Effect of Q10 supplementation on tissue Q10 levels and adenine nucleotide catabolism during high-intensity exercise

The aim of the present study was to investigate the concentration of ubiquinone-10 (Q10), at rest, in human skeletal muscle and blood plasma before and after a period of high-intensity training with or without Q10 supplementation. Another aim was to explore whether adenine nucleotide catabolism, lipid peroxidation, and mitochondrial function were affected by Q10 treatment. Seventeen young healthy men were assigned to either a control (placebo) or Q10-supplementation (120 mg/day) group. Q10 supplementation resulted in a significantly higher plasma Q10/total cholesterol level on Days 11 and 20 compared with Day 1. There was no significant change in the concentration of Q10 in skeletal muscle or in isolated skeletal muscle mitochondria in either group. Plasma hypoxanthine and uric acid concentrations increased markedly after each exercise test session in both groups. After the training period, the postexercise increase in plasma hypoxanthine was markedly reduced in both groups, but the response was partially reversed after the recovery period. It was concluded that Q10 supplementation increases the concentration of Q10 in plasma but not in skeletal muscle.

Mitochondrial function in human skeletal muscle is not impaired by high intensity exercise

The hypothesis that high-intensity (HI) intermittent exercise impairs mitochondrial function was investigated with different microtechniques in human muscle samples. Ten male students performed three bouts of cycling at 130% of peak O2 consumption (V.O2,peak). Muscle biopsies were taken from the vastus lateralis muscle at rest, at fatigue and after 110 min recovery. Mitochondrial function was measured both in isolated mitochondria and in muscle fibre bundles made permeable with saponin (skinned fibres). In isolated mitochondria there was no change in maximal respiration, rate of adenosine 5'-triphosphate (ATP) production (measured with bioluminescence) and respiratory control index after exercise or after recovery. The ATP production per consumed oxygen (P/O ratio) also remained unchanged at fatigue but decreased by 4% (P<0.05) after recovery. In skinned fibres, maximal adenosine 5'-diphosphate (ADP)-stimulated respiration increased by 23% from rest to exhaustion (P<0.05) and remained elevated after recovery, whereas the respiratory rates in the absence of ADP and at 0.1 mM ADP (submaximal respiration) were unchanged. The ratio between respiration at 0.1 and 1 mM ADP (ADP sensitivity index) decreased at fatigue (P<0.05) but after the recovery period was not significantly different from that at rest. It is concluded that mitochondrial oxidative potential is maintained or improved during exhaustive HI exercise. The finding that the sensitivity of mitochondrial respiration to ADP is reversibly decreased after strenuous exercise may indicate that the control of mitochondrial respiration is altered.

Mitochondrial oxidative function in human saponin-skinned muscle fibres: effects of prolonged exercise

1. The influence of prolonged exhaustive exercise on mitochondrial oxidative function was investigated in ten men. 2. Muscle biopsies were taken before and after exercise and mitochondrial respiration investigated in fibre bundles made permeable by pretreatment with saponin. 3. After exercise, respiration in the absence of ADP increased by 18 % (P < 0.01), but respiration at suboptimal ADP concentration (0.1 mM) and maximal ADP-stimulated respiration (1 mM ADP) remained unchanged. 4. In the presence of creatine (20 mM), mitochondrial affinity for ADP increased markedly and respiration at suboptimal ADP concentration (0.1 mM) was similar (pre-exercise) or higher (post-exercise; P < 0.05) than with 1 mM ADP alone. The increase in respiratory rate with creatine was correlated to the relative type I fibre area (r = 0.84). Creatine-stimulated respiration increased after prolonged exercise (P < 0.01). 5. The respiratory control index (6.8 +/- 0.4, mean +/- s.e.m.) and the ratio between respiration at 0.1 and 1 mM ADP (ADP sensitivity index, 0.63 +/- 0.03) were not changed after exercise. The sensitivity index was negatively correlated to the relative type I fibre area (r = -0.86). 6. The influence of exercise on muscle oxidative function has for the first time been investigated with the skinned-fibre technique. It is concluded that maximal mitochondrial oxidative power is intact or improved after prolonged exercise, while uncoupled respiration is increased. The latter finding may contribute to the elevated post-exercise oxygen consumption. The finding that the sensitivity of mitochondrial respiration for ADP and creatine are related to fibre-type composition indicates intrinsic differences in the control of mitochondrial respiration between fibres.

