Training-induced skeletal muscle adaptations are independent of systemic adaptations

To isolate the peripheral adaptations to training, five normal subjects exercised the nondominant (ND) wrist flexors for 41 +/- 11 days, maintaining an exercise intensity below the threshold required for cardiovascular adaptations. Before and after training, intracellular pH and the ratio of inorganic phosphate to phosphocreatine (Pi/PCr) were measured by 31P magnetic resonance spectroscopy. Also maximal O2 consumption (VO2 max), muscle mass, and forearm blood flow were determined by graded systemic exercise, magnetic resonance imaging, and venous occlusion plethysmography, respectively. Blood flow, Pi/PCr, and pH were measured in both forearms at rest and during submaximal wrist flexion at 5, 23, and 46 J/min. Training did not affect VO2 max, exercise blood flow, or muscle mass. Resting pH, Pi/PCr, and blood flow were also unchanged. After training, the ND forearm demonstrated significantly lower Pi/PCr at 23 and 46 J/min. Endurance, measured as the number of contractions to exhaustion, also was increased significantly (63%) after training in the ND forearm. We conclude that 1) forearm training results in a lower Pi/PCr at identical submaximal work loads; 2) this improvement is independent of changes in VO2 max, muscle mass, or limb blood flow; and 3) these differences are associated with improved endurance and may reflect improved oxidative capacity of skeletal muscle.     

Creatine reduces human muscle PCr and pH decrements and P(i) accumulation during low-intensity exercise.

The purpose of this study was to examine with (31)P-magnetic resonance spectroscopy energy metabolism during repeated plantar flexion isometric exercise (Ex-1-Ex-4) at 32 +/- 1 and 79 +/- 4% of maximal voluntary contraction (MVC) before and during a creatine (Cr) feeding period of 5 g/day for 11 days. Eight trained male subjects participated in the study. ATP was unchanged with Cr supplementation at rest and during exercise at both intensities. Resting muscle phosphocreatine (PCr) increased (P < 0.05) from 18.3 +/- 0.9 (before) to 19.6 +/- 1.0 mmol/kg wet wt after 9 days. At 79% MVC, PCr used, P(i) accumulated, and pH at the end of Ex-1-Ex-4 were similar after 4 and 11 days of Cr supplementation. In contrast, PCr utilization and P(i) accumulation were lower and pH was higher for exercise at 32% MVC with Cr supplementation, suggesting aerobic resynthesis of PCr was more rapid during exercise. These results suggest that elevating muscle Cr enhances oxidative phosphorylation during mild isometric exercise, where it is expected that oxygen delivery matches demands and predominantly slow-twitch motor units are recruited.     

Influence of endurance training on muscle [PCr] kinetics during high-intensity exercise

We hypothesized that a period of endurance training would result in a speeding of muscle phosphocreatine concentration ([PCr]) kinetics over the fundamental phase of the response and a reduction in the amplitude of the [PCr] slow component during high-intensity exercise. Six male subjects (age 26 +/- 5 yr) completed 5 wk of single-legged knee-extension exercise training with the alternate leg serving as a control. Before and after the intervention period, the subjects completed incremental and high-intensity step exercise tests of 6-min duration with both legs separately inside the bore of a whole-body magnetic resonance spectrometer. The time-to-exhaustion during incremental exercise was not changed in the control leg [preintervention group (PRE): 19.4 +/- 2.3 min vs. postintervention group (POST): 19.4 +/- 1.9 min] but was significantly increased in the trained leg (PRE: 19.6 +/- 1.6 min vs. POST: 22.0 +/- 2.2 min; P < 0.05). During step exercise, there were no significant changes in the control leg, but end-exercise pH and [PCr] were higher after vs. before training. The time constant for the [PCr] kinetics over the fundamental exponential region of the response was not significantly altered in either the control leg (PRE: 40 +/- 13 s vs. POST: 43 +/- 10 s) or the trained leg (PRE: 38 +/- 8 s vs. POST: 40 +/- 12 s). However, the amplitude of the [PCr] slow component was significantly reduced in the trained leg (PRE: 15 +/- 7 vs. POST: 7 +/- 7% change in [PCr]; P < 0.05) with there being no change in the control leg (PRE: 13 +/- 8 vs. POST: 12 +/- 10% change in [PCr]). The attenuation of the [PCr] slow component might be mechanistically linked with enhanced exercise tolerance following endurance training.     

