Low-load resistance training to task failure with and without blood flow restriction: muscular functional and structural adaptations.

The application of blood flow restriction (BFR) during resistance exercise is increasingly recognized for its ability to improve rehabilitation and for its effectiveness in increasing muscle hypertrophy and strength among healthy populations. However, direct comparison of the skeletal muscle adaptations to low-load resistance exercise (LL-RE) and low-load BFR resistance exercise (LL-BFR) performed to task failure is lacking. Using a within-subject design, we examined whole muscle group and skeletal muscle adaptations to 6 wk of LL-RE and LL-BFR training to repetition failure. Muscle strength and size outcomes were similar for both types of training, despite ~33% lower total exercise volume (load × repetition) with LL-BFR than LL-RE (28,544 ± 1,771 vs. 18,949 ± 1,541 kg, P = 0.004). After training, only LL-BFR improved the average power output throughout the midportion of a voluntary muscle endurance task. Specifically, LL-BFR training sustained an 18% greater power output from baseline and resulted in a greater change from baseline than LL-RE (19 ± 3 vs. 3 ± 4 W, P = 0.008). This improvement occurred despite histological analysis revealing similar increases in capillary content of type I muscle fibers following LL-RE and LL-BFR training, which was primarily driven by increased capillary contacts (4.53 ± 0.23 before training vs. 5.33 ± 0.27 and 5.17 ± 0.25 after LL-RE and LL-BFR, respectively, both P < 0.05). Moreover, maximally supported mitochondrial respiratory capacity increased only in the LL-RE leg by 30% from baseline (P = 0.006). Overall, low-load resistance training increased indexes of muscle oxidative capacity and strength, which were not further augmented with the application of BFR. However, performance on a muscle endurance test was improved following BFR training.

Dichloroacetate reveals the presence of metabolic inertia at the start of exercise in rainbow trout (Oncorhynchus mykiss, Walbaum 1792).

A lag in the increase in oxygen consumption (MO2 ) occurs at the start of sustainable exercise in trout. Waterborne dichloroacetate (0.58 and 3.49 mmol l-1 ), a compound which activates pyruvate dehydrogenase (PDH) by inhibiting PDH kinase in muscle, accelerates the increase in MO2 during the first 10 min of sustainable exercise when velocity is elevated to 75% critical swimming speed in a swim tunnel. There are no effects on MO2 thereafter or at rest. This indicates that a delay in PDH activation ("metabolic inertia") contributes to the lag phenomenon.

Mild Dehydration Does Not Influence Performance Or Skeletal Muscle Metabolism During Simulated Ice Hockey Exercise In Men.

This study determined whether mild dehydration influenced skeletal muscle glycogen use, core temperature or performance during high-intensity, intermittent cycle-based exercise in ice hockey players vs. staying hydrated with water. Eight males (21.6 ± 0.4 yr, 183.5 ± 1.6 cm, 83.9 ± 3.7 kg, 50.2 ± 1.9 ml·kg-1·min-1) performed two trials separated by 7 days. The protocol consisted of 3 periods (P) containing 10 × 45-s cycling bouts at ~133% VO2max, followed by 135 s of passive rest. Subjects drank no fluid and dehydrated during the protocol (NF), or maintained body mass by drinking WATER. Muscle biopsies were taken at rest, immediately before and after P3. Subjects were mildly dehydrated (-1.8% BM) at the end of P3 in the NF trial. There were no differences between the NF and WATER trials for glycogen use (P1+P2; 350.1 ± 31.9 vs. 413.2 ± 33.2, P3; 103.5 ± 16.2 vs. 131.5 ± 18.9 mmol·kg dm-1), core temperature (P1; 37.8 ± 0.1 vs. 37.7 ± 0.1, P2; 38.2 ± 0.1 vs. 38.1 ± 0.1, P3; 38.3 ± 0.1 vs. 38.2 ± 0.1 °C) or performance (P1; 156.3 ± 7.8 vs. 154.4 ± 8.2, P2; 150.5 ± 7.8 vs. 152.4 ± 8.3, P3; 144.1 ± 8.7 vs. 148.4 ± 8.7 kJ). This study demonstrated that typical dehydration experienced by ice hockey players (~1.8% BM loss), did not affect glycogen use, core temperature, or voluntary performance vs. staying hydrated by ingesting water during a cycle-based simulation of ice hockey exercise in a laboratory environment.

AMP-activated protein kinase is required for exercise-induced peroxisome proliferator-activated receptor co-activator 1 translocation to subsarcolemmal mitochondria in skeletal muscle.

