Glucose utilization during exercise: influence of endurance training

Aspalathin alleviates skeletal muscle insulin resistance and mitochondrial dysfunction

Natural compounds may bear promising therapeutic benefits against metabolic diseases such as type 2 diabetes mellitus (T2DM), which are characterized by a state of insulin resistance and mitochondrial dysfunction. Here, we examined the cellular mechanisms by which aspalathin, a dihydrochalcone C-glucoside unique to rooibos, may ameliorate palmitate-induced insulin resistance and mitochondrial dysfunction in cultured C2C12 myotubules. This current study demonstrated that aspalathin remains effective in improving glucose uptake in insulin-resistant skeletal muscle cells, supported by the upregulation of insulin-dependent signaling that involves the activation of insulin receptor (IR) and direct phosphorylation of protein kinase B (AKT). Interestingly, aspalathin also improved mitochondrial respiration and function, which was evident by an increased expression of carnitine palmitoyltransferase 1 (Cpt1), fatty acid transport protein 1 (Fatp1), sirtuin 1 (Sirt1), nuclear respiratory factor 1 (Nrf1), and transcription factor A, mitochondrial (Tfam). Importantly, our results showed that aspalathin treatment was effective in ameliorating the devastating outcomes of insulin resistance and mitochondrial dysfunction that are linked with an undesired pro-inflammatory response, by reducing the levels of well-known pro-inflammatory markers such as interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-alpha), and protein kinase C-theta (PKC-theta). Thus, beyond improving glucose uptake and insulin signaling, the current study brings a new perspective in the therapeutic benefits of aspalathin in improving mitochondrial respiration and blocking inflammation to attenuate the detrimental effect of palmitate in skeletal muscle cells.

Repeated Prolonged Exercise Decreases Maximal Fat Oxidation in Older Men

INTRODUCTION/PURPOSE

Fat metabolism and muscle adaptation was investigated in six older trained men (age, 61 ± 4 yr; V˙O2max, 48 ± 2 mL·kg·min) after repeated prolonged exercise).

METHODS

A distance of 2706 km (1681 miles) cycling was performed over 14 d, and a blood sample and a muscle biopsy were obtained at rest after an overnight fast before and 30 h after the completion of the cycling. V˙O2max and maximal fat oxidation were measured using incremental exercise tests. HR was continuously sampled during cycling to estimate exercise intensity.

RESULTS

The daily duration of exercise was 10 h and 31 ± 37 min, and the mean intensity was 53% ± 1% of V˙O2max. Body weight remained unchanged. V˙O2max and maximal fat oxidation rate decreased by 6% ± 2% (P = 0.04) and 32% ± 8% (P < 0.01), respectively. The exercise intensity that elicits maximal fat oxidation was not significantly decreased. Plasma free fatty acid (FA) concentration decreased (P < 0.002) from 500 ± 77 μmol·L to 160 ± 38 μmol·L. Plasma glucose concentration as well as muscle glycogen, myoglobin, and triacylglycerol content remained unchanged. Muscle citrate synthase and ß-hydroxy-acyl-CoA-dehydrogenase activities were unchanged, but the protein expression of HKII, GLUT4, and adipose triacylglycerol lipase were significantly increased.

CONCLUSIONS

Overall, the decreased maximal fat oxidation was probably due to lower exogenous plasma fatty acid availability and the muscle adaptation pattern indicates an increased glucose transport capacity and an increased muscle lipolysis capacity supporting an increased contribution of exogenous glucose and endogenous fat during exercise.

Dietary energy source and physical conditioning affect insulin sensitivity and skeletal muscle glucose metabolism in horses

REASONS FOR PERFORMING STUDY

Starch rich (S) feeds reduce insulin sensitivity in untrained horses when compared to high fat (F) feeds, but insulin sensitivity is not affected when S or F are fed during exercise training. The effects of S vs. F on training-associated alterations in skeletal muscle glucose metabolism are unknown.

OBJECTIVES

To determine the effects of dietary energy source on training-associated changes in insulin sensitivity, skeletal muscle GLUT4 protein and hexokinase (HK) and glycogen synthase (GS) activities in horses.

