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

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.

Live Imaging of Phosphate Levels in Arabidopsis Root Cells Expressing a FRET-Based Phosphate Sensor

Phosphorous (P) is an essential macronutrient in all organisms serving various fundamental biological processes, and is one of the least available plant nutrients in the soil. The application of inorganic phosphate (Pi) fertilizers is frequent, but it has a high environmental and financial cost. Breeding crops for improved Pi use-efficiency is a promising plant-based solution to pursue a reduction of fertilizer dependency. Availability of tools for monitoring changes of plant cellular Pi concentration in real-time can contribute to advancing knowledge on the molecular basis of Pi transport and homeostasis in plants. Genetically encoded fluorescent sensors have provided new insight on cellular processes. Here, we show that two Pi Fluorescence Resonance Energy Transfer (FRET)-based sensors from the FLIPPi family, the low-affinity FLIPPi-30m and the high-affinity FLIPPi-4µ, can be expressed and analyzed in Arabidopsis thaliana with wild-type background. These FLIPPi sensors had not been tested in plants, but only in mammalian cell lines. We show FRET response and live imaging of Pi levels in seedling roots of Arabidopsis FLIPPi-30m and FLIPPi-4µ lines. Our results reinforce that sensors from the FLIPPi family are valuable tools for studying mechanisms of Pi transport and homeostasis in plants, and for research towards a more sustainable use of Pi fertilization.

Feasibility of absolute quantification for 31 P MRS at 7 T

Purpose

Phosphorus spectroscopy can differentiate among liver disease stages and types. To quantify absolute concentrations of phosphorus metabolites, sensitivity calibration and transmit field (B1+) correction are required. The trend toward ultrahigh fields (7 T) and the use of multichannel RF coils makes this ever more challenging. We investigated the constraints on reference phantoms, and implemented techniques for the absolute quantification of human liver phosphorus spectra acquired using a 10‐cm loop and a 16‐channel array at 7 T.

Methods

The effect of phantom conductivity was assessed at 25.8 MHz (1.5 T), 49.9 MHz (3 T), and 120.3 MHz (7 T) by electromagnetic modeling. Radiofrequency field maps (B1±) were measured in phosphate phantoms (18 mM and 40 mM) at 7 T. These maps were used to assess the correction of 4 phantom 3D‐CSI data sets using 3 techniques: phantom replacement, explicit normalization, and simplified normalization. In vivo liver spectra acquired with a 10‐cm loop were corrected with all 3 methods. Simplified normalization was applied to in vivo 16‐channel array data sets.

Results

Simulations show that quantification errors of less than 3% are achievable using a uniform electrolyte phantom with a conductivity of 0.23‐0.86 S.m⁻¹ at 1.5 T, 0.39‐0.58 S.m⁻¹ at 3 T, and 0.34‐0.42 S.m⁻¹ (16‐19 mM KH₂PO_4(aq)) at 7 T. The mean γ‐ATP concentration quantified in vivo at 7 T was 1.39 ± 0.30 mmol.L⁻¹ to 1.71 ± 0.35 mmol.L⁻¹ wet tissue for the 10‐cm loop and 1.88 ± 0.25 mmol.L⁻¹ wet tissue for the array.

Conclusion

It is essential to select a calibration phantom with appropriate conductivity for quantitative phosphorus spectroscopy at 7 T. Using an 18‐mM phosphate phantom and simplified normalization, human liver phosphate metabolite concentrations were successfully quantified at 7 T.

1H NMR-based metabonomics approach in a rat model of acute liver injury and regeneration induced by CCl4 administration

The administration of carbon tetrachloride (CCl(4)) has been established as a model of toxin-induced acute and chronic liver injury. In the present study, we investigate the progression of the biochemical response to acute CCl(4)-induced liver injury, capturing metabolic variations during both toxic insult and regeneration using NMR-based metabonomic analysis of liver tissue and plasma. A single dose of CCl(4) (1 mL/kg BW) was intraperitoneally administered to male Wister rats sacrificed every 12h up to 72 h post treatment, while healthy animals served as controls. Acquired (1)H NMR spectra of liver tissue extracts and plasma samples were explored with multivariate analysis and the resulted models were correlated with conventional biochemical and histopathological indices of toxicity for monitoring the progression of experimental injury. The metabonomic analysis resulted in discrimination between the subjects under toxic insult (up to 36 h) and those at the regenerative phase (peaked at 48 h). At 72 h normalization of liver's pathology similar to the controls group was apparent. Principal component analysis (PCA) trajectories highlighted the time points of the greater degree of toxic insult and the regenerative state. A number of metabolites such as glucose, lactate, choline, formate exhibited variations suggesting CCl(4) induced impairment in essential biochemical pathways as energy metabolism, lipid biosynthesis and transmethylation reactions. The latter provides new evidence of B12 and folate pathways deficiency, indicative of new mechanistic implications possibly by direct inhibition of B12 dependent enzymes by the chlorinated radicals of CCl(4) metabolism.

Activation of biliverdin-IXalpha reductase by inorganic phosphate and related anions

The effect of pH on the initial-rate kinetic behaviour of BVR-A (biliverdin-IXalpha reductase) exhibits an alkaline optimum with NADPH as cofactor, but a neutral optimum with NADH as cofactor. This has been described as dual cofactor and dual pH dependent behaviour; however, no mechanism has been described to explain this phenomenon. We present evidence that the apparent peak of activity observed at neutral pH with phosphate buffer and NADH as cofactor is an anion-dependent activation, where inorganic phosphate apparently mimics the role played by the 2'-phosphate of NADPH in stabilizing the interaction between NADH and the enzyme. The enzymes from mouse, rat and human all exhibit this behaviour. This behaviour is not seen with BVR-A from Xenopus tropicalis or the ancient cyanobacterial enzyme from Synechocystis PCC 6803, which, in addition to being refractory to activation by inorganic phosphate, are also differentiated by an acid pH optimum with both nicotinamide nucleotides.

A novel analytical method for in vivo phosphate tracking

Genetically-encoded fluorescence resonance energy transfer (FRET) sensors for phosphate (P(i)) (FLIPPi) were engineered by fusing a predicted Synechococcus phosphate-binding protein (PiBP) to eCFP and Venus. Purified fluorescent indicator protein for inorganic phosphate (FLIPPi), in which the fluorophores are attached to the same PiBP lobe, shows P(i)-dependent increases in FRET efficiency. FLIPPi affinity mutants cover P(i) changes over eight orders of magnitude. COS-7 cells co-expressing a low-affinity FLIPPi and a Na(+)/P(i) co-transporter exhibited FRET changes when perfused with 100 microM P(i), demonstrating concentrative P(i) uptake by PiT2. FLIPPi sensors are suitable for real-time monitoring of P(i) metabolism in living cells, providing a new tool for fluxomics, analysis of pathophysiology or changes of P(i) during cell migration.

Hepatic 31P magnetic resonance spectroscopy: a hepatologist's user guide

Hepatic phosphorus magnetic resonance spectroscopy (31P MRS) offers the exciting potential of studying metabolic processes in the human liver in vivo. Many investigators have utilized 31P MRS to research a broad range of metabolic questions, and there is outstanding potential for this imaging modality in the future. However, at times it is difficult to appreciate this potential because most published series have been small, and comparisons between studies are difficult. Indeed, the published literature contains significant variation in data acquisition and data analysis techniques and, perhaps most importantly, the interpretation of the data itself. As MR technology continues to evolve and more studies are being performed, perhaps a greater consensus of study techniques and endpoints will emerge. This review summarizes the present literature on human hepatic 31P MRS.

