Muscle metabolic responses to exercise above and below the "critical power" assessed using 31P-MRS

We tested the hypothesis that the asymptote of the hyperbolic relationship between work rate and time to exhaustion during muscular exercise, the "critical power" (CP), represents the highest constant work rate that can be sustained without a progressive loss of homeostasis [as assessed using (31)P magnetic resonance spectroscopy (MRS) measurements of muscle metabolites]. Six healthy male subjects initially completed single-leg knee-extension exercise at three to four different constant work rates to the limit of tolerance (range 3-18 min) for estimation of the CP (mean +/- SD, 20 +/- 2 W). Subsequently, the subjects exercised at work rates 10% below CP (CP) for as long as possible, while the metabolic responses in the contracting quadriceps muscle, i.e., phosphorylcreatine concentration ([PCr]), P(i) concentration ([P(i)]), and pH, were estimated using (31)P-MRS. All subjects completed 20 min of CP exercise was 14.7 +/- 7.1 min. During CP exercise, however, [PCr] continued to fall to the point of exhaustion and [P(i)] and pH changed precipitously to values that are typically observed at the termination of high-intensity exhaustive exercise (end-exercise values = 26 +/- 16% of baseline [PCr], 564 +/- 167% of baseline [P(i)], and pH 6.87 +/- 0.10, all P < 0.05 vs. 相似文献   

NaHCO3-induced alkalosis reduces the phosphocreatine slow component during heavy-intensity forearm exercise.

During heavy-intensity exercise, the mechanisms responsible for the continued slow decline in phosphocreatine concentration ([PCr]) (PCr slow component) have not been established. In this study, we tested the hypothesis that a reduced intracellular acidosis would result in a greater oxidative flux and, consequently, a reduced magnitude of the PCr slow component. Subjects (n = 10) performed isotonic wrist flexion in a control trial and in an induced alkalosis (Alk) trial (0.3g/kg oral dose of NaHCO3, 90 min before testing). Wrist flexion, at a contraction rate of 0.5 Hz, was performed for 9 min at moderate- (75% of onset of acidosis; intracellular pH threshold) and heavy-intensity (125% intracellular pH threshold) exercise. 31P-magnetic resonance spectroscopy was used to measure intracellular [H+], [PCr], [Pi], and [ATP]. The initial recovery data were used to estimate the rate of ATP synthesis and oxidative flux at the end of heavy-intensity exercise. In repeated trials, venous blood sampling was used to measure plasma [H+], [HCO3-], and [Lac-]. Throughout rest and exercise, plasma [H+] was lower (P < 0.05) and [HCO3-] was elevated (P < 0.05) in Alk compared with control. During the final 3 min of heavy-intensity exercise, Alk caused a lower (P < 0.05) intracellular [H+] [246 (SD 117) vs. 291 nmol/l (SD 129)], a greater (P < 0.05) [PCr] [12.7 (SD 7.0) vs. 9.9 mmol/l (SD 6.0)], and a reduced accumulation of [ADP] [0.065 (SD 0.031) vs. 0.098 mmol/l (SD 0.059)]. Oxidative flux was similar (P > 0.05) in the conditions at the end of heavy-intensity exercise. In conclusion, our results are consistent with a reduced intracellular acidosis, causing a decrease in the magnitude of the PCr slow component. The decreased PCr slow component in Alk did not appear to be due to an elevated oxidative flux.     

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

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

Relationship of muscular fatigue to pH and diprotonated Pi in humans: a 31P-NMR study

Seventeen normal subjects performed maximal wrist flexion exercise with continuous monitoring of forearm muscle pH and H2PO4-, measured with 31P nuclear magnetic resonance, and muscle fatigue, expressed as a percentage of decline in maximal developed force. Four minutes of exercise (flexion duration = 1 s) reduced maximal developed force from 100 to 74 +/- 9% and pH from 6.99 +/- 0.04 to 6.17 +/- 0.33 and increased H2PO4- to 927 +/- 401% of resting levels. In all subjects, linear relationships were noted between developed force and pH (r = 0.90 +/- 0.08) and between developed force and H2PO4- (r = -0.89 +/- 0.08). Doubling the contraction duration to 2 s produced more rapid changes in developed force, pH, and H2PO4- but no change in the relationship of force to pH and H2PO4-. Two minutes of submaximal exercise before maximal exercise significantly reduced pH and increased H2PO4-. During subsequent maximal exercise, the relationship between developed force and H2PO4- remained unchanged. In contrast, the relationship between developed force and pH was shifted leftward; muscle pH remained lower throughout maximal exercise, and developed force remained comparable to that noted during control exercise. These observations suggest that muscle fatigue during intense short-term exercise is primarily caused by an increase in intramuscular H2PO4- rather than by a decrease in intramuscular pH.     