Energy supply and muscle fatigue in humans

Limitations in energy supply is a classical hypothesis of muscle fatigue. The present paper reviews the evidence available from human studies that energy deficiency is an important factor in fatigue. The maximal rate of energy expenditure determined in skinned fibres is close to the rate of adenosine triphosphate (ATP) utilisation observed in vivo and data suggest that performance during short bursts of exercise (<5 s duration) primarily is limited by other factors than energy supply (e.g. Vmax of myosine adenosine triphosphatase (ATPase), motor unit recruitment, engaged muscle mass). Within 10 s of exercise maximal power output decreases considerably and coincides with depletion of phosphocreatine. During recovery, maximal force and power output is restored with a similar time course as the resynthesis of phosphocreatine. Increases in muscle store of phosphocreatine through dietary supplementation with creatine increases performance during high-intensity exercise. These findings support the hypothesis that energy supply limits performance during high-intensity exercise. It is well documented that pre-exercise muscle glycogen content is related to performance during moderate intensity exercise. Recent data indicates that the interfibre variation in phosphocreatine is large after prolonged exercise to fatigue and that some fibres are depleted to the same extent as after high-intensity exercise. Despite relatively small decreases in ATP, the products of ATP hydrolysis (Pi and free ADP) may increase considerably. Free ADP calculated from the creatine kinase reaction increases 10-fold both after high-intensity exercise and after prolonged exercise to fatigue. It is suggested that local increases in ADP may reach inhibitory levels for the contraction process.

Rate of oxidative phosphorylation in isolated mitochondria from human skeletal muscle: effect of training status

Muscle oxidative function has been investigated in subjects with various training status (VO2 max, 41-72 mL O2 kg-1 body wt min-1, n = 10). Mitochondria were isolated from biopsies taken from m. vastus lateralis. Maximal mitochondrial oxygen consumption (QO2) and ATP production (MAPR) were measured with polarographic and bioluminometric techniques, respectively. The yield of mitochondria, calculated from the fractional activity of citrate synthase (CS), averaged 26%. With pyruvate + malate, the respiratory control ratio was 5.7 +/- 0.4 (X +/- SE) and the P/O ratio was 2.83 +/- 0.02, which demonstrates that the isolated mitochondria were functionally intact. QO2 was significantly correlated to aerobic training status expressed as muscle CS activity (r = 0.86), VO2 max (r = 0.84) and lactate threshold (r = 0.83) but not to the fibre type composition. A highly significant correlation (r = 0.93) was observed between ATP production calculated from QO2 and MAPR, but ATP production derived from QO2 was higher than MAPR both for pyruvate + malate (255%) and for alpha-ketoglutarate (23%). QO2 extrapolated to a temperature of 38 degrees C averaged 68 mL O2 min-1 kg-1 wet wt, which is similar to previous findings in vitro and in vivo during the post-exercise period. However, calculated muscle O2 utilization during exercise was three- to fivefold higher than QO2 measured on isolated mitochondria. It is suggested that additional factors exist for activation of mitochondrial respiration during exercise. It is concluded that muscle oxidative function can be quantitatively assessed from the respiration of mitochondria isolated from needle biopsy specimens and that QO2 is closely correlated to whole-body VO2 max.

Phosphocreatine content in single fibers of human muscle after sustained submaximal exercise

The effect of sustained submaximal exercise on muscle energetics has been studied on the single-fiber level in human skeletal muscle. Seven subjects cycled to fatigue (mean 77 min) at a work rate corresponding to approximately 75% of maximal O2 uptake. Biopsies were taken from the vastus lateralis muscle at rest, at fatigue, and after 5 min of recovery. Muscle glycogen decreased from 444 +/- 40 (SE) mmol glucosyl units/kg dry wt at rest to 94 +/- 16. Postexercise glycogen was inversely correlated (P < 0.01) to muscle content of inosine monophosphate, a catabolite of ATP. Phosphocreatine (PCr) in mixed-fiber muscle decreased at fatigue to 37% but was restored above the initial value (106.5%, P < 0.025) after 5 min of recovery. The overshoot was localized to type I fibers. The rapid reversal of PCr is in contrast to the slow recovery in contraction force. Pi increased at fatigue but less than that expected from the changes in PCr and other phosphate compounds. Mean PCr at rest was approximately 20% higher in type II than in type I fibers (86.4 +/- 3.6 and 71.6 +/- 1.8 mmol/kg dry wt, respectively, P < 0.05), but at fatigue similar PCr contents were observed in the two fiber types. Reduction in PCr in all fibers at fatigue suggests that all fibers were recruited at the end of exercise. PCr content in single fibers showed a great variability in samples at rest, exercise, and recovery. The variability was more pronounced than for ATP, and the data suggest that it is due to interfiber physiological-biochemical differences. At fatigue ATP was maintained relatively high in all single fibers, but a pronounced depletion of PCr was observed in a large number of fibers, and this may contribute to fatigue through the associated increases in Pi or/and free ADP. It is noteworthy that the increase in calculated free ADP at fatigue was similar to that after high-intensity exercise.

