Triiodothyronine Nuclear Receptors: Anin VitroComparison of the Binding of Triiodothyronine to Nuclei of Adult Rat Liver, Cerebral Hemisphere, and Anterior Pituitary*

Abstract

An analysis of Scatchard plots of triiodothyronine (T₃) binding to purified nuclei from liver, cerebral hemisphere, and anterior pituitary of adult rats is presented. The data indicate that the binding of T₃ to nuclei from these tissues is linear over the concentration range 50–5000 pM, and show that the binding of hormone to the nuclear receptors is independent, saturable, and obeys Michaelis-Menton kinetics. Analysis of the combined Scatchard data from several experiments reveals that the number of T₃ binding sites per microgram of DNA is greatest for the anterior pituitary (2.0 fmol/μg DNA) followed by cerebral hemisphere (0.8 fmol/μg DNA), and liver (0.2 fmol/μg DNA). The dissociation constants determined for the receptors from the above tissues varies between 0.2–0.4 nM and are in agreement with previously published data from several other laboratories. A comparison of the pH dependence and magnesium ion dependence of nuclear T₃ binding to cerebral hemisphere and liver show that both receptors have pH optima between 7.5 and 8.5 and exhibit identical sensitivity to magnesium ion inhibition. These results indicate that receptors from liver and cerebral hemisphere may be similar. The data are discussed in terms of the known physiological effects of thyroid hormone, particularly with respect to the role of this hormone in the brain. (Endocrinology102: 556, 1978)