Regulation of protein synthesis in heart muscle. I. Effect of amino acid levels on protein synthesis.

Protein synthesis in heart muscle has been studied in the isolated rat heart perfused with buffer containing glucose and various amino acid mixtures. The rate of synthesis was evaluated by measuring incorporation of 14C-phenylalanine into whole heart protein and myosin. Protein degradation was estimated by measuring the fall in the specific activity of the free phenylalanine pool and the release of phenylalanine from the heart. 14C-Phenylalanine was suitable for these studies since it equilibrated rapidly with an intracellular pool of stable size and was not converted to other amino acids. Participation of the intracellular free amino acid pool as an intermediate in the pathway of protein synthesis was established. Synthesis of whole heart protein and myosin was increased about 40% when amino acid levels were increased from 1 to 5 times normal plasma levels. A group of 12 amino acids substituted for the complete mixture in producing this effect. Protein degradation appeared to occur at about twice the rate of protein synthesis during perfusion with normal plasma levels of amino acid. When perfusate levels of amino acid were raised, cellular levels of 12 amino acids increased. In general, intracellular amino acid levels reached stable values within 30 min. These studies indicated that protein synthesis in heart muscle could be stimulated by raising perfusate amino acid levels above those normally found in rat plasma. Although net uptake of isoleucine and leucine was observed, suggesting that these amino acids were used as oxidative substrate, the increased rate of synthesis appeared to be due to a direct effect on the pathway of protein synthesis rather than an indirect effect mediated by changes in energy levels of the cell.