Oximetry in cells and tissues using a nitroxide-liposome system

In order to avoid the complication of reduction of nitroxides in biological media during oxygen measurements, liposomes containing a water-soluble nitroxide, 2,2,6,6-tetramethyl-piperidine-N-oxyl-4-trimethylammonium (Cat1), were used in studies of oxygen consumption by thymus-bone-marrow cells. The superhyperfine structure of Cat1 contained in liposomes was found to be sensitive to oxygen concentration in a fashion similar to that of free Cat1. Measurements of cellular respiration using Cat1 contained in liposomes agreed well with the results obtained using free Cat1. Using this nitroxide-liposome system, the respiration of liver slices was measured successfully, whereas such measurements using free cat1 were complicated by rapid reduction of the nitroxide. This nitroxide-liposome system also could be used in conjunction with a membrane permeable nitroxide and an extracellular broadening agent to measure intracellular and extracellular oxygen concentrations simultaneously.