Effect of various types of acute exercise and exercise training on the insulin sensitivity of rat soleus muscle measured in vitro

Effects of acute exercise varying in duration and intensity, as well as of two training regimes (endurance and sprint training) on the sensitivity of the soleus muscle of rat to insulin was measured in vitro and compared in rats. As an index of the muscle insulin sensitivity the hormone concentration in the incubation medium which would produce half maximum stimulation of lactate production (LA) and glycogen synthesis was determined. A single bout of moderate endurance exercise (60 min treadmill running at 20 m×min^–1, 0° inclination) increased the rate of LA production at the hormone concentrations used and increased the sensitivity of the process to insulin at 0.25 and 2 h but not 24 h after termination of exercise. Similar though less pronounced effects were found after heavy endurance exercise (30 min at 25 m×min^–1, 10°), but sprint exercise (6×10 s bouts at 43 m×min^–1, 0°) had no influence on the insulin sensitivity of the soleus muscle. The rate of glycogen synthesis in vitro was accelerated after endurance exercise, but the sensitivity of this process to insulin was unaffected by the preceding exercise. Endurance training for 5 weeks caused marked enhancement of sensitivity of both LA production and glycogen synthesis to insulin, which persisted for at least 48 h after the last training session. No changes in the soleus muscle sensitivity to insulin were found after sprint training. It is concluded that the increased insulin sensitivity of glucose utilization by skeletal muscle which occurs after endurance exercise and particularly during endurance training can substantially contribute to improved carbohydrate tolerance. Sprint exercise does not produce any changes in muscle insulin sensitivity, at least in the soleus muscle of the rat.Dedicated to the late Professor Stanislaw Kozlowski     

The effect of high-intensity intermittent swimming on post-exercise glycogen supercompensation in rat skeletal muscle

A single bout of prolonged endurance exercise stimulates glucose transport in skeletal muscles, leading to post-exercise muscle glycogen supercompensation if sufficient carbohydrate is provided after the cessation of exercise. Although we recently found that short-term sprint interval exercise also stimulates muscle glucose transport, the effect of this type of exercise on glycogen supercompensation is uncertain. Therefore, we compared the extent of muscle glycogen accumulation in response to carbohydrate feeding following sprint interval exercise with that following endurance exercise. In this study, 16-h-fasted rats underwent a bout of high-intensity intermittent swimming (HIS) as a model of sprint interval exercise or low-intensity prolonged swimming (LIS) as a model of endurance exercise. During HIS, the rats swam for eight 20-s sessions while burdened with a weight equal to 18% of their body weight. The LIS rats swam with no load for 3 h. The exercised rats were then refed for 4, 8, 12, or 16 h. Glycogen levels were almost depleted in the epitrochlearis muscles of HIS- or LIS-exercised rats immediately after the cessation of exercise. A rapid increase in muscle glycogen levels occurred during 4 h of refeeding, and glycogen levels had peaked at the end of 8 h of refeeding in each group of exercised refed rats. The peak glycogen levels during refeeding were not different between HIS- and LIS-exercised refed rats. Furthermore, although a large accumulation of muscle glycogen in response to carbohydrate refeeding is known to be associated with decreased insulin responsiveness of glucose transport, and despite the fact that muscle glycogen supercompensation was observed in the muscles of our exercised rats at the end of 4 h of refeeding, insulin responsiveness was not decreased in the muscles of either HIS- or LIS-exercised refed rats compared with non-exercised fasted control rats at this time point. These results suggest that sprint interval exercise enhances muscle glycogen supercompensation in response to carbohydrate refeeding as well as prolonged endurance exercise does. Furthermore, in this study, both HIS and LIS exercise prevented insulin resistance of glucose transport in glycogen supercompensated muscle during the early phase of carbohydrate refeeding. This probably led to the enhanced muscle glycogen supercompensation after exercise.     

