Effect of repeated stress on a number of plasma amino acids and related compounds in young and old rats

A number of amino compounds were studied in the plasma of young and old rats exposed to an immobilization stress on 3 different days. Baseline values were similar for all compounds between age groups except for slightly higher phenylalanine levels in old animals. Most differences were seen in the stress responses. Stress response intensities for alanine, aspartic acid, methionine, phenylalanine, and tyrosine were generally higher in old rats and recovery times from stress were usually longer. Although the stress responses decreased in young rats for some compounds over the three exposures, old rats showed no or less adaptation during this time. All the other amino acids showed no marked age effects. This study shows that certain plasma amino acids can serve as selective indicators of biological age and that their stress responses might be better markers of the aging process than their resting levels.