P nuclear magnetic resonance studies of isolated rat liver cells

Repeatability of alkaline inorganic phosphate quantification in the skeletal muscle using 31P-magnetic resonance spectroscopy at 3 T

The detection of a secondary inorganic phosphate (Pi) resonance, a possible marker of mitochondrial content in vivo, using phosphorus magnetic resonance spectroscopy (31P-MRS), poses technical challenges at 3 Tesla (T). Overcoming these challenges is imperative for the integration of this biomarker into clinical research. To evaluate the repeatability and reliability of measuring resting skeletal muscle alkaline Pi (Pialk) using with 31P-MRS at 3 T. After an initial set of experiments on five subjects to optimize the sequence, resting 31P-MRS of the quadriceps muscles were acquired on two visits (~4 days apart) using an intra-subjects design, from 13 sedentary to moderately active young male and female adults (22 ± 3 years old) within a whole-body 3 T MR system. Measurement variability attributed to changes in coil position, shimming procedure, and spectral analysis were quantified. 31P-MRS data were acquired with a 31P/-proton (1H) dual-tuned surface coil positioned on the quadriceps using a pulse-acquire sequence. Test-retest absolute and relative repeatability was analyzed using the coefficient of variation (CV) and intra-class correlation coefficients (ICC), respectively. After sequence parameter optimization, Pialk demonstrated high intra-subject repeatability (CV: 10.6 ± 5.4%, ICC: 0.80). Proximo-distal change in coil position along the length of the quadriceps introduced Pialk quantitation variability (CV: 28 ± 5%), due to magnetic field inhomogeneity with more distal coil locations. In contrast, Pialk measurement variability due to repeated shims from the same muscle volume (0.40 ± 0.09mM; CV: 6.6%), and automated spectral processing (0.37 ± 0.01mM; CV: 2.3%), was minor. The quantification of Pialk in skeletal muscle via surface coil 31P-MRS at 3 T demonstrated excellent reproducibility. However, caution is advised against placing the coil at the distal part of the quadriceps to mitigate shimming inhomogeneity.

In-cell NMR: Why and how?

NMR spectroscopy has been applied to cells and tissues analysis since its beginnings, as early as 1950. We have attempted to gather here in a didactic fashion the broad diversity of data and ideas that emerged from NMR investigations on living cells. Covering a large proportion of the periodic table, NMR spectroscopy permits scrutiny of a great variety of atomic nuclei in all living organisms non-invasively. It has thus provided quantitative information on cellular atoms and their chemical environment, dynamics, or interactions. We will show that NMR studies have generated valuable knowledge on a vast array of cellular molecules and events, from water, salts, metabolites, cell walls, proteins, nucleic acids, drugs and drug targets, to pH, redox equilibria and chemical reactions. The characterization of such a multitude of objects at the atomic scale has thus shaped our mental representation of cellular life at multiple levels, together with major techniques like mass-spectrometry or microscopies. NMR studies on cells has accompanied the developments of MRI and metabolomics, and various subfields have flourished, coined with appealing names: fluxomics, foodomics, MRI and MRS (i.e. imaging and localized spectroscopy of living tissues, respectively), whole-cell NMR, on-cell ligand-based NMR, systems NMR, cellular structural biology, in-cell NMR… All these have not grown separately, but rather by reinforcing each other like a braided trunk. Hence, we try here to provide an analytical account of a large ensemble of intricately linked approaches, whose integration has been and will be key to their success. We present extensive overviews, firstly on the various types of information provided by NMR in a cellular environment (the "why", oriented towards a broad readership), and secondly on the employed NMR techniques and setups (the "how", where we discuss the past, current and future methods). Each subsection is constructed as a historical anthology, showing how the intrinsic properties of NMR spectroscopy and its developments structured the accessible knowledge on cellular phenomena. Using this systematic approach, we sought i) to make this review accessible to the broadest audience and ii) to highlight some early techniques that may find renewed interest. Finally, we present a brief discussion on what may be potential and desirable developments in the context of integrative studies in biology.

Novel Sterically Crowded and Conformationally Constrained α-Aminophosphonates with a Near-Neutral pKa as Highly Accurate 31P NMR pH Probes. Application to Subtle pH Gradients Determination in Dictyostelium discoideum Cells

In order to discover new 31P NMR markers for probing subtle pH changes (<0.2 pH unit) in biological environments, fifteen new conformationally constrained or sterically hindered α-aminophosphonates derived from diethyl(2-methylpyrrolidin-2-yl)phosphonate were synthesized and tested for their pH reporting and cytotoxic properties in vitro. All compounds showed near-neutral pKas (ranging 6.28−6.97), chemical shifts not overlapping those of phosphorus metabolites, and spectroscopic sensitivities (i.e., chemical shifts variation Δδab between the acidic and basic forms) ranging from 9.2−10.7 ppm, being fourfold larger than conventional endogenous markers such as inorganic phosphate. X-ray crystallographic studies combined with predictive empirical relationships and ab initio calculations addressed the inductive and stereochemical effects of substituents linked to the protonated amine function. Satisfactory correlations were established between pKas and both the 2D structure and pyramidalization at phosphorus, showing that steric crowding around the phosphorus is crucial for modulating Δδab. Finally, the hit 31P NMR pH probe 1b bearing an unsubstituted 1,3,2-dioxaphosphorinane ring, which is moderately lipophilic, nontoxic on A549 and NHLF cells, and showing pKa = 6.45 with Δδab = 10.64 ppm, allowed the first clear-cut evidence of trans-sarcolemmal pH gradients in normoxic Dictyostelium discoideum cells with an accuracy of <0.05 pH units.