In skeletal muscle, mitochondria exist as two subcellular populations known as subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria. SS mitochondria preferentially respond to exercise training, suggesting divergent transcriptional control of the mitochondrial genomes. The transcriptional co-activator peroxisome proliferator-activated receptor γ co-activator 1α (PGC-1α) and mitochondrial transcription factor A (Tfam) have been implicated in the direct regulation of the mitochondrial genome in mice, although SS and IMF differences may exist, and the potential signalling events regulating the mitochondrial content of these proteins have not been elucidated. Therefore, we examined the potential for PGC-1α and Tfam to translocate to SS and IMF mitochondria in human subjects, and performed experiments in rodents to identify signalling mechanisms regulating these translocation events. Acute exercise in humans and rats increased PGC-1α content in SS but not IMF mitochondria. Acute exposure to 5-aminoimidazole-4-carboxamide-1-ß-ribofuranoside in rats recapitulated the exercise effect of increased PGC-1α protein within SS mitochondria only, suggesting that AMP-activated protein kinase (AMPK) signalling is involved. In addition, rendering AMPK inactive (AMPK kinase dead mice) prevented exercise-induced PGC-1α translocation to SS mitochondria, further suggesting that AMPK plays an integral role in these translocation events. In contrast to the conserved PGC-1α translocation to SS mitochondria across species (humans, rats and mice), acute exercise only increased mitochondrial Tfam in rats. Nevertheless, in rat resting muscle PGC-1α and Tfam co-immunoprecipate with α-tubulin, suggesting a common cytosolic localization. These data suggest that exercise causes translocation of PGC-1α preferentially to SS mitochondria in an AMPK-dependent manner.

Effect of voluntary hyperventilation with supplemental CO2 on pulmonary O2 uptake and leg blood flow kinetics during moderate-intensity exercise.

Pulmonary O2 uptake (V(O2p)) and leg blood flow (LBF) kinetics were examined at the onset of moderate-intensity exercise, during hyperventilation with and without associated hypocapnic alkalosis. Seven male subjects (25 ± 6 years old; mean ± SD) performed alternate-leg knee-extension exercise from baseline to moderate-intensity exercise (80% of estimated lactate threshold) and completed four to six repetitions for each of the following three conditions: (i) control [CON; end-tidal partial pressure of CO2 (P(ET, CO2)) ~40 mmHg], i.e. normal breathing with normal inspired CO2 (0.03%); (ii) hypocapnia (HYPO; P(ET, CO2) ~20 mmHg), i.e. sustained hyperventilation with normal inspired CO2 (0.03%); and (iii) normocapnia (NORMO; P(ET, CO2) ~40 mmHg), i.e. sustained hyperventilation with elevated inspired CO2 (~5%). The V(O2p) was measured breath by breath using mass spectrometry and a volume turbine. Femoral artery mean blood velocity was measured by Doppler ultrasound, and LBF was calculated from femoral artery diameter and mean blood velocity. Phase 2 V(O2p) kinetics (τV(O2p)) was different (P < 0.05) amongst all three conditions (CON, 19 ± 7 s; HYPO, 43 ± 17 s; and NORMO, 30 ± 8 s), while LBF kinetics (τLBF) was slower (P < 0.05) in HYPO (31 ± 9 s) compared with both CON (19 ± 3 s) and NORMO (20 ± 6 s). Similar to previous findings, HYPO was associated with slower V(O2p) and LBF kinetics compared with CON. In the present study, preventing the fall in end-tidal P(CO2) (NORMO) restored LBF kinetics, but not V(O2p) kinetics, which remained 'slowed' relative to CON. These data suggest that the hyperventilation manoeuvre itself (i.e. independent of induced hypocapnic alkalosis) may contribute to the slower V(O2p) kinetics observed during HYPO.

Increase in skeletal-muscle glycogenolysis and perceived exertion with progressive dehydration during cycling in hydrated men.

