Intracellular pH determination by a 31P-NMR technique. The second dissociation constant of phosphoric acid in a biological system

The combination of inorganic phosphate and pyrophosphate 31 P-NMR for the electrodeless pH determination in the 5-12 range

Potentiometry is the primary pH measurement method, but alternatives are sought beyond glass electrodes operative limitations. In nuclear magnetic resonance (NMR) experiments, electrodeless pH sensing is important to track changes along titrations, during chemical reactions or inside compartmentalized environments inaccessible to electrodes, for instance. Although several interesting NMR pH indicators have been already presented, the potential of inorganic phosphate is overlooked, despite its common presence in NMR samples as the buffer main component. Its use for electrodeless pH determination can be expanded by exploiting all its three proton dissociations. This study was aimed at verifying the use of inorganic phosphate 31 P chemical shift to sense pH variations, and at exploring the complementary use of pyrophosphate ions to cover a wide pH range. A simple set of equations is presented to utilize both phosphate and pyrophosphate 31 P chemical shift in combination for accurate pH determination without a glass electrode over the 5-12 pH range, and without affecting the spectrum of other nuclei. The present study demonstrated an average deviation of 0.09 (maximum <0.2) pH unit from glass electrode measurements. The trimethylphosphate can be used as a suitable chemical shift reference for both 31 P and 1 H (also 13 C), with its hydrolysis being significant only at pH > 12. The method was also demonstrated by determining the pKa of three distinct molecules in a mixture and by comparing the results to those obtained when the glass electrode was used to measure the pH. The approach shown here can be easily tuned to different experimental conditions.

Magnetic Resonance pH Imaging in Stroke - Combining the Old With the New

The study of stroke has historically made use of traditional spectroscopy techniques to provide the ground truth for parameters like pH. However, techniques like 31P spectroscopy have limitations, in particular poor temporal and spatial resolution, coupled with a need for a high field strength and specialized coils. More modern magnetic resonance spectroscopy (MRS)-based imaging techniques like chemical exchange saturation transfer (CEST) have been developed to counter some of these limitations but lack the definitive gold standard for pH that 31P spectroscopy provides. In this perspective, both the traditional (31P spectroscopy) and emerging (CEST) techniques in the measurement of pH for ischemic imaging will be discussed. Although each has its own advantages and limitations, it is likely that CEST may be preferable simply due to the hardware, acquisition time and image resolution advantages. However, more experiments on CEST are needed to determine the specificity of endogenous CEST to absolute pH, and 31P MRS can be used to calibrate CEST for pH measurement in the preclinical model to enhance our understanding of the relationship between CEST and pH. Combining the two imaging techniques, one old and one new, we may be able to obtain new insights into stroke physiology that would not be possible otherwise with either alone.

pH detection in biological samples by 1D and 2D 1H-31P NMR

The chemical shifts of several important endogenous phosphorus compounds under different pH conditions were explored, including adenosine-5'-triphosphate, adenosine-5'-diphosphate, adenosine-5'-monophosphate, phosphorylcholine and phosphorylethanolamine. Their ³¹P NMR and ¹H NMR chemical shifts were all pH-sensitive in the similar pH range. Two dimensional (2D) ¹H-³¹P NMR spectra were found helpful to identify these endogenous phosphorus markers in biological samples from rather complicated NMR spectra. Herein, for the first time, a pH sensor based on 2D ¹H-³¹P NMR was established and applied to biological samples analysis with pH values determined in good agreement with those by potentiometric method. Apart from being simple, green, rapid and less sample-consuming, information concerning both the endogenous phosphorus markers and pH status could be attained in a single NMR run, which demonstrated the great potential of this method in rare sample analysis and even disease diagnosis.

Spectroscopic approaches to resolving ambiguities of hyper-polarized NMR signals from different reaction cascades

Ambiguities in identifying transient intracellular reaction intermediates are resolved by site-specific isotope labelling, optimised referencing and response to external perturbations.

Bioactivity of toothpaste containing bioactive glass in remineralizing media: effect of fluoride release from the enzymatic cleavage of monofluorophosphate

Objectives. The aim was to introduce a new methodology to characterize toothpaste containing bioactive glass and to evaluate the effect of release of fluoride ions, by cleaving monofluorophosphate (MFP), on the mineral forming ability of Sensodyne Repair & Protect (SRP). which contains NovaMinTM (bioactive glass, 45S5 composition).

Methods. SRP, NovaMin particles, and placebo paste (PLA) which did not contain NovaMin, were immersed into a remineralization media (RS), which mimics the ionic strength of human saliva, for 3 days with different concentrations of alkaline phosphatase (ALP): 0, 25 and 75 U.L

Results. Hydroxyapatite (HA) formed on the surface of BG alone (after 1 h) and in toothpaste (after 2 h), whereas PLA did not induce any precipitation. ALP cleaved MFP at different rates depending on the enzyme concentration. Increasing the concentration of ALP from 0 and 75 U.L

Effects of adrenaline on glycogenolysis in resting anaerobic frog muscles studied by 31P-NMR

The effects of adrenaline (also called epinephrine) on glycogenolysis in living anaerobic muscles were examined based on time-dependent changes of (31)P-NMR spectra of resting frog skeletal muscles with and without iodoacetate treatments. The phosphate-metabolite concentration and the intracellular pH determined from the NMR spectra changed with time, reflecting the advancement of various phosphate metabolic reactions coupled with residual ATPase reactions to keep the ATP concentration constant. The results could be explained semi-qualitatively as the ATP regenerative reactions, creatine kinase reaction and glycogenolysis, advanced with time showing the characteristic two phases. Thus, it was clarified for living muscles that adrenaline activates the phosphorylase step of glycogenolysis, and the adrenaline-activated glycogenolysis is further regulated at the phosphofructokinase step by PCr and also possibly by AMP. Associated with the adrenaline-activated glycogenolysis in the examined muscles, the P(i) concentration and the intracellular pH, factors affecting the muscle force, changed significantly, suggesting complicated effects of adrenaline on the muscle contractility.

