Kinetics of post-exercise phosphate transport in human skeletal muscle: an in vivo 31P-MR spectroscopy study

Intersubject differences in the effect of acidosis on phosphocreatine recovery kinetics in muscle after exercise are due to differences in proton efflux rates

(31)P magnetic resonance spectroscopy provides the possibility of obtaining bioenergetic data during skeletal muscle exercise and recovery. The time constant of phosphocreatine (PCr) recovery (tau(PCr)) has been used as a measure of mitochondrial function. However, cytosolic pH has a strong influence on the kinetics of PCr recovery, and it has been suggested that tau(PCr) should be normalized for end-exercise pH. A general correction can only be applied if there are no intersubject differences in the pH dependence of tau(PCr). We investigated the pH dependence of tau(PCr) on a subject-by-subject basis. Furthermore, we determined the kinetics of proton efflux at the start of recovery. Intracellular acidosis slowed PCr recovery, and the pH dependence of tau(PCr) differed among subjects, ranging from -33.0 to -75.3 s/pH unit. The slope of the relation between tau(PCr) and end-exercise pH was positively correlated with both the proton efflux rate and the apparent proton efflux rate constant, indicating that subjects with a smaller pH dependence of tau(PCr) have a higher proton efflux rate. Our study implies that simply correcting tau(PCr) for end-exercise pH is not adequate, in particular when comparing patients and control subjects, because certain disorders are characterized by altered proton efflux from muscle fibers.

Insights into muscle diseases gained by phosphorus magnetic resonance spectroscopy

Phosphorus magnetic resonance spectroscopy (P-MRS) has now been used in the investigation of muscle energy metabolism in health and disease for over 15 years. The present review describes the basics of the metabolic observations made by P-MRS including the assumptions and problems associated with the use of this technique. Extramuscular factors, which may affect the P-MRS results, are detailed. The important P-MRS observations in patients with mitochondrial myopathies, including the monitoring of experimental therapies, are emphasized. The findings in other metabolic myopathies (those associated with glycolytic defects or endocrine disturbances) and in the destructive myopathies (the dystrophies and the inflammatory myopathies) are also described. Observations made in normal and abnormal fatigue, fibromyalgia, and malignant hyperthermia are considered. Finally, a summary of the possible diagnostic use of P-MRS in exercise intolerance is provided.

In vivo (31)P-MRS assessment of cytosolic [Mg(2+)] in the human skeletal muscle in different metabolic conditions

Cytosolic free [Mg(2+)] can be assessed in vivo by (31)P-MRS from the chemical shift of beta-ATP. The reliability of in vivo measurements depends on the availability of appropriate in vitro calibration curves obtained by using solutions that mimic the in vivo cytosolic conditions as far as possible. We build a semi-empiric equation that correlates the chemical shift of beta-ATP to free [Mg(2+)] taking into account the amount of Mg(2+) bound to all other ligands in solution. Our experiments resulted in a reliable ten-parameters equation directly usable to assess the cytosolic free [Mg(2+)] of human skeletal muscle at rest, during work and recovery. Our experiments also resulted in a new equation that allows the assessment of cytosolic pH from the chemical shift of Pi taking into account the measured free [Mg(2+)]. To perform simultaneous calculation of free [Mg(2+)] and pH in the skeletal muscle in different metabolic conditions we developed a specific software package available on Internet (http://www.unibo.it/bioclin) together with another program based on the equation previously obtained to calculate cytosolic free [Mg(2+)] in the human brain. The reliability and effectiveness of our equations and software were tested on the calf muscles of healthy volunteers at rest, during work and recovery.

31P MRS measurement of mitochondrial function in skeletal muscle: reliability, force-level sensitivity and relation to whole body maximal oxygen uptake

The reliability, relation to whole-body maximal oxygen uptake (VO(2max)), and force-level sensitivity of (31)P MRS markers of mitochondrial function were studied in 39 normal-weight women. Following 90 s isometric plantar-flexion exercises at 45, 70 and 100% of maximum voluntary contraction, skeletal muscle mitochondrial function was determined from the phosphocreatine recovery time constant (TC(PCr)), the ADP recovery time constant (TC(ADP)), and the rate of change in PCr during the first 14 s of recovery (OxPhos). VO(2max) was measured on a treadmill. Test-retest measurements were obtained in a subset of seven women. Overall, TC(PCr), TC(ADP) and OxPhos were reproducible for all exercises (coefficients of variation = 2.3-19.3%). With increasing force-level, TC(PCr) was prolonged (29.0 +/- 8.2, 31.9 +/- 9.0 and 35.4 +/- 9.5 s), OxPhos was increased (0.159 +/- 0.081, 0.247 +/- 0.090 and 0.310 +/- 0.114), and TC(ADP) was shortened (22.4 +/- 7.9, 21.3 +/- 6.2, and 19.5 +/- 6.7; p < 0.01). All MRS markers of mitochondrial function were correlated with VO(2max) (r = 0.41-0.72; p < 0.05). These results suggest that measurements of TC(PCr), TC(ADP) and OxPhos yield reproducible results that correlate with whole-body VO(2max) and that vary in force-level sensitivity.

Muscle high-energy phosphates in central nervous system disorders. The phosphorus MRS experience

Phosphorus magnetic resonance spectroscopy (MRS) was used to study muscle phosphates metabolism in several brain disorders. Those with primary mitochondrial encephalomyopathies showed the typical pattern of impaired oxidative metabolism at rest and during recovery after exercise. In migraine, Parkinson's disease and alternating hemiplegia muscle MRS observations lend support to a possible mitochondrial dysfunction. Similar observations in multiple sclerosis are probably the result of secondary deconditioning. In post polio syndrome and in some of the hereditary ataxias, elevated intracellular inorganic phosphates may be the result of another, yet unknown, metabolic impairment. Thus, muscle phosphate metabolism may be altered in various central nervous system (CNS) disorders by different metabolic impairments. All these possibilities should be taken into account when evaluating MRS results in brain diseases.