Quantitative mathematical expressions for accurate in vivo assessment of cytosolic [ADP] and DeltaG of ATP hydrolysis in the human brain and skeletal muscle

Magnetic Resonance Spectroscopy affords the possibility of assessing in vivo the thermodynamic status of living tissues. The main thermodynamic variables relevant for the knowledge of the health of living tissues are: DeltaG of ATP hydrolysis and cytosolic [ADP], the latter as calculated from the apparent equilibrium constant of the creatine kinase reaction. In this study we assessed the stoichiometric equilibrium constant of the creatine kinase reaction by in vitro (31)P NMR measurements and computer calculations resulting to be: logK(CK)=8.00+/-0.07 at T=310 K and ionic strength I=0.25 M. This value refers to the equilibrium: PCr(2-)+ADP(3-)+ H(+)=Cr+ATP(4-). We also assessed by computer calculation the stoichiometric equilibrium constant of ATP hydrolysis obtaining the value: logK(ATP-hyd)=-12.45 at T=310 K and ionic strength I=0.25 M, which refers to the equilibrium: ATP(4-)+H(2)O=ADP(3-)+PO(4)(3-)+2H(+). Finally, we formulated novel quantitative mathematical expressions of DeltaG of ATP hydrolysis and of the apparent equilibrium constant of the creatine kinase reaction as a function of total [PCr], pH and pMg, all quantities measurable by in vivo (31)P MRS. Our novel mathematical expressions allow the in vivo assessment of cytosolic [ADP] and DeltaG of ATP hydrolysis in the human brain and skeletal muscle taking into account pH and pMg changes occurring in living tissues both in physiological and pathological conditions.     

Influence of phosphagen concentration on phosphocreatine breakdown kinetics. Data from human gastrocnemius muscle

At the onset of a square-wave exercise of moderate intensity, in the absence of any detectable lactate production, the hydrolysis of phosphocreatine (PCr) fills the gap between energy requirement and energy yield by oxidative pathways, thus representing a readily available source of energy for the muscle. We verified experimentally the relationships between high-energy phosphates and/or their changes and the time constant of PCr concentration ([PCr]) kinetics in humans (tau(PCr)). High-energy phosphate concentration (by (31)P-NMR spectroscopy) in the calf muscles were measured during three repetitions of the rest-to-work transition of moderate aerobic square-wave exercise on nine healthy volunteers, while resting [PCr] was estimated from the appropriate spectroscopy data. PCr concentration decreased significantly (22 +/- 6%) from rest to steady-state exercise, without differences among the three repetitions. Absolute resting [PCr] and tau(PCr) were consistent with literature values, amounting to 27.5 +/- 2.2 mM and 23.9 +/- 2.9 s, respectively. No significant relationships were detected between individual tau(PCr) and mechanical power, fraction or absolute amount of PCr hydrolyzed, or change in ADP concentration. On the contrary, individual tau(PCr) (s) was linearly related to absolute resting [PCr] (mM), the relationship being described by: tau(PCr) = 0.656 + 0.841.[PCr] (n = 9, R = 0.708, P < 0.05). These data support the view that in humans PCr concentration sets the time course of the oxidative metabolism in skeletal muscle at the start of exercise, being one of the main controllers of oxidative phosphorylation.     

