Time Course of Plasma T_3 and T_4 Levels and Tissue Transglutaminase Activity Following Injection of Thyroid Hormones in Tadpoles(Endocrinology)

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Temporal changes in the plasma concentrations of T_3 and T_4 in premetamorphic bullfrog tadpoles (stage V-VI) were determined by radioimmunoassay following intraperitoneal injections of these hormones (0.3 nmol/g body weight), since this example of single injection is frequently used to mimic tissue changes that occur in response to the surge of circulating thyroid hormones during spontaneous metamorphic climax. At 3 hr following T_3 injection, plasma T_3 was 10-20 fold higher than at climax and declined to the climax level by 48 hr. The plasma T_4 level was 20-50 times higher than the climax level 3 hr after T_4 injection, decreasing to the climax level by 72 hr. Significant levels of plasma T_3 were also detected following T_4 administration, declining from 1.5 times climax level at 3 hr to climax level between 6 and 12 hr. This apparent conversion of T_4 to T_3 was suppressed approximately 80% by co-injection of T_4 and iopanoic acid (0.3μmol/g body weight, I.p.), an agent that blocks the degradation of thyroid hormones. Following T_3 injection, the levels of transglutaminase (TG) activity, an indicator of tissue degeneration, showed maximal increases in tail muscle (ca. 5-fold) and caudal spinal cord (ca.6-fold) by 24 hr. In contrast, no change in TG activity was detected in hindlimb bud or lumbar spinal cord, tissues that undergo metamorphic differentiation into adult forms. Thus, despite the supraphy-siological plasma levels of these hormones, changes in TG activity and other short-term tissue responses appear to mimic those occurring during spontanous metamorphosis.
1991-08-15

Time Course of Plasma T_3 and T_4 Levels and Tissue Transglutaminase Activity Following Injection of Thyroid Hormones in Tadpoles(Endocrinology)

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