Haynes L W
Department of Medical Physiology, University of Calgary, Alberta, Canada.
J Gen Physiol. 1995 Sep;106(3):485-505. doi: 10.1085/jgp.106.3.485.
The permeation of monovalent cations through the cGMP-gated channel of catfish cone outer segments was examined by measuring permeability and conductance ratios under biionic conditions. For monovalent cations presented on the cytoplasmic side of the channel, the permeability ratios with respect to extracellular Na followed the sequence NH4 > K > Li > Rb = Na > Cs while the conductance ratios at +50 mV followed the sequence Na approximately NH4 > K > Rb > Li = Cs. These patterns are broadly similar to the amphibian rod channel. The symmetry of the channel was tested by presenting the test ion on the extracellular side and using Na as the common reference ion on the cytoplasmic side. Under these biionic conditions, the permeability ratios with respect to Na at the intracellular side followed the sequence NH4 > Li > K > Na > Rb > Cs while the conductance ratios at +50 mV followed the sequence NH4 > K approximately Na > Rb > Li > Cs. Thus, the channel is asymmetric with respect to external and internal cations. Under symmetrical 120 mM ionic conditions, the single-channel conductance at +50 mV ranged from 58 pS in NH4 to 15 pS for Cs and was in the order NH4 > Na > K > Rb > Cs. Unexpectedly, the single-channel current-voltage relation showed sufficient outward rectification to account for the rectification observed in multichannel patches without invoking voltage dependence in gating. The concentration dependence of the reversal potential for K showed that chloride was impermeant. Anomalous mole fraction behavior was not observed, nor, over a limited concentration range, were multiple dissociation constants. An Eyring rate theory model with a single binding site was sufficient to explain these observations.
通过在双离子条件下测量渗透率和电导率比，研究了单价阳离子通过鲶鱼视锥细胞外段cGMP门控通道的渗透情况。对于通道细胞质侧呈现的单价阳离子，相对于细胞外Na的渗透率比遵循以下顺序：NH4＞K＞Li＞Rb = Na＞Cs，而在+50 mV时的电导率比遵循以下顺序：Na≈NH4＞K＞Rb＞Li = Cs。这些模式与两栖动物视杆通道大致相似。通过在细胞外侧呈现测试离子并使用Na作为细胞质侧的共同参考离子来测试通道的对称性。在这些双离子条件下，相对于细胞内侧Na的渗透率比遵循以下顺序：NH4＞Li＞K＞Na＞Rb＞Cs，而在+50 mV时的电导率比遵循以下顺序：NH4＞K≈Na＞Rb＞Li＞Cs。因此，该通道对于外部和内部阳离子是不对称的。在对称的120 mM离子条件下，+50 mV时的单通道电导率范围从NH4中的58 pS到Cs中的15 pS，顺序为NH4＞Na＞K＞Rb＞Cs。出乎意料的是，单通道电流-电压关系显示出足够的外向整流，足以解释在多通道膜片中观察到的整流现象，而无需引入门控中的电压依赖性。K的反转电位的浓度依赖性表明氯离子是不可渗透的。未观察到异常摩尔分数行为，在有限的浓度范围内也未观察到多个解离常数。具有单个结合位点的艾林速率理论模型足以解释这些观察结果。