Effect of ethanol on metabolism of purine bases (hypoxanthine, xanthine, and uric acid)

There are many factors that contribute to hyperuricemia, including obesity, insulin resistance, alcohol consumption, diuretic use, hypertension, renal insufficiency, genetic makeup, etc. Of these, alcohol (ethanol) is the most important. Ethanol enhances adenine nucleotide degradation and increases lactic acid level in blood, leading to hyperuricemia. In beer, purines also contribute to an increase in plasma uric acid. Although rare, dehydration and ketoacidosis (due to ethanol ingestion) are associated with the ethanol-induced increase in serum uric acid levels. Ethanol also increases the plasma concentrations and urinary excretion of hypoxanthine and xanthine via the acceleration of adenine nucleotide degradation and a possible weak inhibition of xanthine dehydrogenase activity. Since many factors such as the ALDH2*1 gene and ADH2*2 gene, daily drinking habits, exercise, and dehydration enhance the increase in plasma concentration of uric acid induced by ethanol, it is important to pay attention to these factors, as well as ingested ethanol volume, type of alcoholic beverage, and the administration of anti-hyperuricemic agents, to prevent and treat ethanol-induced hyperuricemia.

Fatty acids enhance membrane permeabilization by pro-apoptotic Bax

Fatty acids are known promoters of apoptosis. In the present study, the direct role of fatty acids with regard to their ability to cause membrane permeabilization by Bax was explored. Addition of fatty acids to liposomes in the presence of cations greatly enhanced the permeabilizing activity of Bax, a pro-apoptotic Bcl-2 protein. This provides a putative mechanism for the role of fatty acids in apoptosis. It is not a result of detergent-like properties of fatty acids, since a different micelle-forming amphiphile, dilysocardiolipin, was strongly inhibitory. We also demonstrate that there is a synergistic effect on Bax-induced permeabilization between Ca(2+) and Mg(2+), both on the binding of Bax to liposomes as well as on the induction of the leakage of liposomal contents. Micromolar concentrations of Ca(2+) added externally or submicromolar concentrations of free Ca(2+) present in the medium were sufficient to promote Bax-induced permeabilization synergistically with externally added Mg(2+). These results indicate that Bax can induce leakage from liposomes at ion concentrations resembling those found physiologically. The synergistic effects of Ca(2+) and Mg(2+) were observed with liposomes with different lipid compositions. Thus the action of Bax is strongly modulated by the presence of bivalent cations that can act synergistically, as well as by micelle-forming lipid components that can be either stimulatory or inhibitory.

2,5-Anhydro-D-mannitol increases hepatocyte sodium: transduction of a hepatic hunger stimulus?

To test the hypothesis that decreased hepatocyte ATP is transduced into a hepatic neuronal signal via a change in sodium pump activity, we examined the effect of 2,5-anhydro-D-mannitol (2,5-AM), which stimulates feeding behavior in rats, on intracellular sodium levels using 23Na nuclear magnetic resonance (NMR) spectroscopy. Isolated hepatocytes suspended in agarose beads were superfused with either 2.5 mM 2,5-AM or fructose in the presence of the paramagnetic shift reagent, thulium(III)(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetra(methylenephosphonate)). Superfusion with 2,5-AM decreased hepatocyte ATP and increased intracellular sodium levels compared with superfusion with either fructose or shift reagent alone starting within 15 min of exposure, reaching a maximum level of 120% of baseline by 30 min and declining gradually thereafter over the next 90 min. Superfusion with fructose, which also decreased hepatocyte ATP but by less than half the amount seen with 2,5-AM, had no significant effect on cellular sodium levels. The results support the hypothesis that changes in sodium pump activity could participate in transducing a hunger stimulus associated with hepatocyte energy status into a signal for hunger.

Interactions of dietary fat and 2,5-anhydro-D-mannitol on energy metabolism in isolated rat hepatocytes

The fructose analog 2,5-anhydro-D-mannitol (2,5-AM) stimulates feeding in rats by reducing ATP content in the liver. These behavioral and metabolic effects occur with rats fed a high-carbohydrate/low-fat (HC/LF) diet, but they are prevented or attenuated when the animals eat high-fat/low-carbohydrate (HF/LC) food. To examine the metabolic bases for this effect of diet, we assessed the actions of 2,5-AM on ATP content, oxygen consumption, and substrate oxidation in isolated hepatocytes from rats fed one of the two diets. Compared with cells from rats fed the HC/LF diet ("HC/LF" cells), cells from rats fed the HF/LC diet ("HF/LC" cells) had similar ATP contents but lower oxygen consumption, decreased fructose, and increased palmitate oxidation. 2,5-AM did not decrease ATP content or oxygen consumption in HF/LC cells as much as it did in HC/LF hepatocytes, and it only affected fructose and palmitate oxidation in HC/LF cells. 31P-NMR spectroscopy indicated that differences in phosphate trapping accounted for differences in depletion of ATP by 2,5-AM. These results suggest that intake of the HF/LC diet prevents the eating response and attenuates the decline in liver ATP by shifting hepatocyte metabolism to favor fat over carbohydrate as an energy-yielding substrate.

High-fat diet prevents eating response and attenuates liver ATP decline in rats given 2,5-anhydro-D-mannitol

Administration of the fructose analog 2,5-anhydro-D-mannitol (2,5-AM) stimulates eating in rats fed a low-fat diet but not in those fed a high-fat diet that enhances fatty acid oxidation. The eating response to 2,5-AM treatment is apparently triggered by a decrease in liver ATP content. To assess whether feeding a high-fat diet prevents the eating response to 2,5-AM by attenuating the decrease in liver ATP, we examined the effects of the analog on food intake, liver ATP content, and hepatic phosphate metabolism [using in vivo 31P-NMR spectroscopy (NMRS)]. Injection (intraperitoneal) of 300 mg/kg 2,5-AM increased food intake in rats fed a high-carbohydrate/low-fat diet, but not in those fed high-fat/low-carbohydrate (HF/LC) food. Liver ATP content decreased in all rats given 2,5-AM compared with saline, but it decreased about half as much in rats fed the HF/LC diet. NMRS on livers of anesthetized rats indicated that feeding the HF/LC diet attenuates the effects of 2,5-AM on liver ATP by reducing phosphate trapping. These results suggest that rats consuming a high-fat diet do not increase food intake after injection of 2,5-AM, because the analog is not sufficiently phosphorylated and therefore fails to decrease liver energy status below a level that generates a signal to eat.

Effect of graded hypoxia on the rat hepatic tissue oxygenation and energy metabolism monitored by near-infrared and 31P nuclear magnetic resonance spectroscopy

Alteration in hepatic cellular adenosine triphosphate (ATP) levels has been shown to be a sensitive index for hypoxic damage. Hepatic ATP metabolism can be monitored by 31P nuclear magnetic resonance (NMR). Near-infrared spectroscopy (NIRS) can measure tissue oxyhemoglobin (HbO2), deoxyhemoglobin (Hb), and cytochrome oxidase (Cyt Ox), which reflect ATP production. In this study, hepatic oxygenation parameters have been correlated with ATP metabolism under graded hypoxia. Sprague-Dawley rats underwent laparotomy for liver exposure. NIRS probes and an NMR coil were placed on the liver and the animal was positioned in the NMR magnet. Graded hypoxia was achieved by a stepwise reduction of the fraction of inspired oxygen (FiO2) from 15 to 4%. Recovery between the hypoxic periods was achieved using 30% oxygen. Hepatic tissue oxygenation parameters were measured continuously by NIRS; 31P-NMR spectra obtained at 1 min intervals from energy metabolites and intracellular pH were calculated. All the hypoxic grades produced an immediate reduction in HbO2 with a simultaneous increase in Hb. Cyt Ox was reduced significantly only with FiO2 of </= 10%. 31P-NMR spectra showed a significant decrease in cellular beta nucleoside triphosphate (beta-NTP) only with FiO2 of </= 10%. Significant correlation was seen between beta-NTP and HbO2 (r=0.85), Hb (r=-0.74), and Cyt Ox (r=0.81). Cyt Ox was reduced with intracellular hypoxia and correlated temporally with the reduction of cellular beta-NTP, and therefore could be used as an index for the changes in beta-NTP with hypoxia.