Improved NMR Detection of Phospho-Metabolites in a Complex Mixture

Phosphorylated metabolites are omnipresent in cells, but their analytical characterization faces several technical hurdles. Here, we detail an improved NMR workflow aimed at assigning the high-resolution subspectrum of the phospho-metabolites in a complex mixture. Combining a pure absorption J-resolved spectrum (Pell, A. J.; J. Magn. Reson. 2007, 189 (2), 293-299) with alternate on- and off-switching of the ³¹P coupling interaction during the t₁ evolution with a pure in-phase (PIP) HSQMBC experiment (Castañar, L.; Angew. Chem., Int. Ed. 2014, 53 (32), 8379-8382) without or with total correlation spectroscopy (TOCSY) transfer during the insensitive nuclei enhancement by polarization transfer (INEPT) gives access to selective identification of the individual subspectra of the phosphorylated metabolites. Returning to the initial J-res spectra, we can extract with optimal resolution the full trace for the individual phospho-metabolites, which can then be transposed on the high-resolution quantitative one dimensional spectrum.

In-cell NMR: from metabolites to macromolecules

In-cell NMR of macromolecules has gained momentum over the last ten years as an approach that might bridge the branches of cell biology and structural biology. In this review, we put it in the context of earlier efforts that aimed to characterize by NMR the cellular environment of live cells and their intracellular metabolites. Although technical aspects distinguish these earlier in vivo NMR studies and the more recent in cell NMR efforts to characterize macromolecules in a cellular environment, we believe that both share major concerns ranging from sensitivity and line broadening to cell viability. Approaches to overcome the limitations in one subfield thereby can serve the other one and vice versa. The relevance in biomedical sciences might stretch from the direct following of drug metabolism in the cell to the observation of target binding, and thereby encompasses in-cell NMR both of metabolites and macromolecules. We underline the efforts of the field to move to novel biological insights by some selected examples.

Assessing tissue metabolism by phosphorous-31 magnetic resonance spectroscopy and imaging: a methodology review

Many human diseases are caused by an imbalance between energy production and demand. Magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) provide the unique opportunity for in vivo assessment of several fundamental events in tissue metabolism without the use of ionizing radiation. Of particular interest, phosphate metabolites that are involved in ATP generation and utilization can be quantified noninvasively by phosphorous-31 (31P) MRS/MRI. Furthermore, 31P magnetization transfer (MT) techniques allow in vivo measurement of metabolic fluxes via creatine kinase (CK) and ATP synthase. However, a major impediment for the clinical applications of 31P-MRS/MRI is the prohibitively long acquisition time and/or the low spatial resolution that are necessary to achieve adequate signal-to-noise ratio. In this review, current 31P-MRS/MRI techniques used in basic science and clinical research are presented. Recent advances in the development of fast 31P-MRS/MRI methods are also discussed.

P-31 Mr Spectrum and Histologic Changes after Intrahepatic Arterial Injection of Iodized Oil in Normal and Cirrhotic Rat Liver

Injection of iodized oil (Lipiodol) into the hepatic artery is widely used in the diagnosis and treatment of hepatocellular carcinoma. However, no reports have yet appeared concerning temporal changes in hepatic metabolism following Lipiodol injection. In the present study, Lipiodol was injected into the hepatic arteries of normal and cirrhotic rats, successive P-31 MR measurements were performed, and temporal changes in metabolism were compared with histologic findings. Both normal and cirrhotic rats displayed minimum levels of β-ATP/PME and β-ATP/Pi 5 days after hepatic arterial injection of Lipiodol. However, 10 days after injection these values had reverted to the preinjection levels. The metabolic dysfunction observed in the liver following hepatic arterial injection of 0.3 ml/kg b.w. Lipiodol was transient. Moreover, no distinct differences were observed between P-31 MR changes in normal and cirrhotic rats. Conversely, histologic impairment assessed on the basis of hepatic necrosis ratios was most severe 2 days after hepatic arterial injection in both normal and cirrhotic rats, and this did not coincide with the time of the most pronounced metabolic impairment as inferred from P-31 MR changes.

New advances in MR-compatible bioartificial liver

MR-compatible bioartificial liver (BAL) studies have been performed for 30 years and are reviewed. There are two types of study: (i) metabolism and drug studies using multinuclear MRS; primarily short-term (< 8 h) studies; (ii) the use of multinuclear MRS and MRI to noninvasively define the features and functions of BAL systems for long-term liver tissue engineering. In the latter, these systems often undergo not only modification of the perfusion system, but also the construction of MR radiofrequency probes around the bioreactor. We present novel MR-compatible BALs and the use of multinuclear MRS ((13)C, (19)F, (31)P) for the noninvasive monitoring of their growth, metabolism and viability, as well as (1)H MRI methods for the determination of flow profiles, diffusion, cell distribution, quality assurance and bioreactor integrity. Finally, a simple flexible coil design and circuit, and life support system, are described that can make almost any BAL MR-compatible.

Tight control of mitochondrial membrane potential by cytochrome c oxidase

In the present work we have critically examined the use of the KCN-titration technique in the study of the control of the cellular respiration by cytochrome c oxidase (COX) in the presence of the mitochondrial membrane potential (Δψ(mito)) in HepG2 cells. We clearly show that the apparent high inhibition threshold of COX in the presence of maximal Δψ(mito) is due to the KCN-induced decrease of Δψ(mito) and not to a low control of COX on the mitochondrial respiration. The tight control exerted by COX on the Δψ(mito) provides further insights for understanding the pathogenetic mechanisms associated with mitochondrial defects in human neuromuscular degenerative disorders.

Mitochondrial dysfunction contributes to impaired insulin secretion in INS-1 cells with dominant-negative mutations of HNF-1alpha and in HNF-1alpha-deficient islets