This study investigated the effects of progressive mild dehydration during cycling on whole-body substrate oxidation and skeletal-muscle metabolism in recreationally active men. Subjects (N = 9) cycled for 120 min at ~65% peak oxygen uptake (VO2peak 22.7 °C, 32% relative humidity) with water to replace sweat losses (HYD) or without fluid (DEH). Blood samples were taken at rest and every 20 min, and muscle biopsies were taken at rest and at 40, 80, and 120 min of exercise. Subjects lost 0.8%, 1.8%, and 2.7% body mass (BM) after 40, 80, and 120 min of cycling in the DEH trial while sweat loss was not significantly different between trials. Heart rate was greater in the DEH trial from 60 to 120 min, and core temperature was greater from 75 to 120 min. Rating of perceived exertion was higher in the DEH trial from 30 to 120 min. There were no differences in VO2, respiratory-exchange ratio, total carbohydrate (CHO) oxidation (HYD 312 ± 9 vs. DEH 307 ± 10 g), or sweat rate between trials. Blood lactate was significantly greater in the DEH trial from 20 to 120 min with no difference in plasma free fatty acids or epinephrine. Glycogenolysis was significantly greater (24%) over the entire DEH vs. HYD trial (433 ± 44 vs. 349 ± 27 mmol · kg-1 · dm-1). In conclusion, dehydration of <2% BM elevated physiological parameters and perceived exertion, as well as muscle glycogenolysis, during exercise without affecting whole-body CHO oxidation.

Mitochondrial creatine kinase activity and phosphate shuttling are acutely regulated by exercise in human skeletal muscle.

Energy transfer between mitochondrial and cytosolic compartments is predominantly achieved by creatine-dependent phosphate shuttling (PCr/Cr) involving mitochondrial creatine kinase (miCK). However, ADP/ATP diffusion through adenine nucleotide translocase (ANT) and voltage-dependent anion carriers (VDACs) is also involved in this process. To determine if exercise alters the regulation of this system, ADP-stimulated mitochondrial respiratory kinetics were assessed in permeabilized muscle fibre bundles (PmFBs) taken from biopsies before and after 2 h of cycling exercise (60% ) in nine lean males. Concentrations of creatine (Cr) and phosphocreatine (PCr) as well as the contractile state of PmFBs were manipulated in situ. In the absence of contractile signals (relaxed PmFBs) and miCK activity (no Cr), post-exercise respiratory sensitivity to ADP was reduced in situ (up to 126% higher apparent K(m) to ADP) suggesting inhibition of ADP/ATP diffusion between matrix and cytosolic compartments (possibly ANT and VDACs). However this effect was masked in the presence of saturating Cr (no effect of exercise on ADP sensitivity). Given that the role of ANT is thought to be independent of Cr, these findings suggest ADP/ATP, but not PCr/Cr, cycling through the outer mitochondrial membrane (VDACs) may be attenuated in resting muscle after exercise. In contrast, in contracted PmFBs, post-exercise respiratory sensitivity to ADP increased with miCK activation (saturating Cr; 33% lower apparent K(m) to ADP), suggesting prior exercise increases miCK sensitivity in situ. These observations demonstrate that exercise increases miCK-dependent respiratory sensitivity to ADP, promoting mitochondrial-cytosolic energy exchange via PCr/Cr cycling, possibly through VDACs. This effect may mask an underlying inhibition of Cr-independent ADP/ATP diffusion. This enhanced regulation of miCK-dependent phosphate shuttling may improve energy homeostasis through more efficient coupling of oxidative phosphorylation to perturbations in cellular energy charge during subsequent bouts of contraction.

Acute endurance exercise increases plasma membrane fatty acid transport proteins in rat and human skeletal muscle.

Fatty acid transport proteins are present on the plasma membrane and are involved in the uptake of long-chain fatty acids into skeletal muscle. The present study determined whether acute endurance exercise increased the plasma membrane content of fatty acid transport proteins in rat and human skeletal muscle and whether the increase was accompanied by an increase in long-chain fatty acid transport in rat skeletal muscle. Sixteen subjects cycled for 120 min at ∼60 ± 2% Vo(2) peak. Two skeletal muscle biopsies were taken at rest and again following cycling. In a parallel study, eight Sprague-Dawley rats ran for 120 min at 20 m/min, whereas eight rats acted as nonrunning controls. Giant sarcolemmal vesicles were prepared, and protein content of FAT/CD36 and FABPpm was measured in human and rat vesicles and whole muscle homogenate. Palmitate uptake was measured in the rat vesicles. In human muscle, plasma membrane FAT/CD36 and FABPpm protein contents increased 75 and 20%, respectively, following 120 min of exercise. In rat muscle, plasma membrane FAT/CD36 and FABPpm increased 20 and 30%, respectively, and correlated with a 30% increase in palmitate transport following 120 min of running. These data suggest that the translocation of FAT/CD36 and FABPpm to the plasma membrane in rat skeletal muscle is related to the increase in fatty acid transport and oxidation that occurs with endurance running. This study is also the first to demonstrate that endurance cycling induces an increase in plasma membrane FAT/CD36 and FABPpm content in human skeletal muscle, which is predicted to increase fatty acid transport.

