Calcium channel selectivity for divalent and monovalent cations. Voltage and concentration dependence of single channel current in ventricular heart cells.
Calcium channel selectivity for divalent and monovalent cations. Voltage and concentration dependence of single channel current in ventricular heart cells.
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DOI：
10.1085/jgp.88.3.293
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发表时间：
1986-09
期刊：
影响因子：
--
通讯作者：
Tsien RW
中科院分区：
文献类型：
--
作者：
Hess P;Lansman JB;Tsien RW
用单通道和全细胞记录的方法，研究了豚鼠心肌细胞钙通道的离子通透性。我们用两种方法评估了不同二价阳离子和一价阳离子的渗透性，通过测量一元电流幅度或反转电位(EREV)。根据EREV的全细胞测量，二价离子的相对渗透率顺序为：Ca~(2+)>Sr~(2+)>Ba~(2+)；一价离子遵循Li+大于Na+大于K+大于Cs+的顺序，且比二价离子小得多。这些全细胞测量得到了单通道记录的支持，这些记录显示了在强去极化时通过单个钙通道的清晰的外向电流，EREV的相似值，以及EREV附近的电流-电压关系的相似拐点。EREV测量的信息与开放通道通量或单通道电导的估计相反，后者给出了在110-150 mm载流子情况下，接近0 mV的序列Na+(85ps)大于Li+(45ps)大于Ba2+(20ps)大于Ca2+(9ps)。因此，根据EREV判断，渗透性较高的离子具有较低的离子转移速率。在另一种比较中，通过钙通道的全细胞钠电流减半不到2微米[Ca]o，但需要大于10 mm[Ca]o才能产生半最大单位钙电流。所有这些观察结果似乎与最近关于钙通道渗透机制的假说一致，该假说认为：离子以单列形式通过孔道，沿途与多个结合部位相互作用；选择性在很大程度上取决于离子对结合部位的亲和力，而不是被选择性过滤器排除；只有一个钙离子占据足以阻止单价离子(如Na+)的高电导；钙离子的快速渗透依赖于双占据，由于静电斥力或离子之间的某些其他相互作用，这种双重占据只在毫米摩尔[Ca]o处变得显著；一旦发生双重占位，离子-离子相互作用有助于促进钙离子从孔中快速退出到细胞中。
Single channel and whole cell recordings were used to study ion permeation through Ca channels in isolated ventricular heart cells of guinea pigs. We evaluated the permeability to various divalent and monovalent cations in two ways, by measuring either unitary current amplitude or reversal potential (Erev). According to whole cell measurements of Erev, the relative permeability sequence is Ca2+ greater than Sr2+ greater than Ba2+ for divalent ions; Mg2+ is not measurably permeant. Monovalent ions follow the sequence Li+ greater than Na+ greater than K+ greater than Cs+, and are much less permeant than the divalents. These whole cell measurements were supported by single channel recordings, which showed clear outward currents through single Ca channels at strong depolarizations, similar values of Erev, and similar inflections in the current-voltage relation near Erev. Information from Erev measurements stands in contrast to estimates of open channel flux or single channel conductance, which give the sequence Na+ (85 pS) greater than Li+ (45 pS) greater than Ba2+ (20 pS) greater than Ca2+ (9 pS) near 0 mV with 110-150 mM charge carrier. Thus, ions with a higher permeability, judged by Erev, have lower ion transfer rates. In another comparison, whole cell Na currents through Ca channels are halved by less than 2 microM [Ca]o, but greater than 10 mM [Ca]o is required to produce half-maximal unitary Ca current. All of these observations seem consistent with a recent hypothesis for the mechanism of Ca channel permeation, which proposes that: ions pass through the pore in single file, interacting with multiple binding sites along the way; selectivity is largely determined by ion affinity to the binding sites rather than by exclusion by a selectivity filter; occupancy by only one Ca ion is sufficient to block the pore's high conductance for monovalent ions like Na+; rapid permeation by Ca ions depends upon double occupancy, which only becomes significant at millimolar [Ca]o, because of electrostatic repulsion or some other interaction between ions; and once double occupancy occurs, the ion-ion interaction helps promote a quick exit of Ca ions from the pore into the cell.