The Regulation of Protein Synthesis in Heart Muscle Under Normal Conditions and in the Adriamycin-cardiomyopathy

The regulation of cardiac protein synthesis, in particular messenger-RNA (mRNA) and polyribosome metabolism, has been investigated in normal rat heart muscle and in the adriamycin-cardiomyopathy by using newly developed methods for the isolation, characterization and in-vitro translation of cardiac polyribosomes and mRNA. The obtained data allow the following conclusions: 1. Normal heart muscle has a high content of polyribosomes (865 micrograms/g) and of mRNA (20-60 micrograms/g), and thus a high rate of protein synthesis. 2. The level of cardiac polyribosomes and mRNA is strictly age-dependent and much higher in young animals (2-3 x). This corresponds to a higher cardiac protein synthesis rate in young animals with a growing heart muscle, and shows that the protein-synthetic reserves of heart muscle decrease sharply with age. 3. Withdrawal of food for 1-3 days results in a pronounced decrease (-50% to -70%) of cardiac polyribosomes and mRNA, demonstrating that the cardiac protein synthesis reacts very sensitively to conditions of starving. 4. The cardiac polyribosomes and mRNA are unevenly distributed in the myocyte. The bulk of these substances is present in the cardiac microsomes, and much less is found in nuclei, myofibrils, mitochondria and in the post-microsomal fraction (=cell-sap) of the cardiac muscle. This shows that the major intracellular site of cardiac protein synthesis is the microsomal fraction of the myocyte. 5. A pool of untranslated mRNA was demonstrated to be present in the cell-sap of the myocyte. This mRNA is to some extent translatable in-vitro and appears to represent mRNA sub-pools with two functions: a) mRNA which is partially broken down or in the process of being broken down, and b) intact mRNA which could have a "reserve-function", e.g., by being utilized to increase cardiac protein synthesis under certain conditions. 6. A method of quantitating small amounts of cardiac mRNA (25-50 ng) has been developed which makes it possible to estimate the mRNA content of cardiac biopsies. 7. These methods were utilized to study the relevance of changes in RNA- and protein synthesis in the development of the adriamycin-cardiomyopathy. It appears that severe decreases in cardiac mRNA and polyribosome levels are a key factor in the pathogenesis of the adriamycin-cardiomyopathy. These decreases are probably caused by the direct binding of adriamycin to cardiac DNA and lead themselves to a persisting decrease in cardiac protein synthesis which in view of the short half-lives of the cardiac contractile proteins (5-12 days) causes a gradual loss of cardiac structure and function.