In vivo oximetry using a nitroxide‐liposome system

Liposomes containing the deuterated, charged, aqueous soluble nitroxide 4‐trimethylammonium‐2,2,6,6‐tetramethylpiperidine‐d 16 ‐1‐oxyl (d‐Cat 1 ) were used as probes to measure oxygen concentrations in vivo . Following intramuscular or intraperitoneal injection of the liposome suspension, ICR mice were placed over the surface probe of a low frequency (1.1 GHz) electron paramagnetic resonance (EPR) spectrometer. The linewidth of the deuterated nitroxide is sensitive to changes in the dissolved oxygen concentration: this parameter was calibrated separately so that linewidths measured in the injected mice could be converted into oxygen tensions. This technique detected substantial changes in pO 2 as the oxygen content of the breathing gas was changed from 21 to 85 to 0%. Intravenous injection of the liposomes also is possible, and the liposomes accumulate in the liver and spleen, where detectable, oxygen‐sensitive EPR signals can be measured. © 1991 Academic Press, Inc.