Neural and metabolic mechanisms of excessive muscle fatigue in maintenance hemodialysis patients

Dialysis patients have severe exercise limitations related to metabolic disturbances, but muscle fatigue has not been well studied in this population. We investigated the magnitude and mechanisms of fatigue of the ankle dorsiflexor muscles in patients on maintenance hemodialysis. Thirty-three dialysis patients and twelve healthy control subjects performed incremental isometric dorsiflexion exercise, beginning at 10% of their maximal voluntary contraction (MVC) and increasing by 10% every 2 min. Muscle fatigue (fall of MVC), completeness of voluntary activation, and metabolic responses to exercise were measured. Before exercise, dialysis subjects exhibited reduced strength and impaired peripheral activation (lower compound muscle activation potential amplitude) but no metabolic perturbation. During exercise, dialysis subjects demonstrated threefold greater fatigue than controls with evidence of central activation failure but no change in peripheral activation. All metabolic parameters were significantly more perturbed at end exercise in dialysis subjects than in controls, including lower phosphocreatine (PCr) and pH, and higher P(i), P(i)/PCr, and H(2)PO(4)(-). Oxidative potential was markedly lower in patients than in controls [62.5 (SD 27.2) vs. 134.6 (SD 31.7), P < 0.0001]. Muscle fatigue was negatively correlated with oxidative potential among dialysis subjects (r = -0.52, P = 0.04) but not controls. Changes in central activation ratio were also correlated with muscle fatigue in the dialysis subjects (r = 0.59, P = 0.001) but not the controls. This study provides new information regarding the excessive muscular fatigue of dialysis patients and demonstrates that the mechanisms of this fatigue include both intramuscular energy metabolism and central activation failure.     

Use of phosphocreatine kinetics to determine the influence of creatine on muscle mitochondrial respiration: an in vivo 31P-MRS study of oral creatine ingestion.

Recent human isolated muscle fiber studies suggest that phosphocreatine (PCr) and creatine (Cr) concentrations play a role in the regulation of mitochondrial respiration rate. To determine whether similar regulatory mechanisms are present in vivo, this study examined the relationship between skeletal muscle mitochondrial respiration rate and end-exercise PCr, Cr, PCr-to-Cr ratio (PCr/Cr), ADP, and pH by using (31)P-magnetic resonance spectroscopy in 16 men and women (36.9 +/- 4.6 yr). The initial PCr resynthesis rate and time constant (T(c)) were used as indicators of mitochondrial respiration after brief (10-12 s) and exhaustive (1-4 min) dynamic knee extension exercise performed in placebo and creatine-supplemented conditions. The results show that the initial PCr resynthesis rate has a strong relationship with end-exercise PCr, Cr, and PCr/Cr (r > 0.80, P < 0.001), a moderate relationship with end-exercise ADP (r = 0.77, P < 0.001), and no relationship with end-exercise pH (r = -0.14, P = 0.34). The PCr T(c) was not as strongly related to PCr, Cr, PCr/Cr, and ADP (r < 0.77, P < 0.001-0.18) and was significantly influenced by end-exercise pH (r = -0.43, P < 0.01). These findings suggest that end-exercise PCr and Cr should be taken into consideration when PCr recovery kinetics is used as an indicator of mitochondrial respiration and that the initial PCr resynthesis rate is a more reliable indicator of mitochondrial respiration compared with the PCr T(c).     

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.     

31P magnetic resonance spectroscopy study of phosphocreatine recovery kinetics in skeletal muscle: the issue of intersubject variability

We have analyzed by (31)P MRS the relationship between kinetic parameters of phosphocreatine (PCr) recovery and end-of-exercise status under conditions of moderate and large acidosis induced by dynamic exercise. Thirteen healthy subjects performed muscular contractions at 0.47 Hz (low frequency, moderate exercise) and 0.85 Hz (high frequency, heavy exercise). The rate constant of PCr resynthesis (k(PCr)) varied greatly among subjects (variation coefficients: 43 vs. 57% for LF vs. HF exercises) and protocols (k(PCr) values: 1.3+/-0.5 min(-1) vs. 0.9+/-0.5 min(-1) for LF vs. HF exercises, P<0.03). The large intersubject variability can be captured into a linear relationship between k(PCr), the amount of PCr consumed ([PCr(2)]) and pH reached at the end of exercise (pH(end)) (k(PCr)=-3.3+0.7 pH(end)-0.03 [PCr(2)]; P=0.0007; r=0.61). This dual relationship illustrates that mitochondrial activity is affected by end-of-exercise metabolic status and allows reliable comparisons between control, diseased and trained muscles. In contrast to k(PCr), the initial rate of PCr recovery and the maximum oxidative capacity were always constant whatever the metabolic conditions of end-of-exercise and can then be additionally used in the identification of dysfunctions in the oxidative metabolic pathway.     