Metabolic and ionic coupling factors in amino acid-stimulated insulin release in pancreatic beta-HC9 cells

Fuel stimulation of insulin secretion from pancreatic beta-cells is thought to be mediated by metabolic coupling factors that are generated by energized mitochondria, including protons, adenine nucleotides, and perhaps certain amino acids (AA), as for instance aspartate, glutamate, or glutamine (Q). The goal of the present study was to evaluate the role of such factors when insulin release (IR) is stimulated by glucose or AA, alone or combined, using (31)P, (23)Na and (1)H NMR technology, respirometry, and biochemical analysis to study the metabolic events that occur in continuously superfused mouse beta-HC9 cells contained in agarose beads and enhanced by the phosphodiesterase inhibitor IBMX. Exposing beta-HC9 cells to high glucose or 3.5 mM of a physiological mixture of 18 AA (AAM) plus 2 mM glutamine caused a marked stimulation of insulin secretion associated with increased oxygen consumption, cAMP release, and phosphorylation potential as evidenced by higher phosphocreatine and lower P(i) peak areas of (31)P NMR spectra. Diazoxide blocked stimulation of IR completely, suggesting involvement of ATP-dependent potassium (K(ATP)) channels in this process. However, levels of MgATP and MgADP concentrations, which regulate channel activity, changed only slowly and little, whereas the rate of insulin release increased fast and very markedly. The involvement of other candidate coupling factors was therefore considered. High glucose or AAM + Q increased pH(i). The availability of temporal pH profiles allowed the precise computation of the phosphate potential (ATP/P(i) x ADP) in fuel-stimulated IR. Intracellular Na+ levels were greatly elevated by AAM + Q. However, glutamine alone or together with 2-amino-2-norbornanecarboxylic acid (which activates glutamate dehydrogenase) decreased beta-cell Na levels. Stimulation of beta-cells by glucose in the presence of AAM + Q (0.5 mM) was associated with rising cellular concentrations of glutamate and glutamine and strikingly lower aspartate levels. Methionine sulfoximine, an inhibitor of glutamine synthetase, blocked the glucose enhancement of AMM + Q-induced IR and associated changes in glutamine and aspartate but did not prevent the accumulation of glutamate. The results of this study demonstrate again that an increased phosphate potential and a functional K(ATP) channel are essential for metabolic coupling during fuel-stimulated insulin release but illustrate that determining the identity and relative importance of all participating coupling factors and second messengers remains a challenge largely unmet.

Effects of various catecholamines on high-energy phosphates of rat liver and brain during hemorrhagic shock measured by 31P-NMR spectroscopy

The effects of dopamine, epinephrine and norepinephrine on energy metabolism as well as intracellular pH in rat liver and brain during hemorrhagic shock were examined by in vivo 31P-NMR spectroscopy. The hemorrhagic shock was induced by arterial bleeding to a mean arterial pressure (MAP) of 30-40 mmHg. Upon the induction of hemorrhagic shock, there was a dramatic fall in adenosine triphosphate (ATP) and a rise in inorganic phosphate (Pi) in the liver. The intracellular pH indicated severe acidosis. However, no change in these parameters was observed in the brain during hemorrhagic shock. After infusion of the above catechollamines following 10 min of hemorrhagic shock, MAP increased to 90-100% of its control value. Only dopamine improved hepatic energy metabolism, whereas brain energy metabolism was not affected by any of them. This suggests that dopamine protects liver function during hemorrhagic shock without affecting brain energy metabolism.

Noninvasive measurement of temperature and fractional dissociation of imidazole in human lower leg muscles using 1H-nuclear magnetic resonance spectroscopy

The temperature change of the fractional dissociation of imidazole (α-imidazole) in resting human lower leg muscles was measured noninvasively using 1H-nuclear magnetic resonance spectroscopy at 3.0 and 1.5 T on five normal male volunteers aged 30.6 ± 10.4 yr (mean ± SD). Using 1H-nuclear magnetic resonance spectroscopy, water, carnosine, and creatine in the muscles could be simultaneously analyzed. Carnosine contains imidazole protons. The chemical shifts of water and carnosine imidazole protons relative to creatine could be used for estimating temperatures and α-imidazole, respectively. Using the chemical shift, the values of temperature in gastrocnemius (Gast) and soleus muscles at ambient temperature (21–25°C) were estimated to be 35.5 ± 0.5 and 37.4 ± 0.6°C (means ± SE), respectively (significantly different; P < 0.01). The estimated values of α-imidazole in these muscles were 0.620 ± 0.007 and 0.630 ± 0.013 (means ± SE), respectively (not significant). Alternation of the surface temperature of the lower leg from 40 to 10°C significantly changed the temperature in Gast ( P < 0.0001) from 38.1 ± 0.5 to 28.0 ± 1.2°C, and the α-imidazole in Gast decreased from 0.631 ± 0.003 to 0.580 ± 0.011 ( P < 0.05). However, the values of α-imidazole and the temperature in soleus muscles were not significantly affected by this maneuver. These results indicate that the α-imidazole in Gast changed significantly with alternation in muscle temperature ( r = 0.877, P < 0.00001), and its change was estimated to be 0.0058/°C.

In vivo monitoring of hepatic glutathione in anesthetized rats by 13C NMR

A method for in vivo (13)C NMR monitoring of hepatic glutathione (GSH) in intact, anesthetized rats has been developed. Studies were conducted using a triple-tuned, surgically implanted surface coil designed for this animal model. The coil permitted complete decoupling and sufficient resolution in the (13)C NMR spectrum to monitor the time course of hepatic (13)C-metabolites of intravenously administered 2-(13)C-glycine, particularly GSH at 44.2 ppm and serine signals at 61.1 and 57.2 ppm, respectively. It further allowed concomitant monitoring of high-energy phosphagens and intracellular pH by (31)P NMR. To confirm in vivo NMR peak assignments, we compared high-resolution 2D (1)H[(13)C] heteronuclear multiple quantum coherence and 1D (13)C spectra of hepatic perchloric acid extracts to those of authentic standards. The fractional isotopic enrichment of hepatic (13)C-glycine increased exponentially at a rate of 1.68 h(-1) and reached its plateau level of 81% in 2 h. The (13)C fractional isotopic enrichment of GSH increased exponentially at a rate of 0.316 h(-1) and reached 55% after 4 h of 2-(13)C-glycine infusion, but without achieving a plateau. To confirm that the resonance at 44.2 ppm resulted from GSH, a rat was given an intravenous dose of 2-oxothiazolidine-4-carboxylic acid (OTC), a cysteine precursor that increases intracellular GSH. As expected, with OTC administration the hepatic (13)C GSH-to-glycine peak area increased more than sevenfold.

