Kubokawa M, Sohma Y, Hirano J, Nakamura K, Kubota T
Department of Physiology II, School of Medicine, Iwate Medical University, 19-1, Uchimaru, Morioka, 020-8505, Japan.
J Membr Biol. 2005 Sep;207(2):69-89. doi: 10.1007/s00232-005-0802-3.
Effects of intracellular Mg2+ on a native Ca(2+)-and voltage-sensitive large-conductance K+ channel in cultured human renal proximal tubule cells were examined with the patch-clamp technique in the inside-out mode. At an intracellular concentration of Ca2+ (Ca2+) of 10(-5)-10(-4) M, addition of 1-10 mM: Mg2+ increased the open probability (P(o)) of the channel, which shifted the P(o) -membrane potential (V(m)) relationship to the negative voltage direction without causing an appreciable change in the gating charge (Boltzmann constant). However, the Mg(2+)-induced increase in P(o) was suppressed at a relatively low Ca2+ (10(-5.5)-10(-6) M). Dwell-time histograms have revealed that addition of Mg2+ mainly increased P(o) by extending open times at 10(-5) M Ca2+ and extending both open and closed times simultaneously at 10(-5.5) M Ca2+. Since our data showed that raising the Ca2+ from 10(-5) to 10(-4) M increased P(o) mainly by shortening the closed time, extension of the closed time at 10(-5.5) M Ca(2+) would result from the Mg(2+)-inhibited Ca(2+)-dependent activation. At a constant V(m), adding Mg2+ enhanced the sigmoidicity of the P(o)-Ca2+ relationship with an increase in the Hill coefficient. These results suggest that the major action of Mg2+ on this channel is to elevate P(o) by lengthening the open time, while extension of the closed time at a relatively low Ca2+ results from a lowering of the sensitivity to Ca2+ of the channel by Mg2+, which causes the increase in the Hill coefficient.
采用膜片钳技术的内面向外模式，研究了细胞内镁离子（Mg2+）对培养的人肾近端小管细胞中一种天然的钙（Ca2+）和电压敏感的大电导钾通道的影响。在细胞内钙浓度（[Ca2+]i）为10^(-5)-10^(-4) M时，添加1-10 mM的Mg2+可增加通道的开放概率（P(o)），使P(o)-膜电位（V(m)）关系向负电压方向移动，而不会引起门控电荷（玻尔兹曼常数）的明显变化。然而，在相对较低的[Ca2+]i（10^(-5.5)-10^(-6) M）时，Mg2+诱导的P(o)增加受到抑制。驻留时间直方图显示，添加Mg2+主要通过在10^(-5) M Ca2+时延长开放时间以及在10^(-5.5) M Ca2+时同时延长开放和关闭时间来增加P(o)。由于我们的数据表明，将[Ca2+]i从10^(-5)提高到10^(-4) M主要通过缩短关闭时间来增加P(o)，因此在10^(-5.5) M Ca2+时关闭时间的延长将是Mg2+抑制的钙依赖性激活的结果。在恒定的V(m)下，添加Mg2+会增加希尔系数，从而增强P(o)-[Ca2+]i关系的S形曲线特征。这些结果表明，Mg2+对该通道的主要作用是通过延长开放时间来提高P(o)，而在相对较低的[Ca2+]i时关闭时间的延长是由于Mg2+降低了通道对Ca2+的敏感性，这导致了希尔系数的增加。
