Endurance capacity in maturing mdx mice is markedly enhanced by combined voluntary wheel running and green tea extract.

Duchenne muscular dystrophy is characterized by the absence of dystrophin from muscle cells. Dystrophic muscle cells are susceptible to oxidative stress. We tested the hypothesis that 3 wk of endurance exercise starting at age 21 days in young male mdx mice would blunt oxidative stress and improve dystrophic skeletal muscle function, and these effects would be enhanced by the antioxidant green tea extract (GTE). In mice fed normal diet, average daily running distance increased 300% from week 1 to week 3, and total distance over 3 wk was improved by 128% in mice fed GTE. Running, independent of diet, increased serum antioxidant capacity, extensor digitorum longus tetanic stress, and total contractile protein content, heart citrate synthase, and heart and quadriceps beta-hydroxyacyl-CoA dehydrogenase activities. GTE, independent of running, decreased serum creatine kinase and heart and gastrocnemius lipid peroxidation and increased gastrocnemius citrate synthase activity. These data suggest that both endurance exercise and GTE may be beneficial as therapeutic strategies to improve muscle function in mdx mice.     

Voluntary physical activity alterations in endothelial nitric oxide synthase knockout mice

One of the main factors that control vasoreactivity and angiogenesis is nitric oxide produced by endothelial nitric oxide synthase (eNOS). We recently showed that knocking out eNOS induces an important reduction of mitochondrial oxidative capacity in slow-twitch skeletal muscle. Here we investigated eNOS's role in physical activity and contribution to adaptation of muscle energy metabolism to exercise conditions. Physical capacity of mice null for the eNOS isoform (eNOS-/-) was estimated for 8 wk with a voluntary wheel-running protocol. In parallel, we studied energy metabolism enzyme profiles and their response to voluntary exercise in cardiac and slow-twitch soleus (Sol) and fast-twitch gastrocnemius (Gast) skeletal muscles. Weekly averaged running distance was two times lower for eNOS-/- (4.09 +/- 0.42 km/day) than for wild-type (WT; 7.74 +/- 0.42 km/day; P < 0.01) mice. Average maximal speed of running was also lower in eNOS-/- (17.2 +/- 1.4 m/min) than WT (21.2 +/- 0.9 m/min; P < 0.01) mice. Voluntary exercise influenced adaptation to exercise specifically in Sol muscle. Physical activity significantly increased Sol weight by 22% (P < 0.05) in WT but not eNOS-/- mice. WT Sol muscle did not change its metabolic profile in response to exercise, in contrast to eNOS-/- muscle, in which physical activity decreased cytochrome-c oxidase (COX; -36%; P < 0.05), citrate synthase (-37%; P < 0.06), and creatine kinase (-24%, P < 0.01) activities. Voluntary exercise did not change energy enzyme profile in heart (except for 39% increase in COX activity in WT) or Gast muscle. These results suggest that eNOS is necessary for maintaining a suitable physical capacity and that when eNOS is downregulated, even moderate exercise could worsen energy metabolism specifically in oxidative skeletal muscle.     

Skeletal muscle oxidative capacity, antioxidant enzymes, and exercise training

The purposes of this study were to determine whether exercise training induces increases in skeletal muscle antioxidant enzymes and to further characterize the relationship between oxidative capacity and antioxidant enzyme levels in skeletal muscle. Male Sprague-Dawley rats were exercise trained (ET) on a treadmill 2 h/day at 32 m/min (8% incline) 5 days/wk or were cage confined (sedentary control, S) for 12 wk. In both S and ET rats, catalase (CAT), superoxide dismutase (SOD), and glutathione peroxidase (GPX) activities were directly correlated with the percentages of oxidative fibers in the six skeletal muscle samples studied. Muscles of ET rats had increased oxidative capacity and increased GPX activity compared with the same muscles of S rats. However, SOD activities were not different between ET and S rats, but CAT activities were lower in skeletal muscles of ET rats than in S rats. Exposure to 60 min of ischemia and 60 min of reperfusion (I/R) resulted in decreased GPX and increased CAT activities but had little or no effect on SOD activities in muscles from both S and ET rats. The I/R-induced increase in CAT activity was greater in muscles of ET than in muscles of S rats. Xanthine oxidase (XO), xanthine dehydrogenase (XD), and XO + XD activities after I/R were not related to muscle oxidative capacity and were similar in muscles of ET and S rats. It is concluded that although antioxidant enzyme activities are related to skeletal muscle oxidative capacity, the effects of exercise training on antioxidant enzymes in skeletal muscle cannot be predicted by measured changes in oxidative capacity.     

