EFFECT OF AMINO-ACIDS ALONE OR WITH INSULIN ON MUSCLE AND LIVER PROTEIN-SYNTHESIS IN ADULT AND OLD RATS

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