Dysregulation of muscle lipid metabolism in rats selectively bred for low aerobic running capacity

As substrate for evaluation of metabolic diseases, we developed novel rat models that contrast for endurance exercise capacity. Through two-way artificial selection, we created rodent phenotypes of intrinsically low-capacity runners (LCR) and high-capacity runners (HCR) that also differed markedly for cardiovascular and metabolic disease risk factors. Here, we determined skeletal muscle proteins with putative roles in lipid and carbohydrate metabolism to better understand the mechanisms underlying differences in whole body substrate handling between phenotypes. Animals (generation 16) differed for endurance running capacity by 295%. LCR animals had higher resting plasma glucose (6.58 +/- 0.45 vs. 6.09 +/- 0.45 mmol/l), insulin (0.48 +/- 0.03 vs. 0.32 +/- 0.02 ng/ml), nonesterified fatty acid (0.57 +/- 0.14 v 0.35 +/- 0.05 mM), and triglyceride (TG; 0.47 +/- 0.11 vs. 0.25 +/- 0.08 mmol/l) concentrations (all P < 0.05). Muscle TG (72.3 +/- 14.7 vs. 38.9 +/- 6.2 mmol/kg dry muscle wt; P < 0.05) and diacylglycerol (96 +/- 28 vs. 42 +/- 8 pmol/mg dry muscle wt; P < 0.05) contents were elevated in LCR vs. HCR rats. Accompanying the greater lipid accretion in LCR was increased fatty acid translocase/CD36 content (1,014 +/- 80 vs. 781 +/- 70 arbitrary units; P < 0.05) and reduced TG lipase activity (0.158 +/- 0.0125 vs. 0.274 +/- 0.018 mmol.min(-1).kg dry muscle wt(-1); P < 0.05). Muscle glycogen, GLUT4 protein, and basal phosphorylation states of AMP-activated protein kinase-alpha1, AMP-activated protein kinase-alpha2, and acetyl-CoA carboxylase were similar in LCR and HCR. In conclusion, rats with low intrinsic aerobic capacity demonstrate abnormalities in lipid-handling capacity. These disruptions may, in part, be responsible for the increased risk of metabolic disorders observed in this phenotype.     

Determinants of maximal O(2) uptake in rats selectively bred for endurance running capacity.

O(2) transport during maximal exercise was studied in rats bred for extremes of exercise endurance, to determine whether maximal O(2) uptake (VO(2 max)) was different in high- (HCR) and low-capacity runners (LCR) and, if so, which were the phenotypes responsible for the difference. VO(2 max) was determined in five HCR and six LCR female rats by use of a progressive treadmill exercise protocol at inspired PO(2) of approximately 145 (normoxia) and approximately 70 Torr (hypoxia). Normoxic VO(2 max) (in ml. min(-1). kg(-1)) was 64.4 +/- 0.4 and 57.6 +/- 1.5 (P < 0.05), whereas VO(2 max) in hypoxia was 42.7 +/- 0.8 and 35.3 +/- 1.5 (P < 0.05) in HCR and LCR, respectively. Lack of significant differences between HCR and LCR in alveolar ventilation, alveolar-to-arterial PO(2) difference, or lung O(2) diffusing capacity indicated that neither ventilation nor efficacy of gas exchange contributed to the difference in VO(2 max) between groups. Maximal rate of blood O(2) convection (cardiac output times arterial blood O(2) content) was also similar in both groups. The major difference observed was in capillary-to-tissue O(2) transfer: both the O(2) extraction ratio (0.81 +/- 0.002 in HCR, 0.74 +/- 0.009 in LCR, P < 0.001) and the tissue diffusion capacity (1.18 +/- 0.09 in HCR and 0.92 +/- 0.05 ml. min(-1). kg(-1). Torr(-1) in LCR, P < 0.01) were significantly higher in HCR. The data indicate that selective breeding for exercise endurance resulted in higher VO(2 max) mostly associated with a higher transfer of O(2) at the tissue level.     

Continued divergence in VO2max of rats artificially selected for running endurance is mediated by greater convective blood O2 delivery.

