Possible role of microtubules in thyroid secretion.

Colchicine and other microtubule-active agents have been found to block the release, stimulated by either thyroid-stimulating hormone or by dibutyryl cyclic adenosine 3':5'-monophosphate, of (131)I from previously (131)I-labeled mouse thyroid glands in vitro. The time and concentration characteristics of these inhibitors are consistent with their actions on microtubules in other systems. [(3)H]colchicine was also shown to be bound to a soluble 6S protein of bovine thyroid slices similar to the protein identified in other systems as a microtubular subunit. The demonstrated inhibition of colloid droplet formation and absence of an effect on thyroidal adenyl cyclase or cyclic 3':5'-phosphodiesterase suggests a colchicine-sensitive role for microtubules in colloid endocytosis in the thyroid gland.