[Study of in vivo cellular metabolism by phosphorus 31 nuclear magnetic resonance]

Recent advances in magnetic resonance neurospectroscopy

Over the past two decades, proton magnetic resonance spectroscopy (proton MRS) of the brain has made the transition from research tool to a clinically useful modality. In this review, we first describe the localization methods currently used in MRS studies of the brain and discuss the technical and practical factors that determine the applicability of the methods to particular clinical studies. We also describe each of the resonances detected by localized solvent-suppressed proton MRS of the brain and discuss the metabolic and biochemical information that can be derived from an analysis of their concentrations. We discuss spectral quantitation and summarize the reproducibility of both single-voxel and multivoxel methods at 1.5 and 3-4 T. We have selected three clinical neurologic applications in which there has been a consensus as to the diagnostic value of MRS and summarize the information relevant to clinical applications. Finally, we speculate about some of the potential technical developments, either in progress or in the future, that may lead to improvements in the performance of proton MRS.

[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.

[Spectroscopy of phosphorus in nuclear magnetic resonance. General review of clinical applications to the study of human skeletal muscle]

Phosphorus nuclear magnetic resonance spectroscopy enables the energy metabolism of skeletal muscles to be studied non-invasively in vivo. Relative concentrations of phosphocreatine (PCr), inorganic phosphate (iP), monophosphoric sugars (mP) and ATP, as well as intracellular pH values, are directly accessible through the spectra. The striated muscle is continuously studied at rest, during exercise and during recovery. Exercise-induced changes in pH and mP provide indirect information on glycogenolysis and glycolysis. The speed of PCr resynthesis during post-exercise recovery and the PCr/iP ratio values at rest excellently reflect mitochondrial oxidative phosphorylations. Phosphorus NMR spectroscopy therefore is of interest not only to study the impact, through hypoxia, on muscle energy metabolism of such pathologies as cardiac or respiratory failure, or to study various acquired metabolic muscular diseases, but also and above all, to detect and locate muscular enzyme deficiencies involving glycogenolysis, glycolysis or mitochondrial metabolism, thereby pointing to the diagnosis of congenital, and mainly metabolic, myopathies.

Phosphatic metabolites, intracellular pH and free [Mg2+] in single, intact porcine carotid artery segments studied by 31P-NMR

Superfused porcine carotid artery segments (approximately 7 cm lengths) were analyzed by 31P-NMR spectroscopic methods to characterize the 31P spectrum of arterial smooth muscle and to determine the influence of passive stretch (intraluminal pressurization, 95-100 mmHg) on cellular phosphatic metabolite levels, intracellular pH and free magnesium concentration ([Mg2+free]i). Equilibrated, single, intact arteries were studied under steady-state, constant flow conditions at 37 degrees C. Phosphoethanolamine, phosphocholine, inorganic phosphate (Pi), phosphocreatine (PCr) and nucleoside triphosphates (NTP), primarily ATP, were the principle metabolites detected in the 31P-NMR spectrum of intact arterial smooth muscle. The concentration of these metabolites and intracellular pH, as determined from the referenced chemical shift of Pi, were unaffected by pressurization. The PCr:Pi ratios determined for nonpressurized (flaccid) and pressurized arteries were 1.2 +/- 0.1 and 1.3 +/- 0.3, respectively. Intracellular pH averaged 7.02 +/- 0.02 (mean +/- 1 S.D.) for flaccid arteries vs. 7.03 +/- 0.05 for pressurized arteries. The upfield chemical shift of the beta-ATP peak, which has been described in other types of smooth muscle, was also observed in these experiments. Interestingly, pressurization significantly shifted the resonance position of this peak, which was interpreted to represent a change in [Mg2+free]i. The average [Mg2+free]i of flaccid artery preparations was computed to be 0.54 +/- 0.03 x 10(-3) M, as compared to 0.99 +/- 0.10 x 10(-3) M for pressurized arteries. This change in [Mg2+free]i was evident within the first hour following pressurization and persisted thereafter. These findings suggest that altering the resting length of vascular smooth muscle produces a change in [Mg2+free]i. This shift in free Mg2+ levels may act as a metabolic signal triggering a change in vascular smooth muscle metabolism, an effect which has been reported to occur in smooth muscle in response to stretch.

