Characterization of thyroid hormone transport in synaptosomes from rat brain

In this study we examined the mode of triiodothyronine (T3) and tetraiodothyronine (T4) transport in synaptosomal preparations from cerebral hemispheres of adult rat brain. Our results show that these hormones are transported by different mechanisms: T3 uptake is a saturable process and Hofstee analysis of the data reveals two transport components--a high affinity (Kt approximately 50 pM), low density and a low affinity (Kt approximately 3.1 nM) high density system. The Vmax of both components is influenced by the extracellular/intracellular Na+ gradient. T3 uptake decreases in the presence of ouabain and gramicidin. T3 uptake also shows a temperature dependence and decreases in the presence of KCN. In contrast, T4 uptake is a nonsaturable process and is not influenced by metabolic inhibitors or Na ions. It is proposed that T3 entry into neurons is a carrier-mediated process and depends on Na ions. In contrast, T4 is transported by diffusion which is driven by high extracellular/intracellular differences in T4 concentration, maintained by the high rate of cellular deiodination of T4 to T3, characteristic of this tissue.