Enhanced pyruvate dehydrogenase activity does not affect muscle O2 uptake at onset of intense exercise in humans

How Priming Exercise Affects Oxygen Uptake Kinetics: From Underpinning Mechanisms to Endurance Performance

The observation that prior heavy or severe-intensity exercise speeds overall oxygen uptake ([Formula: see text]O₂) kinetics, termed the "priming effect", has garnered significant research attention and its underpinning mechanisms have been hotly debated. In the first part of this review, the evidence for and against (1) lactic acidosis, (2) increased muscle temperature, (3) O₂ delivery, (4) altered motor unit recruitment patterns and (5) enhanced intracellular O₂ utilisation in underpinning the priming effect is discussed. Lactic acidosis and increased muscle temperature are most likely not key determinants of the priming effect. Whilst priming increases muscle O₂ delivery, many studies have demonstrated that an increased muscle O₂ delivery is not a prerequisite for the priming effect. Motor unit recruitment patterns are altered by prior exercise, and these alterations are consistent with some of the observed changes in [Formula: see text]O₂ kinetics in humans. Enhancements in intracellular O₂ utilisation likely play a central role in mediating the priming effect, probably related to elevated mitochondrial calcium levels and parallel activation of mitochondrial enzymes at the onset of the second bout. In the latter portion of the review, the implications of priming on the parameters of the power-duration relationship are discussed. The effect of priming on subsequent endurance performance depends critically upon which phases of the [Formula: see text]O₂ response are altered. A reduced [Formula: see text]O₂ slow component or increased fundamental phase amplitude tend to increase the work performable above critical power (i.e. W´), whereas a reduction in the fundamental phase time constant following priming results in an increased critical power.

An integrative approach to the regulation of mitochondrial respiration during exercise: Focus on high-intensity exercise

During exercise, muscle ATP demand increases with intensity, and at the highest power output, ATP consumption may increase more than 100-fold above the resting level. The rate of mitochondrial ATP production during exercise depends on the availability of O₂, carbon substrates, reducing equivalents, ADP, P_i, free creatine, and Ca²⁺. It may also be modulated by acidosis, nitric oxide and reactive oxygen and nitrogen species (RONS). During fatiguing and repeated sprint exercise, RONS production may cause oxidative stress and damage to cellular structures and may reduce mitochondrial efficiency. Human studies indicate that the relatively low mitochondrial respiratory rates observed during sprint exercise are not due to lack of O₂, or insufficient provision of Ca²⁺, reduced equivalents or carbon substrates, being a suboptimal stimulation by ADP the most plausible explanation. Recent in vitro studies with isolated skeletal muscle mitochondria, studied in conditions mimicking different exercise intensities, indicate that ROS production during aerobic exercise amounts to 1-2 orders of magnitude lower than previously thought. In this review, we will focus on the mechanisms regulating mitochondrial respiration, particularly during high-intensity exercise. We will analyze the factors that limit mitochondrial respiration and those that determine mitochondrial efficiency during exercise. Lastly, the differences in mitochondrial respiration between men and women will be addressed.

Fitness Level and Not Aging per se, Determines the Oxygen Uptake Kinetics Response

Although aging has been associated to slower V˙O₂ kinetics, some evidence indicates that fitness status and not aging per se might modulate this response. The main goal of this study was to examine the V˙O₂, deoxygenated hemoglobin+myoglobin (deoxy-[Hb+Mb]) kinetics, and the NIRS-derived vascular reperfusion responses in older compared to young men of different training levels (i.e., inactive, recreationally active, and endurance trained). Ten young inactive [YI; 26 ± 5 yrs.; peak V˙O₂ (V˙O_2peak), 2.96 ± 0.55 L·min⁻¹], 10 young recreationally active (YR; 26 ± 6 yrs.; 3.92 ± 0.33 L·min⁻¹), 10 young endurance trained (YT; 30 ± 4 yrs.; 4.42 ± 0.32 L·min⁻¹), 7 older inactive (OI; 69 ± 4 yrs.; 2.50 ± 0.31 L·min⁻¹), 10 older recreationally active (OR; 69 ± 5 yrs.; 2.71 ± 0.42 L·min⁻¹), and 10 older endurance trained (OT; 66 ± 3 yrs.; 3.20 ± 0.35 L·min⁻¹) men completed transitions of moderate intensity cycling exercise (MODS) to determine V˙O₂ and deoxy-[Hb+Mb] kinetics, and the deoxy-[Hb+Mb]/V˙O₂ ratio. The time constant of V˙O₂ (τV˙O₂) was greater in YI (38.8 ± 10.4 s) and OI (44.1 ± 10.8 s) compared with YR (26.8 ± 7.5 s) and OR (26.6 ± 6.5 s), as well as compared to YT (14.8 ± 3.4 s), and OT (17.7 ± 2.7 s) (p < 0.05). τV˙O₂ was greater in YR and OR compared with YT and OT (p < 0.05). The deoxy-[Hb+Mb]/V˙O₂ ratio was greater in YI (1.23 ± 0.05) and OI (1.29 ± 0.08) compared with YR (1.11 ± 0.03) and OR (1.13 ± 0.06), as well as compared to YT (1.01 ± 0.03), and OT (1.06 ± 0.03) (p < 0.05). Similarly, the deoxy-[Hb+Mb]/ V˙O₂ ratio was greater in YR and OR compared with YT and OT (p < 0.05). There was a main effect of training (p = 0.033), whereby inactive (p = 0.018) and recreationally active men (p = 0.031) had significantly poorer vascular reperfusion than endurance trained men regardless of age. This study demonstrated not only that age-related slowing of V˙O₂ kinetics can be eliminated in endurance trained individuals, but also that inactive lifestyle negatively impacts the V˙O₂ kinetics response of young healthy individuals.

Comparative NMR and NIRS analysis of oxygen-dependent metabolism in exercising finger flexor muscles

Muscle contraction requires the physiology to adapt rapidly to meet the surge in energy demand. To investigate the shift in metabolic control, especially between oxygen and metabolism, researchers often depend on near-infrared spectroscopy (NIRS) to measure noninvasively the tissue O₂ Because NIRS detects the overlapping myoglobin (Mb) and hemoglobin (Hb) signals in muscle, interpreting the data as an index of cellular or vascular O₂ requires deconvoluting the relative contribution. Currently, many in the NIRS field ascribe the signal to Hb. In contrast, ¹H NMR has only detected the Mb signal in contracting muscle, and comparative NIRS and NMR experiments indicate a predominant Mb contribution. The present study has examined the question of the NIRS signal origin by measuring simultaneously the ¹H NMR, ³¹P NMR, and NIRS signals in finger flexor muscles during the transition from rest to contraction, recovery, ischemia, and reperfusion. The experiment results confirm a predominant Mb contribution to the NIRS signal from muscle. Given the NMR and NIRS corroborated changes in the intracellular O₂, the analysis shows that at the onset of muscle contraction, O₂ declines immediately and reaches new steady states as contraction intensity rises. Moreover, lactate formation increases even under quite aerobic condition.

