Measuring tissue PO2 with microelectrodes

Blood Oxygen Sensor Using a Boron-Doped Diamond Electrode

The electrochemical behavior of oxygen (O₂) in blood was studied using boron-doped diamond (BDD) electrodes. Cyclic voltammogram of O₂ in a 0.1 M phosphate buffer solution solution containing 1 × 10^-6 M of bovine hemoglobin exhibits a reduction peak at -1.4 V (vs Ag/AgCl). Moreover, the scan rate dependence was investigated to study the reduction reaction mechanism, which was attributable to the reduction of O₂ to H₂O₂ via two electrons. A linear calibration curve was observed in the concentration range of 86.88-314.63 mg L^-1 (R² = 0.99) with a detection limit of 1.0 mg L^-1 (S/B = 3). The analytical performance was better than those with glassy carbon or platinum electrodes as the working electrode. In addition, an application to bovine blood was performed. The O₂ concentration in the blood measured on the BDD electrodes was compared to that measured using an OxyLite Pro fiber-optic oxygen sensor device. Both methods gave similar values of the O₂ concentration in the range of ∼40 to 150 mmHg. This result confirms that BDD electrodes could potentially be used to detect the O₂ concentration in blood.

Hypoxia Imaging As a Guide for Hypoxia-Modulated and Hypoxia-Activated Therapy

Significance: Oxygen imaging techniques, which can probe the spatiotemporal heterogeneity of tumor oxygenation, could be of significant clinical utility in radiation treatment planning and in evaluating the effectiveness of hypoxia-activated prodrugs. To fulfill these goals, oxygen imaging techniques should be noninvasive, quantitative, and capable of serial imaging, as well as having sufficient temporal resolution to detect the dynamics of tumor oxygenation to distinguish regions of chronic and acute hypoxia. Recent Advances: No current technique meets all these requirements, although all have strengths in certain areas. The current status of positron emission tomography (PET)-based hypoxia imaging, oxygen-enhanced magnetic resonance imaging (MRI), ¹⁹F MRI, and electron paramagnetic resonance (EPR) oximetry are reviewed along with their strengths and weaknesses for planning hypoxia-guided, intensity-modulated radiation therapy and detecting treatment response for hypoxia-targeted prodrugs. Critical Issues: Spatial and temporal resolution emerges as a major concern for these areas along with specificity and quantitative response. Although multiple oxygen imaging techniques have reached the investigative stage, clinical trials to test the therapeutic effectiveness of hypoxia imaging have been limited. Future Directions: Imaging elements of the redox environment besides oxygen by EPR and hyperpolarized MRI may have a significant impact on our understanding of the basic biology of the reactive oxygen species response and may extend treatment possibilities.

First-In-Human Study in Cancer Patients Establishing the Feasibility of Oxygen Measurements in Tumors Using Electron Paramagnetic Resonance With the OxyChip

OBJECTIVE

The overall objective of this clinical study was to validate an implantable oxygen sensor, called the 'OxyChip', as a clinically feasible technology that would allow individualized tumor-oxygen assessments in cancer patients prior to and during hypoxia-modification interventions such as hyperoxygen breathing.

METHODS

Patients with any solid tumor at ≤3-cm depth from the skin-surface scheduled to undergo surgical resection (with or without neoadjuvant therapy) were considered eligible for the study. The OxyChip was implanted in the tumor and subsequently removed during standard-of-care surgery. Partial pressure of oxygen (pO2) at the implant location was assessed using electron paramagnetic resonance (EPR) oximetry.

RESULTS

Twenty-three cancer patients underwent OxyChip implantation in their tumors. Six patients received neoadjuvant therapy while the OxyChip was implanted. Median implant duration was 30 days (range 4-128 days). Forty-five successful oxygen measurements were made in 15 patients. Baseline pO2 values were variable with overall median 15.7 mmHg (range 0.6-73.1 mmHg); 33% of the values were below 10 mmHg. After hyperoxygenation, the overall median pO2 was 31.8 mmHg (range 1.5-144.6 mmHg). In 83% of the measurements, there was a statistically significant (p ≤ 0.05) response to hyperoxygenation.

CONCLUSIONS

Measurement of baseline pO2 and response to hyperoxygenation using EPR oximetry with the OxyChip is clinically feasible in a variety of tumor types. Tumor oxygen at baseline differed significantly among patients. Although most tumors responded to a hyperoxygenation intervention, some were non-responders. These data demonstrated the need for individualized assessment of tumor oxygenation in the context of planned hyperoxygenation interventions to optimize clinical outcomes.

