Newman E A
Department of Physiology, University of Minnesota, Minneapolis 55455.
J Neurosci. 1993 Aug;13(8):3333-45. doi: 10.1523/JNEUROSCI.13-08-03333.1993.
The voltage- and K(+)-dependent properties of Müller cell currents and channels were characterized in freshly dissociated salamander Müller cells. In whole-cell voltage-clamp experiments, cells with endfeet intact and cells missing endfeet both displayed strong inward rectification. The rectification was similar in shape in both groups of cells but currents were 9.2 times larger in cells with endfeet. Ba2+ at 100 microM reduced the inward current to 6.8% of control amplitude. Decreasing external K+ concentration shifted the cell current-voltage (I-V) relation in a hyperpolarizing direction and reduced current magnitude. In multichannel, cell-attached patch-clamp experiments, patches from both endfoot and soma membrane displayed strong inward rectification. Currents were 38 times larger in endfoot patches. In single-channel, cell-attached patch-clamp experiments, inward-rectifying K+ channels were, in almost all cases, the only channels present in patches of endfoot, proximal process, and soma membrane. Channel conductance was 27.8 pS in 98 mM external K+. Reducing external K+ shifted the channel reversal potential in a hyperpolarizing direction and reduced channel conductance. Channel open probability varied as a function of voltage, being reduced at more negative potentials. Together, these observations demonstrate that the principal ion channel in all Müller cell regions is an inward-rectifying K+ channel. Channel density is far higher on the cell endfoot than in other cell regions. Whole-cell I-V plots of cells bathed in 12, 7, 4, and 2.5 mM K+ were fit by an equation including Boltzmann relation terms representing channel rectification and channel open probability. This equation was incorporated into a model of K+ dynamics in the retina to evaluate the significance of inward-rectifying channels to the spatial buffering/K+ siphoning mechanism of K+ regulation. Compared with ohmic channels, inward-rectifying channels increased the rate of K+ clearance from the retina by 23% for a 1 mM K+ increase and by 137% for a 9.5 mM K+ increase, demonstrating that Müller cell inward-rectifying channels enhance K+ regulation in the retina.
在新鲜分离的蝾螈缪勒细胞中，对缪勒细胞电流和通道的电压依赖性及钾离子（K⁺）依赖性特性进行了表征。在全细胞电压钳实验中，终足完整的细胞和缺少终足的细胞均表现出强烈的内向整流。两组细胞的整流形状相似，但终足完整的细胞中的电流比另一组大9.2倍。100微摩尔的钡离子（Ba²⁺）将内向电流降低至对照幅度的6.8%。降低细胞外钾离子浓度会使细胞电流-电压（I-V）关系向超极化方向移动，并降低电流幅度。在多通道细胞贴附式膜片钳实验中，终足膜片和胞体膜片均表现出强烈的内向整流。终足膜片的电流比胞体膜片大38倍。在单通道细胞贴附式膜片钳实验中，几乎在所有情况下，内向整流钾离子通道都是终足、近端突起和胞体膜片上唯一存在的通道。在98毫摩尔细胞外钾离子浓度下，通道电导为27.8皮西门子。降低细胞外钾离子浓度会使通道反转电位向超极化方向移动，并降低通道电导。通道开放概率随电压而变化，在更负的电位下降低。这些观察结果共同表明，所有缪勒细胞区域中的主要离子通道是内向整流钾离子通道。通道密度在细胞终足处远高于其他细胞区域。用一个包含代表通道整流和通道开放概率的玻尔兹曼关系项的方程，对置于12、7、4和2.5毫摩尔钾离子溶液中的细胞的全细胞I-V图进行拟合。该方程被纳入视网膜中钾离子动力学模型，以评估内向整流通道对钾离子调节的空间缓冲/钾离子虹吸机制的重要性。与欧姆通道相比，对于钾离子浓度增加1毫摩尔，内向整流通道使钾离子从视网膜清除的速率提高了23%；对于钾离子浓度增加9.5毫摩尔，该速率提高了137%，这表明缪勒细胞内向整流通道增强了视网膜中的钾离子调节。