Nuclear SIRT1 activity, but not protein content, regulates mitochondrial biogenesis in rat and human skeletal muscle.

Silent mating type information regulator 2 homolog 1 (SIRT1)-mediated peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α) deacetylation is potentially key for activating mitochondrial biogenesis. Yet, at the whole muscle level, SIRT1 is not associated with mitochondrial biogenesis (Gurd, BJ, Yoshida Y, Lally J, Holloway GP, Bonen A. J Physiol 587: 1817-1828, 2009). Therefore, we examined nuclear SIRT1 protein and activity in muscle with varied mitochondrial content and in response to acute exercise. We also measured these parameters after stimulating mitochondrial biogenesis with chronic muscle contraction and 5-aminoimidazole-4-carboxamide-1-ß-d-ribofuranoside (AICAR) administration in rodents and exercise training in humans. In skeletal and heart muscles, nuclear SIRT1 protein was negatively correlated with indices of mitochondrial density (citrate synthase activity, CS; cytochrome oxidase IV, COX IV), but SIRT1 activity was positively correlated with these parameters (r > 0.98). Acute exercise did not alter nuclear SIRT1 protein but did induce a time-dependent increase in nuclear SIRT1 activity. This increase in SIRT1 activity was temporally related to increases in mRNA expression of genes activated by PGC-1α. Both chronic muscle stimulation and AICAR increased mitochondrial biogenesis and muscle PGC-1α, but not nuclear PGC-1α. Concomitantly, muscle and nuclear SIRT1 protein contents were reduced, but nuclear SIRT1 activity was increased. In human muscle, training-induced mitochondrial biogenesis did not alter muscle or nuclear SIRT1 protein content, but it did increase muscle and nuclear PGC-1α and SIRT1 activity. Thus, nuclear SIRT1 activity, but not muscle or nuclear SIRT1 protein content, is associated with contraction-stimulated mitochondrial biogenesis in rat and human muscle, possibly via AMPK activation.

Repeated transient mRNA bursts precede increases in transcriptional and mitochondrial proteins during training in human skeletal muscle.

Exercise training induces mitochondrial biogenesis, but the time course of molecular sequelae that accompany repetitive training stimuli remains to be determined in human skeletal muscle. Therefore, throughout a seven-session, high-intensity interval training period that increased (12%), we examined the time course of responses of (a) mitochondrial biogenesis and fusion and fission proteins, and (b) selected transcriptional and mitochondrial mRNAs and proteins in human muscle. Muscle biopsies were obtained 4 and 24 h after the 1st, 3rd, 5th and 7th training session. PGC-1α mRNA was increased >10-fold 4 h after the 1st session and returned to control within 24 h. This 'saw-tooth' pattern continued until the 7th bout, with smaller increases after each bout. In contrast, PGC-1α protein was increased 24 h after the 1st bout (23%) and plateaued at +30-40% between the 3rd and 7th bout. Increases in PGC-1ß mRNA and protein were more delayed and smaller, and did not persist. Distinct patterns of increases were observed in peroxisome proliferator-activated receptor (PPAR) α and γ protein (1 session), PPAR ß/δ mRNA and protein (5 sessions) and nuclear respiratory factor-2 protein (3 sessions) while no changes occurred in mitochondrial transcription factor A protein. Citrate synthase (CS) and ß-HAD mRNA were rapidly increased (1 session), followed 2 sessions later (session 3) by increases in CS and ß-HAD activities, and mitochondrial DNA. Changes in COX-IV mRNA (session 3) and protein (session 5) were more delayed. Training also increased mitochondrial fission proteins (fission protein-1, >2-fold; dynamin-related protein-1, 47%) and the fusion protein mitofusin-1 (35%) but not mitofusin-2. This study has provided the following novel information: (a) the training-induced increases in transcriptional and mitochondrial proteins appear to result from the cumulative effects of transient bursts in their mRNAs, (b) training-induced mitochondrial biogenesis appears to involve re-modelling in addition to increased mitochondrial content, and (c) the 'transcriptional capacity' of human muscle is extremely sensitive, being activated by one training bout.

Exercise training increases sarcolemmal and mitochondrial fatty acid transport proteins in human skeletal muscle.

