The levels of PNPase. Western blot analysis of YC, YE, OC, and OE muscles. Values for PNPase are reported as relative densities that have been corrected for loading-level differences, as judged by α-tubulin levels. PNPase levels increased with age; however, exercise blocked this induction. Values are expressed as means ± SD for six animals per group (**P < 0.01).