Protein and 28S ribosomal RNA fractional turnover rates in the rat heart after abdominal aortic stenosis

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Abstract
The rate of synthesis of myocardial proteins and ribosomal ribonucleic acid (rRNA) was measured during the development of cardiac hypertrophy in rats using a continuous intracardiac infusion of l4 C-tyrosine and 3H-uridine in unanaesthetised animals. Cardiac overload was induced by abdominal aortic stenosis. Left ventricular weight and total myocardial RNA concentration were significantly increased on day 4 after aortic stenosis (+19% and +18% respectively). On day 8 left ventricular weight reached +52% whereas RNA concentration had not increased further (+13%). The fractional turnover rates were calculated using the specific activities of intracellular free tyrosine and free uracil nucleotides (precursors) and those of protein bound tyrosine and 28S rRNA bound uridine monophosphate (products) respectively. The fractional rate of synthesis of proteins and rRNA (expressed as percentage per day) increased from 24% to 45% for proteins and from 25% to 34% for rRNA and peaked by day 2. The RNA activity, expressed as gram of protein synthesised per day and per gram of total RNA, was unchanged on day 1 and reached a maximal value on day 2 (+107%). These results suggest that the pre-existing ribosomal RNA could be underutilised under control conditions and that the boosting of RNA transcription, associated with that of protein translation, is a complementary process rather than a prerequisite for the transition period leading to hypertrophy.