Oximetry in cells and tissues using a nitroxide-liposome system

A Review of Low-Frequency EPR Technology for the Measurement of Brain pO2 and Oxidative Stress

EPR can uniquely measure paramagnetic species. Although commercial EPR was introduced in 1950s, the early studies were mostly restricted to chemicals in solution or cellular experiments using X-band EPR equipment. Due to its limited penetration (<1 mm), experiments with living animals were almost impossible. To overcome these difficulties, Swartz group, along with several other leaders in field, pioneered the technology of low frequency EPR (e.g., L-band, 1-2 GHz). The development of low frequency EPR and the associated probes have dramatically expanded the application of EPR technology into the biomedical research field, providing answers to important scientific questions by measuring specific parameters that are impossible or very difficult to obtain by other approaches. In this review, which is aimed at highlighting the seminal contribution from Swartz group over the last several decades, we will focus on the development of EPR technology that was designed to deal with the potential challenges arising from conducting EPR spectroscopy in living animals. The second half of the review will be concentrated on the application of low frequency EPR in measuring cerebral tissue pO₂ changes and oxidative stress in various physiological and pathophysiological conditions in the brain of animal disease models.

Oxygen permeability of the lipid bilayer membrane made of calf lens lipids

The oxygen permeability coefficient across the membrane made of the total lipid extract from the plasma membrane of calf lens was estimated from the profile of the oxygen transport parameter (local oxygen diffusion-concentration product) and compared with those estimated for membranes made of an equimolar 1-palmitoyl-2-oleoylphosphatidylcholine/cholesterol (POPC/Chol) mixture and of pure POPC. Profiles of the oxygen transport parameter were obtained by observing the collision of molecular oxygen with nitroxide radical spin labels placed at different depths in the membrane using the saturation-recovery EPR technique and were published by us earlier (J. Widomska, M. Raguz, J. Dillon, E. R. Gaillard, W. K. Subczynski, Biochim. Biophys. Acta. 1768 (2007) 1454-1465). At 35 degrees C, the estimated oxygen permeability coefficients were 51.3, 49.7, and 157.4 cm/s for lens lipid, POPC/Chol, and POPC membranes, respectively (compared with 53.3 cm/s for a water layer with the same thickness as a membrane). Membrane permeability significantly decreases at lower temperatures. In the lens lipid membrane, resistance to the oxygen transport is located in and near the polar headgroup region of the membrane to the depth of the ninth carbon, which is approximately where the steroid-ring structure of cholesterol reaches into the membrane. In the central region of the membrane, oxygen transport is enhanced, significantly exceeding that in bulk water. It is concluded that the high level of cholesterol in lens lipids is responsible for these unique membrane properties.

In vivo electron paramagnetic resonance oximetry with particulate materials

Electron paramagnetic resonance (EPR) methods can be used to study tissue pO(2) (PtO(2)) in anesthetized or awake animals (EPR oximetry). The method takes advantage of the fact that some paramagnetic materials have an EPR linewidth that is sensitive to the pO(2) in which the material is located. This article provides an overview of the method of EPR oximetry using implanted particulate materials as the sensors of pO(2). Characteristics of these materials are described to help the reader understand the factors involved in choosing the optimum particulate material. Examples of biological studies are included that show how EPR oximetry may be used on both awake and anesthetized animals.

