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The effects of parathyroid extract (PTE) on the synthesis in vivo of free nucleotide and RNA were compared in rat metaphysial bone and kidney. The incorporation of ³²P into chromatographically pure acid-soluble 5'-AMP and purified bulk RNA was examined at various times after PTE administration. Pulse-labelled RNA was further characterized by sedimentation in sucrose density gradients and by ribomononucleotide analysis.

In both organs the labelling of 5'-AMP and its turnover were accelerated after administration of the hormone. The pool size of free AMP of kidney was approximately 3 times that of bone; neither was affected by PTE. The specific activity of pulse-labelled kidney AMP was always greater, and hormonal stimulation of its labelling was more rapid than in bone.

Despite more extensive precursor labelling, the stimulation of renal RNA synthesis was negligible, and was delayed for several hours, the overall hormonal effect being inseparable from its effect on phosphate entry into the nucleotide precursor pool.

In bone, the hormonal stimulation of RNA labelling was immediate, and continued to increase at a linear rate for up to 12 h. Initially, stimulation of RNA polymerization accounted for the total hormonal effect, while after 4 h an increasing proportion of the total increase in RNA labelling was attributable to enhanced precursor labelling.

Newly synthesized bone RNA differed qualitatively from kidney RNA in its sedimentation properties and composition. Although the labelling of all RNA species and RNA-nucleotides in bone was stimulated by PTE, there was a proportionately greater effect on the labelling of ribosomal RNA, and an apparent shift towards GMP-rich molecules, neither change being manifest in kidney.

It is concluded that while bone and kidney share certain mechanisms, they show changes in RNA biosynthesis in response to parathyroid hormone which are both quantitatively and qualitatively different and which are in accord with the RNA requirements for the respective physiological response of each.