Intracellular calcium chelator BAPTA protects cells against toxic calcium overload but also alters physiological calcium responses

The effect of the membrane-permeant calcium chelator 1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl) ester (BAPTA/AM) on ionomycin-induced cellular calcium overload was studied in single differentiated NH15-CA2 neuroblastoma x glioma hybrid cells. To monitor [Ca2+](i), we used the fluorescent indicator Fura-2. Preincubation of the cells with 3 mu M BAPTA/AM reduced the number of cells showing deregulation of [Ca2+](i) during ionomycin-induced calcium influx. The calcium transients elicited by application of KCI were also severely affected by the chelator. These transients, although varying from cell to cell in shape, amplitude and duration, are well reproducible in individual cells. After incubation of cells for 1 h with 0.3-30 mu m BAPTA/AM the time course of these cellular transients was markedly slowed. At 1 mu M BAPTA/AM, the time constant of decline of [Ca2+](i) was increased by a factor of 4.1 +/- 2.4 (n = 14) and the amplitude was reduced to about 50%. With 30 mu M BAPTA/AM, the K+-induced calcium transients were almost completely inhibited. We conclude that intracellularly loaded calcium chelators may be used for the prevention of [Ca2+](i)-induced cell damage, however, at the expense of a disturbed calcium signalling.