Maturity Onset Diabetes of the Young-type 3 (MODY-3) has been linked to mutations in the transcription factor hepatic nuclear factor (HNF)-1alpha, resulting in deficiency in glucose-stimulated insulin secretion. In INS-1 cells overexpressing doxycycline-inducible HNF-1alpha dominant-negative (DN-) gene mutations, and islets from Hnf-1alpha knock-out mice, insulin secretion was impaired in response to glucose (15 mm) and other nutrient secretagogues. Decreased rates of insulin secretion in response to glutamine plus leucine and to methyl pyruvate, but not potassium depolarization, indicate defects specific to mitochondrial metabolism. To identify the biochemical mechanisms responsible for impaired insulin secretion, we used (31)P NMR measured mitochondrial ATP synthesis (distinct from glycolytic ATP synthesis) together with oxygen consumption measurements to determine the efficiency of mitochondrial oxidative phosphorylation. Mitochondrial uncoupling was significantly higher in DN-HNF-1alpha cells, such that rates of ATP synthesis were decreased by approximately one-half in response to the secretagogues glucose, glutamine plus leucine, or pyruvate. In addition to closure of the ATP-sensitive K(+) channels with mitochondrial ATP synthesis, mitochondrial production of second messengers through increased anaplerotic flux has been shown to be critical for coupling metabolism to insulin secretion. (13)C-Isotopomer analysis and tandem mass spectrometry measurement of Krebs cycle intermediates revealed a negative impact of DN-HNF-1alpha and Hnf-1alpha knock-out on mitochondrial second messenger production with glucose but not amino acids. Taken together, these results indicate that, in addition to reduced glycolytic flux, uncoupling of mitochondrial oxidative phosphorylation contributes to impaired nutrient-stimulated insulin secretion with either mutations or loss of HNF-1alpha.

Matrix alkalinization: a novel mitochondrial signal for sustained pancreatic beta-cell activation

Nutrient secretagogues activate mitochondria of the pancreatic beta-cell through the provision of substrate, hyperpolarisation of the inner mitochondrial membrane and mitochondrial calcium rises. We report that mitochondrial matrix pH, a parameter not previously studied in the beta-cell, also exerts an important control function in mitochondrial metabolism. During nutrient stimulation matrix pH alkalinises, monitored by the mitochondrial targeted fluorescent pH-sensitive protein mtAlpHi or (31)P-NMR inorganic phosphate chemical shifts following saturation transfer. Compared with other cell types, the resting mitochondrial pH was surprisingly low, rising from pH 7.25 to 7.7 during nutrient stimulation of rat beta-cells. As cytosolic alkalinisation to the nutrient was of much smaller amplitude, the matrix alkalinisation was accompanied by a pronounced increase of the DeltapH across the inner mitochondrial membrane. Furthermore, matrix alkalinisation closely correlates with the cytosolic ATP net increase, which is also associated with elevated ATP synthesis rates in mitochondria. Preventing DeltapH increases in permeabilised cells abrogated substrate-driven ATP synthesis. We propose that the mitochondrial pH and DeltapH are key determinants of mitochondrial energy metabolism and metabolite transport important for cell activation.

Role of intracellular buffering power on the mitochondria-cytosol pH gradient in the rat liver perfused at 4 degrees C

The factors regulating the amplitude and the pH gradient between cytosol and mitochondria (DeltapHmito-cyt) were investigated in the isolated rat liver perfused at 4 degrees C. Liver ATP content, pH, and buffering power of cytosolic and mitochondrial compartments were evaluated in situ using phosphorus-31 nuclear magnetic resonance spectroscopy. No DeltapHmito-cyt was detected in the liver perfused without bicarbonate. Permeant weak acid in the perfusate (H2CO3, 25 mM, or isobutyric acid, 25, 50, or 100 mM) acidified both cytosol and mitochondria and revealed a DeltapHmito-cyt from 0.06 to 0.31 pH unit. Nevertheless, the manipulations of the DeltapHmito-cyt were more effective under bicarbonate-free conditions, due to the absence of buffering by H2CO3/HCO-3. In the absence of bicarbonate, the intracellular buffering power was threefold higher in the mitochondria (110 mmol/pH unit at pHmito 7.16) than in the cytosol (44 mmol/pH unit at pHcyt 7.30) and dependent on the matrix and cytosol pH, respectively. These buffering powers were almost double in the presence of bicarbonate. In the bicarbonate-free perfused liver, the respiratory activity was 0.08 +/- 0.02 micromol O2/min. g liver wet weight and the ATP turnover was only 40 +/- 7 nmol/min. g liver wet weight, indicating the weak activity of liver mitochondria when DeltapHmito-cyt was <0.05 pH unit. The ATP turnover during a 50 mM isobutyric acid load was 35 +/- 4 nmol/min. g liver wet weight whereas DeltapHmito-cyt rose to 0.26 +/- 0.02 pH unit and pHmito remained alkaline. Hence, although DeltapHmito-cyt was increased the ATP turnover remained unchanged. This work is the first evaluation of the mitochondrial buffering power in the isolated liver. The DeltapHmito-cyt observed within various acid loads reflected the differential titration of cytosol and mitochondria containing proteins and H2CO3/HCO-3 buffering systems. Moreover, no direct relationship between DeltapHmito-cyt and ATP turnover could be shown.

Observation of two inorganic phosphate NMR resonances in the perfused hypothermic rat heart

The effect of hypothermia on isolated perfused rat hearts was studied with 31P NMR. Hearts were continuously perfused with phosphate-free Krebs-Henseleit buffer while the perfusate temperature was adjusted. Perfusate pH was kept at 7.40 +/- 0.02 throughout the experiments. Using the chemical shift difference between PCr and Pi the intracellular pH was estimated. At 36, 20, and 10 degreesC a cytosolic alkalinization at a pH of 7.05 +/- 0.04, 7.21 +/- 0.05, and 7.40 +/- 0.03 was observed, respectively. At 10 degreesC two Pi resonances were observed with a separation of 0.25 ppm. This resonance corresponded to a Pi resonance of a cellular compartment with a local pH of 7.78 +/- 0.06, likely mitochondrial. This additional resonance disappeared upon warming of the hearts back to 36 degreesC.