Hyperpolarized 13C NMR observation of lactate kinetics in skeletal muscle

The production of glycolytic end products, such as lactate, usually evokes a cellular shift from aerobic to anaerobic ATP generation and O2 insufficiency. In the classical view, muscle lactate must be exported to the liver for clearance. However, lactate also forms under well-oxygenated conditions, and this has led investigators to postulate lactate shuttling from non-oxidative to oxidative muscle fiber, where it can serve as a precursor. Indeed, the intracellular lactate shuttle and the glycogen shunt hypotheses expand the vision to include a dynamic mobilization and utilization of lactate during a muscle contraction cycle. Testing the tenability of these provocative ideas during a rapid contraction cycle has posed a technical challenge. The present study reports the use of hyperpolarized [1-(13)C]lactate and [2-(13)C]pyruvate in dynamic nuclear polarization (DNP) NMR experiments to measure the rapid pyruvate and lactate kinetics in rat muscle. With a 3 s temporal resolution, (13)C DNP NMR detects both [1-(13)C]lactate and [2-(13)C]pyruvate kinetics in muscle. Infusion of dichloroacetate stimulates pyruvate dehydrogenase activity and shifts the kinetics toward oxidative metabolism. Bicarbonate formation from [1-(13)C]lactate increases sharply and acetyl-l-carnitine, acetoacetate and glutamate levels also rise. Such a quick mobilization of pyruvate and lactate toward oxidative metabolism supports the postulated role of lactate in the glycogen shunt and the intracellular lactate shuttle models. The study thus introduces an innovative DNP approach to measure metabolite transients, which will help delineate the cellular and physiological role of lactate and glycolytic end products.

Quantification of skeletal muscle mitochondrial function by 31P magnetic resonance spectroscopy techniques: a quantitative review

Magnetic resonance spectroscopy (MRS) can give information about cellular metabolism in vivo which is difficult to obtain in other ways. In skeletal muscle, non-invasive (31) P MRS measurements of the post-exercise recovery kinetics of pH, [PCr], [Pi] and [ADP] contain valuable information about muscle mitochondrial function and cellular pH homeostasis in vivo, but quantitative interpretation depends on understanding the underlying physiology. Here, by giving examples of the analysis of (31) P MRS recovery data, by some simple computational simulation, and by extensively comparing data from published studies using both (31) P MRS and invasive direct measurements of muscle O2 consumption in a common analytical framework, we consider what can be learnt quantitatively about mitochondrial metabolism in skeletal muscle using MRS-based methodology. We explore some technical and conceptual limitations of current methods, and point out some aspects of the physiology which are still incompletely understood.

Single sodium pyruvate ingestion modifies blood acid-base status and post-exercise lactate concentration in humans

This study examined the effect of a single sodium pyruvate ingestion on a blood acid-base status and exercise metabolism markers. Nine active, but non-specifically trained, male subjects participated in the double-blind, placebo-controlled, crossover study. One hour prior to the exercise, subjects ingested either 0.1 g·kg⁻¹ of body mass of a sodium pyruvate or placebo. The capillary blood samples were obtained at rest, 60 min after ingestion, and then three and 15 min after completing the workout protocol to analyze acid-base status and lactate, pyruvate, alanine, glucose concentrations. The pulmonary gas exchange, minute ventilation and the heart rate were measured during the exercise at a constant power output, corresponding to ~90% O₂max. The blood pH, bicarbonate and the base excess were significantly higher after sodium pyruvate ingestion than in the placebo trial. The blood lactate concentration was not different after the ingestion, but the post-exercise was significantly higher in the pyruvate trial (12.9 ± 0.9 mM) than in the placebo trial (10.6 ± 0.3 mM, p < 0.05) and remained elevated (nonsignificant) after 15 min of recovery. The blood pyruvate, alanine and glucose concentrations, as well as the overall pulmonary gas exchange during the exercise were not affected by the pyruvate ingestion. In conclusion, the sodium pyruvate ingestion one hour before workout modified the blood acid-base status and the lactate production during the exercise.

Oxygen uptake kinetics

Muscular exercise requires transitions to and from metabolic rates often exceeding an order of magnitude above resting and places prodigious demands on the oxidative machinery and O2-transport pathway. The science of kinetics seeks to characterize the dynamic profiles of the respiratory, cardiovascular, and muscular systems and their integration to resolve the essential control mechanisms of muscle energetics and oxidative function: a goal not feasible using the steady-state response. Essential features of the O2 uptake (VO2) kinetics response are highly conserved across the animal kingdom. For a given metabolic demand, fast VO2 kinetics mandates a smaller O2 deficit, less substrate-level phosphorylation and high exercise tolerance. By the same token, slow VO2 kinetics incurs a high O2 deficit, presents a greater challenge to homeostasis and presages poor exercise tolerance. Compelling evidence supports that, in healthy individuals walking, running, or cycling upright, VO2 kinetics control resides within the exercising muscle(s) and is therefore not dependent upon, or limited by, upstream O2-transport systems. However, disease, aging, and other imposed constraints may redistribute VO2 kinetics control more proximally within the O2-transport system. Greater understanding of VO2 kinetics control and, in particular, its relation to the plasticity of the O2-transport/utilization system is considered important for improving the human condition, not just in athletic populations, but crucially for patients suffering from pathologically slowed VO2 kinetics as well as the burgeoning elderly population.

Exercise: Kinetic considerations for gas exchange

The activities of daily living typically occur at metabolic rates below the maximum rate of aerobic energy production. Such activity is characteristic of the nonsteady state, where energy demands, and consequential physiological responses, are in constant flux. The dynamics of the integrated physiological processes during these activities determine the degree to which exercise can be supported through rates of O₂ utilization and CO₂ clearance appropriate for their demands and, as such, provide a physiological framework for the notion of exercise intensity. The rate at which O₂ exchange responds to meet the changing energy demands of exercise--its kinetics--is dependent on the ability of the pulmonary, circulatory, and muscle bioenergetic systems to respond appropriately. Slow response kinetics in pulmonary O₂ uptake predispose toward a greater necessity for substrate-level energy supply, processes that are limited in their capacity, challenge system homeostasis and hence contribute to exercise intolerance. This review provides a physiological systems perspective of pulmonary gas exchange kinetics: from an integrative view on the control of muscle oxygen consumption kinetics to the dissociation of cellular respiration from its pulmonary expression by the circulatory dynamics and the gas capacitance of the lungs, blood, and tissues. The intensity dependence of gas exchange kinetics is discussed in relation to constant, intermittent, and ramped work rate changes. The influence of heterogeneity in the kinetic matching of O₂ delivery to utilization is presented in reference to exercise tolerance in endurance-trained athletes, the elderly, and patients with chronic heart or lung disease.

Mitochondrial activation at the onset of contractions in isolated myofibres during successive contractile periods

At the onset of skeletal muscle repetitive contractions, there is a significant delay in the time to achieve oxidative phosphorylation steady state. The purpose of the present study was to examine the factors that limit oxidative phosphorylation at the onset of contractions. NAD(P)H was measured in real time during two contractile periods (2 min each) separated by 5 min of rest in intact single muscle fibres (n = 7) isolated from Xenopus laevis. The fibres were then loaded with the dye tetramethylrhodamine methyl ester perchlorate (TMRM) to evaluate the kinetics of the mitochondrial membrane potential (Δψ (m)) during two further successive contractile periods. At the onset of contractions in the first period, NAD(P)H exhibited a time delay (14.1 ± 1.3 s) before decreasing toward a steady state. In contrast, Δψ(m) decreased immediately after the first contraction and started to be reestablished after 10.7 ± 0.9 s, with restoration to the pre-stimulation values after approximately 32 s. In the second contractile period (5 min after the first), NAD(P)H decreased immediately (i.e. no time delay) after the first contraction and had a significantly shorter time constant compared to the first contractile bout (3.3 ± 0.3 vs. 5.0 ± 0.2 s, P < 0.05). During the second bout, Δψ(m) remained unchanged from pre-stimulation values. These results suggest: (1) that at the onset of contractions, oxidative phosphorylation is primarily limited by the activity of the electron transport chain complexes rather than by a limited level of substrates; and (2) when the muscle is 'primed' by previous contractile activity, the faster enhancement of the cellular respiratory rate is due to intrinsic factors within the myofibre.

