Estimation of tissue protein synthesis in sheep during sustained elevation of plasma leucine concentration by intravenous infusion.

1. The fractional rate of protein synthesis (FSR) was determined in skeletal muscle, liver, rumen and cardiac muscle of wether sheep by continuous intravenous infusion of L-[4,5-3H]leucine accompanied by infusion of 0, 7.6, 15.2 or 22.8 mmol leucine/h (three sheep per treatment). FSR was calculated assuming plasma (ksp) or intracellular (ksi) leucine-specific radioactivity (SRA) was representative of the leucine precursor pool SRA for protein synthesis. 2. Plasma leucine concentration (plateau) was linearly related to leucine infusion rate, 22.8 mmol/h evoking a tenfold increase in plasma concentration. 3. Difference between plasma leucine SRA and intracellular leucine SRA in all tissues diminished as plasma leucine concentration increased. 4. There were significant differences between ksi and ksp estimates for liver and rumen in control sheep. 5. As leucine infusion rate increased, differences between ksi and ksp diminished in all tissues. With increasing leucine infusion, in liver ksi decreased and ksp was increased, in rumen ksi decreased and ksp was stable, while in cardiac and skeletal muscle ksi and ksp both increased. 6. At a leucine infusion rate of 22.8 mmol/h, mean ksp and ksi respectively were: 11 (SE 2), 13 (SE 1); liver 19 (SE 2), 21 (SE 2); cardiac muscle 3.6 (SE 0.4), 3.8 (SE 0.3); skeletal muscle 4.1 (SE 0.2), 4.5 (SE 0.5) and did not differ significantly in any tissue.