Effect of ethanol and fructose on liver metabolism: a dynamic 31Phosphorus magnetic resonance spectroscopy study in normal volunteers

In vivo 31Phosphorus magnetic resonance spectroscopy (31P-MRS) permits evaluation of dynamic changes of individual phosphorus-containing metabolites in the liver parenchyma, such as phosphomonoester (PME), adenosine triphosphate, and inorganic phosphate (Pi). Intravenous fructose load alters phosphorus metabolites and allows assessment of liver function by 31P-MRS. 31P-MRS data obtained in alcoholic liver disease are however inconclusive. To study the hypothesis that fructose load can be used to investigate metabolic effects of ethanol ingestion, the interaction of different metabolites--i.e., fructose and ethanol--were followed in vivo. Using a 1.5 Tesla magnetic resonance system, six healthy volunteers were examined in three sessions each: a session after administration of (a) fructose only (250 mg/kg) was compared with (b) fructose load after ethanol ingestion (0.8 g/kg). A control experiment (c) was done after ethanol only. Spectra were acquired using one-dimensional chemical shift imaging with a temporal resolution of 5 min. Following a fructose load, the concomitant uptake of ethanol showed drastic changes of individual metabolic steps of the hepatic metabolism (averages +/- standard deviation). While the velocity of the net formation of PME (relative increase 0.46 +/- 0.11 without ethanol vs. 0.61 +/- 0.25 with ethanol) and the use of adenosine triphosphate (-0.13 +/- 0.03 vs. -0.16 +/- 0.03) and Pi (-0.022 +/- 0.009 vs. -0.021 +/- 0.004) were not significantly affected by ethanol uptake, a significant (p < 0.01) reduction of PME degradation (31.3 +/- 9.4 vs. 61.9 +/- 16.9 relative total area) and absence of an overshoot for Pi (10.5 +/- 4.9 vs. -7.1 +/- 5.3 relative area 13 min to 43 min) was observed after ethanol administration. Dynamic 31P-MRS allows the observation of individual steps of hepatic metabolism in situ; fructose metabolism in the human liver is slowed down by concomitant ethanol ingestion after the phosphorylation step of fructose. This could be explained by inhibition of aldolase rather than ethanol-induced changes of the hepatic redox state. Fructose load can be used to study effects of alcohol ingestion and might therefore be useful in patients with alcoholic liver disease.

Predicting the viability of grafted livers in rats through a rapid and sensitive metabolic indicator assessed by 31P-NMR spectroscopy

The present study was undertaken to clarify whether a correlation exists between the hepatic ratio of the beta-phosphorus moiety of ATP (beta-ATP) to inorganic phosphate (Pi), measured by 31P nuclear magnetic resonance spectroscopy 1 h after the reestablishment of portal blood flow, and the survival rate of rats following liver transplantation. This ratio was compared with the arterial ketone body ratio [AKBR (acetoacetate/3-hydroxybutyrate)], which is accepted as a reliable indicator of liver viability. After the transplantation of fresh livers, the 1-week survival rate was 92% and the beta-ATP/Pi ratio was 64% of the normal level. When the liver grafts were subjected to warm ischemia for 25 min or 45 min prior to harvesting, the 1-week survival rate decreased to 43% and 0%, respectively, and the beta-ATP/Pi ratio dropped to 31% and 18% of the normal level, respectively. On the other hand, the AKBR was about 25% of the normal level after transplantation of fresh livers, while it was 37% and 48% after transplantation with 25 min and 45 min of warm ischemia, respectively. However, 4 h after the reestablishment of portal blood flow, the AKBR correlated with the beta-ATP/Pi ratio in both the fresh graft group and the 45-min warm ischemic damage group. These results show that the beta-ATP/Pi ratio provides an accurate evaluation of a graft viability even at an extremely early stage following liver transplantation, and should prove useful for the early diagnosis of primary graft nonfunction after liver transplantation.

Effect of phenylephrine on the compartmentation of inorganic phosphate in perfused rat liver during gluconeogenesis and urea synthesis: a 31P-n.m.r.-spectroscopic study

The transport of Pi between the cytosol and the mitochondria was investigated in perfused rat liver stimulated with phenylephrine and metabolic precursors of glucose and urea: pyruvate, lactate, NH4+ and ornithine. The relative concentrations of phosphorus metabolites in the liver were measured by 31P-n.m.r. spectroscopy. When added simultaneously, phenylephrine and the precursors induced a decrease in the Pi level which in 4-5 min reached a new steady state at 73% of the control level. After 5 min or more of stimulation the ATP level had also decreased. When the stimulation ended, Pi and ATP returned to their initial levels within 15 min. In mitochondria isolated after 5 min of stimulation, Pi was increased more than 2-fold as compared with control mitochondria and, in addition, an accumulation of Pi from the perfusion buffer into the liver was observed. Phenylephrine by itself did not cause any significant changes in the ATP or Pi levels, whereas the glucose and urea precursors in the absence of phenylephrine induced a 9% decrease in Pi, while ATP remained constant. The Pi content of mitochondria isolated under these conditions was not significantly increased as compared with control mitochondria. These results showed that Pi accumulated into the mitochondria by a mechanism possibly involving exchange for malate, and that a major part of the intramitochondrial Pi was invisible by n.m.r.

Compartmentation of inorganic phosphate in perfused rat liver. Can cytosol be distinguished from mitochondria by 31P NMR?

Compartmentation of inorganic phosphate was studied in intact perfused rat liver at 4 degrees C by 31P NMR. It was shown that decreases in cytosolic pH or cytosolic Pi concentration induced the appearance of an additional Pi resonance at low field which was assigned to Pi from an alkaline compartment. Valinomycin (K+ ionophore) induced a further splitting of the lines whereas nigericin (K+/H+ antiport) or potassium cyanide (inhibitor of cytochrome oxidase) had opposite effects. As valinomycin acts mainly on the cytosolic/mitochondrial K+ gradient and KCN on the mitochondrial respiratory chain, it was deduced that the alkaline compartment as revealed by the second Pi resonance was probably mitochondria. Single Pi lines observed on perchloric extracts of livers exhibiting two resonances during cold perfusion confirmed that the split peaks in the intact liver indeed arose from the same molecular species.