Effect of ethanol on the metabolism of primary astrocytes studied by (13)C- and (31)P-NMR spectroscopy

Nuclear magnetic resonance was used as the primary technique to investigate the effect of ethanol (40, 80, and 160 mM) on the levels of high-energy phosphates, glycolytic flux, anaplerotic and oxidative fluxes to the tricarboxylic acid (TCA) cycle, the contribution of the pentose phosphate pathway (PPP), and the uptake and release of amino acids on primary cultures of rat astrocytes. On line (31)P-NMR spectroscopy showed that long-term exposure to ethanol caused a drop in the levels of ATP and phosphocreatine. The ratio between the fluxes through the pyruvate dehydrogenase and pyruvate carboxylase reactions also decreased, whereas the glycolytic flux and the ratio between formation of lactate and glucose consumption increased when cells were exposed to acute doses of ethanol. Flux through the pentose phosphate pathway was not affected. The uptake of cysteine and the release of glutamine were stimulated by ethanol, whereas the release of methionine was inhibited. Moreover, the fractional enrichment in serine was enhanced. The changes in the amino acid metabolism are interpreted as a response to oxidative stress induced by ethanol.

Determination of high-energy phosphate compounds and inorganic phosphate by reversed-phase high-performance liquid chromatography: evaluation of myocardial metabolic status in aerobically perfused and hypoxic mouse heart

The present paper describes a simple HPLC method designed for measuring high-energy phosphate (HEP) compounds in a single run and inorganic phosphate (Pi) in an other short run under the same HPLC conditions. Inorganic phosphate was estimated by using thymidine phosphorylase (EC 2.4.2.4) which catalyzes a reaction involving inorganic phosphate to produce 2-deoxyribose 1-phosphate and thymine. The thymine/Pi stoichiometry was 1. The method provides a reproducible instrument for evaluating myocardial high-energy metabolism under physiological and pathological conditions.

31P and 1H NMR spectroscopic studies of liver extracts of carbon tetrachloride-treated rats

NMR spectroscopy was used to examine hepatic metabolism in cirrhosis with a particular focus on markers of functional cellular hypoxia. (31)P and (1)H NMR spectra were obtained from liver extracts from control rats and from rats with carbon tetrachloride-induced cirrhosis. A decrease of 34% in total phosphorus content was observed in cirrhotic rats, parallelling a reduction of 40% in hepatocyte mass as determined by morphometric analysis. Hypoxia appeared to be present in cirrhotic rats, as evidenced by increased inorganic phosphate levels, decreased ATP levels, decreased ATP:ADP ratios (1.72 +/- 0.40 vs 2.48 +/- 0.50, p < 0.01), and increased inorganic phosphate:ATP ratios (2.77 +/- 0.48 vs 1.62 +/- 0.24, p < 0.00001). When expressed as a percentage of the total phosphorus content, higher levels of phosphoethanolamine and lower levels of NAD and glycerophosphoethanolamine were detected in cirrhotic rats. Cirrhotic rats also had increased phosphomonoester:phosphodiester ratios (5.73 +/- 2.88 vs 2.53 +/- 0.52, p < 0.01). These findings are indicative of extensive changes in cellular metabolism in the cirrhotic liver, with many findings attributable to the presence of intracellular hypoxia.

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.

Mitochondrial permeability transition during hypothermic to normothermic reperfusion in rat liver demonstrated by the protective effect of cyclosporin A

The purpose of this study was to test the hypothesis that mitochondrial permeability transition might be implicated in mitochondrial and intact organ dysfunctions associated with damage induced by reperfusion after cold ischaemia. Energetic metabolism was assessed continuously by 31P-NMR on a model system of isolated perfused rat liver; mitochondria were extracted from the livers and studied by using top-down control analysis. During the temperature transition from hypothermic to normothermic perfusion (from 4 to 37 degrees C) the ATP content of the perfused organ fell rapidly, and top-down metabolic control analysis of damaged mitochondria revealed a specific control pattern characterized by a dysfunction of the phosphorylation subsystem leading to a decreased response to cellular ATP demand. Both dysfunctions were fully prevented by cyclosporin A, a specific inhibitor of the mitochondrial transition pore (MTP). These results strongly suggest the involvement of the opening of MTP in vivo during the transition to normothermia on rat liver mitochondrial function and organ energetics.

Hepatocellular defence against acidosis is preserved after cold storage

BACKGROUND

Primary non-function of liver allografts is related to preservation time, during which hypoxia leads to intracellular accumulation of acid. Preservation-induced failure of hepatocellular pH regulation may play a role in the pathogenesis of primary graft non-function.

METHODS

Using cultured/suspended rat hepatocytes and fluorimetric determination of intracellular pH, we determined whether preservation in University of Wisconsin solution (4 degrees C) impairs hepatocellular defence mechanisms against acidosis.

RESULTS

In non-preserved, 24-h-preserved and 48-h-preserved hepatocytes acidified to pH 6.7-6.8, initial Na+/H+ antiport-mediated H+ fluxes averaged 12 +/- 5, 9 +/- 5 and 12 +/- 5 nmol microL-1 min-1 and initial Na+/HCO3- symport-mediated HCO3- fluxes 7 +/- 2, 7 +/- 3 and 6 +/- 2 nmol microL-1 min respectively (P = NS). Preservation did not affect the inverse relationship between Na+/H+ antiport activity and intracellular pH. Thus, hepatocellular defence against intracellular acidosis is maintained during up to 48 h in University of Wisconsin solution.

CONCLUSION

Altered pHi homeostasis is unlikely to play a role in the pathogenesis of primary non-function of liver allografts.

Cytosolic pH variations in perfused rat liver at 4 degrees C: role of intracellular buffering power

The effect of low temperature on cytosolic pH regulation and buffering capacity was evaluated in the isolated rat liver. The pH changes were followed by phosphorus-31 nuclear magnetic resonance. Cooling from 37 to 4 degrees C, with Krebs-Heinseleit perfusion at an external pH of 7.35, induced an alkaline shift in cytosolic pH (pHcyt) of 0.13 or 0.75 pH units in the presence of bicarbonate, respectively (dpH cys/dT values were 0.004 and 0.022 unit/degrees C. With 4 degrees C perfusion, in the presence or absence of bicarbonate, acute changes of external pH (from 7.40 to 5.90) did not affect pHcyt. In contrast, intracellular loading with isobutyric acid or NH4Cl induced rapid pHcyt variations. The intrinsic buffering power value (10 to 50 slykes) measured in the absence of bicarbonate depended on pHcyt. The larger value was observed for pHcyt 7.30, a value near the pK value of the imidazole group of intracellular proteins at 4 degrees C. The presence of bicarbonate modified the amplitude of the pHcyt change by increasing the total buffering power. It was demonstrated that during hypothermia, ionic carriers are inactivated and the charged forms of molecules are unable to cross the cell membrane; thus, the pHcyt homeostasis depends essentially on intracellular buffering power.