The measurement of oxygen in vivo using EPR techniques

The measurement of pO2 in vivo using EPR has some features which have already led to very useful applications and this approach is likely to have increasingly wide and effective use. It is based on the effect of oxygen on EPR spectra which provides a sensitive and accurate means to measure pO2 quantitatively. The development of oxygen-sensitive paramagnetic materials which are very stable, combined with instrumental developments, has been crucial to the in vivo applications of this technique. The physical basis and biological applications of in vivo EPR oximetry are reviewed, with particular emphasis on the use of EPR spectroscopy at 1 GHz using particulate paramagnetic materials for the repetitive and non-invasive measurement of pO2 in tissues. In vivo EPR has already produced some very useful results which have contributed significantly to solving important biological problems. The characteristics of EPR oximetry which appear to be especially useful are often complementary to existing techniques for measuring oxygen in tissues. These characteristics include the capability of making repeated measurements from the same site, high sensitivity to low levels of oxygen, and non-invasive options. The existing techniques are especially useful for studies in small animals, where the depth of measurements is not an overriding issue. In larger animals and potentially in human subjects, non-invasive techniques seem to be immediately applicable to study phenomena very near the surface (within 10 mm) while invasive techniques have some very promising uses. The clinical uses of EPR oximetry which seem especially promising and likely to be undertaken in the near future are long-term monitoring of the status and response to treatment of peripheral vascular disease and optimizing cancer therapy by enabling it to be modified on the basis of the pO2 measured in the tumour.

Spin label oximetry to assess extracellular oxygen during myocardial ischemia

We describe real-time measurement of myocardial oxygen consumption during ischemia in the intact heart. Measurement of extracellular oxygen concentration during myocardial ischemia by spin label oximetry has been limited by ischemia-induced reduction of the neutral, water-soluble nitroxide TEMPONE. We have overcome this problem by encapsulating the nitroxides. Isolated immature (7-10 d old) rabbit hearts (n = 8) were perfused aerobically within the cavity of a loop gap resonator with bicarbonate buffer containing an oxygen-sensitive, lipid-soluble nitroxide (14N-TEMPO laurate in FC-43 perfluorocarbon micelles) and a much less oxygen-sensitive and positively charged nitroxide (15N-TEMPO choline in multilamellar vesicles) as an internal standard. The ratio of the ESR signal amplitudes of these nitroxides was used as a sensitive index of oxygen concentration. Sequestration of the nitroxides decreased their reduction rate by ascorbate in comparison with nonsequestered nitroxides. Hearts were subjected to 60 min of global no-flow ischemia at 20 degrees C. Extracellular oxygen content (mean +/- SD) during aerobic perfusion was 1195 +/- 55 mumol/liter. The electron spin resonance signal from TEMPO laurate increased with the onset and progression of ischemia, consistent with a decrease in extracellular oxygen, while the signal for TEMPO choline was relatively unchanged. Extracellular oxygen content after 40 and 60 min of ischemia was reduced to 393 +/- 27 mumol/liter (p < .05) and 61 +/- 5 mumol/liter (p < .05), respectively. We conclude that spin-label oximetry can directly and precisely measure myocardial oxygen consumption at constant temperature during ischemia in the intact heart.

Fusinite as a specific probe for the determination of molecular oxygen concentration in cells

The feasibility of using EPR and the paramagnetic derivative of coal 'fusinite' to measure intracellular oxygen concentration in cultured cells in which this substance was internalized in the cytoplasm was examined. First, the possible cytotoxic effects of fusinite on cultured cells were ruled out by both morphological as well as by growth characteristics analyses. After construction of a calibration curve in which the EPR spectral linewidth of this substance was measured in response to known oxygen concentrations, the efficacy of using fusinite in the determination of intracellular oxygen concentration in cells was also tested by flowing different known oxygen gas mixtures outside cultured cells. The results indicate that fusinite is able of measuring the variations in cytoplasmic oxygen concentration that exist in response to the different gas mixtures. In addition, as an example of a possible use of fusinite, data are also presented demonstrating a decrease in cytoplasmic oxygen concentration during respiration in cells with a limited supply of oxygen. In fact, as the oxygen is consumed by the cells, the linewidth of fusinite narrows giving an intracellular oxygen concentration corresponding to zero. From the results obtained, fusinite appears to represent a new extremely precise biophysical cellular oxygen probe which may prove useful in the understanding of the complex interrelationships between oxygen and normal cell physiology and/or pathology.

