Simultaneous measurement of NO· and PO₂ from tissue by in vivo EPR

We describe a technique that utilizes electron paramagnetic resonance (EPR) to measure NO· and pO₂ directly, and non-invasively, from tissue in vivo. Diethyldithiocarbamate (DETC) was injected with iron so as to complex with NO· in the tissue. Gloxy (an oxygen-sensitive, paramagnetic material) was also implanted into the tissue of interest (brain or liver). Because the signals arising from gloxy and NO-Fe-(DETC)₂ did not overlap, they could be monitored and measured simultaneously in vivo. The gloxy was not responsive to NO· and/or DETC. As model systems we either injected SNP (an NO· donor) into animals and monitored NO· and pO₂ simultaneously from brain, or endotoxin (lipopolysaccharide; LPS) was injected in order to induce a septic episode and NO· and pO₂ measured from liver. We found a close correlation between levels of SNP-derived NO· and brain pO₂ in vivo. During sepsis, liver pO₂ decreased dramatically at 300-360 min after endotoxin injection, and this coincided with decreases in mean arterial blood pressure and increased tissue NO· detected. These studies demonstrate the potential usefulness of this technique for making direct in vivo measurements of NO· and pO₂ simultaneously from tissue.