Tricarboxylic acid cycle intermediates during incremental exercise in healthy subjects and in patients with McArdle's disease

1. The importance of the level of tricarboxylic acid cycle intermediates (malate, citrate and fumarate) for energy transduction during exercise has been investigated in six healthy subjects and in two patients with muscle phosphorylase deficiency (McArdle's disease). 2. Healthy subjects cycled for 10 min at low (50 W), moderate [130 +/- 6 W (mean +/- SEM)] and high (226 +/- 12 W) work rates, corresponding to 26, 50 and 80% of their maximal O2 uptake, respectively. Patients with McArdle's disease cycled for 11-13 min at submaximal (40 W) rates, and to fatigue at maximal work rates of 60-90 W. 3. In healthy subjects, phosphocreatine was unchanged during low work rates, but decreased to 79 and 32% of the initial level during moderate and high work rates. In patients with McArdle's disease, phosphocreatine decreased to 82 and 34% of the initial level during submaximal and peak exercise. Muscle lactate increased in healthy subjects during exercise at moderate and high work rates, but remained low in patients with McArdle's disease. 4. In healthy subjects, tricarboxylic acid cycle intermediates were similar at rest and at low work rates (0.48 +/- 0.04 mmol/kg dry weight), but increased to 1.6 +/- 0.2 mmol/kg dry weight and 4.0 +/- 0.3 mmol/kg dry weight at moderate and high work rates. The tricarboxylic acid cycle intermediate level in patients with McArdle's disease was similar to that in healthy subjects at rest, but was markedly reduced during exercise when compared at the same relative intensity. The peak level of tricarboxylic acid cycle intermediates in patients with McArdle's disease was 22% of that in healthy subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of prolonged exercise on the contractile properties of human quadriceps muscle

The contractile properties of the quadriceps muscle were measured in seven healthy male subjects before, during and after prolonged cycling to exhaustion. Special efforts were made to obtain measurements immediately after exercise. The exercise intensity corresponded to about 75% of estimated maximal O2 uptake and time to exhaustion was mean 85 (SEM 9) min. At the end of the cycling heart rate and perceived exertion for the legs were 94% and 97% of maximal values, respectively. Maximal voluntary isometric force (MVC) had decreased after 5 min of exercise to a mean 91 (SEM 4)% of the pre-exercise value (P < 0.05) and decreased further to a mean 82 (SEM 6) and mean 66 (SEM 5)% after 40-min cycling and at exhaustion, respectively. A new finding was that during recovery reversal of MVC occurred in different phases where the half recovery time of the initial rapid phase was about 2 min. The MVC was a mean 80 (SEM 2)% of the pre-exercise value after 30 min and was not affected by superimposed electrical stimulation. Maximal voluntary concentric and eccentric forces decreased to 74% and 80% of initial values at exhaustion (P < 0.05). The kinetics of isometric contraction expressed as the time between 5% and 50% of tension (rise time) and the time between 95% and 50% of tension (relaxation time) were not significantly affected by the prolonged cycling. The electromechanical delay measured as the time between the first electrical stimulus and 5% of tension decreased from a mean 32 (SEM 1) ms at rest to a mean 26.6 (SEM 0.6) ms at fatigue (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of hypoxia on muscle oxygenation and metabolism during arm exercise in humans

