ACTIVATION OF CALCIUM ENTRY BY CYCLOPIAZONIC ACID IN THYROID FRTL-5 CELLS

The aim of the present study was to investigate whether the Ca2+-ATPase inhibitor cyclopiazonic acid (CPA) could empty intracellular Ca2+ stores and activate Ca2+ influx in thyroid FRTL-5 cells. Addition of CPA to Fura-2 loaded cells rapidly increased intracellular free Ca2+ ([Ca2+]i) which then stabilized at a new elevated steady state level. The initial increase was mainly dependent on the release of sequestered Ca2+, but was decreased in Ca2+-free buffer and in depolarized cells. The plateau phase was totally dependent on extracellular Ca2+. Addition of Ca2+ to cells exposed to CPA in Ca2+-free buffer rapidly increased [Ca2+]i. This influx was decreased in depolarized cells and inhibited by SKF 96365. Addition of CPA to cells prior to stimulating the cells with ATP totally abolished the ATP-induced increase in [Ca2+]i. In Ca2+-free buffer, addition of ATP prior to CPA decreased the response in [Ca2+]i evoked by CPA. The results show that emptying intracellular Ca2+ stores with CPA rapidly activates influx of Ca2+ in FRTL-5 cells. Furthermore ATP and CPA appear to release Ca2+, at least in part, from the same intracellular Ca2+ store in these cells.