VO2 kinetics and performance in soccer players after intense training and inactivity

PURPOSE

The study's purpose was to examine the effects of a short-term period with intensified training or training cessation of trained soccer players on VO(2) kinetics at 75% maximal aerobic speed, oxidative enzymes, and performance in repeated high-intensity exercise.

METHODS

After the last match of the season, 18 elite soccer players were, for a 2-wk period, assigned to a high-intensity training group (n = 7) performing 10 training sessions mainly consisting of aerobic high-intensity training (8 × 2 min) and speed endurance training (10-12 × 30-s sprints) or a training cessation group (n = 11) that refrained from training.

RESULTS

For the training cessation group, VO(2) kinetics became slower (P < 0.05) with a larger time constant (τ = 21.5 ± 2.9 vs 23.8 ± 3.2 s (mean ± SD, before vs after)) and a larger mean response time (time delay + τ = 45.0 ± 1.8 vs 46.8 ± 2.2 s). The amount of muscle pyruvate dehydrogenase (17%, P < 0.01) and maximal activity of citrate synthase (12%) and 3-hydroxyacyl-CoA (18%, P < 0.05) were lowered. In addition, the fraction of slow twitch fibers (56% ± 18% vs 47% ± 15%, P < 0.05), Yo-Yo intermittent recovery level 2 test (845 ± 160 vs 654 ± 99 m), and the repeated sprint performance (33.41 ± 0.96 vs 34.11 ± 0.92 s, P < 0.01) were reduced. For the high-intensity training group, running economy was improved (P < 0.05), and the amount of pyruvate dehydrogenase (17%) and repeated sprint performance (33.44 ± 1.17 vs 32.81 ± 1.01 s) were enhanced (P < 0.05).

CONCLUSIONS

Inactivity slows VO(2) kinetics in association with a reduction of muscle oxidative capacity and repeated high-intensity running performance. In addition, intensified training of already well-trained athletes can improve mechanical efficiency and repeated sprint performance.

Prolonged ischaemia impairs muscle blood flow and oxygen uptake dynamics during subsequent heavy exercise

Muscle oxygen uptake ( ˙VO₂,mus) dynamics at the onset of exercise can be affected by prior heavy exercise.We tested the hypothesis that elevated forearm blood flow (FBF) following prior circulatory occlusion would also be associated with accelerated ˙VO₂,mus dynamics during subsequent heavy hand-grip exercise. Ten trained young men performed 5 min of heavy hand-grip exercise at 30% MVC as a control (CON), and four additional heavy bouts after brief recovery from: (1) prior heavy exercise (Heavy A), (2) heavy exercise followed by 2 min occlusion (Heavy B), (3) 15 min occlusion (Heavy C), and (4) 5 min occlusion with 1 min of moderate exercise during occlusion (Heavy D). FBF was measured by ultrasound and arterial venous oxygen content difference was calculated from venous blood samples to estimate ˙VO₂,mus. FBF and ˙VO₂,mus dynamics were quantified from the rise time. All priming conditions elevated FBF immediately before the start of subsequent heavy bout (Heavy A: 207.4 ±92.8, B: 207.8±75.8, C: 135.8±59.2, D: 199.5±59.0 vs. CON: 57.4±16.6mlmin−1, P <0.01). Unexpectedly, prior occlusion reduced FBF and O2 extraction at the onset of subsequent heavy exercise and consequently slowed ˙VO₂,mus dynamics (Heavy C: rise time=95.9±28.9 vs. CON: 58.6±14.3 s, P <0.01). FBF and ˙VO₂,mus dynamics were faster in Heavy A, B and D compared to CON (P <0.05). Overall, there was a positive correlation between the rise times for ˙VO₂,mus and FBF (r² =0.75) indicating that ˙VO₂,mus dynamics during heavy forearm exercise are linked to O₂ delivery in trained young men. To investigate a possible mechanism for slower adaptation of ˙VO₂,mus following ischaemia, the prior occlusion condition was repeated after ingesting a high dose of ibuprofen. This resulted in restoration of the FBF and ˙VO₂,mus to control levels suggesting that a prostaglandin-mediated mechanism after occlusion retarded the adaptation of blood flow and oxygen consumption at the onset of subsequent heavy exercise.

Oxygen uptake kinetics: historical perspective and future directions

Oxygen uptake has been studied in the transitions between rest and exercise for more than 100 years, yet the mechanisms regulating the rate of increase in oxidative metabolism remain controversial. Some of the controversy is a consequence of incorrect interpretations of kinetic parameters describing amplitude and time constant relationships, whereas other factors relate to an incomplete framework for interpretation of experimental results. In this review, a new conceptual 3-dimensional model is proposed to explore the intracellular environment of skeletal muscle in the rest-to-exercise transition. The model incorporates the so-called “metabolic inertia” describing the increases in metabolic substrates and enzyme activation, along with the dynamic changes in intracellular partial pressure of oxygen (PO2). Considerable evidence exists during normal submaximal exercise challenges for an effect of changes in O2 delivery to working muscles affecting the intracellular PO2 (displayed on the x axis) and the high energy phosphate concentration (y axis) during steady-state exercise as well as the transitions from rest to exercise. The z axis incorporates a hypothetical description of metabolic inertia that is enhanced by increased enzyme activation and production of metabolic substrates. Specific examples are given that describe how this axis can affect oxygen uptake kinetics within the context of changing intracellular PO2 and energetic states. Oxidative metabolism at the onset of exercise is regulated by a dynamic balance of O2 transport and utilization mechanisms and is not limited solely by metabolic inertia.

Models of muscle contraction and energetics

How does skeletal muscle manage to regulate the pathways of ATP synthesis during large-scale changes in work rate while maintaining metabolic homeostasis remains unknown. The classic model of metabolic regulation during muscle contraction states that accelerating ATP utilization leads to increasing concentrations of ADP and Pi, which serve as substrates for oxidative phosphorylation and thus accelerate ATP synthesis. An alternative model states that both the ATP demand and ATP supply pathways are simultaneously activated. Here, we review experimental and computational models of muscle contraction and energetics at various organizational levels and compare them with respect to their pros and cons in facilitating understanding of the regulation of energy metabolism during exercise in the intact organism.

