Depolarization of differentiated
neuroblastoma X
glioma (NG108-15) cells with KCl (50 mM) or
veratridine (50 microM) stimulated Ca2+ accumulation, was detected by
quin 2 fluorescence. Intracellular Ca2+ concentrations ([Ca2+]i) were elevated about threefold from 159 +/- 7 to 595 +/- 52 nM (n = 12). Ca2+ entry evoked by high extracellular K+ concentration ([K+]o) was voltage-dependent and enhanced by the
dihydropyridine agonists,
BAY K 8644 and CGP 28 392, in a dose-dependent manner. CGP 28 392 was less potent and less efficacious than
BAY K 8644. The (+) and (-) stereoisomers of
202-791 showed agonist and antagonist properties, respectively. (+)-
202-791 was less potent, but as efficacious as
BAY K 8644. In the absence of KCl,
BAY K 8644 had no effect on Ca2+ entry. Voltage-sensitive
calcium channel (VSCC) activity was blocked by organic Ca2+ channel antagonists (nanomolar range) both before and after KCl treatment and also by divalent
metal cations (micromolar range). High [K+]o-induced Ca2+ accumulation was dependent on external Ca2+, but not on external Na+
ions ([Na]o), and was insensitive to both
tetrodotoxin (3 microM) and
tetraethylammonium (10 microM). In contrast,
veratridine-induced Ca2+ accumulation required [Na+]o, and was blocked by
tetrodotoxin, but not by
nimodipine (1 microM).
Veratridine-induced Ca2+ accumulation was slower (approximately 45 s), smaller in magnitude (approximately 30% of [K+]o-induced Ca2+ entry), and also enhanced by
BAY K 8644 (approximately 50%). VSCC were identified in neuronal hybrid (NG108-15 and NCB-20) cells, but not in glial (C6BU-1), renal epithelial (MDCK), and human
astrocytoma (1321N1) cells. NG108-15 cells differentiated with 1.0 mM
dibutyryl cyclic AMP showed greater VSCC activity than undifferentiated cultures. These results suggest that cultured neural cells provide a useful system to study Ca2+ regulation via
ion channels.