METHODS

After a baseline period on an all forage diet (Phase 1), horses were adapted to high starch (S) or high fat (F) diets (n = 7/group) for 6 weeks (Phase 2) and then completed 7 weeks of exercise training (Phase 3) on the same diets. To measure insulin sensitivity (SI), minimal model analysis of a frequently-sampled i.v. glucose tolerance test was performed at the end of each phase. Middle gluteal muscle biopsies to measure GLUT-4 protein content, muscle glycogen and HK and GS activities were taken before and after euglycaemic-hyperinsulinaemic clamps administered after each phase. Data were analysed by repeated measures ANOVA.

RESULTS

In S, SI was 36% lower (P < 0.05) after Phase 2 when compared to Phase 1 but was unchanged in F. After Phase 3, SI was increased (P < 0.01) in S and F compared to Phase 2 and did not differ (P > 0.05) between diets. Middle gluteal muscle GLUT-4 protein and post clamp HK activity were increased (P < 0.05) in S after Phase 3, with higher (P < 0.01) GLUT4 in S than in F. GS activities were unchanged in both diets.

CONCLUSIONS

Adaptation to S resulted in decreased SI mitigated by moderate physical conditioning. Increased GLUT-4 protein content and HK activity in S may have contributed to higher SI after training.

Effect of training in the fasted state on metabolic responses during exercise with carbohydrate intake

Skeletal muscle gene response to exercise depends on nutritional status during and after exercise, but it is unknown whether muscle adaptations to endurance training are affected by nutritional status during training sessions. Therefore, this study investigated the effect of an endurance training program (6 wk, 3 day/wk, 1-2 h, 75% of peak Vo(2)) in moderately active males. They trained in the fasted (F; n = 10) or carbohydrate-fed state (CHO; n = 10) while receiving a standardized diet [65 percent of total energy intake (En) from carbohydrates, 20%En fat, 15%En protein]. Before and after the training period, substrate use during a 2-h exercise bout was determined. During these experimental sessions, all subjects were in a fed condition and received extra carbohydrates (1 g.kg body wt(-1) .h(-1)). Peak Vo(2) (+7%), succinate dehydrogenase activity, GLUT4, and hexokinase II content were similarly increased between F and CHO. Fatty acid binding protein (FABPm) content increased significantly in F (P = 0.007). Intramyocellular triglyceride content (IMCL) remained unchanged in both groups. After training, pre-exercise glycogen content was higher in CHO (545 +/- 19 mmol/kg dry wt; P = 0.02), but not in F (434 +/- 32 mmol/kg dry wt; P = 0.23). For a given initial glycogen content, F blunted exercise-induced glycogen breakdown when compared with CHO (P = 0.04). Neither IMCL breakdown (P = 0.23) nor fat oxidation rates during exercise were altered by training. Thus short-term training elicits similar adaptations in peak Vo(2) whether carried out in the fasted or carbohydrate-fed state. Although there was a decrease in exercise-induced glycogen breakdown and an increase in proteins involved in fat handling after fasting training, fat oxidation during exercise with carbohydrate intake was not changed.

Models to explain fatigue during prolonged endurance cycling

Much of the previous research into understanding fatigue during prolonged cycling has found that cycling performance may be limited by numerous physiological, biomechanical, environmental, mechanical and psychological factors. From over 2000 manuscripts addressing the topic of fatigue, a number of diverse cause-and-effect models have been developed. These include the following models: (i) cardiovascular/anaerobic; (ii) energy supply/energy depletion; (iii) neuromuscular fatigue; (iv) muscle trauma; (v) biomechanical; (vi) thermoregulatory; (vii) psychological/motivational; and (viii) central governor. More recently, however, a complex systems model of fatigue has been proposed, whereby these aforementioned linear models provide afferent feedback that is integrated by a central governor into our unconscious perception of fatigue. This review outlines the more conventional linear models of fatigue and addresses specifically how these may influence the development of fatigue during cycling. The review concludes by showing how these linear models of fatigue might be integrated into a more recently proposed nonlinear complex systems model of exercise-induced fatigue.