Effects of creatine supplementation on the energy cost of muscle contraction: a 31P-MRS study.

Five women and 3 men (29.8 +/- 1.4 yr) performed dynamic knee-extension exercise inside a magnetic resonance system (means +/- SE). Two trials were performed 7-14 days apart, consisting of a 4- to 5-min exhaustive exercise bout. To determine quadriceps cost of contraction, brief static and dynamic contractions were performed pre- and postexercise. (31)P spectra were used to determine pH and relative concentrations of P(i), phosphocreatine (PCr), and betaATP. Subjects consumed 0.3 g. kg(-1). day(-1) of a placebo (trial 1) or creatine (trial 2) for 5 days before each trial. After creatine supplementation, resting DeltaPCr increased from 40.7 +/- 1.8 to 46. 6 +/- 1.1 mmol/kg (P = 0.04) and PCr during exercise declined from -29.6 +/- 2.4 to -34.1 +/- 2.8 mmol/kg (P = 0.02). Muscle static (DeltaATP/N) and dynamic (DeltaATP/J) costs of contraction were unaffected by creatine supplementation as well as were ATP, P(i), pH, PCr resynthesis rate, and muscle strength and endurance. DeltaATP/J and DeltaATP/N were greatest at the onset of the exercise protocol (P < 0.01). In summary, creatine supplementation increased muscle PCr concentration, which did not affect muscle ATP cost of contraction.     

31P nuclear magnetic resonance study on changes in phosphocreatine and the intracellular pH in rat skeletal muscle during exercise at various inspired oxygen contents

We measured ATP, phosphocreatine (PCr), inorganic phosphate (P_i), and the intracellular pH in rat hindlimb muscles during submaximal isometric exercise with various O₂ deliveries using³¹P nuclear magnetic resonance spectroscopy (³¹P NMR) to evaluate changes in energy metabolism in relation to O₂ availability. Delivery of O₂ to muscles was altered by controlling the fractional concentration of inspired oxygen (F _IO₂) at 0.50, 0.28, 0.21, 0.11 and 0.08 with monitoring partial pressure of oxygen and carbon dioxide, and bicarbonate at the femoral artery. The steady-state ratio of PCr : (PCr + P_i) during exercise decreased as a function ofF _IO₂ even at 0.21. Significant acidification of the intracellular pH during exercise occurred at 0.08F _IO₂. Change in the PCr : (PCr + P_i) ratio demonstrated that the oxidative capacity, i.e. the maximal rate of the oxidative phosphorylation reaction, in muscle was not limited by O₂ delivery at 0.50F _IO₂, but was significantly limited at 0.21F _IO₂ or below. Change in the intracellular pH at 0.08F _IO₂ could be interpreted as an increase in lactate, suggesting activation of glycolysis. Correlation between the PCr : (PCr + P_i) ratio and the intracellular pH revealed the existence of a critical PCr : (PCr + P_i) ratio and pH for glycolysis activation at around 0.4 and 6.7, respectively.     

Muscle fatigue in McArdle's disease studied by 31P-NMR: effect of glucose infusion

In muscle phosphorylase deficiency (McArdle's disease) there is an abnormally rapid fatigue during strenuous exercise. Increasing substrate availability to working muscle can improve exercise tolerance but the effect on muscle energy metabolism has not been studied. Using phosphorus-31 nuclear magnetic resonance (31P-NMR) we examined forearm muscle ATP, phosphocreatine (PCr), inorganic phosphate (Pi) and pH in a McArdle patient (MP) and two healthy subjects (HS) at rest and during intermittent maximal effort handgrip contractions under control conditions (CC) and during intravenous glucose infusion (GI). Under CC, MP gripped to impending forearm muscle contracture in 130 s with a marked decline in muscle PCr and a dramatic elevation in Pi. During GI, MP exercised easily for greater than 420 s at higher tensions and with attenuated PCr depletion and Pi accumulation. In HS, muscle PCr and Pi changed more modestly and were not affected by GI. In MP and HS, ATP changed little or not at all with exercise. The results suggest that alterations in the levels of muscle PCr and Pi but not ATP are involved in the muscle fatigue in McArdle's disease and the improved exercise performance during glucose infusion.     