The influence of hypoxaemia on anaerobic energy production during arm exercise (AE) has been investigated. Six men were studied during maximal AE and during 10 min of sitting submaximal AE under both normoxic (AEN) and hypoxic (AEH, respiratory hypoxia, 12% O2) conditions. Peak pulmonary oxygen uptake (VO2) during maximal AE in normoxia and hypoxia was 2.25 +/- 0.15 and 2.18 +/- 0.14 l min-1, respectively (P < 0.05). The absolute workload was the same during submaximal AEN and AEH and corresponded to 54% of peak VO2 during normoxic maximal AE. To eliminate the potential influence of differences in catecholamine levels on the metabolic response, the submaximal experiments were performed during beta-adrenoceptor blockade. Oxygen deficit was 1.45 +/- 0.26 and 1.67 +/- 0.191 during AEN and AEH, respectively (n.s.). Oxygen extraction at steady state was lower during AEH than during AEN, and assuming a similar O2 demand this corresponds to a 14% higher muscle blood flow during AEH. At the onset of both AEN and AEH, O2 extraction (a-v O2) across the arm increased transiently above that at steady state, the increase being more pronounced during AEN than during AEH (P < 0.05). Muscle oxygenation, measured by near-infrared spectroscopy, demonstrated an initial decrease which was partially reversed as exercise proceeded. The reversal of muscle O2 desaturation was slower in all subjects during AEH (t1/2 = 2.4 +/- 0.2 min) than during AEN (t1/2 = 1.2 +/- 0.2 min; P < 0.01). After 10 min of exercise, arterial blood lactate was higher (P < 0.05) during AEH (5.5 +/- 0.2 mmol l-1) than during AEN (4.9 +/- 0.6 mmol l-1), whereas arterial plasma ammonia (NH3) was similar. The arteriovenous difference for both lactate and ammonia was similar during AEN and AEH. It is concluded that the high anaerobic energy production at the onset of AE is associated with a transient increase in O2 extraction and a transient decrease in muscle oxygenation. The effects of hypoxaemia on peak VO2, oxygen deficit and blood metabolites are less pronounced than previously described during submaximal leg exercise (LE).

Effect of muscle mass on lactate formation during exercise in humans

To elucidate the mechanisms of lactate formation during submaximal exercise, eight men were studied during one- (1-LE) and two-leg (2-LE) exercise (approximately 11-min cycling) using the catheterization technique and muscle biopsies (quadriceps femoris muscle). The absolute exercise intensity and thus the energy demand for the exercising limb was the same [mean 114 (SEM 7) W] during both 1-LE and 2-LE. At the end of exercise partial pressure of O2 and O2 saturation in femoral venous blood were lower and arterial adrenaline and noradrenaline were higher during 2-LE than during 1-LE. Mean arterial blood lactate concentration increased to 10.8 (SEM 0.8) (2-LE) and 5.2 (SEM 0.4) mmol.l-1 (1-LE) after 10 min of exercise. The intramuscular metabolic response to exercise was attenuated during 1-LE [mean, lactate = 49 (SEM 9); glucose 6-P = 3.3 (SEM 0.3); nicotinamide adenine dinucleotide, reduced = 0.17 (SEM 0.02); adenosine 5'-diphosphate 2.7 (SEM 0.1) mmol.kg dry mass-1] compared to 2-LE [76 (SEM 6); 6.1 (SEM 0.7); 0.21 (SEM 0.02); 3.0 (SEM 0.1) mmol.kg dry mass-1, respectively]. To elucidate whether the lower plasma adrenaline concentration could contribute to the attenuated metabolic response, additional experiments were performed on four of the eight subjects with infusion of adrenaline during 1-LE (1-LEE). Average plasma adrenaline concentration was increased during 1-LEE and reached 2-4 times higher levels than during 2-LE. Post-exercise muscle lactate and glucose 6-P contents were higher during 1-LEE than during 1-LE and were similar to those during 2-LE.(ABSTRACT TRUNCATED AT 250 WORDS)

High lactate and NH3 release during arm vs. leg exercise is not due to beta-adrenoceptor stimulation

To investigate the differences in metabolic response between arm exercise (AE) and leg exercise (LE) and to elucidate the underlying mechanisms, seven men were studied during 20 min of AE or LE both with (beta) and without (control, C) nonselective beta-blockade (beta B) (propranolol). The work loads corresponded to 59 and 60% of peak pulmonary O2 uptake (VO2) during LE and AE, respectively. Pulmonary VO2 increased more slowly at the onset of exercise during AEC (half time = 61 +/- 9 s) than during LEC (half time = 35 +/- 3 s) and was not affected by beta B. At the onset of exercise the arteriovenous O2 difference across the exercising limb increased above that of steady state during AEC but not during LEC. This demonstrates that the adjustment of O2 delivery is slower than that of arm VO2 during AE. Despite the smaller exercising muscle mass, the release of lactate and NH3 was about twofold higher during AEC than during LEC. The difference in metabolic response between AE and LE was not altered by beta B. Lactate release was not reduced by beta B but, if anything, tended to increase during both AE and LE (beta vs. C). beta B increased NH3 release during LE (beta vs. C) but not during AE (beta vs. C). We conclude that AE compared with LE at the same relative work load is associated with a greater release of lactate and NH3, indicating a more severe metabolic stress during AE. Furthermore, the present data suggest that the increase in blood lactate at these submaximal exercise intensities is caused by factors other than beta-adrenoceptor stimulation.