Improved insulin-stimulated glucose uptake and glycogen synthase activation in rat skeletal muscles after adrenaline infusion: role of glycogen content and PKB phosphorylation

AIM: Effects of in vivo adrenaline infusion on subsequent insulin-stimulated glucose uptake and glycogen synthase activation was investigated in slow-twitch (soleus) and fast-twitch (epitrochlearis) muscles. Furthermore, role of glycogen content and Protein kinase B (PKB) phosphorylation for modulation insulin sensitivity was investigated. METHODS: Male Wistar rats received adrenaline from osmotic mini pumps ( approximately 150 microg kg(-1) h(-1)) for 1 or 12 days before muscles were removed for in vitro studies. RESULTS: Glucose uptake at physiological insulin concentration was elevated in both muscles after 1 and 12 days of adrenaline infusion. Insulin-stimulated glycogen synthase activation was also improved in both muscles. This elevated insulin sensitivity occurred despite the muscles were exposed to hyperglycaemia in vivo. After 1 day of adrenaline infusion, glycogen content was reduced in both muscles; insulin-stimulated PKB ser(473) phosphorylation was increased in both muscles only at the highest insulin concentration. After 12 days of adrenaline infusion, glycogen remained low in epitrochlearis, but returned to normal level in soleus; insulin-stimulated PKB phosphorylation was normal in both muscles. CONCLUSION: Insulin-stimulated glucose uptake and glycogen synthase activation were increased after adrenaline infusion. Increased insulin-stimulated glucose uptake and glycogen synthase activation after adrenaline infusion cannot be explained by a reduction in glycogen content or an increase in PKB phosphorylation. The mechanisms for the improved insulin sensitivity after adrenaline treatment deserve particular attention as they occur in conjunction with hyperglycaemia.     

Carbohydrate supercompensation and muscle glycogen utilization during exhaustive running in highly trained athletes

Summary Three female and three male highly trained endurance runners with mean maximal oxygen uptake (VO_2max) values of 60.5 and 71.5 ml·kg^–1·min^–1, respectively, ran to exhaustion at 75%–80% of VO_2max on two occasions after an overnight fast. One experiment was performed after a normal diet and training regimen (Norm), the other after a diet and training programme intended to increase muscle glycogen levels (Carb). Muscle glycogen concentration in the gastrocnemius muscle increased by 25% (P<0.05) from 581 mmol·kg^–1 dry weight, SEM 50 to 722 mmol·kg^–1 dry weight, SEM 34 after Carb. Running time to exhaustion, however, was not significantly different in Carb and Norm, 77 min, SEM 13 vs 70 min, SEM 8, respectively. The average glycogen concentration following exhaustive running was 553 mmol· kg^–1 dry weight, SEM 70 in Carb and 434 mmol·kg^–1 dry weight, SEM 57 in Norm, indicating that in both tests muscle glycogen stores were decreased by about 25%. Periodic acid-Schiff staining for semi-quantitative glycogen determination in individual fibres confirmed that none of the fibres appeared to be glycogen-empty after exhaustive running. The steady-state respiratory exchange ratio was higher in Carb than in Norm (0.92, SEM 0.01 vs 0.89, SEM 0.01; P<0.05). Since muscle glycogen utilization was identical in the two tests, the indication of higher utilization of total carbohydrate appears to be related to a higher utilization of liver glycogen. We have concluded that glycogen depletion of the gastrocnemius muscle is unlikely to be the cause of fatigue during exhaustive running at 75%–80% of VO_2max in highly trained endurance runners. Furthermore, diet- and training-induced carbohydrate supercompensation does not appear to improve endurance capacity in such individuals.     

Acute and chronic effects of strenuous exercise on glucose metabolism in isolated, incubated soleus muscle of exercise-trained rats

Male and female Wistar rats were exercise-trained for 6 or 11 weeks respectively, to examine the effects of acute exercise or exercise training per se on insulin-stimulated glucose utilization in soleus muscles isolated and incubated in vitro. The maximal activities of hexokinase and 2-oxoglutarate dehydrogenase were significantly elevated (by greater than 50%) in gastrocnemius muscle of exercise-trained male and female rats, indicating an adaptation to the training regime. No significant differences in any of the variables studied were observed between appropriately matched male and female rats. There were no significant differences in the sensitivity or responsiveness of the rates of lactate formation or glycogen synthesis in soleus muscles isolated from exercise-trained and sedentary animals at rest (exercise-trained animals were studied 40 h after the last exercise bout). On the other hand, acute exercise caused significant changes in soleus muscle glucose metabolism. Basal and insulin-stimulated rates of glycogen synthesis were significantly elevated in soleus muscles incubated from both sedentary and exercise-trained rats immediately after an exercise bout. In addition, the responsiveness of glucose utilization to insulin in soleus muscles from exercise-trained rats was significantly increased after acute exercise. The results indicate that significant changes in the control of glucose metabolism by insulin in soleus muscle occur as a result of an acute exercise bout, while no adaptive changes in insulin sensitivity occur in soleus muscle after exercise training.     

