An autoradiographic assessment of the incorporation of H3-uridine into DNA and RNA of osteoblasts of aging mice.

Intracellular labeling of DNA and RNA after H3-uridine administration was investigated autoradiographically during aging. Five to 78 weeks old mice were injected with 5 muCi of H3-uridine/gm of body weight and were killed from 15 minutes to 30 days later. Five mum decalcified sagittal sections of femora were treated with RNAase, DNAase or appropriate buffers. Autoradiographs were prepared and grain counts were made over diaphyseal periosteal osteoblasts. Both DNA and RNA incorporated H3-uridine. RNA label was 88 to 95% of the total cell label. DNA labeling ranged from 2 to 5%. DNA labeling appeared slightly increased after longer exposure to H3-uridine, whereas, RNA labeling remained relatively unaltered. With increasing age, incorporation into DNA decreased, whereas, RNA label showed a slight increase. A variable amount of non-specific label was undigested by either enzyme and may reflect insoluble conversion and/or degradation products. Apparently, some conversion of uridine into a DNA precursor occurs without loss of tritium label, thus rendering uridine less than totally specific for RNA. Nevertheless, the uptake of H3-uridine is largely indicative of RNA biosynthesis especially in skeletal cells that are not normally highly proliferative.