Leucine metabolism in perfused rat skeletal muscle during contractions

An isolated single rat hindlimb muscle preparation was used to examine leucine metabolism during steady-state conditions as a function of metabolic rate (VO2) and leucine concentration. The rates of muscle leucine uptake and leucine oxidation (measured as alpha-decarboxylation) were dependent on leucine delivery. At a physiological leucine concentration (0.1 mM), leucine uptake and alpha-ketoisocaproic acid (KIC) release during rest was 12.8 +/- 0.4 and 1.86 +/- 0.06 nmol.min-1.g-1 g, respectively. Leucine oxidation was 2.35 +/- 0.11 nmol.min-1.g-1 (n = 24) and if fully oxidized could account for only 3-4% of the resting VO2. This fraction was reduced to approximately 1% during contractions. The rate of leucine oxidation progressively increased, up to two to three times above rest (6-7 nmol.min-1.g-1), during contractions of graded frequency (7.5, 15, 30, 45, and 60 tetani/min) in a manner related to the eightfold increase in VO2 of the mixed fiber muscle. The fraction of muscle leucine uptake that was transaminated (i.e., leucine decarboxylation + KIC release) increased from 33% at rest to approximately 60% during contractions. The increase in leucine oxidation during contractions was probably primarily due to the high oxidative fast-twitch, red muscle mass, whose VO2 was estimated to increase up to 24-fold above rest. On the basis of our observed rates of muscle leucine alpha-decarboxylation, it is reasonable to attribute the rates of whole-body leucine oxidation of nontrained individuals during exercise to leucine oxidation by the working muscle.