Responses of circulating urea cycle and branched-chain amino acids to feeding in adult and aged Fischer-344 rats

Amino acid concentrations in plasma and blood were compared between adult (6 months) and aged (22 months) male rats in fed and food-deprived (fasted) states. Since feeding is known to elevate plasma amino acids, rats were sampled after food had been freely available to them and after food was withheld for 20 hours. Fourteen amino acids increased (8 to 74%) in blood and plasma in fed rats compared to food-deprived rats. Blood concentrations of four amino acids (cystine, aspartate, threonine, and phosphoserine) were increased in aged compared to adult rats, while tyrosine was 10% lower. The interaction between fed state and age was significant for the branched-chain amino acids (BCAA) as they increased 23% in the blood and plasma of fed compared to food-deprived adult rats; however BCAA were not affected by feeding state in the aged rats. Plasma arginine concentration was 12% lower (p<0.001) and citrulline was 20% higher (p<0.005) in aged compared to adult rats, and consequently the citrulline:arginine ratio was elevated by 36% in aged rats. Our results demonstrate that, although the total arginine in whole blood is not altered or is increased with aging, less of the arginine is transported in the plasma of aged compared to adult rats. Furthermore, these results indicate altered metabolism of the urea cycle amino acids and BCAA with aging in the rat. (C) 1997, Editrice Kurtis.