Investigation of human mitochondrial myopathies by phosphorus magnetic resonance spectroscopy

ADP recovery after a brief ischemic exercise in normal and diseased human muscle--a 31P MRS study

The pattern of cytosolic ADP recovery after exercise has not been fully characterized in human skeletal muscle. ADP recovery after brief, ischemic exercise was studied by 31phosphorus magnetic resonance spectroscopy in calf muscles of 33 normal control subjects, four patients with McArdle's disease and 13 patients with mitochondrial myopathy. In normal muscle, the half-time for the initial ADP decline was 0.18 +/- 0.07 min and was unaffected by the pH or the metabolic state at the end of exercise. ADP decreased to below rest values during the second min of recovery in 27 out of 33 control subjects. There was a significant (p < 0.001) linear correlation for both the size (r = 0.65) and duration (r = 0.64) of this ADP undershoot with intracellular pH. Phosphocreatine resynthesis continued during the ADP undershoot. ADP undershoot was also found in patients with mitochondrial diseases (in 11 out of 13), but not McArdle's disease (six patients). Thus ADP recovery follows a complex time course that is partly dependent on pH. Only the initial ADP recovery is independent of pH, which makes it suitable for comparative assessment of muscle mitochondrial function in vivo. As phosphocreatine recovery continues during the ADP undershoot, mitochondrial regulation must be different from that at the onset of recovery. These observations are consistent with variable, changing regulators of mitochondrial metabolism in human skeletal muscle.

Magnetic resonance spectroscopy in congenital heart disease

OBJECTIVE

To determine the feasibility of studying myocardial and skeletal muscle bioenergetics using 31P magnetic resonance spectroscopy (MRS) in babies and young children with congenital heart disease.

SUBJECTS

16 control subjects aged 5 months to 24 years and 18 patients with CHD, aged 7 months to 23 years, of whom 11 had cyanotic CHD, five had cardiac failure, and two had had a Senning procedure.

DESIGN

31P MRS was carried out using a 1.9 Tesla horizontal 65 cm bore whole body magnet to study the myocardium in 10 patients and skeletal muscle (gastrocnemius) in 14 patients, eight of whom were exercised, together with appropriate controls.

RESULTS

In hypoxaemic patients, in skeletal muscle at rest intracellular pH (pHi) was abnormally high [7.06 (SEM 0.04) v 7.04 (0.05), P < 0.01] and showed a positive correlation with haemoglobin (P < 0.03). On exercise, hypoxaemic patients fatigued more quickly but end-exercise pHi and phosphocreatine recovery were normal, implying that an equivalent but smaller amount of work had been performed. End-exercise ADP concentration was lower. On recovery, the initial rate of phosphocreatine resynthesis was low. Skeletal muscle bioenergetics were within normal limits in those in heart failure. In the myocardium, the phosphocreatine/ATP ratio was similar in controls and hypoxaemic subjects, but low in those in heart failure.

CONCLUSIONS

In heart failure, the myocardial phosphocreatine/ATP ratio was reduced, as in adults, while resting skeletal muscle studies were normal. By contrast, hypoxaemic children had normal myocardial bioenergetics, but showed skeletal muscle alkalinity, and energy reserves were more readily depleted on exercise. On recovery, the initially slow phosphocreatine resynthesis rate reflects a low rate of mitochondrial ATP synthesis, probably due to an inadequate oxygen supply. 31P MRS offers a safe, non-invasive method of studying myocardial and skeletal muscle bioenergetics in children as young as 5 months.

Impairment of muscle mitochondrial oxidative metabolism in McArdles's disease

Impairment of muscle glycogenolysis in McArdle's disease (myophosphorylase deficiency) leads to exercise intolerance and exercise-induced myalgia. The pathophysiology of these symptoms is not entirely clear. We used phosphorus magnetic resonance spectroscopy to measure muscle phosphate metabolite concentrations and intracellular pH during brief ischemic exercise and in the period of aerobic metabolic recovery after exercise, with special attention to cytoplasmic adenosine 5'-diphosphate (ADP). In 5 patients with McArdle's disease, calculated muscle intracellular ADP concentrations at the beginning of recovery were higher than in normal control subjects (70-425 mmol/L, control mean: 73 +/- 40 mmol/L, P < 0.05). The half-time for intracellular ADP recovery after exercise, an index of maximal mitochondrial oxidative phosphorylation, was 0.16 +/- 0.07 in normal controls and was independent of metabolic state or intracellular pH. ADP recoveries were abnormally slow in all patients with McArdle's disease (range: 0.32-0.83 min, mean = 0.2 min, P < 0.0001). These results are indicative of a limitation in the rate of oxidative phosphorylation in muscle of patients with McArdle's disease, most likely due to impaired substrate delivery to mitochondria. This impairment of mitochondrial function may contribute to the exercise-related symptoms in McArdle's disease.

Autosomal dominant limb girdle myopathy with ragged-red fibers and cardiomyopathy. A pedigree study by in vivo 31P-MR spectroscopy indicating a multisystem mitochondrial defect

We describe a late-onset autosomal dominant limb girdle myopathy, associated with dilated cardiomyopathy and mental deterioration. In two affected members of the pedigree with histochemical (ragged-red and cytocrome c oxidase - negative fibers) and ultrastructural abnormalities of muscle mitochondria, in vivo muscle phosphorus MR spectroscopy disclosed a slow rate of phosphocreatine resynthesis after exercise. Brain phosphorus MR spectroscopy revealed a defect of the energy metabolism in the two patients and in a third asymptomatic member, as shown by a significantly low phosphocreatine, increased ADP and decreased phosphorylation potential. Molecular analysis of muscle mitochondrial DNA failed to reveal any known mutation, including multiple deletions of the mtDNA which have been associated with some autosomal dominant mitochondrial diseases. The multisystem clinical involvement, the presence of ragged-red fibers and the alterations revealed by in vivo brain and muscle 31P-MRS suggest that this limb-girdle syndrome represents an unusual phenotype of mitochondrial cytopathy.

Localization and amount of myoglobin and myoglobin mRNA in ragged-red fiber of patients with mitochondrial encephalomyopathy

The localization and amounts of myoglobin (Mb) and Mb mRNA in ragged-red fibers (RRF) in skeletal muscle of 6 patients with mitochondrial encephalomyopathy were examined immunohistochemically and by in situ hybridization. The amounts of Mb and Mb mRNA were expressed in terms of optical densities (ODs) of Mb immunostaining and Mb mRNA signals. In nonatrophic RRF, Mb was predominantly seen in the ragged-red region and Mb mRNA signals were increased throughout the sarcoplasm. The amounts of Mb and Mb mRNA in nonatrophic RRF were greater than those in nonatrophic non-RRF. In contrast, the localization and amount in atrophic RRF were similar to those in atrophic non-RRF. Thus, Mb synthesis in nonatrophic RRF may increase to compensate for mitochondrial dysfunction and to supply sufficient oxygen to mitochondria, but this compensatory function may be impaired in atrophic RRF.

The treatment of congenital lactic acidoses

Congenital lactic acidoses form a heterogeneous group of disorders: this paper considers primarily defects of the pyruvate dehydrogenase complex and the respiratory chain. Attempts to treat these disorders are hampered by uncertainty concerning the pathophysiology and by the central role of the enzymes in cellular metabolism. Few strategies are of proven efficacy, though many have been tried, including dietary manipulation, enhancement of residual enzyme activity, artificial electron acceptors and free-radical scavengers. Evaluation of treatment is complicated by the rarity, heterogeneity and unpredictable course of the diseases. Double-blind placebo-controlled trials are needed.