Incorporation rate of glucose carbon, palmitate carbon and leucine carbon into metabolites in relation to enzyme activities and RNA levels in human skeletal muscles.

The activities (Vmax) of hexokinase, glycogen phosphorylase, glucose-6-phosphate dehydrogenase, phosphofructokinase, lactate dehydrogenase, citrate synthase, cytochrome c oxidase, and 3-OH-acyl-CoA dehydrogenase in human skeletal muscles were compared with the in vitro utilization of glucose and palmitic acid assessed under optimal conditions. Statistically significant correlations between substrate fluxes and enzyme activities were found suggesting that the substrate incorporation rate in vitro in some way reflects the capacity of metabolic pathways. The incorporation rate of leucine into muscle proteins was also statistically significantly correlated to the RNA concentration in the muscle tissue. Glycolytic and glycogenolytic enzymes correlated significantly to each other and correlations were also found between aerobic enzymes supporting the validity of constant proportions between certain key enzymes in human skeletal muscles.     

Exercise training increases oxidative capacity and attenuates exercise-induced ultrastructural damage in skeletal muscle of aged horses.

Exercise training improves functional capacity in aged individuals. Whether such training reduces the severity of exercise-induced muscle damage is unknown. The purpose of the present study was to determine the effect of 10 wk of treadmill exercise training on skeletal muscle oxidative capacity and exercise-induced ultrastructural damage in six aged female Quarter horses (>23 yr of age). The magnitude of ultrastructural muscle damage induced by an incremental exercise test before and after training was determined by electron microscopic examination of samples of triceps, semimembranosus, and masseter (control) muscles. Maximal aerobic capacity increased 22% after 10 wk of exercise training. The percentage of type IIa myosin heavy chain increased in semimembranosus muscle, whereas the percentage of type IIx myosin heavy chain decreased in triceps muscle. After training, triceps muscle showed significant increases in activities of both citrate synthase and 3-hydroxyacyl-CoA-dehydrogenase. Attenuation of exercise-induced ultrastructural muscle damage occurred in the semimembranosus muscle at both the same absolute and the same relative workloads after the 10-wk conditioning period. We conclude that aged horses adapt readily to intense aerobic exercise training with improvements in endurance, whole body aerobic capacity, and muscle oxidative capacity, and heightened resistance to exercise-induced ultrastructural muscle cell damage. However, adaptations may be muscle-group specific.     

Imbalance in SOD/CAT activities in rat skeletal muscles submitted to treadmill training exercise

The association between physical exercise and oxidative damage in the skeletal musculature has been the focus of many studies in literature, but the balance between superoxide dismutase and catalase activities and its relation to oxidative damage is not well established. Thus, the aim of the present study was to investigate the association between regular treadmill physical exercise, oxidative damage and antioxidant defenses in skeletal muscle of rats. Fifteen male Wistar rats (8-12 months) were randomly separated into two groups (trained n=9 and untrained n=6). Trained rats were treadmill-trained for 12 weeks in progressive exercise (velocity, time, and inclination). Training program consisted in a progressive exercise (10 m/min without inclination for 10 min/day). After 1 week the speed, time and inclination were gradually increased until 17 m/min at 10% for 50 min/day. After the training period animals were killed, and gastrocnemius and quadriceps were surgically removed to the determination of biochemical parameters. Lipid peroxidation, protein oxidative damage, catalase, superoxide dismutase and citrate synthase activities, and muscular glycogen content were measured in the isolated muscles. We demonstrated that there is a different modulation of CAT and SOD in skeletal muscle in trained rats when compared to untrained rats (increased SOD/CAT ratio). TBARS levels were significantly decreased and, in contrast, a significant increase in protein carbonylation was observed. These results suggest a non-described adaptation of skeletal muscle against exercise-induced oxidative stress.     

