Abstract
Rana catesbeiana tadpoles were injected with [(14)C]thyroxine, and the subcellular distribution of the labeled hormone was determined. At 25 degrees the amount of isotope found in the liver was maximal after 1-2 hr and then rapidly decreased to a relatively constant value. A large percentage of the hormone was found associated with the purified nuclei isolated 24 hr after injection of [(14)C]thyroxine. Injection of [(14)C]thyroxine into tadpoles maintained at 5 degrees resulted in a much slower but constant accumulation of isotope in the liver, with virtually no movement of thyroxine into the cell nucleus. Thyroxine was bound very tightly to the chromatin fraction of the nucleus, but extraction and chromatography revealed no chemical modification of the thyroxine itself. These results suggest the presence of two temperature-dependent processes: one concerned with the transport of thyroxine into the liver cell and a second concerned with the transport of the intracellular thyroxine into the cell nucleus. It is proposed that the latter process is involved in the low-temperature inhibition of thyroxine-induced metamorphosis.