COMPARTMENTAL APPROACH FOR EVALUATION OF PLASMA KINETICS AND (CO2)-C-13-EXHALATION AFTER ORAL LOADING WITH L-[1-C-13]LEUCINE

A seven compartment model was applied for evaluation of oral L-[1-C-13 ]leucine loading tests (38 mu mol/kg body wt.) in healthy volunteers. The model comprises transport and absorption in stomach and gut into a central L-leucine-compartment which is connected to a protein compart ment and to the compartment of the corresponding 2-oxo acid. CO2 relea se from the latter occurs in a fast and a slow compartment into the ce ntral CO2 compartment for exhalation. Using the fmins routine of MATLA B for parameter estimation, a good agreement was obtained between calc ulated and actually measured kinetics of C-13-labelled metabolites and a mean in vivo L-leucine oxidation of 0.365 +/- 0.071 mu mol/kg per m in (n = 5) was computed. Plausibility of the model was checked by pred icting in vivo leucine oxidation rates from primed continuous infusion tests (priming. L-[1-C-13]leucine, 5 mu mol/kg; (NaHCO2)-C-13, 1.2 mu mol/kg; infusion: L-[1-C-13]leucine, 5 mu mol/kg per h). In 5 tested volunteers, the experimental L-leucine oxidation rate amounted to 0.35 8 +/- 0.105 mu mol/kg per min versus predicted 0.324 +/- 0.099 mu mol/ kg per min. Possible causes for some observed intraindividual variatio ns are discussed.