Stimulating rat thyroid FRTL-5 cells with agonists that activate the inositol phosphate cascade results in the release of sequestered calcium and influx of extracellular calcium. In addition, phospholipase A, (PLA,) is activated. Since PLA, is a calcium-dependent enzyme we wanted to investigate the interrelationships between PLA, activity and the entry of calcium. Stimulating 3H-arachidonic acid (3H-AA)-labelled cells with thapsigargin resulted in a substantial release of3H-AA.
This release was totally abolished in a calcium-free buffer. Pretreatment of Fura 2 loaded cells with 4-bromophenacyl bromide, an inhibitor of PLA, activity, decreased the thapsigargin-induced entry of calcium, suggesting a role for PLA, in the regulation of calcium entry. In cells treated with nordihydroguaiaretic acid (NDCA), clotramizole, or econazole, compounds with Iipoxygenase and cytochrome P-450 inhibitory actions, the thapsigargin-induced entry of calcium was decreased in a dose-dependent manner. However, treatment of the cells with indomethacin, a cyclooxygenase inhibitor, had no effect on the thapsigargininduced calcium entry. We also showed that stimulation of the cells with arachidonic acid released sequestered calcium, apparently from the same intracellular pool as did thapsigargin. The results suggested that the calcium-induced PLA, activation and the metabolism of the produced arachidonic acid by a noncyclooxygenase pathway may be of importance in maintaining calcium entry after releasing sequestered Ca2+ in FRTL-5 cells.