A surrogate measure of whole body leucine transport across the cell membrane

Based on a mass-balance model, a surrogate measure of the whole body leucin e transport into and out of cells under steady-state conditions was calcula ted as u/Delta TTR, where u is the infusion rate of(stable label) leucine t racer and Delta TTR is the difference between the tracer-to-tracee ratio of extracellular and intracellular leucine. The approach was evaluated in ten healthy subjects [8 males and 2 females; age, 31 +/- 9 (SD) yr; body mass index, 24.0 +/- 1.6 kg/m(2)] who received a primed (7.58 mu mol/kg) constan t intravenous infusion (7.58 mu mol kg(-1) h(-1)) of L-[1-C-13]leucine over 180 min (7 subjects) or 240 min (3 subjects). Five subjects were studied o n two occasions greater than or equal to 1wk apart to assess reproducibilit y. Blood samples taken during the last 30 min of the leucine infusion were used to determine plasma leucine concentration (129 +/- 35 mu mol/l), TTR o f leucine (9.0 +/- 1.5%), and TTR of alpha-ketoisocaproic acid (6.7 +/- 0.8 %). The latter TTR was taken as the measure of the free intracellular leuci ne TTR. The whole body inward and out ward transport was 6.66 +/- 3.82 mu m ol kg-l min-l; the rate of leucine appearance due to proteolysis was 1.93 /- 0.24 mu mol kg(-1) min(-1). A positive linear relationship between the i nward transport and plasma leucine was observed (P < 0.01), indicating the presence of the mass effect of leucine on its own transport. The transport was highly variable between subjects (between-subject coefficient of variat ion 57%) but reproducible (within-subject coefficient of variation 17%). We conclude that reproducible estimates of whole body transport of leucine ac ross the cell membrane can be obtained under steady-state conditions with e xisting experimental and analytical procedures.