Hypoxic Jumbo Spheroids On-A-Chip (HOnAChip): Insights into Treatment Efficacy

Hypoxia is a key characteristic of the tumor microenvironment, too rarely considered during drug development due to the lack of a user-friendly method to culture naturally hypoxic 3D tumor models. In this study, we used soft lithography to engineer a microfluidic platform allowing the culture of up to 240 naturally hypoxic tumor spheroids within an 80 mm by 82.5 mm chip. These jumbo spheroids on a chip are the largest to date (>750 µm), and express gold-standard hypoxic protein CAIX at their core only, a feature absent from smaller spheroids of the same cell lines. Using histopathology, we investigated response to combined radiotherapy (RT) and hypoxic prodrug Tirapazamine (TPZ) on our jumbo spheroids produced using two sarcoma cell lines (STS117 and SK-LMS-1). Our results demonstrate that TPZ preferentially targets the hypoxic core (STS117: p = 0.0009; SK-LMS-1: p = 0.0038), but the spheroids' hypoxic core harbored as much DNA damage 24 h after irradiation as normoxic spheroid cells. These results validate our microfluidic device and jumbo spheroids as potent fundamental and pre-clinical tools for the study of hypoxia and its effects on treatment response.

O2-Tuned Protein Synthesis Machinery in Escherichia coli-Based Cell-Free System

Involved in most aerobic biochemical processes, oxygen affects cellular functions, and organism behaviors. Protein synthesis, as the underlying biological process, is unavoidably affected by the regulation of oxygen delivery and utilization. Bypassing the cell wall, cell-free protein synthesis (CFPS) systems are well adopted for the precise oxygen regulation analysis of bioprocesses. Here a reliable flow platform was developed for measuring and analyzing the oxygen regulation on the protein synthesis processes by combining Escherichia coli-based CFPS systems and a tube-in-tube reactor. This platform allows protein synthesis reactions conducted in precisely controlled oxygen concentrations. For analysis of the intrinsic role of oxygen in protein synthesis, O2-tuned CFPS systems were explored with transcription-translation related parameters (transcripts, energy, reactive oxygen species, and proteomic pathway analysis). It was found that 2% of oxygen was the minimum requirement for protein synthesis. There was translation-related protein degradation in the high oxygen condition leading to a reduction. By combining the precise gas level controlling and open biosystems, this platform is also potential for fundamental understanding and clinical applications by diverse gas regulation in biological processes.

Retinal oxygen: from animals to humans

This article discusses retinal oxygenation and retinal metabolism by focusing on measurements made with two of the principal methods used to study O₂ in the retina: measurements of PO₂ with oxygen-sensitive microelectrodes in vivo in animals with a retinal circulation similar to that of humans, and oximetry, which can be used non-invasively in both animals and humans to measure O₂ concentration in retinal vessels. Microelectrodes uniquely have high spatial resolution, allowing the mapping of PO₂ in detail, and when combined with mathematical models of diffusion and consumption, they provide information about retinal metabolism. Mathematical models, grounded in experiments, can also be used to simulate situations that are not amenable to experimental study. New methods of oximetry, particularly photoacoustic ophthalmoscopy and visible light optical coherence tomography, provide depth-resolved methods that can separate signals from blood vessels and surrounding tissues, and can be combined with blood flow measures to determine metabolic rate. We discuss the effects on retinal oxygenation of illumination, hypoxia and hyperoxia, and describe retinal oxygenation in diabetes, retinal detachment, arterial occlusion, and macular degeneration. We explain how the metabolic measurements obtained from microelectrodes and imaging are different, and how they need to be brought together in the future. Finally, we argue for revisiting the clinical use of hyperoxia in ophthalmology, particularly in retinal arterial occlusions and retinal detachment, based on animal research and diffusion theory.