In vivo evidence of abnormal mechanical and oxidative functions in the exercised muscle of dystrophic hamsters by 31P-NMR

Mechanical properties and metabolic adaptation to exercise in skeletal muscle of dystrophic hamsters were studied with an in vivo 31P-NMR multistep fatigue test. Three successive 20-min steps with increasing rhythms of tetanic stimulation were followed by a 20-min recovery period. Fatigue in dystrophic hamsters (DH) developed more rapidly and was greater than in normal hamsters (NH); total mechanical performance per min increased step by step in NH while it decreased in DH, showing a progressive mechanical impairment of the dystrophic muscles. ADP and PCr recovery rates were significantly reduced in DH muscles. Acidosis appeared in both DH and NH and persisted in DH throughout the test, suggesting reduced mitochondrial oxidative capacity of the dystrophic muscle. The pH recovery rate was reduced in DH muscles suggesting a reduction in export protons capacity. These results provide evidence of impaired mitochondrial function and intracellular ionic regulation in the dystrophic muscle, associated with the lack of dystrophin and dystrophin-associated glycoproteins in the DH.

A relationship between impaired fetal growth and reduced muscle glycolysis revealed by 31P magnetic resonance spectroscopy

Thinness at birth is associated with insulin resistance and an increased prevalence of non-insulin-dependent diabetes mellitus in adult life. As muscle is an important site of insulin resistance, and because thin babies have reduced muscle mass, thinness at birth may affect muscle structure and function and impair carbohydrate metabolism. We have therefore used 31P magnetic resonance spectroscopy to investigate the bioenergetics of gastrocnemius and flexor digitorum superficialis muscles in 16 normoglycaemic women who had a low ( < or = 23 kg/m3) and 9 women who had a high (> 23 kg/m3) ponderal index at birth. In the flexor digitorum superficialis study anaerobic metabolism was stressed with a constant heavy workload. Low ponderal index subjects fatigued more rapidly (3.3 vs 5.8 min); as phosphocreatine decreased, the accompanying drop in muscle pH was less than in the high ponderal index group. In the first minute of exercise phosphocreatine fell and adenosine diphosphate rose more rapidly (p=0.04 and 0.03, respectively). Gastrocnemius showed a similar trend late in exercise (this exercise was more oxidative, becoming more anaerobic with increasing workload). These changes were not explained by differences in body composition, muscle mass or blood flow. The findings are consistent with a decreased lactic acid and glycolytic adenosine triphosphate production in the low ponderal index group and suggest the possibility that the mechanisms which control substrate utilisation and metabolism in adult life be programmed during prenatal life.

Bryostatin 1, a novel antineoplastic agent and protein kinase C activator, induces human myalgia and muscle metabolic defects: a 31P magnetic resonance spectroscopic study

Bryostatin 1, a novel antineoplastic agent and protein kinase C (PKC) activator, has been found to induce myalgia (muscle pain) 48 h after administration in clinical trials. This is the dose-limiting toxicity and has restricted the duration of therapy in phase I trials. To investigate the mechanisms and try to increase toleration of the drug, we studied calf muscle metabolism of 14 patients at rest and during exercise and subsequent recovery using 31P magnetic resonance spectroscopy (MRS) before and 4 h, 48-72 h and 1-2 weeks following bryostatin therapy. In resting muscle there was a significant (P < 0.001) increase in the phosphodiester/adenosine 5'-triphosphate (PDE/ATP) ratio 48 h post bryostatin and in patients with myalgia compared with pre-bryostatin control studies. Following exercise, patients with myalgia showed significantly slower phosphocreatine (PCr) and ADP recovery half-time (P < or = 0.05) suggesting impaired mitochondrial (oxidative) energy production, possibly due to a direct effect on the mitochondria or secondary to reduced blood flow. The apparent proton efflux rate following exercise was significantly reduced 4 h after bryostatin (P < or = 0.05), suggesting reduced blood flow. The rate of post-exercise reoxygenation was studied in four patients by near-infrared spectroscopy 4 h post bryostatin. In three of these the rate was reduced, consistent with reduced muscle blood flow. Bryostatin 1 appeared to cause a long-lasting impairment of oxidative metabolism and proton washout from muscle, consistent with a vasoconstrictive action. Thus these studies provide evidence for two mechanisms of the dose-limiting toxicity for bryostatin. Prospective studies on the use of vasodilators to improve the tolerance of the drug should be carried out.

Skeletal muscle mitochondrial dysfunction in alternating hemiplegia of childhood

Alternating hemiplegia of childhood is an uncommon disease characterized by repeated, transient attacks of hemiplegia. Its pathophysiology is uncertain, but attention recently has focused on possible mitochondrial abnormalities. Using 31P magnetic resonance spectroscopy, we studied gastrocnemius muscle in 5 patients with alternating hemiplegia, aged 8 to 30 (mean, 18) years, at rest and during incremental aerobic exercise and recovery. There were no significant differences in resting muscle between patients and a control group aged 7 to 42 (mean, 19) years. Exercise performance was grossly impaired in the patients, the mean duration being 30% of normal. The total change in pH during exercise was somewhat less than in control subjects, while the changes in phosphocreatine concentration and intracellular ADP were similar. Thus the average overall rate of fall of phosphocreatine concentration during exercise was three-fold greater than in control subjects. However, the initial rate of ATP turnover at the start of exercise (a measure of muscle mass and efficiency) was not abnormal. During recovery, both the initial rate of phosphocreatine resynthesis and the calculated mitochondrial capacity were reduced by about 35%. This mitochondrial defect probably explains most of the abnormalities seen during exercise.

Lipoic (thioctic) acid increases brain energy availability and skeletal muscle performance as shown by in vivo 31P-MRS in a patient with mitochondrial cytopathy

A woman affected by chronic progressive external ophthalmoplegia and muscle mitochondrial DNA deletion was studied by phosphorus magnetic resonance spectroscopy (31P-MRS) prior to and after 1 and 7 months of treatment with oral lipoic acid. Before treatment a decreased phosphocreatine (PCr) content was found in the occipital lobes, accompanied by normal inorganic phosphate (Pi) level and cytosolic pH. Based on these findings, we found a high cytosolic adenosine diphosphate concentration [ADP] and high relative rate of energy metabolism together with a low phosphorylation potential. Muscle MRS showed an abnormal work-energy cost transfer function and a low rate of PCr recovery during the post-exercise period. All of these findings indicated a deficit of mitochondrial function in both brain and muscle. Treatment with 600 mg lipoic acid daily for 1 month resulted in a 55% increase of brain [PCr], 72% increase of phosphorylation potential, and a decrease of calculated [ADP] and rate of energy metabolism. After 7 months of treatment MRS data and mitochondrial function had improved further. Treatment with lipoate also led to a 64% increase in the initial slope of the work-energy cost transfer function in the working calf muscle and worsened the rate of PCr resynthesis during recovery. The patient reported subjective improvement of general conditions and muscle performance after therapy. Our results indicate that treatment with lipoate caused a relevant increase in levels of energy available in brain and skeletal muscle during exercise.

