THAPSIGARGIN-INDUCED CALCIUM-ENTRY IN FRTL-5 CELLS - POSSIBLE DEPENDENCE ON PHOSPHOLIPASE A(2) ACTIVATION

Stimulating rat thyroid FRTL-5 cells with agonists that activate the i nositol phosphate cascade results in the release of sequestered calciu m and influx of extracellular calcium. In addition, phospholipase A(2) (PLA(2)) is activated. Since PLA(2) is a calcium-dependent enzyme we wanted to investigate the interrelationships between PLA(2) activity a nd the entry of calcium. Stimulating H-3-arachidonic acid (H-3-AA)-lab elled cells with thapsigargin resulted in a substantial release of H-3 -AA. This release was totally abolished in a calcium-free buffer. Pret reatment of Fura 2 loaded cells with 4-bromophenacyl bromide, an inhib itor of PLA(2) activity, decreased the thapsigargin-induced entry of c alcium, suggesting a role for PLA(2) in the regulation of calcium entr y. in cells treated with nordihydroguaiaretic acid (NDGA), clotramizol e, or econazole, compounds with lipoxygenase and cytochrome P-450 inhi bitory actions, the thapsigargin-induced entry of calcium was decrease d in a dose-dependent manner. However, treatment of the cells with ind omethacin, a cyclooxygenase inhibitor, had no effect on the thapsigarg in-induced calcium entry. We also showed that stimulation of the cells with arachidonic acid released sequestered calcium, apparently from t he same intracellular pool as did thapsigargin. The results suggested that the calcium-induced PLA(2) activation and the metabolism of the p roduced arachidonic acid by a noncyclooxygenase pathway may be of impo rtance in maintaining calcium entry after releasing sequestered Ca2+ i n FRTL-5 cells. (C) 1994 Wiley-Liss, Inc.