Impairment of hepatic mitochondrial respiratory function following storage and orthotopic transplantation of rat livers

Prolonged storage of organs for transplant results in tissue damage which may be compounded on reperfusion of the graft tissue. The effect of storage times was examined on hepatic mitochondrial oxygen consumption and activities of complexes I, II-III, IV, and V in mitochondria isolated from rat liver isografts stored for 25 min and 24 h pre- and posttransplantation. While Complex I activity was significantly (P < 0.05) inhibited under all the conditions studied, Complex II-III activity was only significantly (P < 0.05) reduced following transplantation of 24-h stored tissue. Complex IV activity remained unchanged under all the conditions studied. Although Complex V activity was significantly damaged within the first 25 min of ischemia, activity values were partially recovered to control levels following 3 h of reperfusion after transplantation. Prolonged (24 h) storage induced decreases in Complex V activity which were irrecoverable. Mitochondria subjected to 25 min ischemia alone also showed a significant (P < 0.01) decrease in NAD(+)-linked respiratory control indices due to a stimulated state 4 rate. The 24-h storage and transplantation brought about a significantly (P < 0.001) greater inhibition of respiratory control and state 3 respiration. FAD-linked respiration parameters were significantly (P < 0.05) affected in livers subjected to prolonged (24 h) storage or transplantation. These data suggest that a loss of membrane integrity coupled with an inhibition of Complexes I and V and an involvement of Complex II-III in 24-h stored hepatic transplants accounts for mitochondrial respiratory dysfunction in hepatic transplantation injury. No indication of Complex IV damage was found in this study. This study shows that damage to specific mitochondrial complexes occurs as a consequence of hypothermic ischemic injury.

31P-magnetic resonance spectroscopy studies of nucleated and non-nucleated erythrocytes; time domain data analysis (VARPRO) incorporating prior knowledge can give information on the binding of ADP

Human erythrocytes have no nucleus, mitochondria or endoplasmic reticulum, whereas chicken erythrocytes have a nucleus and mitochondria and are closer in internal morphology, to cells such as the hepatocyte. Erythrocytes were used to test the hypothesis that 31P-MRS invisibility of ADP is associated with the presence of intracellular organelles. Simple frequency domain spectral analysis methods showed that all the acid extractable ADP (and ATP) was MR-visible in human erythrocytes. However, such methods gave variable estimates for 31P-NMR spectra of fresh chicken erythrocytes from which no conclusions could be drawn about the MR-visibility of ADP. Only when the data were fitted by a method incorporating prior knowledge of the ATP and ADP peak structure, using the time domain VARPRO method, was it possible to conclude that in fresh chicken erythrocytes, similar to other nucleated cells (liver, muscle), all the acid extractable ADP appeared to be MRS invisible, indicating binding or sequestration by intracellular organelles.

Beneficial effect of alanine on metabolic recovery of fasted livers submitted to cold ischemia

The aim of this study was to investigate the possible beneficial effect on perfused mouse liver of alanine as an exogenous substrate for gluconeogenesis. Livers from fed and fasted animals were perfused with oxygenated Krebs' Henseleit buffer for 30 min, then stored at 4 degrees C in University of Wisconsin solution for 48 h. Then reperfusion at 37 degrees C was performed according to two protocols. In the first one, reperfusion with alanine-free Krebs' Henseleit buffer was used for 1 h. 8 mM (3-(13)C) alanine was then added and perfusion was prolonged for a second hour. In the second one, the first hour of perfusion was omitted and the organs were reperfused directly for an hour in the presence of 8 mM (3-(13)C)alanine. 31P NMR was used to measure the NTP recovery of the livers. At the end of the reperfusions, 13C and 1H NMR spectra of perfusates and of glutamine extracted from these perfusates by HPLC were recorded. These data were analysed according to a model of liver metabolism assuming that the only substrate of the liver was (3-(13)C)alanine and endogenous substrates were metabolizable only through pyruvate. It was found that in the absence of initial alanine at reperfusion, livers from fasted mice recovered less NTP than those of fed ones (40 +/- 4% vs 60 +/- 5%, p <0.01), but not if this substrate is present at the beginning of reperfusion (61 +/- 5% vs 60 +/- 5%). This was confirmed by the amount of labelled metabolites produced. However, the dilution of 13C labelled metabolites by unlabelled ones did not indicate a larger concentration of endogenous substrates in livers from fed mice. The conclusion reached was that the lower pyruvate dehydrogenase activity of livers from fasted mice relatively to that from fed mice could be compensated for by the greater pyruvate concentration provided by alanine for the initial production of NTP after cold ischemia and warm reperfusion.

Differences in nucleotide compartmentation and energy state in isolated and in situ rat heart: assessment by 31P-NMR spectroscopy

Free cytosolic concentrations of ATP, PCr, ADP and 5'-AMP, and the cytosolic [ATP]/[ADP].[Pi] ratio, were determined in isolated and in situ rat hearts using 31P-NMR spectroscopy. Total tissue metabolite concentrations were determined by HPLC analysis of freeze-clamped, perchloric acid-extracted tissue. In in situ myocardium the PCr/ATP ratio was 2.7 +/- 0.2 determined from 31P-NMR data (using either PCr/beta-NTP or PCr/gamma-NTP), and 1.9 +/- 0.1 (P < 0.01) determined from total tissue concentrations. 31P-NMR-determined and total tissue [PCr] were in excellent agreement (49.6 +/- 8.4 and 49.5 +/- 1.0 mumol.g-1 dry wt, respectively), whereas 31P-NMR-determined [ATP] (18.6 +/- 3.2 mumol.g-1 dry wt) was only 71% of the total tissue concentration (26.1 +/- 1.7 mumol.g-1 dry wt, P < 0.01). Isolation and Langendorff perfusion of rat hearts with glucose as substrate reduced total tissue [ATP] and [PCr] and the 31P-NMR-determined PCr/ATP ratio fell to 1.5 +/- 0.1. This value agreed well with the total tissue ratio of 1.4 +/- 0.1, and there was excellent agreement between 31P-NMR-determined and total tissue [PCr] and [ATP] values in the perfused heart. Addition of pyruvate to perfusate increased the 31P-NMR-determined PCr/ATP ratio to 1.7 +/- 0.1 due to elevated [PCr], and there remained excellent agreement between NMR-determined and total tissue [PCr] and [ATP] values. Free cytosolic [ADP] (from the creatine kinase equilibrium) was 5% of total tissue ADP, and free cytosolic [5'-AMP] (from the adenylate kinase equilibrium) ranged from 0.2-0.3% of total tissue 5'-AMP. Bioenergetic state, indexed by [ATP]/[ADP].[Pi], was much lower in isolated perfused hearts (30 mM-1) vs. in situ myocardium (approximately 150 mM-1). In summary, we observe a substantial disproportionality between total tissue PCr/ATP and 31P-NMR-determined PCr/ATP in highly energised in situ myocardium but not in isolated perfused hearts. This appears due to an NMR invisible ATP compartment approximating 29% of total tissue ATP in situ. Additionally, more than 95% of ADP and more than 99% of 5'-AMP exist in bound forms in perfused and in situ myocardium. The physiological significance of these observations is unclear. However, substantial differences between 31P-NMR visible and total tissue [ATP] introduces significant errors in conventional estimation of free cytosolic [ADP], [5'-AMP] and [ATP]/[ADP].[Pi] from in vivo 31P-NMR data.

31P NMR study of acute toxic effects of cadmium chloride on rat liver

In this study, acute effects of cadmium ions (Cd2+) on energy metabolism in rat livers were analyzed in vivo after intravenous administration using 31P NMR. Both inorganic phosphate (Pi) and nucleotide triphosphate (NTP) peaks of in vivo Cd-treated livers gradually decreased over a 6-h period. In the extract, NTP peaks in Cd-treated livers were lower, as in the in vivo experiments, but the Pi peak was significantly higher than the control. The apparent decrease in Pi in in vivo liver treated with Cd could be caused by the reduced visibility of Pi because of its uptake into mitochondria from cytoplasm, accompanied by the uncoupling of oxidative phosphorylation by Cd2+. These results indicated that total Pi in the hepatocytes increases after Cd administration. However, only 10% of Pi was visible in Cd-treated livers in vivo, whereas 34% of Pi was visible in controls. Significant increases in phosphocholine and glycerophosphocholine were also observed in extracts of Cd-treated livers.