A single session of resistance exercise enhances insulin sensitivity for at least 24 h in healthy men

The aim of the present study was to determine whether a single session of resistance exercise improves whole-body insulin sensitivity in healthy men for up to 24 h. Twelve male subjects (23±1 years) were studied over a period of 4 days during which they consumed a standardized diet, providing 0.16±0.01 MJ·kg^–1·day^–1 containing 15±0.1 energy% (En%) protein, 29±0.1 En% fat and 55±0.3 En% carbohydrate. Insulin sensitivity was determined 24 h before and 24 h after a single resistance exercise session (8 sets of 10 repetitions at 75% of 1 repetition maximum for two leg exercise tasks) using an intravenous insulin tolerance test. Insulin sensitivity index was calculated by the decline in arterial blood glucose concentration following intravenous administration of a single bolus of human insulin (0.075 IU·kg^–1 fat free mass). Basal glucose and insulin concentrations were not changed up to 24 h after the resistance exercise. However, a substantial 13±5% improvement in whole-body insulin sensitivity was observed, 24 h after the resistance exercise (P<0.05). This study shows that even a single session of resistance exercise improves whole-body insulin sensitivity for up to 24 h in healthy men, which is consistent with earlier observations following endurance exercise tasks.     

Adrenaline potentiates insulin-stimulated PKB activation in the rat fast-twitch epitrochlearis muscle without affecting IRS-1-associated PI 3-kinase activity

We have previously shown in the rat slow-twitch soleus muscle that adrenaline greatly potentiates insulin-stimulated protein kinase B (PKB) phosphorylation without having an effect alone. However, insulin signalling capacity through the PKB pathway is higher in soleus than in fast-twitch muscles, whereas adrenaline activates phosphorylase more strongly in epitrochlearis. Therefore, the aim of the present study was to investigate the interaction between adrenaline and insulin signalling in the fast-twitch epitrochlearis muscle. Insulin increased insulin receptor substrate-1 (IRS-1)-associated phosphoinositide (PI) 3-kinase activity threefold, and adrenaline did not influence basal or insulin-stimulated PI 3-kinase activity. Insulin but not adrenaline increased PKB activity and phosphorylation of Ser(473) and Thr(308). It is interesting to note that adrenaline potentiated insulin-stimulated PKB activity and PKB Ser(473) and Thr(308) phosphorylation. These effects were mimicked by dibutyryl-cyclic adenosine monophosphate (db-cAMP). Adrenaline and db-cAMP increased glycogen synthase kinase (GSK)-3beta Ser(9) phosphorylation independently of PKB activation and enhanced insulin-stimulated GSK-3beta Ser(9) phosphorylation. Although adrenaline increased GSK-3 phosphorylation (inhibiting activity), phosphorylation of its target sites on glycogen synthase was increased, and adrenaline blocked insulin-stimulated glycogen synthase dephosphorylation of Ser(641) and Ser(645,649,653,657), glycogen synthase activation and glycogen synthesis. Insulin-stimulated glucose transport was not influenced by adrenaline despite the increased PKB activation. In conclusion, as in the slow-twitch soleus muscle, adrenaline potentiates insulin-stimulated PKB activation in the fast-twitch glycolytic epitrochlearis muscle without increasing IRS-1-associated PI 3-kinase activity. Furthermore, adrenaline induces phosphorylation of a pool of GSK-3 that is not involved in the regulation of glycogen metabolism. These results indicate that the combination of adrenaline and insulin may activate novel signalling molecules rather than just summing up their effects on linear pathways.     