Absence of phosphocreatine resynthesis in human calf muscle during ischaemic recovery

Changes in the metabolites phosphocreatine (PCr), Pi and ATP were quantified by 31P n.m.r. spectroscopy in the human calf muscle during isometric contraction and recovery under ischaemic conditions. Time resolution of the measurements was 10 s. During a 30-60 s ischaemic isometric contraction, PCr decreased linearly at a rate of 1.17%/s (relative to the resting value) at a contraction strength equivalent to 70% of the maximal voluntary contraction (MVC) and at a rate of 2.43%/s at 90% MVC. There was a corresponding increase in Pi but the concentration of ATP did not change. pH decreased linearly during contraction by 4.22 and 8.23 milli-pH units/s at 70 and 90% MVC respectively. During a subsequent 5 min interval of ischaemic recovery, PCr, Pi, ATP, phosphomonoesters and calculated free ADP, free AMP and pH retained the value they had attained by the end of contraction with no significant recovery. Thus it is concluded that anaerobic glycolysis and glycogenolysis is halted momentarily on termination of contraction and that PCr is not resynthesized during ischaemic recovery. This paradoxical arrest of glycolytic flow in spite of the very significantly elevated concentration of potent activators such as Pi and free AMP clearly indicates that parameters other than PCr, ATP, Pi, calculated pH, free ADP and free AMP regulate glycolysis and glycogenolysis of human skeletal muscle very efficiently under ischaemic conditions.

Biochemical indicators of hazardous shoulder-neck loads in light industry

Prolonged, repetitive handling of light material is known to increase the risk of shoulder-neck disorders. Biological risk indicators related to musculoskeletal exposure, applicable by the general practitioner in the workplace, could provide an instrument for early intervention and rehabilitation. Eight women were studied, all full-time workers performing assembly tasks associated with a high prevalence of shoulder-neck complaints. All subjects were more tender in the shoulder region than young women in low-risk occupations. Heart rate recordings indicated a low general metabolic load during work. Concentrations in antecubital venous blood of several markers for metabolic stress and cellular damage (lactate, ammonia, hypoxanthine, urate, malondialdehyde, potassium, creatine kinase) were normal for all subjects, and showed no increase during 3 consecutive working days. Thus, the blood markers did not reflect hazardous shoulder-neck exposure.

High anaerobic energy release during submaximal arm exercise

The anaerobic energy release during submaximal arm (AE) and leg exercise (LE) has been estimated from O2 deficit measured at the onset of exercise. Eight male subjects were studied during 8-10 min of arm or leg cycling at the same relative workload (53% of the peak exercise-induced increase in pulmonary oxygen uptake, VO2). The workloads were 78 +/- 4 W during AE and 173 +/- 11 W during LE and VO2 was 1.51 +/- 0.06 1 min-1 for AE and 2.33 +/- 0.15 1 min-1 for LE. The half-time of the VO2 on-response was considerably longer (P < 0.01) during AE (62 +/- 9 s) than during LE (33 +/- 4 s) and the peak blood lactate concentration was higher (P < 0.05) during AE (4.8 +/- 0.5 mmol.l-1) than during LE (3.5 +/- 0.4 mmol.l-1). Oxygen deficit was 1.64 +/- 0.16 and 1.78 +/- 0.16 1 for AE and LE respectively. Oxygen deficit was higher during AE than during LE when related to absolute workload (P < 0.01), or to VO2 at steady state (P < 0.001) or to limb volume (P < 0.001). The proportion of the total energy demand covered by anaerobic energy release at the onset of exercise (0-8 min) was about 54% higher (P < 0.01) during AE than during LE. It is concluded that the energy release to a greater extend is covered by anaerobic processes during AE than during LE.

