes), and have cloned and characterized a functional promoter of this gene for the first time in a non-mammalian vertebrate. In silico analysis of the Fugu GLUT4 promoter identified potential binding sites for transcription factors such as SP1, C/EBP, MEF2, KLF, SREBP-1c and GC-boxes, as well as a CpG island, but failed to identify a TATA box. In vitro analysis revealed three transcription start sites, with the main residing 307 bp upstream of the ATG codon. Deletion analysis determined that the core promoter was located between nucleotides -132/+94. By transfecting a variety of 5´deletion constructs into L6 muscle cells we have determined that Fugu GLUT4 promoter transcription is regulated by insulin, PG-J2, a PPARγ agonist, and electrical pulse stimulation. Furthermore, our results suggest the implication of motifs such as PPARγ/RXR and HIF-1α in the regulation of Fugu GLUT4 promoter activity by PPARγ and contractile activity, respectively. These data suggest that the characteristics and regulation of the GLUT4 promoter have been remarkably conserved during the evolution from fish to mammals, further evidencing the important role of GLUT4 in metabolic regulation in vertebrates.

Regulation of mitochondrial biogenesis and GLUT4 expression by exercise

Endurance exercise training can induce large increases mitochondria and the GLUT4 isoform of the glucose transporter in skeletal muscle. For a long time after the discovery in the 1960s that exercise results in an increase in muscle mitochondria, there was no progress in elucidation of the mechanisms involved. The reason for this lack of progress was that nothing was known regarding how expression of the genes-encoding mitochondrial proteins is coordinately regulated. This situation changed rapidly after discovery of transcription factors that control transcription of genes-encoding mitochondrial proteins and, most importantly, the discovery of peroxisome proliferator-gamma coactivator-1α (PGC-1α). This transcription coactivator binds to and activates transcription factors that regulate transcription of genes-encoding mitochondrial proteins. Thus, PGC-1α activates and coordinates mitochondrial biogenesis. It is now known that exercise rapidly activates and induces increased expression of PGC-1α. The exercise-generated signals that lead to PGC-1α activation and increased expression are the increases in cytosolic Ca(2+) and decreases in ATP and creatine phosphate (∼P). Ca(2+) mediates its effect by activating CAMKII, while the decrease in ∼P mediates its effect via activation of AMPK. Expression of the GLUT4 isoform of the glucose transporter is regulated in parallel with mitochondrial biogenesis via the same signaling pathways. This review describes what is known regarding the regulation of mitochondrial biogenesis and GLUT4 expression by exercise. A major component of this review deals with the physiological and metabolic consequences of the exercise-induced increase in mitochondria and GLUT4.

Possible involvement of AMPK in acute exercise-induced expression of monocarboxylate transporters MCT1 and MCT4 mRNA in fast-twitch skeletal muscle

OBJECTIVE

The regulatory mechanisms responsible for acute exercise-induced expression of monocarboxylate transporters MCT1 and MCT4 mRNA in skeletal muscle remain unclear. 5'-adenosine-activated protein kinase (AMPK) is a key signaling molecule that regulates gene expression at the mRNA level. We examined whether AMPK activation is involved in acute exercise-induced expression of MCT1 and MCT4 mRNA in fast-twitch muscle.

MATERIALS/METHODS

Male Sprague-Dawley rats were subjected to an acute bout of either 5min high-intensity intermittent swimming (HIS) or 6-h low-intensity prolonged swimming (LIS). The effects of acute exercise on the phosphorylation of AMPK (p-AMPK), calcium/calmodulin pendent kinase II (p-CaMKII), p38 mitogen-activated protein kinase (p-p38MAPK), and MCTs mRNA were analyzed in vivo. To observe the direct effects of AMPK activation on MCTs mRNA, the effects of 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR), caffeine, and dantrolene were analyzed in vitro using an isolated muscle incubation model.

RESULTS

The p-AMPK increased in response to both HIS and LIS, although the p-CaMKII and p-p38MAPK were increased only following HIS. Irrespective of exercise intensity, MCT1 and MCT4 mRNA was also transiently upregulated by both HIS and LIS. Direct exposure of the epitrochlearis muscle to 0.5mmol/L AICAR or 1mmol/L caffeine, which activated p-AMPK increased both MCT1 and MCT4 mRNA levels. When pAMPK was inhibited by dantrolene, neither MCT1 nor MCT4 mRNA was increased.

CONCLUSION

These results suggest that acute exercise-induced increases in MCT1 and MCT4 mRNA expression may be possibly mediated by AMPK activation, at least in part in fast-twitch muscle.

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.

VO2 kinetics in supra-anaerobic threshold constant tests allow the visualization and quantification of the O2 saving after cytochrome c oxidase inhibition by aerobic training or nitrate administration

We tested whether the known cytochrome c oxidase (COX) inhibition by nitric oxide (NO) could be quantified by VO(2) kinetics during constant load supra-Anaerobic Threshold (AT) exercises in healthy trained or untrained subjects following aerobic training or nitrate administration. In cycle ergometer constant load exercises supra-AT, identified in previous incremental tests, VO(2) kinetics describe a double exponential curve, one rapid and one appreciably slower, allowing the area between them to be calculate in O(2) l. After training, with increased NO availability, this area decreases in inverse ratio to treatment efficacy. In fact, in 11 healthy subjects after aerobic training for 6-7 weeks, area was decreased on average by 51 %. In 11 untrained subjects, following the assumption of an NO donor, 20 mg isosorbide 5 mononitrate, area was decreased on average by 53 %. In conclusion, supra-AT VO(2) kinetics in constant load exercises permit the quantification of the inhibitory effect NO-dependent on COX after either physical training or nitrate assumption.