Different metabolic responses to exercise training programmes in single rat muscle fibres

Summary The aim of this report is to elucidate the effects of exercise training on metabolic properties of different muscle fibre types of the rat hindlimb. Single muscle fibres were dissected from soleus (SOL) or extensor digitorum longus (EDL) muscles of Wistar strain male rats trained on a treadmill for 16 weeks. Each fibre was typed histochemically (SO, slow-twitch oxidative; FOG, fast-twitch oxidative glycolytic; FG, fast-twitch glycolytic). Then glycolytic and oxidative enzymes (CK, LDH, PFK, PK, SDH, and MDH) activities were measured biochemically. Slow,-type fibres (SO) were hypertrophied following endurance training and fast-twitch fibres (FOG and FG) were hypertrophied following sprint training. In EDL muscles the distribution of the slow-type fibres was reduced following the sprint training. The activity of glycolytic enzymes increased significantly in the fast-type fibres (FOG and FG) following sprint training, while oxidative enzymes activities increased in both fast (FOG and FG) and slow (SO) muscle fibres following the endurance training. Neither glycolytic nor oxidative enzymes' activities always increased equally in all types of fibre following exercise training. Consequently, the metabolic profiles in each type of single muscle fibre were affected differently by different intensities of exercise training. These results suggest that the functional (enzymes activity) and structural (muscle fibre hypertrophy) changes of skeletal muscle fibre following exercise training appeared gradually, and would be controlled by different factors.     

Studies of gene expression and activity of hexokinase, phosphofructokinase and glycogen synthase in human skeletal muscle in states of altered insulin-stimulated glucose metabolism

When whole body insulin-stimulated glucose disposal rate is measured in man applying the euglycaemic, hyperinsulinaemic clamp technique it has been shown that approximately 75% of glucose is taken up by skeletal muscle. After the initial transport step, glucose is rapidly phosphorylated to glucose-6-phosphate and routed into the major pathways of either glucose storage as glycogen or the glycolytic/tricarboxylic acid pathway. Glucose uptake in skeletal muscle involves-the activity of specific glucose transporters and hexokinases, whereas, phosphofructokinase and glycogen synthase hold critical roles in glucose oxidation/glycolysis and glucose storage, respectively. Glucose transporters and glycogen synthase activities are directly and acutely stimulated by insulin whereas the activities of hexokinases and phosphofructokinase may primarily be allosterically regulated. The aim of the review is to discuss our present knowledge of the activities and gene expression of hexokinase II (HKII), phosphofructokinase (PFK) and glycogen synthase (GS) in human skeletal muscle in states of altered insulin-stimulated glucose metabolism. My own experimental studies have comprised patients with disorders characterized by insulin resistance like non-insulin-dependent diabetes mellitus (NIDDM) and insulin-dependent diabetes mellitus (IDDM) before and after therapeutic interventions, patients with microvascular angina and patients with severe insulin resistant diabetes mellitus and congenital muscle fiber type disproportion myopathy as well as athletes who are in a state of improved insulin sensitivity. By applying the glucose insulin clamp method in combination with nuclear magnetic resonance 31P spectroscopy to normoglycaemic or hyperglycaemic insulin resistant subjects impairment of insulin-stimulated glucose transport and/or phosphorylation in skeletal muscle has been shown. In states characterized by insulin resistance but normoglycaemia, the activity of HKII measured in needle revealed any genetic variability that contributes to explain the decreased muscle levels of GS mRNA or the decreased activity and activation of muscle GS in NIDDM patients and their glucose tolerant but insulin resistant relatives. Thus, the causes of impaired insulin-stimulated glycogen synthesis of skeletal muscle in normoglycaemic insulin resistant subjects are likely to be found in the insulin signalling network proximal to the GS protein. In insulin resistant diabetic patients the impact of these yet unknown abnormalities may be accentuated by the prevailing hyperglycaemia and hyperlipidaemia. Endurance training in young healthy subjects results in improved insulin-stimulated glucose disposal rates, predominantly due to an increased glycogen synthesis rate in muscle, which is paralleled by an increased total GS activity, increased GS mRNA levels and enhanced insulin-stimulated activation of GS. These changes are probably due to local contraction-dependent mechanisms. Likewise, one-legged exercise training has been reported to increase the basal concentration of muscle GS mRNA in NIDDM patients to a level similar to that seen in control subjects although insulin-stimulated glucose disposal rates remain reduced in NIDDM patients. In the insulin resistant states examined so far, basal and insulin-stimulated glucose oxidation rate at the whole body level and PFK activity in muscle are normal. In parallel, no changes have been found in skeletal muscle levels of PFK mRNA and immunoreactive protein in NIDDM or IDDM patients. In endurance trained subjects insulin-stimulated whole body glucose oxidation rate is often increased. However, depending on the intensity and frequency, physical exercise may induce an increased, a decreased or an unaltered level of muscle PFK activity. In athletes the muscle PFK mRNA is similar to what is found in sedentary subjects whereas the immunoreactive PFK protein concentration is decreased.     

