Nitrogen retention in rats fed on diets enriched with arginine and glycine. 1. Improved N retention after trauma.

1. Nitrogen retention was measured in adult rats (250-350 g) subjected to the trauma of hind-leg fracture and given diets with or without arginine plus glycine supplementation. Observations were also recorded on excretion of creatine, creatinine, allantoin, and orotic acid. Liver and skeletal muscle transaminase activities were also determined. 2. When traumatized rats weighing approximately 250 g were given a diet with 200 g casein/kg, supplemented with 20 g arginine and 10 g glycine/kg (EC diet) or a casein diet made isonitrogenous with the EC diet by addition of aspartic acid (C diet), a 60-70% increase in N retention was observed for the first 5 d post-injury for animals consuming the EC diet. A soya-bean (S) diet, isonitrogenous to the diet containing 20% casein, supplemented with arginine and glycine was as effective as the EC diet in promoting significantly better N retention of traumatized rats (350 g) in comparison to rats given the C diet. 3. When the dietary casein content was reduced to 100 g/kg, supplements of 10 g arginine and 5 g glycine or 20 g arginine and 10 g glycine/kg did not improve N retention. It is suggested that both protein quality and protein quantity are important following injury. 4. An increased excretion of creatine was observed in traumatized rats given the high-protein diets supplemented with arginine and glycine. No consistent changes were noted for urine creatinine. 5. 5. Urine allantoin levels remained stable after leg-fracture in rats consuming either the C or EC diets. Differences in the levels of urine orotic acid were found during both the pre- and post-injury periods in rats given the C, EC or S diets. 6. The mechanisms responsible for the improved N retention of traumatized rats consuming the high-protein diets with supplements of arginine and glycine may be related to the role of arginine both as a constituent of muscle tissue and as an intermediate in the urea cycle. 7. In traumatized rats fed the C or EC diets, liver transaminase activity increased whereas the transaminase activity in skeletal muscle decreased. These results support the recent concept that the increased excretion of N following injury arises from diminished reutilization of amino acids by muscle tissue without an acute increase in the rate of muscle catabolism.