Effects of glutamine and hyperoxia on pulmonary oxygen uptake and muscle deoxygenation kinetics

The aim of the present study was to determine whether glutamine ingestion, which has been shown to enhance the exercise-induced increase in the tricarboxylic acid intermediate (TCAi) pool size, resulted in augmentation of the rate of increase in oxidative metabolism at the onset of exercise. In addition, the potential interaction with oxygen availability was investigated by completing exercise in both normoxic and hyperoxic conditions. Eight male cyclists cycled for 6 min at 70% VO2max following consumption of a drink (5 ml kg body mass(-1)) containing a placebo or 0.125 g kg body mass(-1) of glutamine in normoxic (CON and GLN respectively) and hyperoxic (HYP and HPG respectively) conditions. Breath-by-breath pulmonary oxygen uptake and continuous, non-invasive muscle deoxygenation (via near infrared spectroscopy: NIRS) data were collected throughout exercise. The time constant of the phase II component of pulmonary oxygen uptake kinetics was unchanged between trials (CON: 21.5 +/- 3.0 vs. GLN: 18.2 +/- 1.3 vs. HYP: 18.9 +/- 2.0 vs. HPG: 18.6 +/- 1.2 s). There was also no alteration of the kinetics of relative muscle deoxygenation as measured via NIRS (CON: 5.9 +/- 0.7 vs. GLN: 7.3 +/- 0.8 vs. HYP: 6.5 +/- 0.9 vs. HPG: 5.2 +/- 0.4 s). Conversely, the mean response time of pulmonary oxygen uptake kinetics was faster (CON: 33.4 +/- 1.2 vs. GLN: 29.8 +/- 2.3 vs. HYP: 33.2 +/- 2.6 vs. HPG: 31.6 +/- 2.6 s) and the time at which muscle deoxygenation increased above pre-exercise values was earlier (CON: 9.6 +/- 0.9 vs. GLN: 8.7 +/- 1.1 vs. HYP: 8.5 +/- 0.8 vs. HPG: 8.4 +/- 0.7 s) following glutamine ingestion. In normoxic conditions, plasma lactate concentration was lower following glutamine ingestion compared to placebo. Whilst the results of the present study provide some support for the present hypothesis, the lack of any alteration in the time constant of pulmonary oxygen uptake and muscle deoxygenation kinetics suggest that the normal exercise induced expansion of the TCAi pool size is not limiting to oxidative metabolism at the onset of cycle exercise at 70% VO2max.

MCA Vmean and the arterial lactate-to-pyruvate ratio correlate during rhythmic handgrip

Regulation of cerebral blood flow during physiological activation including exercise remains unknown but may be related to the arterial lactate-to-pyruvate (L/P) ratio. We evaluated whether an exercise-induced increase in middle cerebral artery mean velocity (MCA Vmean) relates to the arterial L/P ratio at two plasma lactate levels. MCA Vmean was determined by ultrasound Doppler sonography at rest, during 10 min of rhythmic handgrip exercise at ∼65% of maximal voluntary contraction force, and during 20 min of recovery in seven healthy male volunteers during control and a ∼15 mmol/l hyperglycemic clamp. Cerebral arteriovenous differences for metabolites were obtained by brachial artery and retrograde jugular venous catheterization. Control resting arterial lactate was 0.78 ± 0.09 mmol/l (mean ± SE) and pyruvate 55.7 ± 12.0 μmol/l (L/P ratio 16.4 ± 1.0) with a corresponding MCA Vmean of 46.7 ± 4.5 cm/s. During rhythmic handgrip the increase in MCA Vmean to 51.2 ± 4.6 cm/s was related to the increased L/P ratio (23.8 ± 2.5; r2 = 0.79; P < 0.01). Hyperglycemia increased arterial lactate and pyruvate to 1.9 ± 0.2 mmol/l and 115 ± 4 μmol/l, respectively, but it did not significantly influence the L/P ratio or MCA Vmean at rest or during exercise. Conversely, MCA Vmean did not correlate significantly, neither to the arterial lactate nor to the pyruvate concentrations. These results support that the arterial plasma L/P ratio modulates cerebral blood flow during cerebral activation independently from the plasma glucose concentration.

Prior heavy exercise elevates pyruvate dehydrogenase activity and speeds O2 uptake kinetics during subsequent moderate-intensity exercise in healthy young adults

The adaptation of pulmonary oxygen uptake (.VO2) during the transition to moderate-intensity exercise (Mod) is faster following a prior bout of heavy-intensity exercise. In the present study we examined the activation of pyruvate dehydrogenase (PDHa) during Mod both with and without prior heavy-intensity exercise. Subjects (n = 9) performed a Mod(1)-heavy-intensity-Mod(2) exercise protocol preceded by 20 W baseline. Breath-by-breath .VO2 kinetics and near-infrared spectroscopy-derived muscle oxygenation were measured continuously, and muscle biopsy samples were taken at specific times during the transition to Mod. In Mod(1), PDHa increased from baseline (1.08 +/- 0.2 mmol min(-1) (kg wet wt)(-1)) to 30 s (2.05 +/- 0.2 mmol min(-1) (kg wet wt)(-1)), with no additional change at 6 min exercise (2.07 +/- 0.3 mmol min(-1) (kg wet wt)(-1)). In Mod(2), PDHa was already elevated at baseline (1.88 +/- 0.3 mmol min(-1) (kg wet wt)(-1)) and was greater than in Mod(1), and did not change at 30 s (1.96 +/- 0.2 mmol min(-1) (kg wet wt)(-1)) but increased at 6 min exercise (2.70 +/- 0.3 mmol min(-1) (kg wet wt)(-1)). The time constant of .VO2 was lower in Mod(2) (19 +/- 2 s) than Mod(1) (24 +/- 3 s). Phosphocreatine (PCr) breakdown from baseline to 30 s was greater (P < 0.05) in Mod(1) (13.6 +/- 6.7 mmol (kg dry wt)(-1)) than Mod(2) (6.5 +/- 6.2 mmol (kg dry wt)(-1)) but total PCr breakdown was similar between conditions (Mod(1), 14.8 +/- 7.4 mmol (kg dry wt)(-1); Mod(2), 20.1 +/- 8.0 mmol (kg dry wt)(-1)). Both oxyhaemoglobin and total haemoglobin were elevated prior to and throughout Mod(2) compared with Mod(1). In conclusion, the greater PDHa at baseline prior to Mod(2) compared with Mod(1) may have contributed in part to the faster .VO2 kinetics in Mod(2). That oxyhaemoglobin and total haemoglobin were elevated prior to Mod(2) suggests that greater muscle perfusion may also have contributed to the observed faster .VO2 kinetics. These findings are consistent with metabolic inertia, via delayed activation of PDH, in part limiting the adaptation of pulmonary .VO2 and muscle O2 consumption during the normal transition to exercise.