We previously showed that after seven generations of artificial selection of rats for running capacity, maximal O2 uptake (VO2max) was 12% greater in high-capacity (HCR) than in low-capacity runners (LCR). This difference was due exclusively to a greater O2 uptake and utilization by skeletal muscle of HCR, without differences between lines in convective O2 delivery to muscle by the cardiopulmonary system (QO2max). The present study in generation 15 (G15) female rats tested the hypothesis that continuing improvement in skeletal muscle O2 transfer must be accompanied by augmentation in QO2max to support VO2max of HCR. Systemic O2 transport was studied during maximal normoxic and hypoxic exercise (inspired PO2 approximately 70 Torr). VO2max divergence between lines increased because of both improvement in HCR and deterioration in LCR: normoxic VO2max was 50% higher in HCR than LCR. The greater VO2max in HCR was accompanied by a 41% increase in QO2max: 96.1 +/- 4.0 in HCR vs. 68.1 +/- 2.5 ml stpd O2 x min(-1) x kg(-1) in LCR (P < 0.01) during normoxia. The greater G15 QO2max of HCR was due to a 48% greater stroke volume than LCR. Although tissue O2 diffusive conductance continued to increase in HCR, tissue O2 extraction was not significantly different from LCR at G15, because of the offsetting effect of greater HCR blood flow on tissue O2 extraction. These results indicate that continuing divergence in VO2max between lines occurs largely as a consequence of changes in the capacity to deliver O2 to the exercising muscle.     

Enhanced mitochondrial sensitivity to creatine in rats bred for high aerobic capacity.

Qualitative and quantitative measures of mitochondrial function were performed in rats selectively bred 15 generations for intrinsic aerobic high running capacity (HCR; n = 8) or low running capacity (LCR; n=8). As estimated from a speed-ramped treadmill exercise test to exhaustion (15 degrees slope; initial velocity of 10 m/min, increased 1 m/min every 2 min), HCR rats ran 10 times further (2,375+/-80 m) compared with LCR rats (238+/-12 m). Fiber bundles were obtained from the soleus and chemically permeabilized. Respiration was measured 1) in the absence of ADP, 2) in the presence of a submaximally stimulating concentration of ADP (0.1 mM ADP, with and without 20 mM creatine), and 3) in the presence of a maximally stimulating concentration of ADP (2 mM). Although non-ADP-stimulated and maximally ADP-stimulated rates of respiration were 13% higher in HCR compared with LCR, the difference was not statistically significant (P>0.05). Despite a similar rate of respiration in the presence of 0.1 mM ADP, HCR rats demonstrated a higher rate of respiration in the presence of 0.1 mM ADP+20 mM creatine (HCR 33% higher vs. LCR, P<0.05). Thus mitochondria from HCR rats exhibit enhanced mitochondrial sensitivity to creatine (i.e., the ability of creatine to decrease the Km for ADP). We propose that increased respiratory sensitivity to ADP in the presence of creatine can effectively increase muscle sensitivity to ADP during exercise (when creatine is increased) and may be, in part, a contributing factor for the increased running capacity in HCR rats.     

Lower oxidative DNA damage despite greater ROS production in muscles from rats selectively bred for high running capacity

Artificial selection in rat has yielded high-capacity runners (HCR) and low-capacity runners (LCR) that differ in intrinsic (untrained) aerobic exercise ability and metabolic disease risk. To gain insight into how oxygen metabolism may have been affected by selection, we compared mitochondrial function, oxidative DNA damage (8-dihydroxy-guanosine; 8dOHG), and antioxidant enzyme activities in soleus muscle (Sol) and gastrocnemius muscle (Gas) of adult and aged LCR vs. HCR rats. In Sol of adult HCR rats, maximal ADP-stimulated respiration was 37% greater, whereas in Gas of adult HCR rats, there was a 23% greater complex IV-driven respiratory capacity and 54% greater leak as a fraction of electron transport capacity (suggesting looser mitochondrial coupling) vs. LCR rats. H(2)O(2) emission per gram of muscle was 24-26% greater for both muscles in adult HCR rats vs. LCR, although H(2)O(2) emission in Gas was 17% lower in HCR, after normalizing for citrate synthase activity (marker of mitochondrial content). Despite greater H(2)O(2) emission, 8dOHG levels were 62-78% lower in HCR rats due to 62-96% higher superoxide dismutase activity in both muscles and 47% higher catalase activity in Sol muscle in adult HCR rats, with no evidence for higher 8 oxoguanine glycosylase (OGG1; DNA repair enzyme) protein expression. We conclude that genetic segregation for high running capacity has generated a molecular network of cellular adaptations, facilitating a superior response to oxidative stress.     