Effects of high vs. moderate exercise intensity during interval training on lipids and adiponectin levels in obese young females

PURPOSE

We investigate the effects of 12-week interval training of moderate- or high-intensity exercise on blood lipids and plasma levels of adiponectin.

METHODS

Thirty-four obese adolescent females [age = 15.9 ± 0.3 years; BMI and BMI-Z-score = 30.8 ± 1.6 kg/m(2) and 3 ± 0.3, respectively], were randomized to high-intensity interval training (HIIT, n = 11), moderate-intensity interval training (MIIT, n = 11), or a control group (CG, n = 12). Maximal oxygen uptake ([Formula: see text]), maximal aerobic speed (MAS), plasma lipids and adiponectin levels were measured in all subjects before and after training.

RESULTS

Following the training program, in both training groups, body mass, BMI-Z-score, and percentage body fat (% BF) decreased, while [Formula: see text] and MAS increased. Low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, and adiponectin levels were positively altered (-12.6 and -7.4 %; 6.3 and 8.0 %; 35.8 and 16.2 %; high to moderate training program, respectively). Waist circumference, triglyceride and total cholesterol decreased only in HIIT group (-3.5; -5.3 and -7.0 %, respectively, in all P < 0.05). Significant decrease in the usual index of insulin resistance (HOMA-IR) occurred in HIIT and MIIT groups (-29.2 ± 5.3 and -18.4 ± 8.6 %, respectively; P < 0.01).

CONCLUSION

The results show that HIIT positively changes blood lipids and adiponectin variables in obese adolescent girls, resulting in improved insulin sensitivity, as attested by a lower HOMA-IR, and achieving better results compared to moderate-intensity exercise.

Aerobic exercise training induces metabolic benefits in rats with metabolic syndrome independent of dietary changes

OBJECTIVES

We evaluated the effects of aerobic exercise training without dietary changes on cardiovascular and metabolic variables and on the expression of glucose transporter Type 4 in rats with metabolic syndrome.

METHODS

Twenty male spontaneously hypertensive rats received monosodium glutamate during the neonatal period. The animals were allocated to the following groups: MS (sedentary metabolic syndrome), MS-T (trained on a treadmill for 1 hour/day, 5 days/week for 10 weeks), H (sedentary spontaneously hypertensive rats) and H-T (trained spontaneously hypertensive rats). The Lee index, blood pressure (tail-cuff system), insulin sensitivity (insulin tolerance test) and functional capacity were evaluated before and after 10 weeks of training. Glucose transporter Type 4 expression was analyzed using Western blotting. The data were compared using analysis of variance (ANOVA) (p<0.05).

RESULTS

At baseline, the MS rats exhibited lower insulin sensitivity and increased Lee index compared with the H rats. Training decreased the body weight and Lee index of the MS rats (MS-T vs. MS), but not of the H rats (H-T vs. H). There were no differences in food intake between the groups. At the end of the experiments, the systolic blood pressure was lower in the two trained groups than in their sedentary controls. Whole-body insulin sensitivity increased in the trained groups. Glucose transporter Type 4 content increased in the heart, white adipose tissue and gastrocnemius muscle of the trained groups relative to their respective untrained groups.

CONCLUSION

In conclusion, the present study shows that an isolated aerobic exercise training intervention is an efficient means of improving several components of metabolic syndrome, that is, training reduces obesity and hypertension and increases insulin sensitivity.

Exercise restores insulin, but not adiponectin, response in skeletal muscle of high-fat fed rodents

High-fat (HF) diets impair skeletal muscle response to the insulin-sensitizing adipokine adiponectin (Ad) in rodents, preceding the development of insulin resistance. Skeletal muscle insulin response in HF-fed rats can be restored with chronic exercise; whether recovery of skeletal muscle Ad response is necessary for the exercise-induced recovery of insulin-stimulated glucose transport is not known. In the current study, insulin and Ad resistance were induced in rodents with 4 wk of HF feeding (HF4; low-fat fed animals used as control). Rats were then treadmill-exercised (HF5EX1, HF6EX2) or supplemented orally with the pharmacological agent β-guadinoproprionic acid (GPA; HF5GPA1, HF6GPA2) for 1 or 2 wk with continued HF feeding. Insulin and Ad responses (glucose transport and palmitate oxidation, respectively) were assessed 48 h after the last exercise bout ex vivo in isolated solei. Insulin response was impaired following 4 wk of HF feeding and improved with 1 and 2 wk of exercise and β-GPA supplementation (HF5EX1, HF6EX2, HF5GPA1, and HF6GPA2). The recovery of insulin response generally coincided with improved Akt Thr308 phosphorylation in HF5GPA1, HF6EX2, and HF6GPA2, although not in HF5EX1. Ad-stimulated palmitate oxidation was not restored with either treatment. Total protein contents of AdipoR1, AdipoR2, APPL1, and APPL2, as well as total and phosphorylated AMPK and ACC were unaltered by diet, exercise, and β-GPA at the assessed time points. We conclude that the exercise and pharmacologically (β-GPA)-induced recovery of skeletal muscle insulin response after HF feeding is not dependent on the restoration of Ad response, as assessed ex vivo.