In vivo hepatic 31P magnetic resonance spectroscopy in chronic alcohol abusers

BACKGROUND/AIMS

In vivo hepatic 31P magnetic resonance spectroscopy (MRS) can provide information on hepatic energy metabolism, phospholipid substrates, and hepatocyte lipid bilayers. The aim of this study was to ascertain the effects of alcohol ingestion on hepatic 31P spectral variables.

METHODS

Twenty-six chronic alcohol abusers underwent hepatic 31P MRS 6-12 hours after their last alcoholic drink; studies were repeated in 17 individuals following abstinence from alcohol. The reference population comprised 16 healthy volunteers. Ratios of phosphomonoesters (PME), inorganic phosphate, and phosphodiesters (PDE) relative to beta-adenosine triphosphate (ATP) were measured.

RESULTS

In patients with minimal liver injury, recent drinking was associated with a significant elevation in the mean PDE/ATP ratio (P < 0.0001) and an increase in mean PME/ATP, which was not significant; abstinence was associated with reductions in both metabolite ratios. In patients with alcoholic cirrhosis, recent drinking was associated with an elevation in mean PME/ATP (P < 0.05) and an increase in mean PDE/ATP, which was not significant; abstinence was associated with no significant change in PME/ATP but with a reduction in PDE/ATP.

CONCLUSIONS

In the absence of significant liver injury, chronic alcohol abuse is associated with the elevation of PME/ATP, possibly reflecting changes in hepatic redox potential, and of PDE/ATP, most likely reflecting the induction of hepatocyte endoplasmic reticulum. In the presence of cirrhosis, these changes are attenuated and modified.

A magnetic resonance study of the effect of nutritional status on cold-preserved murine liver

BACKGROUND/AIMS

Clinical and experimental studies suggest a link between nutritional status and the recovery of hepatic function after hypoxic and hypothermic insults. This study aimed at determining the metabolic pathways involved in such recovery as a function of nutrition.

METHODS

Livers from fed and fasted mice were perfused with oxygenated Krebs'-Henseleit buffer (RBKB). After depletion of glycogen, 31P and 13C nuclear magnetic resonance spectra were acquired. Livers were flushed with University of Wisconsin solution and stored at 4 degrees C for 0, 24, or 48 hours. At reperfusion with RBKB, recovery of nucleoside triphosphates (NTP) was followed up. After 45 minutes, [3-13C]alanine was added and substrate consumption and metabolic products assessed.

RESULTS

Livers from fed animals recovered more NTP at reperfusion both after 24 hours (85% +/- 11% vs. 67% +/- 7%; P < 0.01) and 48 hours (61% +/- 10% vs. 36% +/- 10%; P < 0.01, respectively) of cold storage. Gluconeogenesis as reflected by [3-13C]alanine consumption was also higher from fed animals. Hepatic glycogen before preservation was low in both groups. Livers from fasted animals contained increased triglyceride levels, but these did not contribute to NTP production at reperfusion.

CONCLUSIONS

Livers from fed mice show an improved recovery after cold ischemia. Glycogen levels are low in these organs, and NTP synthesis must be from substrates other than fatty acids.

Kinetics of creatine uptake in the perfused mouse liver: a 31P-n.m.r. study of transgenic mice expressing creatine kinase (CKBB) in the liver

Transport of creatine in the mouse liver has been investigated in vivo and in the perfused organ. Experiments were carried out with transgenic mice expressing creatine kinase in the liver (brain isoenzyme CKBB; EC 7.2.3.2.) [Koretsky, Brosnan, Chen, Chen and Van Dyke (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 3112-3116] and in the corresponding control mice. The animals were fed a regular chow with or without the addition of 10% creatine (w/w) for 5 days. The kinetics of creatine uptake was measured in the perfused liver by 31P-n.m.r. spectroscopy and biochemical analysis following infusion of creatine at concentrations ranging over 0-15 mM and at an extracellular pH of either 7.40 or 6.40. The results suggest that creatine is actively transported by a pH-dependent mechanism obeying a saturable Michaelis-Menten type of kinetics (Km = 0.80 +/- 0.18 and 5.12 +/- 2.40 mM; Vmax. = 0.57 +/- 0.04 and 1.72 +/- 0.32 mumol.g of liver-1.min-1 at pH 7.40 and 6.40 respectively). Creatine export was evaluated in the perfused liver preloaded with creatine and the results show that less than 2.5 and 5% of the total creatine pool is exported to the perfusate during 80 min of perfusion at pH 7.40 and 6.40 respectively. Taken together, these results seem to explain the observation that creatine accumulates in the mouse liver only when blood creatine is raised by creatine feeding.

1H-2H exchange in the perfused rat liver metabolizing [3-13C]alanine and 2H2O as detected by multinuclear NMR spectroscopy

The exchange of individual protons of hepatic metabolites against the solvent deuterons has been investigated in perfused rat liver. Livers from starved rats were perfused for 20 min with a 10 mM solution of unlabeled or 3-13C-labeled L-alanine in Krebs Ringer bicarbonate buffer, with or without 50% deuterium oxide (2H2O). High resolution 13C NMR analysis of deuterium-induced isotopic shifts and of 2H-13C couplings revealed a differential 1H-2H exchange depending on the chemical nature of the metabolite and on the site of 13C labeling. [3-13C]Aspartate isotopomers showed similar 2H/1H ratios in the C3 and in the C2 carbons while [2-13C]aspartate isotopomers had much smaller 2H/1H ratios in the C2 than in the C3 carbons. Similarly, [2-13C]glutamate isotopomers had 2H/1H ratios significantly smaller in the C2 than in the C3 carbon. These results suggest that the hydration-dehydration reactions of the citric acid cycle, which result in exchange at the C3 carbons of aspartate and glutamate, approach equilibrium with the perfusate faster than the aminotransferases of aspartate and alanine, which induce exchange at the C2 carbons of these amino acids. Taken together, the results obtained are consistent with a heterogeneous solvent exchange environment in the perfused liver.

31P NMR assessment of orthotopic liver rejection in a rat model

31P NMR spectroscopy was used serially to study rejecting (ACl-->LEW) and nonrejecting (ACl-->ACl) orthotopic liver transplants in rats. Recipients were evaluated on post-transplant days 1, 3, 5, 7, 9, and 11. The relative changes in phosphomonoester (PME), inorganic phosphate (Pi), high-energy phosphates and pH were studied. The earliest significant difference between the rejecting and nonrejecting groups was a decrease in the alpha-NTP peak area on Day 5. This was followed by significant decreases in beta-NTP and pH, and increases in PME and Pi on Day 7. High-resolution 31P NMR spectra of perchloric acid extracts demonstrated the PME increase to be due mainly to elevated phosphoethanolamine. Using the parameter (Pi + PME)/(alpha + beta + gamma-NTP), rejecting livers were distinguished from nonrejecting livers at a moderate stage of rejection.

