Basal Level of p70S6k Phosphorylation Is Higher In Aged Rats And Inversely Related To Muscle Atrophy
Authors: Hibbert Jamie E , Salzano Matthew Q , Jones Terry E , Domire Zachary J 
Published: November 2023

IntroductionIt has been shown that older adults have an attenuated hypertrophic response to resistance exercise when compared to young adults. The reason for this attenuation is currently unknown. However, some evidence suggests it could be at the level of mechanotransduction. Remedying this decreased response to resistance exercise could halt and potentially provide an avenue to reverse the age related loss of muscle mass and strength, also known as sarcopenia. p70 ribosomal protein S6 kinase (p70S6k) has been shown to play a role in protein synthesis regulation and muscle growth and phosphorylation of this protein is one of the early responses to mechanical stimulationObjectiveThe purpose of this study is to determine the feasibility of using p70S6k phosphorylation as an indicator of mechanostimulation in skeletal muscle to compare robustness of response in young and old rats. We hypothesize that the young rats will have a more robust s6k phosphorylation response and this response will be positively related to muscle size.Methods9 young (6 mos) and 6 old (32–33 mos) male Fischer F344 x Brown Norway Hybrid rats were anesthetized using isoflurane gas and subsequently underwent a muscle stimulation protocol of the left hind limb The peroneal nerve was stimulated using wire electrodes to induce maximal contraction of the dorsiflexors for three sets of 10 eccentric contractions. The animals were then sacrificed and the muscles of the lower limbs were excised bilaterally. The tibialis anterior muscles were frozen immediately following extraction and later used for Western Blot analysis of p70S6k total protein content and p‐p70S6k (Thr389). The cross‐sectional area (CSA) of the unstimulated extensor digitorum longus was measured immediately as a marker of muscle atrophy.ResultsThe results of the Western Blot analyses were normalized to the mean of the unstimulated limb of the young animals. At baseline, there were no differences between groups in total p70S6k content, but the old animals showed 3.33 fold greater levels of phosphorylated p70S6k (p‐p70S6k). It was found that following stimulation p‐p70S6k increased in the young group 7.16 fold over baseline levels. The old group had 12.3 fold increase in p‐p70S6k when normalized to the young unstimulated leg, but when the change was examined relative to the basal level the change was less extreme (3.65 fold). There was a significant relationship between CSA and both basal and simulated p‐p70S6k in the old animals. However this relationship was inverse what was expected. The animals with the most significant atrophy had the highest level of basal and stimulated p‐p70S6k. When examining the ratio of phosphorylation in the stimulated limb relative to the control limb, there was no relationship to muscle size.ConclusionsIt appears from these data that there is a robust p70S6k phosphorylation response in both young and aged rats in response to mechanical stimulation. However, the relationship between CSA and p‐p70S6k is opposite what was expected in aged rats. Elevated basal levels of p‐p70S6k have not been shown previously. Further investigation is needed to determine what role this could play in reduced response to loading.Support or Funding InformationPartially funded by a Graduate Student Grant‐in‐Aid from the American Society of Biomechanics and a Matching Dissertation Grant from the International Society of Biomechanics