Uptake of 3H-leucine into different fractions of rat skeletal muscle following acute endurance and sprint exercise

Summary In order to study the specificity of the protein synthetic response to different acute exercise loads, 48 male rats served as non-exercised controls or ran at either 0.5 m·s^–1 for 1 h or 1.5 m·s^–1, 10 s20 s workrest, for 1h. Animals were killed and red and white vastus muscles excised from the controls or at 0, 2, 18, 24, or 48 h post-exercise. Muscle slices were incubated in a medium containing 10 Ci l-[4,5-³H]leucine for 30 min. Incorporation of the radionuclide was measured by liquid scintillation (dpm·mg^–1 protein) in the whole homogenate and in four subcellular fractions. The endurance exercise elicited increased uptakes into the mitochondrial fractions of both red and white vastus at 2 and 18 h respectively. However, the mitochondrial uptake was depressed at 24 h in the red and at 2 h in the white vastus. Only in red vastus was incorporation into the soluble protein elevated following endurance exercise. The sprint protocol elicited increased uptake into soluble protein at 2 and 18 h in both red and white vastus and into mitochondrial protein at 18 and 24 h in the white vastus. The shifts in uptake in white vastus occurred in conjunction with depressed uptakes in the total homogenate. These data indicate that both the changes in the type of protein and the time course of amino acid incorporation following acute exercise are related to both the metabolic characteristics of the muscle fibres and the intensity of the exercise.This study was supported by Grants from the Natural Sciences and Engineering Research Council (NSERC A0424 and NSERC A6436)     

Intramyocellular lipid and glycogen content are reduced following resistance exercise in untrained healthy males

Resistance exercise has recently been shown to improve whole-body insulin sensitivity in healthy males. Whether this is accompanied by an exercise-induced decline in skeletal muscle glycogen and/or lipid content remains to be established. In the present study, we determined fibre-type-specific changes in skeletal muscle substrate content following a single resistance exercise session. After an overnight fast, eight untrained healthy lean males participated in a ~45 min resistance exercise session. Muscle biopsies were collected before, following cessation of exercise, and after 30 and 120 min of post-exercise recovery. Subjects remained fasted throughout the test. Conventional light and (immuno)fluorescence microscopy were applied to assess fibre-type-specific changes in intramyocellular triacylglycerol (IMTG) and glycogen content. A significant 27±7% net decline in IMTG content was observed in the type I muscle fibres (P<0.05), with no net changes in the type IIa and IIx fibres. Muscle glycogen content decreased with 23±6, 40±7 and 44±7% in the type I, IIa and IIx muscle fibres, respectively (P<0.05). Fibre-type-specific changes in intramyocellular lipid and/or glycogen content correlated well with muscle fibre-type oxidative capacity. During post-exercise recovery, type I muscle fibre lipid content returned to pre-exercise levels within 120 min. No changes in muscle glycogen content were observed during recovery. We conclude that intramyocellular lipid and glycogen stores are readily used during resistance exercise and this is likely associated with the reported increase in whole-body insulin sensitivity following resistance exercise.     

Comparative effects of hindlimb suspension and exercise on skeletal muscle myosin isozymes in rats