Hepatic metabolism during constant infusion of fructose; comparative studies with 31P-magnetic resonance spectroscopy in man and rats

A protocol of constant infusion of fructose has been carried out both in human volunteers and in the perfused rat liver, aiming at a steady-state blood fructose concentration of 6-8 mM. Localized 31P-NMR spectroscopy and biochemical analyses were used to evaluate the metabolic changes. Comparison of the model experiment and the clinical study allowed an evaluation of this protocol as a clinically relevant assessment of the metabolic function of the liver. The time course of change, as well as the quasi steady-state levels reached during fructose infusion, for phosphomonoesters (PME), ATP and inorganic phosphate (Pi) provided the following results: During fructose infusion, ATP and Pi reached a steady-state level of 74.0 +/- 5.9 and 54.6 +/- 3.3% of control respectively, in the human volunteers. The corresponding data in the rat liver was 71.3 +/- 4.3 and 54.4 +/- 4.3%. Hepatic clearance of fructose was 0.53 and 0.52 ml.g liver-1.min-1 for volunteers and rats, respectively. The time course of intracellular metabolite recovery after fructose could be approximated by a first order kinetic. The rate constants for PME and ATP change were similar during fructose infusion and recovery, while after the discontinuation of fructose infusion, Pi increased with a rate constant significantly greater than during its fructose-induced depletion in human liver (P < 0.005). Thus, this relatively simple clinically applicable protocol seems to be verifiable in the well controlled perfused rat liver model, and it is argued that it may be useful in the clinical evaluation of the metabolic functional capacity of the human liver.

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.

Ischaemic ATP degradation studied by HPLC and 31P-NMR spectroscopy: do the two techniques observe the same ATP pools?

31P-NMR spectroscopy has become the major tool for studying myocardial high energy phosphates. Conflicting results concerning NMR visibility of ATP in ischaemic myocardium were reported. A detailed study was undertaken to resolve this controversy. After cardioplegic arrest, canine hearts were excised and preserved for 24 h at 1 degree C (group 1) or for 6h at 23 degrees C (group 2). ATP breakdown was followed by 31P-NMR spectroscopy in a transmural piece of the anterior wall introduced in the NMR magnet, and by HPLC analysis using serial transmural biopsies from the rest of the anterior wall. At both temperatures, identical relative ATP decay curves were obtained, whether measured by NMR or by HPLC. Absolute quantification of ATP was carried out after varying periods of ischaemia at 1 degree C. The NMR-measured ATP concentration was 106 +/- 8% of the ATP concentration determined by HPLC. From our experiments, we conclude that ATP visibility for 31P-NMR spectroscopy is complete and constant during prolonged periods of hypothermic ischaemia in canine hearts.

Altered phosphorylation status, phospholipid metabolism and gluconeogenesis in the host liver of rats with prostate cancer: a 31P magnetic resonance spectroscopy study

31P magnetic resonance spectroscopy (MRS) in vivo and in vitro was used to study modulation of host liver (HL) metabolism in rats bearing the MAT-LyLu variant of the Dunning prostate tumour. Animals were inoculated either with 10(6) or 10(7) MAT-LyLu cells, or with saline to serve as controls. Carcass weight in tumour-bearing (TB) animals decreased despite similar food and water intake in both groups. Absence of metastatic tumour cells from HL of all TB animals was confirmed by histological examination. Twenty-one days after inoculation, 31P MRS showed a 2.5-fold increase in [Pi]/[ATP] ratios in HL in vivo (P < 0.001) which was confirmed by 31P MRS of liver extracts in vitro (P < 0.005). Phosphodiester to ATP ratios were significantly increased (P < 0.05) in HL in vivo, but absolute PDE levels were similar in both groups. Phosphomonoester to ATP ratios did not change, although absolute phosphomonoester levels in HL were reduced by -41% (not significant). In HL extracts in vitro, sharp reductions in the levels of glucose-6-phosphate (P < 0.05), fructose-6-phosphate (P = 0.05), phosphocholine (P < 0.001), glycerophosphocholine (P < 0.001), and glycerophosphoethanolamine (P < 0.001) were observed. Electron microscopy revealed increased amounts and altered distribution of rough endoplasmic reticulum in HL. These findings show that experimental prostate cancer significantly affects hepatic phosphorylation status, phospholipid metabolism, and gluconeogenesis in the host animal, and demonstrate the value of combined MRS in vivo and in vitro in monitoring HL metabolism in cancer.

Response of normal and reperfused livers to glucagon stimulation: NMR detection of blood flow and high-energy phosphates

The effects of glucagon on blood flow and high-energy phosphates in control and in rat livers damaged by ischemia were studied using in vivo nuclear magnetic resonance (NMR) spectroscopy. Normal livers and livers which had been made ischemic for 20, 40, and 60 min followed by 60 min of reperfusion were studied. Ischemia led to a loss in adenosine triphosphate (ATP) within 30 min. Reperfusion after 20 min of ischemia led to complete recovery of ATP. 60 min of reperfusion after 40 or 60 min of ischemia led to only a 76% and 48% recovery of ATP, respectively. Glucagon, at doses up to 2.5 mg/kg body weight, caused no changes in the inorganic phosphate (P(i)) to ATP ratio in normal livers as measured by 31P-NMR spectroscopy. In livers which had been made ischemic for 20, 40, or 60 min, glucagon caused an increase in the P(i)/ATP ratio of 18%, 40%, and 40%, respectively. 19F-NMR detection of the washout of trifluoromethane from liver was used to measure blood flow. Glucagon-stimulated flow in the normal liver in a dose-dependent manner, with 2.5 mg glucagon/kg body weight leading to a 95% increase in flow. Ischemia for 20, 40, and 60 min followed by 60 min of reperfusion led to hepatic blood flows which were 63%, 68%, and 58% lower than control liver. In reperfused livers, blood flow after glucagon-stimulation was reduced to 56%, 43%, and 48% of control glucagon-stimulated flow after 20, 40, and 60 min of ischemia. These results indicate that ischemia followed by reperfusion leads to decreases in hepatic blood flow prior to alterations in ATP and the response of the liver to glucagon is altered in the reperfused liver.

Determination of absolute phosphate metabolite concentrations in RIF-1 tumors in vivo by 31P-1H-2H NMR spectroscopy using water as an internal intensity reference

The absolute metabolite quantification method of Thulborn and Ackerman [J. Magn. Reson. 55, 357 (1983)] in which the tissue water proton signal is used as an internal intensity standard and its more recent variation in which NMR peak intensities are referenced to that of the natural abundance deuterium signal of water [Li et al., SMRM Abstr. 2, 825 (1988); Song et al., Magn. Reson. Med. 25, 45 (1992) have been implemented to obtain absolute phosphate metabolite concentrations in subcutaneous RIF-1 tumors during untreated growth and following treatment with 5-fluorouracil. The equivalence of these two hydrogen isotopes as intensity standards and the validity of their use in the determination of absolute metabolite concentrations in vivo by NMR has been demonstrated. On matched in vivo and extract tumor samples (n = 5), excellent agreement has been obtained between nucleoside triphosphate concentrations determined by NMR and those derived by HPLC analysis for the control tumors. Following 3 days of untreated growth, absolute concentrations of phosphate metabolites in RIF-1 tumors (n = 10) decreased significantly, except for the Pi concentration which did not vary. For the treated tumors (n = 10) there were no changes in metabolite concentrations except for a decrease in the PCr and, possibly, Pi concentrations. The PCr/Pi ratio in the latter tumors did not change. These observations suggest that changes in absolute metabolite concentrations may be more sensitive indices of response to therapy than changes in metabolite peak amplitude ratios, a parameter commonly used to express in vivo NMR data.

31P magnetic resonance spectroscopy detects a functional abnormality in liver metabolism after acetaminophen poisoning

Eighteen patients with acetaminophen poisoning were studied with 31P magnetic resonance spectroscopy to measure phosphorus-containing metabolites in their livers. The concentrations of all magnetic resonance-detectable metabolites fell in parallel with a decrease in the synthetic ability of the liver, indicated by the prothrombin time ratio (international normalized ratio). In particular, ATP fell to about 20% of its normal concentration in severely affected patients, as did the phosphodiester signal, which is thought to arise mainly from the endoplasmic reticulum in the liver. The correlation between ATP levels and international normalized ratio suggests that the international normalized ratio is a more accurate measure of the number of viable hepatocytes than are other biochemical tests.

