Leucine disposal and oxidation rates in the fetal lamb.

To verify previous indirect evidence suggesting an important role of amino acids as substrates of fetal oxidative metabolism, leucine disposal and oxidation rates were measured in chronically catheterized fetal lambs during the last month of gestation. Under normal physiologic conditions the leucine oxidation rates were 6.43 +/- 1.02 mumol min-1 for fetuses with an average weight of 3.22 +/- 0.07 kg and comparable in magnitude to the fetal rate of leucine accretion. In seven animals studied before and during fasting, the fetal leucine oxidation rate increased with fasting from 5.8 +/- 1.0 to 10.8 +/- 1.3 mumol min-1. These data demonstrate that there is rapid oxidative degradation of leucine by the fetus and that the rate of this process increases in response to maternal fasting.