Implementing oxygen control in chip-based cell and tissue culture systems

Oxygen is essential in the energy metabolism of cells, as well as being an important regulatory parameter influencing cell differentiation and function. Interest in precise oxygen control for in vitro cultures of tissues and cells continues to grow, especially with the emergence of the organ-on-a-chip and the desire to emulate in vivo conditions. This was recently discussed in this journal in a Critical Review by Brennan et al. (Lab Chip (2014). DOI: ). Microfluidics can be used to introduce flow to facilitate nutrient supply to and waste removal from in vitro culture systems. Well-defined oxygen gradients can also be established. However, cells can quickly alter the oxygen balance in their vicinity. In this Tutorial Review, we expand on the Brennan paper to focus on the implementation of oxygen analysis in these systems to achieve continuous monitoring. Both electrochemical and optical approaches for the integration of oxygen monitoring in microfluidic tissue and cell culture systems will be discussed. Differences in oxygen requirements from one organ to the next are a challenging problem, as oxygen delivery is limited by its uptake into medium. Hence, we discuss the factors determining oxygen concentrations in solutions and consider the possible use of artificial oxygen carriers to increase dissolved oxygen concentrations. The selection of device material for applications requiring precise oxygen control is discussed in detail, focusing on oxygen permeability. Lastly, a variety of devices is presented, showing the diversity of approaches that can be employed to control and monitor oxygen concentrations in in vitro experiments.

Mechanotransduction drives post ischemic revascularization through K(ATP) channel closure and production of reactive oxygen species

AIMS

We reported earlier that ischemia results in the generation of reactive oxygen species (ROS) via the closure of a K(ATP) channel which causes membrane depolarization and NADPH oxidase 2 (NOX2) activation. This study was undertaken to understand the role of ischemia-mediated ROS in signaling.

RESULTS

Angiogenic potential of pulmonary microvascular endothelial cells (PMVEC) was studied in vitro and in the hind limb in vivo. Flow adapted PMVEC injected into a Matrigel matrix showed significantly higher tube formation than cells grown under static conditions or cells from mice with knockout of K(ATP) channels or the NOX2. Blocking of hypoxia inducible factor-1 alpha (HIF-1α) accumulation completely abrogated the tube formation in wild-type (WT) PMVEC. With ischemia in vivo (femoral artery ligation), revascularization was high in WT mice and was significantly decreased in mice with knockout of K(ATP) channel and in mice orally fed with a K(ATP) channel agonist. In transgenic mice with endothelial-specific NOX2 expression, the revascularization observed was intermediate between that of WT and knockout of K(ATP) channel or NOX2. Increased HIF-1α activation and vascular endothelial growth factor (VEGF) expression was observed in ischemic tissue of WT mice but not in K(ATP) channel and NOX2 null mice. Revascularization could be partially rescued in K(ATP) channel null mice by delivering VEGF into the hind limb.

INNOVATION

This is the first report of a mechanosensitive ion channel (K(ATP) channel) initiating endothelial signaling that drives revascularization.

CONCLUSION

The K(ATP) channel responds to the stop of flow and activates signals for revascularization to restore the impeded blood flow.

Detection of the oxygen consumption rate of migrating zebrafish by electrochemical equalization systems

A novel measurement system to determine oxygen consumption rates via respiration in migrating Zebrafish (Danio rerio) has been developed. A signal equalization system was adapted to detect oxygen in a chamber with one fish, because typical electrochemical techniques cannot measure respiration activities for migrating organisms. A closed chamber was fabricated using a pipet tip attached to a Pt electrode, and a columnar Vycor glass tip was used as the salt bridge. Pt electrode, which was attached to the chamber with one zebrafish, and Ag electrode were immersed in 10 mM potassium iodide (KI), and both the electrodes were connected externally to form a galvanic cell. Pt and Ag electrodes act as the cathode and anode to reduce oxygen and oxidize silver, respectively, allowing the deposition of insoluble silver iodide (AgI). The AgI acts as the signal source accumulated on the Ag electrode by conversion of oxygen. The amount of AgI deposited on the Ag electrode was determined by cathodic stripping voltammetry. The presence of zebrafish or its embryo led to a decrease in the stripping currents generated by a 10 min conversion of oxygen to AgI. The conversion of oxygen to AgI is disturbed by the migration of the zebrafish and allows the detection of different equalized signals corresponding to respiration activity. The oxygen consumption rates of the zebrafish and its embryo were estimated and determined to be ∼4.1 and 2.4 pmol·s(-1), respectively. The deposited AgI almost completely disappeared with a single stripping process. The signal equalization system provides a method to determine the respiration activities for migrating zebrafish and could be used to estimate environmental risk and for effective drug screening.

