To estimate the relative contribution of L-triiodothyronine (T3) and L-thyroxine (T4) to thyroidal effects, we have measured the concentration of iodothyronine bound to specific hepatic nuclear receptor sites by three different techniques: (a) specific radioimmunoassay after separation of T3 and T4 by preparative paper chromatography; (b) in vivo kinetic approaches as reported previously; and (c) isotopic equilibration. By these three methods, receptor concentration of T3 and T4 in liver was 0.51 ± 0.19 (SD) and 0.08 ± 0.06; 0.52 ± 0.12 and 0.08 ± 0.02; and 0.50 ± 0.13 and 0.10 ± 0.03 pmol/mg DNA, respectively. The percentage contribution of T3 and T4 to total receptor iodothyronine was thus 86.8 ± 9.0 and 13.2 ± 9.4; 86.3 ± 3.5 and 13.7 ± 3.5; and 83.7 ± 5.6 and 16.3 ± 5.6%, respectively. In kidney, specifically bound nuclear T3 and T4 were estimated both by isotopic equilibration and by in vivo kinetic techniques to be 0.28 ± 0.11 and 0.03 ± 0.01 pmol/mg DNA, respectively. Thus T3 constituted 89.4 ± 3.2% of total receptor iodothyronine in this tissue. No other iodothyronines or analogs were bound to the nuclear sites in either tissue. Kidney and liver nuclear T3 concentrations also were identical to values previously reported with in vivo kinetic techniques. Other studies from this laboratory have suggested that thyroid effect is related to the molar concentration of iodothyronine bound to specific nuclear sites, that the sites are similar in various tissues, and that iodothyronine in plasma is in equilibrium with nuclear T3. If these relationships are assumed, T3 contributes between 85 and 90% of thyroidal effects in the euthyroid rat. The remaining 10-15% of thyroidal effect appears to result from the intrinsic activity of T4.
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