Faster protein and ribosome synthesis in hypertrophying heart.

Faster rates of protein synthesis in hypertrophying rat heart were associated with a 25% greater content of ribosomal RNA, termed capacity for protein synthesis. Efficiency of synthesis, nmol phenylalanine/mg RNA X h, was unchanged in hypertrophy induced by aortic banding or thyrotoxicosis. The proportion of total RNA in free ribosomal subunits in hypertrophying hearts was unchanged from that observed in perfused hearts from control rats and from unperfused normal hearts. Accelerated synthesis of new ribosomes was observed within 8 h of the injection of a pharmacologic dose of thyroxine, well before an increase in whole heart protein synthesis was observed. These results indicated that increased protein synthetic machinery, as monitored by content of ribosomes, rather than more efficient initiation or elongation of peptide chains, accounted for the faster rate of protein synthesis and that accelerated ribosome formation was an early and quantitatively important factor in hypertrophy.