Regulation of GLUT4 and Insulin-Dependent Glucose Flux

GLUT4 has long been known to be an insulin responsive glucose transporter. Regulation of GLUT4 has been a major focus of research on the cause and prevention of type 2 diabetes. Understanding how insulin signaling alters the intracellular trafficking of GLUT4 as well as understanding the fate of glucose transported into the cell by GLUT4 will be critically important for seeking solutions to the current rise in diabetes and metabolic disease.

GLUT4 and glycogen synthase are key players in bed rest-induced insulin resistance

To elucidate the molecular mechanisms behind physical inactivity-induced insulin resistance in skeletal muscle, 12 young, healthy male subjects completed 7 days of bed rest with vastus lateralis muscle biopsies obtained before and after. In six of the subjects, muscle biopsies were taken from both legs before and after a 3-h hyperinsulinemic euglycemic clamp performed 3 h after a 45-min, one-legged exercise. Blood samples were obtained from one femoral artery and both femoral veins before and during the clamp. Glucose infusion rate and leg glucose extraction during the clamp were lower after than before bed rest. This bed rest-induced insulin resistance occurred together with reduced muscle GLUT4, hexokinase II, protein kinase B/Akt1, and Akt2 protein level, and a tendency for reduced 3-hydroxyacyl-CoA dehydrogenase activity. The ability of insulin to phosphorylate Akt and activate glycogen synthase (GS) was reduced with normal GS site 3 but abnormal GS site 2+2a phosphorylation after bed rest. Exercise enhanced insulin-stimulated leg glucose extraction both before and after bed rest, which was accompanied by higher GS activity in the prior-exercised leg than the rested leg. The present findings demonstrate that physical inactivity-induced insulin resistance in muscle is associated with lower content/activity of key proteins in glucose transport/phosphorylation and storage.

Effects of 1 day of inactivity on insulin action in healthy men and women: interaction with energy intake

Prolonged periods of limited muscle activity can reduce insulin action. Acute changes in low muscle activity (ie, sitting) have not been assessed. In addition, unless energy intake is reduced during sitting to match low expenditure, the concurrent energy surplus may explain lower insulin action. The objective of the study was to evaluate the acute effect of sitting, with and without energy surplus, on insulin action. Fourteen young (26.1 ± 4.5 years, mean ± SD), nonobese (23.7% ± 7.1% fat), fit (peak oxygen consumption = 49.1 ± 3.3 mL·kg(-1)·min(-1)) men (n = 7) and women (n = 7) completed three 24-hour conditions: (1) an active, no-sitting condition (high energy expenditure of 2944 ± 124 kcal with energy intake matched to expenditure) = NO-SIT; (2) low energy expenditure (sitting) of 2195 ± 121 kcal with no reduction in energy intake (energy surplus) = SIT; and (3) sitting with energy intake reduced to 2139 ± 118 kcal to match low expenditure (energy balance) = SIT-BAL. Insulin action was measured the following morning during a continuous infusion of [6,6-(2)H]-glucose. Data were analyzed using linear mixed-effects models with planned contrasts. Compared with NO-SIT, insulin action, defined as whole-body rate of glucose disappearance normalized to mean plasma insulin, was reduced by 39% in SIT (P < .001) and by 18% in SIT-BAL (P = .07). Insulin action was higher in SIT-BAL compared with SIT (P = .04). One day of sitting considerably reduced insulin action; this effect was minimized, but not prevented, when energy intake was reduced to match expenditure. Strategies to limit daily sitting may reduce metabolic disease risk.

Acute response of blood glucose to short-term exercise training in patients with type 2 diabetes

It is unclear whether the glucose lowering effects of an exercise session are augmented by training. Therefore, we sought to assess the effects of a four-week exercise training program on the acute response of blood glucose to a single exercise session in patients with T2DM. A Quasi experimental design was used. Thirty-four patients with T2DM (18 males) completed a four-week exercise regime consisting of two 1-h supervised sessions and one 30 min unsupervised home session per week. The sessions contained cardiorespiratory and resistance exercises. Blood glucose was measured prior to and after each training session. Resting heart rate (HR), blood pressure (BP), body composition, lipid profile and cardiorespiratory fitness (VO2max) were determined before and after the four week training program. Decreases in blood glucose (pre to post exercise session) over the four weeks were (mean±SD); week 1: 13.3±18.6%, week 2: 19.7±18.5%, week 3: 18.1±20.8%, week 4: 22.8±17.9%. General linear modelling with repeated measures ANCOVA showed that there was a significant (p<0.01) time effect over this period. Additionally, there were small, but significant decreases in resting heart rate (-6.6±10.3 bpm, p=0.001), systolic blood pressure (-5.6±14.9 mmHg, p=0.043) and fat mass (-1.6±3.2%, p=0.024) and an increase in VO(2max) (1.6±3.7 ml/kg/min, p=0.025) over the four weeks. Four weeks of exercise training augments the exercise-induced decrease in blood glucose that occurs in a single exercise session.

The beneficial effects of exercise in rodents are preserved after detraining: a phenomenon unrelated to GLUT4 expression

BACKGROUND

Although exercise training has well-known cardiorespiratory and metabolic benefits, low compliance with exercise training programs is a fact, and the harmful effects of physical detraining regarding these adaptations usually go unnoticed. We investigated the effects of exercise detraining on blood pressure, insulin sensitivity, and GLUT4 expression in spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY).

