Investigation of metabolic myopathies by P-31 MRS using a standardized rest-exercise-recovery protocol: a survey of 800 explorations

Muscle fatigue during high-intensity exercise in children

Children are able to resist fatigue better than adults during one or several repeated high-intensity exercise bouts. This finding has been reported by measuring mechanical force or power output profiles during sustained isometric maximal contractions or repeated bouts of high-intensity dynamic exercises. The ability of children to better maintain performance during repeated high-intensity exercise bouts could be related to their lower level of fatigue during exercise and/or faster recovery following exercise. This may be explained by muscle characteristics of children, which are quantitatively and qualitatively different to those of adults. Children have less muscle mass than adults and hence, generate lower absolute power during high-intensity exercise. Some researchers also showed that children were equipped better for oxidative than glycolytic pathways during exercise, which would lead to a lower accumulation of muscle by-products. Furthermore, some reports indicated that the lower ability of children to activate their type II muscle fibres would also explain their greater resistance to fatigue during sustained maximal contractions. The lower accumulation of muscle by-products observed in children may be suggestive of a reduced metabolic signal, which induces lower ratings of perceived exertion. Factors such as faster phosphocreatine resynthesis, greater oxidative capacity, better acid-base regulation, faster readjustment of initial cardiorespiratory parameters and higher removal of metabolic by-products in children could also explain their faster recovery following high-intensity exercise.From a clinical point of view, muscle fatigue profiles are different between healthy children and children with muscle and metabolic diseases. Studies of dystrophic muscles in children indicated contradictory findings of changes in contractile properties and the muscle fatigability. Some have found that the muscle of boys with Duchenne muscular dystrophy (DMD) fatigued less than that of healthy boys, but others have reported that the fatigue in DMD and in normal muscle was the same. Children with glycogenosis type V and VII and dermatomyositis, and obese children tolerate exercise weakly and show an early fatigue. Studies that have investigated the fatigability in children with cerebral palsy have indicated that the femoris quadriceps was less fatigable than that of a control group but the fatigability of the triceps surae was the same between the two groups. Further studies are required to elucidate the mechanisms explaining the origins of muscle fatigue in healthy and diseased children. The use of non-invasive measurement tools such as magnetic resonance imaging and magnetic resonance spectroscopy in paediatric exercise science will give researchers more insight in the future.

Breakdown of adenine nucleotide pool in fatiguing skeletal muscle in McArdle's disease: a noninvasive 31P-MRS and EMG study

Energy metabolism and electrical muscle activity were studied in the calf muscles of 19 patients with proven McArdle's disease and in 25 healthy subjects. Phosphorus magnetic resonance spectroscopy and surface electromyography (S-EMG) were performed during two isometric muscle contractions of 3 min at 30% maximum voluntary contraction, one performed during normal perfusion and the other during applied ischemia. After about 1 min of ischemic muscle contraction in diseased muscle a significant acceleration in phosphocreatine breakdown was observed, along with a significant decrease in adenosine triphosphate. During both contractions the absence of glycolysis was shown by a significant alkalinization. Furthermore, in patients we observed a greater increase in the S-EMG amplitude than in control subjects. We conclude that early on during moderate exercise, a small number of muscle fibers reach metabolic depletion, indicated by a reduction in the adenine nucleotide pool. An increasing number of motor units, which are still in a high-energy state, are continuously recruited to compensate for muscle fatigue. This functional compartmentation may contribute to the pathophysiology of exercise intolerance in McArdle's disease.

New parameters reducing the interindividual variability of metabolic changes during muscle contraction in humans. A (31)P MRS study with physiological and clinical implications

Interindividual variations in skeletal muscle metabolism make comparative analyses difficult. In this study, we have addressed the issue of capturing the variability of metabolic performance observed during muscle exercise in humans by using an original method of normalization.Metabolic changes induced by various kinds of exercise were investigated using 31P magnetic resonance spectroscopy (MRS) at 4.7 T in 65 normal subjects (23 women and 42 men) and 12 patients with biopsy-proven muscular disorders. Large variations in the extent of PCr breakdown and intracellular acidosis were recorded among subjects and exercise protocols. For all the data pooled, the amplitude of mechanical performance accounts for 50% of these variations. When scaled to the work output, variations of PCr consumption account for 65% of pH changes through a linear relationship. This linear relationship was substantially improved (90%) when both variables were scaled to the square of work output performed (P1 and P2). By capturing most of the initial interindividual variability (90%), P1 vs. P2 relationship represents an ideal standardization procedure, independent of any anthropometric measurements. This relationship also discloses a significant link between the extent of PCr breakdown and intracellular acidosis regardless of exercise protocol. Moreover, changes in the slope of the P1 vs. P2 regression curve, as measured in old subjects and in selected patients, directly reflect alterations of energy production in muscle.

