[Increased liver nuclear triiodothyronine-receptors associated with mitochondrial alpha-glycerophosphate dehydrogenase activity in hyperthyroid rats (author's transl)].

: The effects of triiodothyronine (T3) on binding characteristics of nuclear T3-receptors were examined in rats. The T3-receptors were extracted by 0.4M KCl from the liver nuclei isolated from the following 3 groups of rats: (1) thyroidectomized, (2) thyroidectomized and treated with 230 ng T3/100 g body weight/day for 3 days, and (3) thyroidectomized and treated with 40 micrograms T3/100 g body weight/day for 3 days. Their association constant (Ka) and maximal binding capacity (Cmax) for T3 were determined by Scatchard analyses with and without correction for endogenous T3. The amount of endogenous T3 bound to the nuclear extracts was estimated on the basis of a specific activity of [125I]-T3 injected 2 hours before sacrifice. It was demonstrated that Cmax values corrected for the endogenous T3 were 2.5 times greater in severely hyperthyroid than in hypothyroid rats. By contrast, corrected values for Ka remained unchanged in all 3 groups of rats, although uncorrected values were apparently decreased in the severely hyperthyroid rats. Validity of the correction was supported by the in vitro experiments preincubated with stable T3. The yields of nuclear protein-125I-T3 complex in the procedure of extraction and the contents of DNA per g of liver were nearly the same in the 3 groups. The increase in Cmax of the nuclear receptors was directly related to mitochondrial alpha-glycerophosphate dehydrogenase activity. The results obtained indicate that the Cmax of nuclear T3-receptors is increased in a severely hyperthyroid state, related to hormonal action, and that the correction for endogenous T3 is essential for an accurate estimation of Ka and Cmax.