Inhibitory effect of iopanoic acid on the thyrotropin-stimulated release of cyclic adenosine 3',5'-monophosphate and of 3,5,3'-triiodothyronine from perifused rat thyroids.

Using a perifusion system, we studied the effect of iopanoic acid, an iodinated contrast agent used in oral cholecystography, on the release of cAMP, T3, T4, and rT3 from perifused rat thyroid pieces. A 0.1 mg/ml iopanoic acid solution significantly inhibited the TSH-stimulated release of cAMP (without iopanoic acid, 8175 +/- 373; with iopanoic acid, 5169 +/- 355 fmol/mg thyroid X 3 h, mean +/- SE) and T3 (without iopanoic acid, 971 +/- 32; with iopanoic acid, 659 +/- 32 pg/mg thyroid X 3 h) in the presence of 3-isobutyl-1-methylxanthine. T4 and rT3 releases were not significantly affected. Inhibition of TSH-stimulated T3 release by iopanoic acid was also observed at a concentration of 0.01 mg/ml. Propylthiouracil completely abolished the inhibitory effect of iopanoic acid on TSH-stimulated cAMP release but not on TSH-stimulated T3 release. TSH-stimulated cAMP release was augmented by iodide at a concentration of 1 X 10(-3) M in the presence of 3-isobutyl-1-methylxanthine but suppressed by iodide at a concentration of 1 X 10(-5) M. TSH-stimulated T3 release was suppressed slightly at both concentrations of iodide. These results suggest that iopanoic acid may have an inhibitory effect on the TSH-stimulated cAMP and T3 release from perifused rat thyroids. This effect can probably be attributed to the iodide contained in the agent and to the inhibited intrathyroidal conversion of T4 to T3.