The sensitivity of Xenopus laevis tadpole tail tissue to the action of thyroid hormones

Abstract

Isolated tail tips from Xenopus laevis tadpoles at different developmental stages were cultured in the presence of 3,5,3′-triiodothyronine (T 3) and thyroxine (T 4) at concentrations of 2 × 10 −7 to 4 × 10 −9 M. Tail tissues were found to be sensitive to the actions of both hormones as early as stage 38, which is reached 60 hr postfertilization and approximately 6–12 hr after hatching. Sensitivity to both hormones increases with development, as evidenced by a shortened lag period before the onset of regression, and increased rate of regression, and an increased level of acid phosphatase activity at 50% regression. In all instances, T 3 was found to be more potent than T 4 in terms of eliciting these specific reactions characteristic of metamorphosing tadpole tail tissues. The amount of T 4 in the culture medium that actually entered the tail tips and, hence, was available for evoking metamorphosis was determined. The T 4 concentration in the tail tips varied proportionally with the hormone concentration in the medium. At the lowest medium concentration of T 4 (4 × 10 −8 M), the tissue concentration was 3.4 × 10 −9 M T 4. In all cases, the tail/medium T 4 concentration ratio was between 0.1 and 0.2, indicating that the tail tip is not freely permeable to T 4.