Voltage-dependent noradrenergic modulation of omega-conotoxin-sensitive Ca2+ channels in human neuroblastoma IMR32 cells.
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 (tau 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 alpha 2-adrenergic receptors and selective for omega-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[gamma S] and prevented by GDP[beta S] 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 microM) and developed with mean time constants of 0.56 s (tau on) and 3.6 s (tau 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.
Pollo
A
A
Department of Anatomy and Human Physiology, Torino, Italy.
Lovallo
M
M
Sher
E
E
Carbone
E
E
D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D004056:Differential Threshold; D004305:Dose-Response Relationship, Drug; D004594:Electrophysiology; D019204:GTP-Binding Proteins / Q000502:physiology; D006801:Humans; D009447:Neuroblastoma / Q000378:metabolism* / Q000473:pathology; D009638:Norepinephrine / Q000494:pharmacology / Q000502:physiology*; D010456:Peptides, Cyclic / Q000494:pharmacology*; D011942:Receptors, Adrenergic, alpha / Q000502:physiology; D013997:Time Factors; D014407:Tumor Cells, Cultured; D020960:omega-Conotoxins
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D010456:Peptides, Cyclic; D011942:Receptors, Adrenergic, alpha; D020960:omega-Conotoxins; C053423:Conus magus toxin; D019204:GTP-Binding Proteins; D009638:Norepinephrine