Synthesis and 31P NMR characterization of new low toxic highly sensitive pH probes designed for in vivo acidic pH studies

With the aim to provide sensitive 31P NMR probes of intra- and extracellular pH gradients that may reach cellular acidic compartments in biological systems, new alpha-aminophosphonates were designed to meet basic requirements such as a low pK(a)s and a great chemical difference (Deltadelta(ab)) between the limiting 31P NMR chemical shifts in acidic (delta(a)) and basic (delta(b)) media. A series of six phosphorylated pyrrolidines and linear aminophosphonates were synthesized using aminophosphorylation reactions and were screened for cytotoxicity on cultured Müller cells. Among the compounds not being toxic under these conditions, three molecules were selected since they displayed the best in vitro (in several phosphate buffers and in a cytosol-like solution) properties as 31P NMR acidic pH markers, that is 3, 5 and 9, having the pK(a) values of 3.63, 5.89 and 5.66, respectively. The Deltadelta(ab) values of these pH markers were at least 3 times larger than that of standard 31P NMR probes, with a low sensitivity to ionic strength changes. From these data, it was proposed that 3, 5 and 9 could be used as reporting probes of subtle proton movements in acidic compartments, an area that still remains poorly investigated using non invasive 31P NMR methods.

alpha - and beta -phosphorylated amines and pyrrolidines, a new class of low toxic highly sensitive 31P NMR pH indicators. Modeling of pKa and chemical shift values as a function of substituents

Fourteen linear and cyclic alpha- and beta-aminophosphonates in which the P-atom is substituted by alkoxy groups have been synthesized and evaluated as (31)P NMR pH markers in Krebs-Henseleit buffer. pK(a) values varied with substitution in the range 1.3-9.1, giving potentially access to a wide range of pH. Temperature had a weak influence on pH and a dramatic increase in ionic strength slightly modified the pK(a) of the pyrrolidine diethyl(2-methylpyrrolidin-2-yl)phosphonate (DEPMPH). All compounds displayed a 4-fold better NMR sensitivity than inorganic phosphate or other commonly used phosphonates, as assessed by differences delta(b)-delta(a) between the chemical shifts of the protonated and the unprotonated forms. In isolated perfused rat hearts, a non-toxic concentration window of 1.5-15 mm was determined for three representative compounds. Using empirical linear relationships, the experimental values of pK(a), delta(a), and delta(b) have been correlated with the two-dimensional structure, i.e. the chemical nature of substituents bonded to the secondary amine and P-atom. The data suggest that DEPMPH and its cyclic and linear variants are ideal versatile (31)P NMR probes for the study of tenuous pH changes in biological processes.

Prolonged effect of leukocytosis on reperfusion injury of rat intestine: real-time ATP change studied using (31)P MRS

BACKGROUND

The intestine is one of the most sensitive tissues to ischemia and reperfusion (I/R). Polymorphonuclear neutrophils (PMN) may play an important role in ischemic injury. (31)P magnetic resonance spectroscopy (MRS) has been used to continuously monitor the energy metabolism of an animal in situ. We have applied MRS to study the effect of PMN on the I/R injury of rat intestine.

MATERIAL AND METHODS

In a rat model of 30 min of intestinal ischemia and reperfusion, the number of PMNs was manipulated: group A, control; group B, leukopenia induced by cyclophosphamide; group C, leukocytosis induced by granulocyte colony-stimulating factor (G-CSF). MRS was employed to measure the level of real-time intestinal ATP and pH in vivo.

RESULTS

In group A, ATP rapidly recovered on reperfusion to 61.0 +/- 11.0% of the preischemia level and maintained that level during reperfusion. The other two groups showed similar recovery of ATP at the initial phase of the reperfusion (<10 min). ATP in group B continued to recover, reaching 74.0 +/- 10.0% of the preischemia level. After the initial recovery, ATP in group C deteriorated reaching 46.0 +/- 4.4% of the preischemic level at 150 min of reperfusion. In group A and group B tissue pH decreased on ischemia and recovered on reperfusion in a similar manner. In group C, tissue pH was significantly lower than in other groups during I/R.

CONCLUSION

Leukocytosis induced by G-CSF exerts a prolonged effect on ATP during I/R and leukocyte depletion helps protect against the I/R injury.

Extracellular acidification decreases the basal motility of cultured mouse microglia via the rearrangement of the actin cytoskeleton

The present study was undertaken to examine the effect of extracellular pH (pH(0)) on the locomotor function of murine microglial cells in vitro. We have found that basal motility of microglia, as measured by a computer-assisted video assay, decreased in an acidic, but not in an alkaline environment. Extracellular acidification affected the architecture of F-actin cytoskeleton, inducing bundling of actin and the formation of stress fibers. The change in intracellular pH (pH(i)) resulting from the change in pH(0) seems to be a prerequisite for the motility decrease since other means to decrease pH(i), namely Na(+)-free solution (in the absence of HCO(-)(3)) and nigericin-containing solution, mimicked the extracellular acidification. In contrast to its pronounced effect on basal motility of microglial cells, the motility increase, as induced by the chemoattractant complement 5a (C5a), was not affected by the acidic environment. The relationship of pH(0) to the locomotor function was also studied in a long-term microchemotaxis assay where microglia migrated within a pH gradient. Intracellular acidification induced by lowering pH(0) to 6.0 or removal of Na(+) from the assay medium decreased basal microglial cell migration. The C5a-induced chemotactic migration was moderately decreased by the acidic environment. In conclusion, our results suggest that acidification of the microglial extracellular milieu leads to a decrease in pH(i) and thereby reduces the basal microglial motility and C5a-induced chemotaxis via a rearrangement of the cytoskeleton. We would therefore like to speculate that changes in pH(i) constitute an important control mechanism in regulating the locomotor function of microglia in culture and probably also in the intact tissue.

