Tracer kinetics are of limited value to measure in vivo protein synthesis and degradation rates in muscle of anesthetized rats

Abstract

Measurement of amino acid kinetics using muscle exchange rates of labeled phenylalanine and leucine has been successfully used to estimate in vivo protein synthesis and degradation rates in human forearm and in hindlimb of large laboratory animals. No good method to measure protein breakdown in muscle of small laboratory animals is available, and we therefore investigated whether this technique can be applied to rats. Using [ 3H]phenylalanine-exchange measurements, protein synthesis and degradation rates were measured in muscle of fed and 2-day starved rats. Protein synthesis rates obtained in this way were compared with rates measured with the phenylalanine flooding-dose technique in sham-cannulated (including anesthesia and surgery) fed and fasted rats and in awake fed rats. Using the [ 3H]phenylalanine-exchange method, protein synthesis rates in 2-day starved rats were increased to 292% and protein degradation rates to 217% of the values obtained in fed rats. However, due to a high variation, these changes were not statistically significant. Results obtained with the flooding-dose technique indicate that 2-day starvation reduced protein synthesis rates to 61% of the fed value. However, protein synthesis rates measured with the flooding-dose technique were decreased by 40% in sham-cannulated fed rats in comparison to awake fed rats. An additional 19% reduction in adenosine triphosphate (ATP) concentration in the muscle of the same rats shows that the procedure necessary to apply the exchange measurements to rats has a significant influence on the physiology of the muscle. We therefore conclude that [ 3H]phenylalanine-exchange measurements as applied in this study are of limited value to estimate in vivo protein synthesis and degradation rates of individual tissues in rats.