Non-invasive measurements of O2 availability in human skeletal muscle with near-infrared spectroscopy

The availability of O2 in the human vastus lateralis muscle has been investigated with non-invasive near-infrared spectroscopy (NIRS) using a commercially available unit (RunMan, NIM Inc. Philadelphia). The measuring probe placed above the skin illuminates the underlying tissue and measures the reflected light at two wavelengths (760 and 850 nm). Due to differences in the absorption spectra between HbO2 and Hb the difference in light intensity at these two wavelengths will be a relative index of tissue oxygenation. Prolonged arterial occlusion and static contraction have been studied. Arterial occlusion resulted in a decreased O2 saturation with a half-time of 2.3 +/- 0.2 min (mean +/- SE, n = 4). Restoration of blood flow resulted in a rapid tissue reoxygenation with a half-time of 24 +/- 2 s. Reoxygenation after static contraction occurred with a half time of 19-37 s. The half-time of reoxygenation subsequent to exercise and/or ischemia may be a valuable parameter in sports medicine and in the evaluation of peripheral vascular disease.

Metabolic factors in fatigue

The supply of energy is of fundamental importance for the ability to sustain exercise. The maximal duration of exercise is negatively related to the relative intensity both during dynamic and static exercise. Since exercise intensity is linearly related to the rate of energy utilisation this suggests that energetic deficiency plays a major role in the aetiology of muscle fatigue. Characteristic metabolic changes in the muscle are generally observed at fatigue--the pattern being different after short term exercise (lactate accumulation and phosphocreatine depletion) from after prolonged exercise at moderate intensity (glycogen depletion). A common metabolic denominator at fatigue during these and many other conditions is a reduced capacity to generate ATP and is expressed by an increased catabolism of the adenine nucleotide pool in the muscle fibre. Transient increases in ADP are suggested to occur during energetic deficiency and may be the cause of fatigue. Experimental evidence from human studies demonstrate that near maximal power output can be attained during acidotic conditions. Decreases in muscle pH is therefore unlikely to affect the contractile machinery by a direct effect. However, acidosis may interfere with the energy supply possibly by reducing the glycolytic rate, and could by this mechanism be related to muscle fatigue.

Repetitive static muscle contractions in humans--a trigger of metabolic and oxidative stress?

Repetitive static exercise (RSE) is a repetitive condition of partial ischaemia/reperfusion and may therefore be connected to the formation of oxygen-derived free radicals and tissue damage. Seven subjects performed two-legged intermittent knee extension exercise repeating at 10 s on and 10 s off at a target force corresponding to about 30% of the maximal voluntary contraction force. The RSE was continued for 80 min (n = 4) or to fatigue (n = 3). Four of the subjects also performed submaximal dynamic exercise (DE) at an intensity of about 60% maximal oxygen uptake (VO2max) for the same period. Whole body oxygen uptake (VO2) increased gradually with time during RSE (P less than 0.05), indicating a decreased mechanical efficiency. This was further supported by a slow increase in leg blood flow (P less than 0.05) and leg oxygen utilization (n.s.) during RSE. In contrast, prolonged RSE had no effect on VO2 during submaximal cycling. Maximal force (measured in six additional subjects) declined gradually during RSE and was not completely restored after 60 min of recovery. After 20 and 80 min (or at fatigue) RSE phosphocreatine (PC) dropped to 74% and 60% of the initial value, respectively. A similar decrease in PC occurred during DE. Muscle and arterial lactate concentrations remained low during both RSE and DE. The three subjects who were unable to continue RSE for 80 min showed no signs of a more severe energy imbalance than the other subjects. A continuous release of K+ occurred during both RSE and DE.(ABSTRACT TRUNCATED AT 250 WORDS)

Deficiency of skeletal muscle succinate dehydrogenase and aconitase. Pathophysiology of exercise in a novel human muscle oxidative defect

