Modulation of T4 5′-Monodeiodination in Rat Anterior Pituitary and Liver Homogenates by Thyroid States and Fasting

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  • NAITO KOICHI
    Second Division, Dapartment of Internal Medicine, Kyoto University School of Medicine
  • INADA MITSUO
    Second Division, Dapartment of Internal Medicine, Kyoto University School of Medicine
  • MASHIO YASUO
    Second Division, Dapartment of Internal Medicine, Kyoto University School of Medicine
  • TANAKA KIYOSHI
    Second Division, Dapartment of Internal Medicine, Kyoto University School of Medicine
  • ISHII HITOSHI
    Second Division, Dapartment of Internal Medicine, Kyoto University School of Medicine
  • ISHIKAWA MITSUSHIGE
    Second Division, Dapartment of Internal Medicine, Kyoto University School of Medicine
  • IMURA HIROO
    Second Division, Dapartment of Internal Medicine, Kyoto University School of Medicine

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  • Modulation of T4 5'-monodeiodination in rat anterior pituitary and liver homogenates by thyroid states and fasting.
  • Modulation of T4 5 Monodeiodination in

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In order to clarify the role of the pituitary conversion of L-thyroxine (T4) to 3, 5, 3'-L-triiodothyronine (T3) in regulating thyrotropin (TSH) secretion, the effect of altered thyroid states and fasting on intrapituitary T3 generation was investigated by a paperchromatographic procedure using the anterior pituitary homogenates. Hepatic T3 generation was also studied for comparison.<BR>The rate of pituitary and hepatic T3 generation in normal rats averaged 25.2±12.4 (mean±SE) fmoles T3/mg protein/min and 33.8±12.7 fmoles T3/mg protein/min, respectively. T4 treatment raised the hepatic T3 generation from T4 (46.7±3.1 fmoles T3/mg protein/min) and lowered the intrapituitary T3 generation (4.5±0.2 fmoles T3/ mg protein/min). On the contrary, thyroidectomy slowed the hepatic T4 5'-deiodination (11.0±2.8 fmoles T3/mg protein/min), and accelerated the pituitary T4 5'-deiodination (64.3±1.4 fmoles T3/mg protein/min). In 48h fasted rats, serum T4, T3 and TSH concentrations were all lower than those in fed rats, and both pituitary and hepatic T3 generations were also suppressed.<BR>Thus, altered thyroid states cause an opposite effect on pituitary and liver 5'-monodeiodination, whereas fasting causes similar changes. The findings suggest the existence of an autoregulatory mechanism for thyroid hormone activation within the target tissues.

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