Fig 1. Adaptation to oxidative stress in muscle cells:
the effect of antioxidant supplementation and age
Most studies have provided indirect evidence of an age-related increased production of free radicals in skeletal muscle with an increased production of products of protein, lipid and DNA oxidation and an enhanced oxidative damage to cellular molecules following some forms of exercise.
Studies by Weindruch's laboratory, using oligonucleotide arrays to screen changes in gene expression in resting skeletal muscle of mice and primates, have demonstrated an age-related upregulation of transcripts induced by ROS (reactive oxygen species).
Studies by Bejma and Ji (1999) have provided direct evidence of an age-associated enhanced free radical generation in resting skeletal muscle of aged rats.
This shift in redox state of muscle cells and increased oxidative damage will have profound effects on muscle.
Oxidised proteins will not function efficiently.
This will include proteins with a high percentage of sulphydryl groups such as the myosins, creatine kinase and some ATPases.
In addition, several transcription factors contain redox-sensitive sites, which may be particularly susceptible to damage.
Studies on isolated mitochondria demonstrated that a high proportion, although not all, of the elevated free radicals, which are detected in skeletal muscle of aged rats, were initially generated by the mitochondria and this increase in radical generation was exacerbated following exercise.
In addition, the rate of superoxide anion radical generation by sub-mitochondrial particles from skeletal muscle increases with age.
Various studies have demonstrated that skeletal muscle mitochondria accumulate oxidative damage with age.
For example, a significant accumulation of protein carbonyls and thiobarbituric acid reactive substances (TBARS, a marker of lipid peroxidation) and a significant fall in protein sulphydryl groups is reported to occur in mitochondria from muscle of aged mice.
More controversially, an accumulation of DNA deletions is seen in mitochondria of skeletal muscle with age.
Large mitochondrial deletions have been found to increase as much as 10,000-fold with age in several human tissues and the highest levels have been found in highly oxidative, post-mitotic tissues, particularly skeletal muscle, heart and brain with far lower or undetectable levels in mitotic tissues.
The cellular impact of the accumulation of such abnormalities is unknown.
Such an accumulation of deletions is not sufficient to explain the significant mitochondrial respiratory loss and implicate them as the main cause of age-related skeletal muscle dysfunction.
Focal, segmental electron transport system abnormalities are associated with high levels of mitochondrial deletions in ageing but the abundance of mitochondrial histochemical abnormalities is commonly reported as <1% of total fibres or per area of tissue section although there is some evidence to suggest that electron transport system abnormalities and mtDNA deletions are co-localised with muscle fibre atrophy, fibre splitting and oxidative damage in sarcopenia.
Biochemical analysis demonstrated decreases in NADH dehydrogenase and cytochrome-c-oxidase activities with age in post-mitotic tissues that rely heavily on the electron transport system and oxidative phosphorylation, such as skeletal muscle.
A significant decrease in the activity of mitochondrial encoded cytochrome-c-oxidase activity (COX) is the most commonly reported abnormal electron transport system phenotype and is sometimes observed with an increase in the entirely nuclear encoded succinate dehydrogenase (SDH) activity.
However, even though cytochrome c oxidase is partially encoded on the mitochondrial genome, age related decreases in cytochrome oxidase activity couldn't be attributed entirely to changes in the number on mtDNA deletions.
Oligonucleotide array analysis of resting skeletal muscle from aged mice and primates has demonstrated an age-associated decrease in the expression of genes involved in oxidative phosphorylation and scientists postulate that this is induced by mitochondrial dysfunction in aged animals.
This is supported by the induction of free radical-related transcripts in skeletal muscle of aged monkeys and the accrual of oxidative damage.
A number of studies in humans and rodents have demonstrated that the oxidative capacity of muscles is significantly impaired with age although the extent of these changes seems to be fibre type-specific.
In addition, during the ageing process, abnormal mitochondria are seen to accumulate although the mechanisms behind the defective ability to remove these organelles are not understood.
Thus, the presence of abnormal mitochondria in aged muscle may result a vicious circle of events.
A pathological increase in production of free radicals results in an accumulation of oxidation products and an accumulation of aberrant mitochondria.
Dysfunctional mitochondria produce increased amounts of free radicals and inefficient functioning of mitochondria may play a significant role in the physiological and structural changes seen in muscles of aged mammals.