Forearm metabolic asymmetry detected by 31P-NMR during submaximal exercise

This study evaluated the relationship of skeletal muscle energy metabolism to forearm blood flow and muscle mass in the dominant (D) and nondominant (ND) forearms of normal subjects. 31P-Magnetic resonance spectroscopy was used to determine intracellular pH and the ratio of inorganic phosphate to phosphocreatine (Pi/PCr), an index of energy metabolism. Forearm blood flow and muscle mass were measured by venous occlusion plethysmography and magnetic resonance imaging, respectively. Metabolic measurements and flow were determined at rest and during submaximal exercise in both forearms. After a warm-up period, six normal right-handed male subjects performed 7.5 min of wrist flexion exercise in the magnet (1 contraction every 5 s), first with the ND forearm and then with the D forearm, at 23, 46, and 69 J/min. At rest, there were no differences between forearms in Pi/PCr or pH. However, at each work load the D forearm demonstrated significantly lower Pi/PCr and higher pH than the ND forearm. Blood flow was not significantly different between the forearms at rest or during exercise. Because these subjects were not engaged in unilateral arm training, we conclude that 1) Pi/PCr is lower and pH is higher in the D compared with the ND forearm in normal subjects during submaximal exercise, 2) these differences are independent of muscle mass and blood flow, and 3) the cumulative effect of long-term, low-level daily activity provides an adequate training stimulus for muscular metabolic adaptations.     

Metabolic effects of induced alkalosis during progressive forearm exercise to fatigue.

Metabolic alkalosis induced by sodium bicarbonate (NaHCO(3)) ingestion has been shown to enhance performance during brief high-intensity exercise. The mechanisms associated with this increase in performance may include increased muscle phosphocreatine (PCr) breakdown, muscle glycogen utilization, and plasma lactate (Lac(-)(pl)) accumulation. Together, these changes would imply a shift toward a greater contribution of anaerobic energy production, but this statement has been subject to debate. In the present study, subjects (n = 6) performed a progressive wrist flexion exercise to volitional fatigue (0.5 Hz, 14-21 min) in a control condition (Con) and after an oral dose of NaHCO(3) (Alk: 0.3 g/kg; 1.5 h before testing) to evaluate muscle metabolism over a complete range of exercise intensities. Phosphorus-31 magnetic resonance spectroscopy was used to continuously monitor intracellular pH, [PCr], [P(i)], and [ATP] (brackets denote concentration). Blood samples drawn from a deep arm vein were analyzed with a blood gas-electrolyte analyzer to measure plasma pH, Pco(2), and [Lac(-)](pl), and plasma [HCO(3)(-)] was calculated from pH and Pco(2). NaHCO(3) ingestion resulted in an increased (P < 0.05) plasma pH and [HCO(3)(-)] throughout rest and exercise. Time to fatigue and peak power output were increased (P < 0.05) by approximately 12% in Alk. During exercise, a delayed (P < 0.05) onset of intracellular acidosis (1.17 +/- 0.26 vs. 1.28 +/- 0.22 W, Con vs. Alk) and a delayed (P < 0.05) onset of rapid increases in the [P(i)]-to-[PCr] ratio (1.21 +/- 0.30 vs. 1.30 +/- 0.30 W) were observed in Alk. No differences in total [H(+)], [P(i)], or [Lac(-)](pl) accumulation were detected. In conclusion, NaHCO(3) ingestion was shown to increase plasma pH at rest, which resulted in a delayed onset of intracellular acidification during incremental exercise. Conversely, NaHCO(3) was not associated with increased [Lac(-)](pl) accumulation or PCr breakdown.     

Effects of high-intensity training on muscle lactate transporters and postexercise recovery of muscle lactate and hydrogen ions in women

