Triiodothyronine-stimulated RNA synthesis in primary cultures of adult rat hepatocytes.

We have studied the effect of triiodothyronine (T3) on RNA synthesis by primary cultures of rat hepatocytes in order to ascertain whether hepatocyte transcriptional activity is directly stimulated by this hormone. The results demonstrate that T3 stimulates RNA synthesis as measured by [3H]orotic acid incorporation into RNA and by RNA polymerase activity. The responsiveness of cultured hepatocytes to T3 becomes evident only after a fairly long latency period required for the recovery of T3 nuclear binding sites. The response of RNA synthesis to T3 was absent unless the hepatocytes were simultaneously exposed to insulin and dexamethasone, indicating a permissive role of these factors in the action of T3 on RNA synthesis.