31P NMR of Mg-ATP in dilute solutions: complexation and exchange

1. Monovalent-cation [(CH3)4N+, K(I), Na(I)] ATP, 1 mM in nucleotide, in aqueous solutions at pH 7.2, 24 degrees C, generates 2 different 31P NMR spectra, depending upon the salt content of the solution. At salt concentrations below 10 mM, the 31P NMR signals are chemically-shifted upfield (Na salt: alpha, -11.44 delta; beta, -22.91 delta; gamma, -8.36 delta) and the beta- and gamma-groups are broadened (at half-height: alpha, 3.5 Hz; beta, 9.6 Hz; gamma, 69 Hz). Above 10 mM salt, the signals are shifted downfield and are narrow (Na salt: alpha, -11.09 delta, 1.9 Hz; beta, -21.75 delta, 3.3 Hz; gamma, -6.30 delta, 3.9 Hz). 2. The Na-Mg-ATP complex, corresponding to the composition Na6Mg1ATP2, yields a single set of 31P resonances at concentrations of nucleotide of 100 mM, that upon dilution to 0.2 mM, resolve into 2 sets of ATP resonances characterized by low-field and high-field beta- and gamma-group resonance pairs. This set of ATP resonances, in contrast to the resonance set at 100 mM ATP, are broad (100 mM in ATP: alpha, -10.7 delta, 3.7 Hz; beta, -20.1 delta, 15 Hz; gamma, -5.7 delta, 7.3 Hz. 0.2 mM in ATP: alpha, -10.7 delta, 47 Hz; beta, -18.8 and -21.6 delta, 316 and 274 Hz; gamma, -5.5 and -8.7 delta, 460 and 374 Hz). 3. This new data, in combination with data derived from a survey of metal-ion-ATP studies, are interpreted in terms of ATP dimers, incorporating 2 molecules of ATP and 2 metal cations, that exist in water under the physiological conditions of neutral pH, high salt content [135 mM K(I)] and ATP concentrations in the range of 3 mM. 4. A compilation of 31P in vivo and ex vivo data compared to a reference Mg-ATP chemical shift vs Mg/ATP ratio plot indicates that ATP is not fully Mg-saturated in living systems and that 41% exists as the Mg(ATP)2 complex.

31P-NMR spectroscopy of perifused rat hepatocytes immobilized in agarose threads: application to chemical-induced hepatotoxicity

A system consisting of isolated rat hepatocytes immobilized in agarose threads continuously perifused with oxygenated Krebs-Henseleit (KH) solution has been found to maintain cell viability with excellent metabolic activity for more than 6 h. The hepatocytes were monitored by phosphorus-31 nuclear magnetic resonance (31P-NMR) spectroscopy at 4.7 Tesla, by measurement of oxygen consumption and by the leakage of lactate dehydrogenase (LD) and alanine aminotransferase (ALT). The data obtained were comparable to those found for an isolated perfused whole liver in vitro. The effects of allyl alcohol (AA), ethanol, and 4-acetaminophenol (AP) were examined. A solution of 225 microM AA perifused for 90 min caused the disappearance of the beta-phosphate resonance of adenosine triphosphate (ATP) in the 31P-NMR spectra, a 7-fold increase in LD leakage and a 70% reduction in oxygen consumption. Ethanol (1.0 M) perifused for 90 min reduced the beta-ATP signal intensity ratio by 20%, the phosphomonoester (PME) signal by 50% and inorganic phosphate (Pi) by 33% (P less than 0.05). AP (10 mM) caused only mild liver-cell damage. The results demonstrate that perifused immobilized hepatocytes can be used as a liver model to assess the effects of a wide range of chemicals and other xenobiotics by NMR spectroscopy.

Changes in phosphatidylethanolamine metabolism in regenerating rat liver as measured by 31P-NMR

31P-NMR spectra of regenerating rat liver in vivo show increases in resonance intensities in the phosphomonoester (PME) region and decreases in the phosphodiester (PDE) region as early as 12 h post partial hepatectomy, which return to normal by 8 days. The compounds primarily responsible for these changes have been identified in perchloric acid extracts as the phosphomonoester phosphoethanolamine and the phosphodiester glycerophosphoethanolamine (GPE), indicating altered phosphatidylethanolamine metabolism. A corresponding increase in diacylglycerol (DAG) levels during regeneration indicates a possible role for a phosphatidylethanolamine-specific phospholipase C in cellular proliferation. These results suggest that changes in phospholipid metabolites previously associated with neoplastic tissue can also be induced by normal tissue undergoing rapid cellular proliferation. The spectral changes observed in the regenerating rat liver are similar to changes seen in spectra from the livers of human patients in several disease states, indicating that 31P-NMR may allow non-invasive study of cell turnover in liver disease.

Quantitative P-31 MR spectroscopy of the liver in alcoholic cirrhosis

To determine the cause of reduced urea synthesis in cirrhosis, absolute concentrations of phosphorus metabolites in the human liver have been measured in vivo with magnetic resonance (MR) spectroscopy. One-dimensional chemical shift imaging was used to obtain phosphorus-31 spectra from five healthy volunteers and five patients with alcoholic cirrhosis. A reference standard included in all studies enabled the calculation of absolute concentrations. In contrast to hepatic metabolite ratios, absolute concentrations reveal that in the cirrhotic patients, concentrations of adenosine triphosphate (ATP) were significantly reduced and concentrations of phosphomonoesters slightly reduced. Intracellular pH was unchanged. Histologic evidence suggests that the amount of ATP per cell was unchanged and could not account for the reduced urea production. Instead, urea synthesis depends on the functional liver cell mass, which was reduced by 31% in alcoholic cirrhosis. Quantitative in vivo P-31 MR spectroscopy of liver has potential clinical applications and can supplement the more generally used P-31 metabolite ratios.

Studies on the compartmentation of DOG metabolism in the brain

Using 31P-NMR studies we have observed that 1. 2-Deoxyglucose leads into the brain in vivo and in superfused cortical slices in vitro to a maximum concentration at between 45 and 60 min, when 80% of the material is in the phosphorylated form. 2. The phosphorylated DOG6P disappears from the n.m.r. spectra with a half-life of ca 130 min. 3. Two resonances of DOG6P are observed in the actively metabolising tissue, whereas only one is visible in deproteinised tissue extracts. This suggests that the DOG6P is in two separate compartments which differ in pH. 4. Compartmentation between mitochondria, nerve endings and cytoplasm was concluded to be unlikely from subcellular fractionation studies, but the possibility of compartmentation between neurones and glia could not be so clearly assessed.