The purpose of this study was to investigate the effects of high-intensity interval training (3 days/wk for 5 wk), provoking large changes in muscle lactate and pH, on changes in intracellular buffer capacity (betam(in vitro)), monocarboxylate transporters (MCTs), and the decrease in muscle lactate and hydrogen ions (H+) after exercise in women. Before and after training, biopsies of the vastus lateralis were obtained at rest and immediately after and 60 s after 45 s of exercise at 190% of maximal O2 uptake. Muscle samples were analyzed for ATP, phosphocreatine (PCr), lactate, and H+; MCT1 and MCT4 relative abundance and betam(in vitro) were also determined in resting muscle only. Training provoked a large decrease in postexercise muscle pH (pH 6.81). After training, there was a significant decrease in betam(in vitro) (-11%) and no significant change in relative abundance of MCT1 (96 +/- 12%) or MCT4 (120 +/- 21%). During the 60-s recovery after exercise, training was associated with no change in the decrease in muscle lactate, a significantly smaller decrease in muscle H+, and increased PCr resynthesis. These results suggest that increases in betam(in vitro) and MCT relative abundance are not linked to the degree of muscle lactate and H+ accumulation during training. Furthermore, training that is very intense may actually lead to decreases in betam(in vitro). The smaller postexercise decrease in muscle H+ after training is a further novel finding and suggests that training that results in a decrease in H+ accumulation and an increase in PCr resynthesis can actually reduce the decrease in muscle H+ during the recovery from supramaximal exercise.     

Lipid peroxidation and scavenger enzymes during exercise: adaptive response to training

This study was designed to determine whether endurance training would influence the production of lipid peroxidation (LI-POX) by-products as indicated by malondialdehyde (MDA) at rest and after an acute exercise run. Additionally, the scavenger enzymes catalase (CAT) and superoxide dismutase (SOD) were examined to determine whether changes in LIPOX are associated with alterations in enzyme activity both at rest and after exercise. Male Sprague-Dawley rats (n = 32) were randomly assigned to either trained or sedentary groups and were killed either at rest or after 20 min of treadmill running. The training program increased oxidative capacity 64% in leg muscle. After exercise, the sedentary group demonstrated increased LIPOX levels in liver and white skeletal muscle, whereas the endurance-trained group did not show increases in LIPOX after exercise. CAT activity was higher in both red and white muscle after exercise in the trained animals. Total SOD activity was unaffected by either acute or chronic exercise. These data suggest that endurance training can result in a reduction in LIPOX levels as indicated by MDA during moderate-intensity exercise. It is possible that activation of the enzyme catalase and the increase in respiratory capacity were contributory factors responsible for regulating LIPOX after training during exercise.     

Alteration of regulatory enzyme activities in fast-twitch and slow-twitch muscles and muscle fibres in low-intensity endurance-trained rats

The effect of progressive, low-intensity endurance training on regulatory enzyme activities in slow-twitch (ST) and fast-twitch (FT) muscle fibres was studied in 32 rats. Of those rats 16 were trained on a treadmill at a running speed of 10m · min^–1 5 days a week over an 8-week period. Running time was progressively increased from 15 min to 2 h · day^–1. Of the rats 4 trained and 4 sedentary rats were also subjected to acute exhausting exercise. Enzyme activities of phosphofructokinase 1 (PFKI) from glycolysis, -ketoglutarate dehydrogenase (-KGDH) from the Krebs cycle and carnitine palmitoyltransferase (CPT I and II) from fatty acid metabolism in soleus, tibialis anterior and gastrocnemius muscles were measured in trained and sedentary rats. Enzyme activities of individual ST and FT fibres were measured from the freeze-dried gastrocnemius muscle of 8 trained and 8 sedentary rats. In the sedentary rats the activity of PFK1 in tibialis anterior and soleus muscles was 141% and 41% of the activity in gastrocnemius muscle, respectively. The activity of -KGDH in tibialis anterior and soleus muscles was 164% and 278% of the activity in gastrocnemius muscle, respectively. The activity of CPT I in tibialis anterior and gastrocnemius muscles were at the same level, but in soleus muscle the activity was 127% of that in mixed muscle. Endurance training increased enzyme activities of -KGDH and CPT I significantly (P < 0.05) in gastrocnemius muscle but not in soleus or tibialis anterior muscle. After training both -KGDH and CPT II activities were elevated significantly (P < 0.05) in the ST fibres of gastrocnemius muscle, whereas in FT fibres only -KGDH was increased. For PFK1 activity no significant change was observed in ST or FT fibres. After acute exercise, activities of mitochondrial enzymes -KGDH and CPT I tended to be elevated in all muscles. Thus, low-intensity endurance training induced significant peripheral changes in regulatory enzyme activities in oxidative and fatty acid metabolism in individual ST or FT muscle fibres.     

Effects of endurance exercise training on muscle glycogen accumulation in humans.

