Ionophore A23187 inhibits the release of thyrotropin-stimulated 3, 5, 3'-triiodothyronine from perifused rat thyroid glands.

We have studied the effect of ionophore A23187 on the release of T3 and cAMP from perifused rat thyroid glands and on the morphological changes of the follicle epithelial cells. Ionophore A23187 at a concentration of 5 microM significantly inhibited TSH-stimulated T3 release. Depletion of Ca2+ in the infusion buffer completely abolished the inhibitory effect of ionophore A23187 on TSH-stimulated T3 release. In the presence of TSH and 3-isobutyl-1-methylxanthine, ionophore A23187 at 5 microM did not significantly affect either the release of cAMP during a 3-h perifusion or the cAMP content in the thyroid tissues after 1-h perifusion. By electron microscopy, the follicle epithelial cells of the rat thyroid tissues showed marked responses to TSH after 3-h stimulation. By the addition of 5 microM ionophore A23187, the ultrastructural changes such as phagocytotic uptake of luminal colloid and the formation of intracellular colloid droplets were rarely observed in spite of the presence of TSH. The present study indicates that ionophore A23187 at a concentration of 5 microM inhibits both TSH-induced morphological changes of the follicle epithelial cells and TSH-stimulated T3 release without reducing cAMP level stimulated by TSH in rat thyroid glands.