Metabolic abnormalities in skeletal muscle of patients receiving zidovudine therapy observed by 31P in vivo magnetic resonance spectroscopy

Patients on long-term zidovudine (AZT) therapy experience muscle fatigue and weakness attributed to AZT-induced mitochondrial toxicity in skeletal muscle. To determine if the clinico-pathological abnormalities in these patients correspond to abnormal muscle energy metabolism, we used 31P in vivo magnetic resonance spectroscopy to follow phosphorylated metabolites during exercise. We studied 19 normal volunteers, 6 HIV-positive patients never treated with AZT, and 9 HIV-positive patients who had been treated with AZT for a mean period of 33 mo (range 12-48 mo) and had muscle biopsy-proven AZT-myopathy with abnormal mitochondria. Changes in phosphocreatine, ATP, and intracellular pH in the gastrocnemius muscle were followed during a graded steady state exercise protocol, and the recovery of phosphocreatine was followed on cessation of exercise. We found that graded steady state exercise produced a greater depletion of muscle phosphocreatine levels in the AZT-treated patients, compared to either HIV-positive patients who were not treated with AZT or normal controls. No differences in the effects of steady state exercise on muscle phosphocreatine levels were observed between the control group and the HIV-positive patients who had not been treated with AZT. The results suggest that the effect of AZT on muscle energy metabolism is significant, and similar to the effect observed in patients with known mitochondrial myopathies. Using a well-known model for control of mitochondrial metabolism, the observed differences in steady state phosphocreatine levels during exercise suggest that AZT treatment decreases the maximal work output and the maximal rate of muscle ATP synthesis.

Quasi-linear relationship between Gibbs free energy of ATP hydrolysis and power output in human forearm muscle

The postulated strictly linear descriptions of the rate dependence of oxidative phosphorylation in skeletal muscle on the free energy of ATP hydrolysis (delta GP) over the range of physiological steady states fail to harmonize with reported findings of identical basal respiration rates in mammalian muscles at different delta GP values. The relevance of an extension of the strictly linear description to a description deriving from enzyme kinetics that predicts a sigmoidal dependence was investigated in human finger flexor muscle using 31P-nuclear magnetic resonance spectroscopy. At constant pH 7.0, the experimental variation of adenine nucleotide concentrations with power output, which reflects the rate of oxidative phosphorylation, was compared with predictions by various formulations of adenine nucleotide control of respiration. The quasi-linear sigmoidal description was found to be statistically equivalent but physiologically superior to the strictly linear description. The predicted maximal oxidatively sustained steady-state power output and rate-dependent sensitivity of respiration to changes in delta GP were in agreement both with theoretical considerations and with experimental observations in the present study and other studies of intact mammalian skeletal muscle.

ATP production and mechanical work in exercising skeletal muscle: a theoretical analysis applied to 31P magnetic resonance spectroscopic studies of dialyzed uremic patients

31P magnetic resonance spectroscopy (31P MRS) can yield much information about bioenergetics in skeletal muscle. During mixed aerobic/glycolytic exercise, changes in phosphocreatine (PCr) concentration and pH may be abnormal because of reduced muscle mass or reduced efficiency (which the authors combine here as "effective muscle mass") or because of reduced oxidative capacity. The authors show how these can be distinguished by calculating the nonoxidative and oxidative costs of mechanical work, and also of work per unit of effective muscle mass (measured using the initial rate of ATP turnover). These quantities are substantially time-independent during incremental exercise, and so can be used to compare exercise studies of differing duration. The authors illustrate this analysis by showing that in dialyzed patients with chronic renal failure, the substantial exercise abnormalities seen by 31P MRS are due mainly to a decrease in effective muscle mass, which outweighs the oxidative defect implied by the abnormal PCr recovery kinetics.

Abnormal brain and muscle energy metabolism shown by 31P-MRS in familial hemiplegic migraine

Familial hemiplegic migraine (FHM) is a rare autosomal dominant disorder of unknown pathogenesis characterized by migraine and transitory hemiplegic attacks. We describe a kindred fulfilling the diagnostic criteria for FHM in which: (1) brain phosphorus magnetic resonance spectroscopy (31P-MRS) showed a reduced phosphocreatine content accompanied by high [ADP], high percentage of V/Vmax of ATP biosynthesis and decreased phosphorylation potential; (2) muscle 31P-MRS showed a reduced rate of phosphocreatine recovery after exercise; (3) blood lactate was increased after effort; (4) muscle biopsy showed, in one patient, rare ragged red fibers succinate-dehydrogenase positive and cytochrome c oxidase negative; (5) genetic analysis of muscle mitochondrial DNA did not show any of the two point mutations in the tRNA(Leu(UUR)) associated with the MELAS syndrome (Mitochondrial myopathy, Encephalopathy with Lactic Acidosis and Stroke-like episodes). The defective energy metabolism of brain and muscle found in this pedigree suggests a multisystemic disorder of mitochondrial function in this FHM pedigree.

Twitch response of striated muscle in patients with progressive external ophthalmoplegia, mitochondrial myopathy and focal cytochrome c-oxidase deficiency

Some aspects of the contractile properties of skeletal muscle in patients with progressive external ophthalmoplegia (PEO), mitochondrial myopathy and focal cytochrome c-oxidase deficiency were investigated by studying the twitch response (TR) of the tibialis anterior muscle both at rest and after maximum isometric contraction. The results of needle electromyography were normal in four of the six examined patients, and myopathic in the remaining two. A slowing in muscle relaxation was the most frequently observed abnormality; significantly prolonged muscle contraction times and reduced twitch torque potentiation values after isometric contraction were also detected. TR abnormalities in PEO patients may be due either to a dysfunction of the contractile machinery depending upon impaired muscle energy supply or to altered muscle fiber characterized by the predominance of type I slow fiber. In addition to conventional electromyographic investigations, TR study may be a useful diagnostic tool in PEO patients.

Epilepsia partialis continua associated with NADH-coenzyme Q reductase deficiency

We report the clinical, neuroradiological and biochemical features of a patient with epilepsia partialis continua (EPC). MRI studies disclosed multiple cortico-subcortical areas of abnormal signal intensity. The activity of complex I of the mitochondrial respiratory chain was markedly reduced in skeletal muscle. The biochemical defect was reflected in vivo by a failure of brain and skeletal muscle bioenergetics, as shown by exercise and phosphorus magnetic resonance spectroscopy (31P-MRS) studies. Muscle morphology was repeatedly normal, and molecular genetic analysis of mitochondrial DNA was not informative. On the basis of in vivo and in vitro findings, the observed defect of the mitochondrial respiratory chain was considered the underlying biochemical pathogenesis of the disease. The observation of an oxidative defect in the brain and skeletal muscle of a patient with EPC emphasizes the importance of studying mitochondrial energy metabolism in patients with EPC not associated with primary CNS lesions when clinical and morphological findings suggesting a mitochondrial disorder are lacking. 31P-MRS can be a useful method to uncover deficits of CNS mitochondrial function and provide the indication for further biochemical studies.

Familial myopathy with conspicuous depletion of mitochondria in muscle fibers: a morphologically distinct disease

Three patients (two of them siblings) presented with easy fatiguability and prominent postexercise pain. Muscle biopsy showed that large areas of about one third of the type II fibers were completely devoid of mitochondria. The remaining mitochondria were unusually large in size, but otherwise normal ultrastructurally. In two patients, 31P in vivo MRS showed low phosphocreatine (PCr), high ADP, low phosphorylation potential at rest and slow ADP and PCr recovery after aerobic exercise. This appears to be a pathologically unique form of metabolic myopathy. The cause of the focal mitochondrial depletion is not known. It should be distinguished from the mtDNA depletion syndrome in which muscle mitochondria are not reduced, but proliferate.

