DIHYDROPYRIDINE-SENSITIVE AND OMEGA-CONOTOXIN-SENSITIVE CALCIUM CHANNELS IN A MAMMALIAN NEUROBLASTOMA-GLIOMA CELL-LINE

1. Pharmacological and kinetic properties of high-voltage-activated (HVA) Ca2+ channel currents were studied using the whole-cell and perforated patch-clamp methods in a mouse neuroblastoma and rat glioma hybrid cell line, NG108-15, differentiated by dibutyryl cyclic AMP or by prostaglandin E1 and theophylline. 2. The HVA currents were separated into two components by use of two organic Ca2+ channel antagonists, omega-conotoxin GVIA (omega-CgTX) and a dihydropyridine (DHP) compound, nifedipine. One current component, I(DHP), was blocked by nifedipine (K(d) = 8.2 nM) and was resistant to omega-CgTX. Conversely, the other component, I(omega-CgTX), was irreversibly blocked by omega-CgTX and was resistant to DHPs. Thus, I(DHP) could be studied in isolation by a short application of omega-CgTX, while I(omega-CgTX) could be studied in the presence of nifedipine. 3. The voltage for half-activation of I(DHP) was smaller than that of I(omega-CgTX) by 13 mV. I(DHP) was activated at potentials that were subthreshold for voltage-dependent K+ currents of the cell, whereas I(omega-CgTX) was not. 4. Time courses of activation and deactivation of I(DHP) were faster than those of I(omega-CgTX). 5. Voltage-dependent inactivation was small for both I(DHP) and I(omega-CgTX) at any potential. 6. Ca2+-dependent inactivation of I(DHP) was faster and more prominent than that of I(omega-CgTX). The time course of the Ca2+-dependent inactivation of I(DHP), but not I(omega-CgTX), was slowed as the membrane potential was made more positive between -20 and 30 mV, although amplitude of the current was increased. 7. Alkaline earth metal ions carried the two components of I(HVA) in the same order: Ba2+ > Sr2+ > Ca2+. 8. Metal ions blocked the two components of I(HVA) in the same order of potency: Gd3+ > La3+ > Cd2+ > Cu2+ > Mn2+ > Ni2+. 9. An alkylating agent, N-ethylmaleimide (NEM, 0.1 mM), selectively augmented I(DHP) by 30%. 10. During the course of cellular differentiation induced by dibutyryl cyclic AMP, I(DHP) appeared earlier than I(omega-CgTX). 11. These results indicate that two classes of Ca2+ channels contribute to the HVA currents of this cell line. The DHP-sensitive channel is more apt to generate Ca2+ spikes and Ca2+ plateau potentials than the omega-CgTX-sensitive channel.