Relation between plasma and tissue parameters of leucine metabolism in fed and starved rats

By use of a primed continuous infusion of [1-14C]leucine, we investigated parameters of leucine metabolism in plasma, expired air, and tissues of fed and 48-h starved rats. The ratios of muscle to plasma specific activity of alpha-ketoisocaproate (KIC) in fed and starved rats were not significantly different from 1. The ratio of muscle to plasma specific activity of leucine was also not significantly different from 1 in fed rats, but was significantly lower than 1 in starved rats. The rate of leucine oxidation was 28-34% higher when calculation was based on plasma KIC rather than leucine specific activity. However, starvation significantly increased the rate of leucine oxidation with either specific activity. The rates of leucine incorporation into whole-body protein, calculated as the difference between plasma leucine turnover and oxidation, were unaffected by starvation, but the incorporations into total protein measured directly were significantly decreased in liver and muscle. We conclude that leucine or KIC specific activity in muscle is better predicted by plasma KIC than leucine specific activity, and the difference between rates of plasma leucine turnover and oxidation does not appear to be a valid measurement of leucine incorporation into whole-body protein.