Role of type III iodothyronine 5‐deiodinase gene expression in temporal regulation of Xenopus metamorphosis

To elucidate the role of type III iodothyronine 5‐deiodinase (5‐D) in the temporal regulation of amphibian metamorphosis, the regulation of gene expression of 5‐D and thyroid hormone receptor β (TRβ) in organs of Xenopus laevis was investigated. High levels of TRβ mRNA in the respective organs were observed at the times of their major morphological changes. Expression of the 5‐D gene was highly regulated among the organs during metamorphosis, including up‐regulation in the tail and down‐regulation in the liver. The tail and liver expressed 5‐D gene before their metamorphic changes. These precocious expressions correlated with the lower responsiveness to exogenously added triiodo‐L‐thyronine (T3) for inducing a high level of TRβ mRNA expression. However, the same organs responded to lower doses of T3 to regulate 5‐D gene expression as seen in spontaneous metamorphosis. The induction of 5‐D gene expression was considerably delayed in the intestine, even at an excess dose of T3. Thus, the two genes in a given organ appeared to respond to T3 either with different dose dependencies or with different timetables. The results obtained are also discussed in respect to recent findings in Rana catesbeiana.