Calcium channels in PDGF-stimulated A172 cells open after intracellular calcium release and are not voltage-dependent

Using laser image cytometry and indo-1 fluorescence, we investigated the intracellular free Ca²⁺ concentration ([Ca²⁺]_i) of confluent A172 human glioblastoma cells stimulated by the BB homodimer of platelet-derived growth factor (PDGF-BB). The shape of the calcium transients and the delay time between stimulation and the beginning of the transient varied considerably. The percentage of responsive cells, the peak [Ca²⁺]_i and the duration of the response were directly related to PDGF-BB dose, while the delay time was inversely related; the maximal response occurred at a PDGF-BB concentration of 20 ng/ml. Studies with EGTA and inorganic calcium-channel blockers (Ni²⁺, La³⁺) showed that the increase of [Ca²⁺]_i resulted from initial release of intracellular stores and subsequent calcium influx across the plasma membrane. Opening of calcium channels in the plasma membrane, monitored directly by studying Mn²⁺ quenching of indo-1 fluorescence, was stimulated by PDGF-BB and blocked by La³⁺; the opening occurred 55 ± 10 s after the initial increase in [Ca²⁺]_i. Therefore, in these tumor cells, intracellular release always occurs before channel opening in the plasma membrane. Depolarization of cells with high extracellular [K⁺] did not generally induce calcium transients but did decrease calcium influx. L-type calcium-channel blockers (verapamil, nifedipine, and diltiazem) had little or no effect on the calcium influx induced by PDGF-BB. These results indicate that PDGF-BB induces calcium influx by a mechanism independent of voltage-sensitive calcium channels in A172 human glioblastoma cells.