Voltage-dependent noradrenergic modulation of ω-conotoxin-sensitive Ca²⁺ channels in human neuroblastoma IMR32 cells

Abstract

High-threshold (HVA) Ca²⁺ channels of human neuroblastoma IMR32 cells were effectively inhibited by noradrenaline. At potentials between −20 mV and +10 mV, micromolar concentrations of noradrenaline induced a 50%–70% depression of HVA Ba²⁺ currents and a prolongation of their activation kinetics. Both effects were relieved at more positive voltages or by applying strong conditioning pre-pulses (facilitation). Facilitation restored the rapid activation of HVA channels and recruited about 80% of the noradrenaline-inhibited channels at rest. Re-inhibition of Ca²⁺ channels after facilitation was slow (τ _r 36–45 ms) and voltage-independent between −30 mV and −90 mV. The inhibitory action of noradrenaline was dose-dependent (IC₅₀=84 nM), mediated by α₂-drenergic receptors and selective for ω-conotoxin-sensitive Ca²⁺ channels, which represent the majority of HVA channels expressed by IMR32 cells. The action of noradrenaline was mimicked by intracellular applications of GTP[γS] and prevented by GDP[βS] or by pre-incubation with pertussis toxin. The time course of noradrenaline inhibition measured during fast application (onset) and wash-out (offset) of the drug were independent of saturating agonist concentrations (10–50 μM) and developed with mean time constants of 0.56 s (τ _on) and 3.6 s (τ _off) respectively. The data could be simulated by a kinetic model in which a G protein is assumed to modify directly the voltage-dependent gating of Ca²⁺ channels. Noradrenaline-modified channels are mostly inhibited at rest and can be recruited in a steep voltage-dependent manner with increasing voltages.