Effect of fitness on arm vascular and metabolic responses to upper body exercise

We investigated arm perfusion and metabolism during upper body exercise. Eight average, fit subjects and seven rowers, mean +/- SE maximal oxygen uptake (VO2 max) 157 +/- 7 and 223 +/- 14 ml O2. kg(-0.73).min(-1), respectively, performed incremental arm cranking to exhaustion. Arm blood flow (ABF) was measured with thermodilution and arm muscle mass was estimated by dual-energy X-ray absorptiometry. During maximal arm cranking, pulmonary VO2 was approximately 45% higher in the rowers compared with the untrained subjects and peak ABF was 6.44 +/- 0.40 and 4.55 +/- 0.26 l/min, respectively (P < 0.05). The arm muscle mass for the rowers and the untrained subjects was 3.5 +/- 0.4 and 3.3 +/- 0.1 kg, i.e., arm perfusion was 1.9 +/- 0.2 and 1.4 +/- 0.1 l blood.kg(-1).min(-1), respectively (P < 0.05). The arteriovenous O2 difference was 156 +/- 7 and 120 +/- 8 ml/l, respectively, and arm VO2 was 0.98 +/- 0.08 and 0.60 +/- 0.04 l/min corresponding with 281 +/- 22 and 181 +/- 12 ml/kg, while arm O(2) diffusional conductance was 49.9 +/- 4.3 and 18.6 +/- 3.2 ml.min(-1).mmHg(-1), respectively (P < 0.05). Also, lactate release in the rowers was almost three times higher than in the untrained subjects (26.4 +/- 1.1 vs. 9.5 +/- 0.4 mmol/min, P < 0.05). The energy requirement of an approximately 50% larger arm work capacity after long-term arm endurance training is covered by an approximately 60% increase in aerobic metabolism and an almost tripling of the anaerobic capacity.     

Effects of hyperoxia on skeletal muscle carbohydrate metabolism during transient and steady-state exercise.

This study compared the effects of inspiring either a hyperoxic (60% O(2)) or normoxic gas (21% O(2)) while cycling at 70% peak O(2) uptake on 1) the ATP derived from substrate phosphorylation during the initial minute of exercise, as estimated from phosphocreatine degradation and lactate accumulation, and 2) the reliance on carbohydrate utilization and oxidation during steady-state cycling, as estimated from net muscle glycogen use and the activity of pyruvate dehydrogenase (PDH) in the active form (PDH(a)), respectively. We hypothesized that 60% O(2) would decrease substrate phosphorylation at the onset of exercise and that it would not affect steady-state exercise PDH activity, and therefore muscle carbohydrate oxidation would be unaltered. Ten active male subjects cycled for 15 min on two occasions while inspiring 21% or 60% O(2), balance N(2). Blood was obtained throughout and skeletal muscle biopsies were sampled at rest and 1 and 15 min of exercise in each trial. The ATP derived from substrate-level phosphorylation during the initial minute of exercise was unaffected by hyperoxia (21%: 52.2 +/- 11.1; 60%: 54.0 +/- 9.5 mmol ATP/kg dry wt). Net glycogen breakdown during 15 min of cycling was reduced during the 60% O(2) trial vs. 21% O(2) (192.7 +/- 25.3 vs. 138.6 +/- 16.8 mmol glycosyl units/kg dry wt). Hyperoxia had no effect on PDH(a), because it was similar to the 21% O(2) trial at rest and during exercise (21%: 2.20 +/- 0.26; 60%: 2.25 +/- 0.30 mmol.kg wet wt(-1).min(-1)). Blood lactate was lower (6.4 +/- 1.0 vs. 8.9 +/- 1.0 mM) at 15 min of exercise and net muscle lactate accumulation was reduced from 1 to 15 min of exercise in the 60% O(2) trial compared with 21% (8.6 +/- 5.1 vs. 27.3 +/- 5.8 mmol/kg dry wt). We concluded that O(2) availability did not limit oxidative phosphorylation in the initial minute of the normoxic trial, because substrate phosphorylation was unaffected by hyperoxia. Muscle glycogenolysis was reduced by hyperoxia during steady-state exercise, but carbohydrate oxidation (PDH(a)) was unaffected. This closer match between pyruvate production and oxidation during hyperoxia resulted in decreased muscle and blood lactate accumulation. The mechanism responsible for the decreased muscle glycogenolysis during hyperoxia in the present study is not clear.     

Acute exercise and GLUT4 expression in human skeletal muscle: influence of exercise intensity.