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.

Repeated prolonged whole-body low-intensity exercise: effects on insulin sensitivity and limb muscle adaptations

This study investigates the effect of prolonged whole-body low-intensity exercise on insulin sensitivity and the limb muscle adaptive response. Seven male subjects (weight, 90.2 +/- 3.2 kg; age, 35 +/- 3 years) completed a 32-day unsupported crossing of the Greenland icecap on cross-country skies pulling sleighs. The subjects were studied before and 3 to 4 days after the crossing of the icecap. Subjects came in overnight fasted, and an intravenous glucose tolerance test (IVGTT) was done. A biopsy was obtained from the vastus lateralis and deltoid muscle. On a separate day, a progressive test was performed to establish maximal oxygen uptake. During the crossing, subjects skied for 342 +/- 41 min/d. Peak oxygen uptake (4.6 +/- 0.2 L/min) was decreased (P < .05) by 7% after the crossing and body mass decreased (P < .05) by 7.1 +/- 0.2 kg, of which 4.4 +/- 0.5 kg was fat mass and 2.7 +/- 0.2 kg lean body mass. Glycosylated hemoglobin (5.6% +/- 0.01%) was not affected by the crossing. The IVGTT data revealed that insulin sensitivity (7.3 +/- 0.6 mU x L-1 x min-1) and glucose effectiveness (0.024 +/- 0.002 min-1) were not changed after the crossing. Similarly, the IVGTT data, when expressed per kilogram of lean body mass or body mass, were not affected by the crossing. Citrate synthase activity was higher (P < .05) in the leg (29 +/- 1 micromol x g-1 x min-1) than in the arm muscle (16 +/- 2 micromol x g-1 x min-1) and was unchanged after the crossing. Muscle GLUT4 protein concentration was higher (P < .05) in the leg (104 +/- 10 arbitrary units) than in the arm (54 +/- 9 arbitrary units) and was not changed in the leg, but was increased (P < .05) by 70% to 91 +/- 9 arbitrary units in the arm after the crossing. In conclusion, the increased glucose transporter expression in arm muscle may compensate for the loss of lean body mass and the decrease in aerobic fitness and thereby contribute to the maintenance of whole-body insulin sensitivity after prolonged low-intensity exercise training.

Effect of exercise training on muscle glucose transporter 4 protein and intramuscular lipid content in elderly men with impaired glucose tolerance

This study determined the effects of exercise training on adaptations of skeletal muscle including fibre composition, capillarity, intra-muscular triglyceride concentration (IMTG), as well as glucose transporter 4 protein (GLUT4) and metabolic enzyme activities. Percutaneous muscle biopsies from the vastus lateralis muscle were obtained from non-obese elderly Korean men (n = 10; age range 58-67 years) with impaired glucose tolerance. Subjects performed 12 weeks of endurance exercise training (60-70% of the heart rate reserve). The training program improved the total GLUT4 protein expression (P < 0.01), decreased the IMTG, increased the fatty acid oxidation capacity, and the number of capillaries around type 1 fibres (P < 0.05), whereas no significant alteration was observed around type II fibres. All data are presented as the means together with the standard deviation. The results suggest that endurance training evokes morphological and biochemical changes in the skeletal muscle of elderly men with impaired glucose tolerance that may be considered to limit the development of type 2 diabetes.

Effects of aerobic training on pyruvate dehydrogenase and pyruvate dehydrogenase kinase in human skeletal muscle

This study examined the effects of short- and long-term aerobic training on the stable up-regulation of pyruvate dehydrogenase (PDH) and PDH kinase (PDK) in human skeletal muscle. We hypothesized that 8 weeks, but not 1 week, of aerobic training would increase total PDH (PDHt) and PDK activities compared to pretraining, and this would be detectable at the level of gene transcription (mRNA) and/or gene translation (protein). Resting muscle biopsies were taken before and after 1 and 8 weeks of aerobic cycle exercise training. PDHt and PDK activities, and their respective protein and mRNA expression, did not differ after 1 week of aerobic training. PDHt activity increased 31% after 8 weeks and this may be partially due to a 1.3-fold increase in PDH-E(1)alpha protein expression. PDK activity approximately doubled after 8 weeks of aerobic training and this was attributed to a 1.3-fold increase in PDK2 isoform protein expression. Similar to 1 week, no changes were observed at the mRNA level after 8 weeks of training. These findings suggest that aerobically trained human skeletal muscle has an increased maximal capacity to utilize carbohydrates, evident by increased PDHt, but increased metabolic control sensitivity to pyruvate through increased contribution of PDK2 to total PDK activity.