We evaluated a 22-yr-old Swedish man with lifelong exercise intolerance marked by premature exertional muscle fatigue, dyspnea, and cardiac palpitations with superimposed episodes lasting days to weeks of increased muscle fatigability and weakness associated with painful muscle swelling and pigmenturia. Cycle exercise testing revealed low maximal oxygen uptake (12 ml/min per kg; healthy sedentary men = 39 +/- 5) with exaggerated increases in venous lactate and pyruvate in relation to oxygen uptake (VO2) but low lactate/pyruvate ratios in maximal exercise. The severe oxidative limitation was characterized by impaired muscle oxygen extraction indicated by subnormal systemic arteriovenous oxygen difference (a-v O2 diff) in maximal exercise (patient = 4.0 ml/dl, normal men = 16.7 +/- 2.1) despite normal oxygen carrying capacity and Hgb-O2 P50. In contrast maximal oxygen delivery (cardiac output, Q) was high compared to sedentary healthy men (Qmax, patient = 303 ml/min per kg, normal men 238 +/- 36) and the slope of increase in Q relative to VO2 (i.e., delta Q/delta VO2) from rest to exercise was exaggerated (delta Q/delta VO2, patient = 29, normal men = 4.7 +/- 0.6) indicating uncoupling of the normal approximately 1:1 relationship between oxygen delivery and utilization in dynamic exercise. Studies of isolated skeletal muscle mitochondria in our patient revealed markedly impaired succinate oxidation with normal glutamate oxidation implying a metabolic defect at the level of complex II of the mitochondrial respiratory chain. A defect in Complex II in skeletal muscle was confirmed by the finding of deficiency of succinate dehydrogenase as determined histochemically and biochemically. Immunoblot analysis showed low amounts of the 30-kD (iron-sulfur) and 13.5-kD proteins with near normal levels of the 70-kD protein of complex II. Deficiency of succinate dehydrogenase was associated with decreased levels of mitochondrial aconitase assessed enzymatically and immunologically whereas activities of other tricarboxylic acid cycle enzymes were increased compared to normal subjects. The exercise findings are consistent with the hypothesis that this defect impairs muscle oxidative metabolism by limiting the rate of NADH production by the tricarboxylic acid cycle.

Changes in plasma hypoxanthine and free radical markers during exercise in man

Eight men cycled for about 6 minutes at workloads corresponding to 44 and 72% of maximal oxygen uptake and to fatigue at 98% maximal oxygen uptake. Blood samples from a brachial artery and a femoral vein were taken at rest and during exercise. Hypoxanthine, xanthine and urate in plasma were significantly elevated at fatigue and after 10 minutes of recovery. Only hypoxanthine showed a significant arterio-femoral venous difference. The release of hypoxanthine from the legs increased during the recovery period and was three-fold higher 10 minutes post exercise than at the end of exercise. It is concluded that the marked increase in plasma hypoxanthine which occurs during intensive exercise originates from the working muscle whereas the transformation to xanthine and urate may occur in other tissues. Glutathione, methemoglobin and malondialdehyd (MDA) were used as plasma markers of free radicals. Total glutathione (glutathione + glutathionedisulfide) in blood and plasma increased during intensive exercise and may be indicative of free radical formation. However, MDA was not detectable in plasma during any conditions (less than 0.1 mumol x l-1 plasma) and methemoglobin decreased slightly during exercise. Further studies using more specific techniques are required to determine whether the formation of free radicals is increased after brief intensive exercise.

Regulation of glucose utilization in human skeletal muscle during moderate dynamic exercise

The effect of bicycle exercise (75% of maximal oxygen uptake) on glucose uptake by the inferior limb (LGU) and glycolysis in human skeletal muscle has been investigated. Biopsies were obtained from the quadriceps femoris muscle before exercise, after 5 and 40 min of exercise, and at fatigue [74.9 +/- 4.7 (SE) min]. LGU was 0.05 +/- 0.02 mmol/min at rest, increased approximately sevenfold after 5 min of exercise, and continued to increase linearly during the first 40 min of exercise. Thereafter LGU stabilized at approximately 1.4 mmol/min until fatigue. Intracellular glucose was low at rest but increased sixfold after 5 min of exercise (P less than 0.01 vs. rest); thereafter, intracellular glucose decreased and was not significantly different from the value at rest after 40 min or at fatigue (P greater than 0.05). D-Glucose 6-phosphate (G-6-P) and alpha-D-glucose 1,6-bisphosphate (G-1,6-P2) (inhibitors of hexokinase) increased significantly after 5 min of exercise (approximately 300% G-6-P; approximately 25% G-1,6-P2) and then decreased continuously. The muscle glycolytic rate (glycogenolysis + glucose uptake) averaged 7.7 mmol.kg dry wt-1.min-1 during the first 40 min of exercise and 3.7 mmol.kg dry wt-1.min-1 during the last 35 min of exercise. The contribution of extracellular glucose to muscle glycolysis was estimated to be only 5 and 19% during the initial and latter phases of exercise, respectively. It is concluded that, during the initial phase of exercise, glucose utilization is limited by phosphorylation, probably due to G-6-P-dependent inhibition of hexokinase.(ABSTRACT TRUNCATED AT 250 WORDS)