The purpose of this investigation was to determine whether endurance exercise training increases the ability of human skeletal muscle to accumulate glycogen after exercise. Subjects (4 women and 2 men, 31 +/- 8 yr old) performed high-intensity stationary cycling 3 days/wk and continuous running 3 days/wk for 10 wk. Muscle glycogen concentration was measured after a glycogen-depleting exercise bout before and after endurance training. Muscle glycogen accumulation rate from 15 min to 6 h after exercise was twofold higher (P < 0.05) in the trained than in the untrained state: 10.5 +/- 0.2 and 4.5 +/- 1.3 mmol. kg wet wt(-1). h(-1), respectively. Muscle glycogen concentration was higher (P < 0.05) in the trained than in the untrained state at 15 min, 6 h, and 48 h after exercise. Muscle GLUT-4 content after exercise was twofold higher (P < 0.05) in the trained than in the untrained state (10.7 +/- 1.2 and 4.7 +/- 0.7 optical density units, respectively) and was correlated with muscle glycogen concentration 6 h after exercise (r = 0.64, P < 0.05). Total glycogen synthase activity and the percentage of glycogen synthase I were not significantly different before and after training at 15 min, 6 h, and 48 h after exercise. We conclude that endurance exercise training enhances the capacity of human skeletal muscle to accumulate glycogen after glycogen-depleting exercise.     

Skeletal muscle metabolism during short-term, high-intensity exercise in prepubertal and pubertal girls.

To test the hypothesis that glycolytic metabolism in muscle is attenuated in prepubertal children, (31)P-magnetic resonance spectroscopy was used to determine calf muscle intracellular pH (pH(i)) in nine prepubertal (Pre) and nine pubertal female swimmers (Pub). Maximal plantar flexion work capacity (100% MWC) was established by using a graded exercise test. Between 5 and 10 days later, calf muscle images (magnetic resonance imaging) and phosphorus spectra were acquired at rest, during 2 min of light exercise (40% MWC), and during 2 min of supramaximal exercise (140% MWC) in a 3.0-T NMR system. End-exercise pH(i) was 6.66 +/- 0.11 and 6.76 +/- 0.17 for Pub and Pre, respectively. No significant differences in the mean values for pH(i) or the P(i)-to-phosphocreatine ratio were observed between groups during the protocol; however, an interaction effect was found for the P(i)-to-phosphocreatine ratio during the supramaximal exercise challenge. Cross-sectional area of gastrocnemius was 15.12 +/- 0.46 and 9.37 +/- 0.37 cm(2) for Pub and Pre, respectively (P < 0.05). Differences in muscle size must be considered when interpreting the unlocalized magnetic resonance spectroscopy data. These results suggest that glycolytic metabolism in physically active children is not maturity dependent.     

Training decreases muscle glycogen turnover during exercise

The present study was undertaken to determine the effects of endurance training on glycogen kinetics during exercise. A new model describing glycogen kinetics was applied to quantitate the rates of synthesis and degradation of glycogen. Trained and untrained rats were infused with a 25% glucose solution with 6-³H-glucose and U-¹⁴C-lactate at 1.5 and 0.5 μCi · min⁻¹ (where 1 Ci = 3.7 × 10¹⁰ Bq), respectively, during rest (30 min) and exercise (60 min). Blood samples were taken at 10-min intervals starting just prior to isotopic infusion, until the cessation of exercise. Tissues harvested after the cessation of exercise were muscle (soleus, deep, and superficial vastus lateralis, gastrocnemius), liver, and heart. Tissue glycogen was quantitated and analyzed for incorporation of ³H and ¹⁴C via liquid scintillation counting. There were no net decreases in muscle glycogen concentration from trained rats, whereas muscle glycogen concentration decreased to as much as 64% (P < 0.05) in soleus in muscles from untrained rats after exercise. Liver glycogen decreased in both trained (30%) and untrained (40%) rats. Glycogen specific activity increased in all tissues after exercise indicating isotope incorporation and, thus, glycogen synthesis during exercise. There were no differences in muscle glycogen synthesis rates between trained and untrained rats after exercise. However, training decreased muscle glycogen degradation rates in total muscle (i.e., the sum of the degradation rates of all of the muscles sampled) tenfold (P < 0.05). We have applied a model to describe glycogen kinetics in relation to glucose and lactate metabolism during exercise in trained and untrained rats. Training significantly decreases muscle glycogen degradation rates during exercise. Accepted: 22 May 1998     

