Rando T A, Strichartz G R
Biophys J. 1986 Mar;49(3):785-94. doi: 10.1016/S0006-3495(86)83706-7.
The inhibition by saxitoxin (STX) of single Na channels incorporated into planar lipid bilayers and modified by batrachotoxin (BTX) previously has been shown to be voltage dependent (Krueger, B.K.,J.F. Worley, and R. J. French, 1983, Nature [Lond.], 303:172-175; Moczydlowski, E., S. Hall, S. S. Garber, G. S. Strichartz, and C. Miller, 1984, J. Gen. Physiol., 84:687-704). We tested for such a voltage dependence of STX block of the Na current in voltage-clamped frog nodes of Ranvier. The block by STX of normal Na channels showed no modulation in response to maintained (20 s) changes of the membrane potential or to a train of brief pulses to potentials more positive than the holding potential. However, when the nodal channels were modified by BTX, the train of pulses produced a modulation of the block of the Na current by STX. The modulation of STX block depended on the voltage of the conditioning pulses and this voltage dependence agreed well with that predicted from the single channel studies over the membrane potential range used in those studies. In addition, we found that the voltage dependence of STX block was manifest only at potentials equal to or more positive than required to activate the channels. Most of the apparent differences among data from single channels in bilayers, equilibrium binding studies of STX, and the experiments described here are resolved by the hypotheses that (a) STX binding to open channels is voltage dependent, and (b) the affinities of STX for closed and inactivated channels are independent of voltage, equal, and less than the open channel affinity at potentials less than 0 mV. Whether these hypotheses apply to the STX block of all Na channels or just of BTX-modified channels remains to be determined.
先前已表明，石房蛤毒素（STX）对整合到平面脂质双分子层中且经蟾毒素（BTX）修饰的单个钠通道的抑制作用具有电压依赖性（克鲁格，B.K.，J.F. 沃利，以及R.J. 弗伦奇，1983年，《自然》[伦敦]，303:172 - 175；莫齐德洛夫斯基，E.，S. 霍尔，S.S. 加伯，G.S. 斯特里查兹，以及C. 米勒，1984年，《普通生理学杂志》，84:687 - 704）。我们在电压钳制的青蛙郎飞结中测试了STX对钠电流的阻断是否具有这种电压依赖性。正常钠通道被STX阻断时，在膜电位保持（20秒）变化或一系列短暂脉冲刺激至比钳制电位更正的电位时，未显示出调制作用。然而，当节点通道经BTX修饰后，一系列脉冲产生了STX对钠电流阻断的调制作用。STX阻断的调制作用取决于预处理脉冲的电压，并且这种电压依赖性与在那些单通道研究中所使用的膜电位范围内根据单通道研究预测的结果非常吻合。此外，我们发现STX阻断的电压依赖性仅在等于或高于激活通道所需的电位时才表现出来。双层膜中单个通道的数据、STX的平衡结合研究以及此处描述的实验之间的大多数明显差异，可以通过以下假设得到解释：（a）STX与开放通道的结合具有电压依赖性，以及（b）在电位低于0 mV时，STX对关闭和失活通道的亲和力与电压无关，相等，且小于对开放通道的亲和力。这些假设是否适用于所有钠通道的STX阻断，还是仅适用于BTX修饰的通道，仍有待确定。
