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1 Department of Physiology and Pharmacology, Karolinska Institutet, Stockholm, Sweden
2 Centre for Integrated Systems Biology and Medicine, Nottingham University Medical School, Queen's Medical Centre, Nottingham NG7 2UH, UK
3 AstraZeneca, Cardiovascular and Gastrointestinal Discover Department, Alderley Edge, SK10 4TG, UK
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Abstract |
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1 mmol (kg dry mass)–1 s–1 or less (depending on intrinsic mitochondrial capacity). When measured early during an uninterrupted period of muscle contraction, acetyl group availability is likely to influence SLP under any condition where mitochondria are responsible for a significant proportion of ATP regeneration.
(Received 4 June 2004;
accepted after revision 18 October 2004;
first published online 21 October 2004)
Corresponding author J. A. Timmons: Department of Physiology and Pharmacology, Berzelius Väg 13, Karolinska Institutet, 171 77 Stockholm, Sweden. Email: jamie.timmons{at}fyfa.ki.se
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Introduction |
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When the ‘acetyl group deficit’ was originally discovered (Timmons et al. 1996a) it was proposed that DCA could be reducing SLP through the combined effect of enhancing flux through PDC and/or by providing a ‘stock pile’ of acetyl groups for the tricarboxylic acid cycle in the form of acetylcarnitine (where reversal of the carnitine acetyl-transferase activity rapidly feeds acetyl-coenzyme A to citrate synthase). There is experimental evidence to support all of these eventualities (Timmons et al. 1996a, 1997, 1998a,b; Roberts et al. 2002). We also demonstrated that net NADH accumulation was greater from rest to steady state following DCA pretreatment (Timmons et al. 1997). This indicates that enhanced substrate flux through the Krebs cycle or PDC occurred during the first 60 s of contraction, thus yielding a greater redox drive to the electron transport chain. In our studies, the only real assumption was that DCA resulted in additional oxygen consumption during the rest-to-exercise phase. It has now been confirmed that DCA can speed up intramuscular oxygen consumption during the rest-to-work transition period in perfused single muscle fibres (Howlett & Hogan, 2003).
To precipitate fatigue that might mimic that found in peripheral vascular disease patients, we limited blood flow to 25% of normal, during contraction (Timmons et al. 1996b). We used a 3 Hz twitch stimulation protocol (Timmons et al. 1996b) which achieves an oxygen consumption of 80% of maximum, if blood flow is fully intact (it yields only 25% of 
in practice). The positive characteristics of this canine model include the avoidance of protracted muscle sampling: muscle sampling occurs without the cessation of contraction and it avoids the possibility of inactive (unrecruited) muscle fibres ‘contaminating’ (Constantin-Teodosiu et al. 1996) the metabolic profile of the muscle biopsy sample. We established that reduced SLP following DCA treatment preceded the reduction in muscle fatigue changes, demonstrating that the muscle fatigue appeared to be of metabolic origin (Timmons et al. 1997).
More recently DCA has been shown to improve muscle function during tetanic muscle contraction (with intact blood flow) independent of any changes in SLP (Grassi et al. 2002). Thus an aim of the present study was to re-evaluate these findings using a tetanic stimulation protocol. The protocol was designed to precipitate PCr degradation during the rest-to-steady state period, at a rate of 1 mmol (kg dry mass)–1 s–1. This was because in a parallel met-analysis of the available literature, DCA did not appear to reduce SLP during exercise above this intensity. Thus, a second major aim of this study was to establish whether enhanced acetyl group delivery altered PCr degradation during this intense stimulation protocol. We specifically hypothesized that acetyl group availability may reduce the rate of PCr degradation early during contraction. To confirm this we took a muscle biopsy at the point of peak tension, prior to any fatigue, and without interrupting the stimulation protocol. This would enable us to confirm that, during an intense stimulation protocol, acetyl group availability still limited oxidative ATP regeneration.