To examine the influence of exercise intensity on the increases in vastus lateralis GLUT4 mRNA and protein after exercise, six untrained men exercised for 60 min at 39 +/- 3% peak oxygen consumption (V(O2 peak)) (Lo) or 27 +/- 2 min at 83 +/- 2% V(O2 peak) (Hi) in counterbalanced order. Preexercise muscle glycogen levels were not different between trials (Lo: 408 +/- 35 mmol/kg dry mass; Hi: 420 +/- 43 mmol/kg dry mass); however, postexercise levels were lower (P < 0.05) in Hi (169 +/- 18 mmol/kg dry mass) compared with Lo (262 +/- 35 mmol/kg dry mass). Thus calculated muscle glycogen utilization was greater (P < 0.05) in Hi (251 +/- 24 mmol/kg) than in Lo (146 +/- 34). Exercise resulted in similar increases in GLUT4 gene expression in both trials. GLUT4 mRNA was increased immediately at the end of exercise (approximately 2-fold; P < 0.05) and remained elevated after 3 h of postexercise recovery. When measured 3 h after exercise, total crude membrane GLUT4 protein levels were 106% higher in Lo (3.3 +/- 0.7 vs. 1.6 +/- 0.3 arbitrary units) and 61% higher in Hi (2.9 +/- 0.5 vs. 1.8 +/- 0.5 arbitrary units) relative to preexercise levels. A main effect for exercise was observed, with no significant differences between trials. In conclusion, exercise at approximately 40 and approximately 80% V(O2 peak), with total work equal, increased GLUT4 mRNA and GLUT4 protein in human skeletal muscle to a similar extent, despite differences in exercise intensity and duration.     

Enhancement of arm exercise endurance capacity with dihydroxyacetone and pyruvate

The effects of dietary supplementation of dihydroxyacetone and pyruvate (DHAP) on endurance capacity and metabolic responses during arm exercise were determined in 10 untrained males (20-26 yr). Subjects performed arm ergometer exercise (60% peak O2 consumption) to exhaustion after consumption of standard diets (55% carbohydrate, 15% protein, 30% fat; 35 kcal/kg) containing either 100 g of Polycose (placebo, P) or DHAP (3:1, treatment) substituted for a portion of carbohydrate. The two diets were administered in a random order, and each was consumed for a 7-day period. Biopsy of the triceps muscle was obtained immediately before and after exercise. Blood samples were drawn through radial artery and axillary vein catheters at rest, after 60 min of exercise, and at exercise termination. Arm endurance was 133 +/- 20 min after P and 160 +/- 22 min after DHAP (P less than 0.01). Triceps glycogen at rest was 88 +/- 8 (P) and 130 +/- 19 mmol/kg (DHAP) (P less than 0.05). Whole arm arteriovenous glucose difference (mmol/l) was greater (P less than 0.05) for DHAP than P at rest (0.60 +/- 0.12 vs. 0.05 +/- 0.09) and after 60 min of exercise (1.00 +/- 0.12 vs. 0.36 +/- 0.11), but it did not differ at exhaustion. Neither respiratory exchange ratio nor respiratory quotient differed between trials at rest, after 60 min of exercise, or at exhaustion. Plasma free fatty acid, glycerol, beta-hydroxybutyrate, catecholamines, and insulin were similar during rest and exercise for both diets. Feeding DHAP for 7 days increased arm muscle glucose extraction before and during exercise, thereby enhancing submaximal arm endurance capacity.     

Artificial selection for high-capacity endurance running is protective against high-fat diet-induced insulin resistance

Elevated oxidative capacity, such as occurs via endurance exercise training, is believed to protect against the development of obesity and diabetes. Rats bred both for low (LCR)- and high (HCR)-capacity endurance running provide a genetic model with inherent differences in aerobic capacity that allows for the testing of this supposition without the confounding effects of a training stimulus. The purpose of this investigation was to determine the effects of a high-fat diet (HFD) on weight gain patterns, insulin sensitivity, and fatty acid oxidative capacity in LCR and HCR male rats in the untrained state. Results indicate chow-fed LCR rats were heavier, hypertriglyceridemic, less insulin sensitive, and had lower skeletal muscle oxidative capacity compared with HCR rats. Upon exposure to an HFD, LCR rats gained more weight and fat mass, and their insulin resistant condition was exacerbated, despite consuming similar amounts of metabolizable energy as chow-fed controls. These metabolic variables remained unaltered in HCR rats. The HFD increased skeletal muscle oxidative capacity similarly in both strains, whereas hepatic oxidative capacity was diminished only in LCR rats. These results suggest that LCR rats are predisposed to obesity and that expansion of skeletal muscle oxidative capacity does not prevent excess weight gain or the exacerbation of insulin resistance on an HFD. Elevated basal skeletal muscle oxidative capacity and the ability to preserve liver oxidative capacity may protect HCR rats from HFD-induced obesity and insulin resistance.     

Effect of short-term sprint interval training on human skeletal muscle carbohydrate metabolism during exercise and time-trial performance.

