DiPolo R, Beaugé L
Instituto Venezolano de Investigaciones Científicas, Centro de Biofísica y Bioquímica, Caracas, Venezuela.
J Gen Physiol. 1987 Oct;90(4):505-25. doi: 10.1085/jgp.90.4.505.
We have used dialyzed squid axons to characterize the ouabain- and bumetanide-insensitive Na efflux components and their relation to the operation of the Na/Ca exchange mechanism. In axons dialyzed with solutions containing nearly physiological concentrations of K, Na, and Mg, three components of the Na efflux can be distinguished: Cai-activated, Cao-dependent Na efflux ("reverse" Na/Ca exchange); Cai-activated, Nao-dependent Na efflux; and Cai-independent, ATP-activated, Nao-dependent Na efflux. We have studied the effects of internal alkalinization, Mgi, Cao, and the ATP analogue [gamma-thio]ATP (ATP gamma S) on the different components of the Na efflux. The results show the following: (a) internal alkalinization activates both Cao- and Nao-dependent Na efflux components provided that Cai is present; (b) Mgi inhibits both the Cai-activated, Cao- and Nao-dependent Na efflux components; (c) Cao inhibits the Nao-dependent component by competition for a common site; (d) ATP gamma S activates both Nao- and Cao-dependent Na efflux components only in the presence of Cai; and (e) ATP activates the Nai/Nao and Nai/Cao exchanges, causing a 10-fold increase in the affinity of the reverse Na/Ca exchange toward Cai. In the absence of Cai, ATP stimulates an Nao-dependent Na efflux that is not affected either by internal alkalinization or high Cao. The ATP analogue does not activate the Cai-independent Na/Na exchange system. These experiments demonstrate that the Cai-activated Na/Na exchange is a mode of operation of the Na/Ca exchange mechanism that substantially contributes to Na movement during the activation of the Na/Ca antiporter. The experimental evidence obtained on the Cai-independent Na/Na exchange component shows that this system is not part of the Na/Ca exchange.
我们使用透析后的枪乌贼轴突来表征哇巴因和布美他尼不敏感的钠外流成分及其与钠/钙交换机制运作的关系。在用含有接近生理浓度的钾、钠和镁的溶液透析的轴突中，可以区分出钠外流的三个成分：钙激活、钙依赖的钠外流（“反向”钠/钙交换）；钙激活、钠依赖的钠外流；以及钙不依赖、ATP激活、钠依赖的钠外流。我们研究了内部碱化、镁离子、钙离子和ATP类似物[γ-硫代]ATP（ATPγS）对钠外流不同成分的影响。结果表明：（a）只要存在钙离子，内部碱化就会激活钙依赖和钠依赖的钠外流成分；（b）镁离子抑制钙激活、钙依赖和钠依赖的钠外流成分；（c）钙离子通过竞争共同位点抑制钠依赖成分；（d）ATPγS仅在存在钙离子的情况下激活钠依赖和钙依赖的钠外流成分；（e）ATP激活钠内/钠外和钠内/钙外交换，使反向钠/钙交换对钙离子的亲和力增加10倍。在没有钙离子的情况下，ATP刺激一种钠依赖的钠外流，该外流不受内部碱化或高钙离子的影响。ATP类似物不激活钙不依赖的钠/钠交换系统。这些实验表明，钙激活的钠/钠交换是钠/钙交换机制的一种运作模式，在钠/钙反向转运体激活期间对钠的移动有很大贡献。关于钙不依赖的钠/钠交换成分获得的实验证据表明，该系统不是钠/钙交换的一部分。