Bioenergetics of skeletal muscle in mitochondrial myopathy

31Phosphorus nuclear magnetic resonance spectroscopy was used to examine skeletal muscle in 29 patients with mitochondrial myopathy, 9 male and 20 female. Gastrocnemius was investigated in 15 patients and 30 normal subjects and finger flexor muscle (flexor digitorum superficialis, fds) in 24 patients and 35 normal controls. Both muscles were studied in 10 of the patients. Results were abnormal (outside the full range of normal values) in all but 2 patients. In 86% of patients (25/29) abnormalities were detected in resting muscle. In most cases there was a low phosphocreatine/ATP ratio, high calculated free [ADP] and low phosphorylation potential. At rest, abnormality was detected with equal ease in fds and gastrocnemius. Exercise and recovery increased the sensitivity of MRS in detecting abnormal metabolism. Finger flexion was better tolerated by patients than plantar flexion and gave bigger changes in metabolite concentrations and intracellular pH. Thus, results from fds were more easily differentiated from normal. Exercise duration was significantly shorter than in controls while phosphocreatine depletion was more rapid than normal, consistent with a shortfall in mitochondrial ATP synthesis. Nearly all patients (25/27, 93%) showed abnormalities during recovery from exercise. [ADP] was high during exercise and its recovery was delayed, providing increased drive for oxidative phosphorylation. Phosphocreatine resynthesis during recovery (which reflects oxidative ATP synthesis) was slow both in absolute terms and in relation to [ADP]. Recovery of intracellular pH after exercise was significantly more rapid than normal, consistent with an upregulation of proton efflux.(ABSTRACT TRUNCATED AT 250 WORDS)

Brain and muscle energy metabolism studied in vivo by 31P-magnetic resonance spectroscopy in NARP syndrome

Phosphorus magnetic resonance spectroscopy (31P-MRS) was used to study in vivo the energy metabolism of brain and skeletal muscle in two members of an Italian pedigree with NARP syndrome due to a point mutation at bp 8993 of mtDNA. In the youngest patient, a 13 year old girl with retinitis pigmentosa, ataxia, and psychomotor retardation, there was an alteration of brain energy metabolism shown by a decreased phosphocreatine content, increased [ADP] and decreased phosphorylation potential. The energy metabolism of her skeletal muscle was also abnormal, as shown by resting higher inorganic phosphate and lower phosphocreatine concentrations than in normal subjects. Her mother, a 41 year old woman with minimal clinical involvement, showed a milder derangement of brain energy metabolism and normal skeletal muscle. Findings with MRS showed that this point mutation of mtDNA is responsible for a derangement of energy metabolism in skeletal muscle and even more so in the brain.

A 31P magnetic resonance spectroscopy study of mitochondrial function in skeletal muscle of patients with Parkinson's disease

The activity of complex I of the respiratory chain is decreased in the substantia nigra of patients with Parkinson's disease (PD) but the presence of this defect in skeletal muscle is controversial. Therefore, the mitochondrial function of skeletal muscle in patients with PD was investigated in vivo using 31P magnetic resonance spectroscopy. Results from 7 PD patients, 11 age matched controls and 9 mitochondrial myopathy patients with proven complex I deficiency were obtained from finger flexor muscle at rest, during exercise and in recovery from exercise. In resting muscle, the patients with mitochondrial myopathy showed a low PCr/ATP ratio, a low phosphorylation potential, a high P(i)/PCr ratio and a high calculated free [ADP]. During exercise, stores of high energy phosphate were depleted more rapidly than normal, while in recovery, the concentration of phosphocreatine and free ADP returned to pre-exercise values more slowly than normal. In contrast, the patients with PD were not significantly different from normal for any of these variables, and no abnormality of muscle energetics was detected. Three of the PD patients also had mitochondrial function assessed biochemically in muscle biopsies. No respiratory chain defect was identified in any of these patients by polarography or enzyme analysis when compared with age-matched controls. These results suggest that skeletal muscle is not a suitable tissue for the investigation and identification of the biochemical basis of the nigral complex I deficiency in PD.

Effects of cardiac transplantation on bioenergetic abnormalities of skeletal muscle in congestive heart failure

BACKGROUND

Patients with advanced heart failure have bioenergetic abnormalities of skeletal muscle metabolism during exercise. Using 31P magnetic resonance spectroscopy, we sought to determine whether skeletal metabolic responses to exercise are normalized by orthotopic cardiac transplantation.

METHODS AND RESULTS

Four groups were studied: healthy normal volunteers (n = 9), subjects awaiting heart transplantation (n = 10), subjects < 6 months (mean, 4 months) after transplant (n = 9), and subjects > 6 months (mean, 15 months) after transplant (n = 8). None of the posttransplant patients had biopsy evidence of rejection at the time of study. There were no significant differences in age, preoperative functional class, or symptom duration among the three patient groups. Metabolic responses were monitored in the dominant arm during incremental weight pull exercise and 10 minutes of recovery by 31P magnetic resonance spectroscopy, with measurement of pH and the phosphocreatine (PCr)/(PCr + inorganic phosphate [Pi]) ratio, an index of PCr concentration. In addition, based on recovery data, the rate of PCr resynthesis was calculated as a measure of oxidative metabolism that is independent of work level, recruitment, or muscle mass, and the effective maximal rate of mitochondrial ATP synthesis (Vmax) was determined. Analysis was by ANOVA. There were no differences between groups in pH or PCr/(PCr + Pi) at rest. Compared with the normal control group, the pretransplant group had a decreased exercise duration (11.3 +/- 2.5 versus 15.0 +/- 1.3 minutes, P = .02), a lower submaximal exercise PCr/(PCr + Pi) ratio (0.58 +/- 0.11 versus 0.76 +/- 0.08, P < .05), a reduced PCr resynthesis rate (13 +/- 6 versus 22 +/- 9 mmol/L per minute, P < .05), and a lower calculated Vmax (26 +/- 14 versus 53 +/- 26 mmol/L per minute, P < .05). In the group studied early after transplantation, all the changes noted in the pretransplant group persisted and were if anything somewhat worse. In the group studied late after transplantation, there was a significant improvement in the PCr resynthesis rate compared with the early-posttransplant group (27 +/- 6 late versus 15 +/- 6 mmol/L per minute early, P < .05) and statistically nonsignificant trends toward improvements in submaximal exercise pH (6.86 +/- 0.24 late versus 6.72 +/- 0.24 early) and submaximal PCr/(PCr + Pi) ratio (0.56 +/- 0.14 late versus 0.44 +/- 0.15 early) and Vmax (45 +/- 21 late versus 33 +/- 15 mmol/L per minute early). However, compared with normal subjects, exercise duration and submaximal PCr/(PCr + Pi) were still reduced in the late-posttransplant group.

CONCLUSIONS

Despite successful heart transplantation, skeletal muscle abnormalities of advanced heart failure persist for indefinite periods, although partial improvement occurred at late times. The persistent abnormalities may contribute to the reduced exercise capacity that is present in most patients after transplantation.

