Norepinephrine and Thyrotropin Stimulation of Iodide Efflux in FRTL-5 Thyroid Cells Involves Metabolites of Arachidonic Acid and Is Associated with the Iodination of Thyroglobulin*

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Abstract
Ca2+-dependent and TSH-, norepinephrine (NE)-, and A23187-induced iodide (I-) efflux from FRTL-5 rat thyroid cells is inhibited by quinacrine and trifluoroperazine, agents that inhibit phospholipase A2 activity. Furthermore, I- efflux can be stimulated by an activator of phospholipase A2 activity, melittin. Phospholipase A2 action releases arachidonic acid from phospholipids; arachidonic acid enhances I- efflux in FRTL-5 cells. Inhibitors of arachidonic acid metabolism via the lipoxygenase pathway, 5,8,11,14-eicosatetraynoic acid and nordihydroguaiaretic acid, and via the cytochrome P450-linked epoxygenase pathway, piperonyl butoxide and 2-diethylaminoethyl- 2,2-diphenyl valerate, but not an inhibitor of the cyclooxygenase pathway, indomethacin, can inhibit TSH-, NE-, and A23187-induced I- efflux. TSH, NE, and arachidonic acid stimulation of I- efflux in FRTL-5 cells is associated with increased iodination of thyroglobulin, which is blocked by 10 μM 5,8,11,14- eicosatetraynoic acid and 50 nM piperonyl butoxide. The data thus suggest that TSH- and NE-induced I- efflux from FRTL- 5 thyroid cells involves lipoxygenase and/or epoxygenase metabolites of arachidonic acid, released from phospholipids upon Ca2+-dependent activation of phospholipase A2. Since this process is associated with the iodination of thyroglobulin, TSH- and NE-induced I- efflux in FRTL-5 cells may represent the transport of I- from the cell into the follicular lumen in vivo. (Endocrinology120: 1127–1133, 1987)

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