Oxygen transport in the microcirculation and its regulation

OBJECTIVE

Cells require energy to carry out their functions and they typically use oxidative phosphorylation to generate the needed ATP. Thus, cells have a continuous need for oxygen, which they receive by diffusion from the blood through the interstitial fluid. The circulatory system pumps oxygen-rich blood through a network of increasingly minute vessels, the microcirculation. The structure of the microcirculation is such that all cells have at least one nearby capillary for diffusive exchange of oxygen and red blood cells release the oxygen bound to hemoglobin as they traverse capillaries.

METHODS

This review focuses first on the historical development of techniques to measure oxygen at various sites in the microcirculation, including the blood, interstitium, and cells.

RESULTS

Next, approaches are described as to how these techniques have been employed to make discoveries about different aspects of oxygen transport. Finally, ways in which oxygen might participate in the regulation of blood flow toward matching oxygen supply to oxygen demand is discussed.

CONCLUSIONS

Overall, the transport of oxygen to the cells of the body is one of the most critical functions of the cardiovascular system and it is in the microcirculation where the final local determinants of oxygen supply, oxygen demand, and their regulation are decided.

Adaptive and maladaptive cardiorespiratory responses to continuous and intermittent hypoxia mediated by hypoxia-inducible factors 1 and 2

Hypoxia is a fundamental stimulus that impacts cells, tissues, organs, and physiological systems. The discovery of hypoxia-inducible factor-1 (HIF-1) and subsequent identification of other members of the HIF family of transcriptional activators has provided insight into the molecular underpinnings of oxygen homeostasis. This review focuses on the mechanisms of HIF activation and their roles in physiological and pathophysiological responses to hypoxia, with an emphasis on the cardiorespiratory systems. HIFs are heterodimers comprised of an O(2)-regulated HIF-1α or HIF-2α subunit and a constitutively expressed HIF-1β subunit. Induction of HIF activity under conditions of reduced O(2) availability requires stabilization of HIF-1α and HIF-2α due to reduced prolyl hydroxylation, dimerization with HIF-1β, and interaction with coactivators due to decreased asparaginyl hydroxylation. Stimuli other than hypoxia, such as nitric oxide and reactive oxygen species, can also activate HIFs. HIF-1 and HIF-2 are essential for acute O(2) sensing by the carotid body, and their coordinated transcriptional activation is critical for physiological adaptations to chronic hypoxia including erythropoiesis, vascularization, metabolic reprogramming, and ventilatory acclimatization. In contrast, intermittent hypoxia, which occurs in association with sleep-disordered breathing, results in an imbalance between HIF-1α and HIF-2α that causes oxidative stress, leading to cardiorespiratory pathology.

Self-referencing optrodes for measuring spatially resolved, real-time metabolic oxygen flux in plant systems

The ability to non-invasively measure metabolic oxygen flux is a very important tool for physiologists interested in a variety of questions ranging from basic metabolism, growth/development, and stress adaptation. Technologies for measuring oxygen concentration near the surface of cells/tissues include electrochemical and optical techniques. A wealth of knowledge was gained using these tools for quantifying real-time physiology. Fiber-optic microprobes (optrodes) have recently been developed for measuring oxygen in a variety of biomedical and environmental applications. We have adopted the use of these optical microsensors for plant physiology applications, and used the microsensors in an advanced sensing modality known as self-referencing. Self-referencing is a non-invasive microsensor technique used for measuring real-time flux of analytes. This paper demonstrates the use of optical microsensors for non-invasively measuring rhizosphere oxygen flux associated with respiration in plant roots, as well as boundary layer oxygen flux in phytoplankton mats. Highly sensitive/selective optrodes had little to no hysteresis/calibration drift during experimentation, and an extremely high signal-to-noise ratio. We have used this new tool to compare various aspects of rhizosphere oxygen flux for roots of Glycine max, Zea mays, and Phaseolus vulgaris, and also mapped developmentally relevant profiles and distinct temporal patterns. We also characterized real-time respiratory patterns during inhibition of cytochrome and alternative oxidase pathways via pharmacology. Boundary layer oxygen flux was also measured for a phytoplankton mat during dark:light cycling and exposure to pharamacological inhibitors. This highly sensitive technology enables non-invasive study of oxygen transport in plant systems under physiologically relevant conditions.