Hypernatremic myopathy

We describe a 21-year-old man presenting with proximal muscle weakness associated with hypernatremia. His manifestations other than muscle weakness included dry skin, loss of axillary and pubic hair, decreased libido and loss of thirst sensation. His serum sodium level was elevated to 169-171 mEq./l but all other electrolytes were normal. In addition, serum CK was elevated and an EMG study showed myogenic changes. Endocrinological studies revealed hypothalamic hypopituitarism, while MRI revealed a suprasellar mass. A partial correction of hypernatremia led to an immediate recovery of the muscle weakness as well as a normalization of both the serum CK level and EMG findings, suggesting a direct association between the muscle weakness and hypernatremia. The phosphocreatine/inorganic phosphorus (PCr/Pi) ratios in the resting calf muscle, obtained using 31P magnetic resonance spectroscopy (MRS), were very low during the state of muscle weakness, while they returned to nearly normal values after clinical improvement, suggesting that the muscle weakness in hypernatremic state was caused by a depletion of the intramuscular energy stores, probably due to an overworking Na-K pump to correct the intracellular electrolyte imbalance.

Comparisons of ATP turnover in human muscle during ischemic and aerobic exercise using 31P magnetic resonance spectroscopy

To investigate human muscle bioenergetics quantitatively in vivo, we used 31P magnetic resonance spectroscopy to study the flexor digitorum superficialis of four adult males during dynamic ischemic and aerobic exercise at 0.50-1.00 W and during recovery from aerobic exercise. During exercise, changes in pH and [PCr] were larger at higher power, but in aerobic exercise neither end-exercise [ADP] nor the initial postexercise PCr resynthesis rate altered with power. In ischemic exercise we estimated total ATP synthesis from the rates of PCr depletion and glycogenolysis (inferred using an analysis of proton buffering); this was linear with power output. In aerobic exercise, again we estimated ATP synthesis rates due to phosphocreatine hydrolysis and glycogenolysis (incorporating a correction for proton efflux) and also estimated oxidative ATP synthesis by difference, using the total ATP turnover rate established during ischemic exercise. We conclude that in early exercise oxidative ATP synthesis was small, increasing by the end of exercise to a value close (as predicted) to the initial postexercise rate of PCr resynthesis. Furthermore, a plausible estimate of proton efflux during aerobic exercise can be inferred from the pH-dependence of proton efflux in recovery.

Defect in the lipoyl-bearing protein X subunit of the pyruvate dehydrogenase complex in two patients with encephalomyelopathy

Among the many metabolic encephalomyelopathies caused by deficiencies in the pyruvate dehydrogenase complex (PDHC), nearly all involve its E1 subunit. We describe two new familial cases of PDHC deficiency with encephalomyelopathy, chronic lactic acidemia, and a normal E1 subunit of PDHC but deficiency in another component. Activity of PDHC was measured in cultured skin fibroblasts and skeletal muscle, and immunoblot studies were performed on mitochondrial extracts from skin fibroblasts. Spectra of muscle tissue, obtained in vivo with phosphorus 31 nuclear magnetic resonance, were recorded both at rest and with exercise. The PDHC activity was markedly reduced to 10% to 20% of normal values in both cultured skin fibroblasts and skeletal muscle. Immunoblotting of skin fibroblast mitochondrial extracts showed a specific deficiency in the protein X component of PDHC but normal E1, E2, and E3 components. Spectra obtained with 31P nuclear magnetic resonance showed alterations compatible with those found in mitochondrial myopathies. This is the second description of an encephalomyelopathy associated with a specific absence of the lipoyl-containing protein X component, which has a structural role in the formation of a functional PDHC.

Experimental design of 31P MRS assessment of human forearm muscle function: restrictions imposed by functional anatomy

The restrictions imposed by the functional anatomy of the finger flexor muscles on the experimental design of 31P MRS assessment of human forearm muscle function employing surface coil localization and voluntary exercise were investigated. It was found that 31P MRS metabolic data of finger flexor muscle should be correlated with mechanical data of combined flexion of only the ring and little fingers, rather than all four fingers as has been commonly the case in previously reported studies.

Further impairment of muscle phosphate kinetics by lengthening exercise in DMD/BMD carriers. An in vivo 31P-NMR spectroscopy study

We used phosphorus magnetic resonance spectroscopy (31P-MRS) to study the effect of exercise-induced muscle injury in the calf muscle of 7 DMD/BMD carriers and 6 non-carrier females. All subjects performed 50-80 lengthening contractions with the right calf muscles. 48 h after lengthening exercise non-carriers showed increased sensitivity to pressure in their gastrocnemius accompanied by increased T2 relaxation times and by elevated Pi/PCr ratios at rest. DMD/BMD carriers did not show any effect of lengthening exercise on these measurements. In-magnet exercise revealed in all carriers a reduced initial rate of Pi recovery and an increased time to fully recovery the resting value of intracellular pH. Lengthening exercise further decreased the initial rate of Pi recovery. Non-carriers did not show any variation attributable to lengthening exercise either during in-magnet work or during recovery from exercise. We found that lengthening exercise contractions causes: (1) less muscle injury in carriers compared to non-carriers, (2) even slower rate of Pi recovery, but (3) no effect on Pi recovery in non-carriers. The use of lengthening exercise and measurements of Pi recovery may be a useful method to evaluate the disease process in DMD/BMD.

Quantitative analysis by 31P magnetic resonance spectroscopy of abnormal mitochondrial oxidation in skeletal muscle during recovery from exercise

We use the hyperbolic relationship between cytosolic [ADP] and the rate of phosphocreatine (PCr) resynthesis after exercise to estimate the apparent maximum rate of oxidative ATP synthesis (QMAX). We examine data from some human diseases in which mitochondrial oxidation may be impaired (due to reduced mitochondrial numbers, intrinsic mitochondrial defect or impaired vascular supply). Muscle responds to impaired oxidation by stimulating anaerobic ATP synthesis and/or by increasing [ADP], the stimulus to the mitochondrion. However, these responses interact: [ADP] depends on pH and [PCr], and lactic acid production tends to lower [ADP] (by lowering pH), while proton efflux has the opposite effect. We identify four patterns of results: (A) in mitochondrial myopathy, apparent QMAX is reduced and [ADP] is appropriately increased, because increased proton efflux reduces the pH change in exercise despite increased lactic acid production; (B) in some conditions (e.g., cyanotic congenital heart disease) apparent QMAX is reduced but there is no compensatory rise in [ADP], probably because anaerobic ATP synthesis during exercise is increased without increase in proton efflux; (C) in other conditions (e.g., myotonic dystrophy) [ADP] is increased during exercise but apparent QMAX is normal, suggesting either an increase in proton efflux and/or decrease in anaerobic ATP synthesis during exercise; (D) there are also conditions (e.g., respiratory failure) where, despite impaired oxygen supply, both apparent QMAX and end-exercise [ADP] are normal. We also discuss the metabolic conditions under which end-exercise [ADP] is increased by a mitochondrial defect.(ABSTRACT TRUNCATED AT 250 WORDS)

No evidence of mitochondrial abnormality in skeletal muscle of patients with iron-deficient anaemia

OBJECTIVES

Patients with iron deficiency anaemia complain of decreased exercise capacity. We asked whether this is due to defective oxidative ATP synthesis in skeletal muscle as a consequence of reduced blood oxygen content and/or intrinsic mitochondrial abnormalities.

DESIGN

We used 31P magnetic resonance spectroscopy to examine skeletal muscle bioenergetics in iron-deficient patients and in age- and sex-matched controls.

SETTING

The patients were recruited from the primary care population.

