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1 Department of Physiology & Biophysics, University of California, Irvine, Irvine, CA, USA
* To whom correspondence should be addressed. E-mail: gradams{at}uci.edu.
Sarcopenia is an age related loss of muscle mass and strength. The aged can increase various measures of muscle size and strength in response to resistance exercise (RE) but this may not normalize specific tension. In rats, aging reduces the hypertrophy response and impairs regeneration. In this study, we measured cellular and molecular markers, indicative of muscle hypertrophy, that also respond to acute increases in loading. Comparing 6 and 30 month old rats, the aims were to: 1) determine if these markers are altered with age; 2) identify age sensitive responses to acute RE. The muscles of old rats exhibited sarcopenia involving a deficit in contractile proteins and decreased force generation. The RNA to protein ratio was higher in the old muscles suggesting a decrease in translational efficiency. There was evidence of reduced signaling via components down stream from the insulin/IGF-I receptors in old muscles. The mRNA levels of myostatin and SOCS2, negative regulators of muscle mass, were lower in old muscles but did not decrease following RE. RE induced increases in the mRNAs for IGF-I, MGF, cyclin D1 and SOCS3 were similar in old and young muscles. RE induced phosphorylation of the IGF-I receptor and Akt increased in young but not old muscles while that of S6K1 was similar for both. The results of this study indicate that a number of components of intracellular signaling pathways are sensitive to age. As a result, key anti-catabolic responses appear to be refractory to the stimuli provided by resistance exercise.
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