Skeletal muscles, heart, and lung are the main sources of oxygen radicals in old rats.

The aim of this study was to compare rat tissues with respect to their reactive oxygen and nitrogen species (RONS) generating activities as a function of age. We quantified the RONS generation in vivo in young (6 months) and in old (30 months) male Sprague-Dawley rats using the recently developed spin trap 1-hydroxy-3-carboxy-pyrrolidine, applied intravenously. This spin trap reacts with superoxide radical and peroxynitrite yielding a stable spin adduct which is detectable by means of electron paramagnetic resonance (EPR) spectroscopy in frozen tissues. In old rats RONS generation was significantly increased compared to their young counterparts in the following order: blood<skeletal muscle<lung<heart, but did not change in intestine, brain, liver, and kidney. Experiments with isolated heart mitochondria showed a significant rate of RONS generation in succinate-supplemented mitochondria from old rats while no RONS were detected in mitochondria from young rats. This study identifies heart, lung, and skeletal muscle as the tissues with increased RONS formation as a function of age.