Release of superoxide from skeletal muscle of adult and old mice: an experimental test of the reductive hotspot hypothesis

Increased extracellular generation of reactive oxygen species (ROS) as a result of increasing reliance on glycolytic metabolism by old mitochondria-rich tissues has been claimed to contribute to the propagation of oxidative damage during aging (the reductive hotspot hypothesis), but the process has not been examined experimentally in old animals. Superoxide activity in the extracellular fluid of gastrocnemius muscle and markers of oxidation in blood and the liver were examined in adult and old mice at rest and following a period of demanding isometric contractions. The activity of superoxide in muscle microdialysates did not differ between adult and old mice at rest, but during contractile activity, there was a significant increase in the superoxide activity in microdialysates from adult muscle but no increase in microdialysates from old muscle. At rest, the liver of old mice contained an increased malonaldehyde content and a decreased protein thiol content in comparison with adult mice, but following the contraction protocol, only the adult mice showed significant, transient increases in the serum and liver malonaldehyde content and a decrease in liver glutathione and protein thiol content. Further studies revealed that the lack of superoxide release from contracting muscle of old mice was not due to reduced force generation by these muscles. These data provide no evidence for an increased extracellular superoxide in resting or contracting skeletal muscle of old mice, or that release of superoxide from muscle contributes to oxidation of blood components in the liver in old mice as is predicted from the reductive hotspot hypothesis.