A study of imidazole-based nuclear magnetic resonance probes of cellular pH

A number of imidazole-based compounds were tested for their utility as (1)H NMR molecular probes of intracellular pH. Imidazole, previously found useful as a probe of erythrocyte pH, reported a pH in perfused canine glioma cells that was more than 1 pH unit lower than that reported by inorganic phosphate, consistent with the known lysosomal compartmentation of the molecule. Imidazole acetate, also proposed as an NMR probe of cellular pH, was found not to enter the cells of this study. Histidine was found to be readily taken up by cells and reported a pH consistent with that reported by inorganic phosphate. Using the chemical shift of the histidine H2 proton in cells incubated with 10 mM histidine, cellular pH measurements could be obtained in less than 1 s. This compares quite favorably with the measurement time, typically several minutes, needed to assess in vivo pH by (31)P NMR. The use of histidine as a probe of pH is demonstrated in perfused canine and rat glioma cells subjected to ischemia or to low extracellular pH.

Localized 31P MR spectroscopy of the transplanted human kidney in situ shows altered metabolism in rejection and acute tubular necrosis

The purpose of this study was to investigate the function of transplant kidneys in situ, and to detect pathologic changes, using volume-selective phosphorous NMR spectroscopy (31P MRS). Localized 31P MR spectra were obtained from 37 patients using a whole-body MR scanner with a combination of surface coils, adiabatic excitation pulses, and a modified image-selected in vivo spectroscopy (ISIS) sequence. Seventeen patients with pathologic changes after renal transplant were compared with a control group of 20 patients with no evidence of transplant dysfunction. The transplant kidneys with rejection reaction showed higher ratios of inorganic phosphate (P2i) to adenosine triphosphate-alpha (ATP-alpha) than the normal control group (.4 +/- .16 compared with .22 +/- .11, P = .01) and reduced pH. The spectra of transplant kidneys with tubular necrosis had lower phosphomonoester (PME)/phosphodiester (PDE) ratios than the control group (.65 +/- .35 compared with .96 +/- .5, P = .04). The pathologies of rejection and tubular necrosis could be differentiated from each other by pH (6.93 +/- .1 in rejection versus 7.14 +/- .19 in tubular necrosis, P = .04). Preliminary results indicate that localized image-guided 31P MR spectroscopy of transplant kidneys in situ can detect rejection reactions and acute tubular necrosis noninvasively, providing an incentive for further research.

Effect of brain, body, and magnet bore temperatures on energy metabolism during global cerebral ischemia and reperfusion monitored by magnetic resonance spectroscopy in rats

To record brain temperature for comparison with rectal and temporalis muscle temperatures in preliminary studies before MR spectroscopy experiments, a thermistor was inserted into the basal ganglia in eight anesthetized, ventilated, and physiologically monitored rats. The rats were placed in an MR spectrometer and subjected to 60 min of global cerebral ischemia and 2 h of reperfusion without radiofrequency (RF) pulsing. Body temperature was maintained at 37.5-38.0 degrees C (normothermia) or 36.5-37.0 degrees C (mild hypothermia). Brain temperature during ischemia, which dropped to 31.9 +/- 0.3 (hypothermia) and 33.6 +/- 0.5 degrees C (normothermia), correlated with temporalis muscle temperature (r2 = 0.92) but not with body or magnet bore temperature measurements. Ischemia reduced brain temperature approximately 1.7 degrees C in rats subjected to mild hypothermia (1 degree reduction of body temperature). Parallel MR spectroscopy experiments showed no significant difference in energy metabolites between normothermic and hypothermic rats during ischemia. However, the metabolic recovery was more extensive 20-60 min after the onset of reperfusion in hypothermic rats, although not thereafter (P < 0.05). Mild hypothermia speeds metabolic recovery temporarily during reperfusion but does not retard energy failure during global ischemia in rats.

In vivo (31)P magnetic resonance spectroscopy and morphometric analysis of the perfused vascular architecture of human glioma xenografts in nude mice

The relationship between the bioenergetic status of human glioma xenografts in nude mice and morphometric parameters of the perfused vascular architecture was studied using (31)P magnetic resonance spectroscopy (MRS), fluorescence microscopy and two-dimensional digital image analysis. Two tumour lines with a different vascular architecture were used for this study. Intervascular distances and non-perfused area fractions varied greatly between tumours of the same line and tumours of different lines. The inorganic phosphate-nucleoside triphosphate (P(i)/NTP) ratio increased rapidly as mean intervascular distances increased from 100 microm to 300 microm. Two morphometric parameters - the percentage of intervascular distances larger than 200 microm (ivd200) and the non-perfused area fraction at a distance larger than 100 microm from a nearest perfused vessel (area100), - were deduced from these experiments and related to the P(i)/NTP ratio of the whole tumour. It is assumed that an aerobic to anaerobic transition influences the bioenergetic status, i.e. the P(i)/NTP ratio increased linearly with the percentage of ivd200 and the area100.

The NMR chemical shift pH measurement revisited: analysis of error and modeling of a pH dependent reference

A standard differential calculus-based propagation of error treatment is applied to the traditional chemical-exchange Henderson-Hasselbalch NMR pH model in which the reference shift is pH independent. It is seen naturally from this analysis that (i) the error minimum in derived pH occurs in the region where pH and indicator pKa are equal and that (ii) the dynamic range, or difference between the limiting chemical shifts of acid and base forms of indicator species, determines the insensitivity of the technique to propagation of errors. To extend the useful pH range and utility of NMR pH determination methodology, a more general model is developed in which the internal reference species is also considered as having a pH-dependent chemical shift. Data from standard solution pH titrations are fitted to both models and parameters are estimated for the normally observed family of ionizable phosphorus metabolites (ATP, inorganic phosphate, phosphoethanolamine and phosphocholine) and the xenometabolite 2-deoxyglucose-6-phosphate with either phosphocreatine, the alpha-phosphate of ATP, or H2O taken as the 31P or 1H chemical shift internal reference species as well as with an external reference.