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Methods |
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After an overnight fast, each dog (Animal Breeding Unit, AstraZeneca Pharmaceuticals, Alderley Park, UK; body mass (mean ± S.E.M., 14.7 ± 1.5 kg, n = 10) was premedicated with morphine (10 mg I.M.) 30 min prior to the induction of anaesthesia with sodium pentobarbitone (45 ± 1 mg kg–1) followed by continuous infusion at 0.10 ± 0.01 mg kg–1 min–1, both I.V. (Sagatal, Rhône Merieux, Harlow, UK). The trachea was intubated and the dogs were artificially ventilated (24 cycles min–1, tidal volume 13–15 ml kg–1, model 16/24, Palmer Bioscience, London, UK). Muscle and rectal temperatures were maintained close to 37°C and between 36°C and 38°C, respectively. The right brachial artery was cannulated and systemic blood pressure was recorded using a pressure transducer (PDCR 75, Druck Ltd, Barendrecht, The Netherlands) and an eight-channel chart recorder (Graphtec Linearcorder, Mk 8 WR3500, Nantwich, UK). The left brachial artery and vein were cannulated for collection of arterial blood samples for monitoring blood pH, PCO2 and PCO2 (280 Blood Gas System, Ciba-Corning, Medfield, MA, USA) and venous infusion of heparin, saline or DCA. Blood glucose and lactate were analysed as previously described (Timmons et al. 1998a).
All experiments were carried out in full accordance with the United Kingdom Home Office Animals (Scientific Procedures) Act of 1986, and approved by the AstraZeneca local ethics committee.
Both gracilis muscles were vascularly isolated, leaving only the main arterial and venous blood flows intact. The distal tendon of each muscle was attached to an isometric force transducer (Grass FTC 10, Quincy, Medfield, MA, USA). The popliteal artery was catheterized for recording gracilis muscle perfusion pressure. Heparin was infused (Multiparin, 1 U (kg body weight)–1 min–1) for the duration of the experiment. Each animal was infused with either 45 ml saline (n
= 5) or 300 mg kg–1 DCA (n
= 5, in 45 ml saline) over a period of 45 min. The femoral artery supplying each gracilis muscle was cannulated proximally and distally and attached sequentially to a perfusion pump (Minipuls 3, Gilson, Villiers Le Bel, France). The resting muscle blood flow was fixed by setting the perfusion pump at 6.5 ml min–1. We have previously established this as being equivalent to resting flow rate in the gracilis muscle (Timmons et al. 1996a). This flow rate was maintained for the duration of the experiment and equates to 20% of the normal flow observed when this muscle was stimulated to contract with the blood supply intact (Timmons et al. 1996a). The dose of DCA used in the present study completely transforms PDC to its active form, PDCa, in vivo (Timmons et al. 1997). At the end of the protocol, the animals were humanely killed using a lethal dose of anaesthetic, followed by saturated potassium chloride.
Muscle stimulation parameters
The resting length of the muscle was altered to obtain a standard resting tension. Sixty minutes after the onset of DCA or saline infusion, muscle contraction was induced via electrical stimulation of the obturator nerve (Grass S88 stimulator, Quincy, Medfield, MA, USA). Muscle stimulation was carried out using a tetanic stimulation protocol (75 ms on, once every second, at 10 V (pulse width of 0.2 ms, 30 Hz). This protocol, which results in complete muscle fibre recruitment, was applied for 6 min in total. The stimulation procedure was then repeated with the contralateral muscle.
Muscle sampling and analysis
Each group consisted of five animals and hence ten separate stimulation protocols. A resting muscle biopsy was taken, by superficial excision of the gracilis, using a scalpel, 1 h after the onset of infusion. After 1 min and 6 min of stimulation a thin piece of muscle was excised, during contraction, and rapidly frozen. The number of biopsies obtained at each time point was between 5 and 8 since for each animal, only one resting biopsy was taken for resting metabolite analysis (n = 5) and on a couple of occasions a 6 min biopsy was not successful.
All biopsy samples were then divided into two portions and stored under liquid nitrogen. Subsequently, one portion was freeze dried, dissected free from visible connective tissue and blood, powdered and extracted in 0.5 M perchloric acid containing 1 mM EDTA. Following centrifugation, the supernatant was neutralized with 2.2 M KHCO3 and used for spectrophotometric determination of ATP, PCr, creatine and lactate (Harris et al. 1974). The extract was also used for the determination of free carnitine and acetyl-carnitine by enzymatic assays using radioisotopic substrates, as previously described (Cederblad et al. 1990). Freeze-dried muscle powder was used for the determination of muscle glycogen (Harris et al. 1974). Muscle citrate, malate and fumarate were analysed enzymatically (Bergmeyer, 1974) using a fluorometer (Hitachi F-2000).