Tricarboxylic acid cycle intermediates accumulate at the onset of intense exercise in man but are not essential for the increase in muscle oxygen uptake

It was proposed that a contraction-induced increase in tricarboxylic acid cycle intermediates (TCAI) is obligatory for the increase in muscle oxygen uptake at the start of exercise. To test this hypothesis, we measured changes in muscle TCAI during the initial seconds of intense exercise and used dichloroacetate (DCA) in an attempt to alter the level of TCAI. Five men performed strenuous leg kicking exercise (64+/-8 W) under noninfused control (CON) and DCA-supplemented conditions; biopsies (vastus lateralis) were obtained at rest and after 5, 15, and 180 s of exercise. In CON, the total concentration of three measured TCAI (SigmaTCAI: citrate, malate, and fumarate) increased (p<0.05) by 71% during the first 15 s of exercise. The SigmaTCAI was lower (p<0.05) in DCA than in CON at rest [0.18+/-0.02 vs 0.64+/-0.09 mmol kg(-1) dry weight (d.w.)], after 5 s (0.30+/-0.07 vs 0.85+/-0.14 mmol kg(-1) d.w.), and 15 s of exercise (0.60+/-0.07 vs 1.09+/-0.16 mmol kg(-1) d.w.), but not different after 3 min (3.12+/-0.53 vs 3.23+/-0.55 mmol kg(-1) d.w.). Despite differences in the level of muscle TCAI, muscle phosphocreatine degradation was similar in DCA and CON during the first 15 s of exercise (17.5+/-3.3 vs 25.6+/-4.1 mmol kg(-1) d.w.). Taken together with our previous observation that DCA does not alter muscle oxygen uptake during the initial phase of intense leg kicking exercise (Bangsbo et al. Am J Physiol 282:R273-R280, 2002), the present data suggest that muscle TCAI accumulate during the initial seconds of exercise; however, this increase is not essential for the contraction-induced increase in mitochondrial respiration.

Effects of prior heavy-intensity exercise during single-leg knee extension on v̇o2 kinetics and limb blood flow

The effects of prior heavy-intensity exercise on O2 uptake (V̇o2) kinetics of a second heavy exercise may be due to vasodilation (associated with metabolic acidosis) and improved muscle blood flow. This study examined the effect of prior heavy-intensity exercise on femoral artery blood flow (Qleg) and its relationship with V̇o2 kinetics. Five young subjects completed five to eight repeats of two 6-min bouts of heavy-intensity one-legged, knee-extension exercise separated by 6 min of loadless exercise. V̇o2 was measured breath by breath. Pulsed-wave Doppler ultrasound was used to measure Qleg. V̇o2 and blood flow velocity data were fit using a monoexponential model to identify phase II and phase III time periods and estimate the response amplitudes and time constants (τ). Phase II V̇o2 kinetics was speeded on the second heavy-intensity exercise [mean τ (SD), 29 ( 10 ) s to 24 ( 10 ) s, P < 0.05] with no change in the phase II (or phase III) amplitude. Qleg was elevated before the second exercise [1.55 (0.34) l/min to 1.90 (0.25) l/min, P < 0.05], but the amplitude and time course [τ, 25 ( 13 ) s to 35 ( 13 ) s] were not changed, such that throughout the transient the Qleg (and ΔQleg/ΔV̇o2) did not differ from the prior heavy exercise. Thus V̇o2 kinetics were accelerated on the second exercise, but the faster kinetics were not associated with changes in Qleg. Thus limb blood flow appears not to limit V̇o2 kinetics during single-leg heavy-intensity exercise nor to be the mechanism of the altered V̇o2 response after heavy-intensity prior exercise.

Determinants of Oxidative Phosphorylation Onset Kinetics in Isolated Myocytes

At the onset of constant-load exercise, pulmonary oxygen uptake (VO(2)) exhibits a monoexponential increase, following a brief time delay, to a new steady state. To date, the specific factors controlling VO(2) onset kinetics during the transition to higher rates of work remain largely unknown. To study the control of respiration in the absence of confounding factors such as blood flow heterogeneity and fiber type recruitment patterns, the onset kinetics of mitochondrial respiration were studied at the start of contractions in isolated single myocytes. Individual myocytes were microinjected with a porphyrin compound to allow phosphorescent measurement of intracellular PO(2) (P(i)O(2), an analog of VO(2)). Peak tension and P(i)O(2) were continuously monitored under a variety of conditions designed to test the role of work intensity, extracellular PO(2), cellular metabolites, and enzyme activation on the regulation of VO(2) onset kinetics.

Prior heavy-intensity exercise speeds V̇o2 kinetics during moderate-intensity exercise in young adults

The effect of prior heavy-intensity warm-up exercise on subsequent moderate-intensity phase 2 pulmonary O2 uptake kinetics (τV̇o2) was examined in young adults exhibiting relatively fast (FK; τV̇o2 < 30 s; n = 6) and slow (SK; τV̇o2 > 30 s; n = 6) V̇o2 kinetics in moderate-intensity exercise without prior warm up. Subjects performed four repetitions of a moderate (Mod1)-heavy-moderate (Mod2) protocol on a cycle ergometer with work rates corresponding to 80% estimated lactate threshold (moderate intensity) and 50% difference between lactate threshold and peak V̇o2 (heavy intensity); each transition lasted 6 min, and each was preceded by 6 min of cycling at 20 W. V̇o2 and heart rate (HR) were measured breath-by-breath and beat-by-beat, respectively; concentration changes of muscle deoxyhemoglobin (HHb), oxyhemoglobin, and total hemoglobin were measured by near-infrared spectroscopy (Hamamatsu NIRO 300). τV̇o2 was lower ( P < 0.05) in Mod2 than in Mod1 in both FK (20 ± 5 s vs. 26 ± 5 s, respectively) and SK (30 ± 8 s vs. 45 ± 11 s, respectively); linear regression analysis showed a greater “speeding” of V̇o2 kinetics in subjects exhibiting a greater Mod1 τV̇o2. HR, oxyhemoglobin, and total hemoglobin were elevated ( P < 0.05) in Mod2 compared with Mod1. The delay before the increase in HHb was reduced ( P < 0.05) in Mod2, whereas the HHb mean response time was reduced ( P < 0.05) in FK (Mod2, 22 ± 3 s; Mod1, 32 ± 11 s) but not different in SK (Mod2, 36 ± 13 s; Mod1, 34 ± 15 s). We conclude that improved muscle perfusion in Mod2 may have contributed to the faster adaptation of V̇o2, especially in SK; however, a possible role for metabolic inertia in some subjects cannot be overlooked.

Preexercise metabolic alkalosis induced via bicarbonate ingestion accelerates V̇o2 kinetics at the onset of a high-power-output exercise in humans

The present study investigated the effect of preexercise metabolic alkalosis on the primary component of oxygen uptake (V̇o2) kinetics, characterized by τ1. Seven healthy physically active nonsmoking men, aged 22.4 ± 1.8 (mean ± SD) yr, maximum V̇o2 (V̇o2 max) 50.4 ± 4 ml·min−1·kg−1, performed two bouts of cycling, corresponding to 40 and 87% of V̇o2 max, lasting 6 min each, separated by a 20-min pause, once as a control study and a few days later at ∼90 min after ingestion of 3 mmol/kg body wt of NaHCO3. Blood samples for measurements of bicarbonate concentration and hydrogen ion concentration were taken from antecubital vein via catheter. Pulmonary V̇o2 was measured continuously breath by breath. The values of τ1 were calculated by using six various approaches published in the literature. Preexercise level of bicarbonate concentration after ingestion of NaHCO3 was significantly elevated ( P < 0.01) compared with the control study (28.96 ± 2.11 vs. 24.84 ± 1.18 mmol/l; P < 0.01), and [H+] was significantly ( P < 0.01) reduced (42.79 ± 3.38 nmol/l vs. 46.44 ± 3.51 nmol/l). This shift ( P < 0.01) was also present during both bouts of exercise. During cycling at 40% of V̇o2 max, no significant effect of the preexercise alkalosis on the magnitude of τ1 was found. However, during cycling at 87% of V̇o2 max, the τ1 calculated by all six approaches was significantly ( P < 0.05) reduced, compared with the control study. The τ1 calculated as in Borrani et al. (Borrani F, Candau R, Millet GY, Perrey S, Fuchsloscher J, and Rouillon JD. J Appl Physiol 90: 2212–2220, 2001) was reduced on average by 7.9 ± 2.6 s, which was significantly different from zero with both the Student's t-test ( P = 0.011) and the Wilcoxon's signed-ranks test ( P = 0.014).

