Kinetics of thyroxine (T4) and triiodothyronine (T3) transport in the isolated rat heart

Abstract

The dynamics and kinetics of thyroid hormone transport in the isolated rat heart were examined using the modified unidirectional paired tracer dilution method. The uptake of 125I-thyroxine (125I-T4) and 125I-triiodothyronine (125I-T3) from the extracellular space into heart cells was measured relative to the extracellular space marker 3H-mannitol. The thyroid hormone maximal uptake was 54.4% for 125I-T4 and 52.15% for 125I-T3. The thyroid hormone net uptake was 25.69% for 125I-T4 and 25.49% for 125I-T3. Backflux from the intracellular space was 53.17% for 125I-T4 and 61.59% for 125I-T3. In the presence of unlabelled thyroid hormones, 125I-T4 and 125I-T3 maximal uptakes were reduced from 10.1 to 59.74% and from 34.6 to 65.3%, respectively, depending on the concentration of the unlabelled hormone, suggesting a saturable mechanism of the thyroid hormone uptake by the heart cells, with Km(T4)=105.46 μM and the maximal rate of 125I-thyroid hormone flux from the extracellular space to heart cells (Vmax(T4) = 177.84 nM min-1 for 125I-T4 uptake, and Km(T3) = 80.0 μM and Vmax(T3) = 118.5 nM min-1 for 125I-T3 uptake.