Thyrotropin-Induced Thyroidal Release of 5-Hydroxytryptamine and Accompanying Ultrastructural Changes in Parafollicular Cells*

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Abstract
The action of TSH on parafollicular cells of the bat thyroid was examined. Parafollicular cells were loaded with [3H]5-hydroxytryptamine ([3H]5-HT) by incubation with the precursor [3H]5-hydroxytryptophan in vitro. Subsequent exposure to TSH released [3H]5-HT from the glands, but not [3H]5-hydroxytryptophan. When thyroids were loaded with [3H]norepinephrine, TSH failed to release that amine. The [3H]5-HT-releasing effects of TSH were blocked by dinitrophenol and antimycin A and so were energy dependent. [3H]5-HT was not released by pentagastrin or calcitonin. EM examination of thyroid glands exposed to TSH in vitro revealed degranulation of some parafollicular cells as well as the presence of abnormal appearing microgranules and large intracisternal (within rough endoplasmic reticulum) accumulations of secretory material. These results demonstrate for the first time direct effects of TSH on parafollicular cells. The results are consistent with the hypothesis that 5-HT, a normal constituent of bat parafollicular cells and an activator of follicular cells, may act as an intrathyroid local hormone.