Thyroid Function and Polyamines I. Rapid Fluctuation of Thyroid Ornithine Decarboxylase Activity in Response to Change in Circulating Thyrotropin Level in the Rat

Abstract

Administration of antithyroid drugs in drinking water to rats resulted in an increase in both thyroid weight and ornithine decarboxylase activity, and in serum thyrotropin (TSH) level. These three parameters seemed to increase in parallel by 10-day treatment with the drugs. The enzyme activity increased 8-to 18-fold on the 4th day of methylthiouracil treatment and there was a gradual decline thereafter, while the thyroid weight increased showing a sigmoid curve during the treatment. Nearly a similar change with that in the enzyme activity was observed in the RNA content during the treatment. After thyroxine injection (200μg/kg), the elevated ornithine decarboxylase activity as well as serum TSH level decreased very rapidly. After cycloheximide injection, the enzyme activity decreased more rapidly, the half-life of this enzyme being calculated to be approximately 15 min. These results seem to indicate that the action of TSH results in de novo synthesis of thyroid ornithine decarboxylase, which in turn may be related to regulation of RNA metabolism via poylamines.