Primary mechanisms of thyroid hormone control of amphibian metamorphosis

Abstract

Dramatic adaptive changes in numerous biochemical systems accompany the metamorphosis of the amphibian. These striking morphogenetic and molecular transformations are initiated and controlled by the thyroid hormones. That this differentiation process is providing a valuable experimental system for the study of the mechanism of action of the thyroid hormone is emphasized in a number of recent reviews. Current research progess has centered on the many alterations in RNA and protein metabolism in the developing tadpole liver, the induction of enzymes responsible for the advent of ureotelism, the process of tail resorption, and the remarkable effect of low temperatures on metamorphosis. Common mechanisms that appear to involve genetic control of protein and RNA biosynthesis have emerged to account for the changes observed in such diverse tissues as the liver and the tail. In this paper, we propose a sequence of molecular events during the first 2–3 hours after T 3 is administered to tadpoles by injection. During the first hour, T 3 promotes transport of nucleotides and amino acids into the liver. At the same time, there is a steadily increasing acceleration of protein and RNA biosynthesis that reaches its maximum at 60 min. Further interpretation of the data becomes more complicated because increased prehormonal RNA degradation and de novo nucleotide biosynthesis now impinge on the continuing macromolecular biosynthesis initiated earlier. The net result is the induction of new protein and RNA biosynthesis in the very early period after T 3 injection which eventually lead to the major synthetic events involving most of the intracellular structures of the liver.