Effects of supplemental dietary energy on leucine metabolism in sheep

1. Mixed-breed wethers (40-50 kg), 9 months old, were maintained on high-energy (HED) and low-energy (LED) diets for 2 weeks. 2. After a 15 h fast, a primed-dose constant infusion of L-[U-14C]leucine and alpha-[4,5-3H]ketoisocaproate (KIC) was given. 3. After 2 h, plasma samples were taken and plasma-specific radioactivities of 14C- and 3H-labelled leucine and KIC were measured and analysed by using an open two-pool model. 4. Less than 20% of the total leucine-C entering the circulation was converted to the KIC pool, and 42% of the KIC was converted back to the leucine pool; transamination of the leucine to KIC and reamination of KIC to leucine was much less than in other species. 5. Additional dietary energy resulted in a decrease in tissue protein synthesis, leucine oxidation and interconversion of leucine and KIC. Total leucine-C entry was also lower in sheep given HED, which was most likely due to a suppression of endogenous proteolysis. 6. Plasma glucagon concentration was significantly higher (P less than 0.05) in sheep given LED compared with those given HED. The concentration of glucagon was closely correlated in all treatments with the leucine-C entry (proteolysis + absorbed leucine) and also with KIC-C exit (oxidation).