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.

The efficiency of colostral protein digestion was studied in nine newborn lambs fed one meal of bovine colostrum 3 h after birth. The results were compared with those obtained in two unfed lambs and four lambs fed bovine milk. The protein and peptide composition [immunoglobulins G1 and (IgG1), beta-lactoglobulin, alpha-lactalbumin, caseins and peptides resulting from casein hydrolysis] of digesta, gastrointestinal tissues, blood and urine were determined in samples taken 0.75 or 4 h after feeding. The amounts of ingested proteins in lambs fed colostrum were much higher than in those fed the milk diet, and their abomasal emptying was faster. alpha-Lactalbumin was highly degraded by abomasal and intestinal proteases, whereas beta-lactoglobulin and in particular the immunoglobulins were less sensitive. The gastric emptying of caseins was delayed in and the kinetics of appearance of peptides originating from casein hydrolysis was comparable to that observed in lambs fed milk and in 1-mo-old preruminant calves. Thirty-five percent of dietary amino acids ingested as colostrum were available within 4 h for amino acid metabolism; this percentage was 54% in the milk-fed lambs. In the lambs fed colostrum, these amino acids were provided by beta-lactoglobulin, casein and IgG1 (0.52, 0.43 and 0.30 g/kg body wt, respectively), whereas in milk-fed animals casein and beta-lactoglobulin were the most important sources of these amino acids (0.40 and 0.20 g/kg, respectively).

Our aim was to analyze mechanisms involved in the adaptation of protein metabolism to food deprivation and refeeding in adult rats. Twelve-month-old rats, which had been food-deprived for 113 h and refed for 6 h, were injected subcutaneously with a flooding dose of valine (with 50% [1-13C]-L-valine) to measure in vivo protein synthesis in tibialis anterior, soleus and liver. Protein and RNA contents were also measured. In both muscles, protein mass was maintained during food deprivation. Due to a drop in protein synthetic capacity (Cs), total and myofibrillar protein synthesis rates were reduced in food-deprived rats and were not stimulated by a 6-h refeeding. In contrast, protein levels were maintained lower than RNA levels in liver during food deprivation, and Cs was higher than in fed rats. Protein synthesis rates and ribosomal efficiency were reduced in food-deprived rats. Due to maintenance of protein synthetic capacity, there was a rapid stimulation of liver protein synthesis with refeeding, which induced a significant rise in protein mass (also related to an inhibition of protein degradation). In conclusion, coordinated responses of liver and muscles allowed a sparing of muscle proteins during food deprivation and a rapid recovery of liver proteins during refeeding. Control of ribosome quantity could play a critical role in these adaptations in tissue protein synthesis in adult rats.

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.

RésuméTrois aliments d'allaitement (Témoin, RD et