Holgado A, Beaugé L
División de Biofisica, Instituto de Investigación Médica Mercedes y Martin Ferreyra, Córdoba, Argentina.
Glia. 1995 Jun;14(2):77-86. doi: 10.1002/glia.440140202.
Cultured rat glial cells display a Na(+)-Ca2+ exchange system located at the plasma membrane levels. This was evidenced by the Na+ (i)-dependency of a Na+ (o)-inhibitable influx of Ca2+, or reversal exchange mode. This antiporter has an external site where monovalent cations (K+, Li+, and Na+ were investigated) stimulate the exchange by a chemical action. The monovalent cation is not transported during the exchange cycle. The mechanism of that stimulation agrees with an increase in the apparent affinity of the carrier for Ca2+(o) without effect on the maximal translocation rate. Two models can equally well account for the data: i) the formation of ECa(o) is essential for the binding of the monovalent cation, or ii) the activating cation can bind even when the carrier is free of Ca2+(o). The cations K+ and Li+ produced only stimulation, although that of K+ seem to require actions other than the chemical effect. The response to Na+ was biphasic; this can be fully explained considering that at low concentrations, Na+(o) binds preferentially to the activating monovalent site while at high concentrations it displaces Ca2+ from its external transporting site. Pure type I astrocytes displayed the same Na(+)-Ca2+ exchange mechanism.
培养的大鼠神经胶质细胞在质膜水平表现出一种钠钙交换系统。这通过钠（o）抑制的钙流入对钠（i）的依赖性得以证明，即反向交换模式。这种反向转运体有一个外部位点，单价阳离子（研究了钾离子、锂离子和钠离子）通过化学作用刺激交换。在交换循环中单价阳离子不被转运。这种刺激机制与载体对钙（o）的表观亲和力增加一致，而对最大转运速率没有影响。有两种模型同样能很好地解释这些数据：i）钙（o）的形成对于单价阳离子的结合至关重要，或者ii）即使载体没有钙（o），激活阳离子也能结合。钾离子和锂离子仅产生刺激作用，尽管钾离子的刺激似乎需要化学作用以外的其他作用。对钠离子的反应是双相的；考虑到在低浓度时，钠（o）优先结合到激活单价位点，而在高浓度时它从其外部转运位点取代钙离子，这一点可以得到充分解释。纯I型星形胶质细胞表现出相同的钠钙交换机制。
