Effect of low external calcium on the ERG current of NG108-15 cells

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K+ currents through ERG (ether-à-go-go related gene) channels were recorded in whole-cell voltage clamped NG108-15 neuroblastoma×glioma hybrid cells. The channels were fully activated by low holding potential (VH=−20 mV) and long depolarizing prepulses. Hyperpolarizing pulses elicited inward currents which deactivated after reaching a peak. Lowering [Ca2+]o from 5 to 1.5 or 0.5 mM decreased τ−1, the rate constant of deactivation. The effect can be explained by a shift of the τ−1(V) curve to more negative potentials caused by an increase in surface charge density. Plotting τ−1 against [Ca2+]o for different potentials yielded straight lines; their slope was independent of potential at −140 to −120 mV and decreased at more positive potentials. The time to peak curve and the maximum of the steady-state inward current were also shifted to more negative potentials. In addition, peak ERG inward current increased. Raising [Ca2+]o from 5 to 10 mM accelerated deactivation and decreased the peak current. 5 mM Ba2+ affected τ−1 similarly and inhibited peak current more strongly whereas 5 mM Mg2+ was less potent. As found by Faravelli et al. (J. Physiol. 496 (1996) 13), bath solutions devoid of divalent cations (0 Ca2+, 0 Mg2+, 0.1 or 1.1 mM EGTA) abolished deactivation almost completely. The phenomenon was seen with bath containing either 40 or 6.5 mM K+. Its occurrence was favored by raising the temperature to 34°C. It suggests a particular requirement of channel closing for Ca2+.