Corticoids augment nuclear binding capacity for triiodothyronine in bullfrog tadpole tail fins

The effect of aldosterone and corticosterone on T3 binding to the nuclear fraction of the tail fin of bullfrog tadpoles was investigated in vitro. Both corticoids (10(-7) - 10(-5) M) added to the incubation medium increased T3 binding in a dose-dependent manner. The aldosterone-induced increase in T3-binding was blocked by cycloheximide, an inhibitor of protein synthesis, and actinomycin D, an inhibitor of RNA synthesis. Scatchard analysis revealed that 10(-6) M aldosterone and corticosterone increased the maximum binding capacity for T3 by 60 and 41%, respectively, and that the corticoids did not alter the value of the dissociation constant. The significance of the finding is discussed in relation to metamorphosis.