Tricarboxylic acid cycle intermediates in human muscle during prolonged exercise

Seven subjects cycled to fatigue [75 +/- 5 (SE) min] at a work load corresponding to approximately 75% of their maximal oxygen uptake. Biopsies were taken from the quadriceps femoris muscle at rest and during exercise. Muscle glycogen decreased from a preexercise level of 445 +/- 33 mmol glucosyl units/kg dry wt to 50 +/- 14 at fatigue. The sum of the measured tricarboxylic acid cycle intermediates (TCAI = malate + citrate + fumarate + oxaloacetate) was 0.49 +/- 0.05 mmol/kg dry wt at rest, increased to 4.41 +/- 0.23 after 5 min of exercise, and then decreased continuously to 3.33 +/- 0.29 and to 2.83 +/- 0.27 mmol/kg dry wt after 40 min of exercise and at fatigue (P less than 0.05 vs. 5 min), respectively. The point of fatigue was characterized by an enhanced deamination of AMP (judged by increase in IMP) and reduced contents (vs. 5 min of exercise) of lactate, pyruvate, and alanine. In contrast, acetylcarnitine (reflects the availability of acetylunits) increased threefold at the onset of exercise and was maintained approximately at this level until fatigue. It is concluded that prolonged exercise to fatigue at moderate work loads results in glycogen depletion, energy deficiency (increased AMP deamination), reduced levels of three-carbon compounds and TCAI (compared with the initial phase of exercise) but in maintained levels of acetylunits. The present data indicate that carbohydrate depletion may impair aerobic energy production by reducing the level of TCAI.

Impaired oxidative metabolism increases adenine nucleotide breakdown in McArdle's disease

Two patients with muscle phosphorylase deficiency [McArdle's disease (McA)] were studied during bicycle exercise at 40 (n = 2) and 60 W (n = 1). Peak heart rate was 170 and 162 beats/min, corresponding to approximately 90% of estimated maximal heart rate. Muscle samples were taken at rest and immediately after exercise from the quadriceps femoris. Lactate content remained low in both muscle and blood. Acetylcarnitine, which constitutes a readily available form of acetyl units and thus a substrate for the tricarboxylic acid cycle, was very low in McA patients both at rest and during exercise, corresponding to approximately 17 and 11%, respectively, of that in healthy subjects. Muscle NADH was unchanged during exercise in McA patients in contrast to healthy subjects, in whom NADH increases markedly at high exercise intensities. Despite low lactate levels, arterial plasma NH3 and muscle inosine 5'-monophosphate increased more steeply relative to work load in McA patients than in healthy subjects. The low postexercise levels of lactate, acetylcarnitine, and NADH in McA patients support the idea that exercise performance is limited by the availability of oxidative fuels. Increases in muscle inosine 5'-monophosphate and plasma NH3 indicate that lack of glycogen as an oxidative fuel is associated with adenine nucleotide breakdown and increased deamination of AMP. It is suggested that the early onset of fatigue in McA patients is caused by an insufficient rate of ADP phosphorylation, resulting in transient increases in ADP.

Influence of ATP turnover and metabolite changes on IMP formation and glycolysis in rat skeletal muscle

Deamination of AMP to inosine monophosphate (IMP) and NH3 is thought to be regulated by the observed increases in ADP, AMP, and H+. We have examined this hypothesis by comparing the rate of IMP accumulation in contracting and noncontracting rat skeletal muscle. The rate of IMP formation was high during ischemic contraction, and consistent with previous studies, formation of IMP was associated with high levels of muscle lactate, depletion of phosphocreatine (PCr), and increased levels of ADP and AMP. When the contraction period was followed by 5-min anoxic recovery, the metabolic changes were maintained, but no further IMP or lactate was formed. During long-term (2-4 h) anoxia, the rate of IMP formation was less than 4% of that during contraction, despite similar changes in PCr, lactate, ADP, and AMP. It is concluded that the observed changes in the intracellular chemical environment are not sufficient to explain the high rate of IMP formation during contraction but that a combination of metabolic stress and a high ATP turnover rate is required. It is suggested that a high ATP turnover rate during conditions of metabolic stress results in transient increases in ADP and AMP at the site of ATP hydrolysis and that these activate AMP deaminase and glycolysis. An alternative hypothesis is that these processes are regulated by the increase in cytosolic Ca2+ in a contracting muscle.