Fatty acid oxidation is highly regulated in skeletal muscle and involves several sites of regulation, including the transport of fatty acids across both the plasma and mitochondrial membranes. Transport across these membranes is recognized to be primarily protein mediated, limited by the abundance of fatty acid transport proteins on the respective membranes. In recent years, evidence has shown that fatty acid transport proteins move in response to acute and chronic perturbations; however, in human skeletal muscle the localization of fatty acid transport proteins in response to training has not been examined. Therefore, we determined whether high-intensity interval training (HIIT) increased total skeletal muscle, sarcolemmal, and mitochondrial membrane fatty acid transport protein contents. Ten untrained females (22 +/- 1 yr, 65 +/- 2 kg; .VO(2peak): 2.8 +/- 0.1 l/min) completed 6 wk of HIIT, and biopsies from the vastus lateralis muscle were taken before training, and following 2 and 6 wk of HIIT. Training significantly increased maximal oxygen uptake at 2 and 6 wk (3.1 +/- 0.1, 3.3 +/- 0.1 l/min). Training for 6 wk increased FAT/CD36 at the whole muscle (10%) and mitochondrial levels (51%) without alterations in sarcolemmal content. Whole muscle plasma membrane fatty acid binding protein (FABPpm) also increased (48%) after 6 wk of training, but in contrast to FAT/CD36, sarcolemmal FABPpm increased (23%), whereas mitochondrial FABPpm was unaltered. The changes on sarcolemmal and mitochondrial membranes occurred rapidly, since differences (< or =2 wk) were not observed between 2 and 6 wk. This is the first study to demonstrate that exercise training increases fatty acid transport protein content in whole muscle (FAT/CD36 and FABPpm) and sarcolemmal (FABPpm) and mitochondrial (FAT/CD36) membranes in human skeletal muscle of females. These results suggest that increases in skeletal muscle fatty acid oxidation following training are related in part to changes in fatty acid transport protein content and localization.

Prolonged moderate-intensity aerobic exercise does not alter apoptotic signaling and DNA fragmentation in human skeletal muscle.

Apoptosis in skeletal muscle plays an important role in age- and disease-related tissue dysfunction. Physical activity can influence apoptotic signaling; however, this process has not been well studied in human skeletal muscle. The purpose of this study was to perform a comprehensive analysis of apoptosis-related proteins/enzymes, DNA fragmentation, and oxidative stress in skeletal muscle of humans during an acute bout of prolonged moderate-intensity exercise. Eight healthy, recreationally active individuals (age 20.8 +/- 0.5 yr, Vo(2peak) 51.2 +/- 0.9 ml . kg(-1) . min(-1), BMI 21.5 +/- 0.8 kg/m(2)) exercised on a cycle ergometer at approximately 60% Vo(2peak) for 2 h. Muscle biopsies were obtained at rest as well as at 60 and 120 min of exercise. Although exercise was associated with a significant whole body and muscle metabolic response, there were no significant changes in the content of antiapoptotic (ARC, Bcl-2, Hsp70, XIAP) and proapoptotic (AIF, Bax, Smac) proteins, activity of proteolytic enzymes (caspase-3, caspase-8, caspase-9), DNA fragmentation, or TUNEL-positive nuclei in skeletal muscle. Furthermore, the protein levels of several antioxidant enzymes (catalase, CuZnSOD, MnSOD), concentrations of GSH and GSSG, and degree of ROS generation in skeletal muscle were not altered by exercise. Fiber type-specific analysis also revealed that ARC (P < 0.001) and Hsp70 (P < 0.05) protein were significantly higher in type I compared with type IIA and type IIAX/X fibers; however, protein levels were not affected by exercise. These findings suggest that a single bout of prolonged moderate-intensity aerobic exercise is not sufficient to alter apoptotic signaling in skeletal muscle of healthy humans.

Effect of hyperventilation and prior heavy exercise on O2 uptake and muscle deoxygenation kinetics during transitions to moderate exercise.

