Calcium fluxes in rat thyroid FRTL-5 cells. Evidence for Ca2+ entry after stimulation with ATP

The relationship between ATP-induced uptake of 45Ca2+ and the ATP-induced changes in [Ca2+]i was investigated in rat FRTL-5 thyroid cells. Addition of 1 microCi 45Ca2+/ml together with ATP induced a time- and dose-dependent increase in uptake of 45Ca2+, the uptake being still significantly above control after 30 min. Resting intracellular free Ca2+ levels ([Ca2+]i), measured using Fura-2, was determined to be 60 +/- 14.3 nM (mean +/- SE). ATP induced a rapid, transient increase in [Ca2+]i (785 +/- 56.2 nM) followed by a plateau phase (127 +/- 34.3 nM). In a Ca(2+)-free buffer, the ATP-induced transient was significantly decreased (357 +/- 57.4 nM, p less than 0.05), and the plateau phase was abolished. The results suggested that stimulating FRTL-5 cells with ATP induced an influx of Ca2+, possibly by a mechanism dependent on a transient increase in [Ca2+]i. To further test this possibility, the intracellular Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) was tested. In cells loaded with BAPTA, the ATP-induced uptake of 45Ca2+ was greatly enhanced, while the ATP-induced transient increase in [Ca2+]i was almost totally abolished. In cells stimulated with ATP in a Ca(2+)-free buffer, readdition of Ca2+ after termination of the ATP response induced a rapid increase in [Ca2+]i. Furthermore, addition of Mn2+ to cells stimulated with ATP induced a more rapid quenching of Fura-2, compared to that seen in control cells. The results indicate that stimulating FRTL-5 cells with ATP induces a rapid release of Ca2+ from intracellular stores, followed immediately by an increase in plasma membrane conductance and influx of extracellular Ca2+. The ATP-induced change in [Ca2+]i may function as a signal enhancing influx of extracellular Ca2+, although some other unknown mechanism(s) is also needed.