SUBJECTS

We studied seven symptomatic female iron-deficient patients (aged 32-70 years) with haemoglobin (Hb) concentration, [Hb], 8.0 g dl-1. Six had menorrhagia, the cause in the seventh patient remained undiagnosed. Results were compared with those of 8 healthy female controls (aged 25-48 years) with mean [Hb] 13.7 g dl-1.

RESULTS

The right calf muscle was by studied 31P magnetic resonance spectroscopy in a 1.9 T super-conducting magnet. We measured the intracellular concentrations of phosphocreatine (PCr), inorganic phosphate (Pi), adenosine triphosphate (ATP) and the intracellular pH at rest, during plantar flexion exercise and during recovery from exercise. Exercise duration was reduced in the patients, yet end-exercise PCr/(PCr+Pi) was higher and adenosine diphosphate (ADP) lower than in controls. After exercise, initial PCr recovery was slowed but this was probably because of the lower cytosolic ADP concentration.

CONCLUSIONS

Mitochondrial ATP synthesis was not limited by oxygen supply or an intrinsic mitochondrial defect. Therefore, the reduced exercise capacity seen in iron deficiency could be due to central causes and not to skeletal muscle metabolic abnormalities.

In vivo assessment of mitochondrial functionality in human gastrocnemius muscle by 31P MRS. The role of pH in the evaluation of phosphocreatine and inorganic phosphate recoveries from exercise

In this study we compared the kinetics of phosphocreatine (PCr) and P(i) recovery, and their dependency on cytosolic pH in 38 normal individuals. Spectra were acquired during rest, work and recovery. A time resolution of 10 s was used to obtain detailed information. The kinetics of PCr and P(i) recovery almost overlapped when the lowest value of cytosolic pH reached during recovery (termed the minimum pH) was < 6.95, while they were completely dissociated when the minimum pH was > 6.95. This result is interpreted as indirect in vivo evidence of the kinetic control exerted by ADP on mitochondrial oxidation. Our results represent a rationale for new experimental conditions to be used in clinical routine studies of pathologies due to primary or secondary mitochondrial malfunction.

Skeletal muscle bioenergetics in myotonic dystrophy

Skeletal muscle function of 15 patients with myotonic dystrophy (dystrophia myotonica, DM) was investigated using 31P magnetic resonance spectroscopy to evaluate bioenergetics and intracellular pH at rest and during exercise and recovery. Results from DM patients, normal controls and mitochondrial myopathy patients were compared in order to assess the possible contribution of abnormal mitochondrial metabolism to muscle dysfunction in DM. In resting DM muscle, intracellular pH (pHi) was normal, but there were significant elevations in the concentration ratios of Pi/ATP, phosphomonoesters/ATP and phosphodiesters/ATP. In patients with the most severe exercise intolerance the phosphocreatine/ATP ratio was also reduced. Resting muscle of 11 mitochondrial myopathy patients showed similar changes to those of the most exercise-intolerant DM patients. In exercising DM muscle, energy stores were rapidly depleted as in mitochondrial myopathy. Muscle acidified in all subjects, but in DM the decrease in pHi was less than in normal muscle. Recovery half-times for phosphocreatine, Pi and ADP were normal in DM but slow in mitochondrial myopathy. The initial rate of phosphocreatine repletion after exercise was rapid in DM, consistent with high [ADP], but slow in mitochondrial myopathy in spite of elevated [ADP]. Because recovery is an oxidative process, we conclude that there was no decrease in the oxidative capacity of the muscles in this group of DM patients. In the subjects in whom it could be measured, the rate of recovery of intracellular pH was greater in the 3 DM patients (0.14, 0.15 and 0.16 U/min) than in the 7 normal controls (0.08-0.12 U/min, mean 0.10). The results do not rule out a minor abnormality in glycogenolysis, but they suggest that the failure to acidify normally during exercise is probably due to rapid proton efflux.

Skeletal muscle bioenergetics in the chronic fatigue syndrome

Skeletal muscle bioenergetics and control of intracellular pH have been investigated in 46 patients with chronic fatigue syndrome by phosphorus magnetic resonance spectroscopy. The results have been compared with those from healthy controls and from a group of patients with mitochondrial cytopathies affecting skeletal muscle. No consistent abnormalities of glycolysis, mitochondrial metabolism or pH regulation were identified in the group when taken as a whole, although in 12 of the 46 patients the relationship between pH and phosphocreatine utilisation during exercise fell outside the normal range. Of these, 6 patients showed increased acidification relative to phosphocreatine depletion while 6 showed reduced acidification. These findings do not support the hypothesis that any specific metabolic abnormality underlies fatigue in this syndrome although abnormalities may be present in a minority of patients.

Calf Muscle Metabolism in Venous Insufficiency

Objectives:

To study the effect of deep venous insufficiency on calf muscle energy metabolism.

Design:

A paired study of affected and unaffected calf muscle in seven patients with unilateral lower limb deep venous insufficiency.

Investigations:

Phosphorus-31 magnetic resonance spectroscopy was used to study changes in vivo in pH, phosphocreatine and other phosphorus metabolites during and after exercise.

Results:

Glycolytic activity was increased in the affected muscle at the start of exercise but metabolic recovery following exercise was normal.

Conclusions:

Oxidative phosphorylation is impaired, at least at the start of exercise, but appears normal immediately after exercise. This may be due to inadequate blood flow, perhaps secondary to the rise in intramuscular pressure that occurs with exercise.

Cellular energetics of dystrophic muscle

Cytosolic pH and phosphorus metabolite ratios in skeletal muscle were measured by 31P magnetic resonance spectroscopy in patients with Duchenne muscular dystrophy (DMD) and Becker's muscular dystrophy (BMD) and in Duchenne/Becker carriers. In resting dystrophin-deficient muscle, there was a decrease in phosphocreatine (PCr) and increase in orthophosphate (Pi) relative to ATP, and an increase in calculated free [ADP]. Phosphomonester and phosphodiester were also increased relative to ATP. These changes were largest in DMD, smaller in BMD and small or absent in carriers. Cytosolic pH was increased substantially in DMD, moderately in BMD and slightly but significantly in gastrocnemius of carriers. Raised intracellular pH thus appears to be the most characteristic abnormality in dystrophin-deficient muscle. Responses to erobic exercise were studied in the forearm muscle flexor digitorum superficialis of carriers. PCr depletion during exercise was greater than normal but the fall in pH was disproportionately small, resulting in increased [ADP]. This is likely to result either from reduced anaerobic glycogenolysis to lactic acid or from increased proton efflux (as is seen in mitochondrial myopathy). Detailed analysis suggests: (1) at the start of exercise, calculated lactic acid production was increased, as was the rate of PCr depletion, suggesting that there was no absolute defect of glycogenolysis. (2) At the start of recovery, calculated proton efflux was not increased, although as the pH at the end of exercise was higher than in controls and proton efflux is normally pH-dependent, an up-regulation of proton efflux cannot be excluded. (3) Recovery of PCr, Pi and ADP after exercise were not impaired, suggesting that mitochondrial function is normal.

