An evaluation of the effect of fasting on the exercise-induced changes in pH and Pi/PCr from skeletal muscle

Postmortem 31P magnetic resonance spectroscopy of the skeletal muscle: α-ATP/Pi ratio as a forensic tool?

PURPOSE

Phosphor magnetic resonance spectroscopy ((31)P MRS) is an established method for metabolic examinations of resting and exercising skeletal muscle. So far, there are few MRS investigations of human corpses. The aim of this study was to investigate the temporal postmortem pattern of phosphor metabolites in the adductor magnus muscle and to check the value of MRS as a forensic tool, especially for the determination of the time of death.

MATERIAL AND METHODS

Eight corpses, died of natural cause, were examined (5 males, 3 females; age: 73±7 y, weight 65.8±15.9 kg). A control group of 3 subjects (2 males, 1 female, mean age: 51±24 y, range: 24-69 y, mean body weight: 84.0±16.5 kg) was examined at a single time point as well. (31)P MRS was performed on a 1.5 T MRI (TR 700 ms, TE 0.35 ms, averages 256, flip angle 90°). A standard (31)P/(1)H heart/liver coil was employed (receiver coil diameter 12 cm). The (31)P MRS scans were repeated in intervals of 1 h over a period from 4.5 to 24 h postmortem (p.m.). The core temperature was rectally measured throughout the MRI examination.

RESULTS

The mean core temperature decreased from 36.0°C to 25.7°C. In vivo and ex vivo spectra showed characteristic differences, especially the PCr metabolite was no longer detectable after 10 h p.m. The α-ATP/Pi ratio decreased with time from 0.445 to 0.032 over 24 h p.m.

CONCLUSION

There is a characteristic postmortem time pattern of the phosphor metabolites. Especially the acquired α-ATP/Pi ratio could be described by a significant exponential time course (r(2)=0.92, p<0.001). (31)P MRS might be added to the postmortem imaging methods.

Reproducibility assessment of metabolic variables characterizing muscle energetics in vivo: A 31P-MRS study

The purpose of the present study was to assess the reliability of metabolic parameters measured using (31)P magnetic resonance spectroscopy ((31)P MRS) during two standardized rest-exercise-recovery protocols. Twelve healthy subjects performed the standardized protocols at two different intensities; i.e., a moderate intensity (MOD) repeated over a two-month period and heavy intensity (HEAVY) repeated over a year's time. Test-retest reliability was analyzed using coefficient of variation (CV), limits of agreement (LOA), and intraclass correlation coefficients (ICC). During exercise and recovery periods, most of the metabolic parameters exhibited a good reliability. The CVs of individual concentration of phosphocreatine ([PCr]), concentration of adenosine diphosphate ([ADP]), and pH values recorded at end of the HEAVY exercise were lower than 15%. The CV calculated for the rate of PCr resynthesis and the maximal oxidative capacity were less than 13% during the HEAVY protocol. Inferred parameters such as oxidative and total adenosine triphosphate (ATP) production rates exhibited a good reliability (ICC approximately 0.7; CV < 15% during the HEAVY protocol). Our results demonstrated that measurement error using (31)P-MRS during a standardized exercise was low and that biological variability accounted for the vast majority of the measurement variability. In addition, the corresponding metabolic measurements can reliably be used for longitudinal studies performed even over a long period of time.

Endotoxemia does not limit energy supply in exercising rat skeletal muscle

Although depletion in high-energy phosphorylated compounds and mitochondrial impairment have been reported in septic skeletal muscle at rest, their impact on energy metabolism has not been documented during exercise. In this study we aimed to investigate strictly gastrocnemius muscle function non-invasively, using magnetic resonance techniques in endotoxemic rats. Endotoxemia was induced by injecting animals intraperitoneally at t(0) and t(0) + 24 h with Klebsiella pneumoniae lipopolysaccharides (at 3 mg kg(-1)). Investigations were performed at t(0) + 48 h during a transcutaneous electrical stimulation protocol consisting of 5.7 min of repeated isometric contractions at a frequency of 3.3 HZ. Endotoxin treatment produced a depletion in basal phosphocreatine content and a pronounced reduction in oxidative adenosine triphosphate (ATP) synthesis capacity, whereas the resting ATP concentration remained unchanged. During the stimulation period, endotoxemia caused a decrease in force-generating capacity that was fully accounted for by the loss of muscle mass. It further induced an acceleration of glycolytic ATP production and an increased accumulation of adenosine diphosphate (ADP, an important mitochondrial regulator) that allowed a near-normal rate of oxidative ATP synthesis. Finally, endotoxemia did not affect the total rate of ATP production or the ATP cost of contraction throughout the whole stimulation period. These data demonstrate that, in an acute septic phase, metabolic alterations in resting muscle do not impact energy supply in exercising muscle, likely as a result of adaptive mechanisms.

