Amino acid degradation and effect of leucine on pyruvate oxidation in rat atrial muscle

Both left and right atria from fasted rats produced significant amounts of ¹⁴CO₂ during incubation with U-¹⁴C-labeled leucine, isoleucine, valine, alanine, glutamate, glutamine, aspartate, asparagine, proline, threonine, or lysine. This pattern of amino acid metabolism resembles that of skeletal muscle. Production of ¹⁴CO₂ from [1-¹⁴C]leucine was 2.5-fold greater in atria from fasted than from fed rats and was due to greater α-ketoisocaproic dehydrogenase activity in the tissue from fasted animals. At normal plasma concentrations, leucine reduced the oxidation of glucose and lactate in atria from fasted but not from fed rats by inhibiting pyruvate oxidation and without altering the rate of glycolysis. Leucine also reduced glucose oxidation when added in the presence of ketone bodies or other amino acids and stimulated the release of lactate into the medium. Although the leucine skeleton can be completely oxidized to CO₂ and thus can be serve as an alternative fuel in fasting in place of glucose, oxidation of leucine (like glucose or lactate oxidation) accounts only for a very small fraction of the total oxygen consumption of the resting atria.