Our laboratory recently showed that six sessions of sprint interval training (SIT) over 2 wk increased muscle oxidative potential and cycle endurance capacity (Burgomaster KA, Hughes SC, Heigenhauser GJF, Bradwell SN, and Gibala MJ. J Appl Physiol 98: 1895-1900, 2005). The present study tested the hypothesis that short-term SIT would reduce skeletal muscle glycogenolysis and lactate accumulation during exercise and increase the capacity for pyruvate oxidation via pyruvate dehydrogenase (PDH). Eight men [peak oxygen uptake (VO2 peak)=3.8+/-0.2 l/min] performed six sessions of SIT (4-7x30-s "all-out" cycling with 4 min of recovery) over 2 wk. Before and after SIT, biopsies (vastus lateralis) were obtained at rest and after each stage of a two-stage cycling test that consisted of 10 min at approximately 60% followed by 10 min at approximately 90% of VO2 peak. Subjects also performed a 250-kJ time trial (TT) before and after SIT to assess changes in cycling performance. SIT increased muscle glycogen content by approximately 50% (main effect, P=0.04) and the maximal activity of citrate synthase (posttraining: 7.8+/-0.4 vs. pretraining: 7.0+/-0.4 mol.kg protein -1.h-1; P=0.04), but the maximal activity of 3-hydroxyacyl-CoA dehydrogenase was unchanged (posttraining: 5.1+/-0.7 vs. pretraining: 4.9+/-0.6 mol.kg protein -1.h-1; P=0.76). The active form of PDH was higher after training (main effect, P=0.04), and net muscle glycogenolysis (posttraining: 100+/-16 vs. pretraining: 139+/-11 mmol/kg dry wt; P=0.03) and lactate accumulation (posttraining: 55+/-2 vs. pretraining: 63+/-1 mmol/kg dry wt; P=0.03) during exercise were reduced. TT performance improved by 9.6% after training (posttraining: 15.5+/-0.5 vs. pretraining: 17.2+/-1.0 min; P=0.006), and a control group (n=8, VO2 peak=3.9+/-0.2 l/min) showed no change in performance when tested 2 wk apart without SIT (posttraining: 18.8+/-1.2 vs. pretraining: 18.9+/-1.2 min; P=0.74). We conclude that short-term SIT improved cycling TT performance and resulted in a closer matching of glycogenolytic flux and pyruvate oxidation during submaximal exercise.     

Six sessions of sprint interval training increases muscle oxidative potential and cycle endurance capacity in humans.

Parra et al. (Acta Physiol. Scand 169: 157-165, 2000) showed that 2 wk of daily sprint interval training (SIT) increased citrate synthase (CS) maximal activity but did not change "anaerobic" work capacity, possibly because of chronic fatigue induced by daily training. The effect of fewer SIT sessions on muscle oxidative potential is unknown, and aside from changes in peak oxygen uptake (Vo(2 peak)), no study has examined the effect of SIT on "aerobic" exercise capacity. We tested the hypothesis that six sessions of SIT, performed over 2 wk with 1-2 days rest between sessions to promote recovery, would increase CS maximal activity and endurance capacity during cycling at approximately 80% Vo(2 peak). Eight recreationally active subjects [age = 22 +/- 1 yr; Vo(2 peak) = 45 +/- 3 ml.kg(-1).min(-1) (mean +/- SE)] were studied before and 3 days after SIT. Each training session consisted of four to seven "all-out" 30-s Wingate tests with 4 min of recovery. After SIT, CS maximal activity increased by 38% (5.5 +/- 1.0 vs. 4.0 +/- 0.7 mmol.kg protein(-1).h(-1)) and resting muscle glycogen content increased by 26% (614 +/- 39 vs. 489 +/- 57 mmol/kg dry wt) (both P < 0.05). Most strikingly, cycle endurance capacity increased by 100% after SIT (51 +/- 11 vs. 26 +/- 5 min; P < 0.05), despite no change in Vo(2 peak). The coefficient of variation for the cycle test was 12.0%, and a control group (n = 8) showed no change in performance when tested approximately 2 wk apart without SIT. We conclude that short sprint interval training (approximately 15 min of intense exercise over 2 wk) increased muscle oxidative potential and doubled endurance capacity during intense aerobic cycling in recreationally active individuals.     

Effects of high fat provision on muscle PDH activation and malonyl-CoA content in moderate exercise.