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.

Functional evaluation of hip abductor muscles with use of magnetic resonance imaging

The hip abductor muscles are considered important for gait and biomechanics of the hip joint; however, their specific function has not been defined precisely. The intensity of magnetic resonance imaging signals in skeletal muscle has been reported to increase immediately after exercise. Making use of this phenomenon, we evaluated the hip abductor muscles. Magnetic resonance imaging was performed after isometric exercise of the hip abductor in three positions (20 degrees of abduction, neutral, and 20 degrees of adduction). The abduction force of the hip was measured with a dynamometer, and electromyographic measurements were made simultaneously for the same hip positions. Additionally, magnetic resonance imaging was performed after one-legged stance. As the hip was more adducted, the signal intensity increased on the scans. The values for muscle force, as evaluated with the dynamometer and integrated electromyography, also supported the results. The increase in signal intensity of the gluteus minimus at 20 degrees of abduction and after one-legged stance was significantly greater than that of the gluteus medius (p < 0.0001 and p < 0.0001, respectively). The results of this study indicate that the gluteus minimus muscle, along with the gluteus medius, plays an important role in hip abduction, gait, and stabilization of the pelvis.

Factors which influence alterations of phosphates and pH in exercising human skeletal muscle: measurement error, reproducibility, and effects of fasting, carbohydrate loading, and metabolic acidosis

Previous studies have shown considerable variability in the metabolic response of human skeletal muscle during a standardized exercise protocol. The goal of these studies was to investigate the factors responsible for the broad range of metabolic changes produced by fatiguing exercise. Experiments were performed to quantitate the measurement error of 31P nuclear magnetic resonance spectroscopy of human muscle, the reproducibility of changes within a single subject, and the effects of fasting, carbohydrate loading, and metabolic acidosis. The results show that none of these factors appear to be responsible for the wide variation between subjects. However, the effects of training and genetic factors were not investigated and are likely to be responsible for the substantial variability between subjects.

31P-NMR study of skeletal muscle metabolism in patients with chronic respiratory impairment

To evaluate the energy metabolism of peripheral skeletal muscle during exercise in patients with chronic respiratory impairment, the 31P-nuclear magnetic resonance (NMR) spectra of forearm muscle were investigated in nine patients and nine age-matched control subjects. We calculated the phosphocreatine (PCr) to PCr + inorganic phosphate (PI) ratio, the time constant of PCr recovery and the intracellular pH. The exercise consisted of repetitive hand grips against a 2-kg load every 3 s for 6 min (0.33 W). The patients showed a marked decrease in the PCr/(PCr + PI) ratio and pH in the muscle during exercise in contrast to the control subjects whose PCr/(PCr + PI) showed a minor decrease without any change in pH. The relationship between PCr utilization and pH demonstrated that anaerobic glycolysis switched on earlier in patients with chronic respiratory impairment. A split PI peak was observed in five of nine patients during exercise. The PCr/(PCr + PI) ratio during the last minute of exercise correlated significantly with the vital capacity (% predicted), with the FEV1/FVC, with the body weight, with the maximum strength of hand grip, and with the muscle mass. The results indicate impaired oxidative phosphorylation and the early activation of anaerobic glycolysis in the muscles of patients with chronic respiratory impairment. Several factors related to chronic respiratory impairment, such as disuse, malnutrition and dysoxia, would contribute to the metabolic changes observed in the muscles examined.