Intracellular pH regulation in cultured microglial cells from mouse brain

Intracellular pH (pHi) and the mechanisms of pHi regulation have been investigated in cultured microglial cells from mouse brain using the pH-sensitive fluorescent dye 2',7'-bis-(2-carboxyethyl)-5-(6)-carboxyfluorescein (BCECF). Cells were acidified by a pulse of NH4+ (4-5 min; 20 mM) and the subsequent pHi recovery from an acidification was studied. In HCO3(-)-free saline, pH regulation was dependent on extracellular [Na+] and sensitive to amiloride, indicating the involvement of the Na+/H+ exchanger. In HCO3(-)-containing solution 2 mM amiloride slowed but did not block pHi recovery; the recovery however was dependent on extracellular [Na+] and sensitive to 0.3 mM DIDS, suggesting the presence of Na+/HCO3 cotransporter and/or Na(+)-dependent Cl-/HCO3-exchanger. The involvement of a Na-dependent Cl-/HCO3-exchanger was inferred from the observation that removal of Cl- or application of 1 mM furosemide decreased but did not block the recovery rate. Increasing [K+]0 resulted in an alkalinization by a process that was neither HCO3- nor Na(+)-dependent, nor DIDS- and amiloride-inhibitable. In conclusion, microglial cells express a distinct set of pH regulatory carriers which control for a defined level of pHi. An increase in [K+]0 can offset this level.

Effect of intracellular pH on force development depends on temperature in intact skeletal muscle from mouse

The cellular mechanism of muscle fatigue is still in debate. Opposite conclusions regarding the role of intracellular pH (pHi) in fatigue have been drawn from skinned fiber vs. isolated perfused muscle studies. Because these experiments are typically performed at different temperatures, we tested the hypothesis that temperature alters the effects of pH on force. Tetanic force of isolated mouse extensor digitorum longus was measured at temperatures between 13 and 25 degrees C in either normocapnia (5% CO2) or hypercapnia (25% CO2). Hypercapnia decreased pHi (monitored by 31P nuclear magnetic resonance spectroscopy) by the same amount at both 15 and 25 degrees C. However, inhibition of force by hypercapnia was greater at the lower temperature. A similar pattern of temperature-dependent inhibition of force by pH was observed in glycerinated fibers from rabbit psoas at maximum Ca2+ activation. We conclude that temperature differences are responsible for disparate conclusions on the role of pHi in muscle fatigue. Based on our results, we suggest that changes in pHi may have little or no role in the loss in force production associated with muscular fatigue at physiological temperatures.

Distinct modulation of myocardial performance, energy metabolism, and [Ca2+]i transients by positive inotropic drugs in normal and severely failing hamster hearts

The present study compared the effects of amrinone, dobutamine, dibutyryl cAMP, digoxin, and isoproterenol on mechanical performance, the high energy phosphate metabolites, and the [Ca2+]i transients in normal and cardiomyopathic hamster hearts with severe heart failure. In normal hearts dobutamine, dibutyryl cAMP, and isoproterenol increased left ventricular developed pressure, while amrinone and digoxin did not. However, the amplitude of [Ca2+]i transients was augmented with all drugs. Diastolic [Ca2+]i level was increased with dobutamine and lowered with dibutyryl cAMP and isoproterenol. In cardiomyopathic hearts with severe heart failure, left ventricular developed pressure, the amplitude of [Ca2+]i transients, the phosphorylation potential, and [cAMP]i were significantly depressed and left ventricular end-diastolic pressure and diastolic [Ca2+]i were significantly elevated when compared with normal hearts. Amrinone, dibutyryl cAMP, and isoproterenol improved mechanical performance while increasing [cAMP]i and the amplitude of [Ca2+]i transients, and decreasing diastolic [Ca2+]i. On the other hand, with dobutamine and digoxin diastolic [Ca2+]i was further increased and mechanical performance deteriorated with digoxin. Thus, distinct differences exist in modulation of mechanical performance, high-energy phosphate metabolism, and [Ca2+]i transients by positive inotropic drugs between normal and cardiomyopathic hearts with severe heart failure.

In vivo 31P NMR studies of orientation effects upon rat brain metabolism during mild hypoxia

Energy metabolites in rat brain under the same level of hypoxia were monitored by 31P NMR in both horizontal and vertical magnets. The changes in PCr, Pi, and pHi in the vertical setting toward the end of hypoxia were significantly larger and the recovery in the horizontally held animals was more complete. The results demonstrated quantitatively that the stress of the alignment is superimposed on the stress of hypoxia in the vertical magnet.

31P-MRS measurements of extracellular pH of tumors using 3-aminopropylphosphonate

The extracellular pH (pHex) of tumors is generally acidic. However, it is only recently that noninvasive magnetic resonance spectroscopic (MRS) measurements have determined that the intracellular pH (pHin) of tumor cells in situ is neutral or slightly alkaline compared with that of normal tissues. Thus cells in tumors maintain larger pH gradients than do cells in nontumor tissues. To date, measurements of pHex in tumors have been made using microelectrodes, which preclude measurement of pHex and pHin within the same preparation. In addition, microelectrodes are invasive and have the potential to alter the measured pH values. The present communication describes simultaneous measurement of pHex and pHin in vitro in bioreactor culture and in vivo using 31P-MRS analyses of 3-aminopropylphosphonate (3-APP) and inorganic phosphate. In vitro results indicate that 3-APP is not toxic and that its resonant frequency is sensitive to pH and not significantly affected by temperature or ionic strength. Bioreactor experiments indicate that this compound is neither internalized nor metabolized by cells. Experiments in vivo indicate that 3-APP can be administered intraperitoneally and that RIF-1 tumors maintain a steady-state pHin of 7.25 and a pHex of 6.66. These data have significance to basic tumor cell physiology and to the design of approaches to cancer chemotherapy and hyperthermic therapy, because both of these modalities exhibit pH sensitivity. It is also likely that these techniques will be applicable to localized MRS of other organ systems in vivo.