Effect of moderate exercise immediately followed by induced hyperglycemia on gene expression and content of the glucose transporter-4 protein in skeletal muscles of horses

OBJECTIVE

To determine the effect of a single bout of exercise and increased substrate availability after exercise on gene expression and content of the glucose transporter-4 (GLUT-4) protein in equine skeletal muscle.

ANIMALS

6 healthy adult Thoroughbreds.

PROCEDURES

The study was designed in a balanced, randomized, 3-way crossover fashion. During 2 trials, horses were exercised at 45% of their maximal rate of oxygen consumption for 60 minutes after which 1 group received water (10 mL/kg), and the other group received glucose (2 g/kg, 20% solution) by nasogastric intubation. During 1 trial, horses stood on the treadmill (sham exercise) and then received water (10 mL/kg) by nasogastric intubation. Muscle glycogen concentration and muscle GLUT-4 protein and mRNA content were determined before exercise and at 5 minutes and 4, 8, and 24 hours after exercise.

RESULTS

Although exercise resulted in a 30% reduction in muscle glycogen concentration, no significant difference was detected in muscle GLUT-4 protein or mRNA content before and after exercise. Glycogen replenishment was similar in both exercised groups and was not complete at 24 hours after exercise. Horses that received glucose had significantly higher plasma glucose and insulin concentrations for 3 hours after exercise, but no effect of hyperglycemia was detected on muscle GLUT-4 protein or mRNA content.

CONCLUSION

Under the conditions of this study, neither exercise nor the combination of exercise followed by hyperglycemia induced translation or transcription of the GLUT-4 protein in horses.

Training-induced alterations in glucose metabolism during moderate-intensity exercise

In several species, physical conditioning (training) provokes a large shift in substrate utilisation during submaximal exercise. Few studies in horses have quantitatively examined these effects. Therefore, the effects of exercise training on plasma glucose kinetics during submaximal exercise were examined in 7 horses (5 Thoroughbred, 2 Standardbred; age 3-9 years) that had been paddock-rested for at least 6 months. Two days after determination of maximum aerobic capacity (VO2max), horses ran on a treadmill (4 degree incline) at 55% of VO2max (UT) for 60 min or until fatigue and then completed 6 weeks of moderate-intensity training on a treadmill (5 days/week). Following training and a second VO2max test, the horses completed exercise trials at the same absolute (ABS) and relative (REL) workload in random order, with at least 3 days between tests. After training, VO2max had increased (P<0.05) by 14.9% (mean +/- s.e. pretraining 118.4 +/- 7.4 ml/kg bwt/min; post-training 136.1 +/- 7.8 ml/kg bwt/min). Mean exercise duration was longer (P<0.05) in the ABS trial (57 +/- 1.9 min) than in the UT (46 +/- 3.9 min) and REL (49 +/- 4.6 min) trials. Plasma glucose concentration increased during exercise, and was lower (P<0.05) in ABS than in UT and REL at the end of exercise. Mean glucose rate of appearance (Ra) and disappearance (Rd) were 22 and 21% lower (P<0.05), respectively, in ABS than in UT, but mean glucose Ra and Rd did not differ between the UT and REL trials. Exercise-induced changes in glucagon, epinephrine and norepinephrine were blunted (P<0.05) in ABS, but not REL, when compared to UT. It is concluded that 6 weeks of moderate-intensity training results in a decrease in glucose flux during submaximal exercise at the same absolute, but not relative, workload. The training-induced decrease in glucose flux may, in part, be due to altered plasma concentrations of the major glucoregulatory 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.