AbstractIn developing Rana catesbeiana tadpoles, the timing of the thyroid hormone (TH)-dependent metamorphic responses varies markedly among tissues. Yet at any one time these tissues are exposed to the same plasma concentration of TH, suggesting that TH action is regulated in part at the level of the peripheral tissues. A major factor in TH action is the intracellular level of the active TH, T3. This level is dependent not only on the plasma concentration of TH (mostly T4) but also on the intracellular activities of the type 2 5′-deiodinase (D2) and the type 3 5-deiodinase (D3), which are responsible, respectively, for generating and degrading T3. (D1 is not present in this species.) To determine whether differential expression of D2 and D3 among tissues could be a significant factor in the coordination of metamorphic events, the ontogenic profiles of the two enzyme activities and corresponding messenger RNA levels in most tissues of R. catesbeiana tadpoles have been documented. The profiles of D2 expression in tail, hindlimb, forelimb, intestine, skin, and eye differed markedly at both activity and messenger RNA levels, but it was notable that expression was invariably highest in a given tissue at the time of its major metamorphic change. D2 expression was very low in brain and heart and did not vary during development. D2 was not expressed in liver, kidney, or red blood cells. With the exception of red blood cells, D3 expression was detected in all tissues studied. Furthermore, it was evident that in tissues that expressed both deiodinase genes, the two expression profiles were comparable, indicating a potential for tight control of intracellular T3 levels.Direct evidence of the importance of the intracellular conversion of T4 to T3 for TH-dependent metamorphic events was obtained in tadpoles in which endogenous TH synthesis was blocked with methimazole, and the activities of D2 and D3 were inhibited by iopanoic acid. This treatment inhibited metamorphosis. The inhibition could be overcome by the concomitant administration of replacement levels of T3, but not T4.These results strongly support the view that coordinated development in amphibia depends in part on the tissue-specific expression patterns of the D2 and D3 genes, which ensure that the requisite level of intracellular T3 is attained in a given tissue, regardless of the current level of circulating TH, at the appropriate stage of metamorphosis.
