EPR oximetry as a method for repetitive measurements of in vivo kidney tissue oxygenation (890.2)

Abstract

Kidney tissue oxygenation is heterogeneous with oxygen levels normally being greater in the superficial cortex and lowest in the inner medulla. Decreased oxygenation in the kidney has been implied in the pathology of several diseases. However, there is currently no method available to repetitively monitor kidney oxygenation regionally using minimally invasive procedures. Therefore, we evaluated implantable lithium phthalocyanine (LiPc) probes, which closely correlate between EPR peak‐to‐peak line width and oxygen availability. LiPc probes were implanted in the cortex (left kidney) and medulla (right kidney) in the same mouse and EPR spectra were acquired using an L band scanner (1 GHz). Mice were measured repetitively over 45 days and during inhalation of air (21% oxygen) or an acute measurement of air and nitrogen mixture (10% oxygen). A 1 G/cm gradient was applied in order to separate the signals from the two probes. The signals were derived from 40 consecutive sweeps and a peak‐to‐peak line width comparison of the EPR spectra was used to convert the signal to an applicable oxygen tension in MATLAB. Cortical and medullary oxygenation was stable over the 45 day period (56±7 mmHg and 43±6 mmHg respectively). However, acute 10% oxygen inhalation significantly reduced oxygenation in both tissues (cortex 56±6 to 34±2 mmHg n=15 p<0.05 and medulla 42±5 to 29±3 mmHg n=7 p<0.05). In conclusion, EPR oximetry using LiPc probes implanted in discrete kidney structures can be used to repetitively measure regional tissue oxygenation. This minimally invasive method is particularly well suited for conditions of reduced tissue oxygenation since this increases the signal intensity which enables the quantification of the EPR signal to absolute oxygenation values.