Amiodarone is an antiarrhythmic drug that often induces thyroid disorders. Its effects on several aspects of thyroid function were studied using cultured dog thyroid cells. Within 5-60 min of incubation of cell membranes with amiodarone, there were profound changes in adenylate cyclase activity and TSH receptor binding. Amiodarone specifically decreased TSH-stimulated adenylate cyclase activity, but not the basal or forskolin-stimulated activities, while it increased the binding of125I-labeled TSH to its receptors. Significant effects were seen with 5-10 µM amiodarone, with maximal effects at 50-100 µM, when TSH-stimulated adenylate cyclase activity was completely blocked and the labeled TSH binding increased 4- to 5-fold over control. These effects of amiodarone were reversible, since membranes exposed to 50 µM amiodarone for 1 h exhibited normal binding and cyclase activities, when amiodarone was removed by washing before the assay. The above effects of amiodarone were also observed when cells, instead of membranes, were treated with the drug, although the magnitude of changes was less than in membranes. Lower concentrations of amiodarone (10-25 µM) caused significant inhibition of iodide organification, without affecting iodide uptake, while higher concentrations (50-100 µM) inhibited organification by nearly 75% and uptake by about 20%. Amiodarone (10-100 µM) also inhibited [3H]2-deoxy-glucose uptake and the increase in intracellular calcium concentration in response to TSH and carbachol. In contrast to membranes, treatment of cells with amiodarone caused persistent inhibition of TSH-stimulated cAMP formation and iodide organification even 24-48 h after removal of the drug. However, amiodarone had no effect on cell viability, as judged by trypan blue exclusion and ability to remain attached to the culture dishes. These results suggest that amiodarone has specific inhibitory effects on agonist-stimulated functions in thyroid cells, possibly by interfering with TSH-receptor interactions and also at the level of cholinergic receptors.
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