Pollo A, Lovallo M, Sher E, Carbone E
Department of Anatomy and Human Physiology, Torino, Italy.
Pflugers Arch. 1992 Oct;422(1):75-83. doi: 10.1007/BF00381516.
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.
去甲肾上腺素可有效抑制人神经母细胞瘤IMR32细胞的高阈值（HVA）Ca2+通道。在-20 mV至+10 mV之间的电位下，微摩尔浓度的去甲肾上腺素可导致HVA Ba2+电流降低50%-70%，并延长其激活动力学。在更正的电压下或通过施加强的条件预脉冲（易化作用），这两种效应均可解除。易化作用恢复了HVA通道的快速激活，并在静息状态下募集了约80%被去甲肾上腺素抑制的通道。易化作用后Ca2+通道的再抑制缓慢（τr为36-45毫秒），且在-30 mV至-90 mV之间与电压无关。去甲肾上腺素的抑制作用呈剂量依赖性（IC50 = 84 nM），由α2-肾上腺素能受体介导，且对ω-芋螺毒素敏感的Ca2+通道具有选择性，这些通道代表了IMR32细胞表达的大多数HVA通道。细胞内应用GTP[γS]可模拟去甲肾上腺素的作用，而GDP[βS]或与百日咳毒素预孵育可阻止该作用。在快速应用（起始）和洗脱（终止）药物期间测得的去甲肾上腺素抑制的时间进程与饱和激动剂浓度（10-50 μM）无关，其发展的平均时间常数分别为0.56秒（τon）和3.6秒（τoff）。这些数据可用动力学模型模拟，其中假设G蛋白直接修饰Ca2+通道的电压依赖性门控。去甲肾上腺素修饰的通道在静息时大多被抑制，且随着电压升高可通过陡峭的电压依赖性方式被募集。
