Tracer methodology in whole body and organ balance metabolic studies: plasma sampling is required - A study in post-absorptive rats using isotopically labeled arginine, phenylalanine, valine and leucine

Background and aims: Radioactive and stable amino acid isotopes are frequen tly used in metabolic research. Blood cells contain amino acid transporters , which may influence tracer distribution in blood. The aim of this study w as to determine whether plasma or whole blood specific activity or enrichme nt of amino acid tracers should be used in the calculation of whole body an d organ production rates. Methods: Seven male Wistar rats were infused with L-[2,3-H-3]-Arginine, L-[ 2,6-H-3]-Phenylalanine, L-[3,4-H-3]-Valine, and [L-[4,5-H-3]-Leucine, Whole body and portal drained visceral, hepatic and renal production rates of ar ginine, phenylalanine, valine and leucine were determined in plasma and in whole blood. Results: Amino acid tracers that equilibrate well between plasma and blood cells (for instance phenylalanine, valine and leucine) yield similar whole body production rates when whole blood or plasma is sampled. Also, organ pr oduction rates measured using these amino acid tracers are consistent. Howe ver, a discrepancy exists between the whole body production rate and the su m of PDV, hepatic and renal production rates. When tracers are used that do not equilibrate well between plasma and blood cells (for instance arginine ) the use of whole blood specific activity in the calculations yield overes timations of whole body and organ production rates. Conclusion: From our data we recommend plasma sampling and strongly advise against whole blood sampling in metabolic organ balance studies in which am ino acid tracers are used. (C) 2000 Harcourt Publishers Ltd.