Since iodide (I) inhibits TSH stimulation of cAMP formation, which mediates most of the effects of the hormone, it has been assumed that this accounts for the inhibitory action of iodide on the thyroid. However, TSH stimulation of 32P incorporation into phospholipids and stimulation of thyroid metabolism by other agonists, such as carbachol, phorbol esters, and ionophore A23187, is not cAMP mediated. the present studies examined the effect of iodide on stimulation of glucose oxidation and 32P incorporation into phospholipids by TSH and other agonists to determine if the inhibition of cAMP formation was responsible for the action of iodide. Preincubation of dog thyroid slices for 1 h with iodide (10-4 M) inhibited TSH-, (Bu)2cAMP-, carbachol-, methylene blue-, 12-O-tetradecanoyl phorbol-13-acetate-, ionophore A23187-, prostaglandin E1-, and cholera toxin-stimulated glucose oxidation. I also inhibited the stimulation by TSH, 12-O-tetradecanoyl phorbol- 13-acetate, carbachol, and ionophore A23187 of 32P incorporation into phospholipids. the inhibition was similar whether iodide was added 2 h before or simultaneously with the agonist. I itself sometimes stimulated basal glucose oxidation, but had no effect on basal 32P incorporation into phospholipids. the effects of iodide on basal and agonist-stimulated thyroid metabolism were blocked by methimazole (10-3 M). When dog thyroid slices were preloaded with 32PO4 or [l14C]glucose, the iodide inhibition of agonist stimulation disappeared, suggesting that the effect of iodide involves the transport process. In conclusion, I inhibited stimulation of glucose oxidation and 32P incorporation into phospholipids by all agonists, indicating that the effect is independent of the cAMP system and that iodide autoregulation does not only involve this system. Oxidation and organification of iodide are necessary for the inhibition. the ability of iodide to decrease glucose and 32PO4 transport may play an important role in thyroid autoregulation.
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