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.

Change in chemical shift and splitting of 31P gamma-ATP signal in human skeletal muscle during exercise and recovery

Proton decoupled 31P in vivo NMR spectroscopy of the human finger flexor muscles was performed during exercise and recovery using a 1.5 T whole-body imager. Predominantly the gamma-ATP signal shows a splitting caused by different signal contributions with chemical shifts that vary independently. Studies on the human gastrocnemius and biceps femoris muscle were undertaken to investigate the appearance of the splitting in these muscles as well. In all cases more than one signal contribution was found which might represent the different muscle fibre types and their recruitment pattern following exercise. An analysis of the chemical shifts (delta) of ATP results in changes of up to 0.4 ppm and 0.1 ppm for delta gamma- and delta beta-ATP, respectively. Based solely on the chemical shifts of the ATP 31P signals the tissue pH value following exercise was determined. The result was in good agreement with the value derived from delta Pi.

Magnetic resonance spectroscopy in migraine

31-phosphorus Magnetic Resonance Spectroscopy (MRS) is a technique developed for the non-invasive study of energy metabolism in living subjects. It determines the concentrations of high and low energy phosphates in resting and activated conditions, and of intracellular pH. 31P-MRS has been applied to the study of migraine, both during and in between attacks. Intracellular brain pH remains unchanged during the migraine attack, suggesting that ischemia does not play a relevant role in the origin of the neurological signs. During and in-between attacks, migraineurs display abnormalities in energy metabolism of brain and muscle, consisting of reduced levels of phosphocreatine, reduced cellular-free energy and increased rate of ATP biosynthesis. We suggest that these abnormalities in energy metabolism predispose migraineurs to develop an attack under conditions of increased brain energy demand.

Magnetic resonance spectroscopy studies in migraine

We describe the method of 31phosphorous magnetic resonance spectroscopy and review the results when it is applied to the study of brain and muscle energy metabolism in migraine subjects. Brain energy metabolism appears to be abnormal in all major subtypes of migraine when measured both during and between attacks. Impaired energy metabolism is also documented in skeletal muscle. We suggest that migraine is associated with a generalized disorder of mitochondrial oxidative phosphorylation and that this may constitute a threshold for the triggering of migraine attacks.

Exercise causes oxidative damage to rat skeletal muscle microsomes while increasing cellular sulfhydryls

The physiological and biochemical demands on contracting muscle make this tissue particularly susceptible to molecular and cellular damage. We looked at membrane structures in cardiac and skeletal muscle and in erythrocytes for exercise-induced lipid peroxidation. These tissues were removed from each of the rats used in this study. We also examined and compared the effects of exercise on the redox status of blood plasma, erythrocytes and cardiac and skeletal muscle from the same rats. We used a swim stress protocol to exercise the rats to exhaustion. Some form of chemical modification or oxidative damage to membranes was observed in all of the tissues tested. Cardiac muscle microsomes from exercised rats exhibited increased malondialdehyde and decreased phospholipid (control, 249.1 vs exercised, 120.6 nmols phospholipid/mg protein). Skeletal muscle microsomes showed decreased sulfhydryls, decreased phospholipid (control, 1,276.9 vs exercised, 137.7 nmols phospholipid/mg protein), increased malondialdehyde and greater protein crosslinking after exercise. Erythrocyte membranes also exhibited exercised-induced protein oxidation. However, the total cellular sulfhydryl content remained the same in erythrocytes and cardiac tissue but increased in blood plasma (control, 10.8 vs exercised, 24.7 mumols SH/dl plasma) and skeletal muscle after exercise. We conclude that exercise profoundly effects membrane structures. The body compensates for this lipid peroxidation and protein damage by increasing total cellular sulfhydryls in blood plasma and skeletal muscle which would aid in repair of the damaged membranes.

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.

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.

31P-NMR spectroscopy of skeletal muscle in Becker dystrophy and DMD/BMD carriers. Altered rate of phosphate transport

Muscle energy metabolism was studied by phosphorus nuclear magnetic resonance spectroscopy (31P-NMR) in 6 patients with Becker dystrophy, and in 24 female DMD/BMD carriers (n = 18) and non-carriers (n = 6). At rest all patients showed a high Pi/PCr ratio due to low PCr and high Pi contents, and a high intracellular IpH. 31P-NMR of carriers and non-carriers did not differ from controls. In patients and carriers in-magnet exercise revealed a reduced ability to perform work and Pi/PCr ratios higher than controls for comparable relative levels of steady-state work. Post-exercise Pi recovery was found abnormal in patients and in carriers. The 31P-NMR abnormalities found in the working muscle of both BMD patients and female DMD/BMD carriers indicate a defect of phosphate metabolism that, be it primary or secondary, reflects a deficit of energy metabolism.

The rate of phosphate transport during recovery from muscular exercise depends on cytosolic [H+]. A 31P-MR spectroscopy study in humans

31-Phosphorus magnetic resonance spectroscopy was used to study in vivo the effect of cytosolic [H+] on the kinetics of initial post-exercise recovery of inorganic phosphate (Pi) in human gastrocnemius muscle. Linear correlations were found between the rate of initial phosphate recovery and: a) the minimum value of cytosolic pH reached during recovery, and b) the minimum percentage of divalent anion present. These linear relationships are consistent with the current knowledge of Pi transport, and represent new invariant parameters for the study of muscle pathologies that may involve Pi and/or H+ transport.