Muscle morphological and biochemical adaptations to training in obese Zucker rats

The purposes of the present study were to characterize the histochemical and enzymatic profiles of various hindlimb skeletal muscles, as well as to determine maximal O2 consumption (VO2max) and respiratory exchange ratios (R) during steady-state exercise in the obese Zucker rat. The changes that occurred in these parameters in response to a 6-wk training program were then assessed. Obese rats were randomly assigned to a sedentary or training group. Lean littermates served as a second control. Training consisted of treadmill running at 18 m/min up an 8% grade, 1.5 h/day, 5 day/wk for 6 wk. During week 6, VO2max and R during a steady-state run (74% max) were determined. After 2 days of inactivity, hindlimb muscles were excised, stained for fiber type and capillaries, and assayed for hexokinase, citrate synthase, cytochrome oxidase, and beta-hydroxyacetyl-CoA dehydrogenase. The obese sedentary rats demonstrated greater oxidative enzyme activities per gram of muscle tissue than their lean littermates, greater R values during submaximal exercise of the same relative intensity, and greater absolute VO2max values. Training resulted in a 20-56% increase in oxidative enzymes, a 10% increase in VO2max, and an increase in capillary density in the soleus and plantaris. There was no alteration in R values during exercise at 74% VO2max or in fiber type composition in response to exercise training. Results suggest that the muscle of the obese Zucker rat manifests a greater oxidative capacity than the muscle of its lean littermates. The apparent inability of the obese rat to increase its use of fat during submaximal exercise of the same relative intensity in response to training remains to be elucidated.     

Activation of AMP-activated protein kinase increases mitochondrial enzymes in skeletal muscle.

Muscle contraction causes an increase in activity of 5'-AMP-activated protein kinase (AMPK). This study was designed to determine whether chronic chemical activation of AMPK will increase mitochondrial enzymes, GLUT-4, and hexokinase in different types of skeletal muscle of resting rats. In acute studies, rats were subcutaneously injected with either 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR; 1 mg/g body wt) in 0.9% NaCl or with 0.9% NaCl alone and were then anesthetized for collection and freezing of tissues. AMPK activity increased in the superficial, white region of the quadriceps and in soleus muscles but not in the deep, red region of the quadriceps muscle. Acetyl-CoA carboxylase (ACC) activity, a target for AMPK, decreased in all three muscle types in response to AICAR injection but was lowest in the white quadriceps. In rats given daily, 1 mg/g body wt, subcutaneous injections of AICAR for 4 wk, activities of citrate synthase, succinate dehydrogenase, and malate dehydrogenase were increased in white quadriceps and soleus but not in red quadriceps. Cytochrome c and delta-aminolevulinic acid synthase levels were increased in white, but not red, quadriceps. Carnitine palmitoyl-transferase and hydroxy-acyl-CoA dehydrogenase were not significantly increased. Hexokinase was markedly increased in all three muscles, and GLUT-4 was increased in red and white quadriceps. These results suggest that chronic AMPK activation may mediate the effects of muscle contraction on some, but not all, biochemical adaptations of muscle to endurance exercise training.     

Oxidative stress reversibly inactivates myocardial enzymes during cardiac arrest

Oxidative stress during cardiac arrest may inactivate myocardial enzymes and thereby exacerbate ischemic derangements of myocardial metabolism. This study examined the impact of cardiac arrest on left ventricular enzymes. Beagles were subjected to 5 min of cardiac arrest and 5 min of open-chest cardiac compressions (OCCC) before epicardial direct current countershocks were applied to restore sinus rhythm. Glutathione/glutathione disulfide redox state (GSH/GSSG) and a panel of enzyme activities were measured in snap-frozen left ventricle. To test whether oxidative stress during arrest inactivated the enzymes, metabolic (pyruvate) or pharmacological (N-acetyl-l-cysteine) antioxidants were infused intravenously for 30 min before arrest. During cardiac arrest, activities of phosphofructokinase, citrate synthase, aconitase, malate dehydrogenase, creatine kinase, glucose-6-phosphate dehydrogenase, and glutathione reductase fell by 56, 81, 55, 34, 42, 55, and 45%, respectively, coincident with 50% decline in GSH/GSSG. OCCC effected full recovery of glutathione reductase and partial recovery of citrate synthase and aconitase, in parallel with GSH/GSSG. Phosphofructokinase, malate dehydrogenase, creatine kinase, and glucose-6-phosphate dehydrogenase recovered only after cardioversion. Antioxidant pretreatments augmented phosphofructokinase, aconitase, and malate dehydrogenase activities before arrest and enhanced these activities, as well as those of citrate synthase and glucose-6-phosphate dehydrogenase, during arrest. In conclusion, cardiac arrest reversibly inactivates several important myocardial metabolic enzymes. Antioxidant protection of these enzymes implicates oxidative stress as a principal mechanism of enzyme inactivation during arrest.     