Disruption of muscle energy metabolism due to intense ischaemic exercise: a 31P NMR study in rats

This study uses 31P NMR as a tool for the study of the capacity of recovery of the rat skeletal muscle after an exercise performed during an acute state of ischaemia. The leg muscle of a rat submitted to a 20 minute exercise period one hour after irreversible femoral artery ligation, manifested a dramatic (75%) decrease in phosphocreatine (PC) content, a less pronounced (30%) decrease in ATP, an accumulation of inorganic phosphate (Pi) and an increase in the phosphomonoester (PME) resonances, in addition to acidosis to pH 6.4. An investigation over a 40 minute post-exercise period using 31P NMR and biochemical analysis led to the following observations: 1. The PC and Pi contents of the muscle experienced no further significant changes, remaining at the level reached by the end of the exercise. 2. The ATP content similarly remained at the level reached at the end of this period, the adenylate charge being 0.91 (controls 0.93). 3. The IMP accumulated during ischaemic exercise remained at its high level. It seems likely that this compound contributes in a large part to the resonances in the PME region of the spectra. 4. Intracellular acidosis persisted despite a decrease in lactate content. The most important finding from this study is that the situation created by ischaemic exercise--as revealed by the NMR spectra--is characterized by a blocking of the main biochemical processes (phosphorylations, purine nucleotide cycle, pH regulation). Such a condition, which does not seem to entail lethal cell injury, could thus be used as a basis for the study via 31P NMR of the therapeutic effect of various treatments.     

Four weeks of speed endurance training reduces energy expenditure during exercise and maintains muscle oxidative capacity despite a reduction in training volume

We studied the effect of an alteration from regular endurance to speed endurance training on muscle oxidative capacity, capillarization, as well as energy expenditure during submaximal exercise and its relationship to mitochondrial uncoupling protein 3 (UCP3) in humans. Seventeen endurance-trained runners were assigned to either a speed endurance training (SET; n = 9) or a control (Con; n = 8) group. For a 4-wk intervention (IT) period, SET replaced the ordinary training ( approximately 45 km/wk) with frequent high-intensity sessions each consisting of 8-12 30-s sprint runs separated by 3 min of rest (5.7 +/- 0.1 km/wk) with additional 9.9 +/- 0.3 km/wk at low running speed, whereas Con continued the endurance training. After the IT period, oxygen uptake was 6.6, 7.6, 5.7, and 6.4% lower (P < 0.05) at running speeds of 11, 13, 14.5, and 16 km/h, respectively, in SET, whereas remained the same in Con. No changes in blood lactate during submaximal running were observed. After the IT period, the protein expression of skeletal muscle UCP3 tended to be higher in SET (34 +/- 6 vs. 47 +/- 7 arbitrary units; P = 0.06). Activity of muscle citrate synthase and 3-hydroxyacyl-CoA dehydrogenase, as well as maximal oxygen uptake and 10-km performance time, remained unaltered in both groups. In SET, the capillary-to-fiber ratio was the same before and after the IT period. The present study showed that speed endurance training reduces energy expenditure during submaximal exercise, which is not mediated by lowered mitochondrial UCP3 expression. Furthermore, speed endurance training can maintain muscle oxidative capacity, capillarization, and endurance performance in already trained individuals despite significant reduction in the amount of training.     

Evaluation of exercise muscle energetics by NMR

Magnetic resonance imaging (MRI) is superior to ultrasonography and X-CT especially in density resolution in soft tissue. 31P NMR provides information on metabolism, which has not been obtained in vivo by conventional methods, such as phosphocreatine (PCr), inorganic phosphate (Pi), ATP, and intracellular pH. We used MRI and 31P NMR spectroscopy to study skeletal muscle metabolism of human and rat. These NMR results suggested that 1) estimation of muscle fiber composition, 2) evaluation of muscle ATP turnover and 3) imaging of local muscle fatigue are possible.     

Effect of training on fibre composition and phosphate metabolites in rest measured in vitro in muscles of young pigs.