Muscle metabolic status and acid-base balance during 10-s work:5-s recovery intermittent and continuous exercise

Gastrocnemius muscle phosphocreatine ([PCr]) and hydrogen ion ([H(+)]) were measured using (31)P-magnetic resonance spectroscopy during repeated bouts of 10-s heavy-intensity (HI) exercise and 5-s rest compared with continuous (CONT) HI exercise. Recreationally active male subjects (n = 7; 28 yr ± 9 yr) performed on separate occasions 12 min of isotonic plantar flexion (0.75 Hz) CONT and intermittent (INT; 10-s exercise, 5-s rest) exercise. The HI power output in both CONT and INT was set at 50% of the difference between the power output associated with the onset of intracellular acidosis and peak exercise determined from a prior incremental plantar flexion protocol. Intracellular concentrations of [PCr] and [H(+)] were calculated at 4 s and 9 s of the work period and at 4 s of the rest period in INT and during CONT exercise. [PCr] and [H(+)] (mean ± SE) were greater at 4 s of the rest periods vs. 9 s of exercise over the course of the INT exercise bout: [PCr] (20.7 mM ± 0.6 vs. 18.7 mM ± 0.5; P < 0.01); [H(+)] (370 nM ± 13.50 vs. 284 nM ± 13.6; P < 0.05). Average [H(+)] was similar for CONT vs. INT. We therefore suggest that there is a glycolytic contribution to ATP recovery during the very short rest period (<5 s) of INT and that the greater average power output of CONT did not manifest in greater [H(+)] and greater glycolytic contribution compared with INT exercise.     

Prior exercise delays the onset of acidosis during incremental exercise.

The effects of prior moderate- and prior heavy-intensity exercise on the subsequent metabolic response to incremental exercise were examined. Healthy, young adult subjects (n = 8) performed three randomized plantar-flexion exercise tests: 1) an incremental exercise test (approximately 0.6 W/min) to volitional fatigue (Ramp); 2) Ramp preceded by 6 min of moderate-intensity, constant-load exercise below the intracellular pH threshold (pHT; Mod-Ramp); and 3) Ramp preceded by 6 min of heavy-intensity, constant-load exercise above pHT (Hvy-Ramp); the constant-load and incremental exercise periods were separated by 6 min of rest. (31)P-magnetic resonance spectroscopy was used to continuously monitor intracellular pH, phosphocreatine concentration ([PCr]), and inorganic phosphate concentration ([P(i)]). No differences in exercise performance or the metabolic response to exercise were observed between Ramp and Mod-Ramp. However, compared with Ramp, a 14% (SD 10) increase (P < 0.01) in peak power output (PPO) was observed in Hvy-Ramp. The improved exercise performance in Hvy-Ramp was accompanied by a delayed (P = 0.01) onset of intracellular acidosis [Hvy-Ramp 60.4% PPO (SD 11.7) vs. Ramp 45.8% PPO (SD 9.4)] and a delayed (P < 0.01) onset of rapid increases in [P(i)]/[PCr] [Hvy-Ramp 61.5% PPO (SD 12.0) vs. Ramp 45.1% PPO (SD 9.1)]. In conclusion, prior heavy-intensity exercise delayed the onset of intracellular acidosis and enhanced exercise performance during a subsequent incremental exercise test.     

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

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

Muscle oxidative metabolism accelerates with mild acidosis during incremental intermittent isometric plantar flexion exercise

