Ragsdale D S, Scheuer T, Catterall W A
Department of Pharmacology, University of Washington, School of Medicine, Seattle 98195.
Mol Pharmacol. 1991 Nov;40(5):756-65.
This study examined the actions of phenytoin, carbamazepine, lidocaine, and verapamil on rat brain type IIA Na+ channels functionally expressed in mammalian cells, using the whole-cell voltage-clamp recording technique. The drugs blocked Na+ currents in both a tonic and use-dependent manner. Tonic block was more pronounced at depolarized holding potentials and reduced at hyperpolarized membrane potentials, reflecting an overall negative shift in the relationship between membrane potential and steady state inactivation. Dose-response relationships with phenytoin supported the hypothesis that the voltage dependence of tonic block resulted from the higher affinity of the drugs for inactivated than for resting channels. At -62 mV, approximately 50% of the Na+ channels were blocked by phenytoin at 13 microM, compared with therapeutic brain levels of 4-8 microM. The use-dependent component of block developed progressively during a 2-Hz train of 40-msec-long stimulus pulses from -85 mV to 0 mV. At 2 Hz, verapamil was the most potent use-dependent blocker, lidocaine and phenytoin had intermediate potencies, and carbamazepine was least effective. The use-dependent block resulted from drug binding to open and inactivated channels during the depolarizing pulses and the slow repriming of drug-bound channels during the interpulse intervals. Verapamil, lidocaine, and phenytoin all bound preferentially to open channels, but this open channel block was most striking for verapamil. Use-dependent block was less pronounced at hyperpolarized membrane potentials, due to more rapid repriming of drug-bound channels. The results indicate that type IIA Na+ channels expressed in a mammalian cell line retain the complex pharmacological properties characteristic of native Na+ channels. These channels are likely to be an important site of the anticonvulsant action of phenytoin and carbamazepine. Lidocaine and verapamil, drugs with well characterized effects on peripheral Na+ and Ca2+ channels, are also effective blockers of these brain Na+ channels.
本研究采用全细胞膜片钳记录技术，检测了苯妥英、卡马西平、利多卡因和维拉帕米对在哺乳动物细胞中功能性表达的大鼠脑IIA型Na⁺通道的作用。这些药物以强直和使用依赖性方式阻断Na⁺电流。强直阻断在去极化钳制电位时更为明显，而在超极化膜电位时减弱，这反映了膜电位与稳态失活之间关系的整体负向偏移。苯妥英的剂量-反应关系支持了这样的假说，即强直阻断的电压依赖性是由于药物对失活通道的亲和力高于静息通道。在-62 mV时，13 μM的苯妥英可阻断约50%的Na⁺通道，而治疗时脑内水平为4 - 8 μM。在从-85 mV到0 mV的40毫秒长、频率为2 Hz的刺激脉冲序列中，使用依赖性阻断成分逐渐形成。在2 Hz时，维拉帕米是最有效的使用依赖性阻断剂，利多卡因和苯妥英的效力居中，而卡马西平效果最差。使用依赖性阻断是由于药物在去极化脉冲期间与开放和失活通道结合，以及在脉冲间期药物结合通道的缓慢再激活所致。维拉帕米、利多卡因和苯妥英都优先与开放通道结合，但这种开放通道阻断对维拉帕米最为显著。由于药物结合通道的再激活更快，使用依赖性阻断在超极化膜电位时不太明显。结果表明，在哺乳动物细胞系中表达的IIA型Na⁺通道保留了天然Na⁺通道的复杂药理学特性。这些通道可能是苯妥英和卡马西平抗惊厥作用的重要位点。利多卡因和维拉帕米对周围Na⁺和Ca²⁺通道有明确的作用，它们也是这些脑Na⁺通道的有效阻断剂。