Continuous fluorometric measurement of intracellular pH and Ca2+ in perfused salivary gland and pancreas

Intracellular pH (pHi) has been measured in intact, perfused rat mandibular salivary glands loaded with the fluorescent pH indicator BCECF [2',7'-bis(2-carboxyethyl)-5(6)- carboxyfluorescein]. Glands mounted in the cuvette of a conventional bench-top spectro-fluorometer were perfused for 5 min with the acetoxymethyl ester of BCECF and fluorescence was measured ratiometrically at 6-s intervals. The mean value of pHi in glands perfused with a HCO3(-)-free, N-2-hydroxyethylpiperazine-N'-2- ethanesulphonic acid (HEPES)-buffered solution at 37 degrees C was 7.36 +/- 0.01 (n = 52) which is comparable with values obtained by 31P nuclear magnetic resonance (NMR) spectroscopy. NMR data confirmed that the BCECF loading period was accompanied by a transient acidification of the cells, but there was no significant change in the content of the major phosphorus metabolites. Changes in pHi in response to NH4Cl pulses and acetylcholine stimulation were comparable with results reported previously for isolated acini. Additional, preliminary experiments show that the method can also be used to monitor intracellular Ca2+ (using fura-2) in perfused salivary glands, and can be adapted for studies of the isolated, perfused pancreas.

Fatigue and recovery of phosphorus metabolites and pH during stimulation of rat skeletal muscle: an evoked electromyography and in vivo 31P-nuclear magnetic resonance spectroscopy study

31P-nuclear magnetic resonance spectroscopy and evoked electromyography were applied to rat skeletal muscle to examine the mechanism of muscle fatigue and the recovery of muscle phosphorus metabolites and pH during fatigue. When the sciatic nerve was electrically stimulated at 1 Hz, the contraction force of the gastrocnemius muscle decreased gradually to 46% of the maximal force, accompanied by a decrease in phosphocreatine (PCr) and a corresponding increase in inorganic phosphate (P(i)) and diprotonated inorganic phosphate (H2PO4-). Neither the amplitudes of compound muscle action potentials (CMAP) nor muscle pH changed significantly. At 10-Hz stimulation, contraction force rapidly decreased to 26% of maximal force, accompanied by a decrease in PCr and increases in P(i) and H2PO4-. Muscle pH decreased for a few minutes, then gradually recovered during continued stimulation. The amplitude of the CMAP also decreased for a few minutes and then reached steady values. At 100-Hz stimulation, the contraction force decreased to 6% of the maximal force and there was a decrease in the amplitude of the CMAP. However, the changes in the phosphorus metabolites and pH were transient and recovered to the control value during the stimulation. These results indicated that fatigue at 1 and 100-Hz stimulation was mainly caused by the change in phosphorus metabolite concentrations and electrical failure, respectively, and that fatigue at 10-Hz stimulation might have been due to both of these factors. These results also indicated that electrical failure might have been the cause of the recovery of the phosphorus metabolites and pH during 100-Hz stimulation and of pH during 10-Hz stimulation.

Estimation of the intracellular free ADP concentration by 19F NMR studies of fluorine-labeled yeast phosphoglycerate kinase in vivo

Yeast phosphoglycerate kinase was selectively fluorine-labeled in vivo by inducing enzyme synthesis in stationary phase cells in the presence of 5-fluorotryptophan. Inducible expression was obtained using a galactose-inducible expression vector containing the yeast phosphoglycerate kinase coding sequence. 19F NMR measurements on intact cells showed two resolved resonances, from the two tryptophan residues in the protein, which underwent reversible changes in chemical shift under different metabolic conditions. Measurements in vitro showed that the difference in the chemical shifts of these two resonances was dependent on the adenine nucleotide concentration, in particular the MgADP concentration. A comparison of the spectra obtained in vitro with those obtained from the intact cell indicated that in glucose-fed cells the cytosolic free MgADP concentration was less than 50 microM, which is significantly lower than the concentrations measured in whole-cell extracts.

The hydrophilic spin trap, 5,5-dimethyl-1-pyrroline-1-oxide, does not protect the rat heart from reperfusion injury

The role and site of free radical generation in myocardial ischemia/reperfusion injury were investigated using the hydrophilic spin trapping agent, 5,5-dimethyl-1-pyrroline-1-oxide (DMPO). DMPO (40 mM) proved ineffective in preserving the contractile performance and energy metabolism of Langendorff-perfused rat hearts following ischemia and reperfusion. This result, which is in contrast with the cardioprotection observed with hydrophobic spin traps, suggests that free radicals are generated intracellularly under these conditions.

An in vivo 31P MRS study of patients with liver cirrhosis: progress towards a non-invasive assessment of disease severity

Fourteen patients with liver cirrhosis of differing severity participated in a one-dimensional chemical shift imaging 31P MRS study of the liver. Patients were divided into two groups according to the severity of their liver disease using Child's classification and the aminopyrine breath test (AB test). Seven normal volunteers without liver disease acted as controls. The phosphomonester (PME) peak in normal subjects was 4.77% (95% confidence interval, CI: 4.11-5.42) of total phosphorus. The PME peak was significantly elevated in both mild cirrhosis [5.80% (95% CI: 5.46-6.14), p = 0.0051, vs normal subjects] and severe cirrhosis [9.64% (95% CI: 8.71-10.57), p = 0.0002, vs normal subjects and p = 0.001, vs mild cirrhosis]. There was a significant negative linear correlation (r = 0.88, p < 0.01) of PME with the percentage dose of 14CO2 excreted over 2 h in the AB test. pH values in patients with mild cirrhosis [7.45 (95% CI: 7.35-7.55)] but not severe cirrhosis [7.36 (95% CI: 7.25-7.47)] were significantly elevated (p = 0.04) compared to normal subjects [7.29 (95% CI: 7.17-7.41)]. Comparison of the peak area of PME at TR = 0.5 s against that using TR = 5.0 s in cirrhotic liver suggested no reduction in T1 of phosphorus metabolites in cirrhosis. A relationship between the severity of liver cirrhosis and a relative increase in PME was demonstrated and this was not due to a reduction of T1. This study highlights the clinical potential of 31P MRS as a non-invasive means of assessing the severity of liver cirrhosis.

Effect of glucose and confluency on phosphorus metabolites of perfused human prostatic adenocarcinoma cells as determined by 31P MRS

A series of perfused cell 31P MRS studies were conducted using a well established human prostate adenocarcinoma cell line (DU 145) at different phases of growth, and exposed to varying glucose concentrations during growth. The spectral characteristics of perfused DU 145 cells were compared with the same cells grown in nude mice (xenografts). Perfused DU 145 cells had lower levels of inorganic phosphate and phosphocreative relative to in vivo nude mice xenografts. 31P MR spectra obtained from perfused cells at different phases of growth and exposed to varying glucose concentrations during grown suggest that increases in diphosphodiester levels are associated with high glucose concentrations and confluency. Perfused DU 145 cells grown in 5.5 mM glucose and harvested at log phase of growth best reflected the phosphorus MR spectra of the same cell line grown in nude mice.