The effect of hyperventilation-induced hypocapnic alkalosis (HYPO) and prior heavy-intensity exercise (HVY) on pulmonary O(2) uptake (VO(2p)) kinetics were examined in young adults (n = 7) during moderate-intensity exercise (MOD). Subjects completed leg cycling exercise during (1) normal breathing (CON, P(ET)CO(2) approximately 40 mmHg) and (2) controlled hyperventilation (HYPO, P(ET)CO(2) approximately 20 mmHg) throughout the protocol, with each condition repeated on four occasions. The protocol consisted of two MOD transitions (MOD1, MOD2) to 80% estimated lactate threshold with MOD2 preceded by HVY (Delta50%); each transition lasted 6 min and was preceded by 20 W cycling. VO(2p) was measured breath-by-breath and concentration changes in oxy- and deoxy-hemoglobin/myoglobin (Delta[HHb]) of the vastus lateralis muscle were measured by near-infrared spectroscopy. Adjustment of VO(2p) and Delta[HHb] were modeled using a mono-exponential equation by non-linear regression. During MOD1, the phase 2 time constant (tau) for VO(2p)(tauVO(2p)) was greater (P < 0.05) in HYPO (45 +/- 24 s) than CON (28 +/- 17 s). During MOD2, tauVO(2p) was reduced (P < 0.05) in both conditions (HYPO: 24 +/- 7 s, CON: 20 +/- 8 s). The Delta[Hb(TOT)] and Delta[O(2)Hb] were greater (P < 0.05) prior to and throughout MOD2. The Delta[HHb] mean response time was similar in MOD1 and MOD2, and between conditions, however, the MOD1 Delta[HHb] amplitude was greater (P < 0.05) in HYPO compared to CON, with no differences between conditions in MOD2. These findings suggest that the speeding of VO(2p) kinetics after prior HVY in HYPO was related, in part, to an increase in microvascular perfusion.

Resting metabolic rate and skeletal muscle SERCA and Na+ /K+ ATPase activities are not affected by fish oil supplementation in healthy older adults.

Omega-3 polyunsaturated fatty acids (PUFAs) have unique properties purported to influence several aspects of metabolism, including energy expenditure and protein function. Supplementing with n-3 PUFAs may increase whole-body resting metabolic rate (RMR), by enhancing Na+ /K+ ATPase (NKA) activity and reducing the efficiency of sarcoplasmic reticulum (SR) Ca2+ ATPase (SERCA) activity by inducing a Ca2+ leak-pump cycle. The purpose of this study was to examine the effects of fish oil (FO) on RMR, substrate oxidation, and skeletal muscle SERCA and NKA pump function in healthy older individuals. Subjects (n = 16 females; n = 8 males; 65 ± 1 years) were randomly assigned into groups supplemented with either olive oil (OO) (5 g/day) or FO (5 g/day) containing 2 g/day eicosapentaenoic acid and 1 g/day docosahexaenoic acid for 12 weeks. Participants visited the laboratory for RMR and substrate oxidation measurements after an overnight fast at weeks 0 and 12. Skeletal muscle biopsies were taken during weeks 0 and 12 for analysis of NKA and SERCA function and protein content. There was a main effect of time with decrease in RMR (5%) and fat oxidation (18%) in both the supplementation groups. The kinetic parameters of SERCA and NKA maximal activity, as well as the expression of SR and NKA proteins, were not affected after OO and FO supplementation. In conclusion, these results suggest that FO supplementation is not effective in altering RMR, substrate oxidation, and skeletal muscle SERCA and NKA protein levels and activities, in healthy older men and women.

Carbohydrate refeeding after a high-fat diet rapidly reverses the adaptive increase in human skeletal muscle PDH kinase activity.

Pyruvate dehydrogenase (PDH) regulates oxidative carbohydrate disposal in skeletal muscle and is downregulated by reversible phosphorylation catalyzed by PDH kinase (PDK). Previous work has demonstrated increased PDK activity and PDK4 expression in human skeletal muscle following a high-fat low-carbohydrate (HF) diet, which leads to decreased PDH in the active form (PDHa activity) and carbohydrate oxidation. The purpose of this study was to examine the time course of changes in PDK and PDHa activities with refeeding of carbohydrates after an HF diet in human skeletal muscle. Healthy male volunteers (n = 8) consumed a standardized 3-day Pre-diet with the same energy content as their habitual diet, followed by a eucaloric 6-day HF diet (Pre-diet: 50:30:20%; HF diet: 5:75:20%; carbohydrate/fat/protein). Muscle biopsies were taken before and after the HF diet and at 45 min and 3 h after carbohydrate refeeding with a single high-glycemic index carbohydrate meal (88:5:7% carbohydrate/fat/protein) representing approximately one third of the individual subject's habitual energy intake. PDK activity increased from 0.08 +/- 0.01 Pre- to 0.25 +/- 0.02 min (P < 0.001) Post-HF diet, and decreased with carbohydrate refeeding to 0.17 +/- 0.05 (P = 0.014) and 0.11 +/- 0.01 min (P = 0.006) at 45 min and 3 h, respectively. PDHa decreased from 0.89 +/- 0.20 to 0.32 +/- 0.05 (P = 0.007) mmol x min(-1) x kg wet wt(-1) following the HF diet, and was increased transiently with refeeding at 45 min, but returned to lower values by 3 h (P = 0.025 compared with Pre). The potential mechanism(s) for this attenuation of PDHa activity remains unclear. These data demonstrate that in human skeletal muscle, the adaptive increase in PDK activity following an HF diet is rapidly reversed to Pre-diet activity levels within 45 min to 3 h, and this is accompanied by a short-term increase in PDHa activity.

