Plasma Thyroxine and Triiodothyronine Levels in Spontaneously Metamorphosing Rana catesbeiana Tadpoles and in Adult Anuran Amphibia*

Abstract

We have developed sensitive and reliable radioimmunoassays for T4 and T3 in amphibian plasma and have used these procedures to measure plasma T4 and T3 levels in spontaneously developing Rana catesbeiana tadpoles at various stages of metamorphosis. During premetamorphosis circulating levels of both T4 and T3 were below the limits of detection of the RIA procedures (T4 less than 50 ng/100 ml, T3 less than 5 ng/100 ml). A gradual rise in plasma T3 and T4 became apparent during prometamorphosis, and at the onset of metamorphic climax the levels of both T4 and T3 increased sharply. Peak levels for both hormones were observed in the middle of metamorphic climax (stage XXIII). The circulating T3 level reached a mean peak of 78 ng/100 mg, at least 15 times greater than the level during premetamorphosis. The peak T4 level was 0.5 microgram/100 ml, about a 10-fold increase over the premetamorphosis level. The surge in thyroid hormone secretion lasted only for several days, and during the latter half of metamorphic climax there was a fairly rapid decrease in plasma T4 and T3. By 2 days post-climax the levels had declined to about 20% of their peak values. Free T4 and T3 levels in plasma followed the same general pattern as the total hormone levels during the various stages of tadpole development. In adult R. catesbeiana, plasma T4 and T3 levels were surprisingly low. Similarly low values were observed in Bufo marinus and in Rana pipiens. The very low levels of circulating T4 and T3 both in premetamorphosis tadpoles and in adults suggest that thyroid hormones in anuran Amphibia may be of importance only during the period of metamorphosis.

Developmental expression and hormonal regulation of glucocorticoid and thyroid hormone receptors during metamorphosis in Xenopus laevis

“…Similarly, Tata (1993) was also unable to detect an increase in plasma T 3 until these late prometamorphic stages. Findings in X. laevis are also consistent with measures of plasma T 4 and T 3 in the bullfrog, R. catesbeiana, where significant increases in plasma hormone concentration were not observed until late prometamorphosis/early climax (Regard et al 1978). Thus, the published plasma T 3 and T 4 concentrations in tadpoles during metamorphosis are consistent with our measures of whole-body hormone content, which showed significant increases only during late prometamorphosis.…”

“…Thyroid hormones and TR Our analyses of wholebody T 3 and T 4 showed that both hormones exhibited the expected increases during prometamorphosis and metamorphic climax in X. laevis as has been described for whole-body thyroid hormone content in other species (Niinuma et al 1991, Weber et al 1994, Denver 1998a and for plasma thyroid hormone concentrations in X. laevis (Leloup & Buscaglia 1977) and other anurans (Regard et al 1978, Mondou & Kaltenbach 1979, Weil 1986). We found that peak tissue hormone contents in X. laevis were comparable with those reported for other anurans (Niinuma et al 1991, Weber et al 1994, Denver 1998a.…”

Timing of metamorphosis and the onset of the negative feedback loop between the thyroid gland and the pituitary is controlled by type II iodothyronine deiodinase in Xenopus laevis

“…T he metamorphosis of anurans is controlled by a steadily increasing concentration of thyroid hormone (TH) in tadpoles. TH reaches a peak at the climax of metamorphosis and then falls as the final change, tail resorption, occurs (1,2). This gradual increase in TH concentration is essential for the sequential development of frog tissues and organs (3,4).…”

Structural characteristics of bullfrog (Rana catesbeiana) transthyretin and its cDNA

“…The concentration of thyroid hormones in the blood increases gradually during prometamorphosis, reaches a maximum level at the beginning of metamorphic climax, and then decreases and ceases to be detectable at the end of metamorphosis [3,4]. Thyroid hormone receptors, the genes of which are thought to be among the earliest genes responding to the elevation of thyroid hormone concentration in blood plasma, are upregulated in several tissues during both spontaneous and induced metamorphosis [5,6], and in cell-culture systems [7,8].…”