METHODS

Studied animals were randomized into sedentary, trained (treadmill running/5 days a week, 60 min/day for 10 weeks), 1 week of detraining, and 2 weeks of detraining. Blood pressure (tail-cuff system), insulin sensitivity (kITT), and GLUT4 (Western blot) in heart, gastrocnemius and white fat tissue were measured.

RESULTS

Exercise training reduced blood pressure (19%), improved insulin sensitivity (24%), and increased GLUT4 in the heart (+34%); gastrocnemius (+36%) and fat (+22%) in SHR. In WKY no change in either blood pressure or insulin sensitivity were observed, but there was an increase in GLUT4 in the heart (+25%), gastrocnemius (+45%) and fat (+36%) induced by training. Both periods of detraining did not induce any change in neither blood pressure nor insulin sensitivity in SHR and WKY. One-week detraining reduced GLUT4 in SHR (heart: -28%; fat: -23%) and WKY (heart: -19%; fat: -22%); GLUT4 in the gastrocnemius was reduced after a 2-week detraining (SHR: -35%; WKY: -25%). There was a positive correlation between GLUT4 (gastrocnemius) and the maximal velocity in the exercise test (r = 0.60, p = 0.004).

CONCLUSIONS

The study findings show that in detraining, despite reversion of the enhanced GLUT4 expression, cardiorespiratory and metabolic beneficial effects of exercise are preserved.

Effects of short-term training on insulin sensitivity and skeletal muscle glucose metabolism in Standardbred horses

REASONS FOR PERFORMING STUDY

Increased insulin sensitivity occurs after a period of exercise training, but the mechanisms underlying this training-associated increase in insulin action have not been investigated.

OBJECTIVE

To examine the effects of short-term endurance training (7 consecutive days) and a subsequent period of inactivity (5 days) on whole body insulin sensitivity and GLUT-4 protein and the activities of glycogen synthase (GS) and hexokinase (HK) in skeletal muscle. It was hypothesised that training would increase insulin sensitivity in association with increased GLUT-4 protein and activities of GS and HK, but that these changes would be transient, returning to baseline after 5 days of inactivity.

METHODS

Seven mature Standardbred horses completed training consisting of 7 consecutive days of 45 min of treadmill exercise at a speed that elicited 55% of pretraining maximal aerobic capacity (VO2peak). Insulin sensitivity was determined by rate of glucose disposal (M) during the last 60 min of a 120 min euglycaemic-hyperinsulinaemic clamp (EHC) performed before (-2 days) and at 1 and 6 days following training. VO2peak was measured before (UT) and after (TR) training and the period of inactivity (IA).

RESULTS

Training resulted in a 9% increase in mean VO2peak (P<0.05) that was maintained following inactivity (IA). Mean M values were more than 2-fold higher (P<0.05) in TR than in UT. Mean M was also higher (P<0.05) in IA when compared to UT. GLUT-4 protien abundancewas more than 10-fold higher in TR and IA (P<0.001) than in UT. Pre-EHC GS activity and GS fractional velocity were increased (P<0.05) in TR when compared to UT and IA. Pre-EHC HK activity was increased (P<0.05) in IA when compared to UT and TR. Muscle glycogen was 66% lower (P<0.05) in TR than in UT and IA.

CONCLUSIONS

Short-term training resulted in increases in whole body insulin sensitivity, and GLUT-4 protein content and glycogen synthase activity in skeletal muscle. The enhancements in insulin sensitivity, GLUT-4 protein and glycogen synthase activity were still evident after 5 days of inactivity.

POTENTIAL RELEVANCE

Insulin resistance in equids has been associated with obesity and predisposition to laminitis. Regular physical activity may mitigate risk of these conditions via enhancement of insulin sensitivity and/or control of bodyweight.

Skeletal muscle sorbitol levels in diabetic rats with and without insulin therapy and endurance exercise training

Sorbitol accumulation is postulated to play a role in skeletal muscle dysfunction associated with diabetes. The purpose of this study was to determine the effects of insulin and of endurance exercise on skeletal muscle sorbitol levels in streptozotocin-induced diabetic rats. Rats were assigned to one experimental group (control sedentary, control exercise, diabetic sedentary, diabetic exercise, diabetic sedentary no-insulin). Diabetic rats received daily subcutaneous insulin. The exercise-trained rats ran on a treadmill (1 hour, 5X/wk, for 12 weeks). Skeletal muscle sorbitol levels were the highest in the diabetic sedentary no-insulin group. Diabetic sedentary rats receiving insulin had similar sorbitol levels to control sedentary rats. Endurance exercise did not significantly affect sorbitol levels. These results indicate that insulin treatment lowers sorbitol in skeletal muscle; therefore sorbitol accumulation is probably not related to muscle dysfunction in insulin-treated diabetic individuals. Endurance exercise did not influence intramuscular sorbitol values as strongly as insulin.