BACKGROUND: It has been thought that intramuscular ADP and phosphocreatine (PCr) concentrations are important regulators of mitochondorial respiration. There is a threshold work rate or metabolic rate for cellular acidosis, and the decrease in muscle PCr is accelerated with drop in pH during incremental exercise. We tested the hypothesis that increase in muscle oxygen consumption (o2mus) is accelerated with rapid decrease in PCr (concomitant increase in ADP) in muscles with drop in pH occurs during incremental plantar flexion exercise. METHODS: Five male subjects performed a repetitive intermittent isometric plantar flexion exercise (6-s contraction/4-s relaxation). Exercise intensity was raised every 1 min by 10% maximal voluntary contraction (MVC), starting at 10% MVC until exhaustion. The measurement site was at the medial head of the gastrocnemius muscle. Changes in muscle PCr, inorganic phosphate (Pi), ADP, and pH were measured by 31P-magnetic resonance spectroscopy. o2mus was determined from the rate of decrease in oxygenated hemoglobin and/or myoglobin using near-infrared continuous wave spectroscopy under transient arterial occlusion. Electromyogram (EMG) was also recorded. Pulmonary oxygen uptake (o2pul ) was measured by the breath-by-breath gas analysis. RESULTS: EMG amplitude increased as exercise intensity progressed. In contrast, muscle PCr, ADP, o2mus, and o2pul did not change appreciably below 40% MVC, whereas above 40% MVC muscle PCr decreased, and ADP, o2mus, and o2pul increased as exercise intensity progressed, and above 70% MVC, changes in muscle PCr, ADP, o2mus, and o2pul accelerated with the decrease in muscle pH (~6.78). The kinetics of muscle PCr, ADP, o2mus, and o2pul were similar, and there was a close correlation between each pair of parameters (r = 0.969~0.983, p < 0.001). CONCLUSION: With decrease in pH muscle oxidative metabolism accelerated and changes in intramuscular PCr and ADP accelerated during incremental intermittent isometric plantar flexion exercise. These results suggest that rapid changes in muscle PCr and/or ADP with mild acidosis stimulate accelerative muscle oxidative metabolism.     

Ischemic exercise and the muscle metaboreflex.

In exercising muscle, interstitial metabolites accumulate and stimulate muscle afferents. This evokes the muscle metaboreflex and raises arterial blood pressure (BP). In this report, we examined the effects of tension generation on muscle metabolites and BP during ischemic forearm exercise in humans. Heart rate (HR), BP, P(i), H(2)PO(4)(-), and pH ((31)P-NMR spectroscopy) data were collected in 10 normal healthy men (age 23 +/- 1 yr) during rhythmic handgrip exercise. After baseline measurements, the subjects performed rhythmic handgrip for 2 min. At 2 min, a 250-mmHg occlusion cuff was inflated, and ischemic handgrip exercise was continued until near fatigue (Borg 19). Measurements were continued for an additional 30 s of ischemia. This protocol was performed at 15, 30, 45, and 60% of the subjects' maximum voluntary contraction (MVC) in random order. As tension increased, the time to fatigue decreased. In addition, mean arterial pressure and HR were higher at 60% MVC than at any of the other lower tensions. The NMR data showed significantly greater increases in H(2)PO(4)(-), P(i), and H(+) at 60% than at 15 and 30% MVC. Therefore, despite the subjects working to the same perceived effort level, a greater reflex response (represented by BP and HR data) was elicited at 60% MVC than at any of the other ischemic tensions. These data are consistent with the hypothesis that, as tension increases, factors aside from insufficient blood flow contribute to the work effect on muscle metabolites and the magnitude of the reflex response.     

Skeletal muscle energy metabolism during prolonged, fatiguing exercise.

A depletion of phosphocreatine (PCr), fall in the total adenine nucleotide pool (TAN = ATP + ADP + AMP), and increase in TAN degradation products inosine 5'-monophosphate (IMP) and hypoxanthine are observed at fatigue during prolonged exercise at 70% maximal O(2) uptake in untrained subjects [J. Baldwin, R. J. Snow, M. F. Carey, and M. A. Febbraio. Am. J. Physiol. 277 (Regulatory Integrative Comp. Physiol. 46): R295-R300, 1999]. The present study aimed to examine whether these metabolic changes are also prevalent when exercise is performed below the blood lactate threshold (LT). Six healthy, untrained humans exercised on a cycle ergometer to voluntary exhaustion at an intensity equivalent to 93 +/- 3% of LT ( approximately 65% peak O(2) uptake). Muscle biopsy samples were obtained at rest, at 10 min of exercise, approximately 40 min before fatigue (F-40 =143 +/- 13 min), and at fatigue (F = 186 +/- 31 min). Glycogen concentration progressively declined (P < 0.01) to very low levels at fatigue (28 +/- 6 mmol glucosyl U/kg dry wt). Despite this, PCr content was not different when F-40 was compared with F and was only reduced by 40% when F was compared with rest (52. 8 +/- 3.7 vs. 87.8 +/- 2.0 mmol/kg dry wt; P < 0.01). In addition, TAN concentration was not reduced, IMP did not increase significantly throughout exercise, and hypoxanthine was not detected in any muscle samples. A significant correlation (r = 0.95; P < 0. 05) was observed between exercise time and glycogen use, indicating that glycogen availability is a limiting factor during prolonged exercise below LT. However, because TAN was not reduced, PCr was not depleted, and no correlation was observed between glycogen content and IMP when glycogen stores were compromised, fatigue may be related to processes other than those involved in muscle high-energy phosphagen metabolism.     

