Studies on in vitro effect of serotonin on calcitonin secretion by rat thyroid C cells

Thyroid glands of young rats were incubated for 3 h in Eagle's solution supplemented with 5-hydroxy-l-tryptophan (5-HTP) or with serotonin. Following control incubations or incubations with serotonin, no serotonin could be demonstrated in C cells using immunocytochemical techniques. However, serotonin was demonstrated in the secretory granules of all C cells following incubation with 5-HTP. The secretory function of C cells was evaluated by ultrastructural and immunocytochemical studies, and by calcitonin radioimmunoassays of the incubation medium. Following incubation with 5-HTP, the secretory function of the majority of C cells was inhibited, and calcitonin levels in the media were decreased. Incubation with serotonin produced an increased secretory function of C cells and higher calcitonin levels in the media. The results indicate that serotonin and its direct precursor, 5-HTP, affect calcitonin secretion by rat thyroid C cells by distinct mechanisms.