Aim: Using an in vitro assay we assessed whether the acute exposure to essential amino acids (EAA) enriched mixture (EAAem) is able to activate mTOR signaling pathway (mTOR and p70S6K) after prolonged supplementation with the same mixture in soleus muscle of adult and elderly rats. Method: Adult (9 months of age at the end of treatment, n=10) and elderly (19 months of age at the end of treatment, n=10) male Wistar rats were chronically treated with EAA enriched mixture for 6 months (1.5 gr/kg/day in tap water). For each group 5 untreated rats served as matched control. At the end of treatment rats were grouped as follows (n=5 each): AD adults untreated controls ; EL, elderly untreated controls; AD+EAAem, AD supplemented with EAAem; EL+EAAem, EL supplemented with EAAem; AD+EAAem incubated with EAAem (1% or in Krebs solution for 30 min); EL+EAAem incubated with EAAem. Following treatment the activation level of mTOR and p70S6K was measured by Western blot. Results: The basal level of mTOR and p70S6K activation appeared to be higher in untreated AD compared with EL. Following incubation with EAAem a significant change in the level of p70S6K activation unlike mTOR was observed in EL whereas no change was observed in AD. In chronically treated AD muscles the basal level of p70S6K unlike mTOR activation appeared to decrease and the acute exposure to EAAem produced a significant reduction of mTOR activation. Contrarily to untreated rats, in chronically treated EL muscles the acute exposure to EAAem produced a significant activation of mTOR unlike p70S6K. Conclusion: Results in the young indicate a higher basal level of activation and a reduced responsiveness of the pathway to acute and chronic exposure to EAA enriched mixture. On the contrary, in the elderly, a lower basal level of activation was associated with a higher responsiveness to EAA enriched mixture. In particular although with a different timing, acute exposure activates mTOR signaling even following prolonged supplementation.
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