Reaction kinetics analysis to estimate in vivo decay rate of EPR signals of a nitroxide radical in the brain and the inferior vena cava of rats

Abstract

To clarify the degree of the influence of the peripheral organs on the temporal changes in the electron paramagnetic resonance (EPR) signal intensity of a nitroxide radical permeating the bloodbrain barrier, 3-hydroxymethyl-2,2,5,5-tetramethylprrotidine-l-oxyl (hydroxymethyl-PROXYL), in the brain, temporal changes in the EPR signal in the brain or inferior vena cava of rats were measured by an in vivo 700 MHz radio-frequency EPR spectrometer equipped with a bridged loop-gap resonator ora surface-coil-type resonator. In all measurements, good linearity was observed on semilogarithmic plots of the signal intensity against time after the hydroxymethyl-PROXYL injection. From these plots, the reaction rate and the initial level of hydroxymethyl-PROXYL in the brain and the vena cava were calculated. A mathematical model expressing the nitroxide radical concentration in the brain, which is connected to other organs via the circulatory system, was made. With this model and the results of the EPR measurements, the degrees of influence of the nitroxide reduction in the brain and the other organs were simulated. It was found that the reaction rate (equal to log2/halflife) of hydroxymethyl-PROXYL observed in the brain reflected the reduction of hydroxymethyl-PROXYL there and was not influenced by the reduction in other organs.

EPRI to estimate the in vivo intracerebral reducing ability in adolescent rats subjected to neonatal isolation

“…A previous phantom study confirmed that the FWHM‐to‐gradient ratio could serve as an index of the spatial resolution (9). In our previous study the FWHM‐to‐gradient ratio of the EPR images of the nitroxide‐treated rats was about 2 mm (5, 6, 9).…”

“…A 700 MHz EPRI system was constructed at our institute (as previously described in detail (5, 6, 8–10)) and consisted of a main electromagnet, a pair of magnetic field scan coils, a pair of magnetic field gradient coils, a pair of magnetic field modulation coils, a two‐gap loop‐gap EPR resonator (11) (inner diameter = 41 mm; axial length = 10 mm; unloaded Q = 480), RF circuits for homodyne detection (ca. 700 MHz), and intermediate frequency circuits for lock‐in detection at a modulation frequency of 100 kHz.…”

“…In vivo antioxidant ability (i.e., reducing ability) is subject to a balance between the amount of oxidants and reductants. An in vivo temporal electron paramagnetic resonance imaging (EPRI) technique was recently developed that can be applied to the brain of a rat that has been intravenously injected with a blood–brain barrier (BBB)‐permeable nitroxide radical, 3‐hydroxymethyl‐2,2,5,5‐tetramethylpyrrolidine‐1‐oxyl (hydroxymethyl‐PROXYL) (5, 6). With this imaging technique it is possible to measure the decay rates of a nitroxide radical in multiple regions of the brain simultaneously.…”

Evaluation of Antioxidative Effects of Sesamin on the In vivo Hepatic Reducing Abilities by a Radiofrequency ESR Method

“…When peak-to-peak line width of the ESR signal of TEMPOL exhibits no temporal changes in a target organ, the peak-to-peak height can then be used as the signal intensity. [16][17][18][19][20] In the target organs, changes in the signal intensity correspond to changes in the TEMPOL concentration. When there is good linearity in a semi-logarithmic plot of the signal intensity with high reproducibility after a TEMPOL injection, this is indicative of the pseudo-first-order kinetics of the TEMPOL decrease in the target organ.…”

“…Yokoyama et al proposed that estimations of the reducing ability of nitroxyl radicals could be calculated by using an equation that expresses the nitroxyl radical concentration in the target organ. 19,20 The target organ (Xn) as a function of time (t) approximates Xn = Xn 0 exp (-k0nt), where Xn 0 is the initial value of the concentration and k0n is the first-order rate constant (equal to log 2/half-life). The half-lives of TEMPOL in the target organs were used as the parameter to estimate the decay rate of TEMPOL, as this parameter was not influenced by reduction in the other organs.…”