Premack B A, Thompson S, Coombs-Hahn J
Hopkins Marine Station, Stanford University, Pacific Grove, California 93950.
J Neurosci. 1989 Nov;9(11):4089-99. doi: 10.1523/JNEUROSCI.09-11-04089.1989.
The spatial distribution of transient K current, IA, was studied using a combination of patch-clamp and whole-cell voltage-clamp techniques. The average IA current density in somatic patches is 0.64 times the current density in the entire axotomized cell body, a finding which suggests that the axon hillock or initial segment of the axon has a higher concentration of IA channels than much of soma. The highest density of active channels during the peak IA is 1/micron2 at a membrane voltage of -20 mV. There is no evidence for a gradient in the distribution of IA channels in the cell body, but the channels are not evenly distributed. The variability in the number of channels per patch for multiple patches on the same neuron is much higher than expected for a random distribution. Statistical analysis of the data yields a coefficient of dispersion of 8.1, a value indicating a high degree of clustering. The utility of this statistic for evaluating channel distributions is discussed. Several lines of evidence suggest that the upper limit for the area of IA channel clusters is approximately 250 micron2. Single-channel currents attributed to IA were recorded in the cell-attached configuration. The voltage dependence of channel opening and inactivation are the same as measured in whole-cell voltage-clamp experiments. The single-channel conductance is about 9 pS in normal saline. Patches 9-30 micron2 in areas that contain IA channels are often devoid of other K channel types, suggesting that IA channels can occur in isochannel clusters. IA inactivation follows an exponential time course in all of the neurons examined, but the time constant of inactivation ranges from 25 to 560 msec in different cells. The voltage dependence of activation and inactivation and the reversal potential of the current are approximately the same in all cells. When multiple patches on the same neuron are studied, it is found that IA inactivates exponentially with approximately the same time constant in each patch, regardless of patch area. The data suggest that each neuron expresses predominantly, and perhaps exclusively, a single type of IA channel with distinct kinetic properties. The wide range of IA inactivation time constants observed in different cell suggests that a large number of channel types are available for expression. Possible mechanisms for generating diversity in channel types are discussed.
采用膜片钳和全细胞电压钳技术相结合的方法，研究了瞬时钾电流（IA）的空间分布。体膜片上的平均IA电流密度是整个轴突切断的细胞体中电流密度的0.64倍，这一发现表明轴突丘或轴突起始段的IA通道浓度高于细胞体的大部分区域。在IA峰值期间，活性通道的最高密度在膜电压为-20 mV时为1/μm²。没有证据表明细胞体中IA通道的分布存在梯度，但通道分布并不均匀。同一神经元上多个膜片每片通道数量的变异性远高于随机分布的预期值。对数据的统计分析得出离散系数为8.1，该值表明高度聚集。讨论了该统计量在评估通道分布方面的实用性。几条证据表明，IA通道簇面积的上限约为250μm²。在细胞贴附模式下记录到了归因于IA的单通道电流。通道开放和失活的电压依赖性与全细胞电压钳实验中测得的相同。在生理盐水中，单通道电导约为9 pS。含有IA通道的面积为9 - 30μm²的膜片通常没有其他类型的钾通道，这表明IA通道可以出现在同通道簇中。在所有检测的神经元中，IA失活遵循指数时间进程，但不同细胞中失活的时间常数范围为25至560毫秒。所有细胞中激活和失活的电压依赖性以及电流的反转电位大致相同。当研究同一神经元上的多个膜片时，发现无论膜片面积如何，每个膜片中的IA都以大致相同的时间常数呈指数失活。数据表明，每个神经元主要（或许唯一）表达一种具有独特动力学特性的IA通道类型。在不同细胞中观察到的IA失活时间常数的广泛范围表明，有大量的通道类型可供表达。讨论了产生通道类型多样性的可能机制。