Evidence for mitochondrial dysfunction in patients with alternating hemiplegia of childhood

Phosphorus magnetic resonance spectra of resting muscle were obtained from 4 patients with alternating hemiplegia of childhood. All patients had abnormally high resonance intensities from inorganic phosphate and an abnormally low calculated cytosolic phosphorylation potential. Two of the 4 patients had abnormally low resonance intensities from phosphocreatine and an abnormally high calculated cytosolic free adenosine diphosphate concentration. These abnormalities are indicative of mitochondrial dysfunction. The combination of a central nervous system disorder and evidence of mitochondrial dysfunction in muscle suggests that alternating hemiplegia of childhood may represent a previously unrecognized phenotype of mitochondrial disease.

Oral phosphate supplements reverse skeletal muscle abnormalities in a case of chronic fatigue with idiopathic renal hypophosphatemia

A 57-yr-old man presented with a long history of undiagnosed fatigue but no evidence of bone disease. He was noted to have hypophosphatemia due to an idiopathic phosphaturia. Marked abnormalities of exercising skeletal muscle detected by phosphorus magnetic resonance spectroscopy and by plasma metabolite measurements were consistent with mitochondrial dysfunction. Oral phosphate supplements restored plasma phosphate concentration and muscle biochemistry to normal and produced considerable improvement in symptoms and exercise tolerance, although the phosphate concentration in muscle was only marginally low and increased little by treatment. We conclude that hypophosphatemia should be excluded in unexplained fatigue.

Skeletal muscle metabolism during exercise and recovery in patients with respiratory failure

BACKGROUND

Patients with respiratory failure have early fatiguability which may be due to limitation of oxygen supply for oxidative (mitochondrial) ATP synthesis. Skeletal muscle in exercise and recovery was studied to examine the effect of chronic hypoxia on mitochondrial activity in vivo.

METHODS

The skeletal muscle of five patients with respiratory failure (PaO2 < 9 kPa) was studied by phosphorus-31 magnetic resonance spectroscopy and compared with 10 age and sex matched controls. Patients lay in a 1.9 Tesla superconducting magnet with the gastrocnemius muscle overlying a six cm surface coil. Spectra were acquired at rest, during plantar flexion exercise, and during recovery from exercise. Relative concentrations of inorganic phosphate (Pi), phosphocreatine (PCr) and ATP were measured from peak areas, and pH and free ADP concentration were calculated. For the start of exercise, the rates of PCr depletion and estimated lactic acid production were calculated. For the post exercise recovery period, the initial rate of PCr recovery (a quantitative measure of mitochondrial ATP synthesis), the apparent Vmax for mitochondrial ATP synthesis (calculated from initial PCr resynthesis and the end exercise ADP concentration which drives this process), and the recovery half times of PCr, Pi, and ADP (also measures of mitochondrial function) were determined.

RESULTS

Considerably greater and faster PCr depletion and intracellular acidosis were found during exercise. This is consistent with limitation of oxygen supply to the muscle and might explain the early fatiguability of these patients. There was no abnormality in recovery from exercise, however, suggesting that mitochondria function normally after exercise.

CONCLUSIONS

These results are consistent with one or more of the following: (a) decreased level of activity of these patients; (b) changes in the fibre type of the muscle; (c) decreased oxygen supply to the muscle during exercise but not during recovery. They are not consistent with an intrinsic defect of mitochondrial ATP synthesis in skeletal muscle in respiratory failure.

Energy metabolism in muscle paresis and recovery studied by 31P-MR spectroscopy: a case report

We report a 31P Magnetic Resonance Spectroscopy (31P-MRS) study on the changes of energy metabolism in human leg anterior and lateral compartment muscles with paresis due to compression from a herniated lumbar disc at the L4-L5 level and recovery to normality. A low phosphocreatine to inorganic phosphate ratio due to both decreased phosphocreatine and increased inorganic phosphate contents and a normal intracellular pH were the features of muscles with paresis. Changes of 31P-MRS parameters were followed during 18 weeks of treatment with physical therapy until complete recovery. Results show that 31P-MRS is a useful clinical tool for detecting even small biochemical changes that may occur in muscles and for checking the effects of therapy.

Metabolic changes in reflex sympathetic dystrophy: a 31P NMR spectroscopy study

The lower leg skeletal muscles of 11 patients affected by reflex sympathetic dystrophy were investigated at rest by 31P nuclear magnetic resonance spectroscopy at a fieldstrength of 1.5 T. The results were compared with similar investigations of unaffected lower leg muscles of patients and volunteers. A significant increase was observed for the average tissue pH of the muscles of affected legs as deduced from the chemical shift of the resonance for inorganic phosphate. The average inorganic phosphate/phosphocreatine ratio of these muscles was also increased. The impairment of high energy phosphate metabolism, as deduced from the NMR data, may be caused by cellular hypoxia or diminished oxygen utilization, which would agree with previous findings that oxygen extraction is reduced in extremities affected by reflex sympathetic dystrophy.

31P NMR spectroscopy investigation of muscle metabolism in hemodialysis patients

Calf muscle metabolism of six patients with end-stage chronic renal failure undergoing maintenance hemodialysis and of six control subjects was studied using 31P nuclear magnetic resonance spectroscopy at 4.7 Tesla. Spectra were obtained at rest, during exercise and recovery. At rest, the inorganic phosphate, ATP and phosphocreatine concentrations, and the intracellular pH were similar in both groups of subjects. In the patients, the maximum workload achieved at the end of exercise led to a 84% and 46% depletion of phosphocreatine and ATP, respectively; under this condition, the intracellular pH fell to 6.50 +/- 0.09. In control subjects, a maximum workload caused no change in ATP concentration at the end of exercise, but a phosphocreatine depletion and an intracellular pH fall similar to those observed in the patients. Although the rate of phosphocreatine depletion during exercise was not different in the two groups of subjects, the decrease in intracellular pH was more rapid in the patients than in control subjects. At the end of maximum exercise, the rates of recovery of both phosphocreatine and intracellular pH were significantly reduced in the muscle of hemodialysis patients when compared to normal subjects. These results suggest that, in the calf muscle of hemodialysis patients, energy production via oxidative metabolism is impaired and compensated for by an increase in anaerobic glycolysis.

31P-magnetic resonance spectroscopy assessment of subnormal oxidative metabolism in skeletal muscle of renal failure patients

In hemodialysis patients, erythropoietin increases hemoglobin, but often the corresponding increase in peak oxygen uptake is low. The disproportionality may be caused by impaired energy metabolism. 31P-magnetic resonance spectroscopy was used to study muscle energy metabolism in 11 hemodialysis patients, 11 renal transplant recipients, and 9 controls. Measurements were obtained during rest, static hand-grip, and rhythmic hand-grip; recoveries were followed to baseline. During static hand-grip, there were no between-group differences in phosphocreatine (PCr), inorganic phosphate (Pi), or PCr/(PCr + Pi), although intracellular pH was higher in hemodialysis patients than transplant recipients. During rhythmic hand-grip, hemodialysis patients exhibited greater fatigue than transplant recipients or controls, and more reduction in PCr/(PCr + Pi) than transplant recipients. Intracellular pH was higher in controls than either hemodialysis patients or transplant recipients. Recoveries from both exercises were similar in all groups, indicating that subnormal oxidative metabolism was not caused by inability to make ATP. The rhythmic data suggest transplantation normalizes PCr/(PCr + Pi), but not pH. In hemodialysis patients, subnormal oxidative metabolism is apparently caused by limited exchange of metabolites between blood and muscle, rather than intrinsic oxidative defects in skeletal muscle.

