Thyroid hormone receptor induction by triiodothyronine in tadpole erythrocytes in vivo and in vitro and the effect of cycloheximide and actinomycin-D

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Tadpole erythrocyte nuclei contain specific T 3 binding sites which increase in number during spontaneous or T 3-induced metamorphosis. In the present studies the increase in the number of T 3 binding sites after a T 3 injection appeared to be completely prevented by cycloheximide and actinomycin D, inhibitors of protein synthesis and RNA synthesis, respectively. However, in some experiments the effect was not statistically significant. The increase in sites was prevented only if the inhibitors were administered at 0 or 24 hr after T 3 injection, but not at 48 or 72 hr after T 3. When tadpole erythrocytes were incubated with T 3 in vitro in M199 culture medium, the number of nuclear T 3 binding sites increased within 48 hr. This increase was highly sensitive to inhibition by cycloheximide (maximal at 1 × 10 −6 M) or actinomycin D (maximal at 0.02 μg/ml). These inhibitor concentrations only slightly reduced the incorporation of labeled precursors. The T 3 concentration required to induce a half-maximal increase in binding sites in vitro was about the same as the T 3 concentration at which half the binding sites were occupied. The T 3 binding sites had a high affinity for thyroid hormone analogs which stimulate metamorphosis. These results support the designation of the T 3 binding sites as T 3 receptors. They show that the tadpole erythrocytes respond to T 3 with an increase in the number of T 3 binding sites without the involvement of other tissues. It is proposed that this is a receptor induction involving synthesis of RNA and protein.