[Muscle, fatigue, sports and infection]

PURPOSE

Skeletal muscle can be considered as motors which convert chemical energy into mechanical energy. We can evaluate the intracellular pH and energy state of phosphate-containing metabolites in skeletal muscle of patients complaining fatigue or asthenia, using phosphorus MRS.

MAIN POINTS

Acute infectious disease and extreme endurance exercise may induce a loss of oxidative capacity of muscle tissue. Muscle fatigue is not due only to an insufficient supply of ATP to the energy consuming mechanisms. Phosphorus MRS show a muscle production of toxic metabolites such as lactates, protons and ammonia. These metabolic features induced excessive intracellular acidosis of skeletal muscle and systemic hyperammonia, responsible of fatigue and asthenia.

PERSPECTIVES

Reversal of the excessive acidosis and improvement of the capacity for oxidative ATP synthesis might help to relieve the symptoms of exhaustion/fatigue in these patients.

Aspects of human bioenergetics as studied in vivo by magnetic resonance spectroscopy

We outline the relevant capabilities of in vivo phosphorus MR spectroscopy by discussing some aspects of normal human biochemistry as studied by this technique. The transport of inorganic phosphate from cytosol into mitochondria in the human skeletal muscle was studied by exploiting a new experimental protocol. We found that Pi was transported into mitochondria in the absence of ATP biosynthesis and in the presence of a pH gradient. The control of CoQ on the efficiency of oxidative phosphorylation in the skeletal muscle and brain was studied by administering CoQ to patients with mitochondrial cytopathies due to known enzyme defects. Before CoQ we had detected a relevant reduction of mitochondrial functionality in the skeletal muscle as shown by the reduced rate of phosphocreatine recovery from exercise, and in the occipital lobes by reduced [phosphocreatine] and a high [ADP] and [Pi]. After CoQ all brain variables were remarkably improved. Treatment with CoQ also improved the rate of muscle phosphocreatine recovery from exercise. Our in vivo findings support the hypothesis that the concentration of CoQ rather than the rate of its lateral diffusion in the mitochondrial membrane controls the efficiency of oxidative phosphorylation. Other experiments were undertaken to clarify the functional relationship between cytosolic free [Mg2+] and cell bioenergetics in the intact human brain. In the same group of patients with mitochondrial cytopathies we found decreased delta G of ATP hydrolysis and low cytosolic free [Mg2+]. Treatment with CoQ resulted in improved brain bioenergetics and increased free [Mg2+]. These findings strongly indicate that decreased free magnesium was secondary to defective mitochondrial respiration, and support the hypothesis that cytosolic free [Mg2+] is regulated in the intact brain cell to equilibrate, at least in part, any changes in rapidly available free energy.

Exercise fuel mobilization in mitochondrial myopathy: a metabolic dilemma

In mitochondrial myopathy, severely impaired muscle oxidative capacity poses a dilemma for metabolic regulation in exercise. We inquired whether fuel mobilization during exercise in mitochondrial myopathy is adjusted to the reduced capacity to oxidize substrate, or if fuel is mobilized in excess of oxidative capacity. Hormonal and metabolic responses to 20 minutes of cycle exercise were studied in 4 patients with mitochondrial myopathy working at near maximal effort and in 4 healthy matched controls. On 2 separate days, controls were studied at the same absolute (A) workload (9 +/- 3 W) and the same relative (R) workload (77 +/- 9 W) as performed by the patients. During exercise, average glucose production was higher in patients (28 +/- 5 micromol min(-1) kg(-1)) than in controls at both workloads (A, 12 +/- 1; R, 18 +/- 2 micromol min(-1) kg(-1)). Exercise-induced increases in plasma glucose, growth hormone, epinephrine, norepinephrine, corticotropin, and lactate, and decreases in plasma insulin and pH were also larger in patients compared with findings in controls at both workloads. In conclusion, mitochondrial myopathies are associated with excessive neuroendocrine responses and mobilization of glucose during exercise. These responses augment ATP synthesis but result in progressive accumulation of nonoxidized substrates. Apparently, substrate mobilization and neuroendocrine responses in exercise are linked to oxidative demand rather than to oxidative capacity in working muscle.

Research into individual predisposition to develop acute rhabdomyolysis attributed to fenoverine

A xenobiotic, well tolerated by the majority of treated patients, can cause serious complications in patients with individual susceptibility. Based on the hypothesis that such a phenomenon may occur in rare cases of rhabdomyolysis attributed to fenoverine (DCI), we designed a protocol to look for a genetic predisposition. Six patients were included who had previously had an episode of rhabdomyolysis after taking fenoverine. A seventh patient was added, who had only experienced myalgia without cytolysis. All patients were investigated by the following tests: 31-phosphorus nuclear magnetic resonance spectroscopy, histopathological examination of the muscle, muscle contraction tests and biochemical analysis of the muscle. All patients examined proved to have muscle abnormalities. The pathology found varied greatly from patient to patient: mitochondrial myopathy, lipid storage myopathy, sensitivity to malignant hyperthermia or disorders of oxidative metabolism. The probability of finding by chance such rare muscle disorders associated with the equally rare rhabdomyolysis attributed to fenoverine is practically zero. We conclude that there is a cause and effect link between underlying abnormalities and the muscular cytolysis attributed to fenoverine.