Effect of dichloroacetate on recovery of brain lactate, phosphorus energy metabolites, and glutamate during reperfusion after complete cerebral ischemia in rats

The effects of dichloroacetate (DCA) on brain lactate, intracellular pH (pHi), phosphocreatine (PCr), and ATP during 60 min of complete cerebral ischemia and 2 h of reperfusion were investigated in rats by in vivo 1H and 31P magnetic resonance spectroscopy; brain lactate, water content, cations, and amino acids were measured in vitro after reperfusion. DCA, 100 mg/kg, or saline was infused before or immediately after the ischemic period. Preischemic treatment with DCA did not affect brain lactate or pHi during ischemia, but reduced lactate and increased pHi after 30 min of reperfusion (p < 0.05 vs. controls) and facilitated the recovery of PCr and ATP during reperfusion. Postischemic DCA treatment also reduced brain lactate and increased pHi during reperfusion compared with controls (p < 0.05), but had little effect on PCr, ATP, or Pi during reperfusion. After 30 min of reperfusion, serum lactate was 67% lower in the postischemic DCA group than in controls (p < 0.05). The brain lactate level in vitro was 46% lower in the postischemic DCA group than in controls (p < 0.05). DCA did not affect water content or cation concentrations in either group, but it increased brain glutamate by 40% in the preischemic treatment group (p < 0.05). The potential therapeutic effects of DCA on brain injury after complete ischemia may be mediated by reduced excitotoxin release related to decreased lactic acidosis during reperfusion.

Correlations between in vivo 31P MRS measurements, tumor size, cell survival, and hypoxic fraction in the murine EMT6 tumor

Phosphorus metabolite ratios were measured using 31P magnetic resonance spectroscopy shortly before measurement of cell survival and radiobiologic hypoxic fraction (HF) in EMT6/SF tumors, transplanted into a hindlimb of Balb/c mice. A total of 58 tumors with a volume range of 180 to 1250 mm3 were examined in experiments entailing no anesthesia. Postirradiation tumor cell viability was measured using an in vitro clonogenic assay. Correlations between tumor volume, surviving fraction (SF), HF, phosphorus metabolite ratios, and intracellular pH were computed. Both SF and HF increased significantly with tumor volume as did the metabolite ratios of inorganic phosphorus and phosphomonoesters to nucleoside triphosphates (Pi/NTP and PME/NTP, respectively), as well as Pi to phosphocreatine (Pi/PCr). In comparison to HF, the ratios of Pi/NTP, Pi/PCr, and PME/NTP each yielded significant positive correlations (Kendall correlation coefficients(tau) = 0.25 to 0.33). However, these were not significantly stronger than the correlation between HF and volume (tau = 0.21). Apparent values of tumor pH did not correlate with any other measured parameter. While these results indicate a statistical relationship between HF and the measured metabolite ratios, the widely scattered data, as reflected by magnitude of tau less than 0.35, made metabolite ratios poor predictors of HF in individual tumors.

Concurrent quantification of tissue metabolism and blood flow via 2H/31P NMR in vivo. III. Alterations of muscle blood flow and metabolism during sepsis

In the conclusion of this series of reports, the application of 31P/2H NMR to investigate the pathophysiology of sepsis in rat hindlimb muscle is demonstrated. Sepsis decreased muscle [PCr] by 18%, 18 +/- 4 SD vs 22 +/- 4 SD mmol/kg tissue wet wt (P = 0.01) in control rats but [ATP] was unchanged, 6 mmol/kg tissue wet wt (P = 0.2). The derived free cytosolic [ADP] in the two groups was similar, [ADP]septic = 0.023 +/- 0.004 SD and [ADP]control = 0.021 +/- 0.003 SD mmol/kg tissue wet wt, and not statistically different (P = 0.14). Likewise [Pi] in the septic and control groups was not statistically different, [Pi]septic = 1.1 +/- 0.5 SD and [Pi]control = 1.2 +/- 0.4 SD mmol/kg tissue wet wt (P = 0.2). Septic rats presented the symptom of respiratory alkalosis evidenced by elevated blood pH. Sepsis decreased muscle blood flow by 33%, P = 0.003, but examination of individual subjects did not demonstrate a correlation with the reduction in [PCr]. Thus, a metabolic energy deficit caused by cellular ischemia/hypoxia is not a likely cause of cellular abnormality in rat hindlimb muscle during sepsis.

Alterations in hepatic fructose metabolism in cirrhotic patients demonstrated by dynamic 31phosphorus spectroscopy

Quantitative liver function tests are based on the clearance concept and measure the plasma disappearance of a test compound such as galactose. Metabolism is inferred to be predominantly hepatic, and usually no knowledge is obtained of the true time course of metabolite formation. Dynamic 31phosphorus magnetic resonance spectroscopy after intravenous administration of fructose directly measures hepatic sugar metabolism. To determine the feasibility and the utility of 31P magnetic resonance spectroscopy, we studied the responses of six healthy subjects and nine patients with nonalcoholic cirrhosis to a fructose load. Results were related to the impairment of hepatic function assessed by the galactose-elimination capacity test. Liver spectra were acquired in a 1.5 T whole-body nuclear magnetic resonance unit with a surface coil (9-cm diameter) placed ventrally on the liver; the one-dimensional chemical-shift imaging technique was used to obtain spectra from tissue slices parallel to the surface coil. After a basal spectrum had been obtained, fructose (250 mg/kg) was injected intravenously, and further spectra were collected sequentially every 6 min for 1 hr. Formation of monophosphate esters (9% +/- 5% vs. 20% +/- 8% of total area; p less than 0.01) and utilization of inorganic phosphate (5% +/- 4% vs. 11% +/- 3% of total area; p less than 0.005) were markedly decreased in cirrhotic patients. These measures correlated with the severity of the impairment of liver function measured by the galactose-elimination capacity (r = 0.53 to 0.69; p less than 0.05). We conclude that dynamic 31P magnetic resonance spectroscopy is a safe, clinically feasible test that allows detailed insights into biochemical events in liver disease.

