Spin probe reduction in cells and tissues. 
The reduction rates of different nitroxides in rat lung tissue were measured by electron paramagnetic resonance.  We confirmed that the reduction rate of nitroxide spin probe molecules is coupled with their structure and transport characteristics.  Oxygen was found to slow down the reduction rate of nitroxides in rat lung tissues.  On the basis of these findings it can be concluded that most nitroxide properties described for homogeneous systems--cells and tissue homogenates--are also valid for heterogeneous systems, which is important for the application of nitroxides as metabolically active NMR contrast agents.