Iodide induced suppression of thyrotropin-stimulated adenosine 3',5'-monophosphate production in cat thyroid slices

Cat thyroid slices were studied to investigate their responsiveness to thyrotropin stimulation of cyclic AMP accumulation. Ovine and bovine thyrotropin, in the presence of 2.5 mM aminophylline, induced a dose-dependent increase in the cyclic AMP content of cat thyroid tissue. Half-maximal stimulation of cyclic AMP accumulation was obtained at a thyrotropin concentration of 1-2 mU/ml. The maximal effect of thyrotropin was observed at 10 mU/ml, and was associated with a mean 77 +/- 19-fold increase in thyroidal cyclic AMP. Preincubation of cat thyroid tissue for 2 h with 50 micron NaI resulted in an impairment in the subsequent ability of thyrotropin to enhance cyclic AMP accumulation, without altering the level of cyclic AMP in tissues not exposed to the hormone. Preincubation alone was without effect on thyrotropin stimulation of cyclic AMP, and the inhibitory effect of iodide was prevented by addition of 3 mM methimazole to the preincubation medium. In addition, the time course of thytrotropin stimulation of cyclic AMP accumulation in cat thyroid slices was not significantly altered by the preincubation with excess iodide. These studies provide additional evidence that excess iodide inhibits the adenylate cyclase-cyclic AMP system in thyroid tissue.