Statistics and calculations
All data are reported as mean ± S.E.M. Comparisons between treatments for both absolute concentrations and changes from rest were carried out using analysis of variance (ANOVA). When a significant F value was found a Tukey post hoc test was used to locate the differences. Significance was accepted at the 5% level.
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Results |
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Resting tension did not vary between the groups (range 0.6–0.7 kg (100 g muscle tissue)–1). Peak tension was 10.9 ± 0.4 kg (100 g)–1 in the control group and 10.2 ± 0.5 kg (100 g)–1 in the group pretreated with DCA. There was no difference in the extent of the fatigue demonstrated between treatments (control, 26.5 ± 1.5%, versus DCA, 22.7 ± 3.2%). Muscle force production appeared stabilized by 6 min of contraction consistent with earlier studies (Timmons et al. 1996a, 1997).
Blood flow did not differ between control and DCA treatments. As can be observed from the data in Table 1 DCA had no effect on muscle oxygen consumption, muscle lactate efflux or glucose uptake during the period in which arterio-venous samples were collected (minutes 4–5), although resting muscle glucose uptake tended to be higher following DCA. Arterial plasma glucose concentrations did not differ (control, 5.3 ± 0.2 mM; DCA, 5.8 ± 0.2 mM). Arterial free fatty acid concentrations did not differ (control, 0.17 ± 0.01 mM; DCA, 0.13 ± 0.02 mM). Arterial lactate concentration was reduced by DCA (control, 1.7 ± 0.2 mM; DCA, 0.47 ± 0.05 mM; P < 0.05). Arterial haemoglobin concentration was 14.0 ± 0.2 g dl–1 (control) and 15.5 ± 0.2 g dl–1 (DCA). Oxygen delivery did not significantly differ between the groups at any time point. Likewise, oxygen consumption did not differ between the groups. During contraction venous PCO2 measured during the 1st and 2nd minute (25 ± 2 mmHg) of contraction did not differ from the measurement made between minute 4 and the end of the contraction period (25.5 ± 2 mmHg). There were no differences in venous PCO2 between groups during contraction (DCA group, 26.2 ± 2.5 and 27.1 ± 2 mmHg, respectively).
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As presented previously, DCA pretreatment significantly increases the concentration of acetylcarnitine prior to contraction. There was a clear demarcation in the effects observed with DCA following 1 min of contraction compared with the metabolic profile observed at the end of the stimulation protocol. PCr degradation and lactate accumulation were reduced by 60% and 30%, respectively, after 1 min of the contraction following DCA pretreatment (Table 2). This was under conditions of identical force production and hence presumably ATP turnover. However, at the end of the stimulation period the concentration of PCr and lactate in the DCA group did not differ from the control group. By the end of the stimulation period the acetylcarnitine concentration did not differ between groups. The sum of the measured tricarboxylic acid (TCA) intermediates (citrate + malate + fumarate) did not differ between groups although numerically the concentration tended to be lower at all time points following DCA pretreatment.
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Discussion |
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90% peak whole-body O2 uptake for 90 s, where the rate of PCr degradation was 2–4 times more rapid than in our studies (Timmons et al. 1997, 1998a; Roberts et al. 2002). Indeed, in the studies that have failed to find any evidence for the acetyl group deficit (Howlett et al. 1999b; Grassi et al. 2002; Bangsbo et al. 2002) the average rate of PCr degradation over the first minute of exercise exceeded 1 mmol (kg dry mass)–1 s–1 (Table 3). Interestingly, in both the studies by Howlett et al. (1999a) and Bangsbo et al. (2002) an exercise protocol was used that involved the subjects pausing during the exercise protocol for a muscle biopsy, before undergoing a ‘second’ rest-to-work transition. We have previously explained (Timmons et al. 1998b) that this removes the treatment effect of DCA, since both ‘prior exercise’ and DCA activate PDC. Our present study combined with our meta-analysis confirms that when the initial rate of PCr exceeds a particular threshold, priming oxidative metabolism may not be readily measurable. Exercise protocols which do not rely on a large mitochondrial contribution to ATP regeneration, are unlikely to be influenced by acetyl group availability. Equally, this threshold or ‘break point’ will vary with the training status of the subjects or the intrinsic mitochondrial capacity. Thus, careful consideration must be given to both the exercise model, subject characteristics and the study design, before any reliable conclusions can be reached.