Control of phosphocreatine resynthesis during recovery from exercise in human skeletal muscle

Information about the control of mitochondrial function in skeletal muscle in vivo can be obtained from the relationship between the rate of mitochondrial oxidation and the intracellular concentrations of phosphorus metabolites, although the analysis is complicated by the constraints imposed by the creatine kinase equilibrium. The rate of phosphocreatine (PCr) recovery after exercise measured by 31P MRS is an estimate of net oxidative ATP synthesis. Analysing such data from normal and abnormal human muscle, we show that the approximately exponential recovery kinetics of ADP and PCr imply that the rate of PCr resynthesis has a hyperbolic dependence on [ADP] but remains approximately linear with respect to the concentration of orthophosphate (Pi) and therefore also [PCr] and [creatine]. Both kinds of relationship are consistent with experimental data from exercising animal muscle and also with data from isolated mitochondria which suggest kinetic control of mitochondrial ATP synthesis of [ADP]. These relationships are altered in proven mitochondrial disease. This analysis offers a way to quantify mitochondrial function and its abnormalities in vivo.

The production, buffering and efflux of protons in human skeletal muscle during exercise and recovery

We show how quantitative information about proton handling in human skeletal muscle in exercise and recovery can be obtained by 31P MRS and illustrate this with data from metabolic disorders. Proton production, proton efflux and passive buffering can be distinguished by comparing changes in [phosphocreatine] and pH at the end of exercise and by calculating ATP turnover during ischaemic exercise and in the 'natural experiment' of myophosphorylase deficiency (McArdle's disease). We calculate the effective buffer capacity to be 20-30 mmol/L/pH unit (slykes), somewhat lower than published measurements made in vitro but similar to other values obtained in vivo. This analysis is applied to data from normal muscle and a variety of disease states to estimate proton efflux during recovery and ATP production during exercise: (i) proton efflux during recovery is pH-dependent, reaching a 10 mmol/L/min at pH 6.2, and is increased in some cases of mitochondrial myopathy and in hypertension; (ii) glycogenolytic ATP production during exercise can reach 25 mmol/L/min in normal muscle and correlates approximately with [Pi] at the start of aerobic exercise and throughout ischaemic exercise; (iii) oxidative ATP production can reach 20-25 mmol/L/min and (as during recovery) correlates approximately with [Pi].

Oral glucose lactate stimulation test in mitochondrial disease

We selected 23 patients with neurologic diseases, including 15 boys and 8 girls from 1 month to 10 years of age, who were divided into mitochondrial and nonmitochondrial disease groups. All patients were required to fast and rest for at least 4-10 hours before the test. Glucose was administered orally using a 50% glucose:water solution at a dose of 1.75 mg/kg. Blood samples then were drawn from a retained heparinized venous line at 0, 30, 60, 120, and 180 min and tested for lactate and glucose levels. Of the mitochondrial disease group, 10 of 11 patients had an upward sloping curve of lactate metabolism; the one who had a flat line response was a patient who suffered from a free-fatty-acid metabolic defect presenting with a recurrent Reye-like syndrome. There was a relatively flat line response in the nonmitochondrial disease group except in a patient with alternating hemiplegia whose symptoms responded well to flunarizine therapy. A significant increase in blood lactate content at 60 min after glucose loading occurred in the mitochondrial disease group, in contrast to that of the nonmitochondrial group. Sensitivity and specificity of a 5 mg/dl increase in blood lactate concentration at 60 min was 72.7% vs. 91.7% Moreover, all 4 patients whose blood lactate increased by 13 mg/dl at 60 min were in the mitochondrial disease group.

A 31P-NMR study of muscle exercise metabolism in mdx mice: evidence for abnormal pH regulation

We have studied exercise metabolism in vivo in the mdx mouse model of Duchenne muscular dystrophy with 31P-nuclear magnetic resonance spectroscopy. Intracellular pH, ratios of phosphocreatine (PCr) to ATP and PCr to inorganic phosphate (P(i)) expressed as PCr/ATP and PCr/(PCr+P(i)) as well as tension generated at the Achilles tendon were measured during sciatic nerve stimulation. Tension was similar between the mdx and control strain C57Bl/10ScSn at 10 Hz stimulation but slightly higher than the control at 100 Hz. The PCr/ATP and PCr/(PCr+P(i)) ratios were significantly reduced in mdx vs. control muscle during exercise. Although resting muscle pH in mdx mice is more alkaline than normal muscle, the pH of mdx muscle during exercise is reduced relative to controls, as is the rate of pH recovery. Total lactate is not elevated in the cells and so it is argued that there is a reduction in the capacity to export proton equivalents in muscles of mdx mice which could be caused by an elevation in intracellular sodium. This provides more evidence of impaired ionic regulation in dystrophic muscle and could be used as an index for the evaluation in vivo of therapeutic interventions such as myoblast transfer or gene replacement therapy.

Brain energy metabolism studied by 31P-MR spectroscopy in a case of migraine with prolonged aura

The brain and skeletal muscle oxidative metabolism of a patient with prolonged aura was studied by phosphorus magnetic resonance spectroscopy. We found that the phosphocreatine to ATP ratio in brain was reduced, while the inorganic phosphate to phosphocreatine ratio and the calculated ADP concentration were increased. The phosphorylation potential and percentage of maximal rate of ATP synthesis were also altered. Intracellular pH and inorganic phosphate concentration were normal. In muscle we found a low post-exercise recovery of phosphocreatine. These data indicate an impairment of energy oxidative metabolism both in brain and muscle.

Bio-energetic changes in human gastrocnemius muscle 1-2 days after strenuous exercise

[31P]magnetic resonance spectroscopy was used to study the metabolic sequelae of intense muscular activity in gastrocnemius of seven subjects 1-2 days after a 67-mile bicycle ride. The muscle was examined at rest, during a test exercise and during recovery from test exercise. Post-ride and pre-ride results were compared. At rest, the ratio of phosphocreatine to ATP (PCr/ATP) was increased post-ride; during test exercise PCr/(PCr+Pi) was lower post-ride; and the recoveries of PCr, Pi and PCr/(PCr+Pi) after test exercise were delayed, with decreased 'overshoot' of PCr/(PCr+Pi) (which is due to recovery of Pi to below its resting value). Mild mitochondrial damage (perhaps due to exposure to high cytosolic [Pi] during the bicycle ride) may explain some of these results. In contrast to reports of largely eccentric exercise there was no increase in resting Pi/ATP. We have thus demonstrated perturbations of muscle bio-energetics 1-2 days after strenuous exercise, in the absence of convincing enzymological evidence of muscle damage.

Changes in high-energy phosphates in rat skeletal muscle during acute respiratory acidosis

We used 31P magnetic resonance spectroscopy to study changes in phosphorus metabolite concentrations in rat skeletal muscle during respiratory acidosis (14 and 20% inspired CO2) and recovery. As intracellular pH fell (from 7.05 to 6.75 after 20 min of 20% CO2), intracellular [P(i)] increased by up to 50% while phosphocreatine concentration decreased by up to 8%. The sum of all intracellular phosphates remained constant. [ADP] decreased by up to 40% in accordance with the creatine kinase equilibrium but the phosphorylation potential [ATP]/([ADP][P(i)]) was preserved as a result of increased [P(i)]. This adjustment may be a mechanism for maintaining mitochondrial ATP synthesis despite low pH. Eventually this increase in cellular [P(i)] could lead to slow efflux of P(i) from the skeletal muscle cell contributing to the hyperphosphataemia of acute respiratory acidosis.