Triiodothyronine Stimulates Nuclear RNA Synthesis

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Abstract
The effect of triiodothyronine (T3) administered in vivo on RNA synthesis in rat liver nuclei was studied in vitro. Normal (N) rats were compared to hypothyroid (H) rats, and to hypothyroid rats given 15 .mu.g T3/100 g/bw [body weight] (T rats) for 0.5-24 h prior to sacrifice. Nuclei were prepared by centrifugation through 2.3 M sucrose. DNA dependent-RNA polymerase I activity, total DNA-dependent RNA polymerase, and .alpha.-amanitin-sensitive polymerase were separately assayed by measuring incorporation of [125I]iodo-CTP. RNA was extracted using a phenol-chloroform technique from aliquots of homogenate (unincubated) and from nuclei after in vitro incubation with [125I]iodo-CTP. The Poly-A content of homogenate RNA was assayed by its ability to bind and protect [125I]iodo-Poly-U. In vitro synthesized [125I]labeled RNA was examined by polyacrylamide gel electrophoresis. .alpha.-Glycerophosphate dehydrogenase activity was also measured in mitochondria prepared from the various types of animals. RNA polymerase activity of H rats was reduced in comparison to N rats, was increased within 8-10 h after T3 administration, and was returned to the normal level by 24 h. Both polymerase I and II were increased by 8-10 h after T3 administration. The response was associated with an increase in Poly-A-containing RNA, and was prevented by cycloheximide and .alpha.-amanitin. The RNA synthesized in vitro in nuclei from H rats was qualitatively similar to that synthesized in nuclei of N rats or T3-treated rats. The metabolic response to T3 includes an early and possibly generalized stimulation of liver mRNA synthesis and rRNA synthesis.