Kinetics of Thyroxine (T₄) and Triiodothyronine (T₃) Transport in the Isolated Rat Heart

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 125 I‐thyroxine ( 125 I‐T 4 ) and 125 I‐triiodothyronine ( 125 I‐T 3 ) from the extracellular space into heart cells was measured relative to the extracellular space marker 3 H‐mannitol. The thyroid hormone maximal uptake was 54.4% for 125 I‐T 4 and 52.15% for 125 I‐T 3 . The thyroid hormone net uptake was 25.69% for 125 I‐T 4 and 25.49% for 125 I‐T 3 . Backflux from the intracellular space was 53.17% for 125 I‐T 4 and 61.59% for 125 I‐T 3 . In the presence of unlabelled thyroid hormones, 125 I‐T 4 and 125 I‐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 μM and the maximal rate of 125 I‐thyroid hormone flux from the extracellular space to heart cells ( V max(T4) ) = 177.84 nM min −1 for 125 I‐T 4 uptake, and K m(T3) = 80.0 μM and V max(T3) = 118.5 nM min −1 for 125 I‐T 3 uptake.

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