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As with previous studies, DCA was the tool used in the present study to increase muscle acetyl group availability. DCA is a weak (IC50 > 200 µM) yet selective inhibitor of the kinase pyruvate dehydroxygenase kinase (PDHK) that inactivates PDC. The selectivity profile of DCA for PDHK (over unrelated kinases) is high since DCA is not an ATP analogue. Furthermore, the chronic effect of the di-chloro component of DCA structure has questionable relevance for the short-term administration to skeletal muscle or muscle bioenergetics. It is expected, however, that PDC activation, using DCA or any other PDC activator, will alter systemic pyruvate concentrations and thus influence other metabolites, such as the Krebs cycle intermediates. Indeed, we studied such an effect in the present study and no connection with DCA's effect on PCr sparing was found (Table 2). Such changes are not, however, additional pharmacological actions of DCA and hence when critiquing the usefulness of this experimental tool (Rossiter et al. 2003) greater care should be taken to fully appreciate what reflects the pharmacology of DCA and what simply reflects the biochemical consequences of PDC activation.
The physiological mechanisms responsible for PDC activation in human skeletal muscle are not well characterized. For example, maximal exercise results in a greater PDC activation than submaximal exercise (Constantin-Teodosiu et al. 1991; Howlett et al. 1999a). This probably reflects the greater muscle fibre recruitment, rather than a hypothetical (Spriet & Heigenhauser, 2002) increase in [Ca2+] within already contracting myocytes, as fibre recruitment rather than rate modulation of motor units is utilized by humans during exercise involving large muscle groups. Exemplification of this lack of necessity for a change in [Ca2+] can also be found in the observation that DCA fully activates PDC in resting skeletal muscle (Timmons et al. 1996a). Overall, it is not clear why PDC activation is suboptimal under physiological conditions, especially since full PDC activation does not cause acute hypoglycaemia in humans (Timmons et al. 1998b). It is plausible that at some point in the evolution of metabolic control, excessive PDC activation compromised glucose availability and hence a complex system of four inactivating kinases emerged to provide a survival benefit.
Metabolic control and muscle fatigue development
Hogan et al. (1994) concluded that the initial rate of fatigue development in the contracting gastrocnemius muscle directly related to reduced oxygen availability. This might appear at odds with our earlier study, where the development of muscle fatigue appeared largely due to the metabolic consequences of the acetyl group deficit (Timmons et al. 1997). However, studies by Hogan and colleagues altered oxygen delivery only after the muscle had reached a steady-state force production (Hogan et al. 1994, 1998a,b; Haseler et al. 1998). Thus, under these two conditions, different determinants of muscle fatigue are being studied. We feel that our model is more representative of every day physical activity patterns (Girard et al. 2001) but both models have provided important insight into metabolic control during muscle contraction. If during submaximal exercise, a significant proportion of the OD can be avoided, then this may have performance-enhancing implications for muscle function under low blood flow conditions. DCA is not an ideal long-term pharmacological agent as, for example, it demonstrates a poor pharmacokinetic profile. As newer generation PDC activators emerge (Mayers et al. 2003), with optimized pharmacological properties, then the time will come when the clinical utility of PDC activation for treating exercise intolerance in cardiovascular disease patients can be reasonably addressed.
Recently, Grassi et al. (2002) were able to demonstrate that DCA improved muscle function independently of altered metabolism. It occurred to us that aspects of metabolic control may differ during the rest-to-work transition period if tetanic stimulation was used (rather than twitch contractions). We used a tetanic stimulation protocol that achieved a steady-state oxygen consumption comparable with our previous twitch models (e.g. 3.4–4.2 ml min (100 g wet tissue)–1). Intramuscular lactate concentration and muscle lactate efflux (e.g. 15.3 versus 14.1 µmol min (100 g wet tissue)–1) were also similar to our earlier canine studies. However, despite a comparable steady-state oxidative flux, [PCr] was approximately half of that found during twitch contraction (Timmons et al. 1996b, 1997) indicating that metabolic control of respiration differed. In a previous study, steady-state [PCr] was achieved within 1 min of contraction, following DCA pretreatment (Timmons et al. 1997). In the present study DCA attenuated PCr utilization during the first minute of contraction; however, [PCr] continued to decline until the 6th minute. It is therefore apparent that during tetanic stimulation, the time taken to reach a steady-state [PCr] must be substantially longer than during a twitch protocol (Timmons et al. 1997) despite similar steady-state oxygen consumption. Nevertheless, it would seem clear that DCA pretreatment will not reduce ischaemic muscle fatigue when the final metabolic profile of the muscle is unaltered.