Knee extension dynamometer: a new device for dynamic isokinetic magnetic resonance spectroscopy experiments

In the present study we introduce a new device for exercise magnetic resonance spectroscopy (MRS). It operates in a standard whole-body scanner. Mechanical exertion unit allows maximal 10 degrees to 15 degrees short-arc knee extensions. The device operates hydraulically and is based on isokinetic movement. The force and work conducted are automatically controlled by the electronic control and computer unit. A small surface coil placed on the vastus medialis muscle allows the collection of spectra without interfering spectra from nearby resting muscles. The force used for the extensions can be followed simultaneously as a curve on the screen in the operator's room and the data is transferred to a personal computer for later analysis. Total work and fatigue percentage are also calculated by the device. It also allows the use of different isokinetic exercise protocols. The measurements of force proved reliable in repeat measurements using an isokinetic test device as a control. This device has been used clinically for over a year, is easy to operate, and offers reliable measurements. It is well suited to trials where muscle energy states versus time are followed since it allows noninvasive simultaneous quantification of muscle performance and collecting MRS spectra at rest, during exercise, and in the recovery phase.

[Definition of a diagnostic score of malignant hyperthermia using P-31 magnetic resonance spectroscopy]

OBJECTIVES

To assess the metabolic muscular disorders associated with malignant hyperthermia (MH) using 31-P MRS, in order to develop a diagnostic tool for MH.

STUDY DESIGN

Retrospective analysis of a series of case.

PATIENTS

Group of 39 subjects, including 13 MH susceptible (MHS), and 26 not susceptible (MHN) members of recognized MHS families, according to the results of the in vitro contracture tests.

METHODS

Each subject underwent two protocols, both including rest, exercise and recovery periods. Exercise was performed successively under aerobic and ischaemic conditions.

RESULTS AND DISCUSSION

A significant early acidosis was recorded for the MHS group under both conditions of exercise (normoxia and ischaemia). However, the sensitivity of this parameter (77%) was not high enough to be considered as discriminant. Therefore a MRS score has been defined, corresponding to the sum of metabolic anomalies (acidosis, anomalies in PCr tum-over and in ATP or phosphomonoesters concentrations) recorded throughout both protocols. This score provided satisfactory results for both sensitivity (93%) and specificity (93%).

CONCLUSION

31-P MRS can act as a reliable diagnostic tool for MH.

Phosphorus-31 in vivo magnetic resonance spectroscopy of bone fails to diagnose osteoporosis

The values of in vivo T1 relaxation time (T1) of phosphorus atoms of wrist bone have been measured by phosphorus-31 magnetic resonance spectroscopy (MRS) in 65 menopausal women separated into three groups: (1) age-matched women without any paraclinical or clinical osteoporosis; (2) patients with paraclinical osteoporosis detected only by dual photonic absorptiometry; and (3) women with clinical osteoporosis with vertebral fractures. No significant differences were found in T1 values in the presence of paraclinical or clinical osteoporosis as compared to control values. No relationships were found among the T1, the value of the Z-score, the value of bone mineral content, the age of patients, the number of their children, and the age of menopause. Phosphorus-31 magnetic resonance spectroscopy of the wrist fails to separate osteoporotic from nonosteoporotic women and cannot be clinically used at this time to perform a noninvasive diagnosis of osteoporosis.

Recovery of muscular energy status in chronic alcoholics after 2 weeks of abstinence

Repeated and excessive consumption of alcohol leads to pathophysiological disorders in skeletal muscles. A successful management of this syndrome requires a strict abstinence and a nutritionally adequate diet. We propose here a simple and noninvasive investigation using 31P magnetic resonance spectroscopy (MRS) to monitor the recovery of the basal energy status of eminence thenar muscles from documented chronic alcoholic patients during a controlled 15-day period of abstinence. Cessation of alcohol abuse induced a significant recovery of the PCr/(PCr+P(i)) ratio otherwise depressed before the abstinence. On the contrary, the relative level of free inorganic phosphate decreased, whereas intracellular pH was not affected. These results demonstrate (a) the rapid improvement of basal muscular energy metabolism during abstinence for patients with a chronic and heavy alcohol consumption, and (b) the feasibility of a follow-up of this recovery by serial examinations using 31P MRS.