Iodide Modulation of Ca²⁺ Efflux from Mouse Thyroid

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In a preceding report, we showed evidence that thyrotropin (TSH) stimulates Ca²⁺ efflux from mouse thyroid gland and that TSH stimulation of Ca²⁺ efflux is inhibited by acute administration of excess iodide to mice fed a low iodine diet (Hashizume et al., 1984). The observations sugge ted that iodide inhibits Ca²⁺ efflux through an inhibition of TSH-sensitive adenylate cyclase activity. We found further, that iodide inhibits dibutyryl cyclic AMP (DBC)-stimulated Ca²⁺ efflux. The results isuggeted that iodide influences the step subsequent to the generation of cyclic AMP.
In this report, we studied whether iodide can inhibit Ca²⁺ efflux by a mechanism which is distinct from adenylate cyclase inhibition. The acute administration of excess iodide to mice fed a regular diet did not decrease the basal Ca²⁺ efflux rate in the thyroid. TSH-induced stimulation of Ca²⁺efflux in thyroids obtained from regular diet-treated mice was not modified by iodide administration. Iodide injection to mice fed a low iodide diet, however, decreased the basal Ca²⁺ efflux rate though the content of cyclic AMP in the thyroids was not altered by this treatment. The decreased-Ca²⁺ efflux rate induced by iodide in the low iodine diet-treated thyroids was not modified by TSH in vitro. The results indicate that an acute administration of excess iodide in thyroid inhibits Ca²⁺ efflux not only by an inhibition of adenylate cyclase but also by an inhibitory action which is distinct from the adenylate cyclase inhibiting action of iodide.