A 31P-NMR study of bovine adrenocortical mitochondrial metabolic activities

High-field 31P-NMR spectroscopy has been used to study the metabolic activities of coupled bovine adrenocortical mitochondria in vitro. These differentiated organelles use oxygen as a substrate to support both oxidative phosphorylation and specific steroid hydroxylation reactions. The NMR technique allowed the resolution of two inorganic phosphate signals, attributed to the matrix and external medium phosphate pools, at low and high field, respectively. These signals were used to calculate the respective Pi concentrations and to obtain the pH of the two corresponding compartments. In addition, the NMR spectra displayed resonance signals corresponding to ADP added to the medium and to ATP synthesized during oxidative phosphorylation. NMR analysis of the mitochondrial perchloric acid extracts identified the major phosphate-containing metabolites, namely NADP+, NAD+, phosphocholine, phosphoethanolamine, sn-glycero-(3)phosphocholine, AMP, ADP, ATP and Pi. Upon addition of ADP and malate to the oxygenated suspension, the kinetics of mitochondrial external Pi consumption and of ATP synthesis, along with the intra- and extraorganelle pH variations could be monitored over time periods of approximately 30 min, in the absence and presence of different steroid hydroxylation substrates. A major observation was that oxidative phosphorylation, which takes place in the absence of steroid, was markedly inhibited as soon as steroid hydroxylation was operating. These observations show the potential of 31P-NMR spectroscopy in the study of metabolic activities of isolated intact mitochondrial organelles. Such an approach appears promising for further determination of the underlying mechanisms in the balance between vital oxidative phosphorylation and differentiated steroid hydroxylation which are under hormonal control in adrenocortical mitochondria as well as in other steroidogenic cell systems.

Identification of inositol hexaphosphate in 31P-NMR spectra of Dictyostelium discoideum amoebae. Relevance to intracellular pH determination

A sugar phosphomonoester, myo-inositol hexakisphosphate (phytic acid), has been identified as a major phosphorylated metabolite in Dictyostelium discoideum amoeba. Its intracellular concentration was estimated to be 0.7 mM. The identification was made in perchloric acid extracts on the basis of 31P-NMR chemical shift values and their variations with pH, by addition of authentic compound and by hydrolysis with wheat phytase. Perchloric acid extracts were prepared so as to avoid losses of insoluble salts of polyphosphorylated compounds with divalent cations. The glycolytic intermediate, 3-phosphoglycerate accumulated intracellularly in amoebae incubated in the presence of fluoride. The pH sensitive NMR signal of 3-phosphoglycerate was used to monitor cytosolic pH and a value of pH 7.4 was found in aerobic Dictyostelium amoebae.

Fluoride effects on 31P NMR spectra of macrophages

31P High resolution nuclear magnetic resonance studies have been carried out on the P388D1 tumoral cell line and the BCG elicited alveolar rabbit macrophages both in sedimented cells and in perfused agarose-embedded cells. When the cells were sufficiently oxygenated, the phosphorylated sugars and ATP concentrations attained high levels. The intensity of the peak representing phosphorylated sugars varied inversely with ATP level when macrophagic cells were treated by NaF. The identities of the phosphorylated sugars were revealed by 1H and 31P NMR studies of the P 388D1 cells perchloric extracts.

Growth of a human colonic adenocarcinoma cell line (HT 29) on microcarrier beads: metabolic studies by 31phosphorus nuclear magnetic resonance spectroscopy

A method allowing the growth of a human colon adenocarcinoma cell line (HT 29) on beaded polystyrene microcarriers has been developed by modifying the culture conditions used in monolayer cultures. Under optimized conditions, the cells became confluent 7 days after seeding and reached a density of 2.8 X 10(5) cells/cm2 of microcarrier (65% of the available area occupied). 31P NMR spectra were typically recorded on 300 X 10(6) cells continuously perfused at a flow rate of 15 ml/min in a specially designed NMR chamber in which the microcarrier beads were sequestered within the receiver coil volume. The in vivo spectrum displays a series of resonances assigned to nucleoside triphosphates (ATP and GTP), inorganic phosphate and various phosphomonoesters (mainly glucose-6-P and phosphorylcholine). Diphosphodiester resonances (DPDE, mainly UDP-N-acetyl-glucosamine and UDP-N-acetylgalactosamine) were not detected in the in vivo spectrum and were only apparent in the spectrum of the perchloric acid extract of the cells, indicating that these compounds have a restricted mobility in the intracellular compartment. The intracellular pH of HT 29 cells was 7.2 during the perfusion with a medium buffered at pH 7.3. The internal pH decreased slowly (2 X 10(-3) pH unit/min) during anoxic perfusion, but severe intracellular acidosis occurred after 40 min of ischemia (2.7 X 10(-2) pH unit/min). Sequential recording of 31P NMR spectra has shown that HT 29 cells are able to maintain their high energy phosphorylated compound levels (ATP) when subjected to 100 min of anoxia and 40 min of total ischemia.

