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Sarcopenia could result from the inability of an older individual to recover muscle lost during catabolic periods. To test this hypothesis, we compared the capacity of 5-day-refed 12- and 24-mo-old rats to recover muscle mass lost after 10 days without food. We measured gastrocnemius and liver protein synthesis with the flooding-dose method and also measured nitrogen balance, 3-methylhistidine excretion, and the gene expression of components of proteolytic pathways in muscle comparing fed, starved, and refed rats at each age. We show that 24-mo-old rats had an altered capacity to recover muscle proteins. Muscle protein synthesis, inhibited during starvation, returned to control values during refeeding in both age groups. The lower recovery in 24-mo-old rats was related to a lack of inhibition of muscle proteolysis during refeeding. The level of gene expression of components of the proteolytic pathways did not account for the variations in muscle proteolysis at both ages. In conclusion, this study highlights the role of muscle proteolysis in the lower recovery of muscle protein mass lost during catabolic periods.

This study was undertaken to determine whether the loss of muscle protein mass during aging could be explained by a reduced sensitivity of muscle protein synthesis to feeding and exercise. Male Wistar rats aged 12 and 24 mo were exercised by treadmill running for 4 mo. Protein synthesis was measured by the flooding dose method in tibialis anterior, soleus, and liver of conscious rested, trained rats and age-matched controls in the postprandial or in the postabsorptive state. No marked change with age could be detected in basal muscle protein synthesis. In contrast, protein synthesis was stimulated in adult but not in old rats by feeding in tibialis anterior and by exercise in soleus. In liver, protein synthesis was not modified by age but was stimulated by feeding and by exercise, which improved the response to feeding. We conclude that the impact of nutrition on muscle protein synthesis is blunted in old age, which could contribute to the age-related loss of nutrition-sensitive muscle proteins.

This study was carried out to analyze age-related changes on amino acid and insulin effects on muscle and liver protein synthesis. Conscious male rats, aged 12 (adult) and 24 (old) mo, were infused for 90 min with either saline, amino acids, or amino acids with insulin and glucose. Protein synthesis was measured during the last 15 min of infusion (flooding dose of valine with L-[2,3,4-3H]valine). Gastrocnemius protein mass was 29% lower in old rats than in adults. However, basal muscle absolute synthesis rates were unchanged with age, and fractional synthesis rates (FSR) were increased. Amino acids significantly stimulated muscle FSR to a similar extent (18-20%) in adult (P < 0.01) and old rats (P = 0.03 when variability introduced by muscle atrophy was taken into account by a variance-covariance analysis). Insulin did not elicit any additional effect. Liver protein synthesis did not change with age or in response to infusions. We conclude that, despite an age-related loss of muscle proteins, capacity of muscle protein synthesis to be stimulated is preserved with age.