31P spectroscopy of the human prostate gland in vivo using a transrectal probe

Using a transrectal probe, good quality 31P magnetic resonance spectroscopy of the human prostate was performed safely, consistently, and in a reasonable amount of time (average of 60 min). Initial results indicate that transrectal 31P MRS has the ability to characterize the phosphorylated metabolites of normal, hyperplastic, and malignant prostates. This study demonstrated that malignant prostates are characterized by significantly decreased levels of phosphocreatine (PCr) and increased levels of phosphomonoesters (PME) as compared to healthy prostates.

Energy state of chondrocytes assessed by 31P-NMR studies of preosseous cartilage

The energy state of resting and hypertrophic chondrocytes from growth plate was studied by 31P-NMR spectroscopy of superfused cartilage slices. The presence of phosphocreatine was demonstrated in both cell types, using a repetition time of 3 s. By comparing the decline in the nucleoside triphosphate level after adding blockers of the glycolysis or of the mitochondrial respiration, it was deduced that resting and hypertrophic chondrocytes use both metabolic pathways for energy production, but the glycolysis dominates. Hypertrophic cells rely more on the mitochondrial respiration than the resting cells.

31P nuclear magnetic resonance spectroscopy, histology and cytokinetics of a xenografted hypopharynx carcinoma following treatment with cisplatin: comparison in three sublines with increasing resistance

The changes in the phosphorus metabolism of a xenografted hypopharynx carcinoma (Hyp 1), sensitive to cisplatin (CDDP), were compared to those occurring in two sublines of the tumour, characterised by moderate or high resistance to CDDP (Hyp 1/H and Hyp 1/R) following, i.p. administration of 4, 8 or 12 mg CDDP/kg-1. The investigations were performed by in vivo 31P nuclear magnetic resonance (NMR) spectroscopy. Parallel to the NMR experiments, the cytokinetic and histological alterations in the tumours were studied under the same experimental conditions. No mentionable differences in the levels of the main phosphorus-containing metabolites could be detected between the three tumour lines before treatment. Following application of CDDP, the alterations in the NMR spectra were clearly related to the degree of tumour response. The most sensitive and earliest marker of tumour regression was a decrease in the phosphomonoester/phosphodiester ratio, parallelled by a gradual increase in the phosphocreatine/inorganic phosphorus quotient. In the resistant tumour lines Hyp 1/H and Hyp 1/R non-responding tumours showed alterations in the 31P NMR spectrum which were similar to those observed during uninfluenced tumour growth. Marked changes in the 31P NMR spectrum were always associated with severe cytotoxic lesions following therapy. The results suggest that the changes detected by 31P NMR spectroscopy following chemotherapy with CDDP are response-specific.

Characterization of prostate cancer, benign prostatic hyperplasia and normal prostates using transrectal 31phosphorus magnetic resonance spectroscopy: a preliminary report

We assessed the ability of 31phosphorus (31P) transrectal magnetic resonance spectroscopy to characterize normal human prostates as well as prostates with benign and malignant neoplasms. With a transrectal probe that we devised for surface coil spectroscopy we studied 15 individuals with normal (5), benign hyperplastic (4) and malignant (6) prostates. Digital rectal examination, transrectal ultrasonography and magnetic resonance imaging were used to aid in accurate positioning of the transrectal probe against the region of interest within the prostate. The major findings of the in vivo studies were that normal prostates had phosphocreatine-to-adenosine triphosphate (ATP) ratios of 1.2 +/- 0.2, phosphomonoester-to-beta-ATP ratios of 1.1 +/- 0.1 and phosphomonoester-to-phosphocreatine ratios of 0.9 +/- 0.1. Malignant prostates had phosphocreatine-to-beta-ATP ratios that were lower (0.7 +/- 0.1) than those of normal prostates (p less than 0.02) or prostates with benign hyperplasia (1.1 +/- 0.2, p less than 0.01). Malignant prostates had phosphomonoester-to-beta-ATP ratios (1.8 +/- 0.2) that were higher than that of normal prostates (p less than 0.02). Using the phosphomonoester-to-phosphocreatine ratio, it was possible to differentiate metabolically malignant (2.7 +/- 0.3) from normal prostates (p less than 0.001), with no overlap of individual ratios. The mean phosphomonoester-to-phosphocreatine ratio (1.5 +/- 0.5) of prostates with benign hyperplasia was midway between the normal and malignant ratios, and there was overlap between individual phosphomonoester-to-phosphocreatine ratios of benign prostatic hyperplasia glands with that of normal and malignant glands. To verify the in vivo results, we performed high resolution magnetic resonance spectroscopy on perchloric acid extracts of benign prostatic hyperplasia tissue obtained at operation and on a human prostatic cancer cell line DU145. The extract results confirmed the differences in metabolite ratios observed in vivo. We conclude that transrectal 31P magnetic resonance spectroscopy can characterize metabolic differences between the normal and malignant prostate.

Cyclosporine and liver regeneration studied by in vivo 31P nuclear magnetic resonance spectroscopy

The changes in fructose-1-phosphate (F-1-P), intracellular pH, and ATP content of the liver after a fructose challenge were investigated noninvasively in vivo using phosphorus-31 nuclear magnetic resonance spectroscopy of dog liver four days after a portacaval shunt (PCS) with or without portal venous infusion of cyclosporin (CsA). The F-1-P metabolism was slower in PCS dogs (N = 2) as compared to either the normal (N = 2) or PCS + CsA-treated dogs (N = 3) (P less than 0.05). The intracellular pH temporarily decreased from 7.3 +/- 0.05 to 7.0 +/- 0.05 during the fructose challenge. The regenerative indexes were increased in the PCS + CsA group (P less than 0.01). These data obtained in vivo using 31P-NMR spectroscopy in the liver following a portacaval shunt, suggest that: (1) the energy status of the liver and the metabolic response to fructose are reduced in PCS compared to normal animals and (2) CsA treatment enhances the regenerative response of the liver and prevents the reduction in hepatic function associated with portacaval shunting.