Krause J D, Foster C D, Reinhart P H
Department of Neurobiology, Duke University Medical Center, Durham, NC 27710, USA.
Neuropharmacology. 1996;35(7):1017-22. doi: 10.1016/0028-3908(96)00134-7.
Xenopus laevis oocytes have become a pre-eminent tool for studying cloned ion channels, primarily because they intrinsically express low levels of most types of ion channels. However, when these cells are used for single channel studies, it is essential to determine whether or not oocytes contain even low levels of endogenous ion channels with properties similar to the channel being investigated. We show here that X. laevis oocytes express endogenous large-conductance Ca2(+)-activated K+ channels with properties similar to mammalian isoforms of this channel. The endogenous channels exhibit a voltage-dependence of 12-14 mV per e-fold change in open probability (po), can be activated by micromolar Ca2+ concentrations, and have a single channel conductance of approximately 200 pS in symmetrical 110 mM K+ solutions. Patch clamp experiments indicate that this endogenous channel is present at low densities (approximately 1 channel/3000 microns2). If endogenous channel subunits can form functional tetramers with other exogenous potassium channel subunits, then they will give rise to the expression of a heterogeneous channel population. Therefore, studies involving the heterologous expression of large-conductance Ca2(+)-activated K+ channels in Xenopus laevis oocytes require careful analysis and interpretation.
非洲爪蟾卵母细胞已成为研究克隆离子通道的卓越工具，主要是因为它们本身大多数类型离子通道的表达水平较低。然而，当将这些细胞用于单通道研究时，确定卵母细胞是否即使含有低水平的内源性离子通道，且其特性与所研究的通道相似，这一点至关重要。我们在此表明，非洲爪蟾卵母细胞表达内源性大电导钙激活钾通道，其特性与该通道的哺乳动物同工型相似。内源性通道在开放概率（po）每增加e倍变化时表现出12 - 14 mV的电压依赖性，可被微摩尔浓度的钙离子激活，并且在对称的110 mM钾溶液中的单通道电导约为200 pS。膜片钳实验表明，这种内源性通道以低密度存在（约1个通道/3000平方微米）。如果内源性通道亚基能与其他外源性钾通道亚基形成功能性四聚体，那么它们将导致异质性通道群体的表达。因此，在非洲爪蟾卵母细胞中进行大电导钙激活钾通道异源表达的研究需要仔细的分析和解释。
