Araque A, Cattaert D, Buño W
Instituto Cajal, CSIC, Madrid, Spain.
J Gen Physiol. 1995 Jun;105(6):725-44. doi: 10.1085/jgp.105.6.725.
The effects of Cd2+ on the hyperpolarization-activated K(+)-mediated current called IAB (Araque, A., and W. Buño. 1994. Journal of Neuroscience. 14:399-408.) were studied under two-electrode voltage-clamp in opener muscle fibers of the crayfish Procambarus clarkii. IAB was reversibly reduced by extracellular Cd2+ in a concentration-dependent manner, obeying the Hill equation with IC50 = 0.452 +/- 0.045 mM and a Hill coefficient of 1 (determined from the maximal chord conductance of IAB). Cd2+ decreased the IAB conductance (GAB) and shifted its voltage dependence towards hyperpolarized potentials in a similar degree, without affecting the slope of the voltage dependence. The IAB activation time constant increased, whereas the IAB deactivation time constant was not modified by Cd2+. The IAB equilibrium potential (EAB) was unmodified by Cd2+, indicating that the selective permeability of IAB channels was not altered. IAB was unaffected by intracellular Cd2+. The Cd(2+)-regulation of IAB did not depend on [K+]o, and the effects of [K+]o on IAB were unchanged by Cd2+, indicating that Cd2+ did not compete with K+. Therefore, Cd2+ probably bound to a different site to that involved in the K+ permeability pathway. We conclude that Cd2+ affected the gating of IAB channels, interfering with their opening but not with their closing mechanism. The results can be explained by a kinetic model in which the binding of Cd2+ to the IAB channels would stabilize the gating apparatus at its resting position, increasing the energy barrier for the transition from the closed to the open channel states.
在克氏原螯虾（Procambarus clarkii）的 opener 肌纤维中，采用双电极电压钳技术研究了 Cd2+ 对一种名为 IAB 的超极化激活的 K+ 介导电流（Araque, A., and W. Buño. 1994. Journal of Neuroscience. 14:399 - 408.）的影响。细胞外的 Cd2+ 以浓度依赖的方式使 IAB 可逆性降低，符合希尔方程，IC50 = 0.452 ± 0.045 mM，希尔系数为 1（由 IAB 的最大弦电导确定）。Cd2+ 降低了 IAB 电导（GAB），并使其电压依赖性向超极化电位方向移动，移动程度相似，且不影响电压依赖性的斜率。IAB 的激活时间常数增加，而 IAB 的失活时间常数未被 Cd2+ 改变。IAB 平衡电位（EAB）未被 Cd2+ 改变，表明 IAB 通道的选择性通透性未改变。IAB 不受细胞内 Cd2+ 的影响。IAB 的 Cd(2+)调节不依赖于[K+]o，且 Cd2+ 不改变[K+]o 对 IAB 的影响，表明 Cd2+ 不与 K+ 竞争。因此，Cd2+ 可能与 K+ 通透性途径中涉及的位点不同的位点结合。我们得出结论，Cd2+ 影响了 IAB 通道的门控，干扰其开放但不影响其关闭机制。这些结果可以用一个动力学模型来解释，即 Cd2+ 与 IAB 通道的结合会使门控装置稳定在其静息位置，增加从关闭通道状态转变为开放通道状态的能量屏障。
