Kanazirska M P, Vassilev P M, Ye C P, Francis J E, Brown E M
Endocrine-Hypertension Division, Brigham and Women's Hospital, Boston, Massachusetts 02115, USA.
Endocrinology. 1995 May;136(5):2238-43. doi: 10.1210/endo.136.5.7720673.
The modulation of K+ channels by Ca2+ may have important functional implications in parathyroid cells, since in most endocrine cells they control membrane voltage regulating Ca2+ influx and hormone secretion. To characterize specific channel mechanisms regulating membrane voltage in parathyroid cells, the patch-clamp technique was used to determine the activities of K+ channels at different levels of intracellular Ca2+ concentration (Ca2+i) associated with changes in extracellular Ca2+ concentration (Ca2+o). This study shows that the membranes of dispersed bovine parathyroid cells contain a K+ channel that is activated by elevated Ca2+o through an indirect mechanism (i.e. exposure of the entire cell to high Ca2+o activates the channel despite a low Ca2+ concentration within the pipette solution on the external side of the channel under study). This K+ channel has a unitary conductance of 191 pS and is highly selective for K+, similar to the so-called maxi type of Ca(2+)-activated K+ channel previously defined in a number of other cell types. Like the latter channel, the activity of this channel in excised patches from parathyroid cells is markedly increased when an EGTA-containing buffer on the cytoplasmic face of the membrane is replaced with one containing 0.5 microM Ca2+. Changes in Ca2+ on the intracellular side of the membrane also shift the level of voltage necessary for half-maximal activation of the channel from 103 mV at 0.1 microM Ca2+ to 79 mV and 54 mV at 0.25 and 0.5 microM Ca2+, respectively. When similar studies were carried out using cell-attached patches on parathyroid cells exposed to 0.5, 1.5, or 2.0 mM Ca2+o, the values for half-maximal activation were approximately 105, 56, and 29 mV, respectively. The latter result suggests that in intact parathyroid cells, the channel is exposed to Ca2+i concentrations of about 0.15-0.2, 0.4 and 0.6-0.7 microM at these three extracellular Ca2+ concentrations, values that are in excellent agreement with those previously measured using Ca(2+)-sensitive fluorescent dyes. Thus, parathyroid cells express a maxi type of Ca(2+)-activated K+ channel that is indirectly regulated by Ca2+o, presumably through concomitant changes in Ca2+i. The latter may limit the extent of the cellular depolarization produced in response to elevated Ca2+o in this cell type.
Ca2+ 对钾通道的调节在甲状旁腺细胞中可能具有重要的功能意义，因为在大多数内分泌细胞中，钾通道控制膜电位，调节 Ca2+ 内流和激素分泌。为了阐明调节甲状旁腺细胞膜电位的特定通道机制，采用膜片钳技术测定了与细胞外 Ca2+ 浓度（Ca2+o）变化相关的不同细胞内 Ca2+ 浓度（Ca2+i）水平下钾通道的活性。本研究表明，分散的牛甲状旁腺细胞膜含有一种钾通道，该通道通过间接机制被升高的 Ca2+o 激活（即，尽管在被研究通道外侧的移液管溶液中 Ca2+ 浓度较低，但将整个细胞暴露于高 Ca2+o 可激活该通道）。这种钾通道的单位电导为 191 pS，对 K+ 具有高度选择性，类似于先前在许多其他细胞类型中定义的所谓大电导型 Ca(2+) 激活钾通道。与后者通道一样，当用含有 0.5 μM Ca2+ 的缓冲液替换膜细胞质面含 EGTA 的缓冲液时，甲状旁腺细胞分离膜片中该通道的活性显著增加。膜内侧 Ca2+ 的变化也使通道半最大激活所需的电压水平从 0.1 μM Ca2+ 时的 103 mV 分别变为 0.25 和 0.5 μM Ca2+ 时的 79 mV 和 54 mV。当对暴露于 0.5、1.5 或 2.0 mM Ca2+o 的甲状旁腺细胞进行类似的细胞贴附膜片钳研究时，半最大激活值分别约为 105、56 和 29 mV。后一结果表明，在完整的甲状旁腺细胞中，在这三种细胞外 Ca2+ 浓度下，该通道暴露于约 0.15 - 0.2、0.4 和 0.6 - 0.7 μM 的 Ca2+i 浓度，这些值与先前使用 Ca(2+) 敏感荧光染料测量的值非常一致。因此，甲状旁腺细胞表达一种大电导型 Ca(2+) 激活钾通道，该通道由 Ca2+o 间接调节，推测是通过 Ca2+i 的伴随变化。后者可能会限制这种细胞类型中因 Ca2+o 升高而产生的细胞去极化程度。