Muscle phosphocreatine kinetics in children and adults at the onset and offset of moderate-intensity exercise.

The splitting of muscle phosphocreatine (PCr) plays an integral role in the regulation of muscle O2 utilization during a "step" change in metabolic rate. This study tested the hypothesis that the kinetics of muscle PCr would be faster in children compared with adults both at the onset and offset of moderate-intensity exercise, in concert with the previous demonstration of faster phase II pulmonary O2 uptake kinetics in children. Eighteen peri-pubertal children (8 boys, 10 girls) and 16 adults (8 men, 8 women) completed repeated constant work-rate exercise transitions corresponding to 80% of the Pi/PCr intracellular threshold. The changes in quadriceps [PCr], [Pi], [ADP], and pH were determined every 6 s using 31P-magnetic resonance spectroscopy. No significant (P>0.05) age- or sex-related differences were found in the PCr kinetic time constant at the onset (boys, 21+/-4 s; girls, 24+/-5 s; men, 26+/-9 s; women, 24+/-7 s) or offset (boys, 26+/-5 s; girls, 29+/-7 s; men, 23+/-9 s; women 29+/-7 s) of exercise. Likewise, the estimated theoretical maximal rate of oxidative phosphorylation (Qmax) was independent of age and sex (boys, 1.39+/-0.20 mM/s; girls, 1.32+/-0.32 mM/s; men, 2.36+/-1.18 mM/s; women, 1.51+/-0.53 mM/s). These results are consistent with the notion that the putative phosphate-linked regulation of muscle O2 utilization is fully mature in peri-pubertal children, which may be attributable to a comparable capacity for mitochondrial oxidative phosphorylation in child and adult muscle.     

Skeletal muscle metabolism and work capacity: a 31P-NMR study of Andean natives and lowlanders

Two metabolic features of altitude-adapted humans are the maximal O2 consumption (VO2max) paradox (higher work rates following acclimatization without increases in VO2max) and the lactate paradox (progressive reductions in muscle and blood lactate with exercise at increasing altitude). To assess underlying mechanisms, we studied six Andean Quechua Indians in La Raya, Peru (4,200 m) and at low altitude (less than 700 m) immediately upon arrival in Canada. The experimental strategy compared whole-body performance tests and single (calf) muscle work capacities in the Andeans with those in groups of sedentary, power-trained, and endurance-trained lowlanders. We used 31P nuclear magnetic resonance spectroscopy to monitor noninvasively changes in concentrations of phosphocreatine [( PCr]), [Pi], [ATP], [PCr]/[PCr] + creatine ([Cr]), [Pi]/[PCr] + [Cr], and pH in the gastrocnemius muscle of subjects exercising to fatigue. Our results indicate that the Andeans 1) are phenotypically unique with respect to measures of anaerobic and aerobic work capacity, 2) despite significantly lower anaerobic capacities, are capable of calf muscle work rates equal to those of highly trained power- and endurance-trained athletes, and 3) compared with endurance-trained athletes with significantly higher VO2max values and power-trained athletes with similar VO2max values, display, respectively, similar and reduced perturbation of all parameters related to the phosphorylation potential and to measurements of [Pi], [PCr], [ATP], and muscle pH derivable from nuclear magnetic resonance. Because the lactate paradox may be explained on the basis of tighter ATP demand-supplying coupling, we postulate that a similar mechanism may explain 1) the high calf muscle work capacities in the Andeans relative to measures of whole-body work capacity, 2) the VO2max paradox, and 3) anecdotal reports of exceptional work capacities in indigenous altitude natives.     

