Clarkson C W, Matsubara T, Hondeghem L M
Department of Pharmacology, University of California, San Francisco 94143.
J Mol Cell Cardiol. 1988 Dec;20(12):1119-31. doi: 10.1016/0022-2828(88)90592-5.
The effects of tetrodotoxin (TTX) on cardiac sodium channels in guinea-pig ventricular muscle were investigated. Membrane potential was controlled using a single sucrose gap voltage clamp method, and the maximum upstroke velocity of the ventricular action potential (Vmax) was used as an indicator of drug-free sodium channels. Reduction of Vmax by TTX was found to be both voltage- and time-dependent, similar to the effects of many local anesthetic drugs, with the exception that TTX concentrations high enough to produce significant use-dependent block (e.g. 2 microM), also produced significant tonic block, even at potentials negative to -85 mV. The mechanism underlying use-dependent block was determined by defining the time course of block development at potentials between -40 and +20 mV, and the time course of recovery at -85 mV. In 2 microM TTX, the time course of block development at +20 mV contained two phases, a fast phase (tau less than 3 ms) having a mean amplitude of 8.1 +/- 3.2% of control Vmax, and a slow phase (tau = 429 +/- 43 ms) having an amplitude of 35 +/- 2% of control Vmax (n = 5). Recovery from use-dependent block at -85 mV occurred with a time constant of 324 +/- 58 ms (n = 5). The effects of TTX could be well-described by a modulated receptor model with an estimated 12 mV drug-induced shift of inactivation, and state-dependent dissociation constants of 10, 4 and 0.3 microM for rested, activated and inactivated channels. These same drug rate constants could also be used to adequately simulate the reported effects of TTX on plateau sodium currents in a variant model with slow inactivation kinetics.
研究了河豚毒素（TTX）对豚鼠心室肌心脏钠通道的影响。使用单蔗糖间隙电压钳法控制膜电位，并将心室动作电位的最大上升速度（Vmax）用作无药物钠通道的指标。发现TTX对Vmax的降低具有电压和时间依赖性，这与许多局部麻醉药的作用相似，但不同的是，TTX浓度高到足以产生显著的使用依赖性阻滞（例如2 microM）时，即使在负于-85 mV的电位下也会产生显著的强直阻滞。通过确定在-40至+20 mV电位下阻滞发展的时间进程以及在-85 mV下恢复的时间进程，确定了使用依赖性阻滞的机制。在2 microM TTX中，+20 mV时阻滞发展的时间进程包括两个阶段，一个快速阶段（时间常数τ小于3 ms），其平均幅度为对照Vmax的8.1±3.2%，一个缓慢阶段（时间常数τ = 429±43 ms），其幅度为对照Vmax的35±2%（n = 5）。在-85 mV下从使用依赖性阻滞中恢复的时间常数为324±58 ms（n = 5）。TTX的作用可以用一个调制受体模型很好地描述，该模型估计药物引起的失活位移为12 mV，静息、激活和失活通道的状态依赖性解离常数分别为10、4和0.3 microM。这些相同的药物速率常数也可用于在具有缓慢失活动力学的变体模型中充分模拟TTX对平台期钠电流的报道作用。