The cytosolic concentration of phosphate determines the maximal rate of glycogenolysis in perfused rat liver

Glycogenolysis was studied in glycogen-rich perfused livers in which glycogen phosphorylase was fully converted into the a form by exposure of the livers to dibutyryl cyclic AMP. We monitored intracellular Pi by 31P n.m.r. Perfusion with Pi-free medium during 30 min caused a progressive decrease of the Pi signal to 50% of its initial value. In contrast, exposure of the livers to KCN and/or 2,4-dinitrophenol resulted in a rapid doubling of the Pi signal. Alterations in the intracellular Pi coincided with proportional changes in the rate of hepatic glycogenolysis (measured as the output of glucose plus lactate). The results indicate that the rate of glycogenolysis catalysed by phosphorylase a depends linearly on the hepatic Pi concentration. Hence the Km of phosphorylase a for its substrate Pi must be considerably higher than the concentrations that occur in the cytosol, even during hypoxia.

Bicarbonate-dependent and -independent intracellular pH regulatory mechanisms in rat hepatocytes. Evidence for Na+-HCO3- cotransport

Using the pH-sensitive dye 2,7-bis(carboxyethyl)-5(6)-carboxy-fluorescein and a continuously perfused subconfluent hepatocyte monolayer cell culture system, we studied rat hepatocyte intracellular pH (pHi) regulation in the presence (+HCO3-) and absence (-HCO3-) of bicarbonate. Baseline pHi was higher (7.28 +/- 09) in +HCO3- than in -HCO3- (7.16 +/- 0.14). Blocking Na+/H+ exchange with amiloride had no effect on pHi in +HCO3- but caused reversible 0.1-0.2-U acidification in -HCO3- or in +HCO3- after preincubation in the anion transport inhibitor 4,4'-diisothiocyano-2,2'-disulfonic acid stilbene (DIDS). Acute Na+ replacement in +HCO3- alos caused acidification which was amiloride independent but DIDS inhibitible. The recovery of pHi from an intracellular acid load (maximum H+ efflux rate) was 50% higher in +HCO3- than in -HCO3-. Amiloride inhibited H+ effluxmax by 75% in -HCO3- but by only 27% in +HCO3-. The amiloride-independent pHi recovery in +HCO3- was inhibited 50-63% by DIDS and 79% by Na+ replacement but was unaffected by depletion of intracellular Cl-, suggesting that Cl-/HCO3- exchange is not involved. Depolarization of hepatocytes (raising external K+ from 5 to 25 mM) caused reversible 0.05-0.1-U alkalinization, which, however, was neither Na+ nor HCO3- dependent, nor DIDS inhibitible, findings consistent with electroneutral HCO3- transport. We conclude that Na+-HCO3- cotransport, in addition to Na+/H+ exchange, is an important regulator of pHi in rat hepatocytes.

Noninvasive metabolic analysis of eye bank corneas: a magnetic resonance spectroscopic study

Nondestructive, noninvasive metabolic analysis of corneal donor tissue preserved in modified McCarey-Kaufman medium was achieved using phosphorus-31 magnetic resonance spectroscopy (31 P MRS). Clear corneas (n = 10) with minimal Descemet's folds and an endothelial cell density of 2990 +/- 166 cells/mm2 from donors aged 26 to 89 years were used. Relative amounts of low- and high-energy phosphatic metabolites were quantitated during a 1-h period using the integral of the 31P spectrum. The following indicators of the tissue's energy status were determined: (1) 31P spectral energy modulus (ratio of high- to low-energy metabolites, average 0.95 +/- 0.14); (2) adenosine triphosphate/inorganic orthophosphate (ATP/Pi) ratio, 1.97 +/- 0.55; (3) sugar phosphate/inorganic orthophosphate (SP/Pi) ratio, average 1.07 +/- 0.12; and (4) intracorneal pH, calculated from the resonance shift position of inorganic orthophosphate, average 7.28 +/- 0.05. The 31P MRS technique measures key corneal metabolic processes (maintenance of the high-energy phosphate complement, interrelationship of inorganic orthophosphate and its bioesters, and pH). This study provides baseline data for eventually determining optimal parameters of eye bank corneal tissues and for evaluating the metabolic status, or health, of the cornea.

Intracellular mitochondrial membrane potential as an indicator of hepatocyte energy metabolism: further evidence for thermodynamic control of metabolism

The lipophilic triphenylmethylphosphonium cation (TPMP+) has been employed to measure delta psi m, the electrical potential across the inner membrane of the mitochondria of intact hepatocytes. The present studies have examined the validity of this technique in hepatocytes exposed to graded concentrations of inhibitors of mitochondrial energy transduction. Under these conditions, TPMP+ uptake allows a reliable measure of delta psi m in intracellular mitochondria, provided that the ratio [TPMP+]i/[TPMP+]e is greater than 50:1 and that at the end of the incubation more than 80% of the hepatocytes exclude Trypan blue. Hepatocytes, staining with Trypan blue, incubated in the presence of Ca2+, do not concentrate TPMP+. The relationships between delta psi m and two other indicators of cellular energy state, delta GPc and Eh, or between delta psi m and J0, were examined in hepatocytes from fasted rats by titration with graded concentrations of inhibitors of mitochondrial energy transduction. Linear relationships were generally observed between delta psi m and delta GPc, Eh or J0 over the delta psi m range of 120-160 mV, except in the presence of carboxyatractyloside or oligomycin, where delta psi m remained constant. Both the magnitude and the direction of the slope of the observed relationships depended upon the nature of the inhibitor. Hepatocytes from fasted rats synthesized glucose from lactate or fructose, and urea from ammonia, at rates which were generally linear functions of the magnitude of delta psi m, except in the presence of oligomycin or carboxyatractyloside. Linear relationships were also observed between delta psi m and the rate of formation of lactate in cells incubated with fructose and in hepatocytes from fed rats. The linear property of these force-flow relationships is taken as evidence for the operation of thermodynamic regulatory mechanisms within hepatocytes.

