Comparison of Arterial-Venous Balance and Tracer Incorporation Methods for Measuring Muscle Fractional Synthesis and Fractional Breakdown Rates.

Loss of muscle mass in response to injury or immobilization impairs functional capacity and metabolic health, thus hindering rehabilitation. Stable isotope techniques are powerful in determining skeletal muscle protein fluxes. Traditional tracer incorporation methods to measure muscle protein synthesis and breakdown are cumbersome and invasive to perform in vulnerable populations such as children. To circumvent these issues, a two-bolus stable isotope amino acid method has been developed; although, measured rates of protein synthesis and breakdown have not been validated simultaneously against an accepted technique such as the arterial-venous balance method. The purpose of the current analysis was to provide preliminary data from the simultaneous determination of the arteriovenous balance and two-bolus tracer incorporation methods on muscle fractional synthesis and breakdown rates in children with burns. Five were administered a primed-constant infusion of L-[ 15N]Threonine for 180 minutes (Prime: 8 µmol/kg; constant: 0.1 µmol·kg -1·min -1). At 120 and 150 minutes, bolus injections of L-[ring- 13C6]Phenylalanine and L-[ 15N]Phenylalanine (50 µmol/kg each) were administered, respectively. Blood and muscle tissue samples were collected to assess mixed muscle protein synthesis and breakdown rates. The preliminary results from this study indicate there is no difference in either fractional synthesis rate (mean ± SD; arteriovenous balance: 0.19 ± 0.17 %/h; tracer incorporation: 0.14 ± 0.08 %/h; P = 0.42) or fractional breakdown rate (arteriovenous balance: 0.29 ± 0.22 %/h; tracer incorporation: 0.23 ± 0.14 %/h; P = 0.84) between methods. These data support the validity of both methods in quantifying muscle amino acid kinetics; however, the results are limited and adequately powered research is still required.