1. 1. Cardiac hypertrophy in adult rabbits was induced by subcutaneous injection of isoproterenol. 2. 2. The rate of [3H]leucine incorporation into acid insoluble material was increased and the extent of [32P]phosphate incorporation into several ribosomal proteins was altered. 3. 3. Specifically, a ribosomal protein with a molecular weight of 32,000 from the 40S ribosomal subunit showed a five-fold increase in phosphate incorporation in the hypertrophic heart whereas a protein with a molecular weight of 28,000 from the 60S subunit showed a four-fold decrease. 4. 4. Phosphorylation of ribosome-associated proteins, which could be removed from ribosomes with 0.72 M KCl, was also changed in the hypertrophic hearts. 5. 5. Six major phosphoproteins (with molecular weights 62,000, 49,000, 36,000, 30,000, 20,000 and 12,000) were detected in both the normal and the hypertrophic hearts. 6. 6. Phosphorylation of the 62 K and the 49 K protein was increased by two- and three-fold, respectively, in the hypertrophic hearts, whereas phosphorylation of the 36 K and the 30 K protein decreased by two-fold. 7. 7. The level of phosphorylation of the 20 K and the 12 K protein was not significantly changed in hypertrophic hearts.
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