1. Male Wistar rats were injected with tri-iodothyronine intraperitoneally for either 3 or 5 days. 2. Using 31P nuclear magnetic resonance spectroscopy, skeletal muscle intracellular bioenergetics of the leg were studied at rest and during sciatic nerve stimulation. 3. Tri-iodothyronine caused intracellular alkalinization of the cell, implying that administration of tri-iodothyronine induces an alteration in the set point for the sodium/proton antiport, the chief controller of intracellular pH. 4. During stimulation, there was less acidification of the skeletal muscle cell in the tri-iodothyronine-injected rats, implying either increased intrinsic buffering capacity of the muscle or increased proton efflux possibly due to increased activity of the sodium/proton antiport. 5. Effective proton efflux from the skeletal muscle cell may be sensitive to circulating levels of tri-iodothyronine.

Animals↗

PubMed1993

Abnormalities in skeletal muscle metabolism in cyanotic patients with congenital heart disease: a 31P nuclear magnetic resonance spectroscopy study.

1. Exercise tolerance is impaired in congenital heart disease. To examine the possible contribution of abnormalities in skeletal muscle bioenergetics, we used 31P nuclear magnetic resonance spectroscopy to investigate muscle metabolism in 10 subjects with congenital heart disease with cyanosis (median age 17.3 years) and in eight healthy age-matched control subjects. Spectra were collected from the gastrocnemius muscle at rest and during exercise and recovery. 2. In resting muscle there were significant elevations in cytosolic pH and in the cytosolic concentration of inorganic phosphate in the patients, and a strong positive correlation between cytosolic pH and blood haemoglobin concentration in all subjects. 3. During plantar flexion exercise the patients showed increased phosphocreatine depletion and cytosolic acidification over a shorter duration of exercise. The rise in calculated cytosolic ADP concentration was similar in both groups. 4. After cessation of exercise, the recovery half-times of phosphocreatine, ADP and phosphate were two to three times longer in the patients, and the initial rate of phosphocreatine resynthesis (a measure of the rate of mitochondrial ATP synthesis) was half the control value, consistent with a reduction in the effective maximum rate of oxidative ATP synthesis (expressed per volume of muscle). Also, recovery was faster in the young control subjects than in our earlier studies of older healthy control subjects. 5. The high phosphate concentration in resting muscle and the abnormalities found in exercise and recovery are consistent with a decrease in oxidative ATP synthesis due to reduced oxygen delivery by the blood in chronic hypoxaemia. The correlation between cytosolic pH and haemoglobin concentration remains to be explained.

Adolescent↗

PubMed1993

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.

Adenosine Triphosphate↗

PubMed1993

The effect of iron deficiency on skeletal muscle metabolism of the rat.

Using 31P magnetic resonance spectroscopy we compared skeletal muscle bioenergetics in Wistar rats made chronically anaemic by being fed a diet deficient in iron for 6 weeks with chronically iron deficient animals given a normal diet as well as 5 mg iron dextran at 2 or 7 days before experimentation. Spectra of the gastrocnemius muscle were taken at rest and during stimulation of the sciatic nerve at 2 Hz for 10 min. Relative concentrations of intracellular phosphate (Pi), phosphocreatine (PCr) and ATP were determined. Iron deficiency increased PCr breakdown and production of acid in stimulated skeletal muscle. Recovery of PCr and Pi concentrations after exercise was slow. These metabolic changes are consistent with either a reduction in supply of oxygen to the muscle cell or altered oxidative phosphorylation by the mitochondria. The latter may be mediated by defective function of iron-containing proteins crucial in oxidative phosphorylation and this is suggested both by the observation that treatment with iron, sufficient to correct the anaemia, does not completely reverse the metabolic changes and that there is a different time course for such metabolic improvements and the observed increase in haemoglobin concentration.

Adenosine Diphosphate↗

PubMed1993

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.

Adolescent↗

PubMed1993

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.

Adenosine Triphosphate↗

PubMed1993

Skeletal muscle metabolism in uremic rats: a 31P-magnetic resonance study.

