| Summary: | The hypothesis that age related muscle dysfunction is a result of diminished anabolic stimuli resulting in impaired amino acid uptake was tested as follows. Small muscle fibre bundles isolated from the extensor digitorum longus and the soleus of young, mature and elderly mice were used to investigated the effects of age and dihydrotestosterone (DHT) treatment on the expression and function of SNAT2 and LAT2. C2C12 muscle cells were cultured and tested for both SNAT2 and LAT2 function with DHT and various inhibitors. At all ages investigated, amino acid uptake was significantly higher in slow-twitch fibres than in the fast-twitch fibres. Ageing led to a decrease in amino acid uptake and protein synthesis in both fibre types. The decline was greater in the fast-twitch than in the slow-twitch fibres and was accompanied by a reduction in the expression of both SNAT 2 and LAT2 proteins. Mirroring the uptake results the decrease in SNAT 2 protein was greater in the fast-twitch fibres than in the slow-twitch fibres. The rends seen in SNAT2 uptake and expression were also seen with fluorescence imaging of muscle sections. Ageing had no effect on mRNA levels of LAT2, but has shown a significant reduction in SNAT2. Treating the muscle fibre bundles with physiological concentrations of DHT significantly increased amino acid uptake and SNAT2/LAT2 protein expression in the fast twitch fibres of all elderly group mice. There was a trend toward increased uptake and expression in response to DHT treatment in all groups. Chloroquine and flutamide treatment abolished the increased amino acid uptake induced by DHT treatment. Blocking SNAT2 in C2C12 cells with MeAIB resulted in decreased LAT2 response in starvation conditions, suggesting the role of these as transceptors. It is proposed that reduced anabolic stimuli in ageing leads to impairment of SNAT2/LAT2 recruitment leading to sarcopenia.
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