1. We have used l-[1-13C,15N]leucine as the substrate tracer to study leucine and muscle protein metabolism across the forearm of eight normal fasting adults.

2. The rates of protein synthesis and breakdown, de- and re-amination of leucine, and the oxidative de-carboxylation of its keto acid were calculated directly from the arteriovenous metabolite balances and isotope dilutions as described by the metabolic model.

3. The results were compared with those obtained previously when subjects were fed. The effects of fasting on protein and leucine metabolism were a significant decrease in protein synthesis from 127 (sem 11; n = 6) to 70 (sem 6; n = 12) nmol of leucine min−-1 100 ml−-1 of forearm tissue (P < 0.001) and a marked decrease in leucine catabolism in the forearm muscle.

4. This model has demonstrated that each subject was in negative protein balance across the forearm during fasting while positive during feeding, the mean values being −29(sem 5; n = 12) and + 39(sem 9; n = 6) nmol of leucine min−-1 100 ml−-1 of forearm tissue respectively.

5. These results are sufficiently encouraging to suggest a role for this model in future studies on muscle protein metabolism.

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