Ventricular Nucleic Acid and Protein Levels with Myocardial Growth and Hypertrophy

The myocardial DNA, RNA, actomyosin, and total protein concentrations were determined in rat ventricles ranging between 251 and 1635 mg. Three phases of myocardial growth were evident. Phase 1 (ventricular weights less than 550 mg): DNA concentrations remained relatively constant with increasing ventricular weight. The increasing total DNA per ventricle suggests continuing mitotic activity. Phase 2 (ventricular weights between 550 and 1000 mg): total DNA per ventricle remained relatively constant; with ventricular growth there was a progressive decrease in DNA concentrations. This would be the phase of true myocardial hypertrophy. Phase 3: DNA concentrations continued to slowly decrease although the total DNA per ventricle increased. It would appear that this is the result of a renewed synthesis of DNA. In phases 1 and 2, the RNA concentrations progressively decreased, although the RNA/DNA ratio was increasing. This ratio appeared to reach its maximum value by the end of phase 2 and remain relatively constant in phase 3. Thus in phase 3, the concentrations of DNA and RNA simultaneously decreased to maintain this constant ratio. The ventricular total protein and ventricular actomyosin levels did not change over this range of ventricular weights.