Determinants of SLP – the oxygen deficit versus the acetyl group deficit
Until recently it was believed that the major factor which limited the rate of increase in oxidative phosphorylation during the rest-to-work transition period was oxygen availability (see Hughson et al. 2001). During submaximal contraction, i.e. below peak 
, neither peripheral oxygen diffusion (Grassi et al. 1998a) nor oxygen delivery kinetics (Grassi et al. 1998b) limits oxygen consumption kinetics during the rest-to-work period. Unfortunately, none of these studies measured muscle [PCr] which would have directly demonstrated that muscle SLP was unaltered by enhanced oxygen delivery. Specific mention must also be given to the study by Linnarsson et al. (1974) since it is the most cited evidence that oxygen delivery limits mitochondrial ATP regeneration at the onset of exercise. In this study, hyperbaric hyperoxia did not significantly alter PCr utilization when compared with normoxia, despite extensive citation stating otherwise (Macdonald et al. 1997; Grassi et al. 1998b; Tschakovsky & Hughson, 1999; Parolin et al. 2000; Savasi et al. 2002). Furthermore, it is not possible to determine if the study by Linnarsson et al. (1974) used an appropriate randomized experimental design, where clearly such factors would impact on the lactate production (e.g. glycogen status or experimental duration) and PCr metabolism. It is also difficult to understand why the magnitude of the effect observed on muscle lactate was identical (in opposite directions of course) for both hyperbaric hyperoxia and hypobaric hypoxia – yet clearly hypobaric hypoxia would have a greater effect on blood oxygen content due to the characteristics of the oxyhaemoglobin curve. In more recent studies, hyperoxia reduced the ‘respiratory’ OD during the 3rd to 6th minute of the transition period (Macdonald et al. 1997) but had no effect on intramuscular PCr utilization (Savasi et al. 2002). This suggests that respiratory gas exchange during non-steady-state conditions may not reliably reflect the metabolic status of the contracting muscle tissue.
In contrast to these submaximal studies it was demonstrated that when a canine gastrocnemius preparation was stimulated at peak 
, a more rapid adjustment of blood flow during the rest-to-work transition period reduced the OD (calculated from gas exchange) and muscle fatigue (Grassi et al. 2000). However, there are some problems with the interpretation of this observation. Firstly, in the study by Grassi et al. (2000) the ‘high blood flow’ experimental period always followed the ‘normal blood flow’ period and was accompanied by intra-arterial adenosine infusion. It is also important to appreciate that a minimum recovery period between two successive contraction periods, that leaves acetyl group availability (or the rate of PDC activation) unaffected by the prior exercise, has not been established. It therefore still remains to be seen if oxygen delivery kinetics regulate 
adjustment during rest to peak oxidative flux conditions.
To date, little discussion has been given to the potential source of the ‘extra’ oxygen consumed to off-set the reduction in SLP following DCA administration. Assumptions that it can be measured at the level of respiratory gases and/or across the exercising limbs may be sanguine (Grassi et al. 2002; Rossiter et al. 2003; Koppo et al. 2004). The molar equivalent of molecular oxygen required to ‘compensate’ for the reduced SLP will be modest (due to the P : O ratio being estimated as high as 6). Calculations using the molar volume of oxygen (22.41 ml mmol–1) need also to consider that oxygen is partially dissolved and not exposed to standard temperature–pressure conditions. It is therefore unclear how to accurately translate a molar equivalent of oxygen into an accurate volume of oxygen and conclude that it is measurable under such conditions. The study by Rossiter et al. (2003) also highlights that DCA alters the characteristics of the 
–workload relationship creating further doubt over the applicability of standard analysis methodologies. If PDC activation increases carbohydrate oxidation, then up to 12% less oxygen utilization is required for the same ATP regeneration when using lipid as the fuel. A recent study highlighted several problems associated with the OD calculation, derived from respiratory gas values (Özyener et al. 2003), further indicating that it is not a useful approach to studying the acetyl group deficit. One further recent study (Koppo et al. 2004) failed to design the study to ensure that PDC activation differed between control and DCA-treated conditions (they could not measure PDC status but plasma lactate concentrations during exercise supported the contention that no substantial ‘metabolic’ intervention took place) highlighting the importance of a rational experimental design when investigating the OD.
