Intrarenal pO2 measured by EPR oximetry and the effects of bacterial endotoxin

Contribution of Harold M. Swartz to In Vivo EPR and EPR Dosimetry

In 2015, we are celebrating half a century of research in the application of Electron Paramagnetic Resonance (EPR) as a biodosimetry tool to evaluate the dose received by irradiated people. During the EPR Biodose 2015 meeting, a special session was organized to acknowledge the pioneering contribution of Harold M. (Hal) Swartz in the field. The article summarizes his main contribution in physiology and medicine. Four emerging themes have been pursued continuously along his career since its beginning: (1) radiation biology; (2) oxygen and oxidation; (3) measuring physiology in vivo; and (4) application of these measurements in clinical medicine. The common feature among all these different subjects has been the use of magnetic resonance techniques, especially EPR. In this article, you will find an impressionist portrait of Hal Swartz with the description of the 'making of' this pioneer, a time-line perspective on his career with the creation of three National Institutes of Health-funded EPR centers, a topic-oriented perspective on his career with a description of his major contributions to Science, his role as a mentor and his influence on his academic children, his active role as founder of scientific societies and organizer of scientific meetings, and the well-deserved international recognition received so far.

Clinical EPR: unique opportunities and some challenges

Electron paramagnetic resonance (EPR) spectroscopy has been well established as a viable technique for measurement of free radicals and oxygen in biological systems, from in vitro cellular systems to in vivo small animal models of disease. However, the use of EPR in human subjects in the clinical setting, although attractive for a variety of important applications such as oxygen measurement, is challenged with several factors including the need for instrumentation customized for human subjects, probe, and regulatory constraints. This article describes the rationale and development of the first clinical EPR systems for two important clinical applications, namely, measurement of tissue oxygen (oximetry) and radiation dose (dosimetry) in humans. The clinical spectrometers operate at 1.2 GHz frequency and use surface-loop resonators capable of providing topical measurements up to 1 cm depth in tissues. Tissue pO2 measurements can be carried out noninvasively and repeatedly after placement of an oxygen-sensitive paramagnetic material (currently India ink) at the site of interest. Our EPR dosimetry system is capable of measuring radiation-induced free radicals in the tooth of irradiated human subjects to determine the exposure dose. These developments offer potential opportunities for clinical dosimetry and oximetry, which include guiding therapy for individual patients with tumors or vascular disease by monitoring of tissue oxygenation. Further work is in progress to translate this unique technology to routine clinical practice.

Blood pressure, blood flow, and oxygenation in the clipped kidney of chronic 2-kidney, 1-clip rats: effects of tempol and Angiotensin blockade

Angiotensin II maintains renal cortical blood flow and renal oxygenation in the clipped kidney of early 2-kidney, 1-clip Goldblatt hypertensive (2K,1C) rats. The involvement of Ang II is believed to decline, whereas oxidative stress increases during the progression of 2K,1C hypertension. We investigated the hypothesis that the acute administration of drugs to inhibit reactive oxygen species (Tempol), angiotensin II type 1 receptors (candesartan), or angiotensin-converting enzyme (enalaprilat) lowers mean arterial pressure and increases kidney blood flow and oxygenation in the clipped kidney of chronic 2K,1C rats in contrast to sham controls. Twelve months after left renal artery clipping or sham, mean arterial pressure, renal cortical blood flow, and renal cortical and medullary oxygen tension were measured after acute administration of Tempol followed by enalaprilat or candesartan followed by enalaprilat. The mean arterial pressure of the 2K,1C rat was reduced by candesartan (-9%) and, more effectively, by Tempol (-35%). All of the applied treatments had similar blood pressure-lowering effects in sham rats (average: -21%). Only Tempol increased cortical blood flow (+35%) and cortical and medullary oxygen tensions (+17% and +94%, respectively) in clipped kidneys of 2K,1C rats. Administration of enalaprilat had no additional effect, except for a modest reduction in cortical blood flow in the clipped kidney of 2K,1C rats when coadministered with candesartan (-10%). In conclusion, acute administration of Tempol is more effective than candesartan in reducing the mean arterial blood pressure and improving renal blood perfusion and oxygenation in the clipped kidney of chronic 2K,1C rats.

Application of in vivo EPR for tissue pO2 and redox measurements

The technique of electron paramagnetic resonance (EPR) spectroscopy is more than 50 years old, but only recently it has been used for in vivo studies. Its limited application in the past was due to the problem of high nonresonant dielectric loss of the exciting frequency because of high water content in biological samples. However, with the development of spectrometers working at lower frequencies (1,200 MHz and below) during the last 15 years, it is now possible to conduct in vivo measurements on a variety of animals and isolated organs. This is further facilitated by the development of new resonators with high sensitivity and appropriate stability for in vivo applications. It now has become feasible to obtain new insights into the complex aspects of physiology and pathophysiology using in vivo EPR. Among several important applications of this technique, the in vivo tissue pO(2) (partial pressure of oxygen) and redox measurements seem to be the most extensive use of this technique. In this chapter, we describe the procedure for in vivo pO(2) and redox measurements in animal models.

Repetitive tissue pO2 measurements by electron paramagnetic resonance oximetry: current status and future potential for experimental and clinical studies

Tissue oxygen plays a crucial role in maintaining tissue viability and in various diseases, including responses to therapy. Useful knowledge has been gained by methods that can give limited snapshots of tissue oxygen (e.g., oxygen electrodes) or evidence of a history of tissue hypoxia (e.g., EF5) or even indirect evidence by monitoring oxygen availability in the circulatory system (e.g., NMR methods). Each of these methods has advantages and significant limitations. EPR oximetry is a technique for direct measurement of tissue pO2, which has several advantages over the other existing methods for applications in which the parameter of interest is the pO2 of tissues, and information is needed over a time course of minutes to hours, and/or for repetitive measurements over days or weeks or years. The aim of this article is to provide an overview of EPR oximetry using particulates to readers who are not familiar with this technique and its potential in vivo and clinical applications. The data presented here are from the experiments currently being carried out in our laboratory. We are confident that in vivo EPR oximetry will play a crucial role in the understanding and clinical management of various pathologies in the years to come.

Evaluation of Intra-Renal Oxygenation by BOLD MRI

This is a review of blood oxygenation level-dependent (BOLD) MRI as applied to the kidney. It has been shown that BOLD MRI measurements reflect changes in renal oxygenation, especially in the medulla. Renal medulla functions in a hypoxic milieu and is extremely sensitive to further decrease in blood flow or increase in oxygen consumption. Availability of a non-invasive technique such as BOLD MRI should allow for better understanding of the factors involved in the maintenance of renal oxygenation status, not only in animal models, but also in humans.