Summary The purpose of this study was to ascertain the time course of changes, whilst suspending the hindlimb and physical exercise training, of myosin light chain (LC) isoform expression in rat soleus and vastus lateralis muscles. Two groups of six rats were suspended by their tails for 1 or 2 weeks, two other groups of ten rats each were subjected to exercise training on a treadmill for 9 weeks, one to an endurance training programme (1-h running at 20 m · min^–1 5 days · week^–1), and the other to a sprint programme (30-s bouts of running at 60 m · min^–1 with rest periods of 5 min). At the end of these experimental procedures, soleus and vastus lateralis superficialis muscles were removed for myosin LC isoform determination by two-dimensional gel electrophoresis. Hindlimb suspension for 2 weeks significantly increased the proportion of fast myosin LC and decreased slow myosin LC expression in the soleus muscle. The pattern of myosin LC was unchanged in the vastus lateralis muscle. Sprint training or endurance training for 9 weeks increased the percentage of slow myosin LC in vastus lateralis muscle, whereas soleus muscle myosin LC was not modified. These data indicate that hindlimb suspension influences myosin LC expression in postural muscle, whereas physical training acts essentially on phasic muscle. There were no differences in myosin LC observed under the influence of sprint- or endurance-training programme.     

Improved insulin‐stimulated glucose uptake and glycogen synthase activation in rat skeletal muscles after adrenaline infusion: role of glycogen content and PKB phosphorylation

Aim: Effects of in vivo adrenaline infusion on subsequent insulin‐stimulated glucose uptake and glycogen synthase activation was investigated in slow‐twitch (soleus) and fast‐twitch (epitrochlearis) muscles. Furthermore, role of glycogen content and Protein kinase B (PKB) phosphorylation for modulation insulin sensitivity was investigated. Methods: Male Wistar rats received adrenaline from osmotic mini pumps (≈150 μg kg^?1 h^?1) for 1 or 12 days before muscles were removed for in vitro studies. Results: Glucose uptake at physiological insulin concentration was elevated in both muscles after 1 and 12 days of adrenaline infusion. Insulin‐stimulated glycogen synthase activation was also improved in both muscles. This elevated insulin sensitivity occurred despite the muscles were exposed to hyperglycaemia in vivo. After 1 day of adrenaline infusion, glycogen content was reduced in both muscles; insulin‐stimulated PKB ser⁴⁷³ phosphorylation was increased in both muscles only at the highest insulin concentration. After 12 days of adrenaline infusion, glycogen remained low in epitrochlearis, but returned to normal level in soleus; insulin‐stimulated PKB phosphorylation was normal in both muscles. Conclusion: Insulin‐stimulated glucose uptake and glycogen synthase activation were increased after adrenaline infusion. Increased insulin‐stimulated glucose uptake and glycogen synthase activation after adrenaline infusion cannot be explained by a reduction in glycogen content or an increase in PKB phosphorylation. The mechanisms for the improved insulin sensitivity after adrenaline treatment deserve particular attention as they occur in conjunction with hyperglycaemia.     

Selective glycogen depletion pattern in human muscle fibres after exercise of varying intensity and at varying pedalling rates

1. Glycogen depletion pattern in human skeletal muscle fibres was studied after bicycle exercise of varying intensity performed at different pedalling rates. Work intensities studied were equivalent to 30-150% of V(O) (2) max. with pedalling rates of 30-120 rev/min.2. Glycogen depletion increased dramatically with increasing exercise intensity; depletion was 2.7 and 7.4 times greater respectively at workloads demanding 64 and 84% V(O) (2) max. than at workloads calling for 31% V(O) (2) max. Even greater rates of glycogen utilization occurred at supramaximal loads.3. Slow twitch, high oxidative (ST) fibres were the first to lose glycogen (reduced PAS staining) at all workloads below V(O) (2) max. Progressive glycogen depletion occurred in fast twitch (FT) fibres as work continued. Large quantities of glycogen remained in the muscle after 3 hr of exercise at low exercise intensity. This was almost exclusively found in FT fibres. At workloads exceeding maximal aerobic power, there was an initial depletion of glycogen in both fibre types. Varying the pedalling rate and, thus, the total force exerted in each pedal thrust had no effect on the pattern of glycogen depletion in the fibres.4. Results point to primary reliance upon ST fibres during submaximal endurance exercise, FT fibres being recruited after ST fibres are depleted of glycogen. During exertion requiring energy expenditure greater than the maximal aerobic power, both fibre types appeared to be continuously involved in carrying out the exercise.     