Modulation of streptonigrin cytotoxicity by nitroxide SOD mimics

Nitroxides are cell-permeable, stable radicals that react readily with paramagnetic species such as transition metals or short-lived free radicals, though not generally with diamagnetic molecules. Nitroxides can undergo one-electron selective redox reactions and thereby potentially modify the activity of cytotoxic drugs. Streptonigrin (SN) toxicity requires bioreduction to yield the semiquinone radical, and the toxicity is reportedly mediated by transition metals and oxygen-derived reactive species via redox-cycling of the semiquinone intermediate. The present study shows that (1) nitroxides protected isolated DNA and also aerated or hypoxic bacterial cells from SN toxicity; (2) H2O2 potentiated the hypoxic cytotoxicity of the drug but inhibited the damage to aerated cells; (3) pretreatment of cells with H2O2 conferred some protection, but not when the drug alone was preexposed to H2O2; and (4) desferrioxamine and 2,2-dipyridyl, though neither diethylenetriamino pentaacetate, exogenous catalase, or superoxide dismutase, decreased SN-induced cell killing. The mechanisms by which nitroxides protect from SN toxicity involve both a selective radical-radical reaction with SN semiquinone and the reoxidation of reduced cellular transition metal ions. On the other hand, H2O2 appears to exert two opposing effects: (1) facilitation of cell killing by the Fenton reaction and (2) lowering the cellular level of reducing equivalents, thus inhibiting the bioreductive activation of SN.

Oxygen production and consumption by chloroplasts in situ and in vitro as studied with microscopic spin label probes

A new spin-label oximetry approach able to measure the oxygen partial pressure in complex photosynthetic systems has been developed using bovine serum albumin (BSA)-coated light paraffin oil particles containing cholestane spin label (CSL). Paraffin oil particles protect the spin label against the action of chemically active metabolites. The amplitude of the electron paramagnetic resonance (EPR) signal from CSL measured at a saturating microwave power is sensitive to the concentration of oxygen. We demonstrate here the ability of this method to monitor the kinetics of light-induced oxygen production in situ, i.e., in the interior of a bean leaf. The oxygen release, observed during leaf illumination with continuous light, exhibits an overshoot that correlates with the well-known nonmonotonous behaviour of the Photosystem I reaction center, P700. Short-term illumination of isolated bean chloroplasts, suspended in the presence of the electron mediator methylviologen, induces a reversible uptake of oxygen. However, after prolonged illumination, chloroplasts lose their ability to regenerate oxygen in the dark. The exhaustion of oxygen (and oxygen active forms) is accompanied by the loss of CSL paramagnetism and the capacity to photooxidize P700. Comparison of the kinetics of P700 redox transients with oximetric data demonstrates that oxygen concentration is the essential factor controlling electron transport in leaves and isolated chloroplasts.

In vivo and in vitro EPR oximetry with fusinite: a new coal-derived, particulate EPR probe

The peak-to-peak line width (LW) of the first derivative electron spin resonance (EPR) spectrum of the coal maceral fusinite is reversibly broadened by O2. The extent of broadening per unit of partial pressure of oxygen (pO2) is unusually large, exceeding that of nitroxides by almost two orders of magnitude. This paramagnetic property of fusinite, combined with its very stable physicochemical properties and low toxicity, is shown to be of utility in the measurement of pO2 in vitro and in vivo. Fusinite particles are endocytosed by chinese hamster ovary (CHO) cells in vitro; this is useful for intracellular O2 measurements with commercially available EPR spectrometers operating at 9.1-9.3 GHz. For measurement of oxygen in vivo using low frequency EPR (1.1-1.3 GHz), fusinite provides a sensitive and persistent means to measure pO2 in tissues. Particles implanted into the gastrocnemius muscle of A/J mice remained interstitially in the same position for months with undiminished sensitivity to pO2 and no specific toxic effects.