The 31P NMR visibility of ATP in perfused rat liver remains about 90%, unaffected by changes of metabolic state

The 31P NMR visibility of ATP of the perfused rat liver was tested over a wide range of metabolic conditions, including normoxic and hypoxic perfusions, fructose loads, and various intervals of normothermic ischemia, for both ad libitum fed and 24-h fasted rats. The 31P NMR signal of ATP was compared to the concentration of ATP determined by enzymatic assays on liver biopsies performed at the end of NMR acquisition. In a first series of experiments, the NMR resonance of intracellular ATP was quantitated in absolute terms by applying the 1H NMR water signal as internal reference: during normoxic and hypoxic perfusions, a constant amount of ATP (0.43 +/- 0.19 mM, mean +/- SD), approximately 12% of the cellular ATP, is not detected by NMR. Nevertheless, there is a high correlation (slope = 0.96 +/- 0.09; r2 = 0.93) between the measurements of ATP by 31P NMR spectroscopy and by biochemical analysis. In a second series of experiments, there was a highly significant correlation between the NMR and analytical biochemical measurements of ATP for whole range of metabolic states, i.e., fructose loads (1.0-10 mM) and various intervals of normothermic ischemia (ranging from 2 to 12 min), indicating unchanged ATP visibility. Thus, as opposed to the studies of Murphy et al. [Murphy, E., et al. (1988) Biochemistry 27, 526-528], it is concluded that ATP at 37 degrees C remains almost entirely visible in the perfused rat liver, also during ischemia.

NMR-invisible ATP in heart: fact or fiction?

31P-nuclear magnetic resonance (31P-NMR) spectroscopy is widely used to monitor sequential changes in the nucleoside triphosphate (NTP) pool in intact tissues. Recently, the validity of this technique to quantitate incremental changes in ATP in heart has been challenged. Accordingly, we compared NTP measured by 31P-NMR and by chemical techniques in isolated isovolumic rat hearts at 16 and 56 min of oxygenated perfusion and in hearts subjected to 28 min of hypoxia, with or without 28 min of reoxygenation, and 12 or 28 min of ischemia, with or without 28 min of reperfusion. NTP content was calculated from 31P-NMR spectra using an external standard. At the end of each protocol the heart was freeze-clamped, and NTP and ATP contents were determined by chemical assay. After 16 min of normoxic perfusion the values for NTP and ATP contents measured by both methods in the same hearts were indistinguishable. Results from all seven experimental conditions show no significant difference between methods (P = 0.262). Thus both methods detect the same incremental change in NTP and ATP.

Phosphorus-31 nuclear magnetic resonance of isolated rat liver during hypothermic ischemia and subsequent normothermic perfusion

The effects of prolonged hypothermic ischemia and subsequent normothermic perfusion on the energetic metabolism and intracellular pH (pHin) of isolated rat livers were studied by phosphorus-31 nuclear magnetic resonance spectroscopy. Nucleoside triphosphate (NTP) depletion and intracellular pH were studied within an 18-h-storage phase, by using the following preservation media: Eurocollins (EC), UW Lactobionate (UW) and Bretschneider's solution (HTK). Values obtained after 8-h ischemia were chosen to estimate the performance of the various media: NTP levels were 37 +/- 7%, 10 +/- 5% and 0% of control levels, respectively, in livers stored in UW, HTK and EC solutions. pHin reached values of 7.15 +/- 0.10 in UW and HTK, and 6.96 +/- 0.10 in EC-stored livers. Ischemic damage was assessed by reperfusing the stored organ with Krebs medium: NTP recovery was around 70 +/- 20% for the three solutions used. Recovery of pHin was near the control value (7.23 +/- 0.08), except for EC solution (7.05 +/- 0.20). The main results are that (i) the rates of NTP and pHin decrease are strongly dependent on the nature of the preservation solution, whereas (ii) NTP recovery is not significantly different during post-ischemic reperfusion. With regard to animal survival, UW solution is at present considered largely superior to EC medium for liver preservation. Thus, our data suggest that the rates of NTP depletion and pHin fall during cold preservation could be both considered as better indicators assessing liver injury than the post-ischemic NTP recovery.

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.

The effects of ethanol concentration on glycero-3-phosphate accumulation in the perfused rat liver. A reassessment of ethanol-induced inhibition of glycolysis using 31P-NMR spectroscopy and HPLC

The dose-dependent effect of ethanol on the hepatic metabolism of the perfused rat liver has been investigated by (a) 31P-NMR spectroscopy for the follow-up of intracellular phosphorylated metabolites and (b) HPLC for compounds released in the effluents. Perfusion of livers from fed rats with ethanol induced an increase in the level of sn-glycerol 3-phosphate and net accumulations of 3.30 +/- 0.33 and 0.69 +/- 0.15 mumol x g-1 wet liver were reached after 20 min, for 70 mM and 0.5 mM ethanol, respectively. sn-Glycerol-3-phosphate accumulation was fully detected by 31P NMR as indicated by comparing quantitations based on NMR and biochemical assays. Ethanol administration up to a concentration of 10 mM induced a dose-dependent decrease in the release of lactate + pyruvate by the liver. Lactate release decreased from 1129 +/- 39 to 674 +/- 84 nmol x min-1 x g-1, while pyruvate decreased from 230 +/- 9 to 6.2 +/- 0.4 nmol x min-1 x g-1, after 20 min of perfusion with 10 mM ethanol. Nevertheless, the flux through 6-phosphofructo-1-kinase, as measured by both the accumulation of sn-glycerol 3-phosphate and release of lactate + pyruvate, was not affected in the early phase of ethanol oxidation. Finally, data obtained from oxygen consumption, the release of acetate and the accumulation of sn-glycerol 3-phosphate do not support the involvement of the microsomal ethanol-oxidizing system in the catalysis of ethanol oxidation, even at high doses of alcohol.

A 31P NMR study of mitochondrial inorganic phosphate visibility: effects of Ca2+, Mn2+, and the pH gradient

The effects of external pH, temperature, and Ca2+ and Mn2+ concentrations on the compartmentation and NMR visibility of inorganic phosphate (Pi) were studied in isolated rat liver mitochondria respiring on succinate and glutamate. Mitochondrial matrix Pi is totally visible by NMR at 8 degrees C and at low external concentrations of Pi. However, when the external Pi concentration is increased above 7 mM, the pH gradient decreases, the amount of matrix Pi increases, and the fraction not observed by NMR increases. Raising the temperature to 25 degrees C also decreases the pH gradient and the Pi fraction observed by NMR. At physiologically relevant concentrations, Ca2+ and Mn2+ do not seem to play a major role in matrix Pi NMR invisibility. For Ca2+ concentrations above 30 nmol/mg of protein, formation of insoluble complexes will cause loss of Pi signal intensity. For Mn2+ concentrations above 2 nmol/mg of protein, the Pi peak can be broadened sufficiently to preclude detection of a high-resolution signal. The results indicate that mitochondrial matrix Pi should be mostly observable up to 25 degrees C by high-resolution NMR. While the exact nature of the NMR-invisible phosphate in perfused or in vivo liver is yet to be determined, better success at detecting and resolving both Pi pools by NMR is indicated at high field, low temperature, and optimized pulsing conditions.