Acetyl-CoA provision and the acetyl group deficit at the onset of contraction in ischemic canine skeletal muscle

We examined the effects of increasing acetylcarnitine and acetyl-CoA availability at rest, independent of pyruvate dehydrogenase complex (PDC) activation, on energy production and tension development during the rest-to-work transition in canine skeletal muscle. We aimed to elucidate whether the lag in PDC-derived acetyl-CoA delivery toward the TCA cycle at the onset of exercise can be overcome by increasing acetyl group availability independently of PDC activation or is intimately dependent on PDC-derived acetyl-CoA. Gracilis muscle pretreated with saline or sodium acetate (360 mg/kg body mass) (both n = 6) was sampled repeatedly during 5 min of ischemic contraction. Acetate increased acetylcarnitine and acetyl-CoA availability (both P < 0.01) above control at rest and throughout contraction (P < 0.05), independently of differences in resting PDC activation between treatments. Acetate reduced oxygen-independent ATP resynthesis approximately 40% (P < 0.05) during the first minute of contraction. No difference in oxygen-independent ATP resynthesis existed between treatments from 1 to 3 min of contraction; however, energy production via this route increased approximately 25% (P < 0.05) above control in the acetate-treated group during the final 2 min of contraction. Tension development was approximately 20% greater after 5-min contraction after acetate treatment than in control (P < 0.05). In conclusion, at the immediate onset of contraction, when PDC was largely inactive, increasing cellular acetyl group availability overcame inertia in mitochondrial ATP regeneration. However, after the first minute, when PDC was near maximally activated in both groups, it appears that PDC-derived acetyl-CoA, rather than increased cellular acetyl group availability per se, dictated mitochondrial ATP resynthesis.

Acetyl group availability influences phosphocreatine degradation even during intense muscle contraction

We previously established that activation of the pyruvate dehydrogenase complex (PDC) using dichloroacetate (DCA) reduced the reliance on substrate-level phosphorylation (SLP) at the onset of exercise, with normal and reduced blood flow. PDC activation also reduced fatigue development during contraction with reduced blood flow. Since these observations, several studies have re-evaluated our observations. One study demonstrated a performance benefit without a reduction in SLP, raising a question mark over PDC's role in the regulation of ATP regeneration and our interpretation of fatigue mechanisms. Using a model of muscle contraction similar to the conflicting study (i.e. tetanic rather than twitch stimulation), we re-examined this question. Using canine skeletal muscle, one group was infused with saline while the other was pretreated with 300 mg (kg body mass)(-1) DCA. Muscle biopsies were taken at rest, peak tension (1 min) and after 6 min of tetanic electrical stimulation (75 ms on-925 ms off per second) and blood flow was limited to 25% of normal values observed during contraction. DCA reduced phosphocreatine (PCr) degradation by 40% during the first minute of contraction, but did not prevent the almost complete depletion of PCr stores at 6 min, while muscle fatigue did not differ between the two groups. During intermittent tetanic stimulation PCr degradation was 75% greater than with our previous 3 Hz twitch contraction protocol, despite a similar rate of oxygen consumption at 6 min. Thus, in the present study enhanced acetyl group availability altered the time course of PCr utilization but did not prevent the decline towards depletion. Consistent with our earlier conclusions, DCA pretreatment reduces muscle fatigue only when SLP is attenuated. The present study and our met-analysis indicates that enhanced acetyl group availability results in a readily measurable reduction in SLP when the initial rate of PCr utilization is approximately 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.

Perfused skeletal muscle -- an experimental preparation for many questions!

Perfused mammalian skeletal muscle preparations either in vitro or in situ are one of the options to be considered when planning a physiological research program or project. Such preparations have been and continue to be used to investigate research questions as diverse as skeletal muscle function and metabolism, peripheral vascular function, and an approximation of exercise. When selecting a perfused muscle preparation, both anatomical and physiological organization must be evaluated in the context of the planned experiment. In any experiment, a number of physiologically significant variables can be manipulated, such as the level of flow and the arterial or inflow concentration of a gas or substance to control substrate supply and metabolite removal as well as the stimulation parameters to alter metabolic rate. The choice of blood or an artificial perfusate is of paramount importance because, when compared to blood-perfused preparations, those receiving artificial perfusates show depressed vascular autoregulation among other changes, indicating a decrease in physiological quality. Overall, perfused skeletal muscle preparations can be used to examine many and varied research questions with close to in-vivo quality and a high degree of accuracy and control if blood-perfused.

Influence of l-NAME on pulmonary O2 uptake kinetics during heavy-intensity cycle exercise

We hypothesized that inhibition of nitric oxide synthase (NOS) by NG-nitro-l-arginine methyl ester (l-NAME) would alleviate the inhibition of mitochondrial oxygen uptake (V̇o2) by nitric oxide and result in a speeding of phase II pulmonary V̇o2 kinetics at the onset of heavy-intensity exercise. Seven men performed square-wave transitions from unloaded cycling to a work rate requiring 40% of the difference between the gas exchange threshold and peak V̇o2 with and without prior intravenous infusion of l-NAME (4 mg/kg in 50 ml saline over 60 min). Pulmonary gas exchange was measured breath by breath, and V̇o2 kinetics were determined from the averaged response to two exercise bouts performed in each condition. There were no significant differences between the control (C) and l-NAME conditions (L) for baseline V̇o2, the duration of phase I, or the amplitude of the primary V̇o2 response. However, the time constant of the V̇o2 response in phase II was significantly smaller (mean ± SE: C: 25.1 ± 3.0 s; L: 21.8 ± 3.3 s; P < 0.05), and the amplitude of the V̇o2 slow component was significantly greater (C: 240 ± 47 ml/min; L: 363 ± 24 ml/min; P < 0.05) after l-NAME infusion. These data indicate that inhibition of NOS by l-NAME results in a significant (13%) speeding of V̇o2 kinetics and a significant increase in the amplitude of the V̇o2 slow component in the transition to heavy-intensity cycle exercise in men. The speeding of the primary component V̇o2 kinetics after l-NAME infusion indicates that at least part of the intrinsic inertia to oxidative metabolism at the onset of heavy-intensity exercise may result from inhibition of mitochondrial V̇o2 by nitric oxide. The cause of the larger V̇o2 slow-component amplitude with l-NAME requires further investigation but may be related to differences in muscle blood flow early in the rest-to-exercise transition.