Identical responses of fast muscle to sustained activity by low-frequency stimulation in young and aging rats

Toinvestigate effects of sustained activity on major phenotypicproperties, the left extensor digitorum longus muscle of young (15 wk)and aging (101 wk) male Brown Norway rats was subjected to 50 days ofchronic low-frequency stimulation (CLFS; 10 Hz, 10 h/day).The contralateral muscle served as control. Changes in metabolicenzymes were analyzed by using glyceraldehyde-3-phosphate dehydrogenaseand lactate dehydrogenase as reference enzymes of glycolysis and byusing citrate synthase and 3-hydroxyacyl-CoA dehydrogenase asmitochondrial enzymes representative of aerobic-oxidative metabolism. Myosin heavy chain (MHC) isoforms wereanalyzed by SDS-PAGE. No differences existed between the enzymeactivity profiles of control muscles from young and aging rats. CLFSinduced similar increases in mitochondrial enzymes, as well as similardecreases in glycolytic enzymes. Although the MHC composition of thecontrol muscles in the aging rats displayed a shift toward slowerisoforms, the ultimate changes induced by CLFS led to nearly identicalMHC phenotypes in both young and aging rats. These results demonstrate an unaltered adaptability of skeletal muscle to increased neuromuscular activity in the aging rat.

     

Functional adaptations of rat skeletal muscle arterioles to aerobic exercise training.

This study tested the hypothesis that both structural and functional adaptations of arterioles occur within the skeletal muscle of rats aerobically trained for 8-10 wk with treadmill exercise. The training regimen used has been shown to elicit a 37% increase in plantaris citrate synthase activity but did not result in an elevation in citrate synthase activity in the spinotrapezius or gracilis muscles of rats used in this study. In the in vivo resting spinotrapezius muscle, arteriole diameters were similar in sedentary (SED) and trained (TR) rats. However, large- (1A) and intermediate- (2A) sized arterioles dilated proportionately more in TR than in SED rats during 1- to 8-Hz muscle contractions, even though the passive mechanical properties (circumference-passive wall tension relationships) were similar between groups. Vascular casts demonstrated a trend for an increase in the number of small (3A) arterioles and an approximately 20% increase in the passive diameter of 1A and 2A arterioles in the spinotrapezius muscle of TR rats. In contrast, in the gracilis muscle, arteriole diameters and density were identical in SED and TR rats, but the capillary-to-muscle fiber ratio was approximately 15% higher in TR rats. The results suggest that aerobic exercise training can greatly increase functional vasodilation and induce a slight increase in vascular density in skeletal muscle tissues, even if the oxidative capacity of these tissues is not increased by the training regimen.     

(Over)training effects on quantitative electromyography and muscle enzyme activities in standardbred horses

