Comparison of the Effects of Ouabain and of Iodine Adaptation on the Thyroid Iodide Transport Mechanism

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Abstract
Intact or hypophy-sectomized rats were maintained on low iodine or high iodine regimens to produce iodine-poor or iodine-rich thyroids, which were then studied in vitro. Effects of variation in medium K+ concentration and of ouabain on glandular K+ concentration and thyroid/medium radioiodide concentration ratios (T/M) were assessed. In media of normal K+ content (5 mEq/1), iodine-rich thyroids maintained the same K+ content as did iodine-poor controls, although T/M ratios were far lower in iodine-rich glands. Medium K+ concentrations of 15 and 30 mEq/l did not increase the low T/M ratios of iodine-rich thyroids, although glandular K+ concentrations were increased. The decreases in both K+ and iodide transport induced by ouabain or by exclusion of K+ from the medium were similar in iodine-poor and iodine-rich glands. In both iodine-poor and iodine-rich thyroids, hypophysectomy did not influence the response to variations in extracellular K+ concentration. It is concluded that the autoregulatory inhibition of iodide transport which occurs in the iodine-rich thyroid does not result from a ouabain-like effect on glandular K+ content.