Oxygen uptake kinetics: old and recent lessons from experiments on isolated muscle in situ

The various mechanisms responsible for ATP resynthesis include phosphocreatine (PCr) hydrolysis, anaerobic glycolysis and oxidative phosphorylation. Among these, the latter represents the most important mechanism of energy provision. However, oxidative phosphorylation is characterized by a lower maximal power and a slow attainment of a steady state in response to increased metabolic demand. The rate of adjustment of oxidative metabolism during metabolic transitions, which can be evaluated on the basis of the analysis of O2 uptake (VO2) kinetics, has implications for exercise tolerance and muscle fatigue. Analysis of VO2 kinetics represents a valid tool for the functional evaluation of healthy subjects, athletes and patients. Over the last 35 years experiments conducted on isolated muscle preparations in situ have allowed us to gain insights into several key aspects of skeletal muscle VO2 kinetics. Their main limiting factor resides in an intrinsic slowness of intracellular oxidative metabolism when adjusting to augmented metabolic needs. The rate of adjustment of oxidative phosphorylation in mitochondria can be functionally related to PCr hydrolysis occurring in the cytoplasm.

Inhibition of nitric oxide synthase by L-NAME speeds phase II pulmonary .VO2 kinetics in the transition to moderate-intensity exercise in man

There is evidence that the rate at which oxygen uptake (.VO2) rises at the transition to higher metabolic rates within the moderate exercise intensity domain is modulated by oxidative enzyme inertia, and also that nitric oxide regulates mitochondrial function through competitive inhibition of cytochrome c oxidase in the electron transport chain. We therefore hypothesised that inhibition of nitric oxide synthase (NOS) by nitro-L-arginine methyl ester (L-NAME) would alleviate the inhibition of mitochondrial .VO2 by nitric oxide and result in a speeding of .VO2 kinetics at the onset of moderate-intensity exercise. Seven males performed square-wave transitions from unloaded cycling to a work rate requiring 90 % of predetermined gas exchange threshold with and without prior intravenous infusion of L-NAME (4 mg kg-1 in 50 ml saline over 60 min). Pulmonary gas exchange was measured breath-by-breath and .VO2 kinetics were determined from the averaged response to four exercise bouts performed in each condition using a mono-exponential function following elimination of the phase I response. There were no significant differences between the control and L-NAME conditions for baseline .VO2 (means +/- S.E.M. 797 +/- 32 vs. 794 +/- 29), the duration of phase I (15.4 +/- 0.8 vs. 17.2 +/- 0.6), or the steady-state increment in .VO2 above baseline (1000 +/- 83 vs. 990 +/- 85 ml min-1), respectively. However, the phase II time constant of the .VO2 response was significantly smaller following L-NAME infusion (22.1 +/- 2.4 vs. 17.9 +/- 2.3; P < 0.05). These data indicate that inhibition of NOS by L-NAME results in a significant (19 %) speeding of pulmonary .VO2 kinetics in the transition to moderate-intensity cycle exercise in man. At least part of the intrinsic inertia to oxidative metabolism at the onset of moderate-intensity exercise may result from competitive inhibition of mitochondrial .VO2 by nitric oxide at cytochrome c oxidase, although other mechanisms for the effect of L-NAME on .VO2 kinetics remain to be explored.

Effects of dichloroacetate on VO2 and intramuscular 31P metabolite kinetics during high-intensity exercise in humans

Traditional control theories of muscle O2 consumption are based on an "inertial" feedback system operating through features of the ATP splitting (e.g., [ADP] feedback, where brackets denote concentration). More recently, however, it has been suggested that feedforward mechanisms (with respect to ATP utilization) may play an important role by controlling the rate of substrate provision to the electron transport chain. This has been achieved by activation of the pyruvate dehydrogenase complex via dichloroacetate (DCA) infusion before exercise. To investigate these suggestions, six men performed repeated, high-intensity, constant-load quadriceps exercise in the bore of an magnetic resonance spectrometer with each of prior DCA or saline control intravenous infusions. O2 uptake (Vo2) was measured breath by breath (by use of a turbine and mass spectrometer) simultaneously with intramuscular phosphocreatine (PCr) concentration ([PCr]), [Pi], [ATP], and pH (by 31P-MRS) and arterialized-venous blood sampling. DCA had no effect on the time constant (tau) of either Vo2 increase or PCr breakdown [tauVo2 45.5 +/- 7.9 vs. 44.3 +/- 8.2 s (means +/- SD; control vs. DCA); tauPCr 44.8 +/- 6.6 vs. 46.4 +/- 7.5 s; with 95% confidence intervals averaging < +/-2 s]. DCA, however, resulted in significant (P < 0.05) reductions in 1). end-exercise [lactate] (-1.0 +/- 0.9 mM), intramuscular acidification (pH, +0.08 +/- 0.06 units), and [Pi] (-1.7 +/- 2.1 mM); 2). the amplitude of the fundamental components for [PCr] (-1.9 +/- 1.6 mM) and Vo2 (-0.1 +/- 0.07 l/min, or 8%); and 3). the amplitude of the Vo2 slow component. Thus, although the DCA infusion lessened the buildup of potential fatigue metabolites and reduced both the aerobic and anaerobic components of the energy transfer during exercise, it did not enhance either tauVo2 or tau[PCr], suggesting that feedback, rather than feedforward, control mechanisms dominate during high-intensity exercise.

Muscle oxygenation and pulmonary gas exchange kinetics during cycling exercise on-transitions in humans

Near-infrared spectroscopy (NIRS) was utilized to gain insights into the kinetics of oxidative metabolism during exercise transitions. Ten untrained young men were tested on a cycle ergometer during transitions from unloaded pedaling to 5 min of constant-load exercise below (<VT) or above (>VT) the ventilatory threshold. Vastus lateralis oxygenation was determined by NIRS, and pulmonary O2 uptake (Vo --> Vo2) was determined breath-by-breath. Changes in deoxygenated hemoglobin + myoglobin concentration Delta[deoxy(Hb + Mb)] were taken as a muscle oxygenation index. At the transition, [Delta[deoxy(Hb + Mb)]] was unmodified [time delay (TD)] for 8.9 +/- 0.5 s at <VT or 6.4 +/- 0.9 s at >VT (both significantly different from 0) and then increased, following a monoexponential function [time constant (tau) = 8.5 +/- 0.9 s for <VT and 7.2 +/- 0.7 s for >VT]. For >VT a slow component of Delta[deoxy(Hb + Mb)] on-kinetics was observed in 9 of 10 subjects after 75.0 +/- 14.0 s of exercise. A significant correlation was described between the mean response time (MRT = TD + tau) of the primary component of Delta[deoxy(Hb + Mb)] on-kinetics and the tau of the primary component of the pulmonary Vo2 on-kinetics. The constant muscle oxygenation during the initial phase of the on-transition indicates a tight coupling between increases in O2 delivery and O2 utilization. The lack of a drop in muscle oxygenation at the transition suggests adequacy of O2 availability in relation to needs.

