The Use of EPR for the Measurement of the Concentration of Oxygen in Vivo in Tissues under Physiologically Pertinent Conditions and Concentrations

The aim of this paper is to describe an emerging methodology which appears to offer new capabilities for the measurement of the concentration of oxygen ([O2]) in complex biological systems in vitro and in vivo with the sensitivity and accuracy needed to make these measurements under conditions that are pertinent for physiological and pathological processes. The method uses electron paramagnetic resonance (EPR or, equivalently, ESR); the approach is often termed EPR oximetry. In particular EPR oximetry can make measurements selectively in the intracellular compartment and in tissues in vivo and can detect [O2] as low as 0.1 micromolar. Our use of EPR to measure intracellular [O2] with nitroxides has been described previously (1–2) and therefore this paper will concentrate on the methodology to make measurements in vivo and also, at very low [O2]. We will review the basis of the methodology briefly but will emphasize results in tissues which illustrate the capabilities of this new technique. Because of the newness of this approach the results obtained so far have been aimed at determining and illustrating the potential of this technique, rather than providing detailed information on a specific biological problem. A companion paper illustrates a more detailed application of the new technology, the measurement of [O2] in skeletal muscle (3).