Hormonal control of cardiac protein and amino acid balance

Isolated hearts of fetal mice in organ culture maintain active protein synthesis and protein degradation. Rates of degradation exceed rates of synthesis and as a result, the hearts are in state of negative protein balance as evidenced by net loss of protein and release of amino acids. Several hormones can alter amino acid metabolism and protein balance by altering synthesis or degradation, or both. In cultured fetal mouse hearts, insulin, the most extensively studies of the hormones increases the rate of protein synthesis by 13 +/- 3.7% and decreases the rate of protein degradation by 22 +/- 4.1% (p less than 0.01 for both). Together, these changes account for a 30-40% reduction in the loss of cardiac protein and in the release of phenylalanine from the heart. These changes are accompanied by a decrease of 21 +/- 2.7% in the total activity of the lysosomal proteinase cathepsin D. and by a reduction in the proportion of the enzyme that is present in the nonsedimentable fraction of the issue homogenate. This suggests that the possibility that insulin may function in part by altering lysosomal enzyme activity or availability, or both. The effects of insulin on protein degradation, amion acid release, and cathepsin D activity persist even when protein synthesis has been inhibited by cycloheximide. These results suggest that insulin plays an important role in the control of cardiac protein synthesis and degradation.