This study examined the effects of elevated free fatty acid (FFA) provision on the regulation of pyruvate dehydrogenase (PDH) activity and malonyl-CoA (M-CoA) content in human skeletal muscle during moderate-intensity exercise. Seven men rested for 30 min and cycled for 10 min at 40% and 10 min at 65% of maximal O(2) uptake while being infused with either Intralipid and heparin (Int) or saline (control). Muscle biopsies were taken at 0, 1 (rest-to-exercise transition), 10, and 20 min. Exercise plasma FFA were elevated (0.99 +/- 0.11 vs. 0.33 +/- 0.03 mM), and the respiratory exchange ratio was reduced during Int (0.87 +/- 0.02) vs. control (0.91 +/- 0.01). PDH activation was lower during Int at 1 min (1.33 +/- 0.19 vs. 2.07 +/- 0.14 mmol. min(-1). kg(-1) wet muscle) and throughout exercise. Muscle pyruvate was reduced during Int at rest [0.17 +/- 0.03 vs. 0.25 +/- 0.03 mmol/kg dry muscle (dm)] but increased above control during exercise. NADH was higher during Int vs. control at rest and 1 min of exercise (0.122 +/- 0.016 vs. 0.102 +/- 0.005 and 0.182 +/- 0.016 vs. 0.150 +/- 0.016 mmol/kg dm), but not at 10 and 20 min. M-CoA was lower during Int vs. control at rest and 20 min of exercise (1.12 +/- 0.22 vs. 1.43 +/- 0.17 and 1.33 +/- 0.16 vs. 1.84 +/- 0.17 micromol/kg dm). The reduced PDH activation with elevated FFA during the rest-to-exercise transition was related to higher mitochondrial NADH at rest and 1 min of exercise and lower muscle pyruvate at rest. The decreased M-CoA may have increased fat oxidation during exercise with elevated FFA by reducing carnitine palmitoyltransferase I inhibition and increasing mitochondrial FFA transport.     

Hemodynamics and O2 uptake during maximal knee extensor exercise in untrained and trained human quadriceps muscle: effects of hyperoxia.

To elucidate the potential limitations on maximal human quadriceps O2 capacity, six subjects trained (T) one quadriceps on the single-legged knee extensor ergometer (1 h/day at 70% maximum workload for 5 days/wk), while their contralateral quadriceps remained untrained (UT). Following 5 wk of training, subjects underwent incremental knee extensor tests under normoxic (inspired O2 fraction = 21%) and hyperoxic (inspired O2 fraction = 60%) conditions with the T and UT quadriceps. Training increased quadriceps muscle mass (2.9 +/- 0.2 to 3.1 +/- 0.2 kg), but did not change fiber-type composition or capillary density. The T quadriceps performed at a greater peak power output than UT, under both normoxia (101 +/- 10 vs. 80 +/- 7 W; P < 0.05) and hyperoxia (97 +/- 11 vs. 81 +/- 7 W; P < 0.05) without further increases with hyperoxia. Similarly, thigh peak O2 consumption, blood flow, vascular conductance, and O2 delivery were greater in the T vs. the UT thigh (1.4 +/- 0.2 vs. 1.1 +/- 0.1 l/min, 8.4 +/- 0.8 vs. 7.2 +/- 0.8 l/min, 42 +/- 6 vs. 35 +/- 4 ml x min(-1) x mmHg(-1), 1.71 +/- 0.18 vs. 1.51 +/- 0.15 l/min, respectively) but were not enhanced with hyperoxia. Oxygen extraction was elevated in the T vs. the UT thigh, whereas arteriovenous O2 difference tended to be higher (78 +/- 2 vs. 72 +/- 4%, P < 0.05; 160 +/- 8 vs. 154 +/- 11 ml/l, respectively; P = 0.098) but again were unaltered with hyperoxia. In conclusion, the present results demonstrate that the increase in quadriceps muscle O2 uptake with training is largely associated with increases in blood flow and O2 delivery, with smaller contribution from increases in O2 extraction. Furthermore, the elevation in peak muscle blood flow and vascular conductance with endurance training seems to be related to an enhanced vasodilatory capacity of the vasculature perfusing the quadriceps muscle that is unaltered by moderate hyperoxia.     

Metabolic adaptations to training precede changes in muscle mitochondrial capacity.

To determine whether increases in muscle mitochondrial capacity are necessary for the characteristic lower exercise glycogen loss and lactate concentration observed during exercise in the trained state, we have employed a short-term training model involving 2 h of cycling per day at 67% maximal O2 uptake (VO2max) for 5-7 consecutive days. Before and after training, biopsies were extracted from the vastus lateralis of nine male subjects during a continuous exercise challenge consisting of 30 min of work at 67% VO2max followed by 30 min at 76% VO2max. Analysis of samples at 0, 15, 20, and 60 min indicated a pronounced reduction (P less than 0.05) in glycogen utilization after training. Reductions in glycogen utilization were accompanied by reductions (P less than 0.05) in muscle lactate concentration (mmol/kg dry wt) at 15 min [37.4 +/- 9.3 (SE) vs. 20.2 +/- 5.3], 30 min (30.5 +/- 6.9 vs. 17.6 +/- 3.8), and 60 min (26.5 +/- 5.8 vs. 17.8 +/- 3.5) of exercise. Maximal aerobic power, VO2max (l/min) was unaffected by the training (3.99 +/- 0.21 vs. 4.05 +/- 0.26). Measurements of maximal activities of enzymes representative of the citric acid cycle (succinic dehydrogenase and citrate synthase) were similar before and after the training. It is concluded that, in the voluntary exercising human, altered metabolic events are an early adaptive response to training and need not be accompanied by changes in muscle mitochondrial capacity.     