Prior heavy exercise elevates pyruvate dehydrogenase activity and muscle oxygenation and speeds O2 uptake kinetics during moderate exercise in older adults.

The adaptation of pulmonary oxygen uptake (VO(2)(p)) kinetics during the transition to moderate-intensity exercise is slowed in older compared with younger adults; however, this response is faster following a prior bout of heavy-intensity exercise. We have examined VO(2)(p) kinetics, pyruvate dehydrogenase (PDH) activation, muscle metabolite contents, and muscle deoxygenation in older adults [n = 6; 70 +/- 5 (67-74) yr] during moderate-intensity exercise (Mod(1)) and during moderate-intensity exercise preceded by heavy-intensity warm-up exercise (Mod(2)). The phase 2 VO(2)(p) time constant (tauVO(2)(p)) was reduced (P < 0.05) in Mod(2) (29 +/- 5 s) compared with Mod(1) (39 +/- 14 s). PDH activity was elevated (P < 0.05) at baseline prior to Mod(2) (2.1 +/- 0.6 vs. 1.2 +/- 0.3 mmol acetyl-CoA x min(-1) x kg wet wt(-1)), and the delay in attaining end-exercise activity was abolished. Phosphocreatine breakdown during exercise was reduced (P < 0.05) at both 30 s and 6 min in Mod(2) compared with Mod(1). Near-infrared spectroscopy-derived indices of muscle oxygenation were elevated both prior to and throughout Mod(2), while muscle deoxygenation kinetics were not different between exercise bouts consistent with elevated perfusion and O(2) availability. These results suggest that in older adults, faster VO(2)(p) kinetics following prior heavy-intensity exercise are likely a result of prior activation of mitochondrial enzyme activity in combination with elevated muscle perfusion and O(2) availability.

Fluid and electrolyte supplementation after prolonged moderate-intensity exercise enhances muscle glycogen resynthesis in Standardbred horses.

We hypothesized that postexercise rehydration using a hypotonic electrolyte solution will increase the rate of recovery of whole body hydration, and that this is associated with increased muscle glycogen and electrolyte recovery in horses. Gluteus medius biopsies and jugular venous blood were sampled from six exercise-conditioned Standardbreds on two separate occasions, at rest and for 24 h following a competitive exercise test (CET) designed to simulate the speed and endurance test of a 3-day event. After the CETs, horses were given water ad libitum, and either a hypotonic commercial electrolyte solution (electrolyte) via nasogastric tube, followed by a typical hay/grain meal, or a hay/grain meal alone (control). The CET resulted in decreased total body water and muscle glycogen concentration of 8.4 +/- 0.3 liters and 22.6%, respectively, in the control treatment, and 8.2 +/- 0.4 liters and 21.9% in the electrolyte treatment. Electrolyte resulted in an enhanced rate of muscle glycogen resynthesis and faster restoration of hydration (as evidenced by faster recovery of plasma protein concentration, maintenance of plasma osmolality, and greater muscle intracellular fluid volume) during the recovery period compared with control. There were no differences in muscle Na, K, Cl, or Mg contents between the two treatments. It is concluded that oral administration of a hypotonic electrolyte solution after prolonged moderate-intensity exercise enhanced the rate of muscle glycogen resynthesis during the recovery period compared with control. It is speculated that postexercise dehydration may be one key contributor to the slow muscle glycogen replenishment in horses.

Oral acetate supplementation after prolonged moderate intensity exercise enhances early muscle glycogen resynthesis in horses.