The physics, biophysics and technology of photodynamic therapy

Photodynamic therapy (PDT) uses light-activated drugs to treat diseases ranging from cancer to age-related macular degeneration and antibiotic-resistant infections. This paper reviews the current status of PDT with an emphasis on the contributions of physics, biophysics and technology, and the challenges remaining in the optimization and adoption of this treatment modality. A theme of the review is the complexity of PDT dosimetry due to the dynamic nature of the three essential components -- light, photosensitizer and oxygen. Considerable progress has been made in understanding the problem and in developing instruments to measure all three, so that optimization of individual PDT treatments is becoming a feasible target. The final section of the review introduces some new frontiers of research including low dose rate (metronomic) PDT, two-photon PDT, activatable PDT molecular beacons and nanoparticle-based PDT.

Dynamics of intracellular oxygen in PC12 Cells upon stimulation of neurotransmission

Neurotransmission, synaptic plasticity, and maintenance of membrane excitability require high mitochondrial activity in neurosecretory cells. Using a fluorescence-based intracellular O2 sensing technique, we investigated the respiration of differentiated PC12 cells upon depolarization with 100 mm K+. Single cell confocal analysis identified a significant depolarization of the plasma membrane potential and a relatively minor depolarization of the mitochondrial membrane potential following K+ exposure. We observed a two-phase respiratory response: a first intense spike lasting approximately 10 min, during which average intracellular O2 was reduced from 85-90% of air saturation to 55-65%, followed by a second wave of smaller amplitude and longer duration. The fast rise in O2 consumption coincided with a transient increase in cellular ATP by approximately 60%, which was provided largely by oxidative phosphorylation and by glycolysis. The increase of respiration was orchestrated mainly by Ca2+ release from the endoplasmic reticulum, whereas the influx of extracellular Ca2+ contributed approximately 20%. Depletion of Ca2+ stores by ryanodine, thapsigargin, and 4-chloro-m-cresol reduced the amplitude of respiratory spike by 45, 63, and 71%, respectively, whereas chelation of intracellular Ca2+ abolished the response. Uncoupling of the mitochondria with the protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone amplified the responses to K+; elevated respiration induced a profound deoxygenation without increasing the cellular ATP levels reduced by carbonyl cyanide p-trifluoromethoxyphenylhydrazone. Cleavage of synaptobrevin 2 by tetanus toxin, known to reduce neurotransmission, did not affect the respiratory response to K+, whereas the general excitability of d PC12 cells increased.

Glucose-induced release of nitric oxide from mouse pancreatic islets as detected with nitric oxide-selective glass microelectrodes

Nitric oxide (NO) is believed to play an important role in pancreatic islet physiology and pathophysiology. Research in this area has been hampered, however, by the use of indirect methods to measure islet NO. To investigate the role of NO in islet function, we positioned NO-sensitive, recessed-tip microelectrodes in close proximity to individual islets and monitored oxidation current to detect subnanomolar NO in the bath. NO release from islets consisted of a series of rapid bursts lasting several seconds and/or slow oscillations with a period of approximately 100-300 s. Average baseline NO near the islets in 2.8 mM glucose was 524+/-59 nM (n=12). Raising glucose from 2.8 to 11.1 mM augmented NO release by 429+/-133 nM (n=12, P<0.05), an effect blocked by the NO synthase inhibitor L-NAME (n=3). We also observed that glucose-stimulated increases in NO release were contemporaneous with changes in NAD(P)H and O2 but occurred well before increases in calcium associated with glucose-stimulated insulin secretion. In summary, we demonstrate that NO release from islets is oscillatory and rapidly augmented by glucose, suggesting that NO release occurs early following an increase in glucose metabolism and may contribute to the stimulated insulin secretion triggered by suprathreshold glucose.

Quantifying the L-arginine paradox in vivo

NO and PO(2) microelectrodes were used to quantify the effects of increased availability of L-arginine in an exteriorized rat mesentery and small intestine microcirculatory preparation in n = 16 rats. During short periods of elevated L-arginine added to the superfusion bath, transient changes in perivascular NO or PO(2) were measured at 171 perivascular sites near intestinal arterioles and venules, simultaneously with tissue perfusion using laser Doppler flowmetry (LDF). Excess L-arginine increased perivascular NO over twofold, by 411 +/- 42 nM above the baseline of 329 +/- 30 nM (P < 0.0001), and increased tissue perfusion by 35.5 +/- 7.5% (P < 0.0001). No difference between arterioles and venules was observed in the magnitude or time course of the NO responses. Both increases and decreases in perivascular PO(2) were observed after excess L-arginine, with a similar increase in tissue perfusion by 42.0 +/- 12.3% (P < 0.0001). Our NO measurements confirm that increased bioavailability of L-arginine causes a significant increase in NO production throughout the microcirculation of this preparation, with increased tissue perfusion, and provides direct in vivo evidence for the L-arginine paradox.