Pyruvate overrides inhibition of PDH during exercise after a low-carbohydrate diet

The effects of carbohydrate deprivation on the regulation of pyruvate dehydrogenase (PDH) were studied at rest and during moderate-intensity exercise. An inhibitory effect of a chronic low-carbohydrate diet (LCD) on the active form of PDH (PDHa) mediated by a stable increase in PDH kinase (PDHK) activity has recently been reported (Peters SJ, Howlett RA, St. Amand TA, Heigenhauser GJF, and Spriet LL. Am J Physiol Endocrinol Metab 275: E980-E986, 1998.). In the present study, seven males cycled at 65% maximal O(2) uptake for 30 min after a 6-day LCD. Exercise was repeated 1 wk later after a mixed diet (MD). Muscle biopsies were sampled from the vastus lateralis at rest and at 2 and 30 min of exercise. At rest, PDHa activity (0.18 +/- 0.04 vs. 0.63 +/- 0.18 mmol x min(-1) x kg wet wt(-1)), muscle glycogen content (310.2 +/- 36.9 vs. 563.9 +/- 32.6 mmol/kg dry wt), and muscle lactate content (2.6 +/- 0.3 vs. 4.2 +/- 0.6 mmol/kg dry wt) were significantly lower after the LCD. Resting muscle acetyl-CoA (10.8 +/- 1.9 vs. 7.4 +/- 0.8 micromol/kg dry wt) and acetylcarnitine (5.3 +/- 1.4 vs. 1.6 +/- 0.3 mmol/kg dry wt) contents were significantly elevated after the LCD. During exercise, PDHa, glycogenolytic rate (LCD 5.8 +/- 0.4 vs. MD 6.9 +/- 0.2 mmol x min(-1) x kg dry wt(-1)), and muscle concentrations of acetylcarnitine, pyruvate, and lactate increased to the same extent in both conditions. The results of the present study suggest that inhibition of resting PDH by elevated PDHK activity after a LCD may be overridden by the availability of muscle pyruvate during exercise.     

Effect of extracellular osmolality on cell volume and resting metabolism in mammalian skeletal muscle

The purpose of the present investigation was to establish an in vitro mammalian skeletal muscle model to study acute alterations in resting skeletal muscle cell volume. Isolated, whole muscles [soleus and extensor digitorum longus (EDL)] were dissected from Long-Evans rats and incubated for 60 min in Sigma medium 199 (1 g of resting tension, bubbled with 95% O(2)-5% O(2), 30 +/- 2 degrees C, and pH 7.4). Medium osmolality was altered to simulate hyposmotic (190 +/- 10 mmol/kg) or hyperosmotic conditions (400 +/- 10 mmol/kg), whereas an isosmotic condition (290 +/- 10 mmol/kg) served as a control. After incubation, relative water content of the muscle decreased with hyperosmotic and increased with hyposmotic condition in both muscle types (P < 0.05). The cross-sectional area of soleus type I and type II fibers increased (P < 0.05) in hyposmotic, whereas hyperosmotic exposure led to no detectable changes. The EDL type II fiber area decreased in the hyperosmotic condition and increased after hyposmotic exposure, whereas no change was observed in EDL type I fibers. Furthermore, exposure to the hyperosmotic condition in both muscle types resulted in decreased muscle ATP and phosphocreatine (P < 0.05) contents and increased creatine and lactate contents (P < 0.05) compared with control and hyposmotic conditions. This isolated skeletal muscle model proved viable and demonstrated that altering extracellular osmolality could cause acute alterations in muscle water content and resting muscle metabolism.     

Effect of endurance training on muscle TCA cycle metabolism during exercise in humans.

