Bräu M E, Nau C, Hempelmann G, Vogel W
Abteilung für Anaesthesiologie und Operative Intensivmedizin, Justus-Liebig-Universität Giessen, Germany.
J Gen Physiol. 1995 Apr;105(4):485-505. doi: 10.1085/jgp.105.4.485.
Effects of some local anesthetics were studied in patch clamp experiments on enzymatically demyelinated peripheral amphibian nerve fibers. Micromolar concentrations of external bupivacaine depolarized the excised membrane considerably. The flicker K+ channel was found to be the most sensitive ion channel to local anesthetics in this preparation. Half-maximum inhibiting concentrations (IC50) for extracellular application of bupivacaine, ropivacaine, etidocaine, mepivacaine, lidocaine, and QX-314 were 0.21, 4.2, 8.6, 56, 220, and > 10,000 microM, respectively. The corresponding concentration-effect curves could be fitted under the assumption of a 1:1 reaction. Application from the axoplasmic side resulted in clearly lower potencies with IC50 values of 2.1, 6.6, 16, 300, 1,200, and 1,250 microM, respectively. The log(IC50)-values of the local anesthetics linearly depended on the logarithm of their octanol:buffer distribution coefficients with two regression lines for the piperidine derivatives and the standard amino-amides indicating an inherently higher potency of the cyclic piperidine series. Amide-linked local anesthetics did not impair the amplitude of the single-channel current but prolonged the time of the channel to be in the closed state derived as time constants tau c from closed-time histograms. With etidocaine and lidocaine tau c was 133 and 7.2 ms, and proved to be independent of concentration. With the most potent bupivacaine time constants of wash in and wash out were 1.8 and 5.2 s for 600 nM bupivacaine. After lowering the extracellular pH from 7.4 to 6.6, externally applied bupivacaine showed a reduced potency, whereas at higher pH of 8.2 the block was slightly enhanced. Intracellular pH of 6.4, 7.2, 8.0 had almost no effect on internal bupivacaine block. It is concluded that local anesthetics block the flicker K+ channel by impeding its gating but not its conductance. The slow blocker bupivacaine and the fast blocker lidocaine compete for the same receptor. Lipophilic interactions are of importance for blockade but besides a hydrophobic pathway, there exists also a hydrophilic pathway to the binding site which could only be reached from the cytoplasmic side of the membrane. Under physiological conditions, blockade of the flicker K+ channel which is more sensitive to bupivacaine than the Na+ channel might lead via membrane depolarization and the resulting sodium channel inactivation to a pronounced block of conduction in thin fibers.
在酶解去髓鞘的外周两栖类神经纤维上进行的膜片钳实验中，研究了某些局部麻醉药的作用。微摩尔浓度的外用布比卡因可使分离的膜发生明显去极化。在该制备物中，发现闪烁钾通道是对局部麻醉药最敏感的离子通道。布比卡因、罗哌卡因、依替卡因、甲哌卡因、利多卡因和QX - 314细胞外应用的半数最大抑制浓度（IC50）分别为0.21、4.2、8.6、56、220和>10,000 microM。在1:1反应的假设下，可以拟合相应的浓度 - 效应曲线。从轴浆侧应用导致效力明显降低，IC50值分别为2.1、6.6、16、300、1200和1250 microM。局部麻醉药的log(IC50)值与它们的辛醇:缓冲液分配系数的对数呈线性相关，哌啶衍生物和标准氨基酰胺有两条回归线，表明环状哌啶系列固有地具有更高的效力。酰胺连接的局部麻醉药不会损害单通道电流的幅度，但会延长通道处于关闭状态的时间，该时间从关闭时间直方图推导为时间常数τc。依替卡因和利多卡因的τc分别为133和7.2 ms，且证明与浓度无关。对于600 nM布比卡因，洗入和洗出的时间常数分别为1.8和5.2 s。将细胞外pH从7.4降至6.6后，外用布比卡因的效力降低，而在pH为8.2时，阻滞略有增强。细胞内pH为6.4、7.2、8.0对内部布比卡因阻滞几乎没有影响。结论是局部麻醉药通过阻碍闪烁钾通道的门控而不是其电导来阻断该通道。慢效阻滞剂布比卡因和速效阻滞剂利多卡因竞争相同的受体。脂溶性相互作用对阻滞很重要，但除了疏水途径外，还存在一条仅从膜的胞质侧才能到达结合位点的亲水途径。在生理条件下，对布比卡因比钠通道更敏感的闪烁钾通道的阻滞可能通过膜去极化以及由此产生的钠通道失活导致细纤维中传导的明显阻滞。
