Failure of Organic Iodine Enrichment to Influence the Binding of Bovine Thyrotropin to Rat Thyroid Tissue*

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Abstract
In an effort to clarify the mechanism whereby increases in the thyroid content of organic iodine lead to inhibition of the stimulation of thyroid adenylate cyclase produced by TSH, studies have been conducted of the effect of enrichment with iodine on the specific binding of TSH by receptors in thyroid tissue. Enhancement of thyroid organic iodine content was induced acutely by incubating thyroid lobes of intact iodinedeficient rats in media containing iodide, with or without TSH, in vitro or by administering iodide to intact or hypophysectomized iodine-deficient rats in vivo. Specific binding of 125I-labeled bovine TSH (bTSH) and its displacement by unlabeled bTSH was then assessed in fractions prepared from these and control glands using methods which make possible study of the properties of the saturable binding site for bTSH previously demonstrated in rat thyroid tissue. In none of the models employed did prior enhancement of the thyroid organic iodine content appear to influence either the maximum capacity or affinity of bTSH binding. It is concluded that enrichment of thyroid organic iodine does not result in altered binding of TSH. Since the inhibitory effect of iodine on the response of thyroid adenylate cyclase to TSH is apparently not due to an inhibition of the enzyme itself, it is suggested that iodine acts on the mechanism whereby the binding of TSH is coupled to cyclase activation.

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