Free amino acid patterns and the distribution of 14C from [U-14C]-L-Leucine, [U-14C]-L-Lysine, and [U-14C]-L-Alanine in the tissues of young adult rats.

In studies designed to evaluate the role of the liver and muscle in modifying the plasma amino acid response, three levels of dietary amino acids, 3.6%, 4.8% and 6.0% were fed to three groups of young adult male rats for 2 weeks. After fasting, either [U-14C]-L-leucine, [U-14C]-L-lysine or [U-14C]-L-alanine was administered intragastrically with a portion of diet. After a 4.5 hour fast, rats were killed. Distribution of radioactivity was evaluated in trichloroacetic acid (TCA) supernatants and lipid-extracted precipitates of plasma, liver and gastrocnemius muscle, and in expired CO2. Free amino acids were measured and specific activities of amino acids were determined. Rats fed 6.0% or 4.8% amino acids lost less weight than those fed 3.6%. A high percentage of radioactivity from 14C-leucine and 14C-lysine was recovered in the lipid-extracted precipitate. TCA supernatants from rats fed 14C-leucine contained low levels of radioactivity. A large percentage (60%) of the radioactivity from 14C-alanine was expired in 14CO2. The percentages of alanine found in TCA precipitates were very low. Significant increases in concentration associated with increments in dietary amino acids occurred in plasma free isoleucine, leucine, valine and tyrosine; liver free histidine; and muscle free leucine and threonine. Concentrations of many amino acids were depressed in the muscle of rats fed 4.8% amino acids whereas they increased in response to 6.0%. Protein and free amino acid specific activities indicated no change in rates of protein synthesis. Data from this type of experiment may assist in interpreting the role of the liver and muscle in modifying the plasma amino acid response to dietary amino acids.