Aerobic interval training reduces cardiovascular risk factors more than a multitreatment approach in overweight adolescents

The aim of the present study was to compare the effects of a multidisciplinary approach (MTG) and aerobic interval training (AIT) on cardiovascular risk factors in overweight adolescents. A total of 62 overweight and obese adolescents from Trøndelag County in Norway, referred to medical treatment at St Olav's Hospital, Trondheim, Norway, were invited to participate. Of these, 54 adolescents (age, 14.0 +/- 0.3 years) were randomized to either AIT (4 x 4 min intervals at 90% of maximal heart rate, each interval separated by 3 min at 70%, twice a week for 3 months) or to MTG (exercise, dietary and psychological advice, twice a month for 12 months). Follow-up testing occurred at 3 and 12 months. VO(2max) (maximal oxygen uptake) increased more after AIT compared with MTG, both at 3 months (11 compared with 0%; P<0.01) and 12 months (12 compared with -1%; P<0.01). AIT enhanced endothelial function compared with MTG at both 3 months (absolute change, 5.1 compared with 3.9%; P<0.01) and 12 months (absolute change, 6.3 compared with 1.0%; P<0.01). AIT was favourable compared with MTG in reducing BMI (body mass index), percentage of fat, MAP (mean arterial blood pressure) and increasing peak oxygen pulse. In addition, AIT induced a more favourable regulation of blood glucose and insulin compared with MTG. In conclusion, the novel findings of the present proof-of-concept study was that 3 months of twice weekly high-intensity exercise sessions reduced several known cardiovascular risk factors in obese adolescents more than that observed after a multitreatment strategy, which was initiated as hospital treatment. Follow-up at 12 months confirmed that AIT improved or maintained these risk factors to a better degree than MTG.

Glycogen storage and muscle glucose transporters (GLUT-4) of mice selectively bred for high voluntary wheel running

To examine the evolution of endurance-exercise behaviour, we have selectively bred four replicate lines of laboratory mice (Mus domesticus) for high voluntary wheel running (;high runner' or HR lines), while also maintaining four non-selected control (C) lines. By generation 16, HR mice ran approximately 2.7-fold more than C mice, mainly by running faster (especially in females), a differential maintained through subsequent generations, suggesting an evolutionary limit of unknown origin. We hypothesized that HR mice would have higher glycogen levels before nightly running, show greater depletion of those depots during their more intense wheel running, and have increased glycogen synthase activity and GLUT-4 protein in skeletal muscle. We sampled females from generation 35 at three times (photophase 07:00 h-19:00 h) during days 5-6 of wheel access, as in the routine selection protocol: Group 1, day 5, 16:00 h-17:30 h, wheels blocked from 13:00 h; Group 2, day 6, 02:00 h-03:30 h (immediately after peak running); and Group 3, day 6, 07:00 h-08:30 h. An additional Group 4, sampled 16:00 h-17:30 h, never had wheels. HR individuals with the mini-muscle phenotype (50% reduced hindlimb muscle mass) were distinguished for statistical analyses comparing C, HR normal, and HR mini. HR mini ran more than HR normal, and at higher speeds, which might explain why they have been favored by the selective-breeding protocol. Plasma glucose was higher in Group 1 than in Group 4, indicating a training effect (phenotypic plasticity). Without wheels, no differences in gastrocnemius GLUT-4 were observed. After 5 days with wheels, all mice showed elevated GLUT-4, but HR normal and mini were 2.5-fold higher than C. At all times and irrespective of wheel access, HR mini showed approximately three-fold higher [glycogen] in gastrocnemius and altered glycogen synthase activity. HR mini also showed elevated glycogen in soleus when sampled during peak running. All mice showed some glycogen depletion during nightly wheel running, in muscles and/or liver, but the magnitude of this depletion was not large and hence does not seem to be limiting to the evolution of even-higher wheel running.

Rapid upregulation of GLUT-4 and MCT-4 expression during 16 h of heavy intermittent cycle exercise

In this study, we have investigated the hypothesis that an exercise protocol designed to repeatedly induce a large dependence on carbohydrate and large increases in glycolytic flux rate would result in rapid increases in the principal glucose and lactate transporters in working muscle, glucose transporter (GLUT)-4 and monocarboxylate transporter (MCT)4, respectively, and in activity of hexokinase (Hex), the enzyme used to phosphorylate glucose. Transporter abundance and Hex activity were assessed in homogenates by Western blotting and quantitative chemiluminescence and fluorometric techniques, respectively, in samples of tissue obtained from the vastus lateralis in 12 untrained volunteers [peak aerobic power (V̇o2peak) = 44.3 ± 2.3 ml·kg−1·min−1] before cycle exercise at repetitions 1 (R1), 2 (R2), 9 (R9), and 16 (R16). The 16 repetitions of the exercise were performed for 6 min at ∼90% V̇o2peak, once per hour. Compared with R1, GLUT-4 increased ( P < 0.05) by 28% at R2 and remained elevated ( P < 0.05) at R9 and R16. For MCT-4, increases ( P < 0.05) of 24% were first observed at R9 and persisted at R16. No changes were observed in GLUT-1 and MCT-1 or in Hex activity. The ∼17- to 24-fold increase ( P < 0.05) in muscle lactate observed at R1 and R2 was reduced ( P < 0.05) to an 11-fold increase at R9 and R16. It is concluded that an exercise protocol designed to strain muscle carbohydrate reserves and to result in large increases in lactic acid results in a rapid upregulation of both GLUT-4 and MCT-4.

The biochemical basis of the health effects of exercise: an integrative view

Physical inactivity–gene interactions result in changes in gene expression, leading to phenotypic changes in the skeletal muscle cell. A subpopulation of those genes that show changes in expression during physical inactivity are candidates for the environment–gene interactions that cross a threshold of biological significance such that overt clinical disease occurs. AMP kinase, GLUT4 and myosin heavy chain IIx are proposed as candidates for physical inactivity-modulated genes that have an altered function that may trigger a crossing of a threshold to disease. Future experiments will be needed to test the validity of the ideas presented.

