Effect of Free Fatty Acids and Insulin on Protein Synthesis and Amino Acid Metabolism of Isolated Rat Diaphragms

The effect of free fatty acids and insulin on the incorporation of C-14 from amino acids into muscle proteins was studied in vitro using “cut” or “intact” rat diaphragms. Insulin promoted the incorporation of C-14 from L-gly-cine-2-C-14, L-histidine-2-ring-C-14 and L-leucine-U-C-14 into muscle proteins. Insulin stimulated the uptake of 0.05 mM. leucine-U-C-14 from the medium, but not its oxidation to C-14-O2. Palmitate (3 mM.) decreased the incorporation of C-14 from glycine and histidine into proteins of muscles stimulated with insulin. Octanoate (0.5-3 mM.) decreased the incorporation of C-14 from leucine into proteins. The inhibitory effect of octanoate on protein synthesis was demonstrated in the presence or absence of glucose and/or insulin. DL-β-hydroxybutyrate (15 mM.) decreased the incorporation of C-14 from leucine-U-C-14 into proteins in the presence of insulin. Octanoate (1 mM.) stimulated the uptake of C-14 from leucine-U-C-14 from the medium and its oxidation to C-14-O2. DL-β-hydroxybutyrate (15 mM.) appeared to inhibit both processes, however; the latter may be due to isotope dilution. The nonutilizable amino acid, α-aminoisobutyric acid-C-14, penetrated less readily into the intracellular water of diaphragms incubated with 1 mM. octanoate than into controls. However, since octanoate stimulated the uptake of C-14 from leucine from the medium it appears likely that octanoate inhibited the incorporation of C-14 from leucine into proteins at a step beyond amino acid transport. It is suggested that FFA can influence amino acid metabolism in muscles and decelerate their incorporation into proteins. Limitation of protein synthesis in muscle by increased intracellular FFA may contribute to the supply of amino acids available for neoglucogenesis during stress and carbohydrate deprivation.