Too intensive training may lead to overreaching or overtraining. To study whether quantitative needle electromyography (QEMG) is more sensitive to detect training (mal)adaptation than muscle enzyme activities, 12 standardbred geldings trained for 32 wk in age-, breed-, and sex-matched fixed pairs. After a habituation and normal training (NT) phase (phases 1 and 2, 4 and 18 wk, respectively), with increasing intensity and duration and frequency of training sessions, an intensified training (IT) group (phase 3, 6 wk) and a control group (which continued training as in the last week of phase 2) were formed. Thereafter, all horses entered a reduced training phase (phase 4, 4 wk). One hour before a standardized exercise test (SET; treadmill), QEMG analysis and biochemical enzyme activity were performed in muscle or in biopsies from vastus lateralis and pectoralis descendens muscle in order to identify causes of changes in exercise performance and eventual (mal)adaptation in skeletal muscle. NT resulted in a significant adaptation of QEMG parameters, whereas in muscle biopsies hexokinase activity was significantly decreased. Compared with NT controls, IT induced a stronger adaptation (e.g., higher amplitude, shorter duration, and fewer turns) in QEMG variables resembling potentially synchronization of individual motor unit fiber action potentials. Despite a 19% decrease in performance of the SET after IT, enzyme activities of 3-hydroxyacyl dehydrogenase and citrate synthase displayed similar increases in control and IT animals. We conclude that 1) QEMG analysis is a more sensitive tool to monitor training adaptation than muscle enzyme activities but does not discriminate between overreaching and normal training adaptations at this training level and 2) the decreased performance as noted in this study after IT originates most likely from a central (brain) rather than peripheral level.     

CK and LD isozymes in human single muscle fibers in trained athletes

Individual human muscle fibers from the vastus lateralis were isolated from age-matched endurance-trained and strength-trained athletes and untrained controls. Slow- (ST) and fast-twitch (FT) fibers were assayed for total creatine kinase (CK), CK-MB, total lactate dehydrogenase (LD), the LD isozyme that predominates in the heart muscle of most vertebrates (LD1), and citrate synthase (CS). Regardless of training of the athletes, both CK-MB and CS were higher in ST than in FT fibers. Also, irrespective of fiber type, CK-MB and CS were greatest in the endurance-trained group. A positive correlation existed between CK-MB and CS, relating oxidative capacity of individual fibers with CK-MB. Total CK varied little among the fiber types, trained groups, or controls. Total LD in FT fibers was greater than in ST fibers in all groups, with only ST fibers from the endurance-trained group containing substantial amounts of LD1. These findings suggest that specific training, endurance exercise, causes a favorable metabolic adaptation of CK and LD isozymes at the individual fiber level, allowing for the muscle to cope with increased energy demands during prolonged exercise.     

Enzymatic and mitochondrial responses to 5 months of aerial exposure in the slender lungfish Protopterus dolloi

Mitochondrial respiration and activities of key metabolic enzymes from liver and white skeletal muscle were compared between control aquatic slender lungfish Protopterus dolloi , and those exposed to air for 5 months. Activities of citrate synthase, glycogen phosphorylase, phosphofructokinase and pyruvate kinase in liver were not affected by air-exposure. In muscle, air-exposure reduced citrate synthase and pyruvate kinase activities (relative to tissue wet mass) by 63 and 50%, respectively. Liver carnitine palmitoyl transferase activity (relative to mitochondrial protein) decreased by half following air-exposure, but there was no change in muscle. In mitochondria isolated from muscle, state 3 and state 4 respiration were reduced by 74 and 89%, respectively following air-exposure, but liver mitochondria were not affected. In liver, air-exposure increased activities of ornithine-urea cycle enzymes including glutamine synthase, carbamoyl-phosphate synthase III and arginase, by 1·9- to 4·2-fold. Carbamoyl-phosphate synthase III activity could not be detected in muscle, indicating that urea is not synthesized in this tissue. These data suggest that skeletal muscle metabolism is downregulated in air-exposure, conserving energy and protein during a period when the animals cannot forage. In contrast, ATP production capacities in the liver are maintained, and this may permit expensive urea biosynthesis to continue during aerial exposure.     

