Kinetics of thyroxine (T-4) and triiodothyronine (T-3) transport in the isolated rat heart
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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 I-125-thyroxine (I-115-T-4) and I-125-triiodothyronine (I-125-T-3) from the extracellular space into heart cells was measured relative to the extracellular space marker H-3-mannitol. The thyroid hormone maximal uptake was 54.4 % for I-125-T-4 and 52.15 % for I-125-T-3. The thyroid hormone net uptake was 25.69 % for I-125-T-4 and 25.49 % for I-125-T-3. Backflux from the intracellular space was 53.17 % for I-125-T-4 and 61.59 % for I-125-T-3. In the presence of unlabelled thyroid hormones, I-125-T-4 and I-125-T-3 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 K-m(T4) = 105.46 muM and the maximal rate of I-125-thyroid hormone flax fr...om the extracellular space to heart cells (V-max(T4))= 177.84 nM min(-1) for I-125-T-4 uptake, and K-m(T3) = 80.0 muM and V-max(T3) = 118.5 nM min(-1) for I-125-T-3 uptake.