Phagocytosis of fluorescent beads by rat thyroid follicular cells (FRTL-5): comparison with iodide trapping as an index of functional activity of thyrocytes in vitro.

The ability of FRTL-5 rat thyroid follicular cells to engulf latex beads by phagocytosis was evaluated using flow cytometry and compared to iodide trapping in response to selected growth factors, second messengers, and chemicals. Cell suspensions were analyzed to determine the percentage of fluorescence-positive cells as well as the fluorescence intensity of positive cells. Phagocytosis was stimulated by forskolin, cholera toxin, 8-Br-cAMP, calcitriol, and transforming growth factor-beta. In contrast, phagocytosis was inhibited by insulin, calcium, and aminotriazole, but not by sodium iodide. The results of this study showed that phagocytosis of latex beads was regulated in a manner similar to iodide trapping and could be altered by the addition of numerous compounds. Phagocytic activity was stimulated by both cAMP-dependent and cAMP-independent pathways. Flow cytometric evaluation of phagocytosis of fluorescent latex beads represents a simple, rapid, nonradioactive index of thyroid function in vitro.