Intra‐ and extra‐cellular sources of T₃ binding to putative thyroid hormone receptors in liver, kidney, and gill nuclei of immature rainbow trout, Oncorhynchus mykiss

The sources of extracellular and intracellular 3,5,3'-triiodo-L-thyronine (T3) binding to putative thyroid hormone receptors in liver, kidney, and gill nuclei were determined in vivo for immature rainbow trout at 12 degrees C. Both [131I]T3 and [125I]T4 were injected intraperitoneally, the plasma and tissues were examined at isotopic equilibrium at 20 h, and the proportions of intracellular [125I]T3 and extracellular [131I]T3 saturably bound in the nucleus were determined. Comparable total amounts of T3 were saturably bound in the nuclei of liver (7.2), kidney (8.0), and gill (9.7 moles x 10(-13) .mg DNA-1), but the percentage of nuclear T3 generated within the target cell was greater for gill (76%) than for liver (50%) and kidney (28%). Both gill and liver possess a low Km T4 5'monodeiodinase which could be responsible for the high proportion of the nuclear T3 generated within those tissues.