Putative Nuclear Triiodothyronine Receptors in Tadpole Liver during Metamorphic Climax
It has been shown previously that the maximum binding capacity (MBC) of the putative 3,5,3′-triiodothyronine (T3) receptor in red blood cells (RBCs) and tail tissue of Rana catesbeiana tadpoles is increased during development (1,2), and can be stimulated in RBCs by treatment with thyroid hormone (TH) (3). The present study was performed to determine if the MBC of tadpole liver nuclei is also increased during development or following treatment with TH. Because of the relatively high levels of endogenous TH in tadpoles during climax and the slow rate of dissociation of the hormone-receptor complex (4), the use of an in vivo saturation assay employing [125I]T3 was not feasible. Thus, a method was developed for determining MBC by injecting sufficient stable T3 to just saturate the receptors, and then quantifying total nucleus-bound T3 and plasma T3 concentrations by RIA. Non-saturable (NS) binding was determined by injecting tadpoles with 10 nmol T3. Under these conditions, the contribution of the saturable binding sites to total binding was expected to be insignificant. Thus, when total T3 bound to nuclei was plotted against plasma T3 concentration, the NS binding could be represented as a line drawn between the origin and the point designating the nucleus-bound T3 after injection of 10 nmol T3. The quantity of T3 bound to the saturable sites after injection of a lower dose of T3 was then obtained by subtracting from the value for total nucleus-bound T3, the amount of T3 bound to NS sites at the same plasma T3 concentration. Preliminary studies using isotopic techniques in premetamorphic tadpoles indicated: 1) that comparable values for MBC were obtained when data were analyzed by this method and by Scatchard analysis; 2) that saturation of the nuclear receptors was almost complete following injection of 200 pmol T3/tadpole.
KeywordsThyroid Hormone Scatchard Analysis Liver Cytosol Maximum Binding Capacity Metamorphic Climax
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