Comparison of RNA synthesis by liver nuclei from rats of various ages.

RNA synthesis was studied in nuclei isolated from male Fischer F344 rats of various ages. Both the initial rate and the extent of RNA synthesis were determined in two distinct media that result in the preferential synthesis of either mRNA or rRNA by hepatocytes. Synthesis of mRNA and rRNA, as determined by [3H]-UMP incorporation into acid-insoluble material, was shown to increase 1.6- to 3-fold from 3- to 6-months. The increase in RNA synthesis during maturation was followed by a 1.6 to 2.7-fold decrease from 6- to 31-months of age depending upon the assay medium and time of incubation. The age-related changes in RNA synthesis by rat liver nuclei were shown not to be due to changes in RNase activity, UTP uptake, or UTP degradation. RNA synthesized by liver nuclei from rats of various ages was characterized by SDS--polyacrylamide gel electrophoresis. Slight age-related differences in the size distribution of RNA isolated from liver nuclei were observed when nuclei from rats of various ages were incubated with [3H]-UTP in either assay medium.