Oxidative stress imaging in live animals with techniques based on electron paramagnetic resonance

Abstract

Oxidative stress has been the object of considerable biological and biochemical investigation. Quantification has been difficult although the quantitative level of products of biological oxidations in tissues and tissue products has emerged as a widely used technique. The relationship between these products and the amount of oxidative stress is less clear. Imaging oxidative stress with electron paramagnetic resonance related magnetic resonance imaging, while not addressing the specific issue of quantification of initiating events, focuses on the anatomic specific location of the oxidative stress. Moreover, the relative quantification of oxidative stress of one location against another is possible, sharpening our understanding of oxidative stress. This promises to improve our understanding of oxidative stress and its deleterious consequences and enhance our understanding of the effectiveness of interventions to modulate oxidative stress and its consequences.

FIG. 1

Reactions of nitroxides in biological systems.

FIG. 2

Comparison of oxidative stress measurement using nitroxides as a redox-sensitive probe between EPR, OMRI and MRI. Redox state of the tissue is reflected by the ratio of paramagnetic form of the nitroxide and its reduced non-paramagnetic form, hydroxylamine. All three techniques are based on monitoring the kinetics of nitroxide decay in the tissue. EPR signal measures the absorption of microwave energy by nitroxide, directly reflecting the amount of paramagnetic form present in the sample. OMRI detects changes in the proton relaxation induced by excited nitroxide spins. MRI also measures the proton relaxation, but nitroxide spins act here as contrast agents.

FIG. 3

Redox mapping of tumor. After tail vein infusion of 3-CP, a series of 2D images of the nitroxide from tumors of air- or carbogen-breathing mice were measured using the L-band EPR imaging method. The image data were acquired using a magnetic-field gradient of 150 mT/m at 16 orientations in the 2D plane. Two-dimensional spatial mapping of nitroxide distribution, pseudo-first-order rate constant, and frequency plot of the nitroxide decay constant in the tumor tissue of air-breathing (a1–a3) and carbogen-breathing (b1–b3) mice are shown in the figure. From Ilangovan et al. (77) with permission.

FIG. 4

Nitroxide reduction in rat brain. Panel A: Time-dependent OMRI image of methoxycarbonyl-PROXYL in the head region and image showing the decay rates. Panel B: Semi-logarithmic plot of image intensities of the contralateral hemisphere (○) and ischemic hemisphere (●) images. Panel C: The signal decay rates of the contralateral hemisphere (open bar) and ischemic hemisphere (closed bar). Panel D: Amount of total methoxycarbonyl-PROXYL in the contralateral hemisphere (open bar) and ischemic hemisphere (closed bar) determined using X-band EPR. Each value represents the mean ± SD of four rats. *P < 0.05 compared with the contralateral hemisphere. From Yamato et al. (101) with permission.