Increase of adiponectin receptor gene expression by physical exercise in soleus muscle of obese Zucker rats

Aerobic exercise, including treadmill running has been widely used to treat insulin resistance and type 2 diabetes. We studied the effects of endurance training on gene expression of adiponectin receptor 1 (AdipoR1) in skeletal muscle of obese Zucker rats: the 8-week moderate exercise program consisted of treadmill running at 20 m/min and 0° gradient for 1 h/day, 7 days/week. After 8 weeks, insulin action on glucose disposal rate was measured by glucose–insulin index, the product of the areas under the curve of glucose and insulin during intraperitoneal glucose tolerance testing. In contrast to results for sedentary obese rats, exercise training decreased plasma levels of insulin and glucose as well as the glucose–insulin index in obese rats, indicating the merit of regular moderate exercise for improvement of insulin sensitivity in this insulin-resistant animal model. Also, diabetes-related reductions in mRNA and protein content of AdipoR1 in soleus muscle were observed in obese rats at baseline; they were markedly reversed after the 8-week exercise program. However, such exercise training did not alter plasma levels of insulin and glucose in lean Zucker rats. Also, AdipoR1 gene expression in soleus muscle was not changed by exercise in lean Zucker rats compared with the sedentary, lean littermates. These results suggest that long-term exercise training may reverse reduced AdipoR1 gene expression in soleus muscle and improve insulin sensitivity in the obese Zucker rats. Thus, an endurance exercise training is probably helpful clinically for obese individuals with insulin resistance.     

Effects of gender and prior swim exercise on glucose uptake in isolated skeletal muscles from mice

The purpose of this study was to characterize the effects of prior swim exercise on glucose uptake in isolated skeletal muscles of mice. Male and female mice (C57BL/6) performing 180 min of swimming had significantly decreased glycogen concentration compared to resting controls in soleus, extensor digitorum longus (EDL), and epitrochlearis muscles, regardless of gender. Glucose uptake by isolated muscles was measured using [(3)H]-2-deoxyglucose without insulin or with 180 pmol/l insulin (20, 75, or 200 min post-exercise and sedentary) or 12,000 pmol/l (20 or 200 min post-exercise and sedentary) in the soleus and EDL and without insulin or with 12,000 pmol/l insulin (20 or 200 min post-exercise and sedentary) in the epitrochlearis. Glucose uptake was higher (P < or = 0.01) for female versus male mice at each insulin concentration in the soleus and EDL, but not the epitrochlearis. Although prolonged (180 min) swim exercise did not alter subsequent glucose uptake, a shorter duration exercise protocol (60 min) tested in male mice (20 min post-exercise) led to a 1.5-fold increase in insulin-independent glucose uptake in EDL muscles. However, insulin-stimulated (180 pmol/l) glucose uptake was not altered by 60 min exercise in EDL or soleus. In light of these results, swim exercise is not recommended to evaluate the exercise-induced improvement in insulin-stimulated glucose uptake of muscles of C57BL/6 mice.     

Caffeine and theophylline block insulin-stimulated glucose uptake and PKB phosphorylation in rat skeletal muscles

Aim: Caffeine and theophylline inhibit phosphatidylinositol 3-kinase (PI3-kinase) activity and insulin-stimulated protein kinase B (PKB) phosphorylation. Insulin-stimulated glucose uptake involves PI3-kinase/PKB, and the aim of the present study was to test the hypothesis that caffeine and theophylline inhibit insulin-stimulated glucose uptake in skeletal muscles. Methods: Rat epitrochlearis muscles and soleus strips were incubated with insulin and different concentrations of caffeine and theophylline for measurement of glucose uptake, force development and PKB phosphorylation. The effect of caffeine was also investigated in muscles stimulated electrically. Results: Caffeine and theophylline completely blocked insulin-stimulated glucose uptake in both soleus and epitrochlearis muscles at 10 mm . Furthermore, insulin-stimulated PKB Ser⁴⁷³ and Thr³⁰⁸ and GSK-3β Ser⁹ phosphorylation were blocked by caffeine and theophylline. Caffeine reduced and theophylline blocked insulin-stimulated glycogen synthase activation. Caffeine stimulates Ca²⁺ release and force development increased rapidly to 10–20% of maximal tetanic contraction. Dantrolene (25 μm ), a well-known inhibitor of Ca²⁺-release, prevented caffeine-induced force development, but caffeine inhibited insulin-stimulated glucose uptake in the presence of dantrolene. Contraction, like insulin, stimulates glucose uptake via translocation of glucose transporter-4 (GLUT4). Caffeine and theophylline reduced contraction-stimulated glucose uptake by about 50%, whereas contraction-stimulated glycogen breakdown was normal. Conclusion: Caffeine and theophylline block insulin-stimulated glucose uptake independently of Ca²⁺ release, and the likely mechanism is via blockade of insulin-stimulated PI3-kinase/PKB activation. Caffeine and theophylline also reduced contraction-stimulated glucose uptake, which occurs independently of PI3-kinase/PKB, and we hypothesize that caffeine and theophylline also inhibit glucose uptake in skeletal muscles via an additional and hitherto unknown molecule involved in GLUT4 translocation.     