Proton magnetic resonance imaging and phosphorus-31 magnetic resonance spectroscopy studies of bromobenzene-induced liver damage in the rat

Respiratory-gated proton magnetic resonance imaging was used to study the response of the rat liver in situ to bromobenzene, a classic hepatotoxicant. A localized region of high proton signal intensity in the perihilar region of the liver was seen 24-48 hr after an intraperitoneal injection of bromobenzene. Localized proton magnetic resonance spectra from within this region indicated that the increased proton signal intensity was not due to accumulation of fat in the liver, but primarily due to a longer T2 for the proton resonance of water. This is consistent with acute edema in this localized region. In vivo 31P magnetic resonance spectroscopy studies of the same rat livers in situ were performed. Spectroscopic conditions were determined whereby localized, quantitative 31P spectra could be obtained. Using these methods, 10 mmol/kg bromobenzene was found after 24 hr to cause a number of statistically significant (p less than 0.05) effects: a decrease in adenosine 5'-triphosphate levels from 4.1 +/- 0.5 to 3.0 +/- 0.5 mM, a decrease in phosphodiester levels from 11.3 +/- 0.9 to 9.3 +/- 0.7 mM and an increase in the phosphomonoesters from 3.0 +/- 0.4 to 5.5 +/- 1.2 mM (mean +/- standard deviation). High resolution in vitro 31P spectra of perchloric acid extracts of these rat livers showed that the increased phosphomonoester resonance was due to a selective 4.3-fold increase in phosphocholine. Thus, our in vivo and in vitro 31P magnetic resonance spectra are consistent with the hypothesis that a phosphatidylcholine-specific phospholipase C (generating phosphocholine and diacylglycerol) is activated during tissue damage. Both the imaging and spectroscopy results obtained with bromobenzene closely resemble CCl4-induced liver changes previously reported, and may reflect a generalized response of the liver to any acutely acting toxic chemical.

Effectors of the regulatory protein acting on liver glucokinase: a kinetic investigation

In the absence of fructose 6-phosphate, the regulatory protein of rat liver glucokinase (hexokinase IV or D) inhibited this enzyme, though with a much (15-fold) lower potency than in the presence of a saturating concentration of fructose 6-phosphate. Evidence is provided that this inhibition is not due to contaminating fructose 6-phosphate. In the presence of regulatory protein, sorbitol 6-phosphate, a potent analog of fructose 6-phosphate, exerted a hyperbolic, partial inhibition on glucokinase, the degree of which increased with the concentration of regulatory protein. Plots of the reciprocal of the difference between the rates in the absence and in the presence of sorbitol 6-phosphate versus 1/[sorbitol 6-phosphate] at various concentrations of regulatory protein were linear, and demonstrated that the apparent affinity for sorbitol 6-phosphate increased with the concentration of regulatory protein. Plots of the reciprocal of the difference between 1/v in the presence and in the absence of sorbitol 6-phosphate versus 1/[sorbitol 6-phosphate] were also linear and crossed the axis at a value independent of the concentration of regulatory protein. Fructose 1-phosphate released the inhibition exerted by the regulatory protein in a hyperbolic fashion. The concentration of this effector required for a half-maximal effect increased linearly with the concentrations of sorbitol 6-phosphate and of regulatory protein. These results are consistent with a model in which the regulatory protein exists under two conformations, one form which binds inhibitors and glucokinase, and the other which binds activators, although not glucokinase. Sorbitol 6-phosphate, 2-deoxysorbitol 6-phosphate and mannitol 1-phosphate, all analogs of the open-chain configuration of fructose 6-phosphate, inhibited glucokinase in the presence of regulatory protein at lower concentrations than fructose 6-phosphate, whereas fixed analogs of the furanose form of fructose 6-phosphate were inactive or behaved as activators. This indicated that fructose 6-phosphate in its open-chain configuration is recognized by the regulatory protein. A series of compounds exerted an activating effect. These included, in order of decreasing potency: fructose 1-phosphate, psicose 1-phosphate, ribitol 5-phosphate, analogs of fructose 1-phosphate and of ribitol 5-phosphate and, at much higher concentrations, inorganic phosphate.

Visibility of ATP and ADP in freeze-trapped tissue from perfused rat liver during normoxia and ischemia using 31P-cryo-NMR

The visibility of ATP and ADP to NMR was studied by comparing simultaneous measurements of freeze-trapped tissue sections from perfused rat liver under normoxia and ischemia using a modified 31P-cryo-NMR method and biochemical assay. The 31P-cryo-NMR method provides good time resolution and allows the quantitation of absolute metabolite concentrations. Prior to 31P-cryo-NMR measurements, freeze-trapped tissues were thawed in the presence of cryoprotectant and EDTA. With this sample preparation procedure, the integrity of the plasma and mitochondrial membranes was not maintained, inducing homogeneous microviscosity and chelation of intracellular divalent cations, thereby increasing the visibility of metabolites compared to the in vivo NMR measurement. With ischemic stress, total cellular ATP concentration decreased significantly (P less than 0.001). While ADP concentrations measured by cryo-NMR and biochemical analysis were consistent during normoxia and ischemia, ATP concentrations measured by cryo-NMR were significantly lower (P less than 0.05) than those obtained by biochemical analysis. The amount of invisible ATP (0.42 +/- 0.10 mumol/g wet weight: mean +/- S.E.) did not change after the induction of ischemia. The results of this study suggest that ATP invisibility to cryo-NMR is not due to compartmentation into regions of high paramagnetic ion concentrations or high microviscosity, but is influenced by other factors.

Simple cation-exchange high-performance liquid chromatography optimized to the measurement of metabolites in the effluents from perfused rat livers using refractive index and ultraviolet detectors

A high-performance liquid chromatographic method designed to analyse effluents from perfused organs is described. In the case of rat liver, compounds released by the liver are readily separated and quantitated, using a strong cation exchanger (Aminex HPX 87H), two detectors connected in series (ultraviolet detector at 210 nm and refractive index detector), and by optimizing the concentration of sulphuric acid in the mobile phase. Chromatographic conditions described in the present work enable the quantitation, in a single run, of metabolites derived from the tricarboxylic acid cycle, glycolysis, ketogenesis, adenine nucleotides catabolism and ethanol oxidation. The advantage of this method stems from its ease of implementation, sensitivity and flexibility.

Phosphorus-31 nuclear magnetic resonance of C6 glioma cells and rat astrocytes. Evidence for a modification of the longitudinal relaxation time of ATP and Pi during glucose starvation

31P-NMR spectroscopy has been used to study the energy metabolism and the NMR visibility of ATP and intracellular Pi of the C6 glioma cell line and rat astrocyte grown on microcarrier beads with the following results. 1. In vivo NMR spectra of C6 glioma cells and rat astrocytes indicate that these cells were able to maintain their level of ATP resonances during a long anoxic period (more than an hour). Both cell types were sensitive to ischemia which induced a loss of ATP resonances within 40 min. Glucose starvation induced by 40% decrease in ATP resonances correlated to a 50% increase in the intensity of the Pi signal. These changes corresponded to a new steady state which could be reversed by reperfusing the cells with a glucose-containing medium. 2. In contrast to in vivo data, 31P-NMR analyses of perchloric acid extracts of cells incubated in a glucose-free medium showed that their ATP and Pi contents were unchanged during starvation. The changes of NMR visibility of the metabolites in living C6 cells were correlated to modifications of their macroscopic longitudinal relaxation times, evolving from 0.30 +/- 0.08 s and 6.6 +/- 1.5 s in the presence of glucose to 0.68 +/- 0.26 s and 3.2 +/- 0.9 s in the absence of glucose for ATP and Pi, respectively. The changes of the NMR detectability of ATP and Pi indicate that changes in their microenvironment occur during glucose starvation, suggesting the existence of different pools of these metabolites within the cells. 3. Under various experimental conditions, i.e. anoxia, ischemia and glucose starvation, rat astrocytes in primary culture showed a very similar behavior to that of C6 cells, suggesting a similar adaptability to the nature of the energy supply for both the normal and the malignant cell.