6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase from frog skeletal muscle: purification,kinetics and immunological properties

Fructose 2,6-bisphosphate is the most potent activator of 6-phosphofructo-1-kinase, a key regulatory enzyme of glycolysis in animal tissues. This study was prompted by the finding that the content of fructose 2,6-bisphosphate in frog skeletal muscle was dramatically increased at the initiation of exercise and was closely correlated with the glycolytic flux during exercise. 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase, the enzyme system catalyzing the synthesis and degradation of fructose 2,6-bisphosphate, was purified from frog (Rana esculenta) skeletal muscle and its properties were compared with those of the rat muscle type enzyme expressed in Escherichia coli using recombinant DNA techniques. 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase from frog muscle was purified 5600-fold. 6-Phosphofructo-2-kinase and fructose-2,6-bisphosphatase activities could not be separated, indicating that the frog muscle enzyme is bifunctional. The enzyme preparation from frog muscle showed two bands on sodium dodecylsulphate polyacrylamide gel electrophoresis. The minor band had a relative molecular mass of 55800 and was identified as a liver (L-type) isoenzyme. It was recognized by an antiserum raised against a specific amino-terminal amino acid sequence of the L-type isoenzyme and was phosphorylated by the cyclic AMP-dependent protein kinase. The major band in the preparations from frog muscle (relative molecular mass = 53900) was slightly larger than the recombinant rat muscle (M-type) isoenzyme (relative molecular mass = 53300). The pH profiles of the frog muscle enzyme were similar to those of the rat M-type isoenzyme, 6-phosphofructo-2-kinase activity was optimal at pH 9.3, whereas fructose-2,6-bisphosphatase activity was optimal at pH 5.5. However, the 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase from frog muscle differed from other M-type isoenzymes in that, at physiological pH, the maximum activity of 6-phosphofructo-2-kinase exceeded that of fructose-2,6-bisphosphatase, the activity ratio being 1.7 (at pH 7.2) compared to 0.2 in the rat M-type isoenzyme. 6-Phosphofructo-2-kinase activity from the frog and rat muscle enzymes was strongly inhibited by citrate and by phosphoenolpyruvate whereas glycerol 3-phosphate had no effect. Fructose-2,6-bisphosphatase activity from frog muscle was very sensitive to the non-competitive inhibitor fructose 6-phosphate (inhibitor concentration causing 50% decrease in activity = 2 mol · l^-1). The inhibition was counteracted by inorganic phosphate and, particularly, by glycerol 3-phosphate. In the presence of inorganic phosphate and glycerol 3-phosphate the frog muscle fructose-2,6-bisphosphatase was much more sensitive to fructose 6-phosphate inhibition than was the rat M-type fructose-2,6-bisphosphatase. No change in kinetics and no phosphorylation of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase from frog muscle was observed after incubation with protein kinase C and a Ca²⁺/calmodulin-dependent protein kinase. The kinetics of frog muscle 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase, although they would favour an initial increase in fructose 2,6-bisphosphate in exercising frog muscle, cannot fully account for the changes in fructose 2,6-bisphosphate observed in muscle of exercising frog. Regulatory mechanisms not yet studied must be involved in working frog muscle in vivo.Abbreviations BSA bovine serum albumin - Ca/CAMK Ca²⁺/calmodulin-dependent protein kinase (EC 2.7.1.37) - CL anti-l-type PFK-21 FBPase-2 antiserum - DTT dithiothreitol - EP phosphorylated enzyme intermediate - FBPase-2 fructose-2,6-bisphosphatase (EC 3.1.3.46) - F2,6P₂ fructose 2,6-bisphosphate - I_0,5 inhibitor concentration required to decrease enzyme activity by 50% - MCL-2 anti-PFK-2/FBPase-2 antiserum - M_r relative molecular mass - PEG polyethylene glycol - PFK-1 6-phosphofructo-1-kinase (EC 2.7.1.11) - PKF-2 6-phosphofructo-2-kinase (EC 2.7.1.105) - PKA protein kinase A = cyclic AMP-dependent protein kinase (EC 2.7.1.37) - PKC protein kinase C (EC 2.7.1.37) - SDS sodium dodecylsulphate - SDS-PAGE sodium dodecylsulphate polyacrylamide gel electrophoresis - U unit of enzyme activity     

Dietary supplementation of old mice with flavonoid dihydroquercetin causes recovery of the mitochondrial enzyme activities in skeletal muscles

