Neyton J, Miller C
Graduate Department of Biochemistry, Brandeis University, Waltham, Massachusetts 02254.
J Gen Physiol. 1988 Nov;92(5):549-67. doi: 10.1085/jgp.92.5.549.
Single high-conductance Ca2+-activated K+ channels from rat skeletal muscle were inserted into planar lipid bilayers, and discrete blocking by the Ba2+ ion was studied. Specifically, the ability of external K+ to reduce the Ba2+ dissociation rate was investigated. In the presence of 150 mM internal K+, 1-5 microM internal Ba2+, and 150 mM external Na+, Ba2+ dissociation is rapid (5 s-1) in external solutions that are kept rigorously K+ free. The addition of external K+ in the low millimolar range reduces the Ba2+ off-rate 20-fold. Other permeant ions, such as Tl+, Rb+, and NH4+ show a similar effect. The half-inhibition constants rise in the order: Tl+ (0.08 mM) less than Rb+ (0.1 mM) less than K+ (0.3 mM) less than Cs+ (0.5 mM) less than NH4+ (3 mM). When external Na+ is replaced by 150 mM N-methyl glucamine, the Ba2+ off-rate is even higher, 20 s-1. External K+ and other permeant ions reduce this rate by approximately 100-fold in the micromolar range of concentrations. Na+ also reduces the Ba2+ off-rate, but at much higher concentrations. The half-inhibition concentrations rise in the order: Rb+ (4 microM) less than K+ (19 microM) much less than Na+ (27 mM) less than Li+ (greater than 50 mM). The results require that the conduction pore of this channel contains at least three sites that may all be occupied simultaneously by conducting ions.
将来自大鼠骨骼肌的单个高电导钙激活钾通道插入平面脂质双分子层中，研究钡离子对其的离散阻断作用。具体而言，研究了胞外钾离子降低钡离子解离速率的能力。在含有150 mM胞内钾离子、1 - 5 μM胞内钡离子和150 mM胞外钠离子的情况下，在严格无钾离子的胞外溶液中，钡离子的解离速度很快（5 s⁻¹）。加入低毫摩尔浓度范围的胞外钾离子可使钡离子的解离速率降低20倍。其他可通透离子，如铊离子、铷离子和铵离子也有类似作用。半抑制常数按以下顺序升高：铊离子（0.08 mM）＜铷离子（0.1 mM）＜钾离子（0.3 mM）＜铯离子（0.5 mM）＜铵离子（3 mM）。当胞外钠离子被150 mM N - 甲基葡糖胺取代时，钡离子的解离速率更高，为20 s⁻¹。胞外钾离子和其他可通透离子在微摩尔浓度范围内可使该速率降低约100倍。钠离子也能降低钡离子的解离速率，但所需浓度要高得多。半抑制浓度按以下顺序升高：铷离子（4 μM）＜钾离子（19 μM）＜＜钠离子（27 mM）＜锂离子（＞50 mM）。结果表明，该通道的传导孔至少包含三个位点，这些位点可能同时被传导离子占据。