Increased insulin-stimulated glucose uptake in athletes: the importance of GLUT4 mRNA,GLUT4 protein and fibre type composition of skeletal muscle

In the present study the expression of GLUT4 and fibre type composition were examined in biopsies from skeletal muscle in seven male athletes and eight male sedentary subjects. Estimated maximal oxygen uptake was increased in the trained group when compared with the sedentary group (74.0 ± 3.9 vs. 42.9±5.1 ml kg^-1 min^-1; P < 0.01). A biopsy of vastus lateralis muscle was taken in the fasting state, 36 h after the last bout of exercise. A second muscle biopsy was obtained following 4 h of a hyperinsulinaemic (2 mU kg^-1 min^-1), euglycaemic clamp. The rate of insulin-stimulated glucose uptake was increased in the trained subjects (17.34±0.53 vs. 13.53±0.79 mg kg^-1 min^-1, P < 0.01). In parallel, the steady state levels of GLUT4 protein and mRNA per DNA were higher in muscle biopsies obtained in the basal state from athletes than in sedentary controls, 21 and 71% respectively (P < 0.05). In the total group of participants, GLUT4 protein per DNA in the basal state and insulin-stimulated glucose uptake rate correlated positively, (r = 0.51, P = 0.05). In the insulin-stimulated state we did not find any significant correlation between GLUT4 protein per DNA and glucose uptake rate (r = 0.13, n.s.). No significant relationships between GLUT4 protein abundance per DNA and muscle fibre type distribution were observed. A significantly negative correladon was found between type 2B fibre area and insulin-stimulated glucose uptake (r =–0.63, P < 0.05). In conclusion, the abundance of GLUT4 protein and mRNA, respectively, is increased in skeletal muscle from endurance trained subjects compared to sedentary subjects. However, factors other than GLUT4 immunoreactive protein abundance seem to be determinant for the increased insulin-stimulated whole body glucose uptake in endurance trained subjects.     

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 β-subunit (IRβ) protein level, submaximal insulin-stimulated IRβ 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 Ser⁴⁷³ 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-θ did not vary among groups. The amount of protein tyrosine phosphatase-1B, Src homology phosphatase-2, and protein kinase C-θ associated with IRβ in insulin-stimulated muscle also did not differ among the four groups. The mean of SED and WL53 had a significantly higher IRβ-associated protein tyrosine phosphatase-1B than the mean of WL5 and WL29. The enclosure of multiple changes (decreases in insulin binding, IRβ protein, IRβ 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.     

Biochemical and histochemical adaptation to sprint training in young athletes

The purpose of the present study was to investigate the effects of 8 months of a specific and controlled sprint training programme on three groups of young athletes (two groups of males and one of females). Biopsies of vastus lateralis were taken before and after the period of training. The type percentage and diameter of the fibres, as well as the glycogen content and the activities of the enzymes of glycogen metabolism (glycogen synthase and glycogen phosphorylase), glycolysis (phosphofructokinase, pyruvate kinase, aldolase and lactate dehydrogenase), oxidative metabolism (succinate de-hydrogenase) and creatine kinase and aminotransferases were studied. The results show an increase in the percentage of type I fibres and an increase in the diameter of both fibre types. A significant increase was also observed in glycogen content, and in the activities of glycogen synthase, glycogen phosphorylase, phosphofructokinase, pyruvate kinase, succinate dehydrogenase, aspartate aminotransferase and alanine aminotransferase. We conclude that a long period of sprint training induces a biochemical muscle adaptation to anaerobic exercise. This metabolic adaptation is followed by a morphological adaptation, although this is probably not as specific as the biochemical one.