Cadmium block of squid calcium currents. Macroscopic data and a kinetic model.
Cadmium block of squid calcium currents. Macroscopic data and a kinetic model.
复制标题
DOI：
10.1085/jgp.98.4.751
复制
发表时间：
1991-10
期刊：
影响因子：
--
通讯作者：
Chow RH
中科院分区：
文献类型：
--
作者：
Chow RH
采用全细胞膜片钳技术研究了镉（Cd ~（2+））对鱿鱼神经元Ca ~（2+）电流（伊卡）的阻断机制。控制电流激活S形，更迅速地在更积极的电位，并没有显着。外部Cd 2+高达250 μ M可逆地减少伊卡。对于小的去极化，10 ms步长的电流增加到维持值，类似于对照;但对于Vm大于0 mV，随着Cd 2+阻滞变大，增加后降低。去极化越大，最终阻滞越大。在0 mV时，半阻断浓度为125 μ M。尾电流，测量通道关闭，有一个初始的“钩”时，记录在Cd 2+：电流瞬时增加，然后下降。这表明Cd 2+从某些通道中逃逸，然后在关闭前短暂传导。尾电流的分析表明，Cd 2+不减慢通道关闭。如果Cd 2+是Ca 2+通道的渗透性阻断剂，并且如果通道被Cd 2+占据时可以关闭，则可以解释该数据。Cd 2+渗透通道，但短暂地结合到孔中的位点，阻碍其他离子（例如，Ca2+）。停留时间取决于跨膜电位，内部电位越负，停留时间越短。采用五态模型模拟了反应的稳态和动力学特性。它结合了霍奇金-赫胥黎型M2门控方案和一个网站的Woodhull离子阻塞模型的渗透阻滞剂，包括一个封闭的封闭状态。为了拟合数据，Cd 2+的结合位点必须靠近孔的外端，孔深度为-12.2 RT，并且在孔的每个端部具有屏障。该模型预测，镉2+的进入速率几乎是电压无关的，但退出速率是陡峭的电压依赖性（e倍/17 mV）。分析进一步表明，通道关闭在正常的速度与Cd 2+在孔隙中。
The mechanism of Cd2+ block of Ca2+ currents (ICa) was explored in squid neurons using whole-cell patch clamp. Control currents activated sigmoidally, more rapidly at more positive potentials, and did not inactivate significantly. External Cd2+ up to 250 microM reduced ICa reversibly. For small depolarizations, the current for a step of 10 ms increased to a maintained value, resembling the control; but for Vm greater than 0 mV, the increase was followed by a decrease, as Cd2+ block became greater. Final block was greater for larger depolarizations. At 0 mV the half-blocking concentration was 125 microM. Tail currents, measured as channels close, had an initial "hook" when recorded in Cd2+: currents increased transiently, then decreased. This suggests that Cd2+ escapes from some channels, which then conduct briefly before closing. Analysis of tail currents shows that Cd2+ does not slow channel closing. The data can be explained if Cd2+ is a permeant blocker of Ca2+ channels and if channels can close when occupied by Cd2+. Cd2+ permeates the channels, but binds transiently to a site in the pore, obstructing the passage of other ions (e.g., Ca2+). Dwell time depends on the transmembrane potential, becoming shorter for more negative internal potentials. A five-state model was used to simulate the steady-state and kinetic features. It combines a Hodgkin-Huxley type m2 gating scheme and a one-site Woodhull ionic blockage model for a permeant blocker and includes a closed blocked state. To fit the data, the binding site for Cd2+ had to be near the outer end of the pore, with a well depth of -12.2 RT, and with a barrier at each end of the pore. The model predicts that the Cd2+ entry rate is nearly voltage independent, but the exit rate is steeply voltage dependent (e-fold/17 mV). Analysis further suggests that the channel closes at a normal rate with Cd2+ in the pore.