An Analysis of the Sources and Quantity of 3,5,3′-Triiodothyronine Specifically Bound to Nuclear Receptors in Rat Cerebral Cortex and Cerebellum*

Abstract

Recent studies have indicated that there is rapid in vivo conversion of L-[125I]T4 to L-T3 in rat cerebral cortex and cerebellum. These early studies showed significant quantities of [125I]T3 specifically bound to nuclear thyroid hormone receptors. The amount of [125I]T3 relative to the simultaneously injected [125I]T3 in cell nuclei was greater in the cerebral cortex and cerebellum than in other tissues examined, suggesting a major contribution of locally generated T3 to total nuclear T3 in these organs. The present studies have been performed to assess the relative contribution of both T3 sources, the plasma and local 5′-deiodination to the total mass of T3 specifically bound to the nuclear receptors of cerebral cortex and cerebellum in euthyroid rats in steady state conditions. To determine the maximal binding capacity of the nuclear receptors in these two tissues, rats were injected with increasing quantities of unlabeled T3 along with labeled T3. The maximal binding capacity of the nuclear receptors in cerebral cortex was 266 pg T3/mg DNA (95% confidence limits, 242–290) and that for cerebellum was 95 pg T3/mg DNA (89–101). Euthyroid rats were then given [125I]T4 and [131I]T3 and, after suitable corrections for the contribution of plasma [125I]T3 to nuclear [125I]T3, they were given the locally generated nuclear [125I]T3. Local (Lc) T3(T4), was computed. At 15 h or longer after [125I]T4 injection, Lc T3(T4) accounted for more than 80% of the total nuclear [125I]T3 in cer…