Yeh J Z, Oxford G S
J Gen Physiol. 1985 Apr;85(4):603-20. doi: 10.1085/jgp.85.4.603.
The time-, frequency-, and voltage-dependent blocking actions of several cationic drug molecules on open Na channels were investigated in voltage-clamped, internally perfused squid giant axons. The relative potencies and time courses of block by the agents (pancuronium [PC], octylguanidinium [C8G], QX-314, and 9-aminoacridine [9-AA]) were compared in different intracellular ionic solutions; specifically, the influences of internal Cs, tetramethylammonium (TMA), and Na ions on block were examined. TMA+ was found to inhibit the steady state block of open Na channels by all of the compounds. The time-dependent, inactivation-like decay of Na currents in pronase-treated axons perfused with either PC, 9-AA, or C8G was retarded by internal TMA+. The apparent dissociation constants (at zero voltage) for interaction between PC and 9-AA with their binding sites were increased when TMA+ was substituted for Cs+ in the internal solution. The steepness of the voltage dependence of 9-AA or PC block found with internal Cs+ solutions was greatly reduced by TMA+, resulting in estimates for the fractional electrical distance of the 9-AA binding site of 0.56 and 0.22 in Cs+ and TMA+, respectively. This change may reflect a shift from predominantly 9-AA block in the presence of Cs+ to predominantly TMA+ block. The depth, but not the rate, of frequency-dependent block by QX-314 and 9-AA is reduced by internal TMA+. In addition, recovery from frequency-dependent block is not altered. Elevation of internal Na produces effects on 9-AA block qualitatively similar to those seen with TMA+. The results are consistent with a scheme in which the open channel blocking drugs, TMA (and Na) ions, and the inactivation gate all compete for a site or for access to a site in the channel from the intracellular surface. In addition, TMA ions decrease the apparent blocking rates of other drugs in a manner analogous to their inhibition of the inactivation process. Multiple occupancy of Na channels and mutual exclusion of drug molecules may play a role in the complex gating behaviors seen under these conditions.
在电压钳制、内部灌注的鱿鱼巨轴突中，研究了几种阳离子药物分子对开放钠通道的时间、频率和电压依赖性阻断作用。在不同的细胞内离子溶液中比较了这些药物（泮库溴铵[PC]、辛基胍[C8G]、QX - 314和9 - 氨基吖啶[9 - AA]）的相对效力和阻断的时间进程；具体而言，研究了内部铯（Cs）、四甲基铵（TMA）和钠离子对阻断的影响。发现TMA⁺可抑制所有化合物对开放钠通道的稳态阻断。在用PC、9 - AA或C8G灌注的经链霉蛋白酶处理的轴突中，内部TMA⁺可延缓钠电流的时间依赖性、失活样衰减。当内部溶液中的TMA⁺替代Cs⁺时，PC和9 - AA与其结合位点相互作用的表观解离常数（在零电压下）增加。内部Cs⁺溶液中发现的9 - AA或PC阻断的电压依赖性陡度被TMA⁺大大降低，导致在Cs⁺和TMA⁺中9 - AA结合位点的分数电距离估计分别为0.56和0.22。这种变化可能反映了从Cs⁺存在时主要是9 - AA阻断到主要是TMA⁺阻断的转变。内部TMA⁺可降低QX - 314和9 - AA频率依赖性阻断的深度，但不影响其速率。此外，频率依赖性阻断的恢复不受影响。内部钠离子升高对9 - AA阻断产生的影响在性质上与TMA⁺相似。结果与一种机制一致，即开放通道阻断药物、TMA（和钠）离子以及失活门都从细胞内表面竞争通道中的一个位点或进入一个位点。此外，TMA离子以类似于其对失活过程抑制的方式降低其他药物的表观阻断速率。钠通道的多重占据和药物分子的相互排斥可能在这些条件下观察到的复杂门控行为中起作用。
