Sun Xiao-Ping, Yazejian Bruce, Grinnell Alan D
Department of Physiology, David Geffen School of Medicine at UCLA, Los Angles, CA 90095, USA.
J Physiol. 2004 May 15;557(Pt 1):207-28. doi: 10.1113/jphysiol.2003.060509. Epub 2004 Mar 26.
Single channel properties of Ca(2+)-activated K(+) (BK or Maxi-K) channels have been investigated in presynaptic membranes in Xenopus motoneurone-muscle cell cultures. The occurrence and density of BK channels increased with maturation/synaptogenesis and was not uniform: highest at the release face of bouton-like synaptic varicosities in contact with muscle cells, and lowest in varicosities that did not contact muscle cells. The Ca(2+) affinity of the channel (K(d)= 7.7 microM at a membrane potential of +20 mV) was lower than those of BK channels that have been characterized in other terminals. Hill coefficients varied between 1.5 and 2.8 at different potentials and open probability increased e-fold per 16 mV change in membrane potential over a range of Ca(2+) from 1 microM to 1 mM. The maximal activation rate of ensembled single BK channel currents was in the submillisecond range at > or =+20 mV. The activation rate increased approximately 10-fold in response to a Ca(2+) increase from 1 to 100 microM, but increased only approximately 2-fold with a voltage change from +20 to +130 mV. The fastest activation kinetics of BK channels in cell-attached patches resembled that in inside-out patches with Ca(2+) of 100 microM or more, suggesting that many BK channels are located very close to calcium channels. Given the low Ca(2+) affinity and rapid Ca(2+) binding/unbinding properties, we conclude that BK channels in this preparation are adapted to play an important role in regulation of neurotransmitter release, and they are ideal reporters of local [Ca(2+)] at the inner membrane surface.
在非洲爪蟾运动神经元 - 肌肉细胞培养物的突触前膜中，对钙激活钾通道（BK或大电导钙激活钾通道）的单通道特性进行了研究。BK通道的出现和密度随成熟/突触形成而增加，且分布不均：在与肌肉细胞接触的纽扣状突触膨体的释放面最高，而在不与肌肉细胞接触的膨体中最低。该通道的钙亲和力（在膜电位为 +20 mV时，K(d)= 7.7 microM）低于在其他终末所表征的BK通道。在不同电位下，希尔系数在1.5至2.8之间变化，并且在膜电位从 +20 mV变化至 +130 mV的范围内，每16 mV的膜电位变化，开放概率增加e倍。在≥ +20 mV时，成组单BK通道电流的最大激活速率处于亚毫秒范围。当胞内钙浓度从1 microM增加到100 microM时，激活速率增加约10倍，但随着电压从 +20 mV变化到 +130 mV，激活速率仅增加约2倍。细胞贴附式膜片中BK通道最快的激活动力学类似于胞内钙浓度为100 microM或更高的外翻式膜片，这表明许多BK通道非常靠近钙通道。鉴于其低钙亲和力和快速的钙结合/解离特性，我们得出结论，该制剂中的BK通道适合在神经递质释放调节中发挥重要作用，并且它们是内膜表面局部钙浓度的理想报告分子。