NMR as a metabolic tool

We have briefly reviewed the broad range of applications of NMR spectroscopy to metabolism in tissues and biological fluids. Most of these studies are in the exploratory stage, though the potential of NMR for non-invasive and non-destructive monitoring of certain important substrates and reaction pathways is considerable. The limitations of the technique lie in its relative insensitivity and the rather restricted range of substances that it can detect, as well as the current expense. So far, the main clinically useful applications have been in the diagnosis and monitoring of treatment of certain inborn errors of metabolism, namely those that result in altered energy of pH states or the abnormal accumulation of significant amounts of metabolites in body fluids. It might be expected that as localization techniques improve, clinically useful information will be obtained in a wide range of ischaemic or hypoxic states, e.g. stroke and myocardial infarction. The possibility of producing a detailed spatial image of metabolite concentrations (e.g. ATP), in the way that NMR imaging techniques currently do using features of the water proton resonance, is attractive and the initial results are very encouraging (Bogusky et al, 1986; Bailes et al, 1987; Blackledge et al, 1987).

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.

Methods for the metabolic quantification of regional myocardial ischemia

An adequate balance between oxygen supply and demand is a basic requirement for normal cardiac function. When oxygen supply does not meet the demand, progressive cellular damage occurs leading to cardiac dysfunction and, ultimately, tissue death. While traditionally "ischemia" has been defined as decreased oxygen supply secondary to a decrease in blood flow, and "hypoxia" as decreased oxygen supply secondary to a decrease in oxygen tension, this review defines ischemia in its broader sense, namely as a pathophysiologic state in which there is a lack of oxygen relative to the demand for it. In a large number of experimental studies involving the heart, there is need to promptly recognize the ischemic state, to monitor its course in vivo, and to quantify it. Because of cardiac autoregulatory mechanisms, research methods which attempt to quantify supply (e.g., measurement of myocardial blood flow) and/or demand (e.g., measurement of myocardial oxygen consumption) do not necessarily reflect the status of the balance between supply and demand. An imbalance between myocardial supply and demand is more likely to be reflected by metabolic fluxes and by the accumulation of products specific to the ischemic state. Thus, the purpose of this review is to summarize the various methods available to the cardiac surgical investigator today for the metabolic quantification of myocardial ischemia. Due to the complexity of the heart and its inherent regional differences, myocardial ischemic changes are frequently regional in nature. Thus, this review will address metabolic methods for the regional quantification of myocardial ischemia.

Hepatic metabolism during acute ethanol administration: a phosphorus-31 nuclear magnetic resonance study on the perfused rat liver under normoxic or hypoxic conditions

The effect of ethanol metabolism on the energetic parameters and intracellular pH of the isolated perfused rat liver from fed rats was studied by phosphorus-31 nuclear magnetic resonance spectroscopy. This technique allowed us to analyze nondestructively and in real time the role of low oxygen tension on the possible injurious effect of ethanol on the liver cells. A quantitative analysis of nuclear magnetic resonance data recorded on a perfused rat liver within a 30 mm diameter probe has been performed at 80.9 MHz. Under normoxic and normothermic conditions, the levels of phosphorylated metabolites detected by nuclear magnetic resonance were 2.8, 0.3 and 2 mumoles per gm liver wet weight for ATP, ADP and inorganic orthophosphate, respectively. The cytosolic pH was 7.25 +/- 0.05. During a period of 4 min of hypoxia induced by reducing the perfusion flow rate to 25% of its initial value (i.e., from 12 ml to 3 ml per min per 100 gm body weight), the level of ATP dropped to 2.2 mumoles per gm liver wet weight. Concomitantly, ADP and inorganic orthophosphate increased to 0.6 and 3.3 mumoles per gm liver wet weight. Cytosolic pH fell to 7.02 +/- 0.05. Perfusion of the liver with a Krebs medium containing 70 mM (0.4%) ethanol induced a sharp decrease in intracellular inorganic orthophosphate to reach 1.3 mumole per gm liver wet weight and after a lag time of 4 to 6 min, a decrease in ATP level (2.15 mumoles per gm liver wet weight). A large increase in phosphomonoesters (mainly sn-glycerol 3-phosphate) up to 6 mumoles per gm liver wet weight was also observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Lactic dehydrogenase isozymes, 31P magnetic resonance spectroscopy, and in vitro antimitochondrial tumor toxicity with gossypol and rhodamine-123

Three compounds that share specific antimitochondrial properties are gossypol, rhodamine-123, and lonidamine. We compare the antiproliferative activities of these drugs against six human cell lines derived from breast (T47-D), pancreas (MiaPaCa, RWP-2), prostate (DU-145), colon (HCT-8), and cervix (HeLa) carcinomas. Tumor cells enriched in cathodal LDH isozymes (LDH4 and LDH5) are significantly more sensitive to gossypol and rhodamine-123. When compared for ability to inhibit growth of human marrow in soft agar, 10 microM gossypol shows little effect on colony formation whereas 10 microM rhodamine-123 completely prevents stem cell growth, suggesting that gossypol may have the most favorable therapeutic index. Within 24 h of drug administration, there is a relative increase in intracellular inorganic phosphate pools and a marked decline in soluble high-energy phosphates in sensitive tumor cells, as measured by 31P magnetic resonance spectroscopy. These studies suggest that specific antimitochondrial agents might be selectively administered on the basis of tumor LDH isozyme content and noninvasively monitored for antiproliferative activity by 31P spectroscopy.

