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 (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 microM 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 microM and V(max(T3)) = 118.5 nM min(-1) for (125)I-T(3) uptake. Experimental Physiology (2001) 86.1, 13-18.

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