Mullins L J
J Gen Physiol. 1977 Dec;70(6):681-95. doi: 10.1085/jgp.70.6.681.
A model is developed which requires the binding of 4 Na+ to a carrier before a Ca binding site is induced on the opposite side of the membrane. Upon binding Ca, this carrier translocates Na and Ca. The existence of partially Na-loaded but nonmobile forms for the carrier (NaX, Na2X, Na3X) suffices to explain both the activating and the inhibitory effects of Na on the Ca transport reaction. Analytical expressions for Ca efflux and influx in terms of [Na]o, [Na]i, [Ca]o, [Ca]i, and Em are developed for the Na/Ca exchange system at equilibrium; these provide for a quantitative description of Ca fluxes. Under nonequilibrium conditions, appropriate modifications of the flux equations can be developed. These show a dependence of Ca efflux on [Ca]o and of Ca influx on [Ca]i. The large effect of internal ATP on Ca efflux and influx in squid axons, with no change in net Ca flux, can be understood on the single assumption that ATP changes the affinity of the carrier for Na at both faces of the membrane without providing an energy input to the transport reaction.
建立了一个模型，该模型要求在膜的另一侧诱导出钙结合位点之前，有4个Na⁺与载体结合。结合钙后，该载体转运Na⁺和Ca²⁺。载体存在部分负载Na⁺但不可移动的形式（NaX、Na₂X、Na₃X），这足以解释Na⁺对钙转运反应的激活和抑制作用。针对处于平衡状态的Na⁺/Ca²⁺交换系统，根据细胞外[Na⁺]、细胞内[Na⁺]、细胞外[Ca²⁺]、细胞内[Ca²⁺]和膜电位Em推导了钙外流和内流的解析表达式；这些表达式对钙通量进行了定量描述。在非平衡条件下，可以对通量方程进行适当修改。这些修改表明钙外流依赖于细胞外[Ca²⁺]，钙内流依赖于细胞内[Ca²⁺]。仅需假设ATP改变了载体在膜两侧对Na⁺的亲和力，而不向转运反应提供能量输入，就可以理解细胞内ATP对鱿鱼轴突中钙外流和内流有很大影响，而净钙通量不变这一现象。