The effect of a potent antioxidant, flavonoid dihydroquercetin on the activity of three mitochondrial enzymes in mouse skeletal muscles has been investigated. An ability of this substance to restore the activity of mitochondrial enzymes in old animals was demonstrated. The activities of citrate synthase, NADHcoenzymeQ1-oxidoreductase (complex 1) and cytochromc-oxidase (complex 4) were assessed using spectro-photometric analysis in a quadriceps muscle homogenate. It was shown that the citrate synthase activity decreased moderately and the activities of complexes 1 and 4 in skeletal muscles dropped significantly in old mice. Supplementation of drink water with dihydroquercetin for a few weeks led to an increase of citrate synthase and complex 1 activity (P < 0.1) in muscles of old animals. Activity of complex 4 returned to the level found in the tissue of young mice. Maximal activity of citrate synthase and complex 1 was found in muscles of young mice. Sensitivity of NADH-coenzymeQ1-oxidoreductase to a specific inhibitor rotenone differed in all three groups of mice. Young and old mice exhibited about 95% and 84% of the total sensitivity, respectively, while in old mice receiving dihydroquercetin the sensitivity of complex 1 to the inhibitor increased up to 98%. The biochemical alterations entailed an increase in animals’ mobility as well as an improvement of fur and skin condition. Fatty acid composition of homogenate in muscle tissue of all three groups was also investigated. A reliable decline of the amount of linoleic acid and an increase in stearic and docosanoic acid contents as well as an increase of total amount of fatty acids in muscles of old mice were found. Statistically significant changes in fatty acid composition in muscles of old mice in the control group and in old mice receiving antioxidant were not observed.     

Chronic hypoxia increases insulin-stimulated glucose uptake in mouse soleus muscle

People living at high altitude appear to have lower blood glucose levels and decreased incidence of diabetes. Faster glucose uptake and increased insulin sensitivity are likely explanations for these findings: skeletal muscle is the largest glucose sink in the body, and its adaptation to the hypoxia of altitude may influence glucose uptake and insulin sensitivity. This study tested the hypothesis that chronic normobaric hypoxia increases insulin-stimulated glucose uptake in soleus muscles and decreases plasma glucose levels. Adult male C57BL/6J mice were kept in normoxia [fraction of inspired O? = 21% (Control)] or normobaric hypoxia [fraction of inspired O? = 10% (Hypoxia)] for 4 wk. Then blood glucose and insulin levels, in vitro muscle glucose uptake, and indexes of insulin signaling were measured. Chronic hypoxia lowered blood glucose and plasma insulin [glucose: 14.3 ± 0.65 mM in Control vs. 9.9 ± 0.83 mM in Hypoxia (P < 0.001); insulin: 1.2 ± 0.2 ng/ml in Control vs. 0.7 ± 0.1 ng/ml in Hypoxia (P < 0.05)] and increased insulin sensitivity determined by homeostatic model assessment 2 [21.5 ± 3.8 in Control vs. 39.3 ± 5.7 in Hypoxia (P < 0.03)]. There was no significant difference in basal glucose uptake in vitro in soleus muscle (1.59 ± 0.24 and 1.71 ± 0.15 μmol·g?1·h?1 in Control and Hypoxia, respectively). However, insulin-stimulated glucose uptake was 30% higher in the soleus after 4 wk of hypoxia than Control (6.24 ± 0.23 vs. 4.87 ± 0.37 μmol·g?1·h?1, P < 0.02). Muscle glycogen content was not significantly different between the two groups. Levels of glucose transporters 4 and 1, phosphoinositide 3-kinase, glycogen synthase kinase 3, protein kinase B/Akt, and AMP-activated protein kinase were not affected by chronic hypoxia. Akt phosphorylation following insulin stimulation in soleus muscle was significantly (25%) higher in Hypoxia than Control (P < 0.05). Neither glycogen synthase kinase 3 nor AMP-activated protein kinase phosphorylation changed after 4 wk of hypoxia. These results demonstrate that the adaptation of skeletal muscles to chronic hypoxia includes increased insulin-stimulated glucose uptake.     

