Regulation of Protein Synthesis in Heart Muscle

Abstract Components of the ribosome cycle, subunits and polysomes, were extracted from heart muscle and analyzed on exponential sucrose gradients. When MgCl2 was added in concentrations above 2 mm, subunits were aggregated and sedimented in the position in which monomeric ribosomes would be expected. Subunit peaks were identified by extraction and analysis of RNA. Perfusion of hearts with buffer containing phenylalanine and glucose decreased polysomes and increased levels of ribosomal subunits. These findings, together with a fall in the rate of protein synthesis, indicated that a block in initiation of peptide chains had developed during perfusion of the heart. When 1 or 5 times normal plasma levels of amino acids were added to the buffer, protein synthesis increased and levels of subunits decreased, indicating that initiation had been facilitated. However, levels of subunits and polysomes were not restored to levels in vivo. Protein synthesis in perfused hearts was inhibited by cycloheximide in association with lower levels of subunits and increased polysomes. These findings indicated that steps involved in elongation of peptide chains had been inhibited. When protein synthesis of hearts perfused with buffer containing glucose and amino acids was stimulated by addition of insulin, levels of subunits decreased and levels of polysomes rose, indicating that insulin had accelerated steps involved in initiation of peptide chains.