Simultaneous measurement of intracellular and extracellular oxygen concentrations using a nitroxide-liposome system

The concentration of oxygen within cells is important in many physiological and pathological processes, but such oxygen-dependent phenomena are generally studied as a function of the concentration of extracellular oxygen, due to a lack of suitable methods. Electron parmamagnetic resonance (EPR) oximetric techniques offer an attractive alternative to make such measurements. Previous EPR oximetric studies of extracellular-intracellular oxygen gradients have been hindered, however, by the fact that separate samples, prepared in slightly different ways, were required for individual measurements of extracellular and intracellular oxygen concentrations. In this study we demonstrate a technique that allows simultaneous measurement of intracellular and extracellular oxygen concentrations in a single sample: extracellular measurement is achieved using positively charged nitroxides encapsulated in liposomes, while intracellular oxygen is determined using a membrane-permeable nitroxide along with an extracellular broadening agent. Application of this system to the measurement of oxygen concentrations in suspensions of rat myoblast cells gave results which are consistent with non-simultaneous measurements and which show substantial extracellular-intracellular oxygen gradients in these rapidly respiring cells.

Simultaneous measurements of the intra- and extra-cellular oxygen concentration in viable cells

An EPR method that can measure the intra- and extra-cellular oxygen concentration [O2] simultaneously in vitro has been developed using specially designed nitroxides. In the presence of Fe(CN)6(3-) in the medium, intracellular [O2] is measured by a neutral 15N-nitroxide and extracellular [O2] is measured by a negatively charged 14N-nitroxide, since charged species do not enter cells and the EPR spectrum of a 15N-nitroxide does not overlap with that of a 14N-nitroxide. The method is based in part on the minimal broadening of negatively charged nitroxides by Fe(CN)6(3-) and the very effective broadening of neutral nitroxides by the same paramagnetic ions. Results with this method confirm the existence of gradients in [O2] between the extracellular and intracellular compartments in CHO cells and M5076 tumor cells, even without stimulation of cellular respiration by CCCP. The nature of the barrier that needs to be involved to account for the experimental results raises some significant questions.

In vivo oximetry using a nitroxide-liposome system

Liposomes containing the deuterated, charged, aqueous soluble nitroxide 4-trimethyl-ammonium-2,2,6,6-tetramethylpiperidine-d16-1-oxyl (d-Cat1) were used as probes to measure oxygen concentrations in vivo. Following intramuscular or intraperitoneal injection of the liposome suspension. ICR mice were placed over the surface probe of a low frequency (1.1 GHz) electron paramagnetic resonance (EPR) spectrometer. The linewidth of the deuterated nitroxide is sensitive to changes in the dissolved oxygen concentration: this parameter was calibrated separately so that linewidths measured in the injected mice could be converted into oxygen tensions. This technique detected substantial changes in pO2 as the oxygen content of the breathing gas was changed from 21 to 85 to 0%. Intravenous injection of the liposomes also is possible, and the liposomes accumulate in the liver and spleen, where detectable, oxygen-sensitive EPR signals can be measured.

Oxygen in mammalian tissue: methods of measurement and affinities of various reactions

Oxygen is the primary oxidant in energy-producing biological reactions and is also involved in the synthesis and degradation of many structural and regulatory molecules of physiological importance. This review discusses the advantages and limitations of the currently available methods for measuring oxygen in mammalian tissue and bodily fluids. These include 1) the effects of O2 on the relaxation time of molecules excited by electromagnetic radiation and observed by optical (fluorescence and phosphorescence) and magnetic (nuclear magnetic resonance and electron paramagnetic resonance) techniques, 2) the polarographic and galvanic reduction of oxygen at metal surfaces, 3) in vivo spectrophotometry of intrinsic redox systems, 4) manometry and tonometry, and 5) mass spectroscopy. The values of tissue oxygenation obtained with these techniques are compared with the Michaelis constant values for oxygen of almost 60 oxygen-consuming enzymes involved in mammalian tissue metabolism.