In vivo 31P nuclear magnetic resonance studies of T1 and T2 relaxation times in rat brain and in rat brain tumors implanted to nude mice

31P NMR spin-lattice (T1) and spin-spin (T2) relaxation times of phosphocreatine, ATP, inorganic phosphate, and phosphomonoesters have been measured in vivo at 4.7 T in rat brain and rat brain tumors implanted on nude mice. The relaxation data were acquired using a phase-cycled saturation-recovery spin-echo sequence. The problems associated with the phase modulation of the ATP lines by the homonuclear coupling constants were overcome by using selective refocusing pulses for the T2 measurements. In all the metabolites, large differences (1 to 2 orders of magnitude) are observed between the two relaxation times. T1 values in rat brain tumors are 30 to 90% longer than their counterparts in normal rat brain. T2 values follow the same trend with smaller variations except for phosphocreatine values which seem much less sensitive to the metabolic state of the tissues.

Intracellular pH control in Dictyostelium discoideum: a 31P-NMR analysis

Phosphorus metabolites and intracellular pH have been examined in the slime mold Dictyostelium discoideum by non-destructive 31P-NMR measurements. In a spectrum from a suspension of aerobic amoebae, the major peaks are inorganic phosphate, nucleotide di- and triphosphates. In the corresponding perchloric acid extract, resonances originating from purine and pyrimidine nucleotides are resolved. Adenine nucleotides are the most abundant components, but the other nucleotides are present in significant amounts. In a spectrum from intact spores in a dormant state, only inorganic phosphate and polyphosphates are detected and nucleotides are no longer present in large amounts. Of particular importance is the ability to observe separately in aerobic amoebae the resonance of inorganic phosphate localized in two different cell compartments: the cytosol and the mitochondria. The cytosolic pH and mitochondrial pH have been measured as 6.7 and 7.7, respectively, on the basis of intracellular inorganic phosphate chemical shifts. They are essentially unaffected over a large range of external pH and they are not modified transiently or permanently during the initiation of the developmental program of the organism. A weak acid, such as propionate, which modifies the progression of differentiation by favoring prestalk cells, perturbs intracellular pH gradients by selectively decreasing mitochondrial pH without any effect on cytosolic pH.

Phosphorus-31 nuclear magnetic resonance study of post mortem catabolism and intracellular pH in intact excised rabbit muscle

Phosphorus-31 nuclear magnetic resonance has been used to study the post mortem catabolism of high-energy phosphate compounds and the associated intracellular pH variation in pure fast- and slow-twitch rabbit muscles and in rabbit muscle with mixed fiber types. Comparative results from pure fiber types are reported for the first time. Large amounts of glycerophosphorylcholine (14.1 mumol/g fresh tissue) are found in the internal conoidal bundle (ICB), a pure oxidative slow twitch muscle, whereas the m. psoas major (PM), a pure glycolytic fast twitch muscle and the m. gastrocnemius caput medialis (GCM), with mixed fiber types, are devoid of the same metabolite. The total content of phosphorylated metabolites is constant among the three muscle types. The time-dependent post mortem changes in phosphorylated metabolites display the expected rapid drop in phosphocreatine and a simultaneous increase in intracellular inorganic phosphate. However, the ATP level remains constant during more than 2 h. Rate constants for metabolite breakdown and apparent ATPase activity have been determined. The comparative kinetics of intracellular acidosis at 25 degrees C yield rates of 3.3 X 10(-3) pH unit/min for PM, 2.7 X 10(-3) pH unit/min for GCM and 3.0 X 10(-3) pH unit/min for ICB. Initial intracellular pH values are 7.07, 7.20 and 7.02, respectively. Upon aging, the heterogeneity of the Pi signal reflects the existence of cellular compartments with different internal pH. The results suggest that the more intense low-pH Pi signal arises from the sarcoplasmic reticulum while the less intense resonance would reflect the sarcoplasmic higher pH. The temperature effect on post mortem catabolism in the 15-25 degrees C range has been documented. As expected, phosphocreatine and ATP breakdown increase with temperature but at a higher rate for slow-twitch ICB than for fast-twitch PM.