We tested the theory that links the capacity to perform prolonged exercise with the size of the muscle tricarboxylic acid (TCA) cycle intermediate (TCAI) pool. We hypothesized that endurance training would attenuate the exercise-induced increase in TCAI concentration ([TCAI]); however, the lower [TCAI] would not compromise cycle endurance capacity. Eight men (22 +/- 1 yr) cycled at approximately 80% of initial peak oxygen uptake before and after 7 wk of training (1 h/day, 5 days/wk). Biopsies (vastus lateralis) were obtained during both trials at rest, after 5 min, and at the point of exhaustion during the pretraining trial (42 +/- 6 min). A biopsy was also obtained at the end of exercise during the posttraining trial (91 +/- 6 min). In addition to improved performance, training increased (P < 0.05) peak oxygen uptake and citrate synthase maximal activity. The sum of four measured TCAI was similar between trials at rest but lower after 5 min of exercise posttraining [2.7 +/- 0.2 vs. 4.3 +/- 0.2 mmol/kg dry wt (P < 0.05)]. There was a clear dissociation between [TCAI] and endurance capacity because the [TCAI] at the point of exhaustion during the pretraining trial was not different between trials (posttraining: 2.9 +/- 0.2 vs. pretraining: 3.5 +/- 0.2 mmol/kg dry wt), and yet cycle endurance time more than doubled in the posttraining trial. Training also attenuated the exercise-induced decrease in glutamate concentration (posttraining: 4.5 +/- 0.7 vs. pretraining: 7.7 +/- 0.6 mmol/kg dry wt) and increase in alanine concentration (posttraining: 3.3 +/- 0.2 vs. pretraining: 5.6 +/- 0.3 mmol/kg dry wt; P < 0.05), which is consistent with reduced carbon flux through alanine aminotransferase. We conclude that, after aerobic training, cycle endurance capacity is not limited by a decrease in muscle [TCAI].     

Human skeletal muscle exercise metabolism following an expedition to mount denali

Chronic exposure to high altitude is known to result in changes in the mechanisms regulating O(2) delivery to the contracting muscle. However, the effects of acclimatization on metabolism in the contracting muscle cell remain unclear. In this study, we have investigated the hypothesis that acclimatization would result in a closer coupling between ATP utilization and ATP production and that the improved energy state would be accompanied by a reorganization of the metabolic pathways consisting of an increased oxidative and decreased glycolytic potential. Five men, mean age of 28 +/- 2 (SE) yr, performed a standardized, two-stage submaximal cycling task in normoxia for 20 min at each of 59 and 74% peak O(2) consumption before and 3-4 days after returning from a 21-day expedition to Mount Denali (6,194 m). Acclimatization was without effect in altering the resting values of the adenine nucleotides (ATP, ADP, AMP), inosine monophosphate (IMP), or phosphocreatine (PCr) in the vastus lateralis. During exercise (40 min) after acclimatization compared with preacclimatization, PCr was not as depressed (33.2 +/- 7.1 vs. 40.6 +/- 5.4 mmol/kg dry wt) and IMP (0.289 +/- 0.11 vs. 0. 131 +/- 0.03 mmol/kg dry wt) and lactate (26.1 +/- 6.2 vs. 18.6 +/- 8.8 mmol/kg dry wt) in contracting muscle were not as elevated (P < 0.05). Although no effect of acclimatization was observed for the maximal activity (mol. kg protein(-1). h(-1)) of citrate synthase (4. 76 +/- 0.44 vs. 4.94 +/- 0.45), lactate dehydrogenase was increased by 13% (36.5 +/- 2.6 vs. 41.2 +/- 3.1, P < 0.05). It is concluded that acclimatization results in an improved energy state in the contracting muscle when tested under normoxic conditions; however, these effects are not associated with a higher oxidative potential or a lower glycolytic potential as hypothesized.     

Effect of carbohydrate ingestion on metabolism during running and cycling.

The effects of carbohydrate or water ingestion on metabolism were investigated in seven male subjects during two running and two cycling trials lasting 60 min at individual lactate threshold using indirect calorimetry, U-14C-labeled tracer-derived measures of the rates of oxidation of plasma glucose, and direct determination of mixed muscle glycogen content from the vastus lateralis before and after exercise. Subjects ingested 8 ml/kg body mass of either a 6.4% carbohydrate-electrolyte solution (CHO) or water 10 min before exercise and an additional 2 ml/kg body mass of the same fluid after 20 and 40 min of exercise. Plasma glucose oxidation was greater with CHO than with water during both running (65 +/- 20 vs. 42 +/- 16 g/h; P < 0.01) and cycling (57 +/- 16 vs. 35 +/- 12 g/h; P < 0.01). Accordingly, the contribution from plasma glucose oxidation to total carbohydrate oxidation was greater during both running (33 +/- 4 vs. 23 +/- 3%; P < 0.01) and cycling (36 +/- 5 vs. 22 +/- 3%; P < 0.01) with CHO ingestion. However, muscle glycogen utilization was not reduced by the ingestion of CHO compared with water during either running (112 +/- 32 vs. 141 +/- 34 mmol/kg dry mass) or cycling (227 +/- 36 vs. 216 +/- 39 mmol/kg dry mass). We conclude that, compared with water, 1) the ingestion of carbohydrate during running and cycling enhanced the contribution of plasma glucose oxidation to total carbohydrate oxidation but 2) did not attenuate mixed muscle glycogen utilization during 1 h of continuous submaximal exercise at individual lactate threshold.