An optical multifrequency phase-modulation method using microbeads for measuring intracellular oxygen concentrations in plants

A technique has been developed to measure absolute intracellular oxygen concentrations in green plants. Oxygen-sensitive phosphorescent microbeads were injected into the cells and an optical multifrequency phase-modulation technique was used to discriminate the sensor signal from the strong autofluorescence of the plant tissue. The method was established using photosynthesis-competent cells of the giant algae Chara corallina L., and was validated by application to various cell types of other plant species.

Comparison of EPR oximetry and Eppendorf polarographic electrode assessments of rat brain PtO2

EPR oximetry is a promising, relatively non-invasive method for monitoring the partial pressure of oxygen in tissue (PtO2) that has proved useful in following changes under various physiologic and pathophysiologic conditions. Optimal utilization of the method will be facilitated by systematic comparisons with other available methods. Here we report on the absolute values of rat brain PtO2 using EPR and the more widely used Eppendorf polarographic microelectrode system in the same brain. EPR used an L-band (1.2 GHz) spectrometer and implanted lithium phthalocyanine (LiPc) as the oxygen-sensitive paramagnetic material. Eppendorf measurements were made by a needle probe moved vertically through the cortex at 0.5 mm intervals in three tracks including one adjacent to the location of the LiPc. Several conclusions were drawn, including, (1) the average PtO2 measured by the two methods was similar but EPR reported a significantly higher average PtO2, (2) there was poor correlation between the values in the same animal on the same side of the brain, (3) the Eppendorf reported a larger range of values and (4) the heterogeneity of oxygen levels in the brain and the areas sampled by the two methods provide an adequate explanation for the observed differences.

Tumor oxygenation status as a prognostic marker

Tumor oxygenation status is an independent prognostic indicator in cancer because it influences tumor progression and treatment outcome. Its quantitative value is determined by a number of tumor vascular parameters such as microvascular density, blood flow, blood volume, blood oxygen saturation, tumor tissue pO2, and resistance to oxygen diffusion within the tumor. Over the past several years, considerable time and effort have been invested into developing techniques to effectively and reliably measure the oxygenation status of a tumor. The measurement and interpretation of data obtained with currently available methods is complicated by the heterogeneity in tumor oxygenation. Currently available techniques can be broadly classified into direct invasive methods, direct non-invasive methods, and measurement of surrogate endogenous markers of tumor oxygenation. Of these methods, the Eppendorf pO2 histograph is considered the 'gold standard' and even so has several limitations. Given the importance of tumor oxygenation status in therapy and in predicting disease progression, it is imperative that reliable, globally usable, and technically simplistic methods be developed to yield a consistent, comprehensive, and reliable profile of tumor oxygenation. Until newer more reliable techniques are developed, existing independent techniques or appropriate combinations of techniques should be optimized and validated using known endpoints in tumor oxygenation status and/or treatment outcomes.

Simultaneous measurement of rat brain cortex PtO2using EPR oximetry and a fluorescence fiber-optic sensor during normoxia and hyperoxia

Electron paramagnetic resonance (EPR) oximetry is a promising, relatively non-invasive method of monitoring tissue partial pressure of oxygen (PtO(2)) that has proven useful in following changes in PtO(2) under various physiologic and pathophysiologic conditions. Optimal utilization of the method will be facilitated by systematic comparisons with other available methods. Here, we report on the absolute values and changes of rat brain PtO(2) using EPR oximetry and the OxyLite, an oxygen monitor based on fluorescence quenching, at adjacent locations in the same brain. EPR oximetry utilizes an implanted oxygen-sensitive material and reports tissue PtO(2) at the surface of the material. OxyLite measures PtO(2) using the fluorescence lifetime of a chromophore fixed to the tip of an optical fiber that is inserted into tissue. Measurements were made at a depth of 2-3 mm into the cortex during normoxia and during breathing of carbogen (95% O(2):5% CO(2)) followed by a return to normoxia. We conclude that in this study (1) PtO(2) values reported by the two methods are similar but not exactly the same, (2) both methods can record a baseline and rapid changes in PtO(2), (3) changes in PtO(2) induced by increasing FiO(2) from 0.26 to 0.95 (carbogen) were similar by the two methods and (4) in some rats breathing carbogen, absolute values of PtO(2) were above the sensitive range of the OxyLite method.