Interaction of vitamin E and exercise training on oxidative stress and antioxidant enzyme activities in rat skeletal muscles

It has been shown that free radicals are increased during intensive exercise. We hypothesized that vitamin E (vit E) deficiency, which will increase oxidative stress, would augment the training-induced adaptation of antioxidant enzymes. This study investigated the interaction effect of vit E and exercise training on oxidative stress markers and activities of antioxidant enzymes in red quadriceps and white gastrocnemius of rats in a 2x2 design. Thirty-two male rats were divided into trained vit E-adequate, trained vit E-deficient, untrained vit E-adequate, and untrained vit E-deficient groups. The two trained groups swam 6 h/day, 6 days/week for 8 weeks. The two vit E-deficient groups consumed vit E-free diet for 8 weeks. Vitamin E-training interaction effect was significant on thiobarbituric acid reactive substances (TBARSs), glutathione peroxidase (GPX), and superoxide dismutase (SOD) in both muscles. The trained vit E-deficient group showed the highest TBARS and GPX activity and the lowest SOD activity in both muscles. A significant vit E effect on glutathione reductase and catalase was present in both muscles. Glutathione reductase and catalase activities were significantly lower in the two vit E-adequate groups combined than in the two vit E-deficient groups combined in both muscles. This study shows that vit E status and exercise training have interactive effect on oxidative stress and GPX and SOD activities in rat skeletal muscles. Vitamin E deprivation augmented the exercise-induced elevation in GPX activity while inhibiting exercise-induced SOD activity, possibly through elevated oxidative stress.

Exercise-induced increase in muscle insulin sensitivity

Exercise/muscle contraction activates glucose transport. The increase in muscle glucose transport induced by exercise is independent of insulin. As the acute effect of exercise on glucose transport wears off, it is replaced by an increase in insulin sensitivity. An increase in insulin sensitivity results in a shift in the insulin dose-response curve to the left, with a decrease in the concentration of insulin needed to induce 50% of the maximal response. This phenomenon, which plays a major role in rapid muscle glycogen accumulation after exercise, is not mediated by amplification of the insulin signal. Development of the increase in insulin sensitivity after contractions does not require protein synthesis or activation of p38 MAPK. It does require the presence of a serum protein during the period of contractile activity. The effect of exercise on muscle insulin sensitivity is mimicked by hypoxia and by treatment of muscles with 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside to activate AMP-activated protein kinase. The postexercise increase in sensitivity of muscle glucose transport to activation is not specific for insulin but also involves an increased susceptibility to activation by a submaximal contraction/hypoxia stimulus. The increase in insulin sensitivity is mediated by translocation of more GLUT4 glucose transporters to the cell surface in response to a submaximal insulin stimulus. Although the postexercise increase in muscle insulin sensitivity has been characterized in considerable detail, the basic mechanisms underlying this phenomenon remain a mystery.

Alterations in insulin receptor signalling in the rat epitrochlearis muscle upon cessation of voluntary exercise

The major purpose of this study was to elucidate mechanisms by which decreasing enhanced physical activity induces decreased insulin sensitivity in skeletal muscle. Rats with access to voluntary running wheels for 3 weeks had their wheels locked for 5 h (WL5), 29 h (WL29), or 53 h (WL53); a separate group of rats never had wheel access (sedentary, SED). Relative to WL5, submaximal insulin-stimulated 2-deoxyglucose uptake into the epitrochlearis muscle was lower in WL53 and SED. Insulin binding, insulin receptor beta-subunit (IRbeta) protein level, submaximal insulin-stimulated IRbeta tyrosine phosphorylation, and glucose transporter-4 protein level were each lower in both WL53 and SED than in WL5 and WL29. Akt/protein kinase B Ser(473) phosphorylation was lower in WL53 and SED than in WL5. Protein levels of protein tyrosine phosphatase-1B, Src homology phosphatase-2, and protein kinase C- did not vary among groups. The amount of protein tyrosine phosphatase-1B, Src homology phosphatase-2, and protein kinase C- associated with IRbeta in insulin-stimulated muscle also did not differ among the four groups. The mean of SED and WL53 had a significantly higher IRbeta-associated protein tyrosine phosphatase-1B than the mean of WL5 and WL29. The enclosure of multiple changes (decreases in insulin binding, IRbeta protein, IRbeta tyrosine phosphorylation, and glucose transporter-4 protein) in the epitrochlearis muscle within the 29th to 53rd hour after cessation of voluntary wheel running raises the possibility that a single regulatory event could be responsible for the coordinated decrease.

AMP kinase is not required for the GLUT4 response to exercise and denervation in skeletal muscle

An acute bout of exercise increases muscle GLUT4 mRNA in mice, and denervation decreases GLUT4 mRNA. AMP-activated protein kinase (AMPK) activity in skeletal muscle is also increased by exercise, and GLUT4 mRNA is increased in mouse skeletal muscle after treatment with AMPK activator 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside(AICAR). These findings suggest that AMPK activation might be responsible for the increase in GLUT4 mRNA expression in response to exercise. To investigate the role of AMPK in GLUT4 regulation in response to exercise and denervation, transgenic mice with a mutated AMPK alpha-subunit (dominant negative; AMPK-DN) were studied. GLUT4 did not increase in AMPK-DN mice that were treated with AICAR, demonstrating that muscle AMPK is inactive. Exercise (two 3-h bouts of treadmill running separated by 1 h of rest) increased GLUT4 mRNA in both wild-type and AMPK-DN mice. Likewise, denervation decreased GLUT4 mRNA in both wild-type and AMPK-DN mice. GLUT4 mRNA was also increased by AICAR treatment in both the innervated and denervated muscles. These data demonstrate that AMPK is not required for the response of GLUT4 mRNA to exercise and denervation.

