Thyroidal Iodide Accumulation and Loss<i>in Vitro</i>

Abstract

First order rate constants for 131 exit from rat thyroid lobes in vitro (KTM) were measured directly, using a method whereby the incubating medium was repeatedly replaced at 2-min intervals, and indirectly, by applying a 2-compartment (thyroid-medium) model to the findings from 131I- uptake experiments. In studies of 131I- exit only, KTM was enhanced by increasing the concentration of I- in either the incubating medium or the thyroid lobe, and b3r the presence of C1O44-, dinitrophenol, ouabain or excess K+ in the medium. The increased KTM diminished when excess I-, dinitrophenol or ouabain were removed from the bathing fluid. The effect of ouabain was largely overcome by raising the medium K+ concentration. In 131I- uptake studies, a high concentration of I- or C1O4 in the incubating fluid depressed the unidirectional clearance of 13lI- from the medium by the thyroid (C/m) and raised the KTM, but these findings could not be considered quantitatively reliable because of limitations of the model. Large amounts of I- in the gland lowered C/m to the same extent as they elevated KTM. Dinitrophenol and ouabain, on the other hand, produced little depression of the C/m, but a marked rise in KTM. Excess K+ in the medium moderately decreased C/m and increased KTM; it did not materially influence C/m when added to medium containing ouabain, but diminished KTM. The effects of I- and C1O4- on KTM and C/m can be most easily explained by postulating that they diminish the uptake of 13lI- by the thyroid and its recirculation within the lobe (through saturation and inhibition, respectively, of the I- transport mechanism). Under the conditions of these experiments, dinitrophenol and ouabain, on the other hand, appeared to exert their main influence by decreasing the effectiveness of a barrier (the apical cell membrane?) to 131I- egress from the follicles. Excess K+ apparently counteracted this influence of ouabain; its effect per se may have been more complex. (Endocrinology76: 441, 1965)