The effect of uremia on skeletal muscle metabolism of the rat was examined using 31P-magnetic resonance spectroscopy. Three weeks following either a 5/6 nephrectomy or a sham operation, Wistar rats were placed in a 7T magnet, and the sciatic nerve was stimulated for 10 min. Analysis of spectra allowed calculation of intracellular pH and the relative concentrations of phosphocreatine (PCr), inorganic phosphate (Pi) and ATP. [ADP] was calculated from the creatine kinase equilibrium. There was a significant reduction in the resting intracellular [Pi] despite an elevation in extracellular [Pi], probably due to a reduction in the activity of the membrane Na/Pi cotransporter on account of a reduced sodium gradient. Despite anemia and uremia, there were no significant metabolic abnormalities during exercise and recovery accompanying this substantial reduction in glomerular filtration rate implying that at this level of renal impairment, there is no mitochondrial dysfunction.

Adenosine Triphosphate↗

PubMed1993

Impairment of cardiac function and energetics in experimental renal failure.

Cardiac function and energetics in experimental renal failure in the rat (5/6 nephrectomy) have been investigated by means of an isolated perfused working heart preparation and an isometric Langendorff preparation using 31P nuclear magnetic resonance (31P NMR). 4 wk after nephrectomy cardiac output of isolated hearts perfused with Krebs-Henseleit buffer was significantly lower (P < 0.0001) at all levels of preload and afterload in the renal failure groups than in the pair-fed sham operated control group. In control hearts, cardiac output increased with increases in perfusate calcium from 0.73 to 5.61 mmol/liter whereas uremic hearts failed in high calcium perfusate. Collection of 31P NMR spectra from hearts of renal failure and control animals during 30 min normoxic Langendorff perfusion showed that basal phosphocreatine was reduced by 32% to 4.7 mumol/g wet wt (P < 0.01) and the phosphocreatine to ATP ratio was reduced by 32% (P < 0.01) in uremic hearts. During low flow ischemia, there was a substantial decrease in phosphocreatine in the uremic hearts and an accompanying marked increase in release of inosine into the coronary effluent (14.9 vs 6.1 microM, P < 0.01). We conclude that cardiac function is impaired in experimental renal failure, in association with abnormal cardiac energetics and increased susceptibility to ischemic damage. Disordered myocardial calcium utilization may contribute to these derangements.

Adenosine Diphosphate↗

PubMed1993

Effect of chronic uraemia on skeletal muscle metabolism in man.

Fatigue and lethargy, common symptoms in uraemia, have been attributed to many factors. To assess possible bioenergetic contributions to this, we examined the forearm muscle of five patients in end-stage renal failure using 31P-magnetic resonance spectroscopy. There was a small increase in the ratio of intracellular inorganic phosphate to ATP in resting muscle, suggesting an increased cytosolic phosphate concentration. During exercise, increased phosphocreatine breakdown was accompanied by rapid intracellular acidification and an increase in calculated lactic acid accumulation in the muscle of the uraemic subjects, suggesting glycolysis dominating over oxidative phosphorylation as a source of ATP. After exercise, the half-time of phosphocreatine (PCr) recovery was longer in the uraemic subjects, suggesting diminished mitochondrial function. The initial rate of PCr resynthesis was not significantly decreased, but when account was taken of the high cytosolic ADP concentration (which drives mitochondrial oxidative ATP synthesis) the calculated maximum oxidative capacity was significantly reduced in the uraemic subjects. Thus there was evidence of mitochondrial dysfunction in uraemia due either to limitation of oxygen supply, reduced mitochondrial content, or an intrinsic mitochondrial defect. This resulted in increased phosphocreatine depletion and increased glycolytic ATP production during exercise and there was partial compensation of the mitochondrial abnormality by increased ADP concentration. In three of these patients studied after elevation of haemoglobin with erythropoeitin (from 8 to 12 g/dl), initial phosphocreatine breakdown and lactic acid accumulation during exercise were normalized, while exercise duration and calculated maximum oxidative capacity remained significantly abnormal. This suggests that anaemia contributes to these metabolic abnormalities but does not fully explain them.

Adenosine Triphosphate↗