Biochemical transformation of canine skeletal muscle for use in cardiac-assist devices

Skeletal muscle has an inherent biochemical phenotypic plasticity that provides the possibility for it to be remodeled into a "heart-like" muscle for use in cardiac-assist devices. The purpose of this study was to chronically stimulate skeletal muscle electrically to transform the biochemical capacities of the three major subcellular systems (i.e., metabolic, calcium regulating, and contractile) to resemble those of heart muscle. The latissimus dorsi muscle (LDM) of mongrel dogs weighing 22-27 kg was stimulated via the thoracodorsal nerve at 2 Hz for 6-8 wk. This stimulation protocol reduced the phosphorylase (glycogenolytic) and phosphofructokinase (glycolytic) activities by 70%. The aerobic (citrate synthase activity) and fatty acid oxidative (3-hydroxyacyl-CoA dehydrogenase activity) capacities were not significantly increased by chronic stimulation and remained at about one-fourth those in the canine heart. The calcium-dependent sarcoplasmic reticulum adenosinetriphosphatase (ATPase) activity in the microsomal fraction, which was sixfold greater in the nonstimulated LDM than in the heart, was reduced by electrical stimulation to a level similar to that of the dog heart. The contractile capacity was evaluated by determining the percentage of types I and II fibers, the myofibrillar ATPase activity, and the proportion of myosin isoforms. The transformed muscle was comprised of 93 +/- 2% type I fibers, a myofibrillar ATPase activity similar to that in heart with primarily a slow-twitch muscle myosin isoform. In conclusion, electrical stimulation of canine LDM at 2 Hz for 6-8 wk resulted in two of the three biochemical systems, which confer physiological expression and fatigue resistance to muscle being transformed to resemble those of the myocardium.     

The concentrations of glucose 1,6-bisphosphate and other regulatory metabolites, and the activities of enzymes of the glycogen metabolism in the perfused rabbit psoas muscle

The following parameters were determined in the rabbit psoas muscle after perfusion in the presence of either insulin, propranolol, or isoproterenol: Concentrations of cyclic AMP, glucose 1,6-bisphosphate, fructose 2,6-bisphosphate, glucose-1-phosphate, glucose 6-phosphate, and fructose-1,6-bisphosphate. Maximum and "regulatory" activities of the enzymes glycogen phosphorylase, glycogen synthase, phosphofructokinase, and histone-phosphorylating protein kinase.     

Nonuniform effects of endurance exercise training on vasodilation in rat skeletal muscle.

Endurance exercise training (Ex) has been shown to increase maximal skeletal muscle blood flow. The purpose of this study was to test the hypothesis that increased endothelium-dependent vasodilation is associated with the Ex-induced increase in muscle blood flow. Furthermore, we hypothesized that enhanced endothelium-dependent dilation is confined to vessels in high-oxidative muscles that are recruited during Ex. To test these hypotheses, sedentary (Sed) and rats that underwent Ex (30 m/min x 10% grade, 60 min/day, 5 days/wk, 8-12 wk) were studied using three experimental approaches. Training effectiveness was evidenced by increased citrate synthase activity in soleus and vastus lateralis (red section) muscles (P < 0.05). Vasodilatory responses to the endothelium-dependent agent acetylcholine (ACh) in situ tended to be augmented by training in the red section of gastrocnemius muscle (RG; Sed: control, 0.69 +/- 0.12; ACh, 1.25 +/- 0.15; Ex: control, 0.86 +/- 0.17; ACh, 1.76 +/- 0.27 ml x min(-1) x 100 g(-1) x mmHg(-1); 0.05 < P < 0.10 for Ex vs. Sed during ACh). Responses to ACh in situ did not differ between Sed and Ex for either the soleus muscle or white section of gastrocnemius muscle (WG). Dilatory responses of second-order arterioles from the RG in vitro to flow (4-8 microl/min) and sodium nitroprusside (SNP; 10(-7) through 10(-4) M), but not ACh, were augmented in Ex (vs. Sed; P < 0.05). Dilatory responses to ACh, flow, and SNP of arterioles from soleus and WG muscles did not differ between Sed and Ex. Content of the endothelial isoform of nitric oxide synthase (eNOS) was increased in second-order, fourth-order, and fifth-order arterioles from the RG of Ex; eNOS content was similar between Sed and Ex in vessels from the soleus and WG muscles. These findings indicate that Ex induces endothelial adaptations in fast-twitch, oxidative, glycolytic skeletal muscle. These adaptations may contribute to enhanced skeletal muscle blood flow in endurance-trained individuals.