Changes in thyroidal and plasma iodine compounds during and after metamorphosis of the bullfrog, Rana catesbeiana.

Abstract

Thyroidal thyroxine (T4), 3,5,3′-triiodothyronine (T3), diiodotyrosine (DIT), monoidotyrosine (MIT), and iodide (I−) in metamorphosing tadpoles, froglets, and adult frogs were measured chemically by cation-exchange resin chromatography. Plasma T4, T3, DIT, and 3,3′,5′-triiodothyronine (rT3) were also measured by radioimmunoassay. Plasma protein-bound iodine (PBI) was determined by a modified ultramicro PBI method. T4, T3, MIT, and DIT levels in thyroids tended to increase both during and after metamorphosis. Plasma T4 increased at the end of prometamorphosis, and a sharp increase was found at the onset of the metamorphic climax. At stage XXI, the T4 level reached a peak of about 500 ng/100 ml. Thoreafter the amount of circulating T4 decreased slowly, but even at 7 and 14 days after metamorphosis it remained at a higher level than in the premetamorphic stage. The level of plasma T4 in adult frogs was only 40 ng/100 ml and was undetectable in 4-month-old froglets. Similar changes were found in circulating plasma T3 and its level was less than one-third of T4, especially at the metamorphic climax. Throughout all metamorphic stages, plasma T4 and T3 levels fluctuated remarkably only at the metamorphic climax. Plasma DIT was at the low level of about 200 ng/100 ml and did not change during metamorphosis. Moreover, a small amount of rT3, less than T4 values, was detectable only in tadpoles during the metamorphic climax and in adult frogs. PBI values were at the level of about 2.0 μg/100 ml from stage XV to stage XXII, and thereafter decreased gradually. The PBI level was much higher than the sum of plasma T4, T3, and DIT, and did not correlate with the circulating hormone levels. From these results a relationship between thyroidal and plasma hormone levels and an extrathyroidal monodeiodination of T4 were discussed.