Evidence for receptor‐mediated calcium entry and refilling of intracellular calcium stores in FRTL‐5 rat thyroid cells

The aim of the present study was to investigate the relationship between agonist‐induced changes in intracellular free Ca 2+ ([Ca 2+ ] j ) and the refilling of intracellular Ca 2+ stores in Fura 2–loaded thyroid FRTL‐5 cells. Stimulating the cells with ATP induced a dose‐dependent increase in ([Ca 2+ ] j ). The ATP‐induced increase in [Ca 2+ ] j was dependent on both release of sequestered intracellular Ca 2+ as well as influx of extracellular Ca 2+ . Addition of Ni 2+ prior to ATP blunted the component of the ATP‐induced increase in [Ca 2+ ] j dependent on influx of Ca 2+ . In cells stimulated with ATP in a Ca 2+ ‐free buffer, readdition of Ca 2+ induced a rapid increase in [Ca 2+ ] j ; this increase was inhibited by Ni 2+ . In addition, the ATP‐induced influx of 45 Ca 2+ was blocked by Ni 2+ . Stimulating the cells with noradrenaline (NA) also induced release of sequestered Ca 2+ and an influx of extracellular Ca 2+ . When cells were stimulated first with NA, a subsequent addition of ATP induced a blunted increase in [Ca 2+ ] j . If the action of NA was terminated by addition of prazosin, and ATP was then added, the increase in [Ca 2+ ] j was restored to control levels. Addition of Ni 2+ prior to prazosin inhibited the restoration of the ATP response. In the presence of extracellular Mn 2+ , ATP stimulated quenching of Fura 2 fluorescence. The quenching was probably due to influx of Mn 2+ , as it was blocked by Ni 2+ . The results thus suggested that stimulating release of sequestered Ca 2+ in FRTL‐5 cells was followed by influx of extracellular Ca 2+ and rapid refilling of intracellular Ca 2+ stores.