A recent study demonstrated that the decline in intramuscular PCO2 in a skinned muscle fibre preparation was accelerated during the rest-to-work period following DCA administration (Howlett & Hogan, 2003). Using the Xenopus single fibre preparation, devoid of myoglobin and therefore excluding any complication with this oxygen ‘store’, they were able to provide evidence for more rapid oxygen consumption following DCA administration. In humans, myoglobin represents an oxygen reservoir which is not necessarily in proportional equilibrium with venous saturation during submaximal exercise (Richardson et al. 2001). Given the modest amount of oxygen required to explain the reduction in SLP it is also possible that a small contribution from muscle oxygen stores further limits the possibility of detecting a difference using current in vivo measurement techniques (Grassi et al. 2002; Rossiter et al. 2003).
Conclusions
The present results support the idea that when the initial PCr degradation rate is 1 mmol (kg dry mass)–1 s–1 or less, acetyl group availability measurably impacts on SLP at the onset of exercise, when an early muscle biopsy is obtained. The present data suggest that if the stimulation intensity exceeds the maximum rate of increase in mitochondrial ATP regeneration, prior PDC activation will not prevent [PCr] tending towards zero, nor will it enhance ischaemic skeletal muscle function. Future studies aimed at measuring the net increase in oxygen consumption following PDC activation by DCA must appreciate that the measurement of oxygen mass balance across the muscle (or at the mouth) may not be sufficient. Overall, we would argue that careful examination of the published literature (Table 3), coupled with the present data set, demonstrates that acetyl group deficit is a widely applicable limitation to oxidative phosphorylation during the transition from rest to exercise. This also suggests that the acetyl group deficit may be an important physiological determinant of exercise capacity in health and disease.
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References |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Bergmeyer HU (1974). Methods in Enzymatic Analysis, vol 1, 2nd edn. Academic Press, New York.
Constantin-Teodosiu D, Carlin JI, Cederblad G, Harris RC & Hultman E (1991). Acetyl group accumulation and pyruvate dehydrogenase activity in human muscle during incremental exercise. Acta Physiol Scand 143, 367–372.[Medline]
Constantin-Teodosiu
D, Howell
S
&
Greenhaff
PL (1996). Carnitine metabolism in human muscle fiber types during submaximal dynamic exercise. J Appl Physiol
80, 1061–1064.
Cederblad G, Carlin JI, Constantin-Teodosiu D, Harper P & Hultman E (1990). Radioiostopic assays of CoASH and carnitine and their acetylated forms in human skeletal muscle. Anal Biochem 185, 274–278.[CrossRef][Medline]
Evans
MK, Savasi
I, Heigenhauser
GJF
&
Spriet
LL (2001). Effects of acetate infusion and hyperoxia on muscle substrate phosphorylation after onset of moderate exercise. Am J Physiol Endocrinol Metab
281, E1144–E1150.
Gibala MJ & Saltin B (1999). PDH activation by dichloroacetate reduces TCA cycle intermediates at rest but not during exercise in human. Am J Physiol 277, E33–E38.[Medline]
Girard I, McAleer MW, Rhodes JS & Garland T (2001). Selection for high voluntary wheel-running increases speed and intermittency in house mice (Mus domesticus) J Exp Biol 204, 4311–4320.
Grassi
B, Gladden
LB, Samaja
M, Stary
CM
&
Hogan
MC (1998b). Faster adjustment of O2 delivery does not affect on-kinetics in isolated in situ canine muscle. J Appl Physiol
85, 1394–1403.
Grassi
B, Gladden
LB, Stary
CM, Wagner
PD
&
Hogan
MC (1998a). Peripheral O2 diffusion does not affect VO2 on-kinetics in isolated in situ muscle. J Appl Physiol
85, 1404–1412.
