Modulation of myocardial L-triiodothyronine receptors in normal, hypothyroid, and hyperthyroid rats

To characterize the nuclear receptor believed to mediate the thyroid hormones' actions on the heart, binding of L-[125I]T3 to extracts of myocardial cell nuclei from normal, propylthiouracil, and T4-treated rats has been investigated. Analysis of in vitro iodothyronine binding to this solubilized nuclear preparation revealed multiple saturable, specific binding sites for T3. High affinity binding for T3 (Kd = 4.2 +/- 1.0 X 10(-10) mol/L), and lower affinity (Kd approximately 10(-8) mol/L) binding activity appeared to be independent (Hill plot slope = 1). The mean maximal binding capacity of the high affinity binding site for T3 in euthyroid rats (84 +/- 8 fmol/mg DNA) was increased by approximately 50% in hypothyroidism (120 +/- 6 fmol/mg DNA) and unchanged in hyperthyroidism (88 +/- 25 fmol/mg DNA). The molecular weight of this T3 binding site is estimated to be 50,000-55,000 daltons. The properties of this solubilized binding activity from rat myocardial nuclei are consistent with its putative role as the biologic thyroid hormone receptor. The increase in binding capacity with hypothyroidism suggest regulation by thyroid hormone of its nuclear receptor in myocardium.