Phosphorus-31 nuclear-magnetic-resonance study of phosphorylated metabolites compartmentation, intracellular pH and phosphorylation state during normoxia, hypoxia and ethanol perfusion, in the perfused rat liver

A quantitative analysis of the phosphorus-31 NMR spectra of excised perfused rat liver has been carried out at 80.9 MHz using a 30-mm sample cell. The results indicate that in liver from fed rats, all intracellular ATP is detected by NMR. In contrast, only the cytosolic fractions of Pi and ADP can be observed as indicated by careful analysis of spectra obtained from perchloric acid liver extracts and intact liver under valinomycin perfusion. In well-oxygenated perfused liver the ATP concentration is 7.4 mM. Values of 5.3 mM and 0.9 mM are found respectively for Pi and ADP concentrations in the cytosolic compartment. Cytosolic pH value (pHi) is 7.25 +/- 0.05 and free magnesium concentration 0.5 mM. Addition of 70 mM (0.4%) ethanol to the perfusate of a fed rat liver induces 25% and 38% reduction of ATP and Pi levels, respectively. A large amount of sn-glycerol 3-phosphate is synthesized (up to 11 mM) in the cytosol. After ethanol withdrawal, a large overshoot in cytosolic Pi is observed, which is indicative of a net uptake of Pi across the plasma membrane that occurred during ethanol oxidation. No significant pH variation is observed during ethanol infusion. In perfused liver of rats subjected to 48-h fasts, the concentrations of cytosolic phosphorylated metabolites are 5.3 mM, 0.8 mM and 11.5 mM for ATP, ADP and Pi, respectively. The perfusion of the liver with 70 mM ethanol does not change the adenine nucleotide levels, while the Pi content is decreased by 10%. During a 4-min hypoxia, induced by reducing the perfusion flow rate from 12 ml to 3 ml min-1 (100 g body weight)-1, ATP concentration decreases to 5.8 mM in the fed rat liver. Cytosolic Pi and ADP increase to 8.7 mM and 1.6 mM, respectively. The cytosolic pH evolves to more acidic values and reaches 7.02 +/- 0.05 at the end of the 4-min hypoxic period.

Use of nuclear magnetic resonance spectroscopy to study the effects of ethanol consumption on liver metabolism and pathology

The application of NMR spectroscopy to studies of intact organs represents a powerful technology applicable for investigations of alcohol-induced liver disease in animal models. NMR-assisted studies of ethanol metabolism and its related pathology are now possible utilizing the 13C and 31P nuclei primarily. The major advantage of the technology includes the capacity to investigate biochemical processes at the level of the intact organ which is analyzed as a perfused preparation or in situ in an anesthetized animal model. Quantitative measurements of compounds containing either 31P or 13C nuclei are possible, and the kinetics of precursor incorporation into compounds associated with carbohydrate, lipid, and amino acid metabolism are readily followed. The concentrations of free Mg2+ and the pH of the intact liver can be monitored as a function of external perturbation and/or metabolic disturbances.

Electrical and molecular coupling between sodium and proton fluxes in basolateral membrane vesicles of rat liver

Mechanisms of Na+-H+ exchange in the hepatocyte were studied utilizing isolated basolateral membrane vesicles prepared by two different methods: Evidence was obtained for the existence of molecular coupling of Na+ and H+ fluxes (Na+/H+-antiport) which exhibits saturation kinetics (Km 7 mmol/l Na+) and is inhibited by amiloride (1.0 mmol/l). Although the two membrane preparations showed differences with respect to ionic permeabilities, our data suggest that a relatively high H+ conductance exists in the basolateral plasma membrane. Hence, electrical coupling of conductive H+ and Na+ fluxes in the opposite direction could contribute to net Na+-H+ exchange across the basolateral hepatocyte plasma membrane.

1.5 T in vivo 31P NMR spectroscopy of the human liver using a sectorial resonator

A novel resonator for receiving 31P NMR signals at 1.5 T, which because of its topology has been named a "sectorial resonator," exhibits good radio frequency coupling to the adult human liver. Minimal contamination from nonliver tissue (i.e., muscle) was achieved by the use of depth selective pulses and also because the geometry results in cancellation of signals arising from tissues that are close to the two openings of the structure.

The sectorial loop-gap resonator for 31P NMR of the adult human liver at 1.5 T with surface tissue suppression

A new type of loop-gap resonator, which because of its topology is called a "sectorial resonator," is described in considerable analytical detail. It was used for 31P NMR of the livers of two human adult normal volunteers. An effective strategy for suppression of signals from surface tissue was developed. Depth selection was achieved using a separate excitation coil, permitting a reasonable degree of control of the shape of the sensitive region by adjustment of the dimensions of the excitation coil and of the type of pulse delivered. In addition a new principle for cancellation is described: the topology of the sectorial resonator leads to cancellation signals from tissue masses at the two openings of the structure. Using a phase-alternated repeated 90 degrees FID pulse sequence, good spectra were achieved in 2.5 min that are free from contamination by the phosphocreatine peak.

Carbon-13 and phosphorus-31 NMR study of hepatic metabolism in the perfused rat liver

Phosphorus-31 nuclear magnetic resonance (NMR) has been used to determine non-invasively absolute concentrations of phosphorylated metabolites in the perfused rat liver. It has been shown that the NMR method does detect cytoplasmic ATP and ADP (ATP:ADP ratio of 15 +/- 3) with no contribution from mitochondrial adenine nucleotides. The concentration of ATP was 7.2 +/- 0.3 mM in the cytosol of well-oxygenated liver, after two hours of perfusion with a Krebs-Ringer buffer. Other phosphorylated metabolites were detected, mainly inorganic phosphate (1.1 mumol/g liver wet weight), phosphorylcholine (1.0 mumol/g wet weight), glycerophosphorylethanolamine (0.34 mumol/g wet weight) and glycerophosphorylcholine (0.30 mumol/g wet weight). The intracellular pH measured from the position of the Pi resonance has a value of 7.2 +/- 0.1. It is likely that the detectable Pi originates from the cytosolic compartment since a pH value of 7.4-7.6 would be expected for the mitochondrial matrix. Natural abundance carbon-13 NMR has also been used to follow the glycogen breakdown in situ by measuring the intensity of the glycogen C-1 resonance in the perfused liver spectrum as a function of the perfusion time. The glycogenolytic process has been studied as a function of the glucose content of the perfusate. Rate of glycogenolysis from 2.7 to 0.16 muEq glycosyl units g wet weight-1 min-1 were found when glucose concentration in the perfusate was varied from 0 to 50 mM. The fate of 90% enriched [2-13C] acetate has been studied in the perfused rat liver by 13C-NMR in order to investigate the 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 carbon-13 isotope into glutamine can be linked to the operation of the glutamate-glutamine antiporter and to the high activity of the cytosolic glutamate synthetase. The finding of both active glutaminase and glutamine synthetase activity in the same liver cells is an evidence of the existence of an active glutamine-glutamate futile cycle.