Calcium-induced release of 5-hydroxytryptamine from thyroid lobes in vitro and accompanying ultrastructural changes in parafollicular and follicular cells.

Biogenic amines, including serotonin (5-HT), have been shown to activate follicular cells of the thyroid. 5-HT is stored in bat's parafollicular cells. Previous radioautographical evidence indicates that this 5-HT is present in calcitonin granules. The present study was done to determine if Ca++, the natural stimulus to calcitonin release, would also release parafollicular cell 5-HT and, if so, whether this release would be accompanied by activation of follicular cells. Parafollicular cells were filled with labeled 5-HT by incubation of thyroid lobes of bats or mice with 5-[3H]hydroxytryptophan, the precursor of [3H]5-HT. Thyroid lobes were incubated in vitro in Ca++-free medium containing a chelating agent and were then challenged with Ca++ (0-30 mM). Release of [3H]5-HT was defected beginning at 5 mM Ca++. [3H]5-HT release was roughly proportional to the Ca++ concentration. Calcium challenge also affected thyroid ultrastructure in bats. After 10-min exposure to 30 mM Ca++, there were an increased number and centripetal movement of follicular cell lysosomes, development of apical pseudopods, and formation of colloid droplets. Many lysosomes also developed a crystalline-like matrix. An unique membrane-enclosed, rod-shaped organelle appeared in a small number of follicular cells. Parafollicular cells exposed to high concentrations of Ca++ were often degranulated, although many appeared unchanged. These changes confirm that 5-HT is released from parafollicular cells by elevating the external Ca++ concentration, supporting the hypothesis that 5-HT and calcitonin share storage granules. 5-HT release may mediate the activation of follicular cells by Ca++.