Oxygen uptake kinetics during moderate, heavy and severe intensity "submaximal" exercise in humans: the influence of muscle fibre type and capillarisation

The purpose of the present study was to test the hypothesis that muscle fibre type influences the oxygen uptake (.VO(2)) on-kinetic response (primary time constant; primary and slow component amplitudes) during moderate, heavy and severe intensity sub-maximal cycle exercise. Fourteen subjects [10 males, mean (SD) age 25 (4) years; mass 72.6 (3.9) kg; .VO(2peak) 47.9 (2.3) ml kg(-1) min(-1)] volunteered to participate in this study. The subjects underwent a muscle biopsy of the vastus lateralis for histochemical determination of muscle fibre type, and completed repeat "square-wave" transitions from unloaded cycling to power outputs corresponding to 80% of the ventilatory threshold (VT; moderate exercise), 50% (heavy exercise) and 70% (severe exercise) of the difference between the VT and .VO(2peak). Pulmonary .VO(2) was measured breath-by-breath. The percentage of type I fibres was significantly correlated with the time constant of the primary .VO(2) response for heavy exercise (r=-0.68). Furthermore, the percentage of type I muscle fibres was significantly correlated with the gain of the .VO(2) primary component for moderate (r=0.65), heavy (r=0.57) and severe (r=0.57) exercise, and with the relative amplitude of the .VO(2) slow component for heavy (r=-0.74) and severe (r=-0.64) exercise. The influence of muscle fibre type on the .VO(2) on-kinetic response persisted when differences in aerobic fitness and muscle capillarity were accounted for. This study demonstrates that muscle fibre type is significantly related to both the speed and the amplitudes of the .VO(2) response at the onset of constant-load sub-maximal exercise. Differences in contraction efficiency and oxidative enzyme activity between type I and type II muscle fibres may be responsible for the differences observed.

Oxygen uptake kinetics during two bouts of heavy cycling separated by fatiguing sprint exercise in humans

We tested the hypothesis that O(2) uptake (Vo(2)) kinetics at the onset of heavy exercise would be altered in a state of muscle fatigue and prior metabolic acidosis. Eight well-trained cyclists completed two identical bouts of 6-min cycling exercise at >85% of peak Vo(2) separated by three successive bouts of 30 s of sprint cycling. Not only was baseline Vo(2) elevated after prior sprint exercises but also the time constant of phase II Vo(2) kinetics was faster (28.9 +/- 2.4 vs. 22.2 +/- 1.7 s; P < 0.05). CO(2) output (Vco(2)) was significantly reduced throughout the second exercise bout. Subsequently Vo(2) was greater at 3 min and increased less after this after prior sprint exercise. Cardiac output, estimated by impedance cardiography, was significantly higher in the first 2 min of the second heavy exercise bout. Normalized integrated surface electromyography of four leg muscles and normalized mean power frequency were not different between exercise bouts. Vo(2) and Vco(2) kinetic responses to heavy exercise were markedly altered by prior multiple sprint exercises.

Dichloroacetate accelerates the fall in intracellular PO2 at onset of contractions in Xenopus single muscle fibers

mM DCA, whereas the second group [control (Con); n = 10] was incubated for 30 min in Ringer solution only. After incubation, fibers were electrically stimulated to elicit tetanic contractions (0.5 Hz) for 2 min during which PiO2 was monitored. PiO2 before contractions began was 32.0 +/- 1.8 and 29.0 +/- 1.8 Torr for DCA and Con, respectively, and fell to 6.0 +/- 1.3 and 8.8 +/- 2.4 Torr (no significant difference), respectively, after steady state was reached. The kinetics of the fall, determined by both the time delay (from the start of contractions to the initial decrease in PiO2) and the tau (63% of the change to a steady state in PiO2), were calculated. In DCA cells, the tau was significantly (P < 0.05) faster than Con (22.1 +/- 3.6 vs. 39.7 +/- 5.8 s). In contrast, the time delay was not significantly (P > 0.45) different between the two groups (11.4 +/- 1.7 vs. 12.6 +/- 2.3 s, respectively). The amount of fatigue, reflected by a decrease in force production from initial, was not significantly different between groups. These data suggest that by stimulating pyruvate dehydrogenase with DCA in isolated single skeletal muscle cells, the faster fall in PiO2 is indicative of oxidative metabolism being more rapidly activated. This is the first evidence that oxygen uptake at the onset of contractions may be altered by DCA during moderate- to high-intensity contractile activity.

The acetyl group deficit at the onset of contraction in ischaemic canine skeletal muscle

Considerable debate surrounds the identity of the precise cellular site(s) of inertia that limit the contribution of mitochondrial ATP resynthesis towards a step increase in workload at the onset of muscular contraction. By detailing the relationship between canine gracilis muscle energy metabolism and contractile function during constant-flow ischaemia, in the absence (control) and presence of pyruvate dehydrogenase complex activation by dichloroacetate, the present study examined whether there is a period at the onset of contraction when acetyl-coenzyme A (acetyl-CoA) availability limits mitochondrial ATP resynthesis, i.e. whether a limitation in mitochondrial acetyl group provision exists. Secondly, assuming it does exist, we also aimed to identify the mechanism by which dichloroacetate overcomes this "acetyl group deficit". No increase in pyruvate dehydrogenase complex activation or acetyl group availability occurred during the first 20 s of contraction in the control condition, with strong trends for both acetyl-CoA and acetylcarnitine to actually decline (indicating the existence of an acetyl group deficit). Dichloroacetate increased resting pyruvate dehydrogenase complex activation, acetyl-CoA and acetylcarnitine by approximately 20-fold (P < 0.01), approximately 3-fold (P < 0.01) and approximately 4-fold (P < 0.01), respectively, and overcame the acetyl group deficit at the onset of contraction. As a consequence, the reliance upon non-oxidative ATP resynthesis was reduced by approximately 40 % (P < 0.01) and tension development was increased by approximately 20 % (P < 0.05) following 5 min of contraction. The present study has demonstrated, for the first time, the existence of an acetyl group deficit at the onset of contraction and has confirmed the metabolic and functional benefits to be gained from overcoming this inertia.

Skeletal muscle metabolism is unaffected by DCA infusion and hyperoxia after onset of intense aerobic exercise

This study investigated whether hyperoxic breathing (100% O(2)) or increasing oxidative substrate supply [dichloroacetate (DCA) infusion] would increase oxidative phosphorylation and reduce the reliance on substrate phosphorylation at the onset of high-intensity aerobic exercise. Eight male subjects cycled at 90% maximal O(2) uptake (VO(2 max)) for 90 s in three randomized conditions: 1) normoxic breathing and saline infusion over 1 h immediately before exercise (CON), 2) normoxic breathing and saline infusion with DCA (100 mg/kg body wt), and 3) hyperoxic breathing for 20 min at rest and during exercise and saline infusion (HYP). Muscle biopsies from the vastus lateralis were sampled at rest and after 30 and 90 s of exercise. DCA infusion increased pyruvate dehydrogenase (PDH) activation above CON and HYP (3.10 +/- 0.23, 0.56 +/- 0.08, 0.69 +/- 0.05 mmol x kg wet muscle(-1) x min(-1), respectively) and significantly increased both acetyl-CoA and acetylcarnitine (11.0 +/- 0.7, 2.0 +/- 0.5, 2.2 +/- 0.5 mmol/kg dry muscle, respectively) at rest. However, DCA and HYP did not alter phosphocreatine degradation and lactate accumulation and, therefore, the reliance on substrate phosphorylation during 30 s (CON, 51.2 +/- 5.4; DCA, 56.5 +/- 7.1; HYP, 69.5 +/- 6.3 mmol ATP/kg dry muscle) and 90 s of exercise (CON, 90.6 +/- 9.5; DCA, 107.2 +/- 13.0; HYP, 101.2 +/- 15.2 mmol ATP/kg dry muscle). These data suggest that the rate of oxidative phosphorylation at the onset of exercise at 90% VO(2 max) is not limited by oxygen availability to the active muscle or by substrate availability (metabolic inertia) at the level of PDH in aerobically trained subjects.