Effect of High-Intensity Intermittent Swimming Training on Fatty Acid Oxidation Enzyme Activity in Rat Skeletal Muscle

We previously reported that high-intensity exercise training significantly increased citrate synthase (CS) activity, a marker of oxidative enzyme, in rat skeletal muscle to a level equaling that attained after low-intensity prolonged exercise training (Terada et al., J Appl Physiol 90: 2019-2024, 2001). Since mitochondrial oxidative enzymes and fatty acid oxidation (FAO) enzymes are often increased simultaneously, we assessed the effect of high-intensity intermittent swimming training on FAO enzyme activity in rat skeletal muscle. Male Sprague-Dawley rats (3 to 4 weeks old) were assigned to a 10-day period of high-intensity intermittent exercise training (HIT), low-intensity prolonged exercise training (LIT), or sedentary control conditions. In the HIT group, the rats repeated fourteen 20 s swimming sessions with a weight equivalent to 14-16% of their body weight. Between the exercise sessions, a 10 s pause was allowed. Rats in the LIT group swam 6 h/day in two 3 h sessions separated by 45 min of rest. CS activity in the triceps muscle of rats in the HIT and LIT groups was significantly higher than that in the control rats by 36 and 39%, respectively. Furthermore, 3-beta hydroxyacyl-CoA dehydrogenase (HAD) activity, an important enzyme in the FAO pathway in skeletal muscle, was higher in the two training groups than in the control rats (HIT: 100%, LIT: 88%). No significant difference in HAD activity was observed between the two training groups. In conclusion, the present investigation demonstrated that high-intensity intermittent swimming training elevated FAO enzyme activity in rat skeletal muscle to a level similar to that attained after 6 h of low-intensity prolonged swimming exercise training.

Regulation of the human GLUT4 gene promoter: interaction between a transcriptional activator and myocyte enhancer factor 2A

The GLUT4 gene is subject to complex tissue-specific and metabolic regulation, with a profound impact on insulin-mediated glucose disposal. We have shown, by using transgenic mice, that the human GLUT4 promoter is regulated through the cooperative function of two distinct regulatory elements, domain 1 and the myocyte enhancer factor 2 (MEF2) domain. The MEF2 domain binds transcription factors MEF2A and MEF2D in vivo. Domain I binds a transcription factor, GLUT4 enhancer factor (GEF). In this report, we show a restricted pattern of GEF expression in human tissues, which overlaps with MEF2A only in tissues expressing high levels of GLUT4, suggesting the hypothesis that GEF and MEF2A function together to activate GLUT4 transcription. Data obtained from transiently transfected cells support this hypothesis. Neither GEF nor MEF2A alone significantly activated GLUT4 promoter activity, but increased promoter activity 4- to 5-fold when expressed together. Deletion of the GEF-binding domain (domain I) and the MEF2-binding domain prevented activation, strengthening the conclusion that promoter regulation occurs through these elements. GEF and MEF2A, isolated from nuclei of transfected cells, bound domain I and the MEF2 domain, respectively, which is consistent with activation through these regulatory elements. Finally, GEF and MEF2A coimmunoprecipitated in vivo, strongly supporting a mechanism of GLUT4 transcription activation that depends on this protein-protein interaction.

Differential transcriptional activation of select metabolic genes in response to variations in exercise intensity and duration

Cellular adaptations to endurance training are influenced by the intensity and duration of exercise. To examine the impact of exercise intensity and duration on the acute transcriptional regulation of metabolic genes in red (RG) and white (WG) gastrocnemius muscle, rats completed either low-intensity [ approximately 50% maximal O2 uptake (VO2 max)] treadmill exercise (LIE) for 45 min, LIE for 180 min, or high-intensity ( approximately 75% VO2 max) exercise (HIE) for 45 min. LIE for 45 min activated (P < 0.05) transcription of the pyruvate dehydrogenase kinase-4 (PDK4), uncoupling protein-3 (UCP3), heme oxygenase-1 (HO-1), and hexokinase II (HK II) genes in RG within 1 h after exercise. In WG, transcription of PDK4, UCP3, HKII, and lipoprotein lipase (LPL) was also induced, whereas transcription of the HO-1 gene did not change. In RG, extending LIE duration from 45 to 180 min elicited a similar activation of PDK4 and UCP3 ( approximately 15-fold) but a far greater increase in HO-1 (>30-fold) and HKII transcription (>25-fold). In WG, extending LIE for 180 min induced a much greater and prolonged (through 2- to 4-h recovery) activation of PDK4, UCP3 (both >200-fold), and HO-1 (>10-fold). HIE elicited a similar pattern of gene activation to LIE in both RG and WG, with the exception that HIE triggered >10-fold activation of HO-1 in WG. These data provide evidence that both the intensity and the duration of exercise affect the transcriptional regulation of metabolic genes in muscle in a fiber type-specific manner, possibly reflecting the relative stress imposed by the exercise bout.