Prolonged exercise to fatigue in humans impairs skeletal muscle Na+-K+-ATPase activity, sarcoplasmic reticulum Ca2+ release, and Ca2+ uptake.

Prolonged exhaustive submaximal exercise in humans induces marked metabolic changes, but little is known about effects on muscle Na+-K+-ATPase activity and sarcoplasmic reticulum Ca2+ regulation. We therefore investigated whether these processes were impaired during cycling exercise at 74.3 +/- 1.2% maximal O2 uptake (mean +/- SE) continued until fatigue in eight healthy subjects (maximal O2 uptake of 3.93 +/- 0.69 l/min). A vastus lateralis muscle biopsy was taken at rest, at 10 and 45 min of exercise, and at fatigue. Muscle was analyzed for in vitro Na+-K+-ATPase activity [maximal K+-stimulated 3-O-methylfluorescein phosphatase (3-O-MFPase) activity], Na+-K+-ATPase content ([3H]ouabain binding sites), sarcoplasmic reticulum Ca2+ release rate induced by 4 chloro-m-cresol, and Ca2+ uptake rate. Cycling time to fatigue was 72.18 +/- 6.46 min. Muscle 3-O-MFPase activity (nmol.min(-1).g protein(-1)) fell from rest by 6.6 +/- 2.1% at 10 min (P <0.05), by 10.7 +/- 2.3% at 45 min (P <0.01), and by 12.6 +/- 1.6% at fatigue (P <0.01), whereas 3[H]ouabain binding site content was unchanged. Ca2+ release (mmol.min(-1).g protein(-1)) declined from rest by 10.0 +/- 3.8% at 45 min (P <0.05) and by 17.9 +/- 4.1% at fatigue (P < 0.01), whereas Ca2+ uptake rate fell from rest by 23.8 +/- 12.2% at fatigue (P=0.05). However, the decline in muscle 3-O-MFPase activity, Ca2+ uptake, and Ca2+ release were variable and not significantly correlated with time to fatigue. Thus prolonged exhaustive exercise impaired each of the maximal in vitro Na+-K+-ATPase activity, Ca2+ release, and Ca2+ uptake rates. This suggests that acutely downregulated muscle Na+, K+, and Ca2+ transport processes may be important factors in fatigue during prolonged exercise in humans.     

Plasma hypoxanthine and ammonia in humans during prolonged exercise

In this study we examined the time course of changes in the plasma concentration of oxypurines [hypoxanthine (Hx), xanthine and urate] during prolonged cycling to fatigue. Ten subjects with an estimated maximum oxygen uptake (VO2(max)) of 54 (range 47-67) ml x kg(-1) x min(-1) cycled at [mean (SEM)] 74 (2)% of VO2(max) until fatigue [79 (8) min]. Plasma levels of oxypurines increased during exercise, but the magnitude and the time course varied considerably between subjects. The plasma concentration of Hx ([Hx]) was 1.3 (0.3) micromol/l at rest and increased eight fold at fatigue. After 60 min of exercise plasma [Hx] was >10 micromol/l in four subjects, whereas in the remaining five subjects it was <5 micromol/l. The muscle contents of total adenine nucleotides (TAN = ATP+ADP+AMP) and inosine monophosphate (IMP) were measured before and after exercise in five subjects. Subjects with a high plasma [Hx] at fatigue also demonstrated a pronounced decrease in muscle TAN and increase in IMP. Plasma [Hx] after 60 min of exercise correlated significantly with plasma concentration of ammonia ([NH(3)], r = 0.90) and blood lactate (r = 0.66). Endurance, measured as time to fatigue, was inversely correlated to plasma [Hx] at 60 min (r = -0.68, P < 0.05) but not to either plasma [NH(3)] or blood lactate. It is concluded that during moderate-intensity exercise, plasma [Hx] increases, but to a variable extent between subjects. The present data suggest that plasma [Hx] is a marker of adenine nucleotide degradation and energetic stress during exercise. The potential use of plasma [Hx] to assess training status and to identify overtraining deserves further attention.