Grassi
B, Hogan
MC, Greenhaff
PL, Hamann
JJ, Kelley
KM, Aschenbach
WG
et al. (2002). Oxygen uptake on-kinetics in dog gastrocnemius in situ following activation of pyruvate dehydrogenase by dichloroacetate. J Physiol
538, 195–207.
Grassi
B, Hogan
MC, Kelley
KM, Aschenbach
WG, Hamann
JJ, Evans
RK
et al. (2000). Role of convective O2 delivery in determining VO2 on-kinetics in canine muscle contracting at peak VO2. J Appl Physiol
89, 1293–1301.
Grassi
B, Poole
DC, Richardson
RS, Knight
DR, Erickson
BK
&
Wagner
PD (1996). Muscle O2 uptake kinetics in humans: implications for metabolic control. J Appl Physiol
80, 988–998.
Greenhaff PL & Timmons JA (1998). Interaction between aerobic and anaerobic metabolism during intense muscle contraction. Exerc Sport Sci Rev 26, 1–30.[Medline]
Harris RC, Hultman E & Nordesjö LO (1974). Glycogen, glycolytic intermediates and high energy phosphates determined in biopsy samples of musculus femoris of man at rest. Methods and variance values. Scand J Clin Laboratory Invest 33, 109–120.[Medline]
Haseler
LJ, Richardson
RS, Videen
JS
&
Hogan
MC (1998). Phosphocreatine hydrolysis during submaximal exercise: the effect of FIO2. J Appl Physiol
85, 1457–1463.
Hogan
MC, Gladden
LB, Grassi
B, Stary
CM
&
Samaja
M (1998a). Bioenergetics of contracting skeletal muscle after partial reduction in blood flow. J Appl Physiol
84, 1882–1888.
Hogan MC, Ingham E & Kurdak SS (1998b). Contraction duration affects metabolic energy cost and fatigue in skeletal muscle. Am J Physiol 274, E397–E402.[Medline]
Hogan
MC, Richardson
RS
&
Kurdak
SS (1994). Initial fall in skeletal muscle force development during ischemia is related to oxygen availability. J Appl Physiol
77, 2380–2384.
Howlett RA, Heigenhauser GJF, Hultman E, Hollidge-Horvat MG & Spriet LL (1999a). Effects of dichloroacetate infusion on human skeletal muscle metabolism at the onset of exercise. Am J Physiol 277, E18–E25.[Medline]
Howlett
RA, Heigenhauser
GJF
&
Spriet
LL (1999b). Skeletal muscle metabolism during high-intensity sprint exercise in unaffected by dichloroacetate or acetate infusion. J Appl Physiol
87, 1747–1751.
Howlett
RA
&
Hogan
MC (2003). Dichloroacetate accelerates the fall in intracellular PO2 at the onset of contractions in Xenopus single muscle fibers. Am J Physiol Regul Integr Comp Physiol
284, R481–R485.
Hughson RL, Tschakovsky ME & Houston ME (2001). Regulation of oxygen consumption at the onset of exercise. Exerc Sport Sci Rev 29, 129–133.[CrossRef][Medline]
Koppo K, Wilkerson DP, Bouckaert J, Wilmshurst S, Campbell IT & Jones AM (2004). Influence of DCA on pulmonary VO2 kinetics during moderate-intensity cycle exercise. Med Sci Sports Exerc 36, 1159–1164.
Krogh
A
&
Lindhard
J (1920). The changes in respiration at the transition from work to rest. J Physiol
53, 431–439.
Linnarsson
D, Karlsson
J, Fagraeus L &
Saltin
B (1974). Muscle metabolites and oxygen deficit with exercise in hypoxia and hyperoxia. J Appl Physiol
36, 399–402.
Macdonald
M, Pedersen
PK
&
Hughson
RL (1997). Acceleration of VO2 kinetics in heavy submaximal exercise by hyperoxia and prior high-intensity exercise. J Appl Physiol
83, 1318–1325.
Margaria
R, Cerretelli
P, Di Prampero
PE, Massari
C
&
Torelli
G (1963). Kinetics and mechanism of oxygen debt contraction in man. J Appl Physiol
18, 371–377.