Prevention of glycogen supercompensation prolongs the increase in muscle GLUT4 after exercise

Exercise induces an increase in GLUT4 in skeletal muscle with a proportional increase in glucose transport capacity. This adaptation results in enhanced glycogen accumulation, i.e., "supercompensation," in response to carbohydrate feeding after glycogen-depleting exercise. The increase in GLUT4 reverses within 40 h after exercise in carbohydrate-fed rats. The purpose of this study was to determine whether prevention of skeletal muscle glycogen supercompensation after exercise results in maintenance of the increases in GLUT4 and the capacity for glycogen supercompensation. Rats were exercised by means of three daily bouts of swimming. GLUT4 mRNA was increased approximately 3-fold and GLUT4 protein was increased approximately 2-fold 18 h in epitrochlearis muscle after exercise. These increases in GLUT4 mRNA and protein reversed completely within 42 h after exercise in rats fed a high-carbohydrate diet. In contrast, the increases in GLUT4 protein, insulin-stimulated glucose transport, and increased capacity for glycogen supercompensation persisted unchanged for 66 h in rats fed a carbohydrate-free diet that prevented glycogen supercompensation after exercise. GLUT4 mRNA was still elevated at 42 h but had returned to baseline by 66 h after exercise in rats fed the carbohydrate-free diet. Glycogen-depleted rats fed carbohydrate 66 h after exercise underwent muscle glycogen supercompensation with concomitant reversal of the increase in GLUT4. These findings provide evidence that prevention of glycogen supercompensation after exercise results in persistence of exercise-induced increases in GLUT4 protein and enhanced capacity for glycogen supercompensation.

Weight, adiposity, and physical activity as determinants of an insulin sensitivity index in pima Indian children

OBJECTIVE

To determine whether measures of physical activity are related to an insulin sensitivity index ([ISI] 10(4)/fasting insulin x glucose) independent of weight or adiposity in children.

RESEARCH DESIGN AND METHODS

We conducted a longitudinal study of 90 Pima Indian children (39 boys and 51 girls) at 5 and 10 years of age measuring adiposity (dual-energy X-ray absorptiometry), physical activity behavior (questionnaire: number of activities per week [ACT], average hours per week [TIME]), and energy expenditure (doubly labeled water: physical activity level [PAL]).

RESULTS

In cross-sectional analyses, ACT was correlated with ISI at 5 years of age (r = 0.24, P = 0.02) and at 10 years of age (r = 0.21, P = 0.05), but these relationships were not independent of weight or adiposity. PAL was correlated with ISI at 10 years of age (r = 0.39, P = 0.03) but was not independent of weight or adiposity. Longitudinally, ISI decreased from 5 to 10 years of age, and increases in weight and adiposity were associated with decreases in ISI (r = -0.51 and -0.41, respectively; both P < 0.0001). ACT decreased from 5 to 10 years of age, but children who had smaller decreases in ACT had smaller decreases in ISI, independent of increases in weight or adiposity (partial r = 0.22, P = 0.04 adjusted for either weight or adiposity).

CONCLUSIONS

These data suggest that early establishment and maintenance of an active lifestyle can have a beneficial effect on ISI that is partially independent of changes in weight or adiposity. This is particularly relevant considering the current epidemics of both obesity and type 2 diabetes in children.

Determinants of post-exercise glycogen synthesis during short-term recovery

The pattern of muscle glycogen synthesis following glycogen-depleting exercise occurs in two phases. Initially, there is a period of rapid synthesis of muscle glycogen that does not require the presence of insulin and lasts about 30-60 minutes. This rapid phase of muscle glycogen synthesis is characterised by an exercise-induced translocation of glucose transporter carrier protein-4 to the cell surface, leading to an increased permeability of the muscle membrane to glucose. Following this rapid phase of glycogen synthesis, muscle glycogen synthesis occurs at a much slower rate and this phase can last for several hours. Both muscle contraction and insulin have been shown to increase the activity of glycogen synthase, the rate-limiting enzyme in glycogen synthesis. Furthermore, it has been shown that muscle glycogen concentration is a potent regulator of glycogen synthase. Low muscle glycogen concentrations following exercise are associated with an increased rate of glucose transport and an increased capacity to convert glucose into glycogen. The highest muscle glycogen synthesis rates have been reported when large amounts of carbohydrate (1.0-1.85 g/kg/h) are consumed immediately post-exercise and at 15-60 minute intervals thereafter, for up to 5 hours post-exercise. When carbohydrate ingestion is delayed by several hours, this may lead to ~50% lower rates of muscle glycogen synthesis. The addition of certain amino acids and/or proteins to a carbohydrate supplement can increase muscle glycogen synthesis rates, most probably because of an enhanced insulin response. However, when carbohydrate intake is high (> or =1.2 g/kg/h) and provided at regular intervals, a further increase in insulin concentrations by additional supplementation of protein and/or amino acids does not further increase the rate of muscle glycogen synthesis. Thus, when carbohydrate intake is insufficient (<1.2 g/kg/h), the addition of certain amino acids and/or proteins may be beneficial for muscle glycogen synthesis. Furthermore, ingestion of insulinotropic protein and/or amino acid mixtures might stimulate post-exercise net muscle protein anabolism. Suggestions have been made that carbohydrate availability is the main limiting factor for glycogen synthesis. A large part of the ingested glucose that enters the bloodstream appears to be extracted by tissues other than the exercise muscle (i.e. liver, other muscle groups or fat tissue) and may therefore limit the amount of glucose available to maximise muscle glycogen synthesis rates. Furthermore, intestinal glucose absorption may also be a rate-limiting factor for muscle glycogen synthesis when large quantities (>1 g/min) of glucose are ingested following exercise.