Oral acetate supplementation enhances glycogen synthesis in some mammals. However, while acetate is a significant energy source for skeletal muscle at rest in horses, its effects on glycogen resynthesis are unknown. We hypothesized that administration of an oral sodium acetate-acetic acid solution with a typical grain and hay meal after glycogen-depleting exercise would result in a rapid appearance of acetate in blood with rapid uptake by skeletal muscle. It was further hypothesized that acetate taken up by muscle would be converted to acetyl CoA (and acetylcarnitine), which would be metabolized to CO2 and water via the tricarboxylic acid cycle, generating ATP within the mitochondria and thereby allowing glucose taken up by muscle to be preferentially incorporated into glycogen. Gluteus medius biopsies and jugular venous blood were sampled from nine exercise-conditioned horses on two separate occasions, at rest and for 24 h following a competition exercise test (CET) designed to simulate the speed and endurance test of a 3 day event. After the CETs, horses were allowed water ad libitum and either 8 l of a hypertonic sodium acetate-acetic acid solution via nasogastric gavage followed by a typical hay-grain meal (acetate treatment) or a hay-grain meal alone (control treatment). The CET significantly decreased muscle glycogen concentration by 21 and 17% in the acetate and control treatments, respectively. Acetate supplementation resulted in a rapid and sustained increase in plasma [acetate]. Skeletal muscle [acetyl CoA] and [acetylcarnitine] were increased at 4 h of recovery in the acetate treatment, suggesting substantial tissue extraction of the supplemented acetate. Acetate supplementation also resulted in an enhanced rate of muscle glycogen resynthesis during the initial 4 h of the recovery period compared with the control treatment; however, by 24 h of recovery there was no difference in glycogen replenishment between trials. It is concluded that oral acetate could be an alternative energy source in the horse.

Assessment of Na+/K+ ATPase Activity in Small Rodent and Human Skeletal Muscle Samples.

INTRODUCTION: In skeletal muscle, the Na/K ATPase (NKA) plays essential roles in processes linked to muscle contraction, fatigue, and energy metabolism; however, very little information exists regarding the regulation of NKA activity. The scarcity of information regarding NKA function in skeletal muscle likely stems from methodological constraints, as NKA contributes minimally to total cellular ATP utilization, and therefore contamination from other ATPases prevents the assessment of NKA activity in muscle homogenates. Here we introduce a method that improves accuracy and feasibility for the determination of NKA activity in small rodent muscle samples (5-10 mg) and in human skeletal muscle. METHODS: Skeletal muscle homogenates from mice (n = 6) and humans (n = 3) were used to measure NKA and sarcoplasmic reticulum Ca ATPase (SERCA) activities with the addition of specific ATPase inhibitors to minimize "background noise." RESULTS: We observed that myosin ATPase activity was the major interfering factor for estimation of NKA activity in skeletal muscle homogenates, as the addition of 25 µM of blebbistatin, a specific myosin ATPase inhibitor, considerably minimized "background noise" (threefold) and enabled the determination of NKA maximal activity with values three times higher than previously reported. The specificity of the assay was demonstrated after the addition of 2 mM ouabain, which completely inhibited NKA. On the other hand, the addition of blebbistatin did not affect the ability to measure SERCA function. The coefficient of variation for NKA and SERCA assays were 6.2% and 4.4%, respectively. CONCLUSION: The present study has improved the methodology to determine NKA activity. We further show the feasibility of measuring NKA and SERCA activities from a common muscle homogenate. This methodology is expected to aid in our long-term understanding of how NKA affects skeletal muscle metabolic homeostasis and contractile function in diverse situations.

Incorporation of Omega-3 Fatty Acids Into Human Skeletal Muscle Sarcolemmal and Mitochondrial Membranes Following 12 Weeks of Fish Oil Supplementation.

Fish oil (FO) supplementation in humans results in the incorporation of omega-3 fatty acids (FAs) eicosapentaenoic acid (EPA; C20:5) and docosahexaenoic acid (DHA; C20:6) into skeletal muscle membranes. However, despite the importance of membrane composition in structure-function relationships, a paucity of information exists regarding how different muscle membranes/organelles respond to FO supplementation. Therefore, the purpose of the present study was to determine the effects 12 weeks of FO supplementation (3g EPA/2g DHA daily) on the phospholipid composition of sarcolemmal and mitochondrial fractions, as well as whole muscle responses, in healthy young males. FO supplementation increased the total phospholipid content in whole muscle (57%; p < 0.05) and the sarcolemma (38%; p = 0.05), but did not alter the content in mitochondria. The content of omega-3 FAs, EPA and DHA, were increased (+3-fold) in whole muscle, and mitochondrial membranes, and as a result the omega-6/omega-3 ratios were dramatically decreased (-3-fold), while conversely the unsaturation indexes were increased. Intriguingly, before supplementation the unsaturation index (UI) of sarcolemmal membranes was ∼3 times lower (p < 0.001) than either whole muscle or mitochondrial membranes. While supplementation also increased DHA within sarcolemmal membranes, EPA was not altered, and as a result the omega-6/omega-3 ratio and UI of these membranes were not altered. All together, these data revealed that mitochondrial and sarcolemmal membranes display unique phospholipid compositions and responses to FO supplementation.