Contributions of increased efficiency and capacity of protein synthesis to rapid cardiac growth

Rapid cardiac growth depends upon faster synthesis than degradation of protein. The rate of protein synthesis is determined by the efficiency with which the existing components of the ribosome cycle make protein and by the quantity of the components that are present. The tissue content of RNA is taken as an index of the capacity of synthesis and efficiency is expressed as the amount of protein formed per amount of RNA over a certain time period. The efficiency of synthesis is regulated by hormones, including insulin, agents that increase cAMP, alpha-adrenergic agonists, endothelin I and angiotensin II. In addition, provision of non-carbohydrate substrates and mechanical factors such as stretch and contraction increase efficiency. Impaired energy availability as occurs in anoxic or ischemic muscle decreases efficiency. Increased phosphorylation of ribosomal protein, S6, or of the peptide chain initiation factor, elF-4E, have been suggested as mechanisms to regulate efficiency of mRNA translation. Increased efficiency of synthesis accounts for cardiac growth in the first few days following aortic banding, pulmonary artery constriction and thyroxine administration. Decreased efficiency accounts for cardiac atrophy in heterotopic transplanted hearts during the first 3 days following transplantation. The capacity of synthesis is increased by insulin, thyroid hormone, activators of protein kinase C, agents that increase cAMP, and endothelin-1. Stretch of the ventricular wall and contraction of cultured neonatal myocytes accelerates ribosome formation. An increased rate of ribosomal DNA transcription accounts for accelerated ribosome formation and depends on increased activity of a transcription factor, upstream binding factor (UBF). The activity of UBF is increased either by increased rates of synthesis or by phosphorylation of the protein. Increased capacity of synthesis is a major contributor to rapid cardiac growth in the newborn heart and after several days of pressure overload.