Effect of divalent cations on the binding of 3,5,3'-triiodothyronine to isolated rat liver nuclei.

Abstract

During incubation of rat liver nuclei at 22 C, a fraction of the T3 receptor was released to the medium. Ca2+ and Mg2+ decreased the amount of receptor released, thereby increasing specific T3 binding to the nuclei. However, the total specific binding (i.e. in the nuclei plus the incubation medium) was decreased at concentrations of divalent cations that induced maximal inhibition of receptor release. Neither Ca2+ nor Mg2+ influenced the binding of [125I]T3 to the receptor when a KC1 extract of the nuclei was studied. Therefore, the decrease of total T3 binding when maximal amounts of receptor were chromatin bound was probably due to a decreased affinity of the chromatinbound receptor. Furthermore, in the presence of 1 mM MgCl2 50% of the receptor was released to the incubation medium, and the affinity of the solubilized receptor was 1.7 times higher than the affinity of the receptor that remained associated to the chromatin; when CaCl2 was included in the incubation medium, 91% of the receptor was in the nuclei, and its affinity was lower than the affinity of the nuclear bound receptor in the presence of 1 mM MgCl2 and no Ca2+. These results suggest that divalent cations lower the affinity of the chromatin-bound receptor. Receptor dissociation from the nuclei was studied by diluting 15 times a nuclear suspension, at 0 C for 1 h. Receptor dissociation from the nucleus was enhanced by increasing dilution volume and Na+, Ca2+, or Mg2+ concentrations. In the absence of salt, 25% of the receptor was released from the nuclei. Addition of either CaCl2 or MgC12 up to a concentration of 10 mM increased receptor loss to 50%. In the presence of 0.15 M NaCl, receptor loss was 70% in the absence of divalent cations, and in either 10 mM CaCl2 or MgCl2 it was 90%. The main conclusion from this work is that divalent cations might influence the amount of chromatin-bound T3-receptor complex in two ways: 1) by lowering the affinity of the chromatin- bound receptor for T3, and 2) by increasing the dissociation of the receptor from its chromatin acceptor sites.