31P NMR studies of the intact perfused rat heart: a novel analytical approach for determining functional-metabolic correlates, temporal relationships, and intracellular actions of cardiotoxic chemicals nondestructively in an intact organ model

Intact hearts isolated from adult male, Sprague-Dawley rats were perfused under standardized conditions in an apparatus designed for use in a high-resolution nuclear magnetic resonance (NMR) spectrometer system. Myocardial phosphate metabolite concentrations (ATP, PCr, Pi, and phosphomonoesters) and intracellular pH were determined sequentially at timed intervals coincident with the functional assessments of the intact heart by phosphorus-31 (31P) NMR spectroscopic methods. Myocardial functional and metabolic parameters were unaffected by sustained control perfusion (2 hr). The negative inotropic actions of cadmium were associated with significant changes in the chemical environment of inorganic phosphate (Pi) within the cells. This initial cellular response to cadmium, which correlated with the onset and magnitude of the contractile disturbances, appeared to represent the formation of an acidic, intracellular Pi pool (pH, 6.0). This pH compartment reached a steady state during the period in which maximal changes in contractile function were manifested, and before cellular ATP and PCr concentrations were altered. These findings are consistent with the interpretation that the functional deficits caused by cadmium originated primarily from changes in the chemical environment experienced by intracellular metabolites, rather than changes in the amounts of cellular high energy substrates. In contrast, the time-dependent negative inotropic effects of arsenate were proportional to the loss of cellular ATP stores. Intracellular pH was not affected in these hearts. A distinctive metabolic finding associated with the cardiotoxicity of arsenate was the time-dependent accumulation of previously undetected phosphate metabolites in the arsenate-treated hearts. Efforts to chemically identify these metabolites proved inconclusive; however, existing evidence suggests the possibility that these phosphorus-containing compounds may be arsenophosphate derivatives of naturally occurring cellular metabolites. The present findings provide experimental evidence demonstrating that toxicologic assessments in an intact organ model are feasible using whole organ 31P NMR spectroscopic methods and that meaningful, new insights regarding the biochemical mechanisms responsible for the cardiotoxic actions of xenobiotic agents can be obtained by this analytical approach.

[Detection of peranesthetic malignant hyperthermia by muscle contracture tests and NMR spectroscopy]

To diagnose malignant hyperthermia susceptibility (MHS), caffeine and halothane contracture tests were performed on six patients. One of them, who presented a peroperative crisis, was recognized as MHS; the five others were negative (MHN). By means of 31P-NMR spectroscopy, the muscular energetic metabolism of these patients was studied during and after moderate exercise in normal and moderate ischaemic conditions. Metabolic abnormalities appeared in the MHS patient. It must be concluded therefore that malignant hyperthermia is a latent myopathy. 31P-NMR spectroscopy appeared to be a useful non-invasive tool for screening for this affliction.

Glutamate-glutamine metabolism in the perfused rat liver. 13C-NMR study using (2-13C)-enriched acetate

13C-NMR has been used to follow the metabolism of 13C-enriched substrates in isolated perfused rat liver. The fate of 90% enriched [2-13C]acetate has been studied in the perfused liver in order to investigate mitochondrial metabolism and the interrelations between cytosolic and mitochondrial pools of metabolites. Some compounds of the intermediary metabolism where found to be extensively labelled, e.g. glutamate, glutamine, acetoacetate and beta-hydroxybutyrate. Under our experimental conditions, labelling of glutamate reached a steady-state within 30 min after the onset of perfusion of 20 mM acetate. In addition, the observed incorporation of 13C into glutamine can be linked to the operation of the glutamate-glutamine antiport and to the high activity of cytosolic glutamate synthetase. The finding of both active glutaminase and glutamine synthetase activity in the same liver cells is evidence of the existence of an active glutamine-glutamate futile cycle.