Mayers RM, Butlin RJ, Kilgour E, Leighton B, Martin D, Myatt J et al. (2003). AZD7545, a novel inhibitor of pyruvate dehydrogenase kinase 2 (PDHK2), activates pyruvate dehydrogenase in vivo and improves blood glucose control in obese (fa/fa) Zucker rats. Biochem Soc Trans 31, 1165–1167.[Medline]
Özyener F, Rossiter HB, Ward SA & Whipp BJ (2003). Negative accumulated oxygen deficit during heavy and very heavy intensity cycle ergometry in humans. Eur J Appl Physiol 90, 185–190.[CrossRef][Medline]
Parolin
ML, Spriet
LL, Hultman
E, Matsos
MP, Hollidge-Horvat
MG, Jones
NL
et al. (2000). Effects of PDH activation by dichloroacetate in human skeletal muscle during exercise in hypoxia. Am J Physiol Endocrinol Metab
279, E752–E761.
Richardson
RS, Newcomer
SC
&
Noyszewski
EA (2001). Skeletal muscle intracellular PO2 assessed by myoglobin desaturation: response to graded exercise. J Appl Physiol
91, 2679–2685.
Roberts
PA, Loxham
SJG, Poucher
SM, Constanti-Teodosiu
D
&
Greenhaff
PL (2002). The acetyl group deficit at the onset of contraction in ischaemic canine skeletal muscle. J Physiol
544, 591–602.
Rossiter
HB, Ward
SA, Howe
FA, Wood
DM, Kowalchuk
JM, Griffiths
JR
&
Whip
BJ (2003). Effects of dichloroacetate on VO2 and intramuscular 31P metabolite kinetics during high intensity exercise in humans. J Appl Physiol
95, 1105–1115.
Sahlin K, Ren JM & Broberg S (1988). Oxygen deficit at the onset of submaximal exercise is not due to a delayed oxygen transport. Acta Physiol Scand 134, 175–180.[Medline]
Saltin B (1990). Anaerobic capacity: past, present and prospective. In International Series on Sport Science, vol. 21, Biochemistry of Exercise VII, ed. Taylor AW, Gollnick PD, Green HJ, Ianuzzo CD, Noble EG, Métivier G & Sutton JR, pp. 387–421. Human Kinetics.
Savasi
I, Evans
MK, Heigenhauser
GJF
&
Spriet
LL (2002). Skeletal muscle metabolism is unaffected by DCA infusion and hyperoxia after onset of intense aerobic exercise. Am J Physiol Endocrinol Metab
283, E108–E115.
Spriet LL & Heigenhauser GJF (2002). Regulation of pyruvate dehydrogenase (PDH) activity in human skeletal muscle during exercise Exerc Sport Sci Rev 30, 91–95.[CrossRef][Medline]
Timmons JA, Gustafsson T, Sundberg CJ, Jansson E & Greenhaff PL (1998a). Acetyl group availability is a major determinant of the oxygen deficit in human skeletal muscle during submaximal exercise. Am J Physiol 274, E377–E380.[Medline]
Timmons JA, Gustafsson T, Sundberg CJ, Jansson E, Hultman E, Kaijser L et al. (1998b). Acetyl group availability limits oxidative ATP regeneration at the onset of skeletal muscle contraction in humans. J Clin Invest 101, 79–85.[Medline]
Timmons JA, Poucher SM, Constantin-Teodosiu D, Macdonald IA & Greenhaff PL (1997). The metabolic responses from rest to steady state determine contractile function in ischemic skeletal muscle. Am J Physiol 273, E233–E238.[Medline]
Timmons JA, Poucher SM, Constantin-Teodosiu D, Worrall V, Macdonald IA & Greenhaff PL (1996a). Increased acetyl group availability enhances contractile function of canine skeletal muscle during ischemia. J Clin Invest 97, 879–883.[Medline]
Timmons JA, Poucher SM, Constantin-Teodosiu D, Worrall V, Macdonald IA & Greenhaff PL (1996b). The metabolic responses of canine gracilis muscle during contraction with partial ischemia. Am J Physiol 270, E400–E406.[Medline]
Tschakovsky
ME
&
Hughson
RL (1999). Interaction of factors determining oxygen uptake at the onset of exercise. J Appl Physiol
86, 1101–1113.
Watt
MJ, Heigenhauser
GJF, Stellingwerff Hargreaves
M
&
Spriet
LL (2002). Carbohydrate ingestion reduces skeletal muscle acetylcarnitine availability but has no effect on substrate phosphorylation at the onset of exercise in man. J Physiol
544, 949–956.
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