Transcriptional regulation of ribosomal RNA synthesis during growth of cardiac myocytes in culture

The mechanism(s) by which rRNA accumulates during the growth of cardiac myocytes was investigated. The rates of rDNA transcription were measured in contracting myocytes and compared with nonbeating myocytes depolarized with 50 mM KCl. After 3 days of contraction the absolute rate of rDNA transcription was accelerated by 2-fold as measured by incorporation of [3H]UTP into the external transcribed spacer of preribosomal RNA. Corresponding increases in transcription were observed in isolated nuclei of contracting myocytes as measured by either hybridization of run-on transcripts of preribosomal RNA or activity of RNA polymerase I. The extent to which transcription was stimulated in contracting myocytes accounted for the previously observed acceleration of rRNA synthesis rates. The steady-state levels of preribosomal RNA relative to rRNA were unchanged in contracting myocytes, but the total amount of preribosomal RNA was 1.3-fold greater as a result of increased rRNA content. The increase of preribosomal RNA in proportion to rRNA in contracting myocytes demonstrated that the rate of preribosomal RNA processing was unchanged and that rRNA synthesis is regulated by an accelerated rate of rDNA transcription.