1. Fibre type composition and phosphate metabolites were studied in m. longissimus thoracis (MLT), m. rectus femoris (MRF) and m. triceps brachii (MTB) in trained (N = 6) and sedentary (N = 6) pigs. 2. Samples were analyzed histochemically and by means of in vitro 31P NMR spectroscopy. 3. Training (duration 11 weeks) consisted of treadmill running at a speed of 1.1 m/sec. The daily exercise time of trained animals gradually increased from 10 min during the very first days to 60 min at the end of the 4th week. 4. During the final 7 weeks exercise time remained unchanged. Sedentary animals were not subjected to training. 5. A higher proportion of type beta R fibres in MRF, MTB and MLT and a lower proportion of type alpha W fibres were found in the trained group of animals compared to the control group. 6. In MLT no significant differences in the proportion of type alpha W were observed between both groups. 7. No significant differences in average fibre diameter of muscle fibres were found between groups. 8. No differences in concentration of phosphate compounds were observed between trained and sedentary groups. 9. Muscles with a higher proportion of type IIb fibres in both groups of pigs contained higher amounts of phosphocreatine (PCr) and were also characterized by a higher ratio of PCr to inorganic phosphate (Pi).     

Relating intramuscular fuel use to endurance in juvenile rainbow trout

This study examined fuel depletion in white muscle of juvenile rainbow trout sprinted to fatigue to determine whether the onset of fatigue is associated with a measurable metabolic change within the muscle and whether muscle glycogen levels influence endurance. In this study, "fuels" refer to any energy-supplying compounds and include glycogen, phosphocreatine (PCr), and ATP. Fuel depletion in white muscle was estimated by the calculation of the anaerobic energy expenditure (AEE; in micromol ATP equivalents g(-1)) from the reduction of PCr and ATP and the accumulation of lactate. Progression of fuel use during sprinting was examined by sampling fish before they showed signs of fatigue and following fatigue. Most of the AEE before fatigue was due to PCr depletion. However, at the first signs of fatigue, there was a 32% drop in ATP. Similarly, when fish were slowly accelerated to a fatiguing velocity, the only significant change at fatigue was a 30% drop in ATP levels. Muscle glycogen levels were manipulated by altering ration (1% vs. 4% body weight ration per day) combined with either daily or no exercise. Higher ration alone led to significantly greater muscle glycogen but had no effect on sprint performance, whereas sprint training led to higher glycogen and an average threefold improvement in sprint performance. In contrast, periodic chasing produced a similar increase in glycogen but had no effect on sprint performance. Taken together, these observations suggest that (i) a reduction in ATP in white muscle could act as a proximate signal for fatigue during prolonged exercise in fish and (ii) availability of muscle glycogen does not limit endurance.     

Increased intramyocellular lipids but unaltered in vivo mitochondrial oxidative phosphorylation in skeletal muscle of adipose triglyceride lipase-deficient mice

Adipose triglyceride lipase (ATGL) is a lipolytic enzyme that is highly specific for triglyceride hydrolysis. The ATGL-knockout mouse (ATGL(-/-)) accumulates lipid droplets in various tissues, including skeletal muscle, and has poor maximal running velocity and endurance capacity. In this study, we tested whether abnormal lipid accumulation in skeletal muscle impairs mitochondrial oxidative phosphorylation, and hence, explains the poor muscle performance of ATGL(-/-) mice. In vivo 1H magnetic resonance spectroscopy of the tibialis anterior of ATGL(-/-) mice revealed that its intramyocellular lipid pool is approximately sixfold higher than in WT controls (P = 0.0007). In skeletal muscle of ATGL(-/-) mice, glycogen content was decreased by 30% (P < 0.05). In vivo 31P magnetic resonance spectra of resting muscles showed that WT and ATGL(-/-) mice have a similar energy status: [PCr], [P(i)], PCr/ATP ratio, PCr/P(i) ratio, and intracellular pH. Electrostimulated muscles from WT and ATGL(-/-) mice showed the same PCr depletion and pH reduction. Moreover, the monoexponential fitting of the PCr recovery curve yielded similar PCr recovery times (τPCr; 54.1 ± 6.1 s for the ATGL(-/-) and 58.1 ± 5.8 s for the WT), which means that overall muscular mitochondrial oxidative capacity was comparable between the genotypes. Despite similar in vivo mitochondrial oxidative capacities, the electrostimulated muscles from ATGL(-/-) mice displayed significantly lower force production and increased muscle relaxation time than the WT. These findings suggest that mechanisms other than mitochondrial dysfunction cause the impaired muscle performance of ATGL(-/-) mice.