Effect of malnutrition on aerobic and anaerobic performance of fast- and slow-twitch muscles of rats

The effect of malnutrition on the functional properties of fast- and slow-twitch muscles from rats was studied using aerobic and anaerobic preparations. A 2-day fast and hypocaloric feeding to a weight loss of 25% were used as models of malnutrition. Soleus (slow-twitch) and extensor digitorum longus (EDL) (fast-twitch) muscles were studied using an in situ preparation with the blood supply intact and an in vitro preparation to which cyanide had been added to render the muscles anaerobic. We found that a 2-day fast had little effect on the function of muscles stimulated in situ, whereas anaerobic stimulation produced a decrease in force per gram of muscle weight in the soleus, but not in the EDL, compared with control values. Hypocaloric feeding resulted in a slowed relaxation rate, an increased Fs/Fmax ratio, and an upward shift of the force-frequency curve relative to controls when studied in situ. Under anaerobic conditions, soleus muscles from hypocaloric rats continued to show a slow relaxation rate and demonstrated a loss of force per gram of muscle weight compared with controls, particularly at low stimulation frequencies. EDL muscles from hypocaloric rats had an increased relaxation rate and were able to maintain force with anaerobic stimulation. Soleus and EDL muscles from the fasted and hypocaloric groups had lower activities of phosphofructokinase. We conclude that slow-twitch muscles from malnourished rats are at a disadvantage when required to function under anaerobic conditions. These findings suggest that muscle performance may be impaired in malnourished patients subjected to hypoxia.

In vivo and in vitro 31P-NMR spectroscopy of rat liver treated with halocarbons

Both in vivo and in vitro 31P-NMR spectroscopy were used to demonstrate metabolic changes in rat liver as a function of time after exposure to either carbon tetrachloride (CCl4) or bromotrichloromethane (BrCCl3). The inorganic phosphate resonance, measured in vivo, moves upfield, which is associated with a decrease in cytosolic pH over a 12 or 20 h period (for BrCCl3 or CCl4, respectively). Intoxication by CCl4 or BrCCl3 causes an intracellular acidosis to pH 7.05 or 6.82 (+/- 0.05), respectively. Also, it has been found that halocarbon exposure increases the amounts of phosphomonoesters (PME) detected. High resolution in vitro 31P-NMR spectroscopy studies of perchloric acid extracts of CCl4-treated rat livers indicated a significant increase in the height of the phosphocholine resonance in the PME region 4-5 h after CCl4 exposure.

Observation of fatigue unrelated to gross energy reserve of skeletal muscle during tetanic contraction--an application of 31P-MRS

The mechanism of muscle fatigue was studied by 31P-MRS. During tetanic contraction for 2 minutes(min), the tension measured with a strain gauge and Phosphocreatine(PCr)/Inorganic phosphate(Pi)+ Phosphomonoester(PME) ratio decreased to 31.5 +/- 4.4% of the control value and 0.6 +/- 0.1, respectively. The intracellular pH(pH) also decreased to 6.62 +/- 0.04. Toward the end of the stimulation, the tension decreased to 25.3 +/- 1.9% of the control value. However, during 20min stimulation, the PCr/(Pi+PME) ratio increased to 2.5 +/- 0.5 and the pH to 6.91 +/- 0.04. These results show that muscular fatigue is ascribable not to a decreased level of high energy metabolites required for actomyosin ATPase, but to an increase in the threshold intensity of excitation in excitation-contraction coupling.

Effect of dietary manipulations (fasting, hypocaloric feeding, and subsequent refeeding) on rat muscle energetics as assessed by nuclear magnetic resonance spectroscopy

31P nuclear magnetic resonance (NMR) spectroscopy in vivo and fluorometry were used to measure muscle ATP, total creatine, pH, and Mg2+ in vivo; and to calculate creatine phosphate (CrP), the ratios of CrP/inorganic phosphate (Pi), CrP/ATP, free ADP levels, and the free-energy change in ATP hydrolysis so nutritional effects could be ascertained. These parameters were determined in vivo in resting control, 2-d-fasted, and hypocalorically fed rats and in animals similarly hypocalorically fed and then refed. The ATP, Pi, and intracellular Mg2+ levels were comparable in the four groups. When the fasted and underfed animals were compared with the control and refed animals, there were falls in the ratios of CrP/Pi and CrP/ATP, in the calculated CrP, and the free-energy change of ATP hydrolysis, but a rise in the calculated free ADP. In the hypocaloric group, intracellular pH fell significantly and a large peak was noted in the phosphodiester region. The data are consistent with the hypothesis that ATP levels are maintained at the cost of CrP, suggesting that ATP production is disturbed by aerobic and anaerobic mechanisms.